Improvement of ethylene epoxidation by palladium–silver catalysts in a low-temperature dielectric barrier discharge system

Abstract

This work was to improve the ethylene (C2H4) epoxidation reaction by adding different catalysts in a low-temperature dielectric barrier discharge (DBD) system. The studied catalysts for the ethylene epoxidation were 0.1 wt.% silver (Ag), 0.005 wt.% palladium (Pd), 0.01 wt.% Pd, and 0.01 wt.% Pd-0.1 wt.% Ag catalysts, which were coated on the second half surface of the frosted glass plates, used as a dielectric barrier material. A separate O2/C2H4 feed with a C2H4 feed position fraction of 0.5 was employed to suppress all undesired reactions and to enhance the EO selectivity. The C2H4 separate feed from O2 reduced opportunity of the collision between C2H4 molecules with high energetic electrons, causing the reduction in carbon monoxide (CO) formation, carbon dioxide (CO2) formation, dehydrogenation, C2H4 cracking, and ethylene oxide (EO) oxidation. Interestingly, the presence of any active catalyst (0.1 wt.% Ag, 0.01 wt.% Pd or 0.01 wt.%Pd-0.1 wt.% Ag catalysts) provided the significant increases in the EO selectivity due to the addition of surface reactions on the active sites of the applied catalysts. The bimetallic 0.01 wt.% Pd-0.1 wt.% Ag catalyst in the DBD system exhibited the best synergistic process performance in terms of the highest EO selectivity and yield. The addition of the Pd on the 0.1 wt.% Ag catalyst promoted the reaction of oxametallacycle (OMC) intermediate to form the desired EO and maximized the difference in the activation energies between the desired EO formation and undesired acetaldehyde formation of OMC, resulting in the improved EO selectivity.