Depolymerization Of Kraft Lignin Research Articles

Biobased Vanillin Production by Oxidative Depolymerization of Kraft Lignin on a Nitrogen- and Phosphorus-Functionalized Activated Carbon Catalyst

Biobased Vanillin Production by Oxidative Depolymerization of Kraft Lignin on a Nitrogen- and Phosphorus-Functionalized Activated Carbon Catalyst

Catalytic valorization of Kraft lignin into feedstock chemicals with methyltrioxorhenium (MTO) catalyst in microbial electrochemical cell

In this study, the authors investigate a novel approach to valorize Kraft lignin using the catalyst Methyltrioxorhenium (MTO) in tandem with in-situ produced H2O2 in a Microbial Electrochemical Cell (MEC). This study demonstrates the in-situ oxidation of Kraft lignin using different concentrations of MTO catalyst (2 mM to 8 mM) and H2O2 (5.24 ± 0.40 mM to 8.91 ± 0.70 mM) in three MECs. The depolymerized Kraft lignin samples were characterized using FTIR, CHNS/O, and 1H NMR analysis. The MTO/H2O2 combination showed high selectivity towards the oxidation of Kraft lignin, resulting in both aromatic ring and side chain cleavage reactions and the production of valuable feedstock chemicals. The oxidation also led to a reduction of 68.42 % to 78.18 % in Chemical Oxygen Demand (COD) of lignin. The selective oxidation favored the recovery of Guaiacyl (G) unit-derived feedstock chemicals, with Guaiacol being the most abundant compound (45.04 mg/mL) among the quantified products by HPLC. Additionally, the system demonstrated high efficiency in anodic wastewater treatment, achieving BOD and COD removal rates ranging from 67.68 % to 72.55 %. This method showcases the use of a sustainable system in combination with a selective catalyst to produce valuable products from usually discarded Kraft lignin while simultaneously treating wastewater.

Selective Cleavage of Methylene Linkage in Kraft Lignin over Commercial Zeolite in Isopropanol.

Lignin is an aromatic polymer that constitutes over 30 wt% of lignocellulosic biomass and is the most important source of renewable aromatics in nature. The global paper industry generates more than 70 million tons of Kraft lignin annually. Depolymerization of Kraft lignin to value-added monomers can significantly enhance the profitability of biorefinery. However, the method is impeded by the severe condensation of Kraft lignin during the pulping process, which forms robust C-C bonds and results in low monomer yields. In this study, we present a stepwise approach for producing valuable aromatic monomers from Kraft lignin through the cleavage of both C-O and C-C bonds. The approach initiated with complete cleavage of C-O bonds between lignin units within Kraft lignin through alcoholysis in isopropanol, resulting in a monomer yield of 8.9 %. Subsequently, the selective cleavage of methylene linkages present in the residual dimers and oligomers was achieved with commercial MCM-41 zeolite in the same pot, proceeding with an additional monomer yield of 4.0 %, thereby increasing the total monomer yield by 45 %. This work provides an avenue for increasing the depolymerization efficiency of Kraft lignin.

Bimetallic ruthenium- and zinc-doped beta zeolite for efficiently depolymerizing Kraft lignin

The efficient depolymerization of Kraft lignin is essential for comprehensive utilization of biomass. Herein, we prepared a bimetallic Ru/Zn@Beta zeolite catalyst using a co-impregnation synthesis approach that combined zeolite and metal. The bimetallic Ru/Zn@Beta catalyst efficiently depolymerized Kraft lignin, which has been credited as the electrostatic interaction of Zn Lewis acid sites and Ru hydrogen binding sites on Beta zeolite. Catalytic hydrodeoxygenation of Ru/Zn(1:2)@Beta catalyst for 9 h at 290 °C produced a maximum ethyl acetate soluble product yield of 96% with G-type selectivity reaching 74.7%. In these circumstances, compared to Kraft lignin, which produces a heating value of 26.0 MJ/kg, liquefied lignin possesses a much superior heating value of 30.39 MJ/kg. This research has established a paradigm for increasing lignin depolymerization efficiency by coupling Lewis acid and hydrogenation catalysts.

Oxidative Depolymerization of Kraft Lignin to Aromatics Over Bimetallic V–Cu/ZrO2 Catalysts

Zirconia-supported vanadium–copper catalysts (VCux:yZr) were used for the oxidative depolymerization of softwood LignoBoost Kraft lignin (LB). Various VCux:yZr catalysts were prepared (x:y = 0:1, 1:4, 1:2, 3:4, 1:1, and 1:0) by incipient wetness impregnation, and reactions were performed in alkaline water at 150 °C under an O2 pressure of 5 bar for 10 min. 1H–13C HSQC NMR spectroscopy was used for product identification and quantification. The most promising catalyst was VCu1:2Zr, giving a total monomer yield of 9 wt% and the highest selectivity for vanillin (59%). This catalyst was characterized before and after use by N2 physisorption, XRD, TGA, SEM-EDS, and XPS. Cleavage of the main interunit linkages in LB, including the β-O-4 bonds and recalcitrant C–C bonds, was also observed. The findings of this study demonstrate the potential of the V–Cu/ZrO2 catalyst system in the production of value-added aromatics from technical lignin under relatively mild conditions. This would contribute to the more sustainable use of an underutilized side-stream in forest-based industries, provided catalyst reuse can be successfully demonstrated.

Peroxodicarbonate as a Green Oxidizer for the Selective Degradation of Kraft Lignin into Vanillin.

Lignin, the world's largest resource of renewable aromatics, with annually roughly 50 million tons of accruing technical lignin, mainly Kraft lignin, is highly underdeveloped regarding the production of monoaromatics. We demonstrate the oxidative depolymerization of Kraft lignin at 180 °C to produce vanillin 1 in yields up to 6.2 wt % and 92 % referred to the maximum yield gained from the quantification reaction utilizing nitrobenzene. Using peroxodicarbonate (C2 O6 2- ) as "green" oxidizer for the degradation, toxic and/or harmful reagents are prevented. Also, the formed waste can serve as makeup chemical in the pulping process. Na2 C2 O6 is synthesized in an ex-cell electrolysis of aqueous Na2 CO3 at BDD anodes, achieving a yield of Na2 C2 O6 with 41 %. At least, the oxidation and degradation of Kraft lignin is analysis via UV/Vis and NMR spectroscopy.

Ni-Fe Cocatalysts on Magnesium Silicate Supports for the Depolymerization of Kraft Lignin.

This research aimed to synthesize magnesium silicate (MgSiO3) used as a support for Ni-Fe cocatalysts in the depolymerization of kraft lignin. Magnesium silicate was prepared by a hydrothermal method, followed by metal solution impregnation to obtain lignin depolymerization catalysts. The catalytic efficiency of kraft lignin depolymerization to valued phenolic compounds was studied by varying the ratios of Ni and Fe on the MgSiO3 support. Moreover, other factors such as temperature, reaction time, and catalyst recycling affected both the quality and quantity of the products studied. The results illustrated that the catalyst 10Ni10Fe/MS produced all lignin depolymerization products with the highest yield (14.29 wt %) using reaction conditions of 300 °C and 1 h. In addition, the main products were found to be catechol (11.38 wt %), guaiacol (1.51 wt %), and phenol (0.79 wt %). More importantly, the 10Ni10Fe/MS catalyst showed good reusability even after two recycling processes, and the obtained phenol and guaiacol were found to be 0.63 and 1.01 wt %, respectively.

Catalytic depolymerization of Kraft lignin to high yield alkylated-phenols over CoMo/SBA-15 catalyst in supercritical ethanol.

Lignin is generally considered to be a renewable and sustainable resource of aromatic chemicals. However, the depolymerization of Kraft lignin (KL) for the production of selective phenolic monomers presents a significant challenge due to its highly recalcitrant nature. Therefore, in this work, we investigated the effect of metal sites and acid active sites on Mo/SBA-15, Co/SBA-15 and CoMo/SBA-15 catalysts in supercritical ethanol for the depolymerization of KL to produce phenolic monomers. Ethanol was used as a hydrogen donor solvent instead of using external hydrogen. Results showed that the bimetallic CoMo/SBA-15 catalyst exhibited significantly higher catalytic activity compared to the monometallic, Co/SBA-15, Mo/SBA-15 or bare SBA-15. The highest phenolic monomers yield of 27.04 wt% was achieved at 290 °C for 4 h over CoMo/SBA-15 catalyst. The inter-unit linkages such as β-O-4', β-β and α-O-4' in lignin were considerably cleaved during the catalytic depolymerization in supercritical ethanol. Meanwhile, higher functionality of carbonyl compounds was present in the non-catalytic bio-oil, while more alkylated phenols were produced over CoMo/SBA-15 catalyst. The major phenolic monomers identified in the catalytic bio-oil were 4-ethylguaiacol (9.15 wt%), 4-methylguaiacol (6.80 wt%), and 4-propylguaiacol (2.85 wt%). These findings suggest that the metal sites and abundant acid active sites of CoMo/SBA-15 had a synergistic effect toward the degradation of different linkages of lignin and production of selective phenolic monomers in bio-oils.

Lignin depolymerization for aromatic compounds over Ni-Ce/biochar catalyst under aqueous-phase glycerol

Developing an advanced catalytic system for the purposeful depolymerization of lignin into aromatic compounds has presented a significant prospect for green manufacturing. In this paper, the catalytic process of microporous biochar (BC) derived from lignin supported Ni-Ce catalysts (xNi-Ce/BC) coupling with the aqueous-phase glycerol medium was reported for the depolymerization of Kraft lignin to guaiacol and 4-alkyl guaiacols. Under the optimal conditions, 3Ni-Ce/BC yielded 59.02 % of lignin oil and simultaneously realized the highest yields of guaiacol (243.94 mg/g lignin) and 4-alkyl guaiacols (265.65 mg/g lignin). The characterization results revealed BC promoted the formation of metallic Ni sites and the interaction of Ni with CeO2 drove the generation of Ni-CeO2-x interfaces and oxygen vacancies (OV). These could adsorb and activate the CC and CO bonds of lignin and its depolymerized fragments to form reactive intermediates. Then, the Ni sites activated the aqueous-phase glycerol to form adsorbed H atoms, which then spilled over to the adjacent OV to stabilize reactive intermediates. Subsequently, the optimal distribution between Brønsted acid sites (BAS) and Lewis acid sites (LAS) in 3Ni-Ce/BC enhanced the yields of guaiacol and 4-alkyl guaiacols. The kinetic analysis adopting 2-phenoxy-1-phenylethanol as the β-O-4 bond model demonstrated that 3Ni-Ce/BC significantly reduced both the bond dissociation energy of β-O-4 bond and the apparent activation energy. Finally, the possible reaction mechanism of lignin depolymerization catalyzed by 3Ni-Ce/BC catalyst was proposed. This work provides a feasible method for the simultaneous utilization of lignin wastes and crude glycerol (the by-products of biodiesel market).

Oxidative depolymerization of kraft lignin assisted by potassium tert-butoxide and its effect on color and UV absorption

A chemically mild methodology was demonstrated to obtain decomposed lignin products via oxidative depolymerization. We effectively produced light-colored lignin materials with low molecular weight and narrow molecular weight distribution by simple agitation at ambient temperature and pressure in the presence of potassium tert-butoxide and gaseous oxygen. This oxidative depolymerization dissociated various lignin bonding structures and oxidized aliphatic hydroxyl groups of lignin, expanding conjugated system within the depolymerized lignin molecules. This chemical transformation resulted in the generation of effective chromophoric moieties to enhance the UV absorption property of the resulting depolymerized products, which showed a significant improvement in the sun protection capability.

Solvent Effect in Catalytic Lignin Hydrogenolysis

The solvent effect in the catalytic depolymerization of the three-dimensional network of lignin is discussed based on recent reports in this field. Also, the results of an experimental study on the depolymerization of kraft lignin are presented. The cleavage of ether bonds within the lignin network was promoted using ruthenium and platinum on activated carbon (Ru/C and Pt/C), two common hydrogenolysis catalysts. Methanol was identified as a suitable solvent. Noteworthy, under the chosen reaction conditions, the catalysts showed significant resilience to the sulfur present in kraft lignin. The conversion of kraft lignin to lignin oil was strongly affected by the reaction conditions. Although the Ru/C catalyst provided the highest yield at supercritical conditions, a maximum yield was obtained for the Pt/C catalyst at near-critical conditions. The formation of guaiacol, 4-alkylguaiacols, isoeugenol, and 4-ethyl-2,6-dimethoxyphenol is attributed to the solubility of oligomeric lignin fragments in the solvent and the relative propensity of specific groups to adsorb on the catalyst surface.

Depolymerization of Kraft lignin to liquid fuels with MoS2 derived oxygen-vacancy-enriched MoO3 in a hydrogen-donor solvent system

Depolymerization of Kraft lignin represents an attractive strategy to obtain alternatives to fossil fuels. In this work, via the controlled oxidation of MoS2 precursor, an oxygen-vacancy-enriched MoO3 was synthesized and employed to catalyze the depolymerization of Kraft lignin in a new solvent system without external H2 gas. Systematic characterizations exemplified that the calcination temperature significantly affected the morphology and concentration of oxygen vacancy of prepared MoO3, and the appropriate temperature in this work was proved to be 350 °C. In the optimized hydrogen-donor solvent system that contained 20 mL 1, 4-dioxane, 20 mL isopropanol and 5 mL methanol, MoO3-350 catalyst gave a 87 wt% yield of petroleum ether soluble products, and the higher heating value (HHV) of liquid fuels (33.4 MJ/kg) was dramatically increased compared with the original Kraft lignin (25.23 MJ/kg). Methanol and isopropanol were analyzed to be capable of promoting the solubility of Kraft lignin and supplying hydrogen in the reaction system, respectively. Underpinned by characterizations and products analysis, the schematic representation of possible reaction pathway was given.

Depolymerization of Kraft lignin into liquid fuels over a WO3 modified acid-base coupled hydrogenation catalyst

Lignin depolymerization to prepare valuable liquid fuels and phenolics is an important route to improve its efficient utilization, whereas some intractable challenges remain on account of the recalcitrant three-dimensional structure and the depolymerized intermediates are apt to condensation. Although some catalysts can work on lignin model compounds, the depolymerization rate is unable to compete effectively with the condensation reaction in the actual lignin depolymerization process. Based on this, the selective adsorption and activation of the C-O and C–C bonds, as well as the stabilization of depolymerization intermediates are essential to achieving efficient lignin depolymerization. Herein, a novel W-modified acid-base coupled hydrogenation catalyst was reported for the effective lignin conversion to generate liquid fuels. The Lewis acid sites WOx and TiO2 as well as the base sites MgO were favorable for adsorption and polarization of C–C and C-O bonds, promoting the cleavage of C–C and C-O bonds, while the hydrogenation site NiO activated the H2 molecular and inhibits the condensation reaction of reactive intermediates. Thereby, the cleavage of C-O and C–C bonds was facilitated by selective hydrodeoxygenation, leading to enhanced yields in petroleum ether solubilities and monomers. After reacting at 300 °C for 24 h, 63.03% petroleum ether solubles yield (mainly monomers and dimers) and 21.56% monophenol yield obtained with the HHV increased from 25.41 MJKg−1 to 33.90 MJKg−1, and the reusability was accredited.

Lignin Depolymerization in the Presence of Base, Hydrogenation Catalysts, and Ethanol

Being the major renewable source of bio-aromatics, lignin possesses considerable potential for the chemical industry as raw material. Kraft lignin is a couple product of paper industry with an annual production of 55,000,000 ton/y and is considered the largest share of available lignin. Here we report a facile approach of Kraft lignin depolymerization to defined oligomeric units with yields of up to 70 wt.%. The process implies utilization of an aqueous base in combination with a metal containing catalyst and an alcohol under non-oxidative atmosphere at 300 °C. An advantage of the developed approach is the facile separation of the oligomer product that precipitates from the reaction mixture. In addition, the process proceeds without char formation; both factors make it attractive for industrialization. The suppression of the repolymerization processes that lead to char formation is possible when the combination of metal containing catalyst in the presence of an alcohol is used. It was found that the oligomer units have structural features found in phenol-acetaldehyde resins. These features result from the base catalyzed condensation of lignin fragments with in situ formed aldehydes. Catalytic dehydrogenation of the alcohol provides the latter. This reaction pathway is confirmed by the presence condensation products of Guerbet type reactions.

Catalytic depolymerization of kraft lignin for liquid fuels and phenolic monomers over molybdenum-based catalysts: The effect of supports

Catalytic lignin depolymerization (CCLD) for liquid fuels and phenolic monomers was investigated over various supports including clays (e.g., sepiolite (SEP), attapulgite (ATP), and montmorillonite (MTM)), and oxides (e.g., Al2O3 and SiO2) as well as their supported Mo-based catalysts under supercritical ethanol. The characterization results demonstrated that different supports with diverse structural properties could affect the textural structures, surface Mo5+ content, and acid sites distribution. Clay-based supports had more strong acid sites as compared with Al2O3 and SiO2, which went against the production of lignin oil (LO) and led to form more solid products during CLD experiments. Meanwhile, the obtained petroleum ether-soluble product (PEsp) in LO catalyzed by sole supports was mainly alkyl/alkoxy substituted phenols. Additionally, Mo species (especially Mo5+) significantly increased the yields of LO and PEsp. Mo/SiO2 had the highest surface Mo5+ species, showing the highest LO yield of 85.2%, in which the produced alkyl/alkoxy substituted phenols reached 450.3 mg/glignin. Among the clay-supported Mo catalysts, Mo/SEP presented superior LO (82.3%) and PEsp (70.8%) yields and the generated substituted phenols reached 398.8 mg/glignin. This paper systematically reported the application of green and environmentally friendly clay-based materials in lignin conversion, which provides some key information for the development of clay catalysts for biomass conversion.

Monitoring Molecular Weight Changes during Technical Lignin Depolymerization by Operando Attenuated Total Reflectance Infrared Spectroscopy and Chemometrics.

Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (M w), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex‐situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR‐IR) spectroscopy combined with chemometrics provided temporal changes in M w during lignin depolymerization with high resolution. More specifically, ex‐situ SEC‐derived M w and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR‐IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes.

Catalytic depolymerization of Kraft lignin to liquid fuels and guaiacol over phosphorus modified Mo/Sepiolite catalyst

Phosphorus (P) modified Mo/Sepiolite (Mo/SEP) catalysts (xP-Mo/SEP) are successfully prepared and have been certified to regulate controllably Kraft lignin conversion. The lignin oil yield and corresponding higher heating value (HHV) are obviously reduced after P addition. It attributes to P additive interplays with the skeleton structure of Mo/SEP to generate Al-PO3- and Mo-PO43- species, as a result reducing the relative surface content of Mo5+ species and impairing the strength of acid sites. However, P addition dramatically increases the productivity of phenolic compounds. Particularly, 5P-Mo/SEP has the optimized distribution between Mo5+ sites and acid strength/type. Hence, it produced lignin oil has 32.19 MJ/kg of HHV apparently higher than that of Kraft lignin (22.0 MJ/kg), and in which contains the highest amount of guaiacol monomer, 265 mg/g lignin. This ascribes to the reduction of strong Lewis acid sites over 5P-Mo/SEP surface in comparison with Mo/SEP weakens the alkylation process of reactive intermediates but enhances their demethoxylation and hydrogenation reactions. Furthermore, the plausible reaction pathways of lignin conversion over Mo/SEP and 5P-Mo/SEP are discussed in details.

Catalytic depolymerization of Kraft lignin towards liquid fuels over bifunctional molybdenum oxide based supported catalyst

Catalytic depolymerization of Kraft lignin towards valuable liquid fuels and monomeric phenols has been a significant and extremely attractive target, but it remains a great challenge. Herein, we report a catalytic system consisted of bifunctional molybdenum oxide based supported catalyst for catalytic lignin hydroconversion into alkylated benzenes and phenols. In the meantime, the achieved yield of liquid product was 95% and petroleum ether extracted product was 65% at 300℃ for 12 h over 20%MoOx/ZIF-8@ZIF-67 catalyst. The calorific value was increased from 25.66 MJ/ Kg to 34.31 MJ/Kg. The characterization studies show the incorporation of MoOx species leads to the synergy between redox sites and acid sites. The product analysis and catalytic studies demonstrate its synergism to promote catalytic cleavage of C-O linkages via the coupled hydrodeoxygenation and alkylation reaction. The reasonable catalytic mechanism and redox cycle route of catalyst indicate that the cooperative catalytic system paves the way for high-efficiency waste lignin utilization.

Oxidative depolymerization of lignin using nitric acid under ambient conditions

In this work, we demonstrated an innovative and cost-effective strategy to produce high- value depolymerized kraft lignin (DKL) from softwood kraft lignin (SKL) and hardwood kraft lignin (HKL). The depolymerization of kraft lignin was successfully achieved by an oxidative approach using nitric acid under ambient conditions (room temperature and pressure). This novel approach has the following features: ambient process conditions, no heat required, absence of catalysts or organic solvents, and no further purification process being required. The obtained DKL products were characterized extensively by gel permeation chromatography with ultraviolet detector (GPC-UV), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS), potentiometric titration and elemental analysis. Treatment of SKL or HKL at a KL/nitric acid (HNO3) mass ratio of 1:4 (w/w) at room temperature for 84 h resulted in DKL products with a weight-average molecular weight (Mw) of 1350 Da, much lower than that of the original lignins. In addition, the DKL products displayed a significant increase in carboxylic acid group content, a high solubility in water at low pH and, significantly lower decomposition temperatures compared to the original lignins. It was confirmed that nitration of the lignin structure occurred during nitric acid-oxidative depolymerization, as evidenced by the presence of 5% nitrogen (N) content in the DKL products.

Muconic Acid Production Using Engineered Pseudomonas putida KT2440 and a Guaiacol-Rich Fraction Derived from Kraft Lignin

Industrial lignin such as kraft lignin is an abundant feedstock for renewable chemicals and materials. In this study, a process was developed for depolymerization of kraft lignin followed by an upgrading separation step and further bioconversion of the obtained monoaromatic compounds to muconic acid. First, industrial kraft lignin, Indulin AT, was processed into a guaiacol-rich stream using base-catalyzed depolymerization. This stream was subsequently upgraded using liquid–liquid extraction and evaporation to yield a more concentrated and less inhibitory stream, adapted for bioconversion. Finally, guaiacol was quantitatively converted to muconic acid through bioconversion using an engineered Pseudomonas putida strain containing cytochrome P450 and ferredoxin reductase for guaiacol assimilation and deletion of the native catBC genes for muconic acid production. Isomerization of muconic acid in a fermentation medium depending on pH was also studied.