Synthesis, characterisation and adsorption properties of microporous scandium carboxylates with rigid and flexible frameworks

Conditions for the synthesis of each of the scandium terephthalate frameworks Sc2L3, MIL-53(Sc) (Sc(OH)L) and MIL-88(Sc) (Sc3O(H2O)(2)(OH)L-3) (L = 1.4-benzenedicarboxylate (BDC)) have been established. In addition, the MOFs MIL-100(Sc) (Sc3O(H2O)(2)(OH)L'(2), L' = 1,3,5-benzenetricarboxylate (BTC)) and socMOF (Sc3O(H2O)(3),(NO3-)L ''(1.5), L '' = 3,3',5,5'-azobenzenetetracarboxylate (ABTC)), have been synthesised for the first time with Sc. These materials have been characterised by powder and single crystal X-ray diffraction, H-1, C-13 and Sc-45 MAS NMR, and gas adsorption. MIL-53(Sc) is a highly flexible breathing framework that adopts many different forms, depending on the amount and type of adsorbate included. The structures of the as-prepared solvent-containing form, MIL-53(Sc)-DMF (DMF = dimethylformamide) and of the desolvated, hydrated form, MIL-53(Sc)-H2O, have been determined. The former structure possesses one kind of partially open channel,  the latter includes two kinds of channel, closed and slightly open. The hydrated structure's configuration has not previously been observed for MIL-53 materials. 45Sc MAS NMR is a sensitive probe for the Sc environment: distinctive lineshapes are observed for isolated ScO6 octahedra, corner-sharing chains of ScO4(OH)(2) octahedra and Sc3O(O2C-)(6)(OH, H2O)(3) trimers of octahedra. N-2 adsorption at 77 K indicates that the flexible frameworks MIL-88(Sc) and MIL-53(Sc) show no porosity in their desolvated, closed forms, whilst the rigid frameworks Sc2BDC3 (0.26 cm(3) g(-1)), MIL-100(Sc) (0.72 cm(3) g(-1)) and Sc-ABTC (0.57 cm(3) g(-1)) have so far been prepared with appreciable permanent porosity. For CO2 adsorption on desolvated solids, remarkable behaviour is shown by the flexible MIL-53(Sc), opens in two stages to maximum capacity of >13 mmol g(-1), while the rigid frameworks show uptakes at 196 K (at p/p(o) = 0.40) of 5.5 mmol g(-1) (Sc2BDC3), 21.3 mmol g(-1) (MIL-100(Sc)) and 13.1 mmol g(-1) (Sc-ABTC).