Photocatalytic conversion of alcohols to hydrogen and carbon-containing products: A cleaner alcohol valorization approach

Abstract

Generation of CO2 as byproduct is one of the main issues in the conversion of alcohols to value-added products. This research is aimed to investigate the synthesis of both hydrogen and carbon-containing value-added products from alcohols, as a cleaner strategy that generates no or little CO2. In this regard, methanol, ethanol, and glycerol were selected as representative alcohols and photocatalytic experiments were conducted at different photocatalyst load, pH, and alcohol concentration to investigate, for the first time, the effect of operating parameters on the type, rate, and distribution of liquid and gas products. In the gas phase, although hydrogen and carbon dioxide (CO2) could be generated from the three alcohols at all investigated conditions, methane (CH4) was mainly produced from ethanol and carbon monoxide (CO) was produced only at glycerol concentrations higher than 95%. The maximum amount of CH4 was produced from ethanol at 3 g/L catalyst load, pH of 4, and 5% ethanol concentration. Regarding the liquid phase, formaldehyde and formic acid could be produced from methanol with 95% and 99% selectivity, respectively, while ethanol could be converted to acetaldehyde and formaldehyde (93% and 96% selectivity, respectively). As example, the optimum conditions for acetaldehyde formation were pH 4, 5 g/L of photocatalyst load, and ethanol concentration of 70%. In the case of glycerol, mainly formaldehyde and glyceraldehyde were generated, but with the highest selectivity was found for formaldehyde. Finally, based on the obtained results, reaction mechanisms were proposed for all investigated processes. Overall, this work demonstrates that a thorough optimization of the operating parameters in the photocatalytic conversion of alcohols can lead to a clean generation of hydrogen and carbon-containing value-added products through the valorization of alcohols, while reducing the CO2 emissions.