Partial Atomic Charge Methods for Simulating Porous Frameworks with a Net Charge and their Applications to Gas Separations in Zeolites

Abstract

Computational simulations using empirical force fields are frequently used to model guest-host interactions in porous periodic systems, where the interaction energy is broken into electrostatic and van der Waals contributions. While simulations such as these have been instrumental in progressing our understanding of neutral periodic systems, limitations in deriving partial atomic charges has largely contributed to the difficulty in modeling charged periodic frameworks. However, many nanoporous materials possess frameworks that have a net charge, which is balanced by counter-ions that intercalate through the pores. For example, virtually all zeolites used in practice contain a proportion of Al, which bestows the framework with a negative charge. In this respect, we investigate two methods for the generation of partial atomic charges in periodic systems having a net framework charge. First, we examine the validity of generating REPEAT electrostatic potential fitted charges derived from periodic electronic structure calculations, where a constant background charge is added to neutralize the net charge on the framework without adding neutralizing counter-ions. The second method we explore is the split charge equilibration (SQE) method for very rapid charge generation. In its original formulation, the SQE model cannot be applied to systems with a net charge. In this work, we reformulate the SQE method for non-neutral systems to be treated. The new SQE model, which we call SQEAB, was shown to give equivalent results to those of the original SQE model for neutral systems. For charged frameworks, the model was shown to provide partial atomic charges in good agreement with the DFT derived REPEAT method. Taking advantage of that work, we next focus on the development of a force field for modeling CO2, N2, and CH4 gas adsorption in both neutral and charged zeolites, which we call the AMP (Aluminosilicate MicroPorous) force field. Commonly, the electrostatic potential of zeolites is represented through the use of generic charges, where every atom of the same type in the framework is assigned the same atomic charge. Though this model is fast, it fails to account for structural differences between framework geometries. In this work, we have optimized a set of SQEAB parameters to reproduce the DFT derived electrostatic potentials (ESPs) of a structurally representative set of both neutral and charged zeolite frameworks. Comparing with other popular models, the SQEAB-AMP charges are shown to better reproduce the QM ESP by more than 30%, on average. Gas uptakes obtained using SQEAB AMP charges were found to be within 5% of those obtained using DFT derived charges. We have further optimized a set of Lennard-Jones parameters to be combined the SQEAB-AMP charges that reproduce experimental uptake data in zeolites.