Valorisation of lignocellulose and low concentration CO2 using a fractionation–photocatalysis–electrolysis process

Abstract

The simultaneous upcycling of all components in lignocellulosic biomass and the greenhouse gas CO₂ presents an attractive opportunity to synthesise sustainable and valuable chemicals. However, this approach is challenging to realise due to the difficulty of implementing a solution process to convert a robust and complex solid (lignocellulose) together 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-stage fractionation–photocatalysis–electrolysis process. Lignocellulose from white birch wood was first pre-treated using an acidic 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 μmol_CO g_TiO2⁻¹ (TON_CO = 2.8 ± 0.2; 16% CO selectivity) and 147.7 ± 12.0 μmol_formate g_TiO2⁻¹ 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 kg_lignin⁻¹) and syringaldehyde (1.65 g kg_lignin⁻¹) at the anode, while diluted CO₂ (20 vol%) was converted to CO (20.5 ± 0.2 μmol_CO cm⁻² in 4 h) at a Co(II) porphyrin catalyst modified cathode (TON_CO = 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 using molecular hybrid nanomaterials assembled from earth abundant elements.