Abstract EN:

A molecular simulation study of adsorption of water and xylene isomers in different cationic Faujasites has been undertaken in order to understand how the adsorption selectivity for one or the other isomer is modified by a small amount of preadsorbed water. In the first part of the work, a simple model is proposed for predicting the way cations are distributed among the known crystallographic sites I, I', II and III, in NaY and NaX faujasite zeolites. This model is based on the fact that the cation distribution is dominated by the repulsive cation-cation coulombic interactions. In the case of NaY the predicted distributions are in good agreement with the available experimental data. Monte Carlo simulations have been performed, using specific algorithms such as the parallel tempering technique and a site-to-site hopping procedure which were found necessary to ensure convergence of calculations. These simulations confirm the basic assumption of the cation-filling model. They also reveal a striking sensitivity of the configuration energy to a weak deviation from the most uniform local cation spatial distribution. The existence of the extra site III' experimentally reported in NaX is also observed in these simulations. In the second part of the work, xylene isomers and water adsorption has been studied through Grand Monte Carlo simulations for different Si:Al ratio. Biased particle insertions and deletions were implemented to allow the computation of equilibrium adsorption isotherms of such complex systems. This work is based on a new adsorbate-zeolite potential function named TrAZ potential (Transferable Adsorbate-Zeolite potential). The results reveal a strong sensitivity of the water adsorption isotherms to the cation number and specific location.