Mechanistic Study on Thermally Induced Lattice Stiffening of ZIF-8

The flexibility of the ZIF-8 aperture, which inhibits a molecular cutoff of 3.4 Å, can be reduced by rapid heat treatment to obtain CO-selective membranes. However, the early stages of the structural, morphological, and chemical changes responsible for the lattice rigidification remain elusive. Herein, using and experiments, we determine that a small shrinkage of the unit-cell parameter, ∼0.2%, is mainly responsible for this transformation. Systematic 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 cracks. We show that this condition is uniquely achieved in a short time by exposure of ZIF-8 to a mildly humid environment 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, much higher than that for the thermal degradation of ZIF-8, <80 kJ mol. The latter is promoted by heat treatment in a dry environment at a relatively higher temperature; however, this condition does not shrink the lattice parameters at short exposure time.