Selective adsorption of N2 over CH4 in flexible Sr2+- and Ba2+-UPRM-5 (TEA) titanium silicates: Effect of activation temperature

Abstract

Removal of N2 from CH4 continues to be a challenging endeavor due to the similar properties of both molecules and, particularly, from the process energy consumption point of view. Although adsorption separation processes offer an attractive platform to address the latter, working capacity and selectivity are usually limiting factors. In this work, porous Sr2+- and Ba2+-UPRM-5 titanium silicates adsorbents exhibiting flexible structures were studied to elucidate their applicability for the separation of N2 from CH4 at ambient temperature. These materials showcase surface areas that are an order of magnitude larger than other flexible titanium silicates as a result of the incorporation of tetraethylammonium (TEA) as a structure-directing agent during the hydrothermal synthesis. The adsorbents were fully characterized to consider their response to thermal treatment and its relationship to adsorption selectivity. For Sr2+-UPRM-5 (TEA), the selectivity reversal in favor of N2 at 25 °C occurred upon pre-activation of the adsorbent at 180 °C. However, for Ba2+-UPRM-5 (TEA) this happens at a pre-activation temperature of 320 °C most likely due to its structural thermal stability. Both materials did not exhibit N2 adsorption saturation within a 10 atm range and 25 °C, a reflection of the availability of larger surface areas and a feature suitable for natural gas upgrading at higher pressures. At 25 °C, the maximum observed kinetic selectivity value in favor of N2 and estimated at CH4 and N2 pressures of 0.95 and 0.05 atm was 25, which matches results reported for the commercially available ETS-4 type titanium silicate. However, the UPRM-5 (TEA) adsorbent can be fully activated at a much lower temperature.