Mixed Chlorometallate Ionic Liquids as C 4 Alkylation Catalysts: A Quantitative Study of Acceptor Properties

: The acceptor properties of mixed chlorometallate ionic liquids for isobutane-butene alkylation (C 4 alkylation) reaction were studied. These ionic liquids were prepared by mixing metal chlorides with either triethylamine hydrochloride or 1-butyl-3-methylimidazolium chloride in various molar ratios. Using triethylphosphine oxide as a probe, Gutmann Acceptor Numbers (AN) of the catalysts were determined, and the Lewis acidity of mixed chlorometallate ionic liquids was quantitatively measured. Additionally, AN value was developed to determine the relationship between Lewis acidity and catalytic selectivity. The favorite AN value for the C 4 alkylation reaction should be around 93.0. The [(C 2 H 5 ) 3 NH]Cl–AlCl 3 − CuCl appears to be more Lewis acidity than that of [(C 2 H 5 ) 3 NH]Cl–AlCl 3 . The correlation of the acceptor numbers to speciation of the mixed chlorometallate ionic liquids has also been investigated. [AlCl 4 ] − , [Al 2 Cl 7 ] − , and [MAlCl 5 ] − (M = Cu, Ag) are the main anionic species of the mixed chlorometallate ILs. While the presence of [(C 2 H 5 ) 3 N · M] + cation always decreases the acidity of the [(C 2 H 5 ) 3 NH]Cl − AlCl 3 − MCl ionic liquids. paper, the Lewis acidity of mixed chlorometallate ILs for isobutane alkylation was quantitatively studied by determining the AN values. The relationship between the Lewis acidity of ionic liquids and their catalytic performance of C 4 alkylation has been studied. In addition, the speciation of ions has been determined for [(C 2 H 5 ) 3 NH]Cl − AlCl 3 − CuCl mixed chlorometallate systems. The correlation of the observed changes in the acidity to speciation has also been investigated.


Introduction
In the last decade, the use of chloroaluminate ionic liquids (ILs) to replace conventional acid catalysts has received much attention [1,2]. Due to their versatile properties, ionic liquids have been used in many fields [3]. The applications of chloroaluminate ILs primarily focus on the industrial Friedel-Crafts alkylation, oligomerization, and isomerization reactions of olefins [4]. A well-known example of catalysis in chloroaluminate ILs may be the alkylation of isobutane and butenes (C 4 alkylation) [5]. In this reaction, [C 4 mim]Cl-AlCl 3 [6], [(C 2 H 5 ) 3 NH]Cl-AlCl 3 [7], and amide-AlCl 3 -based [8] ionic liquids were used instead of concentrated sulfuric acid and hyper-toxic hydrogen fluoride. In addition, the isomerization of dicyclopentadienes could be catalyzed by [Hpy]Cl-AlCl 3 chloroaluminate ILs [9,10]. Particularly, a pilot-scale oligomerization of the olefins process has been established by BP (British Petroleum Company plc), which uses the [(CH 3 ) 3 NH]Cl-AlCl 3 ionic liquid as a catalyst. Akzo Nobel also developed an industrial application of the benzene alkylation employing a similar chloroaluminate ionic liquid [11].
Introducing metal salts to chloroaluminate ILs can either change the activity of chloroaluminate anions, or coordinate the added metal ions to chloroaluminate anions [12]. Compared with net chloroaluminate ILs, this feature of mixed chlorometallate ILs might be one of the key benefits for some reactions. For example, a significant catalytic activity could be observed in a Beckman rearrangement reaction of acetophenone oxime, when the mixed metal [(C 2 H 5 ) 3 NH] 2x [(1−x)AlCl 3 + xZnCl 4 ] double salt ILs were adopted. The activity of the ionic liquid was found even higher than that of [(C 2 H 5 ) 3 NH][Al 2 Cl 7 ] or AlCl 3 [13]. Zinurov et al. [14] have found that the route of the n-pentane isomerization could be controlled by using [(CH 3 ) 3 NH]Cl-AlCl 3 /copper-salt mixtures. Additionally, high yields of branched olefin polymers can be obtained by adding TiCl 4 to the [C 4 mim]Cl-AlCl 3 ionic liquid [15]. Yang et al. also found that the mixed chlorometallate [(C 2 H 5 ) 3 NH]Cl-FeCl 3 -CuCl might enhance the conversion and selectivity of the isobutene oligomerization [16]. Perhaps the most famous application of mixed chlorometallate ionic liquids is the Difasol process, which was developed by IFP (Institut Francais Du Petrole) and the [C 4 mim][AlCl 4 ] -EtAlCl 2 -NiCl 2 ionic liquid was employed for olefin dimerization [17,18].
As mentioned above, the C 4 alkylation reaction is one of the most important IL applications. However, the requirement of acidity for the C 4 alkylation reaction is very strict. When the acidity of the catalyst is strong, the product would contain many C 5 −C 7 fractions. In contrast, if the acidity is weak, the C 9+ byproducts would be dominant. In order to improve the catalytic performance of ILs, the super acidity of chloroaluminate ILs usually need to be tuned by other compounds. In the cases of [Bmim]Cl−AlCl 3 and [(C 2 H 5 ) 3 NH]Cl−AlCl 3 , the introduction of CuCl to these chloroaluminate ILs is beneficial to increase the content of the desired trimethylpentanes (TMP, C 8 fractions). Similarly, the presence of AgAlCl 4 in MTBE−AlCl 3 solution often leads to a high TMP selectivity. Adding CuCl to the ether-AlCl 3 system or the amide-AlCl 3 -based ionic liquid analogues also significantly increased the TMP content. Therefore, the transition metal chlorides can significantly improve the catalytic performance of chloroaluminate ILs. This improvement is often interpreted as the decrease of super-acidity of the chloroaluminate ILs [19].
The size, shape, and relative energy of the acid and the base will markedly affect the interaction of a Lewis acid-base pair. It means that the base of a Lewis acid interacting with can determine the acid strength [5]. Although a universal scale of Lewis acidity could not be established [20], several studies on the determination of the quantitative behavior of chloroaluminate ILs have been reported. For example, Thomazeau et al. have measured the acid strength of a series of imidazolium ILs by using UV-vis spectroscopy and Hammett indicators [21]. The acceptor properties of chlorometallate ILs have been studied by Osteryoung [22,23] and Swadźba-Kwaśny [24], respectively. In addition, the Lewis acidity of halometallate-based ILs and the basicity of hydrogen bond could be obtained by X-ray photoelectron spectroscopy [25]. Using infrared spectroscopy and pyridine probe, Kou et al. [26] developed a new method for determining the acid strength of chloroaluminate ILs, and Hu [27] made a more detailed measurement for these ILs. However, the Lewis acidity of the mixed chlorometallate ILs for C 4 alkylation reaction is still ambiguous.
Apart from the changes of acidity, adding metal salts to chloroaluminate ILs would lead to an interaction of the added metal ions and the chloroaluminate anions. In the previous works, we found that the catalytic performance of C 4 alkylation is probably determined by the compositions of the mixed chlorometallate ILs rather than by the Lewis acid strength. That is, the presence of heterometallic chlorometallate anion [CuAlCl 5 ] − might improve the catalytic selectivity. This anion has been detected in the 27 Al NMR spectra of [Bmim]Cl−AlCl 3 −CuCl and [(C 2 H 5 ) 3 NH]Cl−AlCl 3 −CuCl ionic liquids [28][29][30]. However, other researchers argued that the peak of [CuAlCl 5 ] − should be identified as the signal of [Al 2 Cl 6 OH] − [31]. They proposed that Cu(I) could substitute the proton in the [(C 2 H 5 ) 3 NH] + cation and generate [(C 2 H 5 ) 3 N·Cu] + and HCl [19]. Generally, the speciation of chlorometallate ionic liquids are closely related to their Lewis acidity [1,5,32]. Thus, a quantitative study of the Lewis acidity may be helpful to clarify the speciation of the ionic liquids.
We want to make a quantitative investigation on the acidity of mixed chlorometallate ILs by using 31 P NMR chemical shifts. Gutmann Acceptor Number (AN) is considered as an effective methodology for quantitatively determining the Lewis acidity [33]. When triethylphosphine oxide (tepo) was added to the acidic sample, the coordination of tepo with the Lewis acid always induces a change of the 31 P NMR chemical shift, and then the AN can be calculated by the equation: AN = 2.348 × δ inf . In order to obtain the δ inf value ( 31 P chemical shift at infinite dilution of tepo), the chemical shifts of 31 P NMR at several concentrations of tepo need to be measured at first, and then these data should be In this paper, the Lewis acidity of mixed chlorometallate ILs for isobutane alkylation was quantitatively studied by determining the AN values. The relationship between the Lewis acidity of ionic liquids and their catalytic performance of C 4 alkylation has been studied. In addition, the speciation of ions has been determined for the [Bmim]Cl−AlCl 3 −CuCl and [(C 2 H 5 ) 3 NH]Cl−AlCl 3 −CuCl mixed chlorometallate systems. The correlation of the observed changes in the acidity to speciation has also been investigated.

Estimation of δ i,cor at Infinite Dilution
The Gutmann Acceptor Number is an experimental procedure to evaluate the Lewis acidity of molecules. Generally, triethylphosphine oxide (tepo) is very sensitive to the Lewis acidic environment. When tepo is used as a probe molecule, the interaction between tepo and the Lewis acid will cause deshielding of the 31 P chemical shift of tepo. Therefore, the Lewis acidity of ionic liquids could be assessed by 31 P NMR spectroscopy. The AN values of Lewis acidic compounds are usually between the two reference points of the weak Lewis acid hexane (δ = 41.0 ppm, AN = 0) and the strong Lewis acid SbCl 5 (δ = 86.1 ppm, AN 100). Thus, an acceptor number scale for solvent Lewis acidity is established. The acceptor numbers can be calculated from the equation AN = 2.21 × (δ sample -41.0). Higher AN value often indicates that the compound has a greater Lewis acidity. For example, the AN of AlCl 3 is 87 and the AN value is 70 for transition-metal compound TiCl 4 , which all display Lewis acidic properties.
In all studied ILs, the 31 P NMR (Nuclear Magnetic Resonance) signal of tepo was a single peak. Because of the volume susceptibility differences between the studied ILs and hexane, the acceptor numbers of ILs should be calculated by extrapolating the chemical shift to infinite dilution. It means that the 31 P NMR chemical shifts of infinite dilution tepo in the ionic liquid (δ inf ) must be obtained at first. Here, the values of δ inf were determined by extrapolation, and the acceptor numbers were calculated through AN = 2.348 × δ i,cor instead of using AN = 2.21 × (δ sample -41.0). The number of δ i,cor is the susceptibility-corrected value, which is defined as the infinite-dilution chemical shift of the probe molecule in a solvent (i) relative to that of the probe molecule in hexane. For instance, the 31 P NMR chemical shift of tepo with different concentrations in [(C 2 H 5 ) 3 NH]Cl−AlCl 3 and the fitted straight lines are depicted in Figure 1. These data (δ exp ) were fitted by regression analysis to get a linear equation: δ exp = mc tepo + δ i,cor . The values of m and R 2 were collected in Table 1, and the δ i,cor values were finally obtained by the linear regression approach. In this paper, the Lewis acidity of mixed chlorometallate ILs for isobutane alkylation was quantitatively studied by determining the AN values. The relationship between the Lewis acidity of ionic liquids and their catalytic performance of C4 alkylation has been studied. In addition, the speciation of ions has been determined for the [Bmim]Cl−AlCl3−CuCl and [(C2H5)3NH]Cl−AlCl3−CuCl mixed chlorometallate systems. The correlation of the observed changes in the acidity to speciation has also been investigated.

Estimation of δi,cor at Infinite Dilution
The Gutmann Acceptor Number is an experimental procedure to evaluate the Lewis acidity of molecules. Generally, triethylphosphine oxide (tepo) is very sensitive to the Lewis acidic environment. When tepo is used as a probe molecule, the interaction between tepo and the Lewis acid will cause deshielding of the 31 P chemical shift of tepo. Therefore, the Lewis acidity of ionic liquids could be assessed by 31 P NMR spectroscopy. The AN values of Lewis acidic compounds are usually between the two reference points of the weak Lewis acid hexane (δ = 41.0 ppm, AN = 0) and the strong Lewis acid SbCl5 (δ = 86.1 ppm, AN 100). Thus, an acceptor number scale for solvent Lewis acidity is established. The acceptor numbers can be calculated from the equation AN = 2.21 × (δsample -41.0). Higher AN value often indicates that the compound has a greater Lewis acidity. For example, the AN of AlCl3 is 87 and the AN value is 70 for transition-metal compound TiCl4, which all display Lewis acidic properties.
In all studied ILs, the 31 P NMR (Nuclear Magnetic Resonance) signal of tepo was a single peak. Because of the volume susceptibility differences between the studied ILs and hexane, the acceptor numbers of ILs should be calculated by extrapolating the chemical shift to infinite dilution. It means that the 31 P NMR chemical shifts of infinite dilution tepo in the ionic liquid (δinf) must be obtained at first. Here, the values of δinf were determined by extrapolation, and the acceptor numbers were calculated through AN = 2.348 × δi,cor instead of using AN = 2.21 × (δsample -41.0). The number of δi,cor is the susceptibility-corrected value, which is defined as the infinite-dilution chemical shift of the probe molecule in a solvent (i) relative to that of the probe molecule in hexane. For instance, the 31 P NMR chemical shift of tepo with different concentrations in [(C2H5)3NH]Cl−AlCl3 and the fitted straight lines are depicted in Figure 1. These data (δexp) were fitted by regression analysis to get a linear equation: δexp = mctepo + δi,cor. The values of m and R 2 were collected in Table 1, and the δi,cor values were finally obtained by the linear regression approach.

AN Values and C 4 Alkylation Performance
In this work, 31 P NMR chemical shifts of the ILs for alkylation reaction were determined. The chemical shifts of those ILs with weaker Lewis acidity [34,35], such as [(C 2 H 5 ) 3 NH]Cl−CuCl and [(C 2 H 5 ) 3 NH]Cl−ZnCl 2 , benzene−CuAlCl 4 , and ether−CuClAl 4 , were also measured for comparison. For convenience, the apparent molar ratio of organic salt to AlCl 3 was 1:1.6, while the apparent molar ratio of MCl x (M = Cu, Ag, or Zn; x = 1 or 2) to AlCl 3 was 0.5:1. Generally, the metal chlorides would react with the ions of net chloroaluminate ILs. Although some of the transition metal chlorides at this ratio might not be completely dissolved to the chloroaluminate ionic liquids, it did not significantly reduce the catalytic activities of the chloroaluminate ILs. To better understand these changes, we measured the AN values at first, and then the catalytic selectivities of various ILs were compared.
As Table 2 shown, most TMP selectivities of chloroaluminate ionic liquids could be improved by the modification of metal chlorides. Meanwhile, it is found that the transition metal chlorides would reduce the Lewis acid strength of the [Bmim] + −based chloroaluminate IL. For example, the AN value of [Bmim]Cl−AlCl 3 −AgCl is about 2.5 lower than that of [Bmim]Cl−AlCl 3 . The other [Bmim]Cl−AlCl 3 −MCl x (M = Cu, Zn) ionic liquids also have the same trend. If AN over 95, the catalysts can be defined as superacids. For an extreme case, the super-acidity of trifluoromethanesulfonic acid (CF 3 SO 3 H) is not good for the C 4 alkylation reaction (Entry 11: AN > 126, but RON < 85). Similarly, the trifluoroacetic acid (CF 3 COOH) that with strong acidity also lead to a relatively poor product quality (Entry 14: AN > 110, but RON < 87). In contrast, those ionic liquids with weak Lewis acidity (AN < 86), such as CH 3 3 NH]Cl−AlCl 3 −AgCl usually results in much better catalytic selectivity than that of [(C 2 H 5 ) 3 NH]Cl−AlCl 3 (Entries 2 and 3 vs. Entry 1). Thus, the effects of Lewis acidity on the selectivity of alkylation reaction may be much more complicated than the previous conclusions [19].
On the other hand, it is well known that the chloroaluminate ionic liquid appears to be Lewis acidity only when the molar ratio of AlCl 3 to the organic salt is greater than 1:1. Therefore, if we investigated the relationship between the acceptor number of ILs with various mole ratios of AlCl 3 and the alkylation results, it is possible to provide a quantitative scale of Lewis acidity for the C 4 alkylation reaction. Figure 2 depicts this evaluation, which again indicates that the C 4 alkylation reaction is strict to the acidity. Only the AN value of [(C 2 H 5 ) 3 NH]Cl−AlCl 3 is greater than 92.0 (e.g., mole ratio of AlCl 3 to [(C 2 H 5 ) 3 NH]Cl is 1.3), the isobutane can be completely reacted with butenes (olefin conversion > 97%). While the AN value is greater than 95.0, the product quality will be lowered (e.g., [(C 2 H 5 ) 3 NH]Cl−2AlCl 3 ). In general, the favorite AN value for the C 4 alkylation reaction should be around 93.0. On the other hand, it is well known that the chloroaluminate ionic liquid appears to be Lewis acidity only when the molar ratio of AlCl3 to the organic salt is greater than 1:1. Therefore, if we investigated the relationship between the acceptor number of ILs with various mole ratios of AlCl3 and the alkylation results, it is possible to provide a quantitative scale of Lewis acidity for the C4 alkylation reaction. Figure 2 depicts this evaluation, which again indicates that the C4 alkylation reaction is strict to the acidity. Only the AN value of [(C2H5)3NH]Cl−AlCl3 is greater than 92.0 (e.g., mole ratio of AlCl3 to [(C2H5)3NH]Cl is 1.3), the isobutane can be completely reacted with butenes (olefin conversion > 97%). While the AN value is greater than 95.0, the product quality will be lowered (e.g., [(C2H5)3NH]Cl−2AlCl3). In general, the favorite AN value for the C4 alkylation reaction should be around 93.0.
However, when the mole ratio of CuCl to AlCl 3 is great than 0.5, it is found that the AN value of the IL was increased with respect to the 0.5 mole ratio. This result should be attributed to the following reactions (7) and (8).
where tepo·2AlCl 3 is more acidic than the tepo·AlCl 3 , resulting in an increase of the acidity. Quantum theory calculation maybe provides another support of the above explanation. V s,max is an effective parameter for interpreting and predicting the acidic region of ILs. The larger magnitude of V s,max usually implies stronger acidity or interaction [36]. It is found that the V s,max of [(C 2 H 5 ) 3 (6)).
However, when the mole ratio of CuCl to AlCl3 is great than 0.5, it is found that the AN value of the IL was increased with respect to the 0.5 mole ratio. This result should be attributed to the following reactions (7) and (8).

The Speciation of Mixed Chlorometallate Ions
To further clarify the mixed metal ions, effects of AlCl 3 on the acidity of net chlorometallate IL (e.g., [(C 2 H 5 ) 3 [34,[41][42][43]. That is, CuAlCl 4 is the main species of the metal ion-aromatic complexes. When AlCl 3 was added to these solvents, the changed trend of AN values (Figure 5b
In a typical preparation for mixed chlorometallate IL, anhydrous aluminum chloride (~0.16 mol) was first added to a round-bottomed flask that contains 0.1 mol 1-butyl-3-methyl-imidazolium chloride. The reaction was then protected by nitrogen atmosphere at 110 °C. When the chloroaluminate ionic liquid was formed, 0.05 mol metal chloride (e.g., CuCl) was mixed with the above IL. Finally, the mixture was stirred at 110 °C overnight until a homogenous ionic liquid was obtained.
In a typical preparation for mixed chlorometallate IL, anhydrous aluminum chloride (~0.16 mol) was first added to a round-bottomed flask that contains 0.1 mol 1-butyl-3-methyl-imidazolium chloride. The reaction was then protected by nitrogen atmosphere at 110 °C. When the chloroaluminate ionic liquid was formed, 0.05 mol metal chloride (e.g., CuCl) was mixed with the above IL. Finally, the mixture was stirred at 110 °C overnight until a homogenous ionic liquid was obtained.
In a typical preparation for mixed chlorometallate IL, anhydrous aluminum chloride (~0.16 mol) was first added to a round-bottomed flask that contains 0.1 mol 1-butyl-3-methyl-imidazolium chloride. The reaction was then protected by nitrogen atmosphere at 110 • C. When the chloroaluminate ionic liquid was formed, 0.05 mol metal chloride (e.g., CuCl) was mixed with the above IL. Finally, the mixture was stirred at 110 • C overnight until a homogenous ionic liquid was obtained.

31 P NMR Spectroscopy
Triethylphosphine oxide (tepo) was obtained from Sigma-Aldrich Co. (Saint Louis, MO, USA). It was stored in a nitrogen-filled glove box until used. Sample preparation for 31 P NMR spectroscopy experiment was carried out in the glove box. The samples of chlorometallate ionic liquids were weighed, and then tepo (5, 10, or 15 mol%) was mixed with the IL. The sample vial was put in an ultrasonic mixer at room temperature overnight to ensure full dissolution. Before measuring the acceptor number, the ionic liquid/tepo mixture was loaded into a NMR tube. Each NMR tube contained a sealed capillary with deuterated dimethyl sulfoxide for external lock. In addition, 85% phosphoric acid solution was also sealed in a capillary and loaded into the NMR tube, which was used as an external reference (Scheme 1). 31 P NMR measurements were performed on a Bruker WB-400 AMX Spectrometer (Zurich, Switzerland).
The spectra were obtained at 130.32 MHz with a pre-acquisition delay time of 0.5 s. All samples were measured at 25 • C. Additionally, 5, 10, and 15 mol% solutions of tepo in hexane were prepared and measured as described above. From the 31 P NMR chemical shifts measured for tepo in hexane, the δ inf for the infinite dilution of the IL could be obtained by extrapolation. Moreover, the acceptor number that related to this chemical shift can be defined as AN = 0. Triethylphosphine oxide (tepo) was obtained from Sigma-Aldrich Co. (Saint Louis, MO, USA). It was stored in a nitrogen-filled glove box until used. Sample preparation for 31 P NMR spectroscopy experiment was carried out in the glove box. The samples of chlorometallate ionic liquids were weighed, and then tepo (5, 10, or 15 mol%) was mixed with the IL. The sample vial was put in an ultrasonic mixer at room temperature overnight to ensure full dissolution. Before measuring the acceptor number, the ionic liquid/tepo mixture was loaded into a NMR tube. Each NMR tube contained a sealed capillary with deuterated dimethyl sulfoxide for external lock. In addition, 85% phosphoric acid solution was also sealed in a capillary and loaded into the NMR tube, which was used as an external reference (Scheme 1). 31 P NMR measurements were performed on a Bruker WB-400 AMX Spectrometer (Zurich, Switzerland). The spectra were obtained at 130.32 MHz with a pre-acquisition delay time of 0.5 s. All samples were measured at 25 °C. Additionally, 5, 10, and 15 mol% solutions of tepo in hexane were prepared and measured as described above. From the 31 P NMR chemical shifts measured for tepo in hexane, the δinf for the infinite dilution of the IL could be obtained by extrapolation. Moreover, the acceptor number that related to this chemical shift can be defined as AN = 0. Scheme 1. NMR tube for the measurement of 31 P chemical shifts.

C4 Alkylation Reaction
In this work, C4 alkylation refers specifically to the reaction between isobutane and 2-butene. Hydrocarbon materials (>99 wt.%) were all purchased from China National Petroleum Corporation (Lanzhou, China) without further purification. As Scheme 2 shown, C4 alkylation reactions were carried out in a batch reactor (50 mL). The reactor includes a mechanical stirrer, which can provide 1200 r/min agitation. In a typical alkylation procedure, the ionic liquid (~10 mL) was charged to the reactor at first. The mixture of isobutane/2-butene with a 7.5:1 molar ratio was then pumped into the reactor at the rate of 500 mL/h. Meanwhile, the impeller of the reactor began to stir. The reaction temperature was 25 °C, which was controlled by a water bath. The pump was stopped after the pressure of the reactor was higher than 0.4 MPa. However, the impeller kept stirring until the total reaction time was about 30 min. The alkylate and the ionic liquid were unloaded from the reactor and settled for 30 min. In a Claisen flask, the alkylation product was distilled to remove isobutane, and the remainder of the hydrocarbon phase was withdrawn for analysis. The alkylate samples were sent to a gas chromatograph (GC), Hewlett-Packard, 6890, Santa Clara, CA, USA). The GC column was used to quantitatively analyze the product, which was a Supelco Petrocol DH capillary column (Supelco Inc., Bellefonte, PA, USA) (50 m × 0.1 mm × 0.1 mm). The temperatures of the injector and the detector were 180 °C and 200 °C, respectively. The temperature program of GC was listed as follows: (1) holding the column temperature at 40 °C for 2 min; (2) increasing the temperature of column box to 60 °C at a rate of 1 °C/min; (3) increasing the temperature to 120 °C at a rate of 2 °C/min; Scheme 1. NMR tube for the measurement of 31 P chemical shifts.

C 4 Alkylation Reaction
In this work, C 4 alkylation refers specifically to the reaction between isobutane and 2-butene. Hydrocarbon materials (>99 wt.%) were all purchased from China National Petroleum Corporation (Lanzhou, China) without further purification. As Scheme 2 shown, C 4 alkylation reactions were carried out in a batch reactor (50 mL). The reactor includes a mechanical stirrer, which can provide 1200 r/min agitation. In a typical alkylation procedure, the ionic liquid (~10 mL) was charged to the reactor at first. The mixture of isobutane/2-butene with a 7.5:1 molar ratio was then pumped into the reactor at the rate of 500 mL/h. Meanwhile, the impeller of the reactor began to stir. The reaction temperature was 25 • C, which was controlled by a water bath. The pump was stopped after the pressure of the reactor was higher than 0.4 MPa. However, the impeller kept stirring until the total reaction time was about 30 min. The alkylate and the ionic liquid were unloaded from the reactor and settled for 30 min. In a Claisen flask, the alkylation product was distilled to remove isobutane, and the remainder of the hydrocarbon phase was withdrawn for analysis. The alkylate samples were sent to a gas chromatograph (GC), Hewlett-Packard, 6890, Santa Clara, CA, USA). The GC column was used to quantitatively analyze the product, which was a Supelco Petrocol DH capillary column (Supelco Inc., Bellefonte, PA, USA) (50 m × 0.1 mm × 0.1 mm). The temperatures of the injector and the detector were 180 • C and 200 • C, respectively. The temperature program of GC was listed as follows: (1) holding the column temperature at 40 • C for 2 min; (2) increasing the temperature of column box to 60 • C at a rate of 1 • C/min; (3) increasing the temperature to 120 • C at a rate of 2 • C/min; (4) increasing the temperature to 180 • C at a rate of 1 • C/min; (5) holding 180 • C for 2 min.
The qualitative identification of the product was analyzed by means of a mass spectrometer (MS), Hewlett-Packard, 5972 Series II column, Santa Clara, CA, USA).  (4) increasing the temperature to 180 °C at a rate of 1 °C/min; (5) holding 180 °C for 2 min. The qualitative identification of the product was analyzed by means of a mass spectrometer (MS), Hewlett-Packard, 5972 Series II column, Santa Clara, CA, USA).

Scheme 2.
The reaction scheme of C4 alkylation.

Conclusions
Gutmann acceptor numbers of the mixed chlorometallate ILs for C4 alkylation reaction have been determined by using 31 P chemical shifts of tepo. The requirement of the acidity for the C4 alkylation reaction is very strict, and the appropriate AN value should be around 93. If AN is less than 88, the reaction would be difficult to carry out. However, too high AN values (e.g., >95) would reduce the quality of the alkylates.