(503f) Size-Inverse Sieving for Xe/Kr Separation Via Molecular Trapdoor Effect in LTA Zeolites

Xenon (Xe) and Krypton (Kr) are high-value gases with diverse applications, but their separation is energy-intensive and costly, relying primarily on cryogenic distillation of air separation byproducts. Adsorption-based separation offers a more cost-effective alternative; however, current adsorbents suffer from low capacity and/or selectivity. While some materials exhibit molecular sieving for Kr (the smaller molecule), this is inefficient because Kr is the major component in the mixture. Instead, selective adsorption of the minor component, Xe, is more energy-efficient and highly desirable, yet achieving sieving-level selectivity for Xe remains a significant challenge.

In this study, we report a molecular trapdoor mechanism, a concept discovered by us[1-6], to overcome these limitations. By leveraging this mechanism, we designed an LTA trapdoor zeolite capable of size-inverse molecular sieving, achieving unprecedented Xe selectivity and high Xe uptake. Remarkably, we demonstrated invertible sieving-level separation between Xe and Kr by tuning the door-keeping cations, enabling exclusive adsorption of either Xe over Kr or the reverse. This counterintuitive achievement holds great scientific interest and highlights the versatility of the molecular trapdoor approach.

Our findings represent a breakthrough in adsorption-based Xe/Kr separation, establishing a foundation for energy-efficient and highly adaptable gas separation technologies.

References

[1] Shang, J., Li, G., Singh, R., Gu, Q., Nairn, K.M., Bastow, T.J., Medhekar, N., Doherty, C.M., Hill, A.J., Liu, J.Z., and Webley, P.A., Discriminative Separation of Gases by a “Molecular Trapdoor” Mechanism in Chabazite Zeolites. Journal of the American Chemical Society, 2012. 134(46): p. 19246-19253.

[2] Shang, J., Li, G., Gu, Q., Singh, R., Xiao, P., Liu, J.Z., and Webley, P.A., Temperature controlled invertible selectivity for adsorption of N2 and CH4 by molecular trapdoor chabazites. Chemical Communications, 2014. 50(35): p. 4544-4546.

[3] Li, G., Shang, J., Gu, Q., Awati, R.V., Jensen, N., Grant, A., Zhang, X., Sholl, D.S., Liu, J.Z., Webley, P.A., and May, E.F., Temperature-regulated guest admission and release in microporous materials. Nature Communications, 2017. 8: p. 15777.

[4] Shang, J., Hanif, A., Li, G., Xiao, G., Liu, J.Z., Xiao, P., and Webley, P.A., Separation of CO2 and CH4 by Pressure Swing Adsorption Using a Molecular Trapdoor Chabazite Adsorbent for Natural Gas Purification. Industrial & Engineering Chemistry Research, 2020. 59(16): p. 7857-7865.

[5] Tian, Y., Tao, Z., Liu, C., Sun, M., Chang, C., Gu, Q., Li, L., and Shang, J., Adjusting gate-opening behavior in a rigid cage-type “molecular trapdoor” metal–organic framework via anion modulation. Chemical Engineering Journal, 2024. 486: p. 150293.

[6] Tian, Y., Tao, Z., Sun, M., Wang, T., Li, L., Gu, Q., and Shang, J., Tunable Gas Admission via a “Molecular Trapdoor” Mechanism in a Flexible Cationic Metal–Organic Framework Featuring 1D Channels. Small, 2024. 20(27): p. 2400064.