Molecular simulation of p-xylene and m-xylene adsorption in Y zeolites. Single components and binary mixtures study

The properties of p-xylene and m-xylene adsorbed in NaY and KY faujasites h ave been studied by grand canonical Monte Carlo simulation. Biased particle insertions and deletions were implemented to allow the computation of equi librium adsorption isotherms of such complex systems. This work is based on a new adsorbate-zeolite potential function named TrAZ potential (transfera ble adsorbate-zeolite potential). Adsorption isotherms and isosteric heats of adsorption of pure components have been calculated at 423 K and compared to the available experimental data. Grand canonical Monte Carlo simulation s of coadsorption have also been performed. Three binary mixtures correspon ding to different gas-phase compositions have been studied. The p-xylene/m- xylene selectivities predicted from the simulation are in good agreement wi th experiments: the NaY zeolite preferentially adsorbs the m-xylene isomer, whereas the KY faujasite is selective for p-xylene. From a detailed analys is of the xylene adsorption sites, we suggest a molecular mechanism of the cage filling in zeolites NaY and KY. Whatever the loading, our study reveal s a unique adsorption site in the supercage of the zeolite NaY, located in front of the sodium cations in site II. In the case of the faujasite KY, th ree adsorption sites with different adsorbate-zeolite interaction energies are observed. From this microscopic analysis of the cage loading, the obser ved selectivities can be explained by joint energetic and steric effects.