Unravelling Electrochemical Lignin Depolymerization

Abstract

Lignin valorisation via electrochemical depolymerization is a promising approach for commercial application due to its moderate reaction conditions. However, there is no available kinetics model for this reaction. Conventional reaction kinetics equations are inadequate when used for lignin degradation because of the limited kinetics information with respect the reaction mechanism. We suggest to use population balance equations to predict the evolution of molecular weight distribution of lignin with time. Solving the low molecular weight (MW) population balance equations is carried out discretely whereas a continuous solution was implemented for the high MW. Additionally, the model accounts for a recombination reaction for the depolymerized species. The model is capable of predicting the molecular weight distribution of lignin as a function of electrochemical processing time. New experimental results are used to extract kinetics constants for different kraft lignin samples.

Carboxylic Acids Production via Electrochemical Depolymerization of Lignin

“…This confirms the SEC data where the low molecular weight peak increases after a first phase of repolymerization (Figure b and c). Deeper insight into this dual mechanism of electrochemical depolymerization in a similar electrochemical setup has already been gained . Maximal acid concentrations are always achieved after 7 h of electrochemical depolymerization at 3.5 V with the exception of malic acid where comparable amounts were gained with 2.5 and 3.5 V. The overall highest concentration is found for oxalic acid and reaches a value around 500 mg/L.…”

“…To prevent a short circuit, the counter and working electrode were separated by a polymer spacer (148.5 mm×38.6 mm×1.63 mm, Naltex Extruded Netting NO 133280PP‐NAT, DelStar Technologies Inc.). A schematic representation of the reactor can be found in our previous publication . The presence of a polymer spacer instead of a membrane, allows the in‐situ produced oxygen to further react at the cathode to produce hydrogen peroxide and to diminish the resistance of the system due to the membrane.…”

“…Herein, we show that the electrochemical lignin depolymerization approach can produce carboxylic acids with high yields close to 40 wt% and therefore, represents a competitive alternative to catalytic and hydrothermal degradation processes. The electrochemical depolymerization of lignin has been carried out in a homemade “swiss‐roll” electrochemical reactor . A deep insight into lignin depolymerization products is given by means of several analytical techniques, such as size exclusion chromatography (SEC), proton nuclear magnetic resonance spectroscopy (H‐NMR), liquid chromatography coupled to electrospray ionization quadrupole time‐of‐flight mass spectrometry (LC‐ESI‐Q‐ToF‐MS) and total organic carbon determination (TOC).…”

Greener Routes to Biomass Waste Valorization: Lignin Transformation Through Electrocatalysis for Renewable Chemicals and Fuels Production

“…[113] Further,t he reactionr ate coefficientsw ere demonstrated to have al inear relationship with the initial polydispersity and average molecular weighto f the lignin. [113] Later,t hey developed ac onceptual model for lignin degradation in an ECMR andf ound that am embrane pore diameter of 1nmw as optimal, yielding 11 %m onomers, < 2% CO 2 ,a nd water versus 70 %o rganic acids (from continuous degradation of aromatics), 30 %C O 2 ,a nd water in ab atch reactor.M onomer yield increased as af unction of membrane area (optimal 0.01 m 2 )a nd transmembrane pressure (optimal 1.4 bar);g oing beyondt hese optimal valuesw ould only increase capital cost withouta ny significant effect on product yield. [114] Alternatively, Di Marinoe tal.…”

Electrochemical Degradation of Lignin by ROS

“…Therefore, a sustainable chemical industry based on lignin degradation to obtain aromatic platform chemicals has been proposed. Considering the complexity of the lignin molecular structure, the cleavage of the linkage bonds among the phenylpropane units during degradation is essential to improve product selectivity, which might simplify the subsequent separation process of the desired products [4][5][6]. Researchers have tried various methods, including pyrolysis, enzymatic processes, and chemical catalysis methods, to breakdown the linkage bonds in lignin [7][8][9], in which…”