Zeolite adsorption-induced changes

O Brien-Abraham,)., Kanezashi, M., and Un, Y.S. (2008) Effects of adsorption-induced microstrucmral changes on separation of xylene isomers through MFI-type zeolite membranes./. Memhr. Sci., 320, 505-513. 

Another feature that one must also keep in mind, is the possible crystal structure modifications induced by hydrocarbon adsorption, which might result in changes in some catalytic properties. Therefore, it appeared very important to us, to characterize the structural properties of zeolitic-type materials versus chemical, hydrothermal or adsorptive modifications, and subsequently to determine the changes in acidic and catalytic properties. The aim of this paper is to present some examples of our own, in order to illustrate the changes observed for the ZSM-5 (MFI) and SAP0-11 (AEL) type materials (11). 

Under usual conditions, molecules such as hydrogen, deuterium, nitrogen and oxygen are IR-inactive, as with respect to their normal mode there does not occur a change in the dipole moment. However, upon adsorption, for instance at the walls of zeolitic channels and cavities, the symmetry of those molecules is disturbed and a dipole is induced by the local electric field. As a first example. Fig. 31 displays the spectra of D2, Nj and O2 sorbed into zeolite Na4Ca4-A [587] at different pressures and (low) temperatures. Calculated geometries of the sor-bate/sorbent complexes are displayed in subsequent figures (vide infra). 

In general terms, physical adsorption, or physisorption, refers to weak bonding of molecules to surfaces through the interactions of induced or permanent dipoles and/or quadrupoles, whereas chemisorption describes adsorption where transfer of chemical charge between adsorbate and surface takes place. Physisorption is characteristically observed at low temperatures, is not an activated process and is completely reversible. Chemisorption, by contrast, involves the formation of bonds, persists to elevated temperatures and can lead to chemical changes. For the adsorption of molecules on microporous solids, important physisorption interactions include the uptake of simple non-polar molecules such as dinitrogen and dioxygen on cationic forms of zeolites whereas the adsorption of molecules onto acid sites is the most important type of chemisorption, because of its importance in catalysis. 

The development of new or improved processes in catalysis and adsorption were in many cases induced by the development of new catalytic materials and adsorbents. In this context, the synthesis of new aluminosilicates is a continuing challenge in zeolite science. The present review, discussing the synthesis principles of selected more recent zeoUtes, has shown that there is still much room for innovation in this field. It can be expected that by the use of new classes of templates (one recent example is that of the metallocenes) new structures wiU be synthesized in the future. Moreover, with the availability of more and more sophisticated tools for modelling zeolite and template structures and their interactions, it will probably be possible to tailor templates for a given (or a theoretical) zeolite structure. Finally, beside the exploration of new templates and new reaction compositions, the influence of the synthesis conditions on the products should not be overlooked, e.g. changing the reaction parameters from subcriti-cal to supercritical conditions could well have an influence on the materials which are formed. 

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