(40s) Enhance the Water Stability in Cr-BTC Metal-Organic Frameworks

Intense interest in the porous materials for selective adsorption processes. Metal organic frameworks (MOFs) attracting attention over the recent years, which can be manipulated to target specific adsorption behavior by introducing open metal sites and functional groups into the structure. Nowdays, MOF behavior in the presence of water is a topic of significant importance when considering these materials for adsorption applications. Among the water sensitive MOFs, Cr-BTC (Cr₃(1,3,5-benzenetricarboxylate)₂, with a same topological structure as HKUST-1) is a very typical material which can highly-selective and reversible binding O₂ by the open Cr²⁺sites, however, in our study, which cannot survival in the open air even if only trace of water molecules exist. How to prevent the invasion of water molecules, and keep the structure of the Cr-BTC is the focus of our research.

  In this work, the original samples Cr-BTC was synthesized from literature report: Cr(CO)₆, 1,3,5-benzenetricarboxylate acid and anhydrous, air-free N,N-dimethylformamide (DMF) under a dry nitrogen atmosphere. O₂ adsorption volume reach to 14 wt% (very close to the theoretical value 14.4%), but we cannot found any diffraction peaks from the XRD pattern of binding O₂ Cr-BTC sample exposed to the open air, that means the structure was destroyed in the air condition. We suspect the water molecular in the air is the only killer, so the water vapor adsorption was tested under 298 K and 3.16 KPa, and then exposed the sample in the oxygen environment for 1 h. The experiment result showed BET surface of Cr-BTC was decreased dramatically from 1369 m²/g to 51 m²/g, and there is no diffraction peaks from the XRD pattern.

  Surprisingly, when the no O₂ binding Cr-BTC was placed in saturation ethanol (C₂H₅OH) and ethyl bromide (C₂H₅Br) vapor environments separately for 12 h, we can found the XRD diffraction peaks after the two samples exposed in the open air for 1 h, and then they could keep high BET surface area. We also verified this result by DFT and GCMC simulation.