TEMPO-mediated Oxidation Research Articles

Lactose derivatives as potential inhibitors of pectin methylesterases.

The discovery of molecules that can inhibit the action of phytopathogens is essential to find alternative to current pesticides. Pectin methylesterases (PME), enzymes that fine-tune the degree of methylesterification of plant cell wall pectins, play a key role in the pathogenicity of fungi or bacteria. Here we report the synthesis of new lactoside derivatives and their analysis as potential PME inhibitors using three plants and one fungal PME. Because of its structure, abundance and reduced cost, lactose was chosen as a case study. Lactoside derivatives were obtained by TEMPO-mediated oxidation of methyl lactoside, followed by an esterification procedure. Three derivatives were synthesized: sodium (methyl-lactosid)uronate, methyl (methyl-lactosid)uronate and butyl (methyl-lactosid)uronate. The inhibition of the plant and pathogen enzyme activities by lactoside derivatives was measured in vitro, showing the importance of the substitution on lactose: methyl (methyl-lactosid)uronate was more efficient than butyl (methyl-lactosid)uronate. These results were confirmed by docking analysis showing the difference in the interaction between lactoside derivatives and PME proteins. In conclusion, this study identified novel inhibitors of pectin remodeling enzymes.

Nano-lignocellulose from recycled fibres in coatings from aqueous and ethanolic media: effect of residual lignin on wetting and offset printing quality

Abstract Nano-lignocellulose (NLC) and lignin-free nanocellulose (nano-holocellulose, NHC) were used in paper coating to investigate their effect on coating layer quality and offset printing. The NLC was produced by microfluidisation of unbleached secondary fibres while the reference NHC was obtained from the same fibre source after lignin removal (OHEPH bleaching), following the same mechanical process. TEMPO-mediated oxidation of the fibres prior to microfluidisation was applied to increase the electrostatic charge and hydrophilicity of the nanofibrils. The coatings, displaying given surface morphology and energy, were applied on Kraft, printing-grade papers at three grammage levels. The structure of the coated and uncoated (reference) papers were accessed (SEM and AFM) and IGT printing was carried out to determine the print density, print gloss, rub-off resistance, surface energy, roughness, ink transfer, dry pick resistance, water interference and set-off. The results highlight the important effect of residual lignin or type of nanocellulose on the coating layer and the development of offset printing properties. It was observed that roughness was a key factor leading to a deterioration of the print properties, predominantly affecting the NLC coating. Considering the lower hydrophilicity of NLC, an alternative dispersion with water-alcohol mixtures is proposed. By using this dispersing medium, tailorable surface coverage, surface smoothness, ink acceptance and improved printability was achieved. We show that under these conditions and compared to NHC, NLC is equally effective as a coating layer.

Microwave-Assisted Catalyst-Free Oxidative Conversion of a Lignin Model Compound to Value-Added Products Using TEMPO

As oxidative conversion of vanillyl alcohol (VAL) to vanillin (VN) represents an important process for valorization of lignin compounds, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is employed for the first time as an oxidant to oxidize VAL in the absence of catalysts using microwave irradiation (MWI). In comparison with H2O2, persulfate and peroxymonosulfate, TEMPO leads to the significantly higher selectivity of vanillin (SVN) and yield of VN. In addition, MWI is also more effective than conventional hydrothermal process for converting VAL to VN with a much higher SVN. A longer reaction time is highly favorable for VAL conversion to VN by TEMPO using MWI with a much higher SVN and yield of VN. Besides, the SVN and yield of VN from VAL oxidation by TEMPO using MWI could be also significantly improved at the elevated temperature with a higher TEMPO dosage. The optimal condition for VAL conversion to VN by TEMPO using MWI is the usage of 1.5 g of TEMPO for 7.5 min of MWI at 160 °C and the highest yield of VN is 17% with SVN of 99%, which is higher than many reported values of catalyst-involved processes. TEMPO can be also re-used to oxidize VAL to generate VN with quite stable yield of VN and SVN. These results indicate that TEMPO, even without catalysts, can successfully and effectively convert VAL to the value-added phenolic products, such as VN. The findings of this study can be useful for further investigating TEMPO-mediated oxidation of lignin compounds.

Preparation and Characterization of Lignin-Containing Cellulose Nanofibril from Poplar High-Yield Pulp via TEMPO-Mediated Oxidation and Homogenization

In this study, a poplar high-yield pulp [preconditioning refiner alkaline peroxide mechanical pulp (P-RC APMP)] was used to produce lignin-containing cellulose nanofibril (LCCNF) dispersions throug...

Convergent synthesis of the pentasaccharide repeating unit of the biofilms produced by Klebsiella pneumoniae.

A pentasaccharide repeating unit containing α-linked D-glucuronic acid, β-linked D-mannose, corresponding to the repeating unit of biofilms produced by Klebsiella pneumoniae, has been synthesized using a stereoselective [2 + 3] convergent glycosylation strategy. The β-D-mannosidic moiety has been synthesized using a D-mannose-derived thioglycoside by a two-step activation process. Late stage TEMPO-mediated oxidation of the pentasaccharide derivative using phase-transfer reaction conditions furnished the target compound in satisfactory yield.

Ultrafine and carboxylated β-chitin nanofibers prepared from squid pen and its transparent hydrogels

TEMPO-mediated oxidation has been successfully used to prepare carboxylated chitin nanofibers (ChNFs) with purified chitins originating from the outer shells of crab and shrimp (α-form) or tubeworm (β-form). However, the method for obtaining carboxylated ChNFs with squid pen chitin (hydrated β-form) has not been developed yet. It might be due to the existence of the small amount of partial deacetylation (DD ≈ 9%) in the squid pen β-chitins. Herein, ultrafine (2–4 nm in width and several micrometers in length) and carboxylated β-ChNFs were fabricated directly from the squid pen with simultaneously removing of protein, CaCO3 and other non-chitin components by one-step oxidation procedure using ammonium persulfate (APS) and followed ultrasonic disintegration under acid conditions. When 45 wt% APS was used to react with squid pen, the carboxylate content of ChNFs reaches 0.802 mmol/g. Therefore, the β-ChNFs with anionically charged groups (COO−) can be dispersed stably in aqueous solution under basic conditions. Meanwhile, thus-obtained β-ChNFs aqueous solution even with very low concentration (0.8%) can be transformed to transparent, robust and moldable hydrogels by gas coagulation of acetic acid.

Preparation and characterization of cellulose nanofibrils from coconut coir fibers and their reinforcements in biodegradable composite films

Coconut waste husks were effectively utilized in this study as a promising cellulose source for production of purified coir cellulose (PCC) after multiple treatments, e.g., ultrasonic-assisted solvent immersion, alkaline treatment, bleaching, etc. As to upgrade the self-value of coir cellulose based products and further broaden their applications in light of biorefinery, coir cellulose nanofibrils (CCNF) with an average diameter of 5.6 ± 1.5 nm were prepared by selection of a milder TEMPO-mediated oxidation system (TEMPO/NaClO/NaClO2, pH = 4.8) accompanied by subsequent ultrasonic treatment. The cellulose nanofibrils were comprehensively characterized in terms of their functional groups, crystallinity, morphology, and thermal stability. The potential reinforcement of CCNFs as a filler for biodegradable PVA based films was investigated and the main properties including tensile strength, elongation at break, and thermal stability of CCNF/PVA composite films were significantly enhanced especially when 3% of CCNF (based on dry film weight) was applied.

Study of various diameter and functionality of TEMPO-oxidized cellulose nanofibers on paraquat adsorptions

The adsorption of paraquat was examined in the presence of renewable nanomaterials of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) oxidized cellulose nanofibers (TOCNs) with different oxidation time (i.e. TOCN-1h, TOCN-4h, TOCN-8h, and TOCN-24h). First, we monitored the relationships among TEMPO-mediated oxidation times, cellulose surface functional groups, and fiber diameters. We observed the oxidation and the carboxylate contents of cellulose were promptly increased in an hour, leading to the formation of nano-sized cellulose fibers (ca. 20 nm). These cellulose nanofibers were characterized by Fourier-transform infrared (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) method, and wide angle X-ray diffraction (WAXRD). Accordingly, the TOCNs with enlarged specific surface area (ca. 180 m2/g) performed high adsorption efficiency for paraquat (>90%), contributing from the strong attraction force between carboxylate anion on TOCNs and parquet cation. The adsorption behaviors were revealed by intra-particle diffusion (IPD) model. For TOCN-24 h, IPD of adsorbate from the surface to the active sites within the hemicellulose or disorder region was observed, resulting in the increase of the adsorption equilibrium time. We then comprehended the absorption performance was sensitive under different pH environments. The adsorption capacity over 100 mg/g can be reached at a pH value greater or equal to 7. Furthermore, Langmuir isotherm model was fitted to the adsorption behaviors at various temperatures, indicating a single layer adsorption mechanism.

OXYCELLULOSE – MATERIAL FOR NEW APPLICATION AREAS

In this paper, the principles of the action of the selective (NaIO4 and 2,2,6,6-tetramethylpiperidine-1-oxyl-TEMPO radical) and non-selective (H2O2 and KMnO4) oxidation agents on cellulose are presented. Their importance in modifying the existing properties or giving new properties to cellulosic materials, as well as examples of their applications are also given. The oxidation product - oxycellulose presents the base for obtaining a wide range of materials for special applications. Particularly, the influence of TEMPO-mediated oxidation on natural and man-made cellulosic fibers is presented, with a detailed overview on the type and quantity of introduced functional groups, changes in structure, sorption and mechanical properties. Also, we described the use of TEMPOmediated oxidation for the production of cellulosic nano-materials in the form of dispersions, particles and films, as well as the potential application of these nano-structural materials.

A comparative study on the starch-based biocomposite films reinforced by nanocellulose prepared from different non-wood fibers

Nanocellulose was extracted from three kinds of non-wood fibers (bamboo, cotton linter, and sisal) by TEMPO-mediated oxidation and high pressure homogenization. Starch-based composite films containing different kinds of nanocellulose with different content (0–10 wt%) were prepared via solution casting method. The morphology and structure of the three kinds of nanocellulose and their respective effects on the composite films were compared by various characterizations. The impacts of nanocellulose content on the thermal stability and mechanical properties of the composite films were also evaluated. The study found that morphology and chemical composition of the nanocellulose obtained from different sources were almost the same, but there were slight differences in their size and crystallinity. Bamboo nanocellulose had the highest aspect ratio, which enabled it to provide the greatest reinforcing effects on the mechanical properties and barrier properties of the composite films. The addition of nanocellulose improved the mechanical properties of the films but reduced their elongation at break and thermal stability. This study paves the route for choosing the most effective non-wood nanocelluloe source and mixed ratio to produce food packaging with the best performance.

Nanocellulose for gel electrophoresis

HypothesisCellulose nanofibres produced by TEMPO-mediated oxidation can form gels. This study presents a proof-of-concept for gel electrophoresis with nanocellulose (NC). ExperimentsTEMPO-oxidised cellulose nanofibre dispersion is chemically cross-linked by inducing amide linkages to produce gel slabs for electrophoretic separation. Nanocellulose gel slabs 1 cm thick containing Tris/Borate/EDTA (TBE) buffer were casted. Different cross-linker types and ratios are investigated to assess the migration of conventional electrophoresis tracking dyes. FindingsTracking dyes (bromophenol blue and orange G) can diffuse within the gel at different rates and therefore separate. Changing the cross-linker length from EDA to HMDA (C2- to C6-chain) increases the overall network pore size resulting in a faster migration rate for both bromophenol blue and orange G. Increasing the cross-linker concentration stabilises the HMDA-NC gel (no extension) during the electrophoresis run without any effect on the dye migration rate. Increasing the voltage increases the migration rates for both orange G and bromophenol blue. Further development is required to cast the gels evenly and to prevent bubble formation during the cross-linking process. This will enable to effectively separate mixtures of proteins. Nanocellulose gels can become a novel substrate for sustainable biomedical separation and diagnostics by electrophoresis.

Improved Dispersion of Bacterial Cellulose Fibers for the Reinforcement of Paper Made from Recycled Fibers

Bacterial cellulose (BC) can be used to improve the physical properties of paper. However, previous studies have showed that the effectiveness of this improvement is impaired by the agglomeration of the disintegrated BC fibers. Effective dispersion of BC fibers is important to their reinforcing effects to paper products, especially those made of recycled fibers. In this study, carboxymethyl cellulose, xylan, glucomannan, cationized starch, and polyethylene oxide were used to improve the dispersion of BC fibers. With dispersed BC fibers, the paper made of recycled fiber showed improved dry tensile strength. The best improvement in dry tensile index was 4.2 N·m/g or 12.7% up, which was obtained by adding BC fibers dispersed with glucomannan. Glucomannan had the highest adsorption onto BC fibers, i.e., 750 mg/g at 1000 mg/L concentration, leading to the best colloidal stability of BC fiber suspension that had no aggregation in 50 min at 0.1 weight ratio of glucomannan to BC. TEMPO-mediated oxidation of BC was effective in improving its colloidal stability, but not effective in improving the ability of BC fiber to enhance paper dry tensile index while the wet tensile index was improved from 0.89 N·m/g to 1.59 N·m/g, i.e., ~80% improvement.

Lithium Ion Battery Separators Based On Carboxylated Cellulose Nanofibers From Wood

Carboxylated cellulose nanofibers, prepared by TEMPO-mediated oxidation (TOCN), were processed into asymmetric mesoporous membranes using a facile paper-making approach and investigated as lithium ...

Elasticity-Enhanced and Aligned Structure Nanocellulose Foam-like Aerogel Assembled with Cooperation of Chemical Art and Gradient Freezing

Cellulose nanofibril (CNF) aerogels are renewable and biocompatible materials with high porosity and tunable surface chemistry. However, ultralight and ultraporous aerogels remain a great challenge to obtain high elasticity. This work focused on a scalable strategy to create large-scale lamellar-aligned CNF foam-like aerogels and the relationship between structure and mechanical properties. The morphology and mechanical properties of aerogels assembled by original TEMPO-mediated oxidation CNF cross-linking with 1,2,3,4-butanetetracarboxylic acid were investigated for homogeneous freezer freezing and unidirectional gradient freeze-casting. This study successfully fabricated ultralight foam-like aerogels with centimeter-sized and aligned lamellar/porous structure via cooperation of tunable chemical reaction and unidirectional gradient freezing. The resulting aerogels exhibited flyweight densities of 3–4 mg/cm³, enhanced recovery from 70% strain, water adsorption 82.5 times over self-weight at 20 °C and 52.0 times over self-weight at high temperature (100 °C) for 20 cycles. Moreover, the aligned aerogel followed by carbonization showed a differential and anisotropic electrical resistivity.

Highly Dispersible Cellulose Nanofibrils Produced via Mechanical Pretreatment and TEMPO-mediated Oxidation

Cellulose nanofibrils (CNFs) can prepare flexible nanomaterials with its large aspect ratio. Due to the supramolecular, cellulose fibers are difficultly dissociated to CNFs. In order to destroy this supramolecular structure and prepare high dispersible CNFs, the mechanical and chemical treatments are required. This study examined the effects of mechanical pretreatment and TEMPO-oxidation on the properties of CNFs preparing from bleached softwood kraft pulp. The total yield of TEMPO-oxidized cellulose nanofibrils (TOCNFs) reached 85 %. The morphology, carboxyl group content, crystallinity, surface charge, self-assembling properties, and optical transmittance of the corresponding TOCNFs suspension were investigated. The transmittance of TOCNFs suspension was 95.1 % at 600 nm wavelength and its corresponding ζ- potential was -61.3 mV, indicated that the strong electrostatic repulsion between TOCNFs was the most significant factor on the highly transparent TOCNFs suspension in water. Furthermore, the mostly negative charged TOCNFs obtained by freeze drying assembled into the finest and most uniform networked structure. Such highly transparent, functionalized, selfassembled cellulose nanofibrils are favorable in transparent films, flexible displays and electrochemistry materials.

Study of The Reaction Mechanism to Produce Nanocellulose-Graft-Chitosan Polymer.

Cellulose and chitin are the most abundant polymeric materials in nature, capable of replacing conventional synthetic polymers. From them, cellulose nano/microfibers (CNFs/CMFs) and chitosan are obtained. Both polymers have been used separately in graft copolymerization but there are not many studies on the use of cellulose and chitosan together as copolymers and the reaction mechanism is unknown. In this work, the reaction mechanism to produce nano/microcellulose-graft-chitosan polymer has been studied. Recycled cellulose pulp was used, with and without a 2,2,6,6-tetramethylpiperidin-1-oxyl-radical (TEMPO)-mediated oxidation pretreatment, to produce CNFs and CMFs, respectively. For chitosan, a low-molecular weight product dissolved in an acetic acid solution was prepared. Grafted polymers were synthesized using a microwave digester. Results showed that TEMPO-mediated oxidation as the cellulose pretreatment is a key factor to obtain the grafted polymer CNF-g-CH. A reaction mechanism has been proposed where the amino group of chitosan attacks the carboxylic group of oxidized cellulose, since non-oxidized CMFs do not achieve the desired grafting. 13C NMR spectra, elemental analysis and SEM images validated the proposed mechanism. Finally, CNF-g-CH was used as a promising material to remove water-based inks and dyes from wastewater.

Rapid and energy-saving preparation of ramie fiber in TEMPO-mediated selective oxidation system

Although oxidation systems were many and varied, alkali peroxide oxidation system was the only one used for ramie degumming in previous studies. In the work reported here, the possibility of ramie degumming in TEMPO-mediated selective oxidation system was studied. Results revealed that ramie fiber can be extracted by using TEMPO-mediated oxidation treatment (NaClO 2 mmol/g; 15 min; 20 ∼ 25 °C) followed by a low concentration NaOH scouring (2.5%, 100 °C, 60 min) and a reducing process. Fiber degummed in TEMPO-mediated selective oxidation system showed better tensile properties and lower acid group contents compared with that in alkali peroxide oxidation system. The required time for TEMPO-mediated selective oxidation degumming was about 125 min, which was 55 min shorter than hydrogen peroxide system. Moreover, the COD value of wastewater in TEMPO-mediated selective oxidation degumming was 28% lower than that in alkali peroxide oxidation degumming.

Surface modification of TEMPO-oxidized cellulose nanofibrils for composites to give color change in response to pH level

This study aimed to modify the surface of cellulose nanofibrils (CNFs) prepared by TEMPO-mediated oxidation via two approaches: esterification and amination for further reaction with activated bromocresol green (BCG) to prepare CNF-BCG composites capable of changing color to pH level. Both the esterified and aminated CNFs were reacted with the activated BCG to obtain the composites of CNF-BCG1 and CNF-BCG2, respectively. Then, these composites were exposed to different pH levels to observe any color changes. Fourier transform infrared (FTIR) spectroscopy, thermogravimetry analysis (TGA), and UV–Vis spectroscopy were employed to characterize the modified CNFs and CNF-BCG composites. FTIR spectra showed that CNFs have been esterified or aminated, and the modified CNFs had been successfully reacted with the activated BCG to form CNF-BCG1 and CNF-BCG2, respectively. TGA results also supported this result with a greater thermal stability of both CNF-BCG composites than those of the CNFs. The CNF-BCG1 became yellow below pH 2 while it became green above pH 5. Although these CNF-BCG composites showed color changes to relevant pH levels, the CNF-BCG1 had more distinctive color changes than that of the CNF-BCG2. These results suggested that the esterification of CNFs was a better route to the reaction with the activated BCG for the color change, showing a possibility of using the CNF-BCG1 composites for colorimetric application.

Amine functionalization of cellulose nanocrystals for acid–base organocatalysis: surface chemistry, cross-linking, and solvent effects

Sulfuric acid-derived cellulose nanocrystals (CNCs) are investigated as an alternative support for bifunctional heterogeneous organocatalyst materials. CNC catalysts are developed through direct surface functionalization by TEMPO-mediated oxidation and subsequent EDC amide coupling with alkyl diamines, used to impart both acid and base character to CNC surfaces. Sulfate ester content is controlled by partial acid hydrolysis prior to chemical functionalization, optionally reducing the initial sulfur content by approximately 50%. The resultant acidic and basic catalytic moieties are analyzed qualitatively by FT-IR spectroscopy and quantitatively by conductometric titration and elemental analysis. Relatively high degrees of substitution with amine functionality are achieved with diamine loadings around 0.9 mmol/g. This synthesis partially cross-links the CNCs, the extent of which is analyzed by dynamic light scattering of materials functionalized with 2–6 carbon spacer alkyl diamines. These CNC materials are demonstrated to be effective acid–base catalysts for an aldol condensation test reaction in batch liquid phase kinetic experiments. Relatively slow reaction rates were achieved in water:acetone reaction mixtures, in which decreased sulfate ester content is shown to decrease catalytic activity. Significant rate improvements are achieved by elimination of rate limiting transport and miscibility issues by solvent exchange to organic-only acetonitrile:acetone co-solvent reaction mixtures. Cross-linking and limited acid–base cooperativity are shown to decrease activity in CNCs functionalized with too long and too short diamines, respectively. This study demonstrates CNCs as alternative supports for bifunctional heterogeneous organocatalysis with site-specific activity on the order of that of well-studied aminosilica organocatalysts.

Extraction and characterization of cellulose nanofibers and nanocrystals from liquefied banana pseudo-stem residue

Banana pseudo-stems fibers were novel extracted via atmospheric liquefaction combined with TEMPO-mediated oxidation. Cellulose nanofibers (CNF) and nanocrystals (CNC) were subsequently obtained by high-intensity ultrasonic treatment and acidolysis, respectively. Liquefied residue content as well as its constituent varied with liquefaction time. Scanning electron microscopy and fourier transform infrared demonstrate that the progressive removal of non-cellulosic impurities. Morphological studies exhibit that CNF present a range of 3∼10 nm in diameter and 140∼690 nm in length, while CNC have average diameter of 3.5 ± 1.6 nm and a length of 93.3 ± 37.6 nm. XRD analysis indicates that the CNCs have a high crystallinity (95.6%). The thermal stability of the fibers was measured by thermogravimetric analysis. The structures and properties of CNF and CNC give it potential to be used in reinforcing polymeric matrix materials.