Mild Electrocatalytic Hydrodeoxygenation of Lignin-Based Phenolic Monomers in Polar Organic Electrolytes

Abstract

In biomass upgrading process, complete oxygen removal is required to maximize the energy density of the final liquid product as fuel. Conventionally, this process is carried out at high pressure and temperature in the presence of externally supplied hydrogen gas. Electrocatalytic hydrogenation (ECH), which represents a novel approach for synthesis of renewable chemicals from biomass derivatives, is mostly performed in aqueous electrolytes under mild conditions (i.e., ambient pressure and low temperature), which are unfavorable for the deoxygenation.This work demonstrates that electrocatalytic hydrodeoxygenation (ECHDO) can be achieved at mild conditions (1 atm, 40–60 oC) using a stirred slurry catalyst in polar organic electrolytes. Lignin-based phenolic monomers (i.e., cerulignol, creosol, guaiacol, phenol) can be partially and fully deoxygenated to cyclic alcohols and cyclic alkanes, respectively, either individually (as a single reactant) or collectively (as a mixed reactant).Significant cerulignol conversion (~50%) was achieved in the polar organic electrolyte, consisting of methanesulfonic acid (MSA) solution (0.2 M) and 2-propanol (1:1 volume ratio), using slurry Pt/C catalyst after 4 h reaction (T = 50 oC, j = -182 mA cm-2, F.E. ≈ 60%). Propylcyclohexane was the most dominant product with good selectivity (~35%). In this dual catalytic system, Pt/C activates hydrogenation step while MSA solution provides acidity that favors dehydration and/or dehydroxylation steps. Meanwhile, the polar organic solvent (i.e., 2-propanol) improves reactant solubilization and affects proton stabilization, resulting in the higher reactivity of the acidic protons toward dehydration.In comparison to the other polar organic solvents being tested (e.g., ethanol, THF, dioxane, and acetone) in the ECHDO of cerulignol, the use of 2-propanol resulted in the highest reactivity (X = 71%, F.E. = 36%) under the identical reaction conditions. The experimental results clearly indicated that different polar organic solvents give different reactivities, possibly due to the different proton and substrate solvation effects. Overall, complete deoxygenation of phenolic compounds can be done via electrocatalysis at mild conditions and low Pt catalyst concentrations (0.6–2.8 wt.%) in the absence of external hydrogen gas.