Calorimetric and IR-Spectroscopic Study of Methanol Adsorption on Zeolites

Abstract

Abstract The heats of CH3OH adsorption on sodium faujasites and mordenites with different Si/Al ratios in the framework and on zeolites A, ZK-4, and ZK-5 were measured calorimetrically. The IR spectra of adsorbed CH3OH were determined for the same zeolites. The decrease of cation concentration with increasing Si/Al ratio in the framework of faujasites, mordenites, and zeolites A and ZK-4 results in the decrease of CH3OH adsorption heats for the broad coverage range and in weakening of the vOH disturbance for adsorbed CH3OH. For highly dealuminated faujasite, the OH vibrations in adsorbed CH3OH are strongly perturbed due to intermolecular interactions even at small coverages. The adsorption heat is very low in this case. Sodium cations at 8-membered oxygen rings of zeolite ZK-5 are more tightly bonded to the framework and so they are less active adsorption centers for CH3OH molecules as compared to the appropriate cations of zeolite Na-A. The step on the adsorption heat curve for CH3OH on zeolite Na-A in the coverage interval from 3 to 5 molecules/unit cell corresponds to the transition of adsorbed molecules from the interactions with cations in 8-membered oxygen rings to those with cations in 6-rings. The second step at coverages about 6 molecules/unit cell results from the crystallographic nonidentity of adsorption complexes formed by CH3OH molecules with cations at 4-rings for different uptakes. The acidic structural OH groups of decationated mordenite are involved in strong hydrogen bonds with adsorbed CH3OH molecules or protonate them.