Homogeneous and heterogeneous catalytic oxidation of sulfides by H₂O₂ over zinc(II) compounds

It has been recently shown that zinc compounds are effective catalysts for the oxidation of alkyl aryl sulfides to the corresponding sulfoxides in the presence of hydrogen peroxide. In this paper, we have investigated homogeneous and heterogeneous catalytic oxidation of sulfides by H₂O₂ over Zn(NO₃)₂·6H₂O and the metal–organic porous material [Zn₂(bdc)(L-lac)(dmf)]·DMF (where H₂bdc = p-benzenedicarboxylic acid, H₂lac = lactic acid), respectively. The experimental data can be explained by the proposed catalytic cycle which includes the activation of H₂O₂via coordination to Zn(II) ions followed by oxygen transfer step. In homogeneous conditions, the presence of a large amounts of H₂O₂ results in the coordination of two molecules of hydrogen peroxide to Zn(II), so that sulfone is formed via transfer of two oxygen atoms from Zn(H₂O)₄(H₂O₂)₂²⁺ active species. Contrary to the homogeneous system, the use of [Zn₂(bdc)(L-lac)(dmf)]·DMF as catalyst does not lead to the formation of sulfone in the initial period of reaction. This is consistent with the proposed catalytic cycle of sulfoxidation as each Zn(II) center in the crystalline framework is able to activate only one H₂O₂ molecule. Our investigations indicate that the sorption and activation of H₂O₂ molecules by microporous framework [Zn₂(bdc)(L-lac)(dmf)] occur faster than sulfide sorption and oxygen transfer.