Structure–reactivity relationships in the ionic-liquid-catalyzed oxidation of cumene: Cation effects

Cumene oxidation is a key step in the industrial synthesis of phenol via cumyl hydroperoxide (CHP) formation. Ionic liquids (ILs) are attractive as green, metal-free alternatives catalysts for this reaction, but few studies have explored their potential. The study of the relationship between the structure and catalytic performance of ILs is crucial for the development of ionic liquids with excellent performance. In this study, we aimed to establish a theoretical protocol to evaluate the catalytic activity of ILs for cumene oxidation, as well as the structure–activity relationship, using density functional theory calculations. The calculations showed that the catalytic activity of the studied ILs decreased as follows: 1-n-butyl-3-methylimidazolium bromide > 1-butyl-1-methylpyrrolidinium bromide > N-n-butylpyridinium bromide > triethylammonium bromide > 1-n-butyl-2-methylpyrrolinium bromide. Moreover, the polarity, solubility, and maximum electrostatic potential of the ILs were linearly correlated with their catalytic activity. The theoretical predictions were verified by experiment. Additionally, DFT calculation and EPR analysis demonstrated that the catalytic activity of ionic liquids originates from the formation of hydrogen-abstracting radicals. Furthermore, using 1-butyl-1-methylpyrrolidinium bromide as a representative catalyst, we optimized the reaction conditions, a cumene conversion of 24.2 % and a CHP selectivity of > 90 % were obtained. Compared with the aqueous NaOH as a stabilizing agent, the reaction time is reduced by 8 h. This study provides new insights for designing efficient, low-cost, and environmentally friendly catalysts for cumene oxidation.