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 M Lozinska,Paul A Wright,Maarten C Verbraeken,Claire A Murray,Sophie Jamieson,Bela E Bode,David N Miller,Stefano Brandani

doi:10.1002/chem.202101664

Magdalena M Lozinska, Paul A Wright + Show 6 more

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https://doi.org/10.1002/chem.202101664

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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,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.

Highlights

Zeolites find widespread use as adsorbents in a range of commercially-important gas separations involving small molecules, including air separation and hydrogen purification (CO2/H2).[1,2,3],[b] Dr M
The results show that the kinetics and selectivity of CO2 adsorption by zeolite Rho are strongly dependent on the extraframework composition
Cu cations show a strong preference for the single 6ring (S6R) sites, where they are located in trigonal coordination in the plane of the 6Rs, up to ca. 3 per unit cell, some are found in the single 8-ring (S8R) sites in samples with higher Cu content

Summary

Introduction

Zeolites find widespread use as adsorbents in a range of commercially-important gas separations involving small molecules, including air separation (where N2/O2 selectivity is required) and hydrogen purification (CO2/H2).[1,2,3] ,[b] Dr M. Cu2+ cation into Na9.8-Rho has little effect on the CO2 adsorption properties so that it shows hysteresis at low pressures caused by the cation gating effect. In this material there remains around one Na cation per d8r window,[22] so that cation migration is still required to permit uptake. Uptake is much faster for the adsorption of CO2 on the two-phase Cu3.4Na3.0-Rho sample and negligible hysteresis was observed. Type I behaviour is expected for the minority Cu-rich phase, as it is close to the Cu end-member.

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