SOFT AND HARD ACIDITY IN ZEOLITES AND ZEOTYPES: EVALUATION AND CATALYTIC IMPLICATIONS

It has been found by Molecular Orbital calculations, that the energy of the LUMO of a zeolite cluster can be used to measure the softness–hardness acidity. It has been found that the higher the framework Si/AI ratio the softer is the acidity. When applied to the alkylation of aromatics, it has been calculated that the higher the framework Si/AI ratio of the zeolite, the more “orbital controlled” will be the reaction, and therefore a higher para to ortho ratio is expected. This has been confirmed experimentally during the alkylation of toluene and m–xylene by methanol on HY zeolites with different framework Si/AI ratio.