Cesium oxide encapsulation in faujasite zeolites effect of framework composition on the nature and basicity of intrazeolitic species

Abstract

Post-synthetic modification of basic CsNaX and CsNaY zeolites was performed by impregnation with cesium acetate at varoous loadings followed by thermal decomposition of the cesium acetate into oxide. A comparative study of the nature and basic character of intrazeolitic species in CsNaX and CsNaY zeolites is reported. Crystallinities of modified X zeolites are largely retained after activation at 550°C for six hours. Under the same activation conditions modified Y zeoolites are less stable as evidenced by XRD, N 2 sorption, 27Al and 29Si MAS NMR and stepwise thermal desorption of CO 2 (TPD). The modified CsNaY zeolite crystallinities were largely maintained when the activation temperature was lowered to 400°C. The TPD of CO 2 below 500°C allows the differentiation of the structures of guest cesium species occluded in the host CsNaX or CsNaY zeolites. A shift of the desorption peak maximum from 250 to 150°C accounts for a higher basicity of the species within the pores of the CsNaX host than in the CsNaY one. Linear correlations between the amount of desorbed CO 2 and the cesium loading suggest a homogeneously dispersed loading up to 16 and 24 cesium atoms per unit cell for the modified X and Y zeolites, respectively. The formation of oxide (Cs 2O) inside the cages of the CsNaX zeolite is proposed. In the case of the modification of the CsNaY zeolite various structures are discussed involving either the formation of local lattice cesium silicate or aluminate defects or the encapsulation of cesium oxide.