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 Bronsted 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/AI 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 Bronsted acid sites in different channels. By comparing the heat of NH3 desorption in 8-MR (Delta Hdes(NH3-8MR)) of the H-MOR samples with different Si/Al ratios, we found that the acid strength decreased with the increase of Si/AI ratio. A linear relationship between Delta Hdes(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 Bronsted acid sites in 8-MR side pockets (T3-033) could reduce the activation barrier of CO insertion to form acetyl group, which is consistent with the experimental results.