(384b) Mechanistic Study on Thermally-Induced Lattice Stiffening of ZIF-8

The flexibility of the ZIF-8 framework, which inhibits the molecular separation cutoff of 3.4 Å from its pore aperture [1,2,3], was recently reduced by rapid heat treatment to obtain H₂- and CO₂-selective polycrystalline membranes [4]. However, the early stages of the structural, morphological, and chemical changes responsible for this transformation have not been identified. In this presentation, I will discuss systematic investigation of these changes in ZIF-8 exposed to elevated temperature relevant for lattice stiffening. Using ex situ and in situ experiments, we determine that a small shrinkage of the unit-cell parameter, close to 0.2%, is mainly responsible for this transformation [5]. Gas permeation studies show that one needs to achieve this shrinkage without a disproportionately large shrinkage in the grain size of the polycrystalline film to avoid the formation of selectivity-deteriorating grain-boundary defects and cracks. We show that this condition is uniquely achieved in a short time (<10 min) by exposure of ZIF-8 to a mildly humid environment (1.0% H₂O water vapor at 300 ℃) where lattice parameter shrinkage is accelerated by the incorporation of linker vacancy defects, while the shrinkage in grain size is limited. The water-vapor-led incorporation of linker vacancy defects takes place with an energy barrier of 123 kJ mol^-1, much higher than that for the thermal degradation of ZIF-8, <80 kJ mol^-1. The thermal degradation is promoted by heat treatment in a dry environment at a relatively higher temperature, however, this condition does not shrink the lattice parameter at short exposure times.

References

• Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, P. A. Wright, S. Parsons, T. Düren, J. Am. Chem. Soc. 2011, 133, 8900.
• Zhang, R. P. Lively, K. Zhang, J. R. Johnson, O. Karvan, W. J. Koros, J. Phys. Chem. Lett. 2012, 3, 2130.
• Pan, T. Li, G. Lestari, Z. Lai, J. Memb. Sci. 2012, 390–391, 93.
• J. Babu, G. He, J. Hao, M. T. Vahdat, P. A. Schouwink, M. Mensi, K. V. Agrawal, Adv. Mater. 2019, 31, 1900855.
• Hao, D. J. Babu, Q. Liu, A. Schouwink, M. Asgari, L. Wendy, Chem. Mater. 2021 (Just Accepted)