Lignin Samples Research Articles

Modulation of improved lignin structure for conversion of iron oxides on the metal artifact

This research reveals the structural analysis and antioxidant properties of unmodified ethanol organosolv lignin (AH EOL) and modified ethanol organosolv lignin (AHD EOL and AHPB EOL) upon incorporation with 1,5-dihydroxy naphthalene and p-benzoquinone, respectively. The extracted lignin derived from OPF was then evaluated with FTIR NMR, GPC, thermal analyses (TGA and DSC), solubility test, and antioxidant activity. Modified ethanol organosolv lignin incorporated with p-benzoquinone (AHPB EOL) exhibited the smallest lignin matrix, which leads to the highest water solubility and antioxidant activity compared to other ethanol organosolv lignin (EOL) samples (D% AHPB EOL: 24.25 ± 0.13 % > D% AHD EOL: 22.50 ± 0.16 % > D% AH EOL: 20.50 ± 0.22 %). The extracted EOL samples derived from OPF were then utilized in a preliminary rust conversion study of a metal artifact, which indicated that 7 wt% AHPB EOL possessed the highest RT% with 88.81 ± 0.14 %. The XRD and surface analysis of the treated rust also showed that it had transformed the archeological rust into an amorphous phase.

Isolation of β-O-4-Rich Lignin From Birch in High Yields Enabled by Continuous-Flow Supercritical Water Treatment.

The continuous flow supercritical water (scH2O) treatment of Birch wood (T=372-382 °C; t=0.3-0.7 s; p=260 bar) followed by alkali extraction of lignin allowed for the isolation of lignin and lignin carbohydrate complexes (LCCs) with a high number of β-O-4 moieties in the range 29-57/100 Ar (evaluated by quantitative 13C NMR analysis) in yields ranging between 13-19 wt % with respect to the initial wood. A "lightning rod effect" of carbohydrates has been claimed to explain the low degradation of β-O-4 bonds during the process. The structure of the isolated lignin was thoroughly elucidated via comprehensive NMR studies (HSQC, 13C and 31P). A low degree of condensation (DC)<5 % was found for all the lignin samples, which was only slightly dependent on the reaction severity. The number of aliphatic -OH, phenolic -OH, and -COOH groups was in the range 3.37-5.25, 1.41-2.31 and 0.39-0.73 mmol/g, respectively. The number of -COOH groups increased with increased severity, suggesting that oxidation can occur during the scH2O treatment. Furthermore, by simply varying the reaction severity, it was possible to tune important lignin properties, like the molar mass and the glass transition temperature (Tg).

Evaluation of safety and biomedical potential of water-soluble oat lignin Avena sativa L.

The study of the value of lignin for biomedical use is generating growing interest. For the first time, the safety and biological efficacy of lignin from the stems of the oat Avena sativa L. were studied, necessary for a preliminary assessment of its biomedical potential, have been studied. In vitro experiments, a sample of oat lignin exhibited cytotoxicity to the HeLa, A549, and HT-29 cancer cell lines, depending on the concentration. At maximum concentrations 125 and 150 μg/ml, it reduced their survival and increased the level of reactive oxygen species. In vivo experiments, a sample of oat lignin, with acute (from 5 to 250 mg/kg body weight) and chronic (300, 1200 and 2000 mg/kg body weight) administration, did not have a toxic or genotoxic effect on the organs of mice. The biological efficacy of the oat lignin was manifested in activation of repair processes in bone marrow and thyroid gland, a decrease in the level of abnormal spermatozoa in males, stimulation of reproductive performance of females and in increase in research activity and a decrease in the level of anxiety in animals. The results indicate the prospects for further study of the medical and biological potential lignin of the oat.

Surface Wettability of Lignin Materials from Supercritical Water Hydrolysis of Wood

To meet the demands of the evolving circular economy, there is a growing need for renewable resources as base materials for innovative, easily recyclable products. Lignin, the second most abundant biopolymer, has emerged as a promising source of aromatics and reinforcing agent in polymer composites. For the successful manufacturing of homogeneous composite materials, good bonding between the coexisting phases is essential to prevent the formation of voids and agglomerates. Therefore, understanding the surface properties of these materials is crucial for designing optimal composite compounds. In this study, the wettability of lignin-cellulose composites and lignin samples obtained through supercritical water hydrolysis (SCWH) of birch wood is investigated. The contact angle (CA) technique, specifically the sessile drop method, was employed to assess and compare the wettability of SCWH lignin with commercially available lignin and raw birch wood. The results provide insights into their surface energy, adhesion, and hydrophilic or hydrophobic characteristics under processing conditions. All samples exhibited hydrophilicity, with an initial CA approximately 40 °, except for raw birch wood, which had a higher initial CA of ~ 64°. Notably, when lignin is accompanied by significant amounts of cellulose, different trends in CA changes over time were observed. The influence of pressure on the CA between water and these polymers was also analyzed, but no significant impact was detected. This research advances the development of lignin-based materials with tailored surface properties for various industrial applications.

Controlled chemical transformation of lignin by nitric acid treatment and carbonization

This study focuses on understanding the chemical reactions and results of Kraft lignin transformation through nitric acid treatment and subsequent carbonization. With its rich carbon content, lignin stands out as a promising candidate for the manufacturing of high-value carbon materials. The lignin underwent effective nitration, depolymerization, and oxidation under ambient conditions and at 40 °C, while a slight increase in reaction temperature significantly reduced the reaction time. The molecular weight Mw was effectively reduced from 4371 g/mol to 767 g/mol. The acid-treated lignin samples with incorporated nitro groups were further carbonized to create nitrogen-doped carbon structures. The resulting materials show stable nitrogen content (about at 5 wt%) even after carbonization due to the transformation of nitro groups into thermally stable pyridinic moieties, thereby exhibiting enhanced electrocatalytic properties compared to nitrogen-free carbon materials derived from Kraft lignin. The nitric acid-assisted treatment of lignin obviates the need for catalysts, and additional extraction or purification steps for preparing bio-derived carbon precursors, rendering it facile, fast, and cost-efficient.

Preparation of structural colors from lignin: Improving the homogeneity between different raw materials by solvent precipitation fractionation

Structural color materials prepared by lignin colloidal spheres (LCSs) have attracted great interest due to their biocompatibility, biodegradability, multifunctionality, and ability to weaken incoherent scattering. However, the lignin raw materials originating from different biomass resources have distinct heterogeneity leading to a big difference in the size of resultant LCSs via self-assembly, which is a major challenge for the preparation of stable structural colors. In this work, we employ the solvent precipitation method to fractionate the crude lignin samples obtained from corncob, softwood, hardwood, and bamboo, respectively. After solvent precipitation fractionation, the self-assembly behaviors of different lignin samples become similar. The LCSs prepared by fractionated lignin samples show quite similar sizes (212, 202, 215, and 210 nm) compared with the LCSs prepared by crude lignin samples (341, 83, 128, and 253 nm). The quantitative atomic force microscope force spectroscopy demonstrates the improved homogeneity of LCSs prepared by fractionated lignin samples is attributed to similar long-ranged intermolecular forces in self-assembly. By employing LCSs as building blocks, the lignin-based pigments with stable structural colors are successfully prepared. The presented colors are independent of the biomass resource types, which benefits the product stability and large-scale production of lignin-based structural color materials.

Polymerized lignin based carbon nanofiber for high-performance energy generator and storage

Lignin is an ideal precursor for the preparation of lignin-based materials. However, the lack of molecular weight, thermal stability and spatial configuration of lignin hinder the practical application of lignin in lignin-based carbon materials. Herein, a novel and effective strategy for preparing high-performance lignin-based carbon nanofibers (CFs) is designed. Lignin samples with different chemical structures are prepared by polymerization modification and fractionation. This strategy completely gets rid of the dependence on petroleum-based polymers in the electrospinning process, and makes the lignin-based CFs show good performance in energy storage, temperature sensor, and moisture power generation. The specific capacitance and energy of lignin-based supercapacitors reach 357.2 F/g and 31.5 Wh/kg, respectively. The CFs exhibit good flame retardancy and thermal sensitivity, and the response time is 1 s. In addition, moisture power generators assembled with CFs exhibit great potential. The output voltage of moisture power generators has a linear relationship with the number of devices. With the increase of series equipment, the output voltage of 5 series moisture power generators is 4.7 V. This work provides a new approach to prepare high-performance lignin-based carbon nanofibers, which has a good application prospect in the fields of energy storage, sensing and power generation.

Research on modeling the lignin waste molecular structure and pyrolytic reactions using ReaxFF MD method

The lack of clarity in characterizing the chemical structure model of lignin waste hinders the comprehensive understanding of its pyrolysis reaction mechanism. The characterization results revealed that the de–alkalized lignin sample contained larger amounts of aliphatic and aromatic carbons compared to carbonyl carbons. Additionally, the aliphatic carbon molecules exhibited a higher presence of oxygenated carbons. The degree of aromaticity (fa) was determined to be 63.93 %. The resulting single–molecule structural model exhibited a molecular formula of C33H26O11 and a molecular weight of 598.56. Furthermore, the evolutions patterns and formation pathways of primary volatile components (H2, CH4, CO, and CO2) in pyrolysis reactions were revealed. The number of H2 molecules exhibited a positive correlation with the rise in pyrolysis reaction temperature. CH4 molecules exhibited a pattern of initially increasing and subsequently decreasing. The number of CO molecules exhibited a positive correlation with the rise in pyrolysis temperature. Conversely, the number of CO2 molecules demonstrated an initial increase followed by a decrease as the pyrolysis temperature increased. Finally, the generation pathway analysis of pyrolysis products shows that the H2 molecule is composed of two hydrogen atoms bonded together that have been dissociated from the carboxyl group. Alternatively, it can be produced through hydrogenation reactions involving hydrogen atoms detached from the carboxyl group and methyl groups. CH4 is primarily generated through the reaction between –CH3 and free hydrogen ions. CO is primarily produced through the cleavage of the carbonyl group. CO2 is primarily produced through the cleavage of carboxyl and ester groups.

Insights into the Chemical Structure and Antioxidant Activity of Lignin Extracted from Bamboo by Acidic Deep Eutectic Solvents.

Deep eutectic solvents (DESs) composed of choline chloride as hydrogen bond acceptors (HBAs) and six organic acids as hydrogen bond donors (HBDs) were used to extract lignin from bamboo (Phyllostachys edulis (Carrière) J. Houz.). The structures of the DES-extracted lignin samples were analyzed by Fourier-transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-vis), thermogravimetric analysis (TG), and gel permeation chromatography (GPC) to investigate the relationship between the chemical structure of lignin and its antioxidant activity. The results showed that DES treatment removed a large portion of the lignin (73.37-86.38%) from bamboo, and the chemical structure of lignin was changed due to the use of different types of HBDs. The extracted lignin exhibited good UV-vis light shielding properties, thermal stability, and antioxidant activity. Moreover, the total phenolic hydroxyl content of lignins was positively correlated with their antioxidant activity, while the molecular weight of lignins was negatively correlated with their antioxidant activity. Notably, lignin extracted with choline chloride-p-toluenesulfonic acid had the highest phenolic hydroxyl content and lower molecular weight, showing the strongest antioxidant activity (IC50DPPH = 417.69 μg/mL, IC50ABTS = 58.62 μg/mL). This study confirms the high thermal stability, excellent antioxidant activity, and UV shielding properties of lignin extracted with choline chloride-organic acid DESs, suggesting its potential application in the fields of antioxidants and material modifiers.

The presence of nanoparticles in aqueous droplets containing plant-derived biopolymers plays a role in heterogeneous ice nucleation.

Organic matter can initiate heterogeneous ice nucleation in supercooled water droplets, thereby influencing atmospheric cloud glaciation. Predicting the ice nucleation ability of organic matter-containing cloud droplets is challenging due to the unknown mechanism for templating ice. Here, we observed the presence of nanoparticles in aqueous samples of known ice-nucleating biopolymers cellulose and lignin, as well as in newly identified ice-nucleating biopolymers xylan and laminarin. Using our drop Freezing Ice Nuclei Counter (FINC), we measured the median ice nucleation temperature (T50) of xylan and of laminarin droplets of 2 μl to be -14.2 and -20.0 °C, respectively. Next, we characterized these samples using nanoparticle tracking analysis, and we detected and quantified nanoparticles with mean diameters between 132 and 267nm. Xylan contained the largest nanoparticles and froze at higher temperatures. Xylan also dictated the freezing in a 1:1:1:1 mixture with cellulose, lignin, laminarin, and xylan. Filtration experiments down to 300 kDa with the xylan sample indicated that the presence of nanoparticles triggered freezing. Overall, only samples with mean diameters above 150nm froze above -20 °C. Furthermore, we determined the ice-active site densities normalized to particle concentrations, surface area, and mass of the nanoparticles to show that the samples' nucleation site densities are similar to sea spray aerosols and nanometer-sized dust. The identification and characterization of xylan and laminarin as nanometer-sized ice-nucleating substances expands the growing list of organic matter capable of impacting cloud formation and thus climate.

UV–vis spectroscopy as a rapid method for evaluation of total phenolic hydroxyl structures in lignin

Abstract Phenolic hydroxyl groups in lignin are crucial for understanding its structure, reactivity, and potential applications. Various methods have been developed for the determining phenolic groups in lignin. This study focuses on the comparison of a simple, cost-effective, and time-efficient UV–vis ionization difference technique with the highly accurate 31P NMR spectroscopy for analyzing lignin samples of different origins and isolation methods. The results were carefully evaluated, and the strengths and limitations of each method were discussed. Two eco-friendly UV–vis approaches were proposed for a rapid and comprehensive evaluation of the total phenolic-OH groups: one using a strong alkaline solution for analyzing common types of technical lignins, and another employing multipoint wavelength calculations, effective for analyzing softwood lignins regardless of the extraction method. Additionally, the research highlighted the importance of selecting appropriate model phenolic compounds to accurately assess the phenolic hydroxyl group content in lignins using the UV–vis method. Offering straightforward and rapid analysis, with results closely aligning with 31P NMR data, this method is a promising alternative for routine analysis.

Formation of the emerging disinfection byproducts halocyclopentadienes from phenolic compounds after chlorination

Halocyclopentadienes (HCPDs) are an emerging class of alicyclic disinfection by-products (DBPs) with high toxicity in disinfected drinking water. However, their potential precursors remain unclear, which hinders the understanding of their formation and further development of control strategies. In this study, two HCPDs, 1,2,3,4-tetrachloro-1,3-cyclopentadiene (TCC) and 1,2,3,4,5,5-hexachloro-1,3-cyclopentadiene (HCC), were identified in chlorinated lignin and tannic acid samples for the first time. The chlorination of four lignin-like and two tannic-like phenolic model compounds confirmed that guaiacol and digallic acid can produce HCPDs. According to their structures, ortho-substituents of phenolic compounds were speculated to be crucial for HCPDs formation. The simulated disinfection of catechol, 2-ethoxyphenol (2-EOP), 2-propoxyphenol (2-POP) and 3,4-dihydroxy-5-methoxybenzoic acid (DH-5-MBA) with different ortho-substituents demonstrated that three of these compounds can generate HCPDs, except catechol, which further indicates that ortho-substituents, such as the methoxy, ethoxy and propoxy groups, contribute to HCPDs generation. Guaiacol was the simplest compound for generating HCPDs, and possible formation pathways during chlorination were proposed. Seven hydroxy-chlorocyclopentadienes were tentatively identified and are likely important intermediates of HCPDs formation. Additionally, TCC and HCC were confirmed in tap water and chlorinated SRNOM samples with total concentrations up to 11.07 ng/L and 65.66 ng/L, respectively, further demonstrating the wide existence of HCPDs and their precursors. This study reports the clear precursors of HCPDs and provides a theoretical foundation for controlling HCPDs formation in disinfected drinking water.

Characterization of fused aromatic multirings in lignin via synchronous fluorescence spectroscopy and catalytic hydro-depolymerization

The structural analysis of lignin is highly significant in practical terms. We investigated the presence of fused aromatic multirings (FARs) distinct from conventional phenylpropane units in seven lignin types using synchronous fluorescence spectroscopy (SFS) and quantified their relative proportions. Our findings reveal that FARs are present in all lignin samples, primarily composed of 2–6 rings. The relative distribution of FARs in lignin is ranked as follows: 2 rings > 1 ring > 4 rings > 3 rings > 6 rings > 5 rings. The relative contents of these FARs (1–6 rings) exhibit slight variations across all lignin samples. Notably, FARs with 2 rings are the highest at 35 %−53 %, while those with 5 and 6 rings are the lowest, collectively accounting for less than 1.37 %. Moreover, the relative contents of FARs with 1 and 2–6 rings range approximately 24 %−32 % and 68 %−76 %, respectively. Furthermore, the catalytic hydro-depolymerization of lignin was performed and the experimental results demonstrate that the hydro-liquefied oil from lignin contains aromatic compounds with 1–7 rings, and the relative contents of aromatic compounds with 1 ring and 2–7 rings in the oil are 29 % and 71 %, respectively, which is consistent with that of lignin SFS. These findings are also consistent with the existing literature and experimental results. This study offers an explanation for the low yields observed in current depolymerization processes targeting small molecules within lignin and establishes a theoretical foundation for achieving high yields in the preparation of multiring aromatic hydrocarbons.

UV-absorption Capacity of Selected Crude and Functionalized Lignin for Use in Sunscreens

The most commonly used commercial sunscreen agents are aromatic compounds oxybenzone and octinoxate, as they offer a wide range of protection in the UV spectrum. However, oxybenzone has the highest rate of skin irritation, while octinoxate has poor photostability. Oxybenzone is also a genotoxicant and skeletal endocrine disruptor to corals. Appropriate, less toxic and preferentially biorenewable alternatives should be recommended. Lignin, an abundant byproduct of pulp and biorefinery industries, is expected to be an effective replacement for oxybenzone and octinoxate due to its aromatic structure, which, in synergy with a wide variety of functional groups, produces the absorption of energy in the entire UV region. Additionally, lignin is a biocompatible polyphenolic with a strong radical quenching ability, i.e., anti-oxidizing potential. Therefore, it is a promising replacement for synthetic chemicals in cosmetics, sunscreen, and other applications. The UV absorption analysis of selected crude and functionalized lignin samples was conducted, and the results were compared to those of commercially used UV-absorption agents—oxybenzone and octinoxate. The results showed that the total absorption capacity of the lignin samples is lower by 6.4x and 16.3x compared to oxybenzone and octinoxate, respectively; however, it covers a wide absorption range in the UV spectrum. The lignin samples also showed good photostability and color stability, in contrast to the observed yellowing of octinoxate after 3 hours of exposure to sunlight. The UV spectra of some of the lignin samples indicate that their UV absorption capacity was enhanced by as much as 13.31% after exposure to sunlight.

Green Extraction of Reed Lignin: The Effect of the Deep Eutectic Solvent Composition on the UV-Shielding and Antioxidant Properties of Lignin.

Lignin, the second most abundant natural polymer, is a by-product of the biorefinery and pulp and paper industries. This study was undertaken to evaluate the properties and estimate the prospects of using lignin as a by-product of the pretreatment of common reed straw (Phragmites australis) with deep eutectic solvents (DESs) of various compositions: choline chloride/oxalic acid (ChCl/OA), choline chloride/lactic acid (ChCl/LA), and choline chloride/monoethanol amine (ChCl/EA). The lignin samples, hereinafter referred to as Lig-OA, Lig-LA, and Lig-EA, were obtained as by-products after optimizing the conditions of reed straw pretreatment with DESs in order to improve the efficiency of subsequent enzymatic hydrolysis. The lignin was studied using gel penetration chromatography, UV-vis, ATR-FTIR, and 1H and 13C NMR spectroscopy; its antioxidant activity was assessed, and the UV-shielding properties of lignin/polyvinyl alcohol composite films were estimated. The DES composition had a significant impact on the structure and properties of the extracted lignin. The lignin's ability to scavenge ABTS+• and DPPH• radicals, as well as the efficiency of UV radiation shielding, decreased as follows: Lig-OA > Lig-LA > Lig-EA. The PVA/Lig-OA and PVA/Lig-LA films with a lignin content of 4% of the weight of PVA block UV radiation in the UVA range by 96% and 87%, respectively, and completely block UV radiation in the UVB range.

Visible-light driven efficient cleavage of Cα–Cβ bond in lignin models and lignin over porous tubular carbon nitride

The photocatalytic selective breaking of recalcitrant Cα–Cβ bond in real lignin to product value-added chemicals under mild conditions is intriguing but remains challenging. Herein, a sustainable strategy for the efficient and selective breakage of Cα–Cβ bond in lignin β-O-4 model compounds and lignin over porous tubular g-C3N4 (PTCN) photocatalyst was designed under ambient conditions. Compared with bulk g-C3N4, the construction of PTCN not only facilitates photogenerated charge separation and transfer, but also provides more exposed interior active sites, which is conductive to reactant adsorption on the surface of PTCN, thereby resulting in the higher photocatalytic efficiency. With this strategy, 2-phenoxy-1-phenylethanol as the typical model substrate was completely converted into aromatic monomers with Cα–Cβ bond cleavage selectivity of 97.2 %. Both mechanistic studies and theoretical calculations revealed that the photogenerated holes, electrons and O2− respectively played an essential role in the Cβ–H dehydrogenation and Cα–Cβ bond breakage. Native lignin samples (alkaline, pine and wheat straw) were applied to these photocatalytic systems and successfully depolymerized with Cα–Cβ bond cleavage under optimal reaction conditions. This work could provide an in-depth insight into photocatalyst design for the economical and efficient valorization of lignin.

Chemical Composition as the Indicator of Thermally Treated Pine (Pinus sylvestris L.) Wood Colour

This study aimed to determine the influence of increased temperature on the mass loss, chemical composition, and colour of pine wood because of the lack of such information. The colour was measured on samples of wood, extracted sawdust, holocellulose, and lignin isolated from the extracted sawdust of pine heartwood and sapwood. A wood sample labelled 20 °C was considered as wood with the original composition. Subsequently, we verified the measured values with the proposed mixing colour model. Pine heartwood and sapwood samples were thermally treated at temperatures of 100, 150, 200, 220, 240, and 260 °C for 1, 3, and 5 h. It was found that sapwood degraded faster than heartwood. The thermal treatment of wood increases lignin content and decreases holocellulose content, especially at 260 °C. The maximum extractive content of 3.60% was at 1 h and a temperature of 260 °C for both parts of the wood. Lightness values decreased with increasing temperature and time of treatment. The coordinate a* of heartwood showed a positive slope until one hour of treatment duration and a temperature of 240 °C. Then, it decreased for the subsequent duration of treatment. The same course was shown for the coordinate b* of sapwood at a temperature of 200 °C. The proposed model of mixing colours proved that changes in both parts of a wood-extracted substance, holocellulose, and lignin content, were responsible for the changing colour of extracted wood.

Facile Lignin Extraction and Application as Natural UV Blockers in Cosmetic Formulations

Natural compounds are becoming increasingly popular in the fields of pharmaceuticals and cosmetics. One such compound is lignin, a plant-derived aromatic polymer that serves as a natural anti-ultraviolet agent. Conventional methods for extracting lignin from plant materials typically involve performing procedures in harsh environments, such as dissolving it in highly alkaline solutions or subjecting it to treatment in acidic conditions. In this study, lignin was extracted from coconut husk under milder conditions, using neutral solvents and ultrasonic treatment, which allowed us to obtain lignin with significantly improved properties. The developed method facilitated the creation of light-colored lignin, which was employed as a natural ingredient in sunblock cream. Furthermore, for the sake of comparison, lignin was extracted under more rigorous conditions using the traditional method. The research findings confirm that the light-colored lignin sample exhibits a higher level of UV absorption. Furthermore, light-colored lignin demonstrates a synergistic effect when combined with commercial moisturizing creams and sunscreens, leading to a significant enhancement in their SPF performance against both UVA and direct sunlight exposure. This study highlights the potential value of incorporating lignin as a valuable natural ingredient in sunblock and cosmetic products.

The effect of hydrogen bonds on the reactivity of coniferous and deciduous dioxan lignins

The slightly altered samples of dioxan lignin were extracted from coniferous (spruce and juniper) and deciduous wood (aspen) by the Pepper’s method. Lignins were characterized by data of elemental, functional composition, macromolecular and reactive (acid–base and redox) properties. IR spectra of lignins were recorded by the attenuated total refection method. The absorption band of lignins in the 3000–3700 cm−1 region corresponds to the valence vibrations of hydroxyl groups involved in hydrogen bonds. This band was divided into separate Gaussian contours by deconvolution method. The characteristic parameters (energy, length, and relative content) were calculated on the basis of correlation dependencies for each type of H-bonds. The relationship between reactivity (acid–base and redox properties) and the involvement of hydroxyl groups of a natural polymer in H-bonds was shown. The highest reactivity of aspen lignins is caused by the presence of syringyl structures and lower H-bond energies involving reaction centers of macromolecules.

A Complementary Multitechnique Approach to Assess the Bias in Molecular Weight Determination of Lignin by Derivatization-Free Gel Permeation Chromatography.

The growing interest in lignin valorization in the past decades calls for analytical techniques for lignin characterization, ranging from wet chemistry techniques to highly sophisticated chromatographic and spectroscopic methods. One of the key parameters to consider is the molecular weight profile of lignin, which is routinely determined by size-exclusion chromatography; however, this is by no means straightforward and is prone to being hampered by considerable errors. Our study expands the fundamental understanding of the bias-inducing mechanisms in gel permeation chromatography (GPC), the magnitude of error originating from using polystyrene standards for mass calibration, and an evaluation of the effects of the solvent and type of lignin on the observed bias. The developed partial least-squares (PLS) regression model for lignin-related monomers revealed that lignin is prone to association mainly via hydrogen bonding. This hypothesis was supported by functional group-based analysis of the bias as well as pulse field gradient (pfg) diffusion NMR spectroscopy of model compounds in THF-d8. Furthermore, although the lack of standards hindered drawing conclusions based on functionalities, direct infusion electrospray ionization mass spectrometry indicated that the relative bias decreases considerably for higher molecular weight species. The results from pfg-diffusion NMR spectroscopy on whole lignin samples were comparable when the same solvents were used in both experiments; in addition, the comparison between results obtained by pfg-diffusion NMR in different solvents gives some additional insights into the aggregation.