Placement of cations in NaX faujasite-type zeolite using (N,V,T) Monte Carlo simulations

Deep removal of trace C2H2 and CO2 from C2H4 by using customized potassium-exchange mordenite

Adsorptive separation using porous materials is a promising approach for separating alkynes/olefins due to its energy efficiency, while the deep removal of trace amounts of C₂H₂ and CO₂ from C₂H₄ is still very challenging for a commercial adsorbent. Herein, we report a low-cost inorganic metal cation-mediated mordenite (MOR) zeolite with the specific location and distribution of K⁺ cations acting as a goalkeeper for accurately controlling diffusion channels, as evidence of the experimental and simulation results. Deep purification of C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures was first realized on K-MOR with exceptional results, achieving a remarkable polymer-grade C₂H₄ productivity of 1742 L kg^-1 for the CO₂/C₂H₂/C₂H₄ mixture. Our approach which only involves adjusting the equilibrium ions, is both promising and cost-effective, and opens up new possibilities for the use of zeolites in the industrial light hydrocarbon adsorption and purification process.

Interplay of water, extra-framework cations and framework atoms in the structure of low-silicazeolites: the case of the natural zeolite Goosecreekite as studied by computer simulation

Computational methods are described that model accurately the structure of hydrated Ca-bearing zeolites. Using Goosecreekite as a model system we probe the influence of framework ordering, cation siting and hydration of pores on the structure and its stability. We develop a methodology which allows the location of Al within the framework to be determined together with the position of extra-framework cations, in a stepwise fashion, progressing from an anhydrous model, via a dielectric continuum model, to finally, a fully atomistic model of the water in the intrazeolite pore space. Our methods reveal the complex interplay of short- and long-range interactions on the optimal structure of such materials.

Molecular simulation studies of water physisorption in zeolites

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.