Cation ordering and exsolution in copper-containing forms of the flexible zeolite Rho (Cu,M-Rho; M = H, Na) and their consequences for CO₂ adsorption

The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper-exchanged form of Rho and mixed Cu,H- and Cu,Na-forms have been prepared. EPR spectroscopy reveals that Cu²⁺ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu-Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO₂ rapidly at room temperature. Breakthrough tests with 10 % CO₂/40 % CH₄ mixtures show that Cu4.9-Rho is able to produce pure methane, albeit with a relatively low capacity at this pCO₂ due to the weak interaction of CO₂ with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO2 to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na-Rho materials was prepared to improve uptake and selectivity compared to Cu-Rho, and kinetics compared to Na-Rho. Remarkably, Cu,Na-Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na-Rho showed improved performance in CO₂/CH₄ breakthrough tests, producing pure CH4 with improved uptake and CO₂/CH₄ selectivity compared to that of Cu_4.9-Rho.