Nanostructured Nickel/Silica Catalysts for Continuous Flow Conversion of Levulinic Acid to γ-Valerolactone

Selective transformation of levulinic acid (LA) to γ-valerolactone (GVL) using novel heterogeneous catalysts is one of the promising strategies for viable biomass processing. In this framework, we developed a continuous flow process for the selective hydrogenation of LA to GVL using several nanostructured Ni/SiO₂ catalysts. The structural, textural, acidic, and redox properties of Ni/SiO₂ catalysts, tuned by selectively varying the Ni amount from 5 to 40 wt %, were critically investigated using numerous materials characterization techniques. Electron microscopy images showed the formation of uniformly dispersed Ni nanoparticles on the SiO₂ support, up to 30% Ni loading (average particle size is 9.2 nm), followed by a drastic increase in the particles size (21.3 nm) for 40% Ni-loaded catalyst. The fine dispersion of Ni particles has elicited a synergistic metal-support interaction, especially in 30% Ni/SiO₂ catalyst, resulting in enhanced acidic and redox properties. Among the various catalysts tested, the 30% Ni/SiO₂ catalyst showed the best performance with a remarkable 98% selectivity of GVL at complete conversion of LA for 2 h reaction time. Interestingly, this catalyst showed a steady selectivity to GVL (>97%), with a 54.5% conversion of LA during 20 h time-on-stream. The best performance of 30% Ni/SiO₂ catalyst was attributed to well-balanced catalytic properties, such as ample amounts of strong acidic sites and abundant active metal sites. The obtained results show a great potential of applying earth-abundant nickel/silica catalysts for upgrading biomass platform molecules into value-added chemicals and high-energy-density fuels.