Metal-organic frameworks (MOFs) are porous crystalline materials with promising applications in molecular adsorption, separation, and catalysis. It has been discovered recently that structural defects introduced unintentionally or by design could have a significant impact on their properties. However, the exact chemical composition and structural evolution under different conditions at the defects are still under debate.
In the first part, we probed the existence of residual modulator: the commonly-used acetic acid, which controls the formation of defects in UIO-66. We discovered that acetate molecules coordinate to a single metal site monodentately and pair with water at the neighboring position. The acetates are highly flexible which undergo fast libration as well as a slow kinetic exchange with water through dynamic hydrogen bonds. The dynamic processes under variable temperatures and different hydration levels have been quantitively analyzed by SUPER and CODEX experiments. The integration of SSNMR and computer simulations allow a precision probe into defective MOF structures with intrinsic dynamics and disorder.
In another related work, we studied Mg2(dobpdc) (dobpdc4− = 4,4′- dioxidobiphenyl-3,3′-dicarboxylate) that contains accessible coordinatively unsaturated metal sites. We investigated the defect chemistry of Mg2(dobpdc) when synthesized with 4-fluorosalicylic modulators. We illustrated that by varying the concentration of modulator, the linker vacancies can be tuned systematically and the concentration of the ligand substitution defects can be as high as ∼35%. We uncovered the detailed structure of modulated Mg2(dobpdc) and the defects distribution by REDOR solid-state NMR experiments.
Y. Fu, Z. Kang, J. Yin, W. Cao, Y. Tu, Q. Wang, X. Kong Nano Letters, 2019, 19.1618-1624