Reductive Catalytic Fractionation of lignocellulose: insights in the lignin fraction

Our contemporary society highly depends on fossil raw materials that provide us with the necessary carbon skeleton for energy and chemicals production. In the decades to come, our World faces the enormous challenge of making a much-needed transition that minimizes the use of these fossil resources, due to the negative sustainability and climate consequences associated with their consumption. For the production of many chemicals, this means that a transition must be made to a sustainable carbon chemistry, in which the use of renewable carbon sources such as lignocellulose plays an essential role. Lignocellulose is the structural element in the cell wall of a plant, such as grass and wood, and is therefore the most common form of plant biomass on Earth. It is mainly composed of three components, namely the polysaccharides cellulose and hemicellulose and the aromatic polymer lignin. These three components are highly intertwined into a complex matrix in the plant cell wall. Fractionation of lignocellulose in a biorefinery, whereby the various components are separated from each other effectively reducing its complexity, is frequently employed to enable its valorisation. Eventually, this results in separate product flows. A lignocellulose biorefinery is, for example, used in industry to produce paper or 2nd generation bio-ethanol from the (hemi)cellulose fraction. However, the resulting lignin fraction has undergone major structural changes in these industrial biorefineries, hampering its valorisation into a variety of chemicals or materials. Consequently, this lignin fraction is often burned to recover energy. As a result, the full potential of lignin - nature's most abundant source of aromatic molecules - is not exploited. To tackle this problem, lignin-first biorefinery strategies have been developed. Central in these strategies is the active stabilization of lignin fragments, impeding structural changes that negatively impact the valorisation potential of lignin. One of the promising biorefinery strategies applying this lignin-first strategy is the 'Reductive Catalytic Fractionation' (RCF). During RCF, lignocellulose is contacted with an organic solvent, typically an alcohol, at a high temperature (150 - 250 °C) in presence of a heterogeneous redox catalyst in a reducing environment. As a result, lignin is solvolytically extracted from lignocellulose, depolymerized and rapidly stabilized by means of hydrogenation. The resulting lignin fraction typically has a relatively low molecular weight, and consists of phenolic monomers and oligomers. In addition a carbohydrate pulp is obtained that can be further converted into bio-ethanol or many other chemicals. Despite the great potential of this technology, there are still many challenges and unknowns. Little is known about the molecular structure of the phenolic oligomers in RCF lignin, how their structure relates to the structure of the in-planta lignin, and how the lignin structure varies depending on the process conditions used. Deeper insight in these allows better fundamental understanding of RCF and enables to steer the fractionation towards application-dependent lignin products. The first goal of this PhD dissertation was to gain more insight into the structure of the entire RCF lignin oil, including that of the lignin oligomers. By separating the lignin oil into different fractions and using a variety of chromatographic and NMR spectroscopic analytical techniques, a large part of the molecular structure of RCF lignin oligomers was unravelled. It was shown by NMR spectroscopy that the native inter-phenolic lignin linkages are converted to structurally related RCF molecular structures, which were found across fractions of varying molecular weight. In a next step, it was shown by GC x GC analysis that these unique RCF molecular structures are also found in individual RCF lignin dimers and trimers. Combination of the detailed molecular information of GC x GC and the bulk information from NMR spectroscopy, revealed that the structural complexity of RCF lignin oligomers is relatively low and that the inter-phenolic C-C linkages in RCF lignin oligomers originate from the native lignin structure. Subsequently, the formation of RCF lignin structures from native lignin was investigated, as well as the impact of different process conditions such as time and catalyst type on the resulting lignin structure. Mechanistic insights were obtained, which led to the establishment of reaction mechanisms for the conversion of most of the inter-phenolic lignin linkages. Furthermore, it was shown that the conversion of these inter-phenolic lignin linkages strongly depends on the applied process conditions. For example, it was proven that the redox catalyst plays an important role in depolymerizing the extracted lignin. A proper RCF catalyst should not only have a hydrogenation activity to stabilize the depolymerized fragments (e.g. Ni), but should be able to hydrogenolyse the ether linkages, viz. β‑O‑4, between the individual phenolic molecules (e.g. Pd or Ru). Furthermore, it was shown that the physicochemical properties impacting the ultimate valorisation potential of RCF lignin, such as the molecular weight and hydroxyl content, are directly related to the obtained molecular composition. Hence, choosing the right process conditions and redox catalyst is therefore crucial and allows to tune the RCF lignin properties. Finally, a proof of concept of a full lignocellulose-to-chemicals valorisation chain was established, wherein it was demonstrated that RCF lignin oligomers can be used as a precursor in the synthesis of bio-based epoxy resins, because of its favourable high phenolic content and relatively low molecular weight.

Publication year:2021

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