The preparation of mixed matrix membranes (MMMs) for post-combustion CO2 capture and biogas upgrading from PIM-1 (10-90 wt%)/6FDA-DAM heterogeneous blends with ZIF-8 as filler (3-20 wt% with regard to the polymer blend) is described. The heterogeneity of the blend, considered here as an advantage for the dispersion of the filler within the MMMs, has been confirmed by the existence of a single glass transition temperature, consistent with that of 6FDA-DAM. The segregation between the two polymeric phases and the filler dispersion has been studied by Raman spectroscopy, SEM microscopy and EDX analysis. Increasing the amount of PIM-1 in the blend makes the d-spacing of the polymer chains higher and increases gas permeability. When embedding filler nanoparticles of ZIF-8, a better compatibility with 6FDA-DAM than with PIM-1 is observed. The filler locates near the interphase between polymers helping its dispersion. The use of small loadings of ZIF-8 enhances the gas separation performance of the MMMs in terms of permeability and selectivity for 50/50 CO2/CH4 and 10/90 CO2/N-2 mixtures. The best performing membranes are the blends PIM-1/6FDA-DAM 10/90 (w/w) with 10 wt% of ZIF-8, showing a CO2 permeability of 2891 Barrer, a CO2/CH4 selectivity of 26.6 and 2802 Barrer of CO2 with a CO2/N-2 selectivity of 18.1. Importantly, the CO2, N-2 and CH4 permeabilities of the pure polymeric blends can be predicted using both the logarithmic and the Maxwell models. A new coupled Maxwell model has also been developed able to calculate the flow through the blends containing ZIF-8 and to predict the gas separation properties of the filler itself.