Reductive processes stabilise lignin fragments released from lignocellulosic materials. As a result, the fate of lignin changes dramatically. Unlike organosolv processes, which isolate lignin polymers, RCF obtains lignin oil containing a substantial fraction of ‘monomer products’ (i.e. monoaromatic phenolics). They are produced at high individual yields as primary products. We developed a method for lab-scale solvent fractionation of RCF lignin oil using small volumes of environmentally benign solvents to produce multigram lignin fractions comprising products in different molecular weight ranges. This allowed us to determine the structural heterogeneity of lignin products across the entire molecular weight distribution of the RCF lignin oil.
The detailed characterisation of the fractions by HSQC NMR spectroscopy demonstrated the formation of reduced β-O-4 linkages presenting a methylene group at the Cα position. It is accepted that the stabilisation processes primarily occur on lignin monomer intermediates. Although this claim holds, our findings indicated that, via the formation of reduced β-O-4 linkages, lignin stabilisation happens to varying degrees in the entire population of lignin fragments.
We also examined the RCF process from the pulp perspective. We found that the hydrogenation catalyst also plays a crucial role in stabilising monosaccharides and oligosaccharides formed by the hydrolysis of hemicelluloses (pentoses and hexoses). In this context, we have demonstrated that stabilising sugars in the RCF liquor reduces the formation of formic acid in the lignin-first biorefining. We found that the low levels of formic acid were attributed to the preferential hydrogenation of sugars into sugar alcohols in the presence of the Raney Ni. This process prevents sugar degradation and, therefore, hinders the formation of formic acid. As a result, cellulosic pulps with a high degree of depolymerisation are obtained.
From the technical side, developing separation strategies to fractionate the lignin streams by molecular weight ranges constitutes a timely challenge to tackle. We developed a membrane filtration process to refine lignin streams obtained from the H-transfer RCF process. A two-stage membrane cascade achieved the separation and concentration of a fraction rich in monomer products. In our proof-of-concept, we presented a detailed analysis of future developments in the performance required for membrane filtration for lignin refining and, more aspiringly, solvent reclamation.
Considering the reaction engineering of the interdependent processing steps for fractionating the initial biomass feedstock to the intended value-added products, we demonstrated that RCF lignin oils could be upgraded by hydrotreatment in the presence of a phosphidated Ni/silica catalyst under hydrogen pressure. The hydrotreatment of RCF lignin oils rendered two distinct main cuts of branched hydrocarbons (gasoline and diesel). This makes the concept plausible for producing synthetic aviation fuels.
