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The Lignin-First Approach for the Full Valorisation of Lignocellulosic Biomass

Periodic Reporting for period 4 - LIGNINFIRST (The Lignin-First Approach for the Full Valorisation of Lignocellulosic Biomass)

Berichtszeitraum: 2021-09-01 bis 2022-08-31

Transitioning from a fossil-based production system to an environmentally sustainable economy is among the most significant problems in the coming years. Lignocellulosic biomass is one of the most abundant resources found on Earth. It is made up of hemicellulose, cellulose, and lignin. Wood is a lignocellulosic biomass extensively utilised in pulp and paper production and in the construction industry sector. Lignocellulosic biomass is an essential resource for the emerging sustainable bioeconomy.

In everything from sustainable aviation fuels to chemicals, the efficient use of lignocellulosic biomass will reduce carbon dioxide emissions and help combat climate change. To meet goals of the 2030 UN Sustainability Goals, the utilisation of lignocellulosic waste, which is any organic material that is not the principal material that the plants were initially cultivated for, to produce biofuels and chemicals is vital to increase yields of production per hectare. This approach reduces the requirement for additional agricultural lands.

However, the lack of efficient processes for deconstructing the intricate lignocellulose structure hinders the use of lignocellulosic biomass as a circular carbon source for the bioeconomy. For instance, existing methods require multiple stages to transform cellulose into ethanol. They cannot utilise lignin which is 20-30% of biomass composition. So, a biorefinery based on cellulose cannot be considered optimal for how efficiently it uses resources.

The main goal of the LIGNINFIRST research program is to improve the science behind Lignin-First Biorefining (LFB), a unique platform for the integral utilisation of all components of lignocellulosic biomass. We want to understand the interplay between the deconstruction of lignocellulosic biomass by solvent extraction of lignin and the early-stage catalytic conversion of the extracted lignin in the RCF process.

LFB is not a synonym for lignin valorisation. It is a holistic strategy for an atom-efficient, more sustainable utilisation of the lignocellulosic biomass. LFB breaks down lignocellulose through the solvent removal of lignin and hemicellulose to a lesser extent. It produces lignin oils and cellulosic pulps. The process occurs in the presence of a catalyst for stabilising lignin and carbohydrate products. LFB obtains lignin as easy-to-convert products with high molecular homogeneity, obtained without downgrading cellulose pulp quality.

The LIGNINFIRST research program focuses on understanding the mechanism underlying the removal of lignin and hemicellulose from lignocellulosic materials in addition to the catalytic reactions occurring on these components. These reactions are vital to transforming the LFB fractions into sustainable transport fuels and chemicals via catalysis. The fundamental research of LIGNINFIRST helps to improve its value from lignocellulose extracted from crops and wood residues.
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.
The progress obtained from the LIGNINFIRST research programme represented a change in fundamental assumptions. The main question is no longer what lignin is, but which fraction of lignin is the most suitable for an application. Most of the research efforts put into RCF process optimisation have focused on maximising the yield of monomer products and controlling the degree of functionalisation of their alkyl sidechains. We found that the stabilisation imparted by reductive processes preserves the structure of high molecular weight lignin in a more native-like architectural state. Consequently, heterogeneous catalysis can be crucial in generating mid- and high-MW lignin products with controllable structural properties. Such control has thus far been only possible through the rational bioengineering of lignin in planta. However, upon lignin extraction, such precious structural features inserted by bioengineering are often lost. Therefore, the evolution of the RCF concept toward the production of lignin polymer products of controllable structural properties presents itself as an attractive new option in the lignin-first concept.