Understanding of the Oxidation Behavior of Benzyl Alcohol by Peroxymonosulfate via Carbon Nanotubes Activation

Selective oxidation of benzyl alcohol (BzOH) into benzaldehyde (BzH) is very important in synthetic chemistry. Peroxymonosulfate (PMS) is a cheap, stable, and soluble solid oxidant, holding promise for organic oxidation reactions. Herein, we report the catalytic PMS activation via carbon nanotubes (CNTs) for the selective oxidation of BzOH under mild conditions without other additives. A remarkable promotion of BzH yield with a selectivity over 80% was achieved on modified CNTs, i.e., O-CNTs via the radical oxidation process, and the oxygen functionalities for catalysis were comprehensively investigated by experimental study and theoretical exploration. To understand the different surface oxygen species on CNTs for the activation of PMS, density functional theory (DFT) calculations were performed to investigate the adsorption behavior of PMS on various CNTs. The electrophilic oxygen was identified as the electron captor to activate PMS by O-O bond cleavage to form SO5 center dot- and SO4 center dot- radicals. The nucleophilic carbonyl groups can also induce a redox cycle to generate (OH)-O-center dot and SO4 center dot- radicals, but phenolic hydroxyl groups impede the radical process with antioxidative functionality. The carbocatalysis-assisted PMS activation may provide a cheap process for the selective oxidation of alcohols into aldehydes or ketones. The insight achieved from this fundamental study may be further applied to other organic syntheses via selective oxidation.