Acidity and catalytic properties of H · Na-ZSM-8 zeolite: effect of H⁺ exchange, pretreatment condition, and poisoning of stronger acid sites

Abstract

The influence of degree of H⁺ exchange and calcination temperature of H · Na-ZSM-8 on its acidity distribution (measured by the chemisorption and stepwise thermal desorption of pyridine at 373-773 K) and catalytic properties for hydrocarbon conversion (viz., isomerization of o- and m-xylene and disproportionation of toluene) and methanol-to-aromatics conversion reactions have been investigated. Both the acidity and catalytic activity in these reactions increase with the decrease in Na⁺ content and calcination temperature of the zeolite. The effect of poisoning of stronger acid sites with pyridine on the catalytic properties of the zeolite has also been studied. The catalytic activity and selectivity of the zeolite are strongly affected by this poisoning. The hydrocarbon conversion and aromatization reactions occur on strong protonic acid sites of the zeolite. Good correlations between catalytic activity and acidity (measured in terms of the chemisorption of pyridine at 673 K) have been obtained for these reactions. The increase in Na⁺ content and calcination temperature and the poisoning have resulted in the increase in the shape selectivity of the zeolite in the formation of xylene isomers (i.e., the formation of p-xylene is increasingly favored) because of diffusion-reaction interactions. The physical, acidic, and catalytic properties, shape-selectivity behavior, and catalyst deactivation characteristics of ZSM-8 and ZSM-5 zeolites are compared.