Synthesis of methanol and dimethyl ether from syngas over Pd/ZnO/Al2O3 catalysts

Abstract

A Pd/ZnO/Al2O3 catalyst was developed for the synthesis of methanol and dimethyl ether (DME) from syngas with temperatures of operation ranging from 250 °C to 380 °C. High temperatures (e.g. 380 °C) are of interest when combining methanol and DME synthesis with a methanol to gasoline (MTG) process in a single reactor bed. A commercial Cu/ZnO/Al2O3 catalyst, utilized industrially for the synthesis of methanol at 220–280 °C, suffers from a rapid deactivation when the reaction is conducted at high temperature (> 320 °C). On the contrary, a Pd/ZnO/Al2O3 catalyst was found to be highly stable for methanol and DME synthesis at 375 °C. The Pd/ZnO/Al2O3 catalyst was thus further investigated for methanol and DME synthesis at P = 34–69 bar, T = 250–380 °C, GHSV = 5000–18 000 h−1, and molar feeds H2/CO = 1, 2, and 3. Selectivity to DME increased with decreasing operating temperature, and increasing operating pressure. Higher space velocity and H2/CO syngas feed ratios also enhanced DME selectivity. Undesirable CH4 formation was observed, however, it could be lessen through choice of process conditions and by catalyst design. By studying the effect of the Pd loading and the Pd : Zn molar ratio the formulation of the Pd/ZnO/Al2O3 catalyst was optimized. A catalyst with 5% Pd and a Pd : Zn molar ratio of 0.25 : 1 has been identified as the preferred catalyst. Results indicate that PdZn particles are more active than Pdo particles for the synthesis of methanol and less active for CH4 formation. A correlation between DME selectivity and concentration of acid sites has been established. Hence, two types of sites are required for the direct conversion of syngas to DME: (1) PdZn particles are active for the synthesis of methanol from syngas, and (2) acid sites which are active for the conversion of methanol to DME. Additionally, CO2 formation was problematic as PdZn was found to be active for the water-gas-shift (WGS) reaction, under all the conditions evaluated.