Highly permeable ionic liquid membrane by both facilitated transport and the increase of diffusivity through porous materials

Abstract

Ionic liquid membranes have showed low permeance, due to high viscosity when utilized for gas separation, limiting their practical application despite advantages such as high thermal stability and high CO₂ selectivity. In this study, Cu nanoparticles generated by redox reduction with Fe²⁺ ions and porous KIT-6 were utilized for high selectivity and as a permeable membrane. When positively polarized Cu nanoparticles were generated and porous KIT-6 materials were incorporated into ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIM BF₄), the selectivity for CO₂/N₂ and CO₂/CH₄ of the composite membrane was largely enhanced to 16.4 and 23.4, respectively while neat BMIM BF₄ was 5.0 and 4.8, respectively. Furthermore, the CO₂ permeance was also enhanced from 17 to 50.7 GPU, compared to the neat BMIM BF₄ membrane. These enhancements of separation performance is attributed to both the facilitated transport by polarized Cu NPs and the increase of diffusivity by porous materials.

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1. Introduction

Greenhouse gases cause global climate change^1–4 and therefore CO₂ separation using membranes has attracted much attention.⁵ Using membrane separation has many advantages like low cost, high energy efficiency, and large-scale production.^4–9 However, CO₂ separation using membranes has challenges in increasing both permeance and selectivity at high pressures.^6,10 Methods of CO₂ separation through facilitated transport was introduced¹¹ and continue to progress at room temperature ionic liquids (RTILs). Because previous works of RTILs with single gas presented promising results, and ionic liquids (ILs) have many advantages such as no solvent loss and wide operational temperatures,^12–14 using ILs are suitable for CO₂ separation membranes. ILs have imidazolium rings that contains amine groups which act as a carriers of CO₂.¹⁵ CO₂ separation performance using ILs, CO₂/N₂, selectivity and CO₂ permeability of 1-ethyl-3-methylimidazolium dicyanamide were 57 and 1237 barrer while 1-ethyl-3-methyl imidazolium tetrafluoroborate showed 44 and 968 barrer, respectively.¹⁴ CO₂/N₂ selectivity and CO₂ permeability of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide were 15.4 and 1135.8 barrer while those of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide were 19.7 and 1344.3 barrer, respectively.¹⁶ In previous work, we reported 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF₄) to show the separation performance of 5 CO₂/N₂ and 4.8 CO₂/CH₄ selectivities.¹⁷

To enhance CO₂ separation performance of neat RTILs, facilitated CO₂ separation with BMIM BF₄/copper nanoparticles (Cu NPs) composite membranes prepared by adding copper flakes into BMIM BF₄ was reported.¹⁷ These membranes showed good performances for CO₂/N₂ selectivity (11), CO₂/CH₄ selectivity (11), and CO₂ permeance (25 GPU).¹⁷ In addition, we reported a 1-butyl-3-methyl imidazolium nitrate (BMIM NO₃)/copper flake membrane with CO₂/N₂ selectivity (12.25) CO₂/CH₄ selectivity (12.25), and CO₂ permeance (9.8 GPU).¹⁵ These results show Cu NPs is a good carrier for CO₂ separation in RTILs.

Previously, UV-visible absorption spectra of BMIM BF₄/copper flakes show a maximum peak at 675 nm with a broad and asymmetric shape.¹⁷ The UV peak indicates the formation of various sizes of Cu NPs. This process was a top-down method that forms large and various particle sizes. In presented work, a bottom-up process formed relatively uniform and small sized Cu NPs, resulting in an increase of CO₂ separation performance.

The underlying idea in the present work is the utilization of the Fe²⁺ ion to reduce Cu⁺ species. Fe²⁺ species are well-known reducing agents that effectively reduce inorganic ions such as Cu²⁺, Tc⁷⁺ and Cr⁶⁺ as well as organic pollutants.¹⁸ Thus, FeCl₂ and CuCl were utilized for the formation of Cu NPs in neat BMIM BF₄ for the increase of CO₂ separation performance. Further, KIT-6 as porous materials was used for increase of permeance.

2. Experimental

2.1. Materials

The ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF₄) was purchased from C-TRI. Copper chloride and iron chloride were purchased from Sigma-Aldrich Chemical Co. Hydrochloric acid was purchased from Daejung Chemicals & Metals.

2.2. Characterization

BJH data were measured using Autosorb iQ Station 1 (Quantachrome instruments, USA) and analysis gas was nitrogen. Scanning electron microscope (SEM) image was obtained by using JSM-5600LV (JEOL Ltd, Japan).

2.3. Membrane preparation

KIT-6 silica materials were prepared according to methods reported by Kleitz et al.¹⁹ Cu NPs/KIT-6 was prepared by adding 0.44 g CuCl (Aldrich Chemical) and 1.12 g FeCl₂ (Aldrich Chemical) in 62.5 mL of 2 M HCl and stirring for 1 h, and then adding 2 g of KIT-6. Solvent was removed by rotary evaporation for 2 h at 323 K and dried for 1 day at vacuum oven. Cu NPs/KIT-6/BMIM BF₄ composite was coated onto the polysulfone membrane support (Toray Ltd, Japan) using RK Control Coater (model 202, Control Coater RK Print-Coat Instrument Ltd, UK).

2.4. Gas separation performance

The permeances of single gases (N₂, CO₂, CH₄) were measured by a gas bubble meter. Permeance was expressed in unit of GPU (1 GPU = 1 × 10⁻⁶ cm³(STP)/(cm² s cmHg)).

3. Results and discussion

3.1. Porous characteristics

The transmission electron microscopy (TEM) images for the KIT-6 and Cu NPs/KIT-6 are presented in Fig. 1, with (a) showing the porous characteristics of KIT-6. We expect an increase of permeance of the membrane by adding KIT-6. In Fig. 1(b), aggregate particles size of KIT-6 and Cu NPs were 25–90 nm and porous characteristics of KIT-6 was maintained.

Fig. 1 TEM images of (a) KIT-6 and (b) Cu NPs/KIT-6.

3.2. Separation performance

Fe²⁺ can donate an electron to Cu⁺ when FeCl₂ and CuCl are ionized. The reduced Cu was distributed in the form of nanoparticles, and then positively polarized by BMIM BF₄. Positively polarized Cu NPs can be used as carrier for CO₂ separation.^15,17 To determine the gas separation performance, we tested the membrane using single gases (N₂, CH₄ and CO₂) at room temperature and coating on the finger-like shape of a polysulfone support (Fig. 2).

Fig. 2 Scanning Electron Microscopy (SEM) image of polysulfone support.

Fig. 3 show the permeance and selectivity performance values of the composite membrane and CO₂ permeance and selectivities increased with the increasing quantities of Cu NPs/KIT-6. The enhanced selectivity and CO₂ permeance was attributable to two factors: (1) the carrier effect by positively polarized Cu nanoparticles and (2) the increased diffusivity through porous materials. When Cu NPs/KIT-6 was added in a 0.1 weight ratio in BMIM BF₄, the highest performance of CO₂ permeance (51 GPU) and selectivities of CO₂/N₂ (16) and CO₂/CH₄ (23) were demonstrated as shown in Table 1. For more than 0.1 weight ratio of Cu NPs/KIT-6, Cu NPs were no longer well dispersed and aggregated, acting as a barrier. Thus, a certain amount of Cu NPs/KIT-6 resulted in decrease of both permeance and selectivities. The improvement of permeance by the porous materials was also observed in other research. Kim et al. reported using mesoporous TiO₂ hollow nanospheres in mixed matrix membranes for enhancement of CO₂ separation performance.²⁰ The pores on the surface of a sphere and vacant space inside the sphere enabled gases to permeate rapidly.²⁰ Therefore, membrane performance was significantly enhanced in N₂ permeance (85.4) CO₂ permeance (1.97), and CO₂/N₂ selectivity (43.4).²⁰ Carreon et al. used a Bio-MOF membrane for CO₂/CH₄ separation,²¹ and 3 layer bio-MOF showed CH₄ permeance (4.6), CO₂ permeance (11.9 mol m⁻² s pa (*10⁻⁷)), and CO₂/CH₄ selectivity (2.6).²¹ The Carreon group also reported alumina-supported cobalt-adeninate MOF membranes for high CO₂ permeance.²² These membranes showed CH₄ permeance (1.18), CO₂ permeance (4.16 mol m⁻² s⁻¹ pa⁻¹ (*10⁻⁶) and CO₂/CH₄ selectivity (3.5).²²

Fig. 3 (a) Permeance of CO₂, N₂ and CH₄, and (b) selectivities of BMIM BF₄ with varying Cu NPs/KIT-6 contents.

Table 1 CO₂ permeance and selectivities of neat BMIM BF₄ and BMIM BF₄/Cu NPs/KIT-6 composite

	CO2 permeance (GPU)	CO2/N2	CO2/CH4
Neat BMIM BF4[17]	17	5.0	4.8
BMIM BF4/(Cu NPs/KIT-6) 1/0.1	51	16	23

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3.3. Pore size distribution

To investigate surface properties of KIT-6 and Cu NPs/KIT-6, N₂ adsorption-desorption was measured. The results showed a negligible change of structure and surface area in a mixture of Cu NPs and KIT-6 (Fig. 4(a)). Though FeCl₂ and CuCl were introduced into KIT-6, the surface characteristics of KIT-6 were maintained. The average pore size was also constant, however a small decrease of average pore size distribution may be due to Cu partially blocking the KIT-6 pore (Fig. 4(b)). Fortunately, the separation performance showed that the partial blocking of Cu onto KIT-6 has little effect on the CO₂ transport. Actually, the CO₂ permeance increased from 17 to 51 GPU, indicating that pore-blocking phenomena were not generated.

Fig. 4 (a) N₂ adsorption-desorption of KIT-6 and Cu NPs/KIT-6 and (b) pore size distribution.

4. Conclusions

We succeeded in making a highly selective and permeable CO₂ separation membrane consisting of ionic liquid BMIM BF₄/Cu NPs/KIT-6 composite for facilitated CO₂ transport. Cu nanoparticles were generated by utilizing Fe²⁺ capable of electron donation to Cu⁺ in the ionic liquid, and the surface of NPs was positively polarized by interaction with counteranions of BMIM BF₄. Furthermore, the porous KIT-6 was incorporated into BMIM BF₄/Cu NPs composite membrane for enhancement of permeance. As a result, the selectivity for CO₂/N₂ and CO₂/CH₄ of the BMIM BF₄/Cu NPs/KIT-6 composite membrane was largely enhanced to 16 and 23, respectively while those of neat BMIM BF₄ showed 5.0 and 4.8, respectively. Furthermore, the CO₂ permeance was also enhanced from 17 GPU to 51 GPU by the facilitating transport of polarized Cu NPs and increasing diffusivity by porous materials as shown in Scheme 1.

Scheme 1 BMIM BF₄/Cu nanoparticles/KIT-6 composite membrane for facilitating CO₂ transport.

Acknowledgements

This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (20122010100040), funded by the Ministry of Trade, Industry, and Energy of the Korean government. This work was also supported by the Basic Science Research Program (2015011603) and the Korea CCS R&D Center (NRF-2014M1A8A1049313), through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science, ICT and Future Planning.

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Footnote

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

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