We investigated gas adsorption and diffusion in an emerging pillared-bilayer metal−organic framework (MOF), Zn-AIP-AZPY (aip, 5-aminoisophthalic acid; azpy, 4,4′-azobipyridine). This work demonstrated that the crystal structure of this flexible MOF could be controlled by two different activation methods: the thermal activation and the chemical activation. The pore limiting diameter of the thermally activated compound was ∼3.7 Å, whereas that of the fully activated one via methanol extraction was 2.92 Å. Although the aperture size of the fully activated structure was smaller than the kinetic diameter of CO 2 and CH 4 , Zn-AIP-AZPY still presented fair adsorption quantity to these two gases. In situ XRD experiments under various gases at different pressures also suggest a minor breathing effect when Zn-AIP-AZPY was exposed to CO 2 and no breathing effect under CH 4 . These results imply the possible ligand rotation occurring during the adsorption of CO 2 and CH 4 in Zn-AIP-AZPY. The Zn-AIP-AZPY membranes were further prepared using the seeded growth method, and these membranes showed an exceptionally high H 2 permeability (over 10 5 barrer) with a good H 2 /CO 2 selectivity (ideal selectivity beyond 8). A reverse CO 2 /CH 4 selectivity, i.e., CH 4 permeates faster than CO 2 , was surprisingly found. Monte Carlo simulations were conducted for probing the adsorption properties of CO 2 and CH 4 , as well as their adsorption energy landscape in Zn-AIP-AZPY. It was found that this MOF energetically favored the adsorption of CO 2 over CH 4 by nearly 10 kJ/mol. Such a strong adsorption is anticipated to create high energy barriers for CO 2 hopping between neighboring adsorption sites.
Metal−organic frameworks (MOFs) are considered as promising materials for membrane gas separations. Structural defects within a pure MOF membrane can considerably reduce its selectivity and possibly result in a nonselective separation. This work proposes a solution-phase synthesis with dielectric barrier discharge (DBD) plasma to suppress the formation of defects in the pure MOF membrane of CPO-8-BPY. Through comprehensive solid-state characterization with XRD, SEM, XPS, solid-state NMR, and XAFS, DBD plasma is demonstrated to facilitate deprotonation in the H 2 aip linker, which leads to a smaller and more uniform particle size of CPO-8-BPY. The narrow grain size distribution effectively reduces the pinhole-type defects in the pure CPO-8-BPY membrane and endows it with good ideal selectivity for H 2 /CH 4 (α H 2 /CH 4 = 28.2) and N 2 /CH 4 (α N 2 /CH 4 = 5.4). The selectivity for H 2 /CH 4 of this membrane from a mixed-gas permeation test is found to be 15.4. Molecular simulations are also performed to gain insights into the gas transport properties of this MOF. The results suggest that ligand rotation plays an important role in CPO-8-BPY when being applied to the membrane separation of N 2 /CH 4 .
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.