Synthesis and Characterisation of Porous Mixed-Ligand Metal-Organic Frameworks for Sorption Studies

Abstract

Metal-organic frameworks (MOFs) have been identified as promising physisorption candidates because of their unique structures. MOFs are a class of crystalline materials consisting of coordinate bonds between metal ions and organic ligands. The permanent porosity of MOFs enables them to be applied as gas storage and, carbon capture and utilization materials. The main portion of this study describes the synthesis of new porous MOFs from simple building blocks. A total of four (4) organic ligands, namely; N,N’–bis–(3-pyridylmethyl)–benzophone diimide (L1), N,N’–bis–(3-pyridylmethyl)–biphenyl diimide (L2), N,N’–bis(pyridin-4–ylmethyl) naphthalene diimide, N,N’–bis–(3-pyridylmethyl)–biphenyl diimide (L3) and N,N’–bis(gly)–biphenyl diimide (L4) were successfully synthesized and characterized. The second part of this study describes the synthesis of five (5) new MOFs, namely; {[Zn2(OBZ)2(L2)]ꞏ(DMF)3}n (LMMOF01), {[Zn(TPA)(L4)0.5]ꞏ(DMF)2}n (LMMOF02), {[Co3(TPA)3(L1)]ꞏ(DMF)4}n (LMMOF03), {[Co(BYP)(L4)]ꞏ(H2O)}n (LMMOF04) and {[Cu(L4)0.5]ꞏ(DMF)}n (LMMOF05). The MOFs were synthesized from the reactions of the pyridyl N-donor diimide ligands, carboxylate O-donor co-ligands (terephthalic acid (TPA), 4,4-oxybis benzoic acid (OBZ) and 2,2-bipyridy (BPY)) and transition metal salts. The MOFs were characterized using single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD) and variable temperature (VT-PXRD). The thermal stability of MOFs was determined using thermogravimetric analysis (TGA) and hot stage microscopy (HSM). SCXRD revealed that LMMOF01 and LMMOF03 are 3-D, LMMOF02 is 2-D while LMMOF04 and LMMOF05 are 1-D. The 2-D and 3-D MOFs possess channels that are occupied by the solvent molecules. The porosity of the MOFs was tested using carbon dioxide, nitrogen and hydrogen gases.