To break through the permeability-selectivity trade-off in solution-diffusion membranes, novel three-dimensional hollow zeolitic imidazolate framework (ZIF) nanoparticles like open cocoons were built and then in-situ embedded in cross-linked poly(ethylene oxide) backbone by a simple one-step untraviolet polymerization. Those nanoparticles, which were evenly distributed in the polymer, acted as a "CO2 transport promoter" and developed low-resistance inner channels for CO2 Knudsen diffusion in the solution-diffusion membrane. Simultaneously, embedded nanoparticles broke the crosslinking of poly(ethylene oxide), thereby reducing the crystallinity of the polymer and then increasing the flexibility of polymer chains. Therefore, adding open-cocoon ZIF nanoparticles exponentially increased CO2 permeability with small changes in CO2/H-2, CO2/CH4 and CO2/N-2 selectivity. Specifically, adding 10 wt% open-cocoon ZIF nanoparticles in the polymer increased CO2 permeability by - 190%, from - 130 Barrers to - 373 Barrers, with a 15% increase in CO2/H-2 selectivity and a 10% decrease in CO2/N-2 selectivity.