Sorption and diffusion of benzene in H-ZSM-5: effect of Si/Al ratio, degree of cation exchange and pretreatment conditions

Abstract

Sorption and desorptive diffusion of benzene in the presence of helium in H-ZSM-5 zeolites (with different Si/Al ratios and degree of H⁺-exchange and pretreated at different conditions) at 523 K have been studied by the dynamic sorption/desorption technique using a gas chromatograph. Influence of the Si/Al ratio, nature of cation and its degree of exchange, pretreatment temperature, and hydrothermal treatment on both the sorption and diffusion has been observed to be very strong. Dehydroxylation of the zeolite has resulted in a very significant decrease in the sorption, indicating the involvement of the hydroxyl groups responsible for the acidity in the sorption process. However, a large increase in the sorption with the decrease in the degree of H⁺ exchange of H-Na-ZSM-5 reveals that benzene interacts more strongly with Na⁺ cations than with protons. For the zeolites having negligibly small Na⁺-content the diffusion increases with the decrease in the protonic acid sites. On the contrary, the diffusion in H . Na-ZSM-5 increases considerably with the increase in the degree of H⁺-exchange, or with the decrease in the Na⁺-content, which causes increase in the protonic acidity. The strong influence of Na⁺-content is mainly due to the stronger interaction of benzene with Na⁺ ions than with the protons and also may be because of a reduction in the effective channel diameter when H⁺ is replaced by Na⁺. Thus the diffusion is strongly influenced by the interaction of the diffusing species with both the protonic acid sites and nonacid sites, and also by the small changes in the channel diameter.