(45g) Study of Ammonia Transport in Perfluorosulphonated Aquivion® Membranes

Ammonia is one of the most important compounds in the chemical industry and it is a key chemical component for various relevant applications such as pharmaceutical and chemical processes, production of fertilizers, refrigeration systems and as an energy carrier [1–3]. Besides, NH₃ is also a suitable candidate for hydrogen (H₂) storage and transport, thanks to its high storage density and lower energy demand for liquefaction [4].

Many studies, indeed, have recently attempted to find a more efficient and environmentally friendly process for its synthesis and purification, in a reliable and scalable way, while reducing its environmental footprint [5].

In this concern, the membrane-related process might be a really promising technology in such applications for the lower cost combined to high purity and the possibility of a continuous process.

Particularly, perfluoro sulphonated materials play a role both as base materials for polymer electrolyte membranes for low-temperature electrochemical ammonia synthesis and as membranes for effective product separation. Pressure-driven gas separation processes by membranes are in this concern natural candidates for such application, as ammonia high condensability usually provides higher permeability with respect to lighter components, such as N₂ and H₂ because their different solubility and the rate of diffusion [6–8]_.

The characterization of Ammonia transport properties in such materials is therefore of great importance to deeply undisclosed the real potential of their use in such processes, and truly understand the gas transport behavior. Unfortunately, only little experimental data exists of ammonia in perfluorosulphonated membranes, mainly related to Nafion, while other PSFA have been somewhat ignored for such application.

Aiming to partially fill this gap, the present study reports the results of a series of permeation and sorption tests carried out on Aquivion C87-05 (short side chain perfluorosulphonic acid ionomer) with pure ammonia in a temperature range between 20 and 50°C. Moreover, the effect of water on NH₃ separation in Aquivion is investigated through humid permeation experiments, between 0 and 80% RH in order to assess the influence of this parameter on the different gas transport performance. Even though ammonia permeability is significantly larger than those of CO₂, N₂, H₂, CH_4, and H₂S, it is also the one that is influenced the least by the presence of water: NH₃ permeability at RH=80% is 26400 Barrer, only 4 times higher than in that dry condition (5700 Barrer); conversely, the permeability of the other gases experiences more than two order of magnitude increase passing from dry to humid conditions.

In dry conditions, pure Ammonia permeability reached values up to 7000 Barrer at 20°C revealing an increasing trend with upstream pressure (which increases as much the RH increases), while it decreased with temperature, as shown in Fig. 1a. The high ammonia permeability make reasonable to assume that the transport in the membrane occurs through a system of molecular “channels” in the ionomer matrix by means of reversible bonding of NH with Aquivion acidic functional groups [7].

On the other hand, the NH₃ solubility isotherms (Fig. 1b) show a sharp and concave increase with activity followed by a more linear behaviour with pressure. Such change has been observed also for water uptake [9,10] and it is usually related to the peculiar sorption process in Aquivion which is strictly related to the strong interaction of the polar penetrant with the SO₃H group which is suggested to lead, at high activity to a massive swelling of the system and to the formation of a phase segregated structure. In fact, as observed from FT-IR spectra reported in Fig. 2, ammonia is readily dissolved in the polymer and may create interconnected domains that facilitate its transport. The IR characteristic band located in the range from 1300 to 1500 cm^-1 is associated to the ammonium ion or amide group, suggesting the formation of some chemical bonds between NH₃ and Aquivion surface sites, even if the material structure does not change irreversibly. Moreover, it is reported that NH₃ tends to form clusters with sulphonic groups [11–13], detectable in the wavelength between 2700 and 3200 cm^-1, by creating a [-SO₃^- + H⁺(NH₃)] complex.

This theory has been also confirmed by the resolution of a revised Flory Huggins equation, proposed by Futerko and Hsing [14], which can be applied to represent ammonia solubility and proton conductivity in PFSAs, by accounting for the proton transfer between the SO₃H and NH₃ and the formation of a tetramer cluster.

The diffusion coefficient D has been evaluated from the mass uptake curve in the case of Fickian mass transport. NH₃ diffusivity ranges on the same order of magnitude than water and increases with temperature and gas concentration.

The molecular transport of ammonia across the membrane may be dictated by the affinity between the base penetrant and the acid functional groups of the polymer, making NH₃ easily to diffuse [15].

The ideal selectivity for NH₃/N₂ and NH₃/H₂ can be calculated as the ratio between NH₃ and N₂ and H₂ permeability and displayed in the Upper bound Robeson plot (Fig, 4a and 4b).

NH₃ selectivity in Aquivion is one of the highest achieved both with respect to Nitrogen and Hydrogen with values up to 7000 and 1000, respectively.

Furthermore, also the NH₃ permeability obtained from our experimental campaign results to be quite high respect to other PFSA materials present in the literature, despite the temperature difference [6]. It is also worth to notice that a slight decrease in selectivity occurs when passing from dry to humid conditions (RH=80%) for both pairs of gases, even though NH₃ permeability rises.

It is reasonable to assume that ammonia transport in PFSA materials is governed by an interfacial mass transport coupled with a diffusion process across the gas/membrane interface.

The obtained separation performances are found to be significantly better than those of other polymeric membranes proposed for the same separations, as compared in a permeability-selectivity plot. In addition to that, due to the high solubility, ammonia is expected to form a percolated structure of polar channels within the polymer, which is very similar to that usually formed by water.

Finally, the good separation performances obtained in this work and the intrinsic resistance of Aquivion to harsh environments, suggest that this polymer is an ideal candidate for the separation of hydrogen and nitrogen from NH_3.

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