Catalytic way of transforming 2,3-dimethylphenol to para-quinone with the use of vanadium-containing heteropoly acids

Article abstract of DOI:10.1016/j.apcata.2017.09.022

2,3-Dimethyl-p-benzoquinone (2,3-Me₂BQ) is a valuable chemical that is applied as a soft oxidizing and dehydrogenating agent and also as a synthon in preparing different complex products including pharmaceutical and biochemical substances. Keggin- and modified-type aqueous solutions of Mo-V-phosphoric heteropoly acids (Mo-V-P HPAs) with the gross compositions H_3+xPMo_12-xV_xO₄₀ (HPA-x) and H_aP_zMo_yV_x’O_b (HPA-x’), respectively, possessing high oxidation potential and simplicity of regeneration can serve as effective soft oxidants for obtaining such para-quinone from 2,3-dimethylphenol (2,3-Me₂P). The synthesized HPA catalysts with different vanadium content were characterized by a number of analysis techniques, such as ³¹P and ⁵¹V NMR spectroscopy, potentiometry, titrimetry, and pH measurement. It was found that the predominant formation of 2,3-Me₂BQ instead of corresponding diphenoquinone (DPQ) at one-electron oxidation is achieved by a consecutive optimization of reaction conditions, the most important among them being organic solvent and molar ratio of vanadium(V) to substrate. As was shown, the substitution of HPA-x by HPA-x’ allows one to increase the quinone selectivity and to decrease the optimal molar ratio of vanadium(V) to substrate. The highest yield of the desired quinone (97%) at total substrate conversion was obtained by using the biphasic water-benzene system at molar vanadium(V) to substrate ratio of 12. The temperature of 50 °C and an inert atmosphere were established to be the optimal reaction conditions. The aqueous HPA-10′ solution including the highest content of VO₂⁺ ions proved to be the most efficient catalyst among investigated HPAs. Carrying out catalyst regeneration at a separate stage provides the preservation of its activity and selectivity at the initial level for at least ten cycles.