Linkages In Lignin Research Articles

Structure and property variations of mixed hardwood kraft lignins

Kraft lignin as an emerging renewable feedstock can be used to produce fuels, chemicals, and materials. Hardwood kraft lignin bears intrinsic variation due to wood species and the isolation process. The structure and property variation of hardwood kraft lignin could introduce new challenges and opportunities for its application. To better understand such variation, seven kraft lignin samples, originated from southern mixed hardwood (North America), northern mixed hardwood (North America), and Asian mixed hardwood, were isolated from commercial kraft pulping black liquor using both LignoBoost and LignoForce processes. Modern analytical techniques were used to elucidate the characteristics of mixed hardwood kraft lignins, including chemical composition, molecular weight, functional groups, and thermal properties. All lignin samples had a lignin content over 90% (92% to 96%) with one exception, which was northern mixed hardwood kraft lignin with 86% of lignin content and 6% polysaccharides. The elemental and methoxy analyses revealed the expected variation of hardwood kraft lignins with the methoxy content ranging from 0.85 to 1.20 per C9 unit. The weight average molecular weight exhibited a higher variation (from 4800 to 1895 Da) with a descending order of southern mixed hardwood kraft lignins, northern mixed hardwood lignins, and Asian mixed hardwood lignins. The aliphatic hydroxy groups ranged from 14 to 25 per 100 C9 units, and phenolic hydroxy groups ranged from 65 to 112 per 100 C9 units. The catecholic group content ranged from 12 to 34 per 100 C9 units, which is higher than softwood kraft lignin. The lignins isolated from the rapid displacement heating (RDH) pulping process were more condensed than from the regular kraft pulping process. 2D HSQC and quantitative 13C NMR revealed the drastic structure change upon kraft pulp with the low abundance of native lignin linkages and formation of new interunit linkages, such as stilbene, enol ethers, and 1-1’/5’. The S/G ratio was calculated using 2D HSQC spectra correcting for signal shift caused by the catecholic groups.

Green, efficient delignification of hybrid Pennisetum (Pennisetum americanum×P. purpureum) using steam explosion assisted p-toluenesulfonic acid treatment

Effective separation of lignin is a prerequisite for the high-value application of lignocellulosic biomass. In this work, the delignification of hybrid Pennisetum (Pennisetum americanum×P. purpureum, HP) using environmentally-friendly steam explosion assisted p-toluenesulfonic acid (p-TsOH) treatment was investigated. The enhancement mechanism of steam explosion was studied by structural analysis of lignin separated using a double enzymatic hydrolysis method. The results showed that the p-TsOH delignification process could be successfully simulated by a second-order kinetic model. After steam explosion at logR0= 3.89, the activation energy for lignin p-TsOH extraction decreased by 30%, the extraction capacity increased by 60%, and the fractionation yield of lignin reached 89.21%, enabling rapid and near-complete dissolution of lignin. Characterization of physicochemical properties using BET, GPC, and XRD techniques indicated that the specific surface area of HP dramatically increased after steam explosion (from 0.9 to 5.02 m2 g−1), while the lignin molecular weight significantly decreased. The 2D-HSQC spectra further revealed that various linkages (e.g., β-O-4, β-5, and BE) in lignin underwent varying degrees of cleavage after steam explosion, and the depolymerization reaction was dominant. These modifications by steam explosion collectively led to changes in the lignin fractionation process. This work provides a green and efficient method for delignification of HP, which helps to maximize the utilization of HP for biorefinery industries.

Continuous-Flow Microreactor Accelerates Molecular Collisions for Lignin Depolymerization to Phenolic Monomers and Oligomers.

Effective depolymerization of lignin is the most important step for its comprehensive utilization. So far, most of the studies on depolymerization of lignin focused on batch processing, whereas only a few studies relied on the microreactor. In this study, we developed a continuous-flow microreactor for depolymerization of lignin into monomeric and oligomeric compounds. The yields of monomers and oligomers can be adjusted by varying the temperature, pressure, residence time, NaOH dosage, and solvent. Under optimized conditions, the lignin conversion rate was 77.73 wt %, and the monomer yield was 13.26 wt %, with 77.81% being phenolic compounds. In addition, comparative characterizations on the raw lignin and products demonstrated that the oil products were mainly composed of phenolic tetramers and trimers, and the effective cleavage of the β-O-4 linkage of S-type lignin was responsible for the high yield of 2,6-dimethoxyphenol. It indicated that raw lignin could be effectively depolymerized continuously using the continuous-flow microreactor, and it will be a new strategy for comprehensive utilization of lignin to produce fine-chemical intermediates.

Preparation of Co–Co9S8–MoS2 catalyst for efficient deoxygenation of lignin-derived aromatic oxy-compounds into arenes

The formidable challenge for the deoxygenation of lignin-derived aromatic oxy-compounds into arenes under mild conditions is how to engineer a robust catalyst. In this study, Co–Co9S8–MoS2 catalyst was prepared via thermal decomposition, in–situ sulfidation and following by H2 reduction, where Co was highly dispersed and mainly existed in the species of metallic Co and Co9S8. This catalyst exhibited high activity, allowing the reaction temperature for cleaving Caromatic–O bond lowered to 120 °C, which was attributed to that metallic Co accelerated the H2 dissociation and then enhanced the deoxygenation activity. Co–Co9S8–MoS2 also presented a high stability: both the conversion and product selectivity were unchanged after 8 continuous cycles, and the characterization results evidenced almost no structure change and sulfur loss during the deoxygenation reaction. In addition, other 20 aromatic oxy-compounds including acid, ester, guaiacol and dimer lignin linkages were also successfully deoxygenated, and the arenes selectivity could maintain higher than 98 %. This work offered a new strategy for constructing metal–sulfide–MoS2 type catalysts and provided an efficient route towards the practical application in bio-oil upgrading.

Investigation of the effects of ternary deep eutectic solvent composition on pretreatment of sorghum stover

AbstractBiomass‐derived deep eutectic solvents (DESs) have been introduced as promising pretreatment and fractionation solvents because of their mild processing conditions, easy synthesis, and green solvent components from biomass. In recent DES studies, solvent‐based third constituents like water, ethanol, and others improve the processibility of typical binary DESs. However, the impacts of these components are not well understood. Here, two solvent‐based constituents, including water and ethylene glycol, were applied to 3,4‐dihydroxybenzoic acid (DHBA)‐based DES system for improving the conversion efficiency of cellulose‐rich fraction and the properties of lignin fraction. Chemical composition, enzymatic digestibility, degree of polymerization of cellulose and physicochemical properties of lignin were used to evaluate the impact of each third constituent on biomass processing. Ternary ChCl‐DHBA DESs exhibited better performances in delignification, fermentable sugar production, and preservation of β‐O‐4 ether linkage in lignin compared with binary ChCl‐DHBA DES.

Mimicking the Fungal Decay Strategy for Promoting the Bacterial Production of Polyhydroxyalkanoate from Kraft Lignin

Producing polyhydroxyalkanoate (PHA) from lignin through biological conversion has great further potential, but is held by its high heterogeneous characteristic toxicity of the depolymerized products, and low bioconversion of the depolymerized products. In this study, a Fenton-like reaction, which is inspired by fungal decay strategy, was reported to cleave Kraft lignin linkages and produce low toxic mono-aromatic and low molecular organic compounds for microbial conversion. The treatment improved the bioconversion of lignin to PHA by 141.7% compared to Kraft lignin. The bond cleavage of Kraft lignin was characterized by Py-GC/MS and 2D NMR. Seven major depolymerized products were chosen to test their toxicity effect on bacterial fermentation. Furthermore, 920.4 mg of PHA was obtained from 1-L black liquor after Fenton-like reaction treatment. This is a novel attempt mimicking fungal decay strategy coupled with the microbial conversion of lignin into high-value PHA with a sustainable future.

Tuning hydrogenation chemistry of Pd-based heterogeneous catalysts by introducing homogeneous-like ligands

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H2 activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

Selective production of p-hydroxybenzaldehyde in the oxidative depolymerization of alkali lignin catalyzed by copper-containing imidazolium-based ionic liquids

The oxidative depolymerization of alkali lignin (AL) catalyzed by copper-containing imidazolium-based ionic liquids ([C4C1im]CuCl3) was investigated with CH3OH-H2O (9/1, v/v) as solvent under an initial oxygen pressure of 1.5 MPa at 150 °C for 3 h. Compared with other transition-metal chlorides and metal-containing ionic liquids (ILs), [C4C1im]CuCl3 presented a relatively remarkable catalytic capacity for AL oxidative depolymerization due to the cooperative effects of [C4C1im]+ and CuCl3−. [C4C1im]CuCl3 molar concentration (xMcIL) greatly influenced AL conversion and product selectivity. The highest AL conversion of 70.4% was achieved at xMcIL = 0.18 mol.%. For the obtained small-molecule products, the highest selectivity of p-hydroxybenzaldehyde reached 51.3% at xMcIL = 0.72mol.% (corresponding yield of 103.6 mg·g−1). The inter-unit linkages of lignin were effectively cleaved in the oxidative depolymerization catalyzed by [C4C1im]CuCl3, and the H unit in lignin was preferentially destroyed. The molecular interactions and microenvironment of [C4C1im]CuCl3–solvent (CH3OH-H2O) system were investigated with xMcIL varied from 0.05 mol.% to 5.12 mol.% by using attenuated total reflectance-infrared spectroscopy and molecular-dynamics method, respectively. The catalytic properties of [C4C1im]CuCl3 strongly depended on the interactions of [C4C1im]CuCl3 and solvent molecules with changes in microstructure organization. The optimum catalytic capacity of [C4C1im]CuCl3 was obtained when the ion pair formed through hydrogen bonding at xMcIL = 0.18–0.36 mol.%. The solvation of solvent molecules on CuCl3− and the formation of ionic cluster with the hydrogen-bonding self-association of [C4C1im]CuCl3 were unfavorable for the oxidative depolymerization of lignin.

Sunlight-driven superoxide radicals generation from double-modified carbon nitride for efficient lignin β-O-4 bonds photocleavage

Lignin has become the largest renewable aromatic resource, and photocatalytic breaking of lignin linkage bonds has emerged as a promising green oxidation technology initiated by solar energy at room temperature. Wherein, carbon nitride (g-C3N4) photo-cracking of lignin has the advantage of stability and reusability (no photo-corrosion), but is inefficient compared to conventional metal sulfides. In this work, g-C3N4 was dual-modified with Cl doping and high-temperature oxidation, to maximum retain the π-π conjugation that can absorb the benzene ring structure in lignin. The inhibitor experiments and product composition analysis indicated that superoxide radicals acted as the most crucial active species, which was 4 times higher than that of g-C3N4. The attachment sites of doped Cl on carbon nitride were calculated by comparing the density of states with the actual band gap. The as-fabricated g-C3N4-Cl-A5 photocatalyst showed a longer carrier lifetime and higher oxygen adsorption capacity and demonstrated excellent benzaldehyde production (1.74 mg h−1) from the lignin model under sunlight. The work provides insights into the enhanced superoxide radicals generation via photocatalysis and guidance for the selective breaking of lignin β-O-4 bonds.

Draft Genome Sequence of Acinetobacter calcoaceticus TUS-SO1, a Bacterium Capable of Degrading 2-Phenoxyacetophenone.

Acinetobacter calcoaceticus TUS-SO1 degrades 2-phenoxyacetophenone, a model compound for the β-O-4 linkage in lignin. Here, we report the whole-genome sequence of this bacterium. The draft genome comprises 4,284,351 nucleotides and 3,976 coding DNA sequences, with an average G+C content of 38.5%.

Selective photocatalytic aerobic oxidative cleavage of lignin C–O bonds over sodium lignosulfonate modified Fe3O4/TiO2

Lignocellulose shows significantly potential in sustainable conversion to high-quality fuel and value-added chemicals with the demands for realizing the rapid cycle of carbon resources and helping to reach carbon neutrality in nature. Selective tailoring of α-O-4, β-O-4, etc. linkages in lignin has always been viewed as “death blow” for its depolymerization. Herein, novel sodium lignosulfonate (SL) modified Fe3O4/TiO2 (SL-Fe3O4/TiO2) spherical particles have been developed and used as catalysts for selectively photocatalytic oxidative cleavage of organosolv lignin. As expected, 80% selective conversion of lignin in C2–C4 esters has been achieved, while C–O bonds in lignin model compounds can be effectively cleaved. Other than normal hydroxyl radical-mediated photocatalytic depolymerization of lignin over TiO2-based materials, in this contribution, mechanism studies indicate that photogenerated holes and superoxide anion radicals are main active species, which trigger the cleavage of α/β-O-4 bond, and the isotope-labeling study confirms the crucial factor of Cβ–H dehydrogenation in cleavage of β-O-4 bonds.

Unveiling lignin structures and lignin-carbohydrate complex (LCC) linkages of bamboo (Phyllostachys pubescens) fibers and parenchyma cells

Bamboo (Phyllostachys pubescens) is an attractive biomass block to develop biorefining industry, however, less emphasis has been placed on elucidating the chemical linkage variations of lignin and LCC between different bamboo cell walls. Here, purified milled wood lignin (MWLp) and lignin-carbohydrate complex (LCC) were isolated from bamboo (Phyllostachys pubescens) fibers (BF) and parenchyma cells (PC), respectively. The variations of structure features and chemical linkages of lignin and LCC were investigated via FT-IR, 2D HSQC NMR, and 31P NMR techniques. 2D HSQC NMR revealed that β-O-4 alkyl-aryl ether linkages and resinol (β-β) substructure were the main substructures in BF-MWLp and PC-MWLp. β-1 linkages existed in the PC-MWLp (3.18/100 Ar), but not in BF-MWLp. Moreover, tricin, as a flavonoid compound, was only detected in the BF-MWLp. The amount of the syringyl (S) units of PC-MWLp was higher than BF-MWLp. The results indicated that phenyl glycoside (PhGlc) bonds (mainly lignin and xylan) were the predominant chemical linkage type of LCC bonds in BF-LCC and PC-LCC, and the high contents of PhGlc bonds (45.53/100 Ar) were presented in PC. Our finding can provide a reference for the structural variations of lignin and LCC between the different bamboo cell walls.

C-H versus C-O Addition: A DFT Study of the Catalytic Cleavage of the β-O-4 Ether Linkage in Lignin by Iridium and Cobalt Pincer Complexes

The potential energy surfaces of the reactions involved in the catalytic cleavage of 2-phenoxy-1-phenylethanol, a model of the β-O-4 linkage in lignin, by (iprPCP)-Ir, (iprPCOP)-Ir, (iprPCP)-Co and (iprPCOP)-Co complexes have been studied using the M06/6-311G**/LANL2TZ level of theory. Both iridium and cobalt are found to be active towards the cleavage of the β-O-4 linkage, with rate constants of 44.7 s−1 and 5.1 × 106 s−1, respectively. The iridium catalysts prefer the ‘initial C-H addition’ pathway, showing a kinetic preference of 16.8 kcal mol−1 over the ‘direct C-O insertion’ pathway, while the cobalt catalysts prefer the ‘direct C-O insertion’ route which is kinetically favored by 15.7 kcal mol−1 over the ‘initial C-H addition’ pathway. A two-state reactivity occurs along the preferred pathway for the cobalt-catalyzed reaction.

Structural Characterization of the Milled-Wood Lignin Isolated from Sweet Orange Tree (Citrus sinensis) Pruning Residue.

The pruning of sweet orange trees (Citrus sinensis) generates large amounts of lignocellulosic residue. Orange tree pruning (OTP) residue presents a significant lignin content (21.2%). However, there are no previous studies describing the structure of the native lignin in OTPs. In the present work, the "milled-wood lignin" (MWL) was extracted from OTPs and examined in detail via gel permeation chromatography (GPC), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and two-dimensional nuclear magnetic resonance (2D-NMR). The results indicated that the OTP-MWL was mainly composed of guaiacyl (G) units, followed by syringyl (S) units and minor amounts of p-hydroxyphenyl (H) units (H:G:S composition of 1:62:37). The predominance of G-units had a strong influence on the abundance of the different linkages; therefore, although the most abundant linkages were β-O-4' alkyl-aryl ethers (70% of total lignin linkages), the lignin also contained significant amounts of phenylcoumarans (15%) and resinols (9%), as well as other condensed linkages such as dibenzodioxocins (3%) and spirodienones (3%). The significant content of condensed linkages will make this lignocellulosic residue more recalcitrant to delignification than other hardwoods with lower content of these linkages.

Catalytic Strategies and Mechanism Analysis Orbiting the Center of Critical Intermediates in Lignin Depolymerization

Lignin, as a precious resource given to mankind by nature with abundant functional aromatic structures, has drawn much attention in the recent decade from academia to industry worldwide, aiming at harvesting aromatic compounds from this abundant and renewable natural polymer resource. How to efficiently depolymerize lignin to easy-to-handle aromatic monomers is the precondition of lignin utilization. Many strategies/methods have been developed to effectively degrade lignin into monomers, such as the traditional methods of pyrolysis, gasification, liquid-phase reforming, solvolysis, chemical oxidation, hydrogenation, reduction, acidolysis, alkaline hydrolysis, alcoholysis, as well as the newly developed redox-neutral process, biocatalysis, and combinatorial strategies. Therefore, there is a strong demand to systemically summarize these developed strategies and methods and reveal the internal transformation principles of the lignin. Focusing on the topic of lignin depolymerization to aromatic chemicals, this review reorganizes and categorizes the strategies/methods according to their mechanisms, orbiting the center of critical intermediates during the lignin linkage transformation, which includes the critical anionic intermediates, cationic intermediates, organometallic intermediates, organic molecular intermediates, aryl cation radical intermediates, and neutral radical intermediates. The corresponding introduction involves the generation and the transformation chemistry of the critical intermediates via the corresponding C-H/O-H/C-C/C-O chemical bond transformations, leading to the cleavage of the C-C/C-O linkage bonds. Accompanying the brief introduction of lignin chemistry and the final concluding remarks and perspectives on lignin depolymerization, this review aims to provide a current research process of lignin depolymerization, which may provide useful suggestions for this vigorous research field.

Hexaniobate as a Recyclable Solid Base Catalyst to Activate C–H Bonds in Lignin Linkage Boosting the Production of Aromatic Monomers

The targeting cleavage of lignin into value-added aromatic monomers has attracted increasing attention. Although the base-catalyzed depolymerization of lignin has been developed, the use of excess corrosive bases and their poor recyclability limit their industrial implementation. Herein, a catalytic amount of solid base K7HNb6O19 (KNb6) coupled with copper-modified graphitic carbon nitride (Cu/C3N4) shows enhanced catalytic performance for the oxidative cleavage of β-O-4 lignin linkages using molecular oxygen. Due to the synergetic effect between KNb6 and Cu/C3N4, 96% of the β-O-4 ketone model was converted under relatively mild conditions, and phenol (yield: 90%) and other aromatic monomers (yield: 90%) were obtained. Moreover, KNb6–Cu/C3N4 is robust, and its catalytic activity is basically maintained after five cycles. Experimental and theoretical studies (spectroscopic analysis, control experiments, kinetic study, density functional theory calculations, etc.) reveal that the surface basic O atoms of KNb6 can simultaneously activate the Cβ–H bond of β-O-4 ketone to promote the insertion of molecular oxygen, and subsequently, Cu/C3N4 catalyzes the selective cleavage of the Cα–Cβ bond. The catalysts were also active for the oxidative cleavages of other lignin models and oxidized organosolv lignin.

Microwave-assisted catalytic transfer hydrogenolysis of lignin-derived aromatic ethers over Ru/C

Cleavage of aromatic ether bonds through hydrogenolysis is one of the most promising routes for depolymerisation and transformation of lignin into value-added chemicals. Herein, we investigate the catalytic transfer hydrogenolysis (CTH) of benzyl phenyl ether (BPE), as a model molecule of α-O-4 lignin linkages, over commercial Ru/C under microwave irradiation using 2-propanol as H-donor. By tuning the reaction parameters, such as time, temperature and catalyst amount, BPE was fully converted, at 150 °C in 5 min of irradiation, with a related aromatics selectivity of 92%. Phenylethylphenyl ether (PPE), and diphenyl ether (DPE), as a models of β-O-4 and 4-O-5 linkages, respectively, were also considered to this scope and under optimized reaction conditions, >99% and 72% conversion, respectively, were achieved with >89% aromatics selectivity. Remarkably, catalytic tests with moderate conversions clearly showed that the cleavage of etheric C-O bond was the primary step under CTH conditions, while the hydrogenations of the aromatic rings take place in a subsequent step. Overall, the screening of the reaction parameters was crucial to avoid the hydrogenation of aromatic rings. No less significant was the Ru/C stability, which has been recycled at least six times without any noticeable activity loss.

Kinetic Characterization of a Lytic Polysaccharide Monooxygenase Reveals a Unique Specificity for Depolymerization at β-O-4 of Lignin Compounds

The intrinsic role of lytic polysaccharide monooxygenases (LPMOs) for oxidative cleavage of a wide range of polysaccharides like cellulose, starch, chitin, and xyloglucan has been studied extensively. Lignin and its compounds are known to act as electron donors for activation of LPMOs. However, the role of LPMOs in the cleavage of lignin compounds and their complete kinetic analysis has not been well understood. Here, an in-depth kinetic characterization was conducted to study the depolymerization of the phenolic β-O-4 lignin compound, guaicyl glycerol β-guaicyl ether (GGE), by LPMO. Ultrahigh-pressure liquid chromatography, thin-layer chromatography, and NMR analysis of the oxidized reaction mixture confirmed the formation of guaiacol from selective cleavage of GGE at β-O-4 linkage by LPMO. Michaelis–Menten and Lineweaver Burk plot kinetics of LPMOs revealed a Km of 3.566 ± 1.002 mM and a catalytic efficiency (kcat/Km) of 43 × 103 M–1 s–1 for GGE. A complete catalytic mechanism involving O2 and H2O2 was proposed and validated for the LPMO-driven cleavage of GGE. The oxidation reactions involving GGE and LPMO in the N2 atmosphere and competitive inhibition studies using catalase confirmed the role of O2 and H2O2 in completing the catalytic cycle of LPMO. Molecular docking and molecular dynamics simulation studies revealed the interaction of amino residues around the copper active site of LPMO with GGE. Per-residue interaction energy of amino acid depicted the maximum interactive energy contribution by MET1 of LPMO. The presence of GGE around the substrate-binding site further confirms the strong interaction of LPMO and GGE. Hence, the interaction between GGE and LPMO confirmed the role of LPMO in cleaving the β-O-4 linkage of lignin, thus making the LPMO an attractive biocatalyst for use in enzymatic cocktails for lignin valorization.

Oxidative Cleavage of β-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst.

The cleavage of β-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of β-O-4 linkage in lignin model compounds with O2 as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-Nx species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

Steam explosion pretreatment enhancing enzymatic digestibility of overground tubers of tiger nut (Cyperus esculentus L.).

Tiger nut (TN) is recognized as a high potential plant which can grow in well-drained sandy or loamy soils and provide food nutrients. However, the overground tubers of TN remain unutilized currently, which limits the value-added utilization and large-area cultivation of this plant. In the present study, the overground tubers of TN were subjected to enzymatic hydrolysis to produce fermentable sugars for biofuels production. Steam explosion (SE) was applied to modify the physical-chemical properties of the overground tubers of TN for enhancing its saccharification. Results showed that SE broke the linkages of hemicellulose and lignin in the TN substrates and increased cellulose content through removal of hemicellulose. Meanwhile, SE cleaved inner linkages within cellulose molecules, reducing the degree of polymerization by 32.13-77.84%. Cellulose accessibility was significantly improved after SE, which was revealed visibly by the confocal laser scanning microscopy imaging techniques. As a result, enzymatic digestibility of the overground tubers of TN was dramatically enhanced. The cellulose conversion of the SE treated TN substrates reached 38.18-63.97%, which was 2.5-4.2 times higher than that without a SE treatment. Therefore, SE pretreatment promoted saccharification of the overground tubers of TN, which paves the way for value-added valorization of the TN plants.