Unanticipated Effects of Quaternary Ammonium Additives and Brønsted Acids in the Acidic Oxidation of Furfural

Future sustainable processes critically depend on the ability to steer the selectivity of complex reaction pathways toward desired products. The formation of building blocks from biomass-derived furanic precursors, for instance, proceeds along diverging pathways to various diacids in acidic solutions of hydrogen peroxide, which can be affected by the presence of betaine hydrochloride or choline-containing acidic media. The basis of ion effects in such systems remains unclear. Here, we use in situ NMR spectroscopy to track ion-specific effects in the oxidation of furfural by hydrogen peroxide. We use a variety of substrates in conjunction with titration of both acid and additive to address the mechanistic effect of additives. Synergism of acidity and ion pairing enables the formation of fumaric acid (trans-Butenedioic acid) over maleic acid (cis-Butenedioic acid). In this manner, the selectivity can be tuned between 100% maleic acid and 80% fumaric acid, indicating that the zwitterionic additives function as stabilizing structural directors. The additive does not need to contain a quaternary ammonium site, as a sulfonium-containing analogue elicits similar effects. Time-resolved observations of the formation of fumaric acid and maleic acid argue against the isomerization of maleic acid to its thermodynamically more stable isomer under the given conditions. Rather, the formation of fumaric acid and maleic acid appears to occur in competing pathways, where the formation of fumaric acid proceeds along a more extended pathway entailing cis–trans isomerization of an intermediate, enabled by ionic interactions. These findings reveal the tunable ion effect on reaction selectivity in the oxidative upgrading of furfural, providing a new handle for designing efficient oxidation processes in the absence of metal catalysts and organic solvents.