Predicting methane and nitrogen adsorption in ZSM-5 and FAU-13X zeolites with a SAFT-VR-Mie classical density functional theory.

Abstract

We present a classical density functional theory (cDFT) framework based on the SAFT-VR-Mie equation of state for the prediction of gas adsorption in zeolitic materials, using a fully three-dimensional atomistic external potential built directly from crystallographic data. The approach is applied to methane and nitrogen adsorption in two structurally and chemically distinct zeolites: the high-silica ZSM-5 and the cation-rich FAU-13X; with a single fluid-oxygen interaction parameter calibrated on one experimental high-loading point, the model accurately reproduces methane adsorption isotherms over the full pressure range and at three temperatures (273, 313, and 363 K). The same calibration scheme transfers without further adjustment from cation-free to Na-exchanged frameworks and from ZSM-5 to FAU-13X, demonstrating the transferability of the parameters across temperatures, fluids, and zeolite structures. Isosteric heats of adsorption and Henry's law constants derived from the simulated isotherms are consistent with experimental values. Nitrogen adsorption in cation-rich FAU-13X is, however, systematically underestimated; we discuss this limitation in light of the spherical, electroneutral monomeric assumptions inherent to the current SAFT-VR-Mie parameterization. The combination of SAFT-VR-Mie cDFT with realistic three-dimensional external potentials provides a transferable, computationally efficient, and physically rigorous foundation for adsorption modeling in nanoporous materials.