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

Lara Rouco,M Isabel Fernández-García,Alexei M Tyryshkin,Gustavo González-Riopedre,Ana M González-Noya,Marcelino Maneiro

doi:10.3390/app9163430

Lara Rouco, M Isabel Fernández-García + Show 4 more

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https://doi.org/10.3390/app9163430

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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.

Highlights

Lignin comprises polymerized phenolic and aliphatic-hydroxyl groups, being the most abundant renewable source of aromatic polymers in nature [1,2]
Manganese(III)-Schiff base complexes 1–7 were obtained in high yield as outlined in the experimental section
The conversion rates obtained for electrochemical oxidation of VA to VAH are high high considering the mild conditions used in our experiments as compared to the experimental considering the mild conditions used in our experiments as compared to the experimental setups

Summary

Introduction

Lignin comprises polymerized phenolic and aliphatic-hydroxyl groups, being the most abundant renewable source of aromatic polymers in nature [1,2]. The degradation of lignin represented a major target in the pulp industry [3]. In order for the pulp to reach an acceptable brightness level, the residual lignin after the Kraft process was bleached either to remove it from the pulp or, alternatively, to decrease absorption by the residual chromophoric groups as much as possible [4]. Of all the processes involved in the manufacturing of pulp for the paper industry, the bleaching stage to degrade lignin is the step which contributes most to environmental pollution [5,6]. Several alternative bleaching processes have been developed to date including those using chlorine-free oxidants to make this challenging process more environmentally friendly [7,8,9].

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