Thermocatalytic and photocatalytic chemoselective reduction of cinnamaldehyde to cinnamyl alcohol and hydrocinnamaldehyde over Ru@ZnO/CN

Abstract

The selective hydrogenation of α–β unsaturated carbonyl compounds requires a catalyst with a suitable combination of the support and active sites to activate a specific functional group. In this work, ZnO and g-C₃N₄(CN) nanocomposite (ZnO/CN) supported Ru catalysts were synthesized for thermal and photochemical selective hydrogenation of cinnamaldehyde (CAL). Under thermal conditions, formic acid (FA) was employed as a hydrogenating agent in water, and 85% cinnamyl alcohol (COL) selectivity was achieved with nearly complete CAL conversion. The acidity of ZnO activated the CO group of CAL, the basicity of CN facilitated the adsorption of FA, and the decorated Ru assisted the FA to H₂ formation leading to the selective production of COL. A FT-IR study confirmed the effective adsorption of CAL through CO, yielding the selective formation of COL. In contrast, under photochemical conditions, hydrocinnamaldehyde (HCAL) was the selective hydrogenation product that was formed due to the efficient migration of charge carriers at the interface of the Z-scheme heterojunction of CN and ZnO. The synergistic effects at the interface were crucial for the charge transfer mechanism, enhancing the charge carriers' lifetime and enabling excellent charge separation under photocatalytic conditions. Detailed characterization and control reactions were performed to establish the structure–activity relationship and to conclude the plausible reaction mechanism under both conditions.