Molecular simulation of water adsorption in silicalite: Effect of silanol groups and different cations

Using GCMC and MD simulations we investigated the effect of silanol groups and extraframework cations on water adsorption, diffusion and the structure of water in silicalite. The adsorption of water was enhanced with the introduction of defects. In the case of cations this enhancement was much more significant. Below the saturation pressure of water no filling of pores by water molecules was observed. Introduction of silanol nests did not result in significant changes in water structure and self diffusion. On the other hand, the presence of cations decreased self diffusivity of water and changed the water structure observed in the defect-free silicalite. Silanol nests were found to be weak defects and have a limited effect on water adsorption. The model we used in this study satisfactorily predicted adsorption isotherms and heats of adsorption, however, self diffusion coefficients of water were underestimated which is attributed to the rigid treatment of the silicalite lattice.