Synthesis and characterization of highly dispersed cobaltous silicate as a catalyst for selective oxidation of toluene to benzaldehyde

Heterogenization of the homogeneous catalysts is crucial to their separation, recovery and reuse. The CoOx/SiO2 catalysts were prepared by co-precipitation followed by a special treatment using 1-butanol, and their composition, structure, morphology and surface chemical states were systematically characterized. The prepared CoOx/SiO2 catalysts were tested by selective oxidation of toluene to benzaldehyde with N-hydroxyphthalimide using molecular oxygen in hexafluoropropan-2-ol. The CoOx/SiO2 catalysts exhibited outstanding textural and structural properties, and presented a highly dispersion of cobaltous ions even at a Co:Si ratio up to 0.44. It is concluded that the predominant cobalt species could be cobalt silicate. The prepared catalysts efficiently catalyzed the toluene oxidation to benzaldehyde at mom temperature. Under optimized experimental conditions, the conversion of toluene and its selectivity to benzaldehyde were high-up to 98.5% and 62.5%, respectively. It was confirmed that the reaction proceeded via a free radical mode, and the solvent hexafluoropropan-2-ol significantly inhibited the further oxidation of benzaldehyde to benzoic acid. There was no obvious decrease observed in the catalytic performance for the recycled catalyst, indicating an excellent reusable capability of the prepared catalysts. Excellent toluene selective oxidation activity and stable reusability indicated that the CoOx/SiO2 catalysts possess a promising industrial application prospect.