Perfluorocarbon-based supported liquid membranes for O₂/N₂ separation

Highlights

• Perfluorotributylamine (PFTBA) supported on porous alumina was used as a liquid membrane.

• The separation of O₂/N₂ and H₂/N₂ at 40 °C was higher than those of other liquid membranes.

• The O₂/N₂ separation factor was ∼60 with an O₂ permeance of 8 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹.

• The permeance of the membrane correlated with the gas molecular size as H₂ > O₂ > N₂.

Abstract

Air separation is an important commercial process because oxygen and nitrogen are used in several vital industrial processes. This paper reports the implementation of perfluorotributylamine (PFTBA) imbued in porous alumina tubes as a supported liquid membrane to carry out the separation of O₂ and N₂. The PFTBA-supported membrane reached an average O₂/N₂ selectivity of 60 with an O₂ permeance of 8.0 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹. It is shown that the membrane’s selectivity increases as the temperature increases but the stability of the membrane is reduced. Moreover, the separation of H₂ and N₂ was studied to provide insight on the functioning of the membrane and a H₂/N₂ selectivity of 100 was observed with a H₂ permeance of 1.0 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹. The permeance order was H₂ > O₂ > N₂, so the size of the permeating species was found to be an important contributor to the permeance. PFTBA presents an excellent separation performance when compared with conventional supported ionic liquid membranes (SILMs). For instance, 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF₄] showed a maximum O₂/N₂ selectivity of 6 and permeance of 3.4 × 10⁻¹² mol m⁻² s⁻¹ Pa⁻¹.

Introduction

The separation of oxygen from nitrogen in air is an important process that is widely applied in industry. Oxygen is used to enhance the efficiency of several chemical processes such as natural gas combustion, coal gasification, sewage treatment, welding, among others. Nitrogen is used as a low-temperature coolant, an inert diluent, and in the production of ammonia. Conventionally, the separation of O₂/N₂ has been performed by expensive energy intensive processes such as cryogenic distillation and pressure swing adsorption (PSA) [1]. Cryogenic air distillation operates at low temperatures (−170 to −190 °C) to separate O₂/N₂ based on their difference in boiling point [2] resulting in O₂ and N₂ purities of >99% [3], [4]. PSA runs at close to ambient temperatures but at high pressure to separate O₂/N₂ based on their difference in adsorption degree on molecular sieves resulting in O₂ and N₂ purities of 90–95% [3], [5]. Baker et al. [6] reported that alternatives to these methods should: (1) cost less than the cryogenic technique and (2) give gas purity high enough to satisfy application requirements. The separation of hydrogen and nitrogen has not been studied as much, but has application in ammonia synthesis purge streams and synthesis gas composition adjustment [7] and as reported by Bernardo et al. [8] this separation was the first industrial application for polymeric membranes.

Membranes are a promising separation technology to execute process intensification strategies, that is the cutback of production costs by minimizing equipment size, energy consumption and waste production [8]. In the field of membrane science, the separation of O₂ and N₂ [6] is one of the most challenging separations since both gases have similar properties such as molecular weight and size, which are respectively 32 au and 0.346 nm for O₂ and 28 au and 0.364 nm for N₂ [8]. Polymeric membranes have been mainly tested for this separation and Robeson [9] reported that the upper O₂/N₂ selectivity bound is ∼9 with a permeability of 18 barrers (1.3 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹ for a 45 μm thick membrane).

Baker et al. [6] declared that supported liquid membranes (SLMs) are a possible solution to overcome low O₂ fluxes and selectivities. These SLMs consist of an organic liquid immobilized in the pores of a support by means of capillary forces [10] and were reported for the first time in 1967 by Robb and Ward [11]. This first SLM consisted of an aqueous bicarbonate–carbonate solution fixed in a porous cellulose acetate film. This membrane was capable of separating CO₂ and O₂ with a selectivity of 4100. Several authors have reported a variety of active liquids that facilitate the gas transport of several gases. For example, Scholander [12] studied O₂ transport through hemoglobin (Hb) solutions and demonstrated an increase in O₂ transport. Chen et al. [13] developed a liquid membrane using Hb as the carrier in a flat microporous sheet and obtained a maximum O₂ permeance of 1.4 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹ at 720 Pa with a maximum O₂/N₂ selectivity of 18.

In addition, ionic liquids (ILs) have been intensively reported in the SLM literature, primarily due to the physical properties of the liquids, which include negligible vapor pressure, high viscosity, and good chemical and thermal stabilities [14]. ILs have been successfully applied in many separations including CO₂/N₂ [15], CO₂/CH₄ [16], [17], O₂/N₂ [18], H₂/N₂ [19] and H₂/CO [20]. Nevertheless, few ILs have successfully separated O₂ from N₂. For instance, Pez and Carlin [21] used a LiNO₃-liquid membrane for air separation at 430 °C and achieved a maximum O₂ permeance of 3.4 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹ with an O₂/N₂ selectivity of 170. Bara et al. [22] synthesized imidazolium ionic liquids containing fluoroalkyl substituents. The liquid 1-methyl-3-(3,3,4,4,4-pentafluorohexyl)-imidazolium bis(trifluoromethane)sulfonamide supported on a porous polyethersulfone with a thickness of 145 μm achieved a separation of 2.3 at 23 °C with an O₂ permeance of 6.2 × 10⁻¹¹ mol m⁻² s⁻¹ Pa⁻¹. To our knowledge, the maximum IL performance for this separation at room temperature (30 °C) was achieved by Scovazzo and Condemarin [18] using glass fiber disk filters as the support and trimethyl(butyl)ammonium bis(trifluoromethyl)sulfonylimide $\left[{\text{N}}_{(4)111}^{+}\right]$[Tf₂N⁻] as the active IL. They obtained an O₂/N₂ separation factor of 2.4 with an O₂ permeability of 5.0 × 10⁻¹¹ mol m⁻² s⁻¹ Pa⁻¹.

In this paper we report the utilization of perfluorotributylamine (PFTBA) immobilized in a porous alumina support. This liquid was chosen because perfluorocarbons (PFCs) are well known oxygen-transport fluids used in respiratory systems and in cell cultures [23]. For instance, perfluorotributylamine has been reported to have high oxygen solubility capable of dissolving over 38 vol.% O₂ at 25 °C [24]. PFCs are highly stable, immiscible hydrocarbons with total or partial hydrogen replacement by fluorine and have not been reported before for use in liquid membranes. Perfluorocarbons dissolve O₂ molecules physically by loose, non-directional van der Waals interactions. The PFTBA-SLM was compared with an ionic liquid SLM based on [emim][BF₄] for the H₂/N₂ and O₂/N₂ separation at 40 °C and was found to have superior permeance and selectivity.