2-Furoic acid is converted to furan-2,5-dicarboxylic acid in high yield on a mole scale using carbonate-promoted C–H carboxylation.

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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 analog 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 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 to guide the development of other solvent-free transformations. Main Text Solvent-free reactions have been investigated primarily as a means to improve the practicality of conventional solution-phase reactions by reducing waste, accelerating the rate, or simplifying purification. 1-9 The unusual properties of a reaction medium composed of neat reactants also raise the possibility of accessing reactivity that is very difficult or impossible to recapitulate in solution. To this end, we recently showed that carbonate (CO3 2-) can promote C-H carboxylation of very weakly acidic aryl C-H bonds (pKa > 35 in organic solvent) in carefully selected solvent-free alkali salts at elevated temperature. 10-13 By contrast, even under forcing conditions in organic solvent, CO3 2has been found to be incapable of promoting carboxylation of C-H bonds with pKa > 27. 14 The ability to use a simple, regenerable base such as CO3 2is essential if carboxylation is to be applied to scalable chemical synthesis. Solvent-free alkali salts unlock this possibility for some targets, but a better understanding of the reaction medium is needed to realize this potential.

Carbon-neutral liquid fuel generation is essential for decarbonizing sectors that cannot readily electrify. Recently commercialized acetogenic gas fermentation offers an alternative to conventional biofuels that circumvents efficiency limitations and land requirements, provided the requisite H2/CO feedstocks can be generated efficiently using renewable inputs. CO2 electrolysis to CO is under development for this purpose, but suffers from scalability challenges and impurity sensitivity. We describe an alternative that utilizes dispersed alkali carbonates as reverse water-gas shift (RWGS) catalysts to convert H2 and CO2 to an appropriate ratio of CO/CO2/H2 for acetogenic fermentation. Using a fixed bed reactor operating at industrially relevant space velocity, we demonstrate equilibrium RWGS conversion starting at 410 °C that remains stable over days, even with 50 ppm H2S impurity. The combination of carbonate-catalyzed RWGS, water electrolysis, and gas fermentation could convert electricity to ethanol with nearly 50% energy efficiency, providing a compelling option for renewable liquid fuel production.

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