Cyclohexane oxidation: relationships of the process efficiency with electrical conductance, electronic and cyclic voltammetry spectra of the reaction mixture

The cyclohexane oxidation by H2O2 using VO(acac)2 as starting catalyst in the presence of oxalic acid (OA) was studied. The dissociation of OA and VO(oxalate) formed in situ by interaction of VO(acac)2 with OA is the essence of the electrical conductance G elevation (or vice versa 1/G dropping). As follows from the electronic and cyclic voltammetry spectra taken alongside 1/G, the substitution of weak field ligands (acac) of VO(acac)2 by the middle-field (oxalate) ones strengthens the cation-ligand bonds and postpone the irreversible catalyst oxidation. In the absence of OA, 1/G was several times larger than the value intrinsic to VO(acac)2 + OA mixture. The last feature corresponds with the considerable process productivity enhancement in presence of OA. The experimental part of this work was complemented with DFT calculation of the key quantum chemical characteristics as catalyst d-d-splitting, HOMO–LUMO gap and Gibbs energy. Bringing together the experimental and theoretical data led to deduce that the oxidation process efficiency relates, among others, with the modification the outer-sphere electronic configuration of metalocomplexes possibly leading to metal-peroxo species e.g. VO(η2-O2) generation. On the other hand, oxalate anions, besides decreasing 1/G, may facilitate the cations and H2O2 interaction. Mentioned peculiarities may be responsible for the noteworthy yield enhancement in the presence of OA.