Zeolite catalysis approximation

We also expect that more studies will use MD and the Car-Parrinello approach, while the share of studies based solely on energy minimization and cluster approximations will diminish. It seems reasonable to expect that some of the difficulties encountered with embedding will be solved in the next couple of years. If this becomes true, one might expect that embedding, in fierce competition with the Car-Parrinello approach, will be used extensively in studies on zeolite catalysis. For embedding to become a success, it will be necessary to develop force fields that are compatible with the quantum chemistry model that is used in the description of the catalytic site. Such a compatibility seems possible only if these force fields allow for polarization. 

Applications of synthetic zeolites concern three major fields of activity amounting to some 1.3 Mt detergents (A-type zeolites), adsorbents and desiccators (A and X-type zeolites mainly) and finally catalysis (especially Y-type zeolite). In 1998, the world market for these synthetic zeolites was approximately 1.6 G, of which catalysis represented a little over 50 %) (2). As shown on Table 10 which gives these fields of activity according to major geographical regions, the share of catalysis in tons is much less and only represents a little over 12 % ... 

Zeolites exhibit a considerably lower proton (acid site) concentration than liquid acids. For example, 1 g of H2SO4 contains 20 X 10-3 moles of protons, whereas 1 g of zeolite HY, with a Si/Al atomic ratio of five, contain no more than 3 X 10-3 moles of protons. (Note that this is a cmde approximation of the acidic sites available for catalysis, because it assumes that with both materials all protons are available and catalytically active.) Moreover, 1 g of H2SO4 occupies far less volume (i.e., 0.5 cm3) than the equivalent mass of zeolite (4-6 cm3). 

The expected evolution over 5 years (up to the year 2003) is on average a moderate global growth in the order of 1 to 1.5 % per year approximately for catalysis, from 1 to 2 % for adsorbents and desiccators and, for detergents, a slight decrease on average (-1-2 % per year) in the major developed countries (lower queintities of zeolite in detergents). 

Large coffin-shaped zeolite H-ZSM-5 crystals have been chosen as suitable model systems to study the structural motifs and related intergrowth structure, which are also present in smaller powdered zeolite materials used in industrial catalysis. The advantage of large model crystals, which can be prepared with adjustable Si/Al ratio and size, is that the catalytic properties of each growth unit of a zeolite can be studied separately by means of optical microscopy methods, such as UV-vis microspectroscopy and confocal fluorescence microscopy [114, 122]. As both microspectroscopy methods obey the optical Abbe diffraction limit, features can be studied in the size of approximately X/2. The resolution d of an optical microscope is given by... 

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