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Thermally induced phase transformations in cation-exchanged zeolites 4A, 13X and synthetic mordenite and their amorphous derivatives obtained by mechanochemical treatment

Thermal transformations of Li^+, K^+, Cs^+ and NH4^+-exchanged zeolite A, NH4^+-exchanged zeolite X, NH4^+-exchanged synthetic mordenite and the X-ray amorphous materials obtained by ball milling of crystalline phases (the exchanged zeolites A, X and synthetic mordenite) were investigated by different methods such as differential-scanning calorimetry (DSC), differential thermal gravimetry (DTG), X-ray powder diffractometry, FTIR spectroscopy and scanning-electron microscopy (SEM). Thermal stability of the investigated solids depends on the type of zeolite, type of exchanged cation and SiO_2/Al_2O_3 molar ratio in the starting crystalline and amorphous samples. Most crystalline zeolites transform to other crystalline phase(s) without formation of amorphous intermediates, but in some cases of Li^+, K^+-exchanged zeolite A, a transformation to amorphous intermediate proceeds into secondary crystalline phase(s). Both crystalline and amorphized NH4^+-exchanged zeolites A and X transform to a mixture of mullite and amorphous silica, whereas the NH4^+-exchanged synthetic mordenite transforms to amorphous phase only. A special attention is paid to the transformation of crystalline and amorphized samples. (C) 1998 Elsevier Science B.V. [References: 25]

Amorphization; Cation exchange; Phase transformations; Zeolites

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