Synthesis and Thermophysical Properties of Imidazolium-Based

Article pubs.acs.org/jced

Synthesis and Thermophysical Properties of Imidazolium-Based Bronsted Acidic Ionic Liquids Nawshad Muhammad,*,† Zakaria Man,‡ Yasir A. Elsheikh,§ M. Azmi Bustam,‡ and M.I. Abdul Mutalib‡ †

Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia § Department of Chemical and Materials Engineering Faculty of Engineering, King Abdulaziz University, P.O. Box 344, Rabigh, 21911 Saudi Arabia ‡

ABSTRACT: A range of imidazolium-based Bronsted acidic ionic liquids (ILs) have been synthesized by varying the side chain length of the alkyl and alkyl sulfonic group incorporated to the C1 and C3 positions in the imidazolium ring respectively with a fixed HSO4− anion. The synthesized ILs were characterized using NMR for structure confirmation and CHNS for elemental analysis. The thermal properties such as thermal decomposition temperature were determined using thermogravimetric analysis, and several key physical properties such as viscosity and density were measured within a temperature range of 20 °C to 80 °C using an Anton Paar viscometer and densitometer. The density measurement results and established equations were used to calculate the molecular volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients of the synthesized ILs.

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INTRODUCTION Room temperature ionic liquids (ILs) are typically salts that contain at least one organic cation and organic or inorganic anion, and have melting points below 100 °C. Such materials are in demand as alternatives to traditional molecular solvents owing to their desirable properties such as high chemical and thermal stabilities and extremely low flammabilities and vapor pressures.1,2 The wide range of possible cation−anion combinations has enabled ILs to be developed with a specific set of desired physicochemical properties or for specific applications. The present applications of ILs in the field of catalysis are found to be numerous and cover many areas. One of the main areas of interest is to replace the use of traditional liquid acids in catalytic reactions.3 Physiochemical properties of ILs such as melting point, viscosity, density, and miscibility with water was found to vary with the length of the alkyl chain on the cation or the type of anion incorporated to the ILs. It has been reported that 1-alkyl3-methylimidazolium with tetrafluoroborates4 and hexafluorophosphates anions5 formed liquid crystalline when the alkyl chain length exceeded more than 12 carbon atoms. Below this chain length, the ILs formed liquid at room temperature. In another account, the hydrophobic property of an IL was found to favor solvent extraction for product separation that is based on relative solubility between phases. Generally, the cation part of ILs appears to have bulky and uneven structures which result in a low degree of packing symmetry. This results in reduced lattice energy of the salt crystalline formed, thereby decreasing its melting point.6 © 2014 American Chemical Society

In this study a number of Bronsted acidic ILs namely 1methyl-3-(3-sulfopropyl) imidazolium hydrogensulfate (MSPIMHSO4), 1-butyl-3-(3-sulfopropyl) imidazolium hydrogensulfate (BSPIMHSO4), 1-methyl-3-(3-sulfobutyl) imidazolium hydrogensulfate (MSBIMHSO4) and 1-butyl-3-(3-sulfobutyl) imidazolium hydrogensulfate (BSBIMHSO4) were prepared by incorporating a methyl or butyl group at the position of C1 and a propyl or butyl attached with a sulfonate group at the position of C3 within the imidazolium ring with fixed HSO4− anion. The structures of the synthesized ILs were confirmed using NMR, and the elemental analysis was conducted using CHNS. A number of thermophysical properties of the synthesized ILs such as thermal stability, viscosity, and density were measured with respect to temperature for a range between 20 and 80 °C. The effects of the alkyl chain length attached to the imidazolium cation were investigated for thermal stability, viscosity, and density. The values such as molecular volumes, standard entropies, crystal lattice energies, and thermal expansion coefficients were then calculated for all the synthesized ILs.

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METHODS Materials. The following chemicals were obtained from Sigma-Aldrich (Malaysia): 1-methylimidazole (99.5 %), 1butylimidazole (99.0 %), 3-propanesultone (99.0 %), toluene

Received: March 12, 2013 Accepted: February 6, 2014 Published: February 14, 2014 579

dx.doi.org/10.1021/je400243j | J. Chem. Eng. Data 2014, 59, 579−584

Journal of Chemical & Engineering Data

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checked with several ILs previously investigated and published by our research group.8 The Anton Paar viscometer (model SVM3000) and densimeter (model DMA5000) were used accordingly to measure the viscosity and density of the ILs over a temperature range of 20 °C to 80 °C with a temperature control of ± 0.01 °C and uncertainties of ± 0.4 % and ± 3·10−4 g·cm−3, respectively.9

(AR grade), ethyl acetate (AR grade) anhydrous methanol (99.8 %); sulfuric acid (ACS grade, 95.97 %). 1,4Butanesultone was obtained from Fluka (Malaysia). All chemicals were used without any drying or further purification. Synthesis of Bronsted Acidic Ionic Liquids. The preparations of the ILs [(MSPIMHSO4), (MSBIMHSO4), (BSPIMHSO4), (BSBIMHSO4)] were similar to those used in the previous literature, for which a typical reaction is depicted in Figure 1.7 First, 0.03 mol of 1,3-propane and 1,4-

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RESULTS AND DISCUSSION 1. Synthesis of ILs. The results obtained for the 1H NMR spectroscopy and CHNS analysis of all the synthesized ILs as given below confirmed the structure of the respective ILs. MSPIMHSO4. 1H NMR δ (300 MHz): 95.9 8.486 (s, 1H), 7.147 (s, 1H), 7.134 (s, 1H), 4.830 (s, 2H), 4.158−4.186 (t, 2H), 3.697 (s, 3H), 2.648−2.681 (t, 2H), 1.827−1.877 (m, 2H). Anal. Calcd (measured) for C7H14N2O7S2: C = 27.80 (27.85 ± 0.047), H = 4.66 (4.58 ± 0.053), N = 9.26 (9.29 ± 0.081), S = 21.21 (21.26 ± 0.046). MSBIMHSO4. 1H NMR δ (300 MHz): 9.182 (s, 1H), 7.782 (s, 1H), 7.723 (s, 1H), 5.007 (s, 2H), 4.177−4.158 (t, 2H), 3.860 (s, 3H), 2.592−2.634 (t, 2H), 1.835−1.878 (m, 2H), 1.535−1.557 (m, 2H). Anal. Calcd (measured) for C8H16N2O7S2: C = 30.37 (30.32 ± 0.095), H = 5.09 (5.094 ± 0.024) N = 8.85 (8.99 ± 0.0164), S = 20.27 (20.26 ± 0.073). BSPIMHSO4. 1H NMR δ (300 MHz): 9.169 (s, 1H), 7.768 (s, 1H), 7.759 (s, 1H), 4.880 (s, 2H), 4.216−4.337 (t, 2H), 4.099−4.116 (t, 2H), 2.626−2.713 (t, 2H), 2.094−2.121 (m, 2H), 1.697−1.768 (m, 2H), 0.978−1.015 (m, 2H), 0.742− 0.761 (t, 3H). Anal. Calcd (measured) for C10H20N2O7S2: C = 34.87 (34.82 ± 0.05), H = 5.85 (5.89 ± 0.058), N = 8.13 (8.10 ± 0.03), S = 18.62 (18.66 ± 0.04). BSBIMHSO4: 1H NMR δ (300 MHz): 9.156 (s, 1H), 7.775 (s, 1H), 7.704 (s, 1H), 4.975 (s, 2H), 4.131−4.167 (t, 2H), 3.969−3.997 (t, 2H), 2.897−2.913 (t, 2H), 2.346−2.420 (m, 2H), 1.759−1.777 (m, 2H), 1.548−1.580 (m, 2H), 1.198− 1.240 (m, 2H), 0.814−0.823 (t, 3H). Anal. Calcd (measured) for C11H22N2O7S2: C = 36.85 (36.82 ± 0.05), H = 6.18 (6.17 ± 0.058), N = 7.81 (7.91 ± 0.086), S = 17.89 (18.11 ± 0.03) In addition, the results also showed that the purity of the ILs was higher than 99 %. Thermalgravimetric Analysis of ILs. Studies on thermal decomposition (Td) and effects of side chain of imidazoliumbased cation on thermal behavior were carried out on all the synthesized ILs (Figure 2). According to literature, increasing the side chain of the cation will result in decreasing the ILs thermal stability. The reduction in the decomposition temperature could be due either to the decrease in intermolecular interaction or to the initial decomposition of the alkyl substituent.11 However, there were exceptions as reported by many researchers. The finding from this work shows that increasing the side chain generally reduced the thermal stability of the ILs except for a few cases. All the synthesized ILs exhibited good thermal stability with high decomposition temperature. The ILs with alkyl sulfonic functional group as the anions (zwitterions) showed higher thermal stabilities compared to the ones with alkyl sulfonic group side chain with HSO4 as its anion (Table 1). Studies on the effects of the alkyl chain length and the alkyl sulfonic group side chain on decomposition temperature showed that for the imidazolium case with the shortest alkyl chain, that is, CH3 and propyl sulfonic chain in the cation, the decomposition temperature (Td) reduces from 315 °C to 306 °C when the sulfonic chain length was increased from propyl to

Figure 1. A typical pathway for the preparation of some Bronsted acidic ILs.

butanesulfone were mixed and dissolved in anhydrous toluene in a dry round-bottom flask under vigorous stirring followed by dropwise addition of equal molar amounts of N-methyl/ butylimidazole, over a period of 15 min in an ice bath. Upon completing it, the system was gradually heated to room temperature and stirred for 2 h. The resultant mixture was filtered and the precipitated white solid zwitterions, that is, 1methyl/butyl-3-(propyl/butyl-3-sulfonate) imidazolium were collected. The precipitate was washed with ethyl acetate (3 × 30 mL) and then dried in a rotary evaporator at 100 °C for 12 h. The dry sample was then kept under vacuum overnight at 100 °C for further drying. To obtain the desired product of ILs, 0.02 mols of the zwitterions (1-methyl/butyl-3-(propyl/butyl-3-sulfonate) imidazolium) obtained were first dissolved in deionized water under vigorous stirring and then a corresponding stoichiometric amount of concentrated sulfuric acid was added dropwise over a period of 15 min. After completing the addition of acid, the reaction mixture was slowly heated up to 80 °C for 6 h. Then a viscous yellow IL was formed. The IL was then washed with deionized water (3 × 40 mL) followed by overnight drying under vacuum. Characterization. The structures of the ILs were confirmed using 1H NMR (Bruker Avance 300 MHz) spectroscopy in DMSO-d6 while a CHNS-932 (LECO) apparatus was used for the elemental analysis. The water content of the synthesized ILs was determined by coulometric Karl Fischer titration (Mettler Toledo DL 39) with Hydranal Coulomat AG reagent (Riedelde Haen). Thermal Properties. Thermogravimetric measurements were conducted using Perkin-Elmer TGA, Pyris I to investigate the thermal stability of the prepared ILs. A sample of ≤ 10 mg held in a capped Al pan was heated at a heating rate of 10 °C/ min from 50 °C to 600 °C under nitrogen flow. The experimental decomposition temperatures were presented in terms of weight loss (%) and TG (°C). Physical Properties. All instruments used for physical property measurements were calibrated using Millipore-quality water as described elsewhere.8−10 The instruments were also 580



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temperatures, ca. 270 and 240 °C, respectively. Figures 2 panels a and b show the Td traces of the four imidazolium salts. 2. Viscosity Analysis. From an engineering point of view, the viscosity of ILs is of prime importance as it has a major impact on the design of mixing and flow system. In addition, it also affects transport properties such as the diffusion rate.12 At 20 °C, the ILs MSPIMHSO4, MSBIMHSO4, BSPIMHSO4, and BSBIMHSO4 showed absolute viscosities of (1296, 1427, 1000, 1592) mPa·s, respectively. These values are mainly located outside the typical viscosity range reported by earlier researches which is between 66 mPa·s and 1110 mPa·s.12,13 For all the synthesized ILs except for BSPIMHSO4, the increase in the chain length of one or both sides of the alkyl chain with or without the sulfonate link on the cation side will cause an increase in the viscosity. This can be observed by the significant increase in viscosity of the ILs from 1296 mPa·s to 1427 mPa·s when the propylsulfonate chain in MSPIMHSO4 is replaced with a butylsulfonate chain to form MSBIMHSO4. Similarly, changing the propylsulfonate in BSPIMHSO4 to a butylsulfonate to form BSBIMHSO4 resulted in the increase of the ILs viscosity from 1000 mPa·s to 1592 mPa·s respectively. In another observation, the absolute viscosity of the ILs increases from 1427 mPa·s to 1592 mPa·s when the methyl chain of MSBIMHSO4 was changed to a butyl chain thus giving BSBIMHSO4. Nevertheless, when the methyl chain in MSPIMHSO4 was replaced with a butyl chain to form BSPIMHSO4, a considerable decrease in the viscosity, that is, from 1296 mPa·s to 1000 mPa·s, was observed instead. This shows that the IL BSPIMHSO4 demonstrated different viscosity behavior compared to the earlier ILs. This deviation in the viscosity trend indicates that the viscosity behavior of ILs does not necessarily follow a single trend. As for the sulfonate side chain, the viscosity behavior was consistent with the general trend as discussed earlier. This has led to the conclusion that an increase in the sulfonate side chain length could have a great effect in increasing the viscosity of the ILs. Figures 3 and 4 show the effect of temperature on the viscosity and density of the synthesized ILs, respectively. From

Figure 2. Thermogravimetric analysis (TGA) curves of the thermal decomposition of imidazolium ILs: (a) MSPIMHSO4 and BSPIMHSO4; (b) MSBIMHSO4 and BSBIMHSO4.

Table 1. Comparisons on the Thermal Decomposition Temperature of the Synthesized Zwitterions and Their ILs zwitterions/ILs

thermal decomposition temp (Td), °C

MPSIM MSPIMHSO4 MBSIM MSBIMHSO4 BPSIM BSPIMHSO4 BBSIM BSBIMHSO4

370 315 367 306 372 323 360 311

butyl. On the other hand, changing the alkyl chain from CH3 to C4H9, with propyl sulfonic chain in the cation, showed an increasing thermal decomposition temperature from 315 °C to 323 °C. On another account, varying the sulfonic chain from propyl to butyl decreases the Td to 306 °C and 311 °C for MSBIMHSO4 and BSBIMHSO4, respectively. In general, the rate of mass loss for imidazolium type ILs decreases with the increase in the alkyl chain length from methyl to butyl. Apparently, the mass loss of the imidazolium ILs increases when the length of the functional alkyl chain increases from propyl to butyl. Similar observations were reported here: varying the sulfonic chain from propyl to butyl causes the weight loss of the ILs to increase, and thereby making MSBIMHSO4 and BSBIMHSO4 less stable. MSBIMHSO4 and BSBIMHSO4 started to decompose at lower

Figure 3. Viscosities as a function of temperature for functionalized imidazolium-based ILs.

the two figures, it can be clearly seen that both the viscosity and density reduces with an increase in temperature. This is consistent with the earlier findings based on other ILs.13 The influence on viscosity was observed to be more pronounced compared to density particularly at low temperature near the 581



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effect on density was observed to be very small with only about 2 % to 3 % changes over the 60 °C temperature range tested. For example, as presented in Figure 4, the BSBIMHSO4 exhibited a small density drop from 1.3642 g/cm3 to 1.3275 g/cm3 when the temperature was raised from 20 °C to 80 °C, a reduction of only 0.0367 g/cm3. These results indicate that temperature variation has little effect on the IL densities. The above results are found to be consistent with the observation made by14 in their study on different types of ILs. 4. Molecular Volume (Vm). From the density value measured for the ILs, the molecular volume (Vm) for each of them which is the combined volume of the cation and anion can be estimated. The molecular volume (Vm) can be calculated using the eq 1 below at standard temperature and pressure condition.15,16

Figure 4. Densities as a function of temperature for functionalized imidazolium-based ILs.

Vm =

ambient condition. For example, the viscosity of BSBIMHSO4 reduces by about 91 %, when the temperature is changed from 20 °C to 50 °C as shown in Figure 3. This initial significant reduction in viscosity as the temperature was increased from ambient condition has made the application of ILs in catalysis easier at higher temperature conditions in which most reactions were performed. However, for the ILs synthesized, this advantage is true for temperatures up to around 60 °C beyond which no considerable reduction in viscosity was observed. 3. Density Analysis. For density analysis of the synthesized ILs, the same cation base, imidazolium, was used, but changes were made to the side chains that are attached to it (Table 2). It

3

structure code

g/cm

MSPIMHSO4 MSBIMHSO4 BSPIMHSO4 BSBIMHSO4

1.5017 1.4647 1.3469 1.3611

(1) −1

Where M is the molar mass in g mol , NA is Avogadro’s constant in mol−1, ρ is the density in g·cm−3 and Vm is molecular volume in cm3. The calculated values of the molecular volume (Vm) for the synthesized ILs are presented in Table 3. The values calculated were found to be within the range reported17,18 for other types of room temperature ILs, that is, [C2CN BIM]TFMS 4.08 × 10−22, choline hexanoate 3.58·10−22. Table 3. Molecular Volume Vm, Standard Entropy S0, Thermal Expansion Coefficient (αp) and Crystal Energy UPOT of the ILs at 20 °C

Table 2. Density and Viscosity Data for the Present Synthesized ILs (at 20 °C and p = 0.1 MPa)a density (ρ)

M (NAρ)

viscosity (η)

Vm 3

mPa·s

structure code

nm

1296 1427 1000 1592

MSPIMHSO4 MSBIMHSO4 BSPIMHSO4 BSBIMHSO4

0.333 0.357 0.423 0.436

Standard uncertainty u is u(T) = 0.01 °C and the combined expanded uncertainty is uc(ρ) = 3·10−4 g/cm3, uc(η) = 0.4 % mPa·s, (level of confidence = 0.95).

UPOT −1

kJ mol

442.19 434.24 416.21 413.12

thermal expansion coefficient −1

α·10 /(K ) 4

3.81 3.91 4.21 4.25

S0 JK

−1

mol−1

445.03 474.95 557.21 573.42

a

5. Standard Entropy (S0). From the calculated values of the molecular volume (Vm), the standard entropy of the syntehesized ILs could be calculated using the following equation19,20

was observed that when the methyl side chain in MSPIMHSO4 was replaced with a butyl side chain to form BSPIMHSO4, the density dropped from 1.5048 g/cm3 to 1.3504 g/cm3, respectively. Similarly the density dropped when the methyl chain in MSBIMHSO4 was replaced with a butyl chain to form BSBIMHSO4. Also a similar trend where the density dropped from 1.5048 g/cm3 to 1.4678 g/cm3 was observed when the propylsulfonate chain in MSPIMHSO4 was replaced with a butylsulfonate chain to form MSBIMHSO4. However, when a similar change was made, for example, the propylsulfonate chain in BSPIMHSO4 was replaced by butylsulfonate chain to form BSBIMHSO4, the density value increases from 1.3504 g/ cm3 to 1.3642 g/cm3 instead. The increase in the density from BSPIMHSO4 to BSBIMHSO4 could be attributed to the uniformity in spatial arrangement of alkyl groups on the imidazole ring of BSBIMHSO4. On the general effect of temperature on the ILs density, Figure 4 shows that a linear correlation could be established. These linear equations representing the various ILs could then be used for calculating the density of the 4 ILs analyzed, at any temperature. Nevertheless, the magnitude of the temperature

S 0 = 1246.5(Vm) + 29.5

(2) 3

0

where Vm is the molecular volume in nm and S is the standard entropy in J K−1 mol−1. The standard entropy values calculated for each of the synthesized ILs are presented in Table 3. Again, the values of the standard entropy calculated for the synthesized ILs in the study are found to be within similar range as reported for [CnMIM]glycinate ILs with n = 2−6 and its standard entropy ranges from 360.2 J K−1 mol−1 to 498.8 J K−1 mol−1 respectively.3 Similarly a range of standard entropy values have been reported18 for choline carboxylates ILs, that is, 371 J K−1 mol−1 to 475 J K−1 mol−1. Nevertheless, these values are lower compared to those calculated for [C2CN DIM]DOSS (1398.36 J K−1 mol−1) and [C2CN DIM]DDS (1055.74 J K−1 mol−1)17 which have lower densities. Hence it can be concluded that the standard entropy has an inverse relationship with density. 6. Lattice Energy (UPOT). The lattice energy of a salt is reflected by the strength of the interactions between its ions. 582



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ILs generally showed increasing values for the ILs having longer alkyl chain attached either to the imidazoilum ring or to the sulfonic group with an exception noted for the case where the increase in the alkyl chain from MSPIMHSO4 to form BSPIMHSO4 resulted in decreasing value, that is, from 1296 mPa·s to 1000 mPa·s. Similarly the density value was also affected by the alky chain length where a decreasing trend was observed with the increase in the alkyl chain length. Beside the effect from the alkyl chain length, the effect of temperature on both viscosity and density was also investigated. Both properties showed reduction in values with increasing temperature with the effect impacting viscosity more than density. The findings are in line with the reported work elsewhere. The molecular volume (Vm) and thermal expansion coefficient (α) were calculated using the measured values obtained for viscosity and density of the ILs, and they were found to have a direct relationship with density, while the lattice energy (UPOT) and the standard entropy (S0) values showed inverse relationship with density values.

The lattice energies of the ILs could be calculated using the eq 3, which is based on the Glasser theory.21 UPOT = 1981.2(ρ /M)1/3 + 103.8

(3) −1

where UPOT is the lattice energy in kJ mol . The calculated values for the lattice energy for all the synthesized ILs are listed in Table 3. Compared to the normal alkali chloride which has a lattice energy value of 602.5 kJ mol−1,4 the data obtained for the ILs show significantly lower values, i.e., in the range of 413 kJ mol−1 to 442 kJ mol−1. It has been reported that it is not the low packing symmetry of IL molecules that leads to its low lattice energy and thus to low melting points. Also, it has been mentioned that the lattice energies of an IL are in a range that is expected for typical salts and even higher, due to a large dispersive contribution.22 The lower values of melting point and thus lattice energy for ILs were attributed due to the large number of entropic degrees of freedom that is released upon melting; that is, in the solid state, entropy is almost fixed, and upon melting it is released which makes possible the lower values of melting point for the ILs.23 A similar range of calculated values of lattice energy for [CnMIM]alaninate, [CnMIM]glycinate (where n = 2−6), and choline carboxylates ranging from (421 to 456) kJ mol−1, (429 to 469) kJ mol−1 and (434 to 464) kJ mol−1 respectively, were reported elsewhere.18,19 And even lower calculated values were also reported for the ILs [C2CN DIM]DOSS (332 kJ mol−1) and [C2CN DIM]DDS (355 kJ mol−1).17 7. Thermal Expansion Coefficient (αp). Using the values of densities measured for the synthesized ILs, an important thermo-physical property namely the thermal expansion coefficient (α) was calculated accordingly for each IL, and the results are shown in Table 3. The calculation was conducted using the eq 4 as below9,24 αp = −

A1 1 ⎛ δρ ⎞ ⎜ ⎟ = − ρ ⎝ δT ⎠ p A 0 + A1T

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The financial assistance provided by FRGS, MOHE, and PETRONAS Ionic Liquid Centre, Universiti Teknologi PETRONAS (UTP) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Earle, M. J.; Seddon, K. R. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 2000, 72 (7), 1391−1398. (2) Zhou, Y.; Antonietti, M. Synthesis of very small TiO 2 nanocrystals in a room-temperature ionic liquid and their selfassembly toward mesoporous spherical aggregates. J. Am. Chem. Soc. 2003, 125 (49), 14960−14961. (3) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Room temperature ionic liquids and their mixturesA review. Fluid Phase Equilib. 2004, 219 (1), 93−98. (4) Holbrey, J.; Seddon, K. The phase behaviour of 1-alkyl-3methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J. Chem. Soc., Dalton Trans. 1999, 13, 2133−2140. (5) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. Ionic liquid crystals: Hexafluorophosphate salts. J. Mater. Chem. 1998, 8 (12), 2627−2636. (6) Seddon, K. R. In Molten Salt Forum: Proceedings of 5th International Conference on Molten Salt Chemistry and Technology; Dresden, Germany, August 1997; Wendt, H., Ed.; 1998; pp 53−62. (7) Liu, S.-W.; Yu, S.-T.; Liu, F.-S.; Xie, C.-X.; Li, L.; Ji, K.-H. Reactions of α-pinene using acidic ionic liquids as catalysts. J. Mol. Catal A: Chem. 2008, 279 (2), 177−181. (8) Muhammad, A.; Abdul Mutalib, M. I.; Wilfred, C. D.; Murugesan, T.; Shafeeq, A. Thermophysical properties of 1-hexyl-3-methyl imidazolium based ionic liquids with tetrafluoroborate, hexafluorophosphate and bis(trifluoromethylsulfonyl)imide anions. J. Chem. Thermodyn. 2008, 40 (9), 1433−1438. (9) Ziyada, A. K.; Wilfred, C. D.; Bustam, M. A.; Man, Z.; Murugesan, T. Thermophysical properties of 1-propyronitrile-3alkylimidazolium bromide ionic liquids at temperatures from (293.15 to 353.15) K. J. Chem. Eng. Data 2010, 55 (9), 3886−3890. (10) Murshid, G.; Shariff, A.; Lau, K.; Bustam, M.; Ahmad, F. Physical properties and thermal decomposition of aqueous solutions of 2-amino-2-hydroxymethyl-1, 3-propanediol (AHPD). Int. J. Thermophys. 2011, 32 (10), 2040−2049.

(4)

where A0 and A1 are the fitting parameters that have been determined from Figure 4, that is, the plot between density and temperature, while αP, ρ, and T are the thermal expansion coefficient, density, and absolute temperature, respectively. The thermal expansion coefficients (αP), is also known as the volume expansivity. The thermal expansion coefficient values reflect the free volume of ionic liquid; that is, higher thermal coefficient values correspond to higher free volume of ionic liquid. Similar to the other types of imidazolium-based ILs,9,25 relatively low values of thermal expansion coefficient were obtained for these ILs. These lower values could be due to more compact spatial molecular structures of these ILs.

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CONCLUSION In this study, four different types of imidazolium-based Bronstead acidic ILs were synthesized and characterized. The difference in the ILs was due to the variation in chain length of the alkyl group and alkyl sulfonic group attached to the imidazolium ring but with a fixed HSO4− anion. All the ILs structures were confirmed by 1H NMR and elemental analysis using CHNS, and the purity was found to be more than 99 %. The thermal analysis using TGA showed an increasing stability with an increase in the alkyl chain length connected directly to the imidazolium ring from methyl to butyl and vice versa when the length of the alkyl chain attached to the sulfonic group was increased from propyl to butyl. The measured viscosities of the 583



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Synthesis and Thermophysical Properties of Imidazolium-Based

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