Synthesis of mixes ligand metal-organic frameworks for sorption studies

Abstract

Metal-organic frameworks (MOFs) are a class of crystal engineered materials consisting of organic and inorganic units. They are made up of metal ions connected by bridging ligands to form three dimensional (3D) networks. MOFs have attracted widespread interest owing to their diverse applications in gas storage and separation, luminescence, catalysis, non-linear optics as well as magnetism. The first part of this study describes the synthesis and characterisation of five pyridyl N-donor diimide ligands namely; N,N-bis(pyridin-3-ylmethyl)naphthalene diimide (L1), N,N-bis(pyridin-4-ylmethyl) naphthalene diimide (L2), 2,2-bis(pyridin-2-ylmethyl)-[5,5-biisoindoline]-1,1-3,3-tetraone (L3), 2,2-bis(pyridin-4-ylmethyl)-[5,5-biisoindoline]-1,1-3,3-tetraone (L4) and 5,5-carbonylbis(2-(pyridin-3-ylmethyl) isoindoline-1,3-dione) (L5). The second part of this study describes the synthesis of seven (7) new MOFs namely; {[Zn(DCT)(L1)0.5]·(DMF)}n (RMMOF 1), {[Zn(DCT)(L2)0.5]·(DMF)·(H2O)}n (RMMOF 2), {[Co(DCT)(L2)0.5]·(DMF)}n (RMMOF 3), {[Zn5(OBZ)3(DMF)2]·(DMF)4}n (RMMOF 4), {[Cd(NDC)(L4)2]·(DMF)3}n (RMMOF 5), {[Zn0.5(NDC)0.5(L4)0.5]·(DMF)2}n (RMMOF 6) and {[Cu(NDC)(L5)0.5](DMF)3}n (RMMOF 7). The MOFs were synthesized from the reactions of the pyridyl N-donor diimide ligands, carboxylate O-donor co-ligands (6,6-dithiodinicotinic acid (DCT), 4,4-oxybis benzoic acid (OBZ) and 2,6-naphthalene dicarboxylic acid (NDC)) and transition metal salts. The MOFs were characterized using single crystal X-ray diffraction (SCXRD), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) and Brunauer-Emmet-Teller (BET). SCXRD revealed that the MOFs are three dimensional and possess voids that are occupied by solvent molecules. The nature of the voids as well as the void volumes were determined using the Mercury program. RMMOF 7 possesses the largest void space (~33%) compared to the other MOFs. The sorption capacities were determined for RMMOF 4, RMMOF 5, RMMOF 6 and RMMOF 7 using carbon dioxide gas at 195 K and 273 K. RMMOF 7 adsorbs the largest amount of CO2 at 195 K compared to the other MOFs.