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 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 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 using molecular hybrid nanomaterials assembled from earth abundant elements.

µmolCO cm -2 and 18.3±0.4µmolH 2 cm -2 corresponding to a CO selectivity of (67±8)% with a FYCO of (58±8)% and a FYH 2 of (29±7)% (Figure S16 and Table S5).On the anodic side, 9.6±0.1 µmol3,4-MBA cm -2 was formed corresponding to a FY3,4-MBA of (24±4)% assuming a 2-electron oxidation process (Figure S16 and Table S6).Thus, both isolated redox half-reactions can be coupled together to simultaneously reduce low concentration CO2 and oxidize the lignin model substrate.Control experiments were performed under the same conditions where the anolyte did not contain the lignin model substrate where most likely oxygen evolution predominantly occurs.Lower amounts of CO (29.4±0.3 µmolCO cm -2 ) and H2 (6.6±0.2 µmolH 2 cm -2 ) were formed supporting the promotional effect of coupling both half-reactions together to improve the overall productivity on the cathode side, while at the same time generating products beyond the thermodynamically more challenging O2 evolution at the anode side (Figure S4 and Table S5).Table S1.Compositional analysis of white birch, the solid and liquid fraction after white birch treatment with Dioxane/HCl/HCOOH and solid fraction incubation in cellulase.[a] Electrolysis performed with CP|MWCNT without any lignin model substrate in 0.1 M Na2CO3 in 1:1 MeCN:H2O

Figure S6 .Figure S7 .
Figure S6.Flow setup for variable concentration CO2 electrocatalysis.Scheme for CO2 electrocatalysis under flow by controlling the gas composition with mass flow controllers purging the gas through the anolyte followed by online-GC analysis to detect H2 and CO. 1

Figure S8 .Figure S9 .
Figure S8.Scanning electron microscopy of anode.Scanning electron microscopy image of the cross section of CP|MWCNT.
Figure S22.H2 and CO quantification.(a) Calibration curve for H2 (red) and CO (grey) quantification, and (b) spectra of H2 (red) and CO (grey) at different concentrations obtained using gas chromatography.

Figure S23 .
Figure S23.Formate quantification.(a) Calibration curve for formate quantification, and (b) spectra of formate at different concentrations obtained from ion chromatography (retention time 14.2 min).

Table S2 .
Compositional analysis of white birch, the solid and liquid fraction after white birch treatment with Dioxane/HCl/HCOOH and solid fraction incubation in cellulase with respect to the overall composition of white birch.
[a] as glucose.[b] as cellobiose.n.d.stands for not detected.

Table S3 .
Mass fraction recovered from the solid and liquid (or liquor) fraction after white birch treatment with Dioxane/HCl/HCOOH.