Wood cellulose nanofibrils prepared by TEMPO electro-mediated oxidation

A softwood bleached kraft pulp (SBKP) was subjected to electro-mediated oxidation in water with TEMPO or 4-acetamido-TEMPO without any chlorine-containing oxidant. Solid recovery ratios of water-insoluble fractions of the oxidized SBKPs were more than 80%, and C6-carboxylate contents increased up to approximately 1 mmol g⁻¹ after oxidation for 48 h. Significant amounts of C6-aldehyde groups (0.17–0.38 mmol g⁻¹) were also formed in the oxidized SBKPs. The degree of polymerization decreased from 2,200 to 520 and 1,400 by the oxidation for 48 h with TEMPO at pH 10 and 4-acetamido-TEMPO at pH 6.8, respectively. The original cellulose I crystal structure and crystallinity of SBKP were maintained after the oxidation, indicating that all C6-oxidized groups were selectively formed on crystalline cellulose microfibril surfaces. The oxidized SBKPs with carboxylate contents of more than 0.9 mmol g⁻¹ were convertible to individual cellulose nanofibrils in yields of more than 80% by disintegration in water.     

SEC-MALS analysis of cellouronic acid prepared from regenerated cellulose by TEMPO-mediated oxidation

The 4-amino-2,2,6,6-tetramethylpiperidine-1-oxy radical (4-amino-TEMPO)-mediated oxidation was applied to regenerated cellulose, and the obtained cellouronic acid was analyzed by size-exclusion chromatography attached with a multi-angle laser light scattering detector (SEC-MALS). Although the cellouronic acid filtered with the usual 0.1 μm membrane gave a bimodal SEC-elution pattern, the high-molecular-mass fraction was removed by micro filtration of the cellouronic acid solution with a 0.02 μm membrane. It is likely, therefore, that some colloidal particles formed from regenerated cellulose by the TEMPO-mediated oxidation are present as incompletely oxidized residues in the cellouronic acid sample and behave as the high-molecular-mass fraction in the SEC elution pattern. Then, the SEC-MALS analysis was applied to the 0.02 μm membrane-filtered cellouronic acid, and the accurate DPw value of 36 was obtained for cellouronic acid. This DPw value was far lower than that of carboxymethyl cellulose, hydroxypropyl cellulose or alginic acid, resulting from significant depolymerization of cellulose chains during the TEMPO-mediated oxidation. Because the value of DPw 36 for cellouronic acid is close to the leveling-off DP (about 40) of regenerated celluloses obtained by the dilute and heterogeneous acid hydrolysis, the DPw value of cellouronic acid must reflect the solid-state structure of the original regenerated cellulose used in the TEMPO-mediated oxidation.     

Water dispersion of cellulose II nanocrystals prepared by TEMPO-mediated oxidation of mercerized cellulose at pH 4.8

Mercerized wood cellulose was oxidized by 4-acetamide-TEMPO/NaClO/NaClO₂ system at 60 °C and pH 4.8 for 1–5 days. Mostly individual nanocrystals 4–7 nm in width and 100–200 nm in length were obtained by ultrasonication of the oxidized product in water. The nanocrystals had the cellulose II structure, and carboxylate contents of 2.0–2.4 mmol/g, indicating that these carboxylate groups were selectively formed on the cellulose II crystallite surfaces in mercerized cellulose. Moreover, the original wood cellulose and mercerized cellulose were acid-hydrolyzed, and then subjected to the TEMPO-mediated oxidation under the same conditions at pH 4.8 to prepare reference samples. TEM images, light transmittance and rheological properties of water dispersions showed that the nanocrystals prepared from mercerized cellulose by the TEMPO oxidation and sonication in water had the highest dispersibility of individual nanocrystals with less amounts of bundles in water, resulting from the highest carboxylate contents.     

TEMPO-mediated surface oxidation of cellulose whiskers

Cellulose whiskers resulting from HCl acid hydrolysis of tunicin were subjected to TEMPO-mediated oxidation under various conditions and the extent of the resulting oxidation was characterized by Fourier-transform infrared spectroscopy (FT-IR), conductimetry, X-Ray diffraction analysis and transmission electron microscopy (TEM). With degree of oxidation of up to 0.1 the samples kept their initial morphological integrity and native crystallinity, but at their surface the hydroxymethyl groups were selectively converted to carboxylic groups, thus imparting a negative surface charge to the whiskers. When dispersed in water these oxidized whiskers did not flocculate and their suspensions appeared birefringent when viewed between cross polarizers, thus indicating a liquid crystalline behavior.     

TEMPO-mediated oxidation of microcrystalline cellulose: limiting factors for cellulose nanocrystal yield

Cellulose nanocrystal (CNC) production suffers, among other problems, from low yields. The focus of this study was to investigate the universal effect of charge density, centrifugation, and mechanical treatment as limiting causes of yield. Microcrystalline cellulose (MCC) was used as the starting material in order to eliminate the relatively arbitrary yield losses caused by the hydrolysis conditions. To disintegrate MCC into nanocrystals, high surface charge in the form of carboxylic groups was introduced by TEMPO-mediated oxidation, after which the material was mechanically treated, and separated into fine and coarse fractions. The fine fraction collected as supernatant after separation by centrifugation had a yield of 17–20% independent of the mechanical treatment method or time used. The particle sizes of these fractions did not significantly differ from each other, which raises questions on the efficiency of the mechanical treatment (sonication) and centrifugation in traditional CNC production. The results imply that radically new approaches in preparation are needed for truly meaningful increases in the CNC yield.     

Kinetic study on TEMPO-mediated selective oxidation of regenerated cellulose

The kinetics of the TEMPO-mediated oxidation of regenerated celluloses has been studied. It is revealed that the oxidation reaction of the regenerated celluloses by the 2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO)–NaBr–NaOCl system can be approximately described by first-order kinetics with respect to substrate. In the concentration range used the rate constant k is directly proportional to the concentration of TEMPO, while it is proportional to the concentration of NaBr in a relatively lower range and tends to level off at higher concentration. The effect of temperature on the rate constant can be well described by the Arrhenius equation, the apparent activation energy measured is about 66.2 kJ/mol. The effect of the pH of the reaction solution, the crystallinity and morphology of the substrates on the oxidation rate is also discussed.     

TEMPO-mediated surface oxidation of cellulose nanocrystals (CNCs)

Cellulose nanocrystals were successfully oxidized with sodium hypochlorite using catalytic amounts of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical at pH 10 in water. Carboxylate groups were selectively introduced at the surface of the crystals up to a total acid content of 1200 mmol kg^?1 without damaging the integrity of the crystals. The final acid content can easily be tuned by varying the amount of oxidant introduced. The effect of temperature, the quantity of oxidant and co-catalyst on the reaction kinetics were studied. Several methods were used for the characterization of the oxidized material like field emission scanning electron microscopy, diffuse reflectance infrared spectroscopy and thermogravimetric analysis.     

TEMPO-mediated oxidation of bacterial cellulose in a bromide-free system

A partially C6-carboxylated bacterial cellulose (BC) with a high carboxylate content was prepared in a bromide-free system by using 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) as a catalyst. ART-FTIR, X-ray diffraction, solid ¹³C-NMR, TEM analysis, and reaction kinetics measurements were performed to investigate the oxidation reaction of BC. Results show that C6 carboxylate was formed selectively on the microfiber surface without disrupting its highly ordered nanocrystalline structure. Given the extremely low accessibility of hydroxyl groups in d-anhydroglucopyranose units, the reaction can be described by second-order kinetics with very low reaction rate constants. pH exhibited a significant influence on the oxidation of BC and a higher activity at C6 was observed in a neutral medium.     

Optimization of reagent consumption in TEMPO-mediated oxidation of Eucalyptus cellulose to obtain cellulose nanofibers

Eucalyptus cellulose is usually pre-treated by oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), NaBr and NaClO at pH 10.5 and 25 °C before the mechanical process required to obtain cellulose nanofibers (CNFs). In this study, different aspects to improve the effectiveness and sustainability of the TEMPO-mediated oxidation are analyzed. The optimization was carried out at different reaction times by modifying both the concentration of the NaClO and the amount of the catalysts (TEMPO and NaBr). Results show that the carboxyl groups increased up to 1.1 mmol/g with 5 mmol NaClO/g after 50 min, and that the catalyst concentration can be reduced to 0.025 mmol TEMPO/g and 0.5 mmol NaBr/g to minimize costs while maintaining the high fibrillation degree of the CNFs. The kinetic of the reaction can be considered as zero-order with respect to NaClO, and as first order with respect to cellulose. As a result of this work, the catalyst doses are reduced up to 75% compared to the most widely used catalyst doses (0.1 mmol/g TEMPO and 1 mmol/g NaBr), obtaining highly fibrillated CNFs with a lower environmental impact. This reduction of catalyst doses will reduce the costs and facilitate the implementation of CNF production at industrial scale.

Graphical abstract

     

Optimization of cellouronic acid synthesis by TEMPO-mediated oxidation of cellulose III from sugar beet pulp

Microfibrillated cellulose from purified sugar beet pulp was converted into cellulose III by immersion in liquid ammonia. When freed from ammonia, this product was oxidized in water at pH-10 using NaBr, NaOCl and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under various conditions and concentrations. The resulting water-soluble cellouronic acid—i.e. cellulose oxidized at the C6 position- was analyzed by high performance size exclusion chromatography (HPSEC) together with ¹³C NMR spectroscopy. The oxidation parameters, namely reaction time, temperature, NaBr and TEMPO concentrations were varied to determine the optimum reaction conditions. A low TEMPO concentration, a rather fast reaction time and the conducting of the oxidation at 0 °C were critical to obtain pure cellouronic acid in high yield, high purity and high DP.     

Nanocellulose characteristics from the inner and outer layer of banana pseudo-stem prepared by TEMPO-mediated oxidation

Many tonnes of agricultural wastes are generated annually, which contains a relatively high amount of cellulose; banana pseudo-stem is one waste type that is a promising material for nanocellulose production. This research characterised nanocellulose from inner and outer layers of banana pseudo-stem as a preliminary research strategy for designing biodegradable packaging material from banana pseudo-stem nanocellulose. Nanocellulose was successfully prepared through TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl)-mediated oxidation. The extracted nanocellulose from both the inner and outer layers had observed widths of approximately 7–35 nm and long fibrillated fibre. They had high negative zeta potential (lower than ?33.6) that provided good colloidal stability. The purity of the nanocellulose was high as demonstrated by ¹³C solid-state NMR and Fourier transform infrared spectroscopy. Nanocellulose from both layers was significantly more crystalline than the raw materials. Thermal stability of nanocellulose sourced from inner and outer layers was relatively similar, with degradation temperature of approximately 220 °C, which was slightly lower than the degradation temperature of its native form (232 °C for inner layer and 261 °C for outer layer).     

Comparison study of TEMPO-analogous compounds on oxidation efficiency of wood cellulose for preparation of cellulose nanofibrils

The effect of chemical structures of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy radical) derivatives and its analogous compounds on oxidation efficiency of C6 primary hydroxyls of wood cellulose was investigated using the NaClO/NaBr system at pH 10. Because the oxidation takes place selectively on the surfaces of cellulose microfibrils, individualized and surface-oxidized cellulose nanofibrils can be obtained by simple mechanical treatment in water, when sufficient amounts of carboxylate groups are formed homogeneously in cellulose microfibrils. 4-acetamide-TEMPO and 4-methoxy-TEMPO showed efficient catalytic behavior with short reaction times (<4 h) and high carboxylate contents (>1.1 mmol/g) in oxidation of wood cellulose, comparable to TEMPO. Correspondingly, these TEMPO derivatives as well as TEMPO gave high nanofibril yields >56%. On the other hand, the use of 4-hydroxy-TEMPO and 4-oxo-TEMPO resulted in the lowest efficiency in oxidation: oxidation times >24 h, carboxylate contents <0.3 mmol/g, and individualized and surface-oxidized nanofibril yields <2%.     

Reactivity of dissolving pulps modified by TEMPO-mediated oxidation

The reactivity of dissolving pulps towards derivatization or dissolution is a crucial quality parameter and is mainly determined by the accessibility of the hydroxyl groups. When dissolving pulps are produced from paper-grade pulps by cold caustic extraction (CCE), their reactivity is often inferior as compared to commercial prehydrolysis kraft dissolving pulps. It was hypothesized that pulp reactivity can be enhanced by the introduction of small amounts of substituents to facilitate interchain accessibility. In this study, CCE-treated Eucalyptus globulus kraft paper pulp was subjected to TEMPO-mediated oxidation to initiate partial oxidation of the C₆-hydroxyl groups to carboxyl groups. The effect of this pulp modification on the reactivity towards xanthation and the subsequent dissolution in diluted aqueous alkali solution (viscose process) as well as the dissolution in complexing and non-complexing solvents, respectively, was thoroughly examined. The results revealed that the oxidized pulps rich in C₆-carboxylate groups impeded the xanthation reaction obviously because of the reduced availability of hydroxyl groups. When N-methylmorpholine-N-oxide monohydrate was used as a direct solvent, a very high content of C₆-carboxylate groups was found to reduce the solubility of the pulp fibers as less hydrogen bonds can be formed with NMMO·H₂O. In the case of dissolution in the complexing solvent cupriethylenediamine, the dissolution mechanism of cellulose was not deteriorated by the high content of C₆-carboxylate groups. Instead, the oxidation procedure increased the hydrophilic character and the swelling capacity of the outer cell wall layers allowed homogeneous dissolution.     

Dewatering of cellulose nanofibrils using ultrasound

Although cellulose nanomaterials have promising properties and performance in a wide application space, one hinderance to their wide scale industrial application has been associated with their economics of dewatering and drying and the ability to redisperse them back into suspension without introducing agglomerates or lose of yield. The present work investigates the dewatering of aqueous suspensions of cellulose nanofibrils (CNFs) using ultrasound as a potentially low-cost, non-thermal, and scalable alternative to traditional heat-based drying methods such as spray drying. Specifically, we use vibrating mesh transducers to develop a direct-contact mode ultrasonic dewatering platform to remove water from CNF suspensions in a continuous manner. We demonstrate that the degree of dewatering is modulated by the number of transducers, their spatial configuration, and the flow rate of the CNF suspension. Water removal of up to 72 wt.% is achieved, corresponding to a final CNF concentration of 11 wt.% in 30 min using a two-transducer configuration. To evaluate the redispersibility of the dewatered CNF material, we use a microscopic analysis to quantify the morphology of the redispersed CNF suspension. By developing a custom software pipeline to automate image analysis, we compare the histograms of the dimensions of the redispersed dewatered fibrils with the original CNF samples and observe no significant difference, suggesting that no agglomeration is induced due to ultrasonic dewatering. We also perform SEM analysis to evaluate the nanoscale morphology of these fibrils showing a width range of 20 nm–4 um. We estimate that this ultrasound dewatering technique is also energy-efficient, consuming up to 36% less energy than the enthalpy of evaporation per kilogram of water. Together with the inexpensive cost of transducers (<?$1), the potential for scaling up in parallel flow configurations, and excellent redispersion of the dewatered CNFs, our work offers a proof-of-concept of a sustainable CNF dewatering system, that addresses the shortcomings of existing techniques.

     

Degradation of TEMPO-oxidized cellulose fibers and nanofibrils by crude cellulase

The biodegradation behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose fibers (TOCs) suspended in water and TEMPO-oxidized cellulose nanofibrils (TOCNs) dispersed in water by a commercial crude cellulase was studied. Products crude cellulase-treated for 0–7 days were separated into water/ethanol-insoluble and -soluble fractions. Weight recovery ratios and viscosity-average degrees of polymerization of the water/ethanol-insoluble fractions clearly decreased with crude cellulase-treatment time, showing that both TOCs and TOCNs have biodegradability. Water/ethanol-soluble fractions were subjected to size-exclusion chromatography (SEC) with photodiode array (PDA) detection to obtain SEC elution patterns detected by reflective index and UV spectra of each SEC pattern elution slice. SEC–PDA and ¹³C-NMR analyses showed that glucuronosyl unit-containing molecules present on microfibril surfaces in TOCs and TOCNs were primarily cleaved by hydrolyzing enzymes present as contaminants in the crude cellulase to form glucuronic acid as one of the major water-soluble degradation compounds. After the glucuronosyl units in TOCs and TOCNs were degraded and removed from microfibril surfaces by the hydrolyzing enzymes, cellulose chains newly exposed on the microfibril surfaces were rapidly hydrolyzed by cellulases predominantly present in the crude cellulase to form cellobiose. Both TOCs and TOCNs having sodium carboxyl groups are thus biodegradable, but TOCN having free carboxyl groups had clearly low biodegradability by the crude cellulase. Thus, biodegradation behavior may be controllable by controlling the structure of carboxyl group counter ions in TOCs and TOCNs.     

Synthesis and identification of fullerenol prepared by the direct oxidation route

A route was developed for synthesis of fullerenol, which was further identified by electron spectroscopy, infrared spectroscopy, optical microscopy, high-performance liquid chromatography, and mass spectrometry.     

Sonocatalysis of the TEMPO-mediated oxidation of glucosides

Ultrasound is shown to increase the rate of the TEMPO-mediated oxidation of methyl - -glucopyranoside or sucrose in the presence of stoichiometric amounts of sodium hypochlorite in basic aqueous medium. The reaction can then occur without the usually necessary sodium bromide, showing that ultrasound acts at the level of the formation of the nitrosonium ion, the active oxidizing species in the catalytic cycle.     

Depolymerization of cellouronic acid during TEMPO-mediated oxidation

The mechanism of partial depolymerization of cellouronic acid (-1,4-linked polyglucuronic acid sodium salt) during 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of cellulose was studied by treating cellouronic acid with one reagent or a combination of TEMPO-NaBr-NaClO under various conditions. Although NaClO, NaBrO and an alkali at pH 11 brought about partial depolymerization of cellouronic acid, the use of these reagents themselves did not seem to be the primary reason for depolymerization. On the other hand, when all the reagents, i.e. TEMPO-NaBr-NaClO, were applied to cellouronic acid at pH 11, a remarkable decrease in weight-average degree of polymerization (DPw) from 430 to ca. 20 was observed within the initial 10 min. Probably hydroxyl radicals formed from NaBrO and TEMPO at pH 10–11 cause the depolymerization during the oxidation. Some radical scavengers were then used for the TEMPO-mediated oxidation of cellulose in order to suppress the depolymerization. Although the addition of crotonic acid under certain conditions gave cellouronic acid having higher DPw, none of the radical scavengers examined so far could completely prevent the depolymerization. When regenerated celluloses having higher DP were used as the starting materials, cellouronic acids having DPw of more than 1000 were obtained, although they still had large low-molecular-weight fractions.     

Mechanical properties of films made from dialcohol cellulose prepared by homogeneous periodate oxidation

2,3-Dialdehyde celluloses were prepared by homogeneous periodate oxidation in an aqueous solution of methylol cellulose. Since methylol cellulose stays dissolved in water for a certain time before decomposing gradually into regenerated cellulose, the oxidation reaction progressed homogeneously throughout the period. The resulting dialdehyde cellulose achieved an oxidation level of over 90 % in as little as 12 h. Reducing the dialdehyde celluloses with NaBH₄ resulted in water-soluble dialcohol celluloses, which have an open-ring structure at the C2–C3 position. The dialcohol celluloses were characterized using nuclear magnetic resonance spectrometry, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The T_g of the products decreased with increasing oxidation levels. The products might be processable, and unique tensile properties were obtained by cutting the C2–C3 bonds in the glucopyranose rings. The dialcohol celluloses prepared using a cast method yielded clear and transparent films which showed unique mechanical properties by tensile tests depending on the values of oxidation level.