Water Behavior in Hydrophobic Porous Materials

The behavior of water in silicalite and dealuminated zeolite Y (DAY), two highly hydrophobic zeolites, was investigated at different temperatures in the range 100-600 K by molecular dynamics simulations using the Compass forcefield. The full flexibility of water molecules and the zeolite framework was considered. The results show that the behavior of water is more complex in silicalite than in DAY. Three different activation energies for water diffusion were obtained in silicalite in the range 250-600 K compared to two for DAY. The values of these activation energies are discussed in detail and are related to the strength of hydrogen bonds and the zeolite structure. From the radial distribution functions (RDFs), it is shown that water mostly exists in the gas phase at room temperature in silicalite, whereas liquid-like water is observed in DAY. The self-diffusion coefficients of water and the RDFs were obtained as functions of temperature in order to explain the different behaviors of water in the two all-silica zeolites. The influence of loading on the self-diffusion coefficients also was investigated for both crystals.