Ag- and Cu-Promoted Mesoporous Ta-SiO2 Catalysts Prepared by Non-Hydrolytic Sol-Gel for the Conversion of Ethanol to Butadiene

The direct catalytic conversion of bioethanol to butadiene, also known as the Lebedev process, is one of the most promising solution to replace the petro-based production of this important bulk chemical. Considering the intricate reaction mechanism – where a combination of acid-catalyzed dehydration reactions and metal-catalyzed dehydrogenation have to take place simultaneously – tailor-made bifunctional catalysts are required. We propose to use non-hydrolytic sol-gel (NHSG) chemistry to prepare mesoporous Ta-SiO₂ materials which are further promoted by Ag via impregnation. An acetamide elimination route is presented, starting from silicon teraacetate and pentakis(dimethylamido)tantalum(V), in the presence of a pluronic surfactant. The catalysts display advantageous texture, with specific surface area in the 600-1000 m².g^-1 range, large pore volume (0.6-.1.0 cm³.g^-1), an average pore diameter of 4 nm and only a small contribution from micropores. Using an array of characterization techniques, we show that NHSG allows achieved a high degree of dispersion of tantalum, mainly incorporation as single sites in the silica matrix. The presence of these monomeric TaO_x active sites is responsible for the much higher dehydration ability, as compared to the corresponding catalyst prepared by impregnation of Ta onto a pristine silica support. We attempt to optimize the butadiene yield by changing the relative proportion of Ta and Ag and by tuning the space velocity. We also demonstrate that Ag or Cu can be introduced directly in one step, during the NHSG process. Copper doping is shown to be much more efficient than silver to guide the reaction towards the production of butadiene.