In Situ NMR Spectroscopy and Kinetic Modeling Reveal Pathways and Approaches to Controlling Selectivity in Acid-Catalyzed Furfural Oxidation

Furanic compounds have long been known as products that can be formed from carbohydrates in high yields. The need for forming organic precursors and fuels from renewable reactants has hence revitalized the interest in the conversion of furanic precursors to prospective industrial monomers, including acids, diacids, anhydrides, and lactones. These precursors can be formed from furfural in oxidative pathways that are catalyzed by Lewis or Brønsted acids. Only recently, strategies to achieve high selectivity to desired products under mild conditions have emerged. These conditions remain discovered by serendipity and screening, as the pathways in the oxidative upgrading of furanics remain poorly understood and controversially discussed. To enable a rational basis for improving the conversions of furanic compounds, we conduct an extensive mechanistic and kinetic study of the oxidation pathways using hydrogen peroxide in Brønsted acidic medium. Specifically, we employ various types of time-resolved in situ nuclear magnetic resonance (NMR) spectroscopy to gain insights into pathways, reaction mechanisms, energetics, and kinetics. Spin polarization-enhanced (hyperpolarized) NMR using furfural as the substrate is used to elucidate the controversial initial steps channeling furfural into the oxidative pathway. With this approach, a reliable kinetic model is derived that accounts for the time course of 12 main chemicals in the reaction cascade toward at least four principal competing products. The approach sheds unprecedented light on the pathways and their control. Divergent steps that decide the product composition are identified through diligent experimentation, and distinct responses to reaction conditions at these branch points are shown to control selectivity for the competing products.