Efficient production of adipic acid from 2-methoxycyclohexanone by aerobic oxidation with a phosphotungstic acid catalyst

Abstract

Oxidative cleavage reaction of 2-methoxycyclohexanone (2-MCO) to adipic acid (AA) and methanol with O₂ in water solvent was investigated. 2-MCO and AA are one of the lignin-based compounds produced via hydrogenation of guaiacol and an important monomer in industry, respectively. Various vanadium compounds and heteropolyacids were tested as homogeneous catalysts because vanadium compounds, especially phosphomolybdovanadic acids, have been known to be active in various oxidative cleavage reactions with O₂. Simple vanadium-free phosphotungstic acid (H₃PW₁₂O₄₀), which has not been regarded as an oxidation catalyst using O₂ as the oxidant, showed good catalytic activity and excellent selectivity to AA. The carbon-based AA yield reached 74% (86% in molar basis) and this value was higher than those obtained with vanadium-based catalysts. A reuse test and ³¹P NMR confirmed that the H₃PW₁₂O₄₀ catalyst was stable and reusable. Kinetic studies and the reaction test using a radical inhibitor suggested that the reaction mechanism is not auto-oxidation involving free radicals. Instead, the substrate was first activated by one-electron oxidation by H₃PW₁₂O₄₀ catalyst and then reacted with O₂.