Probing Reactions at the Solid-Liquid Interface By Operando Solid-State NMR Spectroscopy

Reactions of biomass-derived carbohydrates and phenolics in porous catalysts take place at solid-liquid interfaces where the local composition of the fluid phase can be very different from that of the bulk solution. Many such reactions take place in semi-aqueous solvent mixtures whose micro-heterogeneity is enhanced inside the catalyst pores. Solid-state NMR spectroscopy can probe molecular dynamics at solid-liquid interfaces, however, the demanding reaction conditions used in hydrothermal biomass transformations are particularly challenging for NMR methods. Customized chemically-resistant high pressure rotors were adapted to withstand elevated temperatures and pressures. Magic-angle-spinning spectra were recorded to monitor the populations of adsorbed molecules and measure the kinetics of their transformations. Examples involving glucose-fructose isomerization in basic zeolites and hydrogenolysis of aryl ethers will be presented, in which selective isotope labeling (¹³C, ²H) was used to enhance sensitivity and provide mechanistic information.