Depolymerizing lignin in electrochemical membrane reactors

A major fraction of today's bulk chemicals, polymers, pharmaceuticals and food additives are ultimately won from fossil oil. When it comes to the search for new technologies to create chemicals from biomass, lignin plays a major role. It consists mainly of aromatic subunits and is the only large-scale renewable source for aromatic compounds. In order to access the lignin fraction of wood, fractionation technologies have to be applied. The solvent that dissolves lignin during fractionation is seldom suited for a subsequent treatment of the lignin. Thus, lignin needs to be transferred from the fractionation liquid into a solvent suited for depolymerization. To this end a promising liquid/liquid extraction to realize this solvent exchange was developed. Once lignin is dissolved in the valorization medium, it can be electrochemically depolymerized by oxidation. To this end, the electrochemical depolymerization of lignin was investigated in terms of the fundamental interrelation between selected process parameters and several physico-chemical properties of lignin and its depolymerization products. In order to intensify this process, the use of different three-dimensional electrodes in an electrochemical membrane reactor for the continuous depolymerization of lignin was shown. To prevent depolymerization products from overreacting, the electrochemical reaction was combined with a membrane separation. The combined application of electrochemical principles, alkaline solvents and membrane technology in the form of electrochemical membrane reactors enabled the generation of low-molecular-weight compounds from lignin.