Experimental and theoretical determination of adsorption heats of CO2 over alkali metal exchanged ferrierites with different Si/Al ratio

Abstract

The adsorption of CO₂ in Li-, Na-, and K-FER was investigated by a combination of volumetric adsorption experiments, FTIR spectroscopy, and density functional theory. Experimental isosteric heats of CO₂, Q_st, depend significantly on the cation size, cation concentration, and on the amount of adsorbed CO₂. The differences observed in experimentally determined isosteric heats were interpreted at the molecular level based on good agreement between experimental and calculated characteristics. The highest interaction energies were found for CO₂ adsorbed on so-called “dual cation sites” in which CO₂ is bridged between two alkali metal cations. The formation of CO₂ adsorption complexes on dual cation sites is particularly important on Na-FER and K-FER samples with higher cation concentration. On the contrary, the differences in Q_st observed for Li-FER samples are due to the changes in the Li⁺ coordination with the framework. The DFT/CC calculations show that the dispersion interactions between CO₂ and the zeolites framework are rather large (about −20 kJ mol⁻¹).