Electro-oxidative depolymerisation of soda and acid lignins catalysed by Ni/NiOOH electrode

The electrochemical depolymerization of lignin, especially if powered by renewable electricity, is a promising technology compared to conventional chemical oxidation because it can operate under mild, safe and eco-friendly reaction conditions, such as room temperature and atmospheric pressure. Among electrochemical approaches, the electro-oxidation of lignin at the anode is the most common one studied. During the electrochemical oxidation of lignin, the surface functionalisation (α-carbonylation) and the cleavage of C–C/C–O bonds are the two main competing reactions. This study aimed to define the operating conditions for the electro-oxidative depolymerisation of soda and acid lignins into valuable monomeric compounds. The performance of nickel oxide hydroxide (Ni/NiOOH) electrode was investigated by cyclic voltammetry (CV) adopting different reactions conditions. Moreover, in order to validate the catalytic performance of the selected electrode, the cyclic voltammetry study was also performed on guaiacol, considered as a model compound of a prominent lignin structural unit. The optimal reaction conditions among those tested were then adopted in the electrolysis of soda P1000 lignin into added-value aromatic compounds. pH 14, 20 g/L lignin and 0.4 V resulted the best reaction conditions among those tested for soda and acid lignins in the CV. Adopting these parameters, the main products of the constant potential electrolysis of soda lignin were sinapic acid, acetovanillone, vanillin and vanillic acid. Both the overall oxidation degree and monomers yield was 1%. The present research aids to future research involving elucidation of solutions aiming to avoid the electrode passivation and to increase the lignin oxidation in order to raise the aromatics yield.