The separation of a propylene/propane mixture into its constituents by distillation is recognized to be one of the most energy-consuming processes in the petrochemical industry1),2). Membrane separation processes are attractive alternatives to distillation as they have the potential to significantly reduce energy consumption. Recently, zeolitic imidazolate framework-8 (ZIF-8) has gained focus as one of the most promising membrane materials for propylene/propane separation3). ZIF-8 is composed of zinc atoms with 2-methylimidazole as a ligand; this combination forms a sodalite zeolite structure with large cavities connected through a small pore aperture4). Zhang et al. reported that the effective aperture size of ZIF-8 for molecular sieve application was between 0.40 nm and 0.42 nm, which corresponds to the van der Waals diameter of propylene and propane, respectively5). Consequently, ZIF-8 has gained recognition as a promising material for the separation of propylene/propane based on the difference in the molecular size of these two species. ZIF-8 membranes are conventionally prepared by the in-situ growth method6)~10), secondary growth method11)~18), and the counter diffusion method19)~21). Pan et al. first reported the effective separation of propylene/propane with ZIF-8 membranes using the secondary growth method, achieving a selectivity of up to 50 for an equimolar mixture of propylene/propane22). Subsequently, Kwon and Jeong15) and Liu et al.18) also reported ZIF-8 membranes with a comparable propylene/propane separation performance. Recently, the counter diffusion approach, which allows for preparation of ZIF-8 selective layers with fewer defects via a simple protocol, has gained prominence. In this preparation, 2-methylimidazole and zinc ions interdiffuse at 237 Journal of the Japan Petroleum Institute, 58, (4), 237-244 (2015)