No abstract

The heterogeneity in adsorption site is a fundamental characteristic of adsorption that is difficult to quantify. Herein, we quantify the site heterogeneity for CO 2 adsorption on NaX by recognizing the characteristics, types, and site occupancies at different loadings. We find an adsorption site geometry of Na••• OCO⊥ O z (where O z denotes an oxygen atom in the zeolite) and that the distance between C and O z is an indicator of heterogeneity. This distance is controlled by the interacting cations on SIII (Na III ), where the energies (numbers) of the dual-, single-, and zero-Na III adsorption sites are −57.2, −43.1, and −29.5 kJ/mol (17, 57, and 26 uc, (per unit cell)), respectively. The adsorption heats and isotherms calculated using heterogeneity-based models agree reasonably with experimental results for both NaX and NaY. An abundance of mechanisms underlying the trends in the adsorption heats and isotherms are elucidated based on the heterogeneity in the adsorption sites. Applying the proposed method to an LTA-4A zeolite shows that the adsorption site geometries do not change and that the distance between C and O z remains an indicator of heterogeneity. These consistencies support the transferability of the proposed method to other zeolites.

The current force fields used in grand canonical Monte Carlo (GCMC) simulations are frequently found to underestimate the low-pressure adsorption of CO 2 on zeolite 13X, which is crucial for engineering applications, including air prepurification and carbon capture from air. In this paper, a series of GCMC simulations are performed with a cation-free 13X model to study the influence of the force field parameters for host−guest interaction pairs on low-pressure adsorption. The unique effects of the equilibrium diameters of the interaction pairs on loading under low pressures are studied, and these effects are classified as one of three affecting modes according to their relative value for the interatomic distance of the interaction pair. Based on these results, six possible trends for a low-pressure adsorption isotherm with a changing equilibrium diameter are predicted. By revealing the different affecting modes caused by the microstructure of the host−guest interaction, the particular effects of the interaction pairs are explained. An efficient method to obtain the force field is thus proposed to develop a more accurate force field for lowpressure adsorption. The obtained force field is validated by comparing its results to experimental loading and isosteric heat data from the literature and by performing adsorption experiments in NaX with different Si/Al ratios.