Increase in the hydrothermal stability of the La-modified ZSM-5 zeolite

Abstract

We carried out a theoretical investigation into the mechanism of steam dealumination of the ZSM-5 zeolite and into the mechanism for the increase in the hydrothermal stability of the La-modified ZSM-5 zeolite. This was done using density functional theory (DFT) with 12T cluster models that simulated the local structures of the zeolite materials. We demonstrate that because of the hydrogen bond interaction between the first adsorbed water molecule and the ZSM-5 zeolite framework, the Al-O bond is weakened and elongated. As the second water molecule is adsorbed, the Al-O bond near the second water molecule is further weakened and eventually broken because of the hydrogen bond interaction between the second adsorbed water molecule and the ZSM-5 zeolite framework. As more water molecules are adsorbed, the other Al-O bonds are broken sequentially resulting in a dealumination of the ZSM-5 zeolite. The introduced lanthanum coordinates with four zeolite framework oxygen atoms, thickens the zeolite framework, moves over the framework Al atom, increases the steric hindrances and partially prevents polar water molecules from attacking the Al-O bond. These actions retard the weakening of the Al-O bonds and improve the hydrothermal stability of the ZSM-5 zeolite. The calculated adsorption and hydrolysis energies of the water molecule further confirm that the presence of lanthanum enhances the hydrothermal stability of the ZSM-5 zeolite. © Editorial office of Acta Physico-Chimica Sinica.