CESIUM OXIDE ENCAPSULATION IN FAUJASITE ZEOLITES - EFFECT OF FRAMEWORK COMPOSITION ON THE NATURE AND BASICITY OF INTRAZEOLITIC SPECIES

Post-synthetic modification of basic CsNaX and CsNaY zeolites was perf ormed by impregnation with cesium acetate at various loadings followed by thermal decomposition of the cesium acetate into oxide. A comparat ive study of the nature and basic character of intrazeolitic species i n CsNaX and CsNaY zeolites is reported. Crystallinities of modified X zeolites are largely retained after activation at 550 degrees C for si x hours. Under the same activation conditions modified Y zeolites are less stable as evidenced by XRD, N-2 sorption, Al-27 and Si-29 MAS NMR and stepwise thermal desorption of CO2 (TPD). The modified CsNaY zeol ite crystallinities were largely maintained when the activation temper ature was lowered to 400 degrees C, The TPD of CO2 below 500 degrees C allows the differentiation of the structures of guest cesium species occluded in the host CsNaX or CsNaY zeolites. A shift of the desorptio n peak maximum from 250 to 150 degrees C accounts for a higher basicit y of the species within the pores of the CsNaX host than in the CsNaY one. Linear correlations between the amount of desorbed CO2 and the ce sium loading suggest a homogeneously dispersed loading up to 16 and 24 cesium atoms per unit cell for the modified X and Y zeolites, respect ively. The formation of oxide (Cs2O) inside the cages of the CsNaX zeo lite is proposed. In the case of the modification of the CsNaY zeolite various structures are discussed involving either the formation of lo cal lattice cesium silicate or aluminate defects or the encapsulation of cesium oxide.