Lignin Feedstock Research Articles

Hexachlorocyclotriphosphazene functionalized lignin as a sustainable and effective flame retardant for epoxy resins

Lignin, a low-cost and abundant polyphenol biomass, has considerable potential to develop sustainable flame retardants for polymers due to its aromatic skeleton and high char-forming capability after combustion. In order to integrate phosphorous and nitrogen elements, herein lignin was facilely modified by hexachlorocyclotriphosphazene through a one-step nucleophilic substitution reaction. The functionalized lignin (Lig-HCCP) was used as an additive to improve the flame retardancy of epoxy resins. The effects of Lig-HCCP with different loading levels on the curing behaviors, rheological characteristics, as well as thermal, mechanical, and flame-retardant properties of the resulting epoxy composites were studied in detail. Notably, the epoxy composite containing 20 wt% Lig-HCCP showed the highest limited oxygen index (LOI) of 28.2% and achieved a UL-94 V-0 rating in the vertical burning test. Furthermore, the results of cone calorimeter tests indicated that the peak heat release rate, heat release rate, and total smoke production of the epoxy composite were decreased by 58.9%, 38.5%, and 63.1%, respectively, compared with the control resin. The flame retardant mechanisms could be ascribed to the P-N synergy and abundant aromatic structures in Lig-HCCP, which contributed to the formation of intumescent and compact char layers. This work demonstrates a simple and promising approach to utilize lignin feedstocks for value-added flame retardants.

LigninGraphs: lignin structure determination with multiscale graph modeling

Lignin is an aromatic biopolymer found in ubiquitous sources of woody biomass. Designing and optimizing lignin valorization processes requires a fundamental understanding of lignin structures. Experimental characterization techniques, such as 2D-heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectra, could elucidate the global properties of the polymer molecules. Computer models could extend the resolution of experiments by representing structures at the molecular and atomistic scales. We introduce a graph-based multiscale modeling framework for lignin structure generation and visualization. The framework employs accelerated rejection-free polymerization and hierarchical Metropolis Monte Carlo optimization algorithms. We obtain structure libraries for various lignin feedstocks based on literature and new experimental NMR data for poplar wood, pinewood, and herbaceous lignin. The framework could guide researchers towards feasible lignin structures, efficient space exploration, and future kinetics modeling. Its software implementation in Python, LigninGraphs, is open-source and available on GitHub.Graphical abstract

Engineering Pseudomonas putida for improved utilization of syringyl aromatics.

Lignin is a largely untapped source for the bioproduction of value‐added chemicals. Pseudomonas putida KT2440 has emerged as a strong candidate for bioprocessing of lignin feedstocks due to its resistance to several industrial solvents, broad metabolic capabilities, and genetic amenability. Here we demonstrate the engineering of P. putida for the ability to metabolize syringic acid, one of the major products that comes from the breakdown of the syringyl component of lignin. The rational design was first applied for the construction of strain Sy‐1 by overexpressing a native vanillate demethylase. Subsequent adaptive laboratory evolution (ALE) led to the generation of mutations that achieved robust growth on syringic acid as a sole carbon source. The best mutant showed a 30% increase in the growth rate over the original engineered strain. Genomic sequencing revealed multiple mutations repeated in separate evolved replicates. Reverse engineering of mutations identified in agmR, gbdR, fleQ, and the intergenic region of gstB and yadG into the parental strain recaptured the improved growth of the evolved strains to varied extent. These findings thus reveal the ability of P. putida to utilize lignin more fully as a feedstock and make it a more economically viable chassis for chemical production.

Fabricating Bifunctional Co−Al2O3@USY Catalyst via In‐Situ Growth Method for Mild Hydrodeoxygenation of Lignin to Naphthenes

AbstractTo enhance the catalytic activity and stability of metal catalysts in the hydrodeoxygenation of lignin derivatives into naphthenes, a bifunctional Co−Al2O3@USY catalyst was fabricated by the reduction of CoAl layered double hydroxide in‐situ grown on the USY zeolite. In the hydrodeoxygenation of guaiacol, a 100.0 % conversion with cyclohexane yield up to 93.6 % was achieved at 180 °C, 3 MPa for 4 h, which should be the hitherto lowest reaction temperature that has been reported over Co‐based metal catalysts. This catalyst was also relatively stable with 5 runs and exhibited excellent catalytic performance in the hydrodeoxygenation of other lignin model compounds and even real lignin feedstock into naphthenes. The high‐efficiency of Co−Al2O3@USY was attributed to the synergistic effect between well‐dispersed small Co nanoparticles and abundant acidic sites on the USY surface, while the outstanding stability was attributed to the anchoring effect of Al2O3 matrix to Co nanoparticles which avoided the leaching of Co species and particle agglomeration. This work provides a potential strategy for the design of an efficient and stable catalyst for lignin utilization.

Local Structure Analysis and Modelling of Lignin-Based Carbon Composites through the Hierarchical Decomposition of the Radial Distribution Function.

Carbonized lignin has been proposed as a sustainable and domestic source of activated, amorphous, graphitic, and nanostructured carbon for many industrial applications as the structure can be tuned through processing conditions. However, the inherent variability of lignin and its complex physicochemical structure resulting from feedstock and pulping selection make the Process‐Structure‐Property‐Performance (PSPP) relationships hard to define. In this work, radial distribution functions (RDFs) from synchrotron X‐ray and neutron scattering of lignin‐based carbon composites (LBCCs) are investigated using the Hierarchical Decomposition of the Radial Distribution Function (HDRDF) modelling method to characterize the local atomic environment and develop quantitative PSPP relationships. PSPP relationships for LBCCs defined by this work include crystallite size dependence on lignin feedstock as well as increasing crystalline volume fraction, nanoscale composite density, and crystallite size with increasing reduction temperature.

Study on the evolution pattern of the chemical structure of Fenton pretreated lignin during hydrothermal carbonization

Two new lignin feedstocks (HLF and DALF) were obtained by Fenton pretreatment of herbal lignin (HL) and de-alkalized lignin (DAL), and hydrothermal carbonization of each lignin sample was carried out at 290 °C with different holding times to investigate the impact of Fenton pretreatment on the physicochemical properties of the hydrothermal materials. The analyses based on 13C solid-state NMR, XPS, FTIR, SEM and fully automated calorimetry showed that the optimal holding time for hydrothermal carbonization of lignin at 290 °C was 30 min. The two types of lignin were treated with Fenton pretreatment to remove some methoxy groups and make the aromatic structural units more compact. Hydrothermal carbonization removed a small amount of αO4 and βO4 bonds from the structure, increased the guaiacyl units [G], decreased the syringyl units [S], and formed more stable 4-O-5 and CC bonds, which in turn polymerized into 10–300 µm irregular spherical carbon particles or porous honeycomb carbon blocks. The calorific values of HLF290-30 and DALF290-30 were increased to 31.04 MJ/kg and 32.17 MJ/kg, respectively, by the combination of Fenton pretreatment and hydrothermal carbonization. Overall, the Fenton pretreatment of lignin enhanced the degree of aromatization and removal of methoxylation during hydrothermal carbonization of lignin. This study provides a sustainable and green strategy for the preparation of high calorific value hydrochar as well as all-round utilization of lignin.

One-Pot Biocatalytic In Vivo Methylation-Hydroamination of Bioderived Lignin Monomers to Generate a Key Precursor to L-DOPA.

Electron‐rich phenolic substrates can be derived from the depolymerisation of lignin feedstocks. Direct biotransformations of the hydroxycinnamic acid monomers obtained can be exploited to produce high‐value chemicals, such as α‐amino acids, however the reaction is often hampered by the chemical autooxidation in alkaline or harsh reaction media. Regioselective O‐methyltransferases (OMTs) are ubiquitous enzymes in natural secondary metabolic pathways utilising an expensive co‐substrate S‐adenosyl‐l‐methionine (SAM) as the methylating reagent altering the physicochemical properties of the hydroxycinnamic acids. In this study, we engineered an OMT to accept a variety of electron‐rich phenolic substrates, modified a commercial E. coli strain BL21 (DE3) to regenerate SAM in vivo, and combined it with an engineered ammonia lyase to partake in a one‐pot, two whole cell enzyme cascade to produce the l‐DOPA precursor l‐veratrylglycine from lignin‐derived ferulic acid.

One-Pot Biocatalytic In Vivo Methylation-Hydroamination of Bioderived Lignin Monomers to Generate a Key Precursor to L-DOPA.

Electron-rich phenolic substrates can be derived from the depolymerisation of lignin feedstocks. Direct biotransformations of the hydroxycinnamic acid monomers obtained can be exploited to produce high-value chemicals, such as α-amino acids, however the reaction is often hampered by the chemical autooxidation in alkaline or harsh reaction media. Regioselective O-methyltransferases (OMTs) are ubiquitous enzymes in natural secondary metabolic pathways utilising an expensive co-substrate S-adenosyl-l-methionine (SAM) as the methylating reagent altering the physicochemical properties of the hydroxycinnamic acids. In this study, we engineered an OMT to accept a variety of electron-rich phenolic substrates, modified a commercial E. coli strain BL21 (DE3) to regenerate SAM in vivo, and combined it with an engineered ammonia lyase to partake in a one-pot, two whole cell enzyme cascade to produce the l-DOPA precursor l-veratrylglycine from lignin-derived ferulic acid.

Microwave-assisted depolymerization of lignin and a biphasic extraction method for the recovery of bio-oil and phenolic monomers

Alkaline lignin and de-alkaline lignin were depolymerized under microwave irradiation in the presence of formic acid and ethanol to produce bio-oil and phenolic monomers. The amount of formic acid was identified as the most critical factor influencing product yields among variables of temperature, retention time, feedstock concentration, and formic acid to feedstock mass ratio. Formic acid acted as both catalyst and in-situ hydrogen donor and it is economically and environmentally advantageous over metal catalysts. An ethyl acetate-water extraction method was developed, which was suitable to recover bio-oil from the depolymerization of lignin with high ash content. Acidic extraction condition favored the product recovery than neutral or basic conditions, giving bio-oil product with much improved quality. This is attributed to the fact that pH adjustment induced ionization of solutes and thus altered the affinity between the solutes and the extraction solvents. This work also revealed that alkaline and de-alkaline lignin feedstocks gave comparable bio-oil yield (ash-free basis) and chemical composition. The bio-oil derived from the de-alkaline lignin has a lower average molecular mass than the alkaline lignin-based bio-oil, likely caused by the presence of alkaline compounds in the alkaline lignin, which could neutralize some formic acid (FA) catalyst, leading to a reduced degree of de-polymerization with the alkaline lignin.

From Lignin to Valuable Aromatic Chemicals: Lignin Depolymerization and Monomer Separation via Centrifugal Partition Chromatography.

Lignin has long been recognized as a potential feedstock for aromatic molecules; however, most lignin depolymerization methods create a complex mixture of products. The present study describes an alkaline aerobic oxidation method that converts lignin extracted from poplar into a collection of oxygenated aromatics, including valuable commercial compounds such as vanillin and p-hydroxybenzoic acid. Centrifugal partition chromatography (CPC) is shown to be an effective method to isolate the individual compounds from the complex product mixture. The liquid–liquid extraction method proceeds in two stages. The crude depolymerization mixture is first subjected to ascending-mode extraction with the Arizona solvent system L (pentane/ethyl acetate/methanol/water 2:3:2:3), enabling isolation of vanillin, syringic acid, and oligomers. The remaining components, syringaldehyde, vanillic acid, and p-hydroxybenzoic acid (pHBA), were resolved by using ascending-mode extraction with solvent mixture comprising dichloromethane/methanol/water (10:6:4) separation. These results showcase CPC as an effective technology that could provide scalable access to valuable chemicals from lignin and other biomass-derived feedstocks.

Microbial hosts for metabolic engineering of lignin bioconversion to renewable chemicals

This review discusses the use of engineered microbes for bioconversion of lignin from plant biomass to produce renewable chemicals. Existing bacterial hosts for lignin bioconversion are discussed, such as Pseudomonas putida KT2440 and Rhodococcus jostii RHA1, and case studies where they have been engineered to generate aromatic and non-aromatic products are described, and the different types of lignin substrates used for these studies. Other bacteria identified as lignin degraders are described, and the prospects for using other bacteria as hosts for metabolic engineering of lignin degradation are discussed. Recent advances in genetic modification of fungi are also discussed, which could lead to the metabolic engineering of lignin-degrading fungi for bioproduct formation from lignin. Prospects and challenges in this field are discussed, as the field moves from the laboratory to industrial application, including: choice of chassis organism, choice of lignin feedstock, the complexity of polymeric lignin breakdown, and considerations for scale-up and choice of bioproduct.

Fast Pyrolysis of Hydrolysis Lignin in Fluidized Bed Reactors

Fast pyrolysis of hydrolysis lignin was studied in fluidized bed units. Hydrolysis lignin, a bioproduct from the lignocellulosic ethanol production process (St1 Cellunolix), was processed in bench scale bubbling fluidized bed (BFB) and pilot scale circulating fluidized bed (CFB) units. Utilization of steam and ethanol as hydrogen sources was tested in a BFB unit. Major technical challenges identified were related to slow reaction rates of lignin degradation and the rapid secondary reactions in the vapor phase resulting in deposit formation and pressure buildup in product gas lines. The carbohydrate content of hydrolysis lignin had a clear correlation to its processability. More challenges with clogging and bed agglomeration were observed with lignin feedstock having lower carbohydrate content. The challenges with the bed agglomeration in the BFB unit were overcome by adding a rapidly rotating mixer in the reactor to break the agglomerates. With the CFB unit, bed agglomeration was not a problem, due to high gas velocities and forces applied to sand and lignin particles. In the BFB unit, the screw feeder was cooled and no significant melting problems were observed. In the CFB unit, melting problems were avoided by feeding the raw material in the cold section of reactor. However, severe increases in the pressure buildup and deposit formation rates were observed in both units. Steam and ethanol was tested, separately, in the BFB unit, to provide excess hydrogen in the system. Based on the product analyses both added hydrogen into the system, but hydrogen ended up mostly in the gas phase. To enhance the hydrogen transfer to the organic liquid, a catalyst active in hydrogen transfer is probably needed.

Towards a new understanding of the retro-aldol reaction for oxidative conversion of lignin to aromatic aldehydes and acids

The retro-aldol reaction is one of the key steps involved in the oxidative conversion of lignin to aromatic aldehydes and acids. In principle, the retro-aldol reaction can proceed in the absence of oxygen. In this work, a new approach based on the influence of oxygen on the oxidation of lignin was investigated. In this approach, the duration of oxygen charged during the reaction was optimized to, for the first time, improve the yield of aromatic aldehydes and acids. The effect of reaction chemistry, time, temperature, and lignin feedstock plays a key role on the yield of aromatic aldehydes and acids. At 140 °C, oxidation of softwood Lignoboost kraft lignin for 40 min results in combined maximum yield of 5.17% w/w of vanillin and vanillic acid. In comparison, using the new approach in which oxygen was charged for only 20 min during the 40 min reaction improved this yield considerably to 6.95%. Further, yield improvement was obtained when applying this approach to different lignin feedstocks. Oxidation also increased the carboxyl content in lignin from 0.49 mmol/g to 1.41 mmol/g which represents a marked improvement. The current study provides new evidence showing that the oxidation reaction is a crucial pathway for lignin valorization.

Could preoxidation always promote the subsequent hydroconversion of lignin? Two counterexamples catalyzed by Cu/CuMgAlOx in supercritical ethanol

In this study, two counterexamples of lignin preoxidation-hydroconversion were reported. First, two lignin feedstocks were preoxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in acetonitrile with various dosages (15%, 30%, and 60%). Then, these preoxidized lignins (HELOs and MWLOs) were hydroconverted in supercritical ethanol catalyzed by Cu/CuMgAlOx. Total yields from HELOs were all higher than those from HEL, indicating the good promotion of DDQ preoxidation on the subsequent hydroconversion of HELOs, especially with the DDQ dosage of 15%. Differently, the promotion effect of DDQ preoxidation on the hydroconversion of MWLOs depended on the DDQ dosage as well as the reaction time. Through the comparison of two counterexamples, this work bursted the myth that preoxidation can always promote the subsequent hydroconversion of lignin, revealed the influence of lignin property, preoxidation degree, and reaction conditions on the subsequent hydroconversion of preoxidized lignin, and presented the new insight into the preoxidation-hydroconversion strategy for lignin.

Biorefining of Pinus pinaster Stump Wood for Ethanol Production and Lignin Recovery

AbstractStump wood is a forest residue having significant valorization potential. The aims of this work were to exploit pine stumps as a carbohydrate and lignin feedstock, and to convert the carbohydrate fraction to bioethanol by simultaneous saccharification and fermentation (SSF). Pinus pinaster stump wood (PPSW) was delignified by the alkaline organosolv (AOS) process. Soda (SA) cooking was used as benchmark process. Higher pulp yields were obtained in AOS treatments, whilst SA pulp showed the highest cellulose content. In the SSF of PPSW pulps, high ethanol concentrations were obtained with high productivity and yield. AOS lignins were purer and had lower molar mass and dispersity index, which are important parameters in high‐value co‐product applications.

Recent Advances in Renewable Polymer Production from Lignin-Derived Aldehydes.

Lignin directly derived from lignocellulosic biomass has been named a promising source of platform chemicals for the production of bio-based polymers. This review discusses potentially relevant routes to produce renewable aromatic aldehydes (e.g., syringaldehyde and vanillin) from lignin feedstocks (pre-isolated lignin or lignocellulose) that are used to synthesize a range of bio-based polymers. To do this, the processes to make aromatic aldehydes from lignin with their highest available yields are first presented. After that, the routes from such aldehydes to different polymers are explored. Challenges and perspectives of the production the lignin-derived renewable chemicals and polymers are also highlighted.

Phenol-derived products from fast pyrolysis of organosolv lignin

Lignin is the largest aromatic bio-polymer feedstock with a huge potential of being refined for aromatic chemical platform and building block. Organosolv process for lignin extraction has a low impact on the environment due to its low chemical requirement, the absence of sulfur, and as it is more practical for chemical recovery. Therefore, refinement of extracted lignin to valuable molecular compounds such as purified lignin and its derivative is the key success for biorefinery industry. In this study, sugarcane bagasse from a local sugar manufacturer in Thailand, noted as BG-lignin, was selected as feedstock for lignin extraction using ethanol and water. The chemical composition of this lignin was slightly different than that of commercially available organosolv lignin, noted as Comm-OS-lignin, in that BG-lignin has more combustible constituents (volatile organic compounds and fixed carbon) of 96.14% and low ash content of 0.09%. The chemical structure of BG-lignin contains a higher number of aliphatic hydrocarbon and aliphatic hydroxy side chains than Comm-OS-lignin, as confirmed by the FT-IR and 1H-NMR analyses. The study on fast pyrolysis of lignin, using Py-GCMS technique, found 2, 3-dihydrobenzofuran as a major constituent at any tested pyrolysis temperature. H-unit, G-unit, and S-unit are distributed in resemble fraction with a H∕G∕S ratio of 1∕1∕0.7 and 0.8∕1∕1 for Comm-OS-lignin and BG-lignin, respectively, at pyrolysis temperature of 400–600 °C, except for H-unit which was enhanced at 700 °C to 28.82% of Comm-OS-lignin and 29.66% of BG-lignin. At a temperature of 400–600 °C, phenolic selectivity of pyrolyzed products, mainly methoxy phenol (Ph-OCH3) and alkylated phenol (R-Ph-OCH3), was done. A higher pyrolysis temperature of 700 °C led to the elimination of aromatic side group and is more favorable in alkyl phenol (R-Ph) and phenol selectivity. However, reduction of the phenolic compound was observed with a higher temperature due to thermal fragmentation of the C–C linkage in the aromatic structure. Products selectivity related to lignin feedstock and pyrolysis temperature could be applied as a conceptual guideline for conducting further study of chemical upgrading process to obtain high-value biochemical products.

Synthesis and Characterization of Lignin-grafted-poly(ε-caprolactone) from Different Biomass Sources

Modification of lignin with poly(ε-caprolactone) is a promising approach to valorize industrial low-value lignins and to advance the bioeconomy. We have synthesized lignin grafted poly(ε-caprolactone) (lignin-g-PCL) copolymers via ring-opening polymerization of ε-caprolactone with different types of lignins of varying botanical sources (G-type pine lignin, S/G-type poplar lignin, and C-type Vanilla seeds lignin) and lignin extraction methods (Kraft and ethanol organosolv pulping). The lignin-g-PCL copolymer showed remarkably improved compatibility and dispersion in acetone, chloroform, and toluene in comparison to non-modified lignins. The structure and thermal properties of the lignin-g-PCL were investigated using Fourier-transform infrared spectroscopy (FTIR), 31P nuclear magnetic resonance (NMR), 2D heteronuclear single quantum correlation (HSQC) NMR, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). We have found that all the technical lignins were reactive to the copolymerization reaction regardless of their plant source and isolation methods. The molecular weights of the synthesized lignin-g-PCL copolymers were positively correlated with the content of aliphatic lignin hydroxyls, suggesting that the copolymerization reaction tends to occur preferentially at the aliphatic hydroxyls rather than the phenolic hydroxyls of lignin. Thermal analyses of the lignin-g-PCL copolymers were studied, and in general, a reduction of melting temperature and crystallinity percentage in comparison to the neat PCL was observed. However, the thermal behavior of lignin-g-PCL copolymers varied depending on the lignin feedstocks employed in the copolymerization reaction.

Promotion of monocyclic aromatics by catalytic fast pyrolysis of biomass with modified HZSM-5

The present study proposes biomass torrefaction combined with catalytic pyrolysis to simplify the liquid products and selectively generate aromatic hydrocarbons. Cedarwood was selected as the representative biomass sample. The torrefaction of cedarwood at 250ºC and 300°C (TB250 and TB300) was carried out to determine the solid product yield and its composition. It was shown that TB250 yielded the highest lignin feedstock and the yield of acetic acid was decreased to 1.88 %. Different transition metals (Ni/Cr) and Mo2N modified HZSM-5 catalysts with different Si/Al ratios were prepared for catalytic pyrolysis. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed to evaluate the effects of several factors on the final aromatic hydrocarbons yields. The results indicated that with a pyrolysis temperature of 700°C, a catalyst-to-biomass ratio of 5, and 5% Ni-Mo2N/HZSM-5 (Si/Al ratio of 25) catalyzed, the yields of benzene, toluene and xylene could be increased to 2.10 %, 3.94 % and 5.54 %, respectively. Resulting from the excellent reduction ability of the modified catalyst, the yields of naphthalene and methylnaphthalene were reduced to 2.58 % and 0.70 %, respectively.

Insights into catalytic valorization of different lignin feedstocks into liquid fuels with microwave heating in hydrogen-donor solvents

Microwave-assisted valorization of alkaline lignin, dealkaline lignin, and lignosulfonate over CuNiAl-based catalysts was investigated in hydrogen-donor solvents. The effect of lignin structures, solvents, and catalysts was investigated concerning the bio-oil yields and properties. A common phenomenon was observed that the highest bio-oil yields were all obtained in methanol over CuNiAl-based oxides under mild conditions. The synergistic and promotional effect of catalyst and solvent was discussed in detail, considering the elemental composition, HHV, and molecular weights of bio-oil. The bio-oil yields improved, and the molecular weights decreased with the increased Cu content in the catalysts, indicating the promotional effect of Cu during the lignin depolymerization. CuNiAl-based catalysts exhibited to be effective to convert lignin to bio-oil with higher HHV and decreased molecular weights, confirming the hydrogenation/hydrodeoxygenation during microwave-assisted hydrogenolysis.