Pillared-Layer Metal-Organic Frameworks (Mofs) as Photocatalysts for Degradation of Dyes

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Addis Ababa University


Metal-Organic Frameworks (MOFs) are highly crystalline and porous materials. In this thesis, three sets of work have been undertaken. The first two involve the synthesis and characterization of pillared-layer MOFs and the third is a study of the photodegradation application of selected pillared-layer MOFs in the textile wastewater treatment sector. In the first work, three pillared-layer Metal-Organic Frameworks (MOFs) were synthesized at room temperature in water/methanol mixed solvents and fully characterized. Fumaric acid was converted to its sodium fumarate salt and used as a linker to form MOFs in combination with the pillar, 4, 4’-bipyridine. The powder x-ray diffraction (PXRD) of two MOFs were found to be in good agreement with simulated diffractograms from single crystal data of related MOFs made at higher temperature using DMF as solvent. The study showed that room temperature synthesis (of such pillared-layer MOFs) could produce mesoporous MOFs in less toxic solvents. This could be an attractive approach to obtain MOFs in a greener way and will increase the applicability. Even though single crystals couldn’t be obtained from all three newly prepared MOFs, quantitative amounts of MOFs could be obtained. Determination of the optical band gaps using Tauc plots revealed that {[Ni2(Ox)2(BPY)]•3.75H2O}n is photo responsive in the visible region, whereas {[Zn2(Fu)2(BPY)]•1.5H2O}n and {[Cu2(Fu)2(BPY)]•H2O}n are responsive in the UV region. Encouraged by the first set of the work, Zn (II) and Cu (II) based pillared-layer MOFs from sodium oxalate linker (and also 2-aminotherephtalate) and 4,4’-bipyridine pillar in water/methanol mixed solvents were synthesized at room temperature and characterized. Powdered MOFs were obtained and their crystallinity was studied using PXRD techniques. DICVOL06 in Expo2014 software was used to index PXRD patterns of the MOFs and it was possible to have crystal parameters. All the MOFs obtained have porosities in the mesoporous region. The optical band gap measurements showed that the {[Zn2(ATA)2(BPY)]•H2O}n is active in the visible region whereas {[Zn2(Ox)2(BPY)]•3.5H2O}n and {[Cu2(Ox)2(BPY)]•0.5H2O}n are active in the UV region. Magnetic property studies of the Cu and Ni-based MOFs revealed that there are long range cooperative spin exchange interactions between paramagnetic metal ions through diamagnetic units implying that the MOFs behaved as low dimensional magnetic materials. In the last sets of the work, the photocatalytic performance of one of the pillared-layer MOF, {[Cu2(Fu)2(BPY)]•H2O}n, has been studied in detail towards degradation of methyl orange as a model dye, which is toxic, nonbiodegradable, and discharged to the environment through wastewater by textile industries. The degradation of this model dye was investigated with UV radiation in the presence of the semiconducting MOF, {[Cu2(Fu)2(BPY)]•H2O}n. The optimized conditions for the photodegradation efficiency of the MOF were achieved after considering the effect of pH, and MOF loading at a fixed dye concentration. 96 % of the 10-ppm methyl orange is photodegraded with 0.25 g/L loading of the photocatalyst at pH 5.2 in 140 min of irradiation time. The photocatalyst was recycled five times with a 96 %, 88 %, 87 %, 80% and 53 % degradation efficiencies. The PXRD analysis of the recycled photocatalyst showed that the crystallinity of the MOF is maintained in the five runs.



Pillared-Layer, Metal-Organic, Frameworks (Mofs), Photocatalysts, Degradation of Dyes