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.