In SIFSIX-3-Zn, the zinc (II) center is octahedrally coordinated to the four nitrogen atoms of the pyz ligands as well as the two SiF62− ions (Fig. 1a). In the equatorial plane, the pyz ligands bridge the zinc ions to produce grids, while the SiF62− groups are coordinated axially and bridge zinc ions, producing an open three-dimensional framework. All pyz planes are parallel to the c-axis to produce 1D channel, as presented by the blue tubular surface using solvent surface with a radius of 2 Å (Fig. 1b). 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).
Density functional theory (DFT) calculations were performed to derive the adsorption potentials of the noble gases inside the channel of SIFSIX-3-Zn, as shown in Fig. 2. Details of the DFT calculations can be found in the Methods. As shown in Fig. 2a and b, the potentials at the two different sites, i.e. Site-1 and Site-2 (Fig. 1b), in the channel are different from each other for all gases. Specifically, the potentials at site-2 surrounded by four pyz linkers are much stronger than that of site-1 surrounded by four SiF62− ions, indicating that the binding of the noble gases are dominated by vdW interactions, rather than electrostatic interactions with the basic SiF62− ions. This agrees well the estimation that strong adsorption/curvature potentials are induced when the molecular size closing to the channel size (3.84 Å), especially for Xe (3.9 Å). The negligible effect of the basic SiF62− ions on enhancing the adsorption of noble gases is different from the cases of CO2 and C2H2 adsorption, in which the SiF62− ions significantly enhance the adsorption due to electrostatic interactions. Moreover, the potential energy for the gas molecules show some variations along the channel direction from Site-1 to Site-2 (Fig. 2c) since the channel surface is formed by different atoms (F, C, N and H) with different atomic radius.
The potential curves of He and Ne are different from those of Ar, Kr and Xe because the vdW potentials have no overlap for the smaller molecules (He and Xe) but strong overlap for the bigger molecules (Ar, Kr and Xe), which is further enhanced with the molecular size increases. The highest potential energy of Ar, Kr and Xe at Site-2 are − 25 kJ/mol, − 40 kJ/mol and − 60 kJ/mol, respectively and therefore results in dramatic differences on gas adsorption behaviors in the MOF due to the significant differences on potential energy.
As shown in Fig. 3, grand canonical Monte Carlo (GCMC) simulations confirms that the designated high adsorption selectivity for noble gases is achieved by utilizing the potential energy differences. First of all, the adsorption abilities of the pure noble gases in SIFSIX-3-Zn follows the order of Xe > Kr > Ar > Ne ≈ He (Fig. 3a), agreeing well with the decreasing order of molecular size, i.e. 3.9 Å (Xe), 3.6 Å (Kr), 3.4 Å (Ar), 2.75 Å (Ne) and 2.6 Å (He) (Fig. 3b) as well as the adsorption potentials revealed by DFT calculations (Fig. 2). The isosteric heats of the gases show the same trend (Fig. 3c-d). Remarkably, the adsorption amount of Xe in SIFSIX-3-Zn is about 3.08 mmol/g at 298 K and 100 kPa, a much higher value than those of other gases, i.e. Kr (1.30 mmol/g), Ar (0.54 mmol/g), Ne and He (0.02 mmol/g). Promising adsorption selectivity for Ar/He, Ar/Ne, Kr/He, Kr/Ne, Kr/Ar, Xe/He, Xe/Ne, Xe/Ar and Xe/Kr based on the pure gas adsorption data are found in SIFSIX-3-Zn (Fig. 4). Remarkably, the adsorption selectivity is as high as 645 and 596 for Xe/He and Xe/Ne gas pairs at 298 K and 10 kPa, respectively (Fig. 4a), indicating the SIFSIX-3-Zn with 3.84 Å 1D channels is promising for equilibrium separation of noble gases.
To further elucidate the effects of the curvature potential in the 1D channels of SIFSIX-3-Zn on equilibrium separation of noble gases at different conditions, we performed detailed studies on Xe/Kr separation, one of the most challenging topics in gas separation. Figure 5 shows pure gas adsorption behaviors of Xe and Kr at different temperatures (273 K, 298 K and 313 K) and pressures up to 1000 kPa. The adsorption amounts of both gases increase with the decrease of temperature with a remarkable adsorption amount of Xe of 2.90 mmol/g at 298 K and 100 kPa (Fig. 5a), approximately 73% of the saturation uptake amount of SIFSIX-3-Zn. As results, increasing the pressure from 100 kPa to 1000 kPa shows little improvement on the adsorption amount of Xe due to the limited sites left for sorption, whereas Kr adsorption is dramatically improved (Fig. 5b). Subsequently, the Xe/Kr sorption selectivity based on pure gas sorption data with equal molar concentration (50/50) decreases with the increase of pressure (Fig. 6a); however, Xe/Kr mixture adsorption studies simulating the realistic conditions for Xe/Kr separation interestingly shows no loss of separation performance with pressure change (Fig. 6a). This clearly shows that the strongly adsorbed Xe molecules can always preferentially occupy the sorption sites in SIFSIX-3-Zn and inhibit the adsorption of less favorable Kr molecules, regardless the molar concentration of Kr in the mixture e.g. 20/80 or 50/50 (Fig. 6a). Moreover, lowering the sorption temperature from 298 K to 273 K in mixed gas with a molar concentration of 20/80 (Xe/Kr) can dramatically enhance the Xe adsorption and Xe/Kr selectivity (Fig. 6a), again, owing to the favorable adsorption of Xe over Kr in the SIFSIX-3-Zn framework (Fig. 6b). Overall, the well-maintained adsorption selectivity in mixed gas conditions demonstrates that SIFSIX-3-Zn is a promising material for Xe/Kr separation.
To evaluate the potential of the identified MOF with strong curvature potential, the separation efficiencies (Xe/Kr selectivity vs Xe adsorption amount) of SIFSIX-3-Zn in mixed gas conditions (20/80) are further compared with other MOFs (Fig. 7). At 298 K and 100 kPa, the Xe/Kr separation efficiency of SIFSIX-3-Zn is comparable to that of NiMOF-74, and greatly surpasses those of CuBTC and IRMOF-1. Remarkably, the Xe/Kr separation efficiency of SIFSIX-3-Zn overcomes all the porous materials reported by far by simply decreasing the sorption temperature from 298 K to 273 K, a much less challenging operation temperature comparing with cryogenic distillation method (<< 165 K). Therefore, equilibrium separation of Xe/Kr using SIFSIX-3-Zn at lower temperatures (e.g. sub-ambient temperature, but still much higher than that in cryogenic distillation conditions) could be a promising approach to balance the separation efficiency and energy input, maximizing the overall Xe/Kr separation efficiency. This is similar with the idea to separate CO2/CH4 using a hybrid separation process combining membrane with low temperature system; however, detailed studies involving capital investments evaluation and operation temperature optimization are required in future [18].