Electrosynthesis of p-Hydroxybenzoic Acid From Phenol and CO2.

Abstract

P-hydroxybenzoic acid (p-HBA), with wide applications in food preservatives, pesticides, pharmaceuticals, and liquid crystalline polyether, is currently synthesized via the Kolbe-Schmitt reaction using anhydrous phenolate (PhO-) and carbon dioxide (CO2). However, this reaction typically suffers from poor selectivity, and requires high temperature/pressure, strongly base, and metal/organic cations for selectivity control. Herein, we report a new method for the electrosynthesis of p-HBA from phenol (PhOH) and CO2 by iodine mediation, achieving 61% yield and 90% selectivity without strong bases and metal cations under ambient temperature and pressure conditions. Mechanism studies indicated that PhOH was first selectively iodinated by iodine cations (I+) generated at the anode to form p-iodophenol (HOPh-I). Subsequently, HOPh-I and CO2 were respectively activated on Ag catalyst with abundant grain boundaries (GB-Ag) through single-electron reduction, and then underwent radical coupling processes to produce p-HBA. Notably, this electrochemical method also successfully converted various oxygen-containing and nitrogen-containing aromatic compounds, including the typical structural units of lignin and its derivatives, into valuable carboxylic acids. This innovative electrochemical approach overcomes the inherent limitations of the traditional Kolbe-Schmitt reaction, offering a promising strategy for the sustainable utilization of CO2 and biomass resources.