Zeolitic imidazolate frameworks (ZIFs) comprise a widely investigated subset of the metal organic framework family. Their potential applications are based on the ability of MOFs to accommodate different gas molecules within the voids in their crystal structure. ZIFs are usually synthesized by precipitation from the solution resulting in the crystals acceptable for the single-crystal diffraction. However, these crystals normally contain solvent molecules in the framework voids, while for the practical purposes it is more important to have solvent-free materials. Desolvation often leads to the large volume change so that the solvent-free ZIFs can be only obtained in the powder form. The same holds for the investigation of the gas sorption in these materials. In situ gas cell available at the MSPD beamline of the ALBA synchrotron allows investigation of the powdered materials in controlled atmosphere in the pressure range from 0 to 50 bars in the temperature range from 80 to 1200K. Combined with high quality of the synchrotron powder diffraction data, it was possible to solve the crystal structure of the various ZIFs including positions and amount of the absorbed gas molecules. While typical result of the gas sorption is simply minor expansion of the unit cell, in certain cases this process results in significant structure modifications. Here we present two examples of these investigations, namely sorption of alkanes in ZIF-8 and structural transformations caused by the CO2 sorption in ZIF-62. In case of ZIF-8 it was found that alkane molecules do not fill the structure void completely. The diffraction data obtained for the propane sorption reveal the location of the adsorbed molecules in the vertices of the tetrahedron within the structure void. CO2 sorption by ZIF-65 causes a series of transformations from hexagonal to cubic and then to orthorhombic structure. Out of these three structures only cubic one was described before, while two others were solved from the powder diffraction data for the first time.

In situ investigation of the gas sorption in the ZIF-type materials.

ACS Spring Meeting 2021

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