A new software package, RASPA, for simulating adsorption and diffusion of molecules in flexible nanoporous materials is presented. The code implements the latest state-of-the-art algorithms for molecular dynamics and Monte Carlo (MC) in various ensembles including symplectic/measure-preserving integrators, Ewald summation, configurational-bias MC, continuous fractional component MC, reactive MC and Baker's minimisation. We show example applications of RASPA in computing coexistence properties, adsorption isotherms for single and multiple components, self-and collective diffusivities, reaction systems and visualisation. The software is released under the GNU General Public License.

A novel united atom force field affords accurate and quantitative reproduction of the adsorption properties of linear and branched alkanes in nanoporous framework structures. The force field was generated by adjusting the parameters so as to faithfully reproduce the experimentally determined isotherms (particularly the inflection points) on MFI-type zeolite over a wide range of pressures and temperatures. It reproduces extremely well the Henry coefficients, heats of adsorption, preexponential factors, entropies of adsorption, and maximum loading. It is shown that the extension of the force field from MFI to other nanoporous framework topologies is successful, that it affords the prediction of topology-specific adsorption properties, and that it can be an effective tool to resolve the many discrepancies among experimental data sets.

Abstract:We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.

We have developed a complete force field that accurately reproduces the adsorption properties of carbon dioxide in a variety of zeolites with different topologies and compositions. The force field parameters were obtained by fitting to our own experimental data and validated with available data taken from the literature. The novelty of this force field is that it is fully transferable between different zeolite framework types, and therefore, it is applicable to all possible Si/Al ratios (with sodium as extra-framework cation) and for the first time affording the prediction of topology-specific and chemical composition-specific adsorption properties.

Molecular simulations were performed to study the adsorption behavior of water in the metal−organic framework Cu−BTC. This is one of the better-known materials of this type that is stable upon water adsorption/desorption. The charge of the framework atoms was fitted to reproduce the available experimental adsorption isotherm. This new set of interaction parameters was used to calculate Henry coefficients as well as the energies, entropies, and enthalpies for the different adsorption sites. Our simulations show that water has a surprisingly large affinity for the metal center in Cu−BTC compared to other that for molecules like carbon dioxide, nitrogen, oxygen, or hydrocarbons. This particular behavior could be further exploited for the separation of water from other compounds.

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Molecular simulations are an important tool for the study of adsorption of hydrocarbons in nanoporous materials such as zeolites. The heat of adsorption is an important thermodynamic quantity that can be measured both in experiments and molecular simulations, and therefore it is often used to investigate the quality of a force field for a certain guest-host (g - h) system. In molecular simulations, the heat of adsorption in zeolites is often computed using either of the following methods: (1) using the Clausius-Clapeyron equation, which requires the partial derivative of the pressure with respect to temperature at constant loading, (2) using the energy difference between the host with and without a single guest molecule present, and (3) from energy/particle fluctuations in the grand-canonical ensemble. To calculate the heat of adsorption from experiments (besides direct calorimetry), only the first method is usually applicable. Although the computation of the heat of adsorption is straightforward for all-silica zeolites, severe difficulties arise when applying the conventional methods to systems with nonframework cations present. The reason for this is that these nonframework cations have very strong Coulombic interactions with the zeolite. We will present an alternative method based on biased interactions of guest molecules that suffers less from these difficulties. This method requires only a single simulation of the host structure. In addition, we will review some of the other important issues concerning the handling of these strong Coulombic interactions in simulating the adsorption of guest molecules. It turns out that the recently proposed Wolf method ( J. Chem. Phys. 1999, 110 , 8254 ) performs poorly for zeolites as a large cutoff radius is needed for convergence.

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. DOI: 10.1103/PhysRevLett.93.088302 PACS numbers: 82.75.Jn, 47.55.Mh, 66.30.-h The effect of confinement on adsorption and diffusion is still poorly understood despite its importance for practical applications. The performance of molecular sieves in separation and catalytic processes depends critically on the match between sieve topology and the shape and size of the adsorbate [1]. It is therefore of considerable industrial importance to explore the adsorption and diffusion of linear and branched alkanes in different topologies using realistic simulations at the microscopic level [2]. Different parameter sets yield values of diffusivities that differ not only quantitatively but also show a different qualitative dependence on the molecular loading [3]. The critical unresolved question follows: Which of these parameter sets is the most physically realistic one? Here, we hope to remedy this situation.Potential parameter sets can be checked only via comparison with experiment. For diffusion the comparison is complicated by large discrepancies between microscopic and macroscopic experimental measurement methods, and even within the same measurement technique there are many disagreements between various studies. However, adsorption results seem to be well established and provide a more solid basis for a detailed comparison between experiment and simulation. Moreover, a large amount of data exists on adsorption of hydrocarbons in siliceous zeolites.Silicalite-1 (Fig. 1) consists of a three-dimensional pore system with straight parallel channels, intersected by zigzag channels [4]. The channels of approximately 6 Å in diameter lead to shape selectivity, especially for the isomers of hexane, which have dimensions close to the silicalite-1 pores. The linear channels intersect with the zigzag channels 4 times per unit cell. Interestingly, for n-heptane and for the branched alkanes in silicalite-1 a kink in the isotherm is observed [5]. This inflection is directly related to the number of intersections in the structure and occurs at exactly four molecules per unit cell. As these inflections are caused by a subtle interplay between the size and configuration of the molecule and two different adsorption sites, it becomes clear that the adsorbent-adsorbate potential size parameter is the most sensitive parameter in the force field.In general, adsorption in any periodic structure has steps or kinks. The reason is that steps and kinks signal transitions between diffe...