Enhancing Ethanol Coupling to Produce Higher Alcohols by Tuning H2 Partial Pressure over a Copper-Hydroxyapatite Catalyst

Catalytic upgrading of ethanol, as a platform molecule from biomass to higher alcohols (C4–12), is a low-carbon route for value-added chemical production. However, the products are generally obtained in low selectivity due to the uncontrollable reactivity of intermediates that cause a complex reaction network. In this study, we show that unsaturated intermediates of aldehydes can be rapidly hydrogenated by surface hydrogen species during the ethanol upgrading process, thereby greatly inhibiting the cyclization reaction of aldehydes. Specifically, the product distributions on the Cu-hydroxyapatite (Cu-HAP) catalyst shift stepwise to higher alcohols from aromatic oxygenates with the partial pressure of hydrogen increasing from 0 to 95 kPa. Kinetic measurements and in situ ethanol infrared results indicated that the intermediates during this process are acetaldehyde and 2-butenal. Combined with physical structure and chemical state analysis of the catalyst, we found that Cu sites catalyze the hydrogenation of the C═C bond of 2-butenal under a hydrogen atmosphere. The C–C coupling of ethanol to higher alcohols over Cu-HAP follows the Guerbet mechanism. In comparison, on bare HAP, n-butanol is formed as a primary product even though little amount of acetaldehyde was detected, indicating that ethanol proceeds mainly in a direct coupling process to yield higher alcohols. This study introduces an efficient ethanol valorization approach that is enabled by subtle control of the intermediate conversion over the Cu-HAP catalyst by the hydrogen partial pressure.