Isomerization of m-xylene on H-ZSM-5: Part I: influence on catalytic activity and selectivity of si/al ratio, degree of cation exchange, deammoniation conditions and poisoning of stronger acid sites

Abstract

Isomerization of m-xylene on H-ZSM-5 zeolites with different Si/Al ratios and degrees of cation exchange, and on the zeolites obtained at different deammoniation conditions from NH₄-ZSM-5 has been investigated in a pulse microreactor. The results are correlated to the number of strong acid sites as measured by the irreversible adsorption of pyridine at 673 K. The reaction on the poisoned zeolite has been studied by selectively blocking the stronger acid sites in the order of their strength with ammonia, quinoline and pyridine at different temperatures in the range 673-773 K. The catalytic activity and selectivity of the zeolite are seen to be strongly affected by the changes in its Si/Al ratio, degree of cation exchange and deammoniation conditions and also by the poisoning of the stronger acid sites of the zeolite by pyridine or quinoline. The selectivity in the Isomerization increases with the decrease in the number of strong acid sites on the zeolite and with the increase in the extent of poisoning of the stronger acid sites. The strength of the acid sites required for catalysing the various parallel reactions occurring in the overall conversion of m-xylene is in the following order :- isomerization < dealkylation < disproportionation.