(30a) Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membrane for Efficient Carbon Capture

Polycrystalline membranes based on metal-organic frameworks (MOFs) are advantageous to their nanoporous counterparts owing to a high degree of structural tunability, relatively fast crystallization kinetics, and higher reproducibility [1]. However, achieving an efficient CO₂/N₂ and CO₂/CH₄ separation, crucial for the energy-efficient carbon capture, has remained a challenge [2]. ZIF-8 possesses a crystallographically-determined pore aperture of 0.34 nm, ideal for CO₂/N₂ and CO₂/CH₄ separations. However, the lattice flexibility and the gate-opening phenomenon in ZIF-8 is a bottleneck in realizing gas-sieving, critical for the membrane-based postcombustion capture, and selectivities have remained below 5 [3]. In this presentation, I will report a novel post-synthetic treatment, which drastically improves the capture performance of the ZIF-8 films [4]. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH₄ > CO₂ > H₂). High CO₂/CH₄, CO₂/N₂, and H₂/CH₄ selectivities of up to 32.5, 29.3, and 175, respectively, and complete blockage of C₃H₆ is achieved. Spectroscopic and X-ray diffraction studies confirm that while the coordination environment and crystallinity is unaffected, lattice distortion and strain are incorporated in the ZIF-8 lattice, increasing the lattice stiffness. Overall, this method can improve the separation performance of several MOF membranes.

References

1. Rangnekar, N. Mittal, B. Elyassi, J. Caro, M. Tsapatsis, Chem. Soc. Rev. 2015, 44, 7128.
2. Yin, J. Wang, Z. Xie, J. Yang, J. Bai, J. Lu, Y. Zhang, D. Yina, J. Y. S. Lin, Chem. Commun. 2014, 50, 3699.
3. Zhang, R. P. Lively, K. Zhang, J. R. Johnson, O. Karvan, W. J. Koros, J. Phys. Chem. Lett. 2012, 3, 2130.
4. J. Babu, G. He, M. T. Vahdat, P. A. Schouwink,M. Mensi, K. V. Agrawal, Submitted, under review.