First structural investigation of a super-hydrated zeolite

Pressure and temperature are basic thermodynamic variables transforming matter from one state to another. Our knowledge of pressure-induced phase transformations of zeolites, however, is very limited compared to the vast number of temperaturedependent studies performed over the past several decades.1-3 This is partly due to the required experimental complexities as well as the analytical ambiguities arising from the porous nature of the materials, which can lead to compositional changes upon interaction with various pressure-transmitting media.4-6 The large variety of zeolite structures is due to the numerous possible linkages of (Al,SiO4)-tetrahedra that bound nanopores of various sizes. The built-in flexibility allows these structures to contract and expand in response to external changes and alters the chemistry occurring in the nanopores. Unusual effects such as negative thermal expansion and cation relocations in zeolite rho have recently been recognized to be driven by temperatureinduced chemical changes.7 Therefore, applying external hydrostatic pressure was also anticipated to alter the chemical environment within the pores. We have reported different phase transitions depending on the type of cations residing inside the pores in zeolite rho.8 Various interaction schemes between cation and pressure-transmitting media have been proposed to drive the observed phase transitions in rho and other zeolites,8-10 but no structural evidence for these pressure-induced chemical changes has been reported to date. We have measured powder diffraction data of natrolite11 as a function of pressure up to 5.0 GPa using a diamond-anvil cell (DAC) and a 200 ím-focused monochromatic synchrotron X-ray beam.12 Upon pressure increase, there is an abrupt volume expansion (ca. 2.5%) between 0.8 and 1.5 GPa. Rietveld refinements using these data showed that this anomalous swelling is due to the selective sorption of water from the alcohol-based pressuretransmitting media into the expanded channels, which gives rise to a “super-hydrated” phase of natrolite, Na16Al16Si24O80â32H2O, distinct from the normal natrolite, Na16Al16Si24O80â16H2O, at ambient conditions.