The Complexity of Comparative Adsorption of C[sub.6] Hydrocarbons at Metal–Organic Frameworks

The relatively stable MOFs Alfum, MIL-160, DUT-4, DUT-5, MIL-53-TDC, MIL-53, UiO-66, UiO-66-NH[sub.2], UiO-66(F)[sub.4], UiO-67, DUT-67, NH[sub.2]-MIL-125, MIL-125, MIL-101(Cr), ZIF-8, ZIF-11 and ZIF-7 were studied for their C[sub.6] sorption properties. An understanding of the uptake of the larger C[sub.6] molecules cannot simply be achieved with surface area and pore volume (from N[sub.2] sorption) but involves the complex micropore structure of the MOF. The maximum adsorption capacity at p p[sub.0] [sup.−1] = 0.9 was shown by DUT-4 for benzene, MIL-101(Cr) for cyclohexane and DUT-5 for n-hexane. In the low-pressure range from p p[sub.0] [sup.−1] = 0.1 down to 0.05 the highest benzene uptake is given by DUT-5, DUT-67/UiO-67 and MIL-101(Cr), for cyclohexane and n-hexane by DUT-5, UiO-67 and MIL-101(Cr). The highest uptake capacity at p p[sub.0] [sup.−1] = 0.02 was seen with MIL-53 for benzene, MIL-125 for cyclohexane and DUT-5 for n-hexane. DUT-5 and MIL-101(Cr) are the MOFs with the widest pore window openings/cross sections but the low-pressure uptake seems to be controlled by a complex combination of ligand and pore-size effect. IAST selectivities between the three binary mixtures show a finely tuned and difficult to predict interplay of pore window size with (critical) adsorptive size and possibly a role of electrostatics through functional groups such as NH[sub.2].