Why a simple vanadate is inefficient as a catalyst in the oxidation of alkanes with H2O2 – the long-standing puzzle is solved

Abstract

In contrast to V(V) complexes with various organic ligands, a simple vanadate without any additive is inactive in neutral medium toward the oxidation of alkanes with H₂O₂. In this work, we discovered that the insufficient activation of H₂O₂ upon coordination to the simple vanadate – the commonly accepted reason for the low catalytic activity of the vanadate – cannot explain this phenomenon. Two main findings are reported here on the basis of DFT calculations. First, the generally accepted Fenton-like mechanism of the generation of the active oxidizing species (HO˙) in a vanadate/H₂O_2(aq)/MeCN system was revisited. A new mechanism based on the tremendous activation of the OOH ligand in the intermediate [V(OO)₂(OOH)(H₂O)] toward the homolytic O–O bond cleavage is not only feasible but significantly more favourable than the Fenton-like pathway. The surprisingly low activation barrier calculated for the HO˙ generation (15.4 kcal mol⁻¹) demonstrates the efficiency of this process. The presence of easily oxidizable non-innocent OO ligands in this intermediate explains such an activation. Second, it was found that the generated HO˙ radicals may be easily captured by the V atom soon after their formation followed by the elimination of the molecular oxygen. This side reaction of the H₂O₂ dismutation efficiently consumes the produced HO˙ radicals decreasing their concentration in the reaction mixture and preventing the following oxidation of alkanes.