Enhancing the efficiency of benzylamine oxidative coupling over N-doped porous carbon-supported CeO2 and ZrO2 nanoparticles

Abstract

A series of N-doped porous carbon-supported CeO₂ and ZrO₂ nanoparticle catalysts were prepared by pyrolysis of the CeZr-NH₂-MOF precursor. These catalysts were designated CeZr-NC-T, where T represents the pyrolysis temperature. The catalytic efficiency of CeZr-NC-T was assessed in the oxidative coupling of benzylamine to imine using oxygen as the sole oxidizing agent. Various techniques were utilized to analyze the crystal structure, specific surface area, relative content of Ce⁴⁺/Ce³⁺, and oxygen vacancies of the catalysts. The results indicated that CeZr-NC-1073 showed outstanding catalytic activity among the tested catalysts, with 97.1% conversion of 4-methoxybenzylamine and 100% selectivity towards the desired N-(4-methoxybenzylidene)-4-methoxybenzylamine at 1 atm oxygen pressure and 383 K for 8 h, using n-decane as the solvent. The superior catalytic properties of CeZr-NC-1073 were mainly attributed to the abundant oxygen vacancies and appropriate number of Ce³⁺ defect sites. Moreover, CeZr-NC-1073 demonstrated good catalytic stability and versatility, as it was not only reusable for up to eight cycles without noticeable reduction in catalytic activity, but also capable of efficiently converting benzylamine substrates with various functional groups. Additionally, the reaction constant ρ of Hammett's equation was determined to be −0.16, predicting the presence of a positively charged intermediate in the aerobic oxidative procedure. These findings provide valuable insights into the development of highly efficient and stable Ce-based heterogeneous catalysts for aerobic oxidative coupling reactions.