(175d) The Cascade Reaction of Ethanol to Butadiene over Silica-Supported Silver-Zirconia Catalysts

Silver and zirconia supported on silica is known to be one of the most effective catalysts for the cascade reaction of ethanol to butadiene, also known as the Lebedev reaction. The reaction includes the dehydrogenation of ethanol to acetaldehyde, condensation of acetaldehyde, and appropriate hydrogenation and dehydration reactions to ultimately produce butadiene. This work focused on studying the role of each component (Ag and ZrO_2, respectively) in the Lebedev reaction by modifying the crystallinity of ZrO₂ and the ratio of Ag to ZrO₂ in the reactor. Characterization of the catalysts was accomplished using X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, electron microscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed COand CO₂. All of the characterization methods were consistent with silica-supported zirconia catalysts presenting the ZrO₂ component in a very highly dispersed phase. Zirconia catalysts, either in bulk (crystalline) or supported (amorphous) forms converted ethanol at 573 K to mostly undesired dehydration products, diethyl ether and ethylene, when Ag was not present. The addition of Ag to bulk ZrO₂ favored the production of acetaldehyde (presumably from the Ag component) without forming an appreciable amount of butadiene (<5%), whereas placing Ag together with silica-supported ZrO₂ exhibited a selectivity of ~35% to butadiene at an ethanol conversion of 20%. The observed change in product distribution dependent on the ratio of Ag to ZrO₂ in the catalyst bed revealed the delicate balance required between the dehydrogenation/hydrogenation function provided by Ag and the acidic/basic function provided by ZrO₂. The spectral features from in situ DRIFTS of adsorbed CO and CO₂, used to probe surface Lewis acid and base sites, respectively, may explain the differences in catalytic performance between bulk and supported ZrO₂.