Hydrogenation of levulinic acid to γ-valerolactone over bifunctional Ru/(AlO)(ZrO)n catalyst: Effective control of Lewis acidity and surface synergy

Abstract

A bifunctional Ru/(AlO)(ZrO)n catalyst was reported for efficient hydrogenation of levulinic acid to γ-valerolactone, an important and environmental-benign upgrading process of bio-platform compound. The optimal Ru/(AlO)(ZrO)0.1 catalyst showed 100% yield to γ-valerolactone in water at 120 °C under 1 MPa H2, one of the best performances among Ru catalysts under mild conditions. And this catalyst also presented good stability and reusability. PXRD, TEM, XPS, CO chemisorption, CO adsorption DRIFT-IR, NH3-TPD and pyridine adsorption IR gave insights into the acidic property and surface synergy of the catalysts. Highly active electron-rich Ru0 sites stabilized by strong metal-support interaction (SMSI) can effectively adsorb and activate molecular H2 and CO bond in levulinic acid. Varying Zr/Al ratio and calcination temperature can rationally control the acid strength and acid sites distribution of support, and (AlO)(ZrO)0.1 calcined at 400 °C exhibited the maximal proportion of Lewis acid sites. Ru0 species and Lewis acid sites synergistically enhanced the conversion of levulinic acid and facilitated the intramolecular dehydration of 4-hydroxypentanoic acid as intermediate to γ-valerolactone (i.e., the rate-limiting step).