Effect of pore size of mesoporous molecular sieves (MCM-41) on Al stability and acidity

Effect of pore size of mesoporous molecular sieves (MCM-41) on Al stability and acidity

THE CHEMICAL ENGINEERING JOURNAL ~,-~.; ,,,IIAill ELSEVIER The Chemical Engineering Journal 64 (1996) 255-263 Effect of pore size of mesoporous mol...

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THE CHEMICAL ENGINEERING JOURNAL

~,-~.; ,,,IIAill ELSEVIER

The Chemical Engineering Journal 64 (1996) 255-263

Effect of pore size of mesoporous molecular sieves (MCM-41 ) on A1 stability and acidity Xiaobing Feng a,1, Jae Sung Lee a,2, Jun Won Lee a, Jeong Yong Lee b, Di Wei a, G.L. Hailer a,. "Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, CT 06520-8286, USA b Department of Electronic Material Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea

Abstract A matrix of mesoporous molecular sieves (MCM-41) with various Si/AI ratios and various pore sizes was synthesized. These were structurally characterized by X-ray diffraction, IR and ZTA1NMR. The relative acidity of these materials was estimated catalytically based on the isomerization selectivity of 2-methyl-2-pentene. The activity and selectivity of this reaction are affected by both the Si/A1 ratio and the pore size at fixed composition. The stability of AI in the wall structure (as tetrahedral AI) is also affected by the pore size, the smallest pore size resulting in the most stable tetrahedral A1. Keywords: Mesoporous molecular sieves; MCM-41; 27A1NMR; Acidity

1. Introduction Considerable literature now exists on the effects of uniform micropores on heterogeneous catalytic activity and selectivity [ 1 ]. We recognize both transport selectivity and restricted transition state selectivity in zeolites [2]. How the shape of uniform mesopores would affect activity and selectivity has remained an unanswerable question until recently because we did not have any such materials available. In any case, it needs to be cast in different terms than those that apply to zeolites because once the pores are larger than the reacting molecules we cannot expect significantly different transport properties for molecules of the same or similar molecular weight, e.g. we might expect the diffusivity of ortho- and para-xylene to be about the same in a 20/~ pore. While there are available a large number ofmesopore ( 2 0 200 /~) oxide catalysts and/or supports, these materials almost always have a wide distribution of pore sizes relative to zeolites which possess very uniform micropores (2-20 /~). It is generally presumed that a mesoporous material with uniform pores would have wide utility in catalysis, although the advantages have not often been articulated. I f a particular * Corresponding author. Present address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15261, USA. 2 Permanent address: Department of Chemical Engineering, Pohang Llniveristy of Science and Technology, San 31 Hyoja-Dong, Pohang, South Korea. 0923-0467/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved P11S0923-0467(96)03143-0

large molecule is to be reacted, a material with only pores that comfortably admit this molecule to the internal surface would have the possibility of maximizing the available surface area per unit volume reactor. This is obvious because a material with pores which were too small or pores that were too large would not be an optimum use of the available surface area per unit volume of reactor. There is some empirical evidence that this is the thrust of improvements in hydrotreating catalysts which often use alumina supports with quite sharp pore size distributions around 100 ,&. In addition, it is possible to imagine certain kinds of shape selectivity in mesopores. For example, if the pores are uniform and there exists a molecule which is cigar shaped with similar functional groups on its side and its end, but with a cross-section that just fits within the pore, then one might expect regio-selectivity in reactions of the functional groups on the side of the molecule. The functional group on the end of the molecule would be more or less inhibited from reaching active sites on the pore walls once the molecule had entered the pore. This kind of shape selectivity will likely be important in certain fine chemical or pharmaceutical catalytic reactions. Here we are interested in a more subtle effect of mesopore size and shape and pose the question, "What is the effect of the radius of curvature of the pore wall on catalysis?" Classical thermodynamics indicates that the surface free energy is a function of the radius of curvature, and while it is rather difficult to deal with experimentally for solid-solid interac-

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tions, the effect on the gas-solid and liquid-solid interfaces is well documented [3]. The above may serve as examples of possible uses of a mesoporous molecular sieves, but clearly it is not possible to define all such applications until such materials have been prepared and characterized. Mobil has patented a family of such mesoporous molecular sieve materials designated MCM-41 [4-7]. These materials are believed to be formed by a liquid-crystal template mechanism [ 8,9 ] and, by varying the template size (the alkyl length in a long chain alkyltrimethylammonium ion), the pore size may be varied in the range of 15--40 A. The usual starting materials are soluble silicates or silica-alumina mixtures. Casci [ 10] has recently reviewed the preparation and potential applications of what he calls "ultra-large pore molecular sieves" and he gives a summary of the several Mobil patents on applications of mesoporous materials. Because it is possible to prepare mesoporous molecular sieve materials of nearly uniform pore sizes (as in the case of crystalline zeolites) and to synthesize these materials with a wide range of pores sizes (which cannot be done with zeolites), we can now seek answers to the question posed above. This question may take different forms. We may incorporate foreign ions into the siliceous MCM-41 structure which will constitute catalytic sites, e.g., AI and Ga as acid sites or Ti and V as oxidation sites. In the former case, we wish to know if the local radius of curvature of the pore wall might alter the acidity of the AI (or Ga) sites and, in the latter case, it might be expected that the oxidation potential will be altered by the radius of curvature. The supposition is that the cation will accommodate to the local wail shape (radius of curvature) by adjusting slightly the bond angles and bond lengths. This is equivalent to changing the orbital hybridization, e.g. s and p orbitals of Si and AI [ 11 ], and is known to effect the acidity of the SiOHAI site [ 12,13]. A second kind of effect of the pore wall radius of curvature might be reflected in the interaction between a small metal cluster and the pore wall. We are using mesoporous molecular sieves (MCM-41) to investigate pore radius of curvature effects on catalytic acidity, oxidation activity, and metal-support interaction. In order to pursue these questions it is first necessary to synthesize, and characterize in some detail, the materials to be used for the catalytic testing of the hypothesis of radius of curvature effects. Here we will concentrate on characterization using XRD, IR and 27AI NMR of AI-MCM-41 materials and demonstrate the effect of pore size oh acidity measured by the rate of a catalytic isomerization of 2-methyl-2-pentene. Radius of curvature effects on oxidation activity and metalsupport interaction will be reported elsewhere.

2. Experimental details 2.1. Chemicals and solutions

The silica source was HiSil-233 ( 150 m 2 g - a, 19 nm particles, pH -- 7, and about 1.4% NaCI), from Pittsburgh Plate

Glass (PPG); tetramethylammonium silicate (0.5 TMA/ SiO 2, 10 wt.% silica), from SACHEM, Inc,; and sodium silicate, N band, 28T, from Fluka Co. The alumina source was PHF alumina [ 14], CYTEC Industries, Inc.; sodium aluminate and aluminum sulfate, Fluka Co. The quaternary ammonium surfactants C~H2~+ ~(CH3)3NBr were purchased from Sigma Co. with n equal to 12, 14, and 16 and from American Tokyo Kasei with n equal to 6, 8, and 10. IRA400(OH) ion exchange resin was obtained from Aldrich Co. Sulfuric acid was purchased from J.T. Baker Chemical Co. The CnH2~ +I(CH3)3NOH (n = 6, 8, 10, 12, 14, 16) solutions were prepared by ion-exchanging the 29 wt.% of CnH2n+I(CH3)3NBr aqueous solution with equal molar exchange capacity of IRA-400(OH) ion exchange resin by batch mixing. The Y-zeolite (Y-54, Union Carbide Corp.) was provided by D.E. Resasco from a standard sample held by Sun Co. 2.2. Mesoporous molecular sieves (MCM-41)

All samples used are listed in Table 1. For the AI-MCM41 samples, 200 g of the appropriate alkyltrimethylammonTable 1 Compositionsand propertiesof MCM-41 samples. The samplenumbersand codesare fromRef. [21 ] and are retainedso that additionalcharacterization found there may be used Sample

Code

SIO2/A1203 Si/AI

Surfactant

214.001 214.002 214.003 214.004 214.005 214.006 214.007 214.008 214.009 214.010 214.011 214.012 214.013 214.014 214.015 221.001 221.002 221.003 221.004 221.005 221.006 221.007 221.008 221.009 221.010 177.002 200.006 200.007 200.008 200.009 200.010 169.003

C06Si/All6 C06Si/A105 C06Si/AI03 C0~i/A101 C06Si/AI0.7 C08Si/AI16 C08Si/AI05 C08Si/AI03 C08Si/AI01 C08Si/AI0.7 Cl0Si/All6 Cl0Si/Al05 Cl0Si/Al03 CIOSi/A101 CIOSi/A10.7 CI2Si/All6 CI2Si/AI05 CI2Si/AI03 CI2Si/AI01 C12Si/AI0.7 C14Si/AII6 C14Si/AI05 CI4Si/AI03 C14Si/AI01 CI4Si/AI0.7 C16Si/AI16 C16Si/AI08 CI6Si/AI04 CI6Si/A102 CI6Si/AIG1 CItSi/AI0.7 Cl6Si/Alco

95/5 85/15 78/22 57/43 45/55 95/5 85/15 78/22 57/43 45/55 95/5 85/15 78/22 57/43 45/55 95/5 85/15 78/22 57/43 45/55 95/5 85/15 78/22 57/43 45/55 95/5 90/10 81/19 69/31 52/48 45/55 co

CeI'II3(CH3)3NOH CtHz3(CH3)3NOH C~'II3(CH~)3NOH CtHx3(CH3)3NOH C~It3(CH3)3NOH CsHIT(CH~)3NOH CsH17(CH~)3NOH CsHI7(CH3)3NOH CsHIT(CH03NOH CsHIv(CH3)3NOH CioH21(CH3)NOH CioH2t(CH3)NOH CioH2t(CH3)NOH CtoI-I21(CH3)NOH CIoH21(CH3)NOH CI2H2s(CH3)3NOH CI2H2s(CH3)3NOH CI2H25(CH3)3NOH Ct2H25(CH3)3NOH CIzH25(CH3)3NOH Ci,tH29(CH3)3NOH C14H29(CH3)3NOH Ct4H2o(CH~)3NOH C1~'129(CH3)3NOH CI,tI'I29(CH3) 3NOH CItH33(CH3)3NOH CI6H33(CH3)3NOH Cx6]-I33( CH3) 3NOH CItH33(CH3)3NOH CI61-133(CH3) 3NOH CItH33(CH3)3NOH CltH33(CH3) 3NOH

16.15 4.82 3.01 1.13 0.70 16.15 4.82 3.01 1.13 0.70 16.15 4.82 3.01 1.13 0.70 16.15 4.82 3.01 1.13 0.70 16.15 4.82 3.01 1.13 0.70 16.15 7.65 3.62 1.89 0.92 0.70 co

X. Feng et al. / The Chemical Engineering Journal 64 (1996) 255-263

ium hydroxide solution was combined with 2 g of PHF alumina, 100 g of tetrarnethylammonium silicate solution, and 25 g of HiSil with stirring. This mixture was placed in an autoclave at 150 °C for 48 h. After cooling to room temperature, the resulting solid product was recovered by filtration on a Buchner funnel, washed with water, and dried in air at ambient temperature. While all of the A1-MCM-41 materials were prepared sodium free, one sample of pure silica MCM-41 was prepared as a structural reference. 40 g of water, 18.7 g of N-brand (sodium silicate, 28.7% silica), and 1.2 g of sulfuric acid were combined with stirring. After stirring the resulting mixture for 10 min, a C16H33(CH3)3NBr template solution was added, and the resulting gel was stirred for 0.5 h. This gel was heated in an autoclave to 100 °C for 144 h. The resulting solid product was recovered and processed by calcining. The pre-dried MCM-41 was loaded into a glass calcination reactor which had a ceramic distributor of inlet gas. The reactor was mounted in an oven which had a programmable temperature controller and a gas flow controller. The sample was heated from ambient temperature to 540 °C in 20 h with a flow of helium, and followed by 5 h of oxygen flow at the same temperature. After cooling to room temperature, the sample was collected and stored for characterization.

2.3. X-ray powder diffraction Low-angle X-ray powder diffraction was performed on samples in the Department of Electronic Material Engineering, Korea Advanced Institute of Science and Technology. Powder X-ray diffraction was performed on a Riguku Dm~B diffractometer with Cu K a radiation.

2.4. Fourier transform infrared spectroscopy All infrared (IR) absorbency data were obtained on Midac (H series) spectrometer using the wafer technique. For the mid-infrared region, the KBr wafer technique was used. Powders containing 1 wt.% MCM-41 in KBr were pressed at 1000 kg cm -2 into a thin wafer with an effective thickness of 90 mg c m - 2. A spectrum was an average of 30 consecutive scans (5 min) recorded between 400 and 1400 e m - mwith a spectral resolution of 0.5 c m - i.

2.5. 29Si and 27Al MAS NMR Both 29Si and 27A1MAS NMR spectra were obtained on a Bruker AM-500 spectrometer at a proton Larmor frequency of 500 MHz. A Doty broad-band double channel 5 mm MAS probe was used with the samples spun at 4 KHz at the magic angle. The pulse width used was 1.5/~s for 29Si (about 20 ° of ~'/2 pulse for 29Si) and 0.3 ~ for 27Al (about 3° of Ir/2 pulse for 2VAl). The recycle delay was 30 s for Si and 5 s for Al. Tetramethylsilane (TMS) and AIC13 (1 M) were used as chemical shift references for 29Si and 2VAl, respectively.

257

2.6. The 2-methyl-2-pentene isomerization 2-Methyl-2-pentene isomerization was performed by injecting the liquid 2-methyl-2-pentene (98%, Aldrich Chemical Co.) into a stream of UHP grade helium at 100 °C with a metering micro-pump (Model 314, ISCO). The vapor was composed of 7% 2-methyl-2-pentene in 25 ml rain- 1 of helium. The 2-methyl-2-pentene vapor passed through 80 mg of catalyst packed in a U-tube reactor with controlled temperature profile, e.g. + 0.1 K. The reactant was injected as a pseudo-pulse by flowing reactant over the catalyst for 5 min. A Perkin-Elmer Sigma 300 gas chromatography with a flame ionization detector was used to perform on line product analysis. The detector temperature was 100 oC. The column was packed with 10% SP-2100 and 80/100 Supelcoport, and the oven temperature was programmed from 35 °C to 45 oC in 10 rain. While performing the GC analysis, the catalyst bed was flushed with UHP helium. The initial reaction rates were evaluated at 180 °C. The reaction rates at time zero were obtained by extrapolating to zero time of the deactivation curve. After finishing the initial rate evaluation, the catalyst was left to deactivate by continuously pumping 2-methyl-2pentene vapor at a temperature of 250 °C for 1 h. The stabilized or "lined-out" catalyst was used to evaluate the steady-state activation energy and activity. The temperature profile used first decreased the temperature from 250 °C to the lowest temperature where the products could no longer be detected, followed by an increase in the temperature to 250 °C again.

3. Results and discussion

Our original synthesis goal was to prepare a matrix of AIMCM-41 catalysts with six different pore sizes (using C6, Cs, C1o, C12, CI4, and C~6 alkyltrimethylammonium ion templates) and five or six SiOJAI203 compositions in the range of 5-55 wt.% A1203. The simplistic expectation was that, at fixed composition and varying pore size, we would be primarily probing the effect of pore radius of curvature. At fixed pore size and varying composition we would be primarily probing acid site density. It was understood that this was an idealized expectation that would require physical characterization of the synthesized catalysts to determine how nearly constant the pore size was when a given template was used in the synthesis but the composition was varied. Likewise, we also sought assurance that the structural composition (Al incorporated into tetrahedral Si sites) would not be affected by the pore size. The very arguments that indicate that the acidity of an SiOHAI site might be affected by the wall radius of curvature suggest that it is likely that the stability of these sites might also be affected by the variation of the surface free energy changes that accompany the change in pore size. The mid-infrared region of 400-1400 e m - 1 encompasses vibrations of the framework structure of silicates and aluminosilicates. Bands are usually identified following an empir-

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icai assignment proposed by Flanigen et al. [ 15] who have classified the mid-infrared vibrations into two types, internai and external. The external vibrations depend on framework structure and are sensitive to topology and special building units in the framework of zeolites, e.g. double rings, but these are not of concern here. The internal vibrations belong to TO4 ( T = S i or A1) tetrahedral units and exhibit only small changes with the framework structure. However, these are sensitive to composition (and bond angles). In Fig. 1, are plotted the frequencies of the asymmetric stretch of the TO4 of Y-zeolite for five Si/Al ratios between 1 and 3 where the frequency moves from about 971 to 1017 c m - ~,respectively. The Si-O bond length is about 1.6 A and the AI-O is of order 0.1 A longer and this results is a more floppy bond (lower vibrational frequency) with increased substitution of Si by AI. Also shown in Fig. 1 is the change in frequency of the asymmetric stretch of the TO4 units of A1-MCM-41 as a function of composition for three nominal pore sizes prepared using C6, C1o and C14 aikyltrimethylammonium ion templates. First we note that the frequency range is blue shifted about 100 cm-~. This is probably the result of the TO4 unit finding itself embedded in a medium with an effective mass less than that of the crystalline zeolites. That is, while the pores of MCM-41 are sufficiently ordered that they diffract X-rays, the walls are practically amorphous [8 ] so that the TO4 will feel the force field of the surrounding few atoms rather than being coupled to the full crystal as in zeolites. In the range of Si/Al from infinity to three, the change in frequency with composition has the same qualitative slope as for the zeolites, but with much less sensitivity. Again, this is surely the result of the short range order of the MCM-41 materials which will give them greater flexibility. At Si/Al ratios greater than three, something very different is happening. One possibility is phase separation into silica-aiumina and alumina which is discussed below. 1120

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functionof compositionfora seriesofC~6templatedAI-MCM41 materials (including the pure silicaSi-MCM-41). In the range of Si/AI from infinity to three (from 100 to 80 wt.% SiO2) where the infrared suggests that there is systematic substitution of Si by AI, we observe a monotonic decrease in the XRD determined wail spacing for a given template (C16 alkyltrimethylammonium ion) (see Fig. 2) from 41 /~ to about 35/~. Note that this is counter to what would be expected based on the difference in the Si--O and AI-O bond lengths. Were there no other changes in structure, the longer AI-O bond length should have resulted in an increase in the pore diameter. It is likely that the increased bond length of AI--O relative to Si--O has been more than compensated by a decrease in the/_OA10 relative to/_OSiO. This is predicted by calculations of van Santen et ai. [ 16] where a Si3 ring structure is compared to a A1Si2 ring structure; while AI substitution results in about a 5% bond lengthening, it is accompanied by at a 20% decrease in the bond angle about the position of A1 substitution. As with the infrared, when AI substitution exceeds 20 wt.% (or Si/AI _<3), we observe a different behavior which, again, may be consistent with alumina being excluded from the walls of the MCM-41 material at higher concentrations, a hypothesis which can be confirmed by 27A1NMR. Fig. 3 shows the 27A1 magic angle spinning NMR of a series of AI-MCM-41 materials before calcination to remove the template, all of which were synthesized with the Cl6 aikyltrimethylammonium ion. Up to 20 wt.% Al2Os, all the AI that can be detected is in a tetrahedrai environment, but at higher wt.% A1203, the incremental AI appears to be octahedral. We assume that this AI in not incorporated into the walls of the MCM-41, but in any case, it is unlikely to contribute to acidity. Fig. 4 shows another series covering the same composition range, but synthesized using C6 alkyltrimethylammonium ion as a template. The pattern of tetrahedrai/octabedral ratio as a function of composition is essentially the same. Fig. 5 shows yet another series of preparations with compositions identical to that in Fig. 4, but synthesized using C~2 aikyltrimethylammonium ion as a template. We have, in fact, synthesized the C6. Cs, Clo, C~2, Ct4

X. Feng et al. I The Chemical Engineering Journal 64 (1996) 255-263

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and CI6 alkyltrimethylammonium ion series with the same result. It appears that the solid solubility of alumina in silica MCM-41 materials is limited to about 20% under our conditions of synthesis. Clearly no such limit exists for the crystalline zeolite structures and no rationalization of this observation for MCM-41 can be given at the moment. Figs. 6-8, show the Ca, Cs and Clo alkyltrimethylammonium ion series after calcination. Thus, Figs. 4 and 6 show the same series of materials before and after calcination. Those materials with a Si/A1 ratio_< 1 appear to have only octahedral A1 following calcination and even the material with Si/ AI = 3, where apparently all of the AI was tetrahedral before calcination, it is mostly octahedral after calcination. Only those samples with Si/A1 ratios of 5 and 16 appear to have retained all of the AI in the tetrahedral positions. However, as the pores are made larger (synthesis with Cs and C~o alkyltrimethylammonium ion templates (see Figs. 7 and 8) ), the tetrahedral AI becomes successively less stable. Thus, while there is no apparent effect of pore size (template) on the stability of tetrahedral A1 as synthesized, after calcination, tetrahedral AI becomes successively less stable as the pore size increases. As our catalytic measure of acidity, we have used the isomerization of 2-methyl-2-pentene, an acid catalyzed reaction suggested by Kramer et al. [ 17]. This reaction may undergo two kinds of isomerization, i.e. a double bond or a

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methyl shift. Double bond isomerization does not require very strong acidity, while the methyl shift only occurs on strong acid sites. If we assume that the double bond isomerization occurs on all acid sites and the methyl shift only on a portion of acid sites which possess acidity above some threshold, then the selectivity ratio of 3-methyl-2-pentene/4methyl-2-pentene (3M2P/4M2P), i.e. the ratio of methyl shift to double bond isomerization, is an approximate measure of the distribution of strong acid site density to the total acid site density. That is, to first order, site density has been norrealized out so that this selectivity is primarily a measure of acid site strength [ 18]. We have used this hypothesis to correlate acid site strength in amorphous silica aluminates with the partial charge on Si as measured by 29Si NMR [ 19] and the observed correlation provides evidence that the hypothesis is correct and useful. Fig. 9 (a) shows the 3M2P/4M2P selectivity as a function of composition for several A1-MCM-41 catalysts, all synthesized by C~ alkyltrimethylammonium ion template. There is a maximum in the rate at about 20-30 wt.% AIzO3. It is probable that the acid strength is low at compositions around 50 wt.% because alumina not incorporated into tetrahedral sites exists, but this alumina may be coordinated to the tetrahedral Ai in the wall. We know from our previous work that placing more AI around a SiOHA1 site lowers the acidity [ 19]. Note also that this alumina is likely octahedral (see

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X. Feng et aL / The Chemical Engineering Journal 64 (1996) 255-263

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Chemical Shift (ppm) Fig. 6. 2~AlNMR spectraof calcinedCe templat~l AI-MCM-41 samples with variousSi/Al ratios. Thesesamplesare the same as in Fig. 4 (before calcination).

27AI NMR) so that Loewenstein's rule does not apply [20], i.e. there may be direct AI-O-AI bonding here which is not permitted for tetrahedral A1. It is more difficult to rationalize the low acid strength of the compositions at 5-10 wt.% A1203. The 27A1 NMR suggests this Al is tetrahedral, and at this composition, each A1 should be more or less isolated and not interacting with other A1 (which, in turn, affects the shape of the ~Si peak and the apparent average chemical shift), a situation that should provide high acid strength. The 29Si mean chemical shift as a function of composition (Fig. 9 (b)) is helpful here. Our previous results on amorphous silica alumina would suggest that all of the compositions between 5 and 30 w t . % ( - 103 to - 104 ppm 29Si chemical shift relative to TMS) should be of high acid strength. While there are some complications with the relaxation time of 29Si [ 21 ], which are affected by bonding to the quadrupolar Al, a simple interpretation which would account for these results would be that at very low Al content, Al is isolated, but more or less buried in the wall and less accessible at compositions below 10 wt.%. This is essentially the argument invoked by Maschmeyer and Rey [22] when they compared the activity of Ti-MCM-41 where the Ti had been incorporated during the initial synthesis compared to a similar catalyst where the Ti was incorporated by grafting to the MCM-41 walls, i.e. all of the Ti was available for catalysis in the latter case. This is somewhat surprising because the walls of MCM-41 are only

about three layers thick and one might have expected that the foreign ion would be excluded to the surface. Fig. 10 summarizes the relative acid strength, as determined by 3M2P/4M2P selectivity, for a full matrix of AIMCM-41 catalysts with five compositions and six pore sizes (estimated from the XRD pore wall spacing and assuming a wall thickness of order 10 ~t, and taken from Kresge et al. [ 8 ] where it was measured as a function of template chain length). There are a couple of immediate observations here: (1) certainly there is an effect of pore size (template used in the synthesis), but the effect never exceeds a factor of five (57 and 78 wt.% SiO2); and (2) the pattern of selectivity appears to be different for low wt.% AI2Oa (less that 15 wt.% A1203) and higher levels of alumina. Almost certainly the pattern of activity at the higher alumina compositions is a convolution of the direct effect of the pore size (radius of curvature) on the acidity of AI incorporated into the pore wall and of the stability of the AI in the wall, i.e. the observations may be dominated by the effect of the alumina phase excluded from the MCM-41 and we do not have enough knowledge about these structure to offer an interpretation. If we restrict our attention the 95 wt.% SIO2/5 wt.% A1203 composition where the phase separation may have a lesser effect, we now see that the effect of pore size is rather small, i.e. it never exceeds a factor of three, and there is a rather

X. Feng et al. / The Chemical Engineering Journal 64 (1996) 255-263

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Chemical Shift (ppm) Fig. 8.27A1NMR spectraof calcinedC,o templatedAI-MCM-41 samples with various Si/Al ratios.

monotonic drop in the apparent acidity as the pore is enlarged from about 15 to 30 ,~. It is possible to estimate the effect of pore diameter effects on local bond angles of SiOHA1 in the wall ofMCM-41 using simple geometric arguments based on cyclic structures (similar to those used in van Santen's models [ 16] ). We have considered these [21], but they are surely too simplistic because they presume a regular crystalline order which we know, based on XRD and 29Si, does not exist. The wall structure is essentially amorphous. A recent molecular dynamics simulation might provide better insight into the effect of pore size on acidity [ 23 ]. There was no direct analysis of the bond angle distribution here, but the distribution of ring sizes (that exist in the amorphous wall structure) was investigated and the average T - O - T bond angle is related to the ring size distribution. In another study, we have observed that an alumina surface which had incorporated Si mostly as four-membered rings provided hydro-thermal stability, but these structures were less acidic than 7-A1203 [24]. A similar finding may apply to MCM-41, but in a somewhat modified form. Six-membered rings are predominantly found in bulk amorphous silica. Previous simulation of silica surfaces (8 deep on a "flat" surface) found a significant increase in the number of three- and four-membered rings near the surface [25] and, not surprisingly, the distribution of rings in the MCM-41 structure appear surface-like. Six models for

MCM-41 have been investigated by Feuston and Higgins [ 23 ]. In one series, the diameter of the pore was held constant (39.4 ]~) while the wall thickness varied at 5.3, 7.7, and 13.0 and in a second series, the hexagonal pore spacing was held constant at 44.6 ,~while the wall thickness varied at 5.9, 8.4, and 10.9 ~. In the latter series, the pore diameter will increase as the wall thickness is decreased and the molecular dynamics simulation predicts an increase in six-, seven-, and eight-membered rings at the expense of five-membered rings in the amorphous wall structure. If these smaller rings account for the increased acidity that we have observed with decreased pore size, then the molecular dynamics simulation results predict an even larger effect when the wall thickness is varied at constant pore size. Here the simulation exhibits increase in the relative five-, six-, seven- and eight-membered rings at the expense of three- and four-membered rings, but of course, most of these may be below the wall surface. Using the new results of Coustel et al. [26], a preparation of different wall thicknesses (varying OH-:SiO2 ratio in the synthesis) at constant pore size (constant template) may be possible and this prediction could then be tested. 4. Conclusions From a structural point of view the 27A1 NMR results are reasonably unequivocal; the stability of tetrahedral AI in

X. Feng et al. / The Chemical Engineering Journal 64 (1996) 255-263

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because of the complications of stability of the A1, effects of alumina phase not part of the wall structure, amorphous nature of the wall structure, etc. More complete physical characterization of these materials is needed and additional measures of acidity based on both equilibrium adsorption of bases as well as other catalytic reactions would be helpful. However, we can conclude that pore size ofMCM-41 is likely to be a design variable for effecting catalysis and this is surely not limited to AI-MCM-41, but affects MCM-41 oxidation sites [27] and the degree of metal-support (MCM-41 as the support) interactions [28], results which we will present elsewhere.

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MCM-41 is greater at smaller pore sizes. There clearly is also an effect of pore size on the catalytic selectivity (and activity) of AI-MCM-41 catalysts, but it is not possible to determine if this is related to effect of pore size on SiOHAI bond angles

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