Design,Synthesis And Photocatalytic Performance Of Novel Bismuth Based Metal-Organic Frameworks(MOFs)Materials

Metal-organic frameworks(MOFs)are a kind of highly ordered porous solid materials composed of metal ions(or metal oxide clusters)and organic ligands.Their characteristics such as high crystallinity,large specific surface area,high porosity,adjustable structure-function,and many functional active sites make them widely used in the fields of gas storage and separation,biomedicine,photoluminescence materials,chemical sensors and catalysis.Among them,the low cost and non-polluting photocatalytic technology that can convert inexhaustible and sustainable solar energy into chemical or electric energy has attracted our special attention.Utilizing the adjustable structure and function of MOFs,the pore structure,light absorption and separation efficiency of photogenerated carriers can be adjusted to optimize the photocatalytic performance of MOFs.Therefore,MOFs have a very good application prospect in photocatalysis.In recent years,a large number of MOFs have been researched in the aspects of photocatalytic degradation,water splitting,reduction of CO2,and selective synthesis of organics.The metal nodes of MOFs reported so far cover transition,main group and lanthanide metals,among which the transition elements such as Ti,Fe,Zn and Zr etc.are the majority.In recent years,bismuth(Bi),a main group metal,has attracted more and more attention.Bi is the largest non-radioactive element on the periodic table.Bi and Bi-based compounds are low-toxic and have been used in stomach medicine,surgical operations,cosmetics and green chemistry,so Bi is also called as the "green" heavy metal.In addition,the electron configuration of Bi atom[Xe]4f145d106s26p3 gives it flexible coordination ability.The coordination number of Bi is from 3 to 12,and is mostly from 7 to 9 in MOFs constructed by Bi and carboxylic acid ligands.Bi3+ cations can coordinate with oxygen,nitrogen or sulfur atoms in organic molecules to form stable chemical bonds.Therefore,Bi-based MOFs with diverse structures can be constructed from Bi and a variety of organic ligands.Similar to most MOFs photocatalysts,Bi-based MOFs also face challenges such as narrow light absorption and weak separation ability of photogenerated carriers,which affect their photocatalytic performance.In addition,the high coordination number of Bi3+ cations in most Bi-based MOFs tends to form a dense structure and low porosity,which is not conducive to the contact between substrate and the active sites of MOFs.In this thesis,we design and synthesize a series of Bi-based MOFs,which not only extend the light absorption by constructing Bi-S bonds and protonation of nitrogen-containing organic ligand,but also improve separation efficiency of photogenerated carriers by utilizing electron withdrawing groups(-COOH)on organic ligands and adjusting the degree of conjugation of ligands.Moreover,we improve the porosity and BET surface area of Bi-based MOFs by changing the arm length of the triphenylamine-based tricarboxylate ligands.In chapter 1,firstly,the concepts,features and main application fields of MOFs materials are described,and the application of MOFs in photocatalysis is discussed in detail.Secondly,the properties and advantages of Bi are described,and then the research object of this thesis is determined to be Bi-based MOFs.Thirdly,the synthesis,structure,research status and especially the latest progress in the field of photocatalysis of Bi-based MOFs are discussed,and the problems of Bi-based MOFs in the application of photocatalysis are summarized.Finally,the solution and the research content of this thesis are clarified.In Chapter 2,a Bi-based MOF(Bi-MBA)with visible light absorption is designed and synthesized with 4-mercaptobenzoic acid(4-MBA)as the organic ligand.Bi-MBA displays a strong visible light absorption with an absorption edge at 500 nm,which is due to the formation of Bi-S bond.Band structure studies show that the conduction band potentials(ECB)of Bi-MBA locates at-1.38 eV,much more negative than that of most Bi-based semiconductors.Theoretical calculations reveal that the combination of Bi and MBA ligand can upshift ECB,and the electron withdrawing group(-COOH)on the organic ligand is conducive to π conjugated delocalization of photogenerated electrons.The photocatalytic performance tests show that Bi-MBA can effectively reduce O2,Cr(Ⅵ)and CO2.The research results in this chapter not only provide a new choice for extending the light absorption of Bi-based MOFs,but also improve the photocatalytic reduction ability of Bi-based photocatalysts.In chapter 3,a novel Bi-based MOF(Bi-MMTAA)is designed and synthesized using a biocompatible molecule,namely 2-mercapto-4-methyl-5-thiazoleacetic acid(MMTAA),as the organic ligand.On the one hand,we continue to extend light absorption by constructing Bi-S bond.On the other hand,carboxylate,mercapto,and thiazolyl groups in MMTAA molecule may be coordinated with Bi,which provides a good platform for studying the coordination behavior of Bi and the effect of the modulation of functional groups on the performance.The detailed research contents in this chapter are as following:(1)In the first part,millimeter-scale Bi-MMTAA single-crystal is cultivated by a reflux method and its crystal structure is determined by single-crystal X-ray diffraction analysis.Similar to the previous results,Bi-MMTAA shows a strong visible light absorption,which is attributed to the formation of Bi-S bond.Theoretical calculations reveal that Bi-MMTAA is a p-type semiconductor,and when Bi-MMTAA is excited by a photon with energy higher than its band gap,electrons can delocalize through the π conjugation,which is advantageous for charge separation and transfer.The performance evaluation shows that Bi-MMTAA can decolorize Rhodamine B(RhB)and oxidize phenylboric acid to phenol under visible light(λ>420 nm)irradiation with superoxide radical(O2-·)as the main reactive oxygen species.In this section,the light absorption of a Bi-based MOF is extended,and the research results also inspire further exploration of Bi-based MOFs both in synthesis and potential applications.(2)In the second part,we find a new method to extend the light absorption of MOFs in the further study of Bi-MMTAA,namely the protonation of nitrogen.Specifically,the protonated Bi-MMTAA(denoted as Bi-MMTAA-H)synthesized by a hydrothermal method displays a significant red-shift in light absorption compared with Bi-MMTAA synthesized by a reflux method,although their crystal structure information are the same.The analysis of nuclear magnetic resonance(NMR),Fourier-transform infrared(FTIR)and Raman spectra confirms that the red-shifted light absorption is caused by the protonation of nitrogen in the thiazole ring in MMTAA on the MOF surface.Moreover,the protonation also improves the separation efficiency of photogenerated carriers and photocatalytic performance for the selective oxidation of α-terpinene to p-cymene.The research results in this section provide a new idea for extending the light absorption and improving the photocatalytic performance of Bi-based MOFs.In chapter 4,we select two triphenylamine-based tricarboxy late ligands(4’,4’",4’""-nitrilotris(([1,1’-biphenyl]-4-carboxylic acid))(H3NBC)and 4,4’,4"-nitrilotribenzoic acid(H3NTB))to build Bi-based MOFs to solve the low porosity caused by large atomic radius and high coordination number of Bi.The synthesized Bi-NBC displays higher porosity and BET surface area due to the longer arm length of H3NBC ligand.In addition,the photophysical properties and separation efficiency of photogenerated carriers can be tuned by the arm length and degree of conjugation of organic ligands.Performance evaluation suggests that Bi-NBC has a better photocatalytic performance than Bi-NTB in the oxidation of phenylboric acid to phenol,which is due to the larger BET surface area,wider light absorption and higher separation efficiency of photogenerated carriers of the former.The research results in this chapter provide a reference for the regulation of the pore structure and photocatalytic performance of Bi-based MOFs by organic ligands.In chapter 5,we summarize the research content,describe the innovation points and put forward our own opinions on the problems and shortcomings of this thesis,and than prospect the next research plan.