Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Functionalized Imidazalium Carboxylates for Enhancing Practical Applicability in Cellulose Processing Airong Xu,*,† Lin Chen,† and Jianji Wang*,‡ †
School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan 471003, P. R. China ‡ School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, P. R. China S Supporting Information *
ABSTRACT: Developing cellulose-based products is highly important because of their low-cost, reproducibility, and biodegradability. However, extensive application for cellulose has been actually hindered due to its well-known insolubility. Herein, some 22 novel functionalized imidazalium carboxylates exhibit tremendously enhanced dissolution capacity for cellulose even without extra energy consumption and are much superior to the previously reported solvents so far. Systematic investigations reveal that the powerful dissolution capacity for cellulose mainly results from the contribution of the imidazolium skeleton cation, not replacing acidic H atoms in imidazolium skeleton by alkyl, binding more allyl in N atoms of imidazolium cation, and binding an electron-donating group in carboxylate anion. Of particular importance, porous cellulose materials with varying micromorphology, for the first time, are reported by tuning the anionic and/or cationic structures of an IL. Moreover, the regenerated cellulose material retains sufficient thermostability and chemical structure. Therefore, this investigation provides a viable strategy for practical application in cellulose conversion into valuable products even without extra heating.
1. INTRODUCTION Cellulose, the most abundant biosynthesized polymer on earth, has been regarded as a potential alternative to oil resource for fuels, chemicals and materials due to its low-cost, reproducibility and biodegradability.1 However, the conversion of cellulose into valuable products still remains challenging in that cellulose is recalcitrant toward dissolution due to its highly ordered structure.2−4 For this reason, efforts to develop efficient solvents for cellulose dissolution has been going on in scientific community. Conventional cellulose dissolution includes the viscose and cuprammonium process which have environmental problems.5,6 The other developed solvents include aqueous N-methylmorpholine-N-oxide solvent, LiCl/ N,N-dimethylacetamide solvent, tetrabutyl ammonium fluoride/dimethyl sulfoxide solvent, N2O4/N,N-dimethylformamide, and LiClO4·3H2O.7−10 However, these solvent systems suffer from such drawbacks as environmental toxicity, inferior dissolution capacity, difficulty in solvent recovery, harsh processing conditions or high cost. Later on, some novel solvents with unique properties were investigated including NaOH/urea(thiourea) aqueous solution,11,12 ionic liquids (ILs),13 dimethyl sulfoxide/1,8-diazabicyclo-[5.4.0]-undec-7ene (DMSO/DBU),14 1,1,3,3-tetramethyl guanidine/ethylene glycol/dimethyl sulfoxide (TMG/EG/DMSO).15 Among these novel solvents, more attention has been paid to ILs due to their unique properties such as negligible vapor © XXXX American Chemical Society
pressure, nonflammability, high chemical and thermal stability, and strong dissolution ability for various organic and inorganic materials.16−21 Especially importantly, the desired properties of ILs can be easily modulated by finely tuning structures of their cations and/or anions. In light of the outstanding properties of ILs as well as the demand for replacing traditional solvents, persistent efforts have been made in developing functionalized ILs which have the ability to efficiently dissolve cellulose. In this regard, the first two reported ILs are 1-N-butyl-3-methylimidazolium chloride and 1-N-allyl-3-methylimidazolium chloride which can gain 10% cellulose solubility at 100 °C and 14.5 wt % cellulose concentration at 80 °C.22,23 Compared with imidazolium-based chloride, imidazolium-based carboxylates and alkylphosphates displayed better dissolution performance for cellulose.24−29 The ease of cellulose dissolution in these ILs primarily results from the effective hydrogen bond disruption in cellulose by the interaction of the anions and cations of the ILs with hydroxyl protons and oxygen of cellulose, respectively. However, neat ILs are well-known to be viscous, leading to the difficult dispersion of cellulose in them. As a result, complete cellulose dissolution generally required long residue time or a rise in temperature which can give rise to a degradation of the Received: April 5, 2018 Revised: May 14, 2018
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DOI: 10.1021/acs.macromol.8b00724 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. Schematic structure of the ILs investigated.
material from cellulose + IL solution was eximined to explore the viability of the modulation of the micromorphologic structure of the cellulose material by tuning IL structure. Additionally, the regenerated cellulose materials were characterized to estimate their thermostability and structure.
ILs and/or cellulose. Some efforts have been made to better the efficacy of cellulose dissolution in ILs. Correspondingly, novel IL + cosolvent systems for cellulose were designed with the outstanding advantages of low dissolution temperature, high cellulose solubility, rapid dissolution rate, and negligible degradation for the regenerated cellulose from them.30−35 However, the issues in relation to IL + cosolvent systems are the lack of an effective recovery technique toward the solvents. To overcome the extant issues in relative to IL-based solvent systems and improve their practical applicability, developing novel ILs endowed with distinguishable advantages such as not only considerably enhanced cellulose dissolution capacity without providing extra energy (e.g., heating or freezing) but also low viscosity and recoverability are of great importance. To this end, 22 functionalized ILs have been prepared (Figure 1). Eight of them have the same 1,3-diallylimidazolium cation [A2mim]+ but varied anions to which different functional groups were bounded. Twelve other ILs have a fixed methoxyacetate anion [CH3OCH2COO]− but varied cationic skeleton structure, alkyl chain length, alkyl chain number, and type of functional group. The choice of [CH3OCH2COO]− is based on its ability to form low-viscosity ILs, since the highly flexible alkoxy chains do not pack as efficiently as alkyl chains, according to molecular dynamics simulations.36 Another 2 ILs are 1-carboxypropyl-3-allylimidazolium chloride [HOOCCH 2CH 2Aim]Cl and 1-propyl-3-allylimidazolium chloride [C3Aim]Cl. Cellulose solubilities in the functionalized imidazalium carboxylate ILs were determined at different temperatures. Furthermore, the comprehensive investigations, based on the effect of cationic skeleton, acidic H atom and allyl number (for imidazolium skeleton), alkyl number, alkyl chain length, functional group type, and functional group bulk on cellulose dissolution together with 13C NMR spectra, have been made to unveil why some ILs easily dissolve cellulose, but the others difficultly do. For the first time, the effect of the IL structure on the micromorphologic structure of the cellulose
2. MATERIALS AND METHODS 2.1. Materials. Microcrystalline cellulose, with a 275 of viscosityaverage degree of polymerization (DP), was purchased from SigmaAldrich Company. 1-Allylimidazole (99%), methoxyacetlc acid (98%), ethoxyacetic acid (98%), glycolic acid (98%), 1-bromoethane (98%), 1,2-dimethylimidazole (98%), allyl chloride (98%), 2-bromoethanol (97%), 2-bromoethyl ethyl ether (97%), and anion exchange resin (Ambersep 900 OH) were from Alfa Aesar. 1-Butyl bromide (99%) and carboxy propyl chloride (98%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Acetic acid (99.5%), propanoic acid (99%), and butyric acid (99%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. 1-Allyl-3-methylimidazolium chloride (99%) was purchased from Linzhou Branch can Material Technology Co., Ltd. 1,2,4,5-Tetramethylimidazole (98%) was purchased from Tokyo Kasei Kogyo Co., Ltd. 1-Butyl-1-methylpiperidinium bromide (99%), N-butyl-N-methylpyrrolidinium bromide (99%), N-butylpiperidine bromide (99%), and 4-methyl-N-butylpyridine bromide (99%) were purchased from Lanzhou Institute of Chemical Physics (LICP) of the Chinese Academy of Sciences. 2.2. Synthesis of ILs. [A2mim]Cl was synthesized and purified using a reported procedure previously.23 Briefly, 1-allylimidazole was reacted with an excess of allyl chloride at 50 °C for 48 h. [A2mim]Cl product was washed with ethyl acetate and then filtrated. The residual ethyl acetate in [A2mim]Cl product was removed by rotary evaporation. The resultant A2mim]Cl was dried at reduced pressure at 50 °C for 24 h at the presence of P2O5. As an example, [A2mim][OH] aqueous solution was gained by passing [A2mim]Cl aqueous solution through a column filled with anion exchange resin. Then, the aqueous [A2mim][OH] solution reacted with equal molar methoxyacetlc acid. The gained aqueous [A2mim][CH3OCH2COO] solution was processed with activated carbon 2−3 times to remove possible residual impurities. [A2mim][CH3OCH2COO] was obtained by removing water by rotary evaporation and then dried at reduced pressure at 55 °C for more B
DOI: 10.1021/acs.macromol.8b00724 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Solubility of Cellulose in 1,3-Diallylimidazolium Carboxylate ILs cellulose solubility (g/100 g of IL)
a
entry
IL
20 °C
25 °C
30 °C
40 °C
50 °C
1 2 3 4 5 6 7 8
[A2im][CH3COO] [A2im][CH3CH2COO] [A2im][CH3CH2CH2COO] [A2im][CH2CHCOO] [A2im][CH3OCH2COO] [A2im][CH3CH2OCH2COO] [A2im][HOCH2COO] [A2im][(C5H4N)COO]
0.5 a 0.2 a 3.3 a a a
15.0 a 1.3 20.3 16.2 3.0 a a
17.0 a 18.2 20.5 20.5 3.5 0.3 a
18.5 a 21.1 21.0 22.0 18.2 0.4 a
19.5 22.8 21.4 23.0 25.7 19.3 1.2 a
Insoluble at given temperature.
than 24 h at the presence of P2O5. All other carboxylate ILs were synthesized using a similar procedure to [A2mim][CH3OCH2COO]. 2.3. Solubility of Cellulose. For [A2mim][CH3OCH2COO] for instance, the cellulose solubility was determined as follows. About 2.0 g of dried [A2mim][CH3OCH2COO] was added to a 20 mL colorimetric tube. The tube was then immersed in an oil bath (DF101S, Gongyi Yingyu Instrument Factory), and the temperature instability of the oil bath was ±0.5 °C. After the temperature reached 20 °C, cellulose (0.1 g cellulose per 100 g of [A 2 mim][CH3OCH2COO]) was added to the tube, and the mixture was stirred. Additional cellulose was added after the solution became optically clear under polarization microscope (Nanjing Jiangnan Novel Optics Co. Ltd.). The above procedure was repeated until no more cellulose was dissolved further for more than 2 h. Cellulose solubility (expressed by g/100 g of [A2mim][CH3OCH2COO]) at 20 °C was calculated based on the amount of [A2mim][CH3OCH2COO] and cellulose added. 2.4. Measurements of 13C NMR Spectra. 13C NMR spectra for neat [A2im][CH3COO], [A2im][CH3OCH2COO] and [CH3CH2OCH2CH2Aim][CH3OCH2COO] in the pure liquid and in [A2im][CH3COO]/cellulose (8.0 wt %), [A2im][CH3OCH2COO]/cellulose (8.0 wt %), and [CH3CH2OCH2CH2Aim][CH3OCH2COO]/cellulose (8.0 wt %) solutions were determined on a Bruker Avance-500 NMR spectrometer at room temperature, respectively. DMSO-d6 was used as an external standard for the 13C NMR spectra. Chemical shifts were given in ppm downfield from TMS. 2.5. Preparation of Cellulose Porous Materials CPM-1, CPM2, CPM-3, and CPM-4. To prepare CPM-1, 5 wt % of cellulose solution was spread onto a glass plate, and the thickness of the liquid film was about 2 mm. The air bubble in the liquid film was taken off in a vacuum oven at room temperature for 30 min. Then, the plate containing the liquid film was immediately immersed in a coagulation bath of water. The coagulated cellulose film was washed repeatedly with distilled water, and then frozen for 2 h in a refrigerator. The frozen cellulose film was freeze-dried using a FD-10 freeze-dryer to obtain CPM-1. The cold trap temperature was below −45 °C and the vacuum pressure was below 0.1 MPa during the freeze-drying process. The cellulose porous materials CPM-2 and CPM-3 were prepared by using the same procedure with CPM-1. The difference is that, in the preparation process of CPM-2 and CPM-3, [A2im][CH3COO] was replaced by [A2im][CH3OCH2COO] and [C4mim][CH3COO], respectively. At the same time, the cellulose porous material CPM-4 was also fabricated using a similar procedure to CPM-1. The difference is that about 2 mm of cellulose solution film was frozen at −80 °C for 2 h before it was immersed in distilled water for coagulation. In addition, detailed characterization information such as scanning electron micrographs (SEM), ATR-FTIR spectra, and thermogravimetric analyses for the regenerated cellulose materials was placed in the Supporting Information.
1,3-diallylimidazolium carboxylate ILs as a function of temperature. Although having the same [A2mim]+ cation, significant differences of cellulose solubility in the ILs are noticeable with varied anionic structure. The ILs with [CH 3 COO] − , [CH3CH2COO]−, [CH3CH2CH2COO]−, [CH2CHCOO]−, [CH3OCH2COO]−, and [CH3CH2OCH2COO]− as anions exhibit powerful dissolution capacity for cellulose. This is mainly due to the increased capacity of the hydrogen bonding of these anions with the OH protons in cellulose because of the electron-donating effect of X group in the [XCOO]− (X = CH3, CH2CH3, CH2CH2CH3, CHCH2, CH2OCH3, CH2OCH2CH3) anion.27,28 However, cellulose is not soluble in [A2im][(C5H4N)COO]. This primarily results from the strong electron-withdrawing effect of C5H4N unit in the [(C5H4N)COO]− anion, leading to the decreased electron cloud density of O1 in [(C5H4N)COO]− anion (see Figure 1). Consequently, the hydrogen bond formation capacity of [(C5H4N)COO]− anion with the OH protons in cellulose decreases. Hence, the hydrogen bonds in cellulose are not disrupted, and cellulose is not dissolved. In addition, the difficulty in access to the hydroxyl of cellulose because of the bulky anionic skeleton of [(C5H4N)COO]− can disable the anion to disrupt the hydrogen bonds in cellulose.29 The replacement of H in [CH3COO]−of [A2im][CH3COO] by OH group also causes a decrease in cellulose solubility. Consequently, the solubility of cellulose in [A2im][HOCH2COO] is much less than that in [A2im][CH3COO] (see Table 1). This results mainly from the decrease of the ability of the hydrogen bonding of [HOCH2COO]− anion with the OH protons in cellulose.27,28 These findings indicate that the anionic structure considerably impacts cellulose dissolution, which is similar to 1-butyl-3-methylimidazolium carboxylate ILs ([C4mim][XCOO], X = H, CH3, HOCH2, H2NCH2, HSCH2, CH3CHOH, (C6H5)).27 It is also interesting to find that, the dissolution capacity of [A2im][CH3COO], [A2im][CH2CHCOO] and [A2im][CH3OCH2COO] for cellulose is tremendously advantageous over the reported ILs such as 1-butyl/allyl--3-methylimidazolium and carbohydrtates,27,28 1,3-dialkylimidazolium formates,241-ethyl-3-methylimidazolium dimethylphosphates,25 and alkylimidazolium-based chlorides.22,23 Among the reported ILs, at 25 °C, only 1-ethyl-3-methylimidazolium dimethylphosphates gives 4.0% cellulose solubility, but cellulose is not soluble in the other ILs. Moreover, a comparison reveals that more allyl substituent in imidazolium cation contributes to higher cellulose solubility (see Figure S1). For example, the cellulose solubility at 25 °C is as high as 15.0% in [A2im][CH3COO] and less than 0.5% in 1allyl-3-methylimidazolium acetate [Amim][CH3COO], and
3. RESULTS AND DISCUSSION 3.1. Influence of Anionic Structure on Cellulose Solubility. Table 1 gives the cellulose solubility values in C
DOI: 10.1021/acs.macromol.8b00724 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Solubility of Cellulose in Methoxyacetate-Based ILs cellulose solubility (g/100 g of IL)
a
entry
IL
20 °C
25 °C
30 °C
40 °C
50 °C
60 °C
1 2 3 4 5 6 7 8 9 10 11 12 13
[A2im][CH3OCH2COO] [C1Aim][CH3OCH2COO] [C2Aim][CH3OCH2COO] [C4Aim][CH3OCH2COO] [(CH3)2Aim][CH3OCH2COO] [(CH3)4Aim][CH3OCH2COO] [CH3CH2OCH2CH2Aim][CH3OCH2COO] [HOCH2CH2Aim][CH3OCH2COO] [C4mim][CH3OCH2COO] [N(C4C1)Py][CH3OCH2COO] [N(C4C1)Pp][CH3OCH2COO] [N(C4)Pyr][CH3OCH2COO] [C1N(C4)Pyr][CH3OCH2COO]
3.3 a 1.1 a c c a a a c c a a
16.2 14.0 20.6b a c c a a a c c a a
20.5 14.2 20.6b a c c a a a c c a a
22.0 16.0 20.6b 1.1 c c 9.7 a a 1.2 c a a
25.7 17.2 20.6b 20.5 21.5 c 14.5 a 19.5 15.2 a 4.0 a
d d d d d c d d 22.0 d a 4.2 a
Insoluble at given temperature. bClot at given temperature. cSolid at given temperature. dWithout determining cellulose solubility.
adding a methyl CH3 in pyridine cation (see Table 2). It was reported that the substitution of the hydrogen atom of 2 position in imidazolium cation by a CH3 resulted in decreased cellulose solubility.29 For instance, the cellulose solubilities in 1butyl-2,3-dimethylimidazolium acetate ([C 4 dmim][CH3COO]) is markedly less than those in [C4mim][CH3COO]. However, attractively, for the allylimidazoliumbased ILs in this work, the cellulose solubility instead increased after the hydrogen atom of 2 position in imidazolium cation was substituted by a CH3. For example, the solubility of cellulose in [(CH3)2Aim][CH3OCH2COO] (21.5%) is observably higher than that in [C1Aim][CH3OCH2COO](17.2%) at 40 °C. This implies that not only the acidic protons but also the allyl in imidazolium cationic skeleton contribute to cellulose dissolution. This is the very reason why [A 2 im][CH3OCH2COO] with 2 allyl units displays better dissolution capacity for cellulose than [C1 Aim][CH 3 OCH 2 COO], [C2Aim][CH3OCH2COO], [(CH3)2Aim][CH3OCH2COO], [C4Aim][CH3OCH2COO], and [C4mim][CH3OCH2COO] with 1 or none allyl unit. This further verifies the above result that the allyl instead of the saturated alkyl in imidazolium cation is more propitious to enhancing cellulose dissolution. Of course, we should also recognize the fact that more acidic hydrogen atoms in cation are also more favorable to enhancing cellulose dissolution. We try to further increase the CH3 number in imidazolium cation to investigate the effect on cellulose dissolution. However, we found that the further increase in CH3 number caused an increase in melting point. For example, [(CH3)4Aim][CH3OCH2COO] with 4 CH3 is solid within 60 °C. The functional group bonded to the nitrogen atom in imidazolium ring influences cellulose solubility. For instance, the replacement of CH 2 CH = CH 2 in [A 2 im] + by CH2CH2OCH2CH3 and CH2CH2OH leads to a decreased cellulose solubility (Table 2). This may be due to the donating electron nature of oxygen in CH3CH2OCH2CH3 and OH groups. The hydroxyl proton of CH2CH2OH may interacts with the oxygen in the anion by hydrogen bonding,37 and the oxygen in CH3CH2OCH2CH3 can interacts with the acidic hydrogen atom in imidazolium ring by forming hydrogen bond.38 The former impairs the capacity of the hydrogen bonding of the oxygen in the anion with the OH proton in cellulose, and the latter decreases the capacity of the hydrogen bonding of the acidic hydrogen atom in imidazolium ring with
cellulose is insoluble in 1-butyl-3-methylimidazolium acetate [C4mim][CH3COO]. This suggests that the diallyl in [A2im]+ plays a crucial role in enhancing cellulose dissolution. The above findings indicate that the binding electrondonating group in carboxylate anion facilitates cellulose dissolution; binding electron-withdrawing groups (e.g., OH and pyridyl) and bulky groups in anion instead impair cellulose dissolution. 3.2. Influence of Cationic Structure on Cellulose Solubility. The solubility values of cellulose in the methoxyacetate-based ILs are summarized in Table 2. Similarly, the cationic structure of the ILs considerably impacts cellulose dissolution as well. To investigate how the influence of the cationic skeleton on cellulose solubility, imidazolium, pyrrolidine, piperidine, and pyridine cationic skeleton were chosen. The cationic skeleton strongly affects cellulose solubility. Generally, the dissolution capacity of the ILs with imidazolium cationic skeleton is vastly superior to those with pyrrolidine, piperidine, and pyridine cationic skeleton. For example, at 50 °C, although [C4mim][CH 3 OCH 2 COO], [N(C 4 C 1 )Py][CH 3 OCH 2 COO], [N(C4C1)Pp][CH3OCH2COO], and [C1N(C4)Pyr][CH3OCH2COO] have the same cationic side chain and [CH3OCH2COO]− anion, as high as 19.5% cellulose solubility is obtained in [C4im][CH3OCH2COO], which decreases to 15.2% in [N(C4C1)Py][CH3OCH2COO], and cellulose is insoluble in [N(C4C1)Pp][CH3OCH2COO] and [C1N(C4)Pyr][CH3OCH2COO]. This is mainly because compared with the saturated pyrrolidinium and piperidium cations ([N(C4C1)Py]+ and [N(C4C1)Pp]+), the conjugated imidazolium cations [C4mim]+ have acidic protons, which are favorable for hydrogen bond forming with the OH oxygen atoms in cellulose.29 The alkyl length in cation also impacts cellulose solubility. As the alkyl length increases, the solubility of cellulose decreases in the order [C 2 Aim][CH 3 OCH 2 COO] > [C 1 Aim][CH3OCH2COO] > [C4Aim][CH3OCH2COO]. 1-Allyl-3methylimidazolium chloride ([Amim]Cl) was reported to exhibit highly efficient cellulose dissolution capacity.23 However, we found that, after substituting the methyl CH3 in [Amim]+ cation by a longer chain propyl CH2CH2CH3, cellulose was insoluble in [AC3im]Cl. The alkyl number in cation also impacts cellulose solubility. In terms of pyridine-based IL, cellulose is insoluble upon D
DOI: 10.1021/acs.macromol.8b00724 Macromolecules XXXX, XXX, XXX−XXX
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atoms of the OH groups in cellulose, the electron cloud density around C7,7′ atoms redistributes, resulting in the remarkable downfield shifts of C7,7′ atoms. Additionally, the almost unchanged signals for C5,5′ atom suggest that the two CH2 units bonded to the nitrogen atoms in imidazolium ring have a very weak (or no) interaction with the OH groups in cellulose and hardly contribute to cellulose dissolution. It is worthy to emphasize that, except for the efficacy of the acidic H2 and H4,4′ atoms in imidazolium cation in cellulose dissolution (decreased 13C NMR signals), the H6 atom in allyl unit also takes part in cellulose dissolution by hydrogen bond interaction with the O atom of the OH in cellulose (decreased 13 C NMR signals). Compared with allyl, the H atoms in the saturated alkyls CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 , and CH2CH2CH2CH3 hardly/weakly interacted with the hydroxyl O atom in cellulose, and thus hardly participated in cellulose dissolution. Therefore, the ILs with imidazolium cation having allyl exhibit stronger dissolution capacity for cellulose than those with imidazolium cation having the same carbon atom saturated alkyl. On the basis of this concept, now that the two allyls intead of alkyls bind to N atoms in imidazolium cation, [A2im][CH3COO] (15.0% at 25 °C) is thought to display a more powerful capacity of cellulose dissolution than [Amim][CH3COO] (
