THERMAL-PROPERTIES OF Y-TYPE ZEOLITES

The thermal and structural properties of two parent NaY zeolites and o f those modified by ion exchange (ReNaY, HNaY, FeNaY) were investigate d by simultaneous thermal analysis (TG-DTA-DTG) and by X-ray diffracti on spectroscopy. Both the intracrystalline water and the zeolite frame work were in our attention. The impurities (Fe) located in the lattice as well as the ions which entered by ion exchange (Re, H, Fe) influen ce the properties of the zeolites. The values of the activation energy of the dehydration process prove that the water molecules are more st rongly bonded in all modified samples than in the parent ones. As comp ared to the NaY zeolites, an increased thermal stability, of about 100 -degrees-C was revealed for ReNaY or of about 180-degrees-C for HNaY, and a decreased stability, of about 50-degrees-C, for FeNaY samples. T he temperature at which the lattice break-down begins T(amf), estimate d by following the X-ray diffraction patterns for samples heated in ai r at temperatures from 300 to 1100-degrees-C, is the temperature which may be related to the structural characteristics of the zeolites, i.e ., to the lattice constant of the uncalcined materials. The XRD studie s reveal the heterogeneity of the crystallites constituting the zeolit e material from both the point of view of the lattice constant values and the thermal stability. As the temperature T(amf), generally, does not coincide with the temperature of the first exothermic peak, T(l), of the DTA curve, we suggest the temperature T(amf) to be taken as an unambiguous measure of the thermal stability.