Adsorption and diffusion of gases in ETS-4

Abstract

Engelhard Titanium Silicate-4 (ETS-4, Na-ETS-4) and its Sr-exchanged variant (Sr-ETS-4) have been synthesized, their crystalline phase identification done by X-ray diffraction (XRD), the particle morphology studied using Scanning Electron Microscopy (SEM), and the thermal stability investigated using Thermogravimetric analysis (TGA). Adsorption equilibria and uptakes of oxygen, nitrogen and methane in Na-ETS-4 and Sr-ETS-4 samples dehydrated at higher temperatures have been measured volumetrically over a wide range of temperatures, pressures, and adsorbate loadings and analyzed in order to establish suitable isotherm and kinetic models. Complete exchange of Na+ with Sr2+ resulted in a significantly rapid uptake of nitrogen compared to methane. However, the large diffusivity ratio favoring nitrogen did not translate into a high kinetic selectivity due to strong methane adsorption. Progressive pore contraction with increasing dehydration temperature resulted in equilibrium reversal in Sr-ETS-4, but it also reduced the diffusivity ratio. Partial Sr-exchange did not bring about the desired synergism of synchronizing the best in equilibrium and diffusivity ratios. Two homogeneous equilibrium isotherm models, namely, Langmuir isotherm, and Multisite Langmuir isotherm, and a heterogeneous model based on a local Langmuir isotherm were used to analyze the measured equilibrium data. Although heterogeneous analysis resulted in some improvement in a few isolated cases, from an overall perspective, the homogeneous analysis appears quite adequate in capturing the experimental equilibrium data and concentration dependence of the micropore diffusivity. Hence, the Multisite Langmuir isotherm and a bidispersed pore diffusion model including concentration dependence of micropore diffusivity according to chemical potential as the driving force for diffusion were examined on limited binary nitrogen-methane data. The model could satisfactorily explain the binary integral uptakes of 50:50 N2:CH4 and 90:10 N2:CH4 mixtures in the Sr-ETS-4 samples dehydrated at 190 and 270 oC. The binary Multisite Langmuir model, however, overpredicted the experimental binary isotherms. The nitrogen-methane separation selectivity in the two adsorbents was revaluated taking into account longer contact times, loading beyond the linear range, and coupled equilibrium and diffusion under binary conditions.