Binary ion-exchanged ETS-4 (Sr/Ba-ETS-4): synthesis, characterization, and selective nitrogen adsorption from methane for natural gas purification

Several variants of ETS-4 were synthesized through single-cation and binary-cation exchange, with a particular emphasis on Sr/Ba-ETS-4. The impact of binary-cation exchange on the structural characteristics was investigated using various analytical techniques, including XRD, FESEM, EDX, ICP-MS, TGA, and FT-IR analysis. The effect of mixed Sr and Ba exchange within ETS-4 on the adsorption, separation, and kinetic behavior of nitrogen and methane gases was explored utilizing the volumetric method at 30°C and up to 100 kPa. Additionally, the investigation extended to examining the influence on diffusion coefficients employing the macropore-micropore kinetic model. ICP-MS results revealed a competitive advantage of barium in the exchange process among incoming cations. Notably, significant discrepancies in the adsorption capacity for nitrogen and methane gases were observed between the Sr-ETS-4 and Ba-ETS-4 samples. Experimental isotherm data suggested that the Sips model offered the most suitable fit, unlike the Langmuir or Freundlich models. Among the Sr/Ba-ETS-4 adsorbents investigated, the adsorbent with the highest barium content in the framework exhibited superior nitrogen adsorption capacity (0.33 mmol/g) and the lowest methane adsorption capacity (0.07 mmol/g), leading to high nitrogen over methane equilibrium selectivity (3.12) and kinetic selectivity (167.6).