The Complexity of Comparative Adsorption of C Hydrocarbons (Benzene, Cyclohexane, -Hexane) 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, UiO-66(F), UiO-67, DUT-67, NH-MIL-125, MIL-125, MIL-101(Cr), ZIF-8, ZIF-11 and ZIF-7 were studied for their C sorption properties. An understanding of the uptake of the larger C molecules cannot simply be achieved with surface area and pore volume (from N sorption) but involves the complex micropore structure of the MOF. The maximum adsorption capacity at p p = 0.9 was shown by DUT-4 for benzene, MIL-101(Cr) for cyclohexane and DUT-5 for -hexane. In the low-pressure range from p p = 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 -hexane by DUT-5, UiO-67 and MIL-101(Cr). The highest uptake capacity at p p = 0.02 was seen with MIL-53 for benzene, MIL-125 for cyclohexane and DUT-5 for -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.