Enhanced carboxylation of furoic salt with CO2 by ZnCl2 coordination for efficient production of 2,5-furandicarboxylic acid

Abstract

C-H carboxylation of furoic acid (FA) with CO₂ is an atom-efficient strategy to produce 2,5-furandicarboxylic acid (2,5-FDCA) from lignocellulose. The existing carbonate-promoted CO₂ carboxylation processes rely on the use of large amounts of expensive Cs₂CO₃ as a deprotonating reagent and molten salt. Substitution of Cs with other cheap and abundant alkali ions (such as K and Na) can reduce the use of Cs, but it faces the problem of a low yield of 2,5-FDCA. This study found that the addition of catalytic amounts of ZnCl₂ as a Lewis acid can increase the yield of 2,5-FDCA in the CO₂ carboxylation reaction of Na/K-FA in a molten salt reaction system. ¹H NMR analysis and DFT calculations confirmed that ZnCl₂ coordinates with the furan ring through electron transfer from the conjugated furan ring to Zn²⁺, thereby activating the H at the C₅ position of Na/K-FA. This coordination lengthened the C₅-H bond and lowered its heterolytic dissociation energy, making it more susceptible to being deprotonated by CO₃^2- and subsequently carboxylated by CO₂. The developed Lewis acid coordination strategy provides a new idea for the efficient construction of C-C bonds between CO₂ and aromatics through carbonate-promoted C-H carboxylation.