Synthesis, magnetism, aqueous-two phase formation and physical properties of novel guanidinium-based magnetic ionic liquids

Abstract

Magnetic ionic liquids (MILs) are a new class of ionic liquids that reveal paramagnetic properties. In this study, a series of novel guanidinium-based organic magnetic ionic liquids were synthesized and characterized by FT-IR and ESI-MS. The magnetic properties of these MILs were investigated using a vibrating sample magnetometer. With decreasing length of the carbon chain of the MILs, magnetic susceptibility was observed to strengthen. MILs also showed extremely strong paramagnetism when the temperature dropped below 25 K. A new concept of a magnetic ionic liquid aqueous-two phase system (MILATPs) was proposed for the first time. Compared to the latest developments in separation systems, the advantages of magnetic separation, organic solvent free and rapid extraction were successfully combined together in the novel proposed MILATPs. Interestingly, all these novel compounds are capable of forming MILATPs and can respond to an external magnetic field to accelerate isolation of two phases. In addition, some physical properties such as density, electrical conductivity and acidity were further studied. These MILs are expected to show great potential for further application in the fields of catalysis, desulfurization, biotechnology, electrochemistry, and especially separation and pretreatment technology.

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Introduction

Room temperature ionic liquids (RTILs) are a class of non-molecular solvents with low melting points (<100 °C), negligible volatility, thermal and chemical stability, wide liquid range, nonflammability and good solubility.^1,2 They are typically comprised of unsymmetrical organic cations and symmetrical/unsymmetrical inorganic/organic anions. The properties of ILs can be modified with different combinations of cations and anions.³ Due to their special properties, ILs have been widely applied in the fields of separation,^4–6 catalysis,^3,7 and electrochemistry.⁸

Magnetic room temperature ionic liquids (MRTILs) are functionalized ILs and exhibit susceptibility to an external magnetic field.^9,10 They are the latest development of RTILs, promising materials with additional advantageous properties than those of typical ILs, such as magnetic, optical¹¹ or electrochromic behavior.¹² Nowadays, MRTILs can be divided into two categories: metal-containing magnetic ionic liquids and pure organic magnetic ionic liquids. Generally, metal magnetic ionic liquids are composed of metal-containing anions, such as iron,⁹ cobalt,¹³ manganese,¹⁴ copper,¹⁵ and so on. Due to their high magnetic response to external magnetic fields and special properties, these MILs have been widely used for the transport, separation and concentration of materials,^16,17 catalysis,¹⁸ sulphide (SO_x) extraction in gasoline¹⁹ or biotechnology.²⁰ Another kind of MIL is non-metal organic MIL based on a radical ion, the first one was discovered and proposed by Yoshida in 2007.²¹ Afterwards, bimagnetic ionic liquid which includes an organic magnetic center was synthesized and efficiently applied in catalysis.²² Up to now there exists so few organic magnetic ionic liquids. More applications of them were never found in the literature.

Aqueous two-phase system (ATPs) as a useful extraction technique was first proposed in the 20th century²³ and has been recognized as an economical and efficient downstream processing method.²⁴ Compared with conventional extraction methods, ATPs is more environmentally benign without any use of harmful volatile organic solvent in the whole process. Therefore, it has attracted much attention in many fields.^25–27 In recent years, ATPs has been successfully applied in the separation and purification of proteins,²⁶ antibiotics,^27,28 metal ions²⁸ and dyes,²⁹ Over the past few years, ATPs consisting of ionic liquids (ILATPs) have drawn increasing attention of researchers. Rogers and his co-workers reported ILATPs with water-structuring salts for the first time in 2003.³⁰ ILATPs have shown many advantages, such as low viscosity, little emulsion, rapid phase separation and environmentally compatible.³¹ It has been successfully applied as a green, non-toxic and sensitive samples pretreatment technique.³²

On the other hand, ILATPs has some disadvantages, for example two aqueous phases hardly efficiently isolate due to small density difference. Some efforts were made such as the change of operating temperatures,³³ the design of functional ionic liquids,³⁴ and the selection of salting-out reagents.³⁵ If one phase has paramagnetic property, two phases should be easily separated under an external magnetic field. In this paper, a series of novel organic magnetic ionic liquids based on guanidinium cation were synthesized. All these novel compounds are able to form magnetic ionic liquid aqueous-two phase system (MILATPs). To the best of our knowledge, there has been no reported literature referring to MILATPs so far. MILATPs is showing great potential in pretreatment and separation field. Compared to the latest development of separation systems, the advantages of magnetic separation, organic solvent free and rapid extraction were successfully combined together in the proposed MILATPs for the first time. This technique is hopefully applied in the microextraction analysis of some environmental pollutant in our further study.

Additionally, acidity of ILs is an important property in widespread applications especially in the field of catalysis.³⁶ Electrical conductivity of ILs is essential in high-temperature fuel cells or other electrochemical devices as well as many other areas.³⁷ Density is also a frequently needed fundamental property.³⁸

In this work, a series of novel guanidinium-based organic magnetic ionic liquids were synthesized and characterized by FT-IR and ESI-MS spectra. The magnetic susceptibilities of MILs are important for further research and have been investigated. Some physical properties such as density, electrical conductivity and acidity were studied. All these novel MILs with uniquely excellent property were expected to show great potential in fields of catalysis, desulfurization, biotechnology, electrochemistry, especially separation and pretreatment technology.

Materials and measurements

All reagents used were of analytical grade or above. 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl free radical (4-OH-tempo) and 1,1,3,3-tetramethylguanidine were purchased from Best Reagent Co., Ltd. (Chengdu, China). Chlorosulfonic acid was obtained from Aike Chemical Reagent Co., Ltd. (Chengdu, China). All the other used chemicals and reagents were supplied by Kelong Chemical Co., Ltd. Ultrapure water was obtained by the UPR-1-5T water purification system from Ulupure Technologh Co. Ltd. (Chengdu, China).

FT-IR spectra of MILs were recorded on a Spectrum Two, L1600300 FT-IR instrument purchased from Perkin Elmer, America. Electrospray ionization mass spectra (ESI-MS) were collected on a amaZon SL mass spectrometer (Germany) and data were processed by Bruker Compass Data Analysis 4.0. Mass value was determined by a FA/JA type electronic analytical balance (uncertainty of ±0.0001 g). Electrical conductivity of MILs was carried out on a Hongyi DDS-12A digital conductivity meter (Shanghai, China) with platinized electrodes whose cell constant was 0.973 cm⁻¹. A CY20A water thermostat (Boxun Industry & Commerce Co., Ltd., Shanghai, China) with an uncertainty of ±0.05 K was applied for keeping constant temperature of this system. Density measurement was conducted by using pycnometers and their temperature control was accomplished by a thermostatic refrigeration recycler (Yuhua Instrument Co.,Ltd) with an uncertainty of ±0.1 K. A Soptop UV-Vis 2800 spectrometer (Beijing, China) was also applied for the acidity measurement. The magnetic properties of MILs 1–4 were investigated by a vibration sample magnetometer (VSM) using MPMS SQUID system (Quantum Design, America).

Synthesis and characterization of organic magnetic ionic liquids

Synthesis of TEMPO-OSO₃⁻ salts. Na[TEMPO-OSO₃] was synthesized according to the literature procedure.³⁹ Certain amount of 4-hydroxyl-tempo was dissolved in dichloromethane at 0–5 °C, equal molar of chlorosulfonic acid was added dropwise. The mixture was continuously stirred magnetically for 12 h at room temperature, washed with dichloromethane 3 times, dried under vacuum (45 °C) and then dissolved in ethanol. Afterwards, equimolar of NaOH dissolved in ethanol was directly neutralized with H[TEMPO-OSO₃]. The product Na[TEMPO-OSO₃] was recrystallized from acetone.

Synthesis of [C_nTMG]Br. Ionic liquids of [C_nTMG]Br (n = 2, 3, 4, 5) were prepared and purified according to the synthesis of imidazolium ionic liquids.⁴⁰ To briefly summarize, the reactions of 1,1,3,3-tetramethylguanidine and alkyl bromide were refluxed in ethanol under 78 °C for 24 h with continuous magnetic stirring. Then residual solvents were removed under vacuum, the obtained products were washed with chloroform.

Synthesis and characterization of organic magnetic ILs. MIL 1–4 were prepared by the metathesis of equal molar of Na[TEMPO-OSO₃] and [C_nTMG]Br in dry acetone, filtered to obtain the solution, then dried under vacuum at 40 °C and repeated 3 times. Afterwards it was washed with chloroform and dried. Whereas MIL 1–4 were reddish or dark-reddish viscous liquids with yield of 79.8% for IL1, 87.5%for IL2, 84.9% for IL3 and 85.1% for IL4, respectively. An aqueous solution of AgNO₃ was used to determine whether residual bromide anion existed and none precipitate was observed. The structures of MIL 1–4 were shown in Fig. 1. All of the MILs have been identified via FT-IR and ESI-MS spectra.

Fig. 1 The structures of magnetic ionic liquids.

Preparation and measurement of ATPs

The binodal curves were determined through the cloud point method in a jacketed glass vessel containing a magnetic stirrer at constant temperature and atmospheric pressure. A MIL solution of known mass fraction was taken into the vessel, then a salt solution of known mass fraction was added dropwise to the vessel until the mixture turned cloudy. The composition of the mixture for each point on the binodal curve was noted and calculated by mass. Afterwards, water was added dropwise to get a clear one-phase system. Repetitive operation was kept on to form the phase equilibrium line. The equilibrium curves data were correlated by the Merchuk equation as eqn (1)

w₁ = a exp(bw₂^0.5 − cw₂³) (1)

where w₁ and w₂ are the IL and salt weight percentages, respectively. a, b and c are constants, obtained by regression of the experimental data.

Density measurement

Densities of four MILs were measured by pycnometer method.⁴¹ Two 10 ml empty pycnometers were marked as 1, 2 and weighed as M_a and M_b, respectively. The effective capacities of the pycnometers at different temperatures were calibrated by weighing the mass of distilled water. Then a certain amount of MIL was added into pycnometer 1 and the total mass was accurately weighed as M′_a. The residual volume of pycnometer 1 was filled with n-hexane. Pycnometer 2 is directly filled with n-hexane. Both pycnometers were placed in the constant temperature water bath at different temperature for 30 min and then taken out, wiped carefully to remove the residual water outside. The total weights of pycnometer 1 and pycnometer 2 were recorded as M_c and M_d. The density of each MIL was calculated as eqn (2)–(5):

ρ_MIL × V_MIL + ρ_hex × V_hex = M_c − M_a (2)

ρ_hex × V₂ = M_d − M_b (3)

ρ_MIL × V_MIL = M′_a − M_a (4)

V_MIL + V_hex = V₁ (5)

where V₁ and V₂ are the calibrated capacities of empty pycnometer 1 and pycnometer 2 at certain temperature, respectively. V_hex and V_MIL are the volume of n-hexane and magnetic ionic liquid in pycnometer 1. ρ_MIL and ρ_hex are densities of magnetic ionic liquid and n-hexane at certain temperature.

Acidity measurement

The Brønsted acid strengths of ILs were determined with UV-Vis spectrum by Hammett method.⁴² The indicator of p-nitroaniline was dissolved in methanol to prepare the indicator solution with 0.1 mmol L⁻¹, followed by the addition of ILs at the concentration of 20 mmol L⁻¹ and then the Hammett function (H₀) was calculated as eqn (6):

H₀ = pK_a + log([I]/[HI]) (6)

where pK_a is the electrolytic constant of p-nitroaniline (pK_a = 0.99), [I] and [HI] are the percentages (%) of unprotonated and protonated p-nitroaniline, respectively.

Conductivity measurement

The conductivity meter was calibrated using a standard KCl solution (0.01 mol L⁻¹). The temperature was maintained constantly at 298 ± 0.05 K by using the thermostatic bath. Conductivities of various concentrations of MILs aqueous solution were measured by adding certain extra amount of ultrapure water into the previous solution.

Results and discussion

Characterization of MILs

FT-IR and ESI-MS spectra of MIL 1. FT-IR (spectrum is shown in Fig. S1†): 3365 cm⁻¹ is the N–H stretching vibration; 2933 cm⁻¹ and 2862 cm⁻¹ are the methyl and methylene saturated C–H stretching vibration in the anion; 1610 cm⁻¹ is the C=N stretching vibration; 1567 cm⁻¹ is the N–H in-plane bending vibration; 1463 cm⁻¹ and 1408 cm⁻¹ are the methyl and methylene C–H in-plane bending vibration; 1216 cm⁻¹ is the C–N stretching vibration; 1060 cm⁻¹ is the N–O stretching vibration; 793 cm⁻¹ is the N–H out-of-plane bending vibration. ESI-MS (spectra is shown in Fig. S2†): 144 for positive ion mode and 251 for negative ion mode.

FT-IR and ESI-MS spectra of MIL 2. FT-IR (spectrum is shown in Fig. S3†): 3354 cm⁻¹ is the N–H stretching vibration; 2973 cm⁻¹ and 2942 cm⁻¹ are the methyl and methylene saturated C–H stretching vibration in the anion; 1609 cm⁻¹ is the C=N stretching vibration; 1568 cm⁻¹ is the N–H in-plane bending vibration; 1463 cm⁻¹ and 1406 cm⁻¹ are the methyl and methylene C–H in-plane bending vibration; 1215 cm⁻¹ is the C–N stretching vibration; 1061 cm⁻¹ is the N–O stretching vibration; 791 cm⁻¹ is the N–H out-of-plane bending vibration. ESI-MS (spectra is shown in Fig. S4†): 158 for positive ion mode and 251 for negative ion mode.

FT-IR and ESI-MS spectra of MIL 3. FT-IR (spectrum is shown in Fig. S5†): 3356 cm⁻¹ is the N–H stretching vibration; 2972 cm⁻¹ and 2900 cm⁻¹ are the methyl and methylene saturated C–H stretching vibration in the anion; 1610 cm⁻¹ is the C=N stretching vibration; 1560 cm⁻¹ is the N–H in-plane bending vibration; 1464 cm⁻¹ and 1408 cm⁻¹ are the methyl and methylene C–H in-plane bending vibration; 1217 cm⁻¹ is the C–N stretching vibration; 1062 cm⁻¹ is the N–O stretching vibration; 791 cm⁻¹ is the N–H out-of-plane bending vibration. ESI-MS (spectra is shown in Fig. S6†): 172 for positive ion mode and 251 for negative ion mode.

FT-IR and ESI-MS spectra of MIL 4. FT-IR (spectrum is shown in Fig. S7†): 3436 cm⁻¹ is the N–H stretching vibration; 2939 cm⁻¹ and 2844 cm⁻¹ are the methyl and methylene saturated C–H stretching vibration in the anion; 1610 cm⁻¹ is the C=N stretching vibration; 1464 cm⁻¹ and 1411 cm⁻¹ are the methyl and methylene C–H in-plane bending vibration; 1216 cm⁻¹ is the C–N stretching vibration; 1062 cm⁻¹ is the N–O stretching vibration; 794 cm⁻¹ is the N–H out-of-plane bending vibration. ESI-MS (spectra is shown in Fig. S8†): 186 for positive ion mode and 251 for negative ion mode.

Study of the magnetization and susceptibility of MILs

The magnetization vs. magnetic field at 5 K and magnetization vs. temperature at applied field of 20 kOe were investigated and shown in Fig. 2. The relation between the magnetization and the applied magnetic field of MILs 1–4 at 298 K was shown in Fig. 3. Magnetic susceptibility rises with the increasing of external magnetic field. All the four MILs were compared and MIL 1 exhibited the largest magnetization value at 298 K. They follow the order: MIL1 > MIL2 > MIL3 > MIL4. Theoretically, radical species on tempo moiety is the magnetic center. Intensity of radical species in the molecule increases with shorter alkyl chain, thus resulting in a stronger magnetic susceptibility. From the relationship of static susceptibility value and the temperature under an applied field of 2 T, [C₂TMG][TEMPO-OSO₃] exhibit a weak paramagnetic property when temperature is above 150 K. When temperature decreases, its magnetization rises sharply to a very strong magnetic susceptibility value as high as 828 emu mol⁻¹ which is about 80 times of the value at room temperature. This phenomenon reveals great potential for its magnetic application under low temperature. In addition, the magnetic behavior of magnetic ionic liquid aqueous two-phase system was demonstrated in Fig. 4. When aqueous two-phase occurred, the magnetic droplet could be attracted by a neodymium magnet and move towards it. Magnetic ionic liquid droplet quickly gathered at the top close to the magnet, as a result, enhanced phase separation efficiency apparently.

Fig. 2 Magnetization vs. magnetic field at 5 K and magnetization vs. temperature at applied field of 20 kOe.

Fig. 3 Magnetic properties of MIL 1–4 at 298 K.

Fig. 4 Magnetic response of MIL droplet to a neodymium magnet.

Effect of inorganic salts on phase separation

A series of inorganic salts were tested for the formation of MILATPs with MILs 1–4. Results demonstrated that MILATPs can be formed by adding appropriate amount of particular salts, such as K₃PO₄, K₂HPO₄, K₂CO₃ or Na₂CO₃, while adding other salts, such as Na₂SO₄, (NH₄)₂SO₄, NaCl, KCl, KOH or NaOH, cannot drive MIL aqueous solution to separate into two phases. Table 1 shows the phase-forming ability of these MILs with different inorganic salts at 298 K.

Table 1 Aqueous two-phase forming abilities of 4 MILs with different salts^a

	K3PO4	K2HPO4	K2CO3	Na2CO3	(NH4)2SO4	Na2SO4	NaCl	KCl	KOH	NaOH
a “Yes” means capable of forming aqueous two-phase; “No” means incapable of forming aqueous two-phase.
MIL 1	Yes	Yes	Yes	Yes	No	No	No	No	No	No
MIL 2	Yes	Yes	Yes	Yes	No	No	No	No	No	No
MIL 3	Yes	Yes	Yes	Yes	No	No	No	No	No	No
MIL 4	Yes	Yes	Yes	Yes	No	No	No	No	No	No

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The binodal curve of MIL [C₅TMG][TEMPO-OSO₃] with inorganic salt K₂HPO₄ is representatively presented in Fig. 5. The points represent experimental data and the line is the calculated value by the Merchuk equation (eqn (1)). The experimental data can fit very well with the calculated data with R² = 0.9992, which indicates that this method can be used to satisfactorily describe the binodal curve of the system presented.

Fig. 5 Binodal curve of MIL [C₅TMG][TEMPO-OSO₃] with K₂HPO₄.

Conductivity

The data of the measured molar conductivity of MIL aqueous solutions are presented in Table 2. The molar conductivity of MIL solution are analyzed by Kohlrausch model as described by eqn (7):

Λ_m = Λ^∞_m − Ac (7)

where Λ_m is the molar conductivity (S cm² mol⁻¹) at the concentration c (mol L⁻¹), Λ^∞_m is the limiting molar conductivity (S cm² mol⁻¹) at infinite dilution, A is a constant connected to the property of electrolyte.

Table 2 Molar conductivities (Λ_m) of binary mixture of magnetic ionic liquid + water at 298 K

10^4c (mol L^−1)	Λm (S cm^2 mol^−1)
	MIL 1	MIL 2	MIL 3	MIL 4
10.0000	97.0	88.8	79.8	72.3
9.3750	98.2	89.8	80.6	72.8
8.8235	99.4	90.6	81.6	73.2
7.8947	100.5	92.0	82.5	73.9
7.1429	102.1	93.0	84.1	74.6
6.2500	104.5	94.4	85.3	75.4
5.5556	105.6	95.2	86.8	76.1
5.0000	107.4	96.2	87.0	76.8
4.4118	108.3	97.2	89.1	77.5
3.7500	110.1	98.7	90.1	78.4
3.2609	111.3	99.7	90.5	78.8
2.8846	112.8	100.9	92.2	79.4
2.5000	114.3	102.0	92.4	80.0
1.9737	116.4	103.4	94.8	81.1
1.6304	117.2	104.3	95.7	81.6
1.2500	118.9	105.6	96.8	82.4
0.9375	120.1	106.7	98.1	83.2
0.7500	121.2	108.0	98.7	84.0
0.6250	122.0	108.9	99.2	84.2

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As demonstrated in Fig. 6, excellent linear fitting was achieved. The conductivity gradually decreases with dilution of the MIL aqueous solution. In addition, the molar conductivity is slightly increased with the decreasing concentration of IL solutions. The limiting molar conductivity (Λ^∞_m) can be used to reflect the conductive capability of solutions. Λ^∞_m of MILs 1–4 were 130.6 S cm² mol⁻¹, 114.9 S cm² mol⁻¹, 106.0 S cm² mol⁻¹ and 88.1 S cm² mol⁻¹, respectively. Obviously, the difference of in water at 298 K is only influenced by the structure of cation. It can be seen that the value of Λ^∞_m decreases with the elongation of carbon chain. According to literature,⁴³ the size of IL is the main factor that influence its conductive ability. With the increase of carbon chain length, the MIL cation size gradually rises thus resulting in a decrease of conductive ability.

Fig. 6 Molar conductivity fitting of different magnetic ionic liquids + water at 298 K. ( [C₂TMG][TEMPO-OSO₃]; [C₃TMG][TEMPO-OSO₃]; [C₄TMG][TEMPO-OSO₃]; [C₅TMG][TEMPO-OSO₃].)

Density of MILs

Density values are listed in Table 3 and shown in Fig. 7. It reveals that densities have linear relationship with temperature. With temperature rising, MIL was expanded and resulted in a density decrease. The densities of MILs follow the order of [C₂TMG][TEMPO-OSO₃] > [C₃TMG][TEMPO-OSO₃] > [C₄TMG][TEMPO-OSO₃] > [C₅TMG][TEMPO-OSO₃]. Obviously, MIL with shorter carbon chain has a relatively higher density.

Table 3 Density values of 4 MILs at different temperature

Temperature (°C)	MIL 1	MIL 2	MIL 3	MIL 4
5	1.1732	1.1664	1.1466	1.1046
15	1.1680	1.1602	1.1376	1.0984
25	1.1629	1.1542	1.1284	1.0921
35	1.1575	1.1470	1.1204	1.0853
45	1.1520	1.1406	1.1120	1.0787

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Fig. 7 Densities of 4 MILs at different temperature [C₂TMG][TEMPO-OSO₃]; [C₃TMG][TEMPO-OSO₃]; [C₄TMG][TEMPO-OSO₃]; [C₅TMG][TEMPO-OSO₃].

Brønsted acidity of MILs

It can be found in Table 4 that the Brønsted acid strengths of the MILs is weak and in the order of [C₅TMG][TEMPO-OSO₃] > [C₄TMG][TEMPO-OSO₃] > [C₃TMG][TEMPO-OSO₃] > [C₂TMG][TEMPO-OSO₃]. For all the four MILs, the anion is the same and their acidities are mainly dependent on the cation. The Brønsted acidity slightly increased with the elongation of carbon chain of the cation. Obviously, MIL with longer carbon chain is easier to dissociate proton and has relatively stronger Brønsted acidity.

Table 4 Brønsted acid strength of gunidinium-based magnetic ionic liquids

Object	Amax (AU)	[I] (%)	[HI] (%)	H0
p-Nitroaniline	1.63	100	0	—
MIL 1	1.36	83.44	16.56	1.69
MIL 2	1.34	82.21	17.79	1.65
MIL 3	1.32	80.98	19.02	1.61
MIL 4	1.30	79.75	20.25	1.59

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Conclusion

A series of novel guanidinium-based organic magnetic ionic liquids were successfully synthesized and characterized by FT-IR and ESI-MS. A new concept of magnetic ionic liquid aqueous-two phase system (MILATPs) was proposed for the first time. Compared to the latest development of separation systems, the advantages of magnetic separation, organic solvent free and rapid extraction were successfully combined together in the novelly proposed MILATPs. Interestingly, all of these novel compounds are capable of forming MIILATPs and can respond to an external magnetic field.

The magnetic properties of these MILs have been investigated and MIL1 exhibits the largest magnetization value at 298 K. Their magnetizations follow the order: MIL1 > MIL2 > MIL3 > MIL4. Consequently, stronger magnetic susceptibility was observed with shorter length of carbon chain of MIL. MIL 1 shows a very strong paramagnetism under rather low temperature. The conductivity gradually decreases with dilution of the IL aqueous solution. Conductive ability and density of MIL slightly decreases with the increase of carbon chain length. The Brønsted acidity is weak and slightly increases with the elongation of carbon chain. These novel MILs were expected to show great potential for further application infields of catalysis, desulfurization, biotechnology, electrochemistry, especially separation and pretreatment technology.

Acknowledgements

Preparation of this paper was supported by the National Scientific Foundation of China (No. 81373284) and the Provincial Scientific Foundation of Sichuan Province, China (No. 2014JY0070).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: FT-IR spectra, ESI-MS spectra, Fig. S1–S8 are available. See DOI: 10.1039/c6ra09879g

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