Triazolium-Based Ionic Liquids â A Novel Class of Cellulose Solvents

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Triazolium-Based Ionic Liquids – A Novel Class of Cellulose Solvents Martin Brehm, Martin Pulst, Joerg Kressler, and Daniel Sebastiani J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b12082 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Physical Chemistry

Triazolium-Based Ionic Liquids – A Novel Class of Cellulose Solvents Martin Brehm1∗ , Martin Pulst1, Jörg Kressler1 and Daniel Sebastiani1

Abstract We present first results on triazolium-based ionic liquids (ILs) as a novel class of nonderivatising solvents for cellulose. Despite their chemical similarity to imidazolium cations, the 1,2,3-triazolium cation lacks the isolated ring proton, leading to reduced formation of N-heterocyclic carbenes (NHCs), and therefore to lower reactivity and less unwanted side reactions. We synthesised six ILs based on 1,2,3-triazolium and 1,2,4-triazolium cations. The acetates are room temperature ionic liquids (RTILs) and dissolve a similar amount of cellulose as the corresponding imidazolium salt. From NMR spectroscopy of the solution, we rule out polymer degradation. The cellulose solubility rises with increasing anion basicity, while being almost independent of the cation. We perform molecular dynamics simulations and compute enthalpies of solvation, which quantitatively fit the experimental solubilities. Trajectory analysis reveals strong hydrogen bonds between acetate anions and cellulose hydroxyl groups, while the cations do not form strong hydrogen bonds to cellulose. Thus, the simulations provide an atomistic explanation of our experimental observations.

1 Institut

für Chemie, Martin-Luther-Universität Halle–Wittenberg, von-Danckelmann-Platz 4, D-06120 Halle (Saale), Germany. Correspondence and requests for materials should be addressed to M.B. (email: [email protected]).

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Introduction Despite their long history of more than one century, 1 ionic liquids (ILs) have gained increasing attention during the last two decades. They have been utilised in many different applications, such as batteries, 2,3 fuel cells, 4 supercapacitors, 5 extraction, 6 electrodeposition, 7 liquid crystals, 8,9 solvents, 10,11 including large-scale industrial processes, 12 for carbon dioxide capture, 13–15 in catalysis, 9,16–18 as explosives 19 and rocket fuel, 20 and even for building telescope mirrors. 21 One milestone in the re-advent of ILs probably happened in 2002, when it was discovered that certain imidazolium-based ILs are able to dissolve cellulose. 22 Since then, numerous studies on dissolving cellulose in ILs have appeared. 23–36 Triazolium salts and triazolium-based ILs have been known for a long time, and have been proposed for various applications, such as polymer electrolytes, 37–39 solvents, 40–42 synthesis, 43 ion conductors, 44–46 extraction, 47,48 membranes, 49 molecular recognition, 50 explosives, 51,52 in “click” chemistry, 53,54 liquid crystals, 55 and capillary electrophoresis. 56 Triazolium-based ILs offer many advantageous properties, such as very good chemical stability under alkaline conditions, 40 antimicrobial and antifungal effect, 57–59 and low cytotoxicity. 60 However, they have never been considered as cellulose solvents up to now, and triazolium acetate ionic liquids have not even been synthesised or characterised at all. With an annual production of 1011 − 1012 t, cellulose is considered the most abundant polymer available on earth, and therefore also a very promising candidate as raw material for a sustainable product cycle in chemical industry. 61 To characterise, process, and modify cellulose, it often needs to be dissolved first, explaining the necessity of good cellulose solvents. 61 It is commonly known that non-derivatising cellulose solvents need to break up the hydrogen bond network of cellulose, 61 which is equivalent to a strong stabilisation of the cellulose strands in solution. Ionic liquids are often characterised as “green solvents” due to their low volatility. 10 Furthermore, when used as cellulose solvents, they lead to significantly reduced amounts of waste water as compared to traditional processes, 61 which even more emphasises their property of being “green” and environmentally benign. Most of the ILs which are proposed as cellulose solvents in literature are based on the imidazolium cation. This comes as no surprise, as it is well known that imidazolium-based ILs are able to form very strong hydrogen bonds. 62–68 Apart from the strong hydrogen bonding ability, imidazolium-based ILs often feature high thermal stabilities, low melting points, and low viscosities, as compared to ionic liquids composed of different cations. 69–73 Despite all these advantageous properties, there is also a drawback with imidazolium-based ILs. In conjunction with strongly basic anions such as acetate (which gives high cellulose solubility due to its very strong 2


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hydrogen bond acceptor strength), the central isolated ring proton of the imidazolium cation can be abstracted by the anion, leading to the formation of N-heterocyclic carbenes (NHCs). 68,74–80 These highly reactive NHCs can either undergo an irreversible dimerisation (by forming a double bond), or form adducts with all other species in the mixture. This phenomenon has already been utilised for some applications, such as the chemisorption of carbon dioxide 78–81 or even the dissolution of elemental sulphur 74 in ILs due to the reaction with intrinsically present NHCs. In the latter case, around 50 mol–% of the cations in [EMIm][OAc] reacted with sulphur to form the carbene adducts within 24 h at 25 ◦ C, emphasising the importance of this reaction pathway. However, this reactivity is clearly unwanted in the case of dissolving cellulose, as it may lead to all kinds of by-products. In the case of cellulose dissolution, it has been reported that NHCs may lead to a molar mass reduction of the cellulose, 61 i. e., degradation of the polymer. More specifically, it was found that [EMIm][OAc] can lead to a degradation of the cellulose backbone and the formation of C–C bonds between cellulose and the imidazolium cation. 27 Recently, it was claimed in the literature that the suppression of NHC formation via structural modification of the IL is infeasible, and that additives such as glycerine should be added instead. 82 In this work, we illustrate the capability of 1,2,3-triazolium and 1,2,4-triazolium salts as cellulose solvents, and report relevant physicochemical properties of the ILs. As only one C–H unit is replaced by a nitrogen atom, the molecular structure of these cations is very similar to the widely used imidazolium cations, which gives rise to the expectation that most physicochemical properties should be comparable (including the cellulose solubility). However, 1,2,3-triazolium cations do not possess an isolated ring proton, and the acidity of the two other ring protons is around four orders of magnitude lower as compared to the imidazolium cation, 53 see Fig. 1. Therefore, the unwanted effect of NHC formation is significantly weaker in 1,2,3-triazolium acetates, greatly reducing the amount of formed by-products and decomposition products when applied as a solvent. The acidity of the two isolated ring protons in the 1,2,4-triazolium cation is even larger than in imidazolium; 53 therefore, we focus on 1,2,3-triazolium-based compounds in this study. Apart from the improved chemical stability under alkaline conditions with respect to imidazolium-based ILs, 40 triazolium-based ILs show antimicrobial and antifungal effects 57–59 and low cytotoxicity, 60 which both could be advantageous in industrial applications. Within our combined experimental and computational study, we synthesised and characterised several triazolium-based ILs (including both 1,2,3-triazolium and 1,2,4-triazolium cations), measured their ability to dissolve cellulose, and subsequently performed molecular dynamics simulations to obtain a microscopic understanding of the differences in the solubilities with respect to cellulose.

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Figure 1 N-heterocyclic carbene (NHC) formation in ionic liquids based on 1,2,4-triazolium (a), imidazolium (b), and 1,2,3-triazolium (c). Carbene equilibrium concentration falls with increasing pKa value 53 of the ring protons from (a) to (c).

The molecular structures of the anions and cations from the ILs considered in this work are shown in Fig. 2, including the ion nomenclature used throughout the article. We write [EMIm]+ for the 1-ethyl-3-methylimidazolium cation, [EMTr123 ]+ for 1-ethyl-3-methyl-1,2,3-triazolium, [EMTr124 ]+ for 4-ethyl-1-methyl-1,2,4-triazolium, [MMTr123 ]+ for 1,3-dimethyl-1,2,3-triazolium, [OAc]− for acetate, [OTf]− for triflate, [NTf2 ]− for bis(trifluoromethane) sulfonimide, [Br]− for bromide, and [I]− for iodide. The top image of Fig. 2 illustrates the hydrogen bond network in cellulose Iβ (most abundant crystal structure), 83 with intramolecular hydrogen bonds shown in blue, and intermolecular hydrogen bonds shown in orange.

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Figure 2 Molecular structures and naming scheme of the molecules/ions investigated within this study, together with the ion nomenclature. Top image shows intramolecular (blue) and intermolecular (orange) hydrogen bonds in cellulose Iβ (most abundant crystal structure); Atom colours: carbon=grey, oxygen=red, hydrogen=white, nitrogen=blue, sulphur=yellow, fluorine=green, bromine=brown, iodine=purple.

Methods Experimental Methods The ionic liquids [EMTr123 ][OAc], [EMTr123 ][Br], [MMTr123 ][OTf], [MMTr123 ][I], and [MMTr123 ][NTf2 ] were prepared from 1-methyl-1,2,3-triazole. [EMTr124 ][OAc] was synthesised from 1-methyl-1,2,4-triazole (received from abcr GmbH). The synthesis involved an alkylation with ethyl bromide, methyl iodide, methyl triflate, or N-methyl-bistriflamide as a first step, which was followed by anion exchange with silver acetate (received from Alfa Aesar) in methanol 5



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to obtain the acetates. Details on the synthesis can be found in the Supporting Information. [EMIm][OAc] (purity ≥ 96.5 % HPLC) and microcrystalline cellulose (20 µm particle size) were purchased from Sigma–Aldrich. We measured the molar mass of the cellulose to be Mv = 37.0 kg mol−1 , which corresponds to a degree of polymerization of DP = 229, determined by viscosimetry at T = 25 ◦ C using an Ubbelohde viscometer from LaborTherm (capillary constant of 0.01) equipped with a VB2 measurement system from Lauda. For these measurements, the cellulose was suspended in 50 mL water followed by the addition of 50 mL copper(II) ethylene diamine solution (1.0 M, purchased from VWR) to yield solutions with different cellulose concentrations ranging between 0.1 and 2.5 mg mL−1 . The molar mass calculation followed the Kuhn–Mark–Houwing relation reported in ref. 84. The cellulose solubility experiments were carried out under an atmosphere of dry nitrogen (water content < 0.5 ppm). 200 mg of the respective IL were heated to T = 80 ◦ C with a specific amount of cellulose (depending on the molar mass of the IL). When the cellulose was completely dissolved after 24 h (as revealed by polarised optical microscopy), more cellulose was added until reaching the solubility limit. The solubility limit was reached when 2 g cellulose per mol IL could not anymore be dissolved within 24 h. The 13 C NMR spectrum of cellulose in [EMTr123 ][OAc] / DMSO-d6 (34 g per mol IL, [EMTr123 ][OAc] : DMSO-d6 ≈ 1 : 2 v/v) was recorded at 27 ◦ C with an Agilent Technologies 400 MHz VNMRS spectrometer operating at 100 MHz for 13 C nuclei. 10,000 scans were acquired and the FID was apodised by an exponential function to obtain a good signal to noise ratio. Differential scanning calorimetry (DSC) was performed under continuous nitrogen flow using a Mettler Toledo DSC 822e module. Aluminum pans were filled with about 5 − 15 mg of sample, and the DSC traces were recorded in the temperature range of −40 ≤ T ≤ 50 ◦ C with a heating rate of 5 K min−1 . Viscosity measurements were performed on an Anton Paar Physica MCR 301 shear rheometer equipped with a CP25-2/TG measurement system having a cone–plate geometry with a diameter of 25 mm, an angle of 2◦ and a gap of 51 µm. The viscosity was measured at different shear rates (10−1 Hz ≤ γ˙ ≤ 102 Hz) in the temperature range of 20 ◦ C ≤ T ≤ 90 ◦ C. Liquid mass densities were measured on a DMA 60 densiometer equipped with a DMA 602 measurement loop from Anton Paar; the measurement temperature was controlled by a thermostat from Julabo. Polarised optical microscopy (POM) was performed with an Axioplan 2 imaging microscope and the images were taken using an AxioCam MRc camera, both from Carl ZEISS Jena. The sample was placed between two cover slips separated by a Teflon spacer. The images were taken upon dissolution of cellulose at room temperature in intervals of 10 s. The water content of the acetate based ionic liquids was determined by Karl Fischer titration with a Mettler Toledo DL 35 titrator. Hydranal Titrant 5 and Hydranal solvent oil (both pur-

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chased from Honeywell / Fluka) were used for the titration. The water contents of [EMIm][OAc], [EMTr123 ][OAc], and [EMTr124 ][OAc] are 0.28 %, 0.82 % and 0.55 %, respectively. Thermogravimetric analysis (TGA) was performed under continuous nitrogen flow with a heating rate of 10 K min−1 in the temperature range of 25 ◦ C ≤ T ≤ 800 ◦ C using a Mettler Toledo TGA/SDTA 851e module. Alumina pans were filled with about 5–15 mg of sample for the measurement. For the long-term thermal stability studies, the three acetates were held at T = 80 ◦ C (i. e., the temperature of the cellulose dissolution experiments) for 12 h. The results are shown in Figure S17 in the SI.

Computational Methods Force-field molecular dynamics simulations of the systems investigated in this work have been performed with the LAMMPS program package, 85 using a Lennard-Jones and Coulomb cutoff distance of 1.0 nm as well as Particle–Particle Particle–Mesh (PPPM) long-range correction for the electrostatic interactions. As a model system for the cellulose strand, a cellulose pentamer was used. For [EMIm]+ , [OTf]− , and [NTf2 ]− , the Canongia Lopes & Pádua (CL&P) force field for imidazolium-based ionic liquids, 86 which is an extension of the OPLS–AA force field, 87 was applied. For the acetate anion, the original OPLS–AA parameters were used. 87 The force field parameters for [Br]− and [I]− were taken from Fyta. 88 Concerning [EMTr123 ]+ , [EMTr124 ]+ , and [MMTr123 ]+ , the bonded and Lennard-Jones parameters were adapted from CL&P, whereas the partial charges have been obtained from quantum chemical calculations via the RESP methodology (the charges are presented Fig. S18 in the SI). All ion charges were scaled down to 0.8 to account for screening effects. 89 For cellulose, the force field parameters from Damm were used. 90 Simulations were set up by placing 192 ion pairs of the ionic liquid and one cellulose pentamer randomly inside a simulation box with Packmol. 91 After an equilibration period, barostate runs in the NpT ensemble were performed at 80 ◦ C to determine the optimal density. After another equilibration period, 10 ns of production run were performed for each system. The average pressure in all production runs was within −0.5 . . . 0.5 MPa, indicating a well converged density. Based on the molecular dynamics simulations, the enthalpy of solvation ∆HSolv for cellulose (i. e., the energy gain when transferring a single isolated cellulose pentamer from vacuum into solvent) was computed by the following protocol. Apart from the simulation of the cellulose pentamer in IL, also simulations of the pure IL and of the cellulose pentamer in vacuum were performed. For all simulations, the average of the total energy hETot i was computed. By subtracting these averages of the latter two simulations from the first one, an estimate for the solvation internal energy change ∆USolv was obtained. According to ∆H = ∆U + p∆V , a work term due to volume change should be included to obtain ∆HSolv . However, as we were only interested in relative differences, and the work term is assumed to be similar for all solvents considered here, we decided to neglect this contribution. The solvation enthalpies obtained by this procedure were 7



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divided by a factor of 5 to give the value per cellulose monomer unit. The resulting densities of the pure IL simulations are reported in Table S1 in the SI. The radial and spatial distribution functions have been computed with TRAVIS, 92 and the plots were created with VMD 93 and xmgrace. 94

Results and Discussion Synthesis and Characterisation We synthesised the triazolium-based ILs [EMTr124 ][OAc], [EMTr123 ][OAc], [EMTr123 ][Br], [MMTr123 ][OTf], [MMTr123 ][I], and [MMTr123 ][NTf2 ]. Together with commercially available [EMIm][OAc], this makes a set of seven ILs investigated in this study. Concerning the physicochemical properties, [EMIm][OAc], [EMTr123 ][OAc], and [EMTr124 ][OAc] were found to be very similar. All three were obtained as viscous liquids at room temperature, no crystallisation occurred upon cooling to T = −40 ◦ C. Therefore, we report that the two newly synthesised triazolium acetates are room temperature ionic liquids (RTILs). Neither one has been characterised in literature before. The remaining four ILs [EMTr123 ][Br], [MMTr123 ][OTf], [MMTr123 ][I], and [MMTr123 ][NTf2 ] were solid at room temperature, and were therefore precluded from further investigations. The purity of [EMIm][OAc], [EMTr123 ][OAc], and [EMTr124 ][OAc] was confirmed by 1 H and 13 C NMR spectroscopy. Possible impurities or water traces were found to be below the NMR detection limit. The NMR spectra are presented in the Supporting Information. We determined the water content of these three samples by Karl Fischer titration, and found it to be below 1 % in all cases (see Experimental Methods). We further characterised the three acetates [EMIm][OAc], [EMTr123 ][OAc], and [EMTr124 ][OAc] in terms of density, viscosity, and decomposition temperature; see Table 1. Densities and viscosities at other temperatures are given in Table T1 and Fig. S16 in the Supporting Information. The densities and viscosities of these three ILs were found to be similar. The decomposition temperature (defined as Td10 : 10 % weight loss) of the two triazolium acetates is around 60 ◦ C lower as compared to the imidazolium salt (as already noted for ILs with other anions in literature before 40 ), which still leaves a wide window of liquid range without risk of decomposition.

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Table 1 Experimentally determined densities, viscosities, and decomposition temperatures (10 % weight loss) for the characterised ionic liquids.

Ionic Liquid [EMIm][OAc] [EMTr124 ][OAc] [EMTr123 ][OAc] [EMTr123 ][Br] [MMTr123 ][OTf] [MMTr123 ][I] [MMTr123 ][NTf2 ]

Density (25 ◦ C) [g·cm−3 ] 1.104 1.109 1.146

Viscosity (80 ◦ C) [Pa·s] 0.0149 0.0116 0.0147

State at RT liquid∗ liquid∗ liquid∗ solid solid solid solid

Decomp. Temp. [◦ C] 210 145 150 190 360 215 385

∗ viscous liquids at RT, no crystallisation upon cooling to T = −40 ◦ C

Cellulose Solubility Similarly to the respective imidazolium salt, we found that [EMTr123 ][OAc] dissolves microcrystalline cellulose already at room temperature as revealed by polarised optical microscopy, see Fig. 3 (a–f). This is remarkable, as only few ionic liquids are able to dissolve cellulose under ambient conditions. 95 A short movie of this process can be found in the Supporting Information. Figure 3 (g) shows the 13 C NMR spectrum of cellulose in [EMTr123 ][OAc] after addition of DMSO-d6 , confirming the successful dissolution. All characteristic NMR resonances of cellulose 96 are observed. Therefore, the presence of oligomers caused by degradation can be ruled out. A solubility limit of 34 g cellulose per mol of IL (= 19.9 w/w %) has been determined for [EMTr123 ][OAc] at T = 80 ◦ C, and a value of 30 g per mol of IL (= 17.5 w/w %) was measured for the respective 1,2,4-triazolium isomer [EMTr124 ][OAc] at the same temperature. The imidazolium analogue [EMIm][OAc] dissolves 36 g cellulose per mol of IL at T = 80 ◦ C, which is in good agreement with common literature values (ranging between 33.3 and 39.7 g per mol of IL). 97–100 Therefore, we find that the cellulose solubility of both triazolium acetates is in the same range as the respective value of the imidazolium acetate. The cellulose solubilities for all seven ILs are given in Table 2 (at 80 ◦ C, expect for [MMTr123 ][I], which was measured at 200 ◦ C due to its high melting point of Tm = 199.4 ◦ C).

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Figure 3 (a–f) Polarised optical microscopy images showing the dissolution process of cellulose in [EMTr123 ][OAc] at room temperature (c ≈ 5 g cellulose per mol [EMTr123 ][OAc]). The micrographs are taken directly after addition and 5 min, 10 min, 20 min, 40 min, and 60 min afterwards, respectively. (g) 13 C NMR spectrum of cellulose in [EMTr ◦ 123 ][OAc] / DMSO-d6 solution at 27 C. The assignment of the resonances is taken from literature. 96

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While the cellulose solubility is found to be very similar for the three acetate-based ionic liquids, the 1,2,3-triazolium salts with the other anions exhibit significantly reduced cellulose solubility in the order of [OAc]− [OTf]− > [Br]− ≈ [I]− > [NTf2 ]− . This leads us to the assumption that the cellulose solubility is primarily governed by the anion, with less contributions from the cation. Anions with smaller pKb value are found to cause larger cellulose solubility, as already observed before in literature for other anions. 29

Table 2 Experimentally determined cellulose solubilities for the ILs and predicted enthalpy of solvation ∆HSolv of a cellulose pentamer from MD simulations, both at 80 ◦ C.

Ionic Liquid

[EMIm][OAc] [EMTr124 ][OAc] [EMTr123 ][OAc] [EMTr123 ][Br] [MMTr123 ][OTf] [MMTr123 ][I] [MMTr123 ][NTf2 ]

Experimental Cellulose Solubility (80 ◦ C) [g per mol IL] w/w % 36.0 21.1 30.0 17.5 34.0 19.9 4.0 2.1 12.0 4.9 ∗ 4.0 1.8∗ < 2.0 < 0.5

Computed ∆HSolv of Cellulose Pentamer [kJ·mol−1 ·unit−1 ] -96.4 -97.9 -97.7 -79.8 -78.0 -71.7 -60.2

∗ measured at 200 ◦ C due to high melting point of T = 199.4 ◦ C m

Based on molecular dynamics simulations of the seven systems (conducted at 80 ◦ C), we computed the enthalpy of solvation ∆HSolv of a cellulose pentamer per monomer unit. The results are also given in Table 2. We observe only a weak dependence on the choice of the cation, but a strong variation for different anions. The enthalpy of solvation is large for the acetate-based ILs, and significantly smaller for the [Br]− , [I]− , [OTf]− , and [NTf2 ]− anions, which is in agreement with the trend of the experimental cellulose solubilities and further strengthens the hypothesis that the anion is the decisive element with respect to cellulose solvation. As dissolution of substances is an equilibrium process (with the solubility product KS as the equilibrium constant), the solubility is directly related to the Gibbs free energy of solvation ∆GSolv via ∆GSolv = ∆HSolv − T · ∆SSolv = −R · T · ln KS . In Fig. 4, we present a plot of ∆HSolv against ln KS , where a linear dependence between both quantities can be seen (note that KS is proportional to the solubility for neutral solutes such as cellulose). This finding shows that the entropy of solution ∆SSolv is nearly identical for all ILs considered here. We conclude that the molecular dynamics simulations are able to predict the correct order of cellulose solubilities.

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Figure 4 Correlation of the experimental cellulose solubility in several ionic liquids with the predicted enthalpy of solvation ∆HSolv per monomer unit of a cellulose pentamer from MD simulations (note the logarithmic scale of the abscissa).

In order to understand the differences in solvation enthalpy for the different ILs, we decomposed the potential energy of the systems along the simulation trajectories into cation–cellulose and anion–cellulose interaction contributions, see Table 3. While the cation–cellulose interactions are all within a range of around −70 to −80 kJ·mol−1 ·unit−1 , the anion–cellulose contributions significantly differ, taking values of around −130 kJ·mol−1 ·unit−1 for the three acetate-based ILs and only around half of this value for the ILs based on other anions. This is another strong indication in support of our claim that the anion is of central importance for the cellulose solubility. Please note that the sum of both interaction energies in Table 3 is not equal to the enthalpies of solvations shown in Table 2, because the latter values include the enthalpy required to create a cavity of suitable size in the solvent, while the sum of interaction energies does not include this contribution. To obtain a microscopic picture of the differences in cellulose solubility, the molecular dynamics trajectories have been analysed. The presumably most important class of interactions between cellulose and ionic liquid solvents are hydrogen bonds; therefore, the hydrogen bonding behaviour was investigated. To this aim, radial distribution functions (RDFs, g(r)) between possible hydrogen bond donors and acceptors were computed. RDF intensities are normalised 12


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Table 3 Decomposition of the potential energy into cation–cellulose and anion–cellulose interaction contributions, averaged along the whole MD trajectories and normalized to one cellulose monomer unit.

Ionic Liquid [EMIm][OAc] [EMTr124 ][OAc] [EMTr123 ][OAc] [EMTr123 ][Br] [MMTr123 ][OTf] [MMTr123 ][I] [MMTr123 ][NTf2 ]

Interaction Energy [kJ·mol−1 ·unit−1 ] Cation–Cellulose Anion–Cellulose -81.9 -138.0 -80.0 -126.8 -77.3 -128.7 -74.9 -78.4 -72.7 -87.9 -71.6 -60.6 -68.2 -80.6

to uniform density, i. e., a value of 1 indicates a particle density which equals the simulation cell average value (e. g., for large distances r). A strong hydrogen bond is characterised by both a peak height g(r) 1 and a relatively small peak distance r, indicating a close contact between hydrogen bond donor and acceptor. Here, we considered all cellulose hydroxyl group protons as well as all aromatic cation ring protons as possible hydrogen bond donors, whereas we considered all cellulose oxygen atoms and all electronegative anion atoms (O for [OAc]− , [OTf]− , and [NTf2 ]− ; N for [NTf2 ]− ; Br for [Br]− ; I for [I]− ) as possible hydrogen bond acceptors. In Fig. 5, RDFs between all hydrogen bond donors of cellulose and hydrogen bond acceptors of the anions (upper panel), as well as between all hydrogen bond donors of the cations and all hydrogen bond acceptors of cellulose (lower panel) are shown. The remarkable first peak of the anion–HCellulose RDF in the upper panel shows that [OAc]− forms very strong hydrogen bonds to cellulose, almost independently of the choice of the cation. [OTf]− forms weaker, but still significant hydrogen bonds, whereas [NTf2 ]− and the two halogenides are no significant acceptors of any hydrogen bonds from cellulose. This is perfectly in line with the experimental solubility results (Table 2), where the acetate-based ionic liquids are by far the best cellulose solvents, followed by the [OTf]− -based IL with intermediate solubility, and the remaining anions with only very small solvating ability. The HCation –OCellulose RDFs (Fig. 5 bottom) show that the cations do not significantly contribute to the hydrogen bonds with cellulose in all cases, indicated by shallow first maxima g(r) ≤ 1 and large first peak distances of almost 300 pm. This finding is confirmed by the experiments, where it was found that the cellulose solubility is almost independent of the choice of the cation. In order to visualise the inter-ionic interactions in the ILs, spatial distribution functions (SDFs) of the three acetate-based ILs are presented in Fig. 6. For a fixed reference orientation of the 13



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Figure 5 Upper panel: Radial distribution functions (RDFs) between H-bond donor atoms of cellulose (hydroxyl protons) and H-bond acceptor atoms of anions; Lower panel: RDFs between H-bond acceptor atoms of cellulose (oxygen) and H-bond donor atoms of cations (ring protons).

cations, the average centre of mass distribution of anions and neighbouring cations is shown as red and green iso-surfaces, respectively. The anions are mainly found in front of the ring protons within the ring plane, emphasising the importance of the hydrogen bond in these compounds. The cations, however, populate the regions above and below the ring (on top configuration). This behaviour can be related to two specific geometric factors: First, most of the in plane positions are preferentially occupied by the anions, such that the cations are left with an on top configuration. Secondly, this on top arrangement represents a π–π stacking geometry, which is energetically favourable. The underlying interaction was found to play a significant role in imidazolium-based ILs before, 101–104 also contributing to the on top preference of the cations.

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Figure 6 Spatial distribution functions (SDFs) depicting the average centre of mass distribution of anions (red) and neighbouring cations (green) around a fixed cation for the three acetate-based ILs.

To obtain an understanding on the distribution of IL close to the cellulose strand, a SDF of [EMTr123 ][OAc] around one monomer unit is depicted in Fig. 7, where [EMTr123 ]+ cations are shown in green, and [OAc]− anions appear in red colour. The corresponding SDFs for [EMTr124 ][OAc] and [EMIm][OAc] were found to be very similar, and are therefore not presented. The top and bottom parts of the illustration give different perspectives of the same distribution. It can be seen that the cations accumulate close to both of the acetale oxygen atoms of the glycosidic bonds (which reside on the same side of the ring plane in cellulose due to the β -glycosidic connection). The anions, on the other hand, mainly populate the opposite side, next to the hydrogen atom of the anomeric carbon. Both findings can be explained with simple electrostatic considerations: The cations are attracted by the two sterically close acetale oxygen atoms, whereas the anions find attractive interaction at the acetale hydrogen atom, which bears a positive charge. The very strong hydrogen bonds between acetate and the hydroxyl groups which were described in the discussion of Fig. 5 are not prominently visible in this type of illustration, because the spatial distribution is strongly broadened due to the thermal motion of the hydroxyl groups.

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Figure 7 Spatial distribution functions (SDFs) depicting the average distribution of anions (red) and cations (green) in [EMTr123 ][OAc] around a monomer unit within the cellulose strand; top and bottom gives two different views of the same distribution.

Conclusions In this combined experimental and computational study, we propose 1,2,3-triazolium-based ionic liquids as a novel class of solvents for cellulose. While the molecular structure of the 1,2,3triazolium cation is very similar to the commonly used imidazolium cation, the former does not possess an isolated ring proton, leading to a pKa value which is around four orders of magnitude larger as compared to imidazolium. This, in turn, results in a significantly reduced formation of N-heterocyclic carbenes (NHCs), such that the amount of by-products due to unwanted side reactions is greatly reduced when the IL is applied as a solvent. Triazolium-based ionic liquids have not been considered as cellulose solvents before, and triazolium acetate ionic liquids have not even been synthesised or characterised at all. Within the scope of this work, we synthesised six different triazolium-based ILs, based on the 1,2,3-triazolium and 1,2,4-triazolium cation, and subsequently assessed the purity of these newly synthesised substances with 1 H and 13 C NMR spectroscopy. We observed that both 1-ethyl-316


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methyl-1,2,3-triazolium acetate ([EMTr123 ][OAc]) and 4-ethyl-1-methyl-1,2,4-triazolium acetate ([EMTr124 ][OAc]) are room temperature ionic liquids (RTILs), and we report their density, viscosity, and decomposition temperature. The physicochemical properties of both compounds are found to be similar to those of 1-ethyl-3-methylimidazolium acetate, which is a known cellulose solvent. Using polarised optical microscopy, we confirm that both triazolium acetates dissolve microcrystalline cellulose already at room temperature. In a 13 C NMR spectrum of cellulose dissolved in [EMTr123 ][OAc], we observed all characteristic resonances of cellulose; therefore, the presence of oligomers caused by degradation can be ruled out. We measured the cellulose solubility of all six synthesised ILs. While the choice of the cation seems to have only minor influence on the cellulose solubility, we find major differences for the different anions, whose ability to dissolve cellulose decays with [OAc]− [OTf]− > [Br]− ≈ [I]− > [NTf2 ]− . This nicely correlates with the trend in pKb values of the anions, as it was already observed in the literature before for other anions. The microscopic solvation structures of the cellulose solutions were revealed by means of molecular dynamics simulations of a cellulose pentamer solvated in seven different ILs, respectively. The computed enthalpies of solvation ∆HSolv for cellulose in the ILs are found to be quantitatively correlated with the experimental cellulose solubilities. The simulation trajectories show very strong hydrogen bonds between the acetate anion and the hydroxyl groups of cellulose (therefore breaking up the hydrogen bond network of cellulose), whereas the other anions with lower basicity form significantly weaker hydrogen bonds. The cations, on the other hand, generally do not possess strong hydrogen bonds to cellulose, which explains the small differences in cellulose solubility when replacing the cation of the ionic liquid. Thus, the experimentally observed dissolution capabilities of this new class of ILs are consistently explained at the atomistic level by our simulations. After reporting first results of triazolium-based ionic liquids as cellulose solvents here, we are planning to perform future studies on this topic, including investigation of the presumably lower reactivity and lower trend to unwanted side reactions of [EMTr123 ][OAc] when compared to [EMIm][OAc].

Acknowledgements M.B. acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG) through project Br 5494/1-1. We thank Andreas Kerth and Jana Eisermann for providing access to their shear rheometer as well as Elvira Börner for her support with the viscosity measurements for molar mass determination of cellulose.

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Supporting Information Experimental details, RESP charges, and NMR spectra (PDF). A movie of cellulose dissolution (MOV).

Conflicts of interest The authors declare no conflict of interest.

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Triazolium-Based Ionic Liquids â A Novel Class of Cellulose Solvents

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