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

Separation of hydrocarbon molecules,
such as benzene/cyclohexane
and o-xylene/m-xylene/p-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] (NIIC-30­(Ph), bpy = 4,4′-bipyridine, ndc2– = 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 NIIC-30­(Ph) demonstrates outstanding
selective sorption properties and becomes a new benchmark for m-/o-xylene separation. In addition, NIIC-30­(Ph) is stable enough to carry out at least three separation
cycles of benzene/cyclohexane mixtures or ternary o-xylene/m-xylene/p-xylene mixtures
both in the liquid and in the vapor phase. Insights into the performance
of NIIC-30­(Ph) are gained from X-ray structural studies
of each aromatic guest inclusion compound.