Basic thermodynamic characteristics and isotherm of ammonia adsorption in NaZSM-5 and LiZSM-5 zeolites

Abstract

This paper presents the results of isotherm and basic (ΔH, ΔF, and ΔS) thermodynamic characteristics of ammonia adsorption in Na zeolites2,87 ZSM-5 and Li3,37 ZSM-5; also the adsorption isotherm is described by the bulk micro-pore filling theory (BFM) equations. The correlation between adsorption-energy characteristics is found, and the molecular mechanism of ammonia adsorption in Na2,87ZSM-5 and Li3,37ZSM-5 zeolites in the whole filling region is revealed. The ammonia adsorbed in NaZSM-5 and LiZSM-5 zeolites is located in the first coordination sphere with Na+ and Li+ cations, forming octa- and hexaammonia complexes. From the adsorption thermokinetics, it was determined that Li+ cations are located in the screened positions of the ZSM-5 zeolite crystal lattice. The adsorption of ammonia molecule results in Li+ cations migration from the zeolite lattice into junctions formed by the intersection of straight and zigzag channels and the formation of ion/molecular complexes of various multiplicity in them. It was determined that the average molar entropy of ammonia adsorption on NaZSM-5 and LiZSM-5 zeolites indicates that the mobility of ammonia molecules in the zeolite is below the liquid phase and close to the mobility of the solid phase, indicating a strong limitation of mobility of ammonia adsorbed on the cations.