Flexible Template-Free RHO Nanosized Zeolite for Selective CO2 Adsorption

Flexible small-pore zeolites are interesting candidates for flue and natural gas processing due to their high sorption capacity and selectivity. It is generally accepted that their high CO2/CH4 selectivity is due to cation gating behavior such as the "trapdoor" effect or cation-controlled molecular sieving. Herein, nanosized RHO-type zeolite containing only inorganic cations (Na+ and Cs+) has been prepared from a colloidal precursor suspension at 90 degrees C for 1 h without the use of expensive organic structure directing agents. The high Cs content significantly improved the thermal stability of the RHO nanocrystals up to 550 degrees C. The flexibility of the RHO cages upon water adsorption/desorption is demonstrated. The dehydration of the nanosized RHO zeolite resulted in two dehydrated RHO phases, the first one presents an enlargement of the cubic unit cell and a transformation from distorted to the more regular d8r units, and the second one presents a reduction of the cubic unit cell and an increase of the distortion of the d8r units. The 8-rings elliptical distortions of the as-synthesized RHO (hydrated form) of 1.87 angstrom changed to 0.94 and 2.16 angstrom for the two dehydrated RHO forms. The flexibility of the nanosized RHO zeolite is due to the ability of the Cs' cation to displace out from and into the D8R window sites which is of great importance for controlling the selective adsorption capacity of the RHO zeolite. The flexible RHO-type nanosized zeolite exhibited great selectivity toward CO2 over CH4. The adsorption capacity is retained after 10 cycles of CO2 adsorption/desorption and the crystalline structure is fully preserved.