Synthesis of Metal Organic Frameworks(MOFs) Based Hybrid Composites and Their Environmental Applications

Author:Han Na Rehana Bibi

Supervisor:zhou jian cheng


Degree Year:2019





In the last few decades,Metal-Organic Frameworks(MOFs)received great research interest in the field of photocatalysis due to their structure tunability,high specific surface area,visible light absorption,easy controllable synthesis and the opportunity of being post-synthetically modified with controllable and functional chemistry.The main applications of materials in the field of photocatalysis include:(1)photocatalytic decomposition of organic pollutants in water;(2)photocatalytic reduction of carbon dioxide to alleviate the greenhouse effect;(3)photocatalytic decomposition of water to produce hydrogen;(4)photocatalytic assisted synthesis of organic Compounds,etc.The key and core of photocatalysis is the design and fabrication of semiconductor catalyst materials.An effective photocatalyst should has efficient light absorption properties,the photo-generated carrier is effectively separated on the catalyst surface,and should suitable for a suitable valence band/guide band position of a chemical reaction.In order to achieve the above effects,scientists have made many attempts to prepare the required photocatalyst.High-performance photocatalytic materials are required.Among them,due to the excellent stability of MOF materials of MIL-125 and UIO-66 in water,these materials and their derivatives are widely used in sewage adsorption and photocatalytic water organic pollutant treatment and photocatalytic hydrogen production.However,the large-scale application of such MOFs materials is limited due to the rapid recombination of photogenerated carriers and the low effective utilization of visible light.Based on the application of photocatalysis in the degradation of dyes and photocatalytic splitting of water to prepare hydrogen energy,this paper designs and synthesizes new photocatalysts such as NH2-MIL-125 MOF derived NH2-MIL-125/TiO2/CdS(X)yolk shell and hollow TiO2/CdS(X)heterostructures,hybrid composite UIO-66-NH2/BiOBr(X)and studied their photocatalytic performances,physicochemical properties such as microstructures,morphologies and optical properties,chemical stability and catalytic mechanisms.Specific research contents include:(1)Synthesized MIL-125 and NH2-MIL-125 MOF materials and apply for the adsorption and purification of organic waste water pollutants.Characterization and structural analysis of the samples were evaluated by X-ray powder diffraction(XRD),scanning electron microscopy(SEM),Fourier transform infrared(FTIR)spectrometry,N2 adsorption/desorption(BET),X-ray photoelectron spectroscopy(XPS)and Zeta potential.The results show that compared with MIL-125,the adsorption selectivity and adsorption capacity of NH2-MIL-125 for methylene blue cationic dyes(MB,model organic pollutants)are greatly improved.The maximum adsorption capacity of MIL-125 reaches 311.39 mg/g(adsorption time 120 minutes),while NH2-MIL-125adsorption capacity reached 405.61 mg/g(adsorption time 20 minutes).Significantly improved adsorption selectivity and adsorption capacity are due to the unique electrostatic interaction between the amino group of NH2-MIL-125 and the dye molecules.In addition,pseudo-second order represents the kinetics adsorption of MB dye on both the adsorbents and follow the Langmuir adsorption isotherm model.Thermodynamic studies have shown that the reaction is a spontaneous endothermic process.(2)A series of visible-light active NH2-MIL-125/TiO2/CdS yolk-shell and hollow H-TiO2/CdS hybrid heterostructures were successfully synthesized through the hydrolysis of NH2-MIL-125 metal-organic framework(MOF)using thioacetamide(CH3CSNH2)and cadmium acetate(Cd(CH3COO)2.2H2O)by post solvothermal method.The obtained heterostructures were applied to H2 photocatalytic production.Amongst the yolk-shell and hollow heterostructures,(NH2-MIL-125/TiO2/CdS(30)and(H-TiO2/CdS(30)exhibited the highest H2 production activity 2997.482μmol.g-1h-1 and 1970.813μmol.g-1h-1,with the apparent quantum efficiency of 4.81%and 2.41%at 420nm.This superior photocatalytic performance of the heterostructures could be as a result of the strong interaction of the component due to the intimate contact,as well as large surface area and porous structures that assisted the mass transfer,thereby forming abundant reactive sites.Moreover,the introduction of CdS nanoparticles into the MOF derivatives enhanced the visible light absorption and improve the separation of charge carriers through heterojunction with well-matched band structure.Furthermore,the H2 production rate of the yolk-shell and hollow heterostructures were18 and 12 times greater than the bare CdS.A plausible mechanism was also proposed for the heterostructures.This work may open up new routes for the development of MOFs derived photocatalysts.(3)The MOF based hybrid composites that are composed of zirconium MOF(UIO-66-NH2)and oxyhalide(BiOBr)were successfully synthesized by co-precipitation method and were applied for the photocatalytic degradation of RhB dye under visible light.The developed BiOBr/UiO-66-NH2 composites exhibited higher photocatalytic activity than the pristine material.In RhB degradation experiments the hybrid composite with 15 wt%of UiO-66-NH2 MOF shows degradation efficiency conversion of83%within two hours of time under visible light irradiation.The high photodegradation efficiency of BUN-15 could be ascribed to efficient interfacial charge transfer at the heterojunction and the synergistic effect between BiOBr/UiO-66-NH2.In addition,an active species trapping experiment confirmed that photo-generated hole+and O2-radicals are the major species involved in RhB degradation under visible light.