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Microporous and Mesoporous Materials 106 (2007) 28–34

Fe-containing mesoporous silicates with macro-lamellar morphology

Yi Lu a,*, Junlin Zheng b, Jinku Liu a, Jin Mu a

a Department of Chemistry, East China University of Science and Technology, Shanghai 200237, PR Chinab Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, PR China

Received 23 July 2006; received in revised form 19 November 2006; accepted 1 February 2007Available online 20 February 2007

Abstract

Fe-containing mesoporous silicates with macro-lamellar morphology were prepared using sodium silicates and Fe(NO3)3 as silica andiron sources in alkaline media, respectively, under the direction of CTAB cationic surfactant. XRD and N2 adsorption results showed thatthe sample had highly ordered hexagonal mesostructures typical of MCM-41. UV–vis and ESR spectra indicated that most of the ironspecies were tetrahedrally incorporated into the mesostructured framework in alkaline media. TEM micrographs manifested themacro-lamellar structures when the SiO2/Fe2O3 = 60 (molar ratio). Additionally, there were many submicron-sized structural defect voidsin the mesostructured lamellas. The morphology character underwent huge changes with the increase of Fe(NO3)3 amounts in the synthe-sis media. When the SiO2/Fe2O3 = 40, tangled microtubules with about several microns in length dominated the final product. Decreasingthe SiO2/Fe2O3 molar ratio to 20, the long cylinders disappeared and 100–600 nm sticks represented 80% of the solid material. The dimen-sions of the resultant sample were reduced with the decrease of SiO2/Fe2O3 molar ratio. The formation mechanism of the macro-lamellarmorphology was also proposed on the basis of the previous reports and our characterization results.� 2007 Elsevier Inc. All rights reserved.

Keywords: Mesoporous; MCM-41; Fe-containing silicates; Lamellar; Morphology

1. Introduction

After more than 10 years of research and developmenton mesoporous materials, it has become a large familyand attracted a great deal of attention as new potentialmolecular sieves and supporting materials [1–3]. Besidesthe widely studied aluminosilicate mesostructures, meso-porous materials containing heteroelements, such as Fe,V, and Ni, have still profound potential for use as catalysts,exchangers, and adsorbents because of their tunable nano-scale pore openings and exceptionally high internal surfaceareas accessible to bulky organic molecules [4,5]. Notably,Fe-containing mesoporous molecular sieves are of particu-lar interest for their unique catalytic activity in variousselective gas-phase reactions, e.g., hydrocarbon oxidation,N2O decomposition, and selective catalytic NO and N2O

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.02.017

* Corresponding author. Tel.: +86 21 64352214.E-mail address: lotusly@ecust.edu.cn (Y. Lu).

reduction in the presence of hydrocarbon or ammonia[5–7]. Fe-containing mesoporous silicates with isolatedframework iron species and highly ordered mesostructures,are the objects of synthesis exploitation.

The synthesis of inorganic structures with nanoscaledimensions, well-ordered mesostructures, and morphologi-cal specificity is of great importance and interest in materi-als science and nanotechnology. Among the variousparticle shapes that have been observed for mesostructuredsilica compositions formed through supramolecular assem-bly pathway, those with sponge-like, tubular, fiber-like,sphere, vesicular, and macro-lamellar hierarchical struc-tures have special chemical significance [8–12]. Because ofthe potential applications in membrane-based separations,selective catalysis, and sensors, the fabrication of meso-structured materials with macro-lamellar hierarchicalstructures was paid much attention on. Oriented transpar-ent films of layered silica-surfactant nanocomposites werefirstly synthesized through spin coating method [13].

Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34 29

Lu et al. used C16TAB (Cetyltrimethylammonium bro-mide), under acidic conditions, to prepare well-ordered,uniform mesostructured films by the sol-gel dip-coatingprocess [14]. However, most of the related lamellar meso-structures reported to date readily collapsed to amorphousoxides upon surfactant removal. Xu and co-workers havealso published a series of papers reporting the synthesisof a so-called highly ordered long-rang lamellar structure,and the mixture of a silicone surfactant and conventionalsurfactants was claimed to be able to direct the formationof the unusual hierarchical mesostructures [15–20]. How-ever, it was pointed out by Yuan et al. that the highlyordered macro-lamellar structure described in the papersof Xu and co-workers was an artefact arising from theultramicrotoming operation in transmission electronmicroscopy (TEM) observation [21].

So it is fascinating and challenging to synthesize meso-porous materials with highly ordered and thermally stablemacro-lamellar structure via simple hydrothermal or sol-gel process. Herein, Fe-containing mesoporous silicateswith macro-lamellar morphology were conveniently pro-duced using sodium silicates and Fe(NO3)3 as silica andiron sources in alkaline media, respectively, under thedirection of CTAB cationic surfactants. The effect of Fecontent on the morphology of final products was investi-gated, and the formation mechanism of the macro-lamellarmorphology was also tentatively explained on the basis ofprevious research results.

2. Experimental

2.1. Synthesis of Fe-containing mesoporous silicates and

MCM-41 silica

Typical synthesis procedure of the Fe-containing meso-porous silicates was as follows: proper amounts of sodiumsilicates (SiO2/Na2O was 1.03) and Fe(NO3)3 were dis-solved in the CTAB template solution, and the molar ratioof Fe2O3/SiO2/CTAB/H2O in the starting system was 1–3/60/28.8/(3000–4800). After stirring for 10 min, the pHvalue was adjusted to about 10 through adding properamount of H2SO4 (2 mol/L) into the mixture slowly. Thefinal mixture was hydrothermally reacted in an autoclaveat 110 �C for 48 h. After filtered, washed and air-dried,the precipitates were then calcined in air at 550 �C for 6 hto remove the CTAB organic templates. The samples withdifferent SiO2/Fe2O3 molar ratios were designated as MFS-60, MFS-40, and MFS-20 (the value refers to the SiO2/Fe2O3 molar ratio). Normal MCM-41 silica were synthe-sized using sodium silicates as silica source, and the molarratio of SiO2/CTAB/H2O in the starting system is 60/28.8/(3000–4800).

2.2. Characterization

The X-ray diffraction (XRD) patterns of the sampleswere recorded using a Shimadzu XD-3A X-ray powder

diffractometer, which employed Ni-filtered Cu Ka radia-tion and was operated at 40 kV and 30 mA. Repeat dis-tance (a0) was calculated from X-ray diffraction data withthe equation a0 = 2d100/30.5. The nitrogen adsorption iso-therms at 77 K were measured using a Micromeritics Tri-star 3000 system. The mesostructures were analyzed fromdesorption branches of the isotherms by the Barrett–Joy-ner–Halenda (BJH) model with Halsey equation for multi-player thickness. Diffuse reflectance UV–vis spectra werecollected on a Shimadzu UV-2510PC spectrophotometer.The powder samples were loaded into a quartz cell, andthe spectra were collected in the 190–800 nm range refer-enced to BaSO4. Electron spin resonance (ESR) spectrawere recorded at room temperature in the X-band modeon a Bruker EMX-8/2.7 spectrometer. The magnetic fieldwas calibrated with a proton resonance meter, and tetracy-anoethylene (g = 2.0077) as reference. Transmission Elec-tron Microscopy (TEM) was recorded on a Hitachi 600-2instrument with accelerating voltage of 100 kV. High reso-lution TEM was recorded on a JEM-3010 (JEOL Japan)instrument with accelerating voltage of 300 kV. TEM spec-imen was prepared by dispersing the powder in alcohol byultrasonic treatment, dropping onto a holey carbon filmsupported on a copper grid, and then dried in air.

3. Results and discussion

3.1. XRD and N2 adsorption results

Fig. 1a shows the XRD patterns of the calcined Fe-con-taining mesoporous silicates MFS-60 and normal MCM-41silica. Well-ordered material is obtained as evidenced bythe XRD pattern, which has resolved (100), (11 0),(200), and (210) reflection peaks characteristic of long-range ordered hexagonal symmetry. At large angle of10–60 � (Fig. 1b), no obvious diffraction peaks correspond-ing to crystalline iron oxides can be observed for the cal-cined sample. Possibly, iron species were introduced intothe framework of ordered mesoporous silica or the particlesize of iron oxides in sample was too small to be detectedby X-ray diffraction. Furthermore, the d100 value ofMFS-60 sample is 4.16 nm, whereas that of normalMCM-41 silica is only 3.98 nm. The increase of d100 valuecan be accounted for by the insertion of larger Fe3+ cations(radius 0.65 A vs 0.26 A for Si4+) in the silica network. Inthe N2 adsorption/desorption isotherm (Fig. 2) of theMFS-60 sample, a steep increase occurs in a type IV iso-therm curve at the relative pressure of 0.30 < p/p0 < 0.40,which can be attributed to the presence of highly regularmesostructures.

The textural parameters of the Fe-containing mesopor-ous silicates MFS-60 and normal MCM-41 silica were sum-marized in Table 1. The BET surface area and pore volumeof the calcined MFS-60 sample are 928 m2/g and 0.91 cm3/g,respectively. Pore size distribution plot is calculatedusing the BJH algorithm employing desorption branch(inset). The pore diameter of the nanochannels is 2.70 nm.

1 875 6432

d100 =4.16

210

Inte

nsity

(a.

u.)

2 θ /degree

100

110200

d100 =3.98

MFS-60

MCM-41

10 15 20 25 30 35 40 45 50 55 60

Inte

nsity

(a.

u.)

2 θ /degree

Fig. 1. Small-angle XRD patterns (a) of MFS-60 sample (bottom) and normal MCM-41 silica (top), and large-angle XRD pattern of MFS-60 (b).

0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

10 1000

5

10

15

Pore

vol

ume

(cm

3 /g)

Pore diameter (nm)

Relative pressure (p/po)

Vol

ume

adso

rbed

(cm

3 /g)

Fig. 2. N2 adsorption isotherm of the Fe-containing mesoporous silicatesMFS-60 (pore size distribution plot derived from desorption branch inset).

Table 1Textural parameters of the Fe-containing mesoporous silicates MFS-60and normal MCM-41 silica

Samples d100

(nm)a0

(nm)Porediameter(nm)

Wallthickness(nm)

SBET

(m2 g�1)Porevolume(cm3 g�1)

MFS-60 4.16 4.80 2.70 2.10 928 0.91MCM-41 3.98 4.60 2.63 1.97 1270 1.23

30 Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34

Calculated from the a0 value and pore diameter, the frame-work wall thickness of MFS-60 is 2.10 nm, which is thickerthan the 1.97 nm framework wall of normal MCM-41 silica.The shrinkage of the pure silica framework is largerthan that of Fe substituted silicate framework in thecalcinations process. The thicker framework wall wouldcontribute to the improved thermal stability of meso-structures.

Besides the inflection characteristic of capillary conden-sation process in regular mesopores, additional uncommontype-H4 hysteresis loops above p/p0 = 0.5 is notable in the

N2 adsorption isotherm of the sample. Although the vol-ume adsorbed in the adsorption and desorption branchesshows comparable trend at higher p/p0 values (p/p0 P0.5), a quite different phenomenon appears aroundp/p0 = 0.45. The adsorption branch shows a progressiveincrease in the volume adsorbed corresponding to a broaddistribution of textural mesoporosity. In contrast, desorp-tion branch shows a pronounced step around p/p0 = 0.45resulting in the closure of the hysteresis loop. It can alsobe observed that the ascending of adsorption isotherm isslightly faster than that of desorption one above p/p0 = 0.5. The plot from desorption data shows anotheradditional 3.70 nm mesopore following the mesopore atthe mean diameter of 2.70 nm. The particular hysteresisloop in N2 adsorption isotherm and 3.70 nm mesopore pre-sented above hints unusual structural characters anddeserves further investigation. This phenomenon will beexplained in combination with the high resolution TEMresult.

3.2. UV–vis and ESR spectroscopy

Iron-containing mesoporous molecular sieves possessfascinating catalytic activity, which is largely dependentupon the state of iron species in mesostructured frame-work. Electronic spectroscopy in the UV–vis region is auseful technique to study the electronic state of isolatedtransition metal ions and aggregated transition metal oxi-des. Fig. 3 is the UV–vis spectra of the Fe-containing mes-oporous silicates MFS-60 (a) and normal mesoporoussilica (b). The UV–vis spectrum of MFS-60 sample showsa strong absorption bands in the �200–300 nm wavelengthrange with two maxima at ca. 219 and 255 nm. A similarhigh-energy absorption band associated with ligand-to-metal charge transfer that is characteristic of isolated tetra-hedral coordination of Fe3+ has been observed for Fe-con-taining mesoporous molecular sieves [22,23]. These twobands have been assigned to the electron transitions of

200 300 400 500 600 700 800

0.2

0.4

0.6

0.8

1.0

1.2

b

Ads

orba

nce

Wavelength (nm)

a

Fig. 3. UV–vis spectra of the Fe-containing mesoporous silicates withmacro-lamellar morphology (a) and normal mesoporous silica (b).

Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34 31

the anion (e.g., O2�) to the t2g and eg orbitals of Fe3+

within [FeO4]� tetrahedral [24]. So it is indicated that thetetrahedral coordination of Fe is dominant in currentmaterial. Previous researchers found that bulk a-Fe2O3

exhibited a broad adsorption band at 320–670 nm with amaximum at 560 nm [25]. The absence of obvious absorp-tion peak above 450 nm suggests that the calcined sample isfree of iron oxides (at least in bulk form). The weakadsorption band between 300 and 450 nm could be attrib-uted to small oligonuclear iron species (FeO)n [26,27]. Incontrast, normal mesoporous silica (b) shows no distinctabsorption band at 200–800 nm. In current synthesis route,most of iron species were tetrahedrally incorporated intothe mesostructured framework in alkaline media.

Fig. 4 presents the ESR spectra of the Fe-containingmesoporous silicates MFS-60 (a), MFS-40 (b), and

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Ads

orpt

ion

deri

vativ

e (a

.u.)

Magnetic field (kG)

ab

c

geff

= 4.3 2.0

Fig. 4. ESR spectra of the Fe-containing mesoporous silicates: (a) MFS-60; (b) MFS-40; (c) MFS-20.

MFS-20(c). In the ESR spectra of these samples, two differ-ent signals at geff values of 4.30 and 2.00 corresponding tothe presence of trivalent iron in two distinct environmentsare observed. With the increase of Fe(NO3)3 dosage in thestarting materials, the intensity of the 4.30 and 2.00 signalsincreased accordingly. On the basis of earlier signal assign-ments [28,29], the transitions at 4.30 and 1.99 was com-monly attributed to trivalent iron in the distorted andsymmetrical tetrahedral framework sites, respectively.Thus, it is inferred from UV–vis and ESR results that triva-lent iron mostly substitutes in the silicate framework.

3.3. Macro-lamellar morphology and embedded voids

Fig. 5 is the TEM micrographs of the Fe-containingmesoporous silicates MFS-60 with different magnifications.In Fig. 5a, the low magnification TEM micrograph of theMFS-60 sample shows a layered lamellar-type structure,where at least three layers can be discerned. The thin sheetsare 5–10 lm in dimension and have erose edges. Lamellarmorphology represents 90% of the solid materials esti-mated from TEM observations. Fig. 5b and c are the highresolution TEM images. In Fig. 5b, the TEM micrographprovides more important and detailed structural charac-ters. The darkness of the flake is not uniform. The darkparts are continuous mesostructures. The gray spots thatdistribute extensively in the lamellar walls can be ascribedto the presence of many submicron-sized, embedded defectvoids, which are irregular shaped and the size distributesbetween 5 and 40 nm. In the higher magnification of thecircled area, as shown in Fig. 5c, there are equi-distant par-allel lines along the breach, with an average spacing at�3.60 nm. The equi-distant parallel lines along can beascribed to the (110) spaces of hexagonally mesostructure.So the d100 value is �4.16 nm after multiplication by the2/30.5, which is in reasonable agreement with the d100 valuefrom XRD pattern. Based on the above-mentioned charac-terizations, it can be concluded that Fe-containing meso-porous silicates with macro-lamellar structure have beensynthesized through this convenient procedure.

Hysteresis loop is usually attributed to the thermody-namic or network effects or the combination of these twoeffects. The presence of extensive embedded voids in themesostructured framework, as shown in TEM micrograph,can effectively explain the uncommon hysteresis loops poresize distribution plot from desorption branch. The defectvoids embedded in the lamellas are surrounded by well-aligned nanochannels of the MCM-41 type. In the adsorp-tion branch, the single steep rise in volume reflects the sizeof nanochannels in capillary condensation. With theincrease of pressure above p/p0 = 0.5, N2 were progres-sively adsorbed into the defect voids through the surround-ing mesopores. In the desorption process of adsorbed N2,since the mesopores surrounding the internal voids are stillfilled with condensed adsorbate, the internal voids cannotbe emptied at the relative pressure corresponding to thecapillary evaporation of them. After the mesopores are

Fig. 5. TEM micrographs of the Fe-containing mesoporous silicates MFS-60 with macro-lamellar morphology: (a) low magnification TEM image; (b)high resolution TEM image; (c) higher magnification of the circled areas.

32 Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34

emptied, the driving force for the evaporation of N2 in theinternal voids is so strong that a lot of cooperative evapo-ration happens at the same pressure. N2 rushes out alto-gether to form an additional inflection in desorptionbranch around p/p0 = 0.45, which is also the cause of theartificial 3.70 nm pore in desorption branch derived poresize distribution plot [30]. The non-physical nature of theartificial 3.70 nm mesopore has also been briefly discussedin the previous literature [31].

3.4. Influence of Fe contents on the morphology

Fe elements replace the Si sites in the crystallization pro-cess of Fe-containing mesoporous silicates. So the dosageof Fe elements would have remarkable impact on the mes-ostructures and morphological specificity. Fig. 6 is theTEM micrographs of the Fe-containing mesoporous sili-cates with the SiO2/Fe2O3 molar ratio of 40 (a) and 20(b). It should be pointed out that the morphology characterunderwent huge changes with the increase of Fe(NO3)3

amounts in the synthesis media. For MFS-40 sample,

tangled microtubules with about several microns in lengthdominate the final product. Upon careful examination, itcan be found that one ends of the mesostructured sheets(the gray parts as shown by arrows) are always dissociatedfrom the tubule bodies. It can be deduced that the tangledtubules come into being by rolling the mesostructured lam-ellas (as sample MFS-60) into a cylinder along the (100)direction. The axes of the tubules are parallel to the 110direction of hexagonal mesostructures. Decreasing theSiO2/Fe2O3 ratio to 20, the long cylinders disappearedand 100–600 nm short tubules represent 80% of the solidmaterials. So the dimensions of the resultant sample arereduced with the decreasing of SiO2/Fe2O3 ratio. The influ-ence of Fe contents on the macroscopic morphology givesimportant inspiration on the formation mechanism of thismacro-lamellar structure.

3.5. Possible formation mechanism

In the previous studies of Mou’s group, a special‘‘delayed neutralization process’’ based on the consider-

Fig. 6. TEM micrographs of the Fe-containing mesoporous silicates MFS-40 (a) and MFS-20 (b).

Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34 33

ation of temporal separation of self-assembly of surfactant-silicate and silica condensation was employed to synthesizehierarchical tubules-within-a-tubule (TWT) MCM-41[9,32–34]. Zheng et al. also synthesized hydrothermally sta-ble mesoporous aluminosilicates with hollow tubular mor-phology via the controlled coassembly of protozeoliticnanoclusters and soluble silica species [35–37]. All thesetubules consist of a micron-sized central pore and coaxialnanochannels (100 spaces) of hexagonal symmetry in thetube wall [35]. The formation mechanisms of these micro-tubules have been discussed in depth. In the beginning,the aluminosilicate-surfactant system was close to thelamellar-hexagonal phase boundary under high pH value,and a little acidification resulted in a mixed lamellar-hexag-onal phase in which layers of hexagonally arranged rodmicelles were separated by bilayers of surfactants andwater. An intermediate lamellar-type structure could beobserved at room temperature with H2SO4/Al2O3 = 5.7(insufficient acidification) [9]. Because the membrane layerswere intrinsically anisotropic, further acidification leadingto the condensation of silicates and charge imbalance onthe membrane surface favored the curvature of the mem-brane along the transrod direction. Thus, tubular morphol-ogy evolved. Tubular morphology is often associated withmore defects and thus larger hysteresis behavior. Extensivevoid defects distributed in the tubule walls. The bendingand closing-up process in a membrane-to-tubule transfor-mation would unavoidably create some stress, and thuspacking defects, between the cylindrical micelles. Whenfurther silica condensation happens, the extra interlayerwater is expulsed to form little puddles. Consequently,many voids were formed as the relic of the water puddles.In brief, the formation of structural defects in the frame-work depends mainly on the amount of incorporated alu-minum and water content [30].

These research results are very advisable for us toexplain the formation mechanism of Fe-containing meso-

porous silicates with macro-lamellar morphology. In pres-ent study, Fe(NO3)3 was added into the synthesis systemto produce Fe-containing mesoporous silicates. After thepH value was adjusted to 10, hydrothermal process pro-ceeded under 110 �C for 48 h. Adopting a similar routeto TWT MCM-41, highly ordered Fe-containing mesopor-ous silicates with macro-lamellar morphology were conve-niently fabricated through careful choice of Fe(NO3)3

contents.The formation mechanism of the Fe-containing meso-

porous silicates with macro-lamellar morphology mightbe as follows: The radius of Fe3+ (0.65 A) is larger thanthat of Al3+ (0.53 A). In the condensation of Fe-silicatemesostructures, the charge imbalance was not strongenough to curve the intermediate lamellar-type structureformed in the early stage of acidification, so the lamellar-type structure was solidified finally after the hydrothermalprocess. The embedded 5–40 nm defect voids were formedas the relic of the water puddles. With the increase of Fecontent (SiO2/Fe2O3 = 40), the charge imbalance on themembrane would accordingly accumulate and served asdriving force to bend the large sheets into tubules alongthe 100 direction. So the TEM image in Fig. 5c shows(110) spaces of hexagonally mesostructure similar tothe TWT tubule walls reported in the previous literature[32]. The dissociated ends in the microtubules, as shownin Fig. 6a, might be for the insufficient driving force tocurve completely. Further increase of the Fe content toSiO2/Fe2O3 = 20, the sheets curved into tubules com-pletely. At the same time, the formation of Fe–O–Fe moi-eties in mesostructured framework should be generallyavoided and thus the microtubules tended to break into100–600 nm short tubules. The different electrostatic prop-erties of Fe and Al elements derived from their intrinsicchemical natures might be the reason for the solidifyingof macro-lamellar morphology of Fe-containing mesopor-ous silicates.

34 Y. Lu et al. / Microporous and Mesoporous Materials 106 (2007) 28–34

4. Conclusions

In summary, highly ordered Fe-containing mesoporoussilicates with macro-lamellar morphology were conve-niently fabricated using sodium silicates and Fe(NO3)3 assilica and iron sources in alkaline media, respectively,under the direction of CTAB cationic surfactant. Iron spe-cies were mainly tetrahedrally incorporated into the meso-structured framework in alkaline media. Through carefulchoice of Fe(NO3)3 contents, the macro-lamellar structurescame into being by a simple hydrothermal synthesis proce-dure and extensive voids were embedded in the mesostruc-tured lamellas. These voids increase the porosity ofunidimensional nanochannels and the diffusion and trans-port of larger molecules become easier in such void systemsand the mixing of molecules across the channels is easier.The different electrostatic properties of Fe and Al elementsderived from their intrinsic chemical natures might be thereason for the solidifying of macro-lamellar morphologyof Fe-containing mesoporous silicates. Combining theproperties of macro-lamellar morphology, more effectiveinterchannel accessibility, and tetrahedrally incorporatedFe species, the Fe-containing mesoporous silicates wouldfind wide applications in membrane-based separation,selective catalysis, and sensors. Further investigation, how-ever, is required to adequately elucidate the influence ofdifferent heteroelements on the morphological specificityand the actual formation mechanism of these hierarchicalmesostructures.

Acknowledgment

Financial support from ECUST’s young scientist foun-dation is gratefully acknowledged.

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