Molecular simulation of adsorption and diffusion of CH4 and H2O in flexible metal-organic framework ZIF-8

•Quantify the spatial distribution of H2O in ZIF-8 cavity and intergranular space.•Quantify CH4 absorption in both dry and pre-humidified ZIF-8 by GCMC simulation.•H2O occupies the preferential adsorption sites of CH4 and reduce adsorption volume.•CH4 has no effect on the structure of H2O, while H2O makes CH4 together more tightly.•The presence of H2O facilitates the diffusion of CH4 at a high loading of CH4.