The effect of regeneration temperature on the adsorptive properties of Na-ETS-4 zeotype in successive adsorption/desorption cycles

In view of the sensitivity of Na-ETS-4 zeotype to temperature, the effect of regeneration temperature on its adsorptive performance was investigated in this work. Adsorption isotherms and uptake kinetics of nitrogen and methane were measured after applying a heat treatment at temperatures of 100, 135, and 165 ^◦C along with vacuum as thermal activation. Increasing the initial activation temperature in the 100–165 ^◦C range led to an increase in adsorption capacity due to the intra-pore water removal. At 100 kPa, methane uptake increased from 0.2 to 0.35 mol/kg and nitrogen uptake increased from 0.07 to 0.16 mol/kg. By applying successive regenerations at 100 and 135 ^◦C, the equilibrium/kinetic adsorption properties did not change. However, by successive regeneration of the adsorbent at 165 ^◦C, the equilibrium capacity (for methane from 0.35 to 0.1, for nitrogen from 0.16 to 0.1 mol/kg at 100 kPa) and diffusion time constant parameter (for methane from 0.02 to 7.16 × 10⁻⁵ s⁻¹) decreased gradually. Also, structural changes of Na-ETS-4 under successive high-temperature (i.e., 165 ^◦C) regeneration were studied by XRD. A gradual reduction of Na-ETS-4 crystallinity was revealed after each regeneration at 165 ^◦C. However, Na-ETS-4 showed stable adsorption performance when using only vacuum as regeneration method.