Ring Activation of Furanic Compounds on Ruthenium-Based Catalysts

We employed a combination of isotopic labeling experiments, density functional theory calculations, and first-principles microkinetic modeling to investigate the mechanism of H/D exchange of furanic platform molecules. Alkylated furans (e.g., 2-methylfuran (2-MF)) exhibit appreciable H/D exchange, but furan and oxygenated furanics (e.g., furfuryl alcohol) do not. Detailed mass fragmentation pattern analysis indicates H/D exchange only occurs at unprotected a-carbons. Simulations show that, in the presence of coadsorbed toluene (solvent), the most likely pathway involves Ru surface mediated scission of the C-O bond in the furan ring at the unsubstituted carbon atom, followed by dehydrogenation, deuteration, and ring-closure steps. The degree of H/D exchange reaction depends mainly on the adsorption strength of exchange intermediates: strongly bound compounds, e.g., furan and furfuryl alcohol, inhibit H/D exchange via site blocking and slow desorption, whereas alkylated furans are sterically repelled by the solvent freeing up catalyst sites for exchange at the unsubstituted a-carbon of the furan ring. The binding strength of exchange intermediates is governed by interaction of the substituent group both with the surface and with the coadsorbed solvent molecules. The proposed H/D exchange mechanism on metal catalysts, which involves the opening of furan ring, is in stark contrast to the Bronsted catalyzed ring activation and suggests a possible pathway for the formation of ring-opening products and for rational selection of solvents.