Ionic

Article pubs.acs.org/jced

Determination of the Hammett Acidity Functions of Triflic Acid/Ionic Liquid Binary Mixtures by the 13C NMR-Probe Method Shuai Zhang, Tao Zhang, and Shengwei Tang* Multi-phase Mass Transfer and Reaction Engineering Lab, College of Chemical Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: Catalyst acidity is among the crucial parameters affecting the direction and degree of acid-catalyzed reactions. The Hammett acidity functions (H0) of binary mixtures of triflic acid (TfOH) and ionic liquids (ILs), namely, [BMim][HSO4], [BMim][TfO], and [BMim][TFA], were measured by the 13C NMR method using mesityl oxide as a probe. The results show that the H0 values of the mixtures can be effectively controlled by tailoring the structure of the IL or tuning the amount of IL added to the system. The −H0 values of the binary mixtures decrease with increasing amount of IL. Mixtures of [BMim][HSO4]/TfOH and [BMim][TfO]/TfOH show minimal changes in acidity (−H0 > 13.00) when the IL mole fraction is less than 0.05, a sharp decline in acidity (−H0 = 13.00 → 8.00) at an IL mole fraction of 0.05 to 0.20, and a relatively stable acidity (−H0 = 8.00 → 5.40) at an IL mole fraction of 0.20 to 0.50. These mixtures share nearly the same H0 value when the IL mole fraction is less than 0.20. The addition of [BMim][TFA] can alter the Hammett acidities of the binary systems more easily than [BMim][HSO4] or [BMim][TfO] at the same concentration. Models to predict the H0 values of the three IL/TfOH binary systems as a function of acid concentration are also proposed.

1. INTRODUCTION Acid catalysts are widely used in industrial reaction processes and play an important role in the field of catalysis. Esterification,1,2 alkylation,3,4 alkane isomerization,5,6 and olefin oligomerization7 are the typical examples requiring a large amount of acidic catalysts. To date, the most widely used acid catalysts in plants include liquid acids, such as H2SO4, HF, and H3PO4. However, these conventional Brønsted acidic catalysts have various drawbacks,8 including being strongly corrosive, generating of large amounts of waste, serious environmental pollution, and so on. Most research done over the last five decades in this field has sought to search for better, more environmentally friendly alternatives to liquid acids. The properties of solid acids, heteropoly acids, and zeolites, for example, have been studied.9−13 Unfortunately, solid catalysts exhibit a rapid decrease in activity with time due to the buildup of coke8,13 and therefore are difficult to use in commercial application. Various chloroaluminate-based catalysts14−17 have also been investigated as catalysts, but these materials are extremely sensitive to moisture. Catalyst acidity, a key parameter affecting the catalytic reaction activity, plays a decisive role in the selectivity toward the desired product, the catalyst lifetime, and acid consumption. Specific acidities are required for some reactions. In recent years, the binary mixtures containing in Brønsted acid ionic liquids (ILs) have been increasingly reported, and have been observed to present several advantages, including prolonged catalyst lifetime, improved conversion, and enhanced catalytic activity, over other catalysts. In 2002, Wasserscheid reported3 that the Brønsted ILs could be used as additives with sulfuric acid during the Friedel− Crafts alkylation of benzene with 1-decene, resulting in a dramatic improvement in product yield. Tang18 used six imidazolium-based © XXXX American Chemical Society

ILs coupled with H2SO4 or triflic acid (TfOH) to improve the C4 alkylation reaction. Wang et al.19 combined TfOH with the SO3H-functionalized imidazolium-based ILs containing BF−4 and TfO− as anions in the C4 alkylation reaction and obtained much better alkylates. Zhang et al.20−23 recently found that trace amounts of the ILs with the SbF6− anion (0.5 wt %) could be used as a buffer agent to enhance the catalytic performance during the alkylation of isobutane with butene and that experimental results indicated the ILs with the SbF6− anion maintained the acid strength of the catalytic system and slowed the formation of acid soluble oils, which are of considerable importance in maintaining catalytic activity.24 The acid strength of coupled systems can be effectively tuned by the addition of ILs to improve the catalytic activity toward the target product. However, the systematic and quantitative research on the acid strength of ILs in mineral acids remains lacking because of the restrictions posed by various measurement methods. Relevant data that can improve the understanding of the regulatory effects and influencing laws of ILs are limited. Therefore, it is of great importance to obtain data on the acid strength of ILs in mineral acid to elucidate the catalytic mechanism of ILs and develop new IL/acid composite catalyst. The methods to determine the acid strength vary in different systems. In dilute aqueous solutions, the acid dissociation constant (pKa) is widely used as an indicative parameter of the acidity or basicity of solutes,25,26 and the pKa can be readily determined from the pH, which is usually measured using a Received: January 6, 2016 Accepted: May 13, 2016

A

DOI: 10.1021/acs.jced.6b00015 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

quantitative strength of the acid cannot be obtained; thus, its application is also limited.37 The NMR-probe method is mainly based on the chemical shifts of the probe molecules, some examples of which are mesityl oxide and triethylphosphine oxide, in the acid to determine acid strength. Welton et al.36 recently studied the H0 of the 1-methyl-3-ethyl hydrogen sulfate imidazole salt [EHim][HSO4]−H2SO4 system by 13C NMR. Kwaśny et al.38,39 also obtained the chemical shifts (δ) of the triethylphosphine oxide in the system [EMim][A]−HA where A = NTf2, TfO, OAs, OAc, or HSO4 by 31P NMR and then determined the Gutmann Acceptor Number (AN, AN = 2.348Δδinf) of the medium investigated. These studies show the NMR-probe method is highly suitable for the quantitative determination of the acid strength in systems containing specific acid ILs. In this work, the 13C NMR spectroscopy based on mesityl oxide as the probe was used to quantitatively measure the acid strength of the coupled systems obtained by adding the ILs, namely, [Bmim][HSO4], [BMim][TfO], and [BMim][TFA] into triflic acid. A series of H0 values were obtained from the TfOH/IL coupled systems with different IL concentrations and anionic species. The ultimate goal of the present acidity determination is to understand how ILs regulate the acid strength and provide basic data for determining the relationship between the acidity and catalytic activity of the acidic reactions

glass electrode. However, in nonaqueous media or concentrated solutions including ILs, determining the pKa may be difficult because of its strong nonideality, and a glass electrode is generally unsuitable for measuring such systems. The alternative approaches, including UV−vis spectroscopy,27−30 infrared or Raman spectroscopy,31−33 and NMR spectroscopy (i.e., 13 C NMR, 31P NMR, 1H NMR),34−39 have been applied to investigate the acid strength of solutes in nonaqueous or concentrated solutions. The Hammett acidity functions (H0)27 is a thermodynamic quantitative measure of the acid strength of the nonaqueous or concentrated Brønsted acids; here, the acid strength is defined as the extent to which the acid protonates a base of known basicity. Lower Hammett values indicate the stronger ability of the acid to transfer a proton. UV−vis spectrophotometry could be used to measure the Hammett acidities by determining the ratio of the molar concentrations of the protonated and unprotonated forms of the indicator in a solvent. However, the method has a strong dependence on indicators, which means the indicators must be changed constantly. A highpurity and colorless system is also required,32,34,36 which suggest its application in ILs that are often the colored liquids is limited. The Lewis or Brønsted acid strength of an unknown acid can be determined using acetonitrile or pyridine as a probe molecule by characterizing the probe molecule in the IR spectrum of the acid and determining its peak position in the absorption bands. This method, however, only allows qualitative analysis and the

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals used (AR grade) were purchased commercially and were used as received unless otherwise noted. Sulfuric acid (AR 95%−98%) and sodium hydroxide (AR, 99%) were purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). Triflic acid and mesityl oxide were obtained from Tokyo Chemical Industry Co., LTD (Tokyo, Japan). The purities were >99.5% and >95%, respectively. 1-Butyl-3-methylimidazolium hydrogen sulfate ([BMim][HSO4]) (99%) was supplied by Shanghai Chengjie Chemical Co., LTD (Shanghai, China). 1-Butyl-3-methylimidazolium trifluoromethanesulfonate ([BMim][TfO]) (98%) and 1-butyl-3-methylimidazolium trifluoroacetic acid ([BMim][TFA]) (98%) were obtained from the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (Lanzhou, China). (See Table 1.) 2.2. Principle of Acidity Measurements via the NMRProbe Method. In 1993, Fărcaşiu et al.34 reported a method

Table 1. Specifications of Chemicals Used CAS no.

source

purity

sulfuric acid

compound

7664-93-9

95%−98%

sodium hydroxide

1310-73-2

triflic acid

1493-13-6

mesityl oxide

141-79-7

1-butyl-3-methylimidazolium hydrogen sulfate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium trifluoroacetic acid

26229713-2 17489966-2

Chengdu Kelong Chemical Reagent Factory Chengdu Kelong Chemical Reagent Factory Tokyo Chemical Industry Co., LTD Tokyo Chemical Industry Co., LTD Shanghai Chengjie Chemical Co., LTD Lanzhou Institute of Chemical Physics

17489994-6

Lanzhou Institute of Chemical Physics

99%

>99.5% >95% 99% 98%

98%

Figure 1. Schematic diagram of Δδ varying as an acidity-dependent parameter. B



Article

Table 2. Correlation of Chemical Shift Differences, Δδ, of Mesityl Oxide with the Known Acidity wt % of H2SO4 0a

0.38 ± 0.02

2.50 ± 0.05

10.06 ± 0.12

25.00 ± 0.21

39.14 ± 0.17

51.39 ± 0.13

70.12 ± 0.26

72.23 ± 0.32

77.56 ± 0.11

82.47 ± 0.23

88.05 ± 0.12

93.58 ± 0.25

95.10 ± 0.08

97.05 ± 0.05

99.77 ± 0.05

TfOH

a

wt % of mesityl oxide 1.56 2.55 3.31 1.17 2.43 3.32 1.06 1.95 2.60 1.79 4.00 5.12 1.01 2.30 3.20 1.02 2.26 3.38 1.70 3.28 4.03 1.59 2.70 3.97 2.01 3.04 3.98 3.00 4.51 5.43 1.62 2.41 3.68 1.53 2.52 3.71 2.10 3.70 4.80 1.49 3.34 4.51 1.61 2.47 3.25 1.55 2.16 2.68 1.71 2.47 2.79

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.01 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.02 0.01 0.03 0.03 0.02 0.02 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.03 0.02 0.01 0.01 0.02 0.02 0.03 0.02 0.01 0.03 0.02 0.02 0.03 0.01 0.02 0.02 0.04 0.01 0.01 0.03 0.02 0.02 0.01

Δδ (ppm)

Δδinf (ppm) (±0.05)

−H0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

33.81

0b

34.71

1.62b

38.75

2.50b

43.92

2.95b

56.48

3.86b

64.47

4.57b

67.88

5.00b

71.89

5.80c

73.75

6.10c

75.26

6.86c

77.33

7.66c

78.74

8.61c

79.87

9.50c

82.20

9.85c

80.62

10.21c

80.77

11.70c

82.32

14.10c

33.77 33.74 33.72 34.60 34.49 34.40 38.36 38.23 37.85 42.06 39.68 37.58 56.35 56.18 56.06 63.67 62.08 62.01 66.81 65.95 65.33 71.06 70.55 69.91 71.85 71.19 70.06 74.44 74.07 73.78 76.90 76.68 76.36 78.43 78.24 77.98 79.50 79.22 79.02 79.97 79.72 79.49 80.40 80.28 80.18 80.64 80.60 80.56 81.93 81.74 81.69

0.05 0.05 0.05 0.05 0.05 0.05 0.09 0.16 0.11 0.18 0.43 0.30 0.05 0.05 0.05 0.28 0.53 0.30 0.08 0.14 0.11 0.05 0.05 0.05 0.13 0.23 0.15 0.06 0.08 0.07 0.05 0.05 0.05 0.06 0.06 0.06 0.05 0.05 0.05 0.06 0.08 0.07 0.05 0.05 0.05 0.05 0.05 0.05 0.15 0.14 0.16

Pure anhydrous acetic acid. b−H0 values in H2SO4−CH3COOH system is from ref 43. c−H0 values in H2SO4−H2O system is from ref 48.

for measuring the acidity of a system using the 13C NMR spectrum of mesityl oxide as the NMR-probe molecule. On the basis of changes in the charge densities of the α and β carbons of the mesityl oxide, the corresponding conjugate acid is transformed by the interaction between protons (H+) and an oxygen atom next to an α carbon (Figure 1). Thus, most of the charge is localized at Cβ, and very little charge is

present at Cα; in turn, the peak position of the Cα changes minimally whereas that of the Cβ varies significantly in the NMR spectrum. The difference between the chemical shifts of two carbon atoms (Δδ) increases as the system becomes more acidic. Herein, Δδ can act as an acidity-dependent parameter from which the acid strength of a system can be measured and quantitated. C



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2.3.2. Preparation of Triflic Acid−Ionic Liquid Mixtures. In a typical procedure, the ionic liquid was dried under high vacuum at 80 °C overnight. Its water content was tested by the

Because the concentration of the mesityl oxide in NMR is much higher than that the UV−vis probes used in the Hammett methodology, the addition of the mesityl oxide to acidic system would logically reduce the acidity of the system itself. Moreover, the difference between the chemical shifts of the two carbon atoms (Δδ) is slightly concentration-dependent. Thus, in a typical experiment, Δδ with different concentrations of the mesityl oxide in an acidic system are extrapolated to infinite dilution, Δδinf, and Δδinf is taken as a reference for the determination of an acidity function by NMR. Considering that every Δδinf corresponds to a specific acid strength, a calibration curve could be constructed by correlating Δδinf with the corresponding H0. The Δδinf of a series of standard acid solutions are obtained from the 13C NMR spectrum with the mesityl oxide as the NMR-probe molecule. The Hammett acidity of an unknown acid can be determined from the calibration curve if the Δδinf of mesityl oxide in the unknown acid is measured. 2.3. Experimental Procedure. 2.3.1. Preparation of Standard Sulfuric Acid Solutions. H2SO4 (95 wt %−98 wt %) was parallel titrated40 three times against a 0.50 mol/L NaOH standard solution41 with a standard uncertainty uc(CNaOH) of 0.003 mol/L so that the exact concentration of the acid is known. The acid was then diluted with deionized water to achieve the desired concentration. The acid and water added were weighed so that an exact concentration of the diluted sulfuric could be calculated. The standard sulfuric acid solutions were also prepared from fuming sulfuric acid (H2SO4 + 50% SO3) and a sulfuric acid aqueous solution (95−98%) when higher mass concentrations (>98 wt %) were required and titrated against a 0.50 mol/L NaOH standard solution. The standard uncertainties of the mass concentrations of the sulfuric acid solutions were analyzed according to the error analysis theory,42 and the masses of sulfuric acid solutions were weighed by an electronic analytical balance with a standard uncertainty uc(m) of 0.0001 g. The uncertainty from the volume of NaOH used uc(V) was 0.05 mL.

Table 4. −H0 Values of [BMim][HSO4]/TfOH with Different IL Concentrations x of [BMim] [HSO4] 0

0.05

0.09

0.15

0.2

0.24

0.3

0.35

0.4

0.45

0.5

0.54

0.6

0.64

0.72

0.75

Figure 2. Standard curve of −H0 with chemical shifts in infinite dilution.

wt % of mesityl oxide 1.71 2.47 2.79 1.37 1.65 3.16 1.40 2.15 4.08 1.99 2.19 2.86 1.53 2.75 3.57 1.19 2.64 3.22 1.24 1.92 2.88 1.29 1.81 2.40 1.03 2.88 3.27 1.13 2.06 3.39 2.40 3.32 4.18 1.17 2.20 3.61 1.17 2.44 3.71 1.19 2.08 3.38 2.33 3.30 4.42 4.53 5.06 5.77

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.01 0.01 0.02 0.02 0.02 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.01 0.01 0.02 0.03 0.02 0.01 0.02 0.04 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.03 0.01 0.02 0.02 0.01 0.01 0.01 0.02 0.02 0.02 0.04 0.02 0.01 0.02 0.01

Δδ (ppm) 81.93 81.74 81.69 81.09 81.03 80.65 80.16 79.93 79.34 78.76 78.68 78.51 77.77 77.48 77.23 77.22 76.78 76.63 76.22 76.06 75.81 75.85 75.81 75.69 74.88 74.38 74.28 72.48 71.95 71.19 70.84 70.24 69.79 70.72 69.96 68.78 67.00 65.38 63.91 61.10 59.68 58.00 43.82 42.99 41.54 34.25 34.15 34.02

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.15 0.14 0.16 0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.05 0.05 0.06 0.07 0.08 0.06 0.06 0.06 0.05 0.05 0.05 0.06 0.07 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.06 0.06 0.09 0.06 0.05 0.05 0.05 0.11 0.15 0.09 0.13 0.20 0.12 0.05 0.05 0.05


−H0

82.32

14.1

81.44

13.2

80.59

10.51

79.32

8.91

78.18

8.06

77.56

7.71

76.54

7.23

76.25

7.12

75.19

6.74

73.12

6.16

72.24

5.87

71.68

5.76

68.4

5.25

62.7

4.51

46.46

2.96

35.09

1.63

Table 3. Reliability of the 13C NMR-Probe Method acid

mass concentration (wt %)

CH3SO3H HClO4 H2SO4

99.0 ± 0.05 71.0 ± 0.20 96.0 ± 0.05

Δδinf (ppm)

−H0exp

76.76 77.95 80.40

7.39 7.99 10.33 D

−H0ref 44

7.75 8.0845 10.0348

error value

relative error

−0.36 −0.08 0.30

4.61% 0.09% 2.99%



Article

3. RESULTS AND DISCUSSION 3.1. Establishment of Calibration Curve. On the basis of the measurement principle described above, a calibration curve correlating Δδinf with H0 is established by measuring a series of Δδinf values determined by extrapolation from the 13C NMR chemical shifts obtained at different mesityl oxide concentrations in sulfuric acid, the acid strength of which are available in the literature. The results are shown in Table 2; here, the standard uncertainties are derived from linear extrapolation. The H0 values included in the calibration curve (Figure 2) ranged from −14 to 0, and eq 1 can be obtained by fitting. The changes in chemical shifts as a function of H+ concentration are very small in systems with H0 values greater than 0 because of the weak acid strength of such systems; thus, the proposed method is unsuitable for determining the acid strength of these systems. In order to describe our work conveniently, −H0 value was used below

Karl Fischer titration. Then a known weight of ionic liquid was added into a 10 mL sample vial. Taking into account the purity, the standard uncertainties were 0.001 for both [BMIm][TfO] and [BMIm][TFA], and 0.002 for [BMIm][HSO4]. The required amount of triflic acid was added to the same vial. Afterward, the sample was sealed, mixed by magnetic stirring, and stored in a desiccator temporarily. 2.3.3. Analytical Method. A certain amount of a binary mixture (2 mL) and mesityl oxide were added to a 5 mL sample vial. Here, the standard uncertainty of the mass concentration of mesityl oxide was determined by weighing uncertainty uc(m) and taking into account the purity of mesityl oxide. The sample vial was sealed with a plastic cap, and the mixture was stirred overnight to ensure complete dissolution of the probe reagent. The liquid was then loaded into an NMR tube (5 mm, borosilicate glass) containing a sealed capillary (0.9 × 0.1 × 100 mm) with DMSOd6 as an external lock. The tube was sealed with a standard cap and Paraffilm. 13C NMR spectra were acquired at 100 MHz using a Bruker Avance III 400 Hz spectrometer with a standard uncertainty uc(δ) of 0.05 ppm. All samples were measured at 27 °C. The differences in the bulk magnetic susceptibility (BMS) between the reference (DMSO-d6) and the sample (the IL/TfOH mixture) were not corrected for only the chemical shift differences were used to measure the acidity. The difference between the measured temperature in NMR spectrometers and the temperature of the sample in the NMR tube were also not corrected for the same reason.

log(−H0) = 0.60 + 0.28log

Δδinf − 33.18 82.12 − Δδinf

(1)

To evaluate the accuracy of the standard curve measured (Figure 2), methanesulfonic acid, perchloric acid, and sulfuric acid were selected as test acids. The experimental and reported acidities of these acids were compared, and the results are shown in Table 3. The results demonstrate that the 13C NMR-probe method has high accuracy with a relative error less than 5%. The H0 value of anhydrous acetic acid is 0, and the corresponding Δδinf is 33.81 ppm. In this study, however, the

Figure 3. 1H NMR spectra (400 MHz, 27 °C, neat, external DMSO-d6 lock) of the [BMim][HSO4]/TfOH system at different triflic acid concentrations. (1) Chemical shifts of protons in the C2 position of the imidazolium ring and (2) chemical shifts of protons in the C4 and C5 positions of the imidazolium ring. E



Article

measured Δδinf in the acidic ILs were less than 33 ppm, which indicates that the acid strength of the ILs used are weaker than that of anhydrous acetic acid. 3.2. Determination of the H0 Values of [BMim][HSO4]/ TfOH Binary Mixtures. [BMim][HSO4]/TfOH mixtures with different IL mole fractions were prepared, and 13C NMR was used to measure the chemical shifts of mesityl oxide in these mixtures. The Δδinf were obtained by extrapolation, and the corresponding H0 were determined according to the standard curve obtained (Figure 2). The results are listed in Table 4. As shown in Table 4, the −H0 values of the binary mixtures gradually decrease with increasing [BMim][HSO4] concentration. When a small amount of [BMim][HSO4] (mole fraction, 0.05) is added to the mixtures, the system retains a strong acidity (−H0 = 13.20). However, continued increases in [BMim][HSO4] rapidly decrease the acid strength of the mixtures. When the IL mole fraction reaches 0.20, the −H0 of the system drops to 8.06 from an initial value of 14.10. When the IL mole fraction is in the range of 0.20 to 0.50, the acid strength of the system changes minimally from 8.06 to 5.87. When the IL mole fraction exceeds 0.50, the −H0 value of the system decreases rapidly once more. The −H0 of the system with an IL fraction of 0.75 is only 1.63. To elucidate the interactions of the ILs with TfOH and analyze the influence of acid concentration on the chemical shifts qualitatively, 1H NMR spectra of [BMim][HSO4]/TfOH binary mixtures with various TfOH mole ratios were measured at ambient temperature (Figure 3). The chemical shifts of the protons in the imidazolium ring move upfield with increasing acid concentration. Peaks in the region marked by red rectangles in the figure indicate the chemical shifts of C2 protons in the imidazolium ring; these protons are the most sensitive to changes in acid strength. When the acid mole fraction in the binary mixture reaches 0.75, the chemical shifts of the C2 protons move down to 8.11 ppm from 9.21 ppm, and protons of C4 and C5 of the imidazole ring peak shift to 7.23 ppm from 7.78 ppm. 3.3. Determination of the H0 Values of [BMim][TfO]/ TfOH Binary Mixtures. The Hammett acidity of the [BMim][TfO]/TfOH system with different mole fractions of the IL were determined by 13C NMR. The results were listed in Table 5. Table 5 reveals that the acidities of the [BMim][TfO]/TfOH system gradually decrease with the increasing amount of [BMim][TfO] added to the system. The system shows a strong acid strength (−H0 = 13.60) when the mole fraction of [BMim][TfO] is 0.05. As the amount of IL continues to increase, however, the Hammett acidity of the system declines quickly, and its acid strength is reduced sharply. In particular, when the IL mole fraction is 0.20, the −H0 of the system drops to 7.99 from its initial value of 14.10. When the IL mole fraction is in the range of 0.20 to 0.50, the acidity of the system is relatively flat and the −H0 value changes within the range of 7.99 to 5.64. When the IL concentration is greater than 0.50, the Cβ peak in the 13C NMR spectrum of the system is difficult to detect, as shown in Figure 4. A similar finding was reported by Fărcaşiu.34 Therefore, the −H0 values of systems with [BMim][TfO] molar fractions of over 0.5 could not be determined in this work. 3.4. Determination of the H0 Values of [BMim][TFA]/ TfOH Binary Mixtures. The −H0 values of different concentrations of the [BMim][TFA]/TfOH binary mixture are presented in Table 6. The acid strength of the system declines with increasing amount of [BMim][TFA]. Unlike in the previous two systems, however, addition of a small amount of [BMim][TFA] results

Table 5. −H0 Values of [BMim][TfO]/TfOH with Different IL Concentrations x of [BMim] [TfO] 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.34

0.40

0.45

0.50

wt % of mesityl oxide 1.71 2.47 2.79 3.20 4.36 4.94 3.67 4.78 5.84 3.00 3.97 5.09 3.21 3.81 4.95 2.83 3.92 4.85 3.43 4.05 4.59 3.27 4.46 5.40 3.25 4.35 5.34 2.78 4.25 5.31 3.00 3.84 5.16

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.02 0.02 0.03 0.02 0.03 0.01 0.02 0.01 0.01 0.04 0.01 0.02 0.01 0.02 0.01 0.02 0.03 0.01 0.02 0.01 0.01 0.02 0.01 0.02

Δδ (ppm) 81.93 81.74 81.69 80.72 80.39 80.25 79.38 78.99 79.38 78.46 78.11 77.73 77.11 76.87 76.46 75.55 75.25 75.05 74.13 73.92 73.73 73.16 72.76 72.40 72.03 71.55 71.11 70.16 69.59 69.38 69.06 68.85 68.61

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.15 0.14 0.16 0.05 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.07 0.06 0.05 0.05 0.05 0.12 0.12 0.16 0.05 0.05 0.05 0.08 0.12 0.10 0.07 0.07 0.06


−H0

82.31

14.10

81.51

13.60

80.59

10.46

79.50

8.94

78.30

7.99

76.24

6.99

75.31

6.66

74.33

6.31

73.46

6.15

70.99

5.64

69.62

5.40

in a rapid decrease in acidity. The −H0 value of the system is only 12.04 at xIL = 0.05. A reduction of one H0 unit indicates a 10-fold decrease in acidity compared with those of the [BMim][TfO]/TfOH and [BMim][HSO4]/TfOH systems at the same concentration. The −H0 of [BMim][TFA]/TfOH declines to 5.73 at xIL = 0.40. When the IL concentration is greater than 0.40, the Cβ peak in the 13C NMR spectra is difficult to observe, as previously noted in the [BMim][TfO]/TfOH system. 3.5. Effect of Anionic Species on the −H0 Values of the [BMim][X]/TfOH Binary Mixtures. ILs containing different anions demonstrate different acid strengths. Thus, the acid strength of the binary systems could be affected by the anionic species added to the system. The ILs used in this work have the same cationic group but different anions. The effect of the anionic species on the −H0 values of the [BMim][X]/TfOH binary mixtures was analyzed by comparing the −H0 values of the three IL/triflic acid binary mixtures studied in this work, and the results are shown in Figure 5. Although the −H0 values of the three binary systems generally decrease with an increasing amount of IL, the trends observed show marked differences. The [BMim][HSO4]/TfOH and [BMim][TfO]/TfOH systems share similar −H0 values when the IL mole fraction is less than 0.20. However, the general trend of the acidity of the [BMim][TfO]/TfOH system decreases with greater speed in F



Article

Figure 4. 13C NMR spectra (400 MHz, 27 °C, neat, external DMSO-d6 lock) of the [BMim][TfO]/TfOH system. (a) Pure mesityl oxide, (b) [BMim][TfO], (c) mesityl oxide in the [BMim][TfO]−TfOH system (xTfOH = 0.90), and (d) mesityl oxide in the [BMim][TfO]/TfOH system (xTfOH = 0.40).

Table 6. −H0 Values of [BMim][TFA]/TfOH with Different IL Concentrations x of [BMim] [TFA] 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

wt % mesityl oxide 1.71 ± 0.02 2.47 ± 0.02 2.79 ± 0.01 1.79 ± 0.02 2.55 ± 0.01 3.59 ± 0.02 1.64 ± 0.01 2.63 ± 0.01 3.27 ± 0.01 2.31 ± 0.02 2.87 ± 0.01 3.60 ± 0.01 1.91 ± 0.02 2.64 ± 0.02 3.61 ± 0.01 1.45 ± 0.01 2.46 ± 0.02 3.65 ± 0.01 3.67 ± 0.02 4.78 ± 0.01 5.84 ± 0.02 1..69 ± 0.01 2.90 ± 0.01 3.48 ± 0.04 3.02 ± 0.01 4.38 ± 0.01 6.84 ± 0.03

Δδ (ppm) 81.93 81.74 81.69 80.73 80.50 80.28 79.54 79.21 79.04 77.81 77.63 77.38 76.69 76.45 76.07 75.22 74.85 74.47 72.79 72.50 72.27 71.90 71.47 71.26 67.23 64.98 62.47

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.15 0.14 0.16 0.07 0.08 0.06 0.06 0.07 0.06 0.05 0.05 0.05 0.06 0.07 0.06 0.06 0.07 0.06 0.06 0.07 0.06 0.05 0.05 0.05 0.28 0.41 0.18


−H0

82.31

14.1

81.18

12.04

80.04

9.57

78.58

8.17

77.4

7.48

75.7

6.79

74

6.28

72.5

5.93

71.5

5.73

Figure 5. Trends of −H0 values with increasing xIL.

acidity of the [BMim][TfO]/TfOH system to some extent. The dilution effect of [BMim][TfO] on TfOH is also more obvious than that of [BMim][HSO4] at the same acid concentration because [BMim][TfO] has larger molar volume than [BMim][HSO4].36 [BMim][TFA] could alter the Hammett acidity of the IL/TfOH system with more ease than [BMim][HSO4] or [BMim][TfO] could at the same concentration because HTFA (−H0 = 2.90) is a weaker acid compared with TfOH (−H0 = 14.10) and H2SO4 (−H0 = 12.00), and TFA− extracts a proton H+ from the superacid TfOH to generate weak acid TFAH.29 Thus, even small amounts of [BMim][TFA] mixed with TfOH can induce significant changes in the acidity of the system. Hence, the −H0 value of the [BMim][TFA]/TfOH binary mixture may be expected to be lower than those of the two other acidic systems under the same concentration The results suggest that the acid strength of the IL/triflic acid system can be easily tuned by careful selection of the anionic

comparison with that of the [BMim][HSO4]/TfOH system with increasing amount of IL. This variation may be attributed to the formation of the [TfO(TfOH)x] complex,46 which is held together by strong hydrogen bond networks47 and weakens the G



Article

Figure 6. (a) Variation in H0 as a function of acid concentrations in the [BMim][HSO4]/TfOH binary mixture. (b) Comparison of experimental and calculated values for the [BMim][HSO4]/TfOH binary mixture.

Figure 7. (a) Variation in H0 as a function of acid concentrations in the [BMim][TfO]/TfOH binary mixture. (b) Comparison of experimental and calculated values for the [BMim][TfO]/TfOH binary mixture.

protonated and unprotonated forms of the indicator in the solvent. C + Assuming CHB = f (C0) × C0

species or amount of IL added to the system, which offers a simple, convenient, and reliable method of accurately tailoring the acid strength of an acidic system. Accurate acidity is of considerable importance in acid-catalyzed processes. 3.6. Models of the −H0 Values of the [BMim][X]/TfOH Binary Mixtures. Obtaining the accurate acidity of IL/mineral acid mixtures with different molar ratios using a mathematical model set up from the limited data is necessary. In this study, models were applied to correlate Hammett acidity with acid concentration using the experimentally determined data. Hammett acidity (H0) essentially embodies the ability of an acid (HA) to protonate an alkaline indicator (B).27 The lower this value, the stronger the ability of HA to transfer protons (H+) to the alkali. The solvent dependence of Hammett acidity has been extensively investigated. Herein, the models established vary with the acidic system studied even when the acid concentration is held constant. On the basis of the definition of H0 H0 = pKHB+ − log

C HB+ CB

B

H0 = pKHB+ − log

C HB+ = pKHB+ − log C0 − log f (C0) CB (3)

Here, pKHB+ = −4 for mesityl oxide Experimental data of the three mixtures were fitted according to eq 3, and the fitted formula is given in eqs 4, 5, and 6. For the [BMim][HSO4]/TfOH binary mixture 34

H0 = −4 − log C0 − 0.75log

2.24 − C02.07 C00.94 − 8.88

(4)

As shown in Figure 6a, the variation in H0 as a function of acid concentrations can be fitted well to eq 4. Comparison of the experimental and calculated values, Figure 6b, reveals an average error of 3.0% and maximum error of 7.8%. For the [BMim][TfO]]/TfOH binary mixture

(2)

H0 = −4 − log C0 − 4.43log

where pKHB+ is the pKa of the indicator in an aqueous solution and. CHB+ and CB are the respective molar concentrations of the H

0.16 − C0−0.45 C00.21 − 1.70

(5)



Article

Figure 8. (a) Variation in H0 as a function of acid concentrations in the [BMim][TFA]/TfOH binary mixture. (b) Comparison of experimental and calculated values for the [BMim][TFA/TfOH binary mixture.

■

As shown in Figure 7a, the variation in H0 as a function of acid concentrations can be fitted well to eq 5. Comparison of the experimental and calculated values, Figure 7b, reveals an average error of 1.3% and maximum error of 3.4%. For the [BMim][TFA]/TfOH system, the fitted formula is given in eq 6 H0 = −4 − log C0 − 2.42log

Corresponding Author

*E-mail: [email protected]. Tel.: 86-28-85405201. Funding

Financial support by the National Natural Science Foundation of China (no. 21276163 and No. 21576168) is acknowledged.

3.58 + C0−0.45 7.22 − C00.83

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

(6)

■

As shown in Figure 8a, the variation in H0 as a function of acid concentration can be fitted well to eq 6. Comparison of the experimental and calculated values, Figure 8b, reveals an average error of 1.0% and maximum error of 2.4%.

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4. CONCLUSIONS The Hammett acidities of binary mixtures composed of TfOH and ionic liquid ([BMim][HSO4], [BMim][TfO], or [BMim][TFA]) were measured by 13C NMR using mesityl oxide as a probe. 13C NMR spectroscopy offers a simple, convenient, and reliable method to determine the Hammett acidity of acidic systems, especially when UV−vis spectroscopy is inapplicable to the system. A measurement error of less than 5% was obtained. Mixtures of [BMim][HSO4]/TfOH and [BMim][TfO]/TfOH showed minimal changes in acidity (−H0 > 13.00) when the IL mole fraction was less than 0.05, a sharp decline in acidity (−H0 = 13.00 → 8.00) at an IL mole fraction of 0.05 to 0.20, and relatively stable acidity (−H0 = 8.00 → 5.40) at an IL mole fraction of 0.20 to 0.50. These mixtures shared nearly the same H0 value when the IL mole fraction was less than 0.20. [BMim][TFA] could alter the Hammett acidities of mixed systems more easily than [BMim][HSO4] or [BMim][TfO] could when applied at the same concentration. Models to predict the changes in Hammett acidity as a function of acid concentration for the three IL/TfOH binary systems described in this work were also obtained. The Hammett acidities of binary mixtures of IL/TfOH can be effectively tuned to achieve good reaction efficiency and selectivity by tailoring the IL amount or anionic species added to the reaction system. This work expands the current understanding on the relationship between acidity and catalytic activity in acidic reactions. I



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