Noble gas separation by a MOF with one-dimensional channels
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.
KeywordsMOFs Noble gas Adsorption Separation
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 . Ne, Ar, Kr and Xe are commercially produced from air by gas liquefaction followed by cryogenic distillation based on the differences in the boiling points (e.g., 27 K for Ne, 87 K for Ar, 120 K for Kr, and 165 K for Xe) , which requires large amount of energy inputs. Therefore, developing novel approaches with lower energy inputs is promising to reduce the separation cost of noble gas production.
One of the promising approaches for low-cost noble gas separation is physisorption onto microporous materials, such as activated carbons (ACs) , zeolites [4, 5] and metal organic frameworks (MOFs) [6, 7, 8, 9, 10, 11]; however, achieving distinguishable equilibrium adsorption abilities among the noble gases for separation is still challenging. Recently, MOFs with open metal sites at the pore surface have shown excellent performance on Xe/Kr separation due to strong interactions between the Xe molecule and accessible open metal sites in the MOFs, indicating that interaction strength plays vital roles on noble gas separation . Furthermore, by screening 670,000 porous material structures for Xe/Kr separation, Simon et al. suggested that highly selective materials should exhibit pore sizes around that of a Xe atom (3.9 Å) to enhance the interaction strength of Xe . This agrees well with the fundamentals that the adsorption potential of molecules in a porous material can be significantly enhanced when the molecular size and pore size closing to each other due to the overlap of van der Waals (vdW) potentials that the molecule “feeling” from the opposite walls of the pore surface, as called “curvature potential” [13, 14].
Results and discussion
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).
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.
In conclusion, we have demonstrated that noble gases can be efficiently separated by the equilibrium separation method utilizing the curvature potential difference of the gas molecules in the well-chosen SIFSIX-3-Zn MOF with ultra-micron sized 1D channels. Future works will focus on further revealing the relationship between the separation efficiency and 1D channel sizes for different noble gas pairs, and screening more MOF materials with 1D channels for the application. Experimental verifications of the prediction results based on theoretical understandings and simulations are also required in future works.
Density functional theory calculations
The periodic boundary condition was considered in all calculations. The Becke exchange plus Lee-Yang-Parr correction (BLYP) exchange-correlation functional  with double numeric polarization (DNP) basis set  and DFT semicore pseudopots (DSPP) are used. A real-space orbital global cutoff of 4.0 Å was applied, and the convergence threshold parameters for the optimization were 2 × 10− 5 (energy), 4 × 10− 3 (gradient), and 5 × 10− 3 (displacement), respectively. To further improve the accuracy of the results, Grimme method for DFT-D approach, which is a semi-empirical combination of DFT approach with pairwise corrections, were used .
where ε0 is the permittivity of the vacuum, and σαβ and εαβ are the collision diameter and well depth, respectively. The LJ cross-interaction parameters were determined by the Lorentz-Berthelot mixing rules.
Grand canonical Monte Carlo (GCMC) simulation
A 8 × 8 × 8 unit cell system of SIFSIX-3-Zn with periodic boundary conditions applied in all three dimensions were constructed, and the framework was treated as rigid with atoms frozen at their crystallographic positions during GCMC simulations. As previously described, the cutoff radius for the LJ interactions was set to 12 Å. For each state point, GCMC simulation consists of 1.0 × 107 steps to guarantee the equilibration, followed by additional 1.0 × 107 steps to sample the desired thermodynamics properties. The following equation was used to define the selectivity for component A relative to component B: S = (αA/αB)(βA/βB), where α and β are the molar fractions of the components in the adsorbed and gas phases, respectively.
This work is supported by National Natural Science Foundation of China (51676079). The funders had no role in study design, data collection and analysis, interpretation of data, or preparation of the manuscript.
Availability of data and materials
YL performed the DFT and GCMC simulations. YL, JL and JH discussed the findings in this paper. YL coordinated the writing of the paper, and all authors contributed to revising the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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