Unveiling the nature of boric acid adsorption by metal-organic frameworks with hexanuclear clusters

A growing demand of boric acid has been forecasted with the rapid development of industries while the world is suffering the consequence from boron-containing wastewater. Metal-organic frameworks (MOFs) with exceptional stability and remarkable surface area have been preliminarily identified as effective boron adsorbents, adsorption mechanism has yet been elucidated though. Herein Zr-6/Hf-6-based NU-1008 and NU-903 were studied for boron removal from water for the first time where around 4.82, 6.27, 3.49 and 6.34 boron were captured per Zr-6/Hf-6 cluster for Zr-NU-1008, Hf-NU-1008, Zr-NU-903 and Hf-NU-903 through a spontaneous endothermic process with fast kinetics by means of chemisorption. Larger capacity of Hf-6-based MOFs than Zr-6-based MOFs can be explained by the weaker electronegativity of hafnium than zirconium. The increased binding energy of Zr 3d, Hf 4f and O 1s as well as reduced binding energy of B 1s suggested the interaction between electron-deficient boron and the hexanuclear clusters (M-6). Owing to the crystalline nature of NU-1008, the boron binding sites were precisely elucidated by single crystal X-ray diffraction where boric acid was attached to the available coordinative sites on the M-6 clusters through the M-6-O-B(OH)(2) connection.