Toward High Performance Liquid Chromatography with Hierarchically Porous Metal Organic Framework Spheres (HP-MOFS).

Metal-organic frameworks (MOFs) have emerged as a promising type of chromatographic stationary phase, owing to their unique size- and shape-selectivity conferred by the material's ordered microporous structure. Currently, the applicability of MOF-based stationary phases is severely restricted by compromised separation efficiencies, a natural result of their nonspherical morphology and resistant mass-transfer toward confined micropores. In order to overcome these limitations, herein, with the UiO-66 framework as an example, we have designed and synthesized hierarchically porous MOF spheres (HP-MOFS), and evaluated their performance as HPLC stationary phases. The HP-MOFS possess spherical morphology and incorporate either dendritic macropores or worm-like mesopores, thereby constructing a hierarchically porous architecture with the inherent micropores. Chromatographic investigations of dendritic macroporous HP-MOFS demonstrated a plate number of 66,300/m for o-xylene─the highest efficiency realized so far for UiO-66 MOF stationary phases. The worm-like mesoporous HP-MOFS also delivered an enhanced efficiency of 43,600 plates/m, outperforming that of nonspherical UiO-66 MOF phases as well as composite MOF@SiO2 materials. Apart from high efficiencies, HP-MOFS exhibited excellent stability in solvents commonly used in column packing and mobile phase preparation, and what's more, maintained a good structural integrity under high operating pressures up to 40 MPa. These findings indicate that HP-MOFS, as a synergetic combination of spherical morphology and hierarchically porous architecture, can effectively improve the performance of MOFs in liquid phase separations. From a precision manufacturing standpoint, the results suggest that it is the pore design and morphology control that play the central role in developing high performance MOF chromatography.