Pressure-induced amorphization of hydrated Na-X zeolite

Abstract

X-ray diffraction patterns of hydrated Na-X zeolite at 1 atm and 22 °C, recovered from high pressure experiments at pressures up to 9 GPa, showed progressive amorphization with increasing pressure. The largest reduction in crystallinity is observed between pressures of 4 and 5 GPa followed by a gradual, continued decrease in ordering up to the maximum pressure. The last vestiges of crystal structure persist to 8 GPa but the sample is completely amorphous at 9 GPa. Heating to 873 K in a differential scanning calorimetry (DSC) apparatus after decompression showed a shift in the broad dehydration endotherm to lower temperatures and a decrease in the endotherm width with increasing pressure. Follow-up X-ray patterns at 1 atm and 295 K after heating showed significant changes in peak intensities and width of the amorphous background signature. These observations are consistent with the negative thermal expansion of Na-X zeolite which results in further compression of the remnant lattice on heating.