Microporous Zeotype Materials

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Zeolites, i.e. microporous aluminosilicate materials with pores smaller than 2 nm, play key roles in the fields of sorption and catalysis [134, 135]. The global annual market for zeolites is several million tons. In the past few decades a large variety of zeolites and related zeotype materials have been produced, whereby transition metal incorporation is extensively used to modulate the catalytic characteristics of these materials. Since the catalytic properties depend on the structure and accessibility of the transition metal sites, a lot of effort is put into probing these sites. Nevertheless, the exact nature of the transition metal incorporation is often strongly debated, since most spectroscopic evidence for isomorphous substitution is indirect. 

Sancho T, Lemus J, Urbiztondo M, Soler J, Pina MP. Zeolites and zeotype materials as efficient barriers for methanol cross-over in DMFCs. Micropor Mesopor Mater 200 115(1) 206-213. 

However, whilst chemical differences between the different classes of microporous materials clearly do exist, it is not yet clear to what extent these affect the mechanism of synthesis. Possibly the products do derive directly from PNBUs but there seems no reason at present to reject the alternative possibility that much of this chemistry goes on at the liquid-solid interfacial growth points rather than (as is implied) independently of the growing crystal. Tt seems unlikely that any completely new concepts will be necessary to explain the formation patterns of zeotypes. However, just as the range of behaviour observed for zeolites requires a flexible (but coherent) mechanistic scheme, this spectrum will need to be further extended in order to allow for the differences in composition, structure, polarity and solution chemistry found in zeotype synthesis. 

Nowadays, the term zeolite includes all microporous solids based on silica and exhibiting crystalline walls, as well as materials where a fraction of Si atoms has been substituted by another element, T, such as a trivalent (T = Al, Fe, B, Ga,. ..) or a tctravalent (T = Ti, Ge,...) metal. Crystalline microporous phosphates are known as zeotypes or as related microporous solids (14, 54). At present, there are 179 confirmed zeoHtc framework types. For the structure types, three-letter codes are used, which were adopted from the name of the first material reported with a specific stmcturc. As an example, FAU is given for the structure of faujasite and its synthetic equivalents X and Y, and MFl for the stracture of ZSM-5 or silicalite-1 (105). Figure 9.11 shows prominent examples of zeolite firameworks, for example, FAU, LTA, and MFI types (pentasil). 

Several hundreds of synthetic zeolites (crystalline mieroporous aluminosilieates) and zeotypes (crystalline mieroporous ferrisilicates, gallosilicates, titanosilicates, isomorphously substituted aluminophosphates, etc.) have been sueeessfully synthesized in recent decades. All these mieroporous materials have tetrahedral coordination of their eentral atoms (Si, Al, P, Fe, etc.), which are interconnected with four oxygen bridges to form a three-dimensional crystal structure. These structures exhibit regular micropores with dimensions up to 1.0 nm and cavities, high surface areas, and adsorption capacities, and shape selectivity toward reactants, products, and transition states.. 

Based on the lUPAC nomenclature, microporous molecular sieves have pores with dimensions up to 2.0 nm in dimension. The mesopore dimension is between 2.0 and 50 nm, while macropores exhibit pores larger than 50 nm. Shown in Fig. 2 are the pore size and schematic structure of several zeolites, zeotypes, and mesoporous molecular sieves. Evidently, a large number of different zeolite or zeotype structures and mesoporous materials are available from which the optimum one for the given application can be selected. 

The diversity of ordered porous solids increases at an astonishing rate, particularly among the readily crystallised MOFs, and continues to olfer novel materials properties. There is no obvious barrier to the synthesis of a myriad of new zeolite, zeotype or hybrid structures. Challenges remain, however. For zeolitic aluminosilicates, the 10 A pore size restriction remains an important barrier, and an enantiomerically pure zeolite is still out of reach. For nonsilicate crystalline microporous solids, thermal and hydrothermal stability, rather than framework geometry, limit their applicability, since fully crystalline germanates and carboxylates with pores in the mesoporous range now exist, and these solids have enormous specific surface areas. In these hybrid solids the ability to choose chirality in the building units indicates that it will be possible to prepare these in chiral form the first examples have already been prepared. 

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