A Closed Cycle for the Carboxylation-Esterification of Furoic Acid with CO2 over Supported Alkali Metal Salt Catalysts

Abstract 2,5-Furan dicarboxylic acid (FDCA) is an important biomass derivative that is commonly used in the synthesis of green polymers. Currently, the carboxylation of furoic acid (FA) to produce FDCA requires the use of large quantities of alkaline reagents to convert FA and CO2 into carboxylates, followed by the displacement of alkali-metal ions from the carboxylates using HCl to obtain FDCA. This process not only involves the use of highly corrosive HCl but also alkaline reagents that are converted into chlorides after the reaction; the recovery of these chlorides relies on bipolar membrane electrodialysis technology. In order to avoid the use of strong acids that corrode equipment and pollute the environment, eliminate the electrodialysis process, and directly regenerate alkali metal salts while simplifying the reaction process, this study has designed a new cyclic process. The process comprises two steps: (1) under the catalysis of an alkali metal salt catalyst (Cs2CO3–CsOPiv/TiO2) supported on spherical titanium dioxide (TiO2) carriers, FA undergoes direct C–H carboxylation to form the corresponding carboxylate intermediates. (2) Using the green methylation reagent dimethyl carbonate (DMC), the carboxylate intermediates are converted in situ to dimethyl furan-2,5-dicarboxylate (FDME), while simultaneously regenerating the alkali metal salt catalyst component. Experimental results indicate that the Cs2CO3–CsOPiv/TiO2 catalyst achieves a 94% conversion rate of FA. The generated carboxylate intermediates are completely converted to FDME with a total yield of 76%.