Studies on electrolytic mediator system (EMS) oxidation of lignin model compounds

概要

In summary, the studies focused on various aspects of electrochemical oxidation of lignin-related
compounds using different mediators and solvent systems. Solubility tests helped identify suitable
solvents for the electrochemical experiments, with CH3CN/H2O (7:3) demonstrating favorable redox
stability for the mediators. CV measurements revealed the influence of solvents on the electrochemical
properties, while the choice of mediator impacted the reaction selectivity. Mediators such as PZH,
NHPI, and ABTS showed promise in mediating the oxidation of specific lignin linkages. Fe(bpy)3 was
found to be an effective mediator for lignin dimer and polymer oxidation, with cleavage and
carbonylation reactions observed. TEMPO-mediated electro-oxidation under alkaline conditions
exhibited high efficiency in oxidizing both dimer and polymer model compounds. Notably, TEMPO
facilitated Cα–Cβ bond cleavage, yielding monomers with specific end-units. However, further
investigations are needed to understand certain observations and optimize reaction conditions. These
findings contribute to a deeper understanding of lignin EMS and offer potential applications for lignin
modification and biomass processing.
The primary findings of this study are as follows:
Significant conversions were observed in the phenolic model compounds containing β−O−4 and β−5
linkages upon direct electro-oxidation at 0.8 V, while only partial changes were noted in the nonphenolic model compounds. However, the direct electro-oxidation of G-DHP did not yield satisfactory
outcomes compared to the dimer model compounds. The intricate and bulky structure of G-DHP
restricts the interaction between the electrode's active sites and the internal architecture of the
macromolecule, leading to minimal oxidation. These findings highlight that the bulk electrolysis
conditions established in this study are not suitable for the direct electrolysis of lignin polymers.
Electro-oxidation of lignin dimer models 1b–3b and the lignin polymer model G-DHP was conducted
with three mediators (PZH, NHPI, ABTS). The mediators demonstrated effective facilitation of the
electro-oxidation reactions, displaying distinct reaction selectivity. PZH exhibited preferential
oxidation of β–O–4 and β–β linkages, NHPI favored β–O–4 linkages, and ABTS preferred β–O–4 and
β–β linkages. The degradation of major lignin linkages involved partial Cα–Cβ cleavage along with
Cα- and/or Cγ-oxidations. Furthermore, the mediators facilitated the conversion of coniferyl alcohol
end-units, with NHPI and ABTS likely mediating Cα–Cβ cleavage through hydrogen atom transfer
(HAT) and electron transfer (ET) mechanisms, respectively. ...