The effect of local defects on water adsorption in silicalite-1 zeolite: A joint experimental and molecular simulation study

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 P-0. 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 P-0. 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/P-0 = 0.2 to a few thousands), until the condensation transition takes place.