Unusual Acid Sites in LSX Zeolite: Formation Features and Physico-Chemical Properties

The advanced approach for the preparation of the NH4 form of highly crystalline LSX zeolite under gentle drying conditions (40 degrees C, membrane pump dynamic vacuum) is discussed. Decationization of this form at moderate temperatures led to the formation of Bronsted acid sites (BASs), whose concentration and strength were characterized by IR spectroscopy. It was found that a maximum concentration of three BASs per unit cell can be achieved at 200 degrees C prior to the initiation of zeolite structure degradation. The proton affinity of BASs is unusual, and aspires 1240 kJ/mol, which is significantly higher compared to faujasites with higher moduli. The increase in temperature of the heat treatment (up to 300 degrees C) resulted in thermal decomposition of BASs and the manifestation of amorphous phase with corresponding Lewis acid sites (LASs) as well as terminal Si-OH groups. Both the destruction of BASs and formation of the LAS-containing amorphous phase are the key reasons for the significant decrease in the adsorption capacity in the micropore region revealed for the sample decationized at 300 degrees C.

Automated summary

This study discusses an advanced method for preparing highly crystalline NH4-form LSX zeolite under gentle drying conditions. Decationization of this form at moderate temperatures leads to the formation of BASs characterized by IR spectroscopy. The maximum concentration of three BASs can be achieved prior to the initiation of zeolite structure degradation at 200 degrees C. The high proton affinity and thermal decomposition of BASs result in a significant decrease in adsorption capacity.