Lignin is a group of cell wall localised heterophenolic polymers varying in the chemistry of the aromatic and aliphatic parts of its units. The lignin residues common to all vascular plants have an aromatic ring with one para hydroxy group and one meta methoxy group, also called guaiacyl (G). The terminal function of the aliphatic part of these G units, however, varies from alcohols, which are generally abundant, to aldehydes, which represent a smaller proportion of lignin monomers. The proportions of aldehyde to alcohol G units in lignin are, nevertheless, precisely controlled to respond to environmental and development cues. These G aldehyde to alcohol unit proportions differ between each cell wall layer of each cell type to fine-tune the cell wall biomechanical and physico-chemical properties. To precisely determine changes in lignin composition, we, herein, describe the various methods to detect and quantify the levels and positions of G aldehyde units, also called coniferaldehyde residues, of lignin polymers in ground plant samples as well as in situ in histological cross-sections.

Maximizing products of high value and minimizing incineration of side-streams is key to realize future biorefineries. In current textile production from forestry, hemicellulose is removed by prehydrolysis before delignification. The resulting prehydrolysis liquor is incinerated in the recovery boiler at low efficiency. This additional burden on the limiting recovery boiler reduces the pulp production. In this study, we demonstrate that prehydrolysis liquor can be upgraded, in 5 steps, to yield aviation fuels. Prehydrolysis liquors were dehydrated to furfural by zeolite catalysis. Furfural was selectively reduced to furfuryl alcohol by Au@NC. Rhenium-catalysed Achmatowicz rearrangement gave a C₅ intermediate susceptible to self [2 + 2] cycloaddition to give the C₁₀ oxygenated precursor. By using a combination of Ru/C and zeolites, full hydrodeoxygenation was achieved. The overall transformation from furfural to hydrocarbons resulted in a 48% carbon yield. The resulting hydrocarbons, containing an anticipated strained four-membered ring, are preferred aviation fuel components. This is an important step to show that aviation fuels can be produced sustainably from existing industrial side-streams. A comparative life cycle assessment was applied to evaluate the environmental impact of the proposed valorization approach, demonstrating benefits in the climate change impact category when implementing this technology in a pulp mill compared to the incineration of pre-hydrolysis liquor scenario.

Here, we show that lignin-first biorefining of poplar can enable the production of dissolving cellulose pulp that can produce regenerated cellulose, which could substitute cotton. These results in turn indicate that agricultural land dedicated to cotton could be reclaimed for food production by extending poplar plantations to produce textile fibers. Based on climate-adapted poplar clones capable of growth on marginal lands in the Nordic region, we estimate an environmentally sustainable annual biomass production of ∼11 tonnes/ha. At scale, lignin-first biorefining of this poplar could annually generate 2.4 tonnes/ha of dissolving pulp for textiles and 1.1 m³ biofuels. Life cycle assessment indicates that, relative to cotton production, this approach could substantially reduce water consumption and identifies certain areas for further improvement. Overall, this work highlights a new value chain to reduce the environmental footprint of textiles, chemicals, and biofuels while enabling land reclamation and water savings from cotton back to food production.

Coconut Coir Pith (CCP) is a relatively unexplored type of lignocellulosic waste from the coconut industry. As a feedstock that is highly enriched in lignin (Klason lignin content of 40.9 wt % found in this study), CCP is a potential source for renewable lignin-derived materials. We have performed a systematic study on the characterization and valorization of lignin from CCP. We have investigated two different valorization approaches: reductive catalytic fractionation (RCF) and soda pulping followed by catalytic hydrodeoxygenation. During RCF, the lignin was converted into monomeric products in 7.6 wt %. Using soda pulping conditions, we were able to isolate lignin from CCP in 74% yield. Subsequent hydrotreatment of the lignin over a Pt/MoO3/TiO2catalyst resulted in the formation of hydrogenated oil in 43 wt % yield, suitable for the production of biobased diesel fuels and lubricant base oils. 

Lignin is an aromatic polymer that constitutes up to 30 wt% of woody biomass and is considered the largest source of renewable aromatics. Valorization of the lignin stream is pivotal for making biorefining sustainable. Monomeric units in lignin are bound via C–O and C–C bonds. The majority of existing methods for the production of valuable compounds from lignin are based on the depolymerization of lignin via cleavage of relatively labile C–O bonds within lignin structure, which leads to yields of only 36–40 wt%. The remaining fraction (60 wt%) is a complex mixture of high-molecular-weight lignin, generally left unvalorized. Here we present a method to produce additional valuable monomers from the high-molecular-weight lignin fraction through oxidative C–C bond cleavage. This oxidation reaction proceeds with a high selectivity to give 2,6-dimethoxybenzoquinone (DMBQ) from high-molecular-weight lignin in 18 wt% yield, thus increasing the yield of monomers by 32%. This is an important step to make biorefining competitive with petroleum-based refineries.

Current pulping technologies only valorize the cellulosic fiber giving total yields from biomass below 50 %. Catalytic fractionation enables valorization of both cellulose, lignin, and, optionally, also the hemicellulose. The process consists of two operations occurring in one pot: (1) solvolysis to separate lignin and hemicellulose from cellulose, and (2) transition metal catalyzed reactions to depolymerize lignin and to stabilized monophenolic products. In this article, new insights into the roles of the solvolysis step as well as the operation of the transition metal catalyst are given. By separating the solvolysis and transition metal catalyzed hydrogen transfer reactions in space and time by applying a flow-through set-up, we have been able to study the solvolysis and transition metal catalyzed reactions separately. Interestingly, the solvolysis generates a high amount of monophenolic compounds by pealing off the end groups from the lignin polymer and the main role of the transition metal catalyst is to stabilize these monomers by transfer hydrogenation/hydrogenolysis reactions. The experimental data from the transition metal catalyzed transfer hydrogenation/hydrogenolysis reactions was supported by molecular dynamics simulations using ReaXFF.

Increases in coniferaldehyde content, a minor lignin residue, significantly improves the sustainable use of plant biomass for feed, pulping, and biorefinery without affecting plant growth and yields. Herein, different analytical methods are compared and validated to distinguish coniferaldehyde from other lignin residues. It is shown that specific genetic pathways regulate amount, linkage, and position of coniferaldehyde within the lignin polymer for each cell type. This specific cellular regulation offers new possibilities for designing plant lignin for novel and targeted industrial uses.

This thesis is focused on the development of strategies for lignocellulosic biomass valorization. The thesis consists of two parts.

The first part of the presented work is related to the catalytic fractionation of biomass (lignin-first approach) and the production of monomeric compounds from lignocellulose. In the first project (Chapter 2) we have established a process to study the transformations occurring during the catalytic organosolv pulping of wood in the presence of Pd/C. This was achieved by performing a fractionation under continuous-flow conditions. In the designed process, the pulping and the transition metal catalyzed reactions were separated in space and time. Thus, the role of the solvolysis and the transfer hydrogenation reactions were studied independently. We discovered that during the solvolysis of wood, a substantial amount of monomeric lignin fragments are released into the solution. The main role of the catalyst is to stabilize these monomers and prevent their repolymerization. Based on the obtained knowledge we developed a new version of the lignin-first approach (Chapter 3). In this process zeolites were used as shape-selective catalysts. We have demonstrated that by tuning the size of pores of the catalyst the undesirable bimolecular reactions can be minimized. Furthermore, the released monomers can be converted into stable products via transfer hydrogenation reactions.  

The second part is related to studies of dimeric and trimeric lignin model compounds. In Chapter 4, the reactivity of the dibenzodioxocin motif, which is considered a main branching point in the lignin structure has been investigated. We have designed a protocol for the catalytic reductive cleavage of lignin model compounds representing this motif, in the presence of Pd/C and benign hydride donors. The cleavage of the dibenzodioxocin structure results in the formation of dimeric biaryl compounds. Unlike monomers, the valorization of lignin-derived dimers is less studied. The last chapter is focused on the transformation of biaryls into highly functionalized synthetic building blocks. This was achieved via a visible light induced dearomative spirolactonization of biaryl carboxylic acids. The synthetic value of the obtained products was demonstrated by the conversion of the products into more complex structures.

Organosolv pulping releases reactive monomers from both lignin and hemicellulose by the cleavage of weak C-O bonds. These monomers recombine to form undesired polymers through the formation of recalcitrant C-C bonds. Different strategies have been developed to prevent this process by stabilizing the reactive monomers (i.e., lignin-first approaches). To date, all reported approaches rely on the addition of capping agents or metal-catalyzed stabilization reactions, which usually require high pressures of hydrogen gas. Herein, a metal- and additive-free approach is reported that uses zeolites as acid catalysts to convert the reactive monomers into more stable derivatives under organosolv pulping conditions. Experiments with model lignin compounds showed that the recondensation of aldehydes and allylic alcohols produced by the cleavage of beta-O-4 ' bonds was efficiently inhibited by the use of protonic beta zeolite. By applying a zeolite with a preferred pore size, the bimolecular reactions of reactive monomers were effectively inhibited, resulting in stable and valuable monophenolics. The developed methodology was further extended to birch wood to yield monophenolic compounds and value-added products from carbohydrates.

The idea of using biaryl structures to generate synthetic building blocks such as spirolactones is attractive because biaryl structures are abundant in biomass waste streams. However, the inertness of aromatic rings of biaryls makes it challenging to transform them into functionalized structures. In this work, we developed photoinduced dearomatization of nonphenolic biaryl compounds to generate spirolactones. We demonstrate that dearomatization can be performed via either aerobic photocatalysis or anaerobic photooxidation to tolerate specific synthetic conditions. In both pathways, dearomatization is induced by electrophilic attack of the carboxyl radical. The resulting spirodiene radical is captured by either oxygen or water in aerobic and anaerobic systems, respectively, to generate the spirodienone. These methods represent novel routes to synthesize spirolactones from the biaryl motif.