ZIF-8 membranes, a type of zeolite imidazolate framework, were synthesized by secondary seeded growth on tubular alpha-Al2O3 porous supports. The presence of small, highly crystalline, microporous crystals with narrow particle size distribution led to continuous thin membranes. The synthesized novel ZIF-8 membranes displayed unprecedented high CO2 permeances and relatively high separation indexes for equimolar mixtures of CO2 and CH4.