Mechanistic insights into methanol carbonylation to methyl acetate over an efficient organic template-free Cu-exchanged Mordenite

Abstract

Currently, acetic acid is produced via the carbonylation reaction of methanol with the main route entailing the use of homogeneous noble metal-based catalysts, which has certain limitations, including the use of polluting alkyl halide promoters and difficulty in catalyst recovery. To overcome these challenges, the exploration of alternative methods utilizing heterogeneous catalysts, particularly zeolites with copper as a redox center, has gained attention. Nonetheless, the conversion and selectivity obtained are sought after to compete against the homogeneous route. Therefore, a deeper understanding of the reaction and mechanism is needed to determine the weak points and overcome them. In this study, we propose the use of time-resolved operando DRIFTS-MS to study the methanol carbonylation reaction over a Cu–HMOR catalyst. The study aimed to propose a reaction mechanism through an investigation of the behavior of the catalyst, including potential identification of the location of the copper redox center in the zeolite. The catalytic performance of the Cu–H-MOR catalyst was also evaluated, demonstrating its activity and stability in the methanol carbonylation reaction. The operando DRIFTS-MS results provide insights into the reaction mechanism and the involvement of the acid and redox centers in the process. Based on the findings, we propose a reaction mechanism for methanol carbonylation on Cu–H-MOR zeolite: (i) methanol dehydration, (ii) CO insertion into methoxide groups, (iii) reaction between dimethyl ether and acetyl groups, and (iv) hydrolysis of methyl acetate. Overall, we believe that this work contributes to a deeper understanding of the heterogeneous route for acetic acid production and offers potential avenues for optimizing the process.