Hydrothermally stable bifunctional catalysts for the aqueous-phase hydrogenation of levulinic acid to γ-valerolactone

Introduction: γ-Valerolactone (GVL) has been identified as one of the most promising platform chemicals derived from lignocellulosic biomass. It is accessible by hydrogenation of levulinic acid (LA) with formic acid (FA) as a hydrogen donor in the aqueous-phase obtained from hydrolysis of wet biomass. However, selective catalysts with high hydrothermal stability and resistance towards aqueous acids are required. Here, stabilization of γ-Al2O3 as support for bimetallic Pt-Co catalysts by silylation and calcination at different temperatures was studied. Furthermore, Al2O3-SiO2 as well as boehmite-SiO2 boehmite were investigated as supports for the Pt-Co catalysts. Materials and methods: Mono- (3 wt.% of Pt or Co) and bimetallic (3 wt.% Pt-9 wt.%Co with Pt loaded first in all samples) catalysts supported on γ-Al2O3, Al2O3-SiO2, and boehmite-SiO2 were prepared and thoroughly characterized by physicochemical methods. The catalysts were studied in the aqueous-phase hydrogenation of LA to GVL using FA as a hydrogen donor, also in the presence of vanillin (V) to replicate real lignocellulosic hydrolysates. Results: The highest GVL yield of 50% was achieved over 3Pt-9Co/γ-Al2O3 after 6 h of reaction. Furthermore, phase transformation of γ-Al2O3 into boehmite was hindered by applying a silylation step with subsequent calcination of two separated samples, one at 1073 K and 773 K, which resulted in lower catalytic activity of 50% of GVL yield (sample calcined at 1073 K) and 62% (sample calcined at 773 K) of GVL yield after 10 h of reaction. A similar phenomenon was observed using Pt-Co catalysts supported on Al2O3-SiO2, over which GVL yield was lower, but the hydrothermal stability was enhanced. On the other hand, boehmite-SiO2 as a support for Pt-Co has a high catalytic activity in LA hydrogenation to GVL with FA, i.e., GVL yield of 93% and the textural properties were largely retained after two consecutive catalytic runs (24 h each), with only a moderate (16%) decrease in specific surface area and no phase transformation of the support. The catalyst also maintained its high hydrothermal stability and activity when converting a more complex model mixture, i.e., including vanillin (V). Conclusions: The results indicate that a boehmite-SiO2 support with a well-balanced composition is a promising and stable support for Pt-Co catalysts for a broad range of hydrothermal biorefinery applications, especially in aqueous-phase processing.