CO2 adsorption on silicalite-1 and cation exchanged ZSM-5 zeolites using a step change response method

The adsorption behavior of CO2 on silicalite-1 and cation exchanged MZSM-5 zeolites (M = H, Na, and Ba) was investigated by a step change response method. Temperature programmed desorption (TPD) and in situ Fourier transform infrared (FTIR) spectroscopy were also performed to study the type of adsorbed species and their thermal stabilities. The adsorption experiments were carried out at temperatures from 323 to 473 K and CO2 partial pressures were below atmospheric pressure. The results indicated that CO2 was adsorbed on silicalite-1 and HZSM-5 by one type of adsorption, and on NaZSM-5 and BaZSM-5 zeolites by at least two types of adsorption over the temperature range studied. One was a weak adsorption probably due to interaction with the framework of ZSM-5, and the other was a stronger adsorption caused by interactions with the Na and Ba cations of ZSM-5. The adsorption behavior for silicalite-1 and HZSM-5 was adequately described by a single site Langmuir adsorption model but for NaZSM-5 and BaZSM-5 a dual site Langmuir model was required. FTIR spectra at various elevated desorption temperatures after adsorption of CO2 at low temperature revealed that CO2 was adsorbed on MZSM-5 and silicalite-1 in several ways and formed carbonate bands, which seemed to be caused by different adsorption sites. The maximum temperatures at which carbonate species were observed were 473 K for silicalite-1, 573 K for HZSM-5 and NaZSM-5 and 673 K for BaZSM-5. The TPD results indicated that the quantity of irreversibly adsorbed CO2 on silicalite-1 and HZSM-5 was undetectable.