(323f) Prediction of Adsorption of CO2 and Hydrocarbons in Zeolites: A Strong Test of Atomically Detailed Models

As an increasingly valuable complement to experiment, computational modeling approaches such as Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) have been used to predict adsorption and diffusion properties ahead of experiment for guest molecules in porous materials like zeolites and interpret experimental observations at the molecular level. Successful implementation of GCMC and MD simulations requires reliable force fields (FFs) to describe the interactions of guest molecules with porous frameworks. Developing accurate and transferable FFs for adsorbed and diffused molecules is challenging.

Since first-principles quantum mechanical (QM) methods are capable of accurately predicting intermolecular interactions, deriving FFs from QM data without experimental input is a promising solution. We previously used this approach to develop a FF for adsorption of CO₂ in siliceous and cationic (Na and K) zeolites. This FF accurately predicts experimentally observed adsorption properties and shows good transferability across different zeolite topologies and Si/Al ratios. In this work, we extend this FF for CO₂ adsorption in ammonium ZSM-5, a zeolite reference material RM 8852, where good reproducibility and high reliability of the experimental isotherm data is available. We also independently performed experiments with RM 8852 by microcalorimetry. The results show that at low pressures good agreement was obtained between predictions and experiments. At high pressures, however, deviations were observed. We demonstrate that the charge-balancing cations in the experimental material are the predominant source of the discrepancy between simulation and experiment at high pressures; the experimental sample treatment causes deammoniation. In addition, accounting for a small amount of non-crystalline mesoporosity in this ZSM-5 brings predictions in much better agreement with experiments.

We also extend this FF to hydrocarbons. Most previous studies on development of FFs for hydrocarbon molecules in zeolite materials focus on prediction of adsorption properties. However, accurately reproducing adsorption data is not sufficient to guarantee the accuracy of predicting diffusivities. We demonstrate an approach to develop FFs that can accurately predict both adsorption and diffusion properties of hydrocarbons in siliceous zeolites.