Control of ZIF-7-III aspect ratio using water-in-oil microemulsion

Zeolitic imidazolate framework-7-III (ZIF-7-III), a well-known metal-organic framework (MOF), has attracted attention as gas separation membrane material. For practical realization, it is necessary to adjust the aspect ratio of ZIF-7-III particles because it can affect the gas separation performance. In this study, we demonstrate the control of the aspect ratio by changing the synthesis condition using microemulsion prepared from n-alkyltrimethylammonium bromide as a surfactant, 1-hexanol as a co-surfactant, heptane as a continuous phase, and water as a dispersed phase. When cetyltrimethylammonium bromide (CTAB) was used as surfactant, ZIF-7-III particles with particle sizes of 10-20 mu m were prepared by changing the water and CTAB concentration. In synthesis using four types of surfactants, the particle size of the resultant ZIF-7-III particles increased from 7 mu m to 26 mu m, and the aspect ratio increased from 4.9 to 11 with an increase in the hydrophobic chain length from C12 to C18. The control of the morphology is attributed to the adsorption of surfactants on hydrophobic (001) facets of ZIF-7-III, which is currently hypothesized to be monolayer or bilayer. The supply of the reactants could be hindered further with the increase of the hydrophobic chain length of the surfactants in either cases. ZIF-7-III particles obtained in this study can be exfoliated into nanosheets, which will accelerate the development of ZIF7-III-based molecular-sieve gas separation membranes.