Highly selective separation of CO2/N2 using [Emim][Tf2N] supported ionic liquid membranes prepared by supercritical fluid deposition
Graphical Abstract
Introduction
CO2 is a definite product of the energy produced by fossil fuels. 60% of global warming is caused by CO2, which has an alarming concentration of 407 ppm in the atmosphere [1]. As a result, the world demands powerful CO2 emission reduction technologies with low energy consumption. Chemical absorption using amine organic solvents (such as monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA), etc.) are the most widely employed approach to remove CO2. However, ammonia solvent absorption technology exposes a lot of drawbacks, including equipment corrosion, solvent evaporation, high stripping and regeneration costs [2]. Membrane-based separation is considered to be eco-friendly, low energy consumption, compact design, low operating cost, and easy integration into existing factories [3], [4]. Additionally, membranes also have the potential to be combined with other technologies to synergistically enhance separation performance.
Ionic liquids (ILs), low-melting salts composed entirely of ions, are regarded as green solvents due to their extremely low vapor pressure [5], easily tunable physical properties [6], [7], [8], great CO2 solubility [9], [10], [11], [12] and high selectivity to other light gases [13], [14], [15], [16], [17], [18]. For example, Jonas et al. [19] reported that the synthesized IL ([PEHA] [Cl]) exhibited very high CO2 adsorption kinetics (16.40 l kg−1 s−1) and CO2 absorption capacity (1.24 mmol/g sorbent (or 2.10 mol/mol IL)), high thermal stability (decomposition at 220 °C), and low cost (37 $/100 G). However, the high viscosity of the bulk ILs hinders the diffusion of CO2 molecules, which was still an obstacle to the direct application of CO2 separation on an industrial scale. To address this issue, scovazzo et al. proposed the SILM technology in 2002 [20], which shortened the diffusion path of CO2 by confining IL in a thin porous inert support. So far, SILM has attracted widespread attention by virtue of its comprehensive advantages of IL and membrane [2], [21], [22], [23], [24]. SILMs are subject to a trade-off between permeability and selectivity, known as Robeson’s upper bound. Thus, the research of SILM has been devoted to obtaining a thinner, defect-free liquid filled layer, and screening for ILs with high CO2 dissolving capacity and diffusion coefficient.
Impregnation and coating methods are very simple, and have always been a common method for the preparation of SILM. The principle of these methods was to use the capillary force of the IL to penetrate into the pores of the support, so that the IL directly contacts the support for a period of time to form an ionic liquid membrane. Obviously, due to the high viscosity and high surface tension of IL, the preparation process demands to spend a long time. Significantly, different approaches have been explored to reduce penetration time. Fortunato et al. [25] employed vacuum on supports to facilitate the immobilization of IL, and Hernandez et al. [26] applied a pressure of 2 bar on IL side to help it penetrate into supports. To decrease the viscosity of IL, Kim et al. [27] diluted it into a solution containing 60 vol% methanol. Although the performance of SILM prepared by all the above-mentioned methods has been dramatically improved, it was still not ideal. Noting that the pore size distribution of the support cannot be completely uniform, and IL always tends to fill the pores with larger pores first, and thus the distribution of IL in the support was not uniform [28]. To ensure that the small pores in the support could be completely filled, a longer preparation time and a larger pushing pressure difference have to be used, yet this would make the formed ionic liquid membrane too thick, which was not conducive to the rapid penetration of CO2.
In 2018, Liu et al. [29] reported for the first time the utilize of the SCFD method to prepare an ionic liquid membrane with asymmetric inorganic γ-Al2O3 as the support. IL dissolved in supercritical CO2 (scCO2) (critical temperature 31.1 °C, critical pressure 7.38 MPa) was enriched in the pores of the support. This enrichment process was called the capillary phase transition of IL, which could only occur in small pores. The as-prepared [Bmim][BF4]-SILMs and [Bmim][Ac]-SILMs displayed CO2 permeances of ~1.5 GPU and ~0.7 GPU, respectively. Electron microprobe analysis exhibited that the IL concentration in the substrate region was lower, while the IL concentration in the top and intermediate layers was higher. Compared with the conventional method, the SCFD method could guarantee that IL quickly filled the effective thin layer with small pores. In the further study [30], quantitative calculation of capillary phase transition was conducted by a modified Young-Laplace equation and the Kelvin equation [31], proving that the surface tension of IL was very importance in capillary phase transition process. With an increasing surface tension, the capillary phase transition became quicker and might occur in larger pores.
Until now, there are only few research reports on the use of SCFD to prepare SILM. The universality of the proposed method should be studied by other experiments, especially the investigation of different ILs. In order to use the difference in pore size between the support to obtain a thinner liquid filling layer, [Emim] [Tf2N] (an ionic liquid with low surface tension) was chosen to prepare SILM in this work. Single-gas permeation through the SILMs were measured in a custom-made apparatus using CO2 and N2. SILM performance was evaluated by the IL loading, the N2 and CO2 permeances and the ideal CO2/N2 selectivity. Effects of four operating parameters (deposition pressure, time, co-solvent amount and IL amount) on the performance of SILM were investigated. The CO2 permeability and CO2/N2 selectivity were comprehensively considered, and the optimal value of the operating parameter was selected. Finally, the pressure resistance and long-term stability were analyzed under different operating conditions.
Section snippets
Materials
Commercial α-Al2O3 membrane tube (substrate) was acquired from Guangdong Jieyang Lishun Technology Co., Ltd with a thickness of 2 mm and an average pore size of 2 µm. 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [Emim] [Tf2N] with a purity higher than 99% was purchased from Shanghai Chengjie Chemical Co., Ltd, which could be used without further purification. The ethanol used as co-solvent was analytical grade and was supplied by Tianjin Fuyu Fine Chemicals Co. Ltd.
Results and discussion
In the current study, we discussed the distribution of IL in the support, as well as the influence of deposition pressure, deposition time, the amount of ethanol, [Emim][ Tf2N] addition amount on SILM loading Δm, CO2 permeability (Pa,CO2), N2 permeability (Pa,N2), ideal CO2/N2 selectivity (αCO2/N2), to obtain the discipline of the effect of separation performance parameters, and achieve the controllable preparation of [Emim][BF4]-SILM. The performance of other ionic liquid-SILM was compared to
Conclusion
In the present study, a tubular γ-Al2O3 with a three-layer structure as a support was developed, and [Emim] [Tf2N]-ionic liquid membrane was supported with the assistance of SCFD. Generally, the low surface tension of [Emim] [Tf2N], the intermediate layer of the support was filled later than the top layer. By shortening the deposition time to 6 h, only the top layer was filled with IL, which significantly improved the CO2 permeability. Simultaneously, the influence of process parameters
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21978043, 21376045, U1662130).
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