NANOPARTICLES IN ZEOLITE SYNTHESIS - Pure · The structure commission of the International Zeolite...
Transcript of NANOPARTICLES IN ZEOLITE SYNTHESIS - Pure · The structure commission of the International Zeolite...
Nanoparticles in zeolite synthesis
Citation for published version (APA):Houssin, C. J. Y. (2003). Nanoparticles in zeolite synthesis. Eindhoven: Technische Universiteit Eindhoven.https://doi.org/10.6100/IR562434
DOI:10.6100/IR562434
Document status and date:Published: 01/01/2003
Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publication
General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverne
Take down policyIf you believe that this document breaches copyright please contact us at:[email protected] details and we will investigate your claim.
Download date: 29. Jun. 2020
NANOPARTICLES IN ZEOLITE SYNTHESIS
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de
Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen op dinsdag 18 maart 2003 om 16.00 uur
door
Christophe Jean-Marie Yves Houssin
geboren te Villedieu-les-Poêles, Frankrijk
Dit proefschrift is goedgekeurd door de promotoren: prof.dr. R.A. van Santen en prof.dr. J.A. Martens Copromotor: dr. B.L. Mojet CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Houssin, Christophe J.Y. Nanoparticles in zeolite synthesis / by Christophe J.Y. Houssin. – Eindhoven : Technische Universiteit Eindhoven, 2003. Proefschrift. – ISBN 90–386–2874–9 NUR 913 Trefwoorden: poreuze materialen ; synthese / zeolieten ; ZSM-5 / nanostructuren / silicalieten / zelforganisatie / kristallisatie / röntgenverstrooiing ; SAXS / kernspinresonantie ; NMR Subject headings: porous materials ; synthesis / zeolites ; ZSM-5 / nanoparticles / silicalites / self-assembly / crystallization / X-ray scattering ; SAXS / nuclear magnetic resonance ; NMR © 2003 by Christophe J.Y. Houssin Printed by Universiteitsdrukkerij Technische Universiteit Eindhoven. The work described in this thesis has been carried out at the Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, The Netherlands. Financial support was provided by NRSC-Catalysis, NWO and the Spinoza fund.
A mes parents,
Contents Chapter 1 Introduction to zeolites and the scope of this thesis 1 Chapter 2 Zeolite nanoslabs: a combined SAXS and TEM study 17 Chapter 3 A 29Si and 27Al NMR study of MFI precursors 31
Chapter 4 In situ SAXS/USAXS investigation on aluminum incorporation 59
in the synthesis of colloidal TPA-ZSM-5 Chapter 5 Zeolite nanoslabs: building blocks for innovative porous materials 85 Summary 97 Samenvatting 99 Résumé 101 Acknowledgments 103 Curriculum Vitae 105
1
1
Introduction to zeolites
and the scope of this thesis
1.1 Background
Despite being discovered as “boiling stones” more than 250 years ago1, zeolites have
received considerable attention only in the last past decades and have today turned into
essential commercial materials.2,3 This is due to their exceptional ability to adsorb large
amounts of water and other molecules in their micropores.
Zeolites are crystalline aluminosilicates with a 3-dimensional, open anion framework
consisting of oxygen-sharing TO4 tetrahedra, where T is Si or Al. Their framework structure
contains interconnected voids that are filled with adsorbed molecules or cations. Zeolite
micropore channels have very well-defined diameters so that bulky molecules will be
excluded from the internal surface. The general empirical formula is:
Mx/m . AlxSi2-xO4 . n H2O
where m is the valence of cations M, n the water content and 0 ≤ x ≤ 1. The flexibility of the
zeolite Si-O-Si bond explains the fact that more than 200 structures have been determined.
Indeed, there is little energetic difference (10-12 kJ/mol) between these remarkable porous
silicates and higher density phases such as quartz. Several properties account for their
2
commercial use: they are strong adsorbents, they show a very high selectivity and they are
excellent solid acid catalysts. Today, synthetic zeolites are employed in a wide range of
industries.4 For example, they are used in the separation of gaseous or liquid compounds.
Detergents represent the highest market by volume because of their high ion-exchange
capacities. However, the high-added value commercial applications of zeolites are
encountered in the petroleum industry. Fluid catalytic cracking (FCC), main process in the
transformation of crude oil into gasoline, mainly uses zeolite Y as catalyst.
1.2 Structure
The fascinating properties of zeolitic materials essentially originate from their
structures. The lack of proper identification techniques hindered longtime the determination
of structures, explaining the slow progress made in the century following their discovery. The
invention of X-ray diffraction at the beginning of the 20th century to probe the structural
properties of materials initiated systematic studies on zeolite identification. A zeolite
topology concept was introduced. It corresponds to the connectivity of the tetrahedra of the
framework through line segments and nodes and represents the highest possible symmetry.
The structure commission of the International Zeolite Association (IZA) provides up to date
classification by framework type. As of November 2002, 136 framework types have been
accepted by this commission and are available on the internet site of the IZA5 or in the Atlas
of Zeolite Framework Types.6
The large majority of zeolite structures are constructed by repeating so-called
secondary building units (SBUs). There are presently 19 SBUs.6 Another way to classify
zeolites is to take into account their pore openings and the dimensionality of their channels.
Thus, one distinguishes small pore zeolites (eight-membered-ring pores), medium pore
zeolites formed by ten-membered rings and large pore zeolites with twelve-membered-ring
pores. Recently an extra-large pore zeolite category has been added.7,8 This classification
simplifies comparisons in terms of adsorptive, molecular sieving and catalytic properties.
Two important and industrially relevant structures are depicted in figure 1.
3
Figure 1. Two examples of zeolite framework and pore system: MFI and MOR topologies.
1.3 Zeolites and Catalysis
Background
When an aluminum atom substitutes a silicon atom, the +III valence of aluminum
introduces a net negative charge in the framework. Cations are then required to preserve
neutrality. Not only this presence of cations allows zeolite crystals to be used in ion-exchange
processes but it creates an acid site if protons act as counterions. However, it would be too
simple to represent acid sites in zeolites by a simple proton like classical homogeneous acids.
Figure 2 shows a typical zeolite acid site. It consists of a hydroxyl group bridging a silicon
atom and an aluminum atom corresponding to a strong Brönsted site and oxobridges
exhibiting Lewis base properties. The acidity of zeolites is very strong, about 1000 stronger
than that of amorphous aluminosilicates. In catalytic applications, high-silica zeolites are
preferred because of the thermal stability of their framework (crucial for regeneration cycles)
4
and high dispersion of acid sites. Moreover, low aluminum content ensures high acidity for
each proton.
Al
H
Figure 2: Active site in zeolites.
Apart from these acidic properties, zeolites are shape selective regarding molecular
adsorption.4 This is due to their pore system that can be one, two or three-dimensional and
contain pores of different sizes which are in the order of molecular dimensions (from about
0.3 to 1.2 nm). The void dimensionality and very high internal surface area (>500 m2/g) are
responsible for catalytic shape selectivity of zeolites. Derouane et al. proposed in the 1980s
the so-called confinement effect in order to describe interactions of adsorbed molecules
within the curved surface of channels and cages of zeolites.9,10 It is obvious that sorbate-
framework interactions and the local framework topology around the active site largely
influence the reactivity of molecules.
Shape selectivity
Shape selectivity was first observed by Weisz and Frilette in 1960.11 Since then this
phenomenon has been thoroughly documented in literature.12,13,14,15,16 Reactions within
zeolites can be inhibited if there is no matching between certain molecules and a sterically
confined environment allowing conversion of reactants. The unique one, two or three-
dimensional pore system of zeolites enables shape selective catalysis. There is a consensus on
the different mechanisms of molecular shape selectivity in zeolite technology:
Reactant selectivity: This occurs when some molecules preferentially enter the zeolite
pore mouth whereas others are rejected because they are too large with respect to the
pore openings. Once a reactant has adsorbed in the zeolite channels, it must diffuse
towards active sites where reactions can occur. This is where the two following
selectivity phenomena can occur.
5
Product selectivity: This occurs when some reaction products or intermediates formed
within the pores are too bulky to diffuse out. They are either converted to smaller molecules
or deactivate the catalyst by blocking of the pores. After reaction has occurred, products must
diffuse away from the micropores which results in a kind of molecular traffic control.
Transition state selectivity: The distinction between transition state selectivity and
product selectivity is not always obvious. It takes place when the transition state cannot be
accommodated in the space available in the intra crystalline volume. A way to differentiate
product and transition state selectivity is to vary the crystal size because only product
selectivity depends on crystal size, whereas transition state shape selectivity does not.
region forshape-selective catalysis
layer for pore mouthcatalysis
dd
cb bbaa
example: ZSM-22example: ZSM-5example: Erionite
tubular pore structureintersecting tube structurecage and narrowwindow structure
interrupted channelinterrupted intersectionhalf cavity with large aperture
Figure 3: Shape-selective environments in different structure types: (a) large molecules have access
to the interrupted cavities and channel intersections of the layer for pore mouth catalysis; (b)
molecules are plugged into the pore apertures; (c) molecules are converted in multiple pore mouths
according to key-lock catalysis; (d) molecules are converted in the intracrystalline shape-selective
environment (after ref. 18).
Martens et al. prompted the existence of pore mouth and key-lock catalysis which
consists of specific adsorption in the pore mouth at the crystal boundaries.17,18 This explains
why long n-alkanes isomerize selectively on a 10-ring bifunctional zeolites (Pt/H-ZSM-22).
This phenomenon differs from the former selectivities discussed above in that reactants do
not undergo bulk adsorption but pore mouth catalysis. Shape selectivity in pore mouth and
key-lock catalysis is illustrated in figure 3.
6
Industrial applications
Since their successful introduction as commercial molecular sieves in 1954, synthetic
zeolites have grown to an estimate $1.6-1.7 billion industry.19 Detergents represent the largest
volume. LTA-type zeolites substitute phosphate compounds in the water softening process in
laundry. The largest market value for zeolites is in refinery catalysis. FCC (Fluid Catalytic
Cracking) catalysts account for more than 95% of zeolite catalyst consumption and consist of
various forms of zeolite Y. MFI-type zeolites are the second most used catalyst, primarily
because they are added to FCC catalysts for octane number enhancement. Zeolites are also
employed in the drying and purification of natural gas, separation of paraffins and
desulfurization processes. Despite being in a relatively early state of development, zeolites
are also used in fine chemicals production such as oxidation and acylation. The main
applications of zeolites are summarized in Table 1.
Process catalyst products
Catalytic cracking Re-Y, US-Y
ZSM-5
Gasoline, fuels
Hydrocracking Y, Mordenite
+ Mo, W, Ni
Kerosene, diesel, Benzene
Alkylation of aromatics ZSM-5, Mordenite p-xylene, ethyl-benzene, styrene
Hydroisomerization Mordenite + Pt, Pd i-pentane, i-hexane
Xylene isomerization ZSM-5 p-xylene
Catalytic dewaxing Mordenite, ZSM-5
+ Ni, noble metals
Improvement of cold flow
properties
Transalkylation Mordenite Xylenes, cumene
MTBE ZSM-5 Aromatics, paraffins
MTG Ga-ZSM-5 Aromatics
Table 1: Main commercial applications involving zeolites.
Zeolite science appears to be a mature science and is still a very dynamic field.
Discoveries of new zeolites continuously open new areas of development. New trends at the
beginning of this century include environmental applications such as De-NOx catalysis and
hydrocarbon storage in vehicles powered with diesel or gasoline engines, and
7
biopharmaceutical applications. Zeolites can also be used in the nuclear industry for
radioactive waste storage. Applications of zeolite material science still play an important role
in many areas of technology.
1.4 Synthesis
Background
Natural zeolites are found in volcanic or metamorphic rocks and their growth involves
geological conditions (low temperature and pressure, low pH (8-9)) and time scale (thousands
of years). Early efforts have been made by Saint Claire de Ville in 1862 to synthesize
zeolites.20 The absence of reliable characterization methods made it impossible to verify that
zeolites were indeed fabricated. The first precise confirmation of zeolite synthesis can be
traced in 1948 when Barrer reported the synthesis of an analogue of mordenite.21 At the same
time Milton and Beck succeeded in synthesizing other zeolite types using lower temperatures
(≈100 °C) and a higher alkalinity.22,23 It led to the discovery of one of the most commercially
successful zeolites which has no natural counterpart, Linde A (LTA). Since then many new
zeolite framework types have been attained thanks to important efforts by oil companies. In
the early 1960s Barrer and Denny were the first to replace inorganic bases in the synthesis
mixture with organic molecules.24 The use of quaternary ammonium salts resulted in an
increase in the Si/Al ratio and the discovery of ZSM-5, being the most important new
structure.25 The quest for higher Si/Al ratios ended in 1978 when Flanigen et al. reported the
synthesis of silicalite-126 which is the all-silica counterpart of ZSM-5. This material shows
remarkable properties because of its hydrophobic and organophilic character. A new class of
materials analogous to zeolites was introduced in the 1980s: microporous
aluminophosphates.27,28 Nevertheless, poor thermal and hydrothermal stability of their metal
substituted analogues hindered their commercial applications. The most noteworthy advance
in crystalline microporous solids has recently been the synthesis of extra large pore zeolites
with more than 12-ring apertures.7,8,29,30
Zeolite synthesis has been extensively reviewed in several books and literature on this
subject is abundant.31,32,33,34 The synthesis of zeolites is carried out under hydrothermal
conditions. An aluminate solution and a silicate solution are mixed together in an alkaline
medium to form a milky gel or in some instances, clear solutions. Various cations or anions
8
can be added to the synthesis mixture. Synthesis proceeds at elevated temperatures (60-200
°C) where crystals form through a nucleation step. The following sections give a general
overview on the parameters governing zeolite synthesis. Emphasis will be given to structure
direction by organic molecules. A schematic representation of zeolite formation process is
given in figure 4.
Basic reactants
Si, AlSolventMineralizer
SDA
Amorphous gelSmall oligomers
precursors
nucleation
Zeolite crystals
crystalgrowth
Basic reactants
Si, AlSolventMineralizer
SDA
Amorphous gelSmall oligomers
precursors
nucleation
Zeolite crystals
crystalgrowth
Figure 4: Simplified zeolite synthesis scheme. SDA stands for structure-directing agent.
a) Molar composition
Although this is not an independent parameter, every zeolite has a specific molar
composition range often represented graphically in a ternary compositional phase diagram
(Na2O, Al2O3 and SiO2). On the other hand, each structure will also impose constraints on the
amount of Al it can incorporate. High-silica molecular sieves such as ZSM-5 can be
synthesized over a wide range of Si/Al ratios (Si/Al from 7 to infinity35).
b) Mineralizer
A mineralizer is a species which enables the formation of a more stable solid phase from
a less stable solid phase via dissolution and crystallization. Supersaturation can be reached by
9
dissolution and these soluble species are then available for nucleation and crystal growth. In
most cases, hydroxyl ions act as mineralizing agents. Indeed, OH- increases the solubility of
silica by depolymerizing amorphous silica particles. Oligomeric species are then present in
solution. Condensation of specific aluminosilicate species, facilitated by the presence of OH-,
occurs and leads to the appearance of the first crystals. In general high pH values increase
crystal growth rates and shorten the nucleation period. Hydroxyl ion concentration can also
influence crystal morphology, crystal yield and final zeolite structure.
Fluoride ions have been used as mineralizers. Silicalite-1 was the first zeolite synthesized
from acidic F- medium. Fluoride anions act similarly to hydroxyl ions without contributing
directly to the pH of the system. Nucleation and crystal growth rates are generally slowed
down resulting in large and high quality crystals. The fluoride ion synthesis route has mostly
been applied in the area of aluminophosphates mainly because it has led to the discovery of
novel aluminophosphates and isomorphously substituted versions that cannot be obtained at
high pH.
c) Inorganic cations
Inorganic cations have been regarded as an important parameter influencing the structure
formed. They are involved in structure direction, solid yield, crystal morphology and purity.
Most of the synthetic analogues of natural zeolites were obtained using alkali and alkaline
earth metal cations. Nucleation and crystal growth can be optimized by the right choice of
inorganic cations.
d) Temperature
Temperature can alter the zeolite structure as well as the induction period and crystal
growth kinetics. The activation energies of zeolite synthesis are quite significant.
e) Silica and alumina sources
Nucleation and growth kinetics can depend on the dissolution of the solid reagents and
formation of aluminosilicates precursors. Kühl found that crystallization of some structures
was dependent on the degree of prepolymerization of the silica source.36 Mintova and
Valtchev recently investigated the colloidal distribution of silicalite-1 synthesis mixtures
containing different silica sources.37 Impurities in silica or alumina sources are likely to
influence crystallization kinetics and framework composition.
10
Structure direction in zeolite synthesis
Structure direction occurs when inorganic or organic molecules are used to direct the
crystallization towards a specific zeolite structure. Structure-directing agents, currently called
templates, are generally: (1) charged molecules which are mostly cations. Inorganic cations
such as Na+, K+, Li+, Ca2+ are frequently used. Organic molecules that are used are usually
tetraalkylammonium, dialkyl and trialkyl amines or phosphonium compounds. (2) neutral
molecules. Water actually plays an important role in the structure direction encountered in
zeolite synthesis. Water molecules act as void fillers in order to stabilize the porous oxide
framework. Interactions of water molecules with cations are part of the template effect and
therefore are of crucial importance. Other molecules include amines, ethers or alcohols. (3)
ionic pairs: Salts (NaCl, KCl, KBr) are occluded into the zeolite framework as guest
molecules, stabilizing the zeolite framework.
The structure-directing role of templates in zeolites can be understood from the
analogy with the formation of clathrasils.38 Guest molecules with size and shape close to
those of the cages were found to accelerate the crystallization of the same product. The
structure of clathrasils is rather independent of the chemical nature of guest molecules.
Structure-direction agents in zeolite synthesis are mostly investigated in high-silica
zeolites.39 In those systems, there is indeed a limited number of variables since low
concentration of alkali metal ions and low Si/Al ratios are used. Moreover, the porous silicon
dioxide framework is mainly uncharged. Defect sites are required to balance the charge of the
cationic structure-directing agent. Interactions between organic molecules and silica are
mostly the Van der Waals forces.
ZSM-5 is one the most important commercial zeolites and has been widely studied.33
This is a very versatile zeolite since it can be made from various organic or inorganic agents
over a wide range of synthesis conditions. Tetrapropylammonium (TPA) has been regarded
as the most efficient template in high-silica ZSM-5 synthesis, thereby appearing as a true
example of structure direction. It indeed enhances nucleation rate and accelerates
crystallization.40 Moreover TPA molecules are located at the channel intersections with their
propyl arms extending into the linear and zig-zag channels. They are tightly encapsulated so
that calcination is required to obtain their removal. This tight entrapment suggests that TPA
molecules are actively involved in the nucleation period and crystal growth. Burkett and
Davis, using 1H-29Si CP, investigated relationships between TPA and silicate species.41,42,43
11
Evidences of close interactions between TPA cations and silicate species have been observed
well before the formation of long-range crystalline structure. They proposed that the key
steps for structure direction were the formation of inorganic-organic composite entities which
may be the precursors of units participating in nucleation and crystal growth. No such
interactions were observed when a molecule lacking of structure-directing properties (i.e.
TMA) was used, suggesting that these intermolecular contacts are specific. At the same time,
Dokter et al. have observed primary silica particles in the range of 1-20 nm by means of
small-angle X-ray/neutron scattering (SAXS/SANS).44 They suggested that these colloidal
particles underwent several aggregation and densification steps leading to the formation of
the first crystals.
Watson et al. have also identified well-defined nanoprecursors in silicalite-1 starting
mixtures.45 Particles with a radius of gyration of 2.8 nm were detected and a cylindrical
model provided the best fit to experimental scattering curves.46 Most importantly, SANS
contrast variation experiments showed that these nanoparticles have a scattering density
almost identical to that of TPA-containing silicalite-1 crystals, suggesting that TPA cations
are occluded in the nanoprecursors.
Hydrophobic hydration
Overlap of hydrophobic
hydration spheres Primary units
2.8 nm
Aggregation
Crystal growth
Nucleation
≈1 nm
5-10 nm
5-10 nm10 nm - microns
N+
HH
H
H HH
HH
H
HH
OO
O
O
HHO
O
Si
OO
O
SiOO
O
Si
Si
Si
SiSi
SiH
H H
HH
H
O
OO
Water solvent
TPA
Hydrophobic hydration
Overlap of hydrophobic
hydration spheres Primary units
2.8 nm
Aggregation
Crystal growth
Nucleation
≈1 nm
5-10 nm
5-10 nm10 nm - microns
N+
HH
H
H HH
HH
H
HH
OO
O
O
HHO
O
Si
OO
O
SiOO
O
Si
Si
Si
SiSi
SiH
H H
HH
H
O
OO
N+
HH
H
H HH
HH
H
HH
OO
O
O
HHO
O
Si
OO
O
SiOO
O
Si
Si
Si
SiSi
SiH
H H
HH
H
O
OO
Water solvent
TPA
Figure 5. Scheme for the crystallization mechanism of Si-TPA-MFI (after ref. 48).
12
De Moor et al., using a combination of in situ SAXS, USAXS and WAXS (ultra-
small and wide-angle X-ray scattering), recently found three particle populations during
TPA-mediated silicalite-1 crystallization. Primary units with a size of 2.8 nm, their
aggregates (≈10 nm) and the growing crystals were identified and their consumption
monitored through the whole course of crystallization.47,48 By varying the alkalinity, it was
found that 2.8 nm particles were always present unlike their aggregates of which the
formation depends on the Si/OH ratio.49 However, the formation of the aggregates appeared
to be an essential step in the nucleation process since it enhances the nucleation rate. It was
suggested that crystal growth probably occurs via the addition of the 2.8 nm primary units to
the growing crystals. Moreover, the same size for primary units were observed using three
other structure-directing agents (a dimer of TPA, a trimer of TPA and trimethylene-bis(N-
hexyl, N-methyl-piperidium)).50 Nanoscale precursors were also found in the crystallization
of other zeolites such as Si-BEA, Si-MTW from gelating systems.51 A mechanism was then
proposed for organic-mediated zeolite synthesis and is depicted in figure 5.
In a series of recent papers,52,53,54,55 Kirschhock et al. described the molecular picture
of the formation of silicalite-1 from TEOS (tetraethylorthosilicate) and an aqueous TPAOH
solution. The first step is the formation of a tetracyclic undecamer at the interface of TEOS
and the aqueous TPAOH solution. An aggregation mechanism then occurs in which three
tetracyclic undecamers form larger units denoted “precursor units”. This particular oligomer
exhibits a characteristic length of 1.35 nm in SAXS. Upon addition of water, stable and well-
defined subcolloidal particles (3.6 nm as characteristic length) were observed. A model
consisting of twelve “precursor units” coupled three by four, having dimensions of 4 × 4 ×
1.3 nm and already containing the MFI topology was proposed. Heating this suspension of
MFI nanoslabs led to the formation of intermediates and eventually to silicalite-1 colloidal
crystals via an aggregation mechanism.55
1.5 Scope of the thesis
The ultimate goal in fundamental research in zeolite synthesis is to arrive at tailor-
made zeolite catalysts with suited pore size, dimensionality and chemical composition for a
given process or reaction. Zeolite synthesis has been mainly an empirical field in which a
large number of experiments were based on the variation of the basic parameters. Even
13
though high-throughput synthesis methods are now being developed56,57,58, the understanding
of the concept of structure direction in microporous materials remains of fundamental
importance for the design of new molecular sieves.39
Over the past decade, new insights into this problem have been brought in.
Experimentally, the use of in situ techniques applied during zeolite synthesis has been
developed. Recent reports agree that subcolloidal particles play an important role during the
nucleation and crystallization of zeolites. The driving force that has prompted this thesis is to
investigate the formation, the nature, the role and the transformations of nanometer-scale
embryonic species during the crystallization of silicalite-1 and ZSM-5. Particularly, this work
aimed at finding unifying concepts regarding the above-mentioned studies using silicic acid
and TEOS as silica sources. TEM, in situ (U)SAXS and NMR were mainly employed to
achieve this goal. This research project involved a cooperation with the group of Prof. Johan
Martens from the Center for Surface Chemistry and Catalysis, KU Leuven, Belgium.
The occurrence of silicalite-1 precursors in clear solutions is investigated in chapter 2.
Special attention has been given to the size of nanoprecursors encountered in several
representative syntheses. The techniques involved in this investigation comprise small-angle
X-ray scattering and transmission electron microscopy.
Chapter 3 is mainly devoted to an extensive NMR study of MFI precursors. 29Si
NMR, SAXS and USAXS were combined to study in situ transformations occurring during
the depolymerization-oligomerization of silicic acid in an aqueous concentrated
tetrapropylammonium hydroxide solution. The results are discussed in terms of intermediate
silicates formed in the oligomerization process. In a second part, specific NMR techniques
were applied to study aluminum incorporation in both the precursors and the final ZSM-5
crystals. The determination of distances between structure-directing agents and silica at early
stages of the synthesis was done with NMR as well.
An introduction in the theory of SAXS and the combined SAXS/WAXS and Bonse-
Hart setup of the ID02 beamline at ESRF is presented in chapter 4. The combination of in situ
SAXS/USAXS allowed us to study the influence of Al framework substitution during the
whole course of ZSM-5 crystallization, from the formation of specific precursors to the
crystal growth.
The possible use of MFI precursors in material synthesis is illustrated in chapter 5.
The method proposed consists of a hierarchical templating scheme in which zeolite building
units assemble into mesostructures via a secondary cooperative templating mechanism.
14
References 1 Cronstedt, A. F. translated by: Schlenker, J. L.; Kühl, G. H., In: Proceedings of the 9th International Zeolite Conference, Montreal 1992, Ed. Von Ballmoos, R.; Higgens, J. B.; Treacy, M. M. J. Butterworth-Heinemann, 1993, 3. 2 Tanabe, K.; Hölderich, W. F. Appl. Catal. 1999, 181, 399. 3 Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, New York, 1997. 4 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 5 http://www.iza-online.org 6 Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework type, Fifth Revised Edition, Elsevier, 2001. 7 Freyhardt, C. C.; Tsapatsis, M.; Lobo, R. F.; Balkus Jr, K. J.; Davis, M. E. Nature, 1996, 331, 295. 8 Yoshikawa, M.; Wagner, P.; Lovallo, M.; Tsuji, K.; Takewaki, T.; Chen, C-Y.; Beck, L. W.; Jones, C.; Tsapatsis, M.; Zones, S. I.; Davis M. E. J. Phys. Chem. B 1998, 102, 7139. 9 Derouane, E. G., Nagy, J. B. Chem. Phys. Letters 1987, 137, 341. 10 Derouane, E. G.; Lucas, A. A.; André, J. M. Chem. Phys. Letters 1987, 137 336. 11 Weisz, P. B.; Frilette, V. J.; Maatman, R. W.; Mower, E. B. J. Phys. Chem. 1960, 64, 382. 12 Csicsery, S. M. ACS Monograph, 1976, 171, 680. 13 Weisz, P. B. Pure Appl. Chem. 1980, 52, 2091. 14 Csicsery, S. M. Zeolites, 1984, 4, 202. 15 Dwyer, J. Chem. Ind. 1984, 7, 229. 16 Weitkamp, J.; Ernst, S. Catal. Today, 1994, 19, 107. 17 Martens, J. A.; Souverijns, W.; Verrelst, W.; Parton, R.; Froment, G.; Jacobs, P. A. Angew. Chem. Int. Ed. 1995, 35, 2528. 18 Goossens, A. M.; Vanbutsele, G.; Martens, J. A. In Fundamentals of Adsorption 6, F. Meunier, Elsevier, Paris, 1998, 31. 19 Smart, M.; Esker, T.; Leder, A.; Sakota, K. Chemical Economics Handbook, SRI International, 1999, 599. 20 Sainte-Claire-Deville, M. H. Compt. Rend. 1862, 54, 324. 21 Barrer, R. M. J. Chem. Soc. 1948, 2158. 22 Milton, R. M. US Patent 2,882,243, 1959. 23 Milton, R. M. US Patent 3,008,803, 1961. 24 Barrer, R. M.; Denny, P. J. J. Chem. Soc. 1961, 971-982. 25 Argauer, R. J.; Landolt, G. R. US Patent 3,702,886 1972. 26 Grose, R. W.; Flanigen, E. M. US Patent 4,061,724 1977. 27 Wilson, S. T.; Lok, B. M.; Flanigen, E. M. US Patent 4,310,440 1982. 28 Wilson, S. T.; Lok, B. M.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. 29 Zhou, Y.; Zhu, H.; Chen, Z.; Chen, M.; Xu, Y.; Zhang, H.; Zhao, D. Angew. Chem. Int. Ed. 2001, 40, 2166. 30 Lin, C. H.; Wang, S. L.; Lii, K. H. J. Am. Chem. Soc. 2001, 123, 4649. 31 Breck, D. W. Zeolite molecular sieves, John Wiley, New York, 1974. 32 Barrer, R. M. Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 33 Jacobs, P. A.; Martens, J. A. Synthesis of high-silica aluminosilicate zeolites, Studies in Surface Science and Catalysis Series; Elsevier Science, New York, 1987, Vol. 33. 34 Szoztak, R. Molecular sieves, Blackie Academic & Professional, 1998.
15
35 Verduijn, J. P.; Martens, L. R. M.; Martens, J. A. US Patent 5,783,321 1995. 36 Kühl, G. H. In 2nd International Conference on Molecular sieve Zeolite, American Chemical Society, Washington, DC, 59, 1970. 37 Mintova, S., Valtchev, V. Microp. Mesop. Mat. 2002, 55, 171. 38 Gies, H.; Marler, B. Zeolites 1992, 12, 42. 39 Lobo, R. F.; Zones, S. I.; Davis, M. E. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 47. 40 Van Santen, R. A.; Keijsper, J.; Ooms, G.; Kortbeek, A. G. T. G. In New developments in Zeolite Science and Technology, Elsevier, Amsterdam, 169, 1986. 41 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4647. 42 Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920. 43 Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 1453. 44 Dokter, W. H.; van Garderen, H. F.; Beelen, T. P. M.; van Santen, R. A.; Bras, W. Angew. Chem. Int. Ed. Engl. 1995, 34, 73. 45 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094-10104. 46 Watson, J. N.; Brown, A. S.; Iton L. E.; White, J. W. J. Phys. Chem. B 1998, 94, 2181. 47 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 48 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 49 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 50 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 51 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11, 36-43. 52 Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P-P.; Feijen, E. J. P.; Grobet, P. J.;
Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens J. A J. Phys. Chem. B 1999, 103, 4960. 53 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. 54 Kirschhock, C. E. A.; Ravishankar, R.; van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. 55 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 56 Akporiaye, D.; Dahl, I. M.; Karlsson, A.; Wendelbo, R. Angew. Chem. Int. Ed. 1998, 37, 609. 57 Holmgren, J.; Bem, D.; Bricker, M.; Gillespie, R.; Lewis, G.; Akporiaye, D.; Dahl, I.; Karlsson, A.; Plassen, M.; Wendelbo, R. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 461. 58 Klein, J.; Lehmann, C. W.; Schmidt, H-W.; Maier, W. F. Angew. Chem. Int. Ed. 1998, 37, 3369.
16
17
2
Zeolite nanoslabs: a combined SAXS and TEM study*
The formation and growth of crystal nuclei of TPA-silicalite-1 (aluminum free ZSM-5) from clear solutions using TEOS and silicic acid as silica sources were studied with small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Information was obtained on the size and shape of nanoscopic precursor particles of silicalite-1. TEM provided a direct observation of slab-like embryonic particles. The combined SAXS and TEM data can be interpreted assuming the presence of nanoslabs of 4 × 2 × 1.3 nm up to 8 × 8 × 1.3 nm depending on the synthesis conditions. Starting solutions have been studied varying the silica source and the cation content (sodium, potassium and TPA). In each case, well-defined particle sizes are observed. Although the nanoparticles differ in size, their shape is very similar and these data strengthen our hypothesis that TPA-silicalite-1 formation is a nanoblock-based aggregation mechanism rather than growth via monomer addition.
* Reproduced in part from: Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Stud. Surf. Sci. Catal. 2001, 135, 135. Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637.
18
2.1 Introduction
Monitoring the early stages of zeolite synthesis still remains a challenge.1 It has long
been known that cationic species (Na+, Al3+ or organic molecules) directly influence the
resulting multi-dimensional crystal lattice.2,3,4, Strong indications of interactions between the
organic molecules and the silicate species in the synthesis mixtures have been observed.5
Consequently, nanoscale species are present well before the formation of long-range order.
But the nature and extent of the interactions between the organic and inorganic components
are not very well understood.6,7 Analytical techniques that can be used to follow the
crystallization include IR,8 Raman,9 DLS,10,11,12 NMR spectroscopy5,8,13 and small-angle
scattering (SAS).14,15,16 SAXS (small-angle X-ray scattering) is a powerful technique which
can provide in situ information on particle size, shape and aggregation in the size range from
1 nm to >100 nm. Moreover, silicate intermediates are so fragile that only non-invasive in
situ measurements will give valuable results. Consequently, in situ time-resolved SAXS
experiments are appropriate to probe zeolite synthesis. Nanoscopic species (3-4 nm) were
recently observed in the formation of the pure silica MFI in presence of TPA from a clear
solution.14 They have been proposed to play a key role in the nucleation and crystal growth.17
The detailed structure of these nanoparticles is beyond the scope of this chapter but particles
prepared from TEOS (tetraethylorthosilicate) and TPA (tetrapropylammonium) have been
extensively investigated by 29Si NMR.8 So far, no comparative studies were conducted on the
TEOS-based and silicic acid-based Si-TPA-silicalite-1 crystallization system. TEOS is an
organic component whereas silicic acid belongs to inorganic silica sources, sometimes used
by industry for zeolite synthesis. The presence of sodium is also important since it is believed
to enhance the dissolution of amorphous silica in alkaline solutions. In fact, sodium facilitates
the transport of hydroxyl anions towards the silica surface. Moreover TPA molecules tend to
strongly adsorb to the silica surface. In this study, the effect of those factors has been
investigated in terms of size of the particles formed in the synthesis mixtures for silicalite-1.
19
2.2 Experimental section
Details of the synthesis procedures are:
- TEOS and TPA system: The synthesis is adapted from Ravishankar et al.18 9 g of TEOS
(Acros, 98%) was added dropwise to a 40% aqueous solution of TPAOH (Alfa) under
vigorous stirring. 9 g of distilled water was then added dropwise after 30 min and the
resulting mixture was stirred continuously for 12h to ensure complete hydrolysis of the silica
source.
- Silicic acid and TPAOH system: The recipe used is based on a patent of Exxon
Chemicals.19 0.411 g of NaOH was dissolved in 15 g of 20% TPAOH in water (Merck),
followed by a spoonwise addition of 4.05 g of silicic acid (Baker, 10.2% H2O). The milky
dispersion was boiled under stirring for 10 min to obtain a clear solution. The mixture was
rapidly cooled down to room temperature in a water bath. Distilled water was added for the
correction for loss of water during boiling. The resulting clear solution was then filtered
through a 0.45 µm syringe filter. The sample was then ready for measurement and never aged
more than 1h. For some solutions NaOH was replaced by an equivalent amount of TPAOH or
KOH. In the latter cases, the dissolution of silicic acid appeared to be more difficult, but 10
min boiling ensured complete dissolution of the silica.
SAXS measurements were performed at the Dutch-Belgian beamline (DUBBLE) at
the ESRF (France) using high-brilliance synchrotron radiation. To perform in situ reactions,
rotating cells were used in which liquid samples could be heated. SAXS patterns were
recorded on the synthesis mixtures described in table 1. All samples were clear and
background solutions were measured for data correction.
Transmission electron microscopy was performed at the Materials Science
Department, Darmstadt University of Technology, Germany. The observations were made
with a Philips CM200 TEM (200 kV, point resolution 0.19 nm) and a JEOL JEM3010 (300
kV, point resolution 0.17 nm) instruments equipped with a Gatan GIF200 electron energy
loss spectrometer. Measurements were performed with all necessary precautions, using a
defocused electron beam, rather low magnification and a video camera connected to the TEM
camera to monitor any possible change in time.
20
S1 S2 S3 S4 TPAOH × × × × TEOS × Silicic acid × × × NaOH × KOH ×
Table 1. Composition of the different starting solutions.
2.3. Results and Discussion SAXS
Figure 1 shows the SAXS patterns of the solutions S1 and S3 (Figure 1a) and S2 and
S4 (Figure 1b). At room temperature several peak maxima can be observed. These starting
solutions all lead to silicalite-1 formation upon heating. They contain a well-defined particle
population with a different size depending on the composition.
The effect of the silica source on the formation of subcolloidal particles is illustrated
in figure 1a. TEOS is an organic and monomeric source of silica whereas silicic acid is
polymeric and can provide monomers, dimers and oligomers. Hydrolysis of TEOS is almost
completed after a few hours stirring at RT but boiling 10 min is needed to dissolve the silicic
acid. TPA molecules interact with silica at a liquid-liquid interface and may function as a
structure-directing agent at very early stages8 when using TEOS, but they strongly adsorb on
the solid surface in the case of silicic acid. This phenomenon influences the releasing of
silicate anions by hydroxide ions from the solid surface. Iler has proposed a mechanism for
dissolution of silica in alkaline aqueous solution in which hydroxyl ions act as catalysts.20
Figure 1a shows two different maxima corresponding to two different characteristic
lengths. Obviously, the use of silicic acid leads to smaller particles than TEOS (3.6 nm and
2.1 nm for S1 and S3 respectively). These results clearly show that two distinct but well-
defined particle populations are formed, depending on the silica source.
21
0
0.1
0.2
0.3
0.5 1.5 2.5
S1
I (a.u.)
S3
Solutions d (nm ) S1 3.6Solutions d (nm)
Figure 1a. SAXS patterns recorded at RT of twoThe scattering vector q is related to the chasubcolloidal particles by the equation: q=2π/d.
0
0.1
0.2
0.3
0.5 1.5 2.5
I (a.u.)
S2
S4
Figure 1b. SAXS patterns recorded at RT of start
Figure 1b shows that the use of sodium o
significantly the size of the particles when using
characteristic length obtained (around 3 nm) is diffe
exclusively used as cation. In fact, alkali ions adsor
have a hydration sphere smaller than TPA, facilitati
surface and giving rise to a different silica dissol
counter ions of the negatively charged surface of t
formation and stability. These two effects may exp
3.5 4.5
q (1/nm )
S3 2.2
S1 3.6 S3 2.1
different starting solutions.racteristic length d of the
3.5 4.5
q (1/nm )
Solutions d (nm) S2 2.9 S4 3
ing solutions using two different alkali ions.
r potassium cations does not influence
silicic acid as silica source. But the
rent from the one found when TPA was
b less strongly to the silica surface and
ng the transport of hydroxide ions to the
ution rate. Alkali ions may also act as
he nanoparticles, then influencing their
lain the size difference. In conclusion,
22
there is experimental evidence that the size of the nanoparticles formed during the early
stages of the silicalite-1 synthesis depends not only on the silica source but also on the type of
cations present next to TPA.
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
-0.6 -0.4 -0.2 0 0.2 0.4
S1
S2
log I (a.u.)
log q (1/nm)
slope -2.2
Figure 2. SAXS patterns after 8 h at 100°C for S1 (125°C for S2).
In an earlier work on the S1 system,8 a model for these nanoparticles was presented in
which the MFI structure is already developed. The particles are slab like with dimensions of 4
× 4 × 1.3 nm. Assuming that the particles observed in the present study are also slab like, the
SAXS signal would correspond to a characteristic length d of 3.7 nm, which is close to the
SAXS experimental results (3.6 nm). The 3 nm dimension in S4 exactly corresponds to a
slab-like particle of 4 × 2 × 1.3 nm which is half of size of the first nanoslabs (S1). Sodium is
shown to influence interactions of these nanoblocks because of its electrostatic charge. SAXS
experiments were done on these nanoparticles (see chapter 4) and it appeared that the Porod
region of the scattering intensity exhibits a slope close to -2. Since the particles formed in S1
are larger, we expect a slope close to -2, which is the value for an infinite sheet. But for the
smallest species (S2 and S3), the Porod slope may be higher and modelisation should be
performed to determine an accurate theoretical slope for a slab of limited size.
Figure 2 shows plots after 8 hours heating of S1 and S2 samples. In the case of S1, the
disappearance of the small particles in favor of the formation of larger entities, not yet
exhibiting Bragg reflections, is observed. These larger nanoparticles are designated as
intermediates. Those intermediates have a Porod slope of –2.2 characteristic of tablet-like
particles. For S2, the increase of intensity after 8h at low q values is due to the scattering of
colloidal crystals already formed. Nanoblocks of dimensions 4 × 2 × 1.3 nm are still present
in the synthesis mixture.
23
In situ temperature dependent dissolution of the silica source in the preparation of the
solution S3 was investigated (Figure 3). SAXS data showed that no particles were formed
after 30 min at 60 °C. It was noted that nanoparticles are growing from 2 to 2.1 nm when
heating few minutes at 100 °C. This growth until an optimum size of 2.1 nm is probably due
to an increasing amount of the soluble silica available with temperature but it indicates that a
very specific size is favoured.
0
0.02
0.04
0.06
0.08
1.5 2 2.5 3 3.5 4 4.5 5
I (a.u.)
q (1/nm)RT
60°C
boiled 10 min.
100°C
Figure 3. In situ time and temperature dependent scattering intensity of Si-TPA-silicic acid
mixture. This solution was first heated 30 min at 60 °C and then to 100 °C. Two spectra were
recorded at 100°C after 3 and 9 min heating.
Transmission electron microscopy
The purpose of the TEM study was to reveal the nature of the particles with
equivalent characteristic length of 2.8-4.3 nm from SAXS and previously detected in MFI-
type zeolite syntheses with DLS, SAXS and neutron scattering.11,12,14,15 Atomic force
microscopy (AFM) on an evaporated suspension of TEOS – TPAOH – H2O revealed a
stepped surface having a characteristic step height of 1.2 ±0.3 nm.21 A detailed structure was
proposed from a 29Si NMR study of the TPA-mediated polycondensation sequence of silicate
monomer using TEOS as silica source.8 29Si NMR spectra provided evidence for the
occurrence of a 33-Si-atom precursor having a characteristic length of 1.35 nm. This
particular specimen is believed to have a size of 1.3 × 1.3 × 1 nm. Addition of water induced
a three by four assembly of this precursor resulting in a very stable suspension of
24
nanoparticles exhibiting a characteristic length of 3.6 nm (Figure 1a). This solution was
spread on a grid for TEM investigations. A large number of nanosized particles was observed
(Figures 4a and 4b). The nanoparticles could survive only few seconds in the electron beam.
From figure 4b, the average in-plane dimensions of individual nanoparticles was estimated to
be 4 × 4 nm, with a thickness of about 1 nm as showed by the dimensions of standing
particles. Occasionally, larger blocks were detected (Figure 4d) which had in-plane
dimensions corresponding to multiples of 4 nm, suggesting they formed through sidewise
aggregation of single nanoslabs observed in figures 4a and 4b. TEM investigations on the
colloidal suspension from room temperature digestion of silicic acid in an aqueous solution of
TPA and sodium hydroxide (solution S2) revealed the presence of rectangular nanoparticles
with in-plane dimensions of 4 × 2 nm (Figure 4c).
Figure 4. TEM images of nanoslabs. a) and b) prepared from TEOS – TPAOH – H2O (solution S1).
c) obtained from silicic acid, TPAOH and NaOH (solution S2). d) occasionally observed larger block
in solution S1.
25
SAXS patterns of solutions S1 and S2 with theoretical scattering functions for nanoslabs
measuring 4 × 4 × 1.3 nm and 4 × 2 × 1.3 nm are displayed in figure 5. Experimental curves
are in excellent agreement with simulated patterns. Consequently the introduction of the in-
plane dimensions derived from the TEM investigations combined with SAXS observations
revealed a thickness of 1.3 nm for the two nanoparticle populations.
Figure 5. SAXS curves of a) solution S1 and b) solution S2. Position and shape agree with simulated
patterns (gray lines) of uniform slab-like entities of dimensions 4 × 4 × 1.3 nm (S1) and 4 × 2 × 1.3
nm (S2).
Formation of TPA-silica composite nanoparticles from the system TEOS – TPAOH –
H2O involves a polymerization process driven by close interactions between TPA cations and
silicate species that shield the propyl groups of TPA cations from the aqueous solution. TPA
molecules are located at the liquid–liquid interface and it has been suggested that the
hydrolysis of TEOS and the structure direction effect take place simultaneously.8 This
complicated process leads to the formation of a peculiar tetracyclic undecamer and eventually
to the occurrence of a 33-Si-atom precursor depicted in figure 6b. A comparative 29Si NMR
study recorded at an early stage of silicic acid digestion and TEOS polymerization in
presence of TPAOH showed that both systems probably proceed under the same silica
polymerization sequence (Figure 6).
26
Figure 6. (a) 29Si NMR spectrum at an early stage of silicic acid digestion in aqueous TPAOH.
Chemical shifts of the 33-Si-atom precursor taken from ref. 8 are indicated in gray lines. (b) For
comparison, the 29Si NMR spectrum of the same precursor in the TEOS system.
In the case of the silicic acid system (solution S2), the signals were rather weak and
significantly broadened (Figure 6a) compared to those in the spectrum of the 33-Si-atom
precursor found in the TEOS system (Figure 6b). This could be due to the fact that silicates
were not solution-borne but still adsorbed on remaining silica. 29Si NMR lines were indeed
not present after removal of the solid phase over a filter with a pore size of 100 nm.
Figure 7. SAXS curves of a) solution S1 prepared with an excess of TPAOH and b) solution S3. The
gray curve shows the simulated pattern for a slab with dimensions of 2.7 × 1 × 1.3 nm.
27
Additional evidence for formation of specific zeolite nanoslabs by self-assembly of the
33-Si-atom precursor irrespective of the silica source are provided from experiments using
higher TPAOH concentrations. Figure 7 displays SAXS experimental and simulated patterns
for solutions prepared from TEOS (solution S1 prepared with an excess of TPAOH) and
silicic acid (replacement of NaOH by TPAOH in S2 i.e. solution S3). The simulated curve
corresponds to a nanoslab with dimensions of 2.7 × 1 × 1.3 nm which matches the size of a
double 33-Si-atom precursor. The asymmetric character of the experimental curves in figures
5 and 7 and their good agreement with slab simulated patterns show that these particles are
not spherical.
Fusion of nanoslabs is hindered at room temperature due to a steric barrier created by
an excess of TPA molecules which are adsorbed on the nanoslabs surface.22 At 100 °C, larger
nanoslabs as identified in figure 2 form by sidewise fusion and lead ultimately to silicalite-1
colloidal crystals. The self-assembly of the 33-Si-atom precursor into different well-defined
and discrete nanoslabs depending on synthesis conditions such as silica source and sodium
content is depicted in figure 8.
S3, S4 S2
S1Aggregate upon heating
Figure 8. Proposed schematic structures for the MFI-type zeosil nanoslabs. The 33-Si-atom precursor
can self-assembly to form stable discrete and organic-inorganic hybrid nanoslabs with dimensions
depending on synthesis conditions.
28
2.4 Conclusion
In this comparative study of the early steps of Si-TPA-silicalite-1 synthesis, the nature of
silica source and inorganic cations were varied. SAXS and TEM revealed to be very powerful
techniques to probe subcolloidal particles formation. It has been shown that, first, these
particles leading to the same zeolite have a well-defined size and, second, they appear to be
tablet-like particles although they differ in size. Based on these results it is suggested that
silicalite-1 crystals form in an organic-mediated synthesis as follows: very well-defined
nanoslabs, which sizes are determined by the silica source and cations, are formed at RT due
to the interactions between the organic template and silicates species. Upon heating,
energetically favorable aggregation occurs and crystal growth is then accomplished by
eliminating water molecules at the interface and oriented attachment.
These peculiar nanoslabs can be used as versatile building units for the synthesis of
innovative porous materials. Synthesis of monoliths, liquid crystals, films or mesoporous
materials are among the potential applications of zeosil nanoslabs.
References
1 Schuth, F. Curr. Opin. Surf. Sc. 2001, 5, 389. 2 Goepper, M; Li, H.; Davis, M. E. J.Chem. Soc., Chem. Commun. 1992, 1665. 3 Gies, H.; Marler, B. Zeolites 1992, 12, 42. 4 Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials: zeolites, clays and nanostructures, Marcel Dekker, New York, 1997, 1-34. 5 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4467. 6 Lok, B. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. 7 Davis, M. E.; Lobo, R. F. Chem. Mat. 1992, 4, 756. 8 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. 9 Cundy, C. S.; Lowe, B. M.; Sinclair, D. M. Faraday Discuss. 1993, 95, 235. 10 Twomey, T. A. M.; Mackay, M.; Kuipers, H. P. C. E.; Thompson, R.W. Zeolites 1994, 14 162. 11 Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites 1994, 14, 568. 12 Schoeman, B. J. Zeolites 1997, 18, 97. 13 Gilson, J. P. In Zeolite Microporous Solids: Synthesis, Structure and Reactivity, E.G. Derouane et al. (eds), Nato ASI Ser., 1992, 352, 19. 14 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086.
29
15 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083. 16 Watson, J. N.; Brown, A. S.; Iton L. E; White, J. W. J. Phys. Chem. B 1998, 94, 2181. 17 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639. 18 Ravishankar, R.; Kirschhock, C. E. A.; Schoeman, B. J.; De Vos, D.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. In Proceedings of the 12th international zeolite conference, Materials Research Society, 1999, 1825. 19 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 20 Iler, R. K. The chemistry of silica, John Wiley and Sons, 1979. 21 Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P-P.; Feijen, E. J. P.; Grobet, P. J.;
Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens J. A. J. Phys. Chem. B 1999, 103, 4960. 22 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027.
30
31
3
A 29Si and 27Al NMR study of MFI precursors*
This chapter examines the huge applications of NMR spectroscopy for probing different features of the structure-direction effect of organic molecules encountered in zeolite synthesis. Silicic acid powder dissolution in a concentrated tetrapropylammonium hydroxide aqueous solution was first followed by 29Si NMR. X-ray scattering was used to follow processes at a colloidal level. The appearance of very well-defined colloidal particles was linked to a specific intermediate already observed in systems using an organic and monomeric silica source. Based on these results, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. The development of new high-resolution methods in NMR offers many possibilities for studying supramolecular assemblies involved in zeolite synthesis. After a classical 27Al NMR study on the incorporation of aluminum in silicalite-1 nanoprecursors, rotational-echo double resonance was applied to measure inorganic-organic interactions in the TPA-mediated synthesis of silicalite-1. Multiquantum 27Al MAS NMR was used in an attempt to estimate the broadening effect due to quadrupolar interaction. An attempt was made to probe the aluminum framework incorporation in the nanoblock-based MFI zeolite synthesis.
* Reproduced in part from: C. J. Y. Houssin; C. E. A. Kirschhock; P. C. M. M. Magusin; B. L. Mojet; P. J. Grobet; P. A. Jacobs; J. A. Martens; R. A. van Santen ’Combined in situ 29Si NMR and Small-Angle X-ray scattering study of precursors in the MFI zeolite formation from silicic acid in TPAOH solutions’ submitted.
32
3.1 Combined in situ 29Si NMR and Small-Angle X-ray scattering study of precursors in the MFI zeolite formation from silicic acid in TPAOH solutions
3.1.1 Introduction Zeolite materials are among the most widely used catalysts in the chemical industry
while being among the least well understood. They are crystalline aluminosilicate molecular
sieves and account for one of the most studied inorganic material family in the last decades.1
Starting back in the 1950s, they have progressively replaced amorphous silica-alumina based
catalysts or other acids like clays in the petrochemical industry for processing of crude oil to
fuels and basic chemicals.2 Zeolite catalysts exhibit unique catalytic activity and selectivity
which originate from their well-defined microporosity.3
The flexibility of the Si-O-Si angle and the small energetic difference between various porous
silica materials explain the ever-growing number of known zeolite structures.4 Despite that
more than 200 different topologies have been found, mainly four zeolite frameworks (LTA,
FAU, MOR and MFI topologies) are involved in industrial applications ranging from
heterogeneous catalysts to adsorbents and ion exchangers.2 Macroscopic properties such as
crystal shape, size, polydispersity and framework composition can be achieved using the right
synthesis procedures, thus avoiding expensive post synthesis modification procedures.
Consequently, the rising number of applications as well as the possibility of discovering new
structures require a better comprehension of their synthesis. The fundamental understanding
of the mechanism that directs the assembly of these unique microporous materials is still
limited because zeolite synthesis processes are difficult to study. First, one of the most
important parameters in zeolite synthesis is certainly the composition of the starting mixtures
from which the crystals will grow. It generally consists of a complicated two-phase mixture
of water, a silica source, alkali cations and organics. Even the impurities as well as
differences among batches of silica sources sometimes have a large impact not only on the
kinetics but also on the nature of the crystalline phase obtained. Secondly, the interactions of
33
silicate species with cations present in precursor solutions remain unclear. Indeed, there has
been a lot of speculation on the role of organic molecules, alkali and alkaline-earth cations.5
Therefore, the prediction of the formation of a given topology based on the presence of a
specific guest molecule is not yet possible. Lastly, prenucleation, nucleation and crystal
growth involve very fragile colloidal species so that non-invasive techniques are best suited
for investigating zeolite synthesis. Only a few methods including NMR, Raman spectroscopy
or light and X-ray scattering can probe such transformations under synthesis conditions.
Chemical analysis through trimethylsilylation and GC analysis has extensively been used in
the past but this technique is too invasive regarding the fragile intermediates that are involved
in zeolite synthesis.
The ability of organic templates to organize SiO4 tetrahedra into very well-defined 3-
dimensional structures has long been recognized, notably for the formation of clathrasils.6
The introduction of organic molecules as guest species in the synthesis of zeolite molecular
sieves led to the discovery of the majority of the synthetic zeolites and contributed to achieve
materials with higher Si/Al ratios. In some instances, there is a strong dependency of the
zeolite topology obtained and the geometry of organic molecules and the zeolite topology.
Therefore, a lot of academic efforts have been devoted to study the nature of interactions
between organic molecules and inorganic precursors.7,8,9,10 There is no chemical bond
formation between template molecules and silicate. This supramolecular assembly process
arises from weak interactions such as van der Waals forces leading to intermediate species
that are not likely to withstand ex situ characterizations, involving isolation or chemical and
physical interventions. Consequently, the system necessitates in situ experiments.
In situ 29Si NMR has proven to be a powerful, non-destructive method for studying
silicate anions in solution.11,12 Actually, 23 oligomeric silicate structures were detected using
both 1D and 2D FT NMR spectroscopy during the 1980s, in silicate solutions containing
alkali and tetraalkylammonium cations.11,12 Recently, Kinrade et al. highlighted the strong
influence of tetraalkylammonium ions on silicate polycondensation.13 They concluded that
tetraalkylammonium ions directed the oligomerization of silicate anions towards the
prismatic hexamer, and mainly to the cubic octamer as final stable structures. Those species
having the tetraalkylammonium cations outside do not exhibit Si-Si exchanges with the
solution such as typically observed in alkali metal ion silicate solutions. The quest for stable
silicate oligomers is going on and has led to the recent discovery of three novel species.14
ZSM-5 is a synthetic high-silica zeolite and has the MFI topology characterized by a
3-dimensional pore system. Its remarkable catalytic and sorption properties are exploited in
34
commercial applications. ZSM-5 synthesis and its crystallization using various template
molecules has been reviewed.15 Notably, this is a very versatile zeolite since it can be
synthesized from a large variety of inorganic and organic agents some of which having hardly
any geometrical fit with the host framework. However, tetrapropylammonium (TPA) is
regarded as the strongest structure-directing organic cation for the MFI topology since it
enhances nucleation rate, fits very well in the channel intersections and allows ZSM-5 to be
synthesized over a wide range of synthesis conditions. The formation of peculiar silicate
oligomers during the hydrolysis of tetraethylorthosilicate (TEOS) in aqueous TPAOH was
recently observed using 29Si NMR.16,17 Two particular new species were encountered as
intermediates in the TPA-mediated self-assembly of colloidal silicalite-1, the all-silica
counterpart of ZSM-5. The first species reported was an undecamer having three five-rings
on a four ring that could polycondensate to form a 33-Si-atom precursor showing MFI
topology. It has been shown that this undecamer could also transform into a pentacyclic
dodecamer by addition of one silicate unit17. The observation that colloidal silicalite-1
crystals do not exhibit systematically missing T-sites led to the conclusion that most
undecamers were converted to dodecamers which are coupled into 36-Si-atom precursors.
After addition of water to the solution, the dominant species was found to be silicalite-1
nanoslabs, built from 12 precursors and having dimensions of 4 × 4 × 1.3 nm as confirmed
with TEM and SAXS.18 A model was then proposed in which silicalite-1 crystallization takes
place via nanoslab aggregation mechanism.19
This aggregation pathway leading to silicalite-1 formation was observed with TEOS
as silica source. In the present study the conversion of a polymeric silica source (silicic acid)
in concentrated TPAOH aqueous solutions will be presented. It was already known that at
room temperature the combination of silicic acid-TPAOH-H2O leads to the spontaneous
formation of subcolloidal particles resembling nanoslabs.20 Nanosized particles were also
observed by De Moor et al. using a combination of SAXS/USAXS/WAXS and a particular
system involving sodium cations besides TPA cations.21,22,23,24,25 The goal of the present work
was to investigate the polymerization process of silicic acid in highly concentrated TPAOH
solutions as a continuation of the recent study on the TEOS-TPAOH-H2O system. 29Si NMR
was used to determine the different silicate species present in solution. Subcolloidal and
colloidal particle populations were investigated using in situ synchrotron SAXS and USAXS
techniques. A reaction pathway for the TPA-mediated formation of silicalite-1 from silicic
acid is proposed.
35
3.1.2 Experimental section
Zeolite synthesis. The silicalite-1 synthesis recipe used is based on a patent of Exxon
Chemicals.26 4.37 g of distilled water was added to 12.71 g of 40% TPAOH in water (Fluka),
followed by a spoonwise addition of 4.05 g of silicic acid (Baker, 10.2% H2O). The molar
composition of the synthesis mixture was 4.41 TPAOH / 10 SiO2 / 117 H2O. The milky
dispersion was stirred 24h to obtain a clear solution and transferred into the sample cell.
Synthesis temperature was 125 °C. Crystallinity was verified by XRD.
NMR. 29Si liquid NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 0°C
operating at 99.36 MHz and using a 7 mm probe head at a spin rate of 500 Hz. In a typical
measurement 512 spectra were accumulated with a pulse length of 3 µs (45° pulse) and a
repetition time of 20s. Chemical shifts were referenced internally to the silicate monomer
resonance.
SAXS and USAXS. The combined SAXS and USAXS experiments were performed on the
high-brilliance beamline ID02A at the European Synchrotron Radiation Facilities, Grenoble,
France.27 This beamline uses a highly monochromatic beam with very low divergence and
small cross section. The SAXS setup consists of a pinhole camera with a beam stop located in
front of an image intensified 2-D CCD camera. Sample to detector distances of 1.5, 2.5 and 6
m were used. Conversion of detector pixels (CCD camera) to the scattering vector q (nm-1)
was performed with the help of a lupolen polyethylene (BASF) sample. The X-ray
wavelength was 0.99 Å. An incident X-ray beam with a cross section of 0.2 X 0.2 mm2 was
used. The high brilliance allowed us to record a SAXS pattern in less than 1s. Data were
corrected for detector response and background using a water reference sample at the same
temperature.
USAXS patterns were recorded using a Bonse-Hart type of X-ray Camera.28,29 The
available range of scattering vector q was 0.001-0.14 (nm-1) with q = (4π/λ)sin2θ and λ = 1.0
Å. A first crystal analyzer [Si(220)] was employed to scan the angle, then the X-ray beam
was collimated in the vertical direction with a second analyzer crystal [Si(111)]. The detector
was a high-dynamic range (≈10 7 counts/sec) avalanche photodiode. The beam size at the
sample was about 1 X 2 mm2. Several scans (4-5) over successive 2θ ranges with sufficient
overlap were recorded with different degrees of attenuation of the incident X-ray beam, so
that the intensities on the detector were in the linear range. For each sample, a rocking curve
spectrum was systematically measured, which consists of the scattering produced by the
36
setup, background scattering and the small intensity fluctuations of the beam. Due to the
strong scattering of silica, the contribution of water scattering was negligible.24 A complete
spectrum could be recorded in 15 min.
Sample Cell. An electrically heated brass holder containing a rotating circular sample cell,
designed in our group25, allowed us to perform in situ measurements. The sample cell rotates
at an approximate rate of 2 rpm thus keeping the solution homogeneous, preventing the
precipitation of zeolite crystals and reducing the sample exposure to a small spot. The sample
was inserted between two mica windows (Attwaters and Sons) separated by a PTFE spacer
(0.5 mm thick). Heating of the sample from RT to the reaction temperature (125 °C) requires
only 2 minutes.
3.1.3 Results and discussion
Initially, we performed 29Si NMR measurements on a system used formerly in our
group24 with the following composition: 10 SiO2 : 1.22 (TPA)O2 : 0.848 Na2O : 117 H2O. It
turned out that, even at low temperatures, silicic acid dissolution was too fast to be followed
with 29Si NMR. When TPA molecules replaced sodium cations, silicic acid dissolution rate
decreased dramatically. This enabled us to follow the entire process using natural abundance 29Si NMR. Differences in amorphous silica powder dissolution rate encountered in alkaline
solutions are due to many factors.30 However, at high pH, this process obeys to a single
mechanism in which hydroxyl ions act as catalysts to release monomeric silicate anions Qo.
Negatively charged surface silanols, neutralized by a cation shell, are attacked by hydroxyl
ions, which then increase the coordination number of silicon atoms. As a consequence, the
underlying siloxane bridges are weakened and silicate monomers can be released in solution.
Wijnen et al. ascribed rate differences to diffusion effects in the cation shell surrounding
amorphous silica.31 This implies that, in our case, TPA cations induced a displacement of
sodium cations from the silicic acid surface and caused an inhibition effect due to a slower
diffusion of hydroxyl ions. TPA cations, more hydrophobic than alkali metal cations, adsorb
very strongly to the silicic acid surface.
Natural abundance 29Si NMR of TPA silicate solutions are displayed in figure 1 at
several times after initial mixing of silicic acid powder and concentrated aqueous TPAOH
solutions. Dissolution of amorphous silica in aqueous alkaline solutions results in a gradual
37
increase in monomer concentration, which leads to oligomerization and further
polycondensation. The hydroxyl ion catalyzes the dissolution process which releases
Si(OH)4. Generally, high-resolution 29Si NMR spectra display individual resonance peaks
that can be grouped into bands corresponding to structurally different sites of the 29Si atom in
the oligomer. In this study, we used the Qn notation for the Si coordination as introduced by
Engelhardt.32 Q designs a silicon atom bonded to 4 oxygen atoms forming a tetrahedron and
the superscript n represents the number of siloxane (Si-O-Si) bridges at the specific 29Si atom
under study. For example Q0 denotes the monomeric anion SiO4- (protonation is not
considered by this notation), Q1 end groups with one siloxane bridge. Q2 silicons with two
siloxane bridges, and so on. Many configurations for Q1…Q4 sites are possible depending on
cyclisation and branching of the silicate skeleton. Even though resonance frequency shifts
arise from the particular environment of the 29Si atoms (connectivity, Si-O-Si angle, Si-O
bond length), the chemical shift of a given Si atom also depends on the pH of the medium,
silica concentration and cation type. Therefore the assignment of signals to silicate structures
must be carried out with extreme care. The procedures are even a matter of debate.
-45-40-35-30-25-20-15-10-505
1h
8h
12h20
11h
15h20
9h
7h
23h
ppm
Q0 Q1Q2cyc Q3
cycQ2 Q3
Figure 1. 29Si NMR of aqueous silicate solutions at different times after initial mixing of silicic acid
powder and a concentrated aqueous TPAOH solution. Q2cyc and Q3
cycl design Q2 and Q3 Si atoms in
cyclic positions respectively.
38
0
300
600
900
1200
1500
1800
0 5 10 15 20 25
Q0
Q2
Q1
Q3
Dissolution time (h)
Inte
grat
ed p
eak
area
Figure 2. Integrated peak area of the Qn resonance bands against dissolution time. Data from for the
spectra shown in figure 1. No Q4 were directly detected with NMR.
The evolution of the integrated peak area of the different resonance bands assigned to
Q0, Q1, Q2 and Q3 is shown in figure 2. It is evident that silica first dissolves to yield the
monomer ions (Q0) until a maximum concentration is reached. The monomer oligomerizes
rapidly to form the dimer with two Q1 silicons and higher molecular weight silicates with Q2
and Q3 silicons. A 29Si NMR spectrum of silicates obtained after 15 min of mixing silicic acid
in a concentrated aqueous solution of NaOH is shown in figure 3. Base concentrations are
similar to those with TPAOH (Figure 1). It is clear that the initial rate of silicic acid
dissolution is much higher compared to the rate observed using TPAOH. In contrast to
spectra of TPA silicate aqueous solutions, broad signals are observed corresponding to Q1,
Q2, Q3 and Q4 unspecified environments. With NaOH, dissolution and condensation of
silicates is much more rapid than with TPAOH.
-45-40-35-30-25-20-15-10-505ppm
Q0 Q1
Q2
Q3
Q4
Figure 3. 29Si NMR spectrum of an aqueous silicate solution 15 min after initial mixing of silicic acid
and a concentrated NaOH solution.
39
Structural information from chemical shift
Three selected 29Si NMR spectra of the silicic acid – TPAOH – H2O system after 7, 8
and 9h of mixing time are reported in figure 4. The silica solution after 1h of dissolution
contains the monomer, the dimer (δ = −8.6 ppm) and the cyclic trimer (δ = −10.1 ppm). After
a considerable time, non-cyclic Q2 species emerge with chemical shifts between δ = −16 and
δ = –18 ppm. The lines at δ = −9.9, −16.5 and −17.3 ppm are assigned to the bicyclic
pentamer. The linear trimer is also present. Most importantly, an additional line is found at δ
= −17.1 ppm. It is attributed to the prismatic hexamer. Indeed, this species tends to be favored
and is very stable in tetraalkylammonium solutions.11,33 But the most informative spectrum is
the one after 8h of mixing (Figure 4b). Besides the five species encountered in figure 4a (the
linear trimer at δ = –8.1 ppm is no longer present) additional lines are observed. The
formation of the tricyclic hexamer (structure IX in ref. 11c) is obvious regarding the
appearance of one Q2 line at δ = –16.2 ppm and three Q3 lines (δ = –16.7, –18 and –25 ppm).
The double bridged cyclic tetramer is found at a very small concentration as shown by its
above-noise but very typical signals at δ = –14.7 and –21.4 ppm. Three signals at δ = –17.6, –
24.8 and –25.2 ppm remain to be assigned. Based on the list of previously observed silicate
oligomers11 very few species can give rise to these resonances. The line at δ = –17.6 ppm
could be attributed to the Q3 site of the hexacyclic octamer (structure XVI in ref. 11c) but this
silicate would give rise to a relatively intense line at –20.5 ppm, which is not observed here.
The tricyclic octamer (structure 16 in ref 11d) and the pentacyclic nonamer (structure 13 in
ref. 11d) are also candidates. However, the absence of resonance lines at around δ = –15.7
ppm makes their presence unlikely. Using 29Si NMR, Kirschhock et al. evidenced the
existence of a new intermediate in the formation of silicalite-1 showing the structure-
directing effect of TPA cations in the system TEOS − TPAOH − H2O.17 This pentacyclic
dodecamer exhibits one 29Si NMR line in the Q2 range and two peaks in the Q3 range at δ = –
89.9, –97.2 and –98.2 (δ referred to external TMS or at δ = –18.5, –25.8 and –26.8 ppm
assuming that the monomer signal is located at δ = –71.4 ppm), respectively. This group of
peaks seems to be shifted a little upfield in the present investigation. In the original work of
Kirschhock et al., the signal at δ = –89.3 ppm (–17.9 ppm referred to Q0 of figure 3 of ref. 17)
can be assigned to the prismatic hexamer.34 In the present study this species gives rise to a
line at –17.1 ppm. This 0.8 ppm shift can possibly be attributed to the use of a different
chemical shift reference since unfortunately monomers were not present in the TEOS –
40
TPAOH – H2O system. The 29Si NMR signals were then referred to external TMS. Assuming
a systematic shift of 0.8 ppm, we expect the pentacyclic dodecamer to give rise to three
peaks, one at δ = –17.7 ppm and two at around –25 ppm, in good agreement with the present
values of –17.6 ppm, –24.8 and –25.2 ppm. The pentacyclic dodecamer is most likely formed
in the system under study. The TEOS and the silicic acid systems result in the growth of the
MFI zeolite structure upon heating. Strong similarities were previously observed on the
colloidal scale in terms of precursor particles irrespective of the use of silicic acid or TEOS as
silica source.18
-26-24-22-20-18-16-14-12-10-8
(a)
(b)
(c)
ppm
Figure 4. 29Si NMR spectra of dissolution of silicic acid in a concentrated aqueous TPA solution after
different reaction times: a) 7h, b) 8h and c) 9h.
Figure 5 shows 29Si NMR spectra during the process of silicic acid digestion by TPAOH
between 11h and 23h of mixing time. It is clear that two additional peaks appear in the Q3
region and that the monomer and dimer are absent. The first additional line is located at δ = –
25.9 ppm which corresponds to a species containing equivalent Q3 centers. Literature data
indicate the tetrahedral tetramer as the unique Q3 symmetric silicate in this region.11 The
signal that appears the latest in the NMR spectra is located at δ = –26.9 ppm. Consulting the
chemical shift data from literature, this peak can be assigned to several completely symmetric
41
silicate anions. The double four-rings (cubic octamers), double five-rings and double six-
rings are expected to give rise to a signal in this region. Knight et al. in their study of aqueous
TMA silicate solutions,13 evidenced the preferential formation of cubic octamer anions.
Indeed, TMA is well known to direct the synthesis of even-membered-ring zeolites.1
However double four-ring silicates have not been observed in TPAOH or TBAOH
solutions.11,33 Only one case is known on the existence of this species in TPAOH solution but
it was achieved under addition of a large amount of dimethylsulfoxide as co-solvent and low
alkalinity.13 Double six(or higher)-ring silicates have never been observed and seem unlikely
to be stable in solution.35 Moreover, the fact that double five-rings have only been observed
in ZSM-5 forming mixtures36 led us to the assignment of the –26.9 ppm line to double five-
rings. The assignment of 29Si NMR signals and comparison with literature data.11,12 is given
in figure 6.
-30-28-26-24-22-20-18-16-14-12-10-8ppm
(a)
(b)
(c)
(d)
Figure 5. 29Si NMR spectra of dissolution of silicic acid in a concentrated aqueous TPA solution at
different reaction times: a) 11h, b) 12h20, c) 15h20 and d) 23h. The arrows refer to the resonances of
the pentacyclic dodecamer discussed in the text.
42
Silicate species Experimental chemical shift
Literature
Monomer
Dimer
Trimer
Cyclic trimer
Bicyclic pentamer
Tricyclic hexamer
Prismatic hexamer
Doubly bridged cyclic tetramer
Double five-ring
Tetrahedral tetramer
Pentacyclic dodecamer
0
8.6 8.6
Q QQ Q
1 1
2 2 8.2 8.2 16.8 16.9
10.2 10.1
Q QQ Q
2 2
3 3 9.9 9.8 16.4, 17.2 16.3, 17.1
Q QQ Q
2 2
3 3 16.2 16.2 16.7, 18, 25 16.8, 17.9, 25
17.1 17.2
Q QQ Q
2 2
3 3 14.7 14.6 21.4 21.4
26 25.5
27 26.9
Q QQ Q
2 2
3 3 17.6 17.7 24.8, 25.2 25, 25.8
11,12
Figure 6. Assignment of 29Si liquid NMR resonances relative to the monomer which lies at δ = –71.4
ppm (referenced to tetramethylsilane) and comparison with literature data11,12 (negative signs of
chemical shifts are omitted).
Prolonged polymerization of aqueous silicate solutions leads to the formation of colloidal
structures besides oligomeric silicates.30 This is evident in figure 5 from the broad features
that progressively appear in the Q3 and Q4 ranges. Due to spectral overlap it becomes
increasingly difficult to get comparable chemical information from the 29Si NMR spectra for
silicates larger than 12 T atoms. A different spectroscopic technique was used to study the
ongoing polycondensation during silicic acid digestion: Small-angle X-ray scattering
(SAXS).
X-ray scattering
USAXS and SAXS allowed to probe in situ formation and consumption of
(sub)colloidal particles as well as silicalite-1 crystal growth from initial mixing of silicic acid
and TPAOH to complete crystallization upon heating. Figure 7A displays small-angle X-ray
43
scattering data obtained from an aqueous TPAOH – silicic acid solution after 23h of mixing
time at room temperature. The size of (sub)colloidal particles under study is reflected in the
characteristic length d which is related to the scattering vector q according to q = 2π/d. A
pronounced hump is observed at q = 2.95 (1/nm) corresponding to a discrete particle
population which we will denote as primary particles. Only one particle population was
found. Their estimated characteristic length, determined from the maximum position of the
hump, is ca. 2.1 nm. The scattering intensity of the 2.1 nm particles is plotted against the
mixing times in figure 7B. It was then possible to correlate the formation of subcolloidal
particles to dissolution time of silicic acid in TPAOH and 29Si NMR spectra shown in figures
4 and 5. Clearly no primary particles were forming during the first 10h. Increasing scattering
intensity is found from 11h on due to the progressive formation of this unique particle
population (Figure 7B).
0
0.01
0.02
0.03
0.5 1 1.5 2 2.5 3 3.5 4
2.95 nm-1I (a.u.)
q (1/nm)
A
0.01
0.02
0.03
5 9 13 17 21 25
I (a.u)
dissolution time (h)
B
Figure7. A) SAXS pattern of a solution of silicic acid and TPAOH after 23h of mixing time.
B) Time dependent scattering intensity at a fixed angle (q = 2.95 nm-1) corresponding with a d value
of 2.1 nm (primary particles). Scattering intensity from nanoparticles was not clearly detected before
10h in B.
SAXS spectra did not significantly evolve after 23h of mixing time. Interestingly, the
formation of nanoparticles sets in right after the appearance of 29Si NMR signals at around δ
= –25 ppm (Figure 5). The tetrahedral tetramer and double five-rings were formed when a
non-negligible amount of primary particles was already present. Consequently, these species
are not involved in the formation of primary particles. Their 29Si NMR peak intensity
increased at the expense of that of the prismatic hexamer. Kirschhock et al. proposed that
three pentacyclic dodecamers obtained in the TEOS – TPAOH – H2O system rapidly
condensed into a specific silicalite-1 precursor.17 This particular precursor has a d value of
44
1.35 nm. Its estimated size is 1.3 × 1.3 × 1 nm. Aging of the same solution led to the
dominant presence of dimers with a d value of 2.2 nm. Upon dilution with water, very stable
nanoslabs were obtained having dimensions of 4 × 4 × 1.3 nm according to TEM, SAXS and
AFM18 by coupling of 3 × 4 precursors. Based on the observation of pentacyclic dodecamer
with 29Si NMR and particles corresponding to double precursors with SAXS, we conclude
that the same molecular steps of Si polymerization from 12 to 72 Si atom species are
encountered in both the TEOS – TPAOH – H2O and the silicic acid – TPAOH – H2O
systems. For as yet unknown reasons, the present experimental conditions render the double
precursors (or primary particles) very stable.
0
100
200
300
400
500
600
700
0 100 200 300 400 500 600 700 800time (min.)
I (a.u.)
2.1 nm
crystals
Figure 8. Evolution of scattering intensity of primary particles and growing crystals during the
synthesis of silicalite-1 from a mixture of silicic acid and aqueous TPAOH at 125 °C. Evolutions of
the scattering intensity for primary particles and crystals were followed respectively by SAXS and
USAXS.
After heating at 125 °C the scattering intensity of the primary particles increases only slightly
(Figure 8). This observation suggests that most of the silicic acid powder has been dissolved
at room temperature. The onset of crystallization was determined with in situ USAXS
measurements as reported previously.21 From the X-ray scattering data, the presence of two
interrelated particle populations was evident: primary particles and growing crystals. The
scattering of the primary particles is almost constant until the formation of the first silicalite-1
crystals (Figure 8). Such behaviour has already been observed in similar syntheses.24 In a
mixed system with TPA and sodium cations, De Moor et al. observed three particle
populations: primary units (2.8 nm), their aggregates (≈10 nm) and growing crystals. By
45
varying the synthesis conditions they were able to clarify the role of each of the particles. The
formation of the 10 nm aggregates depends on the alkalinity of the synthesis mixture but the
2.8 primary units were always present. For high alkalinity such as Si/OH = 2.42 (same as the
present study), no aggregates (10 nm) were obtained whereas low alkalinity appeared to favor
their formation. The data strongly suggested that the growth units were the 2.8 nm
nanoparticles while their aggregates would only act as reservoirs. In the present study, i.e. in
the absence of Na+ cations, the formation of smaller nanoparticles (2.1 nm) is favoured. The
crystal growth rate of silicalite-1 was 0.93 nm/min. De Moor et al.21 recorded a rate of 1.2
nm/min, showing that Na+ cations accelerate the crystal growth. The final crystals were found
to be larger in the present study, suggesting that less nuclei were formed. These observations
suggest that TPA molecules hinder the formation of viable nuclei by keeping away primary
particles from each other, whereas the smaller Na+ counterions favor aggregation of particles
and act as bridging ions.
Proposed mechanism
As shown in figure 4, monomer anions oligomerize very slowly to form dimers,
trimers and cyclic trimers. From the moment the first Q3 Si atoms in the δ = –25 and –28 ppm
range (assigned to pentacyclic dodecamers) are present in solution, the dissolution rate
increases. We attribute this phenomenon to the formation of pentacyclic dodecamers around
TPA cations adsorbed on the silica surface. The formation of 2.1 nm particles follows rapidly
the dodecamer formation. The silica surface becomes more hydrophilic and transport of
hydroxyl ions is facilitated. Appearance of double five-rings seems to arise from the
prismatic hexamer by addition of two Si atoms. In the early 1980s, occurrence of double five-
rings in ZSM-5 synthesis mixtures has led to a model in which they were regarded as zeolite
precursors since MFI framework can be built starting from double five-ring silicates.36
However, based on trimethylsilylation methods and using silicic acid as silica source, further
studies have failed to correlate their enhanced presence with faster nucleation.37 It must also
be noted that those authors37 claimed that a large amount of silica was present as polymeric
silicates that could not be detected by neither chemical trapping nor 29Si NMR. Their
following statement “the presence of zeolite precursor species other than DnR silicates in this
(colloidal) range can not be excluded” is particularly relevant to the present data. The
increasing scattering intensity of the 2.1 nm primary particles and 29Si NMR reveal that silica
is mainly present in very well-defined and stable subcolloidal particles that likely have a
46
silica connectivity related with the MFI topology. The strong decrease of the scattered
intensity from the primary particles with the onset of the crystallization process suggests that
crystal growth occurs via integration of 2.1 nm primary particles or secondary particles
obtained by their aggregation at the crystal surface. A proposed zeolite assembly process
from the dissolution of nutrients to crystal growth is depicted in figure 9. The present study
did not allow to specify the steps from the precursor dimers (2.1 nm), possibly via tablets, to
the final crystals.19
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO O-
SiOO
SiOO
O-
O
O
SiOOO
SiOOO
O-
SiOO
SiOO O-
SiO
OSi
OO O -
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
O
OH-
OH- Crystal growth
Nucleation
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO
O-
SiOO
SiOO O-
SiOO
SiOO
O-
O
a
b
c
d
e
Figure 9. Schematic representation of the proposed mechanism of synthesis of silicalite-1 crystals
from silicic acid powder and a TPAOH solution: a) silica surface with adsorbed TPA. b) pentacyclic
dodecamer. c) double precursor (2.1 nm). d) tablet (observed in the TEOS – TPAOH – H2O system)
but not in the present study (or in non-measurable amount). e) silicalite-1 crystal.
3.1.4 Conclusion
29Si NMR and (ultra)small-angle X-ray scattering were combined to study
transformations taking place in silicalite-1 zeolite synthesis. 29Si NMR allowed the
47
identification of silicate oligomers during the slow dissolution of silicic acid in a concentrated
TPAOH solution. These processes on a molecular scale could be related with events
occurring at a colloidal scale. Followed by the appearance of 2.1 nm primary particles
(monitored by SAXS) and assigned in chapter 2 as nanoslabs, the key steps in the process are
the formation of their probable precursors (pentacyclic dodecamers observed in 29Si NMR
data). These processes accelerate the silica dissolution rate in which zeolite precursors form
at the surface of the amorphous silica and are released upon formation of the nanoslabs. The
quest for zeolite precursors has long been investigated at a molecular level by the means of
NMR. The relevant stable zeolite nanoprecursors are situated at the transition from the
molecular to the subcolloidal scale, as suspected by Keijsper et al. in the early 1980s.37 We
have also shown that double five-rings are observed much later after the formation of the 2.1
nm precursors, suggesting that they are not related to zeolite crystallization. These results
together with recent studies confirm the existence of a common mechanism in which organic-
mediated silicalite-1 crystallization occurs irrespective of the silica source.
3.2 Applications of specific NMR techniques to the study
of nanoslab formation
3.2.1 Introduction
The work reported thus far shows that the mechanisms of zeolite formation are very
complicated. Two hypotheses have originally been proposed in literature: zeolite
crystallization occurs by solid-solid transformation38,39 or via soluble aluminate precursors.40
The first hypothesis is no longer favored. There is now a general agreement that zeolites
crystallize in solution from soluble species even in semi-dry conditions. This has prompted
considerable attention on the study and identification of aluminosilicate anions in alkaline
solutions.41,42,43 As shown in the first section of this chapter, 29Si NMR turns out to be a
powerful tool for probing silicates at early stages of zeolite formation even when starting
from solid silica sources. In the present section, we extended these investigations to include
other NMR methods.
48
As a convenient technique for the evidence of the formation of aluminosilicates, 27Al
NMR spectroscopy was used to follow the evolution of the aluminum connectivity during the
formation of ZSM-5 precursors and the whole course of ZSM-5 crystallization. Based on the
results of the first part of this chapter and the work of Kirschhock et al.16, close contacts
between protons of TPA and silicon atoms are expected irrespective of the silica source. In
some instances, interatomic distances can directly be obtained using a recent and specific
method, rotational-echo double resonance (REDOR).44,45 We were then able to compare the
S-HTPA distance in silicalite-1 nanoslabs spontaneously formed at room temperature with that
encountered in the final TPA-silicalite-1 crystals. Lastly we carried out 27Al multiquantum
MAS NMR experiments on crystals obtained from the TEOS − Al − TPAOH − H2O system
in order to address the issue of random or non-random Al framework incorporation.
3.2.2 Experimental section
Zeolite synthesis. Aluminum containing template solution was first prepared as follows:
aluminum metal powder was dissolved overnight in a concentrated solution TPAOH (Alfa,
40%) and then filtered through a syringe filter (Whatman 0.45 µm).
TEOS or silicic acid was gradually added to the template solution under vigorous stirring at
room temperature. Each step represented an addition of 10% of the final amount of silica. A
20 min delay was applied between each addition in order to allow 27Al NMR measurement.
The final molar compositions were (TPA2O)(SiO2)3.75(Al2O3)0.0375(H2O)30(EtOH)15 and
(TPA2O)(SiO2)5(Al2O3)0.05(H2O)57 for the TEOS and the silicic acid systems, respectively.
SAXS revealed the formation of nanoparticles of dimensions 3.7 nm and 2.1 nm for the
TEOS system and the silicic acid system, respectively. Nanoblock suspensions for REDOR
experiments were prepared as follows: an ion-exchange procedure using Ag2O was
performed on an solution of TPABr in D2O in order to obtained a 37.5% TPAOD solution in
D2O. TEOS and D2O were then added as described in chapter 2.
NMR. 27Al NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 130.3 MHz using
a 4 mm probe head at a spin rate of 1.7 kHz. In a typical measurement 1000 spectra were
accumulated with a pulse length of 3 µs and a repetition time of 1 s. Chemical shifts are
referred to external aqueous AlCl3. 2D 27Al MQ MAS NMR experiments were performed in
the same spectrometer at a spinning rate of 12.5 kHz and a 4 mm probe head, with an
excitation pulse of 10 µs, a conversion pulse of 1.2 µs (both for a rf field strength of 124
49
kHz) and a zero quantum filter consisting of a delay of 20 µs and a 10 µs pulse (for a rf field
strength of 10 kHz). For each t1 900 scans were accumulated, and t1 was incremented 256
times.
3.2.3 Results and discussion 27Al NMR
A series of 27Al NMR spectra of aluminosilicate solutions in which the SiO2
concentration was gradually increased is shown in figures 10 and 11, corresponding to the
TEOS and silicic acid systems, respectively. It is evident that Al connectivity is greatly
affected by the Si/Al ratio. The spectra without silica showed a typical line at δ = 78.9 ppm,
characteristic of the monomeric ion [Al(OH)4]-.
In figure 10, the distinction between Al(0Si) and Al(1Si) may not be straightforward. The
peak assigned to [Al(OH)4]- is known to broaden and shift downfield in presence of
silicates.46 This behaviour was interpreted in terms of loose interactions with silicates. The
same authors identified Al(4Si) between δ = 57 and δ = 58 ppm in very few solutions. The
broad line observed at around δ = 58 ppm (40% silica added) shifting downfield to 55 ppm
(final composition) represents obviously aluminum atoms fully reacted with silicates
(Al(4Si)). The other bands located upfield could then be determined.
40455055606570758085
Q1
Q2
Q3
Q0
Q4
0%
10%
20%
30%
40%
60%100%
%of Si addedfinal ratio : Si/Al=50
ppm
Figure 10. 27Al NMR spectra of the progressive addition of TEOS in an aluminum containing
solution of TPAOH. The relative decrease of intensity at 60% and 100% is related to the fact that
species are more condensed.
50
The broad lines appearing at δ = 73, 71, 65 and 58 ppm are assigned to Al(1Si), Al(2Si),
Al(3Si) and Al(4Si), respectively (nSi refers to the number of siloxane bonds). For relatively
low silica concentrations, three peaks (Al(1Si), Al(2Si) and Al(3Si)) are observed. Those
peaks do not shift. The connectivity of aluminum increases with silica concentration,
suggesting that [Al(OH)4]- or small aluminosilicates appear to complex preferentially with
large silicate species. An explanation could be that highly negatively charged
aluminosilicates react preferentially with silicates having low average negative charge per
silicon, thus highly condensed oligomers such as the pentacyclic dodecamer (See 3.1). The
progressive shifting of the Al(4Si) broad signal is probably due to the ongoing
polymerization process through formation of the 33-Si-atoms precursor and its aggregates
which lead to the presence of very well-defined nanoslabs.47 More importantly, it should be
noted that, from the position of the 27Al NMR signal, the coordination of aluminum atoms
with four silicon atoms (via oxygen bridges) did not change through nucleation and
crystallization of ZSM-5 as shown in figure 12. Since the nanoslabs are the dominant and the
most condensed species after hydrolysis of TEOS, all the aluminum atoms are incorporated in
them and stay there. It was proposed that crystallization occurs via nanoslabs aggregation.19
A different crystallization mechanism like monomer or small oligomers addition is then
excluded.
2030405060708090 ppm
10%
20%30%
40%
50%
60%
100%
%of Si addedfinal ratio : Si/Al=50
Q4
Q3
Q2
Q1
Figure 11. 27Al NMR spectra of the progressive addition of silicic acid in an aluminum containing
solution of TPAOH.
51
Aluminum coordination changes upon addition of silicic acid to an aluminum containing
TPAOH solution are shown in figure 11. The same trends as in the TEOS system are
observed. The final position of the chemical shift for the tetrahedrally coordinated Al atoms
lies at δ = 58.7 ppm. This higher value is probably due to the smaller size of the precursors
(2.1 nm). Unfortunately, no in situ 27Al NMR study of the crystallization of ZSM-5 using
silicic acid has been performed yet as a confirmation of the results shown in figure 12.
01020304050607080
1h
2h
5h
8.5 h
ppm
Figure 12. 27Al NMR spectra of an Al-containing nanoslabs solution (Si/Al=50) heated at 100 °C for
different reaction times. The zeolite crystallization took place in the NMR probe head using a
pressurizable rotor.
REDOR experiments
A solid-state NMR technique was used to investigate the close relationships between
template and silica. As an indication of particular interactions, the distances between silicon
atoms and protons of template molecules were estimated in both the nanoslabs made from the
TEOS system and the silicalite-1 crystals. One of the experimental solid-state NMR
techniques widely used for distance investigation is the so-called REDOR technique.48,49 A
brief description of this experiment is presented below.
Under fast magic angle spinning, usually applied in solid-state NMR, the magnetic
dipole coupling between observed nuclei I (in our case 29Si) and nearest nuclei P (for
example, 1H) is suppressed. This suppression allows a significant improvement of NMR
spectrum resolution and consequently more unambiguous chemical information. But
52
parameters of dipole-dipole coupling also carry some important information about the
investigated system such as distances between interacting spins. Rotational-echo double-
resonance technique (REDOR) reintroduces partially this coupling. In this type of experiment
P (in our case 1H) spins undergo additional inverted pulses in the middle of the sample
rotation period. As a result, complete averaging of dipole-dipole coupling does not appear.
Analysis of REDOR results is based on comparison of spin-echo intensities with and
without refocused 1H pulses. With refocused 1H pulses, this spin-echo intensity is suppressed
due to dipole-dipole coupling. The so-called REDOR fraction (∆S), equal to the normalized
difference between the spin-echo intensity without refocusing pulses (S0) and that with ones
(∆S=(S-S0)/S0)) is used as a measure of this phenomenon.
In the case of isolated I-P pair the REDOR fraction obeys to the equation:
∫ ∫−=− ππ
τπ
2
0
2/
00
0 αββsin)βcosβsinαsin24cos(211 ddDn
SSS
r
where n is the number of rotor cycles in pulse NMR sequence, τr is the sample rotation
period, the angles α and β define the position of internuclear I-P vector with respect to the
sample frame and D is the dipolar coupling constant. Integration provides averaging of all
possible I-P vector positions in a disordered powder sample.
Distance measurement is based on the fact that the dipole constant depends on internuclear
distance rIP:
πµ
πγγ
420
3IP
PI
rD h=
where γI,P is the gyromagnetic ratio for spins I and P; µ0 the universal magnetic constant.
In the case of multiple spins IPn, the situation becomes much more complicated.50 A lot of
additional parameters as I-P-I angles, number of interacting spins, possible spin motion and
so on should be included, which makes distance measurements ambiguous. For this reason,
we used a simplified model in which the REDOR fraction could be described by a simple
monoexponential curve:
)exp(10
0reffnD
SSS τ−=
−
where Deff is a constant, proportional to the dipole constant D mentioned above. This
simplification is possible if we assume that the IPn system is connected with a “spin
temperature bath”. Under dipole coupling, refocusing this bath removes continuously the
53
magnetization from IPn system. This case can be described in a manner similar to the simple
nuclear spin relaxation phenomena that has monoexponential behavior.
Comparison between constant Deff, obtained from a sample with unknown Si-H distances,
and D0 from the reference compound with well-defined distance r0, makes possible to
calculate “effective” Si-H distance in sample under investigation:
0
303
DrD
r effeff =
As reference sample we used 1,3,5,7,9,11,13,15-Octakis
(dimethylsilyloxy)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane with a well-defined Si-H
distance in the dimethylsilyl group, viz. 1.5 Å (figure 13).
Figure 13. reference sample for REDOR (29Si-1H) measurements: 1,3,5,7,9,11,13,15-Octakis (dimethylsilyloxy)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane.
An example of a REDOR curve for silicalite-1 crystals with template molecules inside is
shown in figure 14.
0.00 0.20 0.40 0.60 0.80
1.00 1.20
0 10 20 30 40 50 60
number of rotor cycles
REDOR fraction
Figure 14. Typical REDOR (29Si-1H) curve measured for Silicalite-1 crystals.
In order to exclude any possible influence of protons from non-template molecules,
the precursor solution was prepared in D2O and evaporation of ethanol molecules was
54
performed. Three main peaks can be assigned to the methyl groups and the methylene groups
of TPA by 1H NMR (not shown). The precursor solution was quickly frozen in order to use
the REDOR technique. The formation of solid frozen state was confirmed by observation of
broadening in 1H NMR spectra. All the REDOR studies were carried out at 200 K. Q4 and Q3 29Si peaks in CP-MAS spectrum of the frozen sample have an overall higher intensity that in
the one-pulse excitation spectrum of the liquid sample (Figure 15). This means that 1H-29Si
cross-polarization for silicon atoms in these positions are more effective. Cross-polarization
efficiency depends on Si-H proximity as well as some other factors. The larger intensities in
Q3 and Q4 positions can indicate spatial proximity of protons and silicon atoms in these
positions. Results of effective distance measurements by different REDOR procedures are
shown in table 1:
Figure 15. a) 29Si NMR precursor solution from TEOS – TPAOD – D2O at 288 K, 500 Hz, MAS b) 29Si-1H CP-MAS spectrum of the same solution at 200 K, 2.5 kHz.
Sample Technique Effective Si- HTPA distance (Å)
Silicalite-1 crystals REDOR, 2,5kHz MAS 3.39
Silicalite-1 crystals CP-REDOR, 2,5kHz MAS 3.67
Frozen nanoslab solution CP-REDOR, 3kHz MAS 3.44
Table1. Effective Si-HTPA distance determined from REDOR experiments.
These results evidence close contacts between TPA molecules and silica at the early
stages of zeolite formation. Values for the silicalite-1 crystals are in good agreement with
crystallographic data.51 These interactions, detected prior to the development of long-range
order, are similar to those encountered by TPA cations encapsulated in the silicate-1 crystals.
55
This supports a model in which structure direction involves the preorganization of colloidal
entities resembling the zeolite topology. This is the first direct measurement of interatomic
distances in embryonic species present in zeolite synthesis. Burkett et al. observed van der
Waals interactions based on the efficiency of 1H-29Si CP MAS NMR of freeze-dried samples
obtained at different intervals during the synthesis of ZSM-5.8 The present method does not
require invasive sample treatment and provides a direct interatomic distance.
27Al MQ MAS
Distribution of aluminum in the framework of ZSM-5 is of prime importance. The
occurrence and spatial ordering of Al atoms in the zeolite framework control the properties of
the catalytically active sites. Very similar scattering powers of Si and Al for X-rays and small
crystallite size hinder the efficiency of synchrotron X-ray diffraction. The primary
information available from 27Al MAS NMR is related to the coordination of aluminum.
Generally tetrahedral lattice aluminum in zeolites gives only one signal in the range of 50-65
ppm. It is very difficult to distinguish between two non-equivalent framework Al sites. The
reason for this difficulty is the overlap of lines induced by the second-order broadening and
line shift of 27Al resonance due to quadrupolar interactions. A new 2D multiquantum MAS
(MQ MAS) NMR technique has been developed recently.52 It is then possible to separate the
contributions from the chemical shift (related to the crystallographic site) and quadrupolar
broadening. Han et al. were able to demonstrate the preference of Al atoms for particular T-
sites in as-made and calcined ZSM-5 using 2D 27Al MQ MAS NMR.53 This preference was
observed for a wide range of Si/Al ratios (from 250 to 14). They suggested that Al atoms are
distributed over the 12 possible T-sites with chemical shifts ranging from 51.7 to 56.6 ppm.
We applied the same method for samples prepared from the TEOS – Al – TPAOH –
H2O system having Si/Al ratios of 150, 100 and 50. The 2D 27Al MQ MAS NMR spectrum
of the Si/Al = 50 sample is depicted in figure 15. We were unable to distinguish two or more
crystallographic distinct Al sites for the three different Al contents. Consequently, it is not
possible to determine whether aluminum substitution is random or spatially ordered in the
present case. This study was driven with the hope of resolving a specific T-site for Al
substitution. We would expect that there is a particular pathway of Al incorporation due to
the nanoblock preparation. Nanoslabs indeed exhibit particular Q2, Q3 and Q4 sites but only
Al(4Si) are detected.
56
Figure 16. 2D 27Al MQ MAS NMR of calcined ZSM-5 with Si/Al = 50 made from the TEOS – Al –
TPAOH – H2O system.
3.2.4 Conclusion
Additional NMR experiments gave us some more insight on the formation of MFI
nanoslabs. 27Al NMR revealed the progressive increase of aluminum connectivity with
addition of silica in Al doped TPAOH silica. The exclusive presence of Al (4Si) atoms in
zeolites synthesis starting mixtures irrespective of the silica source makes these systems very
particular. Former studies on the liquid phase of zeolite synthesis did not exhibit such
features.54 The growth of silicalite-1 crystals was followed by in situ 27Al NMR. Results
support a nanoblock-based growth mechanism as indicated by the unique coordination of Al
atoms. Nanoblocks are formed by favorable van der Waals interactions resulting in a
structure direction effect of TPA cations towards silica. These close contacts were directly
detected using the REDOR technique by direct measurement of the Si-HTPA distance, which
appears to be similar to that found in as-made silicalite-1 crystals. 2D 27Al MQ NMR did not
univocally determine the crystallographic position of Al framework substitution via
nanoblock-mediated crystallization. Study of other heteroatoms (Ti or Fe) incorporation
could be more informative since other techniques can be applied.
57
References 1 Szoztak, R. Molecular sieves; Blackie Academic & Professional, 1998. 2 Venuto, P. B. Microporous Mater. 1994, 2, 297-411. 3 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 4 Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework type, Fifth Revised Edition, Elsevier, 2001. 5 Lok, B. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. 6 Gies, H.; Marler, B. Zeolites 1992, 12, 42. 7 Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials: zeolites, clays and nanostructures; Marcel Dekker, New York, 1997, 1-34. 8 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4467-4653. 9 Gougeon, R.; Delmotte, L.; Le Nouen, D.; Gabelica, Z. Microp. Mesop. Mat. 1998, 26, 43. 10 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094. 11 a) Harris, R. K.; Knight, C. T. G. J. Mol. Struct. 1982, 78, 273. (b) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1525. (c) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1539. (d) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457. 12 (a) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Ebert, R.; Lippmaa, E. T.; Alla, M. Z. Anorg. Allg. Chem. 1980, 465, 15. (b) Hoebbel, D.; Garzo, G.; Englehardt, G.; Vargha, A. Z. Anorg. Allg. Chem. 1982, 494, 31. (c) Hoebbel, D.; Vargha, A.; Engelhardt, G.; Usjszaszy, K. Z. Anorg. Allg. Chem. 1984, 509, 85. (d) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites, John Wiley & Sons, New York, 1987. 13 (a) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (b) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. 14 (a) Harris, R. K.; Parkinson, J.; Samadi-Maybodi, A. J. Chem. Soc. Dalton Trans. 1997, 2553. (b) Kinrade, S. D.; Donovan, J. C. H.; Schach, A. S.; Knight, C. T. G. J. Chem. Soc. Dalton Trans. 2002, 1250-1252. 15 Jacobs, P. A.; Martens, J. A. Synthesis of high-silica aluminosilicate zeolites, Studies in Surface Science and Catalysis Series; Elsevier Science, New York, 1987, Vol. 33. 16 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 17 Kirschhock, C. E. A.; Kremer, S. P. B; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 4897-4900. 18 Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637. 19 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 20 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 21 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 22 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088.
58
23 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11, 36-43. 24 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 25 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 26 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 27 Narayanan, T.; Diat, O.; Bösecke, P. Nucl. Instrum. Methods Phys. Res. A 2001, 467-468, 1005-1009. 28 Bonse, U.; Hart, M. In Small-angle X-ray scattering; Gordon and Breach, New York, 1967, 121. 29 Diat, O.; Bösecke, P.; Lambard, J.; De Moor, P-P. E. A. J. Appl. Crystallogr. 1997, 30, 862. 30 Iler, R. K. The chemistry of silica, John Wiley and Sons, 1979. 31 Wijnen, P. W. J. G.; Beelen, T. P. M.; de Haan J. W.; Rummens, C. P. J.; van de Ven, L. J. M.; van Santen, R. A. J. Non-Cryst. Solids 1989, 109, 85. 32 Engelhardt, G.; Jancke, H.; Mage, M.; Pehk, T.; Lippma, E. J. Organometal. Chem. 1971, 28, 293. 33 Mortlock, R. F.; Bell, A. T.; Radke, C. J. J. Phys. Chem. 1991, 95, 372. 34 Knight, C. T. G.; Kinrade, S. D. J. Phys. Chem. B 2002, 106, 3329-3332. 35 Knight, C. T. G.; Kinrade, S. D. Inorg. Chem. 1988, 27, 4253. 36 Boxhoorn, G.; Sudmeijer, O.; van Kasteren, P. H. G. J. Chem. Soc. Chem. Comm. 1983, 1426. 37 Keijsper, J. J.; Post, M. F. In Zeolite synthesis; Ocelli, M. L., Robson, H. E., Eds; ACS Symposium Series 398, American Chemical Society: Washington, D.C., 1989, 28. 38 Flanigen, E. M. Adv. Chem. Ser. 1973, 121, 119. 39 Derouane, E. G.; Detremmerie, S.; Gabelica, Z.; Blom, N. Appl. Catal. 1981, 101, 101. 40 Barrer, R. M. Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 41 Mortlock, R. F.; Bell, A. T.; Radke, C. J. J. Phys. Chem. B 1991, 95, 7847. 42 Fahlke, B.; Mueller, D., Wieker, W. Z. Anorg. Chem. 1988, 562, 141. 43 Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952. 44 Gullion, T.; Schaeffer, J. Adv. Magn. Reson. 1989, 13, 57. 45 Gullion, T.; Schaeffer, J. J. Magn. Reson. 1989, 81, 196. 46 Harvey, G.; Glasser, L. S. In Zeolite synthesis; Ocelli, M. L., Robson, H. E., Eds; ACS Symposium Series 398, American Chemical Society: Washington, D.C., 1989, 49. 47 Kirschhock, C. E. A.; Ravishankar, R.; van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. 48 Gullion, T.; Schaeffer, J. Adv. Magn. Reson. 1989, 13, 57. 49 Gullion, T.; Schaeffer, J. J. Magn. Reson. 1989, 81, 196. 50 Goetz, J. M.; Schaefer, J. J. Magn. Reson. 1997, 127, 147. 51 Van Koningsveld, H.; van Bekkum, H.; Jansen, J. C. Acta Crystallog., Sect. B 1987, B43, 127. 52 Frydman, L.; Hardwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367. 53 Han, O. H.; Kim, C., Hong, S. B. Angew. Chem. Int. Ed. 2002, 41, 469. 54 Swaddle, T. W.; Salerno, J.; Tregloan, P. A. Chem. Soc. Rev. 1994, 23, 319.
59
4
In situ SAXS/USAXS investigation on aluminum incorporation in the synthesis of colloidal TPA-ZSM-5
*
The formation of nanoparticles and growth of TPA-ZSM-5 crystals from clear homogeneous solutions were monitored by use of X-ray scattering techniques. An introduction to small-angle X-ray scattering is given since the understanding of scattering patterns is not always straightforward. The equipment used at the European Synchrotron Radiation Facility (ESRF) will be presented as well as the specially designed sample cell allowing to perform in situ measurements. SAXS and USAXS experiments using synchrotron radiation showed the influence of aluminum on the crystal growth rate and on the size of the aluminosilicate nanoparticles spontaneously formed at RT due to the interactions between silica and the organic template. Increasing aluminum content results in slightly smaller nanoprecursors and a slower crystal growth. The final size of the crystals was found to increase with higher aluminum content indicating that less viable nuclei are formed. The results presented here are consistent with the proposed mechanism for organic-mediated crystallization of low aluminum containing or all-silica MFI, stating that crystal growth is a nanoblock-based aggregation mechanism rather than monomer addition. The data show that aluminum is incorporated in the nanoparticles together with an effect on the nucleation and crystal growth rate. The moderate incorporation of aluminum, however, does not change the pathway of zeolite crystallization.
* Reproduced in part from: C. J. Y. Houssin; P. J. Kooyman; B. L. Mojet; R. A. van Santen ‘In situ SAXS/USAXS investigation on aluminum incorporation in the synthesis of colloidal TPA-ZSM-5’ submitted.
60
4.1 Introduction Zeolites are crystalline aluminosilicates of natural or synthetic origin with many bulk
industrial applications.1 Some of the applications of these microporous solids are related to
their very high surface area, their catalytic sites, their multidimensional pore channels and
their pore size uniformity.2 Although natural zeolites are available, commercial zeolites are
almost exclusively synthetic because a very high degree of purity and uniform particle size
distribution can be obtained from inexpensive materials. Traditionally, zeolite chemists have
focused on the relations between the starting mixtures (Si/Al ratio, alkalinity, organic
molecules) and final product in terms of crystallite size and topology. Consequently, very
little is understood on the mechanism by which these complicated macroscale objects self-
assemble. The possible discovery of new zeolite topologies and the elucidation of the self-
assembly process of microporous materials drives research towards understanding the
assembly mechanism.3 Investigation of the structure-directing effect that controls the
ordering of silica on the nanometer and micrometer scales is also of more general interest.
Biomimicking the process of diatom formation could provide the production of innovative
and advanced materials.4,5
The preparation of high-silica zeolites generally starts from alkaline mixtures containing
silica and alumina sources, structure-directing agents and water.6 Only a few examples of
non-aqueous systems have been reported.7,8 The reactions are sometimes carried out at
atmospheric pressure but more often under hydrothermal conditions. Lately, many efforts
have been undertaken to understand the role of organic structure-directing agents. It has long
been accepted that size, geometry and charge distribution influence the final structure of the
zeolite.9 A lot of efforts have been devoted recently to the study of the interaction between
organic molecules and silicate species.10,11 No covalent bonds are formed but van der Waals
interactions govern the preassembly process. Thus, in the case of tetraalkylammonium ions
these relatively weak interactions are significant enough to favour the formation of specific
silicate species depending on the alkyl chain length. It is generally agreed that structure-
directing agents spontaneously form well-defined oligomeric species in aqueous silicate
solutions at room temperature.12 Consequently, the question arises regarding the relationship
between the ability of organic structure-directing agent to organize the silica into organic-
61
inorganic composite species at the early stages of the synthesis and the final structure of the
zeolite.
Zeolite ZSM-5 is one of the most frequently used microporous crystalline solids in
chemical industry for its unique catalytic properties and separation applications. Furthermore,
silicalite-1, or aluminum free ZSM-5, can easily be synthesized with tetrapropylammonium
(denoted TPA) as structure-directing agent. This all-silica zeolite was the subject of
considerable academic research towards the understanding of organic mediated zeolite
formation, mainly because TPA is believed to be a very efficient structure-directing agent.
Recently, the group of Martens et al. have attempted to follow the polycondensation of silica
in a concentrated TPAOH solution using 29Si NMR.13,14 They suggested that a precursor
species in which the MFI topology is already fully developed was spontaneously formed and
then aggregated to result in stable subcolloidal particles denoted nanoslabs.
In addition to seeking insight into these interactions on a molecular level, information is
also needed on a larger scale when long-range order and crystalloids arise. Crystals can be
detected by traditional X-ray diffraction whereas nucleation is less suitable to be followed by
typical spectroscopic techniques as it involves nanosized species. Therefore there is a gap of
spectroscopic techniques between formation of oligomeric species and crystal growth.
There has been a rapid pace of progress probing nanoscopic species during the all-silica
MFI zeolite synthesis.15 To accomplish this goal, a combination of in situ small-, ultra-small-
and wide-angle X-ray scattering (respectively SAXS, USAXS and WAXS) allowed our
group to probe zeolite synthesis over a large length scale, from a few nanometers to
micrometer dimensions.16,17,18 As mentioned earlier, the weak interactions between structure-
directing agent and silicates result in very fragile intermediates. We demonstrated that this
combination of techniques appeared to be very reliable to follow the formation and
consumption of nanoparticles during the self-assembly of organic mediated zeolite synthesis.
De Moor et al. have observed nanoparticles for the silicalite-1 synthesis, which have been
proposed to play a key role in the nucleation and crystallization.19 Subcolloidal 2.8 nm
primary units are omnipresent in the synthesis mixtures while their aggregation depends on
the alkalinity. The formation of the aggregates is an essential step in the nucleation process.
In each of the above-mentioned cases, silicalite-1 was the zeolite studied but it should be
kept in mind that the large majority of applicable zeolites contain heteroatoms, aluminum
being the most important one. Substitution of Al for Si (as T atoms) results in the formation
of a negatively charged framework that tends to coordinate more strongly to cations. T-O
bond lengths and T-O-T angles are believed to influence the formation, size, charge and
62
colloidal properties of the nanoparticles. The main purpose of the present work is to study the
formation and consumption of these nanoparticles in situ during ZSM-5 synthesis by adding
aluminum to two different representative all-silica synthesis mixtures that formerly have been
studied extensively. The effect of the incorporation of aluminum on the nucleation and crystal
growth are compared to the aluminum-free systems. Despite the great deal of work on the
subject of silicalite-1 synthesis, the present study proposes for the first time in situ
investigation of the influence of heteroatom framework substitution on different length
scales, from the formation of zeolite nanoprecursors to the growth and shape of the final
crystals. Moreover, this study provides more insight into the effect of variation of the silica
source on processes from the formation of colloidal particles to the ultimate size of ZSM-5
crystals. Results are discussed regarding the growing evidence that synthesis of organic
mediated zeolites occurs via self-assembly of specific nanoparticles.
4.2 Small-angle X-ray scattering and synchrotron radiation
The purpose of this section is to present a short introduction of some principles of
small-angle X-ray scattering and a description of an experimental setup used in synchrotron
radiation. The scattering of X-rays is generally used to describe structural ordering of
materials. The interaction of X-rays with matter is basically influenced by the electron-
density distribution and its efficiency increases linearly with the atomic number. In typical X-
ray diffraction experiments, the range over which a certain ordering can be probed is usually
of the order of the wavelength λ of the incident X-ray radiation. Diffraction angles at which
positive interferences occur follow the well-known Bragg relation:
nλ = 2d sin(θ)
Wide-angle X-ray diffraction typically involves angles from 5° to 90°. If some
structural information is needed at larger d-spacings, either the wavelength of the incident
beam should be increased or one has to investigate the system at lower angles. Laboratory
SAXS equipments generally uses the CuKα wavelength (1.54 Å), which would give an angle
of 0.88° for probing distances of 5 nm. Moreover, the scattering efficiency decreases with
longer wavelength. The general scattering of two points is depicted in figure 1.
63
2dsin(θ)
2θ
d
2θ
incident beam
2θ
Ds1
2
Figure 1. Scattering by two point centers.
The vector q is defined as q = (2π/λ) (s2-s1) (bold type indicates vectors) in which the unit
vectors s1 and s2 define the direction of incident and scattered X-rays respectively. The
magnitude of the vector q is directly related to the scattering angle θ:
q = (2π/λ) sin(θ).
The incoherent or Compton scattering (scattering arising from electron transitions in the
irradiated atoms) can be neglected at small-angles.
The main difference between classical XRD and SAXS is that wide-angle scattering is
aimed at studying periodic arrangement of identical scattering centers of particles (usually in
all three dimensions) whereas in small-angle scattering these particles are not ordered
periodically and, in the present experimental conditions, are embedded in a water matrix.
When elementary particles are uniform, a reasonable approximation for the measured
intensity I(q) can be written as20,21:
I(q) = (N/V) P(q).S(q)
where N/V is the number density of particles or individual scatterers in the sample. The form
factor P(q) is related to the geometry of the individual particles. The structure factor S(q)
reflects the spatial distribution and correlation of the scattering particles in the matrix.22 The
general mathematical expression of the form factor using the Debye approximation is20:
64
∫
∫= L
L
ee
drrrg
drrqr
qrrgNIqP
0
2
0
2
2
)(
)sin()()(
where Ie is the scattered intensity per electron, Ne the number of electrons per scatterer and
g(r) the electron density correlation function. This general equation is particularly complex
and analytical solutions have been found for only a limited number of morphologies.23 For
example, in the case of spherical particles of size r0 and electron density ρ in a surrounding
medium of electron density ρ0, the form factor can be described as:
2
30
00020
2
)()cos()sin(
3)()(
−−=
qrqrqrqr
VqP ρρ
The expression for P(q) at very small angles (qr0« 1) compared to the reciprocal value of the
size of the particles (r0) is given by the approximation of Guinier:
)5
exp()()(2
02
20
2 rqVqP −−= ρρ
At large scattering vectors (qr0 » 1), the form factor can be approximated by Porod’s law:
40
20
2
)(1)(
29)(
qrVqP ρρ −=
P(q) decreases then as q-4, which makes log I versus log q-4 plots very convenient.
The quantity S(q) relates to the contribution due to the structural arrangement of particles. In
many experiments, the concentration is such that this contribution cannot be neglected. For
very dilute solutions or synthesis, S(q) is a constant. The mathematical expression of S(q) is
obtained from the pair correlation function g(r) (representing the chance to find another
particle within a distance r) via a Fourier transform21:
65
[ ]∫∞
−+=0
2 )sin(1)(41)( drqr
qrrrgVNqS π
When scattering particles form aggregates with mass fractal properties, S(q) can be evaluated
as:
[ ])()1(sin)11(
)1()(
11)( 1
2/)1(22
0
ξ
ξ
qtgD
q
DDqr
qS fD
ffDf
f
−
−−
+
−Γ+=
where Γ(x) is the gamma function, ξ is the size of the aggregates and Df is the fractal
dimensionality (Df<3). At large scattering angles (r0<q-1<ξ), P(q) ≈ 1 and I(q) ≈ S(q); the
expression of I(q) simplifies as: fDqqI −=)(
Df can then be informative about the way aggregation or transformations occur in the
structure formed.24,25
Surface roughness can also be investigated using SAXS. It has been demonstrated26,27 that
particles with surface fractal properties show a decay of: )6()( sDqqI −−=
where Ds is the surface fractal dimensions. Typically Ds falls in a range between 2 and 3 so
that the power law exponent lies between –3 and –4. Consequently the magnitude of the
power law exponent indicates if the scatterer is a mass fractal or a surface fractal object.
The relation between the scattering pattern from particles with both mass fractal and surface
fractal properties on different length scales is shown schematically in figure 2. For very low q
values, incoherent scattering or bigger structures are observed. At larger q values, the
scattering of the aggregates is dominated by the structure factor S(q), describing the spatial
arrangement of the primary particles inside the aggregates. The crossover point between
scattering at large d-spacings (low q) and the region dominated by the structure factor gives
the size of the aggregates. At even smaller length scales, the scattering pattern is determined
by the form factor of the primary particles. A crossover between the regions where P(q) and
S(q) dominate the scattering intensity is observed from which the size of the primary particles
66
can be measured. In the following sections, we will also use the term ‘characteristic length’
as a general expression which does not need any assumption on the shape of the particles.
1/ξ log q1/r0
log I
-Df
D -6s
S(q)
P(q)
r0
ξ
Figure 2. Schematic representation of an aggregate of size ξ with mass fractal properties which is
built of primary particles of size r0 and the corresponding X-ray scattering pattern.28
SAXS and USAXS high-brilliance beamline at the ESRF
The high-brilliance beamline ID02 was primarily intended to SAXS experiments
using a highly monochromatic beam with very low divergence and small cross-section
(typically 100 µm to 300 µm).29 The beamline is now designed for time-resolved
simultaneous SAXS/WAXS; high-resolution USAXS is also available. ID02 is mainly
dedicated to the study of soft condensed matter. The optics are optimized for experiments
using a fixed wavelength at around 1 Å but a range between 0.73 Å and 1.55 Å is accessible.
There are two experimental hutches devoted to SAXS/WAXS and USAXS respectively. A
general layout of the beamline is depicted in figure 3. Figure 4 shows the motorized table
available in the SAXS/WAXS experimental hutch.
67
Figure 3. Schematic representation of the combined SAXS/WAXS and USAXS setup at beamline
ID02 of the European Synchrotron Radiation Facility, Grenoble, France. The sample position is fixed
and the SAXS detector can be moved automatically in a vacuum tube from 0.75 m to 10 m from the
sample.
Figure 4. Motorized table available in the SAXS/WAXS hutch where three sample cells (see
experimental section) were inserted and could be translated in any direction.
68
The use of high-brilliance radiation allows users to probe microstructures and dynamics of
soft matter and liquid systems from 1 nm to a few microns, and down to a millisecond time
range. A high-resolution Bonse-Hart camera30 is installed in the second experimental hutch of
the beamline (Figure 5). More details about this setup are given in the experimental section of
this chapter.
Figure 5. Bonse-Hart setup at the high-brilliance beamline ID02 at ESRF.
4.3 Experimental Section
Zeolite synthesis. Aluminum containing template solution was first prepared as follows:
aluminum metal powder was dissolved overnight in a concentrated solution of TPAOH (Alfa,
40%) and then filtered through a syringe filter (Whatman 0.45 µm). 27Al NMR of the
resulting clear solution indicates that tetrahedral Al(OH)4- is the only aluminate species
present in the solution.
TEOS system: 9 g of TEOS (Acros, 98%) was added dropwise to a 40% aqueous solution
of TPAOH containing the appropriate amount (for Si/Al= 150, 100 and 50) of the above-
mentioned Al(OH)4- solution under vigorous stirring. After 30 min, 9 g of distilled water was
added dropwise and the resultant mixture was stirred continuously for 12 h to ensure
complete hydrolysis of the silica source.
Silicic acid system: The recipe used is based on a patent of Exxon Chemicals previously
employed in our group.31 0.411 g of NaOH was dissolved in 15 g of 20% TPAOH, already
containing Al(OH)4- as mentioned above, followed by a spoonwise addition of 4.05 g of
silicic acid (Baker, 10.2% H2O). The milky suspension was boiled under stirring for 10 min
to obtain a clear solution. The mixture was rapidly cooled down to room temperature in a
water bath. Distilled water was added for the correction for loss of water during boiling. The
69
resulting clear solution was filtered through a 0.45 µm syringe filter. The sample was then
ready for measurement and never aged more than 1h at room temperature before heating to
the reaction temperature. The reaction temperature was 125 °C.
Sample Cell. An electrically heated brass holder containing a rotating circular sample cell,
designed in our group, allowed us to perform in situ measurements.18 The sample cell rotates
at an approximate rate of 2 rpm thus keeping the solution homogeneous, preventing the
precipitation of zeolite crystals and reducing the sample exposure to a small spot. The sample
was inserted between two mica windows (Attwaters and Sons) separated by a PTFE spacer
(0.5 mm thick). Heating of the sample from RT to the reaction temperature (125 °C) requires
only 2 minutes.
SAXS and USAXS. The combined SAXS and USAXS experiments were performed on the
high-brilliance beamline ID02A at the European Synchrotron Radiation Facilities, Grenoble,
France.29 This beamline uses a highly monochromatic beam with very low divergence and
small cross section. The SAXS setup consists of a pinhole camera with a beam stop located in
front of an image-intensified 2-D CCD camera. Sample-to-detector distances of 1.5, 2.5 and 6
m were used. Conversion of detector pixels (CCD camera) to the scattering vector q (nm-1)
was performed with the help of a lupolen polyethylene (BASF) sample. The X-ray
wavelength was 0.99 Å and an incident X-ray beam with a cross section of 0.2X0.2 mm2 was
used. The high brilliance allowed us to record a SAXS pattern in less than 1s. Data were
corrected for detector response and background using a water reference sample at the
corresponding temperature.
USAXS patterns were recorded using a Bonse-Hart type of X-ray Camera.30,32 The
available range of scattering vector q was 0.001-0.14 (nm-1) with q = (4π/λ)sin2θ and λ = 1.0
Å. A first crystal analyzer [Si(220)] was employed to scan the angle, then the X-ray beam
was collimated in the vertical direction with a second analyzer crystal [Si(111)]. The detector
was a high-dynamic range (≈107 counts/sec) avalanche photodiode. The beam size at the
sample was about 1X2 mm2. Several scans (4-5) over successive 2θ ranges with sufficient
overlap were recorded with different degrees of attenuation of the incident X-ray beam, so
that the intensities on the detector were in the linear range. For each sample, a rocking curve
spectrum was systematically measured, which consists of the scattering produced by the
setup, background scattering and the small intensity fluctuations of the beam. Due to the
strong scattering of silica, the contribution of water scattering was negligible.18 A complete
spectrum could be recorded in 15 min.
70
Figure 5. Rotating electrically heated sample cell used for X-rayscattering experiments. Right: close-up on the rotation gear.
NMR. 27Al NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 130.3 MHz
using a 4 mm probe head at a spin rate of 1.7 kHz. In a typical measurement, 3000 spectra
were accumulated with a pulse length of 3µs and a repetition time of 1s. Chemical shifts are
referred to external aqueous AlCl3.
Electron microscopy. High-resolution transmission electron microscopy (HRTEM) was
performed using a Philips CM30UT electron microscope with a field emission gun as the
source of electrons operated at 300 kV. Samples were mounted on a Quantifoil microgrid
carbon polymer supported on a copper grid by placing a few droplets of a suspension of
crystals in water on the grid, followed by drying at ambient conditions.
4.4 Results
In order to check whether aluminum was incorporated in the zeolite framework, several
experiments have been performed. First, figure 7A shows the 27Al MAS NMR spectrum
recorded for the starting synthesis mixture using TEOS as silica source and a Si/Al ratio of
50. This spectrum shows the exclusive presence of tetrahedrally coordinated aluminum in the
71
clear solution. The lineshape and relatively low intensity observed are typical for
aluminosilicate solutions.33 Figure 7B displays the 27Al MAS NMR spectrum of the calcined
solid obtained from the hydrothermal treatment at 125 °C for 24h of the solution mentioned
above. It shows that aluminum is mostly tetrahedrally coordinated and is part of the zeolite
framework. Secondly, isopropylamine decomposition showed that Brønsted acidity resulting
from the aluminum incorporation was indeed present in the sample. Crystallinity was verified
using XRD and all the samples exhibited the MFI topology. These features were found in
every aluminum-containing sample in this present study. We can then assume that the main
phase obtained was zeolite ZSM-5 for every sample.
-101030507090
54 ppm
ppm
A
-101030507090
53.7 ppm
ppm
B
Figure 7. 27Al NMR spectra of A) starting synthesis mixture containing colloidal nanoparticles with
Si/Al = 50, B) calcined zeolite crystals with Si/Al = 50. Some octahedral aluminum (chemical shift ~0
ppm) has formed during calcination.
Formation of nanoparticles at RT.
It has been recognized that dissolution of silica in concentrated solution of TPAOH leads to
the spontaneous formation of well-defined nanoparticles.12,16,34 Figure 8 shows the small-
angle X-ray scattering curves obtained from two TPAOH aqueous solutions after addition
and dissolution of silica. Those samples do not contain aluminum but are representative for
those with aluminum that will be described and discussed in more detail below. The features
in the scattering curves are clear since only one type of particles is present in every solution.
However the particle populations are different for the different solutions. When using TEOS
as silica source, the precursor solution exhibits a maximum at log q = 0.26 nm-1,
corresponding to primary particles having a characteristic length of approximately 3.6 nm (d
72
= characteristic length and q = 2π/d). Dissolution of silicic acid displays a maximum at log q
= 0.37 nm-1, arising from smaller particles of 2.8 nm as characteristic length. More
interestingly, the slope of the log q vs log I representation of the scattering data from these
solutions provides information on the morphology of the particles. A slope of –2.2 is
observed in the case of TEOS whereas the slope is –2.6 for the colloidal solution born from
silicic acid. Scattering patterns for the synthesis mixtures with three different Si/Al ratios and
using TEOS as silica source are depicted in figure 9. Again, each of these solutions gave rise
to scattering curves showing only one maximum corresponding to a unique particle size
population. Nevertheless, the three maxima are not located at the same position. The q value
of the maximum (at around 1.7 nm-1) increases as the aluminum content increases. Moreover,
the overall intensity decreases at high Al content.
-1.6
-1.3
-1
-0.7
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
TEOS
Silicic acid
slope -2.6
slope -2.2
log q (1/nm)
log I (a.u.)
Figure 8. SAXS pattern of the nanoparticles obtained after silica dissolution using silicic acid and
TEOS as silica sources.
0.02
0.025
0.03
0.035
0.04
0.2 0.7 1.2 1.7 2.2
Si/Al=100
Si/Al= inf
Si/Al=150
I (a.u.)
q (1/nm)
inf. 1.69 3.72150 1.74 3.61100 1.78 3.53
Si/Al q(1/nm) size (nm)
Figure 9. SAXS patterns on the RT dissolution of Al-TPA-TEOS-H2O system using various Si/Al
ratios: infinite, 150 and 100.
73
Zeolite synthesis.
All synthesis mixtures prepared in the present study have been heated at 125 °C in the
rotating cells and lead ultimately to ZSM-5 as checked by XRD. The crystallization of Si-
TPA-MFI from clear aqueous solutions with varying Al content was studied in situ with
SAXS and USAXS. For the SAXS experiments two sample to detector distances have been
chosen to reveal the formation and consumption of colloidal particles in a range from 2 nm to
50 nm. To show the influence of Al content on the formation and consumption of colloidal
particles, figure 10 displays SAXS patterns of three synthesis mixtures using TEOS as silica
source after 25 minutes of heating at 125 °C. One can clearly see in figure 11 that the
scattering at q = 1.7 nm-1 has almost disappeared in favour of a pronounced shoulder at
around 0.6 nm-1. The size of the particles giving rise to this intensity is approximately 10 nm
and they are assumed to be aggregates of the previously described smaller particles formed at
RT. The effect of the Al content on the formation of these aggregates results in a change of
the crossover position between the Porod regime and the structure factor region. This
crossover value is larger at higher Al content as shown in figure 10. Those results are
consistent with the fact that aluminum tends to form slightly smaller particles at RT, directly
resulting in smaller aggregates.
0.15
0.2
0.25
0.3
0.1 0.3 0.5 0.7 0.9
q (1/nm)
I (a.u.)
Si/Al= inf.
Si/Al= 150
Si/Al= 100
Si/Al crossover (q) size (nm) inf. 150 100
0.56 11.20.60 10.50.63 10
Figure 10. SAXS patterns after 25 min heating at 125 °C of the precursors solution (using TEOS as
silica source). The crossover between the regions where the structure factor and the form factor
dominate is graphically determined as shown by the dashed lines. The crossover position gives the
characteristic length of the precursors.
74
Figure 11. Time-resolved SAXS pattern for the heating of a solution of precursors with Si/Al=100.
Figure 12 shows the crystal growth during two series of synthesis using two different
sources of silica in which the Si/Al ratio was varied from 50 to 150. For both series of
syntheses a linear growth of the average crystal size with the reaction time was obtained.
From the observed USAXS patterns it is clear that the crystal growth rate strongly depends
on the Si/Al ratio. The crystal growth rate decreases with increasing aluminum content. In the
case of TEOS the linear growth rate with reaction time was found to be 1.33, 1.27 and 0.40
nm/min for Si/Al = 150, 100 and 50 respectively. Similar trends were observed when using
silicic acid as silica source. However it is also worth noting that the relative decrease of
crystal growth with respect to the same aluminum content is not similar from one silica
source to another.
Crystal shape and size polydispersity
Figure 13 displays the high-resolution electron microscopy images of the products obtained
after the complete crystallization at 125 °C of high silica TPA mediated MFI synthesis
mixtures having various Al contents and using TEOS as silica source. For Si/Al values of 150
and infinite the crystals are spherical, slightly elliptical and do not show intergrowth.
Moreover the crystal surface appears to be rather smooth and regular. Higher aluminum
contents lead to different crystal morphologies as shown in the case of crystals grown from
75
synthesis mixtures having Si/Al ratios of 100 and 50. Indeed, even though the overall shape is
still spherical, the particles are irregularly shaped and seem to be built up from small
crystallites. The size of most of these particles is larger than that of the low aluminum content
crystals and the size distribution is rather polydisperse. Some crystals smaller than 50 nm can
be seen in Figure 13c. Even though figure 13c displays both large crystals and assemblies of
smaller crystallites, only the latter were observed for the highest Si/Al ratio as shown in
figure 13d. Figure 14 shows log I.q4 vs log q plots derived from USAXS data of the synthesis
with TEOS and Si/Al = 150, 100 and 50 at reaction times when crystals have obviously been
formed.
0
50
100
150
200
250
0 300 600 900 1200
Series3
Series2
Series1
Si/Al rate (nm/min.)
time (min.)
size (nm)
150
100
50
1.33
1.27
0.40
A
0
100
200
300
400
0 300 600 900 1200
Series1
Series2
Series3
Si/Al rate (nm/min.)
size (nm)
time (min.)
0.93
0.71
0.26
inf.
150
100
B
Figure 12. Mean crystal diameter as determined from USAXS patterns for ZSM-5 syntheses using
TEOS (A) and silicic acid (B) as silica source with variable Si/Al ratios.
Morphology of the crystals may be derived from the X-ray scattering of their surface at low
angles. Indeed, the scattering patterns resemble the general scattering features of spheres and
76
certainly not of the typical elongated prismatic MFI shape.2 These features were already
observed by de Moor et al. in the synthesis of silicalite-1 from clear solutions using silicic
acid.16 The presence of wiggles suggests that a good monodipersity in size has been obtained
and it was thus possible to follow the growth of zeolite crystals by means of USAXS. The
size of the growing crystals can be determined by fitting calculated patterns resulting from
particles of a certain size to the measured data. This has been extensively discussed in a
previous paper from our group.16 Basically, the main conclusion was that the most accurate
method is to determine the position of the first maxima and minima.
Figure 13. High-resolution TEM micrographs of Si-TPA-MFI crystals for various Si/Al ratios: a) ∞,
b) 150, c) 100 and d) 50.
77
Crystal aggregation. For long reaction times, aggregation of crystals was found using
USAXS (not shown). This results in an increase in intensity at very low q and the estimated
size of those aggregates would be several µm. Nevertheless no crystal aggregates were
observed by SEM after washing and drying of the product.
4.5 Discussion
The time-resolved SAXS scattering data, presented above, give information on the
formation, size, morphology and consumption of colloidal particles in the synthesis mixtures
with a Si/Al ratio varying from infinite to 150 during the crystallization of ZSM-5. USAXS
results allow us to follow the crystal growth and the average size of the growing crystals.
Formation of precursor solution
A first population of particles was observed upon dissolution of silica in concentrated Al
containing TPAOH aqueous solutions. Obviously, the size of the nanoparticles strongly
depends on the source of silica. These differences may be attributed to a different dissolution
mechanism and rate. TEOS is an organic and monomeric source of silica whereas silicic acid
is an inorganic and polymeric form of silica. Moreover the interface where the formation of
these nanoparticles is supposed to occur is different (liquid-liquid and liquid-solid for TEOS
and silicic acid respectively). The eventual presence of alkali ions in the silicic acid case is
also an important factor. Chapter 2 showed that the particle size was 2.1 nm when sodium
ions were substituted by TPA cations12 while particles of 2.8 nm were obtained with both
cations present in the starting solutions. Even though these three clear solutions -TEOS
system, silicic acid system with and without sodium ions- lead to the same zeolite structure,
the nanoparticles or primary units obtained after silica dissolution differ in size. In spite of the
fact that the same size (2.8 nm) was found when using four different templates for the
synthesis of silicalite-117,19, the present study shows that the size of the primary units is not
specific of a certain zeolite topology but rather depends on the synthesis conditions like silica
source and cations content. Consequently, aluminum is also expected to induce some changes
in the colloidal particle formation and evolution.
The nanoparticles formed in the presence of different aluminum contents using TEOS as
silica source have roughly the same characteristic length (ca. 3.7 nm) and the corresponding
scattering patterns show strong similarities to those obtained in the aluminum free system.
78
However some features are different. First the size of the particles slightly decreases with
increasing aluminum content. We found several characteristic lengths with the aluminum
loading: 3.72, 3.61 and 3.53 nm for Si/Al = infinite, 150 and 100 respectively. There are two
possible reasons for this moderately smaller primary unit size. First, the incorporation of an
aluminum atom in the framework of a nanoparticle introduces a negative charge. Therefore it
is possible that the TPA molecules adsorb more strongly to the hydrophobic surface of the
nanoparticles. Second, since TPA is occluded in the nanoparticles a negative charge is
required to compensate the positive charge of the tetraalkylammonium cation. The presence
of aluminum circumvents or reduces the occurrence of a framework defect. Consequently this
effect induces less distortion of the primary unit internal structure thus giving rise to particles
with a slightly different size.
SAXS also provides information on the morphology of the nanoparticles. However,
deriving a particle shape from X-ray scattering data remains a tricky and delicate exercise. In
the present case the high silica concentration can affect the scattering curve because of non-
negligible interparticle interferences. Consequently any attempt to derive morphological
information from SAXS patterns must be combined with other techniques. TEM was
performed on those precursor solutions as shown in chapter 2 and it appeared that the
particles were tablet like with dimensions of 4 × 4 nm and 4 × 2 nm respectively for the
TEOS and silicic acid cases.34 In the present study the slope close to –2 in the log I vs log q
plot indicates flat nanoparticles and the trends are similar when adding aluminum since the
scattering curve shape does not change significantly. Based on NMR experiments, for the
TEOS case the primary units were proposed to consist of tablets of 4 × 4 × 1.3 nm
dimensions. Combined with these NMR results and TEM studies the scattering curves of
such particles fit very well with experimental data. Moreover spherical particles would give
rise to symmetrical scattering patterns, which are not observed in figure 8. In the log I vs log
q representation an infinite flat sheet will give a slope of -2. Here slopes of -2.2 and -2.6 were
obtained for the TEOS and silicic acid recipes, respectively. This is in agreement with the
theoretical model since a tablet of dimension of 4 × 4 × 1.3 nm (use of TEOS) is not an
infinite sheet and a block of dimensions 4 × 2 × 1.3 nm (use of silicic acid) is closer to a cube
(slope -4) but still exhibits enough characteristic flatness to give a slope close to that of a flat
object.
79
Heating of precursors solutions: zeolite synthesis
In all cases, heating of the starting solutions leads to the specific formation of colloidal
particles with a characteristic length of 10-11 nm. Those are thought to be aggregates of the
nanoparticles formed spontaneously at RT.15 This assumption is strengthened by the fact that
the smaller nanoparticles formed with a higher aluminum content leads to smaller aggregates.
As mentioned in an earlier work on the all silica synthesis,12 the slope of the scattering curve
of those aggregates is approximately –2 (log I vs log q plot) suggesting again that they are
tablet like. Nevertheless it was not possible to derive reliable information on the precise
population after few hours at 125 °C since at that time the system contains nuclei, growing
crystals and most probably still some primary units (their scattering can be overshadowed by
that from their aggregates). From figure 11 it is not obvious that the primary units are still
present when the synthesis temperature is reached. Actually after 1 min of heating in the
rotating cell (approximately 60 °C) there is a spontaneous formation of aggregates
independent of the Al content. Aggregation is thermally activated and the presence of
aggregates seems to be crucial since de Moor et al. showed that they correlate with a faster
crystallization.18 Notably the temperature at which they form in a quantitative way
corresponds to the temperature at which we can obtain colloidal ZSM-5 crystals on a
reasonable time scale. Schoeman et al. have indeed found that colloidal ZSM-5 crystals can
be synthesized at 60 °C under the same synthesis conditions, but below this temperature no
crystalline material could be obtained on an acceptable time scale.35 This again illustrates the
importance of these aggregates in the crystallization process also when aluminum is absent.
Two attempts using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory have been
recently described in the literature36,37 to judge the possibility for primary units to form
aggregates and act as building blocks. These studies came up with opposite conclusions.
Schoeman stated that crystal growth occurs via addition of low molecular weight entities,
most likely the silicate monomer36 while Nikolakis et al. supported a ~3 nm subcolloidal
particle aggregation mechanism.37 But none of them assumed that the primary units were
nanoblocks since they used hard spheres as precursor particles. Indeed the interactions
between the nanoparticles, assuming that they are slab like, strongly influence their colloidal
and electrostatic properties. Kirschhock et al. performed DLVO calculations38,39 based on the
TEOS system, which consists of nanoslabs having dimensions of 4 × 4 × 1.3 nm. It was
found that further aggregation at RT was energetically unfavourable. However, upon heating
of the solution at 100 °C, the potential energy of the slab-like intermediates (similar to those
80
observed in figure 10) as a function of particle distance displays a minimum at about 0.75
nm. Thus intermediates remain physically trapped next to each other. They have time to align
for subsequent fusing.
-6.5
-6.2
-5.9
-5.6
-1.7 -1.4 -1.1 -0.8
log (I.q4)
log q
470 min.
450 min.350 min.
315 min.285 min.
A
-6.8
-6.5
-6.2
-5.9
-1.7 -1.4 -1.1 -0.8
log (I.q4)
log q
690 min.
546 min.
450 min.
410 min.
360 min.
B
-7.2
-6.9
-6.6
-6.3
-1.8 -1.5 -1.2 -0.9
1215 min.
1145 min.
1030 min.
1910 min.
880 min.
log (I.q4)
log q
C
Figure 14. I.q4 vs q data plot for the 3 different Si/Al ratios using TEOS as silica source: A) 150 B)
100 C) 50.
Crystal growth rate
The crystal growth rate for TPA-ZSM-5 synthesis using two different silica sources was
found to be strongly dependent on aluminum content. In agreement with the DLS results of
Schoeman et al.40 we found that the crystal growth rate decreased with increasing aluminum
concentration. Moderate addition of aluminum (lower than Si/Al = 50) did not affect the
crystal growth rate in either case but at relatively low Si/Al ratios the growth was
significantly slower. For the TEOS case, the significant decrease occurred at Si/Al = 50 (0.40
nm/min), 1.33 and 0.40 nm/min being the values for Si/Al = 150 and 100. Nevertheless, if
silicic acid was used as silica source, this decrease was already observed at Si/Al = 100 (0.26
nm/min) whereas lower aluminum loading gave higher rates (0.71 and 0.93 nm/min for Si/Al
81
= 150 and infinite, the latter being slightly lower consistent with the results of de Moor et
al.19).
The changes in crystal growth rates can be explained by the effects of the incorporation of
aluminum atoms into the nanoblocks. A relative change of the charge of the nanoblocks
could have a strong effect on the colloidal properties of those nanoprecursors. In a model
where crystal growth occurs by nanoblock aggregation, the potential energy of the precursors
would be greatly modified by the incorporation of one or several aluminum atoms into these
nanoblocks. Moreover, precursors which are formed with silicic acid are more sensitive since
they are smaller. This could explain why the silicic acid series is dramatically slowed down at
a lower aluminum content than the syntheses using TEOS.
Crystals shape and polydispersity
The shape of crystals is an important feature which can influence fundamental properties of
catalysts such as diffusion, stability and mechanical strength. High-resolution transmission
electron microscopy (HRTEM) allowed us to investigate the shape of the ZSM-5 crystals
after crystallization was completed. Despite the fact that the crystals do not show the
characteristic elongated prismatic form of ZSM-5, this absence is a typical characteristic of
small crystals grown from clear solutions via a fast growth. Not only TEM but also log Iq4 vs
log q can give us information on the shape and size polydispersity of those colloidal crystals.
Indeed, de Moor et al. showed that more pronounced first maxima and minima in the log Iq4
vs log q plot suggest a higher degree of monodispersity.16 This statement was based on
calculated patterns using perfect spheres as a model. Deviation from theoretical curves may
arise from irregularly shaped crystals. From figure 14 showing log I.q4 vs log q plots of
growing ZSM-5 crystals with different Si/Al ratios, it is clear that a lower aluminum content
leads to more pronounced first maxima and minima in the form factor patterns. The
observation using TEM gives us two possible explanations for this trend. First the form factor
is influenced by the higher polydispersity when the aluminum content is increased. Secondly,
the particles are irregularly shaped and built up of small crystallites for Si/Al ratios of 100
and 50, but their overall shape is still close to that of spherical particles. Consequently the
first maxima and minima are not very much affected but the form factor oscillations decay
faster. The size polydispersity for high Al contents also reveals a less homogeneous
distribution of viable nuclei in the induction period and non-simultaneous events as
nucleation and crystal growth proceed.
82
4.6 Conclusion
Hydrolysis of TEOS in aluminum containing TPAOH solutions leads to the
spontaneous formation of discrete colloidal particles with a size slightly dependent on Si/Al
ratio similar to the nanoslabs observed in chapter 2 with pure silica syntheses. Aggregation of
these primary units is also not affected by the incorporation of aluminum and is similar to the
all silica synthesis previously studied. However, the higher aluminum containing synthesis
mixtures exhibit slower nucleation kinetics. Additionally, USAXS experiments allowed us to
show that aluminum influences the colloidal ZSM-5 crystal growth rate, size and
morphology. Two different silica sources have been used and these show similar trends.
Aluminum has a strong influence on the addition of primary units to the growing crystals. We
observed the final size of the crystals to increase with increasing aluminum content, which
could be related to a lower amount of viable nuclei formed. Monitoring in situ the evolution
of aluminum containing zeolite precursors through a large length scale has made it possible to
establish directly its influence on nucleation and crystal growth. Additionally, aluminum has
a striking influence on the morphology of ZSM-5 crystals, pointing out its important role
during nucleation and crystallization. These results are in agreement with a common
crystallization pathway in the TPA mediated synthesis of high silica ZSM-5 based on a
nanoblocks aggregation mechanism even when aluminum is present in the synthesis mixture.
Although this paper focuses on aluminum influence on the formation of colloidal ZSM-5
crystals, it will be important to examine other heteroatoms (such as titanium or iron)
incorporation in these remarkable zeolites nanoprecursors.
References 1 Szoztak, R. Molecular sieves; Blackie Academic & Professional, 1998. 2 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 3 Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials: zeolites, clays and nanostructures; Marcel Dekker, New York, 1997, 1-34. 4 Noll, F.; Sumper, M.; Hampp, N. Nano Lett. 2002, 2, 91-95. 5 Vrieling, E. G.; Beelen, T. P. M.; van Santen, R. A.; Gieskes, W. W. C. Prog. Ind. Microbiol. 1999, 35, 39-51.
83
6 Thompson, R. W. In Molecular sieves, Science and technology, Springer, 1998, 1, 1-33. 7 Huo, Q.; Feng, S.; Xu, R. J. Chem. Soc., Chem. Commun. 1988, 22, 1486-1487. 8 Kuperman, A.; Nadimi, S.; Oliver, S.; Ozin, G. A.; Garces, J. M.; Olken, M. M. Nature 1993, 365, 239-42 9 Gies, H.; Marler, B. Zeolites 1992, 12, 42-9. 10 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4467-4653. 11 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C., Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094-10104. 12 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 13 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 14 Kirschhock, C. E. A.; Ravishankar, R.; Van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972-4978. 15 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 16 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 17 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11(1), 36-43. 18 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 19 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 20 Guinier, A.; Fournet, G. Small-angle Scattering of X-Rays, Wiley, New York, Chapman Hall, London, 1955. 21 Teixeira, J. In Stanley, H. E.; Ostrowsky, N. On Growth and Form: Fractal and non-fractal Patterns in Physics, NATO-ASI Series E, 100, Martinus Nijhoff Publishers, Dordrecht, 1986, 145-162. 22 Glatter, O.; Kratky, O. Small-angle X-Ray Scattering, Academic Press, 1982. 23 Pedersen, J.S. Adv. Coll. Interf. Sci. 1997, 70, 171-210. 24 Meakin, P. In Stanley, H. E.; Ostrowsky, N. On Growth and Form: Fractal and non-fractal Patterns in Physics, NATO-ASI Series E, 100, Martinus Nijhoff Publishers, Dordrecht, 1986, 111-135. 25 Olivi-Tran, N.; Thouy, R.; Jullien, R. J. Phys. I France 1996, 6, 557-574. 26 Bale, H. D.; Schmidt, P. W. Phys. Rev. Lett. 1984, 53, 596-599. 27 Schmidt, P. W. J. Appl. Cryst. 1991, 24, 414-435. 28 Dokter, W. H. PhD thesis, Transformations in silica gels and zeolite precursors, Eindhoven University of Technology, 1994. 29 Narayanan, T.; Diat, O.; Bösecke, P. Nucl. Instrum. Methods Phys. Res. A 2001, 467-468, 1005-1009. 30 Bonse, U.; Hart, M. Small-angle X-ray scattering; Gordon and Breach, New York, 1967, 121. 31 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 32 Diat, O.; Bösecke, P.; Lambard, J.; De Moor, P-P. E. A. J. Appl. Crystallogr. 1997, 30, 862. 33 Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952-1954.
84
34 Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637. 35 Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J-E. Zeolites 1994, 14, 557. 36 Schoeman, B. J. Microp. Mesop. Mater. 1998, 22, 9-22. 37 Nikolakis, V.; Kokkoli, E.; Tirrel, M.; Tsapatsis, M.; Vlachos, D. G. Chem. Mater. 2000, 12, 845-853. 38 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 39 Kirschhock, C. E. A.; Ravishankar, R.; Truyens, K.; Verspeurt, F.; Jacobs, P. A.; Martens, J. A. Stud. Surf. Sci. Catal. 2000, 123, 293. 40 Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J-E. Zeolites 1995, 15, 611-619.
85
5
Zeolite nanoslabs: building blocks
for innovative porous materials*
Zeolite nanoparticles containing tetrapropylammonium (TPA) as primary template are assembled into remarkable mesostructures via a secondary cooperative templating mechanism. These crystal-like silicate structures display a uniform morphology. Their spontaneous formation at room temperature shows the possibility of these zeolite nanoblocks becoming units for a new and innovative class of materials.
* Reproduced in part from: C. J. Y. Houssin; B. L. Mojet; R. A. van Santen ‘Remarkable Assembly of Zeolite Nanoprecursors into Crystal-like Mesostructures’ submitted.
86
5.1 Introduction
Mesoporous silicas have emerged as potentially powerful solids for oil refining, fine
chemicals synthesis and separation.1,2 Since their discovery in 19923, hexagonal mesoporous
aluminosilicate sieves have been the subject of considerable academic research.4 The
formation of these ordered mesophases is the result of the interactions between a surfactant
solution and soluble silica. A judicious choice of surfactant and/or cosurfactants allowed
tailoring the architecture and the pore size (2-50 nm). Despite overcoming the pore diameter
restriction of zeolites (≤ 1 nm), these materials are not crystalline i.e. the silica is locally not
very well organized and is closer to amorphous silica. Consequently, these materials cannot
match the high hydrothermal stability and acidity found in zeolites, their crystalline
microporous analogues. In view of these features, a very competitive research activity in this
area is currently directed towards the design of hydrothermally stable and catalytically active
mesoporous silica-based materials. Several attempts such as post synthesis procedures5,
silylation6, thicker walls syntheses7 have been made to improve these materials but both
acidity and stability still remain inferior to zeolites.
Because these limitations are due to the amorphous character of the walls, one might
expect a considerable improvement by rendering the structure of these walls close to those of
zeolites. Recently, two methods have been employed in that regard. The first procedure
involves the transformation of the walls of a conventional hexagonal aluminosilicate into
crystalline areas.8 The second approach consists of assembling zeolites seeds into hexagonal
mesostructures. Both methods have given a remarkable increase in stability and acidity.9,10
De Moor et al. observed the presence of nanoscopic species (few nm) in the reaction
mixture of the pure silica ZSM-5 organic mediated synthesis over the last few years.11,12,13
These nanoparticles, arising from the interactions between the structure-directing agent
(tetrapropylammonium denoted TPA) and the silica, have been proposed to play a key role in
the nucleation and crystal growth.14 Based on NMR experiments from such synthesis
mixtures a model has been presented by Kirschhock et al. in which the MFI structure is
already fully developed.15 Moreover we have extensively studied these nanoparticles in clear
solutions using small-angle X-ray scattering.16 It has been shown that these particles leading
to the same zeolite topology have a well-defined and optimum size depending on the silica
87
source and the cation content. These results combined with TEM showed the RT spontaneous
formation of MFI zeosil nanoslabs with dimensions of 2.7 × 1 × 1.3, 4 × 2 × 1.3 and 4 × 4 ×
1.3 nm (Chapter 2). A model was then proposed in which zeolites growth occurs via nanoslab
aggregation.17
Here, we propose a new method to prepare silica-based materials with both micro and
mesoporosity. The new route consists of organizing nanoparticles that are the primary
building blocks of zeolites with the MFI topology under the influence of a secondary
template that is typically used to synthesize mesoporous silica materials denoted HMS in the
literature.18 Our results indeed showed that a remarkable organization of these nanoparticles
using a larger surfactant has been achieved without destroying them and this self-organization
has been performed at room temperature. These silicate mesostructures were assembled by
adding slowly at RT a MFI nanoslabs solution (leading to silicalite-1 upon heating) to a
mixture of hexadecylamine in ethanol and water. The sample was maintained under gentle
stirring for 2 days. The as-synthesized product was characterized using SEM, XRD, TGA and
nitrogen adsorption.
5.2 Short review on recent advances on synthesis of M41S
mesostructures
The M41S family of mesoporous materials contains several unique structures that can
be indexed to a hexagonal network (MCM-41, SBA-2 and SBA-3), cubic structure (MCM-
48, SBA-1) and lamellar structure (MCM-50).19 The common features of all those
mesoporous materials is that they exhibit a narrow pore size distribution analogous to
crystalline microporous materials, but within the pore dimension of 1.5-20 nm. The initial
member of this family, MCM-41, was first synthesized as aluminosilicate in alkaline media
using a cationic alkyltrimethylammonium surfactant system.3
A better understanding of the synthesis mechanisms is a key point to the rational
design of mesoporous materials. Unlike individual molecule structure-directing agents
encountered in zeolite synthesis, strong organic intermolecular interactions are responsible
for determining the resulting inorganic framework of M41S materials. The original proposed
mechanistic pathways involved liquid crystal templating (LCT).3 However, this simplified
88
model is unlikely to take place since most of M41S syntheses are not performed in the LCT
concentration regime. Few years after the MCM-41 discovery, Stucky et al. proposed a
cooperative self-assembly pathway.20 This mechanism involves silicate-covered cylindrical
micelles that act as template source. Those rods would first be disordered and polymerization
of silica would lead to the cooperative self-assembly of the final mesostructures. These
concepts highlighted the importance of the interactions between silica and surfactants. This is
believed to be the key factor in the control of the M41S mesostructures. Recently several in
situ investigations have been carried out in order to follow the entire process of MCM-41
related material synthesis.21,22,23,24 Frasch et al. studied the changes of the properties of the
micelles that may be induced by silicate species during the formation of mesoporous
hexagonal silica.24 These results led to a model described in figure 1. The key step is
considered to be the formation of silica pre-polymers. The growing of theses pre-polymeric
species takes place in a cooperative manner with a supply of surfactants. Further
polymerization and organization of the silica-micellar entities occur during the aging
sequence leading ultimately to M41S mesostructures.
Figure 1. Proposed mechanism of mesoporous
silica formation (adapted from ref. 24)
89
Originally the M41S family was synthesized using cationic surfactants. These procedures
have been extended to non-ionic surfactants like polyoxyethylene alkyl ethers or amines
(figure 2A). Particularly, the so-called HMS materials were obtained by precipitation at
neutral pH of TEOS and primary long alkyl chain amines.18 It has been postulated that the
formation of the HMS silicas occurs through H-bonding interactions between precursor
silanols and the lone electron pairs of the surfactant head groups as depicted in figure 2B.
Figure 1. Schematic representation of A) Neutral silica-surfactant interactions (SoIo). S represents the
surfactant and I, the inorganic framework. Triangles are solvent molecules. Dashed lines correspond
to H-bonding forces. B) Templating mechanism of formation of HMS mesoporous molecular sieves.
5.3 Experimental section
The nanoparticles are prepared as described in literature using TEOS and TPAOH.15 A
typical preparation is as follows: 10.37 g of TEOS (Aldrich) is mixed with 5 g TPAOH 40%
in water (Alfa) and 5.37 g of distilled water under vigorous stirring. This solution was then
stirred overnight to ensure hydrolysis of TEOS. This solution is slowly added to a surfactant
solution containing 3.24 g of hexadecylamine (Aldrich), 20.7 g of ethanol and 18.2 g of
water. The resulting mixture appeared to be milky but turned into a gel after 20h of gentle
stirring at room temperature. The obtained product was recovered by filtration, washed with
deionized water and ethanol (80% water w/w) and air dried at 60 °C overnight.
Powder X-ray diffraction patterns were recorded on a Rigaku diffractometer using CuKα
radiation. N2 adsorption-desorption isotherms were measured at –196 °C on a Micromeritics
90
Tristar sorptometer. Before measurements, samples were outgassed at 200 °C during 5h. The
pore size distribution was calculated from the desorption branch of the isotherms using the
Barret-Joyner-Halenda (BJH) method.
Scanning electron microscopy (SEM) micrographs were taken with a field emission
gun (FEG) XL30 instrument. IR spectra were recorded using a Nicolet 360FT-IR
spectrometer. Before measurements samples were prepared in the form of thin KBr pellets.
High-resolution transmission electron microscopy (HRTEM) was performed using a
Philips CM30UT electron microscope with a field emission gun as the source of electrons
operated at 300 kV. Samples were mounted on a Quantifoil microgrid carbon polymer
supported on a copper grid by placing a few droplets of a suspension of mesostructured silica
in water on the grid, followed by drying at ambient conditions.
5.4 Results and discussion
The SEM pictures of the as synthesized product are shown in figures 3A and 3B. The
most striking feature is the observation of flat crystal-like structures of a few µm long and
200 nm thick whereas classical mesoporous silicas are known to be amorphous. Only heating
at a minimum temperature of 100 °C for 2 days can lead to the self-assembly of the
nanoblocks to form silicalite-1 crystals. Importantly, the sharp edges and the shape of the
structures remind of ZSM-5 crystals.
The XRD pattern of the as-synthesized sample shown in figure 4 does not show any
Bragg reflection related to a crystalline phase. Nevertheless the XRD spectrum exhibits a
strong d100 reflection corresponding to a d-spacing of 6.5 nm accompanied by higher order
Bragg reflections confirming the relative good long-range order observed with SEM. These
reflections do not correspond to any of the three architectures (hexagonal, cubic or lamellar)
found for the M41S family of aluminosilicates. The HMS silica molecular sieves actually
have not been indexed precisely. 18
91
Figure 3.
A) and B) SEM micrographs of
the as-synthesized silica
mesostructure at different
magnifications.
Figure 4. XRD diffraction of the as-synthesized sample.
To test whether the MFI structure was retained IR experiments on the as-synthesized
product were performed (Figure 5). We indeed found a band between 560 and 570 cm-1
showing the presence of building units containing five-membered rings which is indicative of
the MFI structure. Thermogravimetric analysis (TGA) of the sample exhibited a weight loss
92
of 60% which was attributed to the removal the templates. The most interesting fact is that
both templates are still encapsulated in the material, with ratio close to those in the starting
mixture. However the temperature corresponding to TPA loss (280 °C) is lower to the one
found for of silicalite-1 (320 °C), suggesting that TPA is less tightly held in the present case.
Figure 5: IR spectrum of the as-made silica mesostructure.
Figure 6. TEM image of the as made mesostrucutre.
TEM revealed a layered structure (Figure 6) but the fragility of those remarkable
assemblies makes them very sensitive to the electron beam. We then suspect that TEM
measurements destroyed the mesostructures formed despite the use of lower beam intensity.
93
Figure 7. SEM micrograph of the calcined silica.
0
1
2
3
4
5
6
7
1 3 5 7 9 11 13 15
Pore Diameter (nm)
Pore
Vol
ume
(cm
3 /g)
Figure 8. Pore size distribution of the calcined silica estimated by BJH model.
So far, no efficient calcination route has been found in order to retain this uncommon
supramolecular assembly of zeolite precursors as shown in figure 3. Nevertheless, we
performed N2 adsorption on the product obtained after a normal calcination step used for
mesoporous aluminosilicates. A large specific surface area of 800 m2/g and a pore size of 3.8
nm (Figure 8) were observed. Pinnavaia et al.18 found a pore size of 2.5 nm and a slightly
94
smaller specific surface area using hexadecylamine and TEOS as starting materials. The
collapse of this uncommon mesostructure may be due to the fact that the assembly consists of
loosely bonded nanoblocks all the more that the interactions between hexadecylamine and
silica result from a neutral templating route18 thus preventing the use of elevated
temperatures. The next hurdle will be to strengthen these unique architectures and make them
catalytically active by incorporating heteroatoms.
5.5 Conclusion
In conclusion, highly organized and unprecedented assemblies of zeolite
nanoprecursors were obtained through the use of a secondary neutral template. IR showed
that the zeolite structure was kept during the assembly even though XRD could not detect any
long-range orientation of the building blocks. The present study illustrates that a controlled
choice of surfactants and zeolite precursors may be a general way for the synthesis of
innovative silicate mesostructures.
References 1 Corma, A. Top. Catal. 1997, 4, 249. 2 Schüth, F. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal., 2001, 135, 1-12. 3 Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. 4 Trong On, D.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal. A 2001, 222, 299. 5 Mokaya, R. Angew. Chem. Int. Ed. 1999, 38, 2930. 6 Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. 7 Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Frederikson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. 8 Trong On, D.; Kaliaguine, S. Angew. Chem. Int. Ed. 2002, 41, 1036-1040. 9 Liu, Y.; Zang, W.; Pinnavaia, T. Angew. Chem. Int. Ed. 2001, 40, 1255. 10 Liu, Y.; Pinnavaia, T. Chem. Mater. 2001, 14, 3. 11 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 12 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650.
95
13 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 14 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 15 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 16 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 17 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 18 Tanev, P. T.; Pinnavaia, T. Chem. Mater. 1996, 8, 2068. 19 Biz, S.; Occelli, M. L. Catal. Rev. – Sci. Eng. 1998, 40, 329. 20 Firouzi, A.; Kumar, D.; Bull, L.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. 21 Zhang, J.; Luz, Z.; Godfarb, D. J. Phys. Chem. B 1997, 101, 7087. 22 Zhang, J.; Luz, Z.; Zimmermann, H.; Godfarb, D. J. Phys. Chem. B 2000, 104, 279. 23 Agreen, P.; Linden, M.; Rosenholm, J. B.; Schwarzenbacher, R.; Kriechbaum, M.; Amenitsch, H.; Laggner, P.; Blanchard, J.; Schuth, F. J. Phys. Chem. B 1999, 103, 5943. 24 Frasch, J.; Liebeau, B.; Soulard, M.; Patarin, J.; Zana, R. Langmuir 2000, 16, 9049.
- 96 -
- 97 -
Summary
The study of the early stages of the crystallization of silica-based porous materials is still a very arduous task. The mechanisms by which guest molecules are enclathrated in an organic-inorganic crystalline structure remained very unclear for a long time. This issue also applies for many systems and for a large range of pore sizes. Huge academic efforts have been devoted to the study of the crystallization of zeolites, M41S mesostructures and diatoms, having well-defined pore sizes of 0.6-2 nm, 2-50 nm and 50 nm-1µm respectively. The main question is: How sophisticated structures self-assemble from atoms to organic-inorganic macroscale objects? Although the answer would probably not be the same over all length scales, the ultimate goal of such an understanding is the rational design of porous materials. Chiral porous solids may even be envisioned. In this thesis, the self-assembly process of tetrapropylammonium-mediated MFI zeolites is discussed. As mentioned above, techniques covering a wide range of lengthscales are prerequisites for investigating the whole course of the crystallization process. In situ and non-invasive methods are also required due to the fragileness of the hybrid intermediates. NMR and X-ray scattering have mainly been employed to fulfill these needs. Chapter 2 provides a comparative study of the early steps of silicalite-1 synthesis varying parameters such as silica source and cations content. SAXS and TEM were applied to characterize subcolloidal particles formed during the mixing of TPA and silica. For the first time, it has been shown that these particles have optimum size and they appear to be slab like irrespective of the silica source. NMR showed that they are likely to contain the MFI topology. The combination of NMR and X-ray scattering revealed to be very efficient in probing the early stages of silicalite-1 formation (Chapter 3). Processes on a molecular scale (29Si NMR) could be related to events occurring on a colloidal scale (SAXS) during the dissolution of silicic acid powder in concentrated TPAOH solutions. Besides classical oligomeric silicates encountered in basic aqueous solutions, 29Si NMR allowed the detection of a specific oligomer which assembles into well-defined subcolloidal MFI precursors having dimensions of 2.7 × 1 × 1.3 nm. Applications of new NMR techniques also gave more insight into the nanoparticles observed in ZSM-5 synthesis. The incorporation of aluminum could be followed using 27Al NMR. It appeared that aluminum preferentially migrates to Q4 positions in the nanoslabs. An in situ 27Al NMR synthesis of ZSM-5 showed that aluminum is tetrahedrally coordinated throughout the crystallization, discarding the possibility of crystal growth via monomer or small oligomer addition. Interactions between TPA molecules and silica could be determined by direct distance measurements. It has been shown that the early contacts are similar to those observed in the final crystals where TPA is tightly encapsulated within the porous framework. Combined in-situ synchrotron SAXS/USAXS allowed the study of aluminum incorporation during the complete course of ZSM-5 crystallization (Chapter 4). Hydrolysis of TEOS in aluminum containing TPAOH solution led to the spontaneous formation of discrete colloidal particles with a size slightly dependent on Si/Al ratios. However, this addition did not change the pathways of crystallization. Incorporation of aluminum slows down the nucleation rate and crystal growth. Morphologies were greatly influenced by the Al content.
- 98 -
The present identification of very stable and well-defined nanoparticles can be applied to the synthesis of innovative porous materials (Chapter 5). A new method to prepare silica-based materials via organization of zeolite nanoparticles under the influence of a secondary template has been proposed. The driving force of these efforts was to design silica porous materials exhibiting both micro and mesoporosity. Although removal of template molecules led to the collapse of the structure, high surface area silicas were obtained and the as-synthesized mesostructures are promising.
In conclusion, very well-defined and discrete nanoscale MFI precursors have been identified resulting from the mixing of a organic or inorganic silica source and TPA cations. They show typical dimensions of 2.7 × 1 × 1.3, 4 × 2 × 1.3 and 4 × 4 × 1.3 nm. NMR and X-rays scattering studies support a growth mechanism in which adding units are these unique nanoslabs. We do not discard other mechanism in zeolite crystallization, but this pathway may be general for organic-mediated zeolite synthesis. MFI nanoprecursors can also be applied for the synthesis of hierarchical porous materials.
- 99 -
Samenvatting De bestudering van de vroege kristallisatie stadia van poreuze materialen gebaseerd op silica is nog steeds een erg ingewikkelde taak. De mechanismen waarbij gastmoleculen opgenomen worden in een organisch-anorganische structuur blijven onduidelijk. Dit is ook van toepassing op vele andere systemen met een breed bereik van poriegroottes. Er is enorm veel onderzoeks geleverd om de kristallisatie van zeolieten, M41S mesostructuren en diatomen met goed gedefinieerde poriegroottes van respectievelijk 0.6-2 nm, 2-50 nm en 50 nm-1 µm te bestuderen. De belangrijkste vraag daarbij is hoe deze geavanceerde structuren door de zelforganisatie van atomen tot organisch-anorganische composietmaterialen op macroschaal gevormd worden. Hoewel het antwoord daarop niet eenduidig zal zijn over de gehele lengteschaal, is het uiteindelijke doel van zulk begrip het rationeel kunnen ontwerpen van poreuze materialen. Zelfs chirale poreuze materialen kunnen worden overwogen. In dit proefschrift wordt het zelforganisatiemechanisme van zeoliet met de MFI structuur met behulp van tetrapropylammonium als organisch structuur-bepalend molecuul besproken. Zoals eerder genoemd is het voor de bestudering van het gehele kristallisatieproces noodzakelijk om technieken te gebruiken die een brede lengteschaal kunnen bedekken. In situ en niet-aantastende methodes zijn ook noodzakelijk wegens de breekbaarheid van de hybride intermediaire structuren. NMR (Nuclear Magnetic Resonance) en Röntgendiffractie zijn hoofdzakelijk voor deze doelen gebruikt. Hoofdstuk 2 beschrijft een vergelijkende studie van de eerste stappen in de silicaliet-1 synthese, waarbij parameters als silica-bron en kationengehalte worden gevarieerd. SAXS (Small-Angle X-ray Scattering) en TEM (Transmission Electron Microscopy) werden toegepast om subcolloïdale deeltjes, gevormd tijdens het mengen van TPA en silica, te karakteriseren. Het is voor het eerst aangetoond dat deze deeltjes een optimum grootte bezitten. Ongeacht de silica bron blijken deze deeltjes plaatvormig zijn. Met behulp van NMR is aangetoond dat het waarschijnlijk is dat de deeltjes, MFI topologie bezitten. Het is verder gebleken dat de combinatie van NMR en Röntgendiffractie erg efficiënt is om de vroege stadia van silicaliet-1 formatie te onderzoeken (Hoofdstuk 3). Tijdens de oplossing van siliciumzuur poeder in geconcentreerde TPAOH oplossingen konden processen op moleculaire schaal (29Si NMR) worden gerelateerd aan gebeurtenissen op colloidale schaal (SAXS). Naast de klassieke oligomerische silicaten in basische waterige oplossingen, kon met behulp van 29Si NMR een specifiek oligomeer ontdekt worden dat zich organiseert in goed gedefineerde subcolloïdale MFI-precursors met dimensies van 1.7 x 1 x 1.3 nm. Toepassingen van nieuwe NMR technieken hebben ook meer inzicht gegeven in de nanodeeltjes die kunnen worden waargenomen bij de ZSM-5 synthese. De incorporatie van aluminium kon worden bestudeerd door het gebruik van 27Al NMR. Het bleek dat aluminium bij voorkeur naar Q4 posities in de nanoplaatjes migreert. De in situ 27Al NMR synthese van ZSM-5 liet zien dat aluminium tetraëdrisch gecoördineerd is tijdens de kristallisatie, hetgeen de mogelijkheid van kristalgroei door monomeer- of kleine oligomeer-additie uitsluit. Interacties tussen TPA moleculen en silica konden worden vastgesteld door middel van directe-afstandsmetingen. Het is aangetoond dat de vroege contacten vergelijkbaar zijn met de waargenomen contacten in de eindkristallen, waarbij TPA strak is opgenomen in het poreuze geraamte. Gecombineerde in situ synchrotron SAXS/USAXS heeft het bestuderen van de aluminium incorporatie tijdens de gehele ZSM-5 kristallisatie mogelijk gemaakt (Hoofdstuk
- 100 -
4). Hydrolyse van TPAOH in een aluminiumhoudende TPAOH oplossing leidde tot de spontane formatie van discrete colloïdale deeltjes met een grootte die enigszins afhankelijk is van Si/Al verhoudingen. Echter, deze toevoeging heeft de manier van kristallisatie niet veranderd. De incorporatie van aluminium vertraagt de nucleatiesnelheid en de kristalgroei. Het is gebleken dat het Al gehalte een grote invloed heeft op de morfologie. De identificatie van zeer stabiele en goed gedefinieerde nanodeeltjes kan worden toegepast op de synthese van innovatieve poreuze materialen (Hoofdstuk 5). Een nieuwe methode is voorgesteld om silica-gebaseerde materialen te bereiden via de organisatie van zeoliet-nanodeeltjes onder de invloed van een tweede template. De drijfkracht van deze inspanningen was om poreuze silica materialen te ontwerpen die zowel micro- als mesoporositeit vertonen. Hoewel de verwijdering van de template moleculen tot een ineenstorting van de structuur leidde, zijn silica’s met hoge specifieke oppervlakken verkregen. De aldus verkregen meso-structuren zijn veelbelovend. Samenvattend zijn zeer goed gedefinieerde en discrete nanometer schaal MFI precursoren geïdentificeerd, voortkomend uit het mengen van een organische of anorganische silica bron met TPA kationen. Typische dimensies van de betreffende deeltjes zijn 2.7 x 1 x 1.3, 4 x 2 x 1.3 en 4 x 4 x 1.3 nm. NMR en Röntgendiffractie ondersteunen een groeimechanisme waarbij de samenstellende deeltjes de unieke nanoplaatjes zijn. We sluiten geen ander mechanisme voor zeolietkristallisatie uit, maar deze route zou algemeen kunnen gelden voor zeolietsynthese met behulp van organische structuur-bepalende moleculen. MFI nanoprecursors kunnen ook worden toegepast voor de synthese van nieuwe materialen waarvan de porositeit hiërarchisch is.
- 101 -
Résumé
L’étude des premières étapes de la cristallisation des solides poreux à base de silice n’est pas une tâche aisée. Les mécanismes par lesquels les molécules sont piégées dans une matrice organique-inorganique et cristalline n’ont pas encore été clarifiés. Cela concerne également de nombreux systèmes dont la taille des pores est très variée. D’énormes efforts académiques ont été entrepris concernant l’étude de la cristallisation des zéolithes, des solides mésoporeux et des diatomes ayant une taille de pores variant entre 0.6-2 nm, 2-50 nm et 50 nm-1 µm respectivement. La question majeure est de savoir comment ces structures très compliquées s’assemblent à partir d’entités de taille moléculaire en des composites organiques-inorganiques beaucoup plus grand (plusieurs microns). Même si la réponse n’est probablement la même pour chaque échelle, l’objectif est la fabrication sur mesure de matériaux poreux. La synthèse de matériaux poreux et chiraux peut être envisagée. Cette thèse approfondit la compréhension du mécanisme de synthèse de zéolithes par des molécules organiques. Pour les raisons décrites ci-dessus, des techniques couvrant une large échelle sont préférables pour suivre complètement la cristallisation des zéolithes. Des méthodes in situ et non destructives sont également requises à cause de la fragilité des intermédiaires. La RMN (Résonance Magnétique Nucléaire) et la diffusion des rayons X aux petits angles (SAXS) ont été principalement employées dans ce but. Le chapitre 2 présente une étude comparative des premières étapes de la synthèses de la silicalite-1 en variant des paramètres comme la source de silice ou la nature des cations. SAXS et TEM (microscopie électronique à transmission) ont été employées pour caractériser les particules colloïdales formées lors du contact entre les molécules de TPA et la silice. Pour la première fois, il a été montré que ces particules ont une taille optimum et qu’elles sont plates, indépendamment de la source de silice. L’étude RMN a montré qu’elles contiennent très probablement la topologie MFI. La combinaison de la RMN et de la diffusion des rayons X aux petits angles s’est révélée être très efficace pour étudier les premières étapes de la formation de la silicalite-1 (Chapitre 3). Les processus à l’échelle moléculaire (RMN 29Si) ont pu être reliés aux transformations se déroulant au niveau colloïdal (SAXS) pendant la dissolution de l’acide silicique dans des solutions concentrées de TPAOH. En dehors des oligomères classiques de la chimie de la silice, la RMN du silicium a permis la détection d’un oligomère spécifique qui s’assemble en des précurseurs colloïdaux de la silicalite-1 et dont les dimensions sont: 2.7 × 1 × 1.3 nm. L’application de nouvelles techniques RMN a également clarifié la nature des particules colloïdales observées dans la synthèse de la zéolithe ZSM-5. L’incorporation de l’aluminium a pu être suivie à l’aide de la RMN 27Al. Il s’est avéré que l’aluminium se fixe préférentiellement aux positions Q4 dans les nanoparticules MFI. Une étude in situ RMN 27Al de la synthèse de la zéolithe ZSM-5 a prouvé que l’aluminium a un degré de coordination 4 pendant toute la cristallisation, rendant improbable l’hypothèse d’une croissance des cristaux par addition de monomère ou d’oligomères. Les interactions entre les molécules de TPA et la silice peuvent être directement déterminées en mesurant la distance qui les sépare. Les résultats montrent que ces contacts sont similaires à ceux rencontrés dans les cristaux obtenus où les molécules de TPA sont fermement incorporées dans la structure poreuse. La combinaison des techniques synchrotron telles que SAXS et USAXS ont permis l’étude de l’incorporation de l’aluminum pendant toute la cristallisation de la zeolite ZSM-5
- 102 -
(Chapitre 4). L’hydrolyse du TEOS dans une solution contenant TPAOH conduit spontanément à la formation d’entités colloïdales discrètes dont la taille dépend légèrement du rapport Si/Al. Cependant, cette addition ne change pas le mécanisme de la cristallisation. L’incorporation de l’aluminium ralentit la nucléation et la croissance des cristaux. De plus, le taux d’aluminium influence d’une façon très marquée la morphologie des cristaux obtenus. La présente identification de nanoparticules très stables et bien définies peut avoir des applications dans la synthèse de nouveaux matériaux (Chapitre 5). Une nouvelle méthode pour préparer des matériaux à base de silice en organisant ces nanoparticules à l’aide d’un agent directeur de synthèse secondaire a été proposée. Le but de ces recherches est la fabrication de matériaux à la fois microporeux et mésoporeux. Même si la calcination des agents directeurs de synthèse a conduit à la destruction de la structure, des silices avec une grande surface intérieure ont été obtenues et les matériaux non calcinés restent prometteurs. En conclusion, des nanoparticules bien définies et ayant la topologie MFI ont été identifiées. Elles sont formées par la dissolution de silice dans une solution aqueuse de TPA. Leurs dimensions sont discrètes: 2.7 × 1 × 1.3, 4 × 2 × 1.3 et 4 × 4 × 1.3 nm. La RMN et la diffusion des rayons X aux petits angles confirment l’existence d’un mécanisme de cristallisation dans lequel les unités intervenant dans la croissance des cristaux sont ces uniques entités colloïdales. Nous n’excluons pas un autre mécanisme pour la cristallisation des zéolithes mais il est fort probable qu’il s’applique pour la synthèse des zéolithes en présence de molécules organiques comme agents directeurs de synthèse. Les précurseurs identifiés dans cette étude peuvent être utilisés pour la synthèse de nouveaux matériaux dont la porosité serait hiérarchique.
- 103 -
Acknowledgments
I would like to express my deepest gratitude to Prof. Rutger van Santen for giving me the opportunity to work in his research group. Rutger, thank you very much for the many interesting discussions, your continuous support and the freedom you gave me throughout this work.
Barbara Mojet is acknowledged for her guidance and for providing me with her extensive knowledge in the field of synchrotron radiation, which is the main technique used in this thesis. Barbara, thank you for teaching me all the tricks of synchrotron measurement and data treatment.
I am very grateful to Prof. Johan Martens for welcoming me in his group in Leuven. Johan, without your contribution and the stimulating discussions we had, a lot of the results described in this thesis would not have been obtained. I am also indebted to Christine Kirschhock for her significant involvement in this project and the beautiful pictures of zeosil nanoslabs (see cover). I also thank Prof. Piet Grobet for his guidance during NMR measurements.
I thank Prof. Rob van Veen and Prof. Wilfried Mortier for the time they spent in reading the thesis and for their helpful suggestions. I also thank Prof. Hans Niemantsverdriet, Prof. Jean-Pierre Gilson and Prof. Bert de With for being part of the thesis committee. Jean-Pierre, you were the first who taught me what zeolites and their use were. Thank you for the interesting and inspiring courses, I hope you can see that your teaching efforts were not in vain.
I acknowledge Véronique Buschmann and Patricia Kooymann for their efforts to get beautiful TEM pictures.
I would like to express my gratitude to the “NMR boys”, Pieter Magusin, Eugène van Oers, Yannick Millot and Vadim Zorin for their patience and their professionalism. It turned out that NMR was a powerful complementary technique to SAXS.
Most of the X-ray scattering experiments were performed at the European Synchrotron Radiation Facility (ESRF), France. I am very grateful to Theyencheri Narayanan and Pierre Panine (ID02) and Prof. Wim Bras and Igor Dolbnya (DUBBLE).
I thank Theo Beelen, Dick Lieftink and Engel Vrieling for their assistance during synchrotron measurements and the many discussions on silica chemistry and diatoms.
I thank the current and former members of the department for their welcoming attitude and the nice atmosphere they create in the SKA group.
A number of them I want to mention specifically:
Alina K.: best competitor tied with Mayela G. for seats next to the coffee machine; elected Miss SKA metal catalysis group in 1999 and 2001, second in 2000 and 2002.
Arian O.: best continental driver in England but not the best driver in continental Europe. The first statement might be a consequence of the second. Thanks for all the discussions on zeolite precursors, your many ideas on this topic and the scientific trip to England.
Chrétien H.: best SKA Francophile.
- 104 -
Davy N. and Qianyao S.: synchrotron teammates. The easiest way to convince people to come with you for synchrotron measurements is to promise to show them “Grenoble by night”, which turns out to be: change cell 3, put in cell 1, move rotor x +25 etc etc….
Emiel H.: one of the top candidates for the casting of Men in Black III. Thanks for the interest you showed in my project. I really appreciated your curiosity, questions and help in zeolite synthesis.
Eric Z.: aka the “SKA stuntman”, has taken a decisive turn during his Ph.D. thesis.
(the) French Connection: I could not help but I sometimes had to speak my native language. In that regard, I met several compatriots. Many thanks to Alison, Coralie, Nathalie (my employment advisor), Hélène, Abdel and Xavier. Special thanks go to (newly Ir) Luc v. R. for his supporting and positive attitude. I could not convince him to become an AIO but he made an effort in accepting a TWAIO position. For the own safety of the author, Rafael S. was not included in this section.
Joyce O.: the nicest smile of the Faculty of Chemistry, your secretary office should keep some top-secret files because I often saw a bodyguard there.
Mayela G.: best competitor tied with Alina K. for seats next to the coffee machine; elected Miss SKA metal catalysis group in 2000 and 2002, second in 1999 and 2001.
Marco H.: Another top candidate for the casting of Men in Black III. I am very grateful to you for the time you spent helping me with X-ray measurements. Unfortunately, our dream, a rotating anode equipment, has not come true yet despite your efforts.
Rafael S.: unique member of the Alsacian Connection.
Rob H.: see Joyce O.
Ruben v. D.: alternatively triathlete, SKA borrel organizer, cyclist, SKA activity organizer, chemistry journal coverboy, SKA colloquium organizer, bike repairman…occasionally busy with homogeneous catalysis.
Tiny V.: aka “de SKA voetbaldagblad”; unique specimen “uit De Peel”, once believed that The Netherlands would be “WK kampioenen”. Second best SKA Francophile.
Zhu Q.: funniest chinese person I have ever met. Thanks for your help during these 4 years as officemate. I learnt a lot about Chinese culture and…habits. Zhu, je bent een gezellige chinees. Finally, I would like to thank my family for their warm encouragement and permanent support during all these four years.
- 105 -
Curriculum Vitae
Christophe Houssin was born on the 27th of April 1973 in Villedieu-les-Poêles
(Normandy, France). He got his baccalauréat diploma at Lycée Notre Dame, Avranches in
1991. After three years of preparatory courses at Lycée Victor Hugo in Caen, he entered the
engineering school ENSI-ISMRA Caen in 1994. He graduated as engineer in catalysis,
materials and organic chemistry in 1997. His graduation project on FCC catalysts was
performed at Shell Research in Amsterdam. He obtained a DEA (equivalent to a M.Sc.) in
organic chemistry from Caen University in 1997. He achieved his military duty in 1998 in the
marine infantry. On December 1st 1998, he started his Ph.D. research under supervision of
Prof. Rutger van Santen in the laboratory of Inorganic Chemistry and Catalysis, Eindhoven.
His research project dealt with the understanding of the organic-mediated synthesis of
silicalite-1 and ZSM-5 zeolites. These investigations involved an eight-month stay in the
group of Prof. Johan Martens at the Leuven Catholic University, Belgium. The most
important results of this work are described in this thesis.