Reactivity Index Scale for Interaction of Heteroatomic Molecules with Zeolite Framework

Abstract

The activity of different interacting heteroatomic molecules such as CO2, CO, CH4, NO, NO2, NH3, and H2O along with the zeolite framework is investigated using a range of reactive indexes using density functional theory (DFT). From the values of the local softness and the charge on the hydrogen atom of the bridging hydroxyl, used as the first approximation to the local hardness, it is concluded that the acidities of the zeolite-type model systems, used in the calculations, are dependent on several characteristics which are of importance within the framework of the hard and soft acids and bases (HSAB) principle. We investigated the local softness of the interacting species, namely, CO2, CO, CH4, NO, NO2, NH3, and H2O, to compare their affinity with the zeolite framework cluster models. The recently proposed local hard−soft acid−base principle characterizes the reactive centers of two systems on the basis of equal local softness. We validate the proposition by considering the interaction between systems with different global softness, which further paves the way for proposing a novel qualitative scale “reactivity index” in comparing the activity of reactant species interacting with the zeolite framework. The results were compared with interaction energy calculations using DFT to validate the proposition. The result shows good agreement with the reactivity index scale for heterotomic molecules where the interaction is through the most nucleophilic atoms for CH4, CO2, CO, NH3, and H2O and the results differ for multisite interaction observed in the case of NO2 and NO. The proposed order of activity in terms of both methodologies is CH4 < CO2 < NH3 < H2O < CO.