Synthesis of Acetic Acid on Pd-H4SiW12O40-Based Catalysts by Direct Oxidation of Ethylene

Synthesis of acetic acid by direct oxidation of ethylene on Pd-H4SiW12O4o-based catalystswas studied in a fixed-bed integral reactor and a pulse differential reactor. From the performance of thecatalysts with different compositions and configurations, it is proposed that acetic acid is predominantlyproduced via an intermediate of acetaldehyde. This can be easily confirmed by comparing the productdistributions in the integral and the differential reactors. The active sites for acetic acid formation areconsidered to exist mainly at the boundaries between the H4SiW12O4o and the Pd particles. The Pd-basedcatalysts reduced by H2/N2 have higher activities than those reduced by hydrazine, as explained by thedegree of Pd dispersion obtained from the characteristics of hydrogen chemical adsorption. It was foundthat the Pd-Se-SiW12/SiO2 catalyst with selenium tetrachloride as a precursor was more active than thatwith potassium selenite, and that the acetic acid yield can be greatly increased by adding a suitable amountof dichloroethane (C2H4Cl2/C2H4 mole ratio=0.03) to the reactants.