The gradual depletion of fossil fuel reserves has renewed interest in hydrogen (H₂) as a durable and sustainable energy carrier. However, its successful application on an industrial scale hinges on removing trace contaminants particularly carbon dioxide (CO₂) from mixed gas streams. In this study, the separation of CO₂ from CO₂/H₂ mixtures was investigated using the metal organic framework SIFSIX 3 Cu and Grand Canonical Monte Carlo (GCMC) simulations across a wide range of temperatures and pressures. Two gas compositions (molar ratios CO₂/H₂ of 0.90/0.10 and 0.79/0.21) were examined between 298 K and 370 K. In every case, CO₂ uptake was markedly higher than that of H₂, and the adsorption of both gases decreased with increasing temperature. The Langmuir isotherm model provided an excellent fit to the observed data, indicating monolayer adsorption behavior. The isosteric heats of adsorption were calculated to be about 54 kJ mol⁻¹ for CO₂ and roughly 2 kJ mol⁻¹ for H₂, reflecting the stronger electrostatic attraction between CO₂ molecules and the SiF₆²⁻ sites in the framework. Spatial density distribution analysis revealed that CO₂ molecules preferentially occupy regions near the SiF₆²⁻ units, whereas H₂ molecules are mainly located around pyrazine rings. Overall, these findings highlight the pronounced CO₂ selectivity of SIFSIX 3 Cu even at low concentrations and underline its potential for hydrogen purification in industrial gas separation processes.