We use periodic Density Functional Theory (DFT) method to generate the electrostatic potentials of adsorption materials and use them in Grand Canonical Monte Carlo simulations of fluid adsorption isotherms. This permits us to consider complex solids showing defects and without a priori knowledge of their electrostatic parameters for the Monte Carlo simulations. We apply the method to aluminophosphate and silicate solids of ZON type and evaluate their affinity to adsorb and separate CO₂-N₂(H₂O) mixtures.