The conversion of ZIF-8 into ZIF-7 via post-synthetic modification with benzimidazole has been monitored by quantifying the liberated 2-methylimidazole by chromatography. The reaction kinetics have been adjusted to the shrinking core model, providing the diffusion coefficient of bIm inside the pores and the reaction kinetic constant (2.86 × 10-7 cm2 s-1 and 1.36 × 10-4 cm s-1, respectively). A wide variety of ZIF-7/8 hybrid core-shell frameworks have been obtained during this reaction. The most promising have been characterized by SEM/TEM, TGA, N2 and CO2 adsorption, FTIR and 13C NMR, showing features of the coexistence of both phases inside the frameworks. Their structures have also been simulated, providing comparable XRD and adsorption results. The hybrid material has been used as a filler for PBI mixed matrix membranes (MMMs) applied to H2/CO2 separation, enhancing the performances of the bare PBI polymer and MMMs containing ZIF-8 or ZIF-7 as a filler, with a maximum H2 permeability value of 1921 Barrer and a H2/CO2 selectivity of 11.8.