Rapid mass spectral fingerprinting of complex mixtures of decomposed lignin: Data‐processing methods for high‐resolution full‐scan mass spectra

Lignin is the second most abundant natural biopolymer and its wastes are significant sources for renewable chemicals as an alternative to conventional fossil fuels. Consequently, chemical characterization methods are required to assess the content of valuable chemicals contained in these complex lignin wastes. This short overview summarizes rapid data‐processing methods developed in our laboratory for application to full‐scan raw data from high‐resolution mass spectrometry experiments of decomposed lignin samples. The discussed graphical and statistical methods support the initial classification and elucidation of the main structural features of the lignin components without the need for time‐consuming tandem mass spectrometry analyses.     

Selectively transform lignin into value-added chemicals

Selective transformation of lignin into value-added chemicals is of strategic significance. Phenols, aldehydes, carboxylic acids, alkanes and arenes can be harvested from lignin with high selectivity under appropriate reaction condition.     

(Re)-thinking the bio-prospect of lignin biomass recycling to meet Sustainable Development Goals and circular economy aspects

Lignin, as an abundant natural polymer with interesting mechanical, antimicrobial, and antioxidant properties, has the possibility to produce numerous chemicals and biofuels of current interest. However, the structural recalcitrance, heterogeneity, and complex extraction methods of lignin can hinder its transformation into value-added materials. Therefore, the research community is exploring innovative bioconversion technologies capable of effectively valorizing lignin. Thus, effective bioconversion and deconstruction methods have been recently studied. In this review, we first define lignin as a versatile raw material considering its characteristics, properties, and abundance. Then, lignin valorization is described in terms of the current opportunities and technical challenges. Finally, we discuss the industrial potential of lignin-derived products such as biofuels, biopolymers, biopesticides, and fertilizers. Those lignin-derived products are highly valuable for the energy and food industries, which are two main sectors challenged by the rapid growth of population, urbanization, and consumption. Thus, progress on lignin valorization would represent significant advancements in the Sustainable Development Goals (SDGs) and circular economy aspects.     

Advancements and Perspectives toward Lignin Valorization via O-Demethylation

Lignin represents the largest aromatic carbon resource in plants, holding significant promise as a renewable feedstock for bioaromatics and other cyclic hydrocarbons in the context of the circular bioeconomy. However, the methoxy groups of aryl methyl ethers, abundantly found in technical lignins and lignin-derived chemicals, limit their pertinent chemical reactivity and broader applicability. Unlocking the phenolic hydroxyl functionality through O-demethylation (ODM) has emerged as a valuable approach to mitigate this need and enables further applications. In this review, we provide a comprehensive summary of the progress in the valorization of technical lignin and lignin-derived chemicals via ODM, both catalytic and non-catalytic reactions. Furthermore, a detailed analysis of the properties and potential applications of the O-demethylated products is presented, accompanied by a systematic overview of available ODM reactions. This review primarily focuses on enhancing the phenolic hydroxyl content in lignin-derived species through ODM, showcasing its potential in the catalytic funneling of lignin and value-added applications. A comprehensive synopsis and future outlook are included in the concluding section of this review.     

Selective Utilization of the Methoxy Group in Lignin to Produce Acetic Acid

Selective transformation of lignin into a valuable chemical is of great importance and challenge owing to its complex structure. Herein, we propose a strategy for the transformation of methoxy group (‐OCH₃) which is abundant in lignin into pure highly valuable chemicals. As an example to apply this strategy, a route to produce acetic acid with high selectivity by conversion of methoxy group of lignin was developed. It was demonstrated that the methoxy group in lignin could react with CO and water to generate acetic acid over RhCl₃ in the presence of a promoter. The conversions of methoxy group in the kraft lignin and organosolv lignin reached 87.5 % and 80.4 %, respectively, and no by‐product was generated. This work opens the way to produce pure chemicals using lignin as the feedstock.     

Lignin‐based polymers via graft copolymerization

Lignin is an important source of synthetic materials because of its abundance in nature, low cost, stable supply, and no competition to the human food supply. Lignin, a cross‐linked phenolic polymer, contains a large number of aromatic groups that can be used as a substitute for petroleum‐based aromatic fine chemicals. However, modification of lignin is necessary for its application in advanced materials due to its chemically inert nature and structural complexity. Polymeric modification of lignin via graft copolymerization represents an important avenue for modification because this method forms stable covalent bond linkages between lignin and synthetic functional polymers. In this review, we discuss recent synthetic strategies toward polymeric modification of lignin using graft copolymerization and the special properties and applications of the produced lignin copolymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3515–3528     

Palladium‐Catalyzed Formal Cross‐Coupling of Diaryl Ethers with Amines: Slicing the 4‐O‐5 Linkage in Lignin Models

Lignin is the second most abundant organic matter on Earth, and is an underutilized renewable source for valuable aromatic chemicals. For future sustainable production of aromatic compounds, it is highly desirable to convert lignin into value‐added platform chemicals instead of using fossil‐based resources. Lignins are aromatic polymers linked by three types of ether bonds (α‐O‐4, β‐O‐4, and 4‐O‐5 linkages) and other C?C bonds. Among the ether bonds, the bond dissociation energy of the 4‐O‐5 linkage is the highest and the most challenging to cleave. To date, 4‐O‐5 ether linkage model compounds have been cleaved to obtain phenol, cyclohexane, cyclohexanone, and cyclohexanol. The first example of direct formal cross‐coupling of diaryl ether 4‐O‐5 linkage models with amines is reported, in which dual C(Ar)?O bond cleavages form valuable nitrogen‐containing derivatives.     

The influence of interunit carbon–carbon linkages during lignin upgrading

The cleavage of β-O-4 linkages in lignin can generate monomers with a phenyl propane structure that can easily be upgraded into valuable hydrocarbon biofuels and renewable aromatic chemicals. High-yield lignin monomer production from extracted (or technical) lignin that is produced in a practical way could facilitate the productivity and profitability of biomass conversion processes. However, interunit carbon–carbon (C–C) linkages present in native lignin or formed during lignin condensation in biomass pretreatments dramatically reduce lignin monomer yields. Here, we present a perspective on biological and chemical strategies that have been successfully used to reduce the formation of C–C linkages in native or technical lignin. We analyze the mechanisms involved in these strategies and offer our views on improving the quality of technical lignin resulting from biomass conversion in order to achieve high-yield lignin monomer production.     

Pt/NbPWO双功能催化剂的制备及氢解碱木质素制备芳香单体

Lignin is a natural aromatic polymer that accounts for nearly 30% of lignocellulose and is considered the only renewable aromatic (re)source for producing aromatic chemicals or liquid fuels via the cleavage of C―O ether bonds and C―C bonds. Thus far, the majority of investigations involving the production of valuable compounds via lignin hydrogenolysis have focused on the cleavage of relatively labile C―O bonds only, which restricts the efficiency of hydrogenolysis. Therefore, in this work, a bifunctional Pt/NbPWO catalyst was synthesized using hydrothermal and wet impregnation methods. It was found that aromatic monomers with a yield of 18.02% could be obtained by breaking the C―O and C―C bonds in alkali lignin. This reaction was applicable to breaking the key C―C bonds when the C―O ether bonds were broken in lignin polymers. The hydrogenolysis mechanism most likely involves the abundant Brønsted acid and Lewis acid sites on the catalyst that facilitate C―C bond activation. Additionally, the synergy between the support and Pt species in the Pt/NbPWO catalyst was primarily emphasized.     

Pt/NbPWO双功能催化剂的制备及氢解碱木质素制备芳香单体

木质素是一种天然芳香族聚合物,约占木质纤维素的30%,是唯一通过裂解C―O醚键和C―C键生产芳香族化学品或液体燃料的可再生芳香族资源。迄今为止,对木质素氢解制备有价值化合物的研究主要集中在相对不稳定的C―O键的裂解上,这限制了木质素氢解的效率。采用水热法和湿浸渍法制备了多功能Pt/NbPWO催化剂。通过破坏碱木质素中的C―O键和C―C键,可以得到产率为18.02%的芳香族单体。该反应不仅可以断裂木质素聚合物中醚键,同时也可以断裂部分关键的C―C键。其氢解机理可能是丰富的Brønsted酸和Lewis酸位点参与了C―C的活化。此外,重点分析载体和Pt物种在Pt/NbPWO催化剂中的协同作用。     

NaBH4/I2催化加氢还原碱木质素的研究

从麦草碱法制浆黑液中提取木质素,精制后,以苯乙酮为木质素的模型化合物,对催化剂组成及溶剂进行了考察.在此基础上,以NaBH4/I2为催化剂,无水乙醇为溶剂,对木质素进行加氢还原裂解反应研究.考察了温度和时间对木质素催化加氢效果的影响,采用红外光谱(FTIR)、元素分析及凝胶渗透色谱分析(GPC),表征木质素反应前后结构的变化.凝胶渗透色谱分析表明,加氢还原后木质素的分子量明显降低.采用自动电位滴定法测定反应前后木质素中总羟基含量,反应后木质素中总羟基含量为10.19%.得到了NaBH4/I2催化木质素加氢还原反应的最优条件:以1,2-二氯乙烷和乙醇(2∶1,v/v)作溶剂,m(NaBH4)∶m(I2)=1∶1,温度175℃,反应时间15 h.     

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering,Biorefining and Catalysis

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.     

高沸醇溶剂法提取胡萝卜木质素

以乙二醇水溶液为溶剂,采用高沸醇溶剂法（high boiling solvent,HBS）从胡萝卜中提取木质素。 结果表明,将胡萝卜干粉投入到质量分数为80%的乙二醇中,在210 ℃、料液质量比为1∶6的条件下蒸煮2 h,木质素收率最高（17.79%）。 采用红外光谱（FTIR）、紫外光谱(UV)和核磁共振(1H NMR)等测试技术对其结构进行了表征。 结果表明,高沸醇法提取的胡萝卜木质素具有木质素类化合物的典型结构特征。 通过元素分析和甲氧基含量的测定,得到胡萝卜木质素的C9结构模型为C9H9.62O1.99(OCH3)0.96。 利用Fenton反应产生羟基自由基·OH,研究了胡萝卜木质素对羟基自由基的抑制效率。 结果表明,胡萝卜木质素在较低的浓度（50 mg/L）下对羟基自由基就具有较强的抑制效果,抑制率最高可达64.9%。     

木质素结构及分析方法的研究进展

木质素广泛存在于高等植物中,是仅次于纤维素的地球上第二丰富的生物聚合体,有效地利用自然界中含量丰富的木质素具有重大的意义。然而,由于木质素结构的复杂性,对其具体结构的认识和寻找合适的木质素结构分析方法成为人们长期探索的课题。本文主要阐述了目前对木质素单体的生物合成途径和木质素的化学组成、官能团、单体间的连接方式、木质素模型化合物等木质素结构方面的研究进展,并从降解法和非降解法两个角度介绍了常用的木质素结构分析方法。     

Identifying residues in natural organic matter through spectral prediction and pattern matching of 2D NMR datasets

This paper describes procedures for the generation of 2D NMR databases containing spectra predicted from chemical structures. These databases allow flexible searching via chemical structure, substructure or similarity of structure as well as spectral features. In this paper we use the biopolymer lignin as an example. Lignin is an important and relatively recalcitrant structural biopolymer present in the majority of plant biomass. We demonstrate how an accurate 2D NMR database of approximately 600 2D spectra of lignin fragments can be easily constructed, in approximately 2 days, and then subsequently show how some of these fragments can be identified in soil extracts through the use of various search tools and pattern recognition techniques. We demonstrate that once identified in one sample, similar residues are easily determined in other soil extracts. In theory, such an approach can be used for the analysis of any organic mixtures.     

Nanoengineered ligninolytic enzymes for sustainable lignocellulose biorefinery

Lignin is a key structural component of lignocellulosic biomass with immense potential to replace non-renewable and environmentally unfriendly fossil resources. Structural recalcitrance, heterogeneity, and multifaceted composition of lignin are the major impediments to its gainful biotransformation to a spectrum of bio-based products, biomaterials, and specialty chemicals. In contrast to physicochemical methods, harnessing the biocatalytic potential of the robust ligninolytic armory is considered a greener and more sustainable way for lignin biorefinery. Immobilization of ligninolytic enzymes on different nanoengineered support matrices resulted in designing nanobiocatalytic system with intensified catalytic performance and long-term stability for efficient lignocellulosic biomass valorization. Enzyme incorporation on magnetic nanostructures additionally facilitates facile separation, recovery, and reusability of magnetic nanobiocatalysts. Therefore, developing and implementing immobilized ligninolytic enzyme-based nanoengineered biocatalytic systems constitutes a prodigious and eco-sustainable option to catalyze the deconstruction of lignocellulosic biomass. The multi-enzyme nano-biocatalytic system offers the advantage of direct substrate conversion into the product in a single step owing to its concurrent biocatalytic attributes. This opinion article spotlights current achievements and state-of-the-art developments in engineering ligninolytic enzymes to create a novel biocatalytic system to create greener and sustainable lignocellulose biorefineries ranging from the production of biomaterials to bioenergy.     

Effect of different superficial treatments on structural,morphological and superficial area of Kraft lignin based charcoal

Lignin is a biomass derived from an abundant renewable source, rich in carbon and with potential application in modern society. The goal of this work is to add more value to lignin through its thermal conversion in charcoal, as well contribute to solutions linked to environmental preservation. Charcoal was obtained from Kraft lignin and its surface was modified using chemical (acid attack) and physical (microwave plasma) methods, in order to get charcoals different characteristics. In this work, the prepared charcoals were characterized by field emission gun - scanning electron microscope (FEG-SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and superficial area by BET analyses. Microscopic analyses evidenced morphological differences in the samples as consequence of the used superficial treatments. Raman spectroscopy results point to an increase in the carbon material disorder after chemical and physical treatments. The acid attack of charcoal increased its superficial area by 40% (403 m²/g) in relation to the charcoal without chemical treatment (287 m²/g). Physical treatment based on microwave plasma promoted a further increase in superficial area of 63% (468 m²/g). FT-IR showed that chemically treated charcoals presented more functional groups. Based on these results, it can be verified that the production of activated charcoal from lignin is viable and its superficial area can be increased using acid and plasma treatments, the latter being a more efficient and clean method.     

Effects of lignin on the ionic-liquid assisted catalytic hydrolysis of cellulose: chemical inhibition by lignin

The production of cellulose-derived biofuels and biochemicals, such as bioalcohols and bioplastics, from lignocellulose requires the isolation of cellulose by lignin removal or delignification processes. While the remaining lignin and its phenolic fragments have been reported to inhibit the biological conversion of cellulose, we observed that the catalytic hydrolysis of cellulose also can be inhibited most likely because of an associative interaction between cellulose and lignin. The associative interaction between cellulose and the functional groups of lignin was proven by gel-permeation-chromatography measurement of regenerated mixtures of lignin and cellulose which simulate the lignocellulose-derived cellulose containing lignin as an impurity. Chemical bonds between cellulose and lignin were hypothesized using lignin model compounds containing known functionalities such as hydroxyl, methoxy, phenyl, allyl, and carboxyl groups in order to explain the effects of lignin on the hydrolysis of cellulose. The yield of glucose from cellulose dropped when carboxylic and hydroxyl groups were present possibly because of the formation of ether and ester bonds between the lignin and cellulose. These observations may help develop the chemical processes and therefore convert the inedible biomass resource of lignocellulose-based cellulose containing lignin and its derivatives to the valuable fuels and chemicals.     

Recent advances in epoxy resins and composites derived from lignin and related bio-oils

This short critical review gives an insight on the potential that lignin and its bio-oils present towards the production of thermosetting epoxy polymers and composites. Green and sustainable ways of producing monomers and polymers from renewable sources are critical and lignin, as an underutilized bio-based waste material, presents a high exploitation potential. Due to its versatile and highly functional phenolic structure, the utilization of lignin or its depolymerized fractions (bio-oils) has been investigated in the last years as alternative for fossil-based epoxy resin pre-polymers and crosslinkers. Lignin can in fact be considered as a crosslinker for epoxy resins, especially after appropriate functionalization with amine groups or with additional hydroxyl groups, or it can be modified with epoxide groups towards the replacement of toxic BPA-based epoxy prepolymers. Furthermore, lignin derived pyrolysis or hydrogenolysis bio-oils may offer highly reactive soluble oligomers that after appropriate functionalization could be utilized as bio-based epoxy prepolymers. The lignin-based epoxy resins and composites exhibit similar or even better and novel properties, compared to those of pristine epoxy polymers, thus rendering lignin a highly valuable feedstock for further utilization in the thermoset polymer industry.     

Sequestration and transport of lignin monomeric precursors

Lignin is the second most abundant terrestrial biopolymer after cellulose. It is essential for the viability of vascular plants. Lignin precursors, the monolignols, are synthesized within the cytosol of the cell. Thereafter, these monomeric precursors are exported into the cell wall, where they are polymerized and integrated into the wall matrix. Accordingly, transport of monolignols across cell membranes is a critical step affecting deposition of lignin in the secondarily thickened cell wall. While the biosynthesis of monolignols is relatively well understood, our knowledge of sequestration and transport of these monomers is sketchy. In this article, we review different hypotheses on monolignol transport and summarize the recent progresses toward the understanding of the molecular mechanisms underlying monolignol sequestration and transport across membranes. Deciphering molecular mechanisms for lignin precursor transport will support a better biotechnological solution to manipulate plant lignification for more efficient agricultural and industrial applications of cell wall biomass.