Carbon Tetrafluoride - Sulphur Hexafluoride Adsorption and Separation with Zirconium Metal-Organic Frameworks Decorated with Fluorinated Thiazolium Salts

Abstract

The 8-connected fully-carbocyclic zirconium MOF [Zr6O4(OH)6(H2O)2(HCOO)2(TCPB)2] [Zr_TCPB, H4TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene] has been post-synthetically decorated with the two F-containing thiazolium salts 3-(4-carboxy-2-fluorobenzyl)thiazol-3-ium bromide [(HPhTzF)Br] and 3-(4-carboxy-2-(trifluoromethyl)benzyl)thiazol-3-ium bromide [(HPhTzCF3)Br] via Solvent-Assisted Ligand Incorporation (SALI). After salt anchorage to the metal nodes through their carboxylic function and concomitant water elimination (condensation), two mixed-linker MOFs of general formulae [Zr6O4(OH)5(H2O)(HCOO)2(TCPB)2(PhTzF)]Br (Zr_TCPB_F) and [Zr6O4(OH)5(H2O)(HCOO)2(TCPB)2(PhTzCF3)]Br (Zr_TCPB_CF3) have been obtained. They have been fully characterized in the solid-state and exploited as functional materials for SF6 and CF4 adsorption. The introduction of an F-containing moiety in the pristine MOF improves the fluorinated gas thermodynamic affinity, as witnessed by the increased SF6 and CF4 adsorption enthalpies (Qst) of Zr_TCPB_F and Zr_TCPB_CF3 in comparison to those of bare Zr_TCPB. Intriguingly, the replacement of a simple fluoro (-F) substituent with a trifluoromethyl (-CF3) group on the thiazolium salt switches the MOF preferential adsorption from SF6 to CF4. This behavior is unprecedented to our knowledge, opening new horizons in the discrimination of fluorinated polluting gases in the field of air treatment and cleaning. Both MOFs are selective toward SF6 in SF6/CF4 equimolar binary mixtures, because of the higher SF6 polarizability (due to the much larger and more diffuse central sulfur atom) that translates into stronger dispersion (van der Waals) interactions with the MOFs. The best performance has been found for Zr_TCPB_CF3 (with SSF6/CF4 = 9). Therefore, both materials can be exploited in fluorinated gas separation as further demonstrated by periodic DFT calculations carried out on model systems.