Modified Fenton Oxidation of Cellulose Fibers for Cellulose Nanofibrils

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Modified Fenton Oxidation of Cellulose Fibres for Cellulose Nanofibrils Preparation Qun Li, Aijiao Wang, Keying Long, Zhibin He, and Ruitao Cha ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04786 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Sustainable Chemistry & Engineering

Modified Fenton Oxidation of Cellulose Fibres for Cellulose Nanofibrils Preparation Qun Li,*, † Aijiao Wang,† Keying Long,‡ Zhibin He,§ and Ruitao Cha*, ‖ † Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, 1038 Dagu Nanlu, Hexi District, Tianjin 300457, P. R. China. ‡ Research Institute of Wood Industry, Chinese Academy of Forestry, No. 1 Dongxiaofu, Xiangshan Road, Haidian District, Beijing 100091, P. R. China. § Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3. ‖ Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, CAS Center for Excellence in Nanoscience, National Center for NanoScience and Technology, No. 11 Zhongguancun Beiyitiao, Haidian District, Beijing 100190, P. R. China. Corresponding Author *Email: [email protected] (Q. Li); *Email: [email protected] (R. Cha). ABSTRACT: A novel catalytic oxidation process based on the Fenton reaction (H2O2-FeSO4) was developed to pretreat cellulose fibres for the preparation of cellulose nanofibrils (CNF). In the so-called modified Fenton process, softwood bleached kraft pulp (SWBK) fibres were utilized as individual micro-reactors to carry out efficient in-situ oxidation of cellulose chains, which in turn facilitated nano-fibrillation of fibres in subsequent mechanical treatment. Ferrous ions were pre-loaded into fibre cell wall by adsorption and diffusion, which initiated the catalytic oxidation of cellulose simultaneously inside the fibre cell wall structure when hydrogen peroxide was introduced. The C2, C3 and C6-hydroxyl groups on the glucosyl of cellulose chains were oxidized to carboxyl groups, which could enhance the separation of micro/nanofibrils by increasing the electrostatic repulsion of the fibrils. The carboxyl group content was found to increase from 39 to 56 mmol/kg after

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the oxidation pretreatment. The oxidation also caused breakage of the 1, 4-β-D-glucoside bonds of cellulose chains and dramatically decreased the degree of polymerization (DP) of the cellulose macromolecules. The oxidized SWBK fibres were well dispersed into cellulose nanofibrils in the subsequent homogenization treatment. The obtained CNF had a uniform distribution of cellulose fibrils with an average diameter of less than 100 nm. KEYWORDS: Cellulose nanofibrils, Modified Fenton oxidation, Degree of polymerization, Cellulose fibres, Homogenization


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INTRODUCTION Cellulose nanofibrils (CNF) can be isolated from cellulose-containing materials such as cotton, hemp fibres, wheat straws and wood fibres. CNF has been widely studied in recent years, due to its distinct features such as large surface area, high hydrophilicity, biocompatibility and biodegradability.1-3 These properties are opening new opportunities for CNF applications in areas including food packaging, reinforcement of nanocomposites, biomimetic materials, and flexible electronic devices, etc.4-7 CNF has been successfully prepared with bleached pulp fibres in a laboratory scale through mechanical treatments such as high-pressure homogenization, microfluidization, grinding and ultrasonication.8-12 However, the wastewater pollution and high cost have become a “bottleneck” for further development of CNF.13, 14 Fenton oxidation pretreatment of cellulose pulp fibres to produce a product containing microfibrillated cellulose (MFC) has been reported.15 The MFC produced by Fenton reaction had a stable water-fibre suspension for at least 8 weeks compared to enzymatic pretreated pulps and pulps without subjecting to any pretreatment.15 However, the chemical consumption of a conventional Fenton process is high due to wasteful hydrogen peroxide decomposition. In such a process, the majority of the ferrous ion catalysts and hydrogen peroxide oxidants are in the bulk solution, and the reactive species such as hydroxyl radicals produced from the Fenton reactions may be exhausted by the wasteful side reactions before they reach to the cellulose fibres. As a result, only a small amount of the hydrogen peroxide added to the system is available for the oxidation reaction of cellulose.



An idea to improve the oxidation efficiency of hydrogen peroxide is to have the Fenton reaction take place inside the cell wall structure of cellulose fibres, so that the hydroxyl radicals are formed and utilized in-situ at the reaction sites of cellulose. This can be done by pre-loading ferrous ion catalyst into fibre cell wall structure by adsorption and diffusion before the addition of hydrogen peroxide. Figure 1b schematically illustrates this new oxidation process in comparison with the conventional Fenton process (Figure 1a). Cellulose pulp fibres have good absorbability of metal ions.16,

17

The hypothesis is that the

oxidation efficiency of cellulose will be greatly improved when the pulp fibres are utilized as micro-reactors for the Fenton reactions.

Figure 1 Schematic illustration of the modified Fenton process.

In this work, SWBK fibres were pre-treated with ferrous sulfate solution to let ferrous ions adsorb and diffuse into the fibre cell wall structure, and the excessive ferrous sulfate in the bulk solution was removed by filtration before the introduction of hydrogen peroxide into the system. The wasteful


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decomposition reaction of hydrogen peroxide was largely suppressed in this process due to absence of ferrous ions in the bulk solution. The cellulose oxidation efficiency was evaluated in terms of hydrogen peroxide consumption, the degree of polymerization (DP) and carboxyl content of cellulose, as well as the viscosity of resultant CNF dispersion. The underlying reaction mechanism was also discussed for the oxidation reactions in the modified Fenton process. The so-called modified Fenton method consumes much less hydrogen peroxide than the traditional Fenton method, as the residual hydrogen peroxide can be recycled and reused for the oxidation process. Compared with the classical TEMPO oxidation, the modified Fenton oxidation (Figure 1) does not use any halogen-containing compounds such as sodium chloride and sodium hypochlorite which are hazardous to the environment. Specifically, the pulp consistency used in the modified Fenton oxidation is far higher than that in the TEMPO oxidation process. The actual water consumption of the modified Fenton oxidation is reduced by 95%, compared with the TEMPO oxidation process. Thus, the modified Fenton oxidation has potential advantages in industrial scale applications.18-20

EXPERIMENTAL Materials. Softwood bleached kraft pulp (SWBK) was provided by Celgar Pulp Company, Canada. The pH of the SWBK slurry at 40 oC is 7.42 determined by pH meter (Seven2Go pro, Mettler Toledo, Switzerland). Ferrous sulfate (FeSO4·7H2O), hydrogen peroxide solution (content ≥30%), and copper ethylenediamine solution (CuEn) were obtained from Sigma-Aldrich (Shanghai) Trading Co., Ltd.. All other reagents were of analytical grade and used without further purification.



Oxidation and Homogenization Treatments. In a 500 mL reactor, 1.5 g FeSO4 was completely dissolved in 300 mL deionized water, and then 10 g (o.d.) SWBK was added (Step 1 in Figure 1b). The mixture was stirred at room temperature for 60 min to let Fe2+ ions adsorb and penetrate/diffuse and into fibres. Subsequently, the excess ferrous sulfate solution was removed by filtration (Step 2 in Figure 1b). Afterward, H2O2 solution was added in the pulp, and the oxidation reaction was initiated by the Fe2+ ions inside fibres (Step 3 in Figure 1b). After the excess H2O2 solution was removed by filtration (Step 4 in Figure 1b), the oxidized cellulose fibre was washed with deionized water until the conductivity of the water was less than 5 μS/cm, and then homogenized at 1% (w/w) consistency with a two-chamber high-pressure homogenizer (AH100D, ATS Engineering Inc., Canada.) for 15 times at a pressure of 60 MPa to obtain CNF dispersion. The optical images and the UV-Vis absorption spectra of SWBK and CNF dispersion were shown in Figure S1. The EDS analysis of CNF was shown in Figure S2. The viscosity of the CNF dispersion was determined with a NDJ-1 rotational viscometer, and it was used as an indicator of the nano-fibrillation degree of the CNF dispersion. Scanning Electron Microscopy (SEM). The morphology of the samples was characterized by Zeiss EVO 40 SEM operating at 15 kV. Before the analysis, the samples were mounted on an aluminium holder and coated with gold particles using an ion sputtering instrument to provide adequate conductivity. Transmission Electron Microscopy (TEM). A drop of dilute CNF dispersion was deposited on a carbon-coated grid. After drying under vacuum, it was imaged


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using a Hitachi H800 microscope operated at an accelerating voltage of 100 kV. Atomic Force Microscopy (AFM). A drop of CNF dispersion was placed on the surface of mica sheet for air drying. The morphology of sample was tested with a scanning area of 5.0 m×5.0 m (JSPM-5200, Japan). 13C-NMR.

The

13C-NMR

spectrum was recorded on a Bruker Avance Ⅲ 400

MHz NMR spectrometer.13 The mixture of 7% sodium hydroxide, 12% urea, and D2O was pre-cooled to -5 to -20 oC to dissolve the oxidized cellulose fibres for the NMR analysis.21 X-ray Diffraction (XRD). XRD analysis of the original and oxidized SWBK fibres was performed on an X-ray diffractometer (Philips PW3040/00 X’ Pert MPD system) in the range of 5o to 40o 2θ. Cu Kα radiation and a maximum X-ray power of 40 kV and 30 mA were used. The crystal width was calculated through Jade software according Scherrer formula. Determination of Degree of Polymerization (DP). The DP of the original and oxidized SWBK fibres was measured viscosimetrically with CuEn (copper ethylenediamine solution), and the obtained intrinsic viscosities were converted into the DP values by formula (1)22:

[ (

𝐷𝑃0.905 = 0.75 𝜂𝐶𝑢𝐸𝑛

𝑐𝑚3

)]

𝑔

(1)

Carboxyl Content Determination by Conductometric Titration. The carboxyl content of the original and oxidized SWBK fibres was determined using conductometric titration method by a Metrohm titrator and conductivity



meter.23 The sample was first treated with 200 mL of 0.1 M HCl solution for 30 min, and then thoroughly washed with deionised water. The washed specimen was suspended in 450 mL NaCl solution (0.001 M), and then 2 mL of 0.1 M HCl solution was added, and then titrated with NaOH solution (0.025 M). The amount of adsorbed Fe2+. The amount of absorbed Fe2+ was determined by an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (AA-6800, Shimadzu Corporation, Japan). Nitric acid and hydrogen peroxide were used to digest the sample. In order to determine the distribution of Fe2+ in the inside of fibres, the cross section of the SWBK pulp sheet absorbed Fe2+ after air drying, was examined using a Zeiss EVO 40 SEM equipped with EDS microanalysis software. Oxidation Efficiency of H2O2 Conventional Fenton Process. The conventional Fenton process was shown in Figure 1a. 1.5 g FeSO4 was dissolved in 300 mL deionized water, followed by the addition of 10 g (o.d.) SWBK fibres. The mixture was stirred at room temperature for 30 minutes. Then H2O2 was added at various dosages. The mixture was stirred at 45 oC for 45 min. Then the oxidized cellulose fibres were separated by vacuum filtration. The content of residual H2O2 of the filtrate was determined by iodimetry. Modified Fenton Process. Being different from the conventional Fenton process, the modified Fenton process included 4 steps, as shown in Figure 1b. Step 1 was the same as in the conventional process. In step 2, the pulp slurry was filtered to remove the unadsorbed ferrous ions. Hydrogen peroxide was added


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in step 3 to carry out the oxidation reaction at 45 oC for 45 min. In step 4, the obtained oxidized cellulose fibres were separated by vacuum filtration, and the content of residual H2O2 of the filtrate was determined by iodimetry.

RESULTS AND DISCUSSION The Efficiency of Pre-absorbed Fe2+ inside the Fibres. The Fe2+ can be quickly absorbed onto the surface and permeated into the fibres (Figure 2a). The amount of Fe2+ absorbed on fibres increased steeply at the first 5 min, and then reached to maximum. The value increased from 900 to 1100 ppm as the time increased from 5 min to 10 min. The Fe2+ absorbed in the fibre cell wall would cause the catalytic oxidation (Step 3 in Figure 1b). Since the excess Fe2+ was removed through filter pressing, the H2O2 redox reaction occurred inside the fibres, instead of consuming in pulp suspension. The EDS analysis of SWBK pulp sheet absorbed FeSO4 was shown in Figure 2b. After the process of vacuum-filtration and pressing to remove the free water from fibres, Fe2+ was still evenly distributed on the surface and cross section of fibres in the SWBK pulp sheet. It means that Fe2+ can realize the catalytic oxidation reaction of H2O2 for celluloses inside fibres.



Figure 2. The efficiency of pre-absorbed ferrous catalyst. (a) The effect of absorption time on the amount of Fe2+ absorbed on fibres; (b) EDS analysis of the cross section of SWBK absorbed ferrous catalyst (the red dots represent ferrous).

Conventional versus Modified Fenton Processes. The SWBK fibres were first pretreated by the modified Fenton process (Figure 1b), and then treated with a high pressure homogenizer to fibrillate the oxidized fibres. Figure 3a compares the hydrogen peroxide consumption rate between the modified and conventional Fenton processes. As the hydrogen peroxide dosage increased from 15 to 120 kg/t , the consumption rate of H2O2 decreased dramatically from 58% to 28% in the modified Fenton process, while it increased from 79% to 98% in the conventional Fenton process. When the hydrogen peroxide dosage was higher than 120 kg/t, the hydrogen peroxide consumption rate maintained at a low level of about 28% for the modified process, while it maintained at a high level of 98% for the conventional Fenton process. The high hydrogen peroxide consumption rate in the conventional Fenton process was due to wasteful hydrogen peroxide decomposition caused by the ferrous ions in the bulk solution. Such a high hydrogen peroxide consumption rate for the conventional Fenton process has been reported in an earlier study in which no


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residual hydrogen peroxide was found in the reaction system by the end of the oxidation reaction.15 In the modified Fenton process, ferrous ions were pre-adsorbed on fibres, and no ferrous ion was present in the bulk solution to catalyse the hydrogen peroxide decomposition reaction. Therefore, the modified Fenton process had higher efficiency in oxidizing cellulose fibres per unit consumption of hydrogen peroxide. Oxidation Efficiency of the Modified Fenton Process. The efficiency of cellulose oxidation was evaluated in terms of DP, carboxyl content, mass loss, and the viscosity of resultant CNF dispersion. Result in Figure 3b shows that the modified Fenton process is highly efficient in decreasing the DP of the cellulose fibres. After 30 minutes of reaction under the condition of 150 kg/t H2O2, 45 oC and 20% pulp consistency, the DP of the SWBK fibres decrease from about 800 to about 200. We also determined the iron content of the oxidized cellulose fibres (150 kg/t H2O2, 45 oC, 20% pulp consistency) (Figure 3b). At the beginning of the reaction, owing to the oxidation, the DP of the SWBK fibres decrease, the surface energy and adsorbability of the SWBK fibres reduce. The iron content in the SWBK fibres also drops quickly, and then become mild significantly reduced. After 6 hours of reaction, the iron content of the oxidized cellulose fibres decrease from about 1100 ppm to about 700 ppm. To examine the effect of hydrogen peroxide on cellulose oxidation, in Figure 3c and 3d, the initial hydrogen peroxide concentration in the system was varied, while the reaction temperature, time and pulp consistency were remained constant. Figure 3c shows that the DP of the cellulose fibres



decreased quickly with increasing H2O2 consumption. With about 10 kg/t hydrogen peroxide consumption, the DP of the SWBK fibres decreased from 800 to about 500. The DP decreased further to about 300 at about 22 kg/t hydrogen peroxide consumption. In a conventional Fenton process reported earlier in the literature, the DP decreased from 930 to 450 with a hydrogen peroxide consumption of 50 kg/t.15 The larger response of DP reduction to hydrogen peroxide consumption suggests that the modified Fenton process was more efficient in utilizing the hydrogen peroxide in shortening the cellulose chains. Figure 3c also shows the viscosity of the CNF dispersion obtained after homogenization treatment of the oxidized cellulose fibres at various hydrogen peroxide consumptions. Being opposite to the trend of the DP of the oxidized cellulose fibres, the viscosity of the CNF dispersion increased with increasing hydrogen peroxide consumption in the oxidization pretreatment, when the conditions of the subsequent mechanical treatment remained unchanged. The viscosity was associated with the degree of nano-fibrillation of the fibres. When the nano-fibrillation degree was low, as it was true for the case of non-oxidized fibres, the viscosity reading was low as the non-fibrillated fibres settled down quickly. It is known that the hydroxyl radicals formed by the Fenton reaction can oxidize the hydroxyl groups of cellulose and form new carboxyl groups on the cellulose chains.24 As shown in Figure 3d, the carboxyl group content increased with increasing hydrogen peroxide consumption. While the C2 and C3-hydroxyl groups on the glucosyl of cellulose chains were oxidized to carboxyl groups,15 the DP of the cellulose macromolecules was dramatically


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decreased by the oxidative degradation of 1, 4-β-D-glucoside bonds of cellulose chains because of non-selective oxidation of cellulose by Fenton’s reagent (Figure 3c). The above effects destroyed the network structure of the microfibrils that form the fibre cell wall, resulted in weakened fiber structure which can be easily dissociated under the strong shear forces to obtain nanoscale CNF.25 The modified Fenton oxidation did not have a significant increase of carboxyl group content compared to the TEMPO oxidation. The easy separation of microfibrils in the mechanical treatment is primarily benefited from the degradation of the cellulose macromolecules. However, the carboxyl group content levelled off when the hydrogen peroxide consumption was higher than 40 kg/t, probably due to dissolution of oxidized short-chain cellulose. For this same reason, the resulting CNF from the modified Fenton process had a smaller number of carboxyl groups compared with the CNF obtained by the TEMPO process.26, 27 TEMPO oxidation selectively oxidizes the hydroxyl groups on the C6 of cellulose,28 while the modified Fenton process can also cause oxidation of the hydroxyl groups on the C2 and C3, as well as degradation of the cellulose chains. Figure 3d shows that the mass loss due to degradation and dissolution in the modified Fenton process increased with increasing hydrogen peroxide consumption, suggesting that some of the hydrogen peroxide was consumed in degrading cellulose.



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Figure 3 Effect of H2O2 dosage and consumption on the modified Fenton oxidation. (a) Effect of H2O2 dosage on H2O2 consumption rate (45 min, 45 oC, 20% pulp consistency); (b) Effect of reaction time on the DP and iron content of the oxidized cellulose fibres (150 kg/t H2O2, 45 oC, 20% pulp consistency); (c) Effect of H2O2 consumption on the DP of the oxidized cellulose fibres and the viscosity of the resultant CNF dispersion after homogenization of the oxidized cellulose fibres; (d) Effect of H2O2 consumption on carboxyl content and mass loss (45 min, 45 oC, 20% pulp consistency).

Reactions of Cellulose in the Modified Fenton Process. To study the chemical structure change of cellulose after the oxidation treatment by the modified Fenton process, the treated SWBK fibres were dissolved with a D2O solution of 7% NaOH and 12% urea at -12 oC for

13C-NMR

spectroscopy

analysis. For the NMR spectrum in Figure 4, the chemical shift peak at 162.70 ppm was for the carbon atoms of the carbonyl groups of urea in the solvent, and the peaks appeared at δ103.77, δ79.03, δ75.55, δ75.32, δ73.90 and δ60.71 in the


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spectrum were attributed to the C1, C4, C3, C5, C2 and C6 of the glucosyls of cellulose, respectively.21, 29-31 It is interesting to note that there were three weak peaks appeared at δ167.08, δ168.46 and δ171.08 after the oxidation of the SWBK fibres, which were assigned to the carbon atoms of carboxyl groups at the C2’, C3’ and C6’ positions,26,

32, 33

indicating the oxidation of the hydroxyl

groups at all of these three positions in the modified Fenton process.

Figure 4

13C-NMR

spectra of the cellulose fibres before and after oxidation by the modified

Fenton process.

In the Fenton reaction system, the process of Fenton reaction is as follow steps:34-37 Fe2++H2O2=Fe3++OH﹣+HO• Fe3++H2O2+OH﹣=Fe2++H2O+HO• Fe3++H2O2=Fe2++H++H2O• H2O•+H2O2=H2O+O2↑+HO• During Fenton oxidation, various radicals, including HO• and HOO• would be generated, and Fe2+ and Fe3+ could be converted to one another. The main reason that Fenton reagents possess the ability of catalytic degradation for many organic compounds is the presence of HO• and HOO• with higher oxidation potential.34, 35



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Hydroxyl radicals (HO•) and hydroperoxyl radicals (HOO•) are generated from hydrogen peroxide with the catalysis of ferrous ions. As a strong oxidative agent (E0 = 2.8 V),38,

39

HO• can cause the degradation of polysaccharides,40 and leads to the

decrease of molecular weight (Mw) as well as formation of carboxyl group.31,

35, 41

Fenton reaction has been applied commercially in the oxidation treatment of organics in homogenous systems, because of its eco-friendly nature and low capital investment requirement.34-37 The conventional Fenton process is effective in degrading dissolved or colloidal organic substances in wastewater when the pH of the system is in acidic range.42,

43

However, conventional Fenton process is less efficient in heterogeneous

systems (e.g. cellulose fibres) due to the fact that hydroxyl and hydroperoxyl radicals are instable and they are consumed wastefully by the hydrogen peroxide decomposition reaction before they reach the reaction sites of fibres. Figure S3 shows the morphology of the SWBK fibres pretreated by the conventional Fenton oxidation followed

by

homogenization

treatment

under

the

same

conditions.

Micro/nano-fibrillation of the SWBK fibres was minimal, indicating that the oxidation pretreatment was not effective in weakening the fibres. Indeed, the DP of the SWBK fibres decreased only slightly after the oxidation treatment by the conventional Fenton process. However, plant fibres treated by the conventional Fenton oxidation still retained large quantities of thick fibres, about 17.3-20.2 m in diameter.15 The fibres, which changed little in diameter, retained largely the whole cell wall structure. The results indicate that the cellulose macromolecules within the fiber cell wall are not oxidized evenly and efficiently by the conventional Fenton’s reagent under the experiment conditions, which is not conducive to obtain microfibrils with a good separation during the subsequent high-pressure homogenization process.


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In the modified Fenton process, hydroxyl and hydroperoxyl radicals were formed in-situ on the fibre surface and inside fibre cell wall structure as well, and thus they are available immediately in-situ for the oxidation reaction of cellulose. Therefore, the SWBK fibres were oxidized and degraded efficiently in the modified Fenton process. The severely weakened fibres were fibrillated easily in the homogenization process, and resulted in stable CNF dispersion. Figure 5 shows the reaction mechanism of cellulose in the modified Fenton process. First, HO• radicals attack C2, C3 and C6 to form hydroxyalkyl radicals at these positions, and then an alcohol ketone structure is generated by reaction with oxygen (Reaction 1 and Reaction 2 in Figure 5). This diketone structure can be further oxidized to binary carboxylic acid by HOO-, a nucleophilic agent. The carboxyl groups can dissociate into anionic groups which will increase the electrostatic repulsion of the microfibrils of fibres. However, the carboxyl group content of the SWBK fibres oxidized by the modified Fenton process was relatively low, compared with the TEMPO oxidized cellulose fibres.22, 28, 44-46 Aldehyde groups or carboxylic groups might be generated during the oxidation process of cellulose. However, the aldehyde groups would be readily further oxidized to carboxylic groups when excess of oxidants are present in the system. Therefore, only carboxylic groups on the C2, C3, and C6 positions are found on the glucosyl of cellulose chains.47



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Figure 5 Oxidation reactions and nano-fibrillation of SWBK fibers in the modified Fenton process.

Decrease of cellulose DP was another important effect of the oxidation pretreatment by the modified Fenton process on the nano-fibrillation of the oxidized SWBK fibres in the subsequent homogenization treatment. The resulting aldehyde and keto groups from the oxidation reactions of the hydroxyl groups on C2, C3 and C6 may initiate further degradation reactions such as dehydration and cleavage of the glycosidic linkages (Reaction 3 in Figure 5).48-51 The breakage of the 1, 4-β-D-glucoside bonds in cellulose chain leads to DP decease of the cellulose macromolecules, which weakens the microfibril network

structure

to

facilitate

nano-fibrillation

in

the

subsequent

homogenization process. Indeed, the DP of the SWBK fibres decreased from about 800 to about 200 in 30 minutes in the modified Fenton process (Figure 3b). For comparison, the DP of TEMPO oxidized cellulose was reported to be about 200-500 in the literature.28, 45, 52


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To investigate the effect of oxidation on the crystallinity of cellulose, the SWBK fibres were treated by the modified Fenton process through varying the dosage of hydrogen peroxide to obtain various degrees of polymerization. XRD spectra of the original and oxidized SWBK fibres were shown in Figure 6. All the oxidized fibre samples had a XRD spectrum similar to that for the original SWBK fibres, indicating that the crystal structure was not changed by the oxidation in the modified Fenton process although the DP decreased dramatically in the process. The unchanged of the crystal structure of the cellulose was also supported by the FTIR spectra in Figure S4. This is because that the oxidation reactions occurred mainly in the amorphous regions of cellulose.53 After 2 hours of oxidation under the condition of 150 kg/t H2O2, 45 oC,

and 20% pulp consistency, the crystal width of SWBK changes from 4.2 nm

to 4.1 nm, indicating that the oxidation do almost not influence the crystal width of SWBK.

Figure 6 XRD spectra of the oxidized SWBK fibres at various degrees of polymerization.

In summary, the oxidization treatment of the SWBK fibres by the modified Fenton process created new carboxyl groups which increased the electrostatic



repulsion of fibrils to facilitate the nano-fibrillation of the SWBK fibres in the subsequent homogenization process. The oxidation treatment also caused a dramatic decrease of the DP of the SWBK fibres, which weakened the fibre cell wall structure to encourage fibrillation. Morphology of the CNF Obtained from the Modified Fenton Oxidation– homogenization Process. After the oxidation pretreatment by the modified Fenton process under the conditions of 150 kg/t H2O2, 20% pulp consistency, at 45 oC and 30 min, the SWBK fibres were subjected to homogenization treatment for 15 minutes under 60 MPa pressure to obtain CNF dispersion. Figure 7 shows that the morphology of the SWBK fibres did not change before and after the oxidation treatment (Figure 7a and 7b). After the homogenization treatment of the oxidized SWBK fibres, the obtained CNF dispersion had a uniform distribution of nano cellulose fibrils, with an average diameter of less than 100 nm (Figure 7c and 7d). In contrast, homogenization treatment of the SWBK fibres treated by the conventional Fenton process did not lead to nano-fibrillation of the fibres (Figure S3), demonstrating that the oxidation in the conventional Fenton process was not sufficient to weaken cell wall structure of the SWBK fibres under the conditions.


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Figure 7 SEM images of the original SWBK fibres (a), the oxidized SWBK fibres (b), TEM image of the resulting CNF after oxidation-homogenization treatments (c), and AFM image of the CNF (d). The conditions of the modified oxidation process were 150 kg/t H2O2, 20% pulp consistency, 30 min at 45 oC.

CONCLUSIONS A highly efficient modified Fenton process was developed for the pretreatment of softwood bleached kraft pulp fibres for cellulose nanofibrils preparation. Ferrous ions were pre-adsorbed on the SWBK fibres which functioned as micro-reactors for the Fenton reaction, as well as for the oxidation reactions of cellulose. The oxidation efficiency was high, and the wasteful decomposition of hydrogen peroxide was minimized, as the reactive species were generated and utilized in-situ for cellulose oxidation within the fibre structure. The oxidation reactions caused the carboxyl group content of the SWBK fibres to increase markedly from about 39 to about 56 mmol/kg. Results from 13C-NMR analyses indicated that the hydroxyl groups on the C2 and C3, as well as on the C6 of the glucosyls of cellulose chains were oxidized to carboxyl groups, which could facilitate the fibrillation of the SWBK fibres by increasing the electrostatic



repulsion between fibrils. The oxidation also caused breakage of the 1, 4-β-D-glucoside bonds of cellulose chains and dramatically decreased DP of the cellulose macromolecules. It was found that the DP of the SWBK fibres decreased from about 800 to about 200 in about 30 minutes in the modified Fenton process. The oxidized SWBK fibres were well dispersed into cellulose nanofibrils in the subsequent homogenization treatment. The obtained CNF had a uniform distribution of nano cellulose fibrils with an average diameter of less than 100 nm. This modified Fenton process can be a green alternative method for CNF preparation with plant fibres.

ASSOCIATED CONTENT Supporting Information Electronic Supplementary Information (ESI) available: Homogenizing process and SEM image of SWBK fibres pretreated by the conventional Fenton oxidation, FTIR spectra of the SWBK fibres and CNF oxidized by the modified Fenton reaction.

AUTHOR INFORMATION Corresponding Authors *Tel.: +86 22 60601996. Fax: +86 22 60601996. E-mail: [email protected] *Tel.: +86 10 82545631. Fax: +86 10 82545631. E-mail: [email protected] Notes The authors declare no comprting financial interest.


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ACKNOWLEDGEMENTS Financial support was provided by NSFC (21875050), the special Fund for Independent Innovation and Industry Development in the Core Area in Haidian District of Beijing (255-kjc-020), and the Key Project of Natural Science Foundation of Tianjin (16JCZDJC37700).

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Table of Contents

A novel and green catalytic oxidation process was developed for the preparation of cellulose nanofibrils (CNF).


Modified Fenton Oxidation of Cellulose Fibers for Cellulose Nanofibrils

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