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Dissolution of cellulose in novel carboxylate-based ionic liquids and dimethyl sulfoxide mixed solvents
European Polymer Journal 113 (2019) 89–97
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Dissolution of cellulose in novel carboxylate-based ionic liquids and dimethyl sulfoxide mixed solvents Dawid Kasprzaka, Ewa Krystkowiakb, Izabela Stępniaka, Maciej Galińskia, a b
T
⁎
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, ul. Berdychowo 4, 60965 Poznań, Poland Faculty of Chemistry, Adam Mickiewicz University in Poznań, ul.Umultowska 89b, 61-614 Poznań, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose ionic liquids (ILs) Acetate Piperazinium Piperidinium Pyrrolidinium
A series of substituted piperazinium, piperidinium, and pyrrolidinium acetates were synthesized. Highly effective cellulose solvents have been designed by mixing an aprotic polar solvent, dimethyl sulfoxide (DMSO), with synthesized ionic liquids (ILs). Different weight ratios of corresponding IL/DMSO mixed solvents were prepared to investigate the effect of cosolvent on cellulose solubility. Cellulose solubility in IL/DMSO solvent systems was measured at 25, 50 and, 80 °C. The effect of factors such as solvent composition, process temperature, type of IL’s cation, and alkyl chain length of IL’s cation on cellulose solubility was investigated. Amongst all tested solvents, N,N’-dimethyl-N-ethylpiperazinium acetate ([DMEPpz][Ac]/DMSO) mixed solvent exhibits the best performance in the dissolution of cellulose, reaching up to 13.5 wt% at 80 °C. The dissolved cellulose was regenerated using water as an antisolvent. The structure and morphology of the regenerated cellulose material were characterized by SEM, XRD, and FTIR, respectively.
1. Introduction Cellulose is an abundant biopolymer with fascinating structure and attractive properties such as biocompatibility, biodegradability, and thermal and chemical stability [1,2]. Cellulose is traditionally used in textile and paper-making industries, and recently it is a promising feedstock for biobased products and fuels [3–9]. In many applications, processing of cellulose requires the dissolution of cellulose. However, the intermolecular and intramolecular hydrogen-bonding network in cellulose chains makes this biopolymer hardly soluble in common solvents and limits its wide application [10]. Therefore, it is important to search for effective solvent systems for the dissolution of cellulose. The most important examples of traditional cellulose solvent systems are N-methylmorpholine-N-oxide (NMMO) [11], N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) [12], dimethyl sulfoxide/tetrabutylammonium fluoride (DMSO/TBAF) [13], molten salt hydrate LiClO4·3H2O [14], aqueous solutions of NaOH/urea [15]. All of these systems suffer from various drawbacks and should be replaced mainly because of environmental reasons [16]. In this context, ionic liquids have the potential to replace traditional cellulose solvents due to their dissolving abilities and desirable properties like negligible vapour pressure, low toxicity, high thermal and chemical stabilities, nonflammability, structure tenability, and recyclability [17]. The idea of the processing of cellulose in ionic liquids was born in ⁎
2002 [18]. The research group led by prof. Rogers reported promising results of cellulose solubility in dialkylimidazolium-based ionic liquids with chloride anions. Since then the increased interest in research on the application of ILs, especially imidazolium-based ILs in biomass processing has been observed. Previous studies have shown that compounds composed of cations based on aromatic imidazolium derivatives coupled with halides, carboxylates, or alkyl phosphates anions are the most promising ILs in the field of cellulose processing [19–23]. However, a strong association between the cations and anions makes ILs a highly viscous medium [24]. This property diminishes IL’s solvating power and, as a consequence, hinders the dissolution of large quantities of cellulose. This problem can be overcome by the concept of ILs mixing with polar aprotic cosolvents (such as dimethylsulfoxide (DMSO), dimethylacetamide (DMA), dimethylformamide (DMF), etc.), which was first proposed by Rinaldi et al. [25]. The newly developed solvent systems exhibited lower viscosity, higher dissolving rate, and lower cost compared to neat ILs. The addition of cosolvent accelerates mass transportation by decreasing the system’s viscosity. It also solvates ILs, leading to formation of more free ions, which interact with hydrogenbonding network of cellulose [26,27]. The improvement of the IL’s solvating power leads to the decreasing of the time needed for dissolution of cellulose, even at low temperatures. Ohira et al. found that cellulose could be effectively dissolved in the mixture of DMSO with
Corresponding author. E-mail address: [email protected] (M. Galiński).
https://doi.org/10.1016/j.eurpolymj.2019.01.053 Received 8 October 2018; Received in revised form 19 January 2019; Accepted 23 January 2019 Available online 24 January 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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diethyl-4-nitroaniline were obtained from Fluorochem. Cyclohexane (fluoresc., 99.7%) was obtained from Merck. Dimethyl sulfoxide (P.O. Ch. Poland), acetonitrile (Sigma Aldrich) chromatographic grade, hexane (P.O.Ch. Poland), tetrahydrofuran (Sigma Aldrich) were used as received. Anion exchange resin (Dowex® Monosphere 550 A) was obtained from Sigma Aldrich and washed several times with distilled water before use. NaOH (P.O.Ch, Poland) was used as received. Microcrystalline cellulose powder (MCC) was obtained from SigmaAldrich (20 μm, PD c.a. 250). The cellulose samples were used directly after being dried overnight at 80 °C.
quaternary ammonium-based ILs consisted with alanine anions, at 25 °C [28]. More recently, there have been reports of more IL-based solvent systems which effectively dissolve cellulose at ambient temperature [29–32]. It is also worth mentioning that the dual effects of polar aprotic solvents on the cellulose dissolution ability of IL-based mixed solvents may be observed. Zhang et al. [33] suggested that DMSO improves the dissolution of cellulose at low concentration by improving mass transportation and accelerating the cellulose solvating kinetics. Whereas, at high concentration, the opposite effect is observed. DMSO decreases the solubility of cellulose by weakening the ability of hydrogen bond creation between IL’s ions and cellulose. Polarity is an important property of ionic liquids, because the solubility of the solute, the reaction efficiency as solvent, and the miscibility with other solvents are influenced by the IL properties [34–36]. The classification of ILs with respect to their polarity/polarizability as well as hydrogen-bonding ability is most often performed on the basis of the linear solvation energy relationship proposed by Kamlet and Taft based on UV–Vis spectroscopic measurements [37,38]. This analysis specified the π* parameter to characterize the polarity/polarizability of a given solvent, which scale was selected to run from 0.0 for cyclohexane to 1.0 for dimethyl sulfoxide (DMSO) [39,40]. The solvents capable of hydrogen bond formation have been further classified as hydrogen-bond acceptors and hydrogen-bond donors. The hydrogenbond acceptor property of a solvent depends on its ability to accept a hydrogen atom from a solute to form a hydrogen bond (β), and the hydrogen-bond donor property depends on its ability to donate a hydrogen atom to form a hydrogen bond with a solute (α). The α scale was selected to extend from 0.0 for non-hydrogen-bond donor solvents to ∼ 1.0 for methanol and the β scale from 0.0 for cyclohexane to ∼ 1.0 for hexamethylphosphoramide [40]. Generally, ILs are polar solvents similar to short- or medium-chain alcohols. Anions have the greatest effect on the hydrogen-bond basicity, and the polarity of ILs depend to some extent on the polarity of their environment [20,41,42]. The hydrogen-bond basicity (β) of the solvent system is essential for the dissolution of cellulose [25,43]. Cellulose can be regenerated easily from cellulosic solution by adding antisolvents, such as water, ethanol, methanol, etc. Depending on the regeneration method, there are several possible forms of cellulose-based materials such as powders, films, fibres, gels, and composites [44–49]. If the cellulose processing process is a properly designed, there is a possibility to recycle of IL solvent. In the present work, we have investigated the dissolution of microcrystalline cellulose in the several ionic liquid/dimethyl sulfoxide solvent mixtures. For this purpose, firstly, we synthesized a series of different ionic liquids based on pyrrolidinium, piperidinium, and piperazinium derivatives with acetate anions. Further, each of the synthesized ILs were mixed with DMSO and used as a cellulose solvent. The solvents containing different concentrations of ILs were prepared to investigate the effect of varying amounts of cosolvent on the cellulose solubility. The effect of the temperature, IL’s cation type, and alkyl chain length of IL’s cation on the cellulose solubility was also investigated. Additionally, the structure and properties of the regenerated cellulose material obtained from the representative solvent, N,N′-dimethyl-N-ethylpiperazinium acetate ([EDMPpz][Ac])/DMSO were characterized by using various techniques. The results have been compared with the outcomes obtained for the raw microcrystalline cellulose.
2.2. Characterization The obtained ionic liquids were characterized by 1H and 13C NMR Spectroscopy. NMR spectra were recorded on a Varian VNMR-S 400 MHz spectrometer, in Acetonitriled3 or d6-DMSO. Tetramethylsilane (TMS) was used as an internal standard. The raw microcrystalline cellulose and the cellulose regenerated from IL/DMSO solvent were characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy. The SEM images were taken using a Hitachi S-3400N scanning electron microscope. The XRD analysis was performed using TUR M-62 diffractometer equipment with copper anticathode. Thermogravimetric measurements were carried out using a TGA 4000 Thermogravimetric Analyzer (PerkinElmer, USA). The FT IR measurements were performed with a Bruker Vertex 70 FT IR spectrometer equipped with ATR accessory. Kamlet-Taft measurements were performed on a UV–Vis spectrophotometer Jasco V-650 with a 10 mm path length quartz cuvette, at room temperature. 2.3. Ionic liquids synthesis procedure Ionic liquids were prepared in a two-step reaction which was described in more details elsewhere [50]. The first step was obtaining piperidinium, pyrrolidinium, and dimethylpiperazinium-based bromides: N-ethyl-N-methylpyrrolidinium bromide ([EMPyrr][Br]), Nmethyl-N-propylpyrrolidinium bromide ([MPPyrr][Br]), N-ethyl-Nmethylpiperidinium bromide ([EMPip][Br]), N-methyl-N-propylpiperidinium bromide ([MPPip][Br]), N,N’-dimethyl-N-ethylpiperazinium bromide ([DMEPpz][Br]), and N,N′-dimethyl-N-propylpiperazinium bromide ([DMPPpz][Br]). All bromides were obtained as white precipitates in high yield i.e. 95–98%. In the second stage, an aqueous solution of the corresponding bromide was passing through an anion exchange resin and finally neutralized with acetic acid by acid-base reaction. Fig. 1 presents the reaction scheme for acetate-based ionic liquids preparation. After water vaporization on a rotating vacuum drier, synthesized compounds were dried under vacuum at 60 °C for 24 h. Finally, six ILs
2. Experimental 2.1. Materials N-methylpyrrolidine (MPyrr), N-methylpiperidine (MPip), N,N′-dimethylpiperazine (DMPpz), bromoethane (EtBr), and bromopropane (PrBr) were purchased from Sigma Aldrich and distilled before use. Acetic acid (P.O.Ch, Poland) was used as received. 4-nitroaniline, N,N-
Fig. 1. Synthesis procedure of acetate-based ILs. 90
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were obtained. Two ILs: N-ethyl-N-methylpyrrolidinium acetate ([EMPyrr][Ac]) and N-methyl-N-propylpyrrolidinium acetate ([MPPyrr][Ac]) were obtained as colourless liquids. N-ethyl-N-methylpiperidinium acetate ([EMPip][Ac]), N-methyl-N-propylpiperidinium acetate ([MPPip][Ac]), N,N′-dimethyl-N-ethylpiperazinium acetate ([DMEPpz][Ac]) and N,N′-dimethyl-N-propylpiperazinium acetate ([DMPPpz][Ac]) were obtained as white hygroscopic precipitate. The resulting ILs were obtained in good yield i.e. 90–95%. The amount of water in all synthesized ILs was found to be below 0.1%. The water content was measured by Karl Fischer titration method using a coulometric Karl–Fischer titrator (Metrohm 831 KF). NMR data of the ILs synthesized in this study are presented below:
0.5%) to the mixed solvent until the mixture became optically clear under a polarization microscope. When the cellulose solution remained turbid, even after extended dissolution time (+24 h), it was considered that system has reached the saturation point. It is worth noting that the viscosity of cellulose solutions increases during the addition of further amounts of MCC. As a consequence, the solvation of cellulose by mixed solvents becoming increasingly difficult. Thus, we can say that the increasing viscosity of investigated systems limits their cellulose dissolving ability. Cellulose solubility at given temperature was calculated from the amount of solvent and MCC added. The results were presented as the weight percent of cellulose vs. solvent mass.
2.3.1. N-ethyl-N-methylpyrrolidinium acetate ([EMPyrr][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 1.27 (t, J = 7.27,3H); 1.58 (s, 3H); 2.02–2.12 (m, 4H); 3.03 (s, 3H); 3.42–3.52 (m, 2H); 3.52–3.60 (m, 4H). 13C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 8.94; 21.04; 25.90; 46.62; 58.06; 62.66; 173.09.
2.5. Preparation and characterization of regenerated cellulose film As a representative, a certain amount of microcrystalline cellulose was dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) mixed solvent at 50 °C to obtain a 5 wt% cellulose solution. The homogenous solution was placed on a glass plate with a casting knife and then dipped in water. The regenerated cellulose film was further removed from the solution and washed with deionized water to ensure that IL/ DMSO solvent had been washed out. Finally, the cellulose film was removed from the water, put between two Teflon sheets, and dried at 80 °C for 24 h. The regenerated material was characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy.
2.3.2. N-methyl-N-propylpyrrolidinium acetate ([MPPyrr][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 0.96 (t, J = 7.34,3H); 1.70 (s, 3H); 1.70–1.85 (m, 2H); 2.10–2.20 (m, 4H); 3.03 (s, 3H); 3.25–3.35 (m, 2H); 3.45–3.60 (m, 4H). 13C NMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 10.94; 17.78; 22.12; 25.33; 48.73; 64.83; 65.96; 175.97. 2.3.3. N-ethyl-N-methylpiperidiniumacetate ([EMPip][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppmrelative to TMS): 1.22 (t, J = 7.30, 3H); 1.45–1.62 (m, 4H); 1.52 (s, 3H); 1.70–1.85 (m, 4H); 3.00 (s, 3H); 3.25–3.40 (m, 4H); 3.43 (q, J = 7.28, 2H). 13C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 7.04; 19.25; 20.69; 26.21; 46.16; 57.67; 59.23; 172.21.
3. Results and discussion 3.1. Dissolution of cellulose in the IL/DMSO solvents As mentioned in the Introduction, cellulose is insoluble in a majority of aqueous and organic solvents. However, some polar aprotic solvents including DMSO, DMF, DMAc may actually be considered as cosolvents when mixed with ILs. In this paper, the solubility of cellulose was investigated in mixtures of corresponding ILs with DMSO at three temperatures: 25, 50, and 80 °C. Different weight ratios of corresponding ILs in IL/DMSO mixed solvents were prepared to investigate how the addition of DMSO affects cellulose solubility. The effect of the IL’s cation type and alkyl chain length of IL’s cation on cellulose solubility was also investigated. The results of the cellulose dissolution capacities of the investigated IL/DMSO mixtures, determined at various temperatures are shown in Fig. 2. It is clearly visible that the solubility of microcrystalline cellulose in acetate-based IL/DMSO solvent systems increases with the increasing temperature, regardless of the IL type and IL concentration in IL/DMSO mixture. However, piperazinium-based IL solutions gave the best cellulose solubility performance among all tested systems, at both ambient and elevated temperature. [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) may dissolve maximum 9.5 wt% of MCC at 25 °C and even 13.5 wt% of MCC at 80 °C. On the contrary, pyrrolidinium-based mixtures dissolve cellulose effectively only under elevated temperature, reaching in the case of [EMPyrr][Ac]/DMSO (WIL:DMSO = 25%) 9.5 wt% cellulose solubility at 80 °C. In the context of the maximum possible amount of cellulose, as might be dissolved in IL/DMSO mixture, piperidiniumbased solvents seem to be the least effective systems. However, these solvents exhibited quite close cellulose solubility values over the whole range of the investigated IL weight ratios in IL/DMSO mixtures. This is particularly evident for [EMPip][Ac]/DMSO. As was said before, the efficiency of cellulose dissolution also depends on the amount of IL in IL/DMSO mixed solvents. For most of the investigated systems, MCC solubility increases with the increasing WIL:DMSO, until WIL:DMSO = 25% is reached. The further increase of the IL ratio in IL/DMSO mixture causes the decreasing in the maximum amount of cellulose, which can be dissolved in that mixture. The
2.3.4. N-methyl-N-propylpiperidiniumacetate ([MPPip][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppmrelative to TMS): 0.96 (t, J = 7.30, 3H); 1.55–1.75 (m, 4H); 1.68 (s, 3H); 1.75–1.90 (m, 4H); 3.05 (s, 3H); 3.25–3.45 (m, 6H). 13C NMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 10.92; 15.83; 20.49; 21.53; 25.58; 48.37; 61.38; 65.05; 175.63. 2.3.5. N,N′-dimethyl-N-ethylpiperaziniumacetate ([DMEPpz][Ac]) 1 H NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 1.25 (t, J = 7.22 Hz, 3H) 1.57 (s, 3H) 2.27 (s, 3H) 2.51–2.82 (m, 4H) 3.11 (s, 3H) 3.32–3.48 (m, 2H) 3.48–3.56 (m, 2H) 3.59 (q, J = 7.28 Hz, 2H). 13 C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 7.05, 26.11, 44.47, 47.58, 58.36, 172.82. 2.3.6. N,N′-dimethyl-N-propylpiperazinium Acetate ([DMPPpz][Ac]) 1 H NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 0.91 (t, J = 7.33, 3H); 1.52 (s, 3H); 1.60–1.80 (m, 2H); 2.27 (s, 3H); 2.55–2.80 (m, 4H); 3.08 (s, 3H); 3.35–3.55 (m, 6H). 13C NMR (400 MHz, DMSOd6, δ/ ppm relative to TMS): 10.51; 14.53; 26.31; 44.43; 47.48; 58.98; 172.21. 2.4. Preparation of IL/DMSO solvents Different weight ratios of corresponding ILs in IL/DMSO mixed solvents were prepared to investigate their maximum cellulose dissolution ability at 25, 50, and 80 °C (the weight ratio of IL in the mixed solvent system was denoted as WIL:DMSO). In a typical experiment, a certain amount of microcrystalline cellulose was added into a 4 ml glass scintillation vial that contained 1 g of IL/DMSO mixed solvent. Then the vial was sealed with parafilm and put in a thermostatic block of thermoshaker (Thermo Shaker TS100, NOVAZYM Poland). The mixture was heated and shaken at 600 rpm, at a given temperature until the cellulose has completely dissolved. MCC was gradually added (in portions of 91
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Fig. 2. The effect of temperature and IL weight ratio in IL/DMSO mixed solvents on the cellulose solubility (wt % vs. solvent).
exception is the behaviour of piperazinium-based IL mixtures. The increase of IL concentration to 50% do not significantly affect or improve MCC solubility in [DMEPpz][Ac]/DMSO or [DMPPpz][Ac]/DMSO, respectively. The influence of the alkyl chain length of IL's cation on cellulose solubility was also investigated. It was observed that the solubility of cellulose decreases with the increasing alkyl chain length for all examined mixtures. In fact, an odd-even effect was observed. Cellulose was more soluble in IL composed of cations with even-numbered ethyl chains compared to odd-numbered propyl chains. The similar effect was reported for neat dialkylimidazolium-based ILs [30,51], thus this observation is not surprising. Swatloski et al. attributed this effect to the reduced effective concentration of anions in IL/DMSO mixed solvents employing ILs with longer substituted cations or to the hydrophobic interactions between cations which might reduce their ability to screen the anion/cellulose complexes [18,21]. It is also worth mentioning that cellulose in contact with IL-based solvents could decompose more quickly than separately, under equivalent conditions. Thus, it is necessary to avoid excessive heating that induces cellulose pyrolysis [18]. Fig. 3 shows the picture of 3 samples containing, respectively, the untreated microcrystalline cellulose, [DMEPpz][Ac]/DMSO solvent mixture and 5 wt% cellulose solution in [DMEPpz][Ac]/DMSO mixture after treatment at 120 °C for 48 h. The Kamlet-Taft parameters π* and β of selected fourteen IL/DMSO mixed solvents were determined from the solvatochromic behaviour of two probes 4-nitroaniline (1) and N,N-diethyl-4-nitroaniline (2) [52,53], and were collected in Table 1. The DMSO, applied as cosolvent
Fig. 3. The picture of untreated microcrystalline cellulose (A), [DMEPpz][Ac]/ DMSO solvent mixture (B) and 5 wt% cellulose solution in [DMEPpz][Ac]/ DMSO mixture (C) after treatment at 120 °C for 48 h.
mixed with IL, is polar aprotic solvent (β = 0.76, α = 0.00). For all ILs synthesized in this study, the K-T parameters were determined for IL/ DMSO mixtures with 10 wt% IL content and also for the mixtures with the fixed molar ratio of IL:DMSO = 1:7. The aim of this choice was on the one hand to investigate the influence of IL content in the solvents mixture on the K-T-parameter values, and on the other hand to compare these parameters for various ILs. The molar ratio 1:7 for IL/DMSO mixtures has been chosen because it correspond best to 25 wt% IL content, which indicates the best performance in the dissolution of cellulose, as was demonstrated in the solubility studies performed in 92
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Table 1 The Kamlet-Taft parameters of the IL/DMSO mixtures. Solvent c-C6H12 DMSO [MPPyrr][Ac]/DMSO (10%) [MPPyrr][Ac]/DMSO (26%)c [MPPyrr][Ac]/DMSO (70%)d [MPPip][Ac]/DMSO (10%) [MPPip][Ac]/DMSO (27%)c [DMPPpz][Ac]/DMSO (10%) [DMPPpz][Ac]/DMSO (28%)c [EMPyrr][Ac]/DMSO (10%) [EMPyrr][Ac]/DMSO (24%)c [EMPyrr][Ac]/DMSO (69%)d [EMPip][Ac]/DMSO (10%) [EMPip][Ac]/DMSO (26%)c [DMEPpz][Ac]/DMSO (10%) [DMEPpz][Ac]/DMSO (27%)c a b c d
mol. rat.
1:7 1:1 1:7 1:7 1:7 1:1 1:7 1:7
ν(2)max [cm−1] 27,470 24,180 24,100 24,100 24,150 24,150 24,100 24,150 24,100 24,100 24,040 24,040 24,100 24,100 24,150 24,100
ν(1)max [cm−1]
−ΔΔν(1–2) [cm−1]
30,960 25,640 25,000 24,570 24,330 24,940 24,390 25,000 24,450 25,060 24,510 24,330 24,940 24,450 24,940 24,450
– – 2580 3010 3300 2690 3190 2630 3130 2520 3010 3190 2640 3130 2690 3130
π*
β a
0.00 1.00a 1.02 1.02 1.01 1.01 1.02 1.01 1.02 1.02 1.04 1.04 1.02 1.02 1.01 1.02
0.00a 0.76b 0.92 1.08 1.18 0.96 1.14 0.94 1.12 0.90 1.07 1.14 0.94 1.12 0.96 1.12
By definition. From [59]. Molar ratio IL:DMSO = 1:7. Molar ratio IL:DMSO = 1:1.
this paper. Additionally, K-T parameters of two IL/DMSO mixtures with the fixed molar ratio 1:1 (∼70 wt% IL content) were determined. The polarity/polarizability of IL/DMSO mixtures investigated in this study is generally very high, and the π* value is independent on the IL content in the mixture. The π* values determined of all studied IL/ DMSO mixtures are similar each other and they are slightly higher than that of DMSO and slightly lower than that of water. The π* values for IL/DMSO mixtures (S) were defined according to Eq.:
π ∗ (S) = [ν (S) − −ν (c−C6 H12)] / [ν (DMSO) − −ν (c−C6 H12)],
(1)
where ν (S) corresponds to the wavenumber of the maximum of the long-wavelength solvatochromic absorption band of N,N-diethyl-4-nitroaniline (2) in the IL/DMSO mixture (S). As the reference solvents were used cyclohexane and DMSO, by taking π* (c-C6H12) = 0.00 and π* (DMSO) = 1.00 by definition [37,53]. In contrast to the π* parameter, a basicity (β) of the studied IL/ DMSO mixed solvents depend on the IL content in the mixture and increases with this content increase. All 10 wt% IL content IL/DMSO mixtures has a basicity values of β varying from 0.90 to 0.96, and in case of these with ∼ 25 wt% IL content (the molar ratio of IL:DMSO = 1:7) the basicity is higher, β varying from 1.07 to 1.14. It could be noticed that for the mixtures with the fixed molar ratio, the lowest β values were determined for [MPPyrr][Ac]/DMSO and the [EMPyrr][Ac]/DMSO, which are characterized by the lowest cellulose solubility. The β values for IL/DMSO mixtures were determined by using the enhanced solvatochromic shift (−ΔΔν (1–2)) of 4-nitroaniline (1) relative to homomorphic N,N-diethyl-4-nitroaniline (2):
− ΔΔν (1 − −2) = 1.035·ν (2)max + 2.64 kK − ν (1)max
Fig. 4. The ATR-FTIR spectra the neat [DMEPPz][Ac]/DMSO and [DMEPPz] [Ac]/DMSO/cellulose system (WIL:DMSO = 25%).
to symmetric and asymmetric stretching vibrations of the carboxyl groups, respectively. Peaks associated with these vibrations became slightly weaker upon dissolution of cellulose. The reduction of the vibrations and red-shifting of the peak at the 1576 cm−1 can suggested that hydrogen bonds between the oxygen of IL’s acetate anions and cellulose were formed. We also investigated the interaction between cellulose and DMSO in the presence of [DMEPpz][Ac]. As also shown in Fig. 4, the intensity of the S = O vibration peak at 1021 cm−1 and C-S-C vibration peak at 952 cm−1 decreased upon dissolution of cellulose. Given this fact, it can be deduced that DMSO, apart from the reduction in the viscosity of solvent mixture, could interact with the hydroxyl groups on cellulose, and thereby stabilize the dissolved cellulose chains from the further reforming of their inter- and intramolecular hydrogen bonds [31].
(2)
β = [−ΔΔν (1 − −2)]/2.80,
3.2. Structure and morphology of regenerated cellulose film
where ν (1)max and ν (2)max are observed absorbance maxima for 4nitroaniline (1) and N,N-diethyl-4-nitroaniline (2), respectively [37,52]. The K-T parameters analysis for IL/DMSO mixtures investigated in this study indicates that the hydrogen-bond acceptor property (β) and not the polarity/polarizability (π*) of these solvents are essential from the point of view of searching for the high efficiency cellulose dissolved solvents. The ATR - FTIR spectra of neat IL/DMSO solvent system and saturated cellulosic solution in IL/DMSO solvent is presented in Fig. 4. In the discussion, we have focused on the band in the range of 1600–900 cm−1. The peaks at 1576 cm−1 and 1312 cm−1 correspond
The highest value of cellulose dissolution was obtained for [DMEPpz][Ac]-based ionic liquid. Therefore, [DMEPpz][Ac]/DMSO mixture (WIL:DMSO = 25%), which shows ca. 13 wt% of the cellulose dissolution ability was chosen to prepare the regenerated cellulose film. The cellulose film was prepared by method described in Experimental section. Fig. 5 shows the picture of the untreated microcrystalline cellulose powder (Fig. 5A) and the regenerated dry cellulose film prepared from the solution of 5 wt% cellulose concentration (Fig. 5B). As can be seen, the homogeneous and transparent film was obtained. In addition, this 93
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Fig. 5. Appearance of the untreated microcrystalline cellulose (A) and the regenerated cellulose film (B).
regenerated cellulose film does not contain pores above 1 μm. Additionally, BET surface of the regenerated cellulose film was measured. The specific surface of this material was below 0.1 m2 g−1. Fig. 7 shows the XRD spectra of the untreated microcrystalline cellulose (MCC) and regenerated cellulose (RC) obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C. It was shown that the XRD profiles of microcrystalline cellulose exhibit a typical cellulose I crystalline structure with three characteristic diffraction peaks at 2θ = 14.7–14.8°, 16.15° and 22.5° which are assigned to the (11¯0 ), (110 ) and (200 ) crystallographic planes, respectively. In comparison, the diffraction patterns of regenerated cellulose material show one weak peak at 2θ = 14.4° (1 1¯ 0 crystallographic plane) and two broadened peaks located in the range 2θ = 19.5–21.5°
material displayed flexibility and good mechanical properties. The regenerated material was characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy. The results have been compared with the data recorded for raw microcrystalline cellulose. Scanning electron microscopy (SEM) images of untreated microcrystalline cellulose (MCC) and regenerated cellulose (RC) are shown in Fig. 6. The regenerated cellulose film had highly smooth and uniform surface without obvious flaws (Fig. 6B and D) compared to the rough and scattered surface of untreated cellulosic fibres (Fig. 6A and C). In addition, uniform and dense structure can be seen in the cross-section of regenerated cellulose film. The magnified SEM images suggest that the
Fig. 6. SEM images of untreated microcrystalline cellulose (A, C) and regenerated cellulose (B, D) sample obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/ DMSO (WIL:DMSO = 25%). 94
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Fig. 9. The ATR-FTIR spectra of the untreated cellulose (MCC) and regenerated cellulose (RC). The regenerated cellulose was obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C.
raw cellulose material signifies O–H stretching vibration became broader and weaker upon regeneration from cellulosic solution, suggesting breaking of hydrogen bonds between hydroxyl groups upon dissolution [55,56]. The peak at 1427 cm−1 in regenerated cellulose film is associated with CH2 bending vibration. This band was weakened compared to the peak from untreated cellulose, indicating the destruction of an intra-molecular hydrogen bond involving O6 (oxygen atom of glucose unit) [57]. The absorption band at 890 cm−1 assigned to C-O-C stretching for the glycosidic band is clearly visible in both spectra as well as bands in the range 1155–1030 cm−1.7 A new, small peak observed at 990 cm−1 in FTIR spectra of regenerated cellulose can be assigned to C–O stretching vibration in the amorphous region [45]. Generally, slight blue shifts, increased peak width, enhanced peak strength, and newly occurred small peaks in the fingerprint region of the regenerated cellulose all demonstrate the breaking of hydrogen bonds and the loss of crystallinity in cellulose material [58]. Nevertheless, the similarity of both untreated and regenerated cellulose spectra supports the hypothesis of the inert nature of dissolution process of cellulose in IL/DMSO solvent system.
Fig. 7. The XRD patterns of the untreated cellulose (MCC) and regenerated cellulose (RC). The regenerated cellulose was obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C.
(1 1 0 and 2 0 0 crystallographic plane). The shifts and weakened intensity of peaks, indicating conversion from cellulose I to cellulose II crystalline structure for regenerated cellulose sample. This result showed that the crystal structure of microcrystalline cellulose was destroyed upon dissolution, leading to a more amorphous cellulose structure in the regenerated material. The crystallinity index (CI) was calculated from the peak-fitting data using the following equation [54]:
%CI =
∑ areaofcrystallinepeaks × 100% ∑ areaofcrystallineandamorphouspeaks
(3)
The determined crystallinity index values of original microcrystalline cellulose and regenerated cellulose film equal 49% and 15%, respectively. It reconfirms that the crystal structure of microcrystalline cellulose is destroying upon processing in IL/DMSO solvent. It is also a confirmation of the conclusions drawn from SEM results. In Fig. 8, the TGA (thermogravimetric analysis) curves of original microcrystalline cellulose (MCC) and regenerated cellulose film (RC) are presented. In the case of cellulose film, two regions of weight loss are observed in the TGA curve. As could be seen, the initial part of TGA curve (25–150 °C) presents the weight loss corresponding to the evaporation of the water content as a residual moisture of the sample. The major weight loss is observed above 250 °C, where degradation of the cellulose appears. The regenerated cellulose exhibits a lower onset temperature for decomposition compared to the original cellulose. Lower decomposition temperature may suggest a lower thermal stability caused by more amorphous structure in the samples [54] (The dominance of amorphous form in RC was proved by XRD analysis). The FTIR spectra of both untreated (MCC) and regenerated cellulose (RC) are shown in Fig. 9. The band in the range of 3000–3500 cm−1 for
4. Conclusions A series of substituted piperazinium, piperidinium, and pyrrolidinium acetates were synthesized. Cellulose dissolution in corresponding IL/DMSO mixtures was studied experimentally at 25, 50, and 80 °C. DMSO was used as a cosolvent in mixed solvents. It was observed that cosolvent enhances the performance of IL in the dissolution of cellulose. The effect of IL composition, temperature and alkyl chain length of IL’s cation on cellulose solubility was also investigated. It was observed that the solubility of cellulose in most IL/DMSO solvents increases with the increasing temperature and decreases with the increasing alkyl chain length. [DMEPpz][Ac]/DMSO was the most effective solvent for the dissolution of cellulose from among all studied solvent systems. This mixture solvent dissolves up to 9.5 wt% of MCC at 25 °C, and even 13.5 wt% when the temperature is raised to 80 °C. The highest values of the Kamlet-Taft parameters π* and β were obtained for the mixture that revealed the highest ability for cellulose dissolution. It confirms a close relationship between K-T parameters of cellulose solvents and the efficiency of cellulose solubility. The dissolved cellulosic mass was successfully regenerated from the most powerful solvent system, [DMEPpz][Ac]/DMSO, using water as an antisolvent. SEM, XRD, TGA and FTIR measurements of untreated and regenerated cellulose confirmed the hypothesis of the crystallinity change of cellulose upon its dissolution and as a consequence the amorphous structure gain in regenerated material. Conflict of interest
Fig. 8. Thermal stability profiles of untreated cellulose (MCC) and dry regenerated cellulose film (RC).
The author declares that there is no conflict of interests regarding the publication of this paper. 95
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Acknowledgements
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Dissolution of cellulose in novel carboxylate-based ionic liquids and dimethyl sulfoxide mixed solvents Dawid Kasprzaka, Ewa Krystkowiakb, Izabela Stępniaka, Maciej Galińskia, a b
T
⁎
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, ul. Berdychowo 4, 60965 Poznań, Poland Faculty of Chemistry, Adam Mickiewicz University in Poznań, ul.Umultowska 89b, 61-614 Poznań, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose ionic liquids (ILs) Acetate Piperazinium Piperidinium Pyrrolidinium
A series of substituted piperazinium, piperidinium, and pyrrolidinium acetates were synthesized. Highly effective cellulose solvents have been designed by mixing an aprotic polar solvent, dimethyl sulfoxide (DMSO), with synthesized ionic liquids (ILs). Different weight ratios of corresponding IL/DMSO mixed solvents were prepared to investigate the effect of cosolvent on cellulose solubility. Cellulose solubility in IL/DMSO solvent systems was measured at 25, 50 and, 80 °C. The effect of factors such as solvent composition, process temperature, type of IL’s cation, and alkyl chain length of IL’s cation on cellulose solubility was investigated. Amongst all tested solvents, N,N’-dimethyl-N-ethylpiperazinium acetate ([DMEPpz][Ac]/DMSO) mixed solvent exhibits the best performance in the dissolution of cellulose, reaching up to 13.5 wt% at 80 °C. The dissolved cellulose was regenerated using water as an antisolvent. The structure and morphology of the regenerated cellulose material were characterized by SEM, XRD, and FTIR, respectively.
1. Introduction Cellulose is an abundant biopolymer with fascinating structure and attractive properties such as biocompatibility, biodegradability, and thermal and chemical stability [1,2]. Cellulose is traditionally used in textile and paper-making industries, and recently it is a promising feedstock for biobased products and fuels [3–9]. In many applications, processing of cellulose requires the dissolution of cellulose. However, the intermolecular and intramolecular hydrogen-bonding network in cellulose chains makes this biopolymer hardly soluble in common solvents and limits its wide application [10]. Therefore, it is important to search for effective solvent systems for the dissolution of cellulose. The most important examples of traditional cellulose solvent systems are N-methylmorpholine-N-oxide (NMMO) [11], N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) [12], dimethyl sulfoxide/tetrabutylammonium fluoride (DMSO/TBAF) [13], molten salt hydrate LiClO4·3H2O [14], aqueous solutions of NaOH/urea [15]. All of these systems suffer from various drawbacks and should be replaced mainly because of environmental reasons [16]. In this context, ionic liquids have the potential to replace traditional cellulose solvents due to their dissolving abilities and desirable properties like negligible vapour pressure, low toxicity, high thermal and chemical stabilities, nonflammability, structure tenability, and recyclability [17]. The idea of the processing of cellulose in ionic liquids was born in ⁎
2002 [18]. The research group led by prof. Rogers reported promising results of cellulose solubility in dialkylimidazolium-based ionic liquids with chloride anions. Since then the increased interest in research on the application of ILs, especially imidazolium-based ILs in biomass processing has been observed. Previous studies have shown that compounds composed of cations based on aromatic imidazolium derivatives coupled with halides, carboxylates, or alkyl phosphates anions are the most promising ILs in the field of cellulose processing [19–23]. However, a strong association between the cations and anions makes ILs a highly viscous medium [24]. This property diminishes IL’s solvating power and, as a consequence, hinders the dissolution of large quantities of cellulose. This problem can be overcome by the concept of ILs mixing with polar aprotic cosolvents (such as dimethylsulfoxide (DMSO), dimethylacetamide (DMA), dimethylformamide (DMF), etc.), which was first proposed by Rinaldi et al. [25]. The newly developed solvent systems exhibited lower viscosity, higher dissolving rate, and lower cost compared to neat ILs. The addition of cosolvent accelerates mass transportation by decreasing the system’s viscosity. It also solvates ILs, leading to formation of more free ions, which interact with hydrogenbonding network of cellulose [26,27]. The improvement of the IL’s solvating power leads to the decreasing of the time needed for dissolution of cellulose, even at low temperatures. Ohira et al. found that cellulose could be effectively dissolved in the mixture of DMSO with
Corresponding author. E-mail address: [email protected] (M. Galiński).
https://doi.org/10.1016/j.eurpolymj.2019.01.053 Received 8 October 2018; Received in revised form 19 January 2019; Accepted 23 January 2019 Available online 24 January 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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diethyl-4-nitroaniline were obtained from Fluorochem. Cyclohexane (fluoresc., 99.7%) was obtained from Merck. Dimethyl sulfoxide (P.O. Ch. Poland), acetonitrile (Sigma Aldrich) chromatographic grade, hexane (P.O.Ch. Poland), tetrahydrofuran (Sigma Aldrich) were used as received. Anion exchange resin (Dowex® Monosphere 550 A) was obtained from Sigma Aldrich and washed several times with distilled water before use. NaOH (P.O.Ch, Poland) was used as received. Microcrystalline cellulose powder (MCC) was obtained from SigmaAldrich (20 μm, PD c.a. 250). The cellulose samples were used directly after being dried overnight at 80 °C.
quaternary ammonium-based ILs consisted with alanine anions, at 25 °C [28]. More recently, there have been reports of more IL-based solvent systems which effectively dissolve cellulose at ambient temperature [29–32]. It is also worth mentioning that the dual effects of polar aprotic solvents on the cellulose dissolution ability of IL-based mixed solvents may be observed. Zhang et al. [33] suggested that DMSO improves the dissolution of cellulose at low concentration by improving mass transportation and accelerating the cellulose solvating kinetics. Whereas, at high concentration, the opposite effect is observed. DMSO decreases the solubility of cellulose by weakening the ability of hydrogen bond creation between IL’s ions and cellulose. Polarity is an important property of ionic liquids, because the solubility of the solute, the reaction efficiency as solvent, and the miscibility with other solvents are influenced by the IL properties [34–36]. The classification of ILs with respect to their polarity/polarizability as well as hydrogen-bonding ability is most often performed on the basis of the linear solvation energy relationship proposed by Kamlet and Taft based on UV–Vis spectroscopic measurements [37,38]. This analysis specified the π* parameter to characterize the polarity/polarizability of a given solvent, which scale was selected to run from 0.0 for cyclohexane to 1.0 for dimethyl sulfoxide (DMSO) [39,40]. The solvents capable of hydrogen bond formation have been further classified as hydrogen-bond acceptors and hydrogen-bond donors. The hydrogenbond acceptor property of a solvent depends on its ability to accept a hydrogen atom from a solute to form a hydrogen bond (β), and the hydrogen-bond donor property depends on its ability to donate a hydrogen atom to form a hydrogen bond with a solute (α). The α scale was selected to extend from 0.0 for non-hydrogen-bond donor solvents to ∼ 1.0 for methanol and the β scale from 0.0 for cyclohexane to ∼ 1.0 for hexamethylphosphoramide [40]. Generally, ILs are polar solvents similar to short- or medium-chain alcohols. Anions have the greatest effect on the hydrogen-bond basicity, and the polarity of ILs depend to some extent on the polarity of their environment [20,41,42]. The hydrogen-bond basicity (β) of the solvent system is essential for the dissolution of cellulose [25,43]. Cellulose can be regenerated easily from cellulosic solution by adding antisolvents, such as water, ethanol, methanol, etc. Depending on the regeneration method, there are several possible forms of cellulose-based materials such as powders, films, fibres, gels, and composites [44–49]. If the cellulose processing process is a properly designed, there is a possibility to recycle of IL solvent. In the present work, we have investigated the dissolution of microcrystalline cellulose in the several ionic liquid/dimethyl sulfoxide solvent mixtures. For this purpose, firstly, we synthesized a series of different ionic liquids based on pyrrolidinium, piperidinium, and piperazinium derivatives with acetate anions. Further, each of the synthesized ILs were mixed with DMSO and used as a cellulose solvent. The solvents containing different concentrations of ILs were prepared to investigate the effect of varying amounts of cosolvent on the cellulose solubility. The effect of the temperature, IL’s cation type, and alkyl chain length of IL’s cation on the cellulose solubility was also investigated. Additionally, the structure and properties of the regenerated cellulose material obtained from the representative solvent, N,N′-dimethyl-N-ethylpiperazinium acetate ([EDMPpz][Ac])/DMSO were characterized by using various techniques. The results have been compared with the outcomes obtained for the raw microcrystalline cellulose.
2.2. Characterization The obtained ionic liquids were characterized by 1H and 13C NMR Spectroscopy. NMR spectra were recorded on a Varian VNMR-S 400 MHz spectrometer, in Acetonitriled3 or d6-DMSO. Tetramethylsilane (TMS) was used as an internal standard. The raw microcrystalline cellulose and the cellulose regenerated from IL/DMSO solvent were characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy. The SEM images were taken using a Hitachi S-3400N scanning electron microscope. The XRD analysis was performed using TUR M-62 diffractometer equipment with copper anticathode. Thermogravimetric measurements were carried out using a TGA 4000 Thermogravimetric Analyzer (PerkinElmer, USA). The FT IR measurements were performed with a Bruker Vertex 70 FT IR spectrometer equipped with ATR accessory. Kamlet-Taft measurements were performed on a UV–Vis spectrophotometer Jasco V-650 with a 10 mm path length quartz cuvette, at room temperature. 2.3. Ionic liquids synthesis procedure Ionic liquids were prepared in a two-step reaction which was described in more details elsewhere [50]. The first step was obtaining piperidinium, pyrrolidinium, and dimethylpiperazinium-based bromides: N-ethyl-N-methylpyrrolidinium bromide ([EMPyrr][Br]), Nmethyl-N-propylpyrrolidinium bromide ([MPPyrr][Br]), N-ethyl-Nmethylpiperidinium bromide ([EMPip][Br]), N-methyl-N-propylpiperidinium bromide ([MPPip][Br]), N,N’-dimethyl-N-ethylpiperazinium bromide ([DMEPpz][Br]), and N,N′-dimethyl-N-propylpiperazinium bromide ([DMPPpz][Br]). All bromides were obtained as white precipitates in high yield i.e. 95–98%. In the second stage, an aqueous solution of the corresponding bromide was passing through an anion exchange resin and finally neutralized with acetic acid by acid-base reaction. Fig. 1 presents the reaction scheme for acetate-based ionic liquids preparation. After water vaporization on a rotating vacuum drier, synthesized compounds were dried under vacuum at 60 °C for 24 h. Finally, six ILs
2. Experimental 2.1. Materials N-methylpyrrolidine (MPyrr), N-methylpiperidine (MPip), N,N′-dimethylpiperazine (DMPpz), bromoethane (EtBr), and bromopropane (PrBr) were purchased from Sigma Aldrich and distilled before use. Acetic acid (P.O.Ch, Poland) was used as received. 4-nitroaniline, N,N-
Fig. 1. Synthesis procedure of acetate-based ILs. 90
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were obtained. Two ILs: N-ethyl-N-methylpyrrolidinium acetate ([EMPyrr][Ac]) and N-methyl-N-propylpyrrolidinium acetate ([MPPyrr][Ac]) were obtained as colourless liquids. N-ethyl-N-methylpiperidinium acetate ([EMPip][Ac]), N-methyl-N-propylpiperidinium acetate ([MPPip][Ac]), N,N′-dimethyl-N-ethylpiperazinium acetate ([DMEPpz][Ac]) and N,N′-dimethyl-N-propylpiperazinium acetate ([DMPPpz][Ac]) were obtained as white hygroscopic precipitate. The resulting ILs were obtained in good yield i.e. 90–95%. The amount of water in all synthesized ILs was found to be below 0.1%. The water content was measured by Karl Fischer titration method using a coulometric Karl–Fischer titrator (Metrohm 831 KF). NMR data of the ILs synthesized in this study are presented below:
0.5%) to the mixed solvent until the mixture became optically clear under a polarization microscope. When the cellulose solution remained turbid, even after extended dissolution time (+24 h), it was considered that system has reached the saturation point. It is worth noting that the viscosity of cellulose solutions increases during the addition of further amounts of MCC. As a consequence, the solvation of cellulose by mixed solvents becoming increasingly difficult. Thus, we can say that the increasing viscosity of investigated systems limits their cellulose dissolving ability. Cellulose solubility at given temperature was calculated from the amount of solvent and MCC added. The results were presented as the weight percent of cellulose vs. solvent mass.
2.3.1. N-ethyl-N-methylpyrrolidinium acetate ([EMPyrr][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 1.27 (t, J = 7.27,3H); 1.58 (s, 3H); 2.02–2.12 (m, 4H); 3.03 (s, 3H); 3.42–3.52 (m, 2H); 3.52–3.60 (m, 4H). 13C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 8.94; 21.04; 25.90; 46.62; 58.06; 62.66; 173.09.
2.5. Preparation and characterization of regenerated cellulose film As a representative, a certain amount of microcrystalline cellulose was dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) mixed solvent at 50 °C to obtain a 5 wt% cellulose solution. The homogenous solution was placed on a glass plate with a casting knife and then dipped in water. The regenerated cellulose film was further removed from the solution and washed with deionized water to ensure that IL/ DMSO solvent had been washed out. Finally, the cellulose film was removed from the water, put between two Teflon sheets, and dried at 80 °C for 24 h. The regenerated material was characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy.
2.3.2. N-methyl-N-propylpyrrolidinium acetate ([MPPyrr][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 0.96 (t, J = 7.34,3H); 1.70 (s, 3H); 1.70–1.85 (m, 2H); 2.10–2.20 (m, 4H); 3.03 (s, 3H); 3.25–3.35 (m, 2H); 3.45–3.60 (m, 4H). 13C NMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 10.94; 17.78; 22.12; 25.33; 48.73; 64.83; 65.96; 175.97. 2.3.3. N-ethyl-N-methylpiperidiniumacetate ([EMPip][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppmrelative to TMS): 1.22 (t, J = 7.30, 3H); 1.45–1.62 (m, 4H); 1.52 (s, 3H); 1.70–1.85 (m, 4H); 3.00 (s, 3H); 3.25–3.40 (m, 4H); 3.43 (q, J = 7.28, 2H). 13C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 7.04; 19.25; 20.69; 26.21; 46.16; 57.67; 59.23; 172.21.
3. Results and discussion 3.1. Dissolution of cellulose in the IL/DMSO solvents As mentioned in the Introduction, cellulose is insoluble in a majority of aqueous and organic solvents. However, some polar aprotic solvents including DMSO, DMF, DMAc may actually be considered as cosolvents when mixed with ILs. In this paper, the solubility of cellulose was investigated in mixtures of corresponding ILs with DMSO at three temperatures: 25, 50, and 80 °C. Different weight ratios of corresponding ILs in IL/DMSO mixed solvents were prepared to investigate how the addition of DMSO affects cellulose solubility. The effect of the IL’s cation type and alkyl chain length of IL’s cation on cellulose solubility was also investigated. The results of the cellulose dissolution capacities of the investigated IL/DMSO mixtures, determined at various temperatures are shown in Fig. 2. It is clearly visible that the solubility of microcrystalline cellulose in acetate-based IL/DMSO solvent systems increases with the increasing temperature, regardless of the IL type and IL concentration in IL/DMSO mixture. However, piperazinium-based IL solutions gave the best cellulose solubility performance among all tested systems, at both ambient and elevated temperature. [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) may dissolve maximum 9.5 wt% of MCC at 25 °C and even 13.5 wt% of MCC at 80 °C. On the contrary, pyrrolidinium-based mixtures dissolve cellulose effectively only under elevated temperature, reaching in the case of [EMPyrr][Ac]/DMSO (WIL:DMSO = 25%) 9.5 wt% cellulose solubility at 80 °C. In the context of the maximum possible amount of cellulose, as might be dissolved in IL/DMSO mixture, piperidiniumbased solvents seem to be the least effective systems. However, these solvents exhibited quite close cellulose solubility values over the whole range of the investigated IL weight ratios in IL/DMSO mixtures. This is particularly evident for [EMPip][Ac]/DMSO. As was said before, the efficiency of cellulose dissolution also depends on the amount of IL in IL/DMSO mixed solvents. For most of the investigated systems, MCC solubility increases with the increasing WIL:DMSO, until WIL:DMSO = 25% is reached. The further increase of the IL ratio in IL/DMSO mixture causes the decreasing in the maximum amount of cellulose, which can be dissolved in that mixture. The
2.3.4. N-methyl-N-propylpiperidiniumacetate ([MPPip][Ac]) 1 HNMR (400 MHz, Acetonitriled3, δ/ppmrelative to TMS): 0.96 (t, J = 7.30, 3H); 1.55–1.75 (m, 4H); 1.68 (s, 3H); 1.75–1.90 (m, 4H); 3.05 (s, 3H); 3.25–3.45 (m, 6H). 13C NMR (400 MHz, Acetonitriled3, δ/ppm relative to TMS): 10.92; 15.83; 20.49; 21.53; 25.58; 48.37; 61.38; 65.05; 175.63. 2.3.5. N,N′-dimethyl-N-ethylpiperaziniumacetate ([DMEPpz][Ac]) 1 H NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 1.25 (t, J = 7.22 Hz, 3H) 1.57 (s, 3H) 2.27 (s, 3H) 2.51–2.82 (m, 4H) 3.11 (s, 3H) 3.32–3.48 (m, 2H) 3.48–3.56 (m, 2H) 3.59 (q, J = 7.28 Hz, 2H). 13 C NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 7.05, 26.11, 44.47, 47.58, 58.36, 172.82. 2.3.6. N,N′-dimethyl-N-propylpiperazinium Acetate ([DMPPpz][Ac]) 1 H NMR (400 MHz, DMSOd6, δ/ppm relative to TMS): 0.91 (t, J = 7.33, 3H); 1.52 (s, 3H); 1.60–1.80 (m, 2H); 2.27 (s, 3H); 2.55–2.80 (m, 4H); 3.08 (s, 3H); 3.35–3.55 (m, 6H). 13C NMR (400 MHz, DMSOd6, δ/ ppm relative to TMS): 10.51; 14.53; 26.31; 44.43; 47.48; 58.98; 172.21. 2.4. Preparation of IL/DMSO solvents Different weight ratios of corresponding ILs in IL/DMSO mixed solvents were prepared to investigate their maximum cellulose dissolution ability at 25, 50, and 80 °C (the weight ratio of IL in the mixed solvent system was denoted as WIL:DMSO). In a typical experiment, a certain amount of microcrystalline cellulose was added into a 4 ml glass scintillation vial that contained 1 g of IL/DMSO mixed solvent. Then the vial was sealed with parafilm and put in a thermostatic block of thermoshaker (Thermo Shaker TS100, NOVAZYM Poland). The mixture was heated and shaken at 600 rpm, at a given temperature until the cellulose has completely dissolved. MCC was gradually added (in portions of 91
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Fig. 2. The effect of temperature and IL weight ratio in IL/DMSO mixed solvents on the cellulose solubility (wt % vs. solvent).
exception is the behaviour of piperazinium-based IL mixtures. The increase of IL concentration to 50% do not significantly affect or improve MCC solubility in [DMEPpz][Ac]/DMSO or [DMPPpz][Ac]/DMSO, respectively. The influence of the alkyl chain length of IL's cation on cellulose solubility was also investigated. It was observed that the solubility of cellulose decreases with the increasing alkyl chain length for all examined mixtures. In fact, an odd-even effect was observed. Cellulose was more soluble in IL composed of cations with even-numbered ethyl chains compared to odd-numbered propyl chains. The similar effect was reported for neat dialkylimidazolium-based ILs [30,51], thus this observation is not surprising. Swatloski et al. attributed this effect to the reduced effective concentration of anions in IL/DMSO mixed solvents employing ILs with longer substituted cations or to the hydrophobic interactions between cations which might reduce their ability to screen the anion/cellulose complexes [18,21]. It is also worth mentioning that cellulose in contact with IL-based solvents could decompose more quickly than separately, under equivalent conditions. Thus, it is necessary to avoid excessive heating that induces cellulose pyrolysis [18]. Fig. 3 shows the picture of 3 samples containing, respectively, the untreated microcrystalline cellulose, [DMEPpz][Ac]/DMSO solvent mixture and 5 wt% cellulose solution in [DMEPpz][Ac]/DMSO mixture after treatment at 120 °C for 48 h. The Kamlet-Taft parameters π* and β of selected fourteen IL/DMSO mixed solvents were determined from the solvatochromic behaviour of two probes 4-nitroaniline (1) and N,N-diethyl-4-nitroaniline (2) [52,53], and were collected in Table 1. The DMSO, applied as cosolvent
Fig. 3. The picture of untreated microcrystalline cellulose (A), [DMEPpz][Ac]/ DMSO solvent mixture (B) and 5 wt% cellulose solution in [DMEPpz][Ac]/ DMSO mixture (C) after treatment at 120 °C for 48 h.
mixed with IL, is polar aprotic solvent (β = 0.76, α = 0.00). For all ILs synthesized in this study, the K-T parameters were determined for IL/ DMSO mixtures with 10 wt% IL content and also for the mixtures with the fixed molar ratio of IL:DMSO = 1:7. The aim of this choice was on the one hand to investigate the influence of IL content in the solvents mixture on the K-T-parameter values, and on the other hand to compare these parameters for various ILs. The molar ratio 1:7 for IL/DMSO mixtures has been chosen because it correspond best to 25 wt% IL content, which indicates the best performance in the dissolution of cellulose, as was demonstrated in the solubility studies performed in 92
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Table 1 The Kamlet-Taft parameters of the IL/DMSO mixtures. Solvent c-C6H12 DMSO [MPPyrr][Ac]/DMSO (10%) [MPPyrr][Ac]/DMSO (26%)c [MPPyrr][Ac]/DMSO (70%)d [MPPip][Ac]/DMSO (10%) [MPPip][Ac]/DMSO (27%)c [DMPPpz][Ac]/DMSO (10%) [DMPPpz][Ac]/DMSO (28%)c [EMPyrr][Ac]/DMSO (10%) [EMPyrr][Ac]/DMSO (24%)c [EMPyrr][Ac]/DMSO (69%)d [EMPip][Ac]/DMSO (10%) [EMPip][Ac]/DMSO (26%)c [DMEPpz][Ac]/DMSO (10%) [DMEPpz][Ac]/DMSO (27%)c a b c d
mol. rat.
1:7 1:1 1:7 1:7 1:7 1:1 1:7 1:7
ν(2)max [cm−1] 27,470 24,180 24,100 24,100 24,150 24,150 24,100 24,150 24,100 24,100 24,040 24,040 24,100 24,100 24,150 24,100
ν(1)max [cm−1]
−ΔΔν(1–2) [cm−1]
30,960 25,640 25,000 24,570 24,330 24,940 24,390 25,000 24,450 25,060 24,510 24,330 24,940 24,450 24,940 24,450
– – 2580 3010 3300 2690 3190 2630 3130 2520 3010 3190 2640 3130 2690 3130
π*
β a
0.00 1.00a 1.02 1.02 1.01 1.01 1.02 1.01 1.02 1.02 1.04 1.04 1.02 1.02 1.01 1.02
0.00a 0.76b 0.92 1.08 1.18 0.96 1.14 0.94 1.12 0.90 1.07 1.14 0.94 1.12 0.96 1.12
By definition. From [59]. Molar ratio IL:DMSO = 1:7. Molar ratio IL:DMSO = 1:1.
this paper. Additionally, K-T parameters of two IL/DMSO mixtures with the fixed molar ratio 1:1 (∼70 wt% IL content) were determined. The polarity/polarizability of IL/DMSO mixtures investigated in this study is generally very high, and the π* value is independent on the IL content in the mixture. The π* values determined of all studied IL/ DMSO mixtures are similar each other and they are slightly higher than that of DMSO and slightly lower than that of water. The π* values for IL/DMSO mixtures (S) were defined according to Eq.:
π ∗ (S) = [ν (S) − −ν (c−C6 H12)] / [ν (DMSO) − −ν (c−C6 H12)],
(1)
where ν (S) corresponds to the wavenumber of the maximum of the long-wavelength solvatochromic absorption band of N,N-diethyl-4-nitroaniline (2) in the IL/DMSO mixture (S). As the reference solvents were used cyclohexane and DMSO, by taking π* (c-C6H12) = 0.00 and π* (DMSO) = 1.00 by definition [37,53]. In contrast to the π* parameter, a basicity (β) of the studied IL/ DMSO mixed solvents depend on the IL content in the mixture and increases with this content increase. All 10 wt% IL content IL/DMSO mixtures has a basicity values of β varying from 0.90 to 0.96, and in case of these with ∼ 25 wt% IL content (the molar ratio of IL:DMSO = 1:7) the basicity is higher, β varying from 1.07 to 1.14. It could be noticed that for the mixtures with the fixed molar ratio, the lowest β values were determined for [MPPyrr][Ac]/DMSO and the [EMPyrr][Ac]/DMSO, which are characterized by the lowest cellulose solubility. The β values for IL/DMSO mixtures were determined by using the enhanced solvatochromic shift (−ΔΔν (1–2)) of 4-nitroaniline (1) relative to homomorphic N,N-diethyl-4-nitroaniline (2):
− ΔΔν (1 − −2) = 1.035·ν (2)max + 2.64 kK − ν (1)max
Fig. 4. The ATR-FTIR spectra the neat [DMEPPz][Ac]/DMSO and [DMEPPz] [Ac]/DMSO/cellulose system (WIL:DMSO = 25%).
to symmetric and asymmetric stretching vibrations of the carboxyl groups, respectively. Peaks associated with these vibrations became slightly weaker upon dissolution of cellulose. The reduction of the vibrations and red-shifting of the peak at the 1576 cm−1 can suggested that hydrogen bonds between the oxygen of IL’s acetate anions and cellulose were formed. We also investigated the interaction between cellulose and DMSO in the presence of [DMEPpz][Ac]. As also shown in Fig. 4, the intensity of the S = O vibration peak at 1021 cm−1 and C-S-C vibration peak at 952 cm−1 decreased upon dissolution of cellulose. Given this fact, it can be deduced that DMSO, apart from the reduction in the viscosity of solvent mixture, could interact with the hydroxyl groups on cellulose, and thereby stabilize the dissolved cellulose chains from the further reforming of their inter- and intramolecular hydrogen bonds [31].
(2)
β = [−ΔΔν (1 − −2)]/2.80,
3.2. Structure and morphology of regenerated cellulose film
where ν (1)max and ν (2)max are observed absorbance maxima for 4nitroaniline (1) and N,N-diethyl-4-nitroaniline (2), respectively [37,52]. The K-T parameters analysis for IL/DMSO mixtures investigated in this study indicates that the hydrogen-bond acceptor property (β) and not the polarity/polarizability (π*) of these solvents are essential from the point of view of searching for the high efficiency cellulose dissolved solvents. The ATR - FTIR spectra of neat IL/DMSO solvent system and saturated cellulosic solution in IL/DMSO solvent is presented in Fig. 4. In the discussion, we have focused on the band in the range of 1600–900 cm−1. The peaks at 1576 cm−1 and 1312 cm−1 correspond
The highest value of cellulose dissolution was obtained for [DMEPpz][Ac]-based ionic liquid. Therefore, [DMEPpz][Ac]/DMSO mixture (WIL:DMSO = 25%), which shows ca. 13 wt% of the cellulose dissolution ability was chosen to prepare the regenerated cellulose film. The cellulose film was prepared by method described in Experimental section. Fig. 5 shows the picture of the untreated microcrystalline cellulose powder (Fig. 5A) and the regenerated dry cellulose film prepared from the solution of 5 wt% cellulose concentration (Fig. 5B). As can be seen, the homogeneous and transparent film was obtained. In addition, this 93
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Fig. 5. Appearance of the untreated microcrystalline cellulose (A) and the regenerated cellulose film (B).
regenerated cellulose film does not contain pores above 1 μm. Additionally, BET surface of the regenerated cellulose film was measured. The specific surface of this material was below 0.1 m2 g−1. Fig. 7 shows the XRD spectra of the untreated microcrystalline cellulose (MCC) and regenerated cellulose (RC) obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C. It was shown that the XRD profiles of microcrystalline cellulose exhibit a typical cellulose I crystalline structure with three characteristic diffraction peaks at 2θ = 14.7–14.8°, 16.15° and 22.5° which are assigned to the (11¯0 ), (110 ) and (200 ) crystallographic planes, respectively. In comparison, the diffraction patterns of regenerated cellulose material show one weak peak at 2θ = 14.4° (1 1¯ 0 crystallographic plane) and two broadened peaks located in the range 2θ = 19.5–21.5°
material displayed flexibility and good mechanical properties. The regenerated material was characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared (FTIR) spectroscopy. The results have been compared with the data recorded for raw microcrystalline cellulose. Scanning electron microscopy (SEM) images of untreated microcrystalline cellulose (MCC) and regenerated cellulose (RC) are shown in Fig. 6. The regenerated cellulose film had highly smooth and uniform surface without obvious flaws (Fig. 6B and D) compared to the rough and scattered surface of untreated cellulosic fibres (Fig. 6A and C). In addition, uniform and dense structure can be seen in the cross-section of regenerated cellulose film. The magnified SEM images suggest that the
Fig. 6. SEM images of untreated microcrystalline cellulose (A, C) and regenerated cellulose (B, D) sample obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/ DMSO (WIL:DMSO = 25%). 94
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Fig. 9. The ATR-FTIR spectra of the untreated cellulose (MCC) and regenerated cellulose (RC). The regenerated cellulose was obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C.
raw cellulose material signifies O–H stretching vibration became broader and weaker upon regeneration from cellulosic solution, suggesting breaking of hydrogen bonds between hydroxyl groups upon dissolution [55,56]. The peak at 1427 cm−1 in regenerated cellulose film is associated with CH2 bending vibration. This band was weakened compared to the peak from untreated cellulose, indicating the destruction of an intra-molecular hydrogen bond involving O6 (oxygen atom of glucose unit) [57]. The absorption band at 890 cm−1 assigned to C-O-C stretching for the glycosidic band is clearly visible in both spectra as well as bands in the range 1155–1030 cm−1.7 A new, small peak observed at 990 cm−1 in FTIR spectra of regenerated cellulose can be assigned to C–O stretching vibration in the amorphous region [45]. Generally, slight blue shifts, increased peak width, enhanced peak strength, and newly occurred small peaks in the fingerprint region of the regenerated cellulose all demonstrate the breaking of hydrogen bonds and the loss of crystallinity in cellulose material [58]. Nevertheless, the similarity of both untreated and regenerated cellulose spectra supports the hypothesis of the inert nature of dissolution process of cellulose in IL/DMSO solvent system.
Fig. 7. The XRD patterns of the untreated cellulose (MCC) and regenerated cellulose (RC). The regenerated cellulose was obtained from 5 wt% cellulose dissolved in [DMEPpz][Ac]/DMSO (WIL:DMSO = 25%) at 50 °C.
(1 1 0 and 2 0 0 crystallographic plane). The shifts and weakened intensity of peaks, indicating conversion from cellulose I to cellulose II crystalline structure for regenerated cellulose sample. This result showed that the crystal structure of microcrystalline cellulose was destroyed upon dissolution, leading to a more amorphous cellulose structure in the regenerated material. The crystallinity index (CI) was calculated from the peak-fitting data using the following equation [54]:
%CI =
∑ areaofcrystallinepeaks × 100% ∑ areaofcrystallineandamorphouspeaks
(3)
The determined crystallinity index values of original microcrystalline cellulose and regenerated cellulose film equal 49% and 15%, respectively. It reconfirms that the crystal structure of microcrystalline cellulose is destroying upon processing in IL/DMSO solvent. It is also a confirmation of the conclusions drawn from SEM results. In Fig. 8, the TGA (thermogravimetric analysis) curves of original microcrystalline cellulose (MCC) and regenerated cellulose film (RC) are presented. In the case of cellulose film, two regions of weight loss are observed in the TGA curve. As could be seen, the initial part of TGA curve (25–150 °C) presents the weight loss corresponding to the evaporation of the water content as a residual moisture of the sample. The major weight loss is observed above 250 °C, where degradation of the cellulose appears. The regenerated cellulose exhibits a lower onset temperature for decomposition compared to the original cellulose. Lower decomposition temperature may suggest a lower thermal stability caused by more amorphous structure in the samples [54] (The dominance of amorphous form in RC was proved by XRD analysis). The FTIR spectra of both untreated (MCC) and regenerated cellulose (RC) are shown in Fig. 9. The band in the range of 3000–3500 cm−1 for
4. Conclusions A series of substituted piperazinium, piperidinium, and pyrrolidinium acetates were synthesized. Cellulose dissolution in corresponding IL/DMSO mixtures was studied experimentally at 25, 50, and 80 °C. DMSO was used as a cosolvent in mixed solvents. It was observed that cosolvent enhances the performance of IL in the dissolution of cellulose. The effect of IL composition, temperature and alkyl chain length of IL’s cation on cellulose solubility was also investigated. It was observed that the solubility of cellulose in most IL/DMSO solvents increases with the increasing temperature and decreases with the increasing alkyl chain length. [DMEPpz][Ac]/DMSO was the most effective solvent for the dissolution of cellulose from among all studied solvent systems. This mixture solvent dissolves up to 9.5 wt% of MCC at 25 °C, and even 13.5 wt% when the temperature is raised to 80 °C. The highest values of the Kamlet-Taft parameters π* and β were obtained for the mixture that revealed the highest ability for cellulose dissolution. It confirms a close relationship between K-T parameters of cellulose solvents and the efficiency of cellulose solubility. The dissolved cellulosic mass was successfully regenerated from the most powerful solvent system, [DMEPpz][Ac]/DMSO, using water as an antisolvent. SEM, XRD, TGA and FTIR measurements of untreated and regenerated cellulose confirmed the hypothesis of the crystallinity change of cellulose upon its dissolution and as a consequence the amorphous structure gain in regenerated material. Conflict of interest
Fig. 8. Thermal stability profiles of untreated cellulose (MCC) and dry regenerated cellulose film (RC).
The author declares that there is no conflict of interests regarding the publication of this paper. 95
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Acknowledgements
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