Complete Valorisation of Lignocellulose and Low Concentration CO2 using a Fractionation-Photocatalysis-Electrolysis Process

The simultaneous upcycling of all components in lignocellulosic biomass and the greenhouse gas CO¬2 presents an attractive opportunity to synthesise sustainable and valuable chemicals. However, this approach has not yet been realised, presumably due to the difficulty of implementing a solution process to convert a robust and complex solid (lignocellulose) with a barely soluble and stable gas (CO2). Herein, we present the complete oxidative valorisation of lignocellulose coupled to the reduction of low concentration CO2 through a three-step fractionation-photocatalysis-electrolysis process. Lignocellulose from white birch wood was first pre-treated using an acidic dioxane solution to generate predominantly cellulosic- and lignin-based fractions. The solid cellulosic-based fraction was solubilised using cellulase (a cellulose depolymerising enzyme), followed by photocatalytic oxidation to formate with concomitant reduction of CO2 to syngas (a gas mixture of CO and H2) using a phosphonate-containing cobalt(II) bis(terpyridine) catalyst immobilised onto TiO2 nanoparticles. Photocatalysis generated 27.9±2.0 µmolCO gTiO2–1 (TONCO = 2.8±0.2; 16% CO selectivity) and 147.7±12.0 µmolformate gTiO2–1 after 24 h solar light irradiation under 20 vol% CO2 in N2. The soluble lignin-based fraction was oxidised in an electrolyser to the value-added chemicals vanillin (0.62 g kglignin–1) and syringaldehyde (1.65 g kglignin–1) at the anode, while diluted CO2 (20 vol%) was converted to CO (20.5±0.2 µmolCO cm–2 in 4 h) at a Co(II) porphyrin catalyst modified cathode (TONCO = 707±7; 78% CO selectivity) at an applied voltage of 3 V. We thus demonstrate the complete valorisation of solid and a gaseous waste stream in a liquid phase process by combining fractioning, photo- and electrocatalysis based on earth-abundant molecular hybrid nanomaterials.