Aryl Ether Bonds Research Articles

Hydrogenolysis of guaiacol and lignin to phenols over Ni/Nb2O5[sbnd]HZSM-5 catalyst

Selective hydrogenolysis of CAr-O bonds in lignin to produce aromatic compounds typically necessitates severe conditions. We developed a Ni/Nb2O5HZSM-5 catalyst that facilitates direct cleavage of guaiacol's aryl ether bonds at reduced temperatures (200 °C) and pressure (0.1 MPa H2), achieving a conversion of 89.5 % with the selectivity of phenol at 81.7 %, while retaining its activity after five cycles. The Ni/Nb2O5HZSM-5 exhibits a higher yield of phenol (49.1 mmolphenol·gNi−1·h−1), currently achieving the highest phenol yield among Ni-based catalysts. The addition of Nb2O5 enhances the dispersion of Ni and augments the effective surface area. In addition, the strong interaction of Nb with the HZSM-5 changed the electronic state of Nb and enhanced the resistance of the catalyst to high temperature and mechanical stress. Employing this catalyst for lignin depolymerization in an aqueous medium led to a 17.0 wt% yield of alkyl phenolic compounds. This approach represents an advancement in biomass resource conversion, circumventing the dependency on high-pressure and precious-metal catalysts, and signaling a new trajectory for sustainable biomass utilization in scientific research.

Anion exchange membrane with enhanced alkaline stability through radiation grafting of ETFE for solid polymer electrolytes

AbstractSolid polymer electrolyte membranes are considered as the nub of many electrochemical devices. Given the climate crisis and related concerns, the evolution of new membrane materials to support the sustainable systems is inevitable. Building on recent advances with the radiation technique and polymer chemistry, herein, anion exchange membranes (AEMs), ETFE‐g‐1VIm/4VP, were fabricated through graft copolymerization of vinyl heterocyclic monomer binary mixture, such as 1‐vinylimidazole (1‐VIm) and 4‐vinylpyridine (4‐VP) onto ethylene tetrafluoroethylene (ETFE), a main polymer backbone without aryl ether bonds. The grafting reaction was achieved at 60°C and then followed by quaternization as a subsequent step. The effects of various reaction grafting conditions were investigated. The ETFE‐g‐1VIm/4VP AEM were characterized w.r.t the morphological and structural features. The dense surface of the grafted membranes is proved by field emission‐scanning electron microscopy (FE‐SEM) images, which also show that the vinyl entities are clearly distributed in the prepolymer, which may lead to a continuous ion transport channel. AEMs processed from the highest graft yield showed good hydroxide conductivity at 90°C, reaching 16.9 mS/cm due to the presence of more transport sites. The same membrane has a relatively good alkaline stability, which is studied through weight percentage method and FT‐IR. Hence, we assume that the introduction of multi‐cationic moieties, pyridinium and imidazolium, contributes to the performance of anion exchange membranes and makes a perfect balance, especially the hydrophilicity and hydrophobicity. These data highlight the potential of the copolymer as an anion exchange membrane for wide spectra of electrochemical applications.Highlights AEMs based on ETFE‐g‐1VIm/4VP are developed via radiation grafting. The membrane exhibits remarkable alkaline stability. FT‐IR, SEM, and weight percentage methods were used to prove the alkaline stability. The membrane has the potential to be used for different electrochemical applications.

Novel 4-chlorophenoxyacetate dioxygenase-mediated phenoxyalkanoic acid herbicides initial catabolism in Cupriavidus sp. DL-D2

Microbial metabolism is an important driving force for the elimination of 4-chlorophenoxyacetic acid residues in the environment. The α-Ketoglutarate-dependent dioxygenase (TfdA) or 2,4-D oxygenase (CadAB) catalyzes the cleavage of the aryl ether bond of 4-chlorophenoxyacetic acid to 4-chlorophenol, which is one of the important pathways for the initial metabolism of 4-chlorophenoxyacetic acid by microorganisms. However, strain Cupriavidus sp. DL-D2 could utilize 4-chlorophenoxyacetic acid but not 4-chlorophenol for growth. This scarcely studied degradation pathway may involve novel enzymes that has not yet been characterized. Here, a gene cluster (designated cpd) responsible for the catabolism of 4-chlorophenoxyacetic acid in strain DL-D2 was cloned and identified, and the dioxygenase CpdA/CpdB responsible for the initial degradation of 4-chlorophenoxyacetic acid was successfully expressed, which could catalyze the conversion of 4-chlorphenoxyacetic acid to 4-chlorocatechol. Then, an aromatic cleavage enzyme CpdC further converts 4-chlorocatechol into 3-chloromuconate. The results of substrate degradation experiments showed that CpdA/CpdB could also degrade 3-chlorophenoxyacetic acid and phenoxyacetic acid, and homologous cpd gene clusters were widely discovered in microbial genomes. Our findings revealed a novel degradation mechanism of 4-chlorophenoxyacetic acid at the molecular level.

Highly uncondensed lignin extraction in the novel green L-cysteine/lactic acid cosolvent system

Enhancing the efficient isolation of proto-lignin from lignocellulose using sustainable, environmentally friendly solvents has consistently been a key focus in the lignin-first biorefinery strategy for the past years. At the same time, achieving efficient extraction of highly uncondensed active lignin under acidic conditions has posed an insurmountable challenge. In this study, we develop an L-cysteine-assisted binary acidic cosolvent that efficiently separates the uncondensed lignin under 80 ∼ 90 °C treatment, resulting in a lighter color and a recovery yield of up to 60 %. The corresponding delignification rate reached 82.6 %. The obtained lignin exhibited an ideal retention rate (maximum 84.5 %) of alkyl aryl ether bonds (β–O–4) relative to the native lignin in raw material. The resulting residue was free from cellulose decomposition and yielded over 90 % cellulose with a surface-esterified structure. This study establishes a one-step L-cysteine-assisted stabilization methodology that overcomes the challenges of lignin condensation and allows for the fractionation of lignocellulose into highly uncondensed lignin and high-quality cellulose, thus offering new solutions for biorefinery.

Lattice Strain Engineering on Metal-Organic Frameworks by Ligand Doping to Boost the Electrocatalytic Biomass Valorization.

As an efficient and environmental-friendly strategy, electrocatalytic oxidation can realize biomass lignin valorization by cleaving its aryl ether bonds to produce value-added chemicals. However, the complex and polymerized structure of lignin presents challenges in terms of reactant adsorption on the catalyst surface, which hinders further refinement. Herein, NiCo-based metal-organic frameworks (MOFs) are employed as the electrocatalyst to enhance the adsorption of reactant molecules through π-π interaction. More importantly, lattice strain is introduced into the MOFs via curved ligand doping, which enables tuning of the d-band center of metal active sites to align with the reaction intermediates, leading to stronger adsorption and higher electrocatalytic activity toward bond cleavage within lignin model compounds and native lignin. When 2'-phenoxyacetophenone is utilized as the model compound, high yields of phenol (76.3%) and acetophenone (21.7%) are achieved, and the conversion rate of the reactants reaches 97%. Following pre-oxidation of extracted poplar lignin, >10 kinds of phenolic compounds are received using the as-designed MOFs electrocatalyst, providing ≈12.48% of the monomer, including guaiacol, vanillin, eugenol, etc., and p-hydroxybenzoic acid dominates all the products. This work presents a promising and deliberately designed electrocatalyst for realizing lignin valorization, making significant strides for the sustainability of this biomass resource.

Oxygen bond cleavage and its migration mechanisms during pyrolysis of naomaohu sub-bituminous coal

Investigating the oxygen bond cleavage and its migration mechanisms during the pyrolysis process of sub-bituminous coals is essential for their classification utilization. The present study employed a fixed-bed pyrolysis technique (temperature range: 350–600 °C) to explore the cleavage of oxygen-containing bonds and subsequent behavior of oxygen migration and transformation during the pyrolysis process of Naomaohu sub-bituminous coal (NSBC). X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy results reveal that the organic oxygen in NSBC predominantly exists as the form of C–O bonds, accounting for 22.81 wt% of the total composition, mainly encompassing hydroxyl, carbonyl, ester, and ether groups. The organic oxygen in the light tar is mainly in the form of phenols, reaching 1.71 wt%, of which methyl substituted phenols is predominant at a final pyrolysis temperature of 600 °C. This can be attributed to the presence of more aryl ether bonds and hydroxyl groups in NSBC, which are prone to cleavage at higher temperatures, resulting in the formation of phenoxyl radicals. Additionally, the higher temperature facilitates the weakening of hydrogen bonds between phenolic hydroxyl groups, thereby enhancing the yield of phenolic compounds. The ester compounds present a decreasing trend with increasing temperature, primarily associated with the cleavage temperature of ester linkers being 350 °C, suggesting that an increase in temperature favors the cleavage of ester bonds. In addition, the cleavage of C–O bonds begins to accelerate significantly during the pyrolysis temperature range of 450–500 °C, subsequently accounting for the pronounced proliferation of phenols within this specific thermal interval. The oxygen content in semi-coke decreased with the increase of temperature, from 85.27 wt% at 350 °C to 37.17 wt% at 600 °C, while the oxygen content in the pyrolysis gas increases from 11.63 wt% at 350 °C to 47.53 wt% at 600 °C, indicating that a significant migration of organic oxygen to the pyrolysis gas during the pyrolysis of NSBC. This study can deepen the understanding of the transformation mechanism of oxygen-containing structure in NSBC pyrolysis process, and provide a theoretical basis for high-grade utilization of low-rank coals.

Catalytic carbon–carbon bond cleavage in lignin via manganese–zirconium-mediated autoxidation

Efforts to produce aromatic monomers through catalytic lignin depolymerization have historically focused on aryl–ether bond cleavage. A large fraction of aromatic monomers in lignin, however, are linked by various carbon–carbon (C–C) bonds that are more challenging to cleave and limit the yields of aromatic monomers from lignin depolymerization. Here, we report a catalytic autoxidation method to cleave C–C bonds in lignin-derived dimers and oligomers from pine and poplar. The method uses manganese and zirconium salts as catalysts in acetic acid and produces aromatic carboxylic acids as primary products. The mixtures of the oxygenated monomers are efficiently converted to cis,cis-muconic acid in an engineered strain of Pseudomonas putida KT2440 that conducts aromatic O-demethylation reactions at the 4-position. This work demonstrates that autoxidation of lignin with Mn and Zr offers a catalytic strategy to increase the yield of valuable aromatic monomers from lignin.

Development of a rapid assay for β-etherase activity using a novel chromogenic substrate

Biocatalytic processes play a crucial role in the valorization of lignin; therefore, methods enabling the monitoring of enzymes such as β-etherases, capable of breaking β-O-4 aryl-ether bonds, are of significant biotechnological interest. A novel method for quantifying β-etherase activity was developed based on the β-ester bond formation between a chromophore and acetovainillone. The chromogenic substrate β-(ρ-nitrophenoxy)-α-acetovanillone (PNPAV), was chemically synthesized. Kintetic monitoring of ρ-nitrophenolate release at 410 nm over 10 min, using recombinant LigF from Sphingobium sp SYK-6, LigF-AB and LigE-AB from Althererytrobacter sp B11, yielded enzimatic activities of 404. 3 mU/mg, 72 mU/mg, and 50 mU/mg, respectively. This method is applicable in a pH range of 7.0–9.0, with a sensitivity of up to 50 ng of enzyme, exhibiting no interference with lipolytic, glycolytic, proteolytic, and oxidoreductase enzymes.

Autoxidation Catalysis for Carbon-Carbon Bond Cleavage in Lignin.

Selective lignin depolymerization is a key step in lignin valorization to value-added products, and there are multiple catalytic methods to cleave labile aryl-ether bonds in lignin. However, the overall aromatic monomer yield is inherently limited by refractory carbon-carbon linkages, which are abundant in lignin and remain intact during most selective lignin deconstruction processes. In this work, we demonstrate that a Co/Mn/Br-based catalytic autoxidation method promotes carbon-carbon bond cleavage in acetylated lignin oligomers produced from reductive catalytic fractionation. The oxidation products include acetyl vanillic acid and acetyl vanillin, which are ideal substrates for bioconversion. Using an engineered strain of Pseudomonas putida, we demonstrate the conversion of these aromatic monomers to cis,cis-muconic acid. Overall, this study demonstrates that autoxidation enables higher yields of bioavailable aromatic monomers, exceeding the limits set by ether-bond cleavage alone.

Conversion of highly polymerized lignin into monophenolic products via pyrolysis: A comparative study of acidic and alkaline depolymerization pretreatments using deep eutectic solvents

Converting industrial residual lignin into monophenolic compounds remains a formidable challenge within biorefinery processes, especially when dealing with highly polymerized lignin. This study focuses on the depolymerization of industrial softwood enzyme hydrolysis lignin (SEHL), a by-product of bioethanol production, which possesses a substantial molar mass (M̅w ˃ 10000 g/mol), The approach involves utilizing acidic or alkaline deep eutectic solvent (DES) for pre-depolymerization, followed by rapid pyrolysis, achieving the selective generation of phenolic monomers. Applying acidic or alkaline DES pretreatments initiates depolymerization by breaking the β–aryl ether bonds (β–O–4) in lignin, but consequently triggers distinct reaction pathways. It is found that this pre-depolymerization leads to higher yields of phenolic monomers dominated by 4–methylguaiacol during the subsequent pyrolysis, moreover, the yields resulted from acidic DES-lignin were much higher than those from alkaline DES-lignin. We have comprehensively elucidated the lignin depolymerization process during DES pretreatments through computational simulations and experimental investigations. These efforts have provided valuable insights into the mechanisms involved in the structural changes of DES-lignin. Furthermore, we have established a more profound comprehension of how both acidic and alkaline DES pretreatments enhance the performance of lignin pyrolysis. This study emphasizes the importance of thoroughly understanding the intricate interplay between lignin structure and its thermochemical properties. Such insights are crucial for efficiently valorizing highly polymerized lignin to produce valuable phenolic compounds using the rapid pyrolysis technique.

Cloning, expression, and biochemical characterization of β-etherase LigF from Altererythrobacter sp. B11

Lignin, a complex heteropolymer present in plant cell walls, is now recognized as a valuable renewable resource with potential applications in various industries. The lignin biorefinery concept, which aims to convert lignin into value-added products, has gained significant attention in recent years. β-etherases, enzymes that selectively cleave β-O-4 aryl ether bonds in lignin, have shown promise in lignin depolymerization. In this study, the β-etherase LigF from Altererythrobacter sp. B11 was cloned, expressed, purified, and biochemically characterized. The LigF-AB11 enzyme exhibited optimal activity at 32 °C and pH 8.5 when catalyzing the substrate PNP-AV. The enzyme displayed mesophilic behavior and demonstrated higher activity at moderate temperatures. Stability analysis revealed that LigF-AB11 was not thermostable, with a complete loss of activity at 60 °C within an hour. Moreover, LigF-AB11 exhibited excellent pH stability, retaining over 50 % of its activity after 1 h under pH conditions ranging from 3.0 to 11.0. Metal ions and surface impregnation agents were found to affect the enzyme's activity, highlighting the importance of considering these factors in enzymatic processes for lignin depolymerization. This study provides valuable insights into the biochemical properties of LigF-AB11 and contributes to the development of efficient enzymatic processes for lignin biorefineries. Further optimization and understanding of β-etherases will facilitate their practical application in the valorization of lignin.

Synergy of electronic and steric effects of Br-Ni catalysts for selective hydrogenolysis of diphenyl ether to phenol

Selective hydrogenolysis of 4-O-5 bond in lignin under relatively mild conditions is an important strategy for the production of valuable aromatic products, e.g. phenols, from renewable carbon resources. However, the easy saturation of benzene rings under reductive conditions over metal catalyst reduces the selectivity to aromatics. In this work, we investigated the effect of bromination of supported Ni nanoparticles on the phenol selectivity during the hydrogenolysis of diphenyl ether (DPE), a commonly used lignin model compound. Compared with the unmodified catalysts, Br-Ni/Al2O3 catalyst derived from Ni-Al layered double oxides exhibits enhanced phenol selectivity (37% vs. 15%) under similar DPE conversions (60%). Various characterizations including transmission electron microscopy (TEM), in-situ X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and CO-Fourier transform infrared spectroscopy (FTIR) indicate that Br preferentially located at the terrace site of Ni nanoparticles, deactivating the continuous Ni sites for benzene ring hydrogenation. In addition, the electron-withdrawing effect of Br creates positively charged Ni sites at the corners, facilitating the hydrogenolysis of C–O aryl ether bonds. During the hydrogenation of real lignin, the selective poisoning and electronic effects introduced by Br synergistically increased the yield of phenols from 12.20% on the initial Ni/Al2O3 to 30.47% over the Br-Ni/Al2O3 catalyst. This work provides an advanced strategy for the catalytic valorization of lignin by halogen modified metal-based catalysts.

Elucidation of bioactivity of tricin released by thioacidolysis from wheat straw lignin

Tricin is a complex compound with chemical bonds to phenylpropane units of lignin in gramineous plants, and it is predominantly bound to lignin by β-O-4 ether bonds. Thioacidolysis cleaves the alkyl aryl ether bonds, which releases tricin from the tricin-lignin complex and maintains the natural structure of the tricin. In this study, milled wood lignin (MWL) was isolated from wheat straw by Bjӧrkman’s method, and the MWL was subjected to thioacidolysis to release tricin from the MWL. Medium-pressure preparative liquid chromatography was used for further purification. FT-IR and 1H-NMR analyses showed that the purified fraction was composed mainly of tricin-type flavonoids. The extracted tricin had strong scavenging capacity against 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals, with a half maximal inhibitory concentration (IC50) of 51.9 mg/L. Drug sensitivity paper testing showed that the extracted tricin inhibited the growth of Escherichia coli. The bacteriostatic circle diameters of tricin from wheat straw MWL and quercetin standard were 9.17 and 7.52 mm, respectively. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that the tricin from wheat straw MWL had possible inhibitory effect on lung cancer A549 cells, with a maximum inhibitory performance of 44.5%.

Freeze-thaw assisted maleic acid pretreatment of eucalyptus to prepare cellulose nanocrystals and degraded lignin

A green and effective method is proposed for the pretreatment of eucalyptus by freeze–thaw assisted maleic acid tactic, wherein the effects of freeze–thaw, maleic acid concentration, reaction time, and temperature on the fractionation were examined. Results showed that under optimal conditions (60% maleic acid, 120 °C, and 2 h), a remarkable removal of 74.5% lignin and 95.2% hemicellulose was achieved after freeze–thaw treatment. The resulting cellulose-rich solid residues were further processed with maleic acid to prepare cellulose nanocrystals, which displayed uniform sized rod-like structures and high crystallinity (62.51%). Moreover, maleic acid pretreatment resulted in lignin with low molecular weight (2110–2530) and excellent homogeneity (PDI ≤ 1.86), while maintaining a relatively intact structure. The lignin had high β-O-4 aryl ether bond contents (≥77.5%) and abundant phenolic hydroxyl contents (2.33–3.63 mmol/g). Overall, the process exhibits notable benefits in effectively separating lignocellulose for high valorization.

A mild iodocyclohexane demethylation for highly enhancing antioxidant activity of lignin

Lignin, as a natural antioxidant, shows great potential in food engineering and medicine. However, the inherent macromolecular structure, high polydispersity, and few phenolic hydroxy seriously limit its antioxidant activity. In this work, a mild iodocyclohexane demethylation for highly improving the antioxidant activity of lignin was proposed. The results showed –OCH3 content exhibited an almost linear decrease as a function of treating time, and the demethylation and cleavage of β–aryl ether bonds prompt an obvious increase in phenolic hydroxyl content (4.01 mmol/g) and a significant decline in aliphatic hydroxyl (∼0.03 mmol/g). Meanwhile, attributing to the fragmentation of β–O–4, β–β, and β–5 substructures, the polydispersity of lignin molecular weight decreases from 2.7 to 2.2. As a result, the formed catechol-typed lignin showed an outstanding antioxidant activity, with the radical (DPPH·) scavenging index (inverse of concentration for 50% of maximal effect (EC50) value) over 2 000 mL/mg, much superior to the commercial antioxidants (< 500 mL/mg). Further structure-activity relationship analysis implied that the Ph–OH/–OCH3 ratio might act as a key factor influencing the antioxidant activity of lignin. This mild demethylation demonstrates a facile and effective method for highly enhancing the antioxidant activity of lignin and makes the catechol-typed lignin a green and promising product for practical use in food, medicine, and pharmacy.

Stereoinversion via Alcohol Dehydrogenases Enables Complete Catabolism of β-1-Type Lignin-Derived Aromatic Isomers.

Sphingobium sp. strain SYK-6 is an efficient aromatic catabolic bacterium that can consume all four stereoisomers of 1,2-diguaiacylpropane-1,3-diol (DGPD), which is a ring-opened β-1-type dimer. Recently, LdpA-mediated catabolism of erythro-DGPD was reported in SYK-6, but the catabolic pathway for threo-DGPD was as yet unknown. Here, we elucidated the catabolism of threo-DGPD, which proceeds through conversion to erythro-DGPD. When threo-DGPD was incubated with SYK-6, the Cα hydroxy groups of threo-DGPD (DGPD I and II) were initially oxidized to produce the Cα carbonyl form (DGPD-keto I and II). This initial oxidation step is catalyzed by Cα-dehydrogenases, which belong to the short-chain dehydrogenase/reductase (SDR) family and are involved in the catabolism of β-O-4-type dimers. Analysis of seven candidate genes revealed that NAD+-dependent LigD and LigL are mainly involved in the conversion of DGPD I and II, respectively. Next, we found that DGPD-keto I and II were reduced to erythro-DGPD (DGPD III and IV) in the presence of NADPH. Genes involved in this reduction were sought from Cα-dehydrogenase and ldpA-neighboring SDR genes. The gene products of SLG_12690 (ldpC) and SLG_12640 (ldpB) catalyzed the NADPH-dependent conversion of DGPD-keto I to DGPD III and DGPD-keto II to DGPD IV, respectively. Mutational analysis further indicated that ldpC and ldpB are predominantly involved in the reduction of DGPD-keto. Together, these results demonstrate that SYK-6 harbors a comprehensive catabolic enzyme system to utilize all four β-1-type stereoisomers through successive oxidation and reduction reactions of the Cα hydroxy group of threo-DGPD with a net stereoinversion using multiple dehydrogenases. IMPORTANCE In many catalytic depolymerization processes of lignin polymers, aryl-ether bonds are selectively cleaved, leaving carbon-carbon bonds between aromatic units intact, including dimers and oligomers with β-1 linkages. Therefore, elucidating the catabolic system of β-1-type lignin-derived compounds will aid in the establishment of biological funneling of heterologous lignin-derived aromatic compounds to value-added products. Here, we found that threo-DGPD was converted by successive stereoselective oxidation and reduction at the Cα position by multiple alcohol dehydrogenases to erythro-DGPD, which is further catabolized. This system is very similar to that developed to obtain enantiopure alcohols from racemic alcohols by artificially combining two enantiocomplementary alcohol dehydrogenases. The results presented here demonstrate that SYK-6 has evolved to catabolize all four stereoisomers of DGPD by incorporating this stereoinversion system into its native β-1-type dimer catabolic system.

Miscibility of nitrated poly(phenylene oxide) and styrene–acrylonitrile copolymer blends

AbstractPoly(2,6‐dimethyl‐1, 4‐phenylene oxide) (PPO) is a well‐known high heat‐resistant polymer and miscible with polystyrene. However, it is not miscible with many polymers containing polar groups. In the present study, PPO was nitrated to prepare a nitrated PPO (NPPO) containing polar nitro groups and blended with a copolymer of styrene/acrylonitrile (SAN). Dynamic mechanical analysis (DMA) data showed that NPPO in SAN‐rich phase was miscible with SAN, while NPPO in NPPO‐rich phase was partially phase‐separated from SAN, both over the entire blend composition range from NPPO wt. fractions of 0.1–0.5 investigated for this work. Analyses of Fourier‐transform infrared (FTIR) and ultra‐violet (UV–VIS) spectra indicated that the miscibility of NPPO with SAN was caused by intermolecular hydrogen bonding of CN group of SAN with OH end‐group of NPPO in the NPPO/SAN blends. NPPO's OH end‐group, meta‐nitro 2,6‐dimethyl phenol, was produced during nitration of PPO due to chain scission of aryl ether bond. Besides the intermolecular hydrogen bonding, dipole‐diplo interaction of the CN and nitro groups resulted in dissociation of the CN dimers and trimers to the CN monomers in the NPPO/SAN blends.

Impact of dilute acid treatment on improving the selectivity of lignin and hemicellulose removals from pre-hydrolysis liquor

Pre-hydrolysis liquor (PHL) of the dissolved pulp production process contains sugar and lignin is paper-making wastewater. Although sugar has the potential for value-added applications, the presence of lignin is problematic for the utilization of PHL. To improve the removal of lignin and maintain a high concentration of sugars in PHL, a novel process was introduced that included a dilute acid treatment, activated carbon (AC), and resin treatment of PHL, respectively. The results confirmed that most of the colloidal lignin and part of dissolved lignin were removed by dilute acid post-hydrolysis and the bonds of lignin-carbohydrate complexes were broken, and the adsorption efficiency of AC and resin was improved. In addition, the research found that high temperature can improve the adsorption efficiency of lignin on AC. In the case of without using PAC, the dilute acid treatment can reduce the dosage of AC from 25 g/L to 12.5 g/L and improve the flow rate of PHL through the resin from 0.1 mL/s to 0.8 mL/s in removing lignin to 90 %, while maintaining sugars in PHL. Also, the particle size of the PHL was significantly reduced by dilute acid-post hydrolysis which is probably because of the broken of β-O-4 aryl ether bond of lignin in PHL indicated by the analysis of 2D-HSQC. This work provides a practical technical process for industrial application of PHL and production of sugar solutions (mainly xylose) with high purity.

Highly-efficient conversion of lignin to electricity by nickel foam anode loaded with solid electrocatalysts

A novel liquid flow fuel cell with insoluble metal sulfides-modified nickel foam anodes was developed to achieve direct conversion of lignin to electricity under mild conditions, avoiding the separation problems caused by using soluble electron mediators. CoS has been screened as the most efficient anodic electrocatalyst for lignin oxidation in alkaline medium by Co2+/Co3+ redox couple. By using an alkaline anolyte-acidic catholyte asymmetric design with VO2+/VO2+ redox couple as the cathodic electrocatalysts, the highest peak power density of 176 mW/cm2 was obtained with sodium lignosulfonate as the fuel at 80 ℃. The anode showed excellent stability with a 97% selectivity over oxygen evolution reaction for at least four fed-batch operation without significant loss of the activity. Lignin underwent significant depolymerization and degradation primarily by cleavage of β-O-4′ aryl ether bond, while ring-opening reactions also took place to form aliphatic acids, and even a part of lignin was mineralized to CO2.

Reaction characteristics of metal-salt coordinated deep eutectic solvents during lignocellulosic pretreatment

A binary deep eutectic solvent consisting of choline chloride and glycerol (ChCl/Gly) is proven to be ineffective in lignocellulosic pretreatment, and arbitrarily screening coordination agents makes it difficult to directionally construct a valid solvent. This study investigated the designable diversity of ternary ChCl/Gly coordinated with various metal-salts by elucidating solvent properties and reaction characteristics. Alkaline/acidic strengths and hydrogen-bonding forces were key factors for component fractionation. Pretreatment performance was typically poor for solvents with few acidic sites and active protons (i.e., ChCl/Gly-KCl and MgCl2), as well as those with close proximity of H-bond donor and acceptor abilities (ChCl/Gly-ZnCl2). By contrast, strong alkaline ChCl/Gly-K2CO3 could achieved 72 % of delignification with a remarkable holo-cellulose preservation (70.8–80.9 %). Delignification underwent an independent rate-control of solvent diffusion and surface reaction with an endothermic nature (17.1 kJ/mol of ΔH and 20.3 kJ/mol of apparent activation energy). High temperature was beneficial for mitigating diffusion constraint at the initial pretreatment stage, maximally increasing the reaction rate by 7.7-fold. Moreover, Lewis acidic ChCl/Gly-FeCl3 (or AlCl3) removed 76–84.4 % of hemicellulose and 47.6–61.9 % of lignin while significantly deconstructing the aryl-ether bonds and β-O-4 linkages. But for acidic ChCl/Gly solvents, biomass dissolution was inconformity with the independent diffusion/reaction-controlling process, owing to the excessive hemicellulose degradation and hydrolytic by-product formation (maximally 91 g/kg xylose and 4 g/kg furans). Based on the lignin structural characterization, the potential reaction mechanism related to solvent properties was proposed to develop efficient ternary coordinated deep eutectic solvents.