UC Santa Barbara

A plentiful material found in nature, nonfood lignocellulosic biomass has emerged as a promising renewable source of liquid fuels and valuable chemicals due to its high energy content and abundance of aromatics in the lignin fraction. However, the recalcitrant nature of lignocellulosic biomass hinders its efficient transformation to platform chemicals and fuels. To gain kinetic and mechanistic insight about organic transformations relevant to the valorization of lignin, the reductive cleavage of the aryl ether linkage and the hydrogenation of phenol, catalyzed by the heterogenous catalyst nickel on gamma-alumina, were investigated.Operando Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS-NMR) is a powerful and non-invasive tool for acquiring detailed kinetic information. Recent advances in the design of MAS NMR rotors allows for internal rotor pressures up to 400 bar and temperatures up to 250 °C. The new high temperature and pressure rotors were used to collect operando 1H and 13C spectra during the hydrogenolysis of benzyl phenyl ether (BPE), a lignin model compound, at temperatures up to 250 ºC, both with and without H2 gas. In the absence of H2, the 2-propanol solvent can serve as a source of H2. The reaction generates toluene and phenol with an apparent activation barrier of (80 ± 8) kJ mol-1 at temperatures of 150-170 ºC. Toluene does not undergo further hydrogenation at these temperatures, but phenol undergoes subsequent, slow hydrogenation to cyclohexanol. Benzylic C-O bond cleavage is faster than H/D exchange in the methyl group of toluene, which in turn is faster than phenol hydrogenation. The source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H2/D2 gas rather than the solvent at these temperatures. Although the rate of BPE hydrogenolysis is not strongly solvent-dependent, the selectivity to phenol changes with the choice of solvent: the rate of hydrogenation is significantly faster in solvents with lower polarity. The rates are strongly dependent on the extent of phenol adsorption on the catalyst surface, implicating the balance between phenol solvation and competitive adsorption. For polar protic solvents (typically, alcohols), the rate decreases with the strength of the hydrogen bonding between phenol and the solvent, presumably due to better solvation of phenol. In aprotic solvents (typically, alkanes and aromatics), the rate of phenol hydrogenation correlates inversely with the strength of solvent adsorption on Ni active sites. Consequently, selectivity in aryl ether hydrogenolysis to phenolics can be tuned by solvent selection with little impact on activity.