ARTICLE pubs.acs.org/IECR
Removal of Trace Olefins from Aromatics at Room Temperature Using Pyridinium and Imidazolium Ionic Liquids Yu Sun and Li Shi* The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P.R. China ABSTRACT: [BuPy]BrAlCl3 and [BMIm]BrAlCl3 were used to remove trace olefins from aromatics. At room temperature, almost 100% olefin conversion was obtained using [BuPy]BrAlCl3 as the catalyst. The effects of the cation and the dosage of AlCl3 in the ionic liquid were investigated. It was found that the dosage of AlCl3 had a significant impact on the reaction, whereas the organic cation type mainly influenced the conversion. Anions in [BuPy]BrAlCl3 such as AlCl4 and Al2Cl7 were shown to exist by ESI-MS, and the mechanism of the reaction catalyzed by [BuPy]BrAlCl3 was studied. The investigation of various reaction parameters showed that the process saves time, consumes less energy, separates products more easily, and produces less pollution to the environment than conventional methods to remove trace olefins from aromatics.
1. INTRODUCTION Aromatics, which are mainly derived from processes such as naphtha reforming and thermal cracking, always contain hydrocarbon contaminants including mono-olefins, dienes, and styrenes.1 These trace olefins have high chemical activities to form colloids that will affect the quality of the aromatics and can cause some adverse effects in subsequent processes.2 Therefore, methods to remove trace olefins from aromatics have been studied, for example, hydrotreating and clay treatment. However, the former operates at high temperature, and it is difficult to fully take into account the depth of hydrogenation reactions, which will cause the loss of aromatics. The latter has more problems, such as easy inactivation, short operation cycle, and heavy environmental pollution.3 Consequently, the possibility of exploiting environmentally friendly catalytic technology has been of keen interest in the removal trace olefins from aromatics. Ionic liquids have gained a great deal of attention within the past 20 years as relatively clean catalysts and solvents.4,5 Many organic reactions have been reported to proceed in ionic liquids with excellent yields and selectivity.6 Because of the nonvolatile characteristic of ionic liquids, the product can be separated from the catalyst without any solvent contamination.7,8 In particular, extensive studies on FriedelCrafts alkylation reactions catalyzed by ionic liquids have been done with high catalytic activities.9,10 In the present work, typical ionic liquids, pyridine and imidazole, were used to remove trace olefins from aromatics at room temperature, and the reaction mechanism was investigated. Electrospray ionization mass spectrometry (ESI-MS) was used to characterize the synthesized ionic liquids. The influences of various reaction parameters such as reaction time, reaction temperature, dosage, and type of ionic liquid were investigated. 2. EXPERIMENTAL SECTION 2.1. Materials. N-Methylimidazole (density, 1.03 g/mL; purity, >99%) was purchased from Shanghai Chemical Reagent Co., Ltd., Shanghai, China. Pyridine (density, 0.98 g/mL; purity, g99.5%) and 1-bromobutane (boiling point, 375 K; density, r 2011 American Chemical Society
1.28 g/mL; purity, CP) were purchased from Shanghai Ling Feng Chemical Reagent Company, Shanghai, China. AlCl3 (AR) and CaCl2 (AR) were purchased from Sinopharm Chemical Reagent Company, Shanghai, China. The experimental raw materials were aromatic intermediate products that had not been subjected to clay treatment from the industrial reforming units of Sinopec Zhenhai Refining & Chemical Company. The main olefins were C8C10 long-chain olefins, as detailed in Table 1. 2.2. Synthesis of Ionic Liquid Precursors. N-Butylpyridinium bromide ([BuPy]Br) was prepared as follows: 1-Bromoethane was placed in a three-neck round-bottom flask attached to a reflux condenser and equipped with a magnetic agitator and a glass thermometer, and N-butylpyridinium was added using a dropping funnel at 333338 K in the flask. After 12 h of reaction, the mixture was allowed to cool to room temperature, and the solvent was removed through distillation under a vacuum, with N-butylpyridinium bromide ([BuPy]Br) remaining in the flask. 1-Butyl-3-methylimidazolium bromide ([BMIm]Br) was prepared as follows: N-Methylimidazolium was placed in a threeneck round-bottom flask attached to a reflux condenser and equipped with a magnetic agitator and a glass thermometer, and 1-bromoethane was added through a dropping funnel at 343348 K in the flask. The reactant mixture was then cautiously heated to reflux for several hours until the mixture became yellowish. The mixture was allowed to cool to room temperature, and the solvent was removed through distillation under a vacuum, with 1-butyl-3-methylimidazolium bromide ([BMIm]Br) remaining in the flask. 2.3. Synthesis and Characterization of Ionic Liquids. Butylpyridinium bromochloroaluminate ([BuPy]BrAlCl3, AlIL) was prepared according to the following procedure: A dilute Received: March 16, 2011 Accepted: June 22, 2011 Revised: June 18, 2011 Published: June 22, 2011 9339
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aqueous solution of [BuPy]Br was placed in a glass flask. N2 gas was introduced into the flask to ensure that the synthesis process took place in an inert atmosphere. AlCl3 was added batchwise under constant stirring. The reaction was left stirring overnight at room temperature to ensure the perfect homogenization of the resulting Al-IL. The whole system was kept under a dry nitrogen atmosphere to avoid the hydrolysis of AlCl3. Preparation of 1-butyl-3-methylimidazolium bromochloroaluminate ([BMIm]BrAlCl3) was carried out by a similar method. The ionic liquids used in this work had different molar ratios of AlCl3 to [BuPy]Br or [BMIm]Br. This ratio (X) is defined as X ¼ n1 =ðn1 þ n2 Þ
ð1Þ
where n1 represents the number of moles of AlCl3 and n2 represents the number of moles of [BuPy]Br (or [BMIm]Br). Ions in the ionic liquids were certified by mass spectrometric detection using ESI at an electron energy of 70 ev. 2.4. Removal of Olefins and Analysis. All reactions were conducted in an Erlenmeyer flask. The aromatic and a certain amount of the ionic liquid as a catalyst were added to the flask, and with the help of a magnetic stirrer, the mixture was allowed to react at a certain temperature (293333 K) for a definite period of time (220 min). After the reaction had terminated, the mixture was allowed to settle for 23 h prior to separation of the aromatic with less olefins from the ionic liquid by a separating funnel. Olefinic bonds were quantified by the bromine index (BI).11 The number of grams of bromine absorbed by 100 g of a hydrocarbon or hydrocarbon mixture indicates the percentage of double bonds present. The experimental raw materials were aromatic intermediate products that had not been subjected to clay treatment with a BI value of 1120. In this work, all samples were analyzed with an LC-2 bromine index detector. The olefin
conversion was calculated as Y ¼ ðBI0 BIÞ=BI0 100%
ð2Þ
where Y represents the olefin conversion, BI0 represents the bromine index of the raw material, and BI represents the bromine index of the material treated by the ionic liquid.
3. RESULTS AND DISCUSSION 3.1. Identification of Cations. The cations of the ionic liquids were identified by ESI-MS (Figure 1). The N-butylpyridinium cations had a relative abundance of 100% for a value of m/z = 136.1, whereas the 1-butyl-3-methylimidazolium cations had a relative abundance of 100% for a value of m/z = 139.1. 3.2. Characterization of [BuPy]BrAlCl3. The anions of [BuPy]BrAlCl3 were characterized by ESI-MS (Figure 2). Both mononuclear and polynuclear anionic species were detected. Because of sensitivity to air, except for AlCl4 and a low intensity of Al2Cl7, [BuPy]BrAlCl3 mainly contained oxybromochloroaluminate anion species such as Al2Cl4OH, Al2OCl5, Al4Cl6O3H2, Al2Cl4BrOH, and Al4Cl6BrO. 3.3. Effects of Cation and Dosage on Olefin Removal. Because of their excellent chemical and physical properties, ionic liquids are attracting a great deal of attention as possible replacements for conventional catalysts and solvents. By a judicious combination of cations and anions, it is possible to
Table 1. Composition of the Aromatics in the Raw Materiala from Sinopec Zhenhai Refining & Chemical Company component
component
content (wt %)
toluene
0.238
C9 aromatics
32.622
ethyl benzene
7.885
C10 aromatics
6.81
others
0.238
xylene a
content (wt %)
52.207
Bromine index of 1120.
Figure 2. ESI-MS of anions of [BuPy]BrAlCl3 ionic liquids: X = 0.75.
Figure 1. ESI-MS of ionic liquids with different cations. 9340
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adjust the catalytic performance to the requirements of the reaction, thus creating an almost indefinite set of “designer media”. In the present work, the catalytic activities of [BuPy]BrAlCl3 and [BMIm]BrAlCl3 in the removal of trace olefins from aromatics were investigated and are reported in Table 2. For the same value of X = 0.67, the two types of ionic liquids exhibited excellent effects on the removal of olefins. In addition, nearly 100% conversion of olefins was realized at the [BuPy]BrAlCl3 dosage of 2%, whereas a [BMIm]BrAlCl3 dosage of more than 5% was able to achieve conversions greater than 99%. Pyridine and imidazole each contain six π-electrons. Their electronic structures are in agreement with H€uckel’s (4n + 2) πelectron rule, so pyridine and imidazole can be considered aromatic substances. The nitrogen atom of imidazole is sp2hybridized and provides two electrons to form the large π-bond, Table 2. Effect of Cation Type on the Catalytic Performance of Ionic Liquidsa olefin conversion (%)
a
dosage of
catalyzed by
catalyzed by
catalyst (wt %)
[BMIm]BrAlCl3
[BuPy]BrAlCl3
0.1
11.16
12.49
0.5 1
58.75 83.41
63.04 83.82
2
98.77
100
3
98.86
100
4
98.85
100
5
100
100
6
100
100
Reaction conditions: X = 0.67, T = 293 K, t = 10 min.
whereas all of the π-electrons of pyridine are provided by the double bond. Thus, the electron cloud of pyridine is more evenly distributed than that of imidazole, and the conjugation is more completely in pyridine. Consequently, pyridine has stronger aromatic character than imidazole, and the cations of N-butylpyridinium are more stable than those of 1-butyl-3-methylimidazolium. This means that the anions associated with N-butylpyridinium cations are more stable than those associated with [BMIm]BrAlCl3. Further, according to a previous study,3 the possible mechanism of olefin removal from aromatics is FriedelCrafts alkylation reactions, as discussed further below. Alkylation reactions are governed by the mechanism of the carbenium ion. More stable cations and anions of the ionic liquid used as the catalyst can more easily lead to and stabilize the carbenium ion intermediate. Thus, [BuPy]BrAlCl3 exhibited higher catalytic activities than [BMIm]BrAlCl3. On the other hand, the effects of the dosage of AlCl3 on the catalytic performance of the ionic liquids were investigated and found to be significant (Figure 3). For a given ionic liquid, the catalytic activity has improved as the X value increased. However, for the same dosage of AlCl3, [BuPy]BrAlCl3 always exhibited better performance than [BMIm]BrAlCl3 in terms of catalytic activity. Therefore, it was concluded that the type of organic cation in the ionic liquid mainly influences the conversion, although the dosage of AlCl3 also has a significant impact on the reaction. Anhydrous aluminum chloride can be used as a catalyst in FriedelCrafts alkylation reactions, although there are a series of questions related to long-term use of anhydrous aluminum chloride as a catalyst, such as the large amount of catalyst required and the inhibitors needed.12 Using ionic liquids as new catalysts can overcome these shortcomings and provide higher catalytic activities. Of the ionic liquid catalysts screened
Figure 3. Effects of the dosage of AlCl3 in ionic liquids on the catalytic performance: T = 293 K, t = 10 min. 9341
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Figure 4. Effects of reaction temperature on reaction: X = 0.75, t = 10 min, wcatalyst = 0.5%.
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Figure 6. Plot ln(C/C0) versus time.
Scheme 1. Possible Catalytic Process for the FriedelCrafts Alkylation Reaction of Aromatics
Figure 5. Effects of reaction time on reaction: X = 0.75, T = 293 K, wcatalyst = 0.5%.
(Figure 3), [BuPy]BrAlCl3 was found to have a surprisingly high activity, giving almost 100% conversion of trace olefins. 3.4. Effect of Temperature on the Reaction. Only a slight change in the conversion was observed when the reaction temperature was increased from 293 to 333 K (Figure 4). The catalyst [BuPy]BrAlCl3 exhibited outstanding catalytic properties at the ambient temperature with a dosage of ionic liquid of 0.5%. We concluded that this process consumes less energy. 3.5. Effect of Time on the Reaction. The effect of time on the reaction catalyzed by [BuPy]BrAlCl3 was investigated at ambient temperature by varying reaction time from 2 to 20 min (Figure 5). The conversion was found to increase linearly as the reaction time was prolonged from 2 to 16 min, and then it became constant as the reaction proceeded. The reaction time could be decreased at elevated reaction temperature, but this would be meaningless as the reaction time was short enough. Thus, the optimum reaction time was found to be 16 min, and almost 100% conversion of olefins was reached. 3.6. Kinetics of Olefin Removal. A plot of ln(C/C0) versus reaction time, where C0 is the initial concentration of olefins and C is the instantaneous concentration at some moment, showed excellent linearity (Figure 6). Thus, the reaction is clearly quasifirst-order in olefins. The first-order rate constant calculated from the plot in Figure 6 is k = 0.096 min1, which implies a high activity of the chloroaluminated ionic liquid in the alkylation reaction.
Wicelinski and Gale characterized chloroaluminate ionic liquids by fast-atom-bombardment mass spectrometry (FAB-MS).13 AlCl4, an unexpectedly low intensity of Al2Cl7, and various oxychloro and hydroxychloro species of Al could always be detected in both [BuPy]ClAlCl3 and [EMIm]ClAlCl3. Mantz et al. also confirmed that basic melts contained the chloride and tetrachloroaluminate anions.14 As shown in Figure 3, AlCl4 and Al2Cl7 were detected in [BuPy]BrAlCl3. The amount and species of oxychloro and hydroxychloro ions of Al present in ionic liquids are uncertain and depend on the environment, whereas AlCl4 and Al2Cl7 exist inherently in the ionic liquids regardless of the ambient conditions. Therefore, we concluded that AlCl4 and Al2Cl7 play important roles in the function of [BuPy]BrAlCl3 as a catalyst. The reasons for the low intensity of Al2Cl7 might be that Al2Cl7 is highly susceptible to fragmentation in the presence of traces of moisture or oxygen. On the basis of previous studies and the above results, a possible reaction mechanism was devised (Scheme 1).7,15 According to this mechanism, HCl is produced by the reaction of the ionic liquids and adventitious water in the air (eq 3). An activated proton is produced by the reaction of HCl with an anionic species such as Al2Cl7. From Scheme 1, one can see that the reaction catalyzed by [BuPy]BrAlCl3 is a FriedelCrafts reaction, which is one of the most thoroughly studied and recognized catalytic processes. Because of the stable anions of this system, carbonium ions are easily generated and are extremely stable, which is beneficial for the reversible reaction (eq 5) to occur in the positive direction. According to this reaction mechanism, a catalyst with stable anions and acidity will be advantageous to the FriedelCrafts 9342
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reaction. Most importantly, this reaction mechanism provides the principles for choosing and creating the most effective catalyst for the removal of trace olefins from aromatics.
4. CONCLUSIONS Butylpyridinium bromochloroaluminate ionic liquid was found to efficiently catalyze the removal of trace olefins from aromatics. At room temperature, with a molar ratio of AlCl3 to [BuPy]Br (X) of 0.75 and a [BuPy]BrAlCl3 dosage of 0.5%, almost 100% conversion of olefins was achieved, and a reaction time of 16 min was sufficient to achieve this goal. The high activity of this catalyst can be explained on the basis of the aromatic character of the pyridine ring and the acidity of the ionic liquid produced in this system, as generalized in the proposed reaction mechanism. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 021-64252274.
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