Cation Ordering and Exsolution in Copper-Containing Forms of the Flexible Zeolite Rho (Cu,M-Rho; M=H, Na) and Their Consequences for CO 2 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 Cu ²⁺ 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 CO ₂ rapidly at room temperature. Breakthrough tests with 10 % CO ₂ /40 % CH ₄ mixtures show that Cu _4.9 -Rho is able to produce pure methane, albeit with a relatively low capacity at this p _CO2 due to the weak interaction of CO ₂ with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO ₂ 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 CO ₂ /CH ₄ breakthrough tests, producing pure CH ₄ with improved uptake and CO ₂ /CH ₄ selectivity compared to that of Cu _4.9 -Rho.
(© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)