Electrochemical Polymer and Polymer Additive Synthesis from Biobased Vanillin and Syringaldehyde
Authors: Kunkel Robin , Müller Dominik , Schmiedl Detlef , Cremers Carsten , Tübke Jens 
Published: May 2020

In recent years the field of biobased polymers and materials has drawn much attention due to oil depletion and environmental concerns regarding petrochemical processes [1, 2]. Lignin is the world’s second most abundant renewable biopolymer. In comparison to cellulose, the most abundant biopolymer, the potential of lignin is sparsely valorized. Currently, lignin is considered as a waste stream in the pulp and paper industry and is used almost only energetically on a 107 ton per year scale [3, 4]. However, recent findings showed that lignin can be electrochemically depolymerized selectively to the monomeric compounds vanillin and syringaldehyde [5]. Due to lignin’s abundance and good mechanical as well as thermal behavior of the depolymerization products, they have the potential to become key intermediates in the field of high performance biobased polymers and polymer additives [6].
Cathodic pinacolization of these monomers offers a green and CO2-neutral pathway for controlled molecular weight increase, leading to attractive bi- or multifunctional phenolic compounds for polymeric applications [7]. Feasibility studies of the electrochemical synthesis of hydrovanilloin [8] and polyvanillin [9] at lead electrodes by pinacolization of vanillin and divanillin showed already promising results. Herein, we investigated the cathodic pinacolization of the lignin monomers vanillin and syringaldehyde as well as polyvanillin synthesis from divanillin in detail.
First, the electrochemical hydrodimerization of vanillin and syringaldehyde (equation 1) in alkaline aqueous solution was studied in an H-type cell investigating electrode material, current density and applied charge. The electrochemical reduction of vanillin was monitored by capillary electrophoresis with diode array detection (CE-DAD). The electrode materials lead, zinc and glassy carbon were investigated. We found that zinc can replace the currently used toxic electrode material lead with almost same reaction rates, product distribution and yields up to 72 % of isolated dimer. Electrode degradation was found for lead, whereas zinc and glassy carbon remained stable. The pinacolization product hydrovanilloin was found as the main product for all three electrode materials at current densities of 30 mA cm-2 in alkaline aqueous solution. However, also significant amount of vanillyl alcohol production was found at glassy carbon at 30 mA cm-2 increasing with higher current densities. Only a slight increase of vanillyl alcohol production was found at lead at higher current densities and no increase was found at zinc. We assume cathodic depolarization at zinc and lead electrodes by the competing hydrogen evolution reaction, hindering the potential to drop lower and thus suppressing vanillyl alcohol production. For syringaldehyde equal yields of 72 % were obtained for the dimeric pinacolization product 1,2-Bis(4-hydroxy-3,5-dimethoxyphenyl)glykol. However, the reaction temperature had to be increased from room temperature to 60 °C due to solubility reasons. Solid products were characterized via several methods, e.g. HPLC-DAD-MS, FT-IR, 1H-NMR and 13C-NMR.
Further, polyvanillin was electrochemically synthesized at zinc electrodes by pinacolization of divanillin, which had been produced by enzymatic conversion of vanillin with horse-radish peroxidase (equation 2). The degree of polymerization of the isolated biopolymer was monitored via gel permeation chromatography (GPC). Weight average molecular weights up to 3292 g mol-1 were achieved. Changing the current densities for the polymerization showed no difference in the molecular weight distribution upon full divanillin conversion. The polymer was further characterized by several methods, e.g. FT-IR, TGA and DSC, confirming its structure and good thermal behavior.
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