Characterization of basicity in alkaline cesium-exchanged X zeolites post-synthetically modified by impregnation: A TPD study using carbon dioxide as a probe molecule

Interesting basic zeolites have been prepared by impregnating CsNaX zeolite with various loadings of cesium acetate in order to improve, after calcination, the basic character of the solid and to benefit from the cage effect in fine chemistry. In the range of 0 up to 26 cesium atoms per unit cell, X-ray powder diffraction shows that the crystallinity is largely retained after the generation of the basic species. The microporous volume values indicate that an interesting space is available for fine chemistry. For characterizing the surface chemical properties of the basic post-synthetically modified CsNaX zeolites, the stepwise thermodesorption of CO2 (TPD) has been studied. Experimental conditions allowing an accurate determination of the basic species are described. The modified zeolites with various loadings have been compared under these conditions. For loadings below 16 cesium atoms per unit cell, a linear relationship is established between the amount of desorbed CO2 (molecules per unit cell), up to 550°C, and the impregnated cesium loading (atoms per unit cell). This result supports the hypothesis that the active cesium species are homogeneously located inside the cages of the CsNaX zeofite. The slope of the straight line (0.44) indicates that one cesium oxide per zeolite cage is generated, leading upon CO2 adsorption to a carbonate species (Cs2CO3). For loadings higher than 16 cesium atoms per unit cell, a saturation is observed for the amount of desorbed CO2 up to 550°C along with an increase of the amount of desorbed CO2 above 550°C corresponding to a deposition of basic species on the external surface of the solids.