Colloids and Surfaces

A: Physicochemical and Engineering Aspects 179 (2001) 177–184

Investigating the state of Fe and La in MCM-41mesoporous molecular sieve materials

Yueping Kuang a, Nongyue He b,c,*, Jian Wang d, Pengfeng Xiao b,Chunwei Yuan b, Zuhong Lu b

a Department of Chemistry, Hunan Nonnal Uni6ersity, Changsha 410006, PR Chinab National Laboratory of Molecular and Biomolecular Electronics, Southeast Uni6ersity, Nanjing 210096, PR China

c Department of Chemistry, Xiangtan Uni6ersity, Xiangtan 411105, PR Chinad Nanjing Analytical Instrument Factory, Nanjing 210017, PR China

Abstract

Fe-containing, La-containing, siliceous and aluminum silicate MCM-41 mesoporous materials were synthesizedusing water glass as silica source. The states of Fe(III) and La(III) species were investigated by thermogravimetricanalysis (TGA), powder X-ray diffraction (XRD), framework infrared (FTIR), electron spin resonance (ESR) andMossbauer spectroscopic techniques. It was shown that Fe(III) or La(III) species was incorporated into framework.Because the Fe–O or La–O bond is longer than Si–O bond, Fe(III) or La(III) species was limited to insert intoframework and transformed from tetrahedrally coordinated state to octahedrally coordinated state upon calcinationto remove template. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Iron; Lanthanum; MCM-41 mesoporous materials; Zeolite molecular sieves; X-ray diffraction (XRD); Thermogravimet-ric analysis (TGA); Infrared (IR); Electron spin resonance (ESR); Mossbauer spectra

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1. Introduction

The average pore size of microporous zeolitemolecular sieve materials is �0.7 nm. Their ap-plications were limited when used as sorbents,hosts in host-guest systems, as well as catalyticmaterials for large organic and inorganic com-pounds. Therefore, many efforts have been madeto synthesize zeolite molecular sieve materialswith enlarged pore sizes in the past 2 decades.

VPT-5 [1–7], JDF-20 [8] and Cloverite [9,10] withwindow sizes of about 1.5 nm were synthesized.Another conventional approach to prepare molec-ular sieves materials with larger pore sizes is theso-called ‘secondary synthesis method’ — microp-orous zeolite molecular sieves such as A, X, Ywere chemically treated with a (NH4)2SiF6 solu-tion or steamed. But the pore sizes of thesetreated materials did not narrowly distribute [11].A hexagonal mesoporous molecular sieve familyof crystalline aluminosilicates, designated asMCM-41, was first reported in 1992 [12,13] byresearchers at the Mobil Company. Its average

* Corresponding author. Fax: +86-25-7712719/3619983.E-mail address: nyhe@seu.edu.cn (N. He).

0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (00 )00654 -3

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184178

pore diameter can be adjusted between 1.8 and 10nm. This work led to new materials for use assorbents, catalyst materials and host materials forhost-guest chemistry in studies of electron transferphotosensitizers [14], semiconductors [15], poly-mer wires [16–19], conducting carbon wires [20],sensing devices [21], materials with non-linear op-tic properties [22], quantum sized clusters [22,23],etc.

Fe-containing molecular sieve materials arevery important catalysts [24]. La-supported zeolitemolecular sieves are also well-known catalysts.Siliceous, aluminum silicate, and Fe- or La-con-taining MCM-41 mesoporous materials can beeasily synthesized using water glass as siliconsource [25]. The synthesis and the investigation ofthe thermal and hydrothermal stability of thosematerials have been reported in detail in ourprevious report [25], but the states of Fe or Laspecies were left not characterized in detail. Wewill present evidences of the incorporation of Fe,La into the framework (channel wall) of thesesamples by Thermogravimetric Analysis (TG),framework vibration Fourier transform infrared(FT-IR) spectroscopic method, powder X-ray dif-fraction (XRD), electron spin resonance (ESR)and Mossbauer spectroscopic techniques.

2. Experimental

The Fe-containing mesoporous materials (des-ignated as FeSiMCM-41) with different Fe con-tents were prepared according to the followingsynthesis procedures: 11 g distilled water, 0.4 gsulphuric acid (95%) and optionally, differentamounts of Fe(NO3)3 9H2O, varying from 0 to0.71 g, were mixed and stirred for 10 min. Then8.6 g water glass (20.3% SiO2, 6.7% Na2O) wereadded. After stirring the formed mixture for 10min, the appropriate amount template solution(25% C16H33(CH3)3NBr, CTABr) was added. Theformed gel was stirred for 30 min before adding6.7 g distilled water. Then the gel was transferredinto a stopped teflonlined steel bottle and heatedwithout stirring at 373 K for 7 days. After cooledto room temperature, the resulting solid productwas recovered by filtration, extensively washed

with distilled water, and dried in air at ambienttemperature. The preparation of La-containingmesoporous materials, LaSiMCM-41, was similarto that mentioned above except for substitutinglanthanum nitrate for ferric nitrate. For compari-son, the Al-containing sample, A1SiMCM-41,was also synthesized by means of adding a givenamount of sodium aluminate into the gel mixtureinstead of ferric nitrate.

The samples in synthesized or calcined formwere characterized by TG on a thermoflex ana-lyzer (Rigaku) up to 1073 K using a heating rateof 20 K min−1 and about 10 mg samples open toair, low-angle powder X-ray diffraction (Rigaku,D/max-gA) with Cu–Ka radiation. Frameworkvibration Fourier transform infrared spectra wererecorded on a Nicolet 510 P FTIR spectrometerusing the KBr pellet technique. Electron spin res-onance spectra were investigated on a Bruker EP200-D-SCR instrument. The Mossbauer spectrawere recorded on a constant acceleration Moss-bauer spectrometer with a source of 57Co in Pdmatrix. The adsorption of nitrogen at 77 K wasconducted on a Micromeritics ASAP2000 instru-ment. The composition of samples was obtainedon a Jarrell–Ash 1100 inductively coupled plasmaquantometer.

3. Results and discussion

Data for the as-synthesized materials with dif-ferent Si/Me (Me=Al, Fe and La) bulk ratiosdetermined by chemical analysis are given inTable 1. The numbers in parentheses attached tosample designations are the SiO2/Me2O3 ratios(Me=Al, Fe, La) in framework; in reference toMCM-41, Me means Al, Fe, La in the synthesizedsamples. Not only the Fe or La species are foundin these solid products, but also the content of Feor La increases with the content of these species ingel mixtures. The powder XRD patterns of theseas-synthesized samples (Figs. 1 and 2) and theircalcined forms (Fig. 3) show two to four peaks inthe region of 1–8° 2u indexed on a hexagonallattice typical of MCM-41 materials [12,13]. Wehave, therefore, come to the conclusion that in-corporation of Fe(III) or La(III) does not change

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Table 1Composition of gel mixtures and as-synthesized samples

Samples As-synthesized samplesMolar composition of the gel mixtures

SiO2 Al2O3CTAB Fe203 La2O3 Na2O H2O SiO2/Al2O3 SiO2/Fe2O3 SiO2/La2O3

1.0 –b – –SiMCM-41 0.20.25 57.0 – – –AlSiMCM-41(118)a 1.00.25 0.008 – – 0.2 57.0 118 – –

1.0 0.015 – 0.2 57.00.25 58A1SiMCM-41(58) – –1.0 0.030 – – 0.2A1SiMCM-41(32) 57.00.25 32 – –

1.0 – 0.008 –FeSiMCM-41(124) 0.20.25 57.0 – 124– 0.015FeSiMCM-41(62) –0.25 0.2 57.0 – 62– 0.030 – 0.20.25 57.0FeSiMCM-41(32) – 32 –

– – 0.008LaSiMCM-41(147) 0.20.25 57.0 – – 147– – 0.015 0.20.25 57.0LaSiMCM-41(83) – 83– – 0.030 0.2 57.0 –LaSiMCM-41(48) 480.25

a Numbers in parentheses are the SiO2/Me2O3 ratios (Me=Al, Fe, La) in framework; in reference to MCM-41, Me means Al,Fe, La in the synthesized samples.

b ‘–’ means the components are absent or negligible.

the hexagonal structural form of MCM-41 meso-porous materials. However, what are the states ofFe(III) and La(III) species in these samples?

For the La–O bond (0.254 nm) much longerthan Si–O bond (0.161 nm), the incorporation ofLa(III) into the frameworks of microporousmolecular sieves is usually thought impossible.However, The incorporation of La(III) into theframework of ZSM-5 was reported by Inui et al.[26] and Wang et al. [27] based on the XRD andframework IR spectra of their synthesized ZSM-5samples. After the introduction of La(III) into thegel mixture to synthesize ZSM-5, a shift of XRDpeaks toward low-angle direction and a shift offramework IR vibration bands to lowerwavenumbers for the synthesized samples wereobserved and were attributed to the substitutionof longer La–O bond for the shorter Si–O bond[27]. It is evident from Fig. 1 that compared withthe XRD peaks for as-synthesized SiMCM-41, theXRD peaks for the assynthesized LaSiMCM-41samples shift to low 2u and the framework IRspectra in Fig. 4 for the same samples showvibration bands at lower wavenumbers than thosefor SiMCM-41. Combining the above results withthat the channel wall of MCM-41 type meso-porous materials is amorphous [13,28] and, there-fore, no exact structural demand to hinder the

Fig. 1. XRD patterns for as-synthesized for (a) SiMCM-41; (b)LaSiMCM-41(147); (c) LaSiMCM-41(83); (d) LaSiMCM-41(48).

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184180

Fig. 2. XRD patterns for as-synthesized (a) SiMCM-41; (b)FeSiMCM-41(124); (c) FeSiMCM-41(62); (d) FeSiMCM-41(32).

nents of MCM-41-type mesoporous materials willdecompose and be removed at different tempera-tures. Thus TG is one of the powerful tools toinvestigate the state of inorganic components inMCM-41 samples. The thermogravimetric curvesof all the above as-synthesized samples are shownin Figs. 6 and 7. Two steps can be clearly ob-served in the TG curves of as-synthesizedSiMCM-41. The low-temperature step (450–620K) was attributed to the decomposition of tem-plate interacting with siloxy group sites, while thestep in high-temperature range was attributed tothe condensation of OH groups and the decompo-sition of template associated with aluminum spe-

Fig. 3. XRD patterns for calcined (a) SiMCM-41; (b)A1SiMCM-41(58); (c) FeSiMCM-41(62); (d) A1SiMCM-41(83).

incorporation of La(III) into channel wall, wehere suppose that La(III) species has been incor-porated into the as-synthesized LaSiMCM-41samples.

Compared with La(III), the substitution ofFe(III) for Si easily takes place in many micropo-rous molecular sieves [13]. Similar to LaSiMCM-41 samples, the XRD peaks in Fig. 2 foras-synthesized FeSiMCM-41 samples shift tolower 2u than that for the as-synthesized SiMCM-41. On the other hand, the framework IR vibra-tion bands in Fig. 5 for the same as-synthesizedFeSiMCM-41 samples shift towards lowerwavenumbers than those for the as-synthesizedSiMCM-41. As mentioned above for theLaSiMCM-41 samples, these results clearly sug-gest that some Si–O bonds have been replaced bythe longer Fe–O bond (0.197 nm). We will furtherdiscuss the states of Fe(III) and La(III) in thesesamples.

According to the previous reports [13,28], tem-plates associated with different inorganic compo-

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184 181

Fig. 4. Framework vibration FT-IR spectra for as-synthesizedsamples. (a) SiMCM-41; (b) LaSiMCM-41(147); (c)LaSiMCM-41(83); (d) LaSiMCM-41 (48).

MCM-41, a new step (530–620 K for FeSiMCM-41, 600–710 K for LaSiMCM-41, respectively)appears in the TG curves for the as-synthesizedLaSiMCM-41 and FeSiMCM-41, respectively,supporting again the incorporation of Fe(III) orLa(III) into the channel wall of these mesoporousmaterials. These results from TG analysis are inconsistence with those from XRD and Frame-work IR characterization mentioned above.Moreover, that the step associated with La(III)species exhibits a higher temperature range thanthe step associated with Fe(III) species indicates astronger interaction between template and La(III)species than that between template and Fe(III)species.

Although no other more effective characteriza-tion techniques but XRD and FT4R are reported

Fig. 6. Thermogravimetric Analysis of as-synthesized samples.(a) SiMCM-41; (b) FeSiMCM-41(124); (c) FeSiMCM-41(62);(d) FeSiMCM-41 (32).

Fig. 5. Framework vibration MIR spectra for as-synthesizedsamples. (a) SiMCM-41; (b) FeSiMCM-41(124); (c) FeS-iMCM-41(62); (d) FeSiMCM-41 (32).

cies in channel wall (framework) [13,28]. The alu-minum species in SiMCM-41 should come fromthe silica source, water glass, as an impurity. Asimilar phenomenon was also observed bySchmidt et al. [28].

From Figs. 6 and 7, we find that perhaps as aresult of introduction of Fe(III) or La(III) into

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184182

Fig. 7. Thermogravimetric Analysis of as-synthesized samples.(a) SiMCM-41; (b) LaSiMCM-41(147); (c) LaSiMCM-41(83);(d) LaSiMCM-41 (48).

species mainly locate in framework positions. Thiscan be further demonstrated by the Mossbauerspectroscopy investigation. Mossbauer spec-troscopy is a very useful tool to study the state ofFe species in zeolite molecular sieve materials. Ifin tetrahedrally coordinated state, Fe(III) specieswill give a isomer shift (IS) value smaller than 0.3mm s−1 in contrast to the IS value greater than0.3 mm s−1 typical of octahedrally coordinatedFe(III) species [24]. Fig. 9 shows the Mossbauerspectrum for FeSiMCM-41(62) (Fig. 9a), the ISvalue for this sample is 0.28 and no apparentcomponent with IS value greater than 0.3 is de-tected, indicating the Fe(III) species mainly existin framework in trahedrally coordinated condi-tion, suggesting again the Fe(III) species in theas-synthesized FeSiMCM-41 exist in a tetrahe-drally coordinated environment.

All the above experimental results have provedthe incorporation of Fe(III) or La(III) species, atleast partially, into channel wall of as-synthesizedFe-containing or La-containing MCM-41 sam-ples. However, comparing the lost contents corre-sponding to the steps associated with Fe(III) orLa(III) species (see Figs. 6 and 7), we find that thelost contents increase with the increment ofFe(III) or La(III) contents for samples possessingrelatively low Fe(III) or La(III) contents, but no

Fig. 8. ESR spectra for as-synthesized (a) FeSiMCM-41(124);(b) FeSiMCM-41(62); (c) FeSiMCM-41 (32); and (d) calcinedFeSiMCM-41(62) at 813 K in air for 1 h; (e) calcined FeS-iMCM-41(62) at 813 K in air for 4 h.

to characterize the states of La(III) species inzeolites, the structural arrangement of iron speciesin zeolite molecular sieve materials is also easilydetected by other methods such as ESR [29,30]and Mossbauer spectra [24] of Fe-containing zeo-lite molecular sieve samples. As depicted in Fig. 8,two different signals occur in the spectra of theas-synthesized FeSiMCM-41 samples. The signalwith g=4.3 was assigned to Fe(III) in distortedframework tetrahedral coordination whereas thesignal with g=2.0 belongs to Fe(III) species in ahighly symmetric octahedrally coordinated envi-ronment [29,30]. Taking it into consideration thatthe signal of g=2.0 is much more sensitive to thecontent of Fe(III) species in a octahedrally coordi-nated environment than in tetrahedral coordina-tion, Fig. 8 shows us that the introduced Fe(III)

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184 183

Fig. 9. Mossbauer spectra for (a) as-synthesized FeSiMCM-41(62) and (b) calcined FeSiMCM-41(62) at 813 K in air for 4h.

do not demonstrate a same trend. Compared withthose for FeSiMCM-41(62), the bands for FeS-iMCM-41(32) even slightly shifts towards highwavenumbers.

As contrast with the behavior of Fe(III) orLa(III) species in Fe-containing or La-containingsamples upon TG reported here, the lost contentassociated with Al species in A1SiMCM-41 sam-ples increases almost linearly with the Al contentsin the synthesized A1SiMCM-41 samples (notshown here), which was also reported by Schmidtet al. [28]. It is supposed that the differenceamong the Me–O bonds (Me=Si, Al, Fe and La)is responsible for the different thermogravimetricbehaviors of these as-synthesized samples. Thelength of Al–O bond (0.175 nm) is slightly longerthan that of Si–O bond, making it easy for Al toinsert into framework. However, Fe–O bond(0.197 nm) or La–O (0.254 nm) is much longerthan Si–O bond, the incorporation of Fe(III) orLa(III) into channel wall to replace Si–O bondwill severely distort the tetrahedron MeO4 and is,therefore, more difficult than the incorporation ofAl species. Thus the content of Fe(III) or La(III)species in framework is very limited, no matterwhat is the total content of these species in theas-synthesized samples.

The longer Fe–O or La–O bond not onlylimits the incorporation of Fe(III) or La(III) intoframework (channel wall), but also destabilizesthe Fe(III) or La(III) species in framework. Uponcalcination at 813 K in air to remove template,the Fe(III) species in FeSiMCM-41 samples grad-ually transformed from tetrahedrally coordinatedstate to octahedrally coordinated state [25], ac-companied by the disappearance of ESR signalwith g=4.3 (see Fig. 8d and e) and the increaseof Mossbauer IS value from 0.28 to 0.43 mm s−1

(Fig. 9b), meaning that Fe(III) species mostlymigrated from framework to the outer surface ofchannel wall. La(III) species also transformedfrom framework state to nonframework state [25].However, as shown in Fig. 10 no detectableLa2O3phase is found in calcined LaSiMCM-41(83) (Fig. 10b). In contrast to the appearance ofLa2O3 phase in the XRD pattern for the La(III)loaded SiMCM-41 sample prepared by mixing

Fig. 10. XRD patterns for (a) La2O3; (b) calcined LaSiMCM-41(La2O36.1%) (813 K, 4h); (c) La2O3/SiMCM-41(La2O3

6.1%).

linearity relationship between the lost contentsand the Fe(III) or La(III) contents in these as-syn-thesized samples is found. This shows us thegreater the Fe(III) or La(III) content, the moredifficult the incorporation of these species intoframework. Especially, even though the Fe(III)content in FeSiMCM-41(32) is almost twice asmuch as that in FeSiMCM-41(62), the lost con-tents associated with Fe(III) species in these twosamples are nearly equal to each other. The situa-tion can also be observed when investigating theframework vibration FT-IR spectra for as-synthe-sized samples (see Figs. 4 and 5). Although thebands for these samples generally shift to lowwavenumbers with the increase of Fe(III) orLa(III) content, the bands for FeSiMCM-41(32)

Y. Kuang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 179 (2001) 177–184184

La2O3 and SiMCM-41 and possessing the similarLa2O3 content as that in LaSiMCM-41(83) (seeFig. 10c), it is drawn that after calcination thedeveloped La(III) species will highly disperse onthe surface of channel wall, very similar to thesituation of Fe(III) species in FeSiMCM-41 sam-ples after calcination [25].

Acknowledgements

This project was financially supported by theNatural Science Foundation of P.R. China, theNatural Science Foundation of Hunan Province,PR China, the Science Foundation of ChinesePost-doctoral Programs and the National Labora-tory of Solid State Microstructures, Nanjing Uni-versity, Nanjing 210093, PR China.

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