Direct synthesis of methyl acetate and acetic acid from syngas over tandem catalysts composed of a Cu-based catalyst and a Cu-exchanged mordenite zeolite

Abstract

Direct conversion of syngas to methyl acetate (MA) and acetic acid (AA) was investigated using a tandem catalyst system combining Cu-based catalysts for methanol synthesis and mordenite (MOR) zeolite for dimethyl ether (DME) carbonylation. The catalysts were loaded as separate layers in a fixed-bed flow reactor and used to convert syngas to MA and AA via methanol and DME at 230 °C and 5 MPa. Combination of a Cu catalyst with unmodified MOR afforded DME with high selectivity (87%), but the selectivity for MA was low at 2%; however, replacement of the zeolite with a Cu ion-exchanged MOR increased the total selectivity for MA and AA thirtyfold without affecting hydrocarbon formation. A Cu ion-exchanged MOR with both moderate Cu loading and SiO₂/Al₂O₃ ratio showed the highest activity, indicating that Cu ions and Brønsted acid sites are both essential for efficient DME carbonylation. Characterization of the tandem catalysts revealed that Cu⁺ ions present within the eight-membered ring channels of MOR enhanced CO adsorption and insertion into DME, boosting catalytic activity. The effects of reaction temperature, the weight ratio of the two catalyst layers and the gas hourly space velocity were also investigated, and the maximum total selectivity of MA and AA reached 93.7%.