Influence of Acid Strength on the Reactivity of Dimethyl Ether Carbonylation over H‑MOR

Ethanol synthesis that consists of carbonylation of dimethyl ether (DME) to methyl acetate (MA) and hydrogenation of MA is considered as a potential and sustainable technology. The aim of this study was to explore the influence of strength of Brønsted acid sites in 8-membered ring (8-MR) of H-mordenite (H-MOR) on the reactivity of DME carbonylation. A series of H-MOR samples with Si/Al ratios ranging from 8 to 26 was obtained by treating with oxalic acid and ammonium hexafluorosilicate. A new method, Fourier transformed infrared spectra of temperature-programmed desorption (FTIR-TPD) of NH3 was used to determine the strength of Brønsted acid sites in different channels. By comparing the heat of NH3 desorption in 8-MR (ΔHdes(NH3‑8MR)) of the H-MOR samples with different Si/Al ratios, we found that the acid strength decreased with the increase of Si/Al ratio. A linear relationship between ΔHdes(NH3‑8MR) and the turnover frequency of MA formation was established, indicating the stronger acidity in 8-MR accelerates the formation rate of MA. Theoretical calculations suggested that stronger Brønsted acid sites in 8-MR side pockets (T3-O33) could reduce the activation barrier of CO insertion to form acetyl group, which is consistent with the experimental results