Ind. Eng. Chem. Res. 2006, 45, 525-529
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Butyl-3-methylimidazolium Chloride Preparation in Supercritical Carbon Dioxide Zhenhuan Zhou, Tao Wang,* and Huabin Xing State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China
A novel process for preparing an ambient-temperature ionic liquid precursor (1-butyl-3-methylimidazolium chloride) was developed. The reaction of 1-methyl imidazole with butyl chloride was conducted in supercritical carbon dioxide (scCO2) and followed by in situ scCO2 extraction for purifying the product, 1-butyl-3methylimidazolium chloride. The yield of 1-butyl-3-methylimidazolium chloride could be >90%, according to the reaction in a scCO2 reaction environment. The residual reactants could be, in situ, removed completely using scCO2 extraction after the required reaction time. The conversion of the reaction is dependent positively and strongly on the pressure. The kinetics of the reaction in scCO2 could be described using the two-order irreversible reaction model. The reaction rate constant (k) is linearly dependent on the density of CO2 in the logarithmic coordinates. The 1H nuclear magnetic resonance (1H NMR) spectral data of the prepared 1-butyl3-methylimidazolium chloride product was satisfactorily consistent with its standard structure. The Fourier transform infrared (FT-IR) spectroscopy results show that the products are as pure as that made via the conventional reaction in the organic solvent/recrystallization method. 1. Introduction For ecological and economic concerns, ambient-temperature ionic liquids such as 1,3-dialkylimidazolium salts have shown good potential as attractive alternatives to the conventional organic solvents.1,2 Although originally investigated in the electrochemistry,3 1,3-dialkylimidazolium salts ionic liquids, which are nonvolatile and good solvents for a variety of organic and inorganic solutes, are currently being explored as environmentally benign solvent substitutes for the conventional volatile solvents in many fields, including the fields of chemical synthesis,4 liquid/liquid separations,5-7 dissolution,8 catalysis,9-11 bio-catalysis,12 and polymerization.13 However, the conventional preparation processes of 1,3dialkylimidazolium salts ionic liquids must use a large quantity of the alkyl halides/organic solvents.14 To synthesize the precursors (1,3-dialkylimidazolium halides), the reactions were conducted in a nitrogen atmosphere, to exclude water and oxygen. An organic solvent, such as the alkyl halide itself, 1,1,1trichloroethane, ethyl acetate, or toluene, was often used. When the reaction was terminated, it was necessary to remove the excess solvent and unreacted reactants by heating the mixture under a vacuum. The purification of the products is realized by the recrystallization from the mixture of acetonitrile and ethyl acetate. Although the ionic liquids are environmentally benign, their present preparation processes are concerned with the excessive use of volatile organic solvents and are quite complex. There is an urgent need to develop a solvent-free reactionpurification process for the preparation of 1-alkyl-3-methyl imidazolium halides. It is reported that the microwave-assisted preparation of 1,3dialkylimidazolium halides could reduce the reaction time from several hours to a few minutes, and avoid the use of excessive alkyl halides or organic solvents as the reaction medium.15 However, the continuous microwave heating may result in overheating, and lead to the formation of the colored products.16 Varma17 synthesized the ionic liquid, using an ultrasonic energy source. The reaction required shorter reaction time and lower * To whom correspondence should be addressed. Tel: +86 10 62773017. Fax: +86 10 62770304. E-mail:
[email protected].
Figure 1. Schematic diagram of supercritical CO2 reaction system. Legend is as follows: 1, CO2 cylinder; 2, cooling bath; 3, high-pressure pump; 4, buffer; 5, back-pressure regulator; 6, reactor; 7, magnetic stirrer; 8, valve; and 9, separator.
reaction temperature without the excessive the reactants or any other solvents, in contrast to the reaction under the conventional heating conditions. However, the removal of the residual halide by washing with the ethyl acetate/ether and drying under the vacuum can also lead to some environmental problems. Supercritical carbon dioxide (scCO2) is an environmentally benign solvent and has shown advantages for many separation and chemical reaction applications.18 The solubility of CO2 in the ionic liquids is relatively high, and the solubility of the ionic liquids in the scCO2 is very low. It was also reported that the wide variety of the solutes could easily be extracted from the ionic liquids using scCO2.19,20 In this work, a new method to prepare 1-butyl-3-methylimidazolium chloride (shown in Scheme 1), which is the important precursor for the preparation of ambient-temperature ionic liquids, was developed using CO2 as the reaction medium and scCO2 extraction as the purification technique. With CO2 as the solvent, 1,3-dialkylimidazolium chloride can be synthesized and purified without organic solvents. 2. Experimental Section N-methylimidazolium (99% pure) and 1-butyl chloride (99% pure) was purchased from Acros Organics. Ethyl acetate (99.7% pure) and acetonitrile (99% pure) were obtained from the Beijing Chemicals Factory (Beijing, PRC). CO2 (99.9% pure) and N2
10.1021/ie050947f CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
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Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006
Scheme 1. 1-Butyl-3-methylimidazolium Chloride Synthesis Reaction
(99.9% pure) were obtained from Beijing Analytical Apparatus Co. (Beijing, PRC). All chemicals were used without further purification. All experiments were conducted in a batch reactor, which is depicted in Figure 1. The experimental setup consisted of a 100mL stainless steel autoclave (VPH-100, Hai’an Petroleum Instruments, Jiangsu, PRC), a 100-mL separator with a cooling jacket, a high-pressure pump (model 2J-X 5/45, Hangzhou Zhijiang Science Instruments, Hangzhou, PRC) with a cooler attached, a back-pressure regulator (BY50, Xinyi Co., Beijing, PRC), and a magnetic stirrer. The reactor was equipped with pressure gauges and thermometers. The pressure was controlled by a back-pressure regulator with an accuracy of (0.1 MPa. The reactor was heated with an electrothermal jacket. The experimental temperature was measured with type-K thermocouples and was controlled within an accuracy of (1 °C. At the beginning of the experiments, the reactants (N-methylimidazolium and 1-butyl chloride) were first put into the reactor. The reactor then was sealed and heated to the desired temperature. CO2 was charged into the reactor. All the experiments were performed with magnetic stirring at a rate of 200 rpm. The pre-experiments showed that the mass-transfer resistance could be eliminated at the stirring rate. After the desired reaction time, the valve (denoted by the number 8 in Figure 1) was opened very slowly and carefully. The pressure of the reactor can be adjusted by the back-pressure regulator (denoted by the number 5 in Figure 1). The residual reactants were extracted by the scCO2 into the separator, which was cooled by the cycled cold water from the cooling bath (denoted by the number 2 in the figure). The extraction parameters, the temperature, pressure, and time were varied at different levels. The flow rate of CO2 in the dynamic extraction was maintained in the range of 1.52.0 L/min. The extract, which contains unreacted alkylimidazolium and butyl chloride, was absorbed with 50 mL toluene in the separator at the room temperature and analyzed using a gas chromatograph (HP 6869, Hewlett-Packard, Palo Alto, CA) that was equipped with a capillary column (Hewlett-Packard, model HP 19091F-112) and a flame ionization detector. The ionic liquid in the reactor was collected and weighed carefully. In contrast, 1-butyl-3-methylimidazolium chloride was also synthesized according to the conventional routine and purified via the recrystallization process. The two reactants were combined in a 100-mL round-bottomed flask/reflux condenser. Butyl chloride should be in excess. The reaction was conducted in a nitrogen atmosphere. The reaction was conducted at the desired temperature with vigorous stirring. After the desired time, the reaction mixture was cooled to the ambient temperature to precipitate the product. The liquid phase, which was mainly the excess butyl chloride, was removed carefully. The crude 1-butyl-3-methylimidazolium chloride was dissolved in the ethyl acetate, and recrystallized by adding acetonitrile dropwise into the solution. The 1-butyl-3-methylimidazolium chloride then was dried under vacuum to remove the volatile solvent residue. The purity of the produced 1,3-dialkylimidazolium chloride was checked via Fourier transform infrared (FT-IR) spectroscopy (model FTIR-8201PC, Shimadzu) and 1H nuclear magnetic resonance (1H NMR) (600 MHz, CDCl3, 20 °C).
The conversion, selectivity, and yield were calculated according to the molar quantities of the species before and after the reaction using the following equations. The conversion of each species (XR) was calculated by
XR )
M0R - MR M0R
× 100%
(1)
The yield of each species (YR) was calculated by
YR )
MIL M0R
× 100%
(2)
where M0R represents the molar quantities of the reactants before reaction, MR represents the molar quantities of the reactants after the reaction, R denotes the reactants (N-methyl imidazolium (MIm) or the butyl chloride (BuCl)), and IL denotes the product (ionic liquid ([Bmim]+Cl-)). The extraction efficiency for the residual reactant (butyl chloride) was calculated by
EBCl )
ME × 100% M0
(3)
where M0 is the amount of unreacted butyl chloride and ME is the amount of butyl chloride extracted by scCO2. Each experiment was repeated three or more times. The results that have been reported represent the average of the trials and had standard deviations of less than (1.5%. 3. Results & Discussion Table 1 lists the experimental results of the synthesis of 1-butyl-3-methylimidazolium chloride using CO2 as the reaction medium and scCO2 extraction as the purification technique. For the comparison, the results of the control experiments with the conventional synthesis and purification routine are also listed. As shown in Table 1, the proposed preparation procedure, which uses CO2 as the reaction medium and scCO2 extraction as the purification technique, provided a much higher yield of the target product than that by the conventional procedure. The main advantage of the proposed procedure is that it does not need the excess butyl chloride as the solvent. This leads to not only high yield but also an environmentally benign process. The effect of the CO2 pressure on the synthesis reaction was also investigated. The reactions were performed at 70 °C. CO2 was charged into the reactor and the pressure was set at different levels. After the desired reaction time, the reaction mixture was extracted with scCO2 at 70 °C and 15.0 MPa with a flow rate of 1.5 NL/min. The extraction time was 2 h. The data from these experiments are shown graphically in Figure 2. With the same reaction time, the reaction was accelerated as the CO2 pressure increased, especially when the CO2 pressure was higher than the critical pressure of CO2. The result indicated that the reaction rate was enhanced significantly by conducting
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 527 Table 1. Results of Preparation for 1-Butyl-3-methylimidazolium Chloride entry
temperature (°C)
reaction time (h)
mole ratio, BuCl/MIm
reaction atmosphere
purification method
XMIm (%)
YBuCl (%)
1 2 3 4 5 6 7
40 60 80 40 60 70 80
24 12 8 24 12 12 8
1.52 1.61 1.88 1.05 1.09 1.05 1.05
N2 (0.1 MPa) N2 (0.1 MPa) N2 (0.1 MPa) CO2 (0.5 MPa) CO2 (0.5 MPa) CO2 (0.5 MPa) CO2 (0.5 MPa)
recrystallizationa recrystallizationa recrystallizationa scCO2 extractionb scCO2 extractionb scCO2 extractionb scCO2 extractionb
81.2 93.9 95.7 84.7 92.5 95.5 96.1
53.4 58.3 50.9 80.7 84.9 90.9 91.5
a Recrystallization conditions were as follows: the crude solid product was dissolved in the ethyl acetate (20 mL) at room temperature; then, [Bmim]Cl was recrystallized by adding acetonitrile (15 mL) dropwise into the ethyl acetate solution and dried under vacuum at temperature T ) 80 °C for 24 h. b Conditions for scCO extraction were as follows: T ) 80 °C, pressure of P ) 8.0 MPa, time ) 2 h, scCO flow rate ) 1.5-2.0 NL/min. 2 2
Figure 2. Conversion of the synthesis of 1-butyl-3-methylimidazolium chloride versus CO2 pressure at 70 °C.
the reaction in scCO2. This resulted from the fact that the reaction pressure markedly influences the reaction rate in scCO2. As reported by Bowing and Jess,21 the produced [Bmim]+Clresulted an additional liquid phase (lower ionic liquid phase) as the [Bmim]+Cl- synthesis reaction proceeded. As they concluded, this ionic liquid phase can be regarded as the passive phase; that is, the occurrence of the ionic liquid phase does not effect the synthesis reaction in the upper phase. Bowing and Jess21 also reported that this reaction was a two-order reaction. According to their results, the synthesis reaction was modeled as a pseudo-single-phase two-order reaction. For the synthesis reaction, the kinetics was described using an irreversible primitive reaction model, which is shown as eq 4.
-
dCR ) kCMImCBCl dt
(4)
The molar ratio of the reactants (1-methyl imidazole and butyl chloride) was 1:1; therefore,
(5)
The integral result of eq 5 is as follows:
x ) kCR0t 1-x
parameter
Value
pressure (MPa) rate constant, k (L mol-1 h-1)
0.1 2.1
(6)
5.0 4.8
8.0 5.9
10.0 6.5
15.0 7.9
where x is the total conversion, t the reaction time, k the reaction rate constant, and CR0 the initial concentration of the reactants. Some results of fitting experimental data with eq 6 are shown in Figure 3. For the experimental data at 70 °C and different pressures, the fitting error was
