The Structure−Activity Relationship and Physicochemical Properties of

We prepared three series of novel protic ionic liquids (PILs) based on acetamide and a Brønsted acid (HX, where X is CF3COO−, CH3COO−, or HSO4−...
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J. Phys. Chem. C 2010, 114, 20007–20015

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The Structure-Activity Relationship and Physicochemical Properties of Acetamide-Based Brønsted Acid Ionic Liquids Feng Wu,†,‡ Jin Xiang,† Renjie Chen,*,†,‡ Li Li,†,‡ Junzheng Chen,† and Shi Chen†,‡ School of Chemical Engineering and EnVironment, Beijing Institute of Technology, Beijing Key Laboratory of EnVironmental Science and Engineering, Beijing, 100081, China, and National DeVelopment Center of High Technology Green Materials, Beijing 100081, China ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: October 12, 2010

We prepared three series of novel protic ionic liquids (PILs) based on acetamide and a Brønsted acid (HX, where X is CF3COO-, CH3COO-, or HSO4-) by a simple atom-economic neutralization reaction. We investigated their physicochemical properties, such as acidic scale, viscosity, ionic conductivity, and thermal characteristics, as well as the structure-activity relationships of these PILs. Carboxylic acid formation is only observed in the acetamide sulfate complex system by FT-IR. The ionic conductivities for most of the samples are between 10-3 and 10-1 S/m at room temperature and at low viscosity. Acetamide trifluoroacetate (ATFA) with a molar ratio of 5/5 (acetamide/trifluoroacetic acid) has an ionic conductivity of 0.25 S/m, and its viscosity is only 10 cP at 25 °C. The samples exhibit better properties at higher temperatures. ATFA with molar ratios of 7/3 and 8/2 have ionic conductivities of 1.13 and 1.07 S/m at 80 °C, respectively. Moreover, most of the prepared samples possess relatively moderate thermal stabilities (up to 106 °C for ATFA) and a wide liquid range (down to -69 °C for ATFA). All these properties make these acetamide-based PILs of interest as reaction media, as catalysts in organic synthesis, or as electrolytes in fuel cells. 1. Introduction Recently, room-temperature ionic liquids (ILs) have gained increasing attention in many fields, including synthetic and catalytic chemistry, electroplating, and batteries, because they possess several peculiar physicochemical properties, such as a good solvating ability for a wide range of substrates and catalysts, wide liquid range, acceptable ionic conductivity, high thermal stability, and negligible vapor pressure.1-5 Protic ionic liquids (PILs) that consist of combinations of Brønsted acids and bases are a subclass of ILs and retain the characteristics that are typical of ILs. The PILs possess the properties of high proton conductivity, super acidity, and excellent chemical and thermal stability. Moreover, proton conducting materials have been of interest to many researchers because of their possible application in electrochromic devices or fuel cells,6,7 chemical sensors,8 and acid catalytic reactions.9-11In recent years, a series of novel PILs based on ammonium/inorganic or organic acid,12-15 ZIL/HTFSI,16 lactams/inorganic or organic acid,17 H2NC3H6mim/Tf2N,18 and 2-MPy/trifluoroacetic acid19 have been reported. For instance, Angell et al.12 were the first to react organic amines with inorganic or organic acids to form PILs, and their conductivities matched those of liquid electrolytes without solvents and were 15 S/m at 25 °C and 47 S/m at 100 °C. Yoshizawa et al.16 reported novel binary ILs based on a zwitterionic liquid (ZIL) and HTFSI. These ZIL/acid mixtures were expected to be thermally stable proton conductors under water-free conditions, but their ionic conductivities were only about 10-2 S/m at room temperature. Primary amine organic anions of the forms RNH3 and R(OH)NH3 have been combined with organic anions of the forms RCOO- and R(OH)COO- and * To whom correspondence should be addressed. Tel: +86 10 68918766. Fax: +86 10 68451429. E-mail: [email protected]. † Beijing Institute of Technology, Beijing Key Laboratory of Environmental Science and Engineering. ‡ National Development Center of High Technology Green Materials.

inorganic anions to produce PILs. These samples have low viscosity and high ionic conductivity at room temperature, and their values may reach to 17 mPa · s and 4.38 S/m, respectively. However, many of these PILs have stability problems as they form amides through a condensation reaction.14 Lactams, such as caprolactam and butyrolactam, were reacted with Brønsted acids (HBF4, CF3COOH, phCOOH, ClCH2COOH, HNO3, or H3PO4) by a simple and atom-economic neutralization reaction with no reverse reaction. Lactam-based PILs are relatively cheaper and less toxic, but they exhibit unsatisfactory ionic conductivity of less than 0.144 S/m (25 °C).17 Recently, [CH3CH2CH2NH3+][CF3COO-] (TFAPA) has been synthesized and its application in intermediate temperature PEMFCs was investigated. However, the temperature needed to prepare TFAPA is around -20 °C, which means a complex operation is required at great expense.15 Amides (RCONHR), such as acetamide (CH3CONH2), with an intrinsically low toxicity, are relatively cheaper and safer than imidazolium-based and amine-based PILs. From the molecular structure of these amide compounds, the O atom is partially negatively charged and the N atom is partially positively charged. When they are mixed with a lithium salt (LiX), the hydrogen bonds (-CdO · H-N-) in the RCONHR molecules and the association between Li and X in the LiX lattice are weakened due to a Columbic interaction between Xand Oδ-, forming a new group of liquid electrolytes.20-22 We propose that those amide compounds probably interact with protic salts (HX) to form new PILs. The application of acetamide in Li+-conducting ILs for use in electric double-layer capacitors has been reported by our group,20 but its application in PILs has never been reported. In this work, acetamide trifluoroacetate (ATFA), acetamide acetate (ATAA), and acetamide sulfate (ATSA) with nine different molar ratios (from 1:9 to 9:1) were synthesized at room

10.1021/jp104905p  2010 American Chemical Society Published on Web 11/05/2010

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CHART 1: Possible Structures of ATAA and ATFA. X ) CH3COO- (ATAA) and CF3COO- (ATFA)

Wu et al. TABLE 1: Physicochemical Properties of PILs, Including Their Water Ratio (W), Acidic Scale (A), Kinetic Viscosity (η), and Ionic Conductivity (σ) A (H0)

temperature, and their physicochemical properties were investigated in detail.

η (cp)

σ (S/m)

PILs

W (%)

25 °C

25 °C

60 °C

25 °C

60 °C

80 °C

ATFA5 ATFA6 ATFA7 ATFA8 ATFA9 ATAA5

0.47 0.32 0.38 0.33 0.25 0.22

4.33 4.30 4.38 4.70 4.82 5.19

10.0 13.7 19.7 24.0a

3.0 3.7 4.5 5.1 5.4 2.7

0.25 0.23 0.19 0.09 0.03 0.008

0.74 0.76 0.71 0.61 0.50 0.023

1.07 1.16 1.12 1.07 0.87 0.057

b

7.7

Solid state at 25 °C, but after heating it to a liquid and then keeping the temperature at 25 °C, it can be a stable liquid.We attribute it to its two melting transition behaviors, as shown in Table 2. b Solid state at 25 °C. a

2. Experimental Section 2.1. Preparation of Acetamide-Based PILs. Abbreviations for the acetamide-based PILs are shown in Chart 1. ATFA with various molar ratios were prepared by mixing acetamide (Acros Organics, 99%) and trifluoroacetic acid (American Scharlau Chemie, 99.5%) slowly at a certain molar ratio (1:9-9:1, acetamide/trifluoroacetic acid) under vigorous stirring at room temperature and in a flask within 30 min. The reaction was carried out for about 5 h at room temperature for the liquid product and at 80 °C for the solid product. Finally, the synthesized samples were immediately sealed to prepare to test. ATAA and ATSA at various molar ratios were synthesized using the same process. Acetic acid (Beijing Chemical Works, 99.5%) and sulfuric acid (Beijing Chemical Works, 98%) were used as received. ATFA8 (8:2), ATFA9, ATAA7, ATAA8, ATAA9, and ATSA9 were solid products, whereas the other samples were colorless liquids and ATSA (4-8) were very ropy at room temperature. Acetamide was dried in an oven at 70 °C for 24 h before use. 2.2. Analysis and Measurement. The water content of the PILs was determined by Karl Fischer titration using a Cou-Lo Aquamax KF moisture meter. Typically 1 mL of liquid sample or 1 mL of 20 w/w% solution of the solid sample in dilute and dry DMF was injected for analysis. The density of the PILs was measured by the weight method at 25 °C. The acidic scale of the PILs was measured using an Agilent UV-1201 spectrophotometer with a basic indicator. The kinetic viscosity of the PILs was measured using a glass capillary viscometer in a water bath from 25 to 60 °C. The PILs flowed through the glass capillary viscometer between two scaled lines at a certain temperature, and the time was measured. At least three measurements were taken and an average time value was used to calculate the kinetic viscosity. Fourier transform infrared spectroscopy (FTIR) measurements were carried out using a Nicolet 6700 FTIR spectrometer (America) between 4000 and 400 cm-1. Thermal behavior was investigated using a TA Instruments MDSC 2910 differential scanning calorimeter (America). About 10 mg of sample was sealed in a special aluminum pan, and then the pan was transferred into a DSC sample holder. The pan containing the PILs was first cooled to -90 °C and then heated to 90 °C at rate of 10 °C · min-1 under N2. Thermal stability measurements were performed with a thermogravimetric analyzer (Netzsch TG209 F1, Germany) from room temperature to 500 °C at a scan rate of 20 °C · min-1 under N2. The ionic conductivity of PILs was examined using a DDS11A conductivity electrode with a cell constant of 1.0 cm-1 attached to a CHI 600C electrochemical workstation in a frequency range of 10-105Hz at various temperatures. The PILs were carefully sealed into the electrode to avoid the influence of the environment. Configurations of the acetamide coordinated with H+ ions were optimized with the BLYP functional of nonlocal DFT using a DNP (double numerical with polarization)

TABLE 2: Thermal Properties of PILs, Including Their Decomposition Temperature (Td), Glass Transition (Tg), Devitrification Temperature (Tc), Melting Point (Tm), and Unassigned Phase Transition (Tp) PILs

Td

Tg

ATFA5 ATFA6 ATFA7 ATFA8 ATFA9 ATAA5 ATSA5

75 82 96 96 106 83 170

-69 -69 -67 -69 -73 -39 -66

Tc

Tm

-3

15a 15a,60b 77 -3

Tp

-36 34

a Melting transition (Phase I) existed in both ATFA7 and ATFA8. b Melting transition (Phase II) transition existed in ATFA8.

basis set as implemented in the DMol3 module of the Materials Studio 4.0 program. The Mulliken charges as well as the bond length of the acetamide and acid before and after coordination were calculated with this program. 3. Results and Discussion In this investigation, three novel PILs were produced from acetamide and a Brønsted acid. The physicochemical properties of the ATFA5, ATFA6, ATFA7, ATFA8, ATFA9, and ATAA5 ionic liquids are listed in Table 1, namely, their water content, acidic scale, kinetic viscosity, and ionic conductivity. Their thermal properties are also listed in Table 2 and comprise Td, Tg, Tc, Tm, and Tp. In the following section, the interaction between the cations and anions as well as the physicochemical and thermal properties listed in Tables 1 and 2 are discussed. 3.1. Interaction between Acetamide and the Brønsted Acid. The interactions between acetamide and lithium salts, such as LiClO4, have been extensively studied using FT-IR spectra, and a possible structure for the acetamide/LiClO4 composite ionic liquid electrolyte has also been proposed in our previous work.20 Figure 1a shows IR spectra for the acetamide/CF3COOH complex with different molar ratios between 1600 and 1720 cm-1. The bands at 1687 and 1641 cm-1 are assigned to the CdO stretching mode and NH bending mode of acetamide, respectively. The CdO stretching band shifts to 1667 cm-1 and the NH bending band shifts to 1617 cm-1 after mixing CF3COOH with acetamide, and this is similar to the acetamide/ LiClO4, acetamide/LiBOB, and acetamide/LiTFSI systems.20,22,23 Therefore, it is reasonable to assume that the interaction between CF3COOH and acetamide may be similar to the acetamide/ lithium salt complex (see Chart 1). We suggest that, when acetamide is mixed with CF3COOH, the partially negatively charged O atom and the partially positively charged N atom in

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Figure 1. (a) FT-IR spectra of ν(CO) and δ(NH) of acetamide for ATFA at various molar ratios in the range of 1600-1720 cm-1. (b) Structural parameters and geometries optimized for ATFA. (c) FT-IR spectra of ν(CO) of CF3COOH for ATFA at various molar ratios in the range of 1720-1900 cm-1. (CF3COOH has no IR absorption in the range of 1600-1720 cm-1.)

Figure 2. (a) Structural parameters and geometries optimized for ATAA. (b) FT-IR spectra of ν(CO) and δ(NH) of acetamide and ν(CO) of CH3COOH for ATAA at various molar ratios in the range of 1600-1720 cm-1.

acetamide can accelerate the ionization of trifluoroacetic acid. The hydrogen bond in acetamide is gradually weakened because the more active proton in CF3COOH competes with it and free X- bonds are combined with the N atom. This interaction results in a red shift of the CdO stretching mode and NH bending mode, which shift from 1687 to 1667 cm-1 and from 1641 to 1617 cm-1, respectively. Because of the interaction between CF3COOH and acetamide, the absorption of the CdO stretching vibration for CF3COOH shifts to 1760 cm-1 from 1784 cm-1, which is its normal absorption15 before the introduction of acetamide (see Figure 1c). This further indicates the formation of an ionic liquid. Figure 2b shows the IR spectra of the acetamide/CH3COOH complexes with different molar ratios between 1600 and 1720 cm-1. For acetamide/CH3COOH, the CdO stretching mode of acetamide shifts from 1687 to 1648 cm-1 and the NH bending mode of acetamide shifts from 1641 to 1605 cm-1, which is similar to the acetamide/CF3COOH system, and it may have a similar structure (see Chart 1). However, unlike the acetamide/CF3COOH system, the CdO stretching vibration band for CH3COOH blue shifts gradually as the acetamide content increases and shifts slightly from 1703 to 1708 cm-1. The difference may originate from the weak electron-withdrawing methyl in CH3COOH. It should be noted

Figure 3. (a) Possible process of the hydrolysis of acetamide in sulfuric acid. (b) FT-IR spectra of the hydrolysis process of acetamide for ATSA at various molar ratios in the range of 1600-1720 cm-1. (H2SO4 only has a shoulder absorption band around 1631 cm-1 assigned to the proton hydrate mode.)

that the red shift of the CdO stretching mode band and the NH bending mode of acetamide for the acetamide/CH3COOH system is greater than that for the acetamide/CF3COOH system. We attribute this to the difference in coordination structures. For the acetamide/H2SO4 complex, the IR spectra are more complicated (as shown in Figure 3b). On the one hand, the CdO and NH vibrations of acetamide are present for the acetamide-H2SO4 ) 9:1, 8:2, 7:3, 6:4, and 5:5 complexes. Furthermore, the samples with excessive acetamide, such as ATSA8 and ATSA9, exhibit similar trends to the above two systems. Nevertheless, when the molar ratio of H2SO4 is increased as in the samples ATSA7, ATSA6, and ATSA5, the NH bending mode disappears and an absorption peak appears around 1700 cm-1 for ATSA6 and ATSA5. We assume that ATSA8 and ATSA9 have a similar structure to the above two systems, whereas ATSA7, ATSA6, and ATSA5 have different structures because of the hydrolysis of acetamide (see Figure 3a). It is well known that amides hydrolyze under acidic conditions, and the hydrolysis mechanism of acetamide is widely

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Wu et al. resonance form B. As a result, the C-N bond has more doublebond character in the liquid composite than in the solid acetamide. On the contrary, CdO bonding would be more like a single bond, and the position of this band, as mentioned above, will shift toward a lower frequency with increasing composite acid content. Figure 4 also compares the IR spectra of the C-N stretching mode of the acetamide/CF3COOH complex with different molar ratios. It is clear that the C-N stretching band keeps blue shifting as the acid content increases, which is in good agreement with the spectral variation of the CdO band. The spectra shown here demonstrate that the acylamino group has an important role in the interaction between the acid and acetamide and the formation of a complex system. The ATFA, ATAA, and ATSA samples with excessive acetamide possess similar structures where the acylamino groups work as complexing agents for the cations and the anions because of their polarity (CdO and NH2). They are thus capable of coordinating with cations and anions. However, most ATSA samples may hydrolyze under strongly acidic conditions and form NH3+, acetic acid, and ammonium bisulfate. Because of this hydrolysis of acetamide in ATSA, we mainly discuss the ATFA and ATAA complex systems in the following section. 3.2. Theoretical Calculations. We performed quantum chemical calculations by optimizing the geometries of the organic molecules and the coordinating ions. In fact, geometry optimizations for the interaction between acetamide and LiTFSI were performed using the DMol3 module of the Cerius2 program in our previous work, and the energy level of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of acetamide were optimized and calculated.21 The LUMO energy of acetamide is low and its HOMO is mainly located around the carbonyl oxygen, which indicates that H-O coordination occurs easily during the interaction between the Brønsted acid and the organic compounds. The calculated charge distributions are listed in Table 3. They show that the negative charge on the carbonyl oxygen is larger than that on nitrogen in acetamide, confirming the fact that H+ ions coordinate to carbonyl oxygens. Further information about the interaction between acetamide and acids is included in Table 3. On the one hand, the Mulliken charge of the carbonyl oxygen in acetamide becomes more negative, whereas that of the nitrogen atom in acetamide is less negative after coordination with H+. This explains the resonance equilibrium form of acetamide. On the other hand, the Mulliken charge of the hydroxyl oxygen in the acid becomes more negative, whereas that of the carbonyl oxygen in the acid is less negative after coordination with H+, which implies that a hydrogen bond is formed between the hydroxyl oxygen in the acid and the amino group in the acetamide (NH · · · O).

Figure 4. FT-IR spectra of ν(CN) of acetamide for ATFA at various molar ratios and the resonance form of acetamide.

accepted.24 The disappearance of the NH vibration is probably due to the functional group NH3+, which has been proposed for primary amine-based and lactam-based PIL systems.14,17 This assumption is confirmed by the fact that hydrogen bonding between HSO4- and -NH3+ is observed as a peak at 3071 cm-1. The introduction of -NH3+ may lead to absorption peaks for the acetamide CdO stretching mode at 1708 and 1700 cm-1 for ATSA5 and ATSA6, respectively. It seems that ATSA6, which has two CdO stretching vibration peaks at 1700 and 1659 cm-1, and ATSA7, which only has one CdO stretching vibration peak at 1655 cm-1, are transition states from structure B to structure C (see Figure 3a). On the other hand, the CdO stretching and NH bending vibrations of acetamide all disappear for the acetamide-H2SO4 ) 4:6, 3:7, 2:8, and 1:9 complexes. Instead of these vibrations, two new absorption bands at around 1710 and 1616 cm-1 are present. The absorption peaks around 1710 cm-1 are attributed to the CdO stretching mode of acetic acid, which is a product of the hydrolysis reaction (see Figure 3a). To characterize the vibration bands around 1616 cm-1, we investigated the IR spectrum of a mixture of 1 mol of acetic acid and 3 mol of sulfuric acid (see Figure 3b), and an absorption peak at a similar position was observed. We, therefore, suggest that the vibration bands around 1616 cm-1 result from the interaction between acetic acid and excess sulfuric acid. This assumption is further based on the fact that a shoulder absorption band assigned to the proton hydrate mode should present around 1631 cm-1 for pure H2SO4,25 which was also observed in our test, but it disappears in the presence of acetic acid due to the expected molecular interaction. Considering the resonance forms of acetamide (see Figure 4), H+-oxygen coordination in the acetamide/acid composite system may lead to an increase in the amount of acetamide of

TABLE 3: Calculated Mulliken Charges for Acetamide, Acids, and the Coordinations between Them Mulliken charges a

C

a

N

H1a

H2a

-0.353

0.168

0.184

structure

O

CH3CONH2 CH3COOH CF3COOH CH3CONH2(H+ -OOCCF3) CH3CONH2(H+ -OOCCH3) CH3CONH2(H+ -OOCCF3)2

-0.463 -0.524 -0.530 -0.596

0.480 0.472 0.514

-0.343 -0.348 -0.325

0.221 0.225 0.227

0.184 0.179 0.195

CH3CONH2(H+ -OOCCH3)2

-0.593

0.499

-0.331

0.234

0.188

a

0.440

Hb

Ob

Oc

0.257 0.283 0.355 0.315 0.345 0.338 0.306 0.306

-0.415 -0.375 -0.479 -0.490 -0.463 -0.434 -0.487 -0.456

-0.416 -0.338 -0.373 -0.412 -0.363 -0.356 -0.410 -0.398

Atoms in the acylamino group. b Atoms in the hydroxyl group in acid. c Atom in the carbonyl group in acid.

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TABLE 4: Selected Bond Lengths of Acetamide, Acids, and the Coordinations between Them bond length (×10-1 nm) structure

r(CdO)

a

r(C-N)

CH3CONH2 CF3COOH CH3COOH CH3CONH2(H+ -OOCCF3) CH3CONH2(H+ -OOCCH3) CH3CONH2(H+ -OOCCF3)2

1.232 1.250 1.248 1.261

1.362 1.363 1.352

CH3CONH2(H+ -OOCCH3)2

1.260

1.353

CH3CONH2(H+ -OOCCF3)3

1.267

1.347

CH3CONH2(H+ -OOCCH3)3

1.265

1.353

(CH3CONH2)2(H+ -OOCCF3)

1.248 1.259 1.243 1.252

1.347 1.358 1.354 1.373

(CH3CONH2)2(H+ -OOCCH3)

interaction r(CdO)

b

r(CO · · · H)

r(NH · · · O)d

1.615 1.757 1.753 1.753 1.826 1.845 1.627 1.728 3.738 1.736 1.816 4.308 1.478 2.849 1.681 3.504

2.364 2.218 2.325

c

1.380 1.204 1.219 1.211 1.216 1.208 1.209 1.214 1.216 1.217 1.218 1.223 1.218 1.218 1.226 1.215 1.223

2.115 2.425 2.374 2.006 1.913

a In acetamide. b In acid. c The interaction between the carbonyl oxygen in acetamide and the hydroxyl hydrogen in acid. d The interaction between the amino group in acetamide and the hydroxyl oxygen in acid.

The bond lengths of various organic compounds and their coordination behavior in the acetamide/CH3COOH and acetamide/CF3COOH systems before and after coordination with the H+ ion are shown in Table 4. The variations in bond length for all systems are consistent with the IR spectroscopy results; that is, coordination elongates the CdO bond but shortens the C-N bond in acetamide. The CdO bond length in CF3COOH is longer, whereas that in CH3COOH is shorter after coordination. As a result, the position of the CdO band shifts to lower frequency with an increase in the acetamide content for the former and the opposite is true for the latter. The structures of various organic compounds and possible structures of the acetamide (acid)n (n ) 1-2) coordination compounds are shown in Figures 1b and 2a. It seems that a larger number of acetamide or acid molecules in these coordination compounds is not possible because of the long CO · · · H bond (i.e., 0.3738 nm for CH3CONH2(H+ -OOCCF3)3). A greater red shift is observed for the CdO stretching mode band of acetamide in the acetamide/CH3COOH system than that in the acetamide/ CF3COOH system. We, therefore, conclude from the variation of the CdO length of acetamide (0.126 nm for CH3CONH2(H+ OCCH3)2 and 0.125 nm for CH3CONH2(H+ -OOCCF3)) that one acetamide molecule in the acetamide/CH3COOH system coordinates with two CH3COOH molecules, whereas it coordinates to one CF3COOH in the acetamide/CF3COOH system. 3.3. Thermal Properties of the PILs. The thermal properties of representative PILs are given in Table 2, and their TGA traces and representative DSC traces are shown in Figure 5a,b. ATFA5 and ATAA5 have decomposition temperatures of approximately 75 and 83 °C (10% decomposition), respectively, and ATFA9 is thermally stable until about 106 °C. As shown in Figure 5a, all compounds show a single-stage decomposition behavior except for ATSA5, which shows a two-step decrease in mass. ATSA5 shows very low weight loss up to 160 °C and decomposes rapidly between 170 and 210 °C and between 320 and 385 °C. By comparison with the TGA trace of acetamide, the first weight loss stage is mainly due to dissociation of the compound as well as the decomposition of the acetamide. The second stage is due to the decomposition of ammonium bisulfate and sulfuric acid. In addition, the dotted lines in Figure 5a represent the mass loss rate during the TGA measurement. As

Figure 5. TG (a) and DSC (b) curves for representative samples.

the molar ratio increases for the ATFA samples ATFA5-ATFA9, it is clear that the peak of the mass loss rate gradually moves to a higher temperature, which may mean better thermal stability. As shown in Table 2, all the samples shown have a Tm below 100 °C or no Tm at all, so they can be classified as protic ionic liquids. Furthermore, the melting point of acetamide is above 80 °C, whereas the melting points of acetamide/acid systems are obviously lower than 80 °C, such as 15 °C for ATFA7. This result indicates that there is a strong interaction between the acid and acetamide. It is worth noting that all the samples exhibit a glass transition and most of them even have a very low glass transition temperature (Tg) (such as -69 °C for ATFA5), which is a good indication that they may have desirable physicochemical properties, such as high conductivity and low viscosity.14 The Tg values of the acetamide-trifluoroacetic acid complex system approach each other as the acetamide concentration increases (see Table 2), and the Tg value of ATFA9 is -73 °C. ATFA5 and ATFA6 only show a Tg thermal transition, and an exothermic Tc peak is only observed for ATFA7. By comparison with ATFA 7 and ATFA9, the two endothermic peaks related to the melting phase transitions for ATFA8 are

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H0 ) 3.3 + log([MY]/[MYH+])

Figure 6. UV-vis absorbances of ATFA at various molar ratios.

probably due to the similar ATFA 7 phase (Phase I) and excess acetamide (Phase II), indicating the existence of a liquid-solid coexistence regime between 15 and 60 °C. The phase transition, designated as Tp in Table 2, likely results from the different hydrogen bonding configurations within the sample.14 3.4. Acid Scale of the PILs. PILs usually exhibit superstrong acidity at room temperature, but this property is seldom reported because a conventional pH measurement is unable to detect the acid scale of the PILs quantitatively. Some researchers have tried to obtain a relative acidity for PILs by the use of a Hammett function and a UV-vis absorption method or by 1H NMR characterization. The Hammett function (H0) was measured on an Agilent UV-1201 spectrophotometer with a basic indicator used with methyl yellow (pKa ) 3.3).17 Methyl yellow (MY, 0.03 mmol L-1) and acetamide-based PILs (80 mmol L-1) were dissolved in N-methyl pyrrolidone, and H0 was calculated according to eq 1

(1)

where [MY] is the absorbance of the unprotonated form of the methyl yellow and [MYH+] is the absorbance of the protonated form of the methyl yellow. As shown in Table 1, the order of acidity with H0 values for the two PILs that have the same molar ratio (5:5) is ATFA5 (4.33) > ATAA5 (5.19), suggesting that the Brønsted acidity of the acetamide-based PILs is apparently dependent on the variety of anions, which agrees well with the acidity characterization of lactam-based PILs, as observed by Du.17 Figure 6 shows the typical absorption spectra of pure methyl yellow and ATFA at various molar ratios. The spectra of ATFA1 and ATFA2 were not obtained because white smoke was observed when we added solvent. Because of the presence of acidic PILs in methyl yellow, the absorbance of the unprotonated form of the methyl yellow decreases. In contrast, the protonated form of the indicator could not be observed because of its small molar absorptivity, which results in the decrease of absorbance of methyl yellow. It is worth mentioning that ATFA6 shows a decrease of absorbance to the greatest degree, suggesting that the strongest acidity of ATFA around the molar ratio of 1:1 is probably related to the structure of one acetamide associating one CF3COOH, which has been discussed above. 3.5. Viscosity of the PILs. Figure 7a,b shows the viscosity behavior of ATAA and ATFA with different molar ratios and at various temperatures. The viscosity decreases as the temperature increases over the entire range of molar ratios studied. Among all the acetamide-based complex samples, the ATAA complexes have lower viscosity (i.e., 7.7 cP for ATAA5 at 25 °C) than the ATFA complexes. Moreover, the ATFA (acetamide-CF3COOH) complexes also exhibit comparatively

Figure 7. Relationship between viscosity and concentration of acetamide in ATAA (a) and ATFA (b) at various temperatures and the Arrhenius plots of viscosity for some representative samples (c).

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Figure 8. Arrhenius plot of viscosity for some representative samples, scaled by Tg, consists of Lg(η) against Tg/T. The solid line represents “strong ” behavior. The shadow region represents the location of PILs that were reported by Greaves,14 whereas the PILs that were reported by Angell27 are located above the dotted line.

lower viscosities, and 10 cP is determined for ATFA5 at 25 °C, which is lower than that for the lactam-CF3COOH ionic liquids at the same molar ratio (11 cP).17 In fact, the CH3CH2CH2NH3+ cation has also been reported to combine with the CF3COO- anion, and it was found to be a solid at room temperature.15 There is no doubt that both the cations and the anions significantly affect their physicochemical properties. For the PILs with the same CF3COO- anionic environment, the lactam cation has a ring structure and is bigger in size, whereas the CH3CH2CH2NH3+ cation has no acyl structure comparing with the acetamide cation. This means that the size reduction of the cation and the addition of the acyl group may lead to a modification of the interaction between the cation and the anion. The similarity in viscosity for the PILs with the same acetamide cation is due to the low viscosity of acetic acid and trifluoroacetic acid. Figure 7a,b also shows that the viscosity of ATFA and ATAA increases as the molar ratio of acetamide increases, and this implies that their viscosity is mainly controlled by acetamide. Arrhenius plots of the viscosity for some PILs from 25 to 60 °C are shown in Figure 7c. It is quite obvious from this figure that the temperature dependency of viscosity fits the Arrhenius law well. The activation energy for their viscosity is as follows: 29.10 kJ/mol for ATFA5, 31.40 kJ/mol for ATFA6, 35.42 kJ/ mol for ATFA7, 36.57 kJ/mol for ATFA8, and 24.70 kJ/mol for ATAA5. Their calculated activation energies are directly related to their viscosities; in other words, a high activation energy results in high viscosity. Angell and Greaves believed that the fragility of glass-forming ILs can be described by the Arrhenius relationship of viscosity in the low-temperature region where Tg/T is between 0 and 1.26 Strong liquids show a linear increase in Lg (viscosity) with Tg/T, whereas the viscosity of fragile liquids should decrease with increasing temperature at a much faster rate than that predicted by Arrhenius behavior. From the Arrhenius plot that we scaled by Tg, as shown in Figure 8, all the representative samples have good fragility and most of them are even more fragile than the ILs reported by Angell27 and Greaves,14 indicating their great decrease in viscosity with increasing temperature. ATFA and ATAA exhibit surprisingly low viscosity, which may be the lowest viscosity recorded, under the same conditions, among the PILs, and this is comparable to the most commonly used organic solvents. 3.6. Ionic Conductivity of the PILs. Ionic conductivity was measured for the two series of PILs between 20 and 80 °C. The data in Table 1 show that ATFA has the highest ionic

Figure 9. Arrhenius plots of conductivity for ATAA (a) and ATFA (b) at various molar ratios and some representative samples (c).

conductivity at a molar ratio of 5:5 at 25 °C (0.25 vs 0.144 S/m for the lactam-CF3COOH17 and 0.1 S/m for BMIm [CF3COO]28) because of its lower viscosity and smaller cation size, which may enhance the rate of ion mobility. It is also apparent that the viscosity of ATFA and ATAA has no significant influence on their ionic conductivity. ATFA has a higher viscosity than ATAA, but ATAA gives a lower ionic conductivity than ATFA. We attribute this to the stronger proton-donating ability of trifluoroacetic acid comparing with that of acetic acid. Furthermore, it is worth noting that ATFA has better conductivity properties at a molar ratio of 5:5 at higher temperatures (1.07 S/m at 80 °C) than [CH3CH2CH2NH3+][CF3COO-] (less than 0.5 S/m at 80 °C).15 This further proves that the acyl group has a special effect on the interaction between the cation and the anion. Theoretically, ATFA5 should have better ionic conductivity than ATFA6 at 80 °C, but the results did not reflect this, which may be attributed to the easier decomposition of ATFA5 at high temperature. As expected, we found that ATFA performs excellent ionic conductivity as the temperature increases for the compounds with molar ratios of [acetamide]/[CF3COOH]

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TABLE 5: Activation Energy (Ea) of Ionic Conductivity for Some Representative Samples

a

PILs

ATFA5

ATFA6

ATFA7

Ea (kJ/mol)

24.2

26.4

28.7

ATFA8 44.4a

ATFA9 26.4b

52.4c

ATAA5 28.6d

26.2

Between 20 and 55 °C. Between 60 and 80 °C. Between 20 and 50 °C. Between 55 and 80 °C. b

c

d

from 6:4 to 9:1 because of their low Tg values. For instance, the conductivity of ATFA 9, which also possesses relatively good thermal stability, can reach to 0.87 S/m at 80 °C. Arrhenius plots of ionic conductivity from 20 to 60 °C are shown in Figure 9a,b. First, ATAA is more conductive at the molar ratio of 3:7 at room temperature, which is consistent with the theoretical prediction that one acetamide is more likely to coordinate with two CH3COOH molecules. However, the conductivity increases when the concentration of acetamide decreases in ATFA because of the great decrease in viscosity. Second, with increasing temperature, the conductivity of most PILs increases according to the Arrhenius law. However, the Arrhenius plots of samples, such as ATAA1, ATAA2, ATFA1, and ATFA9, are clearly not linear, which possibly results from the temperature dependency of the proton-donating ability of acetic acid in ATAA1 and ATAA2 and the low thermal stability of ATFA1 as well as the phase transition of ATFA9. For a hopping-like conduction mechanism, the conductivity should follow the Arrhenius law according to eq 2

( )

σ ) σ0 exp

-Ea RT

Figure 10. Walden plots of ATFA (F) and ATAA (A) at various ratios at low temperatures, where Λ is the conductivity per mole of charge and η is the dynamic viscosity. The solid line represents “good ILs” behavior. The shadow region represents the location of PILs that were reported by Greaves,14 whereas the PILs that were reported by Angell27 are located above the dotted line.

(2)

where σ0 (S/cm) is the initial conductivity, R is a constant of the equation, and Ea (kJ/mol) is the activation energy. The conductivities of some representative samples from 20 to 80 °C are shown in Figure 9c. When the acetamide/CF3COOH molar ratio is 8:2 or 9:1 the Arrhenius curves show two slopes, and when the acetamide/CH3COOH molar ratio is 5:5, the Arrhenius curves show an inflection point at 80 °C. The appearance of two slopes is consistent with a liquid-solid phase transition, as shown by the DSC results, and we believe that the inflection point occurs because of the initiation of ATAA5 decomposition. The activation energy calculated from the log(σ) ) f(1/T) straight line for those samples is listed in Table 5. Most notably, a low activation energy and high conductivity are obtained for the low-viscosity samples in the liquid-phase region and in the liquid-solid coexistence region. Greaves14 and Angell26,27 used Walden plots to classify ionic liquids as nonionic, poor, good, or superionic. Walden plots of these two series of PILs at various ratios and at low temperature are shown in Figure 10, and the solid line represents “good” ionic behavior. Good ionic liquids are expected to have negligible vapor pressure and to have a conductivity increase that is directly related to the increase in fluidity. Plots below the solid line represent weak transfer from acid to the base or transfer with associated ions. The PILs in our work are mostly poor ILs, according to these Walden plots. However, in comparison to the ILs investigated by Greaves14 and Angell,27 the samples of ATFA locate in a similar region and locate very close toward the dotted line. These two PILs show different trends close to the solid line at various ratios. With an increase in the molar ratio, ATAA moves closer to the solid line, whereas ATFA moves further away initially until a molar ratio of 5:5 and then moves closer to the solid line for reasons that are not completely clear. Nevertheless, these differences demonstrate

that the factors that influence the ionic conductivity in these two novel PILs are different. 4. Conclusion Three series of novel protic ionic liquids based on acetamide and Brønsted acids with a low cost and toxicity were prepared and characterized. Their water ratio, acidic scale, viscosity, ionic conductivity, thermal properties, and infrared spectra were investigated in detail. Carboxylic acid formation was only observed in an acetamide sulfate complex system, and this process was proved through FTIR. In the acetamide/CF3COOH and acetamide/CH3COOH systems, however, the acylamino groups in acetamide work as complexing agents for the cations and the anions because of their polarity (CdO and NH2). They are thus capable of coordinating with cations and anions, resulting in the special structure of acetamide-based PILs, which is quiet different from that of amine-based PILs. Because of the effect of acylamino groups, many samples possess a wide liquid range, high conductivity, and low viscosity together with relatively good thermal stability. The acetamide-CF3COOH composite with a molar ratio of 9:1 has favorable physicochemical properties and is a potential electrolyte for intermediate temperature PEMFCs. Its decomposition temperature and ionic conductivity can reach 106 °C and 0.87 S/m (80 °C), respectively. However, its thermal stability may not be good enough for application in intermediate temperature fuel cells, and the improvement of this property is under investigation in our group. Furthermore, we can obtain an acidity scale, viscosity, or ionic conductivity by controlling the concentration of acetamide in these complex systems, and they are promising media or catalysts for organic synthesis. Acknowledgment. This work was supported by the National Key Program for Basic Research of China (No. 2009CB220100), the National 863 Program (No. 2007AA03Z226), the National

Acetamide-Based Brønsted Acid Ionic Liquids Science Foundation of China (NSFC, 20803003), and the New Century Educational Talents Plan of Chinese Education Ministry (NCET-10-0038). References and Notes (1) Reiter, J.; Velicka, J.; Mika, M. Electrochim. Acta 2008, 53, 7769– 7774. (2) Sugimotoa, T.; Atsumi, Y.; Kikuta, M.; Ishiko, E.; Kono, M.; Ishikawa, M. J. Power Sources 2009, 189, 802–805. (3) Kim, J. D.; Hayashi, S.; Mori, T.; Honma, I. Electrochim. Acta 2007, 53, 963–967. (4) Nakamoto, H.; Watanabe, M. Chem. Commun. 2007, 2539–2541. (5) Poole, C. F.; Poole, S. K. J. Chromatogr., A 2010, 1217, 2268– 2286. (6) Egashira, M.; Todo, H.; Yoshimoto, N.; Morita, M. J. Power Sources 2008, 178, 729–735. (7) Iojoiu, C.; Martinez, M.; Hanna, M.; Molmeret, Y.; Cointeaux, L.; Lepretre, J. C.; El Kissi, N.; Guindet, J.; Judeinstein, P.; Sanchez, J. Y. Polym. AdV. Technol. 2008, 19, 1406–1414. (8) Liang, C. D.; Yuan, C. Y.; Warmack, R. J.; Barnes, C. E.; Dai, S. Anal. Chem. 2002, 74, 2172–2176. (9) Zhu, H. P.; Yang, F.; Tang, J.; He, M. Y. Green Chem. 2003, 5, 38–39. (10) Gui, J. Z.; Cong, X. H.; Liu, D.; Zhang, X. T.; Hu, Z. D.; Sun, Z. L. Catal. Commun. 2004, 5, 473–477. (11) Zhang, L.; Xian, M.; He, Y. C.; Li, L. Z.; Yang, J. M.; Yu, S. T.; Xu, X. Bioresour. Technol. 2009, 100, 4368–4373. (12) Xu, W.; Angell, C. A. Science 2003, 302, 422–425.

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