Lignin Monomer Production Research Articles

Enhancing α-etherification of lignin in Eucalyptus diol pretreatment to improve lignin monomer production

In this work, α-etherification of lignin in diol pretreatment was selectively enhanced at mild temperature for lignin isolation and subsequent valorisation. More than 90 % of lignin was removed from Eucalyptus at 120 °C in diol (ethylene glycol and 1,4-butanediol) pretreatment, resulting in >90 % cellulose conversion in 24 h at 7.5 FPU/g glucan cellulase loading. Subsequent catalytic transfer hydrogenolysis of the isolated lignins with Ru/C in ethanol gave 15 % monomer yield on native lignin basis, 5 times of that from the technical ethanol process (170 °C). HSQC NMR analysis revealed that diol pretreated lignin (120 °C) contained ∼23 % α-etherified β–O–4 interunit bonds, indicating that lignin degradation (i.e. cleavage of β–O–4 bonds) was suppressed via etherification by grafting a hydroxyl group at the α position of lignin. This finding was consistent with the isolated lignin (120 °C) had less number of phenolic OH and higher molecular weight via31P NMR and GPC analysis. 31P NMR analysis also revealed that diol isolated lignin contained more number of aliphatic OH than ethanol-isolated lignin, which increased lignin solubility and maintained the high yield (>80 %) of isolated lignin from Eucalyptus at 120 °C as expected. In summary, diol pretreatment of woody biomass can effectively isolate more lignin for hydrogenolysis to valued-added monomers without compromising the isolated yield of lignin and hydrolysis yield of remained cellulose.

Comparative Transcriptomics Unveil the Crucial Genes Involved in Coumarin Biosynthesis in Peucedanum praeruptorum Dunn.

Peucedanum praeruptorum Dunn is a commonly used traditional Chinese medicine that is abundant in furano- and dihydropyrano coumarins. When P. praeruptorum reaches the bolting stage, the roots gradually lignified, and the content of coumarins declines rapidly. Non-bolting has always been a decisive factor for harvesting the P. praeruptorum materials. To evaluate the amount of coumarin components in unbolted and bolted P. praeruptorum, the variations of praeruptorin A, praeruptorin B, praeruptorin E, peucedanocoumarin I, and peucedanocoumarin II were determined. Additionally, 336,505 transcripts were obtained from the comparative transcriptome data. Among them, a total of 1,573 differentially expressed genes were screened out. To identify the critical genes involved in coumarin biosynthesis, comparative transcriptomics coupled with co-expression associated analysis was conducted. Finally, coumarin biosynthesis-related eighteen candidate genes were selected for the validation of qPCR. Additionally, a phylogenetic tree and the expression profile of ATP-binding cassette (ABC) transporters were constructed. To clarify the main genes in the regulation of coumarin biosynthesis, the interaction network of the co-expression genes from thirteen modules was constructed. Current results exhibited the significant increment of praeruptorin A, praeruptorin B and praeruptorin E in the bolted P. praeruptorum. Although, peucedanocoumarin I and peucedanocoumarin II were slightly increased. Besides the content of coumarins, the essential genes involved in the coumarin biosynthesis also exhibited an overall downward trend after bolting. Three peroxidases (PRXs) involved in the production of lignin monomers had been demonstrated to be downregulated. PAL, C4H, HCT, COMT, CCoAOMT, and some ABC transporters were dramatically downregulated at the bolting stage. These results indicated that the downregulation of coumarin biosynthetic genes in the bolted P. praeruptorum ultimately reduced the formation of coumarins. However, the mechanism through which bolting indirectly affects the formation of coumarin still needs extra functional verification.

Effect of Cobalt(II) on Acid-Modified Attapulgite-Supported Catalysts on the Depolymerization of Alkali Lignin

With an aromatic structure, lignin has great potential for the production of high value-added chemicals. In this study, Co/m-ATP catalyst was prepared by the co-impregnation method to depolymerize lignin with inexpensive attapulgite as a carrier, 1,4-dioxane as a solvent, and ethanol as an in situ hydrogen source. Meanwhile, formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization. The effects of Co particle loading and reaction conditions (e.g., temperature and reaction time) on lignin conversion and product yield were investigated, resulting in a low char yield (6.6 wt %) and high bio-oil yield (63.1 wt %) when alkali lignin was depolymerized over the Co/m-ATP catalyst (cobalt loading of 15 wt %) at 220 °C for 8 h. Therefore, this catalyst with the advantage of low cost and high catalytic activity would be a potential catalyst during the depolymerization of lignin.

Techno‐economic and life‐cycle assessment of a wood chips‐based organosolv biorefinery concept for production of lignin monomers and oligomers by base‐catalyzed depolymerization

AbstractMore wood for technical valorization is to be expected in Europe over the coming years, due to climate change and the bark beetle. To further independence from fossil‐fuel resources a novel approach to the base‐catalyzed depolymerization (BCD) of organosolv lignin into monomers and oligomers, while maximizing yields and minimizing coke formation, was scaled up in a theoretical wood‐based biorefinery with a yearly input of 40 000 t of wood chips. Other process steps were modeled from the literature. A novel work‐up approach was evaluated under laboratory conditions, implemented, and compared with a simulation‐based pervaporation technique. The life‐cycle assessment (LCA) showed that the biorefinery provided a significantly lower global warming potential (GWP) (excluding biogenic carbon) than its fossil counterparts. Moreover, the majority of impacts on the other midpoint categories was also smaller than for the fossil reference. However, after allocating the GWPs, it was evident that subsequent conversion of the C6 fraction to value‐added products is necessary for optimal results. The discounted cash flow analyses for the biorefinery setups (40 to 400 kt year–1) showed that they were not profitable with prices based on fossil references. However, when using higher prices/t from the literature, such as €1615 for monomers, €2000 for oligomers, and €510 for C6 sugars, a positive net present value could be reached at approximately 100 000–120 000 t processed annually. Hence, the conclusion was that a biorefinery utilizing wood not only brings environmental benefits and a reduction in the use of fossil resources but can also be financially beneficial. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd

Systematic evaluation of fractionation and valorization of lignocellulose via two-stage hydrothermal liquefaction

Converting lignocelluloses through conventional hydrothermal liquefaction results in a complex mixture of products due to the compositional heterogeneity of the feedstocks. Sequential hydrothermal liquefaction (SeqHTL) was therefore proposed to address this drawback by selectively fractionating biomass in a stepwise manner. In this work, a two-stage SeqHTL process was designed to isolate sugars in the first stage and to enhance the production of sugar-derived biochemicals and lignin monomers in the second stage from poplar biomass. As a unique feature, the SeqHTL process enabled 24.3% conversion yield of hemicellulose to xylose in the first stage at 180 ℃. In addition, the yields of furfural and lignin monomers were improved by 152.9% and 58.7% for the second stage at 280 ℃ in comparison with the one-stage process operating at the same temperature, respectively. The fractionation of the SeqHTL process reduced the coupling reactions between sugar and lignin components in the feedstock at the first stage, reducing the formation of char and high-molecular-weight compounds at the second stage. Besides, the effects of additives (alcohol, acid, and salt) on the hydrothermal conversion of poplar were investigated in terms of product yields, reaction pathways, surface chemistry and morphology of the solid products. Results showed that the presence of ethanol significantly promoted the production of lignin monomers, while phosphoric acid facilitated the formation of levulinic acid. Overall, this study demonstrates that by properly utilizing subcritical water properties, the SeqHTL platform through simultaneous fractionation and conversion, provides a potential for the stepwise production of bio-based chemicals from lignocellulosic biomass.

Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon–Carbon Linkages

Summary Conversion of lignin into monocyclic hydrocarbons as commodity chemicals and drop-in fuels is a highly desirable target for biorefineries. However, this is severely hindered by the presence of stable interunit carbon–carbon linkages in native lignin and those formed during lignin extraction. Herein, we report a new multifunctional catalyst, Ru/NbOPO4, that achieves the first example of catalytic cleavage of both interunit C–C and C–O bonds in one-pot lignin conversions to yield 124%–153% of monocyclic hydrocarbons, which is 1.2–1.5 times the yields obtained from the established nitrobenzene oxidation method. This catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic arenes over repeated cycles. The mechanism of the activation and cleavage of 5–5 C–C bonds in biphenyl, as a lignin model adopting the most robust C–C linkages, has been revealed via in situ inelastic neutron scattering coupled with modeling. This study breaks the conventional theoretical limit on lignin monomer production.

Towards high-yield lignin monomer production

This article discusses the formation of robust C–C linkages in lignin and proposes ways to avoid it.

Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization

Practical, high-yield lignin depolymerization methods could greatly increase biorefinery productivity and profitability. However, development of these methods is limited by the presence of interunit carbon-carbon bonds within native lignin, and further by formation of such linkages during lignin extraction. We report that adding formaldehyde during biomass pretreatment produces a soluble lignin fraction that can be converted to guaiacyl and syringyl monomers at near theoretical yields during subsequent hydrogenolysis (47 mole % of Klason lignin for beech and 78 mole % for a high-syringyl transgenic poplar). These yields were three to seven times those obtained without formaldehyde, which prevented lignin condensation by forming 1,3-dioxane structures with lignin side-chain hydroxyl groups. By depolymerizing cellulose, hemicelluloses, and lignin separately, monomer yields were between 76 and 90 mole % for these three major biomass fractions.

ChemInform Abstract: Organic Solvent Effects in Biomass Conversion Reactions

AbstractReview: 257 refs.

Organic Solvent Effects in Biomass Conversion Reactions.

Transforming lignocellulosic biomass into fuels and chemicals has been intensely studied in recent years. A large amount of work has been dedicated to finding suitable solvent systems, which can improve the transformation of biomass into value-added chemicals. These efforts have been undertaken based on numerous research results that have shown that organic solvents can improve both conversion and selectivity of biomass to platform molecules. We present an overview of these organic solvent effects, which are harnessed in biomass conversion processes, including conversion of biomass to sugars, conversion of sugars to furanic compounds, and production of lignin monomers. A special emphasis is placed on comparing the solvent effects on conversion and product selectivity in water with those in organic solvents while discussing the origins of the differences that arise. We have categorized results as benefiting from two major types of effects: solvent effects on solubility of biomass components including cellulose and lignin and solvent effects on chemical thermodynamics including those affecting reactants, intermediates, products, and/or catalysts. Finally, the challenges of using organic solvents in industrial processes are discussed from the perspective of solvent cost, solvent stability, and solvent safety. We suggest that a holistic view of solvent effects, the mechanistic elucidation of these effects, and the careful consideration of the challenges associated with solvent use could assist researchers in choosing and designing improved solvent systems for targeted biomass conversion processes.

The cell biology of lignification in higher plants.

Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying. Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level. The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.

Lignin monomer production integrated into the γ-valerolactone sugar platform

We demonstrate an experimental approach for upgrading lignin that has been isolated from corn stover via biomass fractionation using γ-valerolactone (GVL) as a solvent.

Metabolic engineering for p‐coumaryl alcohol production in Escherichia coli by introducing an artificial phenylpropanoid pathway

The plant polymer lignin is the greatest source of aromatic chemical structures on earth. Hence, the chemically diverse lignin monomers are valuable raw materials for fine chemicals, materials synthesis, and food and flavor industries. However, extensive use of this natural resource is hampered by the large number of different lignin monomers and the complex and irregular structure of lignin, which renders current processes for its chemical or enzymatic degradation inefficient. The microbial production of lignin monomers from renewable resources represents a promising alternative to lignin degradation, which could meet the demand for aromatic chemical structures. In this study, we describe the functional introduction of an artificial phenylpropanoid pathway into Escherichia coli, achieved by transferring several genes from plants and microbes. The established chimeric pathway efficiently converts l-tyrosine into the lignin precursor molecule p-coumaryl alcohol.

Enhanced Lignin Monomer Production Caused by Cinnamic Acid and Its Hydroxylated Derivatives Inhibits Soybean Root Growth

Cinnamic acid and its hydroxylated derivatives (p-coumaric, caffeic, ferulic and sinapic acids) are known allelochemicals that affect the seed germination and root growth of many plant species. Recent studies have indicated that the reduction of root growth by these allelochemicals is associated with premature cell wall lignification. We hypothesized that an influx of these compounds into the phenylpropanoid pathway increases the lignin monomer content and reduces the root growth. To confirm this hypothesis, we evaluated the effects of cinnamic, p-coumaric, caffeic, ferulic and sinapic acids on soybean root growth, lignin and the composition of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) monomers. To this end, three-day-old seedlings were cultivated in nutrient solution with or without allelochemical (or selective enzymatic inhibitors of the phenylpropanoid pathway) in a growth chamber for 24 h. In general, the results showed that 1) cinnamic, p-coumaric, caffeic and ferulic acids reduced root growth and increased lignin content; 2) cinnamic and p-coumaric acids increased p-hydroxyphenyl (H) monomer content, whereas p-coumaric, caffeic and ferulic acids increased guaiacyl (G) content, and sinapic acid increased sinapyl (S) content; 3) when applied in conjunction with piperonylic acid (PIP, an inhibitor of the cinnamate 4-hydroxylase, C4H), cinnamic acid reduced H, G and S contents; and 4) when applied in conjunction with 3,4-(methylenedioxy)cinnamic acid (MDCA, an inhibitor of the 4-coumarate:CoA ligase, 4CL), p-coumaric acid reduced H, G and S contents, whereas caffeic, ferulic and sinapic acids reduced G and S contents. These results confirm our hypothesis that exogenously applied allelochemicals are channeled into the phenylpropanoid pathway causing excessive production of lignin and its main monomers. By consequence, an enhanced stiffening of the cell wall restricts soybean root growth.

Phylogenetic, Molecular, and Biochemical Characterization of Caffeic Acid o-Methyltransferase Gene Family in Brachypodium distachyon

Caffeic acid o-methyltransferase (COMT) is one of the important enzymes controlling lignin monomer production in plant cell wall synthesis. Analysis of the genome sequence of the new grass model Brachypodium distachyon identified four COMT gene homologs, designated as BdCOMT1, BdCOMT2, BdCOMT3, and BdCOMT4. Phylogenetic analysis suggested that they belong to the COMT gene family, whereas syntenic analysis through comparisons with rice and sorghum revealed that BdCOMT4 on Chromosome 3 is the orthologous copy of the COMT genes well characterized in other grass species. The other three COMT genes are unique to Brachypodium since orthologous copies are not found in the collinear regions of rice and sorghum genomes. Expression studies indicated that all four Brachypodium COMT genes are transcribed but with distinct patterns of tissue specificity. Full-length cDNAs were cloned in frame into the pQE-T7 expression vector for the purification of recombinant Brachypodium COMT proteins. Biochemical characterization of enzyme activity and substrate specificity showed that BdCOMT4 has significant effect on a broad range of substrates with the highest preference for caffeic acid. The other three COMTs had low or no effect on these substrates, suggesting that a diversified evolution occurred on these duplicate genes that not only impacted their pattern of expression, but also altered their biochemical properties.

Immunolocalization of two ligninO-methyltransferases in stems of alfalfa (Medicago sativa L.)

Caffeic acid 3-O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) catalyze parallel reactions that are believed to be involved in the biosynthesis of lignin monomers. Antisera specific for alfalfa (Medicago sativa L.) COMT or CCOMT were raised against the enzymes expressed inEscherichia coli, and were used for immunolocalization studies in lignifying alfalfa stem tissue. Both COMT and CCOMT were localized to xylem parenchyma cells, as assessed by light microscopy and immunocytochemistry. Electron microscopy revealed that both enzymes were located in the cytoplasm of xylem parenchyma cells, and to a lesser extent, in the cytoplasm of phloem cells. There was no significant difference in the localization pattern of COMT and CCOMT, suggesting that the two enzymes may be part of a metabolic grid leading to production of lignin monomers in lignifying tissue of mature alfalfa stem internodes.

Cinnamyl-alcohol dehydrogenase, a molecular marker specific for lignin synthesis: cDNA cloning and mRNA induction by fungal elicitor.

Cinnamyl-alcohol dehydrogenase (CAD; EC 1.1.1.195) catalyzes the final step in a branch of phenylpropanoid synthesis specific for production of lignin monomers. We have isolated a full-length cDNA clone encoding CAD, as a molecular marker specific for lignification, by immunoscreening a lambda gt11 library containing cDNAs complementary to mRNA from elicitor-treated cell cultures of bean (Phaseolus vulgaris L.). The clone comprises a single long open reading frame of 1767 base pairs, 31 base pairs of 5' leader, and 152 base pairs of 3' untranslated sequence. The deduced 65-kDa CAD polypeptide has several features that are strongly conserved in alcohol dehydrogenases. Addition of fungal elicitor to cell cultures stimulates CAD transcription, which leads to a remarkably rapid, but transient, accumulation of CAD mRNA, with no detectable lag and maximal levels after 1.5 hr. Southern blot analysis of bean genomic DNA indicates that elicitor-induced CAD is encoded by a single gene. The regulatory significance of the rapid activation of this CAD gene and the possible existence of a second, divergent CAD gene involved in lignification during xylogenesis are discussed.