Enhancement of Ethane Selectivity in Ethane–Ethylene Mixtures by Perfluoro Groups in Zr-Based Metal-Organic Frameworks

A series
of zirconium dicarboxylate-based metal-organic frameworks
(Zr MOFs) of the UiO-66 (tetrahedral and octahedral cages) or MIL-140
(triangular channels) structure type were investigated for the separation
of ethane/ethylene mixtures. The adsorption, investigated both experimentally
and computationally, revealed that the size and type of pores have
a more pronounced effect on the selectivity than the aromaticity of
the linker. The increase in pore size when changing from benzene to
naphthalene (NDC) dicarboxylate ligand makes UiO-NDC less selective
(1.3–1.4) than UiO-66 (1.75–1.9) within the pressure
range (100–1000 kPa), while the three-dimensional (3D) pores
of the UiOs favor the adsorption of ethane due to the interactions
between ethane with more spacers than in the case of the 1D channels
of MIL-140s. The impact of the functionalization revealed a very interesting
increase of selectivity when two perfluoro groups are present on the
aromatic ring (UiO-66-2CF3) (value of 2.5 up to 1000 kPa).
Indeed, UiO-66-2CF3 revealed a unique combination of selectivity
and working capacity at high pressures. This is due to a complex adsorption
mechanism involving a different distribution of the guest molecules
in the different cages associated with changes in the ligand/perfluoro
orientation when the pressure increases, favoring the ethane adsorption
at high pressures.