M. Lasperas et al., CESIUM OXIDE ENCAPSULATION IN FAUJASITE ZEOLITES - EFFECT OF FRAMEWORK COMPOSITION ON THE NATURE AND BASICITY OF INTRAZEOLITIC SPECIES, Microporous materials, 7(2-3), 1996, pp. 61-72
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.