Oxygen selective iron and cobalt–metalloporphyrin polymers – Extraordinary selectivity at low temperature

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Highlights

  • Polymers made up of porphyrin ring building blocks with coordinated Fe and Co ions were synthesized.

  • The Co-POP was shown to be oxygen selective at 273.15 K (selectivity of up to 1.3).

  • Both Fe-POP and Co-POP were oxygen selective at 91.35 K (selectivity of up to 14.7).

  • Nitrogen gas excluded from the pores below a critical temperature.

  • Repeated gas sorption measurements at 273.15 K showed no drop in adsorption capacity.

Abstract

Oxygen binding properties of a recent class of porphyrin-based porous organic polymers (POPs) were investigated. POPs containing iron (Fe-POP) and cobalt (Co-POP) were synthesized, characterized and evaluated. Adsorption studies under ambient and cryogenic conditions (91–303 K) were studied. Interestingly, these POPs showed selective adsorption of O2 over N2 of up to 1.3 at 273 K, and most significantly, the oxygen uptake was found to be stable and reversible over the range of temperatures studied. Furthermore, the polymers also exhibited a molecular sieving effect, which showed increasing nitrogen exclusion between 110 K and 160 K and led to an extraordinary O2:N2 selectivity of up to 14.7 at 91 K. The high oxygen loading (13 wt% at 1 atm) together with the high selectivity suggests great potential for use of these materials in hybrid adsorption–cryogenic air separation systems.

Introduction

Naturally occurring metal complexes, such as haemoglobin and myoglobin, play an important role in the transportation of oxygen in biological systems. Transition metal complexes of porphyrin, cyclam and phthalocyanine, have previously been studied in efforts to mimic the oxygen binding capabilities of these molecules [1], [2], [3], [4]. Early efforts in the development of oxygen binding porphyrin structures concentrated on the effects of employing different functional groups attached to the porphyrin ring on the oxygen binding mechanism [2]. This was achieved through various methods such as immobilization of the porphyrin on supports [5], [6], [7], [8], [9], the use of ‘picket fence’ porphyrins [10], [11], capped porphyrins [12], [13], [14], [15] and other variations [16], [17], [18], [19]. Although these porphyrin materials showed selective adsorption for oxygen, their oxygen capacities were inherently low as they were non-porous. Nevertheless, those studies provided an insight into the chemistry of oxygen binding.

More recently, synthetic metalloporphyrins have been more widely studied for other applications, such as in catalysis and therapeutics [20], [21], [22]. Tailoring of these molecules, via incorporation of different central metal ions and porphyrin ring linkages, can provide a wide range of functionalities and physical properties, which can potentially be used in selective gas adsorption and separation applications.

In 1994, Abrahams et al. were the first to utilize porphyrins as building blocks to construct a 3D crystal framework structure [23]. Although the crystallinity of the structure was lost on solvent removal, the engineering of porphyrin-based structures was demonstrated. Kosal et al. reported the utilization of a functionalized porous porphyrin polymer, PIZA-1, which exhibited size and shape-selective adsorption characteristics [24]. Adsorption on a range of organic solvents, such as amines and alcohols, as well as water was reported.

Feng et al. synthesized organic frameworks with Zr nodes and metalloporphyrin ligands [25]. A high surface area and pore volume metalloporphyrin metal organic framework (MOF), PCN-222(Fe), was reported [25]. The presence of Zr ions was also shown to contribute to the high stability of the MOF [25]. However, only nitrogen adsorption experiments at 77 K were reported [25]. Wang et al. synthesized a range of Ni-porphyrin based porous polymers using various covalent linkages [26]. Polymers with surface areas of 778–1711 m2/g were obtained [26]. The hydrogen, methane and carbon dioxide gas adsorption isotherms were measured, and a CO2/N2 selectivity as high as 19 was reported [26]. Modak et al. synthesized a Fe-porphyrin based organic polymer linked via covalent bonding with several aromatic di-aldehydes [27]. The polymers reported showed a high surface area (875 m2/g) and a CO2 capacity of 19 wt% at 273 K and 1 bar [27].

Fateeva et al. investigated the effect of incorporating several metal ions in the porphyrin framework of MIL-141 and its effect on the O2/N2 selectivity [28]. An oxygen selectivity of up to 1.4 at 303 K and 4 bar was reported for these porphyrin structures. Low pressure data on this material was not reported. Ma et al. reported a metalloporphyrin framework polyhedron which showed oxygen gas and nitrogen gas capacity of 45 cm3/g and 5 cm3/g respectively at 77 K [29]. The high selectivity towards oxygen was attributed to the pore size of the material (∼3.5 Å).

Of all the transition metals, complexes of iron and cobalt have been the most studied for oxygen binding applications. Other transition metals complexes which had shown oxygen binding ability included complexes of manganese [30], [31], copper [32], nickel [33], and chromium [34]. As iron can be found in haemoglobin for the transportation of oxygen gas, the use of iron in metal complexes for oxygen binding has been extensive. In these molecules, the iron atom exists in a 5-coordinate Fe2+, and forms a Fe3+O2− complex upon oxygenation. The control of this oxygenation reaction reversibly is the important step in developing oxygen selective adsorbents. In order to replicate the conditions present in haemoglobin, large functional group chains were used to sterically hinder the oxygen molecules and ensure that they approach the Fe-porphyrin ring in the preferred orientation. These functionalized porphyrin complexes were shown to reversibly bind oxygen at ambient conditions [13]. Recently, a MOF with Fe metal centre, Fe2(dobdc).4DMF, was reported to show oxygen selectivity at 25 °C [35].

Cobalt Schiff bases, such as Co-salen and Co-fluomine, had been extensively studied for their oxygen affinity at ambient temperatures. Several studies also attempted to incorporate these complexes into porous materials to improve their stability, and increase the specific surface area for adsorption or catalysis applications [36], [37]. There are two types of cobalt Schiff bases: Type A complexes can adsorb up to 0.5 mol of O2/mol of cobalt; and Type B complexes can adsorb up to 1 mol of O2/mol of cobalt [1]. The difference between the two types is in the coordination number of the cobalt metal centre. Cobalt complexes with cyanide ligands were also shown to exhibit oxygen selectivity, however, it was suggested that these complexes may be susceptible to moisture [38], [39], [40]. A MOF employing cobalt metal centres, [(Co2bpbp)2bdc](PF6)4, was also reported to show preferential adsorption of oxygen gas over nitrogen gas, with negligible nitrogen uptake [41].

In most of the studies on porphyrin-based materials, oxygen gas adsorption isotherms were not studied extensively as this was not often the focus of the work. In the present work, organic framework polymers made up of porphyrin rings linked via terephthalaldehyde groups were synthesized with Fe and Co as the central metal ion in the porphyrin ring. Iron and cobalt ions were employed because of their binding affinity towards oxygen molecules, with a view to establishing the applicability of these materials to air separation. The polymers were characterized and their oxygen and nitrogen isotherms measured over a range of temperatures in order to deduce separation feasibility and thermodynamic information. The effect of metal loading on the adsorption properties of these materials was also investigated.

Section snippets

Materials

Pyrrole (98%), terephthalaldehyde (99%), and cobalt(II) chloride hexahydrate (98%) were purchased from Sigma–Aldrich. Ferrous chloride tetrahydrate (99.0%) and ferric chloride anhydrous (98.0%) were purchased from Merck. Glacial acetic acid (100%), methanol (99.8%), acetone (99.5%), tetrahydrofuran (THF) (99.7%) and chloroform (99.8%) were purchased from Chemsupply. The metal chlorides were heat treated to remove the water of crystallization where possible, while all other reagents were used as

SEM

Powder XRD analysis of the POPs revealed that all the polymers synthesized were amorphous, similar to those reported in the literature [27]. SEM images of these POPs (Fig. 1a–f) showed the formation of fairly uniform, solid, globular particles with sizes between 1 and 2 μm for Co-POPs and between 0.3 and 0.6 μm for Fe-POPs. It was noted that the fb-POP, Fe-POP-O, and the Co-POPs exhibited a more uniform particle size distribution, while the sizes of the Fe-POP-L and Fe-POP-H particles were more

Conclusions

Porphyrin porous organic polymers with iron and cobalt ions as central metal ions in the porphyrin rings were synthesized. The POPs showed selectivity for oxygen over nitrogen (1.3) at ambient temperatures. The oxygen selectivity increases with increased cobalt loading, which was further supported by the difference in the oxygen heat of adsorption calculated for the Co-POP-H (20 kJ/mol) and fb-POP (16 kJ/mol). Furthermore, the constant volume LDF mass transfer coefficient also indicated a

Acknowledgements

The authors acknowledge the funding provided by the Australian Government through its CRC program to support this CO2CRC research project.

The authors wish to acknowledge financial assistance provided through Australian National Low Emissions Coal Research and Development (ANLEC R&D) for project number 3-0510-0044. ANLEC R&D is supported by Australian Coal Association Low Emissions Technology Limited and the Australian Government through the Clean Energy Initiative.

The authors gratefully

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