Quantum Chemical Study of low-pressure adsorption in zeolitic materials

Zeolites are mainly used as heterogeneous catalysts, with an increasing use as gas-separation adsorbents. Their properties as adsorbents are described by adsorption isotherms and isosteric heats of adsorption. Two methods are presented that predict isosteric heats of adsorption of gaseous molecules at low coverages. Both methods are based on the quantum mechanical (QM) evaluation of the interaction between the molecule adsorbed and the cationic site treated as an embedded cation. The first method uses Boltzmann statistics, whereas the other method assumes that adsorption at low pressure can be described with a Langmuir model. These two procedures yield very comparable isosteric heats for N2 and O2 in Ca-A and Ca-LSX. Their comparison allows us to underline the strong heterogeneity of N2 adsorption in Ca-LSX and the effect of temperature on the equilibrium constant of adsorption.