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

Abstract

Natural gas upgrading requires the separation of its major components, predominantly CH4 and CO2, 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 CO2 from CH4 over a wide temperature and pressure range (298-373 K and 0-25 bar). Both zeolites exhibit impressive CO2/CH4 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 H2O. 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.