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

Abstract

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 C₃ and C₄ hydrocarbons, including propane, propylene, isobutane and n-butane at a temperature of 25 °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 (p₀) 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 p₀ differs significantly for each gas and to various extents for various ZIFs. Therefore, it is possible that the distinct values of p₀ 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.