Molecular simulation studies of water physisorption in zeolites Angela Di Lella, ab Nicolas Desbiens, a Anne Boutin, a Isabelle Demachy, a Philippe Ungerer, b Jean-Pierre Bellat c and Alain H. Fuchs* d Received 31st July 2006, Accepted 4th October 2006 First published as an Advance Article on the web 17th October 2006 DOI: 10.1039/b610621h We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342–353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si : Al ratio in the range 1.53–3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite- based separation processes. 1. Introduction The interest in the properties of water confined in the nan- ometer-scale channels and pores of zeolites and other inor- ganic open framework materials dates back to the pioneer work of Barrer. 1 From a practical point of view, water plays a key role in many applications such as ion-exchange and liquid phase separation. It has been observed for instance that the presence of pre-adsorbed water in the porous solid affects the adsorption selectivity with respect to the hydrocarbon mixture that one wants to separate. 2 Most often, traces of water tend to decrease the adsorption selectivity observed in the dry solid, and can thus severely handicap an intended process. From time to time though, selectivity enhancement was observed. 3–5 The mechanism producing these effects is poorly understood. Being able to understand and predict the effect of water on fluid (such as hydrocarbons) adsorption is considered as a key challenge in the adsorption community. 6 From the fundamental point of view, water in zeolite represents a model system for a wide range of experimental as well as theoretical investigations, aimed at understanding the effect of confinement on the structure, 7 dynamics 8,9 and thermodynamics 10–14 of molecular fluids. Understanding the behaviour of nano-confined water is also relevant to such important problems as protein stability, 15–19 transport in ion channels, 20–22 nano-fluidic devices 23 and the thermodynamics of colloidal assemblies. 24,25 Recently, the arrangement of water molecules and nonframework cations in faujasite zeolite at different water loadings has been the subject of experimental investigations, in which the existence of confined water com- plexes, such as hexamers, has been disclosed. 26,27 Adsorption properties in aluminosilicate zeolites are closely related to the location of nonframework cations and to their accessibility to adsorbed molecules. The partition of these cations among the different sites may change during the course of the molecular guest adsorption process. Grey et al. 28 observed sodium cation redistribution upon hydrofluorocar- bons adsorption in faujasite zeolite, by combining NMR and X-ray powder diffraction. Mellot-Draznieks and Cheetham 29 have carried out a neutron scattering study of CFCl 3 adsorp- tion in NaY and observed cation redistribution together with a new and previously unknown cation location. Cation redis- tribution was also observed in the xylene–BaX guest–host system. 30 Cation migration upon water dehydration was observed, using X-ray diffraction in Cs-faujasite, 31 Ba-Philipp- site, 32 Cd-Heulandite 33 and Clinoptilolite. 34 It should be stressed that only cationic zeolites are being considered here. The issue of water chemisorption on acidic protons (Brønsted sites) is beyond the scope of this work. Molecular simulations have played an important role in understanding the adsorption, diffusion and the synthesis in zeolitic materials. Whereas ten years ago adsorption simula- tions were limited to noble gases or small alkane in purely siliceous zeolites, progress in the simulation techniques allows a Laboratoire de Chimie Physique, Ba ˆtiment 349, UMR 8000 CNRS and Universite ´ Paris-Sud, F-91405 Orsay, France b Institut Franc ¸ ais du Pe ´trole, 1-4 Avenue de Bois Pre´au, F-92852 Rueil-Malmaison Cedex c Laboratoire de Recherche sur la Re ´activite ´ des Solides, UMR 5613 CNRS, Universite´ de Bourgogne, B.P. 47870, F-21078 Dijon Cedex, France d Ecole Nationale Supe ´rieure de Chimie de Paris (ENSCP) and Universite ´ Pierre et Marie Curie-Paris 6, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France. E-mail: alain.fuchs@enscp.fr 5396 | Phys. Chem. Chem. Phys., 2006, 8, 5396–5406 This journal is  c the Owner Societies 2006 INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics