Mechanism of Alkaline Lignin Oxidation Using Laccase-methyl Syringate Mediator System

Abstract

The mechanism of alkaline lignin oxidation in presence of laccase-methyl syringate (MS) mediator is discussed in terms of morphological changes that take place during exposure of the lignin to the phosphate buffer solution (pH=6.5) for 72hr at 70 o C. The SEM analysis of lignin before and after enzymatic treatment reveals the morphological changes explained by the interaction of the lignin surface groups with laccase-methyl syringate system. The BET analysis confirms that this interaction causes the change in the surface area from 2.75 to 5.50cm 2 /g. The corresponding pore-size distribution in lignin sample treated with laccase-methyl syringate is much broader in comparison to the untreated lignin and the pores within 25-150nm range are detected as a result of the BJH analysis. The electrochemical study of lignin, lignin with laccase, and lignin with laccase in presence of the mediator in the buffer solution has been performed in the potential range from -0.3 to +1.0V vs. Ag, AgCl, Cl -reference electrode. The cyclic voltammetry confirms reversible oxidation-reduction behavior of the methyl syringate natural mediator in anaerobic and aerobic environment. Specifically in anaerobic conditions three oxidation anodic peaks (0.265V, 0.474V and 0.884V) and two reduction peaks (0.421V and 0.103V) were detected out of which the oxidation peak at 0.474V was assigned to the formation of MS• radical. In aerobic conditions the methyl syringate demonstrates two oxidation peaks (0.473V and 0.812V) and two reduction peaks (0.410V and 0.135V). The mechanism of MS radical stability in oxygen vs. anaerobic environment is proposed based on formation of MS• radicals.

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

“…Although copper-containing oxidases such as laccases can degrade lignin, they are not as effective for ECO due to their low electrochemical reduction potential [131][132][133][134] and their large size [135] and will require lower molecular weightm ediators as electron carriers.Y ao et al conducted two studies to investigate the effects of am ediator (methyl syringate) and ac ombined laccase-mediator system (LMS) (Myceliophthora thermophila [134] and Trametes versicolor [136] )o nt he electrochemical degradation of alkali lignin deposited on ag lassy carbon electrode disc. Interaction of the lignin with the LMS caused morphological changes on the lignin structure and significantly increased the surface area.…”

“…Studying lignin ECO in aerobic and anaerobic environments, four major mechanisms were proposed.I nt he absence of LMS under aerobic conditions, direct oxidation of lignin can be promoted by the oxygen reduction reaction( ORR). [134,136] In the presence of laccase (a known biocatalyst), surfacep henolic groups on the lignin structure can also be oxidized at high potentials and can be enhanced in aerobic environments aided by ORR. [136] In the presence of just the mediator,u nder aerobic conditions, both the surface phe-nolic groups and the more internal b-O-4 linkages can be oxidized as the small mediator radical formed at lower potential can access the internal lignin linkages via mesopores.…”

“…iv.T he complexity and bulkiness of thel ignin polymerp oses as ignificant challenge in the application of electrocatalysis forlignin degradation. More specifically,limited access of the lignin linkages by active electrode sites as ar esult of the structural complexity and bulkinessoft he substrate can hindert he selectivity towards specific products.W ith the growing application of redox mediators such as TEMPO [137] or methyl syringate [134,136] in electrocatalysis to transport the redox electrons to the target redox site, this issue can be circumvented. Other methods such as modification and organic activation of electrode surfaces can also be beneficial in addressing this issue.…”

“…Although copper‐containing oxidases such as laccases can degrade lignin, they are not as effective for ECO due to their low electrochemical reduction potential and their large size and will require lower molecular weight mediators as electron carriers. Yao et al.…”

“…Yao et al. conducted two studies to investigate the effects of a mediator (methyl syringate) and a combined laccase–mediator system (LMS) ( Myceliophthora thermophila and Trametes versicolor ) on the electrochemical degradation of alkali lignin deposited on a glassy carbon electrode disc. Interaction of the lignin with the LMS caused morphological changes on the lignin structure and significantly increased the surface area.…”