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Chem. Mater. 1996, 8, 1840-1852
Catalysis by Crystalline Mesoporous Molecular Sieves Abdelhamid Sayari Department of Chemical Engineering and CERPIC, Universite´ Laval, Ste-Foy, Qc, Canada G1K 7P4 Received December 5, 1995. Revised Manuscript Received March 14, 1996X
Crystalline mesoporous molecular sieves may be prepared under a wide range of conditions in the presence of cationic, anionic, gemini, or neutral surfactants. These mesostructured materials include pure and modified silicates, other metallic oxides and sulfides as well as aluminophosphates. Air calcination of silicate-based mesoporous molecular sieves with stable frameworks (MCM-41, MCM-48, SBA-n, MSU-n) affords materials with extremely high surface areas. Their pore size may be adjusted from ca. 20 to more than 100 Å using different strategies. Because of their unique flexibility in terms of synthesis conditions, pore size tuning, and framework composition, these materials have been targeted for a number of potential applications, particularly in catalysis. The present review deals with the fastgrowing area of catalysis by crystalline mesoporous materials. Three topics are discussed separately: (i) acid catalysis, (ii) redox catalysis, and (iii) miscellaneous applications. Particular attention is put on the patent literature, and some opportunities in the field of catalysis over these materials are pointed out.
Introduction Since their discovery in 1991-92 by Mobil’s researchers1,2 the so-called M41S family of crystalline mesoporous materials attracted the attention of many scientists working in areas such as the synthesis of zeolites and related materials, catalysis, and materials science. In a very short period of time a large number of potential applications of these materials have been developed in the area of catalysis, separation,3 and advanced materials.4-10 Early progress in this field was reviewed by Casci.11 Three main subgroups of M41S materials were first reported by Mobil’s group. They consist of a hexagonal phase referred to as MCM-41, a cubic phase (space group Ia3d) known as MCM-48, and a nonstable lamellar phase.1,2,12-15 Since then, three additional phases were reported. The first one is a cubic phase with the space group Pm3h n.12,13 The second phase, referred to as SBA-2, displays a three-dimensional hexagonal symmetry (P63/mmc) with supercages instead of unidimensional channels.16 The third phase designated as MSU-n consists of a highly disordered hexagonal-like array of channels with diameters in the nanometer range.17 The preparation methods of M41S materials are reminiscent of the synthesis of zeolites.18 The main difference is that a surfactant is used as template.1,2 The preparation of M41S materials takes place under mild conditions, typically below 120 °C, in the presence of anionic, cationic, gemini or neutral surfactants, under either basic or acidic conditions.12-17,19 Moreover, a number of synthesis strategies first applied to crystalline mesoporous silicates were extended to phosphates,13,20-23 sulfides,13 and a large number of oxides other than silica.12,24-28 Because of the similarity between the different M41S phases and known liquid-crystal phases, the first mechanisms proposed for the synthesis of these materials X
Abstract published in Advance ACS Abstracts, July 15, 1996.
S0897-4756(95)00585-0 CCC: $12.00
were coined liquid-crystal templating mechanisms.1,2,29-33 Despite some discrepancies, at present there is a broad consensus that the formation of M41S materials in the presence of charged surfactants takes place via a cooperative mechanism where the electrostatic interactions between the inorganic and surfactant ions play a key role in determining the morphology of the obtained mesophase.1,2,12,13,33-37 This is basically a three-step process (Figure 1). The first step, driven by electrostatic interactions, is the formation of ion pairs between polydentate and polycharged inorganic species on the one hand and the surfactant on the other. The ion pairs then self-organize into a mesophase, having most often a liquid-crystal structure, i.e., hexagonal, lamellar, or cubic. The structure of the mesophase depends on the composition of the mixture, the pH, and the temperature.13 The last step is the condensation of the inorganic species leading to a rigid structure. Figure 1 is a schematic representation of this mechanism. In the presence of neutral surfactants, hydrogen bonding instead of the electrostatic interactions become of crucial importance for the formation of the organic/inorganic mesophase. In addition, Yanagisawa et al.38 and Inagaki et al.39-41 invented crystalline mesoporous silicates and aluminosilicates designated as FSM-16. These materials which have a number of common features with MCM-41 silicates were prepared using a layered kanemite polysilicate in the presence of long-chain alkyltrimethylammonium cations. Further studies dealing with these materials were carried out by Vartuli et al.29 and Chen et al.42 The pore sizes (or interlayer distances) of M41S materials are easily adjustable from ca. 20 to about 100 Å. This can be achieved in three diffrent ways: (i) by changing the length of the alkyl chain of the surfactant molecule,1,2 (ii) by adding expander molecules such as 1,3,5-trimethylbenzene1,2,13,43 which dissolve in the hydrophobic region of the micelles, thus increasing their size, or (iii) by aging a sample prepared at low temper© 1996 American Chemical Society
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Chem. Mater., Vol. 8, No. 8, 1996 1841
Figure 1. Proposed synthesis mechanism.36
ature (e.g., 70 °C) in its mother liquor at higher temperature (e.g., 150 °C) for different periods of time.44 Moreover, the pore size of MCM-41 silicates may be adjusted by post-synthesis silylation.45,46 Calcination of stable phases (MCM-41, SBA-n, MSUn, and MCM-48) affords materials with extremely high surface areas, often exceeding 1000 m2/g, all of which is readily accessible. Moreover, these materials have another important degree of flexibility. They can be easily modified by incorporation of different cations, thus leading to materials with acidic or redox properties. This paper is a short review of the fast growing area of catalysis by crystalline mesoporous materials. Because of their multilevel flexibility in terms of synthesis conditions, pore size tuning and “framework” composition, they were tested in a number of potential catalytic applications. These may be divided into three categories: (i) acid catalysis, (ii) redox catalysis, and (iii) other catalytic applications. In this review, particular emphasis will be put on the patent literature with the caveat that statements in this type of literature are often illustrated only by short examples of experimental data. These statements must be considered with caution. Acid Catalysis Acid sites in MCM-41 silicates can be generated either by isomorphous substitution of trivalent cations such as Al or B for Si or by adding an acidic ingredient such as a heteropolyacid, an ultrastable Y-(USY) or a Alcontaining ZSM-5 zeolite. A summary of the preparation, characterization and applications of these catalysts is given below. MCM-41 Aluminosilicate Catalysts. The synthesis of aluminum-containing MCM-41 molecular sieves was reported in both the patent43,47,48 and the open1,2,49-58 literature. MCM-41 aluminosilicates were prepared under hydrothermal conditions, typically at 70-150 °C
during 1-10 days. Various sources of silica were used, including HiSil, Ultrasil, Cab-O-Sil, tetramethylammonium silicate and sodium silicate. Likewise, a large number of aluminum sources were used. The list of such sources includes Catapal B alumina, sodium aluminate, aluminum sulfate, aluminum isopropoxide, aluminum orthophosphate, and aluminum acetylacetonate. As for the best aluminum source for the preparation of pure phase of aluminum-rich MCM-41, different workers arrived at different conclusions. Janicke et al.51 reported that aluminum isopropoxide is a much better precursor than Catapal B. Using the isopropoxide, they were able to prepare samples with Si/Al ratios down to 16, with aluminum being entirely in tetrahedral positions. Luan et al.55 found that when Catapal alumina or sodium aluminate is used, almost all Al in the final material is six-coordinated and does not belong to the silicate framework. They reported, however, that aluminum sulfate leads to total incorporation of Al in tetrahedral sites up to very high loadings (Si/Al ) 2.5). On the contrary, Schmidt et al.52,53 and Borade and Clearfield54 prepared Al-rich samples using sodium aluminate. These discrepancies may be explained, at least partly, on the basis that the preparation methods used by various groups were different. More recently, Fu et al.59,60 prepared MCM-41 aluminosilicates using a two-step approach. Instead of using gel precursors followed by hydrothermal crystallization, they first prepared well-defined aluminosilicate oligomers to be used as precursors. These oligomers consisted of aluminosilicate polyanions AlxSi8-x(OH)xO20-x4- (0 e x e 4) with cubiclike structure similar to the double four-ring (D4R) silicate, Si8O208-. In a second step, these precursors were precipitated with a surfactant and then treated with water vapor at 110 °C for 3 days. This method not only affords MCM-41 aluminosilicates with variable Si/Al ratios down to the lowest possible ratio of 1/1 but also offers additional flexibility in the design of new materials by using suitable building blocks. The main techniques used for the characterization of MCM-41 aluminosilicates were X-ray diffraction (XRD), 27Al and 29Si MAS NMR, transmission electron microscopy (TEM), infrared spectroscopy of adsorbed pyridine and temperature-programmed desorption (TPD) of ammonia. XRD data (Figure 2) showed that incorporation of aluminum brings about a dramatic decrease in the intensity of the diffraction peaks.54,55 This was also accompanied by significant broadening of the pore distribution.50 Previous Raman,49 IR,49 and 29Si MAS NMR1,2 studies showed that the MCM-41 silicate walls are amorphous with a wide range of T-O-T bond angles. The presence of Al in such a highly distorted environment in addition to the limited flexibility of the O-Al-O angle compared to O-Si-O may generate a more defective structure with a broader pore size distribution. 27Al MAS NMR is by far the most widely used technique to distinguish between “framework” and extraframework aluminum. Luan et al.55 studied a series of noncalcined samples with Si/Al ratios in the range 2.5-60 prepared using aluminum sulfate. They found a linear relationship between the absolute intensity of the 27Al NMR peak and the Al content. Similar data were also reported by other workers.50,54,61 How-
1842 Chem. Mater., Vol. 8, No. 8, 1996
Figure 2. X-ray diffraction patterns of Al-MCM-41 samples.55 The numbers on the right-hand side indicate the Si/Al ratios.
ever, compared to typical aluminosilicate zeolites, Alcontaining MCM-41 silicates exhibit a stronger tendency to dealumination during the removal of surfactant by calcination. Dealumination is mostly due to hydrolysis of framework aluminum by steam generated during the combustion of the surfactant. Corma et al.50 found that direct calcination of Al-rich MCM-41 at 540 °C in air gives rise to material with significant amounts of extraframework aluminum, smaller pores and lower Bronsted acid site density compared to samples treated first in N2 and then in air at the same temperature. The two-step activation procedure has two advantages: (i) the local temperature is lower and (ii) much less water vapor is formed. Luan et al.55,62 found that calcination of as-synthesized samples (Na+ form) with Si/Al ratios in the range 10-70 in air at 550 °C does not generate extraframework Al species. However, it brings about a broadening of the 53 ppm 27Al NMR peak due to decreased symmetry. On the contrary, proton exchanged samples were found to be prone to dealumination, presumably because the small H+ cation cannot satisfy the framework charge balance efficiently. A series of Al containing MCM-41 samples with Si/ Al ratios in the range 62-2.5 were synthesized in our laboratory using sodium aluminate as described in the literature.54 NMR data indicated that for as-synthesized as well as for calcined samples at least 90-95% of all Al in all samples was located in tetrahedral positions.54 Other pertinent data concerning the composition and BET surface areas of these samples are summarized in Table 1. It is seen that the BET surface area decreases sharply as the Si/Al ratio drops below 10, indicating that as far as Al incorporation is concerned, 27Al NMR data may be misleading. Indeed, using TEM, Kloetstra et al.56 found that in samples with low bulk Si/Al ratios, most of the aluminum was part of a separate dense phase displaying a tetrahedral environment. The presence of such a dense phase is
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consistent with the fact that the surface area and the pore volume of their samples were rather low and also with the sharp decrease in BET surface area of our samples for Si/Al ratios below 10 (Table 1). This indicates that the conventional one-pulse NMR technique does not discriminate between tetrahedral Al in the MCM-41 framework and tetrahedral Al in the socalled dense phase.56 It is therefore recommended that a combination of NMR, adsorption, and TEM measurements should be carried out to characterize the state of Al in mesoporous aluminosilicates, particularly in samples with high Al content. The thermal and hydrothermal stability of a sample with Si/Al ) 26 was investigated in our laboratory. Sample batches were heated for 3 h in dry air or in pure water vapor in the temperature range 550-850 °C. Table 2 shows that in dry air the extent of dealumination increases as the treatment temperature increases; however, as shown by XRD and N2 adsorption measurements the crystallinity and the pore structure are preserved. Under hydrothermal conditions, extensive dealumination takes place even at 550 °C, and the structure collapses above 650 °C. Corma et al.50 and Chen et al.49 characterized the acidity of Al-MCM-41 using TPD of ammonia. Both studies reached the conclusion that the acidity of Al containing MCM-41 is comparable to that of amorphous silica-alumina, and much lower than the acidity of zeolites such as USY or H-mordenite. Indeed, as mentioned earlier various techniques including Raman, FTIR, and 29Si NMR data led to the conclusion that despite their long-range order, M41S mesoporous silicates and aluminosilicates exhibit essentially amorphous walls.1,2,49 As for catalytic applications, aluminum-containing MCM-41-based materials were tested in a number of petroleum refining processes. Corma et al.64 used a NiMo-impregnated Al-MCM-41 catalyst containing 12 wt % MoO3 and 3 w % NiO for hydrocracking of vacuum gas oil. They found that the MCM-41-based catalyst has higher hydrodesulfurization activity and also higher hydrodenitrogenation activity than NiMo loaded on USY or on amorphous silica-alumina. The higher performance of NiMo/MCM-41 was attributed to its high surface area and also to its mesoporous structure which is not only freely accessible to large molecules but also favors a high dispersion of catalytically active ingredients. Despite its lower acidity, the NiMo/MCM-41 catalyst was also found to have higher activity in mild hydrocracking of gasoil than USY or amorphous silicaalumina based catalysts. Roos et al.65 carried out microactivity tests (MAT) in the presence of aluminosilicates MCM-41 using hexadecane as a model feed. An equilibrated commercial fluid catalytic cracking (FCC) catalyst was used for comparison. It was found that at a given conversion, Al-MCM-41 produced a higher amount of gaseous products indicative of more severe cracking. It also yielded more olefins and lower amounts of branched hydrocarbons. The patent literature provides a wealth of information about the potential use of MCM-41 based catalysts for various petroleum refining processes. Cracking and hydrocracking applications have been dealt with in several patents.66-75 The hydrogen form of Al-MCM-
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Chem. Mater., Vol. 8, No. 8, 1996 1843 Table 1. Some Properties of Al-MCM-41a Si/Al ratio
a
sample
gel
product
Si-MCM-41 Al-MCM-41-100 Al-MCM-41-50 Al-MCM-41-25 Al-MCM-41-10 Al-MCM-41-5 Al-MCM-41-2
∞ 100 50 25 10 5 2
62.5 35.5 26 7.4 3.4 2.5
d100 spacing (Å)
SBET (m2/g)
pore volumeb (cm3/g)
34.0 40.1 40.5 39.8 40.5 43.2
1340 1447 1441 1465 1318 823 132
1.01 1.42 1.42 1.42 1.24 0.75 0.18
From refs 61 and 63. b Pore volume according to the MP method. Table 2. Thermal and Hydrothermal Stability of Al-MCM-41 (Si/Al ) 26)a thermal treatment temperatureb 550 650 750 850
(°C)
SBET
(m2/g)
1465 1230 1252 1320
pore
volc
(cm3/g)
1.42 1.16 1.13 1.03
hydrothermal treatment Al(T)d 92 61 49 45
(%)
SBET (m2/g)
pore volc (cm3/g)
974 1005 231 170
0.78 0.75 0.30 0.22
a From ref 61. b 3 h in dry air for thermal treatment and in water vapor for hydrothermal treatment. c Pore volume according to the MP method. d Tetrahedral aluminum (%).
41 mixed with Al2O3 binder in a ratio of 65:35 wt% was used for the cracking of a straight-run naphtha at 540 °C and ca. 3 atm.69 The results showed that at the same conversion (43-45%), the MCM-41-based catalyst produces more C3-C5 olefins (74 vs 54%) and much less light gas and linear hydrocarbons (11 vs 29%) than medium-pore ZSM-5 zeolite. In addition, MCM-41 exhibited higher selectivity toward valuable isobutane and isopentanes which can be further upgraded via alkylation by olefins or via dehydrogenation into isoalkenes. The fluid catalytic cracking activity of an extruded catalyst comprised of 35% Al-MCM-41 and 65% silicaalumina-kaolin clay matrix was compared to that of a similar catalyst containing 35% USY.70 The results showed that the MCM-41 based catalyst is more active and more gasoline selective than the USY-based catalyst. It also displays higher selectivity toward C5 olefins. Apalian et al.71 studied the hydrocracking of a heavy wax over a MCM-41 catalyst prepared as follows. The acid form of Al-MCM-41 was combined with alumina in the proportions of 65:35%. The mixture was extruded and calcined. Nickel (5.8%) and tungsten (29.1%) were then added by incipient wetness coimpregnation. This catalyst was found to be more active than fluorided NiW/Al2O3. At low conversion (
