Efficient transfer hydrogenation of alkyl levulinates to γ-valerolactone catalyzed by simple Zr–TiO2 metal oxide systems

Zr–TiO2 synthesized heterogeneous catalysts could efficiently convert ethyl levulinates (ELs) to γ-valerolactone (GVL) using isopropanol (2-PrOH) as H-donor. Obtained catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), High revolution transmission electron microscope (HR-TEM), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma optical emission spectroscopy (ICP-OES), NH3/CO2 temperature programmed desorption (NH3/CO2-TPD), pyridine-infrared spectroscopy, H2 temperature-programmed reduction (H2-TPR), and N2 adsorption and desorption measurements. In total, 10 wt% Zr–TiO2 with average nanoparticle sizes (ca. 4–6 nm) exhibited optimum catalytic activity after optimization of reaction temperature, reaction time, catalyst loading, as well as solvent effect. GVL yield reached 74% with 79% EL conversion at 190 °C for 5 h over 10 wt% Zr–TiO2 in 2-PrOH. The high catalytic activity could be attributed to an appropriate proportion of acidic/basic sites, high Brønted/Lewis acid ratio, and large surface areas. Both acidic and basic sites lead to a synergistic effect on the concurrent activation of H-donor and substrate. The major side product ethyl 4-hydroxypentanoate (EHP) and other byproducts were found. GVL yield achieved from methyl levulinate (ML) and levulinic acid (LA) were 65% and 20%, respectively. Catalyst deactivation was observed due to coke deposits on the catalyst's surface. The spent catalyst proved to be reusable to recover almost completely its initial activity after calcination (300 °C, 2 h). A plausible reaction mechanism is presented on the basis of characterization results.