Hierarchically Ultrasmall Hf-Based Mof: Mesopore Adjustment and Reconstruction by Recycle Using Acid Etching Strategy

Benefiting from unique properties including faster mass transfer, more exposed active sites and large molecule recognition, hierarchically porous metal-organic frameworks (HP-MOFs), in the form of ultrasmall nanoparticles (NPs) and having self-assembled interparticle mesopores, have garnered greater attention in recent years that given to typical MOFs. However, the controllable synthesis of HP-MOFs on the nanoscale remains a significant challenge. Herein, we describe the results of a study that led to the development of the first, facile top-down acid etching method for preparing the nanosized (~15 nm) mesoporous MOF, HP-UiO-66-NH2(Hf). The sizes of mesopores in this material can be effectively tuned by simply controlling the concentration of propionic acid (PA) utilized in the preparation solution. Remarkably, the mesoporous structure of HP-UiO-66-NH2(Hf) exhibits excellent chemical stability under acid/base conditions and it can be readily constructed by using at least 4 times recycled PA etching solution. In addition, HP-UiO-66-NH2(Hf) displays high size-selectivity for adsorption of enzymes. Moreover, the nanocomposite generated by complexation with glucose oxidase (GOD), GOD@HP-UiO-66-NH2(Hf) has a higher catalytic activity and a significantly enhanced stability (recycled for at least 7 times) in promoting oxidation of glucose compared to its MOF counterpart, GOD@UiO-66-NH2(Hf) and native GOD. This study provides a potentially general strategy to design and construct stable nanosized HP-MOFs which have applications to large molecule encapsulation and catalysis.