Deactivation mechanism for dimethyl ether carbonylation: Elucidating the role of Brønsted acid sites within different channels

Microporous zeolites as solid acid catalysts are indispensable in petroleum industry, methanol to olefin (MTO) and fine-chemical synthesis etc. The considerable Brønsted acidity and well-defined micro-porosity contribute to great performance. But meanwhile, the steric constrains and strong acidity inevitably result in coking, which poisons active sites and block the diffusion pathways, hindering the industrialization of zeolites. Carbonylation of dimethyl ether (DME) to methyl acetate is a key step for a new eco-friendly route of ethanol synthesis from syngas. Mordenite (H-MOR) has been applied for catalyzing DME carbonylation in industry because of extremely high selectivity and considerable activity. However, it suffers rapid deactivation due to coke deposits, which are generally accepted to occur at Brønsted acid sites (BAS) in twelve-membered ring (12-MR). But contradictory deactivation behaviors in some cases inspire us to reveal the deactivation mechanism in DME carbonylation.
In this work, we mainly focus on dissecting the role of BAS at eight-membered ring (B_8-MR) and B_12-MR of MOR in deactivation. The concentration of BAS in different channels were precisely modulated. Based on speculated mechanism of deactivation, in-situ FTIR, GC-MS, TPO-MS etc. were applied to determine the predominant deactivation pathway during the early-stage of DME carbonylation, in which ketene or its derivates were evidenced as the main coke intermediates. Further kinetic studies established the specific roles of Brønsted acid sites especially acidity in 8-MR. This new insight into the deactivation mechanism will provide inspirations for further design and modification of robust zeolite catalysts associated with carbonylation reaction.