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

Abstract

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,Naforms 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.

A forgotten participant in pore deblocking of zeolites: dicarbonate in NaMeA zeolites, Me = Na, K, Rb, Cs

“…3 Cation gating appeared in different cationic form zeolites as cationic displacements in the presence of CO 2 or its mixtures. 3–6 The flexibility-driven effects are observed in the same systems in parallel. More precisely, the unusual growth of CH 4 adsorption of Me′MER zeolites (Me′ = Na, K, Cs with different Si/Al modulus) after the minor addition of CO 2 to the gas has been interpreted as a consequence of the transformation of the 8R windows.…”

Toward the Synthesis of New Zeolite Structures in the Presence of Cesium: Zeolite MMU-1

“…Zeolite RHO has shown great potential for use in such technologies because of its very high CO 2 /CH 4 selectivity and high CO 2 adsorption capacity . In addition, its flexible framework with tunable pore sizes depending on the extraframework cations offers additional control over its adsorption properties and “trapdoor” effects. − Other eight-membered-ring zeolites have also demonstrated potential for CO 2 adsorption applications, namely Zeolite-F, MER, and GIS …”

Engineering RHO Nanozeolite: Controlling the Particle Morphology, Al and Cation Content, Stability, and Flexibility

“…The distortion or ellipticity (Δ) of the D8Rs can be described by the average of the shortest and longest O–O distances within the cross-section of the D8R . The ability to easily tune the pore size and shape by simply varying the extra-framework cation content allows for the facile tuning of the zeolite’s properties for applications such as catalysis and adsorption. ,− In particular, due to the ability of RHO zeolites to selectively adsorb CO 2 over nonpolar molecules such as CH 4 and N 2 , the last decade has seen renewed investigation and further characterization of the structural behavior of RHO focussing on the extra-framework cation content and adsorption performance of the zeolites. ,,,− …”