HIGH PRESSURE-INDUCED PENETRATION OF ALCHOOL AND WATER MOLECULES IN ALL-SILICA ZEOLITES: THE CASE OF Si-CHABAZITE

The behavior of pure Si-chabazite1 (CHA framework type, s.g. R-3m, a=13.5454 c=14.7635) compressed in the penetrating fluid m.e.w. - a mixture of methanol:ethanol:water (16:3:1) - was investigated by in situ synchrotron X-ray powder diffraction (XRPD) in the pressure range Pamb - 4.82 GPa. The characterization of the Si-chabazite at ambient pressure, performed with XRPD and thermo-gravimetric analysis, indicates a water content of 2.5 molecules per unit cell, confirming the hydrophobic nature of this material. No phase transitions were induced by compression. A cell volume decrease of about 3.7% was observed up to 4.8 GPa. The structural refinements indicate the penetration of both alcohol and water molecules in the zeolite porosities, even at the lowest investigated pressure (0.1 GPa). Interestingly, a segregation between methanol and ethanol was observed. In fact, on the basis of the interatomic distances and of the steric hindrances, the two alcohol types cannot occupy simultaneously the same CHA cage (Fig.1). The process is completely reversible, both concerning the recovering of the original cell volume and the removal of the guest alcohol and water molecules. The reversibility of the intrusion process - already detected by porosimetric experiments performed by using pure water2 - suggests that this materials can be used as a “spring device”, able to store and restore the mechanical energy. Preliminary results will be shown also on the effects of the changes of the solution composition on the penetration pressure. The study is part of the project (www.zapping-prin.it), financed by Italian MIUR. 1Diaz-Cabañas, M.J.; Barret, P.A.; Camblor, M.A. Chem. Commun. 1998, 1881-1882. 2Trzpit, M.; Soulard, M.; Patarin, J., Chem. Letters Vol.36, 2007, 8, 980-981.