Received Date: October 18, 2016; Accepted Date: October 21, 2016; Published Date: October 28, 2016
Citation: Jaganathan JR, Sivapragasam M, Wilfred CD (2016) Thermal Characteristics of 1-Butyl-3-Methylimimidazolium Based Oxidant Ionic Liquids. J Chem Eng Process Technol 7: 309. doi: 10.4172/2157-7048.1000309
Copyright: © 2016 Jaganathan JR, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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This study investigates the thermal properties of a series of 1-butyl-3-methylimizolium based ionic liquids (ILs) possessing various oxidative anions such as dichromate (Cr2O72-), nitrate (NO3-), hydrogen sulphate (HSO4- ), tetrafluoroborate (BF4-), perchlorate (ClO4-) and hexafluorophosphate (PF6-). The synthesized ILs was defined by their thermal properties in terms of glass transitions, melting and decomposition temperatures. The effect of oxidant anions on activation energy of ILs were computed by non-isothermal kinetic methods i.e., through were of Ozawa and Kissinger’s methods. The most thermally stable IL was seen to be the transition metal-containing IL [bmim] Cr2O72- where it recorded the highest activation energy of 11.65 kJ/mole using Ozawa method and 15.71 kJ/mole using Kissinger method compared to other ILs. The lowest activation energy was obtained for [bmim]BF4 and [bmim] PF6. Based on the detailed thermal behavior of this high temperature oxidant ILs, it can potentially be beneficial for desulfurization applications.
Activation energy; DSC; Kinetics; Oxidant ionic liquids; TGA
Ionic Liquids (ILs), best described as “designer solvents”, have been known to be easily tunable by a selection of ionic species so as to own a plethora of chemical and physical functionalities. By combining an oxidative anion with an organic cation such imidazolium (Figure 1), oxidative ILs can be tuned to be of function in separation and extraction , catalysis [2,3] electrochemistry , analytics  and as solvents for many different processes [6,7]. For example, various anions like hydrogen sulphate (HSO4 -), tetrafluoroborate (BF4-) and hexafluorophosphate (PF6-) paired with 1-butyl-3-imidazolium (bmim) cations are widely used for oxidation of aromatic aldehydes  and oxidative desulfurization . Pyridinium dichromate  and quinolinium dichromate  based ILs have been also used as an oxidant in the oxidation of aldehydes. Past decades have seen a combination of oxidative desulfurization with imidazolium based ILs drawing a lot of attention [12-15]. Conventionally, ILs was used as an extractant; acetic acid as a catalyst and H2O2 as the oxidant. However, high concentrations of H2O2, is potentially explosive [15,16]. This common method of desulfurization is widely used but less robust in treating benzothiophenes and dibenzothiophenes; nonetheless imidazolium based ILs, are said to possess very good extractive properties on these sulfur containing compounds . In achieving deep desulfurization, stringent conditions such as high temperatures, high pressure and high hydrogen consumption are required . Hence, knowledge on the stability of ILs at high temperatures is a vital factor in preventing early degradation so as to eliminate existing discrepancies. Thermal stability of ILs depends primarily on the anion, with the effect of the cation being less significant . In this study we focus on analyzing the thermal behavior of a series of bmim based ILs with various oxidant anions such as Cr2O72-, ClO4-, NO3- (strongly coordinating anions); and HSO4 -, BF4-, PF6- (weakly coordinating anions). The effect of various oxidant anions on the activation energy (Ea) of ILs was studied by applying the Ozawa and Kissinger non-isothermal kinetic method. This is in lieu with the recommendations by the ICTAT Kinetics Committee (2011)  and Starink  that mention these kinetic methods to be one of the best iso-conversion methods available to compute activation energy based on accuracy and reliability of the results. The Kissinger method was also said to offer a significant improvement in the accuracy of the activation energy values compared to the Ozawa method. Calculating Ea is important to have a better understanding on the kinetic behavior and thermal sensitivity of any material. The minimum energy required to decompose a material at various stages can be obtained from the Ea. Cao and Mu  comprehensively reported the thermal analysis of 66 ILs (comprising of 19 cations and 20 anions) using the pseudo-zero order rate expression to obtain Ea values with five different levels of onset decomposition temperatures. However, no data was reported on Ea using Ozawa and Kissinger methods. Another study by Fereira et al.  also lacked information on aforesaid parameters but they obtained thermal stability of ILs using a polynomial function which is associated with heat capacity. Studies by Navarro et al. , emphasized on calculating Ea of binary ILs via first order model by applying Arrhenius’s Law. This work then aims to fill the gap of finding Ea of oxidant ILs based on the two models proposed; to which our knowledge has not been reported elsewhere.
The following chemicals have been used in this experiment; 1-methyl-3-butyl-imidazolium chloride (Merck, >98%), 1-methyl-3- butyl-imidazoliumbromide (Merck, >98%), Sodium tetrafluoroborate (Fluka, >98%), sodium hexafluorophosphate (Fischer, >98%), sodium hydrogen sulphate (Merck), Chromium trioxide (Sigma, >99%), sodium perchlorate (Sigma, >98%), silver nitrate (Merck Millipore), dichloromethane (Merck Millipore), decolorizing charcoal and acetonitrile (Merck Millipore). All of these chemicals were used as obtained without further purification.
Synthesis of ILs
1-methyl-3-butyl-imidazolium tetrafluoroborate, [bmim]BF4 was synthesized as per literature procedure . The [bmim]Br (10.95 g, 0.05 mol) was dissolved in 100 mL of water in a double necked round bottom flask. NaBF4 (5.49 g, 0.05 mol) was added to the mixture and the final mixture was stirred for 24 hrs at room temperature. The resultant mixture was extracted with DCM to filter out bromide salt. Due to the hydrophilicity of [bmim]BF4, the IL was dissolved in water phase leaving white sodium bromide precipitate in DCM which was filtered out. This process was repeated twice to ensure the complete removal of bromide salt. This was confirmed using silver nitrate titration. Organic solvents were then removed on a rotary evaporator under reduced pressure and [bmim]BF4 were obtained in good yield (72%). Other remaining ILs, [bmim]PF6 (70% yield), [bmim]HSO4 (68% yield), [bmim]Cr2O7] (55% yield), [bmim]ClO4 (72% yield) and [bmim]NO3 (65% yield), were synthesized via similar metathesis reaction [25-29].
Proton nuclei was measured by Nuclear Magnetic Resonance (1H NMR spectrometer Bruker Avance II Plus 500 MHz). 1H NMR spectrum for [bmim] HSO4 - is exhibited as (500 MHz, [D6]Acetone): δ: 8.98 (s,1 H), 7.75 (d, 1 H), 7.70 (d, 1 H), 4.34 (t, 2H), 4.06 (s, 3H), 1.92 (q, 2H), 1.39 (m, 2H), 0.94 (t, 3H). 13C NMR (500 MHz, [D6] Acetone): δ: 135.76, 123.37, 122.11, 49.17, 35.51, 31.17, 18.61, 12.52. Fourier Transform Infrared Spectroscopy was performed on Perkin- Elmer 6700 FT-IR in the range of 4000-400 cm-1. Water content was determined by Karl Fischer Coulometer, (Mettler Toledo) and was maintained under 1000 ppm. Thermal analysis was carried out by thermogravimetry analysis (TGA, Perkin-Elmer STA 6000) and Differential scanning calorimetry (DSC, Mettler Toledo).
FT-IR Identification of ILs
Characteristic peaks for all synthesized ILs are displayed in Table 1. FT-IR spectrograph of bmim[HSO4] is displayed in Figure 2.
|3158 and 3119(C-H stretching fromimidazole ring),2964 and 2938C-H,CH2 and CH3 stretching peak on lateral chain of imidazole ring,1574 and 1467(skeletal vibration peaks of imidazole ring)1170(in-plane bending vibration peak ofC-H fromimidazole ring)||||832, 1015, 1033 and 1045 for B-F stretching vibration1305(S=O stretching), 1198 (antisymmetric SO2 stretching) and 1164 (SOH bending modes)1088, 1347 (antisymmetric NO stretching)930, 875, 765 and 730||[12,24-27]|
Table 1: Table showing characteristics of Ionic liquids.
Glass Transition (Tg)
Melting point (Tm) and glass transition temperature (Tg) of ILs were determined by differential scanning calorimetry (DSC, Mettler Toledo), heating range from -150°C to 130°C. Highest Tg value was observed for [bmim]Cr2O7 which is 14.5°C and the lowest for [bmim] BF4 (Table 2) Melting endothermic peaks were only observed for [bmim]PF6 and [bmim]NO3.
|>Tg/°C(lit)||>Melting (DSC)Tm/°C||>Tm/°C(lit)||>Onset Degradation from TGA
|>Tonset/(°C)(lit)||>Charred Residue at 800°C|
Table 2: Table showing characteristics of Ionic liquids along with Tg, Tm and Tonset.
Decomposition of ILs
Samples were heated from 50°C-800°C at 10°C/min under N2 atmosphere. (Tonset) which represents the first temperature where any measurable weight loss was observed (Figure 3). Generally, most ILs exhibited high decomposition temperatures ranging from (300-400°C). It can be clearly observed from Figure 3 that [bmim]Cr2O7 is thermally stable even up to 800°C with 73% weight loss despite a low initial degradation step. This goes to show that the cation has little effect on decomposition temperature. In an investigation by Cao and Mu , it was concluded that the metal coordinating nature on the anion has the effect to lower thermal degradation points. Crosthwaith et al.  stated that the decomposition temperature primarily depended on the coordinating nature of the anions in the ILs. We are also able to deduce that the decomposition temperature (Tcomp) is much lower for highly coordinating anions, as observed in Table 2. The ILs, [bmim]Cr2O7 and [bmim]NO3 had the lowest degradation temperature which is due to the strongly coordinating anions compared to weakly coordinating anion species like BF4-, PF6-, ClO4- and HSO4 -. This is congenial with results from Crosthwaith et al.  where decomposition temperature for poorly coordinating anions such as bis(trifluoromethylsulfonyl)imide [Tf2N] and tetrafluoroborate [BF4] were low. In a series of pyridinium and imidazolium based ILs, it was found that the nature of cation had a small effect on the decomposition temperature (Tcomp). On the other hand, the formation of thermally stable metal oxide by [bmim] Cr2O7 may account to greater stability which produced 27.49% charred residue at 800°C. The degradation temperature of the remaining ILs which had weak coordinating anions increased in the order of [bmim]ClO4 < [bmim]HSO4 < [bmim]BF4 < [bmim]PF6. The effect of anion on density is relatively obvious and it increases with an increase in size of anion . ‘Inter molecular’ attraction also contributed to high degradation temperatures. For example, the PF6- anion has an octahedral shape which is more symmetrical compared to the tetrahedral ClO4- anion, thus making the van der Waals attractive forces stronger between PF6- compared to ClO4- anion. This correlates with a high density value (Table 3) obtained for [bmim]PF6 (1.33 g/cm3) compared to that of [bmim]ClO4 (1.26 g/cm3). Halogen containing anions such as [BF4- ] and [PF6- ] are unstable and tend to decompose to hydrogen fluoride (HF) in the presence of water and disrupts the purity of ILs. Another relaying factor affecting thermal stability of ILs is the nucleophilicity and the hydrophobicity of the corresponding anions . The effect of various heating rates for [bmim]NO3 is shown in Figure 4. At high heating rates, the degradation peak shifted to a higher temperature for all cases and ILs were thermally stable at a high heating rate. Data extracted from Figure 4 are the vital factors in computing degradation kinetics of ILs in the next section.
|ILs||Linear Correlation Factor(R2)||Average Ea by Ozawa (kJ/mole)||Linear Correlation Factor(R2)||Average Ea by Kissinger (kJ/mole)|
Table 3: Table showing linear correlation factors of Ionic liquids.
Degradation kinetics of ILs
The Ozawa and Kissinger method were applied to quantify the value of Ea despite various available methods for the evaluation of the non-isothermal kinetic parameters. An advantage of these methods is that it does not require knowledge of the reaction mechanism and has reliable accuracy of Ea .
Arrhenius equation generally can be written as follows;
Where Ea is the activation energy for decomposition, K0 is the pre-exponential factor and R is the gas constant. The non-isothermal decomposition is characterized by a constant heating rate. The relationship between the sample temperature T and the heating rate can be written as;
Kissinger and Ozawa equations can be written with the highest rate of decomposition at maximum peak approximations, respectively;
Tp is maximum decomposition temperature, β; heating rate, Β values were obtained by TGA at several heating rates. Heating rates of 10°C/min, 15°C/min, 20°C/min and 25°C/min were used in this study.
The value of Ea can be computed by Ozawa and Kissinger’s method for any particular degree of decomposition which can be determined from the linear dependence of ln β versus 1/Tp (Figure 5 - Ozawa) and ln(β/T2) versus 1/Tp (Figure 6 - Kissinger) at different heating rates without knowledge of the reaction order. E value can be directly obtained at a fixed conversion with the slope being- Ea /R. Based on results in Table 4, it can be said that the Ea obtained by Kissinger method recorded slightly higher results than the Ozawa method, but remained in a comparable range. [bmim]BF4 and [bmim]PF6 had similar yet lowest Ea values compared to other ILs. Increasing trend in Ea was also observed upon switching from fluorinated to oxygenated anion. This could be explained by the formation of oxides by the anion at various stages which acted as a thermal barrier providing a complicated pathway for breakdown to take place during the heating process. This in turnresulted in an increase in the decomposition temperature. However, the Ea for [bmim]Cr2O7 is the highest compared to all other oxidant despite exhibiting low degradation temperature. Such complexed thermal profile is most likely due to the presence of more than one crystallographic structures of dichromate ion as discussed by  and the formation of metal oxides which led to greater thermal resistance.
|Bmim Cr2O-7||Dark brown||1.43|
|Bmim HSO4||Mild Brown||1.28|
|Bmim ClO4||Dark Brown||1.26|
|Bmim NO3||Mild Brown||1.39|
|Bmim BF4||Light Brown||1.31|
Table 4: Table showing Appearance and Densities of Ionic liquids.
Thermal characteristics of 1-butyl-3-methylimimidazolium based oxidant ILs were studied by TGA and DSC. Non-isothermal kinetic technique by Ozawa and Kissinger’s was applied to determine the change in Ea for various oxidant anions. Ea for ILs with fluorinated anion were found to be lower than the oxygenated containing anions. Increasing trend in Ea and decomposition temperatures were observed as the oxygen atoms in the anion increases. The density due to molar mass of anion was also believed to contribute to greater thermal stability of ILs.
The present work has been supported by the Centre of Research in Ionic Liquids (CORIL), Universiti Teknologi, Petronas, Malaysia.