Effects of the conditions of the TEMPO/NaBr/NaClO system on carboxyl groups, degree of polymerization, and yield of the oxidized cellulose

Abstract

Dissolving pulp from Pinus caribaea was oxidized by means of the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxide radical)/NaBr/NaClO system. Effects of the conditions including pH, NaClO dosage, and time on the carboxyl group content, degree of polymerization (DP), and solid recovery of oxidized cellulose were determined. A pH of 10 to 10.5 was found to be optimum for selectively oxidizing cellulose, and carboxyl groups were up to 0.75 mmol/g. However, increasing pH from 9 to 12 facilitated cellulose depolymerization by consistently indicating a gradual drop in DP, thus resulting in cellulose loss. In addition, oxidation was accelerated by the addition of 1 mmol/g to 6 mmol/g NaClO; however, further addition did not enhance the carboxyl groups. The maximum value of carboxyl groups was more dependent on NaClO dosage and governed by the crystal structure of the raw material. To obtain oxidized cellulose with a higher yield and DP, NaClO dosage could be controlled at 4 mmol/g to 6 mmol/g, while the reaction time was limited to 6 h to 8 h.

Full Article

Effects of the Conditions of the TEMPO/NaBr/NaClO System on Carboxyl Groups, Degree of Polymerization, and Yield of the Oxidized Cellulose

Changmei Lin,^a Tong Zeng,^a Qinhua Wang,^a Liulian Huang,^aYonghao Ni,^a,b Fang Huang,^a Xiaojuan Ma,^a,* and Shilin Cao ^a,*

Dissolving pulp from Pinus caribaea was oxidized by means of the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxide radical)/NaBr/NaClO system. Effects of the conditions including pH, NaClO dosage, and time on the carboxyl group content, degree of polymerization (DP), and solid recovery of oxidized cellulose were determined. A pH of 10 to 10.5 was found to be optimum for selectively oxidizing cellulose, and carboxyl groups were up to 0.75 mmol/g. However, increasing pH from 9 to 12 facilitated cellulose depolymerization by consistently indicating a gradual drop in DP, thus resulting in cellulose loss. In addition, oxidation was accelerated by the addition of 1 mmol/g to 6 mmol/g NaClO; however, further addition did not enhance the carboxyl groups. The maximum value of carboxyl groups was more dependent on NaClO dosage and governed by the crystal structure of the raw material. To obtain oxidized cellulose with a higher yield and DP, NaClO dosage could be controlled at 4 mmol/g to 6 mmol/g, while the reaction time was limited to 6 h to 8 h.

Keywords: Cellulose; TEMPO/NaBr/NaClO; Carboxyl groups; DP; Yield

Contact information: a: College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China; b: Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, E3B5A3, Canada;

* Corresponding authors: 1212juanjuan@163.com; scutcsl@163.com

INTRODUCTION

As one of the cellulose derivatives, oxidized cellulose is widely used in hemostatic, film, filter, carbon aerogels, and textile material, among other applications (Wang et al. 2017; Wu and Cheng 2017; Abe et al. 2018; Lal and Mhaske 2018). In the past decades, many studies have been focused on oxidation of cellulose, with an objective to develop the material properties of cellulose by introduction of aldehyde and carboxyl groups (Kim et al. 2000; Saito et al. 2010). The catalytic oxidation method using stable nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was found to be one of the promising procedures for oxidation of cellulose; besides, NaBr and NaClO were used as catalyst and as a primary oxidant in conjunction with TEMPO (Habibi et al. 2006; Okita et al. 2010; Salminen et al. 2017).

The TEMPO mediated system is expected to catalyze the oxidation of primary alcohol groups to carboxyl groups in aqueous media; and it also has been applied in the preparation of nanofibrillated celluloses (Kuramae et al. 2014). In such applications, anionically charged functional groups are introduced onto the cellulose fiber surface to form strong electrostatic repulsion and therefore greatly enhance the ease of separation of individual microfibrils (Isogai et al. 2011). Specifically, the C6 primary hydroxyls of cellulose are selectively oxidized to become C6 carboxylated groups, and the oxidized

products mostly have the homogeneous chemical structure of sodium (1→4)-β-D-poly-glucuronate or Na salt of cellouronic acid consisting of D-glucuronosyl units alone (Saito et al. 2009; Isogai et al. 2011). However, remarkable depolymerization is inevitable in the TEMPO/NaBr/NaClO system under alkaline conditions. The degree of the polymerization of oxidized cellulose was far lower than that of the original cellulose (Saito and Isogai 2004; Funahashi et al. 2017; Sang et al. 2017). Of course, the methods used for DP determination might account for some of the sharp decrease; the presence of C6 aldehyde groups would initiate β-elimination in the copper ethylenediamine hydroxide solution and therefore result in a lower DP compared to the true value (Isogai et al. 2011).

As mentioned above, TEMPO can selectively oxidize primary hydroxyl groups of cellulose to carboxyl groups, and cellulose loss is inevitable. However, DP is one of the most important determinants for the strength, length, and flexibility of individual cellulose fibrils and has a direct relationship with the properties of their applications (Fukuzumi et al. 2013).

In the present work, the dissolving pulp prepared from pine wood was treated by TEMPO/NaBr/NaClO under various conditions. The focus was to investigate the effects of the conditions on the yield, DP, and carboxyl groups. The results are expected to help in the optimization of reaction conditions that are suitable for the subsequent application.

EXPERIMENTAL

Materials

Wood (Pinus caribaea) dissolving pulp was used as the original cellulose sample and was obtained from a local pulp mill (Fuzhou, China). The TEMPO (2,2,6,6-tetramethylpiperidine-1-oxide radical) was purchased from Aladdin (Shanghai, China). Sodium hypochlorite and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Hydrochloric acid was obtained from Lanxi Xuri Chemical Co., Ltd. (Lanxi, China). Anhydrous ethanol, sodium bicarbonate, and sodium chloride were purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Deionized water was made in the laboratory. The degree of polymerization of the starting cellulose sample was 480, while the carboxyl groups were not detected.

Methods

Cellulose oxidization

In the present work, the oxidized conditions of NaClO dosage, pH, and time were studied, while the NaBr and TEMPO were kept constant. The oxidation procedure generally followed the literature methods (Hirota et al. 2009; Shinoda et al. 2012; Fukuzumi et al. 2013; Gamelas et al. 2015). A total of 5 g pulp was dispersed in 500 mL deionized water and formed a 1 wt% pulp slurry. Next, 0.08 g TEMPO and 1 g NaBr were dissolved in 20 mL water and then added in sequence into the pulp slurry. The NaClO with different dosages (1 to 16 mmol/g dissolving pulp) was added drop-wise to the suspension to start the oxidation reaction. During the process, the pH of the reaction system was monitored and adjusted by uninterruptedly dropping NaOH solution within the range of 9 to 12. After a period of reaction (3 to 15 h), the reaction was quenched by adding excess ethanol (10 mL). Then, HCl (Hydrochloric acid) (1 mol/L) was used to adjust the pH of the solution at 7. The reaction product was filtered through a filter and

washed thoroughly with ethanol and water until the Cl^– no longer was present in the filtrate. The ratio between the retained cellulose in the filter and the cellulose added was defined as solid recovery (yield, %).

Carboxyl group content determination

The carboxyl group content of TEMPO-oxidized cellulose was determined by headspace gas chromatography (Agilent 7890B, Germany) equipped with a headspace sampler (Thermo ScientificTM TriPlusTM 300, USA). The carboxylic acid group on TEMPO-oxidized cellulose reacts with sodium bicarbonate to produce CO₂ by acid treatment; the detailed procedure was described in previous studies (Chai et al. 2003; Li et al. 2015).

DP determination

The intrinsic viscosity of the oxidized cellulose samples was measured at 25 ºC with a capillary-tube viscometer (DP-02, Beijing hengchengyuke Co. Ltd, China) using cuproethylenediamine (CED) as the solvent, and these values were converted to viscosity average degree of polymerization (DP) by the reported method (Ma et al. 2013).

Fourier transform infrared (FTIR) spectra

Fourier transform infrared (FTIR) spectra of cellulose and TEMPO-oxidized cellulose were processed using a Thermo Electron Corporation Nicolet 380 spectrometer (VERTEX 70, Bruker, Germany). The sample was prepared by the KBr tableting method with a ratio of 1:100. The spectra were obtained viaoperating at a nominal resolution of 4 cm^-1 and co-adding 100 scans in the region between 400 cm^-1 and 4000 cm^-1.

X-ray diffraction (XRD)

The measurements of X-ray diffraction were operated on a high resolution X-ray diffractometer (Ultima IV, Japan). The patterns with Cu-Ka radiation (wavelength: 0.154 nm) at 40 kV and 40 mA were recorded in the region of 2θ =10° to 60° at a scanning rate of 5°/min and the step width of 0.02°. The crystallinity of the samples was determined based on the Segal method (Segal et al. 1959; French 2014).

RESULTS AND DISCUSSION

The Reagent Effects

The variables in the reaction system include concentrations of the reactants, pH, and the temperature. As described in the literature, the increase in the amount of TEMPO and NaBr sped up the reaction rate, but the carboxyl group content of TEMPO-oxidized cellulose was almost unchanged.

The NaBr and TEMPO are used only in catalytic amounts in the whole reaction process (de Nooy et al. 1996; Saito and Isogai 2004). In the present study, the temperature was kept constant in the whole experiment. Because temperature induces severe degradation effects (de Nooy et al. 1996; Sang et al. 2017), only the NaClO, pH, and time effects were investigated.

Table 1. Carboxyl Group Formation and Cellulose Degradation Changes with Various Reagents (pH 10.5, Reaction Time 6 h)

Table 1 shows the effect of reagents on the carboxyl group formation and cellulose degradation. The TEMPO or NaBr applied alone barely induced a cellulose reaction, indicated by trace carboxyl group formation and invariable DP change. However, NaClO under alkaline medium degraded cellulose, which is shown by a lower DP (379 vs. 479); in fact, NaClO in alkaline is characteristic of a pulp bleaching system. It was reported that NaClO at pH 9 to 11 may cause the depolymerization of cellouronic acid (Shibata and Isogai 2003). Although cellulose cleavage occurred, carboxyl groups were not found in the cellulose after NaClO treatment without TEMPO and NaBr, as NaClO did not selectively oxidize the cellulose hydroxyl group of C6. However, treatment with the TEMPO/NaBr/NaClO system led to a remarkable carboxyl group formation and cellulose depolymerization. As compared between the different systems, only the TEMPO/NaBr/NaClO system selectively oxidized cellulose, while the NaClO in the alkaline medium had only some oxidizing effects on cellulose.

Figure 1 shows the FTIR spectra of the cellulose and oxidized cellulose. As well as cellulose, the oxidized cellulose showed a broad absorption around 3500 cm^-1, which can be ascribed to the stretching of H-bonded -OH groups; while the peak at 2900 cm^-1 is due to C-H stretching.

Fig. 1. FTIR spectra of cellulose and oxidized cellulose

The peak appearing at 1633 cm^-1 in the two samples could be attributed to the bending mode of absorbed water in cellulose (Das et al. 2010). The 1429 cm⁻¹ vibration could be assigned to CH₂scissoring, while the 1376, 1317 and 1048 cm⁻¹ vibrations were attributed to C-H bending, CH₂ rocking, and C-O stretching, respectively (Abidi et al. 2014).

Unlike original cellulose, the oxidized cellulose spectra showed a pronounced peak at 1734 cm^-1, which is attributed to the presence of carboxyl groups acquired during the TEMPO oxidation reaction (Poletto et al. 2014). The appearance of carboxyl groups indicated that the hydroxyl groups at the C6 position of the cellulose molecules were converted to carboxyl groups.

As compared between the different systems, only the TEMPO/NaBr/NaClO system selectively oxidized cellulose, while the NaClO in the alkaline medium had only some oxidizing effects on cellulose.

Effect of pH

Figure 2 shows the effects of pH on the oxidized cellulose. The carboxyl group formation had a strong correlation to pH. The carboxyl group content substantially increased from 0.45 mmol/g to 0.73 mmol/g as the pH varied from 9 to 10.5. With further increase of the pH to 12, the carboxyl content gradually declined. The pH governs the reaction mechanism and therefore affects the carboxyl formation. In contrast to findings that a higher oxidation rate was found at a pH 8 to 10 when the TEMPO/NaBr/NaClO system was used to oxidize an α-D-glucopyranoside sample (de Nooy et al. 1995), a higher oxidation rate was found at pH 10 to 10.5 when dissolving pulp cellulose was used.

Fig. 2. The effects of pH on the oxidizing cellulose (NaClO 5 mmg/mol, time 6 h)

The hydroxyl radicals produced by TEMPO and NaClO lead cellulose to degrade during the oxidation process at pH 10 to 11 (Bragd et al. 2001; Shibata and Isogai 2003). A study about the effects of pH on the depolymerization of cellouronic acid demonstrated that the system at pH 9 caused partial depolymerization of cellouronic acid; in this case, the DP decreased from 430 to 200 (DP almost leveled off) within the initial 30 min. Correspondingly, application of this system at pH 11 caused a severe degradation, in which the DP drastically decreased to 20 in the initial 10 min. Increasing the pH to a high level was favorable for cellouronic depolymerization. Similar effects were probably involved with cellulose degradation in the present system the authors studied. A gradual DP decrease and cellulose loss was observed with an increase of pH from 9.5 to 12, as can be seen from Fig. 2. Strictly, the DP slowly decreased when the pH was raised above 11; this agreed with the results from potato starch (de Dooy et al. 1995). Because of the mild degradation revealed from the lingering variation of DP, the solid recovery decreased slightly as well. However, the carboxyl groups declined consistently, indicating the decreased oxidation selectivity of the TEMPO/NaBr/NaClO system at this higher pH level. A pH of 10 to 10.5 proved most favorable for selective oxidation and protection of cellulose from severe degradation. Although pH higher than 10.5 had a comparable oxidation effect, the selectivity was decreased.

Effect of Sodium Hypochlorite Dosage

As shown in Fig. 3, a consistent increase of carboxyl content was observed with increased NaClO addition. Specifically, the carboxyl group formation was a function of NaClO dosage; as NaClO dosage ranged from 1 mmol/g to 6 mmol/g, the carboxyl groups increased substantially from 0.05 mmol/g to 1.09 mmol/g. Moreover, the relationship between NaClO dosage and carboxyl formation appeared linear. In contrast, when the NaClO dosage increased to 6 mmol/g and above, the carboxyl group content increased slowly and even remained stagnant, from 1.09 mmol/g to 1.38 mmol/g. The maximum value of carboxyl groups is more than two times that reported for cotton linter oxidized cellulose (Saito and Isogai 2004); however, it is comparable with that reported of kraft pulp (Saito et al. 2009; Wu et al. 2017). It was reported that the maximum contents of C6 oxidized groups can vary depending on the cellulose I crystal width when the cellulose is treated with an excess amount of NaClO in the TEMPO system (Okita et al. 2010).

Fig. 3. The effect of NaClO dosage on the carboxyl group formation (pH 10.5, time 6 h)

The oxidation mainly occurred on the surface of the cellulose fiber, while the C6 of cellulose in the amorphous regions and the surface of crystalline are susceptible to the TEMPO/NaBr/NaClO system (Sang et al. 2017). The increase of the crystallinity index might account for the lingering carboxyl group formation. Figure 4 depicts the X-ray diffraction patterns of the sample and the crystallinity index. The oxidation process still kept the cellulose I of oxidized cellulose; meanwhile, the crystallinity index increased from 71.4% to 78.4% with increased NaClO addition. The crystallinity index change was almost in agreement with the change in the carboxyl group content. The results further confirmed that the oxidization resistance was attributable to the high crystallinity index and low accessibilities to the reagents (Dong et al. 1996; Saito et al. 2004).

Fig. 4. XRD patterns and CI of the cellulose sample and oxidized cellulose (CI: Crystallinity index)

Fig. 5. The effect of NaClO dosage on the DP and solid recovery (6 h, pH 10.5)

Figure 5 depicts the effects of NaClO dosage on the DP and solid recovery. The DP and solid recovery of oxidized cellulose decreased with increasing NaClO dosage. A mild TEMPO system with only 1 mmol/g NaClO decreased the DP from 480 (starting cellulose sample) to 196. It was indicated that not only did the C6 take part in the oxidizing reaction where the primary hydroxyl was converted to carboxyl, the C1 also was involved in and initiated β-elimination and therefore cellulose cleavage (Shinoda et al. 2012). Furthermore, the DP evidently decreased in the case of NaClO ranging from 1 to 6 mmol/g; afterward, the DP decreased slowly. In contrast, the cellulose loss was negligible when NaClO dosage was less than 3 mmol/g; afterward, some of the oxidized cellulose was loss and showed a gradual decrease of yield.

The NaClO addition tended to optimize carboxyl groups but at the expense of cellulose degradation and even cellulose loss. Cellulose loss mainly occurred when the NaClO dosage exceeded 3 mmol/g. Fortunately the cellulose loss could be controlled beyond 10% in the case of NaClO dosage less than 6 mmol/g, where the carboxyl group increased to 1.09 mmol/g and DP was controlled at 90. To obtain oxidized cellulose with high DP and low loss, it was suggested that the NaClO dosage should be controlled at 4 to 6 mmol/g.

Effect of Reaction Time

The carboxyl content increased with increasing reaction time (Fig. 6), until a plateau was observed at 12 h or longer, while the DP and yield gradually decreased. To obtain a higher content of carboxyl groups and higher DP, the reaction time could be controlled at 4 h to 6 h. Compared to the results from Fig. 3, an extension of reaction time barely enhanced the maximum amount of carboxyl group. For example, the amount of carboxyl group was approximately 0.73 mmol/g when 6 h of reaction proceeded; with extension of time to 15 h, the maximum value was up to 0.75 mmol/g. However, with a slight increase in the dosage of NaClO, the amount of carboxyl group substantially increased. It was indicated that the amount of carboxyl group could be more dependent on NaClO dosage.

Fig. 6. Time effects (NaClO 4.5 mmol/g, pH 10.5)

CONCLUSIONS

1. The applied pH of the TEMPO/NaBr/NaClO system had a substantial influence on carboxyl group formation; the peak selective oxidation of C6 primary hydroxyls was found to occur at pH 10.5.
2. The carboxyl groups could be introduced into the wood dissolving pulp at levels up to 1 mmol/g with the cellulose loss lower than 10%. Excessive addition of NaClO could not significantly enhance carboxyl content, whereas severe cellulose degradation resulted.
3. To obtain oxidized cellulose with a relatively high carboxyl group, DP, and yield, the NaClO dosage in the TEMPO/NaBr/NaClO system should be controlled at 4 mmol/g to 6 mmol/g and the pH limited to within the range of 10 to 10.5.

ACKNOWLEDGMENTS

The authors acknowledge the support from the Canada Research Chairs Program, the National Natural Science Foundation of China (Grant Nos. 31500488 and 31770632), and the Natural Science Foundation of Fujian Province (Grant Nos. 2016J01089 and 2016H6004). The authors also acknowledge the funding support from education department of Fujian for the “Excellent talents support plan in Fujian Universities”.

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Article submitted: March 12, 2018; Peer review completed: May 5, 2018; Revised version received: May 28, 2018; Accepted: June 1, 2018; Published: June 14, 2018.

DOI: 10.15376/biores.13.3.5965-5975