Physical adsorption of an inert gas such as nitrogen or argon is commonly used to characterize zeolites and other porous materials. In high-resolution adsorption (adsorption in micropores), it has long been known that nitrogen begins to adsorb at a substantially lower relative pressure than argon, despite the similar sizes of the two absorbates. The standard explanation for this difference in relative pressure is that nitrogen has a quadrupole moment, which strengthens its interactions with the zeolite framework atoms, whereas argon does not. In this study, we investigate this standard explanation by simulating the adsorption of nitrogen at 77 K and argon at 87 K in siliceous zeolites of framework types LTA, FAU, and MFI using grand canonical Monte Carlo. For nitrogen, we tested models with a quadrupole formed by three collinear point charges, as well as models in which the effect of the quadrupole moment was removed by setting the point charges to zero. We find that the influence of the quadrupole moment is heavily dependent on the magnitude of the zeolite framework charges in the model. For a model with relatively weak framework charges, the quadrupole moment makes very little difference, but with stronger framework charges, nitrogen with its quadrupole moment begins to adsorb at much lower pressures than nitrogen without its quadrupole moment, accounting for most of the spread between nitrogen and argon. Since the larger-magnitude charges we studied are more in line with theoretically-derived values, we conclude that the standard explanation is largely correct. We recommend that zeolite models use partial charges with reasonable theoretical justification and that are checked against the accuracy of simulations of adsorption of quadrupolar gases.
Hackett, C., & Hammond, K. D. (2018). Simulating the effect of the quadrupole moment on the adsorption of nitrogen in siliceous zeolites. Microporous and Mesoporous Materials, 263, 231–235. https://doi.org/10.1016/j.micromeso.2017.12.018