Computational and experimental investigations of the catalytic transfer hydrogenation of methyl levulinate with ethanol on ZrO2

The reduction of levulinic esters to γ-valerolactone is a key step in the valorization of biomass-derived compounds. This thesis investigates the formation of angelica lactones (ALs), hypothesized as key intermediates in the catalytic transfer hydrogenation of methyl levulinate (ML) with ethanol over tetragonal zirconia (t-ZrO₂) catalyst. Both experimental and computational investigations are integrated to investigate this process. The experimental studies involve Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) to analyze surface species and the mechanism behind the transformation of ML into ALs. These findings are complemented by computational studies based on Periodic Density Functional Theory (DFT), providing fundamental insights into molecular adsorption modes, reaction intermediates structures, and spectroscopic properties. DRIFTS results reveal that ML forms levulinate-like species on the zirconia surface, even at room temperature, with more pronounced effects as the temperature rises. At the reaction temperature of 250 °C, the DRIFTS spectra of ML and angelica lactone (αAL) are nearly identical, indicating that either ML is converted into ALs or αAL is reverted to a common intermediate shared with ML, possibly a levulinate-like one. On the computational side, DFT simulations model the adsorption of key molecules on the catalyst surface, revealing strong interactions between the ester group of ML and ALs with the t-ZrO₂ (101) facet, which results in significant molecular distortion, consistent with experimental findings. The simulations also revealed the role of surface hydroxyl groups in facilitating the cyclization of ML into αAL while preventing surface poisoning. Simulated vibrational spectra further helped interpreting the specific surface species observed in the experimental DRIFTS spectra. These findings suggest that both ALs and levulinate-like species can account for the DRIFTS spectrum of ML at the reaction temperature.