Tuning the properties of the metal-organic framework UiO-67-bpy via post-synthetic N-quaternization of pyridine sites.

A new UiO-67-type zirconium metal organic framework (MOF) material UiO-67-bpy-Me (bpy = 2,2-bipyridine-4,4'-dicarboxylic acid, Me = methyl) was prepared by N-quaternization of the pyridine sites in UiO-67-bpy. After N-quaternization, the pristine neutral framework turned cationic while its high thermal and chemical stabilities were primarily preserved. Fast and enhanced anionic dye adsorption was observed in UiO-67-bpy-Me. In addition, despite the decrease in surface area and pore volume, UiO-67-bpy-Me exhibited an evident increase in CO2 uptake compared to UiO-67-bpy. The enhancement was ascribed to the strong interactions between CO2 and the N-quaternized framework. More importantly, as the N-quaternization has changed the electronic structure of the organic linker. UiO-67-bpy-Me showed optical absorption up to ca. 800 nm with a large red shift of 450 nm compared to the pristine UiO-67-bpy (ca. 350 nm). The extended optical absorption may lead to more efficiency in light utilization. A proof-of-concept demonstration showed that UiO-67-bpy-Me could more efficiently catalyze methyl-orange degradation under UV-Vis light irradiation than the pristine UiO-67-bpy. These findings demonstrate that N-quaternization could serve as a facile post-synthetic modification method to tune the chemical/physical properties of free pyridyl-containing MOFs.