Synthesis Of Metal-organic Framework Compounds Carboxylate Ligands, Characterization, Stability And Performance-based Hydrogen Storage

Metal-organic frameworks (MOFs) have great application potentials in various fields including hydrogen storage. However, many MOF materials are sensitive to moistures in the air, and are not suitable for practical use. In chapter two of this thesis,1,4-phenylenediacetic acid (H2PDA) and two common nitrogen ligand4,4’-bipyridyl (BIPY) or1,4-di (pyridine-4-yl) benzene (DPB) were applied as mixed ligand,and four novel coordination polymers (Cu(HPDA)2(BIPY)1(1D),Cu(PDA)(BIPY)2(H2O)2(2D),Cu2(PDA)2(DMF)2-2H2O3(2D),Zn(PDA)(DPB)4(3D)and a known MOF Zn(PDA)5were obtained.1,2, and5are water stable.3and4are air stable. They are all more stable than MOF-5which is based on1,4-benzenedicarboxylic acid (H2BDC)5is based on four-coordinated Zn(Ⅱ), and is very stable towards H2O, which is contrary to the common view that6-coordianted structures are more stable than4-coordinated structures. We found that the water stability depends on bond strength and also bond polarity. If the bond is not very strong, but quite polar, it will be more susceptible to the attack of water than a not so polar bond.The effects of synthetic conditions on the structures of MOFs were studied. Anions of metal salts and solvent polarity were found to have influence on the structure of the products. The polarity of the solvent also controls the degree of deprotonation of carboxylic acids. The molar ratios of the metal ions to ligands were found less important. We demonstrated that by reducing solvent polarity, adding acid and template molecules or coordinating molecules, single crystals or large size MOFs can be obtained. The thermal stabilities of1-4and the fluorescence properties of4were studied. This work is beneficial to synthesizing water-stable MOFs and growing single crystals.In chapter three of this thesis, we synthesized three known MOFs (Zn2(FMA)2(BIPY)·nDMF6,Zn2(BDC)2(BIPY)·DMF7,Zn3(BPDC)2(BIPY)·3DMF·H2O8using three rigid carboxylic acids, fumaric acid (H2FMA),1,4-benzenedicarboxylic acid (H2BDC) and1,4-biphenyldicarboxylic acid (H2BPDC),4,4’-bipyridyl (BIPY) and zinc (Ⅱ) nitrate. They are all2-fold interpenetration structures. We found6is air stable, and more stable than7and8which is unstable in air. We found the acidity of the bidentate carboxylic acids are related with the stability of the MOF structure, but the charges of the O of the corresponding carboxylate are more directly related with the stability of MOFs. The greater the charge density, the greater the stability of the MOF structure.Hydrogen storage studies of6and8indicated that N2is not suitable for the measurements of the specific areas of MOFs which have very small pores since N2is not able to enter into some small pores, but H2is able. When pores are small, hysteresis occurs in the adsorption/desorption isothermal curves. High pressure can also induce hysteresis. In addition, we found solvent exchange followed by heating under vacuum is a more efficient method to remove the solvent molecules in the pores of MOFs than simply heating under vacuum. This work is helpful for synthesizing water stable MOF materials and hydrogen storage studies. We also studied the thermal stability of6-8and discovered that6and8are wide band semiconductors.