Structure and Properties of Al−MSU−S Mesoporous Catalysts

Ind. Eng. Chem. Res. 2003, 42, 3989-4000

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Structure and Properties of Al-MSU-S Mesoporous Catalysts: Structure Modification with Increasing Al Content Stephen A. Bagshaw,*,† Stephan Jaenicke,‡ and Chuah Gaik Khuan‡ Materials Technologies Group, Industrial Research Ltd., P.O. Box 31310, Lower Hutt, New Zealand, and Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Republic of Singapore

Al-MSU-SFAU mesoporous molecular sieve catalysts with Al contents ranging from 2.5 to 50 mol % and different pore sizes have been prepared from solutions of nanoparticular zeolite “seeds” and C12, C14, and C16 templates. Significantly higher amounts of tetrahedrally coordinated Al are incorporated into the Al-MSU-SFAU structure over typical Al-MCM-41-type materials. At high Al loadings, Al-MSU-SFAU materials exhibit better hexagonal pore ordering than Al-MCM-41 materials, but pore diameters are observed to decrease with increasing Al content. Al-MSU-SFAU materials contain significant volumes of irregular mesovoids randomly distributed within the primary particles, the volumes of which increase with increasing Al content. Catalytic cumene cracking activity is very high over the low Al content Al-MSU-SFAU materials but decreases in direct correlation with the increasing Al content. Hence, reduced accessibility to the Al centers, or decreasing catalytic activity of the Al centers as the Al content increases, is suggested as a result of the formation of small-pore, high-Al zeolite seeds of zeolite A or sodalite, where Al centers are not accessible to larger cumene reagents. Introduction The development of large-pore molecular sieve and solid-acid catalysts has proceeded at great pace over the past 20 years. The driving force behind this development remains the desire to create active and selective catalysts for the processing of heavy cycle oils, waxes, residues, and new high molecular weight fine chemicals.1 The discovery of the M41S silicate family of materials was a major breakthrough for this area of scientific and technological development.2 Not only did that work produce a new family of ordered mesoporous materials but it created paradigms for the conceptualization and preparation of inorganic materials, and indeed the general manipulation of matter. This new materials chemistry was also very timely. Development of traditional zeolite synthesis routes had not succeeded in reliably producing large-pore catalysts3 of the types required by industry for future precracking, cracking, hydrocracking, isomerization, and alkylation reactions. The new M41S technology offered some promise in filling this void; however, the widespread introduction of regular M41S-type materials into industrial catalysis has not materialized. Two probable reasons for this are the low acid activities of the materials and their poor stabilities under reaction and regeneration conditions when compared to crystalline zeolite and AlPO4-type materials.4 One of the possible ways of improving the activities and stabilities of the new mesoporous type materials has long been identified to lie in the crystallization of the walls of the materials.1,5 The walls between pores in mesostructured materials are generally considered to be amorphous, or rather to lack short-range crystal* To whom correspondence should be addressed. E-mail: [email protected]. † Industrial Research Ltd. ‡ National University of Singapore.

lographic order.5 Crystallization, or indeed an approximation to it, of the pore walls can be deemed to be an important issue that has been the theme of less research effort than might have been expected. It is supposed that pore-wall crystallization would improve the thermal and hydrothermal stabilities of mesostructured materials.6 It would also conceivably augment the catalytic activities of isomorphously substituted ions, such as Al3+, Ga3+, or Ti4+, by increasing the coordination symmetries of the ions and perhaps also allowing higher concentrations of these species to be incorporated into the lattices than is currently possible.7 To this end, an interesting new route to the synthesis of ordered, highly stable, and acidic mesoporous catalysts was recently reported by Liu et al.8 The new method purported to form precrystalline zeolite seeds of zeolite Y9 and then, using the established S+Imicellar templating system,2 subsequently to assemble these into the so-called Al-MSU-SFAU hexagonal mesostructured material. Seeds of two other zeolite structure types,9 MFI and *BEA, have also been successfully transformed into hexagonal mesostructures using similar methods simultaneously described by Liu et al.10 and by Zhang and co-workers.11 Mesostructured materials prepared from these different zeolite seed solutions suggest great promise for high-stability solid-acid catalytic applications.8,10,11 While these methods perhaps show some derivation from early reports of the conversion of zeolites into mesostructures, or indeed vice versa, those early routes tended to result only in intimate mixtures of separate domains of the zeolite and the mesostructure.12,13 Indeed, a more recent report suggested that pore-wall crystallization of mesoporous aluminosilicates can be achieved by using the mesostructure as the silicate source in a modified zeolite synthesis.14 Analysis of that data, however, strongly suggested that the mesostructure was simply being dissolved and subsequently

10.1021/ie021050a CCC: $25.00 © 2003 American Chemical Society Published on Web 07/22/2003

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Table 1. Textural Characteristics of Al-MSU-S Catalysts samplea

Al % prod. (1%b

unit cell (a0) (0.1/nmc

surface area (5/m2 g-1 d

pore volume (0.02/cm3 g-1 e

pore diameter (0.2/nmf

wall thickness (0.2/nmg

C16 50% C16 25% C16 20% C16 10% C16 6.7% C16 5% C16 2.5% C14 50% C14 25% C14 10% C14 5% C14 2.5% C12 10% C12 5% C12 2.5% Si-MCM-41 Al-MCM-41 10%

55 24 20 12 7 7 3 54 25 11 6 4 10 6 3 0 NDj

NPh 4.4 4.2 4.3 4.4 4.2 4.3 NP 3.8 3.8 4.0 3.9 3.9 4.0 3.9 5.1 5.8

40 260 430 620 750 815 1035 210 315 720 1020 1120 705 910 1185 1015 720

0.16 0.31 0.37 0.80 0.82 0.79 0.95 0.22 0.29 0.81 0.98 0.93 0.68 0.79 1.04 0.91 0.53

NP 1.4 1.4 2.2 2.4 2.6 3.0 1.2 1.6 2.0 2.4 2.5 2.2 2.0 2.0 3.3 3.2

N/Ai 3.0 2.8 2.1 2.0 1.6 1.3 N/A 2.2 1.8 1.6 1.4 1.7 2.0 1.9 1.8 2.6

a C X % represents X mol % Al-MSU-S samples prepared using C (CH ) N+Br- templates. b mol % Al of calcined final products n n 3 3 determined using EDAX. c Unit cell calculated from the interplanar spacing using the formula a0 ) 2d100/x3. d Brunauer-Emmette f Teller surface area. Total pore volume calculated at P/P0 ) 0.98. Pore diameter calculated by applying the Barrett-Joyner-Halenda model to the desorption branch. g Pore wall thickness calculated by subtracting the pore diameter from the unit cell dimension, a0. h NP: for XRD measurement, indicates no peaks observed, for pore size measurement, indicates nonporous. i N/A: not applicable. j ND: not determined.

recrystallized in situ, once again producing separate domains of mesostructure and zeolite. The new Al-MSU-SFAU method, on the other hand,8 strongly indicated that nanoscopic, aluminosilicate building blocks exhibiting significant crystalline traits, but which were not fully crystallized, were formed in the zeolite seed solutions.8 The moderate alkalinity, short synthesis time and, in the Al-MSU-SFAU case in particular, the stirred nature of the reaction mixture all served to restrict the sizes of these species by not allowing them to grow into typical zeolite crystallites. It was suggested,8 and subsequently supported in this work, that if a sample of this zeolite seed solution was isolated, a crystalline but nonzeolitic sodium silicate type material was identified. This indicated that indeed the silicate formed was protozeolitic rather than zeolitic in nature. The condensation of the building blocks into larger zeolite crystals was intercepted by the addition of a micellar template. This caused the zeolite seeds to assemble into an ordered mesostructure around these templating structures. The materials that were isolated exhibited hexagonal pore symmetry, significantly higher levels of tetrahedral Al coordination, and greater hydrothermal stability than Al-MCM-412,15 frameworks that were assembled either from monomeric or noncrystalline oligomeric species (sodium silicate and silica).2 Recently, this new area was reviewed, and the important advances in the development of these semicrystalline mesostructured materials were described.16 Zeolites with different structure types can be prepared over a very wide range of Si/Al ratios17 from approximately infinity [e.g., 0 mol % Al in zeolite silicalite (MFI)] to 1:1 [e.g., 50 mol % Al in zeolites of low-silica X (FAU), zeolite A (LTA), or sodalite (SOD)]. Different Si/Al ratios have downstream effects not only on the thermal and hydrothermal stabilities of the zeolites but also on their hydrophobicities, cation-exchange capacities, and catalytic activities, and indeed the type of zeolite that can be formed from a given reagent mixture.18 Al-MCM-41-type materials, on the other hand, when prepared by one-step direct Al incorporation methods,2 can generally be prepared up to Al loadings of about 5-7 mol %.8 More Al than this, and the

formation of octahedral extraframework aluminum (EFA), and decreases in the structural regularity of the hexagonal mesostructure become significant. Although some of these characteristics have been improved by the delayed neutralization-condensation method described by Kim et al.,19 this method is more labor-intensive and has not become mainstream. Therefore, Al-MSU-SFAU materials discussed here will be compared to Al-MCM-41 materials prepared by the standard onestep direct incorporation method.2 Liu et al.8 described the formation of Al-MSU-SFAU with 10 mol % Al loading but did not expand on the possibility of extending the system to lower or to higher Al loadings and the effect this might have on the system. In this contribution, we have prepared a range of Al-MSU-SFAU materials with Al loadings from 2.5 to 50 mol % and different pore diameters prepared from C12, C14, and C16 templates. The effect of the increased Al loading on the mesoporous and chemical structures and the catalytic cracking activities of the materials is discussed, in particular the change from the formation of LZY seeds to LTA seeds as the Al content in the synthesis gel is increased. Brief conclusions are drawn regarding the likely suitability of these materials for industrial application. Experimental Section 1. Preparation of Al-MSU-S Materials. A range of aluminosilicate Al-MSU-S samples, with different Al loadings and different pore sizes, was prepared from zeolite FAU-(LZY) seed solutions following the general procedure outlined by Liu et al.8 The Al concentration of the starting synthesis gel and subsequently the final catalyst materials was modified by changing the amount of Al reagent, NaAlO2, added to the zeolite seed solution. Al loadings of 50, 25, 20, 10, 6.7, 5, and 2.5 mol % were selected. Al incorporation into the final materials was not 100% quantitative (Table 1), but it was within acceptable variation from the values expected. At higher Al contents, we will develop the hypothesis of the formation of zeolite seeds other than LZY, such as LTA, so we henceforth remove the descriptor, FAU, from the

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Al-MSU-S8,10 nomenclature. Typically, NaOH and sodium aluminate (54.0 wt % Al2O3, 39.0 wt % Na2O, and 1.2-1.3 Na2O/Al2O3) were dissolved in distilled water. A sodium silicate solution (29.5 wt % SiO2, 14.7 wt % Na2O, and 55.5 wt % H2O) was added slowly under vigorous stirring. As the sodium silicate was added, a viscous aluminosilicate gel was formed, the viscosity of which depended on the Al concentration. A clear solution was formed at the lowest Al loadings. The zeolite seed solution was subsequently formed by stirring this gel at room temperature for at least 1 h and then heating under reflux with moderate stirring at 100 °C overnight. The hexagonal Al-MSU-S mesostructure was formed by adding, under moderate stirring, appropriate amounts of the aluminosilicate zeolite seed solution to a solution of alkyl trimethylammonium bromide (RTAB) template in distilled water at room temperature. This mixture was further stirred for at least 1 h to homogenize the gel. The pH of the system was then lowered to pH 9-10 by adding dropwise the correct amount of a dilute H2SO4 solution under stirring. Further vigorous stirring was required to break up and homogenize the gel that formed during acid addition. These gels were also stirred for at least 1 h to homogenize. The importance of the homogenization steps to the formation of high-quality materials cannot be overstated. The importance of obtaining the correct pH also cannot be overstated. If the pH is adjusted too low, the templating effect will be all but lost through charge mismatching effects and reduced hydrolysis and condensation of the silicate. The final gel composition can be represented as 0.09:0.90: x:0.20:0.65:100-200 NaOH/Na2SiO3/NaAlO2/RTAB/ H2SO4/H2O, where x ) 0.0225-0.45. The H2O content needed to be increased from 100 to 200 mol equiv for the 25 and 50 mol % Al samples to allow for the dispersion of the aluminosilicate gels. This was required because of the increased viscosity that occurred in the gel that was apparently generated by larger zeolite seed particles that were produced in the higher Al reaction mixture. The final gels were then crystallized in sealed polypropylene bottles for 36 h at 100 °C under static conditions. The as-prepared white crystalline precipitates were filtered, washed well, and dried at 80 °C overnight. This resulted in hexagonal aluminosilicate mesostructures that are denoted as Cn-X Al-MSU-S, where n ) 12, 14, or 16 and X ) 50, 25, 20, 10, 6.7, 5, or 2.5 mol % Al. As we will discuss later, at high Al contents it may not be appropriate to use the FAU descriptor to describe the structures of the zeolites that are formed in the zeolite seed solutions. In this particular study, the order of NH4+ for Na+ ion exchange was reversed from that of Liu et al.8 to more closely replicate standard zeolite practice.17 While this may not indeed produce the most hydrothermally stable materials as suggested,8 it allowed for more quantitative ion exchange and also resulted in less structural degradation during the ion exchange. Ion exchange prior to calcination regularly resulted in materials with decreased pore ordering and increased mesovoid formation, resulting from localized structural dissolution. The dried, as-prepared samples were calcined at 540 °C for 7 h to remove the template. The calcined catalysts were then ion-exchanged under stirring with a 1.0 mol L-1 NH4NO3 solution at 100 °C overnight (1 g per 50 mL of solution), filtered, washed,

and dried to prepare NH4+-exchanged materials. This ion exchange was repeated if required. The catalysts were subsequently activated by thermal decomposition of the exchanged NH4+ to H+ in situ in the catalytic reactor. 2. Characterization. Powder X-ray diffraction (XRD) patterns of Al-MSU-S mesostructures were obtained with a Philips PW 1700 series APD diffractometer using Co KR radiation (λ ) 0.178 896 nm), equipped with an automatic variable divergence slit (sample beam length 12.5 mm), and using a step-scan mode with a step of 0.03° 2θ and an acquisition time of 5 s. N2 isotherms of calcined samples were obtained with a Micromeritics ASAP 2010 comptometer at -196 °C after degassing at 250 °C and 10-6 mmHg for at least 4 h. 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were obtained with a Varian Unity 500 spectrometer using a DOTY Scientific multinuclear probe and 5 mm zirconia rotors: 29Si resonance frequency, 99.745 MHz; pulse width, 4 µs; recycle delay time, 400 s; spinning speed, 8 kHz; referenced to tetramethylsilane (TMS) assigned 0 ppm chemical shift. The usual notation Qx is used for the Si speciation, where x denotes the number of Si atoms surrounding a central Si atom bridged by oxygen atoms. 27Al spectra were recorded at 130.245 MHz with 1 µs pulse width, 1 s recycle delay time, and 10 kHz spinning speed. Spectra were recorded with an external reference of a totally octahedral 1.0 M Al(NO3)3 solution assigned 0 ppm chemical shift. Scanning electron micrographs (SEMs) were obtained from carbon-coated powdered samples with a Philips SEM 505 microscope with a LaB6 filament and a low-voltage anode operating at 4 keV. Si/Al ratios in the final materials were determined using the energy-dispersive analysis of X-rays (EDAX) capability of the SEM instrument. No internal referencing was applied. Thermal analyses were performed with a Polymer Laboratories PL-STA thermogravimetric balance. A total of 15-20 mg of sample was heated in air from room temperature to 800 °C at a heating rate of 5 °C min-1. Ammonia temperature-programmed desorption (NH3 TPD) analyses were obtained with an in-house fabricated tubular furnace and a quartz sample cell attached to a gas chromatograph (GC). Samples were activated in air at 350 °C and then exposed to NH3 gas at room temperature. The adsorbed NH3 was then desorbed at increasing temperature with a ramp of 5 °C min-1 in a stream of He gas of 20 mL min-1. 3. Cumene Cracking Reactions. The catalytic activity of the Al-MSU-S catalysts in the H+ form was tested using the gas-phase isopropylbenzene (cumene) cracking reaction, between 150 and 400 °C, at atmospheric pressure, in a fixed-bed flow reactor (quartz) system. The Al-MSU-S catalyst samples were pressed into self-supporting disks, the disks were crushed, and 400-600-µm-sized pieces were retained. The catalyst (ca. 0.2 g) was loaded into the reactor followed by approximately 0.5 g of 80 mesh SiC particles on top of the catalyst to aid hot-zone evaporation of the cumene. He gas flow at 20 mL min-1 through a bubbler containing analytical-reagent cumene, maintained at room temperature, delivered a constant cumene concentration to the catalyst. Gas products were analyzed by GCflame ionization detection (FID) (HP 5890 Series II) using a Chromosorb 20% FFAP, 1.4 m × 1/8 in. diameter packed column and a temperature program from 120 to 175 °C to remove liquid products from the column.

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Figure 1. XRD patterns of Al-MSU-S materials prepared with a C16 template and various Al loadings as indicated and calcined at 540 °C for 7 h. XRD patterns have been offset along the vertical axis, on the same scale, for clarity.

Individual products were identified by comparison with the retention times of likely standards under the same analysis conditions. Prior to the catalytic reactions, the catalysts were activated in situ at 350 °C for 3 h in a stream of He (20 mL min-1). Good mass balances were obtained from the catalytic reaction, with the products being propene, benzene, unreacted cumene, and traces of other cracking products toluene and ethylbenzene and the alkylation product diisopropylbenzene. Results and Discussion 1. Mesostructure Analysis. 1.1. XRD. XRD patterns of calcined C16-templated Al-MSU-S samples in the Na form are presented in Figure 1. Patterns for both C16- and C14-templated materials tended to exhibit between three and five well-defined hk0 reflections that can be indexed to the p6m hexagonal lattice (a0 ) 2d100/ x3),2 until the Al content exceeded about 20 mol %. Above this level, the XRD patterns became less welldefined, indicating loss of mesostructural order. Samples prepared from the C12 template exhibited reduced order relative to the larger template samples, which is characteristic for the use of the smaller more soluble template. X-ray patterns obtained to higher angles of 2θ did not reveal the presence of reflections due to the formation of crystalline zeolite phases,8 indicating that the zeolite seed particles do not have sufficient size to generate coherent reflections. The intensities and definitions of the individual reflections observed from Al-MSU-S materials were lower than those typically observed for Si-MCM-41 materials2 but were better than those of Al-MCM-41

materials prepared by established direct incorporation methods.2,20 The hk0 reflections of the Al-MSU-S materials appeared at angles higher than those typically observed for Al-MCM-41; interestingly, however, they appeared in positions similar to those of Si-MCM-41. These observations are consistent with the results reported by Liu et al.8 and raise interesting issues regarding the long-range structure and the unit cell assembly of Al-MSU-SFAU compared with Al-MCM-41. In Al-MCM-41, Al incorporation always leads to an expansion of the unit cell, with X-ray reflections being observed at lower angles relative to the siliceous variant.20 In Al-MCM-41 also, greater structural disorder is induced by the incorporation of Al into the mesostructure, which increases with higher levels of Al incorporation.20 Indeed, beyond approximately 5 or 10 mol % Al incorporation into typical MCM-41 materials, mesopore symmetry is generally all but lost.8,16,20 It should be recognized, however, that Al-MCM-41 prepared by the much more labor-intensive pH neutralization method of Kim et al.19 does exhibit very regular hexagonal structures at Al contents beyond 10 mol %. In the Al-MSU-S system,8 the observation of X-ray reflections at angles similar to those of Si-MCM-41 indicates that Al incorporation into this material does not cause expansion of the unit cell (Table 1). It seems appropriate to argue, however, that the incorporation of Al into the same sized unit cell must cause modification of one or another of the unit cell properties relative to Si-MCM-41.2 Changes in the intensities of the lowangle reflections of mesostructured materials can be the result of a number of factors, which include particle (domain) size or fidelity of the pore structure.21 It is generally accepted, however, that incorporation of Al into typical mesostructure frameworks induces defect formation and subsequent reduction of the long-range structure and hence the pore ordering.20 It is inferred from the well-ordered X-ray diffraction data obtained both here and elsewhere8 that Al-MSU-S mesostructures possess greater long-range structural order than typical Al-MCM-41 of similar Al content. We observed from time to time that if the zeolite seed gel was heated too hard or too long, higher angle reflections in the X-ray pattern became apparent. These higher angle reflections did not tend to correspond to those of a zeolite, but rather they were indicative of some other crystalline or semicrystalline aluminosilicate species. Hexagonal mesostructures formed from these phases were always less ordered than those formed from solutions that did not exhibit these higher order reflections. 1.2. Nitrogen Sorption Analysis. The N2 sorption isotherms of Al-MSU-S (Figure 2) exhibited interesting type IV isotherms22 with strongly rectangular-type H4 hysteresis loops22 at high relative pressures (0.4751.0). The low relative pressure end (0.0-0.1) of the isotherm in the low Al materials suggested the potential presence of microporosity; however, t-plot analyses did not suggest that microporosity was present except in the highest Al sample, which, in turn, exhibited no mesoporosity. Only this sample (C16; 50% Al) exhibited any microporosity (0.05 cm3 g-1); however, there was no evidence that this porosity was due to zeolitic pores. Very well-defined capillary condensation steps22 were observed at P/P0 ) 0.2-0.4, with a trend to lower relative pressure of the inflection point of the step with

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Figure 2. N2 sorption isotherms of calcined Al-MSU-S materials prepared with a C16 template and various Al loadings as indicated. Isotherms have been offset along the vertical axis, on the same scale, for clarity.

smaller templates and also, more interestingly, with increasing Al content. No hysteresis was observed in the region of the capillary condensation step, indicating that N2 sorption within the framework-confined pores was reversible.22 Modification of the position and slope of the capillary condensation step by different templates indicated that a true templating effect analogous to that of the M41S system was also active in the Al-MSU-S system. Similar isotherm modification through increasing Al content also indicated pore diameter reduction. The type H4 hysteresis is commonly observed in lamellar materials with slitlike pore structures such as pillar interlayered clays22 and in materials with cage or bubblelike pores22 where the pore windows are smaller than their major diameters. Extended highpressure hysteresis can be observed in HMS-type mesoporous materials,23 which appear to possess textural or interparticular porosities. Those hysteresis curves are not rectangular and, in fact, bear little resemblance to those observed for Al-MSU-S materials. We conclude, therefore, that the cause of the hysteresis loops in Al-MSU-S is different from that of HMS-type materials.8,23 The rectangular nature of the Al-MSU-S hysteresis loop, along with the sharp closure of the isotherm branches at P/P0 of approximately 0.475, was an indication of the presence of irregular or bottlenecked pores. The pores that produced these features have a broad size range that is at least an order of magnitude larger than the framework-confined pores described by the lowpressure capillary condensation steps. The initial analyses also suggested that those larger irregular pores were

Figure 3. SEMs of calcined (A) C16-templated Al(2.5)-MSU-S and (B) Al(10)-MSU-S materials. Scale bar ) 10 µm.

intraparticular rather than interparticular as in the case of HMS. The most interesting feature of the Al-MSU-S N2 sorption data, however, was that both the frameworkconfined mesopore diameters and pore volumes were consistently smaller than those found in either Si- or Al-MCM-41 prepared with the same templates. Furthermore, the mesopore diameters and volumes and the specific surface areas all exhibited steady contraction with increasing Al loading (Table 1). This feature is probably due to the particular way the zeolite seeds assemble in the final Al-MSU-S materials. The consequences of this will be discussed further. 1.3. SEM. SEM images of two Al-MSU-S samples are presented in Figure 3. These images indicate the presence of particles that do not display any particular crystalline habit or morphology. The primary particles are very small and appear to be aggregated into larger secondary particles of 10-20 µm in diameter. At low Al content (Figure 3A), these particles exhibit a definite spherical morphology over a broad size range. As the Al content was increased, however (Figure 3B), the particles became irregular and no particular morphology can be determined with any certainty. 2. Short-Range Structural Analysis. 2.1. SolidState 27Al NMR Spectroscopy. As previously stated, the main goal of the new synthesis method was the

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Figure 4. 27Al MAS NMR spectra of calcined Al-MSU-S materials prepared with a C16 template and various Al loadings as indicated. Asterisks indicate spinning sidebands. Spectra are offset along the vertical axis on the same scale for clarity.

introduction of greater crystalline traits into the lattice of the aluminosilicate mesostructure. To investigate the outcome of this, the coordination of Al atoms in the structures was probed by 27Al MAS NMR spectroscopy of both as-prepared and calcined materials (Figure 4). While this method is not strictly quantitative, it certainly permits qualitative and relative determination of Al coordination and concentrations in different samples.24 We observed that both as-prepared and calcined samples with greater amounts of Al in the synthesis gels indeed produced spectra with more intense resonances. This suggested that the Al nuclei contained in the final materials were in coordination environments ordered enough to be observed by the 27Al MAS NMR experiment and that this coordination symmetry was generally maintained as the Al concentration was increased. The spectra of the as-prepared materials (not presented) containing 2.5-50 mol % Al showed very strong, narrow resonances at around 60 ppm and weak, broad resonances centered around -10 ppm. The 60 ppm signals were assigned to Al(O)4 joined to four Si(O)4 units in highly symmetrical tetrahedral (Td) coordination.24 The broad, low-intensity resonance centered around -10 ppm was assigned to Al(O)6 species in poorly symmetrical octahedral (Oh) coordination, most likely arising from noncrystalline EFA. No other resonances, such as five-coordinate or symmetrical octahedral Al, were observed.24,25 The intensities of the Al(Oh) signals in the calcined samples (Figure 4) were similar to those in the as-prepared samples, while the intensities of the Al(Td) signals were reduced. This suggested that there was some modification of the Al(Td) species making them “NMR invisible”24 but did not necessarily suggest conversion of framework Al(Td) species into EFA species. It is known that calcination of aluminosilicate materials can render a proportion of the Al atoms within a solid lattice invisible to NMR via various

relaxation processes, which may be interpreted as the formation of such EFA material. However, the retention of high-intensity Al(Td) resonances in the spectra does suggest that a large proportion of the Al present remained in its original coordination symmetry. Semiquantitative analysis of the peak areas pointed to a similar conclusion. While the intensities of the resonances decreased, the chemical shifts of the Al(Td) resonances remained constant at 50-60 ppm after calcination. This observation was important in terms of the development of crystalline structural traits and was consistent with that reported by Liu et al.,8 who reported Al(Td) signals around 60 ppm for their Al-MSU-S materials. The much more intense and very sharp Al(Td) resonance over the weaker and very much broader EFA resonance was a good indication that the majority of the Al atoms observed in the NMR experiment are incorporated into the Al-MSU-S lattices in framework-confined positions of tetrahedral coordination. The retention of the strong upfield chemical shift also indicated that the surrounding silicate environment is symmetrical. 24 This is to say that the structure appears to display more of the desired “crystalline” traits or short-range atomic order than previously observed in “amorphous” Al-MCM-41, which does not exhibit short-range atomic order.2,20 Regular Al-MCM-41-type mesostructures tend to produce materials with low-intensity 27Al MAS NMR resonances around 42-46 ppm20 and often intense and sharp Al(Oh) resonances around 0 ppm. Such chemical shifts are indicative of poorly symmetric tetrahedral and symmetric octahedral coordination environments, respectively. Indeed, the Al(Td) species often exhibit strong conversion during calcination to octahedral symmetry or, in other words, EFA formation. Al-MCM-41 prepared by postsynthesis alumination, on the other hand,26 tends to exhibit highly symmetrical Al signals around 50-60 ppm, which is indicative of tetrahedrally symmetrical Al. In those cases, the Al atoms can be thought of as being attached to the pore surface rather than incorporated into the walls. 27Al NMR data obtained here indicated a significant improvement in the atomic order of at least the Al atoms in the new Al-MSU-S over direct incorporation of Al-MCM-41. 2.2. Solid-State 29Si NMR Spectroscopy. The intended improved crystallinity of the Al-MSU-S materials over Al-MCM-41-type systems was not unambiguously demonstrated in the 29Si NMR spectra. The spectra of as-prepared samples (not presented) tended to exhibit broad and poorly resolved mixtures of Q3 and Q4 Si signals, with the Q4 Si signals increasing in intensity with increasing Al content. The broad nature of the spectral lines can be ascribed to the proposition that the so-called zeolite seed building blocks used to assemble the mesostructured materials are small or poorly crystalline. If this is the case, then a number of different Si coordination environments will be evident that will not be well resolved by the 29Si MAS NMR experiment. The outcome is that the average zeolitic structure would appear to be poorly crystalline, and as such the Si resonances would be broadened or merged. This is a common result in poorly crystallized zeolite samples or in zeolites with very small crystallites.27 Overall, however, a trend did appear in the NMR data, suggesting that the Si Q3/Q4 ratio decreased with increasing Al content and suggesting the formation of more highly condensed lattices with increasing Al.

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Figure 5. 29Si MAS NMR spectra of calcined Al-MSU-S materials prepared with a C16 template and various Al loadings as indicated. A spectrum of a typical Al-MCM-41 sample is included for comparison. Spectra are offset along the vertical axis, on the same scale, for clarity.

Calcination of the Al-MSU-S materials tended to increase the Q3/Q4 ratio, but as can be seen in the spectra in Figure 5), the Q3 resonances remained intense. This suggested that the population of Si-OH species decreased with increasing Al content. As in the case of the as-prepared samples, the spectra were broad and not well resolved, precluding reliable quantitative analysis and suggesting that the zeolite crystallites remained small and lacked very long-range order.27 In typical silicate mesostructures, the amorphous nature of the frameworks implies that there is a continuum of SiOx symmetries resulting in spectra that reflect various mixtures of Q2, Q3, and Q4 Si coordination. The amorphous structures will possess high defect populations, therein causing the population of structural Q3 silanol species to be significantly higher than the crystalline zeolitic materials. Such results are often observed in Al-MCM-41, with significant intensity observed in the Q3 Si signal even after calcination (Figure 5). It will be noticed that the 29Si NMR spectra of the Al-MSU-S and Al-MCM-41 materials are very different. This is a clear indication that the atomic structures of the two aluminosilicate materials are very different. The inference can be drawn that, because of the spectral differences and the likelihood of the presence of small zeolite particles, the Al-MSU-S structure is indeed assembled from nanoscopic zeolitic particles. 2.3. Thermogravimetric Analysis. Differential thermal analysis of various zeolites prepared with organic templates has been shown to provide information on the

Si/Al ratio of the materials and also the coordination environment of the template within the zeolite framework.28 It seemed possible that the apparently improved short-range order introduced into the Al-MSU-S system could modify the surface interactions between the template and the silicate framework, thereby offering improved structural information. Figure 6 displays the thermogravimetric analysis/differential thermogravimetry (TGA/DTG) curves of a range of as-prepared samples with different Al contents prepared with C16 templates. The curves of all samples exhibited a sharp endotherm and weight loss of typically 3-5 wt % at approximately 50 °C, as a result of desorption of water weakly adsorbed within hydrophobic or weakly bonding environments. A broad and shallow endotherm with a corresponding broad weight loss extended from approximately 100 to 200 °C. This feature corresponds to desorption of 10-12 wt % water that was hydrogen bound more strongly within the micelle environment of the occluded template, hence, the broad nature of the feature. Following this, a weak exotherm was observed as a shoulder to the low-temperature side of a very strong exotherm, at approximately 250 °C. This lowtemperature exotherm appeared to shift to lower temperature and became broader as the Al concentration increased. Indeed, in the 25% sample, this exotherm was quite separate from the larger exotherm, while in the 50% Al sample, which we have seen was highly disordered and not mesoporous, this low-temperature exotherm was absent. This feature is not generally observed in MCM-41-type materials. A very sharp, high-intensity exotherm was then observed at approximately 300 °C, with a corresponding sharp weight loss. This exotherm shifted to slightly higher temperature as the Al concentration was increased and appeared to become broader in the less wellordered samples. On the high-temperature side of this intense exotherm, a weak shoulder was observed in some samples. The intensity of this shoulder also appeared to increase slightly as the Al concentration was increased and was accompanied by a very shallow weight loss. The total weight loss due to template removal from the Al-MSU-S samples was between 30 and 18%, decreasing with increasing Al content and disorder in the respective samples. Finally, a broad, low-intensity exotherm that extended from approximately 450 to 700 °C was observed. This aspect of the DTG curve shifted to a higher temperature range as the Al concentration was increased and was also associated with a shallow weight loss. No further features were observed beyond 700 °C, indicating no further structural rearrangements occurring at the temperatures examined here. As the Al content of the samples increased, the lowtemperature exotherm became slightly more prominent and the temperature at which it appeared decreased. The low temperature of the exotherm would appear to preclude any decomposition of the template, so appears that it might simply be due to further removal of tightly bound water within the occluded template structure or to structural reordering of the template within that same environment. The group of exotherms from 200 to 400 °C can be attributed to loss of CTAB template molecules that are respectively weakly occluded within the silicate structure and strongly bound to the silicate surface. In MCM-41-type materials, the two higher temperature exotherms are generally well differentiated

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Figure 6. TGA/DTG curves from room temperature to 800 °C of as-prepared Al-MSU-S materials prepared with a C16 template and various Al loadings as indicated.

and have been ascribed respectively to the decomposition of primary amines (presumably sourced from the ammonium template) via the Hoffman amine elimination reaction and to the decomposition and cracking of the hydrocarbon chains.29 The remaining broad and weak exotherm can be attributed to combustion of the remaining hydrocarbon fragments. Overall, the TGA

analysis does not offer new insights into the interaction between the template and the “new” Al-MSU-S structure. 2.4. NH3 TPD. The NH3 TPD curves of two Al-MSU-S samples (6.7 and 20 mol % Al) were compared with that of an industrial silica-alumina catalyst (Figure 7). Each curve exhibited a sharp increase from

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Figure 7. NH3 TPD curves of two representative H+-Al-MSU-S catalyst samples, 6.7 and 20 mol % (Si:Al ) 15:1 and 5:1, respectively). The NH3 TPD curve of an industrial silica-alumina catalyst (Si/Al ratio unknown) is included for comparison.

Scheme 1

approximately 120 °C to maxima at 190 °C for Al(6.7)-MSU-S and 225 °C for Al(20)-MSU-S. Tmax for the silica-alumina catalyst was approximately 230 °C. This result suggested that the surface acidity of the Al(20)-MSU-S was similar to that of the silica-alumina catalyst, while the that of the Al(6.7)-MSU-S catalyst was weaker. The shapes of the various curves were also slightly different. The lower Al catalyst showed a sharp peak at 190 °C and thus a lower total area under the curve, which is consistent with the lower total acidity that would be expected to be present in the lower Al catalyst. The higher Al catalysts exhibited significant tailing of the desorption curves to high temperature, indicating that there was a wider distribution of acid sites on the two high Al catalysts. The higher areas under these curves were also consistent with the larger number of acid sites that would be present on the higher Al materials. In terms of the particular zeolite structures supposed to be formed in the zeolite seed reactions, the NH3 TPD results do not assist in this determination. The results do indicate, however, that the surface acid sites are accessible to ammonia but that they do not exhibit the intense acidity of zeolites such as LZY or USY. 3. Catalytic Cumene Cracking. The general cumene cracking reaction scheme and both cracking and alkylation products are given in Scheme 1. Propene and benzene are the major products of the cracking reaction. Minor cracking and alkylation products such as toluene, ethylbenzene, and diisopropylbenzene can also be formed depending on the type and strength of the acidity and the contact time over the particular catalyst. Figure 8 shows the non-normalized cumene cracking conversions

Figure 8. Non-normalized cumene cracking conversions between 150 and 400 °C over H+-Al-MSU-S catalysts with various Al loadings as indicated, 40 min on stream.

of Al-MSU-S catalysts prepared from C16 templates with increasing Al contents, after 40 min of reaction time, at different temperatures. The highest conversion was achieved over the Al(2.5)-MSU-S sample, while the highest Al concentration sample Al(50)MSU-S exhibited the lowest conversion. There was a steady decrease in cracking activity as the Al content in each catalyst was increased, but note that the surface areas and pore volumes also decreased with higher Al contents (Table 1). At lower reaction temperatures and thus lower conversions (150-300 °C), the reaction was almost 100% selective to propene and benzene over all of the catalysts. Above 300 °C, a minor alkylation byproduct, diisopropylbenzene, was observed, while at higher conversions (400 °C), increased levels of the alkyl-cracking products toluene and ethylbenzene were observed in the product stream. These trends were consistent over all of the samples prepared from different templates. 3.1. Pore Diameter, Pore Volume, and Surface Area Effects. Increasing the Al content had not only the effect of reducing pore ordering but also the effect of decreasing the effective pore sizes, pore volumes, and surface areas of the catalysts (Table 1). It is likely that these structural modifications had some bearing on the catalytic conversion efficiencies, and this is very clearly evidenced in the decrease in the apparent catalyst turnover number (not presented) as the Al content was increased. Conventional wisdom would have it that the catalytic activity should increase with increasing Al content, if those Al atoms are present in active sites. This should hold if at least the pore structures and surface areas were similar over the range of catalysts. While the catalytic data obtained here indicated that such a trend is not the case for Al-MSU-S materials, it did appear that the cumene cracking conversions over Al(2.5)MSU-S and Al(5)-MSU-S, at least, approached those of crystalline ZSM-5-type zeolites. A plausible reason for this decrease in activity would be that the Al atoms became less accessible to the cumene reagent as the Al content was increased. Such a conclusion could be supported by analysis of the pore size and surface area data. As the amount of Al was increased, a contraction of the pore diameters, volumes, and surface areas was evident. One hypothesis that could be advanced for this behavior is an increase in the dimensions of the primary zeolite seed particles with increased Al reagent, result-

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ing in less efficient packing of these particles around the templating micelles. It would also follow that the most part of each individual particle containing the catalytic Al atoms will be used to form the pore wall, and thus the Al atoms will not be located at the eventual pore surfaces. This in and of itself would not be a problem because the pore diameters of zeolite LZY are large enough to allow the diffusion of cumene to active sites. Remember, however, that the nitrogen sorption analysis did not indicate the presence of any microporosity, suggesting that the zeolite seed particles are not large enough to exhibit the diffusional characteristics of true zeolite crystals. The smallest mesopore size measured was still larger than the dimensions of the cumene molecule, so it follows that the overall diffusion would not be rate limiting. Additionally, as the Al content increased, so did the volumes of the intraparticle mesovoid volumes. These mesovoids will have some impact on the cumene diffusion through the catalyst samples. The effect, however, could be that of increased diffusion rates, implying shorter mean-free paths through the catalyst particles and hence shorter catalyst contact time and lower conversion rates. In this work, we have not sought to separate these effects but simply to observe and report on them. 3.2. Al Content Modification. The 27Al NMR (Figure 4) and elemental analyses (Table 1) both indicated that the amount of Al incorporated into the Al-MSU-S samples was close to quantitative, with respect to the starting gel. Furthermore, the 27Al NMR data suggest that the amount of EFA formed might not be significant. The analysis, however, could not identify the structures, or indeed lack thereof, of the zeolite seeds. As discussed earlier, as the Al content of the synthesis gel is increased, it becomes less appropriate to imply that the zeolite seeds will form the FAU structure.9 It is likely, in fact, that above 10 mol % Al the zeolite seed structures will be closer to that of the small-pore, high-Al zeolite LTA.9 While the FAU structure can be maintained at high Al content (zeolite X and LSX),30 the synthesis of these zeolites is less trivial than that of LZY. It is thus considered that the zeolite structure will change with synthesis conditions. This might explain the access to acid sites by NH3 but the reduced access by cumene. Indeed, the NH3 TPD results suggest that the surface acidity of the higher Al catalyst samples is indeed stronger and more abundant than the lower Al samples (Figure 7) but that these acid sites are not accessible to cumene. Cumene will not be able to access the internal acid sites of LTA, and thus catalysis will occur only at acid sites on the mesopore walls, hence external to the zeolite seed particles. 4. Formation Mechanism of Al-MSU-S Mesostructures. The assembly of Al-MSU-S appears to be conceptually similar to the assembly of MCM-41 materials from monomeric species.2 Essentially, colloidal or nanoscopic zeolite seed silicate particles bearing negative charge exchange at the surface of cationic (CTAB) templating assemblies. The silicate particles then condense into the cross-linked hexagonal mesostructure. For this assembly process to achieve a well-defined hexagonal structure, the silicate species will very probably possess spherical or near-spherical symmetry in order to pack efficiently. As the inorganic gel phase in the present system is made more Al rich, the silicate

building units not only become more anionically charged, but they also grow to larger size and change their structures away from FAU toward LTA. During this growth, there is the strong probability that the particles will also become less symmetrical in shape. This, in turn, will lead to less efficient ion exchange at the template surface, less ordered packing, and eventual loss of the mesostructure templating effect. This hypothesis is supported by the experimental observation of clear synthesis gels and good mesostructural order at low Al concentrations and high-viscosity opaque white synthesis gels and poor mesostructural order at high Al contents. The significant observations that the pore diameters of the Al-MSU-S materials investigated here were smaller that those of the MCM-41 materials and that the pore diameters exhibited steady contraction with increasing concentration of Al in the lattice are also explained by particle packing arguments. A significant difference between a typical mesostructure assembly and the assembly of Al-MSU-S is evident at this point. Mesostructures are generally assembled from monomeric or oligomeric silicate species that assemble very efficiently at the templating micelle surface.2,19,20 However, in the Al-MSU-S system,8 the building blocks are nanoscopic, and larger, “crystalline” silicate particles of measurable size, certainly at higher Al contents. These large particles will not pack as efficiently around the template micelles as monomeric species. Indeed, they will need to pack in such a way that the particles will encroach upon the space occupied by the template, thereby forcing the template molecules to adopt less symmetrical and smaller diameter conformations, occupy less space, and in the final analysis produce smaller pores. This is further reflected in the pore wall thicknesses that can be calculated from unit cell and pore diameter values. Values for the thickness of the pore wall (Table 1) show increasing thickness as the Al content of the reaction gel is increased. This is indicative of either larger building blocks, i.e., zeolite seeds, or the formation of multiple layers of seeds around any given template micelle. From other observations of the system, we prefer the hypothesis that the materials are assembled from larger building blocks, thus forming less ordered mesostructures. Lin et al.31 produced a hypothesis to explain the occurrence of the H4 type hysteresis in Al-MCM-41type materials, which invoked the formation of irregular intraparticular mesovoids. These mesovoids were orders of magnitude larger than the framework-confined pores but were contained within the primary particles, not formed between particles as in the HMS system. These types of structures seemed to be restricted to synthesis routes where pH neutralization was employed,19 such as that used in the preparation of Al-MSU-S. The original Al-MSU-SFAU report8 showed hysteresis loops similar to those presented here, as does the extended pH titration method of Kim et al.19 for the synthesis of highly ordered Si- and Al-MCM-41. The intraparticular mesovoids that are described by these hysteresis loops are irregular in shape and size but appear to possess diameters 10-100 times larger than those of the framework-confined pores. The volumes of these mesovoids appear to be directly related to the H2O/ template ratio in the synthesis but also show correlation to the Al content in the materials. In the report by Lin et al.31 preparing Al-MCM-41, they observed maxi-

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mum void defect volumes at H2O/template ratios of 80250:1. In the present work, the H2O/template ratio was controlled around the middle of that range at 110:1. As the Al loading increased, the volumes described by the hysteresis loops also tended to increase. Conclusions Well-ordered hexagonal mesoporous aluminosilicate Al-MSU-S catalysts that exhibit significant crystalline traits can be prepared using a hydrothermal synthesis route that assembles preformed nanoscopic zeolite seed particles. The assembly of the mesostructure proceeds in a fashion similar to that of M41S, where anionically charged aluminosilicate species exchange at the cationic micellar template surface, cross-link, and assemble thereafter. The mesostructure seems to be significantly more crystalline than typical M41S materials, thereby allowing the amount of incorporated Al to be tailored over a significantly wider range than MCM-41. The pore diameters of the Al-MSU-S materials are smaller than those of corresponding Al-MCM-41 materials prepared with the same templates. This is thought to be due to packing constraints created by the large zeolite seed particles. The substituted Al atoms are located within the zeolite seed particles in positions of tetrahedral coordination and have significantly higher symmetry than Al atoms in Al-MCM-41. The surface acidity increases with increasing Al content, but it is not as strong as that of zeolite Y and is closer to that of amorphous silica-alumina catalysts. However, the Al atoms become inaccessible to larger reagents, such as cumene, as the Al content is increased. This is explained by a change in the zeolite seed structure from FAU to LTA as the Al content is increased. Overall, the catalytic cumene cracking is higher than that observed over Al-MCM-41 and approaches that of MFI-type zeolite catalysts at low Al loadings. While these materials may offer certain advantages in terms of shape selectivity for larger feedstocks than cumene, their activities suggest that they will be unlikely to outperform existing industrial zeolite catalysts at least in cracking and hydrocracking applications and silica-alumina in terms of cost. Acknowledgment The authors gratefully acknowledge the assistance of the Royal Society of New Zealand ISAT Bi-lateral Relations and Activities Program. Literature Cited (1) Corma, A. Chem. Rev. 1997, 97, 2373. Climent, M. J.; Corma, A.; Guil-Lo´pez, R.; Iborra, S.; Primo, J. From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis. J. Catal. 1998, 175, 70. Trong On, D.; DesplantierGiscard, D.; Danumah, C.; Kaliaguine, S. Perspectives in Catalytic Applications of Mesostructured Materials. Appl. Catal. A 2001, 222, 299. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834. (3) Balkus, K. J.; Gabrielov, A. G.; Sandler, N. Molecular Sieve Synthesis using Metallocenes as Structure Directing Agents. Mater. Res. Symp. Proc. 1995, 368, 369.

(4) Occelli, M., Robson, H. E., Eds. Zeolite Synthesis; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. (5) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Studies on Mesoporous Materials I. Synthesis and Characterization of MCM-41. Microporous Mater. 1993, 2, 17. (6) Behrens, P. Mesoporous Inorganic Solids. Adv. Mater. 1993, 5, 127. (7) Kosslick, H.; Lischke, G.; Walther, G.; Storek, W.; Martin, A.; Fricke, R. Physicochemical and Catalytic Properties of Al-, Ga-, and Fe-substituted Mesoporous Materials Related to MCM-41. Microporous Mater. 1997, 9, 13. Sun, Y.; Yue, Y.; Gao, Z. Synthesis and Characterization of AlMCM-41 Molecular Sieves. Appl. Catal. A 1997, 161, 121. (8) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Steam-stable Aluminosilicate Mesostructures Assembled from Zeolite Type Y Seeds. J. Am. Chem. Soc. 2000, 122, 8791. (9) Baerlocher, Ch.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework types, 5th ed.; Elsevier: Amsterdam, The Netherlands, 2001. (10) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Steam-stable MSU-S Aluminosilicate Mesostructures Assembled from Zeolite ZSM-5 and Zeolite Beta Seeds. Angew. Chem., Int. Ed. 2001, 40, 1255. (11) Zhang, Z.; Han, Y.; Zhu, L.; Wang, R.; Yu, Y.; Qiu, S.; Zhao, D.; Xiao, F.-S. Strongly Acidic and High-temperature Hydrothermally Stable Mesoporous Aluminosilicates with Ordered Hexagonal Structure. Angew. Chem., Int. Ed. 2001, 40, 1258. (12) Kloetstra, K. R.; van Bekkum, H.; Jansen, J. C. Mesoporous material containing framework tectosilicate by pore-wall recrystallisation. Chem. Commun. 1997, 2281. (13) Huang, L.; Guo, W.; Deng, P.; Xue, Z.; Li, Q. Investigation of Synthesizing MCM-41/ZSM-5 Composites. J. Phys. Chem. B 2000, 104, 2817. (14) Trong-On, D.; Kaliaguine, S. Ultrastable and Highly Acidic, Zeolite-coated Mesoporous Aluminosilicates. Angew. Chem., Int. Ed. Engl. 2002, 41, 1036. (15) Janicke, M. T.; Landry, C. C.; Christiansen, S. C.; Birtalan, S.; Stucky, G. D.; Chmelka, B. F. Low Silica MCM-41 Composites and Mesoporous Solids. Chem. Mater. 1999, 11, 1342. (16) Liu, Y.; Pinnavaia, T. J. Assembly of Hydrothermally Stable Aluminosilicate Foams and Large-pore Hexagonal Mesostructures from Zeolite Seeds under Strongly Acidic Conditions. J. Mater. Chem. 2002, 12, 3179. (17) Robson, H., Ed. Verified syntheses of zeolitic materials, 2nd ed.; Elsevier: Amsterdam, The Netherlands, 2001. (18) Breck, D. W., Ed. Zeolite Molecular Sieves; Wiley: New York, 1974. (19) Kim, J. M.; Kwak, J. H.; Jun, S.; Ryoo, R. Ion Exchange and Thermal Stability of MCM-41. J. Phys. Chem. 1995, 99, 16742. (20) Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. Mesopore Molecular Sieve MCM-41 Containing Framework Aluminum. J. Phys. Chem. 1995, 99, 1018. (21) Peiser, H. S.; Rooksby, H. P.; Wilson, A. J. C. X-ray Diffraction by Polycrystalline Materials; Chapman and Hall: London, 1960. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (23) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. R. Textural Mesoporosity and the Catalytic Activity of Mesoporous Molecular Sieves with Wormhole Framework Structures. J. Am. Chem. Soc. 1999, 121, 8835. (24) MacKenzie, K. J. D.; Smith, M. E. Multinuclear solid-state NMR of inorganic materials; Pergamon: Oxford, 2002. (25) Bagshaw, S. A.; Pinnavaia, T. J. Mesoporous Alumina Molecular Sieves. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (26) Mokaya, R.; Jones, W. Efficient Post-Synthesis Alumination of MCM-41 using Aluminum Chlorohydrate Containing Al Polycations. J. Mater. Chem. 1999, 9, 555. (27) Thomas, J. M.; Klinowski, J.; Fyfe, C. A.; Gobbi, G. C.; Ramadas, S.; Anderson, M. W. In Intrazeolite Chemistry; Stucky G. D., Dwyer, F. G., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983; Vol. 218, p 159.

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(28) Soulard, M.; Bilger, S.; Kessler, H.; Guth, J. L. Thermoanalytical Characterization of MFI-type Zeolite Prepared Either in the Presence of OH- or of F- Ions. Zeolites 1987, 7, 463. (29) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. Comprehensive Study of Surface Chemistry of MCM-41 Using 29Si CP/MAS NMR, FTIR, Pyridine-TPD, and TGA. J. Phys. Chem. 1997, 101, 6525. (30) Khu¨l, G. H. Crystallization of Low-Silica Faujasite (SiO2/ Al2O3 ∼ 2.0). Zeolites 1987, 7, 451.

(31) Lin, H.-P.; Wong, S.-T.; Mou, C.-Y.; Tang, C.-Y. Extensive Void Defects in Mesoporous Aluminosilicate MCM-41. J. Phys. Chem. 2000, 104, 8975.

Received for review December 30, 2002 Revised manuscript received June 4, 2003 Accepted June 11, 2003 IE021050A