Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at High Pressures

Natural gas upgrading requires the separation of its major components, predominantly CH 4 and CO 2, at the high pressures found at the wellhead. Here we demonstrate that the Na + -tetraethylammonium form of ZSM-25 and PST-20, the fourth and fifth generations of the RHO family of zeolites with embedded isoreticular structures, can be efficiently employed to separate CO 2 from CH 4 over a wide temperature and pressure range (298–373 K and 0–25 bar). Both zeolites exhibit impressive CO 2 /CH 4 selectivity (>20) at 298 K and 25 bar, relatively large adsorption capacity (4.0 and 3.6 mmol g –1, respectively) even at 373 K and 25 bar, and long-term durability even in the presence of H 2 O. These superior separation properties could be linked to the limited framework flexibilities of ZSM-25 and PST-20 mainly arising from the larger ratio of embedded to scaffold cages in the higher generation of the RHO family.