An introduction to model compounds of lignin linking motifs; synthesis and selection considerations for reactivity studies

C.W.L., P.C.J.K. and P.J.D. would like to thank the European Union (Marie Curie ITN “SuBiCat” PITN-GA-2013-607044, C.W.L. also thanks the framework of the Dutch TKI-BBEI project “CALIBRA”, reference TEBE117014. P.C.J.K. also thanks the EPSRC (critical mass grant EP/J018139/ 1). C.S.L. thanks the Leverhulme Trust Early Career Fellowship (ECF‐2018‐480) and the University of St Andrews.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.Publisher PDFPeer reviewe

The use of residual dipolar coupling for conformational analysis of structurally related natural products

The authors would like to acknowledge the EPSRC for funding.Determining the conformational preferences of molecules in solution remains a considerable challenge. Recently, the use of residual dipolar coupling (RDC) analysis has emerged as a key method to address this. Whilst to date the majority of the applications have focused on biomolecules including proteins and DNA, the use of RDCs for studying small molecules is gaining popularity. Having said that, the method continues to develop and here we describe an early case study of the quantification of conformer populations in small molecules using RDC analysis. Having been inspired to study conformational preferences by unexpected differences in the NMR spectra and the reactivity of related natural products, we showed that the use of more established techniques was unsatisfactory in explaining the experimental observations. The use of RDCs provided an improved understanding which, following use of methods to quantify conformer populations using RDCs, culminated in a rationalisation of the contrasting diastereoselectivities observed in a ketone reduction reactionPostprintPeer reviewe

Copper-mediated conversion of complex ethers to esters : enabling biopolymer depolymerisation under mild conditions

Authors acknowledge the China Scholarship Council (G.X. studentship), the University of St Andrews (G.X. and J.R.D.M. studentships) and the EPSRC-funded CRITICAT Centre for Doctoral Training (studentship to I.P.; EP/L016419/1) for PhD funding. C.S.L. thanks the Leverhulme Trust Early Career Fellowship (ECF-2018-480) and the University of St Andrews.Selective processing of the β-O-4 unit in lignin is essential for the efficient depolymerisation of this biopolymer and therefore its successful integration into a biorefinery set-up. An approach is described in which this unit is modified to incorporate a carboxylic ester with the goal of enabling the use of mild depolymerisation conditions. Inspired by preliminary results using a Cu/TEMPO/O2 system, a protocol was developed that gave the desired β-O-4-containing ester in high yield using certain dimeric model compounds. The optimised reaction conditions were then applied to an oligomeric lignin model system. Extensive 2D NMR analysis demonstrated that analogous chemistry could be achieved with the oligomeric substrate. Mild depolymerisation of the ester-containing oligomer delivered the expected aryl acid monomer.Publisher PDFPeer reviewe

Selective primary alcohol oxidation of lignin streams from butanol-pretreated agricultural waste biomass

We would like to thank the CRITICAT Centre for Doctoral Training for financial support [Ph.D. studentship to IP; Grant code: EP/L016419/1] and BBSRC Global Challenges Research Fund Impact Acceleration Account at St Andrews BB/GCRFIAA/20. CSL thanks the Leverhulme Trust for funding an Early Career Fellowship.Chemically modified lignins are important for the generation of biomass-derived materials and as precursors to renewable aromatic monomers. A butanol-based organosolv pretreatment has been used to convert an abundant agricultural waste product, rice husks, into a cellulose pulp and three additional product streams. One of these streams, a butanol-modified lignin, was oxidized at the γ position to give a carboxylic acid functionalized material. Subsequent coupling of the acid with aniline aided lignin characterization and served as an example of the flexibility of this approach for grafting side chains onto a lignin core structure. The pretreatment was scaled up for use on a multi-kilogram scale, a development that enabled the isolation of an anomeric mixture of butoxylated xylose in high purity. The robust and scalable butanosolv pretreatment has been developed further and demonstrates considerable potential for the processing of rice husks.PostprintPeer reviewe

Pre-treatment of lignocellulosic feedstocks using biorenewable alcohols::towards complete biomass valorisation

Here, we report on the ability of the biomass derived solvents ethanol and, in particular, n-butanol to fractionate lignocellulose into its main components. An organosolv system consisting of n-butanol containing 5% water and 0.2 M HCl at reflux was found to remove effectively the lignin and hemicellulose components of lignocellulosic biomass leaving a cellulose pulp suitable for enzymatic hydrolysis to simple sugars. Using a hardwood beech pulp as an example, essentially complete conversion of the cellulose component to reducing sugars was achieved with a cellulase loading of 22 FPU per g. Analysis of the solubilised hemicellulose fractions revealed that they consist almost exclusively of alkyl xylosides and mannosides which could serve as valuable synthetic building blocks. Additionally, the mild conditions (<120 °C) and high alcohol content of the pre-treatment solvent suppressed lignin degradation reactions and allowed for the isolation of high quality lignins in good yields. Detailed HSQC NMR analysis of the isolated lignins revealed that they still contained large amounts of β-aryl ether units, especially α-ethoxylated and α-butoxylated β-O-4 units, making them particularly suitable for depolymerisation to mono-aromatic chemicals. This was demonstrated using a recently reported acidolysis method utilizing ethylene glycol which gave monomer yields of between 7.4 and 18 wt%. The yields for n-butanol lignins were at least four fold higher than those obtained from a current generation technical organosolv lignin under comparable conditions

Investigation of the chemocatalytic and biocatalytic valorization of a range of different lignin preparations: The importance of β-O-4 content

A set of seven different lignin preparations was generated from a range of organosolv (acidic, alkaline, ammonia-treated, and dioxane-based), ionic liquid, autohydrolysis, and Kraft pretreatments of lignocelluloses. Each lignin was characterized by 2D HSQC NMR spectroscopy, showing significant variability in the β-O-4 content of the different lignin samples. Each lignin was then valorised using three biocatalytic methods (microbial biotransformation with Rhodococcus jostii RHA045, treatment with Pseudomonas fluorescens Dyp1B or Sphingobacterium sp. T2 manganese superoxide dismutase) and two chemocatalytic methods (catalytic hydrogenation using Pt/alumina catalyst, DDQ benzylic oxidation/Zn reduction). Highest product yields for DDQ/Zn valorization were observed from poplar ammonia percolation-organosolv lignin, which had the highest β-O-4 content of the investigated lignins and also gave the highest yield of syringaldehyde (243 mg L -1 ) when using R. jostii RHA045 and the most enzymatic products using P. fluorescens Dyp1B. The highest product yield from the Pt/alumina hydrogenation was observed using oak dioxasolv lignin, which also had a high β-O-4 content. In general, highest product yields for both chemocatalytic and biocatalytic valorization methods were obtained from preparations that showed highest β-O-4 content, while variable yields were obtained with preparations containing intermediate β-O-4 content, and little or no product was obtained with preparations containing low β-O-4 content

Metal triflates for the production of aromatics from lignin

This work was funded by the European Union (Marie Curie ITN ‘SuBiCat’ PITN-GA-2013-607044, PJD, CWL, NJW, PCKL, KB, JGdeV) as well as EP/J018139/1, EP/K00445X/1 grants (NJW and PCJK) and an EPSRC Doctoral Prize Fellowship (CSL).The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields, under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy to handle metal triflates (M(OTf)x). Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2-acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol, or methyl aromatics obtained by catalytic decarbonylation. Notably, when the former method was ultimately tested on lignin, especially Fe(OTf)3 proved very effective and the phenolic C2-acetal products were obtained in an excellent, 19.3±3.2 wt % yield.PostprintPeer reviewe

Linkage abundance and molecular weight characteristics of technical lignins by attenuated total reflection-FTIR spectroscopy combined with multivariate analysis

The authors gratefully acknowledge the support of the Smart Mix Program of the Netherlands Ministry of Economic Affairs and the Netherlands Ministry of Education, Culture and Science.Lignin is an attractive material for the production of renewable chemicals, materials and energy. However, utilization is hampered by its highly complex and variable chemical structure, which requires an extensive suite of analytical instruments to characterize. Here, we demonstrate that straightforward attenuated total reflection (ATR)‐FTIR analysis combined with principle component analysis (PCA) and partial least squares (PLS) modelling can provide remarkable insight into the structure of technical lignins, giving quantitative results that are comparable to standard gel‐permeation chromatography (GPC) and 2D heteronuclear single quantum coherence (HSQC) NMR methods. First, a calibration set of 54 different technical (fractionated) lignin samples, covering kraft, soda and organosolv processes, were prepared and analyzed using traditional GPC and NMR methods, as well as by readily accessible ATR‐FTIR spectroscopy. PLS models correlating the ATR‐FTIR spectra of the broad set of lignins with GPC and NMR measurements were found to have excellent coefficients of determination (R2 Cal.>0.85) for molecular weight (Mn, Mw) and inter‐unit abundances (β‐O‐4, β‐5 and β‐β), with low relative errors (6.2–14 %) as estimated from cross‐validation results. PLS analysis of a second set of 28 samples containing exclusively (fractionated) kraft lignins showed further improved prediction ability, with relative errors of 3.8–13 %, and the resulting model could predict the structural characteristics of an independent validation set of lignins with good accuracy. The results highlight the potential utility of this methodology for streamlining and expediting the often complex and time consuming technical lignin characterization process.Publisher PDFPeer reviewe

Use of bisulfite processing to generate high-β-O-4 content water-soluble lignosulfonates

This work was supported by EPSRC grants (EP/1518175), the Industrial Biotechnology Innovation Centre (IBioIC) (DMB Ph.D. studentship) and an EPSRC Doctoral Prize Fellowship (CSL).With lignin-first biorefineries likely to become a reality, controlled depolymerization of high-quality lignin streams to high value products has become a priority. Using bisulfite chemistry, access to a high-β-O-4 content water-soluble lignosulfonate has been achieved, allowing follow-on procedures in water to be conducted. We show that phenolic β-O-4 units preferential-ly react under acidic bisulfite conditions, whilst non-phenolic β-O-4 units react much more slowly. Exploiting this improved chemical understanding and inherent selectivity, a softwood lignosulfonate has been prepared in which phenolic β-O-4 α-sulfonation has occurred leaving significant native β-O-4 content. Use of an O-benzoylation protocol with lignin coupled with advanced 2D NMR methods has allowed detailed analysis of this and other commercial and industrial lignosulfonates. Conversion of the native β-O-4 to benzylic- oxidized β-O-4 units was followed by a selective reductive cleavage to give a premium aromatic monomer in pure form.Publisher PDFPeer reviewe

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