Elsevier

Journal of Catalysis

Volume 370, February 2019, Pages 304-309
Journal of Catalysis

A new sight on the catalytic oxidation kinetic behaviors of bamboo cellulose fibers under TEMPO-oxidized system: The fate of carboxyl groups in treated pulps

https://doi.org/10.1016/j.jcat.2019.01.003Get rights and content

Highlights

  • The catalytic oxidation behavior of C6-OH group in treated pulps was deeply studied.

  • The time-dependent oxidation is mainly affected by TEMPO and NaOCl’s concentration.

  • A kinetic description of the generated carboxyl in treated pulps was firstly declared.

Abstract

This article conducted an investigation on the selective oxidation of C6-OH in cellulosic chains during TEMPO-oxidized process of bamboo pulps under different conditions, from which the kinetic behaviors and model for describing the fate of carboxyl groups in treated pulps were studied. It was found that the end concentration of carboxyl groups in treated pulps is mainly affected by the time-dependent TEMPO and NaOCl concentration. Meanwhile, the NaBr dosage and process temperature could also affect the reaction rate of carboxylate generation. Based on the experimental data directly from the determination of carboxyl groups, the proposed kinetic model could describe the TEMPO oxidation process of bamboo pulps at various process conditions, and there is a good correlation (R2 = 0.9608) between the measured and predicted data. According to the model, a two-stage kinetic description with different orders about the carboxyl generation based on the modelling was firstly declared in this paper.

Introduction

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-mediation has been recognized as one of the most effective and selective oxidants for the primary hydroxyl group at C6 in cellulose [1]. Not only can the physical and chemical properties be enhanced, but also the further modified products showed distinctive characters, such as high elastic modulus, high specific surface area, low density, non-abrasive nature, easy surface functionalization, biodegradability, and biocompatibility and therefore wide applications in fields of nano-electronics, biomedicine, biosensor and even biofuels [1], [2], [3]. Since different degree of oxidation from hydroxyl to carboxylate will have major effect on process performance, cellulose properties and further applications, it is necessary to investigate on the kinetic behavior of TEMPO oxidation process.

Based on the theoretical mechanism (as shown in Fig. 1) of TEMPO-mediated oxidation process involving the formation of HOBr from NaOCl followed by formation of TEMPO+, and oxidation from C6 primary hydroxyl into carboxyl groups, previous kinetic study about the regenerated cellulose showed first-order with respect to the substrate [4]. For methyl R-D-glucopyranoside (MGP), the oxidation with TEMPO/hypochlorite/bromide was also first order with respect to MGP, TEMPO and NaBr. Moreover, [OCl] had no influence on the observed reaction rate, and the rate constant of oxidation step from the intermediate, i.e. hydrated aldehyde, is about 7 times of that from alcohol [5], [6]. In another oxidation case of MGP, it was also found that the reaction rate-controlling step was the oxidation of the primary hydroxyl groups with the nitrosonium ion [7]. For lignin-containing mechanical pulps, kinetic results of the oxidation process indicated that the formation of carboxylic acid was first-order with respect to TEMPO and NaBr. The catalysts TEMPO and NaBr have little effect on the maximum attainable carboxyl content on mechanical pulp [8]. Investigation on the oxidation process of cotton fibers was divided into two steps: primary wall and secondary wall, both of which also showed first-order. However, the reaction rate of secondary wall is only one-sixth of that in primary wall [9]. Recent study on the oxidation kinetic of bleached softwood kraft pulp employed four models, two of which were based on shrinking core theory with different rate-determining steps, i.e. diffusion and/or reaction, and two of which were homogeneous first-order reaction models with respect to the concentration of hypochlorite ions and primary hydroxyl groups on cellulose [10]. Compared to previous models, it involved the mass transfer of hypochlorite ions which was sufficiently fast to not affect the rate of the reaction, but it is possible that the mass transfer of the catalytic compounds limited the reaction rate in the later phases of the reaction. Based on the corresponding assumptions, above kinetic modelling of oxidation processes with various subjects was first order, which was based on the experimental data of carboxylic groups obtained indirectly from either the NaOH consumption [4], [5], [6], calorimetric method [11], or conductometric titration technique [12]. As was known, since the cellulose macromolecules in fibers would be degraded and then dissolved in the solution with the oxidation processes [10], the above experimental results reflected the change of COOH content in not only undissolved cellulose which is the final oxidizing product, but also the dissolved cellulose which belongs to the invalid generation. Therefore, the kinetic modelling based on the direct experimental data of the carboxyl groups generation only in undissolved cellulosic fibers and analysis on the generation behavior is much necessary for the accurate process control. Moreover, the reaction orders and active energy derived from the prediction model can also accurately reflect how hard the TEMPO-oxidation process of the undissolved cellulosic fibers is.

In this paper, the oxidation behaviors of lignocellulosic fibers in TEMPO-NaBr-NaOCl solution media were studied. Based on the experimental data and relationship between carboxyl generation and process conditions, the kinetic modelling with different orders for initial and rapid stage of oxidation were firstly proposed. With this kinetic model, the generation of carboxyl groups can be predicted, and the mechanism involved in the oxidation process was declared.

Section snippets

Chemical and materials

The fully bleached bamboo kraft pulp was kindly provided by a pulp mill in Southwestern China. The main properties of the pulps are shown in our recent work [13]. Besides, the carboxyl content is 0.023 mmol/g. All analytical-grade chemicals including TEMPO, sodium hypochlorite (ca. 6.0% active chlorine), potassium bromide, sodium hydroxide, hydrochloric acid, sodium bicarbonate, ethanol and etc., used in this paper were all obtained from commercial sources.

Oxidation of bamboo pulps by TEMPO-NaBr-NaOCl system

The oxidation process was referred to

The content of carboxyl groups in pulps

The content of carboxyl groups in pulps was determined according to our previous work, which was performed with a headspace sampling (HS 87.60) coupled with gas chromatography (Agilent, 7890A, USA) with a thermal conductivity detector (TCD) [15]. The employed HS-GC method was based on phase reaction conversion technique and the methodology can be written asFibers-COOH+HCO3-Fibers-COO-+H2O+CO2

Thus, based on the analysis of the generated CO2, the carboxyl groups in cellulosic fibers can be

Effect of the addition of TEMPO agents

The results of carboxyl formation with different TEMPO addition have been described in Fig. 2. It can be known that the formation tendency of carboxyl groups can be divided into three oxidation stages: initial stage, rapid stage and supplemental stage. At initial stage with the reaction time of 0–25 min, the oxidation has a low reaction rate; while at rapid stage, the generation rate of carboxyl group increased sharply and thus the content of carboxyl groups in treated pulps reached to the

Conclusion

Carboxyl generation during TEMPO/NaBr/NaOCl oxidation process of bamboo cellulosic fibers has been deeply studied. Results showed that the time-dependent TEMPO and NaOCl concentration are the major parameters affecting the end concentration of carboxyl groups in treated pulps. Based on the experimental data directly from the determination of carboxyl group, the proposed model has a good correlation (R2 = 0.9608) between the measured and predicted data. Besides, a two-stage kinetic description

Acknowledgments

This work was supported by the Foundation of Key Laboratory of Pulp and Paper Science (No. KF201810) and Zhejiang Provincial Natural Science Foundation of China (No. LQ18C160003).

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