Diformylxylose as a new polar aprotic solvent produced from renewable biomass

Lignin is a promising candidate for the replacement of fossil-based materials, due to its natural abundance and aromatic structure. This same structure poses major challenges to lignin's exploitation for material development. The harsh conditions generally needed for its isolation make it susceptible to uncontrollable side reactions that limit its further upgrading and/or functionalization. Here, by using aldehyde-assisted fractionation (AAF), we have extracted lignin while avoiding condensation reactions and, at the same time, introduced new functional groups on the lignin in a single step. By using a multifunctional aldehyde such as terephthalic aldehyde (TALD), we used acetal functionalization to prevent the dehydration and condensation of lignin's β-O-4 linkage and introduced an aldehyde on the lignin backbone. By tuning the amount of TALD during the extraction process, we were also able to precisely control the degree of chemical functionalization. We then exploited the reactivity of the newly functionalized biopolymer to increase phenolation reactions yields in both acid and basic environment, in order to provide better lignins for incorporation in phenol formaldehyde resins. These TALD-lignins were more reactive than Kraft lignin or mild organosolv lignin extracted in an acid solution of 80% ethanol in water, which could facilitate their use for developing more sustainable resins and materials.

Extraction and controlled functionalization of lignin for materials applications

Stefania Bertella

Lignin is a renewable aromatic polymer that due to its abundance and unique chemical structure is a promising candidate to replace aromatic materials that are currently sourced from fossil oil. The same structure of lignin poses however drawbacks for its valorization. The high temperature and strong pH that are generally used to isolate lignin, make this polymer prone to reactions of condensation during its extraction. This translates into isolated lignins where most of their native functional groups are eliminated in favor of the formation of interunit carbon-carbon linkages. The lack of control on lignin's chemical functionalities ultimately hinders the use of this polymer for the development of new materials. Many research groups have been trying to tackle these challenges by developing new processes that could allow for the efficient isolation of lignin without compromising on its functional groups. Amongst these, the Aldehyde-Assisted Fractionation (AAF), which relies on the formation of acetal groups between lignin's ß-O-4 units and an added aldehyde, was proven to be efficient in isolating high yields of lignin, that could be then quantitatively depolymerized into small aromatic monomers. Inspired by this research, the aim of my work is to take advantage of the AAF to quantitatively isolate lignin from lignocellulose, at the same time introducing on the biopolymer scaffold new functional groups that could allow for more effective development of lignin-based materials. My work is based on a Multifunctional-AAF, in which the multifunctional aldehydes can react with the lignin via the formation of acetals, at the same time leaving on the biopolymer additional functionalities. In the first part of my thesis, I show how the extraction and simultaneous functionalization of lignin can be achieved by using terephthalic aldehyde (TALD). In addition to successfully functionalizing the lignin, I demonstrate how the degree of functionalization can be precisely controlled and monitored via 1H NMR, yielding a material with high reactivity towards phenolation. In the second part, I show how the isolation and controlled functionalization of lignin can be translated to the use of other aldehydes, namely glyoxylic acid (GA), to functionalize lignin with carboxylates, quantified via 31P NMR. The isolated lignin offers in this case unique avenues for the development of more sustainable surfactants. In the last chapter of the thesis, I show that the Multifunctional-AAF can be expanded to isolate and functionalize lignin with multiple functionalities by using mixtures of aldehydes (TALD and GA), demonstrating again that each functional group can be easily quantified. These lignins, extracted with different ratios of the two functional groups, showed dissimilar properties in terms of solubility and reactivity towards gelatin. We used these lignins to develop lignin-gelatin based hydrogels, showing how the bi-functionalization was pivotal for the formation of hydrogels with improved rheological, mechanical, self-healing and adhesive properties. In summary, this thesis shows the development of a new approach for lignin extraction and simultaneous valorization. The versatility of this process enables more efficient functionalization, allowing the obtainment of lignins which are "tailor-made" for their end use. The control over lignin's chemical structure and functionalization will pave the way for the use of lignin in high-end application.

EPFL2022

Production and characterization of stabilized lignin

Masoud Talebi Amiri

Lignocellulosic biomass is a sustainable source of renewable carbon, and it is the most abundant form of terrestrial biomass. The three main constituents of lignocellulosic biomass are cellulose, hemicellulose, and lignin. The monomers from these biopolymers can be valuable feedstocks for a future sustainable chemical industry, and they include glucose from cellulose, predominantly xylose from hemicellulose, and aromatic molecules from lignin. Even though both glucose and xylose are already established feedstocks for further upgrading in biorefineries, the upgrading of lignin to higher value-added chemicals has not achieved the same success, despite the tremendous need for renewable aromatic molecules. Developing methods for the high-yield depolymerization of lignin can dramatically increase the efficiency and profitability of biorefineries. The main limiting factor in producing high-quality lignin is the inter-unit carbon-carbon linkages in the chemical structure of lignin, and also the formation of these bonds during the extraction process, known as repolymerization or condensation. In the first part of my thesis, I study the possibility of avoiding the condensation of lignin by preventing the formation of C-C linkages. This is achieved by adding formaldehyde, as a protection group, to the pretreatment of lignocellulosic biomass. The results show a near theoretical yield of monomers from the stabilized lignin after depolymerization by hydrogenolysis. This method was then further expanded by screening other protecting groups in order to develop a comprehensive picture of the chemistry involved in this process, and to optimize the process notably to be able to yield a narrower product distribution. Even though this method provides high-quality lignin, there is a need for developing a fractionation process for large-scale production of isolated lignin. This can facilitate the implementation of the process in biorefineries and also further studies on developing new upgrading pathways. The second part of this thesis presents a protocol that fractionates lignocellulosic biomass into pure parts of three main constituents in large laboratory-scale with high extraction yields. The results show extraction yields higher than 95% for each biopolymer. The highly digestible cellulose fraction and high-quality lignin can be depolymerized into their constituent monomers with conventional methods. However, the hemicellulose fraction is obtained as functionalized sugars due to its reaction with aldehyde during pretreatment. The third part of my work presents a non-destructive method for the prediction of the potential yield of monomers from the depolymerization of isolated lignin. A novel 2D heteronuclear single quantum coherence nuclear magnetic resonance (2D-HSQC NMR) spectroscopy technique can be used for simultaneous identification and quantification of chemical groups in the structure of lignin. Standard 2D-HSQC NMR spectroscopy does not provide accurate quantification for polymers and oligomers such as lignin due to errors that are principally caused by differences in relaxation times for different parts of the oligomer chain. However, these errors can be avoided by extrapolation to time-zero 13C HSQC (HSQC0) for series of spectrums, acquired with different repetition times. The prediction results show agreement within a few percentage points with experimental results determined by gas chromatography (GC) after hydrogenolysis.

EPFL2019

Development of Catalytic Umpolung

Jonathan Brand

The development of efficient synthetic methodologies is crucial to access complex molecules in an economic, eco-friendly and safe way. The reversal of the intrinsic reactivity of functional groups, called Umpolung, allows alternative synthetic strategies and can improve synthetic efficiency. In this thesis, two new Umpolung methods have been investigated. First a novel approach for the direct alkynylation of aromatic C-H bonds has been developed to access aromatic alkynes. Secondly, novel bifunctional (thio)urea carbene catalysts have been synthesized and their use in asymmetric intermolecular heterocouplings has been investigated. Aromatic alkynes are compounds of utmost prominence as synthetic intermediates and for application in materials science. Classical synthetic methods require a prefunctionalization of the aromatic ring in addition to the alkynylation step. An alternative emerging strategy to introduce triple bonds is based on the Umpolung of acetylenes by the use of electrophilic acetylene reagents. In addition, the use of metal catalysis has recently arisen to couple aromatic C-H bonds with electrophiles in a single step. However, direct alkynylation has been only scarcely investigated. In this thesis, alkynyl hypervalent iodines, highly electrophilic acetylenes, were used for the metal-catalyzed direct alkynylation of aromatic C-H bonds. Indoles were alkynylated in C3 position using AuCl as catalyst and TIPS-EBX (1- [(triisopropylsilyl)ethynyl]-1λ3,2-benziodoxol-3(1H)-one) as source of electrophilic acetylene. The reaction proceeded under mild open-flask conditions (room temperature under air, commercial solvents). Importantly, it was the first example showcasing the superiority of alkynyl benziodoxolones over alkynyl iodoniums in alkyne transfer reactions. On contrary to other alkynylation methods, which have either a limited scope or require indole protection, this reaction tolerated a large range of functional groups and worked for unprotected indoles. Furthermore in the presence of pyridine, pyrroles were alkynylated in C2 position in high yields. This development was important in light of the instability of unprotected halogeno- pyrroles, which makes their use in cross-coupling reactions difficult. A one-pot 2- alkynylaniline cyclization/direct alkynylation was next developed. The influence of substituents on the acetylene, the iodoxole ring structure and the benzene ring substitution on the alkynyl benziodoxolones were examined. This study revealed that bulky silyl groups were ideal alkyne substituents. Preliminary mechanistic investigations indicated that the reaction proceeded either via a π activation mechanism or via an oxidative AuI/AuIII mechanism. iii Unfortunately it has not yet been possible to distinguish between both mechanisms. The alkynylation of electron-rich arenes was next explored. N,N-dialkylanilines were alkynylated with high para regioselectivity using iPrOH as solvent. This transformation represented the first one-step synthesis of para-alkynylanilines and was then extended to 1,3,5- trimethoxybenzenes.

EPFL2012