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Keywords:

Science & Technology, Physical Sciences, Chemistry, Physical, Chemistry

Abstract:

An equilibrium distribution of exchangeable cations in zeolites might reflect the Boltzmann distribution law, the occupancy ratios of the cation sites being related to the energy level differences and the absolute temperature. Dehydrated calcium mordenite (Ca3.33Al7.82Si40.2O96) was chosen to follow the variation of the Ca ion localization with temperature. Two crystals were studied: A at 25 and 350°C (accidentally rehydrated) and B at 345 and 635°C with single crystal x-ray diffraction methods. The Ca ions are distributed among four nonequivalent sites. A shift of the population parameters with temperature is observed, the sites yielding a better coordination to the cations becoming more and more stable. This indicates a change in the energy level of the sites. At all temperatures, site I is the most stable site, located in the center of a highly twisted eight-ring and coordinated to six framework oxygens. Sites III and VI have a one-sided coordination to four framework oxygens and site IV is located off-center on the eight-ring, coordinated to only two oxygens. The stability sequence of these sites, as calculated from the energy level differences is I > VI > IV > III at temperatures below 345°C, site IV showing considerable destabilization at 635°C. The presence of water molecules at 350°C resulted in stabilization of sites I and IV by increasing the coordination of the cations.