Hydrodeoxygenation of Levulinic Acid over Supported Catalysts

Abstract

Levulinic acid (LA), which can be produced from the sugar fractions of lignocellulosic biomass, is a promising sustainable platform molecule that can play a major role in future biorefineries. The work described was aimed at the development of heterogeneous catalysts for the selective conversion of LA by hydrodeoxygenation (HDO) reactions. In addition, the aim was to gain a better fundamental understanding of the relation between catalyst structure and performance. To this extent, we studied the influence of the support acidity on catalytic performance of supported Ru catalysts (i.e., supported on Nb2O5, TiO2, H-β and H-ZSM5) in the hydrodeoxygenation of levulinic acid (LA) in different solvents (i.e., dioxane, 2-ethylhexanoic acid (EHA) and neat LA). The Ru/TiO2 gave excellent selectivity to GVL (97.5%) at 100% conversion and was remarkably stable even under severe reaction conditions. Ru/H-ZSM5 showed a 45.8% yield of pentanoic acid (PA) and its esters in dioxane, which was the first example of this one-pot conversion directly from LA. Furthermore, a detailed study of the catalysts by 27Al 3Q MAS NMR and FT-IR shows that dealumination is the main reason of catalyst deactivation for two zeolite-supported ruthenium catalysts (Ru/H-β and Ru/H-ZSM5) in 2-ethylhexanoic acid. For Ru/H-ZSM5, the symmetric, tetrahedral framework aluminum species (FAL) are found to be mainly converted into distorted tetrahedral FAL species, with limited loss of aluminum to the solution. A severe loss of both FAL and extraframework aluminum (EFAL) species to the liquid phase was observed for Ru/H-β instead. The strong acid sites of Ru/H-ZSM5 are more stable than those of Ru/H-β, which explains the better catalytic performance in the hydrodeoxygenation of LA to PA of the former. The synthesis of Ru/H-ZSM5 was optimized for direct pentanoic acid production from LA. The influence of cation form of ZSM5, ruthenium precursor and Si/Al ratio) on metal dispersion and acidity could be related to catalytic performance in this reaction. A highly active bifunctional 1 wt% Ru/H-ZSM5 catalyst was developed by a simple approach to give an improved PA yield of 91.3% after 10 h reaction time and a PA productivity of 1.157 molPA gRu-1 h-1, which is the highest reported to date. A significant fraction of Ru was located inside the zeolite pores, providing the desired proximity between hydrogenation function and strong acid sites, which is essential for this reaction. Finally, nano-alloys proved to be highly active, selective and stable catalysts for the conversion of LA to GVL. The Au-Pd and Ru-Pd random alloys were prepared by a modified impregnation (MIm) method and characterized by a combination of advanced microscopic and spectroscopic techniques. The Au-Pd/TiO2 (MIm) material showed a dramatic ~27-fold increase in activity (i.e., TOF 0.102 s-1) compared to its monometallic counterparts, which was attributed to an electronic modification of Pd. The 1% Ru-Pd/TiO2 (MIm) catalyst was not only exceptionally active (TOF 0.638 s-1), but also showed excellent, sustained selectivity to GVL (99%). The dilution and isolation of Ru active sites by Pd is thought to be responsible for this superior catalytic performance. Alloying furthermore greatly improved the stability of both bimetallic catalysts.