Continuous Synthesis of γ‑Valerolactone in a Trickle-Bed Reactor over Supported Nickel Catalysts

Various Ni-based catalysts were tested in the continuous liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in a trickle-bed reactor using water as solvent with the aim to develop an economic and environmentally friendly way for the GVL synthesis. For this purpose, various synthesis methods were used to prepare Ni-based catalysts, which were first screened in batch reactors. Characterization by X-ray diffraction, temperature-programmed reduction, electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy showed that slow precipitation using urea resulted in a good Ni dispersion. The dispersion also improved at lower Ni loading, and smaller Ni particles mostly showed an enhanced catalytic performance for the synthesis of GVL. 5 wt % Ni/Al₂O₃ prepared by wet impregnation showed the highest specific activity for the hydrogenation of LA to GVL (90% LA conversion and 75% GVL yield) featuring an average Ni particle size of 6 nm. Some deactivation of the catalysts was observed, probably due to transformation of γ-Al₂O₃ to boehmite and sintering of the Ni particles. In addition, reoxidation of Ni particles may additionally lead to deactivation as concluded by comparison with screening studies in batch reactors.