Cascade Reaction of Ethanol to Butadiene over Multifunctional Silica-Supported Ag and ZrO2 Catalysts

Although butadiene is currently a byproduct of naphtha cracking, interest in producing butadiene from biobased ethanol has increased because of the lower environmental impact of the ethanol to butadiene reaction. This work explores a multifunctional catalyst system composed of silica-supported Ag and ZrO2 used for the cascade reaction of ethanol to butadiene at 573 K. The Ag and ZrO2 components were synthesized on separate support particles, enabling the characterization of each component without interference from the other. High selectivity to butadiene (65%) at high ethanol conversion (75%) was achieved with an appropriate ratio of Ag and ZrO2 in the reactor. Silver catalyzed the initial dehydrogenation of ethanol to acetaldehyde, while ZrO2 catalyzed the C–C coupling and subsequent dehydration reactions. The silica-supported ZrO2 exhibited superior selectivity relative to bulk ZrO2 in the Ag-promoted ethanol to butadiene reaction. Results from Zr K-edge X-ray absorption spectroscopy and UV–vis spectroscopy showed that ZrO2 was highly dispersed on the silica support over a range of loadings. Infrared spectroscopy of adsorbed pyridine, CO, and CO2, and kinetics of probe reactions 1-butene double bond isomerization, 2-propanol decomposition, and ethanol hydrogenation of acetone were used to compare the acid–base nature and chemical reactivity of silica-supported ZrO2 to bulk ZrO2.