Increased CO2/N2 selectivity by stepwise fluorination in isoreticular ultramicroporous metal–organic frameworks

Exploration of porous adsorbents with high CO2/N2 selectivity is of great significance for reducing CO2 content in the atmosphere. In this study, a series of isoreticular ultramicroporous fluorinated metal–organic frameworks (MOFs) were prepared to explore the benefits of fluorinated ultramicropores in improving CO2/N2 selectivity. Gas adsorption measurements revealed that the increase in the number of fluorine atoms in a ligand molecule leads to the increased CO2 uptakes and CO2/N2 selectivity. Theoretical calculations indicate that the interaction between the fluorine atoms and adsorbed CO2 molecules enhances the CO2-philicity, offering useful insight into the improvement of CO2/N2 selectivity in isoreticular frameworks.


Synthesis of 2-fluoroterephthalic acid (H21FBDC)
H21FBDC was synthesized according to the reported literature 1 with minor modifications.

Fig. S2
Fig. S2 Various modes of coordination of 1FBDC ligand between two paddlewheel-type Zn dimers.The occupancy factor of fluorine atom in 1FBDC linker is set to 1/3 of that of hydrogen atoms.Hydrogen atoms and DABCO pillars are omitted for clarity.Colour code: Zn: green; N: blue; O: red; C: grey; F: orange.

Fig. S3
Fig. S3 Various modes of coordination of 2,5-difluoro-BDC (2FBDC) ligand between two paddlewheel-type Zn dimers.The occupancy factor of fluorine atoms in 2FBDC linker is the same as that of hydrogen atoms.Hydrogen atoms and DABCO pillars are omitted for clarity.Colour code: Zn: green; N: blue; O: red; C: grey; F: orange.

Fig. S4
Fig. S4 Illustration of centre-to-centre distance between the closest atoms on an opposite side of 1D channel in (a) DMOF-0F, (b) DMOF-1F, and (c) DMOF-2F.Given that the molar ratio of xFBDC and tmBDC ligand is 1:1, we first measured the centre-to-centre distances between a fluorine atom of xFBDC (for x = 1 or 2) or a hydrogen atom of BDC (i.e., x = 0)and a hydrogen atom of a methyl group of tmBDC on an opposite side; 8.54 Å for DMOF-0F (x = 0), 8.73 Å for DMOF-1F (x = 1), and 8.58 Å for DMOF-2F (x = 2).Next, we subtracted the van der Waals radii of hydrogen and fluorine atoms, which are 1.20 and 1.47 Å, respectively, 2 from the above centre-to-centre distances.Therefore, it is apparent that the estimated pore sizes (6.14, 6.06, and 5.91 Å for DMOF-0F, DMOF-1F, and DMOF-2F, respectively) are comparable to each other and are in good agreement with the values estimated from the N2 sorption measurements (ca.6 Å; Fig.3b).

Fig. S9
Fig.S9PXRD patterns of samples after the CO2 adsorption/desorption measurement together with those after activation (Fig.S6).

Fig. S11
Fig. S11 Simulated interactions between the fluorine atom and adsorbed CO2 molecule in (a) DMOF-1F and (b) DMOF-2F when there exist 6 CO2 molecules in a channel in the unit cell.The CO2 molecules forming a short CF•••C(CO2) contact (red dotted lines) are emphasized by enlarged representations.

Table S1
Summary of porosity and CO2/N2 selectivity in ultramicroporous fluorinated MOFsTableS2Simulated interaction types and interatomic distances in Fig.6