(17e) Electron Beam Induced Modification of ZIF-8 Membrane Permeation Properties

Ionizing radiation is widely used in material processing applications ranging from polymer functionalization to medical equipment sterilization. Recent reports show that metal-organic frameworks (MOFs) undergo significant structural transformation under electron and X-ray irradiation. However, irradiation-induced modification of MOFs in gas separation applications has not been explored.

Here we report that electron irradiation can be used to modify the gas permeation properties of a prototypical MOF, ZIF-8, as demonstrated by improved CO₂/N₂ and CO₂/CH₄ selectivity in electron irradiated ZIF-8 membranes. ZIF-8 membranes were exposed to an electron flood gun operation at 2 kV for various time and single gas permeances of H₂, CO₂, N₂ and CH₄ were measured. The irradiated membranes show reduced permeation for all gases, among which CO₂ is least affected at short exposure time. After irradiation for 1 min, the CO₂/N₂ and CO₂/CH₄ selectivity of the ZIF-8 membrane increases to 6.3 and 4.2 and corresponds to 3.8- and 3.2-fold increase, respectively. The gas separation performance of the membrane is stable during 23-80 °C temperature cycling over two weeks. After longer treatment of 5 and 100 min, permeances of all gases decrease dramatically in the ZIF-8 membranes, suggesting significant structural changes. X-ray scattering suggests dose-dependent amorphization in e-beam irradiated ZIF-8 and XPS indicates that formation of N-H bonds may contribute to the improved selectivity for CO₂.