Improved dissolution of cellulose in quaternary ammonium hydroxide by adjusting temperature

Abstract

A remarkably improved dissolution of cellulose in 40 wt% tetra-n-butylammonium hydroxide has been realized with a decrease in temperature to 16 °C.

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It is reported that natural cellulose shows characteristics of both hydrophobicity and hydrophilicity in its molecular chain^1,2 and crystal structure.³ In addition, the formation of the cellulose crystal is favored by both hydrogen bonding and hydrophobic association.⁴ The latter, which is known as van der Waals forces, may play a more important role.^2,5 It is known that little changes in the amphiphilicity of the polymer may have a significant impact on its properties such as solubility, especially for high molecular weight polymers. Therefore, the solubility of cellulose can be facilitated in solvents that also are amphiphilic,^1,6 which is known as the Lindman hypothesis. Accordingly, cellulose is difficult to dissolve in most aqueous and non-aqueous solvents due to its unusual amphiphilicity. However, the development of efficient solvents for cellulose has been pursued for its high potential applications. In addition, various solvent systems have been developed, such as lithium chloride/N,N-dimethylacetamide (LiCl/DMAc),⁷ N-methylmorpholine-N-oxide (NMMO),⁸ inorganic molten salt hydrates,⁹ ionic liquids (ILs),^10,11 and NaOH/urea.¹²

Recently, quaternary ammonium hydroxide aqueous solutions, such as tetra-n-butylammonium hydroxide (TBAH) and tetra-n-butylphosphonium hydroxide (TBPH), have been reported as an efficient solvent for cellulose.¹³ Compared with non-aqueous solvents, a quaternary ammonium hydroxide aqueous solvent is relatively economical, efficient and green. Half of this solution is composed of water, which is the most common and environmentally friendly solvent in the world. Compared with other aqueous solvents such as NaOH/urea, the process of dissolving cellulose in a quaternary ammonium hydroxide aqueous solution is mild and facile, due to its room temperature dissolution. In addition, the solubility of cellulose in quaternary ammonium hydroxide is much higher.¹³

It seems that microcrystalline cellulose displays a fast dissolution in 60 wt% TBAH aq. solution under mild stirring at 25 °C. However, it takes a longer time to be dissolved in 40 wt% TBAH aq. solution and the obtained solution shows a fuzzy appearance without the assistance of additives.¹⁴ Herein, we find that temperature has an obvious effect on the dissolution of cellulose in 40 wt% TBAH aq. solution and a remarkably improved dissolution of cellulose in this solvent has been demonstrated experimentally by lowering the temperature to 16 °C. Moreover, much less work in concentrating is required, which makes 40 wt% TBAH aq. solution a more efficient and economical solvent for cellulose.

376 mg of microcrystalline cellulose (Aladdin, C104843) was added into 5 mL TBAH aq. solution (Alfa Aesar, L02809, 40 wt% or 60 wt%, ESI†) and stirred at 30 °C for 10 min. Then, the solution was transferred into a bath with controlled temperature (Table 1) and was stirred for 60 min. The mixture was then stored and stirred at 28 °C for 30 min.

Table 1 Experimental design for the effect of temperature on the dissolution of cellulose in 40 wt% TBAH aq. solution

Sample Number	Conc. of TBAH in aq. solution (wt%)	Temperature (°C)
6028	60	28
4028	40	28
4020	40	20
4016	40	16

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Fig. 1 shows the differences in transparency of the solution with different temperatures of dissolution. Sample 6028 has high clarity, indicating a good dissolution of cellulose. However, the solution of sample 4028 and 4020 is turbid and grey. This is consistent with the reported literature.^13,14 When the temperature is lowered to 16 °C, the obtained cellulose/40 wt% TBAH solution also shows a good transparence similar to that of the sample 6028. Herein, the changes from bad to good solubility of microcrystalline cellulose in the 40 wt% TBAH aq. solution is realized by lowering the temperature from 28 °C to 16 °C. Temperature plays a key role in controlling the dissolution of cellulose in 40 wt% TBAH aq. solution. The yellowing of sample 6028 and sample 4016 may be attributed to the formation of chromophores from the oxidized and degraded carbohydrate material under alkaline conditions.^15,16 Similarly, the solution of cellulose dissolved in a mixture of 40 wt% TBAH and crown ether (18-crown-6) also shows such an interesting coloration phenomenon.¹⁴

Fig. 1 Images and turbidity of the (original) initial state of the solution of microcrystalline cellulose in a 40 wt% TBAH aq. solution at 28 °C, the solution of cellulose dissolved in (6028) 60 wt% TBAH aq. solution at 28 °C for 1 h, (4028) 40 wt% TBAH aq. solution at 28 °C for 1 h, (4020) 40 wt% TBAH aq. solution at 20 °C for 1 h and (4016) 40 wt% TBAH aq. solution at 16 °C for 1 h. The turbidimeter was calibrated using a formazine turbidity standard solution.

The morphology of cellulose dissolved in a TBAH aq. solution was characterized by TEM (Fig. 2). As can be observed, both sample 6028 (Fig. 2a) and sample 4020 (Fig. 2b) show a fiber-like morphology. The width and length of both cellulosic fibres are in the nano-scale and micro-scale, respectively, which is similar to that found for cellulose dissolved in a NaOH/urea aq. solution.^17,18 This indicates that the well crystallized region in cellulose has not yet been dissolved under such conditions. Interestingly, for sample 4016 (Fig. 2c and d), a mass of cobweb-like cellulose with good flexibility can be observed. The width of cellulose is about 1 nm. In addition, the length of cellulose is less than 300 nm. The molecular chain length of cellulose used in this experiment was observed to be 100–250 nm (Fig. S1†) and estimated to be 125 nm by the degree of polymerization (DP = 120, the length of cellobiose residue is 1.038 nm (ref. 19)). It has also been reported that natural cellulose shows an unexpected flexibility at a single-chain level.²⁰ These results suggest that the crystalline structure of the original microcrystalline cellulose has been distinctively destroyed during its dissolution in a 40 wt% TBAH aq. solution at 16 °C within 60 min. Cellulose can be dissolved to an almost molecular level under this condition.

Fig. 2 TEM images of cellulose dissolved in (a) 60 wt% TBAH aq. solution at 28 °C for 60 min, (b) 40 wt% TBAH aq. solution at 20 °C for 30 min and (c and d) 40 wt% TBAH aq. solution at 16 °C for 60 min. The concentration of cellulose is 6 mg mL⁻¹. A thin layer of the dilute cellulose solution is suspended on a holey carbon film and dried in air at 19 °C for about 10 min.

The dissolved cellulose was precipitated by mixing with hot water and collected by filtration through an organic membrane. Wide-angle X-ray diffraction (WAXD) was used to detect the transformation in the crystal form of cellulose during the dissolution process (Fig. 3). The original microcrystalline cellulose shows a typical diffraction pattern of cellulose-I.²¹ However, all the cellulose dissolved in TBAH aq. solution exhibit three typical diffraction peaks of cellulose-II at 2θ = 12.1°, 19.8° and 22.0° for its (11̄0), (110) and (020) planes, respectively,²² which indicates the destruction of the cellulose-I crystals during the dissolution of cellulose in a 40 wt% or 60 wt% TBAH aq. solution. However, for sample 4028 or sample 4020, there is an additional peak at 2θ = 22.6° corresponding to the (200) plane of cellulose-I, indicating the partial dissolution of cellulose under those conditions. The results from FT-IR spectroscopy also suggest the transformation of the hydrogen bond network associating with O₆, which also indicates the change in the crystal structure of cellulose (ESI†).

Fig. 3 WAXD curves of (TBAH) freeze dried TBAH, (MC) microcrystalline cellulose, cellulose after dissolution in (6028) 60 wt% TBAH aq. solution at 28 °C, (4028) 40 wt% TBAH aq. solution at 28 °C, (4020) 40 wt% TBAH aq. solution at 20 °C and (4016) 40 wt% TBAH aq. solution at 16 °C, recorded on an X'Pert Pro MPD (PANalytical B.V.) X-ray diffraction using a reflection method with Cu K_α radiation (λ = 1.5406 Å) at 40 kV and 40 mA.

The effect of temperature on the dissolution of cellulose in a 40 wt% TBAH aq. solution is helpful to understand the mechanism of cellulose dissolution. It has been reported that the ionic structures of NaOH²³ and LiOH¹⁸ are relatively stable due to the slow exchange between bulk water and coordinated water at low temperature. This stability is important for attaching NaOH hydrates to the cellulose chains, wherein a stronger hydrogen-bonded network can form at a lower temperature.²⁴ With the addition of urea, the dissolution of cellulose is accredited to a dynamic self-assembly process and a worm-like inclusion complex could be self-assembled to prevent the regeneration of cellulose.^12,18 According to the DSC results of a TBAH aq. solution (Fig. S3†), the 40 wt% TBAH aq. solution has a melting peak at about 29.5 °C. This means that 16–20 °C is the “low temperature” for 40 wt% TBAH aq. solution wherein its ionic structure is relatively stable. Moreover, a similar inclusion complex was formed, as shown in Fig. 2b. It seems that TBAH can serve as both the role of NaOH and urea. In addition, 60 wt% TBAH aq. solution does not show any melting peak above 0 °C and its ionic structure may not be stable at 28 °C. Unlike sample 4020, there is also no shielding layer on the cellulose in sample 6028 that prevents the cellulose from regeneration, as shown in Fig. 2a. However, the solution of sample 6028 had high clarity and was stable at room temperature. There may be other mechanism.

Fig. 4 shows the binary phase diagram of the TBAH aq. solution, which is considered to be a typical phase diagram of a partially miscible binary system. For the 40 wt% TBAH aq. solution, there is a synchronous formation of both an α phase (water rich phase) and β phase (TBAH rich phase) along with a decrease in temperature from 29.5 °C. The contents and concentrations of both the α and β phase depend on the temperature of the solution. Consistently two kinds of crystal with different ED patterns have been observed in the 40 wt% TBAH aq. solution, as shown in Fig. S4.† Therefore, with a decrease in temperature, the concentration of the β phase increases gradually and 16 °C seems to be the critical point wherein the concentration of the β phase is suitable for dissolving cellulose. Thus, cellulose in both the samples 4016 and 6028 were dissolved in the high concentration TBAH aq. solution.

Fig. 4 Binary phase diagram of the TBAH aqueous solution with its concentration ranging from 0 wt% to 60 wt% according to the DSC results of the TBAH aq. solution.

Fig. S5† shows a molecular model of the elemental fibril of cellulose-I_β with a crystal surface of (11̄0), (110) and (200). Both the (11̄0) and (110) surfaces reveal a weaved pattern of hydrophobicity and hydrophilicity,³ which can be regarded as an amphiphilic surface. This unique property differs from most other solvents, making cellulose stable in the solution. However, the (200) surface is simply hydrophobic due to the hydrogen atoms of the C–H bonds on the axial positions of the glucopyranose ring. When the concentration of TBAH is high enough, the TBAH solution also shows hydrophobic properties due to the alkyl residue of the TBAH molecules. The similar properties will reduce the interface mismatch between the (200) surface and the solvent. Interestingly, for sample 4016, there are conjugated hydrophobic and hydrophilic phases in the solvent, which may further reduce the interface mismatch between either the (11̄0) surface or (110) surface and the solvent. This may be the reason for the remarkably improved dissolution of cellulose in 40 wt% TBAH aq. solution at 16 °C, as shown in Fig. 2c and d. Then, the TBAH hydrates can penetrate into cellulose, bind to the chains and establish a dynamic equilibrium between the bonding cellulose and water, which is similar to what happens in the NaOH/urea solvent system.^24–26

Conclusions

In summary, we have found that temperature plays a key role in controlling the dissolution of cellulose in a 40 wt% TBAH aq. solution. Cellulose can be efficiently dissolved in a 40 wt% TBAH aq. solution at 16 °C, wherein a transparent aqueous solution of cellulose can be obtained. Cobweb-like cellulose demonstrates that cellulose can be dissolved to an almost molecular level under that condition. The remarkably improved dissolution of cellulose may be accredited to the distinctive amphiphilicity of the solvent. The advantages of lower concentration of TBAH, and faster and better dissolution make it a good solvent towards the environmentally-friendly dissolution of natural cellulose and its high-valued applications.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (no. 51303151), the National Key Technology R&D Program of China (no. 2011BAE11B01) and the Science and Technology Planning Project of Sichuan Province (nos 2013RZ0036, 2014GZ0099).

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra04247j

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