Mind the Gap! Tailoring Sol-gel Ceramic Mesoporous Coatings on Labile Metal-organic Frameworks through Kinetic Control.

Surface engineering of metal-organic frameworks (MOFs) with a mesoporous silica coating can improve MOF mechanical properties and provide an easy way to decorate MOF nanoparticles with organic or biological molecules though silane chemistry or electrostatic interactions, while retaining open access to MOF porosity. Silica coating would be highly beneficial for employing MOFs in a wide range of applications such as catalysis or drug delivery. However, obtaining a stable, controlled core-shell structure using MOF nanoparticles as seed is challenging because of their intrinsic chemically labile nature. Here we analyze the factors that destabilize the core of the Zeolitic Imidazolate Framework-8 (ZIF-8) MOFs during the sol-gel deposition of a mesoporous silica shell causing a partial or total etching of the MOF material. Silicates in solution are found to scavenge Zn2+ ions removing them from the ZIF structure and causing a partial or complete dissolution of the ZIF seed. By carefully tuning the silicate concentration in solution simultaneous control can be obtained over both the ZIF-8 dissolution and the silica condensation kinetics, resulting in the growth of a uniform mesoporous silica shell while preserving the integrity of ZIF-8. The core-shell nanoparticles obtained show a compact core shell structure with no gap between the MOF core and the silica shell, even after calcination, while the crystalline ZIF-8 structure is retained. Overall, a general synthetic approach is presented for producing nanocomposite core-shell materials which can be applied to other MOF labile seed to design new hierarchical materials.