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 phase s (the exchanged zeolites A, X and synthetic mordenite) were investiga ted by different methods such as differential-scanning calorimetry (DS C), differential thermal gravimetry (DTG), X-ray powder diffractometry , FTIR spectroscopy and scanning-electron microscopy (SEM). Thermal st ability of the investigated solids depends on the type of zeolite, typ e of exchanged cation and SiO2/Al2O3 molar ratio in the starting cryst alline and amorphous samples. Most crystalline zeolites transform to o ther crystalline phase(s) without formation of amorphous intermediates , but in some cases of Li+, K+-exchanged zeolite A, a transformation t o amorphous intermediate proceeds into secondary crystalline phase(s). Both crystalline and amorphized NH4+-exchanged zeolites A and X trans form to a mixture of mullite and amorphous silica, whereas the NH4+-ex changed synthetic mordenite transforms to amorphous phase only. A spec ial attention is paid to the transformation of crystalline and amorphi zed samples. (C) 1998 Elsevier Science B.V.