Nano-gate opening pressures for the adsorption of isobutane, n-butane, propane, and propylene gases on bimetallic Co-Zn based zeolitic imidazolate frameworks

In this article, zeolitic-imidazolate framework-8 (ZIF-8) and its mixed metal CoZn-ZIF-8 were synthesized via a rapid microwave method. The products were characterized by Raman spectroscopy, XPS, XRD, EDX, TEM, NanoSEM, TGA, and DSC. The gas adsorption properties of samples were determined using C-3 and C-4 hydrocarbons, including propane, propylene, isobutane and n-butane at a temperature of 25 degrees C. The adsorption equilibrium and kinetics of these gases on various ZIFs were studied. It was noted that ZIF-8 and mixed metal CoZn-ZIF-8 samples start to adsorb these gases after certain pressures which are believed to result in the opening of their nano-gates (i.e., 6-membered rings) to allow the entry of gas molecules. The nanogate opening pressure value (p(0)) for each ZIF towards different gases was determined by fitting adsorption equilibrium data against a modified form of the Langmuir adsorption isotherm model. It was observed that the value of p(0) differs significantly for each gas and to various extents for various ZIFs. Therefore, it is possible that the distinct values of p(0) afford a unique technique to separate and purify these gases at the industrial scale. The overall mass transfer coefficient values of the adsorption process were also investigated.