Author:

Abstract:

Our current economy mainly relies on fossil-based chemicals, materials and fuels to cover the needs of our affluent society. However, the exploitation of fossil resources is inextricably linked to environmental concerns, global warming and geopolitics. Shifting towards a renewable carbon feedstock is highly desirable, yet very challenging. In particular lignocellulosic biomass has great potential as renewable feedstock for the production of organic chemicals. Lignocellulose, originating from photosynthetic CO2 captured by plants, compromises three biopolymers, namely cellulose, hemicellulose and lignin. While the first two constituents are sugar-based, lignin is a complex aromatic polymer composed of phenylpropanoids, making it the most abundantly available renewable source of aromatics. Deconstruction of lignocellulosic biomass proceeds in a biorefinery, with new lignin-first biorefinery schemes also targeting the valorization of the complex lignin fraction. One such strategy is Reductive Catalytic Fraction (RCF), which generates a low molecular weight depolymerised lignin oil, in addition to a carbohydrate pulp. By fine-tuning the RCF process conditions, a lignin oil can be obtained with a high yield of phenolic monomers, dimers and oligomers, carrying high levels of phenolic and aliphatic hydroxyl groups. Typically, RCF phenolic monomers contain a para-propanol group and methoxy substituents in ortho position relative to the phenolic hydroxyl. As a result phenol (H), guaiacyl (G) and syringyl (S) aromatic units exist with zero, one or two ortho-methoxy groups, respectively. Although these substituted phenolic platform molecules can be used directly as raw material, tailored upgrading strategies are essential to modify their molecular structure according to the desired application. A particular alluring structural motive, which is non-native to lignin-derived aromatics, are tertiary amines. Tertiary amines are precious functional groups that fulfil key roles in medicines, agrochemicals, surfactants and polymer materials. Incorporation of nitrogen containing groups into lignin-derived phenolics via amination is therefore highly desirable. Such amination route should preferably be sustainable, catalytic and selective. With the high content of aliphatic hydroxyl groups within RCF lignin phenolics, the aliphatic C-O bond offer an attractive point of entry to generate C-N bonds. Catalytic hydrogen borrowing amination (HB) is a strategy that aims to convert C-O bonds into C-N bonds via in situ generation of a more reactive carbonyl intermediate. Despite the great potential of such strategies, dedicated heterogeneous catalytic methodologies to construct tertiary amine functionalized phenylpropylamines from lignin aromatics are still largely missing. Moreover, current upgrading strategies mainly focus on the well-characterized monomer fraction, whereas the complex mixture of dimers and oligomers, making up to 70 wt% of the RCF lignin oil, is overlooked. A first objective of this doctoral thesis was to develop a heterogeneous catalyzed HB strategy for lignin-derived phenolics towards tertiary phenylalkylamines. Catalytic screening using a model system with dimethylamine (DMAn) reactant and lignin model compound 3-(3,4-dimethoxyphenyl)-1-propanol (1V) put forward Cu-ZrO2 as a superior catalyst, outperforming expensive noble metal catalysts. A prominent role was reserved for hydrogen, as low pressure hydrogen (1 bar) reduced the formation of undesirable amides and secondary amine side products. A high yield of 92% towards the targeted N,N-dimethylamino product was obtained, and besides DMAn, the methodology could readily be extend to other secondary linear and cyclic alkyl amines reactants. A reaction scheme was proposed based on kinetic experiments and throughout (side) product identification with reactant DMAn disproportionation and alcohol amidation being the main side reaction. The stability and reusability of the Cu-ZrO2 catalyst was assessed by surface techniques, such as ToF-SIMS, XPS and N2O chemisorption, revealing a reduction of the number of active Cu surface sites upon catalyst recycling as a major cause for catalyst deactivation. This was readily counteracted by thermal reduction of the catalyst, thereby largely restoring the initial catalytic activity. A second objective was the extension of the developed protocol so to generate tertiary amines from actual RCF lignin monomers, dimers and oligomers. Relative to the previously used model compound 1V, kinetic experiments proved that the specific combination of an ortho-methoxy moiety next to a phenolic hydroxyl greatly reduced the substrate conversion. O-demethylation of the ortho-methoxy group, yielding strongly catalytic inhibiting 1,2‑dihydroxybenzene derivatives (catechols) was identified as a cause. Applying a neutral silica support instead of amphoteric zirconia and fine-tuning of the process parameters, such as increased temperature (210 °C) and hydrogen pressure (3 bar), lead to decent tertiary amine yields (83-84 %) for the HB of lignin monomers dihydroconiferyl (1G) and dihydrosinapyl alcohol (1S). In a final step, actual refined RCF lignin oil fractions derived from spruce wood were subjected to the Cu-SiO2 catalyzed HB protocol with DMAn. In addition to the N,N-dimethylamino product from monomer 1G (57 %), successful amination of six RCF dimer was proven and there molecular structure was revealed by GC x GC - TOF/MS. 2D 1H - 13C HSQC NMR and 31P‑NMR revealed that about 70% of the aliphatic hydroxyl moieties were converted with a 93% selectivity towards the tertiary amine. As shown by 2D NMR, whereas dimeric and oligomeric end-unit hydroxyls displayed good conversion, inter-unit aliphatic hydroxyls remained largely unaffected during the HB, likely due to steric hindrance. As a proof of concept, antioxidant measurements of the synthetized phenolic tertiary amines and their side product, highlighted the very good antioxidant capacity. The lignin-derived tertiary amine compounds out-performed a commercial phenolic amine antioxidant during an in vitro ABTS antioxidant assay, thereby demonstrating a first potential application for these functionalized lignin phenolics.