Enhancement in Lewis Acidity-Basicity of the Ce–O Pairs on CeO <sub>2</sub> via Incorporation of Ga Facilitating Ketonization of Propionic Acid
Zijun Yang, Shuang Ding, Jiayu Zhao, Jiankang Zhao, Jinyu Han, Qingfeng Ge, Xinli Zhu
Abstract
The Lewis acid–base property of amphoteric metal oxide catalysts is crucial for catalyzing the ketonization of carboxylic acids. Herein, Ga-doped CeO 2 catalysts were synthesized via a coprecipitation method and investigated for vapor phase ketonization of propionic acid by combining experiments and density functional theory (DFT) calculations. A Ga species was dissolved in the CeO 2 matrix to form a CeGaO x solid solution with the Ga/Ce ratio up to 1/3 (sample Ce 75 Ga 25 O x ), and excess Ga species were aggregated from the CeGaO x solid solution to form Ga 2 O 3 domains. The Ce–O–Ga interaction in a solid solution facilitates the formation of oxygen vacancies (O v ) and enhances the acidity and basicity of Ce–O pairs in proximity to the Ga–O v center, as confirmed by DFT calculations. Infrared spectroscopy of propionic acid adsorption and Langmuir–Hinshelwood kinetic analysis indicated that monodentate propionate instead of more stable bidentate propionate is the most abundant reactive intermediate and C–C coupling is the rate-determining step toward ketonization. The enhanced acidity-basicity of the Ce–O pair enhances the dissociative adsorption of propionic acid in the monodentate configuration and facilitates the α-H abstraction of propionate, resulting in 2.1 times improvement in the intrinsic ketonization rate on Ce 75 Ga 25 O x (62.4 mmol·g cat –1 ·min –1 at 330 °C) relative to pure CeO 2 . These findings identified the surface reactive intermediate and demonstrated that incorporation of Ga into CeO 2 enhances the acidity-basicity of Ce–O pairs vicinal to the Ga–O v center, providing a facile approach to improve the activity of metal oxides for ketonization of carboxylic acids.