Eco-friendly polysorbate aqueous solvents for efficient dissolution of lignin

Abstract

Herein green, low energy consuming and inexpensive solvents (polysorbate/H₂O (Tween-80/H₂O)) were developed, which could be readily prepared, instantaneously dissolve lignin without any heating, and hardly disrupt the structure of lignin. The facile lignin dissolution can be ascribed to the interaction between the Tween-80 chain and molecular chain of lignin.

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With the steady exhaustion of petroleum resources, developing alternatives to petroleum resources has become an inevitable trend. In this regard, low-cost and biorenewable bioresources such as lignin have received great attention.¹ Lignin is a potential feedstock to produce value-added chemicals as an alternative to the petrochemical industry² and to prepare high performance composites.³ However, the production of lignin-based products generally first involves selection of efficient solvents which are used to isolate lignin from lignocellulosic biomass. Currently, the dominant global method to isolate lignin from lignocellulose bioresources is the kraft pulping process which accounts for about a 90% share of the total production capacity.⁴ The other rarely used processes include steam explosion,⁵ ball-milled method,⁶ and so on. Over the past few years, due to their distinct advantages such as chemical and thermal stability, non-detectable vapor pressures and chemical tunabilities etc.,⁷ imidazolium-based ionic liquids (ILs) have been developed as lignin solvents⁸ and isolation/extraction of lignin from lignocellulose.⁹ However, imidazolium-based ILs have proved to be highly toxic and poorly biodegradable.¹⁰ Recently, bio-derived choline-based ILs has been proposed to pretreat rice straw and enhance enzymatic hydrolysis of rice straw.¹¹ To address the high cost issue of ILs, cholinium lysine and dialkylimidazolium-based IL aqueous solution has been recently used to remove lignin from rice straw¹² and dissolve lignin.¹³

This work emphasizes on developing nontoxic, low energy consuming, low viscosity and inexpensive lignin solvents. To achieve this objective, a green and inexpensive feedstock Tween-80 is utilized. At the same time, another greenest and safest solvent H₂O was used to decrease not only viscosity but cost.

Table 1 shows the solubilities of lignin in Tween-80 and Tween-80/H₂O solvents at 25 °C. Lignin is insoluble in Tween-80. Interesting, after H₂O was add to Tween-80, lignin became soluble. Moreover, the Tween-80/H₂O solvents displayed excellent dissolution behavior for lignin at the mass ratio range of H₂O to Tween-80 from 0.13 : 1 to 2.70. To the best of our knowledge, such efficient and green lignin solvents have never been documented before. However, the further addition of H₂O in Tween-80 decreases lignin solubility, which could be attributed to the decrease in the concentration of Tween-80 of Tween-80/H₂O solvent. This is also an indication that Tween-80 in Tween-80/H₂O solvent dominates lignin dissolution.

Table 1 Solubility of lignin in Tween-80/H₂O solvents at 25 °C^a

R	Solubility (gram per 100 g of solvent)
a R is the mass ratio of H2O to Tween-80.
0	Insoluble
0.05 : 1	1.0
0.13 : 1	>65
0.25 : 1	>63
0.43 : 1	>63
0.5 : 1	>60
0.76 : 1	>70
0.92 : 1	>63
1.09 : 1	>59
1.2 : 1	>36
1.35 : 1	28
1.50 : 1	26
1.99 : 1	14.5
2.70 : 1	8.0
4.55 : 1	3.5

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In order to examine the effect of H₂O during the dissolution of lignin in the Tween-80/H₂O solvents, as an example, measurements of absorption wavelength λ for lignin in Tween-80/H₂O/lignin solution were carried out in that the increase or decrease in λ of lignin can generally reflect the interaction between lignin and solvent. Fig. 1 shows the dependence of λ on the mass ratio R of H₂O to Tween-80 in the Tween-80/H₂O/lignin solution at room temperature. The lignin λ (the basic UV spectrum of typical lignin with an absorption peak at about 280 nm, which was associated to non-conjugated phenolic groups (aromatic rings) of lignin¹⁴) is hardly variable even if R exceedingly increases from 1 : 1 to 5 : 1. This indicates that, the dissolution of lignin in the Tween-80/H₂O solvent is due to the interaction between Tween-80 and lignin, not H₂O and lignin, which is in agreement with the above conclusion that Tween-80 in the Tween-80/H₂O solvent dominates the dissolution of lignin.

Fig. 1 UV/Vis spectra of lignin in Tween-80/H₂O(R = 5 : 1)/lignin solution and Tween-80/H₂O(R = 1 : 1)/lignin solution at 25 °C.

To further examine the role of H₂O in Tween-80/H₂O solvent. ¹³C NMR spectra of neat Tween-80 and Tween-80 in Tween-80/H₂O(R = 1.5) solution were determined (see Fig. S1 and S2†). Evidently, compared to neat Tween-80, the addition of H₂O in Tween-80 does not results in the change in chemical shifts of C atoms of Tween-80, suggesting that the role of H₂O in Tween-80/H₂O solvent is to disperse/dilute Tween-80 molecules. At the same time, ¹³C NMR spectra of Tween-80 in Tween-80/D₂O(R = 1.5) solvent and Tween-80/D₂O(R = 1.5)/lignin(8%) solution were also determined (see Fig. S3 and S4†) to further investigate the effect of Tween-80 during dissolution process of lignin. After the dissolution of lignin in Tween-80/D₂O(R = 1.5) solvent, chemical shifts of C atoms of Tween-80 decreased. This could be ascribed to the fact that the weak interaction between Tween-80 and lignin molecules promoted the dissolution of lignin, and Tween-80 was then shielded by lignin molecules, which caused the signal of the C atom of Tween-80 moved upfield (a decrease of chemical shift). This further indicates that Tween-80 in the Tween-80/H₂O solvent dominates the dissolution of lignin.

The lignin regenerated from Tween-80/H₂O(R = 0.5)/lignin solution by addition of water after 1 h of dissolution at 25 °C was characterized by TGA and FTIR spectroscopy.

FTIR spectra of the original and the regenerated lignin are shown in Fig. 2. The two spectra are quite similar and no new peaks are observed in the regenerated lignin, indicating that no chemical reaction takes place during the dissolution and regeneration processes of the lignin. FTIR spectra of the original and the regenerated lignin are similar to the results reported in the literatures.¹⁵ The absorption band at 3429 cm⁻¹ in the regenerated lignin is assigned to the stretching vibration of O–H of phenolic OH and aliphatic OH. The absorption band at 2945 cm⁻¹ is assigned to the stretching vibration of C–H of CH₃ and CH₂. The absorption band at 2845 cm⁻¹ is assigned to the stretching vibration of C–H of OCH₃. The absorption bands at 1600 cm⁻¹, 1515 cm⁻¹ and 1425 cm⁻¹ are assigned to the stretching vibration of C–C of aromatic skeleton. The absorption band at 1460 cm⁻¹ is assigned to the in-plane asymmetric deformation vibration of C–H of CH₃ and CH₂. The absorption band at 1270 cm⁻¹ is assigned to the stretching vibration of C–O of guaiacyl type. The absorption band at 1218 cm⁻¹ is assigned to the stretching vibration of C–O(H) + C–O(Ar) phenolic OH + ether. The absorption band at 1136 cm⁻¹ is assigned to the aromatic C–H in-plain deformation for syringyl type. The absorption band at 1030 cm⁻¹ is assigned to the stretching vibration of C–O(H) + C–O(C) of 1st order aliphatic OH + ether. The absorption bands at 855 cm⁻¹ and 810 cm⁻¹ are assigned to the out-of-plane deformation vibration of aromatic C–H of guaiacyl type.

Fig. 2 FT-IR spectra: (a) the original lignin; (b) the regenerated lignin from Tween-80/H₂O(R = 0.5)/lignin solution by addition of water after 1 h of dissolution at 25 °C.

TGA curves of the original and regenerated lignin are shown in Fig. 3. TGA curves of the regenerated lignin and the original lignin are nearly overlapped at the temperature range from 25 °C to 380 °C. The regenerated lignin exhibits a similar onset temperature (272 °C) for the decomposition compared to the original lignin, indicating that the lignin regenerated from Tween-80/H₂O(R = 0.5) solvent has good thermal stability, and does not degrade during the dissolution and regeneration.

Fig. 3 Thermal decomposition profiles: (a) the original lignin; (b) the regenerated lignin from Tween-80/H₂O(R = 0.5)/lignin solution by addition of water after 1 h of dissolution at 25 °C.

In conclusions, the findings of this research have very important implications for future practice. The novel and efficient lignin solvents circumvent the problems faced in the conventional solvents such as high viscosity, high cost, high energy consuming or toxicity.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (No. 21373078), the Science and Technology Innovation Team Training and Development Plans, Henan University of Science and Technology (No. 2015XTD008), and the Henan Province Science and Technology Research Plan (No. 152102210274).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24570b

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