Calorimetric study of methanol adsorption in LiZSM-5 and CsZSM-5 zeolites

Abstract

This paper presents isotherms and basic (ΔH, ΔF, and ΔS) thermodynamic characteristics of methanol adsorption in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites obtained by the calorimetric method. For measurements of isotherms and differential heat of adsorption, a system consisting of a universal high-vacuum adsorption unit and an attached differential modified Tian-Calvet type microcalorimeter, DAC-1-1A, which directly quantifies and qualifies the nature and forces of adsorption interaction, was used. The adsorption isotherm is described by the equations of the volumetric micropore filling theory (VMOT). The correlation between the adsorption-energy characteristics is found, and the molecular mechanism of methanol adsorption in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites throughout the filling region is revealed. Methanol adsorbed in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites is located in the first coordination sphere with cations Li+ and Cs+, forming tetra- and trimeric complexes. It is found that the charge density significantly affects the adsorption mechanism, the adsorption energy, and the number of adsorbed molecules. It was determined that the average molar entropy of ammonia adsorption on LiZSM-5 and CsZSM-5 zeolites indicates that the mobility of methanol molecules in the zeolite is below the liquid phase and close to the mobility of the solid phase, indicating a strong inhibition of mobility of methanol adsorbed on cations. This work shows how calorimetric data can complement crystal structure results and detect subtle adsorbent/adsorbate interactions at the molecular level.