Shape-Induced Selective Separation of Ortho-substituted Benzene Isomers Enabled by Cucurbit[7]uril Host Macrocycles

Abstract

The separation of benzene derivatives is energy intensive and laborious as a result of the overlapping physicochemical properties of these isomers. Here, we report on the separation of ortho-disubstituted benzene isomers using cucurbit[7]uril (CB7) aqueous solution with more than 92% selectivity. Thermodynamic and kinetic analysis proves that the ortho-isomer has stronger binding ability and slower decomplexation rate constant than the para-and metaisomers when hosted by CB7. Optimized host-guest models indicate that the ortho-isomer with the smallest aspect ratio well matches the spherical interior cavity of CB7, resulting in highly stable complexes. Furthermore, laboratory scale-up experiments using commercial xylenes and C8 aromatic fraction of pyrolysis gasoline proved that CB7 is able to separate ortho-xylene (OX) with a remarkable selectivity of up to 83%. We believe that this work accentuates the role of molecular recognition studies using macrocyclic hosts to improve the quality and energy bill of critical industrial separations.

Creating porosity in a trianglimine macrocycle by heterochiral pairing

“…The separation of xylene isomers is challenging because they have similar molecular structures and physical properties. Recently, molecular materials such as cucurbit [7]uril 38 and the polymorphic azobenzene cage 39 have been studied as adsorbents for the separation of xylene isomers. The largest included sphere along the free sphere path (D if ) calculated by Zeo++ 31,40 in activated 1-rac is 4.26 Å, which is close to the molecular size of para-xylene (pX) (4.2 Â 6.8 Å) (Table S1, ESI †).…”

An Adaptive Resorcinarene Hemicarcerand

“…These macromolecules show outstanding selectivity in the separation of compounds with similar physical properties, like boiling points or chromatographic behavior, such as xylenes, [2] chloropyridines, [3] and monochlorotoluenes [4] . Unlike the structurally related cucurbiturils, for which a comparable selectivity is reported, [5] nonporous adaptive crystals feature a certain degree of structural flexibility to adjust their cavity shape according to the guest molecule. Oxatubarenes, the most recently developed structural analogs, even undergo conformational conversions by flipping structural units to vary their cavity size [6] …”

Steric Hindrance in Metal Coordination Drives the Separation of Pyridine Regioisomers Using Rhodium(II)‐Based Metal–Organic Polyhedra

“…[17][18][19][20] These materials are amenable to selective host-guest recognition in liquid-based separation methodologies (e.g.l iquid-liquid extraction), which are known in industry for requiring relatively little energy. [21][22][23] Our group has recently shown that nanoscale cuboctahedral Rhodium-based Metal-Organic Polyhedra (Rh-MOPs), aclass of permanently porous metal-organic cages assembled from 24 (functionalised) 1,3-benzenedicarboxylate linkers and 12 dirhodium paddlewheels, [24] can also be harnessed for molecular separation, albeit through an alternative mechanism to host-guest recognition. Unlike other coordination cages,R h-MOPs present twelve Rh II paddlewheels,e ach of which has an exohedral open metal site that can be used to bind molecules with coordinating groups (Figure 1).…”

“…These materials are amenable to selective host‐guest recognition in liquid‐based separation methodologies (e.g. liquid‐liquid extraction), which are known in industry for requiring relatively little energy [21–23] …”