Kinetic investigations of unusual solvent effects during Ru/C catalyzed hydrogenation of model oxygenates

Abstract

A systematic study of solvent effects in low-temperature hydrogenation of various model oxygenates in bio-oils (such as 2-butanone, 2-pentanone, and phenol) with Ru/C catalyst is presented. Protic (e.g., water and C1–C4 primary alcohols), aprotic polar (e.g., γ-butyrolactone, acetonitrile, and tetrahydrofuran), and aprotic apolar (e.g., cyclohexane and n-heptane) solvents were investigated. It was found that for 2-butanone hydrogenation, Ru/C catalyst shows the highest hydrogenation activity in water, followed by alcohols, while in aprotic apolar solvents (i.e., cyclohexane and n-heptane), a remarkably lower hydrogenation activity was observed. In sharp contrast, Ru/C catalyst shows little hydrogenation activity in aprotic polar solvents (i.e., γ-butyrolactone, acetonitrile, and tetrahydrofuran). To explain these dramatic effects, both classical measures of polarity and others based on different solvatochromic scales were considered. For 2-butanone hydrogenation, a clear correlation between the initial hydrogenation rate and hydrogen-bond donor capability (α) of the solvents was observed. This is consistent with previously reported hypothesis that the strong interaction between protic solvents and 2-butanone by hydrogen bonding lowers the activation energy barrier and leads to high hydrogenation rate. For aprotic polar solvents, it seems plausible that such solvents could strongly adsorb onto the catalyst surface and block the Ru active sites, thereby inhibiting the hydrogenation rate. In the case of aprotic apolar solvents, both solvent–substrate and solvent–catalyst interactions are insignificant, and thus, a reasonable hydrogenation rate was observed in these solvents. Kinetic modeling of 2-butanone hydrogenation in water as a solvent indicates that the power-law and Langmuir–Hinshelwood (with the second hydrogenation step as the rate-determining step) models provide the best fits to experimental rate data. Further, the estimated activation energy for 2-butanone hydrogenation in water is 33.4kJ/mol, which is lower than the value of 68kJ/mol reported from density functional theory (DFT) calculations that assume the formation of the hydroxybutyl intermediate and its subsequent hydrogenation as the rate-determining step.