Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al2O3, NiMo/Al2O3, NiW/Al2O3 catalysts as well as the effects of solvent, H-2 pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al2O3, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H-2 are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al2O3, NiMo/Al2O3 and NiW/Al2O3 catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al2O3 shows the highest activity among the tested conventional catalysts with 97 % selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al2O3 catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H-2 availability (i.e., low H-2 surface coverage) and toluene formation is favored. In supercritical n-heptane with high H-2 availability (i.e., high H-2 surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H-2 surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.