A comparative study of the adsorption of water and methanol in zeolite BEA: a molecular simulation study

Abstract

Grand canonical Monte Carlo simulations were carried out to study the equilibrium adsorption concentration of methanol and water in all-silica BEA zeolite and HBEA zeolites with different Si/Al ratios over a wide range of temperatures and loadings. These zeolites have oval-shaped channels with one side longer than the other. Water sorption into the hydrophobic BEA zeolite had a sharp transition with its sorption going from zero to near full capacity over a very small pressure range. Methanol sorption was much more gradual with respect to pressure. With the addition of hydrophilic sites for the HBEA zeolites by decreasing the Si/Al ratio, adsorption at lower pressures increased significantly for water and methanol. At higher loadings, water and methanol adsorption were found to behave in fundamentally different ways. Water structures in the zeolite channels formed hydrogen-bonded chains while maximising contact with the surfaces on the longer edges of the zeolite channels. Methanol molecules, in contrast, formed very few hydrogen bonds between themselves, with their hydroxyl groups primarily binding with surface of the shorter edge of the zeolite channels and their methyl groups located near the middle of the zeolite channels. The addition of hydrophilic groups in the HBEA zeolites strongly influenced positions of the methanol hydroxyl groups at high loadings, but did not have a significant effect on water structure.

Keywords::

• molecular simulation of zeolite
• water
• methanol

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. Van T. Nguyen and Phuong T.M. Nguyen gratefully acknowledge support in the form of International Travel Awards from University of Queensland Graduate School.