Influence of carbon support properties in the hydrodeoxygenation of vanillin as lignin model compound

Abstract

Hydrodeoxygenation of lignin is a fundamental step in biomass valorisation for the reduction of the oxygen content and the synthesis of a wide range of products. Vanillin is a lignin model compound containing an aldehydic group attached to an aromatic ring. Pd nanoparticles are particularly active in the hydrogenation of carbonyl groups and the subsequent hydrogenolysis of the alcoholic bond, especially when the nanoparticles are supported on carbon-based materials. This work aims to study the effect of carbonaceous supports with different chemical-physical properties on both activity and selectivity in the hydrogenation of vanillin to vanillyl alcohol and the subsequent hydrodeoxygenation to creosol. Pd nanoparticles were synthesised by sol immobilisation technique and deposited on four supports (three activated carbons – Norit, KB and G60 – and a carbon nanofiber – HHT) with different graphitisation order and oxygen functionalisation. The catalysts were thoroughly characterised with transmission electron microscopy, X-ray photoelectron spectroscopy and N2 adsorption. The conversion of vanillin was correlated to the carbon graphitisation order and the concentration of oxygen functionalities of the support, while the production of ether through reaction between vanillyl alcohol and the solvent (isopropanol) was correlated to the presence of carboxylic groups on the support surface. Full conversion to creosol was obtained with all the catalysts in short reaction times (Pd/G60, creosol yield > 99.5 %) and in mild reaction conditions. In addition, the Pd/HHT catalyst showed good stability upon reuse in 5 consecutive reactions.