Molecular simulation studies of water physisorption in zeolites

Abstract

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 ratio 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.

Evaluation of various water models for simulation of adsorption in hydrophobic zeolites

“…Figure 2 compares experimental [23,61] and simulation data for water adsorption in both MFI structures at 300 K. For this particular study, we use the Tip4p model at the same simulation conditions as that of previous groups [11,19], assigning a partial charge of 1.4 e to the silicon atoms of the framework (and therefore a partial charge of 2 0.7 e to the oxygen atoms). Although the adsorption isotherms for both frameworks have a similar shape, for a given loading the pressure differs up to 60 MPa.…”

“…The most popular model is Tip4p [26], which has been used by Beauvais et al [18] to describe the location of the sodium cations in FAU in the presence of water; by Di Lella et al [19] to calculate the adsorption isotherms and cation distribution in FAU and MFI; by Trzpit et al [20] and Yang et al [21] to study water diffusion in MFI or by Cailliez et al [22] to study the interaction of water with the possible defects of the zeolite MFI. Desbiens et al [11,23] computed adsorption isotherms in MFI with different partial charges for the framework atoms not only with the Tip4p water model [26], but also with the Tip5p [27], MSPC/E [28] and the polarisable DEC [29] model.…”

Cation redistribution upon dehydration of Na58Y faujasite zeolite: a joint neutron diffraction and molecular simulation study

“…The parameters are taken from Jaramillo et al [12] for the zeolite framework (in particular, Si and Al atoms are described as "T atoms" carrying identical charges) and from Dang et al [13] for Na + cations, together with the TIP4P water model of water [14]. This force field was successfully used in previous studies of water adsorption in Y and X zeolites [4,5,6,15,16]. Periodic boundary conditions are used, electrostatic interactions are computed using Ewald summation and a cut-off equal to half the box length is used to compute Lennard-Jones interactions.…”

“…One of the reasons for the discrepancy between cation location results is the strong sensitivity of cation distribution to experimental conditions (temperature, dehydration conditions, nature and quantity of sorbed molecules …) [1,3]. The presence of water has been shown to drastically modify the cation location in zeolites and in particular in faujasite zeolite [4,5,6,7,8]. The experimental determination of cation location in hydrated sample is complicated because of partial occupancies affecting low symmetry sites.…”

“…Water adsorption has been previously studied by Monte-Carlo simulation in several sodium faujasites with Si:Al ratio of 1.53, 2.69 and 3 with a unique rigid zeolitic framework and cation redistribution have been evidenced [6].…”