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Enhancement of the 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 was investigated for the separation of ethane/ethylene mixtures. The adsorption, investigated both experimentally and computationally, revealed that the size and the 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 (BDC) 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 to 1000 kPa), while the 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.