Behavior of Water in the Hydrophobic Zeolite Silicalite at Different Temperatures. A Molecular Dynamics Study

Abstract

Classical molecular dynamics simulations of water adsorbed in silicalite, a hydrophobic all-silica zeolite, were performed at different temperatures in the range 100−580 K, to explore possible phase transitions and to compare the behavior of adsorbed water with that of bulk water or water confined in nanopores of different geometry. We used a potential model including full flexibility both of water molecules and of the silicate framework. The results show an unexpected complexity. At very low temperatures (below 225 K), water appears to be mostly in form of amorphous solidlike clusters among which a slow molecule interchange occurs, giving rise to a single-file like diffusion on the time scale of our simulations. At intermediate temperatures, in the approximate range of 225−350 K, the behavior of water is almost liquidlike, whereas at higher temperatures, there are evidences of a vaporlike features, in agreement with the suggestions of previous theoretical and experimental works. This behavior is discussed by considering, among others, the average distribution of water in the channels, the size and lifetime of the hydrogen bonded clusters, and the water−water interaction energy. The results are compared with the available experimental data, previous simulations, and statistical mechanical studies.

Dynamical behavior of one-dimensional water molecule chains in zeolites: Nanosecond time-scale molecular dynamics simulations of bikitaite

“…12. To include lattice vibrations and deformations in the simulated system, a flexible zeolite framework model developed in this laboratory 14,28 has been used. Moreover, new empirical potential functions have been elaborated to represent Li ϩ -water interactions, as the ones previously proposed for simulating aqueous solutions containing lithium ions did not reproduce the structure of water in bikitaite.…”

“…As in our previous papers, [12][13][14] water was assumed to interact with Si and Al atoms via a Coulomb potential only, and the potential functions between the oxygen atoms of the framework and the oxygen or hydrogen atoms of water were derived from a simplified form of the corresponding O-O and O-H ones for water-water interactions.…”

“…As for water in silicalite, 14 we evaluated the lifetime of the clusters, which is shorter than 1 picosecond for temperatures above 600 K. Contrary to the case of water in silicalite, 14 these lifetimes do not diminish for larger clusters, but are all similar, and the largest usually belongs to the largest cluster, because the clusters are formed by a ballistic effect of multiple collisions and not by attractive interactions as in silicalite.…”

“…[7][8][9] Among the different approaches, the molecular dynamics ͑MD͒ simulation technique seems to be very promising in order to gain a better microscopic description for the behavior of water in porous materials. [10][11][12][13][14] A recent example is the study of the phase transitions of water in slit pores. 15 An extreme case of confined water is onedimensional water chains, which are present in some zeolites like bikitaite, 16 whose study has been performed [17][18][19][20][21] by using the Car-Parrinello MD ͑CPMD͒ first principles method.…”

Pressure-induced water intrusion in FER-type zeolites and the influence of extraframework species on structural deformations

“…[30][31][32][33]. In general, different behaviors can be observed, depending on various physical parameters of the matrix, such as pore size, pore system, channel dimensionality (one-, bi-or three-dimensional) [34][35][36][37], and on the hydrophobic/hydrophilic character of the zeolite [33,38].…”