Highly efficient reversible adsorption of NO₂ in imidazole sulfonate room temperature ionic liquids

Abstract

The highly efficient reversible adsorption of NO₂ in room-temperature ionic liquids is reported for the first time, making a platform for promising applications.

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Controllable dissolution of nitrogen oxides (NO_x) is of great importance for the design and application of hypergolic fuel, organic synthesis (such as oxidation, nitration and nitrosation),^1–4 and NO_x sensing.⁵ As a main source of air pollution, NO_x contains huge amount of wasted energy which is difficult to reuse.⁶ Though species as NO₂ and N₂O₄ have been theoretically proved to be ideal oxidizer,⁷ it is problematic in handling and application,^2,8 unless they are easily stored and reacted as our wishes. Here, imidazole sulfonate ionic liquids (ILs) were found to be ideal solvent for NO₂, and, what's more, the adsorption of NO₂ in such ILs was reversible upon simple operations. The study of such reversible adsorption process provides not only a highly efficient method to separate NO₂, but also a promising and fascinating way to utilize the wasted energy in NO_x.

With the advantages of ideal interfacial properties, negligible vapor pressure, thermal stability, favorable viscosity, miscibility with water and organic solvents, good abilities to extract metal ions and compounds, and environmentally friendly,^9–13 ILs have been used in fields such as separation, bioanalytical preconcentration and detection, organic synthesis, and catalysis.^14–19 Therefore, there has been an explosion of interest in the research of ionic liquids in recent years.¹³ The capture or sensing of gasses as N₂, CO₂, SO₂, and NO₂ in ILs has been studied,^5,20–24 however, to the best of our knowledge, the reversible adsorption of NO₂ and the interaction between cations and NO₂ or N₂O₄ molecule in ILs have not been reported yet, although such process is of huge application potential in the fields as synthesis and energy utilization.

In our study, the imidazole sulfonate room-temperature ILs including 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim]OTf), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([Bmim]OTf), and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate ([HMIM]O-TF) were chosen to be the solvent. The reversible adsorptions of NO₂ in these room-temperature ILs were found, respectively. In addition, N₂O₄ was produced after the adsorption in each IL.

As shown in Fig. 1, for [Hmim]OTf, the color of the solution turned cyan blue after the adsorption of NO₂ at the pressure of 40 kPa and the temperature of 35 °C (Fig. 1, insert left). Compared with the UV-vis adsorption spectrum of [Hmim]OTf (Fig. 1, blue line), six hyperfine split peaks at the wavelengths of 324.6 nm, 335.3 nm, 346.0 nm, 358.8 nm, 372.2 nm, and 387.9 nm, respectively, appeared after the adsorption (Fig. 1, red line), resulting from the vibrational electronic transition of N₂O₄.^25–28 The same color change appeared with [Emim]OTf and [Bmim]OTf after adsorbing NO₂ at the same condition.

Fig. 1 UV-vis spectra of [Hmim]OTf before (blue line) and after (red line) adsorption of NO₂. Insert: the photograph of [Hmim]OTf with NO₂ in it (left) and after release of NO₂ (right). The adsorption pressure was 40 kPa.

However, different from [Hmim]OTf and [Bmim]OTf, a new peak at 417.0 nm appeared on the spectrum of [Emim]OTf after adsorbing NO₂ as well as the six fine peaks (Fig. 1 and S1 in ESI†), which should be ascribed to the UV-vis adsorption of NO₂ monomer.^26–28 The great tendency of NO₂ monomer to become the dimer led to the formation of N₂O₄. Meanwhile, the structure of π bond of N₂O₄ is prone to form π_cation–π interaction with imidazole cation in the ILs,²⁹ which could be a main factor contributed to the adsorption state of NO₂ in these ILs. The cyan blue color was probably due to the π_cation–π interaction and the ion interaction induced charge transfer. The disintegration of N₂O₄ was easier among the molecules of [Emim]OTf than in the other two ILs, since the smaller steric hindrance of alkyl on [Emim]OTf made room for the stronger molecular motion of N₂O₄. Therefore, the ratio of NO₂ monomer was bigger in [Emim]OTf than in [Bmim]OTf and [Hmim]OTf, leading to the obvious additional adsorption peak at the wavelength of 417.0 nm (Fig. S1a in ESI†). This fact also referred that the six split peaks around 320–390 nm attributed most to N₂O₄.

The Fourier transform infrared (FTIR) spectra of [Emim]OTf, [Bmim]OTf, and [Hmim]OTf were recorded before and after the adsorption of NO₂, respectively (Fig. S2 in ESI†). After the adsorption of NO₂, a new transmission peak at 1665 cm⁻¹ was recorded with [Emim]OTf and [Bmim]OTf, respectively (Fig. S2a and S2b in ESI†). Meanwhile, for [Hmim]OTf the new peak appeared at 1662 cm⁻¹. These peaks should be ascribed to the a-stretch vibration of N₂O₄.³⁰ The slight red-shift of IR adsorption in [Hmim]OTf was probably because that the inductive effect and the steric hindrance of –hexyl constrained the a-stretch vibration, leading to the vibration strength. While the same effect of –ethyl and –butyl were not as strong as –hexyl. Accompany with the UV-vis spectra, the FTIR spectra (Fig. S2 in ESI†) confirmed the existence of N₂O₄, indicating the successful adsorption and stabilization of NO₂ by the imidazole sulfonate ILs.

With the help of π_cation–π interaction between N₂O₄ and the imidazole sulfonate IL molecules, the adsorbed N₂O₄ could remain stable in these ILs in the closed container. After 2 months, the colors of the solutions still remained unchanged. Once open to the air at 25 °C, NO₂ will gradually release from the solvent. The color of the solution turned from dark cyan blue to yellow and to transparent eventually. The time of release depends on the solvent. Generally, in [Emim]OTf, it spent about 2 hours, in [Bmim]OTf it was 5 hours, and 20 hours for [Hmim]OTf, indicating reversible adsorption–desorption properties.

To investigate the reversible adsorption–desorption process, vacuum conditions were applied so that the N₂O₄ and NO₂ dissolved in the ILs could release immediately. Two containers (one for gas storage and the other for adsorption) were connected together with a valve. A vacuum pump and a pressure sensor were also connected to the system so that the gas pressure could be controlled. At the initial pressure of NO₂ as 40 kPa, the adsorption processes were conducted in 4 g [Emim]OTf, [Bmim]OTf, and [Hmim]OTf, respectively. With the help of vacuum-assisted desorption, pressure changes during 4 cycles of adsorption–desorption processes in each IL were recorded as shown in Fig. 2. The pressures of the connected system, which was positively related to the remained amount of NO₂, decreased rapidly indicating the fast adsorption of NO₂ in these ILs. The adsorption equilibrium achieved after about 100 s in [Emim]OTf and [Bmim]OTf (Fig. 2a and b), respectively, and 280 s in [Hmim]OTf (Fig. 2c). The longer adsorption time in [Hmim]OTf probably resulted from the bigger steric hindrance of –hexyl on Hmim molecule than –ethyl on Emim and –butyl on Bmim (Fig. 2, insert), which made it more difficult for NO₂ to enter the space among [Hmim]OTf molecules and located on the best adsorption site.

Fig. 2 The pressure change upon the adsorption–desorption cycles of NO₂ in [Emim]OTf (a), [Bmim]OTf (b), and [Hmim]OTf (c) at 35 °C. Insert is the structural formula of each IL.

During the second adsorption round, the equilibrium achieved later compared with the first one in all the ILs. It spent about 130 s in [Emim]OTf (Fig. 2a), 175 s in [Bmim]OTf (Fig. 2b), and 450 s in [Hmim]OTf (Fig. 2c), respectively. Meanwhile, more NO₂ was adsorbed in each IL, which was probably because the adsorbed NO₂ in the first adsorption–desorption process made path and room in IL molecules for the followed NO₂ molecules, just like what happens in molecular imprinting. The more amount of NO₂ adsorption led to the longer adsorption time in each IL. As shown in Fig. 2, the molecular-imprinting like processes achieved their equilibriums at 360 s in [Emim]OTf (the second adsorption–desorption round), 630 s in [Bmim]OTf (the third round), and 900 s in [Hmim]OTf (the second round), respectively, showing that the bigger the steric hindrance of alkyl group on the imidazole cation, the longer the imprinting time was. This fact, together with the FTIR results, indicates that the adsorption of NO₂ (taking form of dimer) was through the interaction with the imidazole parts of ILs. After the imprinting process, some of NO₂ molecules were deeply adsorbed by the ILs and could not be driven out by simply applying vacuum condition, so that they could not be replaced by the NO₂ molecules in the next round. Therefore, the adsorption amount of NO₂ decreased in the next adsorption round, but still more than the first round without the molecular-imprinting like process.

Both of the pressure changes in the adsorption–desorption processes and the molecular-imprinting like processes illustrate that the adsorption of NO₂ in the imidazole sulfonate ILs is reversible and controllable, unlike in water and the other solvents. The solubility of NO₂ in each IL was evaluated at different initial pressures. There is the linear relationship between the dissolved amount of NO₂ in unit gram of IL (C_NO2, calculated from the pressure change shown in Fig. S3 in ESI†) and the initial pressure (P₀), which is P₀ = 0.46 C_NO2 − 2.28 (R² = 0.95) in [Emim]OTf; P₀ = 0.61 C_NO2 − 4.05 (R² = 0.99) in [Bmim]OTf; and P₀ = 0.67 C_NO2 − 5.00 (R² = 0.98) in [Emim]OTf, respectively (Fig. 3). The good but simple linear ship is in accordance with Henry's Law, indicating the inexistence of complex chemical reactions the solute and the solvent during the adsorption. Additionally, the average amount of NO₂ adsorbed in these three ILs at 60 kPa was 32.2 mg g⁻¹, which means 15.7 mL NO₂ at the standard state could be adsorbed into 1 g solvent.

Fig. 3 The relationship between the amount of NO₂ adsorbed in unit gram of IL (C_NO2), in different ILs (left corner), and the initial pressure (P₀).

Besides, the reversible adsorption of NO₂ in the ILs was studied with the temperature changing from 35 °C to 65 °C. With the initial pressure of 42 kPa, the solubility of NO₂ in the ILs decreased as the temperature grew up (Fig. S4 in ESI†). The equilibrium pressure at 45 °C, 55 °C, and 65 °C were almost the same in [Emim]OTf (Fig. S4a in ESI†), but different from each other in [Bmim]OTf and [Hmim]OTf, especially in the latter (Fig. S4b and S4c†). Moreover, the equilibrium pressures at 65 °C in the three ILs were almost the same (Fig. S4 in ESI,† 29.3 kPa for [Emim]OTf, 31.4 kPa for [Bmim]OTf, and 30.9 kPa for [Hmim]OTf). The different variation rate of the equilibrium pressures towards the temperature in the different ILs is proposed to be resulted from the different molecular motion of N₂O₄ in each IL. In [Emim]OTf, the steric hindrance is smaller than the other two solvents, leading to the smaller inter-molecular fraction. Therefore, the frequency and strength of the motion of N₂O₄ molecules in [Emim]OTf increased rapidly as the temperature grew up, which is in accordance with the UV-vis spectrum (Fig. S1 in ESI,† the extra peak appeared at 417.0 nm). As a result, the internal energy of the N₂O₄-[Emim]OTf increased rapidly, which means the adsorption enthalpy change (ΔH) increased rapidly with the temperature growth. Thus the adsorption equivalent shifted to the left rapidly, leading to the rapid equilibrium-pressure growth with the temperature growth. On the other hand, in [Hmim]OTf the bigger hindrance led to the slower changing rate of the reaction enthalpy, so that the equilibrium pressure grew slower. As the temperature grew, the increase of the frequency and strength of N₂O₄ molecular motion gradually slowed down, since the bigger constraint resulting from the molecular body of each IL. Therefore the motion in different IL eventually tended to be almost the same level, leading to nearly the same equilibrium pressures in three ILs at 65 °C. In addition, NO₂ exists in equilibrium with the colourless N₂O₄ as an exothermic process with enthalpy change of about −57 kJ mol⁻¹.^31,32 NO₂ will be favoured at high temperatures. This single equilibrium existed in all of the three ILs, thus did not interfere with the change of equilibrium pressures. The temperature related adsorption proves the proposed adsorbing mechanism again and provides important data for the application in the scenes of real work.

To sum up, NO₂ can be reversibly adsorbed in imidazole sulfonate room temperature ILs and takes form of dimer among IL molecules. Unlike in water and the other solvents, the molecular-imprinting like process was found during the adsorption–desorption cycles in these ILs. The solubility of NO₂ in different ILs depends on the initial pressure, temperature, and the steric hindrance of alkyl on imidazole cation. The well adsorbed NO₂ can be conveniently used in the fields like fuel cell design and the possible organic synthesis, which opens a new way to separate and utilize the wasted energy in nitrogen oxides. We believe these initial results will make our life cleaner and the energy be utilized more efficiently in varieties of chemical industries.

Acknowledgements

This work was financially supported by the Project of Jiangsu province research prospective study (BY2013072-07).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials, methods, the UV-vis spectra of [Emim]OTf and [Hmim]OTf before and after adsorption of NO₂, the FTIR spectra of ILs before and after adsorption of NO₂, the amount of the pressure changes during the adsorption at different initial pressures, and the pressure changes upon the adsorption of NO₂ in the ILs at different temperatures. See DOI: 10.1039/c4ra05268d

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