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| - A Density Functional Theory Study of the Mechanism of Free Radical Generation in theSystem Vanadate/PCA/H2O2
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| - Experimental studies by Shul'pin and co-workers have shown that vanadate anions in combination withpyrazine-2-carboxylic acid (PCA ≡ pcaH) produce an exceptionally active complex that promotes the oxidationof alkanes and other organic molecules. Reaction of this complex with H2O2 releases HOO• free radicals andgenerates V(IV) species, which are capable of generating HO• radicals by reaction with additional H2O2. Theoxidation of alkanes is initiated by reaction with the HO• radicals. The mechanism of hydrocarbon oxidationwith vanadate/PCA/H2O2 catalyst has been studied using density functional theory. The proposed modelreproduces the major experimental observations. It is found that a vanadium complex with one pca (PCA ≡pcaH) and one H2O2 ligand is the precursor to the species responsible for HOO• generation. It is also foundthat species containing two pca ligands and an H2O2 molecule do not exist in the solution, in contradictionto previous interpretations of experimental observations. Calculated dependences of the oxidation rate oninitial concentrations of PCA and H2O2 have characteristic maxima, the shapes of which are determined bythe equilibrium concentration of the active species. Conversion of the precursors requires hydrogen transferfrom H2O2 to a vanadyl group. Our calculations show that direct transfer has a higher barrier than pca-assisted indirect transfer. Indirect transfer occurs by migration of hydrogen from coordinated H2O2 to theoxygen of a pca ligand connected to the vanadium atom. The proposed mechanism demonstrates the importantrole of the cocatalyst in the reaction and explains why H2O2 complexes without pca are less active. Our workshows that the generation of HOO• radicals cannot occur via cleavage of a V−OOH bond in the complexformed directly from the precursors, as proposed before. The activation barrier for this process is too high.Instead, HOO• radicals are formed via a sequence of additional steps involving lower activation barriers. Thenew mechanism for free radical generation underestimates the observed rate of hexane oxidation by less thanan order of magnitude; however, the calculated activation energy (67−81 kJ/mol) agrees well with thatdetermined experimentally (63−80 kJ/mol).
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