Metal–Organic Frameworks for Highly Selective Separation of Xylene Isomers and Single-Crystal X-ray Study of Aromatic Guest–Host Inclusion Compounds

Abstract

Separation of hydrocarbon molecules, such as benzene/cyclohexane and <i>o</i>-xylene/<i>m</i>-xylene/<i>p</i>-xylene, is relevant due to their widespread application as chemical feedstock but challenging because of their similar boiling points and close molecular sizes. Physisorption separation could offer an energy-efficient solution to this problem, but the design and synthesis of sorbents that exhibit high selectivity for one of the hydrocarbons remain a largely unmet challenge. Herein, we report a new heterometallic MOF with a unique tortuous shape of channels decorated with aromatic sorption sites [Li₂Zn₂(bpy)(ndc)₃] (<b>NIIC-30(Ph)</b>, bpy = 4,4'-bipyridine, ndc^2- = naphthalene-1,4-dicarboxylate) and study of its benzene/cyclohexane and xylene vapor and liquid separation. For an equimolar benzene/cyclohexane mixture, it is possible to achieve a 10-fold excess of benzene in the adsorbed phase. In the case of xylenes, microporous framework <b>NIIC-30(Ph)</b> demonstrates outstanding selective sorption properties and becomes a new benchmark for <i>m</i>-/<i>o</i>-xylene separation. In addition, <b>NIIC-30(Ph)</b> is stable enough to carry out at least three separation cycles of benzene/cyclohexane mixtures or ternary <i>o</i>-xylene/<i>m</i>-xylene/<i>p</i>-xylene mixtures both in the liquid and in the vapor phase. Insights into the performance of <b>NIIC-30(Ph)</b> are gained from X-ray structural studies of each aromatic guest inclusion compound.