Oxidation of Native Cellulose for the Preparation of Cellulose Nanofibers

Downloaded by CITY UNIV OF HONG KONG on October 25, 2017 | http://pubs.acs.org Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch010

Chapter 10

TEMPO/Laccase/O2 Oxidation of Native Cellulose for the Preparation of Cellulose Nanofibers Jie Jiang and Yimin Fan* Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Jiangsu Key Lab of Biomass-based Green Fuel & Chemicals, College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China *E-mail: [email protected].

The environment friendly 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/laccase/O2 (TLO) system was used to oxidize cellulose without any chlorine-containing oxidant and further to prepare cellulose nanofibers by the following mechanical treatment. The one-way reaction mechanism (TEMPO-to-TEMPO+-to-reduced TEMPO) of TLO system was assumed, in which TEMPO could not regenerate with laccase and O2. The TEMPO+ molecules formed in this system further oxidized cellulose, along with degrading laccase molecules. Thus, large amounts of laccase and TEMPO and long oxidation time were required to introduce 0.6 mmol/g amount of C6-carboxylate groups to different kinds of cellulose materials. The content of C6-carboxylate groups could be further increased to over 1.0 mmol/g when the oxidation was applied again to the purified oxidized cellulose, and the yield of nanofibers obtained from the resulting sample by sonication was high.

© 2017 American Chemical Society Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Introduction Cellulose is one of the most abundant renewable biopolymers on earth, which was consisted of glucose linked in a linear chain via β-1,4 glycosidic bonds (1, 2). The structure of native cellulose is known to be hierarchical, the cellulose microfibrils consist of 30-40 glucan chains with the diameters of about 3 nm. In recent years, cellulose nanofibers (CNFs) prepared from cellulose microfibrils have been reported and CNFs have attracted more and more attention. At first, CNFs were isolated by Herrick and Turbak using homogenization treatment (3, 4). To reduce the high energy consumption, some pretreatments of cellulose fibers were developed, such as enzymatic hydrolysis (5), acid hydrolysis (6), carboxymethylation with low degrees of substitution (7), and catalytic oxidation using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (8–10). All these pretreatments were carried out in water without using any organic solvent. Among them the TEMPO mediated oxidation was a promising route making the extraction of CNF much easier with selective oxidation of primary alcohols on the surface of native cellulose to corresponding carboxylate and aldehyde groups (9, 10). The TEMPO mediated oxidation included TEMPO/NaBr/NaClO system at pH 10 (8–10). TEMPO/NaClO/NaClO2 system under neutral conditions at ~60 °C (11), and TEMPO electro-mediated oxidation under ambient conditions (12). The most effective was the TEMPO/NaBr/NaClO system which introduced more than 1 mmol/g C6-carboxylate groups in several hours. And the oxidized cellulose could be converted to individual CNF with homogeneous diameter of about 3 nm by sonication in water in high yield (8–10). However, the primary oxidant of this system, NaClO, and the catalyst, NaBr, might do harm to environment as the liquid waste after washing process. The enzyme, laccase, one of the blue-copper oxidases (13), that performed monoelectronic oxidation of suitable substrates, such as phenols, aromatic or aliphatic amines, to the corresponding radicals at the expense of molecular oxygen (14). The oxidation of non-phenolic substrates with higher redox potential (>0.8 V) or huge size that could not directly enter the enzymatic active site was also possible with a mediator used together with laccases. The mediator was usually a small molecule that acted as electron shuttle, once oxidized by laccase, it diffused away from the enzyme active site and in turn oxidized the target substrates (15–17). TEMPO was one of the most effective mediators reported (18). The TEMPO/laccase/O2 (TLO) system originally was used for alcohols and phenols (19–21). Recently Aracri et al. (22–24) applied this system to sisal cellulose and studied the improvement in wet and dry strengths of paper sheets. The increased carboxylate content of oxidized cellulose was much lower (almost 0.3 mmol/g), and as a result, individual CNF could not be obtained in considerable yield from the oxidized cellulose. This chapter focuses on the oxidation of cellulose and the preparation of CNF using TLO system. By optimizing the oxiditon condition, the TLO oxidized cellulose (TLO-cellulose) with abundant carboxylate groups was obtained and then CNF were successfully prepared. Additionally, the one-way reaction mechanism of TLO system was assumed (25). 192 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


TLO Oxidation of HBKP and Possible Reaction Mechanism The oxidation of HBKP (hardwood bleached kraft pulp) cellulose using TLO system was carried out by suspending HBKP, TEMPO and laccase in acetate buffer at pH 4.5, and introducing excessive O2 to the mixture according to our previous study (25). During the oxidation, the activity of laccase was monitored. In 0.1 M acetate buffer with different concentration of TEMPO, the laccase activity clearly decreased when the stirring time extended (Figure 1A). The activity decreased to almost 50% of the original after stirring the mixture containing 5 mM TEMPO for 10 h and to 20% for 24 h. When the concentration of TEMPO increased to 30 or 50 mM, the activity of laccase rapidly decreased to almost 0% after stirring for 15 h, indicating high concentration of TEMPO caused serious inactivation of laccase. In the TEMPO and laccase mixture, laccase oxidized TEMPO to the Noxoammonium form (TEMPO+), and TEMPO+ molecules accumulated. However, as the former research indicated that the primary amine groups of chitosan reacted with TEMPO+ and chitosan depolymerized to low-molecular-mass compounds (26), the primary amine groups of laccase molecules probably also reacted with TEMPO+, and then laccase molecules were degraded and lost activity. Figure 1B illustrates the change of laccase activity with HBKP and different amount of TEMPO (5 mM to 50 mM) during TLO oxidation at a constant amount of laccase (3 U/mL). During the oxidation, the inactivation of laccase also occurred and was similar to that in Figure 1A. As partial TEMPO+ molecules were consumed to oxidize HBKP, the remained activities of laccase in Figure 1B were little higher. Figure 1C presents the changes of different amount of laccase activity during oxidation of HBKP in TLO system at a constant TEMPO concentration of 50 mM. The inactivation or degradation of laccase could not be avoided, even when the concentration of laccase increased to 8.2 U/mL. In the TLO system, laccase oxidizes TEMPO to TEMPO+, which can in turn oxidize primary hydroxyl groups on the surface of cellulose to aldehyde and carboxylate groups. But at the same time, laccase was degraded. So the reactions between the formation of aldehyde and carboxylate groups and degradation of laccase molecules compete with each other when TEMPO+ existed. To obtain oxidized cellulose with high carboxylate content, the TLO oxidation conditions were further studied. When HBKP was oxidized by the TLO system in acetate buffer at pH 4.5 and room temperature for 24 h at a constant amount of laccase (3 U/mL) with different amount of TEMPO (5 mM to 50 mM), the carboxylate content increased substantially from 0.07 to 0.24 mmol/g (Figure 1D). The higher the concentration of TEMPO was, the more carboxylate groups formed. The yields of oxidized cellulose were stable at about 80%, which was almost pure cellulose, indicating that the hemicelluloses in the original HBKP were degraded during oxidation and then removed as water-soluble fractions in the washing process. When 4.0 to 8.2 U/mL of laccase were added to the mixture of TEMPO (50 mM) and HBKP for 48 h, the carboxylate contents of oxidized cellulose was almost constant at 0.55 mmol/g (Figure 1E). And the yield increased slightly from 75% to 82%. Figure 1F showed the effect of oxidation time on carboxylate contents and yields of oxidized cellulose with constant amount of TEMPO (50 mM) and laccase 193 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


(5 U/mL). Similar to laccase amount, the carboxylate content kept increasing with prolonged oxidation time and was stable at 0.62 mmol/g after stirring for more than 96 h. The yields of oxidized cellulose were almost stable at 80% when the reaction time extended to more than 48 h.

Figure 1. Changes of laccase activity (A) in TEMPO solution in pH 4.5 acetate buffer at concentration of 2 U/mL, (B) in TEMPO buffer solution with HBKP at concentration of 3 U/mL and (C) in 50 mM TEMPO buffer sulotion with HBKP at different concentration of laccase .Carboxylate contents and yields of TLO-HBKP at (D) laccase concentration of 3 U/mL with various TEMPO concentrations for 24 h, (E) TEMPO concentration of 50 mM with various laccase concentrations for 48 h, and (F) laccase and TEMPO concentrations of 5 U/mL and 50 mM, respectively, for 0–140 h (25). 194 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


As a result, the optimum TLO oxidation conditions were established as 1 g HBKP in 100 mL buffer solution at pH 4.5 with 50 mM TEMPO and 5 U/mL laccase, and the oxidation lasted for 96 h. However, some discrepancy was observed in the results above. As shown in Figure 1C, laccase sharply lost activity in 12 h, but the carboxylate content of TLO-HBKP continued increase with extended time even up to 140 h.

Figure 2. (A) Absorbance changes of TEMPO solution during the TLO oxidation of HBKP. (B) The relative amounts of TEMPO and TEMPO+ in the mixtures (25). During the oxidation, TEMPO was first transformed to TEMPO+, and the formed amount of TEMPO+ directly affected the oxidation efficiency. Figure 2A shows the UV-vis spectra of the reaction solutions during oxidation. The changes of absorbance at 245 nm and 300 nm indicated the transformation of TEMPO to TEMPO+ (27). The changes of the relative absorbance of TEMPO and TEMPO+ were switched to relative amount values in Figure 2B. In the first 4 h, more than 80% of TEMPO were converted to TEMPO+. Lower amount of formed TEMPO+ was detected because of some part was consumed in oxidizing cellulose and laccase. At the same time the transformation rate of TEMPO to TEMPO+ was much faster than the inactivation of laccase. The results were not reasonable when explained with the conventional mechanism, by which with no active laccase, the TEMPO+ molecules were supposed to not form any more. But in the tested range, the carboxylate content still increased even up to 140 h (Figure. 1F). 195 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 3. Carboxylate and aldehyde content of oxidized HBKP under optimized condition with fresh TEMPO and laccase (TLO-HBKP), with recycled TEMPO (TrLO-HBKP) and with 24 h-reacted TEMPO and laccase mixture (TLdO-HBKP).

Further experiments and more detailed data were required to make the mechanism clear. Figure 3 shows the carboxylate and aldehyde contents of samples with different processes. Under optimized experiment condition, the carboxylate and aldehyde content of TLO-HBKP reached 0.60 mmol/g and 0.24 mmol/g respectively. However, if the used TEMPO solution after 96 h was recycled and used again with newly added laccase, the increased carboxylate content of TrLO-HBKP was much smaller than that of TLO-HBKP, which indicated that no enough effective amount of TEMPO+ were formed. And in the former oxidation, to successfully oxidizing hydroxyl groups to aldehyde or carboxylate groups, most TEMPO+ oxidized by laccase exchanged to hydroxyl amine form (TEMPOH). So TEMPOH molecules seemed to consist most of TEMPO molecules. On the other side when adding laccase and TEMPO first, and adding HBKP after 15 h with no detected active laccase, the formed carboxylate and aldehyde groups of TLdO-HBKP was improved to 0.33 mmol/g and 0.15 mmol/g, which were obviously higher than TrLO-HBKP case. In the first 15 h, TEMPO+ was formed in large amount and then oxidized cellulose in the later 96 h. According to these results, the most likely oxidation mechanism was shown in Figure 4. The oxidized laccase firstly oxidize TEMPO to TEMPO+. At the same time, TEMPO+ degraded laccase molecules and oxidized cellulose. But the oxidation rate of cellulose seemed to be very slow, that made the oxidation time to last 96 h. So the oxidation was surpposed to be one-way conversion and after oxidizing cellulose or laccase molecules the TEMPOH molecules could not be regenerated to TEMPO (25). This was the reason why such large amounts of TEMPO and laccase were required in TLO system. In the former literature (22–24, 28–30), when catalytic amounts of TEMPO and laccase were used to 196 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


oxidize cellulose, the resulting oxidized celluloses contained very small amount of carboxylate groups but large amount of aldehyde groups. The possible reason was that the oxidation system did not have so much TEMPO+ to conduct the next step, converting aldehyde group to carboxylate groups.

Figure 4. Possible oxidation mechanism of C6-primary hydroxyl groups of HBKP by the TLO system in buffer solution at pH 4.5 (25).

Further Optimization of TLO Oxidation and the Preparation of Cellulose Nanofibers The carboxylate content of TLO-HBKP under optimized condition was 0.60 mmol/g, which performed the yield of CNF at almost 65%. To improve the oxidation efficiedncy of TLO system and the yield of CNF, further experiments were designed shown in Table 1. With the use of different resources of laccase, the carboxylate and aldehyde content were all around 0.60 mmol/g and 0.25 mmol/g. When the laccase was added in batches at 0 h and 48 h with half the amount, the carboxylate content of oxidized cellulose was obviously higher. And if the TLO-HBKP of optimized condition underwent TLO oxidation for once more, the carboxylate content of twice oxidized HBKP was over 1 mmol/g, which was comparable to the chemical oxidized cellulose by TEMPO/NaBr/NaClO system (25). In this case, the aldehyde content decreased to 0.056 mmol/g. The XRD patterns of original HBKP and TLO-HBKP under optimized condition is shown in Figure 5A. The crystalline index (C.I.) and crystal width of cellulose I were almost unchanged after oxidation. It supposed that the carboxylate groups were formed on the surface of crystalline domains indicated that had no influence on the inner molecules. 197 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Table 1. Carboxylate and aldehyde contents of samples with different oxidation conditions Sample

Condition

Carboxylate content (mmol/g)

Aldehyde content (mmol/g)

Original HBKP

/

0.098

0.016

With

laccase1

0.60

0.24

With

laccase2

0.62

0.24

TLO-HBKP-2

Add

laccase1

0.77

0.20

TLO-HBKP-3

Twice oxidized by TLO oxidation

1.1

0.056

TLO-HBKP


TL2O-HBKP

in batches

L1: laccase from Trametes versicolor.

L2: laccase from Pleurotus grtreatus.

Figure 5. (A) XRD patterns of HBKP and TLO-HBKP, (B) AFM images of CNF prepared from TLO-HBKP. After sonication, the cellulose dispersion of TLO-HBKP was centrifuged to remove the unfibrillated fraction. The CNF dispersion was further diluted with distilled water to the concentration of almost 0.001% (w/v) and observed with AFM microscope. Figure 5B showed that although some CNFs formed bundles and network structures without being completely isolated because of the relatively lower carboxylate content (0.62 mmol/g), the yield of CNF was high at about 65%. The lengths and widths of the CNFs were > 1 µm and 4 – 8 nm, respectively. It was clear that CNFs could be successfully prepared from HBKP by TLO oxidation and sonication in water without using any chlorine-containing oxidant. It was shown that CNFs of HBKP could be successfully prepared by TLO oxidation system. To verify the universal applicability of TLO oxidation, different kinds of cellulose materials were oxidized with the system. Because of the differences in morphology and structure, the oxidation efficiencies of each cellulose were not the same. But the carboxylate and aldehyde contents of all oxidized cellulose were improved markedly to more than 0.5 mmol/g and around 0.2 mmol/g respectively (Table 2), indicating that the TLO system were applicable to different cellulose materials for the preparation of CNFs. 198 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Table 2. Carboxylate and aldehyde contents of different oxidized cellulose after TLO oxidation TLO-HBKP

TLO-SBKP

TLO-BBSP

TLO-BBaP

Carboxylate content (mmol/g)

0.57 ± 0.12

0.54 ± 0.075

0.63 ± 0.13

0.56 ± 0.094

Aldehyde content (mmol/g)

0.24 ± 0.042

0.18 ± 0.034

0.24 ± 0.036

0.32 ± 0.031

SBKP: softwood bleached kraft pulp; BBSP: bagasse bleached sulfate pulp; BBaP: bleached bamboo pulp.

Conclusion When applying the TLO system to oxidize cellulose materials, the laccase molecules were unstable in the presence of TEMPO+, which reacted with amino groups of laccase together with oxidizing cellulose. In 12 h, almost all the laccase lost its activity, but the formation of carboxylate group was continued until 140 h. As a result, large amount of TEMPO and laccase and long oxidation time were required to oxidize cellulose to introduce almost 0.6 mmol/g of carboxylate groups. It was assumed that one-way formation of TEMPO+ from TEMPO occurred in the system and TEMPO could not be regenerated from TEMPOH directly by laccase and O2. When HBKP was subjected to TLO oxidation with 50 mM TEMPO and 5 U/mL of laccase for two times, the carboxylate content of the resulting TLOHBKP could reach 1.1 mmol/g, from which CNFs were supposed to be obtained in high yield. The TLO system was also applied to other cellulose materials successfully.

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Oxidation of Native Cellulose for the Preparation of Cellulose Nanofibers

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