Effect of Substituents on Molecular Reactivity during Lignin Oxidation by Chlorine Dioxide: A Density Functional Theory Study.

Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction's activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.

Polydopamine-Modified Cellulose Nanofibril Composite Aerogel: An Effective Dye Adsorbent.

In this study, a new cellulose nanofibril (CNF) composite aerogel was fabricated using a green and facile mussel-inspired coating strategy. First, the CNF hydrogel was crosslinked by calcium ion followed by immersion in dopamine solution. Second, the surface of CNF was modified using polydopamine (PDA) to obtain PDA@CNF (PCNF) composite aerogel. The PCNF composite aerogels had large surface areas (368.15 m2/g) and low bulk density (27.2 mg/cm3). The composite aerogel exhibited improved mechanical properties, which were almost three times compared with those of CNF aerogel. Moreover, PCNF composite aerogel had good resilience under a wet state. The PDA functional layer remarkably enhanced the adsorption capacities of the composite aerogel for methylene blue (MB). The maximum adsorption of MB was 208 mg/g at an initial dye concentration of 50 mg/L. The adsorption isotherm and kinetic behaviors of the composite aerogel were consistent with Langmuir and pseudo-second-order models. In addition, the PCNF composite aerogels had a high adsorption capacity over a wide pH range. The reuse experiment showed that the removal efficiency of the composite aerogel remained higher than 85% after five cycles. Therefore, PCNF composite aerogels may have potential application in wastewater treatment due to its environmental sustainability and low energy consumption.

Effects of the Preferential Oxidation of Phenolic Lignin Using Chlorine Dioxide on Pulp Bleaching Efficiency.

Chlorine dioxide is widely used for pulp bleaching because of its high delignification selectivity. However, efficient and clean chlorine dioxide bleaching is limited by the complexity of the lignin structure. Herein, the oxidation reactions of phenolic (vanillyl alcohol) and non-phenolic (veratryl alcohol) lignin model species were modulated using chlorine dioxide. The effects of chlorine dioxide concentration, reaction temperature, and reaction time on the consumption rate of the model species were also investigated. The optimal consumption rate for the phenolic species was obtained at a chlorine dioxide concentration of 30 mmol·L-1, a reaction temperature of 40 °C, and a reaction time of 10 min, resulting in the consumption of 96.3% of vanillyl alcohol. Its consumption remained essentially unchanged compared with that of traditional chlorine dioxide oxidation. However, the consumption rate of veratryl alcohol was significantly reduced from 78.0% to 17.3%. Additionally, the production of chlorobenzene via the chlorine dioxide oxidation of veratryl alcohol was inhibited. The structural changes in lignin before and after different treatments were analyzed. The overall structure of lignin remained stable during the optimization of the chlorine dioxide oxidation treatment. The signal intensities of several phenolic units were reduced. The effects of the selective oxidation of lignin by chlorine dioxide on the pulp properties were analyzed. Pulp viscosity significantly increased owing to the preferential oxidation of phenolic lignin by chlorine dioxide. The pollution load of bleached effluent was considerably reduced at similar pulp brightness levels. This study provides a new approach to chlorine dioxide bleaching. An efficient and clean bleaching process of the pulp was developed.

Efficient degradation of lignin by chlorine dioxide and preparation of high purity pulp fiber.

High-purity pulp fibers can be obtained by using chlorine dioxide to oxidize lignin. However, organic halogen compounds (AOX) are generated from chlorination side reactions during the lignin oxidation process. In this study, phenolic lignin model compounds with different substituents were selected. The effects of substituent position on the production of free radicals and oxidative ring opening in benzene rings were analyzed. It was found that the structural transformation of lignin and the reaction consumption of ClO2 were significantly changed under high concentration of ClO2. The molar consumption ratio of compound to ClO2 was increased from 1:2 to 1:3. Quinone, an intermediate product that promotes the formation of phenoxy radicals, was found to be stabilized in the reaction. This is attributed to that the benzene ring of lignin is activated through long-range electrostatic interactions. The formation of free radicals and the oxidative ring-opening reaction of benzene rings were facilitated. The efficient oxidation of lignin by ClO2 was fulfilled. Chlorination reactions of lignin were suppressed at elevated oxidation efficiency. The pollution load of wastewater was significantly reduced. AOX generation was reduced by 69.27 %. This provides a new method for efficient oxidative degradation of lignin and preparation of high purity pulp fiber.

Unveiling the Dissolution Regularities of the Lignin-Carbohydrate Complex in Bamboo Cell Walls during Alkali Pretreatment.

Bamboo is a promising biomass resource. However, the complex multilayered structure and chemical composition of bamboo cell walls create a unique anti-depolymerization barrier, which increases the difficulty of separation and utilization of bamboo. In this study, the relationship between the connections of lignin-carbohydrate complexes (LCCs) within bamboo cell walls and their multilayered structural compositions was investigated. The chemical composition, structural properties, dissolution processes, and migration mechanisms of LCCs were analyzed. Alkali-stabilized LCC bonds were found to be predominantly characterized by phenyl glycoside (PhGlc) bonds along with numerous p-coumaric acid (PCA) linkage structures. As demonstrated by the NMR and CLSM results, the dissolution of the LCC during the alkaline pretreatment process was observed to migrate from the inner secondary wall (S-layer) of the bamboo fiber cell walls to the cell corner middle lamella (CCML) and compound middle lamella (CML), ultimately leading to its release from the bamboo. Furthermore, the presence of H-type lignin-FA-arabinoxylan linkage structures within the bamboo LCC was identified with their primary dissolution observed in the S-layer of the bamboo fiber cell walls. The study results provided a clear target for breaking down the anti-depolymerization barrier in bamboo, signifying a major advancement in achieving the comprehensive separation of bamboo components.

A facile strategy to fabricate antibacterial hydrophobic, high-barrier, cellulose papersheets for food packaging.

As a traditionally used packaging material, natural cellulose-based paper has poor barrier properties to water and oxygen, which severely limits its wide application in food packaging. In this study, we report a new sustainable approach to producing hydrophobic, high-barrier, and antibacterial packaging materials from cellulose paper. In this process, commercially available microcrystalline cellulose was first modified by long-chain stearic acid to form hydrophobic microcrystalline cellulose ester and then mixed with stearic acid as filler in the subsequent surface coating of bagasse fibre paper. The microcrystalline cellulose ester/stearic acid-coated paper (MSP) exhibited good water repellency and oxygen barrier activity due to a continuous hydrophobic film that formed, which completely covered the pores of the original bagasse fibre paper. The coated MSP sample also showed excellent dimensional stability in water and a good wet tensile strength of 16 MPa. In addition, poly(hexamethylene guanidine) (PHMG) was chemically grafted onto the free carboxyl groups of the MSP surface layer, and the resulting MSP-g-PHMG samples exhibited excellent antibacterial activity against Escherichia coli and Listeria monocytogenes. The biodegradable cellulose-based MSP-g-PHMG sample significantly delayed the decay of raspberry during storage, indicating its potential application in food packaging.

Distinct microplastics abundance variation in root-associated sediments revealed the underestimation of mangrove microplastics pollution.

Mangrove sediment is acknowledged as the critical sink of microplastics (MPs). However, the potential effect of mangrove root systems on the MPs migration in sediment remains largely unknown. Here, our study characterized the spatial distribution of MPs trapped in root hair, rhizosphere, and non-rhizosphere zones, and analyzed their correlations with physicochemical properties of sediments. The significantly increased MPs abundances toward root systems shed light on the distinct effect on the migration of MPs exerted by mangrove root systems. Partial least squares path modeling (PLS-PM) analysis revealed that pore water content and pH influenced the abundances of different MP characteristics (shape, color, size, and type) and further promoted the accumulation of MPs toward the root systems. In different mangrove areas from landward to seaward, other sediment properties (median grain size, clay content, and salinity) also controlled MP distribution. Additionally, smaller-sized MPs (<1000 µm) were more easily transported to the root systems. Our study emphasizes the importance of considering root systems effect when investigating the mechanisms of MPs distribution and migration in mangrove sediments.

Simultaneous achievement of efficient hemicellulose separation and inhibition of lignin repolymerization using pyruvic acid treatment.

The efficiency of organic acid treatment in the conversion of lignocellulosic biomass fractions has been widely recognized. In this study, a novel green pyruvic acid (PA) treatment is proposed. The higher separation efficiency of eucalyptus hemicellulose was obtained at 4.0% PA and 150 °C. The hemicellulose separation yield was increased from 71.71 to 88.09% compared to glycolic acid (GA) treatment. In addition, the treatment time was significantly reduced from 180 to 40 min. The proportion of cellulose in the solid increased after PA treatment. However, the accompanying separation of lignin was not effectively controlled. Fortunately, a six-membered ring structure was formed on the diol structure of the lignin ß-O-4 side chain. Fewer lignin-condensed structures were observed. High-value lignin rich in phenol hydroxyl groups were obtained. It provides a green path for the simultaneous achievement of efficient hemicellulose separation and inhibition of lignin repolymerization using organic acid treatment.

Rapid and mild fractionation of hemicellulose through recyclable mandelic acid pretreatment.

The development of organic acid pretreatments from biological sources is essential to facilitate the progress of green and sustainable chemistry. In this study, the effectiveness of mandelic acid pretreatment (MAP) was analyzed for eucalyptus hemicellulose separation. 83.66% of xylose was separated under optimal conditions (temperature: 150 °C; concentration: 6.0 wt%; time: 80 min). The hemicellulose separation selectivity is higher than acetic acid pretreatment (AAP). The stable and effective separation efficiency (56.55%) is observed even after six reuses of the hydrolysate. Higher thermal stability, larger crystallinity index and optimized surface element distribution in the samples were demonstrated by MAP. Lignin condensation is effectively inhibited through MAP, as determined from the structural of different lignin. In particular, the demethoxylation of lignin by MA was found. These results open up a new way to construct a novel organic acid pretreatment for separating hemicellulose with high efficiency.

Pretreatment of poplar with eco-friendly levulinic acid to achieve efficient utilization of biomass.

Organic acid pretreatment is an effective method for green separation of lignocellulosic biomass. However, repolymerization of lignin seriously affects the dissolution of hemicellulose and the conversion of cellulose during organic acid pretreatment. Therefore, a new organic acid pretreatment, levulinic acid (Lev) pretreatment, was studied for the deconstruction of lignocellulosic biomass without adding additional additives. The preferred separation of hemicellulose was realized at Lev concentration 7.0%, temperature 170 °C, and time 100 min. The separation of hemicellulose increased from 58.38% to 82.05% compared with acetic acid pretreatment. It was found that the repolymerization of lignin was effectively inhibited in the efficient separation of hemicellulose. This was attributed to the fact that γ-valerolactone (GVL) is a good green scavenger of lignin fragments. The lignin fragments in the hydrolysate were effectively dissolved. The results provided theoretical support for creating green and efficient organic acid pretreatment and effectively inhibiting lignin repolymerization.

Polydopamine-Reinforced Hemicellulose-Based Multifunctional Flexible Hydrogels for Human Movement Sensing and Self-Powered Transdermal Drug Delivery.

The preparation of bio-based hydrogels with excellent mechanical properties, stable electrochemical properties, and self-adhesive properties remains a challenge. In this study, nano-polydopamine-reinforced hemicellulose-based hydrogels with typical multistage pore structures were prepared. The nanocomposite hydrogels exhibit stable mechanical properties and show no significant crushing phenomenon after 1000 cycles of cyclic compression. Its ultimate tensile strain was 101%, which is significantly higher than that of native skin. The shear adhesion strength of the hydrogel to skin tissue reaches 7.52 kPa, which is better than fibrin glue (Greenplast) (5 kPa), and the excellent adhesion property prolongs the service time of the hydrogel in biomedicine applications. The impedance of the hydrogel was reduced and the electrical conductivity was increased with the addition of nano-polydopamine. The prepared nanocomposite hydrogel can detect various body movements (even throat vibrations) in real time as a motion sensor while being able to rapidly load cationic drugs and facilitate transdermal introduction of electrically stimulated drug ions as a drug patch. It provides theoretical support for the fabrication of hemicellulose-based hydrogels with excellent properties through molecular design and nanoparticle reinforcement. This has important implications for the development of next-generation flexible materials suitable for health monitoring and self-administration.

Novel dual-action vanillic acid pretreatment for efficient hemicellulose separation with simultaneous inhibition of lignin condensation.

Aromatic acids play a selective role in the separation of hemicellulose. Phenolic acids have demonstrated an inhibitory effect on lignin condensation. In the current study, vanillic acid (VA), which combines the characteristics of aromatic and phenolic acids, is used to separate eucalyptus. The efficient and selective separation of hemicellulose is achieved simultaneously at 170 °C, 8.0% VA concentration, and 80 min. The separation yield of xylose increased from 78.80% to 88.59% compared to acetic acid (AA) pretreatment. The separation yield of lignin decreased from 19.32% to 11.19%. In particular, the ß-O-4 content of lignin increased by 5.78% after pretreatment. The results indicate that VA, as a "carbon positive ion scavenger", it preferentially reacts with the carbon-positive ion intermediate of lignin. Surprisingly, the inhibition of lignin condensation is achieved. This study provides a new starting point for the development of an efficient and sustainable commercial technology by organic acid pretreatment.

Highly selective separation of eucalyptus hemicellulose by salicylic acid treatment with both aromatic and hydroxy acids.

Aromatic and hydroxyl acid treatments demonstrate their respective characteristics for the separation of lignocellulosic biomass. In this study, the effect of salicylic acid (SA-A) treatment on the separation of eucalyptus components with both aromatic and hydroxyl acid properties was analyzed. The optimal conditions were SA-A concentration 9.0%, reaction temperature 140 °C and time 75 min. The separation yield of xylose was 85.93%. The separation of cellulose and lignin was inhibited by SA-A treatment in contrast to the separation by glycolic acid and p-toluenesulfonic acid treatment. Moreover, SA-A treatment resulted in a larger fiber crystallinity index and higher thermal stability. The SA-A-treated samples contained lignin that was rich in ß-O-4 and hydroxyl groups. The degradation and condensation of lignin was inhibited. The selectivity of aromatic acids for separating hemicellulose and protecting the lignin structure using hydroxy acids was demonstrated. Thus, new and efficient organic acid treatments can be developed.

Effect of temperature on simultaneous separation and extraction of hemicellulose using p-toluenesulfonic acid treatment at atmospheric pressure.

Hemicelluloses were effectively separated using p-toluenesulfonic acid (p-TsOH) treatment at high temperature. High temperature and pressure promoted hydrolysis of hemicellulose, which limited its value upon recovery. In this study, bagasse hemicellulose was separated and extracted by p-TsOH treatment at atmospheric pressure. The effects of temperature, p-TsOH concentration, and time on hemicellulose separation and extraction were investigated. The optimal conditions were 80 °C, 3.0% p-TsOH, and 120 min. The separation and extraction yield of hemicellulose was 73.23% and 36.02%, respectively. Extraction hemicellulose with 95.60% purity was obtained. In addition, the dissolution mechanism of hemicellulose was analyzed. Degradation of ß-glycosidic bonds was inhibited. Benzyl ether bond between carbohydrates and lignin was selectively cleaved. The skeleton structure of xylan in hemicellulose was protected while the functional groups of branch chain were severely damaged. It provides a valuable theoretical basis for the efficient separation and extraction of hemicellulose.

Efficient separation of bagasse lignin by freeze-thaw-assisted p-toluenesulfonic acid pretreatment.

Lignin separation is an important procedure that benefits multiple industries and in particular biomass transformation efforts. In this study, bagasse lignin was separated by freeze-thaw-assisted p-toluenesulfonic acid (p-TsOH) pretreatment. The optimal conditions were freezing temperature -60 °C, freezing time 8.0 h, thawing temperature 15 °C, p-TsOH concentration 60%, pretreatment temperature 70 °C, and time 20 min. Lower acid concentrations and temperatures were used compared with traditional p-TsOH pretreatment. The efficiency and selectivity of lignin separation were improved. It was attributed to freeze-thawing, which provided a more efficient physical channel for the effective penetration of p-TsOH. The separation, extraction and purity of lignin were improved to 89.76%, 78.22% and 77.89%, respectively. High separation, high extraction, high purity and large molecular weight lignin samples were obtained. In addition, the recovery and reuse of p-TsOH was enhanced. This provided a new method for the efficient and clean separation of lignin.

Structural evolution of lignin after selective oxidation of lignin-carbohydrate complex by chlorine dioxide.

Lignin-carbohydrate complexes (LCCs) are important from the perspective of the anti-depolymerization barrier of lignocellulosic biomass, as it limits the high-value utilization of lignocellulosic biomass resources. In this study, the unit structure of lignin in the LCC has been investigated in depth. Oxidation of a selective lignin unit by chlorine dioxide revealed that the LCC structures are enriched with xylanase-macroporous resins, and the structure that was not oxidized in LCC was also identified. At a chlorine dioxide concentration of 90.93 mg/L, 1 g of LCC lignin is oxidized by 0.7 g chlorine dioxide. The purified residual hemicellulose lignin was mainly H-type. The ß-O-4' signal was the strongest for the bond between lignin and carbohydrates. Therefore, it is speculated that most of the residual lignin in bamboo-alkali hemicellulose exists in the form of non-phenolic structural units. This study provides a reference for further studies on the specific structures of LCC.

Efficient removal of residual lignin from eucalyptus pulp via high-concentration chlorine dioxide treatment and its effect on the properties of residual solids.

In fact, effectively removing lignin from pulp fibers facilitates the conversion and utilization of cellulose. In this study, the residual lignin in eucalyptus pulp was separated using a high concentration of chlorine dioxide. The effects of chlorine dioxide dosage, temperature, and time on lignin removal were investigated. The optimal conditions are chlorine dioxide dosage 5.0%, reaction temperature 40 °C, and reaction time 30 min. The lignin removal yield is 88.21%. The removal yields of cellulose and hemicellulose are 2.28 and 17.00%, respectively. The treated eucalyptus pulp has higher fiber crystallinity and thermal stability. The carbon content on the fiber surface is significantly reduced. The results show that lignin is removed by efficient oxidation, and the degradation of carbohydrates is inhibited using high concentrations of chlorine dioxide at low temperatures and short reaction times. This provides theoretical support for high value conversion of cellulose.

Enhancement of separation selectivity of hemicellulose from bamboo using freeze-thaw-assisted p-toluenesulfonic acid treatment at low acid concentration and high temperature.

The cellulose-rich residual solids are obtained with p-toluenesulfonic acid (p-TsOH) treatment. However, better fractionation of hemicellulose and separation is difficult to obtain during treatment. This study aims at investigating the separation selectivity of bamboo hemicellulose using freeze-thaw-assisted p-TsOH (F/p-TsOH) treatment. The desired separation effect was achieved at freezing temperature -40 °C, freezing time 20 h, p-TsOH concentration 3.0 %, treatment temperature 130 °C and time 80 min. 93.26 % hemicellulose separation was found, which was 32.88 % higher than that of conventional p-TsOH treatment. Furthermore, the separation yield of lignin decreased significantly from 69.29 % to 13.98 %. The distinct lignin characteristic absorption peaks were found, while that of hemicellulose was difficult to observe. The fiber crystallinity index increased from 50.42 to 56.55 %. Furthermore, greater selectivity for hemicellulose separation was achieved. The results provide a new research thinking for efficient fractionation of lignocellulosic biomass by organic acid treatment.

High-efficiency separation of hemicellulose from bamboo by one-step freeze-thaw-assisted alkali treatment.

The selectivity of alkali treatment (AT) for hemicellulose separation is reduced due to the alkali solubility of lignin. It was improved using freeze-thaw-assisted alkaline treatment (FT/AT). In this study, bamboo hemicellulose was separated via a one-step freeze-thaw-assisted alkali treatment (OFT/AT). The effects of freezing temperature, freezing time, alkali concentration, and treatment time on bamboo components were studied. The separation yield of hemicellulose was 73.26%, compared to 64.00% using conventional FT/AT. The separation of lignin and cellulose was inhibited as alkali concentration decreased from 7.0% to 5.0%. The extraction yield of hemicellulose increased from 46.35% to 56.12%. Structural analysis of extracted hemicellulose revealed the effective inhibition of the breakage of the xylose backbone and arabinose side chain of hemicellulose. This indicated that the molecular structure of extracted hemicellulose was relatively complete. It provides theoretical support for the efficient separation of hemicellulose by AT.

Structural enrichment and identification of lignin-carbohydrate complex in alkaline stabilized system.

Effectively breaking down its anti-depolymerization barrier is key to the conversion and utilization of biomass resources. The anti-depolymerization barrier is mainly composed of a lignin-carbohydrate complex (LCC), which hinder the high-value utilization of biomass. In an alkaline environment, phenyl glycosidic bonds in LCC exist stably. Therefore, it is of great significance for the structural analysis of LCC in alkaline environment. In this study, LCC rich in carbohydrates was extracted from alkaline hemicellulose by using the xylanase-macroporous resin method, the effects of enzyme dosage and treatment time on LCC enrichment were discussed. The results showed that the LC bond of LCC samples was dominated by phenyl glucoside bond, the composition of lignin showed the rule of S > H > G. This method could fulfill the requirements of lignin enrichment and structural integrity. This provides a new method for solving the problems of LCC signal masking and low detection sensitivity.