(629c) First Principles Derived, Transferable Force Fields for CO2 Adsorption in Cation-Exchanged Zeolites

The development of accurate force fields is vital for predicting adsorption in porous materials. Previously, we introduced a first principles-based transferable force field for CO₂ adsorption in siliceous zeolites (Fang et al. J. Phys. Chem. C 2012, 116, 10692). In this study, we extend our approach to CO₂ adsorption in cationic zeolites which possess more complex structures. Na-exchanged zeolites are first chosen for demonstrating the approach. These methods account for several structural complexities including Al distribution, cation positions and cation mobility, all of which are important for predicting adsorption. The simulation results are validated with high-resolution experimental measurements of isotherms and microcalorimetric heats of adsorption on well-characterized materials. The choice of first-principles method has a significant influence on the ability of force fields to accurately describe CO₂-zeolite interactions. The PBE-D2 derived force field, which performed well for CO₂ adsorption in siliceous zeolites, does not do so for Na-exchanged zeolites; the PBE-D2 method overestimates CO₂ adsorption energies on multi-cation sites that are common in cationic zeolites with low Si/Al ratios. In contrast, a force field derived from the DFT/CC method performed well. Agreement was obtained between simulation and experiment not only for LTA-4A on which the force field fitting is based, but for other two common adsorbents, NaX and NaY. Recently, we have expanded this approach to other cations including divalent Ca-exchanged zeolites as well as Ca/Na mixed zeolites. The effect of residual H₂O on CO₂ adsorption will also be considered.