The evolution of nano- and micro-sized ZIF-11 (nZIF-11 and ZIF-11, respectively) when embedded into a PBI polymeric matrix is studied. The prepared membranes, with loadings up to 55 wt%, have been characterized through several techniques (XRD, SEM, FTIR, TGA, ¹³C NMR and XPS) and the changes in the morphology of the fillers upon combination with PBI, as well as in the chemical environment of their main atoms (interactions between the linker of the filler and the benzyl rings of the polymeric bIm units) are discussed. All the membranes have been tested at temperatures ranging between 70 and 200 °C to study their H₂/CO₂ separation performance. The integration of both types of MOF in the polymeric phase improves not only the hydrogen permeability but also the selectivity in comparison with the pure polymer in all cases. H₂ permeability increases due to a better diffusion of the penetrants, while CO₂ adsorption on the MOF and solution in the polymer decreases. The best result obtained corresponds to the membrane with 55 wt% loading of ZIF-11, with 495 Barrer of H₂ permeability and a H₂/CO₂ selectivity of 7.0.