Intrusion-extrusion of water/alcohol molecules in Si-chabazite: structural interpretation of the energetic performance

The demand for low cost, efficient and sustainable energy is ever increasing. Since fifteen years, absorption or energy storage using pure-silica zeolites has been developed and opened a new field of applications for these exciting microporous solids (Eroshenko et al., 2001) . The basic idea is that to penetrate water into a hydrophobic porous material a certain pressure must be applied. During this forced penetration (intrusion), the mechanical energy can be converted into interfacial one. Depending on various physical parameters related to the porous material-such as its pore system, its dimensionality and pore size-when the pressure is released (extrusion) the “zeosil-water” system is able to restore, dissipate or absorb the supplied mechanical energy accumulated during the compression step with a more or less significant hysteresis. As a consequence, it displays a spring, shock-absorber or bumper behavior, respectively. In the present work, the energetic performances of the pure-silica Si-chabasite (CHA) zeolite were preliminary tested. The water/alcohol intrusion of synthetic zeolite Si-chabazite-CHA framework type, s.g. R-3m, a = 13.5454, c = 14.7635 Å (Diaz-Cabañas et al., 1998) was explored in the pressure range Pamb-4.82 GPa by in situ synchrotron X-ray powder diffraction (XRPD) experiments, using a mixture of methanol:ethanol:water = 16:3:1 (mew) as penetrating Ptransmitting medium. No phase transitions were observed in the investigated P-range. The cell volume decreases of about 3.7% up to 4.8 GPa and the Pamb volume is almost completely regained upon pressure release. The structural refinements indicate a significant penetration (corresponding to about 270 extraframework electrons) of the P-transmitting medium even at 0.1 GPa, the lowest investigated pressure. On the basis of the interatomic distances and of the steric hindrances, both alcohol and water molecules seem to penetrate the zeolite porosities. However, impressively, a segregation between methanol and ethanol was observed, since the two molecules cannot occupy simultaneously the same CHA cage. The reversibility of the intrusion process, with the extrusion of the medium molecules upon P release was already noted by porosimetric experiments performed by using pure water (Trzpit et al., 2007). Our and literature results suggest that Si-CHA can be used as a “spring device”, able to store and restore mechanical energy. Preliminary results on the effects of changes of the medium composition on the intrusion/extrusion process will be shown, as well. Acknowledgements: The study is part of the project ZAPPING, financed by Italian MIUR (www.zapping-prin.it). Diaz-Cabañas, M.J., Barrett, P.A., Camblor, M.A. (1998): Synthesis and structure of pure SiO2 chabazite: the SiO2 polymorph with the lowest framework density. Chem. Commun., 1998, 1881-1882. Eroshenko, V., Regis, R.-C., Soulard, M., Patarin, J. (2001): Energetics: A new Field of Application for Hydrophobic Zeolites. Am. Chem. Soc., 123, 8129-8130. Trzpit, M., Soulard, M., Patarin, J. (2007): The Pure Silica Chabazite: A High Volume Molecular Spring at Low Pressure for Energy Storage. Chem. Letters, 36, 980-981.