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To expedite the valorisation of lignin as a sustainable component in materials applications, rapid and generally available analytical methods are essential to overcome the bottleneck of lignin characterisation. Where features of a lignin's chemical structure have previously been found to be predicted by Partial Least Squares (PLS) regression models built on Infrared (IR) data, we now show for the first time that this approach can be extended to prediction of the glass transition temperature (Tg), a key physicochemical property. This methodology is shown to be convenient and more robust for prediction of Tg than prediction through empirically derived relationships (e. g., Flory‐Fox). The chemometric analysis provided root mean squared errors of prediction (RMSEP) as low as 10.0 °C for a botanically, and a process‐diverse set of lignins, and 6.2 °C for kraft‐only samples. The PLS models could separately predict both the Tg as well as the degree of allylation (%allyl) for allylated lignin fractions, which were all derived from a single lignin source. The models performed exceptionally well, delivering RMSEP of 6.1 °C, and 5.4 %, respectively, despite the conflicting influences of increasing molecular weight and %allyl on Tg. Finally, the method provided accurate determinations of %allyl with RMSEP of 5.2 %. [ABSTRACT FROM AUTHOR]

To fully exploit kraft lignin's potential in material applications, we need to achieve tight control over those key physicochemical lignin parameters that ultimately determine, and serve as proxy for, the properties of lignin-derived materials. Here, we show that fractionation combined with systematic (incremental) modification provides a powerful strategy to expand and controllably tailor lignin property space. In particular, the glass transition temperature (Tg) of a typical kraft lignin could be tuned over a remarkable and unprecedented 213 °C. Remarkably, for all fractions the Tg proved to be highly linearly correlated with the degree of derivatisation by allylation, offering such tight control over the Tg of the lignin and ultimately the ability to 'dial-in' this key property. Importantly, such control over this proxy parameter indeed translated well to lignin-based thiol–ene thermosetting films, whose Tgs thus covered a range from 2–124 °C. This proof of concept suggests this approach to be a powerful and generalisable one, allowing a biorefinery or downstream operation to consciously and reliably tailor lignins to predictable specifications which fit their desired application. [ABSTRACT FROM AUTHOR]

Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (Mw), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex‐situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR‐IR) spectroscopy combined with chemometrics provided temporal changes in Mw during lignin depolymerization with high resolution. More specifically, ex‐situ SEC‐derived Mw and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR‐IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes. [ABSTRACT FROM AUTHOR]

Background: The aromatic compounds vanillin and vanillic acid are important fragrances used in the food, beverage, cosmetic and pharmaceutical industries. Currently, most aromatic compounds used in products are chemically synthesized, while only a small percentage is extracted from natural sources. The metabolism of vanillin and vanillic acid has been studied for decades in microorganisms and many studies have been conducted that showed that both can be produced from ferulic acid using bacteria. In contrast, the degradation of vanillin and vanillic acid by fungi is poorly studied and no genes involved in this metabolic pathway have been identified. In this study, we aimed to clarify this metabolic pathway in Aspergillus niger and identify the genes involved. Results: Using whole-genome transcriptome data, four genes involved in vanillin and vanillic acid metabolism were identified. These include vanillin dehydrogenase (vdhA), vanillic acid hydroxylase (vhyA), and two genes encoding novel enzymes, which function as methoxyhydroquinone 1,2-dioxygenase (mhdA) and 4-oxo-monomethyl adipate esterase (omeA). Deletion of these genes in A. niger confirmed their role in aromatic metabolism and the enzymatic activities of these enzymes were verified. In addition, we demonstrated that mhdA and vhyA deletion mutants can be used as fungal cell factories for the accumulation of vanillic acid and methoxyhydroquinone from guaiacyl lignin units and related aromatic compounds. Conclusions: This study provides new insights into the fungal aromatic metabolic pathways involved in the degradation of guaiacyl units and related aromatic compounds. The identification of the involved genes unlocks new potential for engineering aromatic compound-producing fungal cell factories. [ABSTRACT FROM AUTHOR]

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. One stop shop: The structural characterization of technical lignins is typically challenging, requiring extensive, high‐end instrumentation such as high‐field NMR spectrometers and gel permeation chromatography equipment. For routine analyses we show that attenuated total reflectance‐FTIR spectroscopy combined with chemometric approaches, especially partial least squares regression, can provide equivalent information on molecular weight and interunit abundances in a fraction of the time. [ABSTRACT FROM AUTHOR]

Lignin is a potentially abundant renewable resource for the production of aromatic chemicals, however its selective depolymerisation is challenging. Here, we report a new catalytic system for the depolymerisation of lignin to novel, non-phenolic monoaromatic products based on the selective β-O-4 primary alcohol dehydrogenation with a Cp*Ir-bipyridonate catalyst complex under basic conditions. We show that this system is capable of promoting the depolymerisation of model compounds and isolated lignins via a sequence of selective primary alcohol dehydrogenation, retro-aldol (Cα–Cβ) bond cleavage and in situ stabilisation of the aldehyde products by transfer (de)hydrogenation to alcohols and carboxylic acids. This method was found to give good to excellent yields of cleavage products with both etherified and free-phenolic lignin model compounds and could be applied to real lignin to generate a range of novel non-phenolic monomers including diols and di-acids. We additionally show, by using the same catalyst in a convergent, one-pot procedure, that these products can be selectively channelled towards a single di-acid product, giving much simpler product mixtures as a result. [ABSTRACT FROM AUTHOR]

Lignin is an attractive renewable feedstock for aromatic bulk and fine chemicals production, provided that suitable depolymerization procedures are developed. Here, we describe a tandem catalysis strategy for ether linkage cleavage within lignin, involving ether hydrolysis by water-tolerant Lewis acids followed by aldehyde decarbonylation by a Rh complex. In situ decarbonylation of the reactive aldehydes limits loss of monomers by recondensation, a major issue in acid-catalyzed lignin depolymerization. Rate of hydrolysis and decarbonylation were matched using lignin model compounds, allowing the method to be successfully applied to softwood, hardwood, and herbaceous dioxasolv lignins, as well as poplar sawdust, to give the anticipated decarbonylation products and, rather surprisingly, 4-(1-propenyl)phenols. Promisingly, product selectivity can be tuned by variation of the Lewis-acid strength and lignin source. [ABSTRACT FROM AUTHOR]

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance. [ABSTRACT FROM AUTHOR]

Detailed insight into the structure and composition of industrial (technical) lignins is needed to devise efficient thermal, bio- or chemocatalytic valorisation strategies. Six such technical lignins covering three main industrial pulping methods (Indulin AT Kraft, Protobind 1000 soda lignin and Alcell, poplar, spruce and wheat straw organosolv lignins) were comprehensively characterised by lignin composition analysis, FT-IR, pyrolysis-GC-MS, quantitative 31P and 2D HSQC NMR analysis and molar mass distribution by Size Exclusion Chromatography (SEC). A comparison of nine SEC methods, including the first analysis of lignins with commercial alkaline SEC columns, showed molar masses to vary considerably, allowing some recommendations to be made. The lignin molar mass decreased in the order: Indulin Kraft ＞ soda P1000 ＞ Alcell ＞ OS-W ∼ OS-P ∼ OS-S, regardless of the SEC method chosen. Structural identification and quantification of aromatic units and inter-unit linkages indicated that all technical lignins, including the organosolv ones, have considerably been degraded and condensed by the pulping process. Importantly, low amounts of β- ether linkages were found compared to literature values for protolignin and lignins obtained by other, milder isolation processes. Stilbenes and ether furfural units could also be identified in some of the lignins. Taken together, the insights gained in the structure of the technical lignins, in particular, the low β-O-4 contents, carry implications for the design of lignin valorisation strategies including (catalytic) depolymerisation and material applications. [ABSTRACT FROM AUTHOR]

A two-step approach to the conversion of organosolv, kraft and sugarcane bagasse lignin to monoaromatic compounds of low oxygen content is presented. The first step consists of lignin depolymerization in a liquid phase reforming (LPR) reaction over a 1 wt% Pt/γ-Al2O3 catalyst at 225 °C in alkaline ethanol–water. The first LPR step resulted in a decrease in lignin molecular weight of 32%, 57% and 27% for organosolv, kraft and bagasse lignin, respectively. GC analysis of the depolymerized lignin reaction mixture furthermore showed the formation of alkylated phenol, guaiacol and syringol-type products in 11%, 9% and 5% yields from organosolv, kraft and bagasse lignin, respectively. The lignin-oil that was isolated by extraction of the ethanol–water solution was subjected to a subsequent hydrodeoxygenation (HDO) reaction in the second conversion step. HDO of the lignin-oil was performed in dodecane at 300 °C under 50 bar hydrogen pressure over CoMo/Al2O3 and Mo2C/CNF catalysts. GC analysis of the product mixture obtained after the two-step LPR–HDO process revealed the formation of, amongst others, benzene, toluene, xylenes and ethylmethylbenzenes. Of the total observed monomeric products (9%), 25% consisted of these oxygen-free products. Notably, such products cannot be obtained by direct HDO of lignin. HDO of the lignin-oil at 350 °C resulted in the conversion of all tris-oxygenated products, with 57% of the observed monomeric products now identified as mono-oxygenated phenolics. [ABSTRACT FROM AUTHOR]

Hydrodeoxygenation (HDO) studies over carbon nanofiber-supported (CNF) W2C and Mo2C catalysts were performed on guaiacol, a prototypical substrate to evaluate the potential of a catalyst for valorization of depolymerized lignin streams. Typical reactions were executed at 55 bar hydrogen pressure over a temperature range of 300-375 °C for 4 h in dodecane, using a batch autoclave system. Combined selectivities of up to 87 and 69 % to phenol and methylated phenolics were obtained at 375 °C for W2C/CNF and Mo2C/CNF at >99 % conversion, respectively. The molybdenum carbide-based catalyst showed a higher activity than W2C/CNF and yielded more completely deoxygenated aromatic products, such as benzene and toluene. Catalyst recycling experiments, performed with and without regeneration of the carbide phase, showed that the Mo2C/CNF catalyst was stable during reusability experiments. The most promising results were obtained with the Mo2C/CNF catalyst, as it showed a much higher activity and higher selectivity to phenolics compared to W2C/CNF. [ABSTRACT FROM AUTHOR]

The cobalt-catalyzed oxidation of lignin and lignin model compounds using molecular oxygen in ionic liquids proceeds readily under mild conditions, but mechanistic insight and evidence for the species involved in the catalytic cycle is lacking. In this study, a spectroscopic investigation of the complexes involved during this process was conducted using in situATR-IR, Raman, and UV-Vis spectroscopy. A plausible mechanism was proposed that explains the role of the ionic liquid and the other reaction conditions necessary to achieve high catalytic activity. Direct spectroscopic evidence for the species involved in the catalytic cycle was obtained. In addition, substrate consumption and product formation during the oxidation of several lignin model compounds, such as veratryl alcohol, cinnamyl alcohol, and a model compound with a β-O-4 linkage, was directly monitored. The reaction proceeds viathe coordination of alcohol-containing substrates to the Co followed by formation of a Co-superoxo species. The presence of hydroxide is necessary for coordination of the alcohol to occur. Hydrogen peroxide that forms as a reaction by-product underwent rapid disproportionation to yield water and molecular oxygen. Involvement of the various intermediates was further confirmed by 18O2labeling studies. The properties of the ionic liquid greatly influence the catalytic activity both by stabilizing reactive intermediates and by favoring the coordination of the substrate to the cobalt over the direct oxidation of the cobalt without substrate. [ABSTRACT FROM AUTHOR]

The cobalt-catalyzed oxidation of lignin and lignin model compounds using molecular oxygen in ionic liquids proceeds readily under mild conditions, but mechanistic insight and evidence for the species involved in the catalytic cycle is lacking. In this study, a spectroscopic investigation of the complexes involved during this process was conducted using in situ ATR-IR, Raman, and UV-Vis spectroscopy. A plausible mechanism was proposed that explains the role of the ionic liquid and the other reaction conditions necessary to achieve high catalytic activity. Direct spectroscopic evidence for the species involved in the catalytic cycle was obtained. In addition, substrate consumption and product formation during the oxidation of several lignin model compounds, such as veratryl alcohol, cinnamyl alcohol, and a model compound with a β-O-4 linkage, was directly monitored. The reaction proceeds via the coordination of alcohol-containing substrates to the Co followed by formation of a Co-superoxo species. The presence of hydroxide is necessary for coordination of the alcohol to occur. Hydrogen peroxide that forms as a reaction by-product underwent rapid disproportionation to yield water and molecular oxygen. Involvement of the various intermediates was further confirmed by 18O2 labeling studies. The properties of the ionic liquid greatly influence the catalytic activity both by stabilizing reactive intermediates and by favoring the coordination of the substrate to the cobalt over the direct oxidation of the cobalt without substrate. [ABSTRACT FROM AUTHOR]

The article presents a study which examined the catalytic lignin volarization to provide a perspective that would prompt the development of new catalysts and process to valorize lignin for the production of renewable chemicals. Possible reactions of chemicals obtained during degradation of processed lignin streams is explained. It is stated that molecular weight and amount of lignin differ from plant to plant.

The Front Cover shows that the traditional queuing up of samples for step‐by‐step structure analysis of technical lignins with HSQC NMR, 31P NMR and GPC, all requiring considerable amounts of time and/or access to high end equipment, can be avoided by taking an alternative, fast lane to gain the same quantitative structure information from a combination of readily accessible IR spectroscopy combined with chemometrics. Once the chemometrics model is in place, one single ATR‐FTIR measurement could then, in principle, replace the entire suite of traditional analysis methods. This would be particularly advantageous for routine structural characterization, such as those required during lignin fractionation processes, in quality control applications or when, for example, access to high end NMR equipment is limited. More information can be found in the Full Paper by Lancefield et al on page 1139 in Issue 6, 2019 (DOI: 10.1002/cssc.201802809). [ABSTRACT FROM AUTHOR]

The author looks at the potential of lignins, one of the main components of non-edible, lignocellulosic biomass, as sustainable raw materials for renewable fuels, chemicals and materials. Topics covered include the importance of ligin for the economic viability of current and future biorefinery operations, the challenges for its efficient valorisation, and the growing interest of the scientific community in lignin's applications and catalytic conversions.

Review: 427 refs. [ABSTRACT FROM AUTHOR]