Facilitated CO₂ transport and barrier effect through ionic liquid modified with cyanuric chloride

Abstract

A 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIM⁺BF₄⁻) ionic liquid membrane containing cyanuric chloride as a carrier can be utilized for facilitated CO₂ transport. The cyanuric chloride complex was characterized by Fourier transform (FT) infrared spectroscopy, thermogravimetric analysis, viscosity measurements, and FT-Raman spectroscopy. The BMIM⁺BF₄⁻/cyanuric chloride membrane showed a high CO₂ permeance of 19.2 GPU, in addition to excellent ideal selectivities of 11.0 and 10.7 for CO₂/N₂ and 10.7 for CO₂/CH₄ mixtures, respectively. A barrier effect was observed for CH₄ and N₂, while the amine groups on the imidazolium ring and cyanuric chloride selectively interacted with CO₂ molecules, thus facilitating their transport.

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Global warming due to greenhouse gas (such as CO₂) emission continues to be one of the major environmental problems.^1,2 Carbon dioxide separation has been of specific interest to the research community, and many separation processes have been developed, including cryogenic separation, adsorption, and absorption.³ Compared to the traditional methods, membrane separation technology has various merits such as high energy efficiency, ease of operation, and environment friendliness.^4,5 However, achieving high selectivity as well as high permeability remains one of the challenges to be overcome in the development of suitable membranes.⁶ As a solution, facilitated transport membranes (membranes containing carriers⁷) have been introduced. The carrier utilized in facilitated transport forms a reversible complex with the specific gas (CO₂ or olefin gas), thereby increasing both selectivity and permeability.⁸

The imidazolium ring in ionic liquids (ILs) contains amine groups, which are known to be CO₂ carriers.^9–13 In addition, imidazolium-containing ILs have unique properties such as nonvolatility, high thermal stability, high ionic conductivity, and tunable solvation ability.^14–17 For these reasons, imidazolium-containing ILs have been used for facilitated CO₂ transport membranes. We have previously reported the CO₂ separation performance of membranes containing 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIM⁺BF₄⁻) and copper nanoparticles (CuNPs). Such membranes utilized both the amine functionality of the imidazolium ring and the interface of the dipole on the CuNP surface caused by the anion of imidazolium. This membrane demonstrated reasonable separation performance, with a CO₂ permeance of 17 GPU (1 GPU = 1 × 10⁻⁶ cm³ (STP) (cm⁻² s⁻¹ cm⁻¹ Hg⁻¹)) and an ideal selectivity of 5.0 and 4.8 for CO₂/CH₄ and CO₂/N₂, respectively.¹⁸ We have also recently reported the CO₂ separation performance of PVP/potassium fluoride (KF) membranes that utilized the interaction between K⁺ ions and CO₂ molecules. The PVP/KF membrane performance showed a CO₂ permeance of 28.0 GPU and an ideal selectivity of 4.1 for CO₂/N₂.¹⁹ However, conventional membranes utilizing metal nanoparticles generally showed low gas permeability, which remains one of the most important challenges for commercialization.⁷

Scheme 1 Structure of cyanuric chloride.

Herein, we present a new strategy for developing facilitated transport membranes based on ILs in concert with cyanuric chloride. Cyanuric rings having amine groups are expected to interact reversibly with CO_2, while other gas molecules such as CH₄ and N₂ can be blocked by the barrier effect of the cyclic ring (Scheme 1). The synergistic effect of both the amine group of the ionic liquid and the amine group of cyanuric chloride was investigated. The decomposition characteristics of the BMIM⁺BF₄⁻/cyanuric chloride complexes were assessed by TGA and are shown in Fig. 1 Cyanuric chloride decomposed over the temperature range 50–110 °C, and BMIM⁺BF₄⁻ evaporated over the temperature range 315–470 °C. The TGA curve of the prepared BMIM⁺BF₄⁻/cyanuric chloride mixture showed two shoulders at 90–190 °C (weight loss ∼9%) and 320–480 °C, indicating that the first dip was attributed to cyanuric chloride decomposition. The BMIM⁺BF₄⁻/cyanuric chloride mixture decomposed slowly and at a higher temperature than did neat cyanuric chloride, indicating the interaction between cyanuric chloride and BMIM⁺BF₄⁻.

Fig. 1 TGA data for BMIM⁺BF₄⁻, cyanuric chloride, and BMIM⁺BF₄⁻/cyanuric chloride.

For a detailed analysis of the C=N band of cyanuric chloride complexed to BMIM⁺BF₄⁻, the FT-IR spectrum was deconvoluted, as shown in Fig. 2 The C=N bond attributable to cyanuric chloride had one peak at 1492 cm⁻¹. When cyanuric chloride was incorporated into BMIM⁺BF₄⁻, the C=N peak shifted to 1511 cm⁻¹ due to the interaction of carbon atom to have positive polarity with anions of BMIM⁺BF₄⁻ in cyanuric chloride. Interactions between cyanuric chloride and BMIM⁺BF₄⁻ in the BMIM⁺BF₄⁻/cyanuric chloride mixture were also investigated by FT-Raman spectroscopy.

Fig. 2 FT-IR spectra for BMIM⁺BF₄⁻, cyanuric chloride, and BMIM⁺BF₄⁻/cyanuric chloride mixture (1/0.1).

The Raman spectra in the regions of the BF₄⁻ stretching bands are shown in Fig. 3. The peaks for free ions, ion pairs, and ion aggregates of BF₄⁻ appeared at 765, 770, and 774 cm⁻¹, respectively.¹⁸ The peak of neat BMIM⁺BF₄⁻ was appeared predominately at 772.5 cm⁻¹, indicating that the existence of ion aggregates and ion pairs. When cyanuric chloride was added, the peak shifted to 765 cm⁻¹ because of the release of free BF₄ from the BMIM⁺BF₄⁻of IL. This result suggests the interaction of BMIM⁺BF₄⁻ with cyanuric chloride. A change in the viscosity of BMIM⁺BF₄⁻ was observed after cyanuric chloride was added (Table 1). When cyanuric chloride was incorporated into BMIM⁺BF₄⁻, the viscosity increased to 129.5 cP, while that of neat BMIM⁺BF₄⁻ was 92.9 cP. These results suggest that cyanuric chloride was well dispersed in BMIM⁺BF₄⁻.

Fig. 3 FT-Raman spectra in the BF₄⁻ stretching region for neat BMIM⁺BF₄⁻ and BMIM⁺BF₄⁻/cyanuric chloride mixture.

Table 1 Viscosity of neat BMIM⁺BF₄⁻ and BMIM⁺BF₄⁻/cyanuric chloride

	Neat BMIM^+BF4^−	BMIM^+BF4^−/cyanuric chloride
Viscosity (cP)	92.9	129.5

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Finally, the separation performance of the BMIM⁺BF₄⁻/cyanuric chloride membrane was investigated. Values for CO₂ selectivity and permeance were measured for CO₂/N₂ and CO₂/CH₄ mixtures (Table 2). The CO₂, CH₄, and N₂ permeance of a pure BMIM⁺BF₄⁻ membrane were 17.0, 5.0, and 3.4 GPU, respectively.

Table 2 Permeance and selectivity performance of neat BMIM⁺BF₄⁻ and BMIM⁺BF₄⁻/cyanuric chloride (1/0.1)

	Permeance (GPU)			Selectivity
	N2	CH4	CO2	CO2/N2	CO2/CH4
Neat BMIM^+BF4^−^18	3.4	5.0	17.0	5.0	4.8
BMIM^+BF4^−/cyanuric chloride	1.7	1.8	19.2	11.0	10.7

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Scheme 2 BMIM⁺BF₄⁻/cyanuric chloride membrane for CO₂ separation.

Thus, the ideal selectivities for the CO₂/CH₄ and CO₂/N₂ mixtures were 4.8 and 5.0, respectively, because CO₂ interacts only with the amine groups of the imidazolium ring. When cyanuric chloride was incorporated into BMIM⁺BF₄⁻, the permeances of N₂, CH₄, and CO₂ were 1.7, 1.8, and 19.2 GPU, respectively. Thus, the ideal selectivities of the CO₂/CH₄ and CO₂/N₂ mixtures were 11.0 and 10.7, respectively. The ideal selectivity and CO₂ permeance of the BMIM⁺BF₄⁻/cyanuric chloride membrane were both higher than those of neat BMIM⁺BF₄⁻. These improvements can be attributed to two factors: (1) the carrier effect of the amine in cyanuric chloride and (2) the barrier effect for N₂ and CH₄ caused by the cyanuric chloride ring. As a result, the BMIM⁺BF₄⁻/cyanuric chloride membrane showed superior CO₂ separation performance because of the combination of facilitated CO₂ transport and the barrier effect as shown in Scheme 2.

Conclusions

A newly designed membrane based on both facilitated CO₂ transport and a barrier effect was demonstrated using cyanuric chloride in concert with ionic liquids. The BMIM⁺BF₄⁻/cyanuric chloride membrane showed an improvement in the ideal selectivity for CO₂/CH₄ and CO₂/N₂ mixtures (with an observed CO₂ permeance of 19.2 GPU) over neat BMIM⁺BF₄⁻. The improved separation performance of up to 170% was attributed to both facilitated transport and the barrier effect. The amine functionalities of cyanuric chloride and BMIM⁺BF₄⁻ both interact with CO₂ to facilitate transport, while the cyclic ring of cyanuric chloride can act as a barrier for N₂ and CH₄ molecules.

Acknowledgements

This work was supported by an Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry and Energy (20122010100040). This work was also supported by the Basic Science Research Program (2013021962) and Korea CCS R&D Center through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental method, characterisation and material. See DOI: 10.1039/c4ra00945b

‡ These authors contributed equally to this work as first authors.

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