(619a) Novel Al-Rich Chabazite Compositions Enable Ar PSA

Adsorbent session

Novel Al-rich Chabazite Compositions Enable Ar PSA

Anthony Vallace¹, Grant Kester¹, William Casteel², Garret Lau²,

Roger Whitley², and Charles Coe¹

¹ Department of Chemical Engineering, Villanova University, Villanova, PA 19085

² Adsorption Technology Center, Air Products and Chemicals, Inc. 7201 Hamilton Blvd, Allentown, PA 18195

Carbon molecular sieves enable kinetic based gas separations. It has been shown that single phase chabazite (CHA) in a mixed cation form (Na/KCHA (X_Kz~21%)) having an Si/Al (SAR) range of 1.35 to 1.6 has both high oxygen diffusivity (D/r²) and high kinetic selectivity of O₂ over Ar. Due to the kinetic nature of the separation, the O­₂/Ar kinetic selectivity of Al-rich CHA was shown to be a strong function of the potassium cation exchange level as well as the SAR of the CHA at low and ambient temperatures. These CHA adsorbents were prepared by interzeolite conversion of Y-type zeolites in both powder and beaded form. Various pretreatments including ion exchange, caustic treatment, calcination, and steaming were performed on beaded and powder forms of NaY. A range of gel composition were studied to map the synthesis space for converting the Y-type zeolite to KCHA (1.44 -2.41). The SAR of the CHA, formed by this interzeolite conversion is controlled by the Si/Al atomic ratio, the total alkalinity, and the water content. It was shown that the SAR and Si/OH in the starting gel strongly affected the CHA product SAR; whereas the water content strongly affected the product phase (higher water content leads to densification). Decreasing the SAR(gel) or Si/OH leads to leaching of more SiO₂ into solution, which in turn leads to a lower SAR product. The consequence of low SAR and Si/OH gels is densification to an undesired P-zeolite (GIS-type) if the gel is too basic. Determining synthetic conditions for controlling the framework SAR is crucial because the mixed cation Na/K CHA is unselective for SAR’s>1.7. Activation studies needed for successful adsorption measurements clearly show that the Al content and K⁺ exchange level strongly affect the adsorbent’s hydrothermal stability. O₂ and Ar isotherms at 175K were obtained for various pure and mixed cation forms of CHA at different SARs prepared from beads and powders of NaY. After it was determined that 3K/unit cell with an SAR of 1.6 was the optimal CHA composition for removing O₂ from Ar, kinetic and equilibrium ion exchange studies were used to optimize the batch ion exchange conditions for both a single and double contact Na⁺ exchange to obtain the desired potassium level. The adsorption properties at 175K for the single and double contact Na exchanged chabazites were consistent with those of 3KNaCHA(1.6) prepared from a stoichiometric K⁺ back exchange of NaCHA(1.6), validating the batch exchange method. The adsorption properties of different pure and mixed alkali cation forms of CHA(1.4-2.4) at 175K show a significant decrease in Ar adsorption in 3KNaCHA at SAR<1.6. Kinetic adsorption data collected at 30°C also shows an optimal O₂/Ar kinetic selectivity for the 3KNaCHA(1.6) composition. Simulation results predict that these novel Al-rich compositions provide similar Ar recovery and double the Ar productivity, compared to CMS, for an ambient temperature pressure swing adsorption (PSA) process.