Communication: Evidence of structural phase transitions in silicalite-1 by infrared spectroscopy.

The adsorption of trichloroethylene, perchloroethylene, and p-xylene on a MFI (Mobile-FIve) zeolite is studied using in situ FTIR spectroscopy at 298 K. Spectra of self-supported zeolites in contact with increasing pressures of pure gas were recorded at equilibrium in the mid-infrared domain. Analysis of the evolution of the shape and location of vibrational bands of the zeolite as a function of the amount adsorbed allowed the observation of structural modifications of the adsorbent for the first time by infrared spectroscopy.

Stark spectrum simulation for X2Y4 molecules: application to the ν12 band of ethylene in a high-silica zeolite.

The influence of an electric field of silicalite-1-zeolite on the FTIR vibrational absorption spectrum of ethylene has been simulated and compared to experimental spectra. The presence of silicalite-1 produces a global shift and a change of the structure of vibrational bands. To explain the global shift of the ν(12) band (CH(2) scissor mode) and therefore to estimate an effective average field produced by silicalite-1, Stark calculations were performed. These calculations were based on a tensorial formalism implemented in the D(2h)TDS-ST package [M. Sanzharov, M. Rotger, C. Wenger, M. Loëte, V. Boudon, and A. Rouzée, J. Quant. Spectrosc. Radiat. Transf. 112, 41 (2011)]. The value of the field obtained using tensorial formalism (8-11 GV/m) is compared with values obtained using ab initio calculations. A theory of the molecular alignment in the electric field using tensorial formalism is also developed to model the interaction of ethylene in contact with a zeolite environment.

New insights in the formation of silanol defects in silicalite-1 by water intrusion under high pressure.

The "water-silicalite-1" system is known to act as a molecular spring. The successive intrusion-extrusion cycles of liquid water in small crystallites (6 × 3 × 0.5 µm(3)) of hydrophobic silicalite-1 were studied by volumetric and calorimetric techniques. The experiments displayed a decrease of the intrusion pressure between the first intrusion-extrusion cycle and the consecutive ones, whereas the extrusion pressures remained unchanged. However, neither XRD studies nor SEM observations revealed any structural and morphological modifications of silicalite-1 at the long-range order. Such a shift in the value of the intrusion pressure after the first water intrusion-extrusion cycle is attributed to the creation of silanol groups during the first water intrusion. Detailed FTIR and solid-state NMR spectroscopic characterizations provided a molecular evidence of chemical modification of zeolite framework with the formation of local silanol defects created by the breaking of siloxane bonds.

Thermal effects of water intrusion in hydrophobic nanoporous materials.

Liquid water intrusion in hydrophobic nanoporous silicalite-1, a pure siliceous zeolite, in isothermal conditions under high pressure produces an endothermic effect. After intrusion, confined water in zeolite pores is in a different state from that of the liquid bulk water. Such forced intrusion also chemically modifies the material and tends to render it slightly more hydrophilic.