Insights into the Pyridine-Modified MOR Zeolite Catalysts for DME Carbonylation

Pyridine-modified mordenite (MOR) zeolite catalysts have attracted great attention in recent years due to their unique shape selectivity within eight-membered ring (8-MR) side pockets for dimethyl ether (DME) carbonylation to methyl acetate (MA) and syngas conversion to ethylene. Herein, aimed at elucidating pyridine modification–carbonylation activity relationships and developing high-performance catalysts, we investigated the adsorption/desorption behaviors of pyridine on MOR zeolites with varying Si/Al ratios and their impact on DME carbonylation. Instead of the previously proposed selective adsorption of pyridine in 12-MR channels, pyridine is revealed to penetrate into 8-MR side pockets of MOR zeolites and interact with acidic hydroxyls therein. Upon heating, pyridine in pockets desorbs preferentially, likely arising from the lower stability of pyridine adspecies in constrained spaces. This well explains the observed increment of carbonylation activity following the increase of pretreatment temperature. Unprecedentedly, high MA yield (7.2 mmol/(h g)) has been achieved on pyridine-modified MOR (Si/Al = 13.8) under controlled pyridine desorption conditions, resulting from the joint contributions of better diffusion properties and larger amounts of active acid sites. Moreover, the catalytic activity of Brønsted acid sites within 8-MR pockets is demonstrated to be inhomogeneous, closely associated with their locations.