Insertion and Confinement of H₂O in Hydrophobic Siliceous Zeolites at High Pressure

The insertion of H₂O in the siliceous zeolites TON (Theta-one) and MFI (Mobil Five) was studied at pressures up to 0.9 GPa by synchrotron X-ray diffraction, infrared spectroscopy, and Monte Carlo modeling. TON (orthorhombic, Cmc2₁) and MFI (monoclinic, P2₁/n) have 1D and 3D pore systems, respectively. H₂O insertion was quantified by a combination of structure refinements and Monte Carlo modeling. Complete pore filling is observed at 0.9 GPa in the high-pressure forms of TON (orthorhombic, Pbn2₁) and MFI (orthorhombic, Pnma). This corresponds to more than twice as many H₂O molecules per SiO₂ unit in the 3D pore system of MFI than in the 1D pore system of TON. This results in a greater swelling of the MFI system as compared to the TON system upon insertion. In both cases, both experiments and modeling indicate that the density of water in the pores is close to that of bulk water at the same pressure. A greater degree of molecular disorder is observed in the 3D H₂O network of MFI. Infrared spectroscopy indicates a weakening of the hydrogen bonds associated with geometrical constraints because of confinement. The majority of the H₂O molecules are extruded on pressure release, indicating that this insertion is reversible to a great extent, which gives rise to the molecular spring properties of these materials.