Lignocellulosic biomass is a sustainable source of renewable carbon, and it is the most abundant form of terrestrial biomass. The three main constituents of lignocellulosic biomass are cellulose, hemicellulose, and lignin. The monomers from these biopolymers can be valuable feedstocks for a future sustainable chemical industry, and they include glucose from cellulose, predominantly xylose from hemicellulose, and aromatic molecules from lignin. Even though both glucose and xylose are already established feedstocks for further upgrading in biorefineries, the upgrading of lignin to higher value-added chemicals has not achieved the same success, despite the tremendous need for renewable aromatic molecules.
Developing methods for the high-yield depolymerization of lignin can dramatically increase the efficiency and profitability of biorefineries. The main limiting factor in producing high-quality lignin is the inter-unit carbon-carbon linkages in the chemical structure of lignin, and also the formation of these bonds during the extraction process, known as repolymerization or condensation. In the first part of my thesis, I study the possibility of avoiding the condensation of lignin by preventing the formation of C-C linkages. This is achieved by adding formaldehyde, as a protection group, to the pretreatment of lignocellulosic biomass. The results show a near theoretical yield of monomers from the stabilized lignin after depolymerization by hydrogenolysis. This method was then further expanded by screening other protecting groups in order to develop a comprehensive picture of the chemistry involved in this process, and to optimize the process notably to be able to yield a narrower product distribution.
Even though this method provides high-quality lignin, there is a need for developing a fractionation process for large-scale production of isolated lignin. This can facilitate the implementation of the process in biorefineries and also further studies on developing new upgrading pathways. The second part of this thesis presents a protocol that fractionates lignocellulosic biomass into pure parts of three main constituents in large laboratory-scale with high extraction yields. The results show extraction yields higher than 95% for each biopolymer. The highly digestible cellulose fraction and high-quality lignin can be depolymerized into their constituent monomers with conventional methods. However, the hemicellulose fraction is obtained as functionalized sugars due to its reaction with aldehyde during pretreatment.
The third part of my work presents a non-destructive method for the prediction of the potential yield of monomers from the depolymerization of isolated lignin. A novel 2D heteronuclear single quantum coherence nuclear magnetic resonance (2D-HSQC NMR) spectroscopy technique can be used for simultaneous identification and quantification of chemical groups in the structure of lignin. Standard 2D-HSQC NMR spectroscopy does not provide accurate quantification for polymers and oligomers such as lignin due to errors that are principally caused by differences in relaxation times for different parts of the oligomer chain. However, these errors can be avoided by extrapolation to time-zero 13C HSQC (HSQC0) for series of spectrums, acquired with different repetition times. The prediction results show agreement within a few percentage points with experimental results determined by gas chromatography (GC) after hydrogenolysis.