New tools in lignocellulosic chemistry

The PhD project is inserted within the broad field of lignocellulose chemical characterization and comprehensive utilization, as promoted by the biorefinery approach. Lignocellulose is an extremely structurated natural material made up of three main biopolymers: cellulose, hemicellulose, and lignin. Cellulose consists of linear chains of beta(1-4) linked D-glucopyranose units which, when found in cell wall, is difficult to break down into glucose because of its extensive inter- and intra-molecular H-bonded network and highly organized crystalline structure. Hemicellulose is a carbohydrate heteropolymer composed of several different sugars including five-carbon and six-carbon which is easily broken down into its building blocks. Lignin is a complex and irregular polymer network, composed of randomly cross-linked phenylpropanoid units, and acts as a glue holding cellulose and hemicellulose together. The biorefinery concept is analogous to today’s petroleum refinery that produces multiple fuels, power and chemical products from petroleum. Biorefinery systems generally work by processing a bio-based feedstock input to create fuel, chemicals, feed or power/heat as an output. Lignocellulose biorefinery generally includes three fundamental steps: first, a pretreatment to fractionate the recalcitrant lignocellulose structure; secondly, an enzymatic hydrolysis of the isolated cellulose moiety to obtain fermentable sugars; and third, the fermentation, to produce cellulosic ethanol or other bio-based chemicals. Because of the resistant structure of crystalline cellulose and natural composite structures of lignocellulosics, efficient pretreatment technologies are needed prior to the enzymatic hydrolysis. The PhD project find its common thread in the development and application of an integrated analysis protocol that encloses different chromatographic and spectroscopic techniques, and the utilization of innovative solvent media (i.e., ionic liquids) for the functionalization and subsequent chemical analysis of otherwise undetectable substrates such as unprocessed, native lignocellulose. This novel approach requires mild reaction conditions for the derivatization reactions and leaves an overall unaltered substrate, thus avoiding any chemical and structural modification due to components extraction. The only harsh pretreatment required is several hours of milling, needed to reduce the particle size and cellulose crystallinity to help the ionic liquid to diffuse into the interior of the lignocellulose. Summarizing, the PhD thesis is based on the following topics: i. Exhaustive chemical characterization of lignins extracted from different woody and herbaceous materials by chromatographic and spectroscopic analysis. ii. Characterization of unprocessed lignocellulose substrates exploiting the striking solubilizing power of ionic liquids to obtain derivatized specimens subjectable to chromatographic and spectroscopic analysis. iii. Detection of lignin-carbohydrate complexes in native herbaceous plants, again accomplished by solubilization in ionic liquid, appropriate functionalization, and subsequent chromatographic analysis. iv. Optimization of the lignin extraction procedure from husk and its functionalization for the production of value-added fillers addressed to the preparation of novel biocomposites. v. Thermal, morphological and structural characterization of poly(3-hydroxybutyrate)-based biocomposites prepared by casting from chloroform solution of the polymer and different functionalized lignins. vi. Assessment of the state of conservation of archaeological woods excavated from underwater shipwrecks. During the PhD research, these different aspects were compenetrated and organized into three different projects, namely: 1. Annual plants: characterization and lignin-carbohydrate complexes detection. 2. Rice husk lignin recovery and its effect as a filler in the thermal behaviour of poly(3-hydroxybutyrate)-based biocomposites. 3. Archaeological waterlogged woods characterization. The second project has been developed within a joined research activity of the University of Milano-Bicocca, the Italian Pulp and Paper Research Institute (SCCP, Milan) and ISMAC CNR (Milan).