Non-hydroxyl radical dominated catalytic oxidation of lignin-derived vanillyl alcohol by H2O2 upon ionic liquids for vanillin production

Abstract

Oxidative conversion of lignin-derived vanillyl alcohol (VA) into value-added vanillin (VN) represents an important move towards sustainable valorization of lignin resource. However, the oxidation upon hydrogen peroxide often suffers from side reactions like over-oxidation, limiting the reaction efficiency. The key step to enhance VN yield from VA oxidized by H2O2 lies in the generation of selectively active oxidizing species as an alternative to hydroxyl radicals. To this end, we demonstrate a homogeneous oxidation of VA for VN acquisition using H2O2 in ionic liquid (IL) triethylammonium acetate [Et3NH][CH3COO] catalyzed by cobalt (II). The generation of hydroxyl radical was proved to be inhibited by [Et3NH][CH3COO], and the oxidative high valent metal–oxygen complexes Co(IV) = O with higher selectivity might be involved to promote the conversion efficiency upon the coordination of IL anion with cobalt ion. Compared to the oxidation proceed in non-IL system, the IL system contributed to higher reaction efficiency. 11.85 % VN yield was achieved for 81.65 % conversion rate of VA at 60 ℃ for 2 h in atmosphere with the addition of 0.04 mmol of Co2+ in [Et3NH][CH3COO].