Promoting zirconia with carbon: Enhanced hybrid ZrO2/C catalyst for the ketonization of diluted aqueous acetic acid

CO2 electroreduction (CO2ER) fuelled by renewable electricity in state-of-the-art flow-cells with solid-state electrolytes (SSE) produces aqueous solutions of C1-C2 carboxylic acids with concentrations up to 15-30 wt %, but the synthesis of longer-chain compounds remains elusive. Multicarbon e-fuels/e-chemicals could be instead obtained by upgrading acetic acid (AA) solutions produced by CO2ER with well-established processes (e.g., ketonization, consecutive aldol condensation/aromatization, hydrogenation) provided that the employed catalysts withstand water inhibition. In this work, novel hybrid catalysts based on ZrO2 deposited over a high surface area carbon support were successfully applied for the first time to the continuous-flow, gas-phase ketonization of AA to acetone (AC) with increasing amounts of steam in the feed. The catalysts were prepared by a simple incipient wetness impregnation of a commercial carbon and thoroughly characterized by N2 porosimetry, XRD, XPS, HRTEM, SEM-EDS, Raman spectroscopy, and TGA, to uncover structure-activity relationships. Steam co-feeding, even in low amounts, rendered bulk ZrO2 completely unactive due to the preferential adsorption of water, resulting in blockage of the active sites required for AA ketonization. On the other hand, the carbon support effectively hindered water adsorption on ZrO2 even with highly diluted feeds (e.g., 15 wt % of AA in water), thus playing a fundamental role in the catalytic cycle by creating a hydrophobic environment at the boundary with small (∼5 nm) and highly dispersed ZrO2 nanoparticles. The comparison with the available literature showed that the AC productivity achieved in optimized conditions with the carbon-promoted ZrO2 is the highest reported so far.