Ultrahigh Size Exclusion Selectivity for Carbon Dioxide from Nitrogen/Methane in an Ultramicroporous Metal–Organic Framework

Abstract

Separations based on molecular size (molecular sieving) are a solution for environmental remediation. We have synthesized and characterized two new metal–organic frameworks (MOFs) (Zn 2 M; M = Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. We explore the synthesis of these MOFs and the role that the DMSO/H2O/DMF solvent mixture has on the crystallization process. We further explore the crystallographic data for the DMSO and methanol solvated structures at 273 and 100 K; this not only results in high-quality structural data but also allows us to better understand the structural features at temperatures around the gas adsorption experiments. Structurally, the main difference between the two MOFs is that the central metal in the trimetallic node can be changed from Zn to Cd and that results in a sub-Å change in the size of the pore aperture, but a stark change in the gas adsorption properties. The separation selectivity of the MOF when M = Zn is infinite given the pore aperture of the MOF can accommodate CO2 while N2 and/or CH4 is excluded from entering the pore. Furthermore, due to the size exclusion behavior, the MOF has an adsorption selectivity of 4800:1 CO2/N2 and 5 × 1028:1 CO2/CH4. When M = Cd, the pore aperture of the MOF increases slightly, allowing N2 and CH4 to enter the pore, resulting in a 27.5:1 and a 10.5:1 adsorption selectivity, respectively; this is akin to UiO-66, a MOF that is not able to function as a molecular sieve for these gases. The data delineate how subtle sub-Å changes to the pore aperture of a framework can drastically affect both the adsorption selectivity and separation selectivity.

Cold, Hot, Dry, and Wet: Locations and Dynamics of CO2 and H2O Co-Adsorbed in an Ultramicroporous MOF

“…The adsorption of water molecules proximate to MOF metal centers can trigger a distortion of the molecular structure or a change in CO 2 adsorption capacity. , 67 Zn NMR can probe the influence of water upon the local Zn environment. The only NMR-active isotope of zinc is the quadrupolar spin 5/2 67 Zn nucleus, which has a significant quadrupole moment of 122(10) mb, low gyromagnetic ratio (1.68 rad·T –1 ·s –1 , 6.3% that of 1 H), and poor natural abundance of 4.1%. − Consequently, 67 Zn NMR experiments on MOFs are challenging and uncommon. ,,− The 67 Zn static and MAS NMR spectra of as-made and activated ZnAtzOx at 21.1 T are depicted in Figure , with additional data in Figures S14 and S15. Experimental and calculated 67 Zn NMR parameters are summarized in Table S8.…”

Incorporating Microporous Zn3 and Zn2Cd MOFs into Pebax/PVDF Mixed Matrix Membranes for Improved Carbon Dioxide Separation Performance

“…This is expected given the ca. two-fold increase in adsorption capacity of Zn 2 Cd vs Zn 3 . For PVDF-based MMMs, the two MOFs are able to increase the adsorption capacity, but there is little difference in the adsorption capacity between the MOFs.…”

“…Carbon capture can be accomplished by utilizing materials/systems that excel at separating carbon dioxide from a complex mixture, storing carbon dioxide, or simultaneously separating and storing carbon dioxide from a complex mixture. ,− Materials that focus solely on separating carbon dioxide (i.e., kinetic-based separations) require that carbon dioxide–which has the smallest kinetic diameter between nitrogen, oxygen, and carbon dioxide–traverses a material faster than the other gases in a matrix. − Kinetic separations have several advantages. For DAC, the partial pressure of carbon dioxide is low (ca.…”

“…Given the importance of carbon capture technology and the benefits of kinetic-based separations, the design of materials capable of kinetically separating carbon dioxide from a complex mixture is of great interest. With that in mind we have been working on porous materials, specifically metal–organic frameworks (MOFs), whose pore apertures are designed for kinetic separations . MOFs consist of metal ions interconnected by organic linkers.…”

Single‐Crystal to Single‐Crystal Transformations: Stepwise CO2 Insertions into Bridging Hydrides of [(NHC)CuH]2 Complexes

“…However, the volumes of the crystal unit cells increased by only 2.3 % [1b] . Analysis of the void space of 1 using an estimated probe radius of 1.7 Å for CO 2 did not show sufficient space in the crystal lattice to readily accommodate CO 2 [31] (Figure S5). This result suggests a complex pathway by which CO 2 diffuses within the crystalline lattice towards the [Cu 2 H 2 ] core for reaction.…”