Linker Engineering of Iron-Based MOFs for Efficient Visible-Light-Driven Water Oxidation Reaction

Iron-based metal-organic frameworks (MOFs) have been reported to display notable activity for photocatalytic water oxidation reaction (WOR). Previous reports published managed to grasp some of the important structural factors to develop activity, but no rigorous study has been accomplished toward the impact of the organic linkers on the WOR photoactivity. In this study, nine MOFs of the MIL-88B structure with different linkers were synthesized. The linkers were separated into two categories: those with electron-donating groups and those with electron-withdrawing substituents. MOFs fabricated from the latter linkers exhibited higher efficiency for WOR compared to the former, even though they demonstrated a lower visible light absorption ability. It was considered that the difference in reactivity originated from side reactions of the linkers toward reactive intermediates created during WOR, such as hydroxyl radical (OH center dot). Hydroxylation of terephthalic acid during WOR was confirmed by( 1)H NMR. Since the highest activity, five times greater than pristine MIL-88B, was obtained from tetrafluoroterephthalic acid-based MIL-88B, it was concluded that adding electron-withdrawing groups could prevent or slow the hydroxylation rate of the linkers by reducing the electron density of the benzene ring, thus promoting the photocatalytic WOR reaction.