Controlled fabrication of core-shell silica@chiral metal-organic framework for significant improvement chromatographic separation of enantiomers.

ABSTRACT

Chiral metal-organic frameworks (CMOFs) have been explored as potential chiral stationary phases (CSPs) for chiral high performance liquid chromatography (HPLC). However, their application is still hindered by the low column efficiency, high back pressure and the difficulty in column packing due to the irregular shapes and wide size distributions of CMOF particles. Here we report an efficient one-pot method for the immobilization of chiral MOF [Cu2((+)-Cam)2Dabco] (Cu2C2D) onto microspheric silica particles, generating a uniform core-shell microsphere with SiO2@CMOF@CMOF morphology as CSP packing material. Significantly, the shell thickness and the corresponding column efficiency could be rationally regulated by controlling the growth cycles of [Cu2((+)-Cam)2Dabco]. The structure of developed core-shell microspheres were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), N2 adsorption experiments, infrared spectroscopy (FT-IR) and thermogravimetric analysis. Mechanism involved in the chromatographic separation is the multi-interactions including hydrophobic and hydrogen-bonding, etc. Based on these interactions, successful separation could be achieved among different types of racemic compounds such as carboxylic acid, ketones and phenols under normal phase liquid chromatography (NPLC) condition. In addition, the relative standard deviation (RSD%) value of the retention time for TNPTO was below 1.1% (n = 5), indicating the good repeatability and stability of the chiral SiO2@Cu2C2D-2 column for HPLC enantioseparation. The results reveal that the CMOF coating approach is convenient to fabricate CSP with pre-designed functions of good recognition performance and to facilitate the evolution of CSP in chiral HPLC. Furthermore, the SiO2@Cu2C2D-2 column was successfully employed for the determination of the enantiomeric excess value for TNPTO in the asymmetric Michael addition reaction.