Separation of Hexane Isomers on Rigid Porous Metal Carboxylate-Based Metal-Organic Frameworks

Screening separation studies with equimolar quaternary mixtures of hexane isomers, namely, n-hexane/3-methylpentane/2,3-dimethylbutane/2,2-dimethylbutane (n-HEX/3MP/23DMB/22DMB), in the temperature range of 343-423 K and partial pressures up to 10 kPa have been performed using three rigid porous metal (III/IV) carboxylate-based metal organic frameworks (MOFs) with different composition and topology. These carboxylate-based MOFs are denoted as MIL-100(Cr), MIL-125(Ti)-NH2 and MIL-127(Fe). Adsorption study results revealed that all the isomers were adsorbed on MIL-100(Cr) and MIL-125(Ti)-NH2, with adsorption amounts reaching 24 and 12 wt%, respectively, and the order of adsorption being similar to the normal boiling point of the compounds, that is, n-HEX > 3MP > 23DMB > 22DMB, with selectivity of adsorption never exceeding three (i.e. selectivities between the extreme compounds n-HEX and 22DMB). It is worth noting that adsorption of branched isomers is practically excluded on MIL-127(Fe), which exhibits a strong molecular sieve effect of the adsorbed n-HEX with the loading capacity reaching a maximum of 7 wt%. Finally, an alternative strategy, which involved probing the effect of post-synthetic functionalization on the coordinatively unsaturated metal sites of the MIL-100(Cr) using functional group ethylamine or N-methylethylenediamine showed no significant differences in terms of separation.