2 nd FEZA School Paris, 1-2 September, 2008
description
Transcript of 2 nd FEZA School Paris, 1-2 September, 2008
Eni Refining & Marketing Division
1
2nd FEZA SchoolParis, 1-2 September, 2008
Structural Characterization of Zeolites and Structural Characterization of Zeolites and Related Materials by X-Ray Powder DiffractionRelated Materials by X-Ray Powder Diffraction
Roberto Millini, Stefano Zanardi
Eni Refining & Marketing Division
2
TOPICS
• X-RAYS
• X-RAY POWDER DIFFRACTION
• METHODS
PHASE IDENTIFICATION
PATTERN INDEXING
UNIT CELL PARAMETERS REFINEMENT
CRYSTALLINITY
CRYSTALLITE SIZE
• CONCLUSIONS
Eni Refining & Marketing Division
3
What are X-rays?
Electromagnetic radiation with wavelength, Electromagnetic radiation with wavelength, , in the region 0.01 , in the region 0.01 – 100 Å– 100 Å
In the electromagnetic spectrum, X-rays are placed between UV In the electromagnetic spectrum, X-rays are placed between UV and γ-radiationsand γ-radiations
RADIO MICROWAVE IR VISIBLE UV X-RAY γ-RAY
5·109 1·104 500 250 0.5 5·10-41·107
λ (nm)
2.48·10-7 0.124 2.48 4.96 2480 2.48·1061.24·10-4
E (eV)
Eni Refining & Marketing Division
4
Production of X-rays
HV source
+ -
-+
X-rays
evacuated tube
anode
heated W filament
electrons
Only 1% of the energyOnly 1% of the energyproduces X-rays!produces X-rays!
99% is lost as heat99% is lost as heat
Eni Refining & Marketing Division
5
Photon Energy (keV)
Inte
nsit
y (
cou
nts
1
03)
Kβλ = 0.184374 Å
Kα1
λ = 0.2090100 Å
Bremsstrahlung(80 – 90%)
Characteristic X-rays(10 – 20%)
The X-ray spectrum of W
Emax = Ee- (87 keV)
Eni Refining & Marketing Division
6
X-ray diffraction
Scattering occurs when there is a perfectly elastic collision among photons and electrons: the photons change their direction without any transfer of energy
If the scatterers (atoms) are arranged in an ordered manner (crystal) and the distances among them are similar to the wavelength of the photons, the phase relationship becomes periodic and interference diffraction effects are observed at various angles.
X-raysX-rays InterferenceInterference
Eni Refining & Marketing Division
7
d
λ
X-ray diffractionThe Bragg’s lawThe Bragg’s law
A C
B
θ
D
The difference in path between the waves scattered in B and D is equal to
AB+BC = 2dsinθ
If AB+BC is equal to a multiple of λ, the two waves combine themself with maximum positive interference; therefore:
nλ = 2dsinθ
the fundamental relationship in crystallography, known as Bragg equation
Eni Refining & Marketing Division
8
X-ray diffractionsingle crystal vs. powdersingle crystal vs. powder
X-rays
Eni Refining & Marketing Division
9
integration
X-ray powder diffraction (XRD)
XRD pattern
Eni Refining & Marketing Division
10
InstrumentationBragg-Brentano diffractometerBragg-Brentano diffractometer
Ss1
s2DS
SP
RS
AS
D
S = X-ray source
DS = divergence slit
SP = sample
RS = receiving slit
D = detectorθ
2θ
SDS
SP
RS
DS = X-ray source
DS = divergence slit
SP = sample
RS = receiving slit
D = detector
AS = antidivergence slit
s1, s2 = Soller slits
Eni Refining & Marketing Division
11
The XRD pattern
Kα1
Kα2
peakanisotropy
intensityI = k · Lp · P · A · F2
position23.13° 2θ, d = 3.845 Å
Eni Refining & Marketing Division
12
Information contained in the XRD pattern
Background
Scattering fromsample-holder, air, …
Amorphous phase,disorder, …
Incoherent scattering(Compton, TDS, …)
Sample
Eni Refining & Marketing Division
13
Information contained in the XRD pattern
Position
Lattice parametersSpace group
Qualitative phase analysisPhase purityThermal expansionCompressibilityPhase change
Reflections
Intensity
Crystal structure:Atomic positionsOccupancyThermal factorsTextureCrystallinity
Quantitative phase analysis
Profile
InstrumentalSample
Crystallite sizeStressStrain
Eni Refining & Marketing Division
14
ZeolitesFramework types vs. materialsFramework types vs. materials
Each open 4-connected 3D net, with (approximate) AB2 composition, where A is a tetrahedrally connected atom and B is any 2-connected atom, constitutes a framework typeframework type, which is defined by a 3-letter code assigned by the IZA Structure Commission
“The 3-letter codes describe and define the network of the corner sharing tetrahedrally coordinated framework atoms … [and] should not be confused or equated to actual materialsmaterials.”
“The framework types do not depend on composition, distribution of the T-atoms, cell dimensions or symmetry.”
Several materials may possess Several materials may possess the same framework typethe same framework type
Eni Refining & Marketing Division
15
ZeolitesPeculiar propertiesPeculiar properties
• Variable composition of the framework (e.g., Si, Ge, Si/Al, Si/B, Si/Ga, Si/Ge, Si/Ti, Al/P, Si/Al/P)
• Variable stoichiometry (e.g. Si/Al = 1 – ∞)
• Variation of the nature and concentration of the extra-framework species (inorganic cations and/or organic species)
Each change of the basic structure Each change of the basic structure produces a new materialproduces a new material
All these phenomena induce the change of:
• the dimensions of the unit cell, hence the positions of the Bragg reflections
• the intensities of the reflections
Eni Refining & Marketing Division
16
SAMPLE
XRD characterization
INDEXINGIDENTIFICATION
FRAMEWORKCOMPOSITION
CRYSTALLINITY
CRYSTALLITESIZE
STRUCTUREDETERMINATION
STRUCTUREREFINEMENT
XRD NEW PHASE
KN
OW
N P
HA
SE
Eni Refining & Marketing Division
17
XRD characterizationPhase identificationPhase identification
Each crystalline phase is characterized by a XRD pattern constituted by a set of reflections with well-defined positions (2θ (°) or d (Å)) and
relative intensities (I/I0·100)
The XRD pattern is the fingerprint of the crystalline phase
Eni Refining & Marketing Division
18
XRD characterizationPhase identificationPhase identification
INPUT DATA
A list of 2θ (or d) – relative intensities [(I/I0)·100] of the reflections
METHODS
• Automated search in databases: the PDF2 (Powder Diffraction File, by ICDD) contains some 200,000 measured and calculated patterns
• Atlas of Zeolite Framework Types: the Structure Commission of IZA periodically publishes the Atlas of the Zeolite Framework TypesAtlas of the Zeolite Framework Types and a Collection of Simulated XRD Powder Patterns for ZeolitesCollection of Simulated XRD Powder Patterns for Zeolites; all the information are available on the web (http://www.iza-structure.org/databases/), with the possibility to simulate the XRD pattern with custom-defined parameters
• Search on the open and patent literature: the “last chance” when the other methods fail
IF THE SEARCH IS UNSUCCESSFUL, WE ARE IN THE PRESENCE IF THE SEARCH IS UNSUCCESSFUL, WE ARE IN THE PRESENCE OF A NEW CRYSTALLINE PHASEOF A NEW CRYSTALLINE PHASE
Eni Refining & Marketing Division
19
XRD characterizationThe PDF2 fileThe PDF2 file
Eni Refining & Marketing Division
20
XRD characterizationPhase identificationPhase identification
Automated search on PDF2 database of a complex mixture of zeolite phases
1. The XRD pattern
2. Definition of the background
3. Peak search
4. Identification of Phase 1
5. Identification of Phase 2
6. Identification of Phase 3
7. Identification of Phase 4
Eni Refining & Marketing Division
21
XRD characterizationPhase identificationPhase identification
The phase composition (framework and/or extraframework species) influences positions and relative intensities of the reflections, making sometimes difficult the automated phase identification
Eni Refining & Marketing Division
22
ERB-1 (B-containing MWW)
XRD characterizationPhase identificationPhase identification
as-synthesizedNH4
+-exchanged
intercalated with: quinuclidineethylenglycoli-PrOH
R. Millini et al., Microporous Mat., 1995
Eni Refining & Marketing Division
23
as-synthesized
calcined
ERB-1 (B-containing MWW)
XRD characterizationPhase identificationPhase identification
R. Millini et al., Microporous Mater., 1995
Eni Refining & Marketing Division
24
Indexing the XRD pattern
The structural characterization of an unknown crystalline phase firstly requires the determination of the unit cellunit cell and of the symmetry
elements associated to one of the 230 space groups230 space groups
The indexing process tries to find the solution to the relation:
ddhklhkl = f( = f(h, k, l, a, b, c,h, k, l, a, b, c, αα, , ββ, , γγ))
The form of the equation depends on the crystal system:
from the simple cubic system:
d*d*22hkl hkl = (= (hh22 + + kk22 + + ll22)a*)a*22
… to the complex triclinic system:
d*d*22hkl hkl ==hh22a*a*22++kk22bb*2*2++ll22c*c*22+2+2hkhka*b*cosa*b*cosγγ*+2*+2hlhla*c*cosa*c*cosββ*+2*+2klklb*c*cosb*c*cosαα**
Eni Refining & Marketing Division
25
Indexing the XRD patternThe cubic systemThe cubic system
h k l d(obs) d(calc) res(d) 2T.obs 2T.calc res(2T) 1 2 2 0 8.63321 8.63721 -0.00400 10.238 10.233 0.005 2 3 1 1 7.36358 7.36584 -0.00226 12.009 12.005 0.004 3 3 3 1 5.60587 5.60457 0.00130 15.795 15.799 -0.004 4 5 1 1 4.70244 4.70150 0.00094 18.855 18.859 -0.004 5 4 4 0 4.31898 4.31861 0.00037 20.547 20.549 -0.002 6 6 2 0 3.86310 3.86268 0.00042 23.003 23.005 -0.003 7 5 3 3 3.72596 3.72550 0.00046 23.862 23.865 -0.003 8 5 5 1 3.42090 3.42085 0.00005 26.025 26.026 -0.000 9 6 4 2 3.26457 3.26456 0.00001 27.295 27.295 -0.00010 6 6 0 2.87871 2.87907 -0.00036 31.040 31.036 0.00411 5 5 5 2.82066 2.82090 -0.00024 31.696 31.693 0.003
a = 24.4297(23) Å
V = 14579.9(41) Å3
a = dhkl · (h2 + k2 + l2)1/2
Eni Refining & Marketing Division
26
Laboratory XRDλ = 1.54178 Å
Synchrotronλ = 1.1528 Å
Indexing the XRD patternA lower symmetry case: ERS-7 (ESV)A lower symmetry case: ERS-7 (ESV)
R. Millini et al., Proc. 12th IZA, 1999
Eni Refining & Marketing Division
27
Indexing the XRD patternA lower symmetry case: ERS-7 (ESV)A lower symmetry case: ERS-7 (ESV)
The program TREORTREOR was used for indexing the complex XRD pattern.
The input is simple:
• the d (or 2θ) values of the first 20 – 30 lines
• the maximum UC volume (negative if all the systems should be checked, otherwise only the cubic, tetragonal, orthorhombic and hexagonal are considered)
• the maximum β angle for monoclinc system
• some specific input parameters if more information are available from other sources
Eni Refining & Marketing Division
28
Indexing the XRD patternA lower symmetry case: ERS-7 (ESV)A lower symmetry case: ERS-7 (ESV)
The output consists of a number of possible solutions, all characterized
by specific figure of merits
The consistency of the best solution should be checked
1 or more unindexed reflections indicate the presence of impurities or that the solution is not reliable
FOMs
Eni Refining & Marketing Division
29
Indexing the XRD patternA lower symmetry case: ERS-7 (ESV)A lower symmetry case: ERS-7 (ESV)
Once a reliable UC is found, the possible space groups are searched through the inspection of the systematic absences, i.e. the classes
of reflections absent for symmetry
The following systematic extinctions were detected:
h00: h = 2n+1 0k0: k = 2n+1 00l: l = 2n+1
hk0: h = 2n+1 0kl: k+l = 2n+1
possible space groups:
Pn21a or Pnma
Eni Refining & Marketing Division
30
Indexing the XRD patternProblemsProblems
• Diffractometer and sample. The experimental setup should be accurately checked and the sample accurately prepared
• Data collection strategy. The results are strongly related to the accuracy in the determination of d (or 2θ); requiring all the first 20 – 30 lines, those located in the low-angle region (usually present in the XRD patterns of zeolites) are more critical to measure
• Overlap of the reflections. As the UC dimensions increase and the symmetry decreases the number of reflections increases; therefore, high-resolution powder diffraction data are necessary
• Phase purity. The presence of a second phase (even in trace amounts) makes difficult the indexing process; the reflections of the second phase (if unknown) can be identified by inspecting other samples synthesized in a similar way.
Eni Refining & Marketing Division
31
Unit cell parameters refinement
The accurate determination of the UC parameters is important because they depend on the chemical composition of zeolites. In fact:
• Zeolites can be synthesized in a wide Si/Al rangewide Si/Al range or it can be modulated by post-synthesis treatments (e.g. dealumination by steaming)
• The framework composition can be varied by isomorphous isomorphous substitutionsubstitution, i.e. by replacing (at least partially) Al and/or Si by other trivalent (e.g. B, Ga, Fe) and tetravalent (e.g. Ge, Ti) elements
The determination of the real framework The determination of the real framework composition is important because from it depend composition is important because from it depend
the properties of the materialthe properties of the material
Eni Refining & Marketing Division
32
Unit cell parameters refinement
Different analytical (e.g. Cs+-exchange) and spectroscopic (e.g. MAS NMR, FT IR) techniques have been proposed but XRD proved to be, in many cases, the most effective
The XRD methods are based on the observation that:
the incorporation of a heteroatom (i.e. an element different from Si) the incorporation of a heteroatom (i.e. an element different from Si) in the framework produces an expansion or a contraction of the UC in the framework produces an expansion or a contraction of the UC parameters, depending on its size respect to Si (provided that no parameters, depending on its size respect to Si (provided that no changes of the T-O-T angles occur)changes of the T-O-T angles occur)
Eni Refining & Marketing Division
33
Unit cell parameters refinementLeast-squares fit on the interplanar spacings of selected reflectionsLeast-squares fit on the interplanar spacings of selected reflections
The computer programs based on this classical approach minimize the sum of the squares of the quantity:
Q(hkl)obs - Q(hkl)calc
where:
Q(hkl) = 1/d2 = 4(sin2θ)/λ2
Input data (minimal):
• hkl indices and corresponding d (Å) or 2θ (°) for a certain number of reflections
Output:
• UC parameters and volume with the associated e.d.s.’s
• calculated d and/or 2θ and the difference respect to the experimental value(s) (for each reflection)
Eni Refining & Marketing Division
34
Unit cell parameters refinementLeast-squares fit on the interplanar spacings of selected reflectionsLeast-squares fit on the interplanar spacings of selected reflections
The method is easy and can be used even when the crystal structure of the phase under investigation is unknown; however, the reliability of the results depends on the complexity of the XRD pattern and on the quality of the input data
The main problems arise when:
• a non-strictly monochromatic radiation is used (e.g., CuKα1/CuKα2)
• the reflections are affected by severe overlapping phenomena
• the geometry of the diffractometer is not accurately adjusted (angular shift)
• the sample is not accurately prepared (sample displacement)
The use of a reference material (e.g. Si SRM 640b) as an external or, better, internal standard is suggested. In this way, the measured 2θ values can be corrected by the Δ2θ shifts measured on the reflections of the standard
Eni Refining & Marketing Division
35
Unit cell parameters refinementFull-profile fitting methodsFull-profile fitting methods
The use of full-profile fitting procedures has to be preferred when possible, namely when reliable structural information are available for the phase(s) under investigation
The goal of these methods is the reproduction of the experimental XRD pattern through the appropriate parametrization and refinement of the structural and instrumental parameters
On this concept is based the well known:
Rietveld MethodRietveld Method
Eni Refining & Marketing Division
36
The Rietveld Method
• Developed in the late years 1960s by H. M. Rietveld for refining neutron powder diffraction data
• At the end of years 1970s, it was extended to the refinement of XRD pattern
It is not a method for solving the crystal structure of a given phase but only for the refinement of a reasonable structural
model derived from other sources
During the least-squares refinement, the function minimized is:
R = Σiwi(YiO – YiC)2
where:
YiO and YiC are the observed and calculated intensities at step i
wi the weight assigned at each step and generally equal to 1/YiO
Eni Refining & Marketing Division
37
The Rietveld MethodThe refinement involves the variation of:
Scale factor
Scale factor
Instrumental parameters
Instrumental parameters
(Wavelenght)
(Polarization)
Angular shift
Background intensities
Peak-profile coefficients
FWHM vs 2θ
Peak asymmetry
(Wavelenght)
(Polarization)
Angular shift
Background intensities
Peak-profile coefficients
FWHM vs 2θ
Peak asymmetry
Structural parameters
Structural parameters
a, b, c, α, β, γ
Atomic coordinates
Site occupancy
Thermal factors
a, b, c, α, β, γ
Atomic coordinates
Site occupancy
Thermal factors
Correction parameters
Correction parameters
Primary extinction
Surface adsorption
Preferred orientation
Sample displacement
Primary extinction
Surface adsorption
Preferred orientation
Sample displacement
Eni Refining & Marketing Division
38
The Rietveld MethodApplicationsApplications
• Structure refinementStructure refinement
• Accurate determination of UC parametersAccurate determination of UC parameters
• Quantitative phase analysis (including Quantitative phase analysis (including quantification of the amorphous phase)quantification of the amorphous phase)
Eni Refining & Marketing Division
39
The Rietveld MethodStructure refinementStructure refinement
Rough structural model required, produced by applying different strategies:
Direct methods, Patterson, …
Identification of an isostructural phase with known structure
Use of difference Fourier methods to investigate phases of known structure
Trial & Error methods
Computer modeling techniques
Eni Refining & Marketing Division
40
The Rietveld MethodStructure refinementStructure refinement
K
Na
W
EMS-2: Na2K2Sn2Si10O26·6H2O isostructural with the mineral natrolemoynite: Na4Zr2Si10O26·9H2O
S. Zanardi et al., Microporous Mesoporous Mater., 2007
Eni Refining & Marketing Division
41
The Rietveld MethodStructure refinementStructure refinement
Location of hexamethonium dications in EU-1 (EUO)Model built by molecular modeling
R. Millini et al., Microporous Mesoporous Mater., 2001
Eni Refining & Marketing Division
42
The Rietveld MethodQuantitative phase analysisQuantitative phase analysis
PHASEConc. (wt%)
EXP. FOUND
CaSO4·2H2O 28.1 26.7
CaSO4·0.5H2O 4.3 5.3
CaSO4 6.6 6.1
α-Al2O3 2 2.5
CaCO3 (calcite) 45 45
SiO2 (quartz) 4 2
CaC2O4·H2O 10 12.3
Standardless quantitative phase analysis is possible even on relatively complex mixtures of crystalline phases
R. Millini, unpublished results
Eni Refining & Marketing Division
43
The Rietveld MethodDetermination of UC parametersDetermination of UC parameters
The application of the Rietveld Method is preferred when the determination of the UC parameters should be performed on complex XRD patterns, provided that an accurate structural model is available
The Rietveld programs take into account (and can refine):
• the use of non-strictly monochromatic radiation (e.g., CuKα1/CuKα2)
• severe overlapping phenomena of the reflections
• the geometry of the diffractometer (angular shift)
• moderate sample displacement deriving from a non-optimal preparation of the sample
It is not necessary to use an internal standard, but the data collection strategy should be accurately designed in terms of: 2θ range, step size, counting time
Eni Refining & Marketing Division
44
Unit cell parameters refinementCase study: assessing Ti and B incorporation in the silica frameworkCase study: assessing Ti and B incorporation in the silica framework
MFI
B
BOR-CAcid Catalyst
Ti
TS-1Oxidation Catalyst
Incorporation of Ti in: MFI (TS-1), MFI/MEL (TS-2, TS-3)
Incorporation of B in: RTH (BOR-A), BETA (BOR-B), MFI (BOR-C), MFI/MEL (BOR-D), MWW (ERB-1), EUO, LEV, MTW, ANA
Eni Refining & Marketing Division
45
Incorporation of B in MFI framework
The contraction of the UC parameters is expected when the small B3+ ions are incorporated in the zeolite framework
To unambiguously assess the incorporation of the heteroatom, the UC parameters of samples with increasing B3+ content should be accurately determined
PROBLEM
The XRD pattern of the orthorhombic MFI-type zeolites is very complex (it contains 500+ reflections below 50°2θ). Only a few single reflections can be used for the least-squares refinement of the UC parameters
Eni Refining & Marketing Division
46
Incorporation of B in MFI framework
The experiments confirmed that the UC parameters linearly decrease as the B3+ content increases
Vx = VSi – VSi[1 – (dB3/dSi
3)]x
HYPOTHESIS
The contraction of the UC volume is due only to the smaller dimensions of the [BO4] tetrahedron respect to [SiO4] and no change of the T-O-T angles occurs:
VSi = 5345.5 Å3, dSi = 1.61 Å (typical Si-O bond length in zeolites), dB = 1.46 Å (mean tetrahedral B-O bond length in the mineral reedmergnerite, NaBSi3O8):
Vx = 5345.5 – 1359.2xM. Taramasso et al., Proc. 5th IZA, 1980
Eni Refining & Marketing Division
47
Incorporation of Ti in MFI framework
The expansion of the UC parameters is expected when the large Ti4+ ions are incorporated in the zeolite framework
UC parameters and volume were firstly determined by least-squares fit on the interplanar spacings of selected reflections
Eni Refining & Marketing Division
48
Incorporation of Ti in MFI framework
G. Perego et al., Proc. 7th IZA, 1987
Eni Refining & Marketing Division
49
Incorporation of Ti in MFI framework
The data produced by the least-squares fitting procedure are scattered from the regression curve, because the severe overlap of some reflections made difficult the accurate determination of the peak positions
A significant improvement of the quality of the data are expected by applying the Rietveld Method
• Low angular region excluded because of the high asymmetry of the reflections
• High angular region excluded because of the very low intensitiy and the excessive overlap of the reflections
R. Millini et al., J. Catal., 1992
Eni Refining & Marketing Division
50
Incorporation of Ti in MFI framework
R. Millini et al., J. Catal., 1992
Eni Refining & Marketing Division
51
Incorporation of Ti in MFI framework
The experiments confirmed that the UC parameters linearly increase as the Ti4+ content increases
Vx = VSi – VSi[1 – (dTi3/dSi
3)]x
The expansion of the UC volume is due only to the larger dimensions of the
[TiO4] tetrahedron respect to [SiO4] and no change of the T-O-T angles occurs:
VSi = 5339.4 Å3, dSi = 1.61 Å (typical Si-O bond length in zeolites),
Vx = 5339.4 + 2110.4x
dTi = 1.79 Å
Tetrahedral Ti-O bond lengths BaTiO3 in the range 1.63 – 1.82 Å
R. Millini et al., J. Catal., 1992
Eni Refining & Marketing Division
52
Incorporation of Ti in MFI frameworkThe same method was applied on high-resolution synchrotron powder diffraction patterns collected on samples treated at 400 K under vacuum and sealed in capillaries under vacuum
C. Lamberti et al., J. Catal., 1999
Laboratory data
Synchrotron data
R. Millini et al., J. Catal., 1992
Eni Refining & Marketing Division
53
Incorporation of Ti in MFI framework• Determination of the Ti content in the framework with an accuracy
of 2 – 3 %
• Quantification of the extraframework Ti species (e.g. anatase, SiO2-TiO2 glassy phases, …)
• Determination of the maximum Ti content in MFI framework (2.5 atoms%)
G. Perego et al., Molecular Sieves –Science and Technology Vol. 1, 1998
Eni Refining & Marketing Division
54
Determination of the crystallinity
Useful for determining:
• the kinetics of crystallization of a given phase• the stability of a phase after thermal/hydrothermal treatments• the variations eventually occurred on zeolite catalysts
CR = [Σ(I)/Σ(I0)]·100
20 21 22 23 24 25
2-Theta [°]
20 21 22 23 24 25
2-Theta [°]
20 21 22 23 24 25
2-Theta [°]
Eni Refining & Marketing Division
55
Determination of the crystallinityThe method is easy to apply but:
• the crystallinity values are non absolute, being relative to the reference sample
• it may give wrong or unrealistic results if not correctly applied
In particular:
• the composition (framework and extraframework) of the reference and the unknown samples should be similar
• preferred orientation phenomena should be avoided
• the data collection strategy should be suitably selected
• the intensity data should be corrected for the decay of the intensity of the X-ray beam (measured by an external reference intensity standard)
If even one of these conditions is not respected, the If even one of these conditions is not respected, the results are meaninglessresults are meaningless
Eni Refining & Marketing Division
56
Determination of the crystallinity
Same framework structure
but
Different framework and extraframework composition
Different relative intensitiesof the reflections
Same extraframework composition
but
Different framework composition
Slightly different framework structure
Eni Refining & Marketing Division
57
Determination of the crystallite sizeThe term crystallitecrystallite is intended as coherent scattering domaincoherent scattering domain
It may not correspond to a geometrically well-defined particle, because it can be composed by two or more coherent scattering domains deriving from the presence of defects, fractures, …
Electron microscopy techniques (SEM, TEM) are useful for determining the particle size but, in many cases, the aggregation of the crystallites may render difficult the correct evaluation of the their size
1000 Å
Eni Refining & Marketing Division
58
Determination of the crystallite size
The breadth of a reflection is due to instrumental and sample factors
The instrumental breadthinstrumental breadth is that characterizing the reflections of the XRD pattern collected on infinite perfect crystals; it depends on the type and geometry of the diffractometer
The sample factorssample factors include: crystallite size, presence of defects (stacking faults, dislocations), microstrains due to the presence of inclusions incompatible with the crystalline lattice, the fluctuation of stoichiometry among different domains, surface relaxation typical of nanosized materials
If the breadth of the reflection is due to size effects only, the crystallite size D can be computed with the Scherrer equation:
D = K·λ/(β·cosθ)
where the constant K ~ 0.9, β is the FWHM of the reflection
Eni Refining & Marketing Division
59
Determination of the crystallite size
Dhkl = 0.9·λ/(β·cosθhkl)
D is usually referred to a given hkl reflection:
It is common practice to consider the effective value of β as:
β = (B2 – b2)1/2
where B is the measured FWHM of the hkl reflection and b the corresponding instrumental breadth
In the case of zeolites, the presence of defects is probably the main cause affecting the correct evaluation of the crystallite size
The Scherrer equation is useful not for determining the absolute crystallite size but for evaluating its relative variations
Eni Refining & Marketing Division
60
Determination of the crystallite size
1000 Å1000 Å
Eni Refining & Marketing Division
61
Crystallinity and crystallite sizeCase study: thermal stability of zeolite Beta catalystCase study: thermal stability of zeolite Beta catalyst
Polimeri Europa uses a zeolite Beta catalyst in its cumene and ethylbenzene technologies, based on the direct alkylation of benzene with propylene and ethylene, respectively
It is important to determine the thermal stability (in terms of loss of crystallinity and framework dealumination) of the catalyst for better defining the regeneration conditions
Eni Refining & Marketing Division
62
POLYMORPH A POLYMORPH B
Tetragonal, P4122a 12.5, c 26.4 Å
Monoclinic, C2/ca b 12.5√2, c 14.4 Å
114°J.M. Newsam et al., Proc. R. Soc. London, 1988.
The zeolite Beta structure
Eni Refining & Marketing Division
63
Newsam et al., Proc. R. Soc. London A 420 (1988) 375.
Polymorph A 50%
Polymorph B 50%
The zeolite Beta structure
Eni Refining & Marketing Division
64
[1] Perez-Pariente et al., Appl. Catal. 31 (1987) 35; J. Catal. 124 (1990) 217.
[2] Liu et at., J. Catal. 132 (1991) 432.
Thermal stability of zeolite Beta is controversial:
Tmax 550°C [1]
Tmax < 760°C with limited dealumination and structural collapse [2]
HH++-BETA-BETAHH++-BETA-BETA
CharacterizationCharacterizationCharacterizationCharacterization
650°C650°C650°C650°C 750°C750°C750°C750°C 850°C850°C850°C850°C 900°C900°C900°C900°C
Thermal stability of zeolite Beta
Calcinations: 5 hrs in air
Eni Refining & Marketing Division
65
Thermal stability of zeolite Beta
008 600
Complete breakdown of the structure: > 850°C
Loss of crystallinity: < 20% at 850°C
Progressive decrease of the average crystallite size, more pronounced when computed on the sharp 008 reflection
Is it really a size effect?Is it really a size effect?R. Millini et al., Proc. 14th IZC, 2004
Eni Refining & Marketing Division
66
Thermal stability of zeolite BetaAssessing effective framework compositionAssessing effective framework composition
Vx = VSi – VSi[1 – (dAl3/dSi
3)]x
Vx = 4076.8 Å3 (experimental)
dAl = 1.75 Å
dSi = 1.61 Å
x = 0.071 (from NH3 titration, 0.091 from elemental analysis)
VSi = 3996 Å3
Indices of sharp reflections according to the tetragonal model
of polymorph A
Eni Refining & Marketing Division
67
Thermal stability of zeolite BetaAssessing effective framework compositionAssessing effective framework composition
Known VSi, dAl and dSi, from the experimental Vx value the x molar
fraction of Al in the zeolite Beta framework is computed
Elementalanalysis
0.091
NH3 titration
0.071
0.068
0.065
0.056
The progressive dealumination of the framework produces The progressive dealumination of the framework produces structural defects, which also contribute to the broadening of structural defects, which also contribute to the broadening of
the reflectionsthe reflections
Eni Refining & Marketing Division
68
Conclusive remarks
XRD techniques are very powerful, allowing the accurate structural characterization of polycrystalline samples to be performed
As for all the other analytical, spectroscopic, …, techniques the achievement of reliable results depends both on the skills of the researcher and on the availability of high quality experimental data
Standard laboratory instruments are sufficient for solving most of the structural problems
The achievement of reliable results strongly depends on the accurate setup of the diffractometer, on the appropriate preparation of the sample and on the use of the most suitable data collection strategy
DON’T WASTE YOUR TIME ON BAD DATA
Eni Refining & Marketing Division
69
Structure determinationThe knowledge of the crystal structure of a material is fundamental
for understanding and even predicting its properties
Usually determined by single crystal X-ray diffraction, if specimens of suitable dimensions (> 50 μm for standard laboratory diffractometers, > 5 μm when operating with synchrotron radiation) are available Zeolites usually crystallize in form of powder composed by very small crystallites, even with submicronic dimensions
X-ray powder diffraction data only are available
2 m 100 nm
Eni Refining & Marketing Division
70
Structure determination from XRD data
Reciprocal spacemethods
Direct spacemethods
All require chemical and basic structural information:
UC parameters and space group
Chemical composition (elemental analysis)
Framework density (helium pycnometry)
Tetrahedra per UC (n = (V ρ)/(Mw 1.6603))
Independent T-atoms (e.g. 29Si MAS NMR)
Eni Refining & Marketing Division
71
Structure determination from XRD dataBasic information: the case of ERS-7Basic information: the case of ERS-7
Chemical composition:
Na0.04R0.08(Si0.89Al0.11)O2
Total density: 2.04 g·cm-3
R + H2O = 15.5 wt% (TGA)
Na = 1.2 wt% (AA)
Density: 1.70 g·cm-3
Unit cell volume: 2821 Å3
48.1 T-sites/unit cell
6 to 12 independent T-sites
Primitive orthorhombic cell
a = 9.81, b = 12.50, c = 23.01 Å
Space group: Pna21 or Pnma
No significant SHG signal suggests Pnma
5 10 15 20 25 30 35 40
2-Theta [°]
= 1.1528 Å
INDEXING (TREOR90)
Eni Refining & Marketing Division
72
Structure determination from XRD dataReciprocal space methodsReciprocal space methods
The methods are those used for structure solution from single crystal X-ray diffraction data: Patterson function, heavy-atom method, isomorphous replacement, anomalous dispersion, direct methods
The intensities of all the reflections in the XRD pattern are extracted by using automatic profile fitting programs and the structure factors are calculated and used as input data for structure solution programs
The main problems of these approaches (very successful for single crystal data) are related to:
• the uncertainties in the intensity values when severe overlapping of the reflections occurs
• the data set is considerably smaller than that obtained from single crystal
Eni Refining & Marketing Division
73
Structure determination from XRD dataReciprocal space methodsReciprocal space methods
The direct methods approach was used for determining the structure of some zeolites, including, for instance:
ITQ-12 (ITW): C2/m, 3 T-atoms, V = 1354 Å3
Yang X.B. et al. J. Am. Chem. Soc., 126, 10409 (2004)
ITQ-22 (IWW): Pbam, 16-T atoms, V = 6737 Å3
Corma A. et al. Nature Materials, 2, 493 (2003)
MCM-35 (MTF): C2/m, 6-T atoms, V = 2121 Å3
Barrett P.A. et al. Chem. Mater., 11, 2919 (1999)
The wider application of the reciprocal space methods is somewhat limited by the complexity of the XRD pattern
Eni Refining & Marketing Division
74
Structure determination from XRD dataDirect space methodsDirect space methods
When the classical crystallographic approaches fail, a starting structural model has to be built up by:
the identification of an isostructural material with known crystal structure (es. EMS-2, the synthetic Sn-counterpart of natrolemoynite, a rare microporous zirconium silicate)
the use of difference Fourier methods to investigate derivatives of known phases(location of adsorbed molecules in zeolite pores)
model building (trial & error)(es. UMZ-5 (UFI), SSZ-59 (SFN), MCM-22 (MWW), …)
computer modeling techniques(automated model building schemes, such as simulated annealing or tempering, global optimization algorithms, FOCUS, …)
Eni Refining & Marketing Division
75
Energy (keV)
Inte
nsit
y (
a.u
.)
Production of X-rays
X-ray are produced through two different mechanisms:
1. Bremsstrahlung (braking radiation)
Emax = Ee-
Eni Refining & Marketing Division
76
Production of X-rays
X-ray are produced through two different mechanisms:
2. Characteristic X-ray radiation
Energy (keV)
Inte
nsit
y (
a.u
.) KβKα
K
L
M
N
Kβ
Kα
80 - 90%
10 - 20%
Eni Refining & Marketing Division
77
Production of X-raysThe K The K spectrumspectrum of Cu of Cu
K(1s) 8979
L1(2s)
L2(2p1/2)L3(2p3/2)
M1(3s)
M2(3p)
M3(3d)
1097
952933
122
76
0 (eV)
Kα1 Kα2 Kβ
E = c·h/λ
Kα1 = 1.54056 Å
Kα2 = 1.54439 Å
Kβ = 1.39222 Å
Eni Refining & Marketing Division
78
c
aO
b
βγ
α
Fundamental crystallographic data
• Unit Cell (UC): the smallest part of the crystal which maintains the properties of the crystal itself; the entire crystal can be constructed by translating the UC along the three directions. It is defined by the unit cell parameters: the lengths of the sides [a, b, c] and the angles [α, β, γ]
• Crystal system
• Space group