A molecular simulation study is reported for CO2 adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Al3+). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ < Al3+). At low pressures, cations act as preferential adsorption sites for CO2 and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO, adsorption. Furthermore, the adsorption selectivity of CO2/H-2 mixture increases as Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ approximate to Al3+. At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H2O, the selectivity decreases drastically because of the competitive adsorption between H2O and CO2, and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.