Reductive Electrochemical Synthesis of Polyvanillin from Biobased Vanillin

In recent years the field of biobased polymers and materials has drawn much attention due to oil depletion and environmental concerns regarding petrochemical processes. The potential of lignin, the world's second most abundant biopolymer, is sparsely valorized. Currently lignin is obtained from the Kraft-process in paper and pulp industry on a million-ton scale and is almost only used energetically providing process heat [1]. One promising valorization route is to produce vanillin from lignin, which can be obtained with appropriate yields and high selectivity via classical thermal catalytic or more recently by electrochemical oxidation [2]. Electrochemical conversion of vanillin to biobased polymers would be an attractive route enabling an overall green process. A feasibility study of the total vanillin-based polymer polyvanillin was shown in 2012 [3]. Polyvanillin was produced by electrochemical pinacolization of divanillin, which can be easily obtained enzymatically by oxidative phenol coupling of vanillin. However, to further establish a reliable electrochemical process for divanillin polymerization, a more detailed understanding of the reaction mechanism and electrochemical parameters such as current density, applied charge or electrode material on polyvanillin formation is required. As a first step, the electrochemical reduction of vanillin was investigated in an H-type cell. Different cathode materials were screened regarding their activity for aldehyde reduction and product distributions were analyzed. As a second the electrochemical polymerization of divanillin was investigated. Influence parameters such as current density and applied charge on the molecular weight distribution (MWD) were studied. Cathode materials Zn and glassy carbon (GC) were found as stable, non-toxic and cheap alternatives for the electroreduction of vanillin and divanillin with comparable activity beside the currently used toxic and unstable Pb cathodes. Pinacolization product was found as the main reaction pathway for all three electrode materials. However, a significant amount of side product formation (alcohol) was observed at high current densities for Pb and GC. Molecular weights up to 2650 g mol-1 were achieved until a final state of polymerization set in. MWD was independent on current density for Zn up to 60 mA cm-2, whereas a decrease of MWD was found for Pb and GC at these high currents.