Separation of CO2 and CH4 by Pressure Swing Adsorption Using a Molecular Trapdoor Chabazite Adsorbent for Natural Gas Purification

Adsorption materials play a key role in determining the efficiency of the pressure swing adsorption process. The selectivity of CO 2 over CH 4 on most zeolite adsorbents drops dramatically with increasing pressure, and few adsorbents work for CO 2 /CH 4 separation at high pressures. Here, we show that the high selectivity (81) can be retained with our “molecular trapdoor” adsorbent—a potassium exchanged chabazite with an Si/Al ratio of 1, which can selectively admit gas molecules by regulating its cation-kept eight-membered ring (8MR) doorways. Analysis of equilibrium adsorption of CO 2 and CH 4 showed an exceptionally high selectivity (81) of CO 2 over CH 4; when the temperature is lower than a threshold value, CH 4 molecules cannot access the interior structure of chabazite via K + blocked 8MRs. We confirmed this characteristic experimentally by adsorption equilibria of CO 2 and CH 4 up to 10 bar. We investigated the behavior of this adsorbent in a five-step pressure-vacuum swing adsorption (PVSA) cycle to separate CO 2 and CH 4 from a natural gas stream (15 vol % CO 2 balanced with CH 4 ) at a temperature of approximately 293 K and at a feed pressure of 20 bar. The effect of adsorption time on the separation performance was investigated since the adsorbent is known to have kinetic limitations. Our experiments show that the CH 4 product with 100% purity and over 90% recovery can be obtained by using the cyclic adsorption process with our novel molecular trapdoor chabazite.