Non-conventional hydrogen sources for the catalytic hydrogenation of biomass-derived levulinic acid to produce γ-valerolactone under mild conditions

Abstract

The use of biomass as a renewable energy source has been studied as an alternative to traditional and non-renewable energy sources in order to reduce the greenhouse gas emissions and decarbonize the economy. Levulinic acid (LA) is an important biomass-derived compound, that can be easily obtained by acid hydrolysis from biomass. LA can be transformed into high valuable chemical compounds by catalytic route, as for example in γ-valerolactone (GVL), which has important applications as additive or precursor of biofuels. The transformation of LA to GVL requires a hydrogen source and a proper catalyst. In this work, monometallic catalysts based on Ru and Ni as well as bimetallic RuNi supported on a zeolite have been synthesized and characterized by XRD, XPS and TEM. The hydrogenation of LA into GVL has been studied with these Ni/Ru catalysts using non-conventional hydrogen sources and the results have been compared with the traditional hydrogen source (pressurized molecular hydrogen). The alternative hydrogen sources used are: i) a system based on metallic Zn and water, where Zn can decompose water into hydrogen but it is oxidized to ZnO, ii) a combined system that uses hydrogen produced by photoelectrochemical (PEC) water splitting (WS) using a nanostructure of TiO₂. The bimetallic catalyst led to the highest yield to GVL when Zn was used in the reaction, but the yields achieved are not remarkable (less than 25 %). Conversely, the monometallic catalyst based on Ru was the one that exhibited the highest yield to GVL when pressurized and PEC hydrogen were used. In this case, yields to GVL exceeding 95 % were achieved at 30 °C although the reaction time required was lower when pressurized hydrogen was used. Interestingly, a new system for PEC WS has been proposed, improving the contact between hydrogen and levulinic acid. This new system decreases the induction time, and enhances the GVL formation, especially at low reaction times. Finally, these catalysts are stable after reaction regardless of the source of hydrogen employed.