Biomass‐derived degraded lignin and cellulose serve as possible alternatives to fossil fuels for energy and chemical resources. Fast pyrolysis of lignocellulosic biomass generates bio‐oil that needs ...further refinement. However, as pyrolysis causes massive degradation to lignin and cellulose, this process produces very complex mixtures. The same applies to degradation methods other than fast pyrolysis. The ability to identify the degradation products of lignocellulosic biomass is of great importance to be able to optimize methodologies for the conversion of these mixtures to transportation fuels and valuable chemicals. Studies utilizing tandem mass spectrometry have provided invaluable, molecular‐level information regarding the identities of compounds in degraded biomass. This review focuses on the molecular‐level characterization of fast pyrolysis and other degradation products of lignin and cellulose via tandem mass spectrometry based on collision‐activated dissociation (CAD). Many studies discussed here used model compounds to better understand both the ionization chemistry of the degradation products of lignin and cellulose and their ions' CAD reactions in mass spectrometers to develop methods for the structural characterization of the degradation products of lignocellulosic biomass. Further, model compound studies were also carried out to delineate the mechanisms of the fast pyrolysis reactions of lignocellulosic biomass. The above knowledge was used to assign likely structures to many degradation products of lignocellulosic biomass.
Lignin is the most abundant natural resource of aromatic moieties and the second most abundant natural biopolymer. Analytical techniques that obtain as much information as possible on the exact ...structural content of lignin species are essential for developing efficient processes that transform highly complex lignin wastes into value chemicals and biofuels. For mass spectrometric analysis of lignin samples, usually electrospray ionization, atmospheric pressure chemical ionization, or atmospheric pressure photoionization are used as ionization techniques. Matrix-assisted laser desorption/ionization (MALDI) is less frequently applied but offers a much more rapid screening option for lignin mixtures. In this study, we compared several common MALDI matrices for analysis of alkali lignin and discovered that different chemical matrices exhibited very different ionization efficiencies and selectivity with respect to the structures of the lignin-related compounds as well as the presence of heteroatoms. Importantly, the results highlight that the choice of matrix strongly determines the analytical coverage of molecular species in the complex lignin degradation mixtures.
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This perspective addresses efficiency and selectivity of high-temperature lignin liquefaction processes conducted in various reaction media as sub- and supercritical fluids. The challenges in ...efficient and selective production of high-value organic monomers from lignin are reviewed critically, along with analytical protocols essential for their accurate recovery after lignin degradation. The current approaches targeting the formation of phenolic monomers from lignin are discussed in terms of their repolymerization, a process that decreases the reaction selectivity and yield of the dominant phenolic monomers. The potential to solve this grand challenge is analyzed in terms of acid and/or protic cosolvent application, reduction of the reaction temperature, “quenching” of the reactive lignin depolymerization intermediates, and presence of heterogeneous catalysts, such as zeolites, metals, and metal oxides, sulfides, and phosphides.
There is increasing interest in the application of ionic liquids for the pretreatment and fractionation of lignocelluloses. In this study, a series of functional acidic ionic liquids (ILs) with ...various heterocyclic organic cations were synthesized. Corn stalks were successfully fractionated into lignin, hemicelluloses, and cellulose when ultrasonically pretreated with ILs at 70 °C for 3 h, and subsequently treated with alkaline extraction. High yields of IL-isolated lignin (18.3% to 19.6%) and (8.3% to 14.6%) were obtained using ILs in the absence and presence of water, respectively. The yield of cellulose ranged from 40.0 to 77.0% from IL treatments, whereas the yield of hemicelluloses ranged from 1.1% to 17.3%. Enzymatic hydrolysis of the isolated cellulose residual produced 89.2% to 94.9% reducing sugar with 77.8% to 86.1% glucose, which corresponded to 80.5% to 91.4% enzymatic conversion of cellulose. Syringol and vanillin were found as the main lignin degradation products.
Microbial degradation of lignocellulosic biomass is primarily affected by the composition and structure of biomass, as well as enzyme activities that are influenced by the presence of in-process ...degradation products. This study focuses on the latter, and demonstrates that cellulase activity of Neurospora discreta is stimulated in the presence of in-process soluble lignin degradation products. Two types of biomass - cocopeat and sugarcane bagasse, with contrasting lignin content and cellulose structure were tested at two biomass loadings each. At the higher biomass loading, cocopeat showed the highest amount of hydrolyzed cellulose and cellulase activity, despite its low cellulose content and recalcitrant cellulose structure. A strong positive correlation was revealed between the amount of in-process degraded lignin and cellulase activity, indicating a stimulatory effect on cellulase, which contradicts most previous literature. Furthermore, the causal relationship between the amount of degraded lignin and cellulase activity was established in a model system of commercial cellulase and standard soluble lignin. This work could pave the way for using biomass loading as a process lever to enhance cellulose hydrolysis in microbial conversion of lignocellulosic biomass.
Endoxylanase (Tx-Xyl) 3D-structure, quenching rate constants (
K
Q) and binding constants (
K
A) to phenolic compounds.
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► Phenolic compounds inactivate endoxylanase by non competitive ...multi-site inhibition mechanism. ► The microenvironment of tryptophan residues of the enzyme changes through phenolic compound binding. ► Enzyme–phenolics interaction affinity was higher with increasing phenolic hydroxyl content.
Phenolic compounds generated from lignin degradation during the pre-treatment step in the process of producing bioethanol from lignocellulosic biomass are known to be inhibitory to enzymatic hydrolysis and fermentation. The inactivation mechanism of a GH11 endoxylanase (Tx-Xyl) by several phenolic compounds varying in their hydroxyl and methoxyl radical content was investigated. Apparent kinetic inactivation parameters were measured as an approximate index of the inhibitory effects. All the tested aromatic compounds had strong negative impact on enzyme activity and kinetic analysis revealed non competitive multi-site inhibition mechanism. The interactions between Tx-Xyl and the phenolic compounds were further studied by steady-state (tryptophan) fluorescence spectroscopy. Changes in
λ
max of emission and quenching of fluorescence intensity indicated changes in the microenvironment of tryptophan residues. In agreement with the kinetic parameters, the fluorescence derived binding constants evidenced higher enzyme–phenolics interaction affinity with increasing phenolic hydroxyl radical content, suggesting clear correlations of such radicals with the inhibitory effects. Results indicated that the inhibitory effects of phenolic compounds on Tx-Xyl activity are most likely brought about by conformational alterations of the enzyme protein inducing steric inactivation.
Conversion of lignin into smaller molecules provides a promising alternate and sustainable source for the valuable chemicals currently derived from crude oil. Better understanding of the chemical ...composition of the resulting product mixtures is essential for the optimization of such conversion processes. However, these mixtures are complex and contain isomeric molecules with a wide variety of functionalities, which makes their characterization challenging. Tandem mass spectrometry based on ion–molecule reactions has proven to be a powerful tool in functional group identification and isomer differentiation for previously unknown compounds. This study demonstrates that the identification of the phenol functionality, the most commonly observed functionality in lignin degradation products, can be achieved via ion–molecule reactions between diethylmethoxyborane (DEMB) and the deprotonated analyte in the absence of strongly electron-withdrawing substituents in the
ortho
- and
para
-positions. Either a stable DEMB adduct or an adduct that has lost a methanol molecule (DEMB adduct-MeOH) is formed for these ions. Deprotonated phenols with an adjacent phenol or hydroxymethyl functionality or a conjugated carboxylic acid functionality can be identified based on the formation of DEMB adduct-MeOH. Deprotonated compounds not containing the phenol functionality and phenols containing an electron-withdrawing
ortho
- or
para
-substituent were found to be unreactive toward diethylmethoxyborane. Hence, certain deprotonated isomeric compounds with phenol and carboxylic acid, aldehyde, carboxylic acid ester, or nitro functionalities can be differentiated via these reactions. The above mass spectrometry method was successfully coupled with high-performance liquid chromatography for the analysis of a complex biomass degradation mixture.
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The potential of wheat straw for ethanol production after pretreatment with O
3
generated in a plasma at atmospheric pressure and room temperature followed by fermentation was investigated. We found ...that cellulose and hemicellulose remained unaltered after ozonisation and a subsequent washing step, while lignin was degraded up to 95% by O
3
. The loss of biomass after washing could be explained by the amount of lignin degraded. The washing water of pretreated samples (0–7 h) was analyzed for potential fermentation inhibitors. Approximately 30 lignin degradation products and a number of simple carboxylic acids and phenolic compounds were found, e.g., vanillic acid, acetic acid, and formic acid. Some components had the highest concentration at the beginning of the ozonisation process (0.5, 1 h), e.g., 4-hydroxybenzladehyde, while the concentration of others increased during the entire pretreatment (0–7 h), e.g., oxalic acid and acetovanillon. Interestingly, washing had no effect on the ethanol production with pretreatment times up to 1 h. Washing improved the glucose availability with pretreatment times of more than 2 h. One hour of ozonisation was found to be optimal for the use of washed and unwashed wheat straw for ethanol production (maximum ethanol yield, 52%). O
3
cost estimations were made for the production of ethanol at standard conditions.
The effect of lignin degradation products liberated during chemical hydrolysis of lignocellulosic materials on xylose-to-xylitol bioconversion by
Candida guilliermondii FTI 20037 was studied. Two ...aromatic aldehydes (vanillin and syringaldehyde) were selected as model compounds. A two-level factorial design was employed to evaluate the effects of pH (5.5–7.0), cell concentration (1.0–3.0
g
l
−1), vanillin concentration (0–2.0
g
l
−1) and syringaldehyde concentration (0–2.0
g
l
−1) on this bioprocess. The results showed that in the presence of vanillin or syringaldehyde (up to 2.0
g
l
−1) the cell growth was inhibited to different degrees with a complete inhibition of the yeast growth when the mixture of both (at 2.0
g
l
−1 each) was added to the fermentation medium. The xylitol yield was not significantly influenced by vanillin, but was strongly reduced by syringaldehyde, which showed a more pronounced inhibitor effect at pH 7.0. The yeast was also able to convert vanillin and syringaldehyde to the corresponding aromatic acids or alcohols and their formation was dependent of the experimental conditions employed.
Comparative studies on the structures of residual and dissolved lignins isolated from pine kraft pulp and pulping liquor have been undertaken using the 1H−13C HMQC NMR technique, GPC, and sugar ...analysis to elucidate the reaction mechanisms in kraft pulping and the lignin reactivity. A modified procedure for the isolation of enzymatic residual lignins has resulted in an appreciable decrease in protein contaminants in the residual lignin preparations (N content < 0.2%). The very high dispersion of HMQC spectra allows identification of different lignin moieties, which signals appear overlapped in 1D 13C NMR spectra. Elucidation of the role of condensation reactions indicates that an increase in the degree of lignin condensation during pulping results from accumulation of original condensed lignin moieties rather than from the formation of new alkyl−aryl structures. Among aryl−vinyl type moieties, only stilbene structures are accumulated in lignin in appreciable amounts. Benzyl ether lignin−carbohydrate bonds involving primary hydroxyl groups of carbohydrates have been detected in residual and dissolved lignin preparations. Structures of the α-hydroxyacid type have been postulated to be among the important lignin degradation products in kraft pulping. The effect of the isolation method on the lignin structure and differences between the residual and dissolved lignins are discussed. Keywords: Residual lignin; kraft lignin; kraft pulping; NMR; 2D HMQC technique; lignin−carbohydrate complex; condensation reactions; lignin degradation products
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