Electrochemical Decomposition of Wheat Straw Lignin into Guaiacyl‐, Syringyl‐, and Phenol‐Type Compounds Using Pb/PbO₂ Anode and Alloyed Steel Cathode in Alkaline Solution

Abstract

An electrocatalytic method was applied to decompose unit structures of wheat straw lignin (WSL) through oxidation on Pb/PbO2 and reduction on alloyed materials cathode with different catalytic activity in alkaline solution. Six of guaiacyl‐type (G) compounds were chosen as target products for evaluating their properties and efficiencies of all selected cathodes in WSL decomposition. A series of alloyed materials were examined as catalysts for the electrocatalytic decomposition, thereinto, the selected catalysts were characterized and evaluated by a systematic study for hydrogenation of WSL in NaOH solution. The effects of reaction parameters, such as pH, current density, lignin concentration and temperature, on product yields were investigated. Gas chromatography (GC) monitored maximum yields of 1.134 and 1.248 g/kg‐lignin of guaiacol and acetovanillone using FeW9Cr4V5Co3 as cathode, 0.364, 3.797, and 1.147 g/kg‐lignin of 4‐ethylguaiacol, vanillin, and vanillyl acetone using 304 alloyed steel as cathode, 1.040 g/kg‐lignin of 4‐propyl guaiacol using FeW9Cr4Mo3V as cathode, respectively. Based on the maximum yield of guaiacyl‐type compounds, 304 alloyed steel gave the best results with respect of lignin decomposition at the optimum conditions. In addition, gel permeation chromatography (GPC) demonstrated that the average molecular weights (Mw) of the lignin decreased in the process of all reactions and the unit structures cut down from lignin were electro‐catalytically hydrogenated to guaiacyl‐, syringyl‐, and phenol‐type compounds. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13117, 2019

Electrochemical Lignin Conversion

“…However, the complex structure of lignin makes it very difficult to dissolve in common organic solvents. [82] Alkaline solutions (e. g., NaOH, KOH) have been widely used as the electrolyte in many lignin electrooxidation studies [10,31,44,48,59,[83][84][85][86][87][32][33][34]37,[40][41][42][43] due to increased solubility for lignin and high electron/ion conductivity. However, electrolysis in aqueous solution is limited by the water splitting reaction at 1.23 V, severely restricting the potential window for electrochemical reactions.…”

“…Various cathodic electrodes have been utilized, including mercury pool, [137] Raney nickel, [138,139,141] copper, [34] ruthenium supported on activated carbon cloth, [147] ruthenium or palladium, [138,143] and alloyed steel. [87] Pacut and Kariv-Miller reported a cathodic reduction of (C aryl À O) of 4-O-5 lignin model (diphenyl ether) in the presence of mercury pool as a cathode and tetrabutylammonium salts as an electrolyte in aqueous and mixed organic-aqueous solutions. [137] A mixture of both hydrogenation and cleavage products were observed.…”

“…Li and co-workers reported electrochemical depolymerization of wheat straw lignin using a Pb/PbO 2 anode with varying alloyed materials as the cathode in alkaline solution. [87] The yields of G-type lignin compounds (guaiacol, 4-ethylguaiacol, 4propyl guaiacol, vanillin, acetovanillone, and vanillyl acetone) from wheat straw lignin depolymerization in different operating conditions are summarized in Table 3. Meanwhile, S-type (2,6dimethoxyphenol, 4-hydroxy-3,5-dimethoxybenzaldehyde, 1-(4hydroxy-3,5-dimethoxyphenyl)ethan-1-one, 4-hydroxy-3,5dimethoxybenzoic acid), and phenol-type (phenol, 1-(4hydroxy-2-methylphenyl)ethan-1-one, 4-hydroxy-3-propylben-zoic acid, isobutyl 4-hydroxybenzoate, butyl 4-hydroxybenzoate) compounds were also observed in depolymerized products.…”

“…Yield of products for degradation of lignin over different types of cathodes. [87] Run Cathode pH Current density All reactions were carried out for duration of 1 h.…”

“…[108] Only limited efforts have focused on hybrid electro-oxidative and electroreductive depolymerization of lignin. [34,59,87,152,153] The advantage of a hybrid approach results from simultaneous generation of reactive oxygen species and hydrogen in situ, which can be used for electro-oxidation and electro-reduction of lignin and its intermediates. [59] Li and co-workers investigated a hybrid electrochemical degradation of bamboo lignin using a Cu cathode and Pb/PbO 2 anode in sodium hydroxide solution, [34] and identified vanillin, syringaldehyde and p-coumaric acid as the main products.…”