Toward rational design of stable, supported metal catalysts for aqueous-phase processing: Insights from the hydrogenation of levulinic acid

Aqueous-phase hydrogenation of levulinic acid (4-oxopentanoic acid) and 2-pentanone was considered to probe the activity and stability of supported Ru catalysts during ketone hydrogenation in water. Selected supports include activated carbon, amorphous SiO₂, γ-Al₂O₃, and TiO₂. For both probe molecules, intrinsic hydrogenation activity was independent of support identity and Ru particle size. Moreover, the presence of a secondary functional group in levulinic acid (i.e., a carboxyl group) and the resulting dissociated protons do not appear to perturb the activity of Ru sites in water. LA hydrogenation thus appears kinetically equivalent to that of 2-pentanone, and both occur with an average turnover frequency of 0.11 s⁻¹ under our experimental conditions (323 K, 24 bar H₂, 0.5 M LA/2-Pentanone). Supported Ru clusters were susceptible to both irreversible and reversible modes of deactivation. The former was attributed to particle sintering, which is accelerated in bulk water relative to gas phases. The extent of particle growth is relatively insensitive to solution pH and depends primarily on the nature of the support, decreasing in the order SiO₂ > C ≈ TiO₂ > γ-Al₂O₃. Reversible activity losses are also support-dependent; however, the underlying cause could not be identified conclusively. Its extent correlates with the dominant surface charge of the support under experimental conditions. As such, trends in reversible and irreversible deactivation both appear to be governed by the bulk electronegativity of the support.