Missing Linker Defects in a Homochiral Metal-Organic Framework: Tuning the Chiral Separation Capacity

Efficient chiral separation remains a very challenging task due to the identical physical and chemical properties of the enantiomers of a molecule. Enantiomers only behave differently from each other in the presence of other chiral species. Homochiral metal-organic frameworks (MOFs) have received much attention for their promising enantioseparation properties. However, there are still challenges to overcome in this field such as high enantiomeric separation. Structural defects play an important role in the properties of MOFs and can significantly change the pore architecture. In this work, we introduced missing linker defects into a homochiral metal-organic framework [Zn₂(bdc)(l-lac)(dmf)] (ZnBLD; bdc = 1,4-benzenedicarboxylic acid, l-lac = l-lactic acid, dmf = N,N'-dimethylformamide) and observed an increase in enantiomeric excess for 1-phenylethanol of 35% with the defective frameworks. We adjusted the concentration of monocarboxylic acid ligand l-lactic acid by varying the ratio of Zn²⁺ to ligand from 0.5 to 0.85 mmol. Additionally, a defective framework was synthesized with propanoic acid as modulator. In order to elucidate the correlation between defects and enantiomeric excess, five characterization techniques (FTIR, TGA, ¹H NMR, ICP, and PXRD) were employed. Full width at half-maximum analysis (fwhm) was performed on the powder X-ray diffraction traces and showed that the higher concentration of monocarboxylic acid MOFs were isostructural but suffered from increased fwhm values.