Effect of the SiO2 support on the catalytic performance of Ag/ZrO2/SiO2 catalysts for the single-bed production of butadiene from ethanol

Abstract

A ternary Ag/ZrO2/SiO2 catalyst system was studied for single-step conversion of ethanol to butadiene by varying the catalyst composition (Ag, Ir, or Pt metal component, Ag/ZrO2 loading, and choice of SiO2 support) and operating conditions (space velocity and feed gas composition). Exceptional catalytic performance was achieved over a 1%Ag/4%ZrO2/SiO2-SBA-16 catalyst leading to 99% conversion and 71% butadiene selectivity while operating under mild conditions (325 °C, 1 atm, and 0.23 h−1). Several classes of silica—silica gels, fumed silicas, mesoporous silicas)—were evaluated as catalyst supports, and SBA-16 was found to be the most promising choice. The SiO2 support was found to significantly influence both conversion and selectivity. A higher SiO2 catalyst surface area facilitates increased Ag dispersion which leads to greater conversion due to the accelerated initial ethanol dehydrogenation reaction step. By independently varying Ag and ZrO2 loading, Ag was found to be the main component that affects ethanol conversion. ZrO2 loading and thus Lewis acid sites concentration was found to have little impact on the ethanol conversion. Butadiene selectivity depends on the concentration of Lewis acid site, which in turn differs depending on the choice of SiO2 support material. We observed a direct relationship between butadiene selectivity and concentration of Lewis acid sites. Butadiene selectivity decreases as the concentration of Lewis acid sites increases, which corresponds to an increase in ethanol dehydration to ethylene and diethyl ether. Additionally, adding H2 to the feed had little effect on conversion while improving catalytic stability; however, selectivity to butadiene decreased. Finally, catalyst regenerability was successfully demonstrated for several cycles.