In this work grand canonical Monte Carlo simulations were performed to study gas separation in three pairs of isoreticular metal-organic frameworks (IRMOFs) with and without catenation at room temperature. Mixture composed of CO2 and H-2 was selected as the model system to separate. The results show that CO2 Selectivity in catenated MOFs with multi-porous frameworks is much higher than their non-catenated counterparts. The Simulations also show that the electrostatic interactions are very important for the selectivity, and the contributions of different electrostatic interactions are different, depending on pore size, pressure and mixture composition. In fact, changing the electrostatic interactions can even qualitatively change the adsorption behavior. A general conclusion is that the electrostatic interactions between adsorbate molecules and the framework atoms play a dominant role at low pressures, and these interactions in catenated MOFs have much more pronounced effects than those in their non-catenated counterparts, while the electrostatic interactions between adsorbate molecules become evident with increasing pressure, and eventually dominant.