Effects of secondary mesoporosity and zeolite crystallinity on catalyst deactivation of ZSM-5 in propanal conversion

Abstract

Cooperative effects of secondary mesoporosity and zeolite crystallinity on the deactivation of ZSM-5 were investigated during propanal conversion. Varied secondary mesoporosities were introduced into a commercial ZSM-5 by alkaline desilication using solutions with different NaOH concentrations (0.1–0.5 M). The results showed that the mesoporosity gradually increased with the concentration of NaOH, while the intrinsic zeolitic microporosity decreased. This indicated that the alkaline desilication for mesopore generation is a destructive technique that sacrifices the zeolite crystallinity. In propanal conversion, ZSM-5 showed a longer catalyst lifetime as the external surface area increased (or as the zeolite framework thickness decreased) in the mild desilication regime (NaOH concentration ≤ 0.3 M). The enhanced catalyst lifetime could be attributed to the facilitated diffusion of coke precursors out of the zeolite structure. However, when the zeolite crystallinity or microporosity of ZSM-5 was decreased too much from excessively severe alkaline treatments (NaOH concentration > 0.3 M), deactivation of the catalyst became again faster. The result indicates that the crystallinity or the microporosity of ZSM-5 is also important in suppressing coke formation. This is in line with earlier reports showing that coke formation itself is a shape-selective reaction and significantly inhibited in the constrained space of the unique 10-membered micropore channels of ZSM-5. The present results imply that the generation of large mesoporosity (enhancing the diffusion of coke precursors) while keeping the zeolite crystallinity intact (suppressing coke formation by shape-selectivity) is highly desirable for designing a zeolite catalyst with an enhanced catalyst lifetime.