Optimizing flexible zeolite Rho for unrivalled argon purification

Abstract

The preparation of pure Ar from air requires removal of O₂ from a crude Ar stream produced by cryogenic distillation. Whereas their similar boiling points make further separation by distillation expensive, the difference in their molecular sizes makes separation by flexible zeolites an attractive route to kinetic separation. Modifying the cation content of the flexible and cubic zeolite Rho enables precise tuning of a single window size for unprecedented kinetic separation of O₂ from Ar. Li-Rho (unit cell Li_9.8Al_9.8Si_38.2O₉₆) is a very selective adsorbent due to its small pore size, but the highly distorted framework results in very slow O₂ diffusion. By replacing some of the Li⁺ with H⁺, Cs⁺, or Zn²⁺, the window can be enlarged and in some cases the window cation occupancy reduced, increasing O₂ diffusion rates whilst retaining high O₂/Ar selectivity. The optimum unit cell composition among those examined was found to be Li_6.2Zn_1.8-Rho (O₂/Ar selectivity = 813, D/r²(O₂) = 0.37 s⁻¹).