Mesoporous zeolite

Although these examples show the possible immobilization on clays and mesoporous zeolites, the most widely used support for salen complexes has... 

By demetallation, one constituent is preferentially extracted from a preformed zeolite material to form mesoporous zeolite crystals. Existing and emerging demetallation strategies basically comprise dealumination, detitanation, and desilication. 

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Table 2.1 Benefits of mesoporous zeolites prepared by desilication in catalytic applications with respect to the purely microporous counterparts.

Zeolites form a class with tremendous variety. Besides the microporous solids described in the above, mesoporous materials have been synthesized. A breakthrough were the MCM-41 mesoporous zeolites with pores of typically 3 nm. Later, many related materials have been reported allowing fine-tuning of pore sizes. A recent example is the synthesis of materials with pores in the lOnm range with satisfactory uniformity and stability (Sun etai, 2001). 

Ikeue, K., Yamashita, H., and Anpo, M. (1999) Photocatalytic reduction of C02 with H20 on titanium oxides prepared within the FSM-16 mesoporous zeolite. Chemistry Letters, 28 (11), 1135—1136. 

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

Keywords Microwave synthesis Mesoporous zeolite Carbon template Sn-MFI Baeyer-Villiger Oxidation... 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titanium-containing Mesoporous Zeolite... 

The preparation of mesoporous zeolites has long been a desired goal, since it can provide for more accessible catalysts. Recently, Pinnavaia et al. made a... 

Figure 1.5 Schematic illustration of the synthesis principle for crystallization of mesoporous zeolite single crystals. The individual zeolite crystals partially encapsulate the nanotubes during growth. Selective removal of the nanotubes by combustion leads to formation of intracrystalline mesopores. Reproduced from Schmidt et alP51 by permission of American Chemical Society...

Schmidt, I., Boisen, A., Gustavsson, E., Stahl, K., Pehrson, S., Dahl, S., Carlsson, A. and Jacobsen, C. J. H. Carbon nanotube templated growth of mesoporous zeolite single crystals, Chem. Mater., 2001, 13, 4416-4418. 

Chiral Mo complexes bearing ligands derived from a (2S,4R)- or (2S,4S)-4-hydroxyproline compound (13a and 13b) have been tethered to the internal surface of a mesoporous zeolite USY (251). The supported asymmetric Mo catalyst was tested for the enantioselective epoxidation of allylic alcohols. 

Very recently, Lui and co-workers215 made an attempt to characterise dispersed HPW on mesoporous zeolites silicate-1 and meso-silicate-1, by SS 31P NMR spin-lattice relaxations. They concluded that, on the regular silicalite-1 surface, HPA molecules exist as small agglomerates, while on the mesosilicalite-1 surface about 46% of the HPA molecules are successfully dispersed to the mesoporous channel surfaces. The structural integrity of the surface-exchanged HPA molecules is preserved. Coupled with reaction data, it can be concluded unambiguously that the well-dispersed HPA molecules are the active centres for isomerisation of 1-butene. 

In this context, some comments on protein crystallisation can be made. The process of crystallisation can be viewed as one of self-assembly of the quaternary structure, although the constituent units now have a well-defined arrangement in space, in contrast to their less rigid shape in liquid crystalline mesophases. Indeed, twisted structures are very commonly found in globular protein crystals, which are reminiscent of the hyperbolic forms of micro- and mesoporous zeolites, described in Chapter 2. 

The similar dimensions of many globular proteins and novel mesoporous zeolites offer fascinating possibilities for bio-organic-inorganic composites. For example, the inorganic mesoporous alumino-silicates have been proposed as templates for protein crystallisation [12], Conversely, proteins could be used to template mesostructured inorganic materials. 

However, despite a significant decrease in the micropore volume, the organic chain content obtained with the non-mesoporous zeolite whatever the silylating agent, is very low with respect to the intracrystalline surface. This could be explained by a channel blockage occurring near the pore entrance. This hypothesis could also explain the incomplete transformation of the amino group previously linked to the mineral structure. 

Reddy, K.M. and Song, C.S. Synthesis of mesoporous zeolites and their application for catalytic conversion of polycyclic aromatic hydrocarbons. Catalysis Today, 1996, 31, 137. 

Song, C. and Reddy, K.M. Mesoporous zeolite-supported Co-Mo catalyst for hydrodesulfurization of dibenzothiophene in distillate fuels. American Chemical Society, Division of Petroleum Chemistry, 1996, 41, 567. 

As discussed above, the models presently available for the analysis of porous carbons may yield misleading results for the pore size distribution of OMCs. This also applies to other materials. For example, for purely mesoporous zeolites, some models wrongly indicate the presence of micropores [36]. Thus, it is preferable to verify the presence of micropores by model-less methods, such as a-plots. In a-plots, the amount of nitrogen adsorbed on the sample of interest (i ads.) is compared for all data points to the amount adsorbed on a nonporous standard (ttg. Fig. 18.11). The quantity is the amount of nitrogen adsorbed on... 

The titanium-substituted mesoporous zeolite, Ti-MCM-41 has recently been synthesized and shown to catalyze oxidations of bulky substrates with HjOi or TBHP [57], e.g. reactions IS and 16. 

Phenol forms by consecutive transformation of benzaldehyde probably via benzoic acid which, however, quickly transforms inside the zeolite along different possible routes (Scheme 1). The formation of phenol requires the formation of large intermediates and thus is favoured in mesoporous zeolites. The lower formation of phenol in P zeolite than in MH, however, derives from the faster side reactions in P zeolite due to its higher acidity. 

By this reasoning, some polymer does escape from these shallow pores, where its character may be influenced. Other polymer grows from the fragment exterior. In this way, pore size does still influence polymer character, and nearly all the surface does contribute. Recent work with mesoporous zeolites also suggests that some egress of polymer from pores is possible [388,390-393,395,399]. 

Zeolites can also be used in polymerizations. Tetrahy-drofuran has been polymerized in high yield by acetic anhydride in combination with a dealuminated zeolite Y to a product with a narrow molecular weight distribution.213 H-ZSM-5 also works in the presence of a trace of water.214 Acrylonitrile has been polymerized in the mesoporous zeolite MCM-41, using potassium persulfate as a catalyst, then converted to graphite.215 Polymerization of methyl methacrylate in MCM-41 with an azo initiator gave a polymer with a number average molecular weight of 360,000... 

Several ways have been used to place finely dispersed heteropolyacids on supports. Polyaniline has been used as a support for H4SiW12O40.310 Heteropolyacids have been placed on silica,311 in mesoporous zeolites,312 and even... 

The first mesoporous zeolite single crystal (ZSM-5) was synthesized by Jacobsen et al. [20] and since that time a particular attention has been devoted to characterization of this material [ 83, 84 ]. Combination of nitrogen adsorption isotherms, scanning electron... 

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