Suppressing Defect Formation in Metal–Organic Framework Membranes via Plasma-Assisted Synthesis for Gas Separations

Abstract

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 .

X-ray Diffraction and Molecular Simulations in the Study of Metal–Organic Frameworks for Membrane Gas Separation

“…Copyright 2019 American Chemical Society. (b–d) were reproduced with permission from ref . Copyright 2019 American Chemical Society.…”

“…In a subsequent study, Lin and Kang employed highly detailed molecular simulations to probe the effects of linker rotation on N 2 and CH 4 transport behavior in CPO-8-BPY . In gas permeation tests, they found that a CPO-8-BPY membrane grown on an alumina disk was selective for N 2 over CH 4 (selectivity = 5.4; Figure b); however, permselectivity estimates based on the assumption of a rigid structure were close to unity.…”

“…The challenges involved in developing MOF membranes for gas separation include the fabrication of defect-free polycrystalline MOF membranes or MOF-containing mixed-matrix membranes (MMMs) while maintaining precise control over gas permeation properties. Defect-free MOF membranes can be fabricated using the conventional seeded growth method, electrochemical deposition, plasma-assisted synthesis, and interfacial synthesis. − The latest methods have all but eliminated defects and made it possible to manufacture MOFs on inexpensive and scalable substrates, such as polymer hollow fibers. Gas permeation properties and the structural deformation of MOF membranes have been investigated using experimental and computational methods, including structural characterization on MOFs in the presence of specific gases (CO 2 , N 2 , CH 4 , C 3 H 6 , and C 3 H 8 ).…”

Seed assisted synthesis of an anionic metal–organic framework membrane for selective and permeable hydrogen separation

“…19,20 In recent years, some MOF-based membranes have been assessed in the recovery of hydrogen. 10,21–54 For instance, the first MOF membrane of Cu 3 (BTC) 2 with open copper sites was fabricated for hydrogen separation. 21 Later on, ZIF-7 with a small pore aperture of 3.0 Å was synthesized for selective permeation of hydrogen from other gases.…”

Vapor-Phase Adsorption of Xylene Isomers and Ethylbenzene in MOF-74 Thin Films

“…Despite the plethora of fundamental studies, the practical application of MOFs is limited as they are typically obtained as polycrystalline micropowders with very low processability . MOF thin films have the potential to significantly advance energy, catalysis, separations, and chemical/biological sensing applications as they offer possibilities otherwise not feasible with traditional bulk powders. − Therefore, the need for innovative methods to afford MOF thin films that improve the scalability, quality, and generality is necessary. − Indeed, several synthetic techniques have been developed to afford MOF thin films; however, there remain scientific challenges. − As in the case of zeolites, the extrinsic properties such as crystal orientation and microstructure (i.e., film thickness, roughness, particle distribution) may ultimately play a decisive role in the overall performance. − The scientific literature on the study of MOFs as bulk powders is insightful in identifying MOF candidates with significant promise for targeted applications, including kinetic separations . To evaluate the functionality of a thin film, an optical, electrical, or physical signal transduction mechanism is required. − Integration of MOF thin films on a solid surface offers a physical interface for such signal transduction .…”