Grafting (S)-2-Phenylpropionic Acid on Coordinatively Unsaturated Metal Centers of MIL-101(Al) Metal-Organic Frameworks for Improved Enantioseparation.

Chiral metal-organic frameworks (cMOFs) are emerging chiral stationary phases for enantioseparation owing to their porosity and designability. However, a great number of cMOF materials show poor separation performance for chiral drugs in high-performance liquid chromatography (HPLC). The possible reasons might be the irregular shapes of MOFs and the low grafting degree of chiral ligands. Herein, MIL-101-Ppa@SiO was synthesized by a simple coordination post-synthetic modification method using (S)-(+)-2-Phenylpropionic acid and applied as the chiral stationary phase to separate chiral compounds by HPLC. NH-MIL-101-Ppa@SiO prepared via covalent post-synthetic modification was used for comparison. The results showed that the chiral ligand density of MIL-101-Ppa@SiO was higher than that of NH-MIL-101-Ppa@SiO, and the MIL-101-Ppa@SiO column exhibited better chiral separation performance and structural stability. The binding affinities between MIL-101-Ppa@SiO and chiral compounds were simulated to prove the mechanism of the molecular interactions during HPLC. These results revealed that cMOFs prepared by coordination post-synthetic modification could increase the grafting degree and enhance the separation performance. This method can provide ideas for the synthesis of cMOFs.