Electrochemical Conversion of the Lignin Model Veratryl Alcohol to Veratryl Aldehyde Using Manganese(III)-Schiff Base Homogeneous Catalysts

Abstract

Lignin and other colored structures need to be bleached after the Kraft process in the pulp industry. Development of environmentally-safe bleaching catalysts or electrocatalysts constitutes an attractive strategy for selective removal of lignin. Seven manganese(III)-complexes with Schiff base ligands 1–7 were synthetized and characterized by different analytical and spectroscopic techniques. The tetragonally elongated octahedral geometry for the manganese coordination sphere and the global µ-aquo dimeric structure were revealed by X-ray diffraction (XRD) studies for 1, Mn2L12(H2O)2(N(CN)2)2 (N(CN)2 = dicyanamide). Complexes 1–4 behave as more efficient peroxidase mimics as compared to 5–7. Electrochemical oxidation of the lignin model veratrylalcohol (VA) to veratrylaldehyde (VAH) is efficiently catalyzed by a type of dimanganese(III) complexes in a chlorine-free medium. The electrocatalytic reaction proceeds through the oxidation of chloride into hypochlorite at alkaline pH along with the formation of hydrogen from water as a subproduct.

Electrocatalytic Lignin Oxidation

“…The phenolate oxygen is a hard donor that is known to stabilize the higher oxidation state of the metal ion, whereas the imine nitrogen is a softer donor with a tendency to stabilize the lower oxidation state. 61 In 2019, Maneiro and co-workers developed a "MnP-like" catalytic system for the electrochemical oxidation of the lignin model veratryl alcohol using manganese−Schiff base complexes (Figure 8B). 61 In the paper, seven manganese−Schiff complexes with basic ligands were synthesized and evaluated.…”

“…61 In 2019, Maneiro and co-workers developed a "MnP-like" catalytic system for the electrochemical oxidation of the lignin model veratryl alcohol using manganese−Schiff base complexes (Figure 8B). 61 In the paper, seven manganese−Schiff complexes with basic ligands were synthesized and evaluated. The tetragonally elongated octahedral geometry for the manganese coordination sphere and the global μ-aquo dimeric structure were characterized by X-ray crystallography.…”

“…Manganese­(III) Schiff base complexes have been shown to be effective catalysts for the oxygenation of both saturated and unsaturated hydrocarbons. , Strongly chelating oxygen−nitrogen−nitrogen−oxygen (ONNO) Schiff base ligands are able to stabilize the manganese ion in various oxidation states since the two donor atoms of the chelating Schiff base exert opposing electronic influences. The phenolate oxygen is a hard donor that is known to stabilize the higher oxidation state of the metal ion, whereas the imine nitrogen is a softer donor with a tendency to stabilize the lower oxidation state …”

“…In 2019, Maneiro and co-workers developed a “MnP-like” catalytic system for the electrochemical oxidation of the lignin model veratryl alcohol using manganese–Schiff base complexes (Figure B) . In the paper, seven manganese–Schiff complexes with basic ligands were synthesized and evaluated.…”

High-Value Chemicals from Electrocatalytic Depolymerization of Lignin: Challenges and Opportunities

“…In another study, electrochemical oxidation of veratryl alcohol is reported when a variety of manganese(III) complexes with Schiff base ligands Mn 2 L 2 (H 2 O) 2 (N(CN) 2 ) 2 were used as electrocatalysts [ 108 ]. The effect of different parameters such as temperature, reaction pH, concentration, redox activity, turnover number of catalyst, etc., on the reaction kinetics was evaluated.…”