Molecular-sieving Separation of p-Xylene with Metal-organic Frameworks

Abstract

Metal-organic frameworks (MOFs) consisting of organic linker such as terephthalic acid or fumaric acid and cation such as Al³⁺ or Zr⁴⁺ were prepared by the hydro- and solvo-thermal methods. MOFs were characterized by XRD, N₂ adsorption-desorption isotherm and FE-SEM. The prepared MOFs were mostly crystalline and phase-pure. MOFs were used as the column packing for chromatographic separation of p-xylene from a xylene mixture. With MOFs prepared with large linker molecules, xylenes diffused into the micropores and eluted fundamentally according to kinetic molecular size. With MOFs of low crystallinity or very small micropores, xylenes passed through spaces between MOF particles and separation of p-xylene was not possible. With MOFs consisting of fumaric acid and Zr⁴⁺ (MOF-801), o- and m-xylenes eluted quickly and p-xylene eluted later, because o- and m-xylenes passed through spaces between MOF particles, whereas p-xylene diffused into the micropores of MOF-801. Temperature dependence of separation with MOF-801 revealed that at temperatures lower than 140 °C, xylenes could not diffuse into the micropores. At 170 °C, the smallest p-xylene diffused into the micropores and thus p-xylene could be separated from o- and m-xylenes by the molecular-sieving mechanism. MOF-801 was robust against repeated exposure to xylenes and heat treatment at high temperature for elution of adsorbed xylene.