Factors Affecting the Selective 5-Hydroxymethylfurfural Electrooxidation to 5-Formyl-2-furancarboxylic Acid over Amorphous CoO _<i>x</i> Catalyst

The utilization of renewable energy and mild synthesis conditions has recently garnered increased interest for the electrochemical conversion of 5-hydroxymethylfurfural (HMF) into high-value chemicals. However, the deep oxidation of the alcohol group (−OH) while protecting the aldehyde group (−CHO) of HMF made the selective electrooxidation to 5-formyl-2-furancarboxylic acid (FFCA) challenging. Herein, three overall operating factors (applied potential, temperature, and HMF concentration) governing partial and deep oxidation of the alcohol group of HMF were investigated; thereby, the selective formation of FFCA in borate electrolyte (pH 9.5) over CoOx electrocatalyst was achieved. At room temperature, the applied potential could regulate the partial to deep oxidation ratio of the −OH group of HMF, thereby allowing high selectivity of FFCA but leading to a low total Faradaic efficiency (FE) at the high applied potential. The temperature was the most significant factor affecting product distribution, causing both cathodic shifts in onset potential and deep −OH group oxidation, resulting in higher FFCA selectivity at high temperature. The HMF concentration was found to have an insignificant influence on the −OH group oxidation process but caused HMF electrooxidation (HMFOR) to be more effective. Accordingly, HMFOR on CoOx in a pH 9.5 electrolyte at 50 °C achieved a total FE of 70% and yielded FFCA as the main product with a selectivity of 60%. Our study provides insight into the simple approach to control the oxidation process of the −OH group of HMF, hence facilitating future research in this domain, especially in refining reaction conditions for the selective large-scale synthesis of targeted products.