Extra-framework Subject

Recently the effect of steaming on the presence of framework and extra-framework aluminium and its effect on catalytic activity has been the subject of a considerable research effort. 

Zeolites have also transformed the field of chiral catalysis, because of the possibility of introducing extra-framework active sites in the pores of these materials. There is, however, one subject yet to be investigated-the synthesis of chiral zeolite structures and/or chiral framework sites. 

Microcalorimetric studies of several zeolites (H-mordenite, USY, H-ZSM-5), treated in such a way as to contain a noticeable amount of extra-framework aluminum, have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the I wis sites (Freundlich isotherm) and linear for the Brdnsted sites (Temkin isotherm) [82,83]. In most cases, the catalytic activity is related to the number of Brdnsted sites rather than Lewis acid sites. However, the influence of acidic Lewis sites in catalytic reactions over zeolites is still subject to controversy and cannot be neglected. 

There is stiU a dispute as to whether the catalytic activity of iron-containing zeotype materials, for example, Fe-ZSM-5, should be attributed to isomorphously substituted framework iron or to extra-framework iron oxide or iron hydroxide species that are highly dispersed in the material. These extra-framework iron species are present for two reasons, either because they were not incorporated into the framework during the synthesis or because they were ejected from the framework during postsynthesis treatments (such as calcination or other heat treatments). The unresolved issue of the origin of catalytic activity continues to be the subject of research, whereby state-of-the-art characterization techniques are being applied. 

In some instances the materials were also repeatedly subjected to this two-step treatment to enhance the degree of dealumination, e.g., in [89]. In the case of zeolites resistant to acids due to high framework Si/Al ratios, e.g.,mordenite and ZSM-5, acid leaching may also precede the hydrothermal treatment [ 10,23]. In this case, healing of framework hydroxyl nests left by aluminum release is the lattice-stabilizing process. The hydrothermal stability of steamed Y zeolites was found to increase upon removal of extra-framework aluminum by extraction with acids [94]. 

The dealumination of zeolite Y has been studied intensively during the past two decades, mainly for the purpose of developing new FCC catalysts. The changes in acidity upon dealumination of zeolite Y samples has received much attention from the theoretical point of view (refs.1-11). The catalytic consequences of the depletion of the framework with aluminium and, in particular, the changes in product selectivity with the framework aluminium (Al ) and extra-framework aluminium (Al F) content of the zeolite are extremely important in FCC. Reviews on this subject are available (refs.12-14). 

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