Hierarchical Micro- and Mesoporous Zn-Based Metal-Organic Frameworks Templated by Hydrogels: Their Use for Enzyme Immobilization and Catalysis of Knoevenagel Reaction.

Encapsulation of enzymes in metal-organic frameworks (MOFs) is often obstructed by the small size of the orifices typical of most reported MOFs, which prevent the passage of larger-size enzymes. Here, the preparation of hierarchical micro- and mesoporous Zn-based MOFs via the templated emulsification method using hydrogels as a template is presented. Zinc-based hydrogels featuring a 3D interconnecting network are first produced via the formation of hydrogen bonds between melamine and salicylic acid in which zinc ions are well distributed. Further coordination with organic linkers followed by the removal of the hydrogel template produces hierarchical Zn-based MOFs containing both micropores and mesopores. These new MOFs are used for the encapsulation of glucose oxidase and horseradish peroxidase to prove the concept. The immobilized enzymes exhibit a remarkably enhanced increased operational stability and enzymatic activity with a kcat /km value of 85.68 mm s-1 . This value is 7.7-fold higher compared to that found for the free enzymes in solution, and 2.7-fold higher than enzymes adsorbed on conventional microporous MOFs. The much higher catalytic activity of the mesoporous conjugate for Knoevenagel reactions is demonstrated, since the large pores enable easier access to the active sites, and compared with that observed for catalysis using microporous MOFs.