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

Magdalena Malgorzata Lozinska, Sophie Jamieson, Maarten Christiaan Verbraeken, David Noel Miller, Bela Bode, Claire Murray, Stefano Brandani, Paul Anthony Wright*

*Corresponding author for this work

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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 Cu4.9-Rho.
Original languageEnglish
Article number202101664
Pages (from-to)13029-13039
Number of pages12
JournalChemistry - A European Journal
Issue number51
Early online date6 Aug 2021
Publication statusPublished - 9 Sept 2021


  • Copper zeolites
  • CO2 adsorption
  • CO2 separation
  • Exsolution
  • Zeolite Rho


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