Pd/TiO2 and Oxygen Vacancy: A Dual Driver for Highly Efficient and Selective Hydrogenation of 2-Ethyl-2-Hexenal.

The selective hydrogenation of 2-ethyl-2-hexenal to 2-ethylhexanal is crucial for the green synthesis of 2-ethylhexanoic acid, with the key challenge being the selective hydrogenation of C═C while preserving C═O. Although Pd/TiO2 catalysts with oxygen vacancies show excellent hydrogenation performance, the mechanism for selective C═C hydrogenation in α,β-unsaturated aldehydes remains unclear. This study employs density functional theory (DFT) to investigate the effect of oxygen vacancies in Pd/TiO2 catalyst on the selective hydrogenation of 2-ethyl-2-hexenal. Results show that oxygen vacancies reverses interfacial charge transfer, shifting it from 0.34 |e| from Pd to TiO2 to 0.46 |e| from TiO2 to Pd. This generates electron-rich Pd species and upshifts Pd d-band center by 0.09 eV, thereby enhancing catalytic activity. 2-Ethyl-2-hexenal adsorbs on Pd via C═C, with strong π-d hybridization facilitating Pd electron injection into C═C π* orbital, thereby activating the bond. Conversely, the weak interaction between the C═O and Pd inhibits electron injection, thereby suppressing C═O hydrogenation. The energy barriers for the rate-determining steps of C═C and C═O hydrogenation are 0.50 and 0.88 eV, respectively, demonstrating the excellent C═C hydrogenation selectivity of Pd/Def-Vo-TiO2. Compared with the perfect surface, oxygen vacancies lower the energy barrier for the main product (2-ethylhexanal) by 60%, while those for the byproducts (2-ethyl-2-hexenol and 2-ethyl-1-hexanol) are reduced by 39% and 30%. This differential reduction further enhances the selectivity toward 2-ethylhexanal. This work provides a theoretical basis for achieving highly selective C═C hydrogenation in 2-ethyl-2-hexenal, supporting the rational design of selective hydrogenation catalysts via defect engineering.