Alcohol Adsorption onto Silicalite from Aqueous Solution

Abstract

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.

A priori predictions of type I and type V isotherms by the rigid adsorbent lattice fluid

“…This mild hydrophilicity then leads to type I isotherms for water on ZSM-5 and can also be attributed to the presence of extra framework charged defects. Similarly for ethanol, inflections at low pressures have been reported in both simulation and experimental work (Dubinin et al 1989;Xiong et al 2011), yet often the behaviour is regarded as type I, due to lack of data under these conditions (Oumi et al 2002). Nonetheless, regardless of the exactness of the predictions, the inflections as shown in Figs.…”

Studying two-dimensional zeolites with the tools of surface science: MFI nanosheets on Au(111)

“…In fact this is clear for this case in which the presence of methanol is evident by the LF phonon shift even before the C-O stretching vibration is visible in the spectrum, suggesting that the C-O bond is oriented parallel to the surface plane for low coverages. Isotherms for methanol adsorption on bulk silicalite at 303 K have been published by Dubinin et al [16] and later simulated by Monte Carlo methods, [28][29][30] Carlo simulations underestimate the methanol adsorption at these low pressures and they attribute this to the presence of defects in the structure in the experimental case, which are absent in the simulations. [28][29][30] A rapid increase in methanol adsorption starts at pressures higher than ~ 6 Torr.…”

Adsorption of HMF from Water/DMSO Solutions onto Hydrophobic Zeolites: Experiment and Simulation

“…This indicates that the pore structure of silicalite-1 has a significant effect on the adsorption of larger molecules (such as HMF) but only a minor effect on smaller molecules (such as DMSO). [37] Finally, deviations between experiments and simulations, caused from the force fields used in calculating the solvation free energy, may lead to a shift in the fugacity-composition relationship and, therefore, in adsorption isotherms, as observed for DMSO adsorption using different models. Specifically, adsorption is very sensitive to defects that could lead to a significant increase at low loadings in experiments.…”

“…[42] In addition, framework flexibility and surface adsorption, which were not considered in our simulations, could result in higher loadings. [37] Finally, deviations between experiments and simulations, caused from the force fields used in calculating the solvation free energy, may lead to a shift in the fugacity-composition relationship and, therefore, in adsorption isotherms, as observed for DMSO adsorption using different models. Overall, the simulations are generally in reasonable agreement with experiments using the PARA silicalite-1 structure.…”

“…excess chemical potential of species i, which is the solvation free energy of species i [35] and also that calculated using the expanded ensemble (EE) [36] method. In the original GCMC-EE method, [37] EE simulations were used to calculate m 1 i and then used to relate f i to x i with Equation (4). Then, GCMC [38] simulations were used to calculate the extent of adsorption by specifying the chemical potential (fugacity), volume, and temperature.…”