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 CO32- 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.
