Alcohol Adsorption onto Silicalite from Aqueous Solution

A methodology based on the combination of grand canonical Monte Carlo (GCMC) and expanded ensemble (EE) simulations has been used to study the adsorption of alcohols from dilute aqueous solution onto the silicalite zeolite. The chemical potential of the guest alcohol molecules in bulk aqueous solutions is calculated by the EE method, and the adsorption isotherms of the alcohols are calculated using GCMC simulations. This approach results in adsorption isotherms that relate the loadings to external concentrations without using an equation of state, experimental data, or performing computationally expensive simulations such as two-phase Gibbs ensemble Monte Carlo (GEMC). Also, the method is force-field-consistent, in that the same force field is used for the bulk fluid and the adsorption calculations, and the resulting isotherms can be directly compared with experimental data. We have established the validity of the method used by comparing the calculated adsorption isotherms with experimental data for methanol and ethanol on silicalite, both from aqueous solution and from the vapor phase. Calculated heats of adsorption are also in reasonable agreement with experimental data. In addition, these simulations provide information on the location of the alcohol adsorption sites. We also observe that the presence of water increases the adsorption of alcohols at dilute concentrations onto silicalite; therefore, an accurate estimate of amount of water adsorbed in the nanoporous substrate is important to the prediction of alcohol adsorption from aqueous solution. The method presented here can be used to study the adsorption of any molecular species on any nanoporous adsorbent.