Microwave Solid-Phase Synthesis of Ni-Co Ferrites and their Activities in Liquid-Phase Oxidation of m-Xylene

Abstract

In this paper, the activity of mono- and di-substituted cobalt and nickel ferrites prepared by microwave solid-phase synthesis was studied in the reaction of liquid-phase oxidation of m-xylene to m-toluic acid. It was established that among the tested samples, the di-substituted ferrites of spinel structure with the composition of Ni_0,6Co_0,4Fe₂O₄ have the shortest induction period and the highest initial rate of oxygen absorption. In order to clarify the role of the active surface of the Ni_0.6Co_0.4Fe₂O₄ catalyst and the volumetric conversions of m‑xylene in the nucleation stage of primary xylyl radicals, the dependence of the induction period was studied. Furthermore the rate of oxygen absorption was investigated as a function of the time of introduction of the inhibitor into the oxidized m-xylene at different ratios of the total size of the heterogeneous surface to the volume of m‑xylene loaded into the reactor (S/V). The dependence of the selectivity of the process to the targeted product, m-toluic acid, on the specific surface area of the catalyst samples was established and it was shown that selectivity to the by-product, m-methylbenzyl alcohol, increased and selectivity to m-toluic acid decreased in the case of cobalt ferrite with a relatively low specific surface area.It was shown that at the first stage reaction (5–10 min.), ie., in conditions of an as yet undeveloped radical chain reaction, in the presence of disubstituted ferrites of the composition (Ni_1–xCo_xFe₂O₄), in comparison with monosubstituted ferrites, a higher initial rate of formation of the primary oxidation product of m-xylene — m-methylbenzylhydroperoxide (main intermediate product obtained during formation of m-toluic acid) is observed. The correlation of the activity of mono- and disubstituted Ni-Co ferrites, in the conditions of developed heterogeneous catalytic conversion of m‑xylene, disubstituted Ni-Co ferrites are also characterized by higher rate of heterogeneous nucleation of free radicals and heterogeneous catalytic conversion of the intermediate product — m-toluic aldehyde to m‑toluic acid. This, in turn, provides a high degree of selectivity for the targeted product with the yield of 79.8 %.