In this work we fabricated alumina samples by the freeze-casting technique using tert-butanol as the solvent. The prepared materials were examined by scanning electron microscopy and X-ray microtomography. Next, they were coated with sol–gel silica films by dip-coating. Permeability tests were carried out in order to assess the permeation behavior of the materials processed in this study. We observed that the sintering time and alumina loading showed a remarkable effect on both the structural properties and flexural strength of the freeze-cast samples. Nitrogen adsorption tests revealed that the silica prepared in this study exhibited a microporous structure. It was observed that the presence of silica coatings on the alumina surface decreased the CO₂ permeance by about one order of magnitude. Because of the similar kinetic diameters of nitrogen and carbon dioxide, the CO₂/N₂ system showed a separation efficiency that was lower than that observed for the He/CO₂ and He/N₂ systems. We noticed that increasing the feed pressure improved the separation capacity of the obtained materials.