Structural Elucidation of Thiophene Interaction with Ionic Liquids by

Similarly, a stacking structure of 2/1 TS/BMIMBF4 repeating units was formed in the solution of thiophene and BMIMBF4, and a stacking structure of 1/1...
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J. Phys. Chem. B 2004, 108, 19510-19517

Structural Elucidation of Thiophene Interaction with Ionic Liquids by Multinuclear NMR Spectroscopy Biing-Ming Su,* Shuguang Zhang, and Z. Conrad Zhang* Akzo Nobel Chemicals Inc., 1 LiVingstone AVenue, Dobbs Ferry, New York 10522-3401 ReceiVed: March 3, 2004; In Final Form: September 22, 2004

Ionic liquids were found to be highly selective for extractive removal of aromatic sulfur compounds from fuels at room temperature. The efficiency of ionic liquids for the absorption of aromatic sulfur is dependent on the size and structure of both cations and anions of the ionic liquids. In this work, room temperature 1H, 19 F, 11B, and 31P NMR spectroscopy was used to study the interaction of thiophene as a model aromatic sulfur-containing compound with ionic liquids of 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 1-n-buty-3-methylimidazolium tetrafluoroborate (BMIMBF4), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). The molar ratio of thiophene to each ionic liquid was varied by including saturated absorption of thiophene in ionic liquids. Pronounced changes were observed in NMR chemical shifts of the protons from the imidazolium cations and thiophene (TS), and NMR chemical shifts of fluorine, boron, and phosphorus nuclei of the anions. At the maximum absorption of thiophene in each ionic liquid, the NMR results indicate that a relatively ordered and extended stacking structure of 4/1 TS/BMIMPF6 repeating units was formed in the solution of thiophene and BMIMPF6. Similarly, a stacking structure of 2/1 TS/ BMIMBF4 repeating units was formed in the solution of thiophene and BMIMBF4, and a stacking structure of 1/1 TS/EMIMBF4 repeating units was formed in the solution of thiophene and EMIMBF4. NMR results obtained from this study also showed that the chain length or size of the alkyl group on the imidazolium cation and the property and size of the anion determined the absorption capability of thiophene by various ionic liquids with imidazolium cations.

Introduction Ionic liquids are easy to handle and present several characteristic advantages over conventional organic solvents. They are nonvolatile, nonflammable, and thermally stable. Some ionic liquids have been used in liquid/liquid extractions, gas separations, electrochemistry, and catalysis. One of the ionic liquid applications related to green chemistry is to remove organic and heavy metal polluting sources or contaminants from aqueous media or fuels.1-3 Due to growing environmental pressures and regulatory requirements, oil refineries are facing increasing technical challenge to remove trace sulfur-containing compounds in the transportation fuels. In our recent works,4 it was shown that ionic liquids, such as 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4), effectively absorbed a large amount of S-containing aromatic compounds, such as thiophene (TS) and methylthiophene. It was further demonstrated that the absorption capacity of the ionic liquids for sulfurcontaining compounds was dependent on the molecular structure of the sulfur compounds and on the size and structure of the ionic liquids. In particular, the absorption capacity of the ionic liquids for thiophene was strikingly high.4 The molar ratios of thiophene absorbed in the ionic liquids were found to be approximately 3.5/1, 2.4/1, and 0.9/1 for BMIMPF6, BMIMBF4, and EMIMBF4, respectively.4 Although the chemical reaction * To whom correspondence should be addressed. B-M.S.: e-mail, [email protected]; fax, (914)-693-5780; phone, (914)-674-5138. Z.C.Z.: e-mail, Zongchao.Zhang@ AkzoNobel-Chemicals.com; fax, (914)-693-2029; phone, (914)-674-5034.

between thiophene and the ionic liquids can be excluded because the absorbed thiophene can be quantitatively removed from the ionic liquids by distillation, the nature of the interaction and structural features of the thiophene absorbed by ionic liquids was not clear. Several review and studies5-8 had shown that these neat ionic liquids formed a more or less extended stacking structure with alternating cations and anions. However, no further studies had been carried out to investigate the structural change of these neat ionic liquids when an aromatic compound, such as thiophene, was added. In this work, 1H, 19F, 11B, and 31P NMR spectroscopy was used to study these three different systems to understand the structural effects of the ionic liquids on the absorption capability for thiophene. NMR spectroscopy was also used to follow the local structural changes of the ionic liquids at various levels of thiophene absorption. In the literatures, several studies about similar ionic liquids were done using 1H NMR, but few report is about 19F, 11B, and 31P NMR. Experimental Section NMR Samples Preparation. The syntheses of BMIMPF6, BMIMBF4, and EMIMBF4 have been described elsewhere.4 The melting points of the ionic liquids are +5, -80, and +5 °C for BMIMPF6, BMIMBF4, and EMIMBF4, respectively. Samples with various thiophene concentrations in the ionic liquids were prepared by adding thiophene to the ionic liquids to reach specified molar ratios. Saturated thiophene in each ionic liquid was prepared by adding an excess amount of thiophene to the neat ionic liquid, followed by removal of the excess thiophene from a separate phase. In NMR measurements, approximately 0.3 mL of liquid with a predetermined thiophene/ionic liquid

10.1021/jp049027l CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004

Thiophene Interaction with Ionic Liquids

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Figure 1. Structures and proton NMR signals assignment of thiophene and 1-n-butyl-3-methylimidazolium hexafluorophosphate (system I), 1-n-buty-3-methyllimidazolium tetrafluoroborate (system II), and 1-ethyl-3-methylimidazolium tetrafluoroborate (system III).

molar ratio was added to a 5 mm NMR tube (Wilmad 535-PP). A stem coaxial capillary tube (Wilmad WGS-5BL) loaded with CD2Cl2 (99.9 atom % D) was then inserted into the 5 mm NMR tube. The deuterium in CD2Cl2 was used for the external lock of the NMR magnetic field/frequency and the residual CHDCl2 in CD2Cl2 was used as the 1H NMR external reference at 5.320 ppm. When 1H NMR data are obtained in this way, the reference signal of CHDCl2 will remain as a constant and not be affected by change in sample concentration. NMR Instrumentation and Experiments. NMR data were collected on a Varian Unity Inova-500 MHz NMR spectrometer (Varian Associates, Palo Alto, CA) with the resonance frequency of 1H NMR at 499.902 MHz, 19F NMR at 470.330 MHz, 11B NMR at 160.388 MHz, and 31P NMR at 202.348 MHz, respectively. An external reference of CF3C6H5 was used for 19F NMR at -63.900 ppm, BF :diethyl ether complex was used 3 for 11B NMR at 0.000 ppm, and (C6H5O)3P(dO) was used for 31P NMR at -17.500 ppm. Results and Discussion To investigate the interaction and the specific local structure of thiophene and ionic liquid, three ionic liquid systems I-III (Figure 1) were studied by 1H, 19F, 11B, and31P NMR spectroscopy. Effect of the Interaction between Thiophene and Ionic Liquids on the 1H NMR Spectrum. Assignments of the 1H NMR signals corresponding to the protons on thiophene and to those on the imidazolium cation of ionic liquids for these systems are also shown in Figure 1. Tables 1-3 show the 1H NMR chemical shifts of thiophene and ionic liquid at various TS/ionic liquid molar ratios for BMIMPF6, BMIMBF4 and

Figure 2. 1H NMR signals assignment of the ring protons in TS and BMIMPF6 at various mole ratios of TS/BMIMPF6. (The top spectrum is the neat TS, and the bottom spectrum is the neat BMIMPF6. The proton assignment to each signal is referred to system I in Figure 1.)

EMIMBF4, respectively. An array of representative expanded 1H NMR spectra and signal assignment for protons in aromatic rings of thiophene and imidazolium cation of system I, i.e., TS/ BMIMPF6, are shown in Figure 2. 1H NMR chemical shifts of the ring protons in neat thiophene (top spectrum) are more upfield than those in neat BMIMPF6 liquid (bottom spectrum), indicating that the average electron density of each ring carbon in neat thiophene is higher than that in neat BMIMPF6 ionic liquid. The ring protons of BMIM+ cation in BMIMPF6 are gradually moved upfield when the concentration of thiophene was increased (Figure 2, spectra from bottom to top). Concomitantly, the proton NMR signals of 1-n-butyl and 3-methyl groups in the BMIM+ cation of BMIMPF6 were also moved upfield when the concentration of thiophene was increased (as shown in Table 1, proton 1 to proton 7). On the contrary, the ring protons of thiophene are moved downfield when the concentration of BMIMPF6 was increased (Figure 2, spectra from top to bottom). To explain the trend of chemical shift change of thiophene and BMIM+ cation in the mixture, several possible contributing factors,9 such as (a) aromatic ring current effect (i.e., π-π interaction), (b) hydrogen bonding effect due to the H-bond donation of BMIM+ cation to the sulfur atom of thiophene, (c) C-H-π interaction between cation and thiophene, (d) anion effect, (e) dilution effect, and (f) electrostatic field effect, are considered and discussed individually in the following paragraphs.

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TABLE 1: 1H NMR Chemical Shifts of Protons on Thiophene and 1-n-Butyl-3-methylimidazolium Hexafluorophosphate in Each TS/BMIMPF6 Solution (Figure 1, System I) molar ratio of TS/BMIMPF6 neat BMIMPF6 0.5/1.0 1.0/1.0 1.5/1.0 2.0/1.0 3.0/1.0 3.5/1.0 neat thiophene

H1 6.939 6.914 6.892 6.872 6.834 6.823 6.651

H2 7.235 7.198 7.164 7.132 7.067 7.049 6.781

H3

H4

H5

H6

H7

H8

H9

H10

0.715 0.684 0.662 0.645 0.632 0.613 0.608

1.152 1.093 1.047 1.010 0.982 0.938 0.925

1.703 1.612 1.537 1.477 1.431 1.362 1.341

4.040 3.917 3.813 3.731 3.669 3.578 3.543

3.771 3.665 3.567 3.484 3.417 3.315 3.279

7.326 7.199 7.078 6.977 6.898 6.780 6.732

7.290 7.157 7.033 6.928 6.846 6.724 6.676

8.366 8.213 8.081 7.978 7.902 7.800 7.726

The aromatic ring current effects in aromatic and antiaromatic compounds have been well studied and documented in the past.10-12 It is known that NMR signals of a compound situated above or below the shielding cone of the aromatic ring will move upfield (i.e., shielding effect), whereas those for a compound situated outside the shielding cone will move downfield. In this system, both thiophene and BMIM+ cation have an aromatic ring. The aromatic ring current effect in a neat thiophene is stronger than that in a neat BMIMPF6 liquid because thiophene molecules are tightly packed whereas BMIM+ cations are separated by PF6- anions. The bulky PF6- anions prevent BMIM+ cations from coming too close and greatly reduce the aromatic current effect.7 Therefore, the NMR chemical shift of a neat thiophene is more upfield than a neat BMIMPF6 liquid. In a mixture of BMIMPF6 and thiophene, the thiophene molecules are able to approach the BMIM+ cation. The trend of chemical shift change observed in our spectra indicates that the BMIM+ cation is located in the shielding cone of thiophene ring current and the thiophene is located near the deshielding zone of BMIM+ ring current. This local structure arrangement caused NMR chemical shift of BMIM+ to move upfield and the signal for thiophene to move downfield. The formation of hydrogen bond between the acidic hydrogen of BMIM+ and thiophene is weak and typically introduces downfield chemical shift of BMIM+ cation13 instead of the upfield shift, as was observed in our study. In addition, thiophene is not a good H-bond acceptor because the unpaired electrons of sulfur participate in the aromatic ring current and may carry a partial positive charge in its resonance structure when the aromatic ring current carries a partial negative charge. The C-H-π interaction between the BMIM+ cation and thiophene aromatic ring could also induce an upfield chemical shift of BMIM+ cation with different extent on each aromatic proton instead almost equal, as was observed in our study. The PF6- anion is a potential hydrogen acceptor, but it will introduce a downfield shift.7,9 In addition, it was known9 that neither PF6- nor BF4- anion is such a good acceptor as Cl- or AlCl4-. The dilution of BMIMPF6 by a neutral molecule like thiophene is not expected to break apart the strong Coulombic interaction between BMIM+ and PF6- ions significantly by a simple insertion between them. Indeed, many organic molecules, particularly the aliphatic ones, have been found to be incapable of dissolving in the ionic liquids.4 Therefore, dilution does not simply occur without a favorable molecular interaction between the dissolved molecules and the ionic liquids. The electrostatic field effect is important when electrons surrounding the resonating nucleus were displaced by a chemically bonded polar atom such as fluorine. Studies of the electric filed effect had been reported in several open chain and cyclic fluorine-substituted alkanes.11 This effect is not a dominant factor in our systems because the fluoro-anions are not chemically bonded to the imidazolium cations or thiophene molecule.

Figure 3. Variation of 1H NMR chemical shift difference of protons in BMIMPF6 at various TS/BMIMPF6 molar ratios relative to the neat BMIMPF6. (Hx-Hxneat is the chemical shift difference of proton x at various molar ratios of TS/BMIMPF6 and neat BMIMPF6, respectively. Assignment of proton x in BMIMPF6 is referred to system I in Figure 1.)

The electric field effect arisen from the physical interaction between fluoro-anions and cations or thiophene is also not a dominant factor in determining the absorption capacity of thiophene in these systems. Otherwise, a similar chemical shift effect on the imidazolinium cation will be observed in the nonaromatic systems such as alkanes or aliphatic mercaptans. Instead, the aliphatic compounds are barely soluble in these ionic liquid system4 and no chemical shift effect can be observed. With the consideration of all these possible factors and the observed NMR results, the aromatic ring current effect is the dominant factor to influence the trend of chemical shift change of cation and thiophene. Comparison of 1H NMR Results of Systems I-III. When thiophene and neat ionic liquid are mixed, the thiophene molecules approach and surround the imidazolium cation due to the aromatic ring current interaction. Compared to those in corresponding neat ionic liquid, the relative 1H NMR chemical shift of the protons in imidazolium cation with increasing TS/ ionic liquid molar ratio are shown in Figures 3-5 for systems I-III, respectively. Results obtained from these three systems show that the upfield shift effect (i.e., negative value) is strongest for the ring protons in the imidazolium cation followed by the methyl and methylene protons next to the nitrogen. The further away the protons are from the ring as in 1-n-butyl or 1-ethyl

Thiophene Interaction with Ionic Liquids

Figure 4. Variation of 1H NMR chemical shift difference of protons in BMIMBF4 at various TS/BMIMBF4 molar ratios relative to the neat BMIMBF4. (Hx-Hxneat is the chemical shift difference of proton x at various molar ratios of TS/BMIMBF4 and neat BMIMBF4, respectively. Assignment of proton x in BMIMBF4 is referred to system II in Figure 1.)

Figure 5. Variation of 1H NMR chemical shift difference of protons in EMIMBF4 at various TS/EMIMBF4 molar ratios relative to the neat EMIMBF4. (Hx-Hxneat is the chemical shift difference of proton x at various molar ratios of TS/EMIMBF4 and neat EMIMBF4, respectively. Assignment of proton x in EMIMBF4 is referred to system III in Figure 1.)

groups, the weaker the upfield shift effect becomes. This phenomenon suggests that thiophene molecules must face and locate close to the ring of the imidazolium cation of the ionic liquid. A comparison of the changes in 1H NMR chemical shift of the ring protons in the imidazolium cation in each ionic liquid is also shown in Figure 6. The maximum absorption capacity for thiophene per mole of ionic liquid obtained from real measurement4 and described in the Experimental Section is

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Figure 6. Variation of 1H NMR chemical shift difference of the ring protons in the imidazolium cation of each ionic liquid system at various TS/(ionic liquid) molar ratios relative to each respective neat ionic liquid. (EMIMBF4-x, BMIMBF4-x, and BMIMPF6-x represent proton x on the imidazolium cation in systems III, II, and I in Figure 1, respectively.)

approximately 3.5 for BMIMPF6, 2.4 for BMIMBF4, and 0.9 for EMIMBF4. These experimental values are slightly off the integer values of 4, 2, and 1 and could be attributed to trace impurities present in each starting ionic liquid. 1H NMR upfield shifts (Figure 6) are approximately -0.20, -0.35, and -0.65 ppm, respectively, for systems III, II, and I when the absorption capacity of thiophene reached the maximum in each system. This more or less proportional result suggests that the numbers of thiophene molecules surrounding each imidazolium cation are approximately 1, 2, and 4 for systems III, II, and I, respectively. The nearly parallel and converged curves of the ring protons in each system (Figure 6) also indicate that the effect of overall thiophene molecules on each ring proton of the respective imidazolium cation is similar. It follows that the overall (average) geometrical arrangement of thiophene molecules surrounding the imidazolium cation must be more or less symmetric in each TS/ionic liquid system. This result suggests that the C-H-π interaction between the acidic hydrogen of cation and thiophene is not as important as the π(imidazolium ring)-π(thiophene ring) interaction. Otherwise, the trend of chemical shift change of ring protons will not be parallel and converged. Apparently, the aromatic ring current effect (π-π interaction) is the dominant factor to determine the chemical shift trend in all these three ionic liquid systems. A comparison of change in 1H NMR chemical shift of the ring protons of thiophene in each ionic liquid system is shown in Figure 7. The results show that all ring protons of thiophene are moved downfield (i.e., positive value), with the strongest effect on the protons in the carbon bonded directly to the sulfur for all systems when the relative ionic liquid concentration increases. The downfield chemical shift effect on thiophene in the EMIMBF4 system is the strongest, followed by BMIMPF6 and BMIMBF4 systems. The diverged curves of the ring protons of thiophene in each ionic liquid system suggest that the thiophene molecule interacts asymmetrically with the anion (i.e., PF6- or BF4-) even though the overall geometrical arrangement of thiophene relative

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Figure 7. Variation of 1H NMR chemical shift difference of the ring protons in thiophene of each ionic liquid system at various TS/(ionic liquid) molar ratios relative to each respective neat ionic liquid. (EMIMBF4-x, BMIMBF4-x, and BMIMPF6-x represent proton x on thiophene in systems III, II, and I in Figure 1, respectively.)

to the imidazolium cations is symmetrical. It is apparent that the size (or chain length) of the alkyl group of imidazolium cation and the size and property of anion affects the coordination number and geometric arrangement of thiophene molecules relative to the cations and the anions of ionic liquids. Several studies5-8 have suggested that most neat ionic liquids at room temperature form a more or less stacking structure with alternating cations and anions. 1H NMR study by Headley et al.13 also showed that the interaction of the ring protons in 1-nbutyl-3-methylimidazolium salt with the relatively small and basic BF4- anion is more intimate than that with the fairly large, polarizable, and less basic PF6- anion. Our 1H NMR results for systems I-III (protons 8-10 in Tables 1 and 2 and protons 6-8 in Table 3 of neat liquid) also showed the similar trend where EMIMBF4 has the most downfield chemical shift

followed by BMIMBF4 and BMIMPF6. These results show that the intimacy of interaction between the ring protons of imidazolium cation and anion is influenced not only by the size (or type) of the anion but also by the size (or chain length) of alkyl group in the imidazolium cation. Due to this difference in intimacy, it is apparent that the distance between the cation and anion is the shortest for the EMIMBF4 system, followed by that in the BMIMBF4 system, and with the longest distance in the BMIMPF6 system. Therefore, the stacking structure of the cation and anion is tightest for the neat EMIMBF4 system, followed by BMIMBF4 and BMIMPF6 systems. With the different interacting force between the cation and anion in various ionic liquids, the maximum numbers of thiophene molecules allowed to surround the imidazolium cation can be varied. This observation is in good conformity with ionic or inorganic salts that typically exist in predictably packed structures having maximum coordination numbers as determined by the relative sizes of cation and anion. A main difference between an ionic liquid and a classical mineral salt relates to the dynamic mobility of the constituent ions in the former in its liquid state. Hence, ionic liquids are also frequently referred to as molten salts. 19F, 11B, and 31P NMR Studies. 19F, 11B, and 31P NMR spectroscopy were also used to investigate the interaction of anion with thiophene in various ionic liquids. The chemical shifts of these nuclei were measured at various TS/ionic liquid molar ratios, and the results are listed in Tables 4-6 for systems I-III, respectively. Table 4 shows 31P and 19F NMR chemical shifts and the P-F coupling constants of PF6- anion at various TS/BMIMPF6 molar ratios. Tables 5 and 6 show 11B and 19F NMR chemical shifts of BF4- anion at various TS/ionic liquid molar ratios in TS/BMIMBF4 and TS/EMIMBF4 systems, respectively. Two different 19F NMR chemical shifts of BF4were observed for systems II and III, reflecting fluorine bonding to the isotope B-11 (80.2%) and B-10 (19.8%). The absolute 19F NMR chemical shifts of BF - in the EMIMBF system are 4 4 more upfield than those in the BMIMBF4 system whereas the opposite is observed for 11B NMR chemical shifts (Tables 5 and 6). These results suggest that the negative charge of BF4anion in the EMIMBF4 system is spread more around fluorine atoms and less around boron as compared to those in the BMIMBF4 system. It also suggests that the BF4- anion in the EMIMBF4 system is more basic and interacts more intimately with the imidazolium cation than in system BMIMBF4. This result is consistent with the 1H NMR finding that the interaction

TABLE 2: 1H NMR Chemical Shifts of Protons on Thiophene and 1-n-Butyl-3-methylimidazolium Tetrafluoroborate in Each TS/BMIMBF4 Solution (Figure 1, System II) molar ratio of TS/BMIMBF4 neat BMIMBF4 0.5/1.0 1.0/1.0 1.5/1.0 2.0/1.0 2.4/1.0 neat thiophene

H1

H2

6.881 6.868 6.847 6.829 6.827 6.651

7.219 7.181 7.136 7.101 7.092 6.781

H3

H4

H5

H6

H7

H8

H9

H10

0.606 0.603 0.601 0.594 0.590 0.590

1.040 1.019 0.998 0.974 0.960 0.959

1.605 1.545 1.498 1.455 1.427 1.426

4.059 3.957 3.880 3.815 3.773 3.772

3.782 3.690 3.618 3.554 3.513 3.512

7.437 7.323 7.229 7.146 7.091 7.090

7.398 7.287 7.195 7.116 7.064 7.062

8.727 8.587 8.487 8.414 8.372 8.370

TABLE 3: 1H NMR Chemical Shifts of Protons on Thiophene and 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Each TS/EMIMBF4 Solution (Figure 1, System III) molar ratio of TS/EMIMBF4

H1

H2

neat EMIMBF4 0.20/1.00 0.50/1.00 0.86/1.00 neat thiophene

6.978 6.963 6.945 6.651

7.346 7.317 7.282 6.781

H3

H4

H5

H6

H7

H8

1.368 1.342 1.290 1.244

4.202 4.168 4.092 4.024

3.892 3.861 3.792 3.728

7.539 7.508 7.426 7.343

7.497 7.455 7.368 7.282

8.817 8.776 8.689 8.610

Thiophene Interaction with Ionic Liquids

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TABLE 4: 31P and 19F NMR Chemical Shifts for Phosphorus and Fluorine of 1-n-butyl-3-methylimidazolium Hexafluorophosphate in Each TS/BMIMPF6 Solution molar ratio of TS/BMIMPF4 neat BMIMPF6 0.5/1.0 1.0/1.0 1.5/1.0 2.0/1.0 3.0/1.0 3.5/1.0

31

P NMR chemical shift (ppm)

coupling constant JP-F (Hz) from 31P NMR

-144.347 -144.219 -144.128 -144.045 -143.985 -143.894 -143.879

19

F NMR chemical shift (ppm)

coupling constant JF-P (Hz) from 19F NMR

-68.068 -67.720 -67.475 -67.296 -67.169 -66.982 -66.949

710.827 710.827 711.336 711.350 711.336 711.590 711.590

710.460 710.460 711.681 711.681 711.680 711.680 711.680

TABLE 5: 11B and 19F NMR Chemical Shifts for Boron and Fluorine of 1-n-butyl-3-methylimidazolium Tetrafluoroborate in Each TS/BMIMBF4 Solution molar ratio of TS/BMIMBF4

11

B NMR chemical shift (ppm)

19

F NMR chemical shift (ppm) bonded to B-11

-1.548 -1.384 -1.279 -1.205 -1.153 -1.152

neat BMIMBF4 0.5/1.0 1.0/1.0 1.5/1.0 2.0/1.0 2.4/1.0

19

F NMR chemical shift (ppm) bonded to B-10

-147.513 -146.983 -146.612 -146.457 -146.314 -146.311

-147.461 -146.929 -146.612 -146.402 -146.259 -146.257

TABLE 6: 11B and 19F NMR Chemical Shifts for Boron and Fluorine of 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Each TS/EMIMBF4 Solution molar ratio of TS/EMIMBF4 neat EMIMBF4 0.20/ 1.00 0.50/1.00 0.86/1.00

11

B NMR chemical shift (ppm) -1.504 -1.459 -1.375 -1.307

of the cation and anion in EMIMBF system is stronger than in the BMIMBF4 system. The changes in the 19F NMR chemical shift of the fluoroanions with increasing TS/ionic liquid molar ratios for systems I-III are compared in Figure 8. When the absorption for thiophene reaches the maximum, the 19F NMR chemical shifts of ionic liquids with BF4- anion had downfield shifts (i.e., positive values) of approximately +0.70 ppm and +1.20 ppm

Figure 8. Variation of 19F NMR chemical shift difference of the fluorine-containing anion in each ionic liquid system at various TS/ (ionic liquid) molar ratios relative to each respective neat ionic liquid. (The numbers -10 and -11 after symbols EMIMBF4 and BMIMBF4 represent the fluorine of BF4- anion bonded to isotope of B-10 and B-11, respectively. The anion in BMIMPF6 is PF6-.)

19

F NMR chemical shift (ppm) bonded to B-11 -147.965 -147.790 -147.487 -147.251

19

F NMR chemical shift (ppm) bonded to B-10 -147.915 -147.741 -147.432 -147.199

for systems EMIBF4 and BMIMBF4, respectively. This result suggests that there is approximately twice the amount of thiophene molecules surrounding the BF4- anion in the BMIMBF4 system than in the EMIMBF4 system, which is in close agreement with the results of experimental absorption measurement. The 19F NMR chemical shift variation curve of PF6- anion does not follow the same trend as BF4- because they are different anion species. Due to the significantly larger phosphorus atom than boron, the maximum number of thiophene molecules surrounding the PF6- anion could reach 4. Figure 9 shows that the maximum 11B NMR downfield shift (∼+0.4 ppm) of the BMIMBF4 system is about twice that (∼+0.2 ppm)

Figure 9. Change in 11B NMR chemical shift of BF4- anion in EMIMBF4 and BMIMBF4 systems at different TS/(ionic liquid) molar ratios relative to the respectively neat ionic liquid.

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Figure 10. Proposed structures of various thiophene/ionic liquid systems. (The structure in left is for the EMIMBF4 system with a repeating unit of 1/1 TS/EMIMBF4; the structure in middle is for the BMIMBF4 system with a repeating unit of 2/1 TS/BMIMBF4; and the structure in right is for the BMIMPF6 system with a repeating unit of 4/1 TS/BMIMPF6. The orientation of Et- or n-Bu group on the imidazolium cation is flexible and will point to the direction to achieve the most stable state of the arrangement.)

of the EMIMBF4 system. The results again indicate that the number of thiophene molecules surrounding the BF4- anion in the BMIMBF4 system is approximately twice that in the EMIMBF4 system. The 31P NMR chemical shift of PF6- also moves downfield when the molar ratio of TS/BMIMPF6 increases, as shown in Table 4. The downfield chemical shift effect of 19F, 11B, and 31P NMR for BF4- and PF6- anions in all three systems suggests that these anions are situated outside the shielding cone of the thiophene molecules. The results further suggest that the sulfur with partial positive charge in the thiophene molecule should direct toward the anions outside the thiophene plane. The local structures of thiophene molecules in BMIMPF6, BMIMBF4, and EMIMBF4 ionic liquids are proposed below by taking into account all structural information obtained by multinuclear NMR measurements. Structures of Various Thiophene/ionic Liquid Systems. System I: TS in BMIMPF6. On the basis of all NMR data, the structure of system I can be constructed as a stacking structure with the repeating 4/1 TS/BMIMPF6 units, as shown to the right of Figure 10. The thiophene molecules form a pseudotetrahedron geometry surrounding the BMIM+ cation. The planes of all thiophene molecules are facing but situated outside the shielding cone of the BMIM+ cation with the sulfur of thiophene molecules pointing toward the anion of PF6-. The PF6- anion is situated between two BMIM+ cations and also surrounded by four thiophene molecules in tetrahedral geometry. The BMIM+ cation is surrounded and situated in the shielding cone of thiophene molecules. A cube is drawn in the structure to aid a 3D viewing. With this symmetrical structure, the upfield chemical shifts of all ring protons in the BMIM+ cation induced by thiophene molecules are equivalent and result in the parallel and converged shift effect, as shown in Figure 6. For the thiophene molecule, the sulfur atom preferably pointing to the PF6- anion induces an asymmetric interaction with the anion. Due to this asymmetrical interaction, the downfield chemical shift effect of two types of protons in each thiophene will diverge, as shown in Figure 7 even though the overall interaction of thiophene with the imidazolium cation is symmetrical. System II: TS in BMIMBF4. For system II, the stacking structure with repeating units of 2/1 TS/BMIMBF4 is shown in the middle of Figure 10. Two thiophene molecules, one above and one below, surrounding the BMIM+ cation form a slanted

linear structure. The planes of both thiophene molecules are also facing the BMIM+ cation with the sulfur pointed to the anion of BF4-. Due to the smaller anion size of BF4- than PF6and a tighter stacking structure of BMIMBF4 than BMIMPF6 ionic liquid, only two thiophene molecules surrounding the BMIM+ cation are allowed. There are also two thiophene molecules surrounding the BF4- anion, as shown by 11B and 19F NMR results. The overall stacking structure of system II is a zigzag symmetric structure with repeating units of 2/1 TS/ BMIMBF4. Due to the alternating location of thiophene molecules between cations and anions, the upfield chemical shifts of the ring protons in the BMIM+ cation induced by thiophene molecules are also parallel and converged, as shown in Figure 8. The chemical shift variations of two types of thiophene protons also diverge due to the preferred orientation of the sulfur atom pointing to the anion. System III: TS in EMIMBF4. For system III, the stacking aggregate is shown in the left of Figure 10 with repeating units of 1/1 TS/EMIMBF4. There is only one thiophene molecule with each EMIM+ cation. The plane of the thiophene molecule faces the EMIM+ cation with the sulfur atom pointing to the BF4anion. Due to the even tighter packing structure of the EMIMBF4 ionic liquid than the BMIMBF4 and BMIMPF6 ionic liquids, only one thiophene molecule could be accommodated by each BMIM+ cation. The 11B and 19F NMR results also show that there is only one thiophene molecule with each BF4- anion. With the alternating arrangement of thiophene molecules between each pair of EMIMBF4 ionic liquid, the overall chemical shift changes of the ring protons in BMIM+ cation will be averaged out and show a parallel and converged curve, as shown in Figure 8. In addition, the EMIM+ cation in system III probably exhibits a more facile rotation about the perpendicular axis of the cation plane as compared to systems I and II, where two and four thiophene molecules surround the cation. The strongest divergence of the 1H NMR chemical shift difference of the two types of protons in thiophene for system III suggests that this system has the tightest packing structure. Conclusions Multinuclear NMR measurement of absorbed thiophene in three ionic liquids, BMIMPF6, BMIMBF4, and EMIMBF4,

Thiophene Interaction with Ionic Liquids showed gradual structural changes of the new liquid systems upon the addition of thiophene to the neat ionic liquids. The extent of the change in the ionic liquid structure is strongly correlated to the size and chain length of the alkyl group (i.e., n-butyl or ethyl) in the imidazolium cation and the type and size of anions (i.e., PF6- or BF4-). The maximum structural change is limited to the saturated absorption of thiophene in the ionic liquids. The NMR results are therefore in excellent conformity with our previous measurement based on mass related to the maximum absorption capability.4 Due to the aromatic ring current interaction between the imidazolium cation and the thiophene molecules, the aromatic molecules are strongly absorbed by ionic liquids. Therefore, thiophene molecules are accommodated into the ionic pair structure of the ionic liquids in significantly high molar ratios, enabling the selective and extractive removal of aromatic sulfur compounds from fuels. The local structures of thiophene in the three ionic liquids are elucidated on the basis of the multiple nuclei 1H, 19F, 11B, and 31P NMR results. A unique feature of ionic liquids as salts is their mobility and flexibility, which allow for a facile restructuring of the ionic liquids in the process of thiophene dissolution. The restructuring process is primarily driven by the interaction of thiophene with the imidazolium cations of the ionic liquids, and the maximum absorption capacity of thiophene by ionic liquids is primarily determined by the size and structure of both cations and anions. Acknowledgment. We thank the Multiple BUs of Akzo Nobel Chemicals Inc. for support and permission to publish this work.

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