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 O-2 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 similar to 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 similar to 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. [GRAPHICS] .