Effective electro-oxidation of hydroxymethylfurfural using the electrografted immobilized aminoxyl radical

Abstract

2,5-Furandicarboxylic acid (FDCA) is an eco-friendly substitute of fossil fuel-derived polymers that can be produced from biowaste. One of the methods for its production is electrochemical oxidation, through which hydroxymethylfurfural (HMF)–a readily available lignocellulosic compound from biorefineries as well as the pulp and paper industry–can be converted into FDCA. The primary challenge of this process is catalyst design, which encompasses characteristics such as reaction rate, conditions, and product yield in determining the overall performance. Herein, a reactive analogue of an aminoxyl radical (TEMPO) was prepared through organic synthesis and immobilized through the electrografting method as an anodic catalyst, with the cathode available for pairing with desirable reduction reactions. Through facile electrochemical methods, the immobilization procedure allows the traditionally homogeneous TEMPO catalyst to function heterogeneously, eliminating the need for catalyst separation and recycling. The attained catalyst demonstrated excellent performance: the synthesized electrografted TEMPO showed a low onset potential (1.45 V vs. RHE) and a high faradaic efficiency (>90%) and was catalytically active even in a mildly alkaline environment–a distinct advantage over metal-based catalysts as HMF decomposes in high pH electrolytes. Furthermore, due to the versatility of aminoxyl radicals, this catalyst can be adapted to other oxidative organic electrochemical reactions in electrolyzer anodes.