Insight into Dimethyl Ether Carbonylation Reaction over Mordenite Zeolite from in-Situ Solid-State NMR Spectroscopy

Abstract

Carbonylation of dimethyl ether (DME) with CO over H-mordenite (H-MOR) zeolites from 423 to 573 K was studied by in-situ C-13 solid-state NMR spectroscopy. The reaction was monitored separately in the 8-membered ring (MR) and in the 12-MR channels of the zeolite under identical conditions. The experimental results indicated that both 8-MR and 12-MR channels were capable of producing methyl acetate product but with remarkably different selectivities. At low-reaction temperature, surface acetyl species (CH3CO-) as stabilized acylium cation was solely identified in the 8-MR channels, which was confirmed by measurement of its characteristic chemical shift anisotropy (CSA) parameters. Additionally, the intermediate role of the acetyl species was evidenced by the fact that it could react with DME to form methyl acetate. This demonstrated the theoretically proposed specificity of the 8-MR channels for generation and stabilization of the acetyl intermediate which was considered as the rate-limiting step of carbonylation reaction. While in the 12-MR channels, the acetyl intermediate was not found during the reaction, and formation of hydrocarbons was favored. The absence of acetyl intermediate might account for the lower reactivity of the 12-MR channels to generate methyl acetate product.