UiO-66@SiO2 core–shell microparticles as stationary phases for the separation of small organic molecules

Composite particles containing the Zr-based metal–organic framework (MOF) UiO-66 were prepared using microwave-assisted solvothermal synthesis. Scanning electron microscopy, infrared spectroscopy, powder X-ray diffraction and nitrogen physisorption studies confirmed the deposition of 100–300 nm microporous particles with the UiO-66 topology on the surface of mesoporous 5 μm and non-porous 2.1 μm silica particles. The core–shell particles exhibited a unique flow-dependent separation selectivity (FDSS) effect which allows changes in both the retention and separation selectivity of small molecules by simple variation of the mobile phase flow rate under isocratic conditions. The impact of the loading of UiO-66 as well as the porosity of the underlying silica core (mesoporous and non-porous) on the FDSS effect was evaluated. The prepared adsorbents were also tested for the normal-phase (NP) and reversed-phase (RP) separation of xylene isomers, substituted benzenes and polyaromatic hydrocarbons (PAHs). Efficiencies of up to 32 400 plates per m (styrene, k 1.59) and 37 200 plates per m (anisole, k 2.90) were achieved under NP and RP modes, respectively. The results demonstrate the potential of novel MOF-based stationary phases for the separation of closely related compounds (e.g. positional isomers).