The Effect of Local Defects on Water Adsorption in Silicalite-1 Zeolite:  A Joint Experimental and Molecular Simulation Study

Abstract

We report a joint experimental and molecular simulation study of water condensation in silicalite-1 zeolite. A sample was synthesized using the fluoride route and was found to contain essentially no defects. A second sample synthesized using the hydroxide route was found to contain a small amount of silanol groups. The thermodynamics of water condensation was studied in these two samples, as well as in a commercial sample, in order to understand the effect of local defects on water adsorption. The molecular simulation study enabled us to qualitatively reproduce the experimentally observed condensation thermodynamics features. A shift and a rounding of the condensation transition was observed with an increasing hydrophilicity of the local defect, but the condensation transition was still observed above the water saturation vapor pressure P0. Both experiments and simulations agree on the fact that a small water uptake can be observed at very low pressure, but that the bulk liquid does not form from the gas phase below P0. The picture that emerges from the observed water condensation mechanism is the existence of a heterogeneous internal surface that is overall hydrophobic, despite the existence of hydrophilic "patches". This heterogeneous surface configuration is thermodynamically stable in a wide range of reduced pressures (from P/P0 = 0.2 to a few thousands), until the condensation transition takes place.

Desalination of seawater ion complexes by MFI-type zeolite membranes: Temperature and long term stability

“…Some simulation studies [36][37][38][39] have shown that a high pressure (e.g. 60-100 MPa) is required to infiltrate water into the purely siliceous pores of MFI zeolites (e.g.…”

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

“…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.…”

Thermodynamic study of water confinement in hydrophobic zeolites by Monte Carlo simulations

“…More recently, the notion of surface hydrophobicity was used [5,6]. It should be understood as a simple extension of the concept of hydrophobicity for large solutes described above.…”

“…This enables to build up a variety of purely siliceous open framework materials containing cages and channels of 0.4 to 2 nm width, with essentially no internal surface defects. In the all-silica frameworks such as silicalite-1 or β zeolite, water uptake in the gas phase at ambient conditions was found to be extremely small [6]. For this reason, these nanoporous materials were termed hydrophobic.…”