Structural study of water/alcohol mixtures adsorbed in MFI and MEL porosils

Abstract

Ethanol and other biofuels produced during biomass conversion must be separated from the fermentation broth (mainly water) before being used as a fuel. This can be addressed by adsorption-based separation using porous materials. The key objective of this study is to obtain a molecular understanding of water and alcohol adsorption in pure-silica zeolites, particularly in silicalite-1 (MFI-type zeolite) and silicalite-2 (MEL-type zeolite). Molecular simulation techniques are used for this purpose. They provide information on the configuration of the fluids, and so the microscopic network structure of the adsorbed polar molecules can be characterized by using a specific criterion of hydrogen bonding formation. We conducted Grand-Canonical Monte Carlo simulations to compute the adsorption isotherms of pure short alcohols and water and from the liquid alcohol/water binary mixtures throughout the composition range. Despite MFI and MEL being structurally very similar, we found differences in adsorption, which are related to the underlying molecular behavior. While water intrusion occurs by applying pressure due to stronger water-water than water–silicalite interactions, notable water adsorption from the mixture occurs first by hydrogen bond formation with the adsorbed alcohols and then by self-association. A higher degree of water clustering in MEL compared to MFI zeolite, promoted by its straight channels, leads to relatively lower uptakes of water in the latter zeolite (in favor of alcohol molecules).