Controlled synthesis of CuxMn3.66−xMo3O12 with the citrate sol–gel method for the selective liquid-phase toluene oxidation to benzaldehyde by air

Abstract

A systematic study on the synthesis of the CuxMn3.66−xMo3O12 using citrate sol–gel technique is presented. The influence of the copper substitution, citric acid concentration and pH of the precursor solution on the exclusive formation of Cu–Mn–Mo polymetallic oxide are studied. The physicochemical properties of as-prepared oxides are characterized by XRD, ICP-MS, SEM, TEM, XPS, FT-IR, BET, and H2-TPR, and their dependence on the conditions of synthesis is explored. It is found that these three synthesis factors remarkably alter the crystal phases and surface structures. The catalytic behaviors of synthesized catalysts for liquid-phase toluene oxidation are evaluated with toluene-catalyst mixture under uniformly distributed conditions. The evaluation reveals that the ratios of Mn3+/Mn2+ as well as Oads/Olat on catalysts play a crucial role in the activity and selectivity. Factors effecting reaction parameters, such as reaction temperature, residence time, catalyst weight, promoter weight, airflow rate and the volume ratio of substrate to solvent, are investigated in detail. The reusability of the catalyst is examined through conducting repeat runs with the same catalyst, exhibiting no significant decline in its activity even after 8 runs. A toluene conversion of 60% with 57.4% selectivity toward benzaldehyde is observed after 4 h over the high-crystalline CuxMn3.66−xMo3O12 nanoparticles.