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 (FDCA2–). 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.