Aqueous Electrocatalytic Hydrogenation Depolymerization of Lignin β-O-4 Linkage via Selective Caryl-O(C) Bond Cleavage: The Regulation of Adsorption.

Abstract

The cleavage of the benzene-oxygen (C_aryl-O(C)) bond of the lignin β-O-4 linkage is expected to relieve condensation of the degradation product and improve the product value. Nevertheless, the electrochemical breaking of the C_aryl-O(C) bond has not been achieved yet due to the high dissociation energy (∼409 kJ mol^-1) and the easy over-reduction of aromatic compounds. Here, we report an aqueous electrochemical reduction strategy for breaking C_aryl-O(C) bonds via the regulation of molecular adsorption. The density functional theory (DFT) calculations and quartz crystal microbalance (QCM) measurements reveal that the residual Cu(I) in the CuO electrocatalyst enhances the adsorption of the 2-phenoxy-1-phenylethyl alcohol (PPE) by the C_aryl-O(C) bond and lowers the energy barrier of the protons attacking the oxygen atom in the β-O-4 linkage. Thus, compared to the Cu electrocatalyst (with a hydroquinone yield of 47.4% and a benzyl alcohol yield of 24.8%), the CuO nanorod exhibits a much higher yield of hydroquinone (95.3%) and benzyl alcohol (88.6%) at a potential of -0.4 V vs reversible hydrogen electrode (RHE) in an undivided cell. Moreover, the reaction pathway and the cleavage of the C_aryl-O(C) bond are identified through a combination of in situ synchrotron-radiation Fourier transformed infrared spectroscopy (SR-FTIR) and DFT calculations. This effective method is utilized for poplar lignin electrolysis, yielding 10.9 wt % of guaiacylglycerol, with an outstanding selectivity of >63.0%. This work provides an efficient and mild method of cleavage of C_aryl-O(C) bonds in lignin valorization.