Activation-Controlled Structure Deformation of Pillared-Bilayer Metal–Organic Framework Membranes for Gas Separations

Abstract

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.

Highly CO2 Selective Metal–Organic Framework Membranes with Favorable Coulombic Effect

“…This intensity reduction is an indication of gas adsorption, wherein the gas molecules occupying the pores decreased the overall contrast associated with the density of electrons from the MOF structure. [ 21 ] The intensity changes in XRD patterns obtained under N 2 and CH 4 were not as pronounced as those obtained under CO 2 (Figure S4, Supporting Information). These in situ XRD measurements suggest that the uptake of CO 2 in CAU‐10‐H exceeded the uptake of N 2 and CH 4 .…”

“…The setup was detailed in the authors’ previous work. [ 21 ] Prior to obtaining measurements, the membrane samples were placed in a sealed cell and degassed under vacuum (<0.05 torr) at room temperature for 24 h.…”

Thin Film Growth of 3D Sr‐based Metal‐Organic Framework on Conductive Glass via Electrochemical Deposition

“…Due to the wide variety of applicative measures of thin film of MOFs, different deposition methodologies are employed when different kinds of substrates are being used such as alumina, silicon wafers, Ag/Au nanosheets, copper foil, and glass slides [12–17] . These reported methods are only successful for the deposition of MOFs on specific type of surface‐modified and non‐conducting substrates [18–20] .…”

Efficient and Accurate Charge Assignments via a Multilayer Connectivity-Based Atom Contribution (m-CBAC) Approach

“…A fast-growing class of nanoporous materials, metal–organic frameworks (MOFs), has shown versatile properties such as large surface area, selective adsorption, low heat capacity, and high tunability, which have made them attractive candidates for a variety of applications including gas adsorption and storage, − catalysis, − pharmaceuticals, , energy storage, , and gas purification. − To date, tens of thousands MOFs have been reported and more than hundreds of thousands have also been hypothetically predicted. − Considering such large material space, molecular simulation techniques can play an important role in facilitating the exploration of promising candidates, given various adsorptive and diffusive properties of small molecules in the materials can be efficiently estimated …”