High Specific Surface Area Silica Nanorod Supported Bimetallic NiCu Effectively Catalyzes Levulinic Acid Hydrogenation to γ-Valerolactone

Bimetallic NiCu catalysts supported on mesoporous SiO₂ nanorods were prepared by impregnation method with the help of complexing agents. The introduction of various complexing agents including citric acid (CA), polyvinyl pyrrolidone (PVP), triethanolamine (TEOA), triethylamine (TEA) and ammonia (AMH) significantly affected the metal particle size, surface chemical state, redox and acidic properties. The NiCu/SiO₂–PVP catalyst had the smallest metal nanoparticle sizes of 2.3 nm among the catalysts. The variation of metal particle size resulted in the differences of the metal-support interaction, which further caused the changes of the redox property. Acidic property was another indispensable factor to promote this reaction, and the types of complexing agents did not change the acidic strength distribution but the total number of that. The apparent activation energy on NiCu/SiO₂–PVP was 26.55 kJ/mol, which was obviously lower than that on NiCu/SiO₂–AMH (37.49 kJ/mol). Consequently, the NiCu/SiO₂–PVP catalyst exhibited the best activity compared to other catalysts in the gas phase catalytic hydrogenation of levulinic acid with formic acid as the hydrogen source, achieving the highest levulinic acid conversion of 76.9% and γ-valerolactone selectivity of 91.6%. It also showed good catalytic stability within 20 h time-on-stream.