Tree bark in the biorefinery: implementing lignin-first principles

Shifting away from fossil resources constitutes an enormous challenge for our society. For energy purposes, alternatives like wind and solar power are becoming increasingly attractive options. For organic chemicals, materials and fuels, biomass could provide a renewable alternative. In this context, biorefineries are being developed, aimed at converting biomass ultimately into chemicals, materials and fuels. Lignocellulose, the most abundant form of biomass on earth, is an interesting feedstock for a biorefinery. Lignocellulose is the structural element of plant cell walls and consists of three biopolymers, viz. cellulose, hemicellulose and lignin. It is ubiquitous in the plant kingdom (e.g. wood, grasses, shrubs) and also present in residues from agriculture or forestry. As these residues are abundantly available at low cost, they constitute a favorable feedstock for a biorefinery. Tree bark is a major waste stream from wood processing industries, and a source of lignocellulose biomass. Currently, bark is mainly burned for energy recuperation or used as mulch in horticulture. Valorizing bark in a biorefinery thus presents an alluring possibility. In this respect, decent knowledge of the bark feedstock is indispensable. For that reason, the barks of six relevant species, viz. poplar, black locust, red oak, willow, Corsican pine and larch, were thoroughly characterized in this dissertation. Anatomical analysis illustrated the structural heterogeneity and various cell types in the different barks. Chemical compositional analyses highlighted the large differences between species. All barks had a substantial lignocellulose content, however, the fraction of (hemi)cellulosic carbohydrates was low (35-44 wt%). The lignin content was rather high (22-45 wt%), and it was found to have a low S/G ratio for hardwood barks (0.3-0.7), and a G type lignin for softwood barks. The fraction of suberin, an aliphatic polyester, was highest is black locust bark (10 wt%). Besides structural components, the studied barks had a high extractives content (14-30 wt%). As barks generally have a smaller carbohydrate fraction, typical carbohydrate-oriented lignocellulose biorefining strategies (e.g. second generation ethanol, pulp and paper) are less suited for barks. Such lignocellulose biorefining strategies aim at removing lignin as efficiently as possible. However, as lignin is prone to undergo repolymerization reactions, the resulting isolated lignin is highly condensed and unreactive, and thus less suited for further upgrading. To tackle this, lignin-first biorefining strategies that focus on lignin conversion prior to the carbohydrate fraction, have been developed. Such lignin-oriented lignocellulose biorefining could prove very useful for bark valorization, given their typically higher lignin content. One promising example of a lignin-first biorefining strategy is the 'Reductive Catalytic Fractionation' (RCF). In an RCF process, lignocellulose is contacted with an organic solvent (mixture), like methanol, at elevated temperature in presence of an heterogeneous redox catalyst (e.g. Pd/C, Ru/C) in a reducing environment. This effectuates the solvolytic extraction of lignin fragments from the lignocellulose matrix, its further depolymerization and, at the same time, chemical stabilization of the formed intermediates through reductive catalysis. The role of the metal catalyst is hereby crucial, as the reduction of lignin fragments effectively lowers their reactivity towards repolymerization reactions. The outcome of the RCF strategy is thus a low molecular weight lignin oil, amenable for subsequent conversion into chemicals, and a (hemi)cellulose pulp suited for further valorization. In contrast to this reductive strategy (i.e. RCF), also a review of the literature on oxidative lignin conversion was provided. Such oxidative pathways can provide an interesting tool for the formation of highly functionalized, valuable lignin products. The catalytic systems for the oxidative conversion of dimeric lignin model compounds and isolated lignins were summarized and critically discussed. Next, several challenges regarding the substrate, catalyst and operating conditions were highlighted, and a future perspective on lignin oxidation was offered. Finally, comparing reductive and oxidative lignin-first processing illustrated the advantages of the reductive process (i.e. RCF) for bark biorefining. In this dissertation, an evaluation was made of the RCF strategy when using bark as feedstock. First, the bark of black locust (Robinia pseudoacacia) was studied. Given the substantial suberin content in this bark, focus was put on both the lignin and the suberin fraction: RCF enabled the extraction and depolymerization of both biopolymers. The thus obtained oil phase contained lignin-derived phenolic mono-, di- and oligomers, as well as suberin-derived, long-chain (bifunctional) aliphatic monomers. The process severity (i.e. temperature and reaction time) was found to govern the extend of both suberin and lignin depolymerization. Parameters involving catalytic hydrogenation (i.e. catalyst type and loading, H2 pressure) did not influence the suberin depolymerization, but affected the stabilization of the lignin fragments and consequently the phenolic monomer yield. Comparing RCF of black locust bark with black locust wood, revealed the resistance to delignification and the lesser extent of lignin depolymerization in bark. Next, the substrate scope for the RCF biorefinery was expanded by evaluating the barks of ten different species under identical RCF conditions. By using both raw and extractive-free barks, it was found that the extractives minimally affect the lignin conversion. Between different species however, the product outcome varied strongly. The lignin monomer yield ranged from 2 to 19 wt% lignin, with a high selectivity towards 4-n-propanol-subtituted monomers. Next to lignin products, also catechols, resorcinols, pyrogallols and ring-opened flavonoids were observed, likely from the depolymerization of condensed tannins present in barks. Interestingly, the redox catalyst was found to be crucial not only for generating lignin monomers, but also for certain condensed tannins products, notably the ring-opened flavonoids.

Jaar van publicatie:2020

Toegankelijkheid:Open