Gas permeation studies in supported ionic liquid membranes

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Abstract

Room Temperature Ionic Liquids (RTILs) based on the 1-n-alkyl-3-methylimidazolium cation were immobilised in polymeric membranes, in order to study the potential of using supported ionic liquid membranes (SILMs) for CO2/N2 and CO2/CH4 gas separations. Different aspects were investigated, namely: the evaluation of the SILMs stability using two membrane supports which differ in terms of their hydrophobicity; and the effect of using RTILs with cations of different alkyl chains and also with different anions, on the permeability and selectivity of pure and humidified pure gases as well as gas mixtures. H2, O2, N2, CH4 and CO2 gas permeabilities were determined and CO2/N2 and CO2/CH4 ideal selectivities were calculated and compared with data available in the literature described by the Robeson upper bound correlation. The effect of the presence of water vapour in different gas streams of N2, CH4 and CO2 was also studied. Finally, CO2/N2 and CO2/CH4 binary mixtures (50%, v/v) were prepared and the selectivity obtained was compared with the ideal selectivity. The results show that the SILMs prepared with the most hydrophobic support are more stable than those based on the hydrophilic support, and have a high affinity for CO2 when compared with other gases. This behaviour was observed both for pure and gas mixtures, at low pressures. The high selectivity values obtained for CO2 indicate that these SILMs may be considered for CO2 separation processes.

Introduction

The use of supported liquid membranes (SLMs) for gas/vapor separations has been widely studied during the last 16 years [1]. In a SLM, the membrane pores are impregnated with a selected solvent, and transport of the permeating species occurs according to a solution-diffusion mass transfer mechanism. Due to the fact that the diffusion of species in liquids is faster when compared with diffusion in solids, it is expected that the permeability across liquid membranes becomes higher than when using solid membranes. Even though SLMs are considered attractive for gas separations, their application in industry is still limited mainly due to problems in their stability and long-term performance. The stability of the impregnated solvent within the membrane pores may be affected at high temperatures by solvent depletion through evaporation, or due to adverse operating conditions (e.g. the transmembrane pressure should not be higher than the breakthrough pressure so that the solvent is not expelled from the membrane pores). One of the most interesting strategies for improving the stability of SLMs seems to be the use of Room Temperature Ionic Liquids (RTILs) as the immobilised phase within the pores, due to the fact that these compounds have a negligible vapour pressure. This feature eliminates the problem of solvent evaporation that typically occurs in SLMs, allowing for obtaining liquid membranes with a high stability [2], [3], [4], [5], [6], [7]. RTILs are compounds that typically consist of bulky organic cations, and inorganic anions, and have the special characteristic of being liquids at room temperature. Some of the typical cations and anions present in a RTIL are shown in Fig. 1. RTILs exhibit a large variety of properties, such as negligible vapour pressure, thermal stability at high temperatures (above 300 °C), and reduced solubility in various solvents which has additionally made their use as the immobilised phase in supported liquid membranes (SLMs) very attractive [8], [9], [10], [11].

RTILs have received increasing interest in applications involving carbon dioxide separations, due to the large solubility of CO2 in selected RTILs [12]. Among the large diversity of RTILs, those based in the imidazolium cation typically present a large solubility for CO2. Additionally, these RTILs based on the imidazolium cation, may exhibit an even higher solubility for CO2 by selecting an appropriate RTIL anion, which also plays an important role in the gas solubility due to a weak Lewis acid/base complexation that occurs between CO2 and the RTIL anion [13]. Additionally, the solubility of CO2 in RTILs is also expected to increase with an increase in the alkyl chain length of the RTIL cation [14]. In this way, it is possible to design tailor-made membranes with a defined selectivity for a specific application combining different alkyl chain lengths of the RTIL cation and different anions. Among the diverse gas mixtures, the most attractive seem to be the CO2/N2 and CO2/CH4 separations, associated respectively with the purification of flue gas streams and natural gas processes [15].

There are several works available in the literature where supported ionic liquid membranes were studied for potential applications in gas separations [16], [17], [18], [19], [20], [21], [22]. Scovazzo et al. [16] determined the pure gas permeability of N2, CH4 and CO2 and the corresponding ideal selectivities through a porous hydrophilic polyethersulfone support with different RTILs immobilised. After representing the data obtained in a Robeson-plot the authors concluded that these permeabilities/selectivities of SILMs were competitive or even superior to other membrane materials. The facilitated transport of CO2 and SO2 through SILMs was studied by Luis et al. [17]. The permeabilities of air, CO2 and a mixture of SO2/air were measured using different SILMs, and ideal selectivities were calculated. It was concluded that SILMs can be very selective to CO2 and SO2. Regarding the separation of CO2/He, Ilconich et al. [18] determined the pure gas permeability of CO2 and He, and ideal selectivity and membrane stability at different temperatures, up to 125 °C. The ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide was immobilised in two different porous polymeric supports and it was observed that membranes prepared with polysulfone supports were stable at high temperatures. Mixed-gas permeabilities and selectivities for the CO2/CH4 and CO2/N2 gas pairs were determined by Scovazzo et al. [19], and it was observed that the selectivity for a gas mixture slightly decreases when compared to the ideal selectivity. The CO2/CH4 mixture was also studied by Hanioka et al. [20], where, by using a SLM based on a task specific ionic liquid for CO2, a highly stable and selective membrane for CO2 separation (during more than 260 days) was obtained.

Despite the large number of publications concerning supported ionic liquid membranes and the examination of their potential for gas separations, there is still a need for a better understanding of the phenomena taking place. Actually, there is not a clear understanding of the effect of different membrane supports, with diverse chemical nature (typical supports are hydrophilic), on gas permeability and selectivity when using relevant gas mixtures. The effect of the ionic liquid cation structure (namely the length of their side alkyl chain) and the type of ionic liquid anion are other aspects which have not been investigated. Moreover, in the work developed by Fortunato et al. [3], [4] it was observed that the presence of water influences the stability of SILMs due to formation of water clusters within the ionic liquid. Therefore, another important aspect that needs to be clarified, which is not usually taken into account, is the effect of the water vapour content in the gas stream to be processed, on the selectivity of these SILMs.

This work proposes to investigate the following aspects: (1) the design of supported ionic liquid membranes by immobilisation of different Room Temperature Ionic Liquids in polymeric membranes; (2) the evaluation of the stability of the SILMs and the selection of the best membrane support regarding its nature (hydrophobicity); (3) the effect of using different alkyl chain lengths of the RTIL cation and different anions on gas permeability; (4) the effect of the presence of water vapour on the supported ionic liquid membrane performance; (5) the behaviour of the membranes designed, for separation of selected gas mixtures, CO2/N2 and CO2/CH4.

Section snippets

Polymeric porous membranes

The supported ionic liquid membranes were prepared using two different polymeric porous membranes as supporting material, both made of polyvinylidene fluoride (PVDF), with a similar pore size but with different chemical natures: one being hydrophobic (provided by Millipore Corporation, USA) and the other one being more hydrophilic (provided by Pall Corporation, USA). These membranes are characterized by their high chemical resistance, being previously used in other works as a supporting

Stability of SILMs

In order to assess the stability of the SILMs developed and to proceed to the selection of the best membrane support, different RTILs were immobilised within two distinct polymeric membranes—hydrophilic PVDF and hydrophobic PVDF. The membrane weight as a function of time is represented in Fig. 4a and b, for the different RTILs immobilised in the hydrophilic and hydrophobic membranes, respectively, for a N2 applied pressure of 1 bar. It is assumed that the membrane weight loss is only due to the

Conclusions

It has been shown that the supported ionic liquid membranes prepared with Room Temperature Ionic Liquids (RTILs) based on the 1-n-alkyl-3-methylimidazolium cation are stable, especially those based in the most hydrophobic support, possibly due to a better chemical affinity between the RTILs and the supporting membrane.

The effect of using different alkyl chain lengths of the RTIL cation, as well as different anions, on the pure gas permeability was studied. An increase in the gas permeability

Acknowledgements

The authors would like to thank Prof. Susana Luque (Universidad de Oviedo, Spain) for the assistance with the RTILs viscosity measurements. The authors also acknowledge Prof. António Lopes, Instituto de Biologia Experimental e Tecnológica (IBET, Portugal), for the assistance with the contact angle measurements and analysis, and to Eng. Nuno Costa for the assistance in the humidified gas experiments. Luísa Neves would like also to acknowledge the financial support of Fundação para a Ciência e

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