Interfacial Synthesis of Free-Standing Metal–Organic Framework Membranes

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Abstract

A method based on a liquid–liquid interfacial coordination mechanism has been developed for the synthesis of free-standing metal–organic framework (MOF) membranes. MOF precursors, zinc nitrate [Zn(NO3)2] and terephthalic acid (TPA or H2BDC), as well as catalyst triethylamine (TEA), were dissolved in two immiscible solvents, dimethylformamide (DMF) and hexane. The reaction of Zn(NO3)2 and TPA (at a 2:1 molar ratio) in DMF was catalyzed by TEA in hexane at the solvent interface thus forming a free-standing membrane. A region of reactant concentrations critical to membrane formation was identified; a free-standing membrane could only be formed in the region of high Zn/TPA and low TEA concentrations. The combination of low Zn/TPA and high TEA concentrations yielded MOF particles that precipitated from DMF. The membranes were characterized by SEM and XRD and found to be asymmetric. SEM results showed that the top layer was particulate, whereas the bottom layer had a sheet-like morphology, which was further revealed by XRD data as 3D Zn4O(BDC)3 (also known as MOF-5) and 2D ZnBDC·DMF (MOF-2) for the top and bottom, respectively. Other transitional zinc–carboxylate structures between the two MOFs, namely Zn3(OH)2(BDC)2 (MOF-69c) and Zn5(OH)4(BDC)3, were also observed with small signal intensities revealing a transition of coordination modes between zinc ions and BDC groups from 3D to 2D at the membrane cross-section. This was caused by a change of the TEA diffusion rate during the synthesis process, which might change the pH and alter the membrane growth. From both SEM and XRD characterizations, MOF-2 is the dominant material in the overall composition. Nitrogen adsorption tests showed an average Langmuir surface area of 709 m2/g for the membrane, demonstrating its potential for gas separation applications.

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