Active site identification and selective adsorption mechanism of QHU-6 based on boron species

Metal-organic frameworks (MOFs) show promises as boron adsorbents, but insufficient insight into adsorption mechanisms as a result of the unclear active site. In this study, a series of QHU-6 MOFs with identical structures were synthesized by tuning the amount of 1,4-cyclohexanedicarboxylic acid ligand. Characterization via SXRD, FT-IR, and XPS revealed Al–OH groups on unsaturated aluminum centers as active sites. The selective adsorption of specific boron species and the correlation between active site features and performance were revealed by 11 B NMR, Raman spectroscopy, and DFT calculations. The results showed that QHU-6 (0.5) achieved a boron adsorption capacity of 5.1 mmol·g −1 at 298 K, pH 10, and 0.5 M boron concentration, over two times higher than the commercial resin IRA743 (2.07 mmol·g −1 ). The adsorbents performed efficiently across both high and low boron concentrations, owing to their ability to selectively recognize and adsorb boron based on solution boron speciation. This work not only advances high-performance boron adsorbents but also deepens fundamental understanding of boron adsorption.