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In situ time resolved synchrotron powder diffraction study of mordenite

Dip. di Scienze della Terra, Sez. di Mineralogia, Petrologia e Geofisica, Università di Ferrara, I–44100 Ferrara, Italy

^* Corresponding author, e-mail: mrs{at}dns.unife.it

The step by step thermal dehydration process of the zeolites mordenite from Pashan (Poona, India) (Na_3.51K_0.14Ca_1.89Mg_0.09Sr_0.01)[Fe³⁺_0.03Al_7.40Si_40.53O₉₆] · 27.26H₂O, has been studied in situ by synchrotron powder diffraction. The time-resolved experiment was performed using a translating imaging plate system. The structure refinements by full profile Rietveld analysis were performed in the Cmcm space group in the temperature range from 25 to 830°C. The results of structure refinements indicate that the slight cell-volume contraction ( 1.9%) is related to the release of water molecules from the channels: above 375°C, the water loss lead to an enlargement of the 8-membered ring parallel to [010], which occurred by a flattening of T3-O9-T3 and T4-O4-T3 bridges. The dehydration process is reflected not only in the content of water molecules in the zeolite channel, but also in the temperature behaviour of the unit cell parameters. The cell parameters b and c decreased regularly as the temperature rose, whereas a decreased up to 400°C then increased up to 630°C, and finally decreased until the end of experiment. The removal of water molecules was accompanied by a spreading of the initial Ca sites into many positions bonded to the framework oxygens. The increased interaction with the framework oxygens of Ca sites was intimately related to the distortion of the 12-ring which is in turn related to the lengthening of the a cell parameter.

Key-words: in situ powder diffraction, X-ray synchrotron radiation, zeolites dehydration, mordenite.

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