Combined lignin defunctionalisation and synthesis gas formation by acceptorless dehydrogenative decarbonylation

The valorization of lignin, consisting of various phenylpropanoids building blocks, is hampered by its highly functionalized nature. The absence of the γ-carbinol group in an unnatural C2 β-O-4 motif compared to the native lignin C3 β-O-4 motif provides great opportunities for developing new valorization routes. Thus efficient defunctionalisation approaches that transform the C3 β-O-4 motif into a simplified C2 β-O-4 motif are of interest. Based on a study with a series of model compounds, we established a feasible application of an iridium-catalysed acceptorless dehydrogenative decarbonylation method to efficiently remove the γ-carbinol group in a single step. This defunctionalisation generates valuable synthesis gas, which can be collected as a reaction product. By this direct catalytic transformation, a yield of ∼70% could be achieved for a C3 β-O-4 model compound that was protected from undergoing retro-aldol cleavage by alkoxylation of the benzylic secondary alcohol in the α position. A phenylcoumaran model compound containing a γ-carbinol group as well as a benzylic primary alcohol also proved to be reactive under dehydrogenative decarbonylation conditions, which can further contribute to the reduction of the structural complexity of lignin. Notably, the liberation of synthesis gas was confirmed and the signals for the defunctionalized C2 β-O-4 motif were observed when this dehydrogenative decarbonylation approach was applied on organosolv lignins. This selective defunctionalized lignin in conjunction with the formation of synthesis gas has the potential to enhance the development of profitable and sustainable biorefineries.</p

Chemicals from lignin by diol-stabilized acidolysis:Reaction pathways and kinetics

The product selectivity, production rates and the required process conditions are important for technology development. Selective lignin depolymerisation on the prime β-O-4 motif provides an opportunity to obtain valuable functionalized phenolic monomers. Diol-stabilized acidolysis of lignin with sulfuric acid, triflic acid or triflate salts is a proven β-O-4 cleavage methodology that forms acetals by trapping of released reactive aldehydes. For future scale-up, a better understanding of the prime reaction pathways and how these can be controlled upon changing reaction parameters is required. By using β-O-4 model compounds and ytterbium(iii) triflate as catalyst, starting material conversion and product formation including two key intermediates, the diol adducts (in this study, ethylene glycol as the diol) and the vinyl ethers, were accurately monitored, allowing for detailed kinetic modelling. Over the selected temperature range (80-150 °C), higher temperatures led to higher overall carbon balance and selectivity for the main desired acetal product. The kinetic modelling allowed for establishing a detailed reaction network with activation energies and rate constants. These collectively led to new insights into the key steps involved in the diol-stabilized β-O-4 motif acidolysis and how the reaction selectivity can be manipulated by controlling the reaction temperature, and the ethylene glycol and water content. The elucidation on reaction kinetics and networks constitutes a further step in the design of a diol-stabilized lignin acidolysis process

Unravelling stereoisomerism in acid catalysed lignin conversion:An integration of experimental trends and theoretical evaluations

For the effective valorization of lignin, which is a significant component in agricultural residues, its reactivity needs to be understood in detail. Selective acid-catalysed depolymerisation of the lignin β-O-4 linking motif with stabilization of the formed aldehydes by diols is a promising approach to obtain phenolic monomers in high yields. However, the lignin β-O-4 linking motif exists in both the erythro and threo isomeric forms, and very little information is available on the influence of stereochemistry on the efficiency of the lignin diol-stabilised acidolysis. This is especially true for the set of intermediates in which the presence of stereochemistry persists. In this study, the stereoisomer ratios of two key intermediates, namely the diol (here ethylene glycol) adducts and C2-vinyl ethers, are monitored carefully in ytterbium(iii) trifluoromethanesulfonate [Yb(OTf)3]-catalysed conversion of an erythro β-O-4 model compound. The reactions showed the preferential formation and consumption of the ethylene glycol adduct in the erythro configuration, and the favored formation of trans C2-vinyl ether. Multiscale computational methods (including classical reactive molecular dynamics simulations and quantum chemistry calculations) were applied to elucidate the catalytic origins of the observed stereo-preferences and suggested that a proto-trans intermediate complex is stabilised by a hydrogen bond network connecting the carbocation, ethylene glycol, and the anionic [OTf]− species. The synergistic combination of experiments and computational studies disclosed the stereo-preference and the underlying mechanism in triflate-catalysed acidolysis, especially the catalytic role of [OTf]−, which can be helpful for a further improvement of the chemical process

Sequential Catalytic Modification of the Lignin alpha-Ethoxylated beta-O-4 Motif To Facilitate C-O Bond Cleavage by Ruthenium-Xantphos Catalyzed Hydrogen Transfer

Lignin is an abundant natural biopolymer that has the potential to act as a renewable feedstock for valuable aromatic compounds via selective catalytic depolymerization. In recent years, elegant, mild, catalytic hydrogen neutral C-O bond cleavage methodologies have been developed on model compounds yielding acetophenone derivatives. However, none of these have been reported to be effective once applied to lignin. One of the reasons for this is the highly functionalized nature of the native lignin beta-O-4 motif; which is often not taken into account in the beta-O-4 model compounds used for methodology development. In this work, we demonstrate the development of a stepwise modification protocol on lignin beta-O-4 model compounds to overall yield a partially defunctionalized beta-O-4 motif. This was achieved by making use of an a-ethoxylated beta-O-4 motif that is readily available from ethanosolv extraction of lignocellulosic biomass. This specific motif allowed us to apply selective copper catalyzed aerobic oxidation and subsequent rhodium catalyzed decarbonylation of the primary hydroxyl group in the y position. The obtained partially defunctionalized beta-O-4 lignin motif allowed effective homogeneous ruthenium catalyzed hydrogen neutral C-O bond cleavage (>99% of 3,4-dimethoxyacetophenone and >99% of guaiacol). The stepwise modification strategy was extended to walnut ethanosolv lignin, demonstrating that the specific structural motifs are accessible from such a readily available lignin. Overall, this work illustrates that the structure of lignin can be strategically modified to allow access to otherwise inaccessible specific aromatic compounds via selective depolymerization methodologies

Experimental studies on a combined pyrolysis/staged condensation/hydrotreatment approach to obtain biofuels and biobased chemicals

Fast pyrolysis is an efficient technology to convert lignocellulosic biomass to a liquid product. However, the high contents of oxygenated compounds and water hinder the direct utilization of pyrolysis oils. Here, we report an upgrading concept to obtain liquid products with improved product properties and enriched in valuable low molecular weight chemicals and particularly alkylphenols. It entails two steps, viz. i) pyrolysis with in-situ staged condensation at multiple kg scale followed by ii) a catalytic hydrotreatment of selected fractions using a Ru/C catalyst. Of all pyrolysis oil fractions after staged condensation, the product collected in a condenser equipped with an electrostatic precipitator (ESP) at 120 °C was identified as the most attractive for hydrotreatment when considering product yields and composition. The best hydrotreatment results (Ru/C, 350 °C, 100 bar H2, 4 h) were achieved using beechwood and walnut shells as feedstock, resulting in a high oil yield (about 64 wt% based on pyrolysis oil fraction intake) with a higher heating value of about 37 MJ/kg and enriched in alkylphenols (about 16 wt%). Overall, it was shown that the type of biomass (beech sawdust, walnut granulates, and pine/spruce sawdust) has a limited impact on liquid and alkylphenols yields which implies feedstock flexibility of this integrated concept

Revealing the fate of the phenylcoumaran linkage during lignin oxidation reactions

This work was funded by the EP/J018139/1, EP/K00445X/1 grants (N.J.W. and P.C.J.K.), an EPSRC Doctoral Prize Fellowship (C.S.L.), and the European Union (Marie Curie ITN “SuBiCat” PITN-GA-2013-607044, C.W.L., N.J.W., P.C.J.K.).The fate of most lignin linkages, other than the β-O-4, under selective oxidation conditions is largely unknown. In this work we use advanced β-5 lignin model compounds to identify the fate of phenylcoumaran units in a softwood lignin during oxidation with DDQ. By using model compounds combined with detailed characterisation of the oxidised lignin polymer using HSQC and HMBC NMR we show that phenylcoumarones are a major product, and therefore constitute a novel non-native β-5 linkage in oxidised lignins. Additionally, the reactivity of these units in lignin led us to further investigate their connectivity in lignin, showing that they are found as both phenolic and etherified units. The findings and approach developed here will help improve the efficiency of selective oxidative lignin depolymerisation processes, particularly those aimed at the upgrading of softwood lignin in which phenylcoumarans are a major linkage.PostprintPeer reviewe

Biobased Chemicals:1,2,4-Benzenetriol, Selective Deuteration and Dimerization to Bifunctional Aromatic Compounds

1,2,4-Benzenetriol (BTO), sourced from the carbohydrate-derived platform chemical 5-hydroxylmethylfurfural (HMF), is an interesting starting point for the synthesis of various biobased aromatic products. However, BTO readily undergoes dimerization and other reactions under mild conditions, making analysis and isolation challenging. To both control and utilize the reactivity of BTO to produce biobased building blocks, its reactivity needs to be better understood. Here it was found that specific BTO aromatic C-H bonds are reactive toward deuterium exchange with D2O, which appears pronounced under acidic conditions at room temperature and can lead to the selective formation of BTO with an aromatic ring that contains one or two deuterium atoms, the first at the five and the second at the three position. By exposure to air, it was shown that BTO forms a 5,5'-linked BTO dimer [1,1'-biphenyl]-2,2',4,4',5,5'-hexaol (1) and subsequently a hydroxyquinone containing dimeric structure 2',4,4',5'-tetrahydroxy-[1,1'-biphenyl]-2,5-dione (2). Additionally, condensed dimer dibenzo[b,d]furan-2,3,7,8-tetraol (3) can be relatively easily accessed. The controlled formation of these symmetric and asymmetric multifunctional dimers illustrates diverse possibilities for BTO to be converted to valuable biobased aromatic compounds. Deuterium exchange was attributed to electrophilic aromatic substitution because this reactivity was found to be independent of oxygen and acid mediated. On the contrary, the dimerization was dependent on the presence of oxygen and thus likely involves radical intermediates. Thus this report overall displays different accessible reaction pathways for BTO that can be exploited for the production of BTO-derived compounds

Advanced model compounds for understanding acid-catalyzed lignin depolymerization : identification of renewable aromatics and a lignin-derived solvent

This work was funded by the EP/J018139/1, EP/K00445X/1 grants (NJW and PCJK), an EPSRC Doctoral Prize Fellowship (CSL), and the European Union (Marie Curie ITN ‘SuBiCat’ PITN-GA-2013-607044, CWL, NJW, PCJK, PJD, KB, JdeV).The development of fundamentally new approaches for lignin depolymerization is challenged by the complexity of this aromatic biopolymer. While overly simplified model compounds often lack relevance to the chemistry of lignin, the direct use of lignin streams poses significant analytical challenges to methodology development. Ideally, new methods should be tested on model compounds that are complex enough to mirror the structural diversity in lignin but still of sufficiently low molecular weight to enable facile analysis. In this contribution, we present a new class of advanced (β-O-4)-(β-5) dilinkage models that are highly realistic representations of a lignin fragment. Together with selected β-O-4, β-5, and β–β structures, these compounds provide a detailed understanding of the reactivity of various types of lignin linkages in acid catalysis in conjunction with stabilization of reactive intermediates using ethylene glycol. The use of these new models has allowed for identification of novel reaction pathways and intermediates and led to the characterization of new dimeric products in subsequent lignin depolymerization studies. The excellent correlation between model and lignin experiments highlights the relevance of this new class of model compounds for broader use in catalysis studies. Only by understanding the reactivity of the linkages in lignin at this level of detail can fully optimized lignin depolymerization strategies be developed.PostprintPeer reviewe

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

The development of fundamentally new valorization strategies for lignin plays a vital role in unlocking the true potential of lignocellulosic biomass as sustainable and economically compatible renewable carbon feedstock. In particular, new catalytic modification and depolymerization strategies are required. Progress in this field, past and future, relies for a large part on the application of synthetic model compounds that reduce the complexity of working with the lignin biopolymer. This aids the development of catalytic methodologies and in-depth mechanistic studies and guides structural characterization studies in the lignin field. However, due to the volume of literature and the piecemeal publication of methodology, the choice of suitable lignin model compounds is far from straight forward, especially for those outside the field and lacking a background in organic synthesis. For example, in catalytic depolymerization studies, a balance between synthetic effort and fidelity compared to the actual lignin of interest needs to be found. In this Review, we provide a broad overview of the model compounds available to study the chemistry of the main native linking motifs typically found in lignins from woody biomass, the synthetic routes and effort required to access them, and discuss to what extent these represent actual lignin structures. This overview can aid researchers in their selection of the most suitable lignin model systems for the development of emerging lignin modification and depolymerization technologies, maximizing their chances of successfully developing novel lignin valorization strategies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Unravelling stereoisomerism in acid catalysed lignin conversion: An integration of experimental trends and theoretical evaluations

For the effective valorization of lignin, which is a significant component in agricultural residues, its reactivity needs to be understood in detail. Selective acid-catalysed depolymerisation of the lignin β-O-4 linking motif with stabilization of the formed aldehydes by diols is a promising approach to obtain phenolic monomers in high yields. However, the lignin β-O-4 linking motif exists in both the erythro and threo isomeric forms, and very little information is available on the influence of stereochemistry on the efficiency of the lignin diol-stabilised acidolysis. This is especially true for the set of intermediates in which the presence of stereochemistry persists. In this study, the stereoisomer ratios of two key intermediates, namely the diol (here ethylene glycol) adducts and C2-vinyl ethers, are monitored carefully in ytterbium(iii) trifluoromethanesulfonate [Yb(OTf)3]-catalysed conversion of an erythro β-O-4 model compound. The reactions showed the preferential formation and consumption of the ethylene glycol adduct in the erythro configuration, and the favored formation of trans C2-vinyl ether. Multiscale computational methods (including classical reactive molecular dynamics simulations and quantum chemistry calculations) were applied to elucidate the catalytic origins of the observed stereo-preferences and suggested that a proto-trans intermediate complex is stabilised by a hydrogen bond network connecting the carbocation, ethylene glycol, and the anionic [OTf]− species. The synergistic combination of experiments and computational studies disclosed the stereo-preference and the underlying mechanism in triflate-catalysed acidolysis, especially the catalytic role of [OTf]−, which can be helpful for a further improvement of the chemical process