Study on thermal stabilities and symmetries of chemisorbed species formed on K-zeolites upon CO2 adsorption by TPD and in situ IR spectroscopy

Abstract

In the current study, K-zeolites with different structure, Si/Al ratio and morphology have been prepared and then characterized by different techniques including in situ IR spectroscopy upon CO2 sorption and CO2–TPD with the aim of understanding the nature of basic sites present on their surface acting as catalytic sites in aldol condensation reaction. Results showed that depending on the zeolite structure, pore size and Si/Al ratio, two categories of basic sites could be present in potassium modified zeolites. Symmetries of chemisorbed CO2 on these sites are different and comparing the results of TPD and in situ IR spectroscopy; it can be concluded that highly symmetric species (e.g., monodentate carbonates) have higher thermal stability than low symmetric adsorbed species (e.g., bidentate carbonates). It was found that in the zeolite with relatively smaller pore size or less accessible pores (e.g., MFI), second type of adsorbed species is more popular, while highly symmetric species tend to form on large pore zeolites and on materials with some mesoporosity (e.g., BEA or dealuminated FAU zeolites). It is observed that almost all the bidentate species are desorbed at 80 °C, while monodentate species are thermally stable at least up to 130 °C. Based on combination of experimental data obtained from TPD with IR spectroscopy results, origin and assignment of the TPD peaks were discussed.