To efficiently separate CO₂ from multi-species gas streams, sorbents must have high CO₂ selectivity, and CO₂ mobility within the materials must not be a limiting factor. Zeolites are one attractive option as sorbents due to their variety of structures, availability, and thermal and chemical stability. To investigate the influence of pore geometry on adsorption and selectivity, we have used Grand Canonical Monte Carlo to study the adsorption of CO₂, N₂, and CO₂/N₂ mixtures onto three zeolites with identical chemical composition (SiO₂,) but different structures; silicalite, ITQ-3, and ITQ-7. These zeolites preferentially adsorb CO₂ over N₂ in both single component and mixture adsorption. The CO₂/N₂ selectivities observed in the three siliceous zeolites studied vary strongly as the adsorbent's crystal structure changes, with the selectivity in ITQ-3 being the largest. We have thus concluded that pore geometry strongly affects adsorption in the different zeolites. We present here the complementary studies of non-isotropic CO₂ diffusion within these materials. Molecular Dynamics simulations have been used to compute CO₂, N₂ and CO₂/N₂ mixtures self-diffusion, corrected diffusion, and transport diffusion within ITQ-3, ITQ-7, and silicalite. Significant diffusion occurs, and carbon dioxide molecules explore large regions in all three zeolites. Rates of diffusion vary between zeolites, decreasing with increased pore loading in ITQ-7 and silicalite but showing a non-monotonic dependence in ITQ-3. In a complementary study we have also identified the preferred CO₂ and N₂ sites within silicalite, ITQ-3, and ITQ-7.