Mild Oxidation of Toluene to Benzaldehyde by Air

We report here the green oxidation of toluene to benzaldehyde by air under mild conditions in >99% selectivity at 35% conversion at 180 °C. This is an important reaction in both theoretical and practical research studies, but little progress has been made in the past five decades. Either the selectivity or the conversion is very low, and the sum of selectivity and conversion is hardly more than one hundred. In previous investigation, we have constructed a highly efficient system of catalysis, i.e., Pickering-like emulsion of toluene in water stabilized by amphiphilic catalysts HDPA-FeOx/Al2O3, hexadecyl phosphonic acid-modulated iron oxides, which work in an enzyme-like mechanism for oxidation of toluene to benzaldehyde. However, the oxygen pressures more than 3 MPa needed for high conversion impede their practical uses. In this work, we modify the catalytic system for the green oxidation of toluene to benzaldehyde using 1 MPa of air as an oxidant. First, the catalytic center of HDPA-FeOx/Al2O3 is optimized as PPA-CuFeOx/Al2O3 with promoted activity of crystal oxygen toward toluene oxidation. Readily available phenylphosphonic acid (PPA), in place of HDPA, shows good performance in the new catalytic system. Second, 5 vol % acetone/water is used as a mixed solvent of the reaction, which markedly modifies the oxygen dissolving and transferring, the access of the toluene molecule to the catalyst, and the recovery rate of catalytic center. The acetone molecule itself, however, is proved not converted and can be recycled in the catalytic process. Comprehensively, the catalytic system including multifactor synergism and good intrinsic safety developed in the current work has realistic feasibility for benzaldehyde production from toluene green oxidation in high-performance and atomic economy.