Electrocatalytic reduction of lignin related phenols in a stirred slurry reactor for green synthesis of renewable chemicals and fuels

Electrocatalytic hydrogenation-hydrogenolysis (ECH) is a prospective route for valorization of lignin derivatives, mainly for synthesis of organic chemicals. This environmentally benign process enables the integration of biorefinery and renewable electrical energy for clean fuels and chemicals production. Electrochemical water and/or proton reduction facilitates in situ, continuous generation of chemisorbed hydrogen on an electrocatalyst surface. However, most conventional ECH studies were operated at low current densities with low Faradaic efficiencies, owing to diffusion limitations of the organic molecules to the electrode surface. Polar organic electrolyte, which could facilitate the solubility of non-polar organic substrates in aqueous electrolyte, has not been extensively studied for the ECH purposes. This work presents the ECH of lignin model compounds (e.g., guaiacol and phenol) using dispersed metal catalysts (e.g., Pt/C, Ru/C, Pd/C) in diverse aqueous electrolytes under mild conditions (25–60 oC, 1 atm). The stirred slurry electrochemical reactor (SSER) configuration enables ECH operation at high current densities (> |100 mA cm⁻²|) and efficiencies (>50%) due to the improved mass, heat, and electron transfers between the reacting molecules and catalyst particles. Different catholyte-anolyte pair effects were investigated under potentiostatic and galvanostatic conditions whereby the electrocatalyst activity was found to be dependent on electrolyte pH and composition. In the process development, electrocatalytic reduction of bio-oil substrates was conducted using organic solvent-mixed acidic electrolytes for mild depolymerization. Acid-acid and neutral-acid catholyte-anolyte pairs were efficient for ECH of guaiacol and phenol, capable of resulting in high conversions (>90%) and efficiencies (>70%). This electrolyte pair combination enables the synergy of electrocatalyst and electrolyte, which could improve Faradaic efficiency and extend the pH-dependent catalyst options. Polar organic solvents (e.g., isopropanol) improve the reactant solubilization and affect proton stabilization for enhancing dehydration reaction, however they could also hinder substrate reactivity owing to competitive adsorption on the catalyst and suppressed ionic activities in the electrolyte. Electrocatalytic hydrodeoxygenation (HDO) of alkyl guaiacols in the mixed electrolytes could produce cycloalkanes at the low temperatures, suggesting the potential of ECH routes for the synthesis of alkane fuels, besides the value-added chemicals. Finally, challenges and opportunities for future development of electrocatalytic pathways for lignin valorization are discussed.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Lignin depolymerization and monomeric evolution during fast pyrolysis oil upgrading with hydrogen from glycerol aqueous phase reforming

A novel approach to Fast Pyrolysis Oil (FPO) upgrading with hydrogen from glycerol aqueous phase reforming (APR) was conducted in a biphasic solution. FPO contains both monomer and polymer compounds which rich in oxygen, giving high acidity and low stability. Hydrogen demanding reaction of depolymerization and hydrodeoxygenation (HDO), often called upgrading, is required to improve FPO properties by converting these compounds to hydrocarbon monomers. APR reaction of glycerol where glycerol is reacted with water to produce hydrogen is one of the renewable choices to obtain hydrogen. Prior to upgrading of FPO, catalyst screening and reaction optimization were studied using phenol as a model compound. Upgrading of FPO with in situ glycerol APR was conducted with Pt/C, facilitating hydrogen production (APR) and utilization (hydrogenation), and H-ZSM-5, facilitating dehydration reaction. n-Decane was added to the reaction as a co-solvent to prevent the condensation of the non-polar fragments of FPO which led to coke formation. Upon upgrading the weight average molecular weight (Mw), polydispersity index (PDI), and oxygen to carbon (O/C) ratio of FPO decreased. The highest hydrocarbon yield (7.7 FPO basis or 34.6 lignin basis) was obtained by combining Pt/C and H-ZSM-5 catalysts with n-decane as a co-solvent. Evidence of progressive depolymerization and sequential demethoxylation, hydrogenation, and deoxygenation during upgrading were observed in the products. © 2022 Elsevier Lt