Photocatalytic pre-treatment for lignin enzymatic depolymerization

Homogeneous lignin fractions coming from industrial ultrafiltration processes were subjected to a photocatalytic oxidation process in order to reduce their molecular weight and produce more accessible substrates for the following enzymatic treatment. Early results indicate that due to the effectiveness of the photocatalytic step which accounts for a reduction of the mean molecular weight of treated lignins, the enzymatic production of small degradation compounds is favoured. 1. Scope The oxidation of lignin for the production of valuable chemicals is an interesting application of photocatalysis, particularly as integrated process with other oxidation technologies.1 In the present work the photocatalytic oxidation of lignin is studied as a pre-process for enzymatic depolymerization, in order to reduce the average molecular weight and enhance the performance of the enzymatic degradation. 2. Results and discussion In this work raw lignin has been physically separated by industrial sequential ultrafiltrations on different membranes in order to obtain more homogeneous starting materials for the following degradation processes. The photocatalytic oxidation of lignin was performed in aqueous basic solution using nanocrystalline ZnO supported on glass and UV-A radiation. After 28 h of photocatalytic treatment in a batch reactor lignin results partially degraded (UV spectroscopy, A365 decreased by 25%) and GPC analyses indicate a reduction of both the number-average molecular weight (Mn decreased by >20%) and the polydispersity index (PDI decreased by >30%). Early results on the enzymatic digestion of the partially degraded lignin indicate an enhanced production of small degradation compounds. 3. Conclusions These early results indicate that the photocatalytic process can be an interesting pre-treatment for lignin degradation. The assessment of the two consecutive processes, exploiting the high activity of the photocatalytic oxidation combined to the selectivity of the enzymatic treatment, will pave the way to the development of a fully integrated approach for new lignin valorization processes. Acknowledgements This work has been performed as part of the ValorPlus Project that has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no FP7-KBBE-2013-7- 613802. References 1. Shao-Hai Li, Siqi Liu, Juan Carlos Colmenares, Yi-Jun Xu, Green Chem. 2016, 18, 594-607