Lignin valorization: From molecules to materials

Lignin is a highly complex phenolic matrix that acts as a binder in plants conferring them structural integrity and strength, and is one of the three major subcomponents of lignocellulosic biomass. Although burning lignin is still considered a valuable contribution in saving fossil sources, the exploitation of this extremely abundant natural polymer in terms of higher value-added applications is very appealing as it represents the only viable source to produce aromatic compounds as fossil fuels alternative. Due to the very broad composition in terms of molecular weight of the raw material, a pretreatment strategy becomes necessary for an efficient lignin valorization as macromolecular building block for polymeric materials or as precursor for aromatic small molecules. To this end, a physical fractionation has been performed in this work, where Lignin (ProtobindTM1000) in a water/ethanol solution is subjected at first to microfiltration under vacuum in order to eliminate the insoluble residues. The permeate then undergoes a cross-flow filtration process using two subsequent membranes with cut-off of 3 kDa and 1 kDa. All the retentates and permeates have been fully characterized by GPC, GC-MS, ESI-MS, DSC, TGA and FT-IR. This procedure is an essential tool for a thorough exploitation of the main three different fractions recovered, namely a high, an intermediate and a low molecular weight fraction. The first one is characterized by the presence of high molecular weight polymers and is used without further chemical modification for developing bio-based polymeric materials; the last one can be separated by chromatography into small aromatic molecules for preparative organic chemistry; whereas the middle fraction, characterized by an intermediate molecular weight, is the ideal starting material for oxidative depolymerization assays. On this fraction, a new cascade process has been investigated involving at first a chemical/photochemical step aiming at a partial conversion of macromolecules to low molecular weight intermediates followed by a biocatalytic step performed by different classes of O2-dependent laccases (EC 1.10.3.2) in the presence of TEMPO as a mediator. Promising results have been obtained and extensive research is now in progress.