Abstract
Noble gas separation by microporous materials is a promising alternative to energy-intensive cryogenic distillation method by reducing the separation cost; however, developing novel microporous materials with excellent noble gas separation performance is still challenging due to closing chemical and physical properties among the gases. In this study, we propose to separate the noble gases (He, Ne, Ar, Kr and Xe) utilizing a metal organic framework (MOF), named SIFSIX-3-Zn, with ultra-micron sized 1-dimenssional (1D) channels (3.84 Å). Density functional theory (DFT) calculations reveal that the 1D channels provide significant adsorption potential differences among the noble gas molecules in various sizes: the larger the molecular size, the stronger the adsorption potential. Grand canonical Monte Carlo (GCMC) simulations verify that the MOF exhibits exceptional equilibrium separation performance of noble gases. Remarkably, Xe/He and Xe/Ne adsorption selectivity can be as high as 645 and 596, respectively, at 298 K and 10 kPa. While Xe/Kr selectivity in mixed gas is around 12 with a Xe adsorption amount of about 2.27 mmol/g at 273 K and 100 kPa, making SIFSIX-3-Zn one of the promising materials for equilibrium separation of Xe/Kr mixtures.
Highlights
Noble gases, i.e. helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe), play vital roles in our daily lives, ranging from lighting and medicine to cryogenic refrigerants [1]
We demonstrate that the van der Waals (vdW) potentials in the 1D channels of SIFSIX-3-Zn vary from noble gas molecules in different sizes, resulting in dramatic differences on gas adsorption abilities in the metal organic framework (MOF) for effective noble gas separation
The 1D channel exhibits overall smooth inner surface with some variations induced by the surface atoms in different atomic radius, which are distinguished by Site-1 nearby the pyz ligands and Site-2 nearby the SiF62− ions (Fig. 1b)
Summary
I.e. helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe), play vital roles in our daily lives, ranging from lighting and medicine to cryogenic refrigerants [1]. One of the promising approaches for low-cost noble gas separation is physisorption onto microporous materials, such as activated carbons (ACs) [3], zeolites [4, 5] and metal organic frameworks (MOFs) [6,7,8,9,10,11]; achieving distinguishable equilibrium adsorption abilities among the noble gases for separation is still challenging. MOFs with open metal sites at the pore surface have shown excellent performance on Xe/Kr. The adsorption potential varies from molecules in various sizes and porous materials, e.g. MOFs, with thousands of different topologies [13]. Non-uniform sized pores in a material with intersecting cages (Fig. 1a) usually introduce diverse adsorption potentials to molecules, a drawback to achieve high sorption ability at low pressure range if no functional group exist [13]. Porous materials with uniform sized 1-dimensional (1D) channels (Fig. 1b) are attractive
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