Effect of alkali metal addition on catalytic performance of Ag/ZrO2/SBA-16 catalyst for single-step conversion of ethanol to butadiene

Abstract

This paper describes how adding Na and K to a 4Ag/4ZrO₂/SBA-16 catalyst enhances catalytic performance for single-bed conversion of ethanol to butadiene. While adding Na and K leads to a slight decrease in conversion (i.e., ∼10% loss), the production of desired butadiene is significantly increased with up to 50% improvement in productivity for the 4Ag/4ZrO₂/SBA-16 catalyst promoted with 0.5% Na. The reasons for this improvement are a beneficial decrease in Lewis acid site concentration and higher Ag dispersion when Na or K are added, which results in decreased activity involving ethanol dehydration to ethylene and diethyl ether. A remarkable butadiene selectivity of 75% was achieved while maintaining high conversion (i.e., 90%) with 0.5Na/4Ag/4ZrO₂/SBA-16 catalyst. A 72-hour catalyst lifetime study shows that because of higher coke formation from polymerization of desired butadiene, catalyst deactivation occurs more rapidly with the 0.5Na/4Ag/4ZrO₂/SBA-16 (55% loss in conversion) than with 4Ag/4ZrO₂/SBA-16 (45% loss in conversion). However, this does not alter the advantageous effect of Na addition because the butadiene yield remained higher throughout the study period for 0.5Na/4Ag/4ZrO₂/SBA-16. A key finding is that during the reaction, Na limits sintering of Ag particles and promotes selective coking of the acid sites responsible for ethylene and diethyl ether formation.