Highly Efficient CeO2–MoO3/SiO2 Catalyst for Solvent-Free Oxidative Coupling of Benzylamines into N-Benzylbenzaldimines with O2 as the Oxidant

The development of noble-metal-free catalysts is currently a topic of paramount research investigation in the chemical industry. In this work, a promising CeO2–MoO3/SiO2 catalyst was synthesized for the oxidation of a wide variety of benzylamines with O2 as the oxidant under solvent-free conditions. For comparison, the efficiency of CeO2 and CeO2–MoO3 catalysts were also studied for the oxidation of benzylamine under identical conditions. The physicochemical properties of the catalysts were thoroughly analysed using X-ray diffraction, Raman spectroscopy, Barrett–Joyner–Halenda pore size distribution, Brunauer–Emmet–Teller (BET) surface area, Fourier-transform infrared spectroscopy, ammonia-temperature programmed desorption (NH3-TPD), and X-ray photoelectron spectroscopy techniques. Characterization studies revealed that the CeO2–MoO3/SiO2 catalyst shows favourable structural, textural and acidic properties, which are due to strong interaction between the Ce–Mo mixed oxide and the SiO2. In particular, the CeO2–MoO3/SiO2 sample exhibits smaller crystallite size and highest BET surface area compared with the CeO2–MoO3 and bare CeO2. Raman analysis revealed that the CeO2–MoO3/SiO2 sample exhibits greater number of oxygen vacancy defects. The NH3-TPD studies indicated that the CeO2–MoO3/SiO2 sample has large amount of acidic sites. Owing to the beneficial properties, the CeO2–MoO3/SiO2 sample exhibited an outstanding catalytic performance for the aerobic oxidation of various benzylamines. The achieved benzylamine conversions are ~16, 65 and 98 %, respectively, for CeO2, CeO2–MoO3 and CeO2–MoO3/SiO2 samples. Despite the nature of the catalyst used and the reaction conditions employed, almost ~100 % selectivity for the dibenzylimine product was found in the present study. Remarkably, the CeO2–MoO3/SiO2 catalyst can be repeatedly used up to 5 cycles without any considerable variation in the benzylamine conversion and imine product selectivity. The CeO2–MoO3/SiO2 catalyst showed an outstanding efficiency in the oxidation of benzylamine, which is attributed to abundant acidic sites, large number of oxygen vacancies, and superior BET surface area.