Influence of cation Na/Ca ratio on adsorption in LTA 5A: a systematic molecular simulation study of alkane chain length.

Recent adsorption isotherms of n-alkanes on Ca,Na-LTA-type zeolite afford development of a force field describing the interactions between calcium and n-alkanes in configurational-bias Monte Carlo simulations. The force field of Calero et al. (J. Am. Chem. Soc. 2004, 126, 11377-11386) is able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. Here, we extend upon this type of force field by including calcium-type ions. The force field was fitted to reproduce the calcium and sodium cations positions on LTA 5A and the experimental adsorption properties of n-alkanes over all range of temperatures and pressures. This opens up a vast amount of experimental data on LTA 5A, both on adsorption and diffusion. Furthermore, evaluation of half a century of reported n-alkane adsorption data on LTA-type zeolites indicates that there are many inconsistencies between the various data sets, possibly as a result of (i) undisclosed calcium and sodium contents, (ii) less than perfect drying of the hygroscopic zeolite, and (iii) coadsorption of contaminants such as vacuum grease. Having obtained our force field, and confirmed its reliability on predictions outside the calibration set, we apply the force field on two "open" problems: (a) the heats of adsorption and Henry coefficient as a function of chain length and (b) the effect of cations in LTA-type zeolites. The molecular simulations shed new light on previous experimental findings, and we provide rationalizations on the molecular level that can be generalized to the class of cage/window-type nanoporous materials.

A coarse-graining approach for the proton complex in protonated aluminosilicates.

We have developed a computational framework for the adsorption of linear alkanes in protonated aluminosilicates. These zeolites contain trace amounts of water that form hydrated proton complexes. The presence of hydrated protons makes the simulations at the fully atomistic level difficult. Instead of constructing an elaborate and complex model, we show that an approach based on a coarse-graining of the proton-complex accurately describes the available experimental isotherms, Henry coefficients, heats of adsorption, and oxygen-proton distances. Our approach is supported by MP2 quantum mechanical simulations. The model gives remarkably good agreement with experimental data beyond the initial calibration set.

Force field parametrization through fitting on inflection points in isotherms.

We present a method to determine potential parameters in molecular simulations of confined systems through fitting on experimental isotherms with inflection points. The procedure uniquely determines the adsorbent-adsorbate interaction parameters and is very sensitive to the size parameter. The inflection points in the isotherms are often related to a subtle interplay between different adsorption sites. If a force field can predict this interplay, it also reproduces the remaining part of the isotherm correctly, i.e., the Henry coefficients and saturation loadings.

Incommensurate diffusion in confined systems.

Molecular simulations corroborate the existence of the disputed window effect, i.e., an increase in diffusion rate by orders of magnitude when the alkane chain length increases so that the shape of the alkane is no longer commensurate with that of a zeolite cage. This window effect is shown to be characteristic for molecular sieves with pore openings that approach the diameter of the adsorbate. Furthermore, the physical compatibility between the adsorbate and the adsorbent has a direct effect on the heat of adsorption, the Henry coefficients, the activation energy, and the frequency factors.