Grand Canonical Monte Carlo Simulation Study of Water Adsorption in Silicalite at 300 K

Abstract

This molecular simulation work focuses on the adsorption of water in a priori hydrophobic silicalite-1, a microporous ordered silica. The water-water interactions are described with the SPC model, while water-silica interactions are calculated in the framework of the PN-TrAZ model. The water adsorption isotherm at 300 K, the configurational energies, and the isosteric heat of adsorption are calculated by the grand canonical Monte Carlo (GCMC) simulation method. The thermodynamic integration scheme allows one to calculate the grand potential along the adsorption isotherm. The adsorption results are compared with experiments, showing only qualitative agreement. Indeed, the simulations do not reproduce the expected hydrophobicity of silicalite (Eroshenko, V.; Regis, R.-C.; Soulard, M.; Patarin, J. C. R. Phys. 2002, 3, 111). This indicates that common models used to describe confined polar molecules are far from being operative. In this work, it is shown, on the basis of periodic ab initio calculations, that confined water molecules in silicalite have a dipole value roughly 10% smaller than that in the bulk liquid phase, indicating that the environment felt by a confined water molecule in silicalite pores is not equivalent to that in the bulk liquid. This suggests that effective intermolecular potentials parametrized for bulk water are inefficient to describe ultraconfined water molecules. Reducing the SPC water dipole moment by 5% (i.e., decreasing water partial charges in magnitude) in GCMC calculations does allow reproducing the experimental water/silicalite isotherm at 300 K.

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

“…Hydrocarbons and common molecules such as nitrogen or carbon dioxide adsorbed in zeolites are examples of systems that can easily be studied with this kind of methods, obtaining adsorption data that match the experimental data [9,10]. Similar approaches have been used for water in zeolites, but with less success [11,12]. This is attributed to the need for long equilibration cycles, and to the fact that small changes in the potential parameters largely influence the computed values [13].…”

“…The SPC [31] water model has also been widely used for adsorption of water in zeolites. Hence, Pellenq et al [24] used this model in MFI, including polarisation in all the atoms of the system, to study polarisation effects, configuration energy and heats of adsorption, and Puibasset et al [12] used this simple model of water in conjunction with a more complex, polarisable potential for the interactions of water -zeolite in MFI to study the adsorption and formation of water clusters [31]. Other models for water such as Tips2 [32] have also been used by Halasz et al [25] to compute the adsorption isotherms of water in different FAUs, and similar studies with similar techniques and models have been performed in other types of zeolites, for example in zeolite A (LTA) [33], heulandite and CLI [34].…”

Molecular Scale Modeling of Membrane Water Treatment Processes

“…However, there have been a number of simulations investigating the behavior of water within siliceous zeolites [55][56][57][58][59][60][61][62]. The most popular zeolites for simulation are silicalite or dealuminated zeolite Y (DAY).…”

“…Consequently, when considering the diffusion of water within zeolites, the use of QM techniques is not appropriate because of the limited timescale that is accessible. Although unable to provide information about dynamic properties, the Monte Carlo (MC) techniques, especially the Grand Canonical MC, can provide valuable information that is difficult to obtain through other methods, such as adsorption isotherms [59][60][61]71].…”

Isotherms of Fluids in Native and Defective Zeolite and Alumino-Phosphate Crystals: Monte-Carlo Simulations with “On-the-Fly”ab initioElectrostatic Potential

“…The LennardJones parameters for aluminophosphates [21,24] and silicates [22] oxygen atoms are different yet defined from the same group for consistency. They have been validated vs experimental data of alkanes and water adsorption isotherms [21][22][23][24][25][26][27].…”