The step-by-step thermal dehydration process of the zeolite epistilbite from Osilo (Sardinia, Italy), (Ba0.01Sr0.03K0.08Na1.04Ca2.29) [Al5.80Si18.20O48]15.8H2O, has been studied in situ by synchrotron radiation powder diffraction. The time-resolved expt. was performed using a translating imaging plate system. Rietveld refinements, based on the av. C2/m epistilbite structure, were carried out on 26 consecutive powder patterns in the temp. range from 323 K to 791 K, the upper stability limit of epistilbite in our expt. Three dehydration steps occur at about 370, 470, and 573K. The first step is mainly related to the loss of the H2O mols. in the W3 site not directly coordinated to extra-framework cations ("only hydrogen bonded" water). In the intermediate region between step (a) and (b), epistilbite retains only about 14 water mols., corresponding to those coordinated to Ca (and Na) ions. Gradual W3, W2, and W1 water losses in step (b) reduce the residual water to about 9 H2O mols., which is retained up to step (c). This is the min. water content to maintain the six-coordination of Ca2+ ions. Further water expulsion leads to a first-order phase transition to a collapsed structure (epistilbite B), which has been refined in the C2/m space group. The deformation pattern of the T-O-T angles and the rearrangement of the extraframework Ca (and Na) cations in epistilbite B suggest a remarkable control exerted by the Si, Al ordering on the thermal stability of epistilbite, whose structural breakdown occurs at very low temp. compared to the related mordenite and ferrierite structures.
Dehydration dynamics of epistilbite by in situ time resolved synchrotron powder diffraction
CRUCIANI, Giuseppe;MARTUCCI, Annalisa;
2003
Abstract
The step-by-step thermal dehydration process of the zeolite epistilbite from Osilo (Sardinia, Italy), (Ba0.01Sr0.03K0.08Na1.04Ca2.29) [Al5.80Si18.20O48]15.8H2O, has been studied in situ by synchrotron radiation powder diffraction. The time-resolved expt. was performed using a translating imaging plate system. Rietveld refinements, based on the av. C2/m epistilbite structure, were carried out on 26 consecutive powder patterns in the temp. range from 323 K to 791 K, the upper stability limit of epistilbite in our expt. Three dehydration steps occur at about 370, 470, and 573K. The first step is mainly related to the loss of the H2O mols. in the W3 site not directly coordinated to extra-framework cations ("only hydrogen bonded" water). In the intermediate region between step (a) and (b), epistilbite retains only about 14 water mols., corresponding to those coordinated to Ca (and Na) ions. Gradual W3, W2, and W1 water losses in step (b) reduce the residual water to about 9 H2O mols., which is retained up to step (c). This is the min. water content to maintain the six-coordination of Ca2+ ions. Further water expulsion leads to a first-order phase transition to a collapsed structure (epistilbite B), which has been refined in the C2/m space group. The deformation pattern of the T-O-T angles and the rearrangement of the extraframework Ca (and Na) cations in epistilbite B suggest a remarkable control exerted by the Si, Al ordering on the thermal stability of epistilbite, whose structural breakdown occurs at very low temp. compared to the related mordenite and ferrierite structures.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
