Biobased Polymer Synthesis from Vanillin in an Electrochemical Flow Reactor

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The development of new synthesis routes for the production of biobased polymers is an urgent matter in the context of climate change. One possible promising key intermediate is vanillin (4-hydroxy-3-methoxybenzaldehyde), as it is one of a few aromatic biobased compounds, which is already available on an industrial scale. Vanillin can be produced sustainably with potential yields of up to 7% from lignin, a side product in the paper and pulp industry with amounts of over 107 tons per year. Therefore, much effort was spent from the scientific community for the preparation of a broad range of new vanillin-based polymers [1].One attractive synthesis route is the electrochemical reductive production of polyvanillin, a total vanillin-based polymer. Polyvanillin is synthesized by electrochemical pinacolization of divanillin (5-5´bisvanillyl), which is easily available by enzymatic conversion of vanillin with horseradish peroxidase and hydrogen peroxide (Scheme 1). Recent studies from others [2] as well as from our working group [3] showed the feasibility of this synthesis route and investigated the impact of electrochemical reaction parameters in batch cells.As a next step, we here transferred the electrochemical reduction of divanillin to polyvanillin from an electrochemical batch reactor into a divided plan parallel flow reactor in recycle mode. The reductive conversion of the aldehyde groups was monitored by an on-line UV-Vis setup and the molecular weight increase was followed by size exclusion chromatography. Further, 2D-NMR (HSQC, 1H/13C) was used to identify structural features of the produced polyvanillin samples. The impacts of different electrochemical impact parameters, such as applied charge, current density, flow velocity and concentration were investigated. Current densities of over 50 mA cm-2 were readily achievable and weight-averaged molecular weights up to 4300 g mol-1 were reached.This study shows an innovative electrochemical approach for the synthesis of new biobased polymers in an electrochemical flow reactor offering a total green process.