Liquid-phase oxidation of 2,6-di-tert-butylphenol with Cu-impregnated MCM-41 catalysts in the presence of alkali metals

Mesoporous silicate MCM-41, with a uniform pore diameter of ca. 35 Angstrom, was used as a support for impregnated Cu catalysts for liquid-phase oxidation of 2,6-di-tert-butylphenol (BOH) in the presence of a base such as KOH. The oxidation products were 4,4'-dihydroxy-3,3',5,5'-tetra-tert-butyl-biphenyl (H(2)DPQ) and 3,3',5,5'-tetra-tert-butyl-4,4'-diphenoquinone (DPQ). An alkali such as a potassium salt, which promotes the phenol oxidation activity of copper ion-impregnated MCM-41 (Cu/MCM-41) catalyst, was found to be more effective as an additive to the reaction solution than impregnation on the Cu/MCM-41 catalyst. The added alkali was found to play a role in generating the corresponding phenolate anion by dissolving a BOH molecule. H2DPQ is formed through tautomerization of an intermediate dimer obtained by the C-C coupling of the corresponding phenoxy radicals. DPQ is formed via the consecutive oxidation of H2DPQ and/or via the oxidative dehydrogenation of the intermediate dimer. The BOH molecules in the mesopores of Cu/MCM-41, rather than the BOH in bulk solution in the presence of CuCl2, were found to favor H(2)DPQ production. Cu/MCM-41 with added potassium [(K-Cu)/MCM-41] was much more active for phenol oxidation than Cu-impregnated NaZSM-5 with added potassium [(K-Cu)/NaZSM-5] or the corresponding NaY [(K-Cu)/NaY], each catalyst having only uniform micropores. This result indicates that the oxidation of sterically bulky BOH occurs mainly at the active sites in the mesopores and is difficult to carry out in the micropores of(K-Cu)/NaZSM-5 and (K-Cu)/NaY zeolites because of the steric bulkiness of the oxidation products, H2DPQ and DPQ. The liquid-phase adsorption amounts of BOH on NaZSM-5 and NaY were found to be comparable to that on MCM-41. The shape selectivity by the oxidation products in the micropores of (K-Cu)/NaZSM-5 and (K-Cu)/NaY zeolites was thus suggested to inhibit the BOH oxidation activities of both catalysts, based on the results of the oxidation reaction and liquid-phase adsorption of BOH. (C) 1999 Academic Press.