Phase Behavior That Enables Solvent-Free Carbonate-Promoted Furoate Carboxylation

Solvent-free chemistry has been used to streamline synthesis, reduce waste, and access novel reactivity, but the physical nature of the reaction medium in the absence of solvent is often poorly understood. Here we reveal the phase behavior that enables the solvent-free carboxylation reaction in which carbonate, furan-2-carboxylate (furoate), and CO₂ react to form furan-2,5-dicarboxylate (FDCA^2-). This transformation has no solution-phase analogue and can be applied to convert lignocellulose into performance-advantaged plastics. Using operando powder X-ray diffraction and thermal analysis to elucidate the temperature- and conversion-dependent phase composition, we find that the reaction medium is a heterogeneous mixture of a ternary eutectic molten phase, solid Cs₂CO₃, and solid Cs₂FDCA. During the reaction, the amounts of molten phase and solid Cs₂CO₃ diminish as solid Cs₂FDCA accumulates. These insights are critical for increasing the scale of furoate carboxylation and provide a framework for guiding the development of other solvent-free transformations.