Aerosol-assisted sol–gel synthesis of mesoporous Ag–Ta–SiO2 catalysts for the direct upgrading of ethanol to butadiene

Abstract

The Lebedev process, or the direct catalytic conversion of bioethanol to butadiene, offers an up-and-coming sustainable alternative to the petrochemical route toward this high-demand C4 hydrocarbon. Since the reaction mechanism involves a cascade of dehydrogenation, hydrogen transfer and dehydration steps, a bifunctional catalyst combining both redox (for the dehydrogenation reaction) and acid (for hydrogen transfer and dehydration reactions) functionalities is required. Multi-step preparation methods are typically implemented to obtain tailored bifunctional catalysts, and one of the challenges is to balance the two functions to maximize the BD yield. Here, we disclose a straightforward, one-step, and continuous preparation method of Ta-doped SiO₂ loaded with Ag nanoparticles by coupling sol–gel chemistry with aerosol processing. Combining tantalum ethoxide, silver nitrate, hydrolysed tetraethyl orthosilicate and pluronic F127 as templating agent in the aerosol process leads to mesoporous bifunctional catalysts featuring a specific surface area between 310–370 m² g⁻¹, a pore volume of ca. 0.5 mL g⁻¹ and an average pore diameter of 5 nm. As attested by a variety of characterization techniques, the method leads to the homogeneous incorporation of highly dispersed tantalum species in the silica matrix, thereby creating the required acidic sites. These new catalysts have higher dehydration activity, as compared to the corresponding reference catalysts prepared by classical impregnation. Concomitantly, relatively small silver nanoparticles are stabilized (∼15 nm). The relative Ta and Ag loading can be tuned easily. In the ethanol to butadiene reaction, these aerosol-made catalysts achieve a butadiene yield of ca. 25% by optimizing the relative loadings of Ta and Ag, outcompeting the corresponding formulations prepared by impregnation.