Functional Design Of Metal-Organic FrameworksMaterials And Study Of Their Photocatalytic Properties

Energy crisis and environmental pollution are two major crises in today’s society which gradually become the main factors restricting economic development.As a renewable energy source,solar energy has the advantage of being inexhaustible and is regarded as a strong alternative to fossil fuels.Therefore,photocatalytic technology came into being.The efficient utilization of solar energy through photocatalytic technology,such as the decomposition of water to produce clean energy H2,or the reduction of CO2 to produce methane,can solve the problem of energy shortage.In addition,photocatalytic technique is also applied to the degradation of organic pollutants,adsorption of heavy metal ions,or green synthesis of industrial chemical raw materials,which will relieve the pressure of environmental pollution effectively.Traditional semiconductors include organic and inorganic semiconductor.The structure of inorganic semiconductor is fixed,not easy to modify for functional transformation,while the organic semiconductor’s energy conversion efficiency is low,with poor stability,low mechanical strength and poor recyclability.Therefore,it would be promising in the development of new type photocatalysts if organic and inorganic materials can be combined.Metal organic frameworks(MOFs)is a porous material with infinite network structure formed by coordination of metal ions or metal oxygen clusters and organic ligands.As a new functional molecular material,MOFs material has the following advantages:large specific surface area,diverse pore structure,modifiable pore surface,adjustable structure and function,unsaturated metal coordination sites,etc.Therefore,MOFs displays great application prospect in in the storage and separation of gases,screening of mixed gases,luminescent materials,drug carriers and heterogeneous catalysis.In recent years,more and more reports have been made on the use of MOFs as photocatalyst.MOFs materials have unique advantages in photocatalytic applications,which are manifested in three aspects:(1)they are structure-functionally adjustable,and the light absorption range can be adjusted by changing or modifying metal sites and organic ligands;(2)the existence of large pores and well-arranged pore structure is conducive to the introduction of guest molecules.Therefore,they can directly contact the active sites on MOFs,which will shorten the electron transmission distance and improving the carrier migration efficiency;(3)the exposed unsaturated metal sites can participate in the photocatalytic reaction as active sites and promote the photocatalytic reaction.At present,there are reports on photocatalytic hydrogen production,oxygen production,total water production,CO2 reduction and organic synthesis of MOFs materials.However,the photocatalytic mechanism of MOFs is still focused on electron-hole separation,which was proposed from the traditional semiconductors.The exciton effect and energy transfer process are rarely reported.Compared with traditional photocatalytic semiconductor materials,MOFs has more advantages in the field of photocatalysis due to its flexible regulation at the molecular level.In addition,the ordered arrangement of the organic and inorganic parts in MOFs provides a convenient platform for studying the interfacial charge migration and photocatalytic mechanism of catalysts,which is helpful for us to understand the photocatalytic mechanism of MOFs from the microscopic perspective.Finally,MOFs is the product of sequential connection between inorganic and organic compounds,which has the characteristics of both inorganic and organic compounds.This ordered coordination provides an accurate model for the study of charge transfer and energy transfer between the two.As is known to all,the electron-hole binding energy in organics is large and electron-hole pairs are difficult to separate,so it is more likely to produce excitons to play a role.Due to the large amount of organic connection ligands,the exciton effect in MOFs is more significant than that in inorganic semiconductors.However,the exciton effect and energy transfer process in the photocatalytic process of MOFs are seldom reported.As an effective unit of traditional photocatalyst C3N4,triazine ring plays an important role in the photocatalytic performance of C3N4.At the same time,there are few Bi based MOFs being synthesized,and the Bi based MOFs used as photocatalyst are rare.The ligand defects of MOFs are very suitable for the introduction of foreign organic compounds.Therefore,we not only synthesized triazinyl MOFs and Bi-based MOF materials,but also used traditional Zr-based MOF materials as a template for the insertion of foreign organic molecules to study their photocatalytic hydrogen production,organic synthesis and application in heavy metal adsorption.The specific contents are as follows:In chapter one,we first introduced the current energy crisis and environmental pollution in China,and introduced the semiconductor photocatalytic technology,a green,economic and clean technology,as an effective means to solve the above two problems,and listed some traditional semiconductor photocatalysts.Then we explained the basic structure of MOFs material,introduced the common MOFs material classification and the application of MOFs material.After that,we focused on the photocatalytic mechanism of MOFs materials,including electron transfer process and energy transfer process,and the current applications of MOFs in photocatalysis.Then we introduced how to design an excellent MOFs photocatalyst,including functional design,post modification and so on.Finally,the significance of the topic and the research content of this paper are introduced.In chapter two,Bi-TATB was synthesized from 2,4,6-tri(4-carboxyl phenyl)-1,3,5-triazine(H3TATB).At the same time,in order to compare the effect of triazine on the photocatalytic performance of MOFs,another Bi based MOFs material,Bi-BTC,was synthesized using H3BTC without triazine as the organic ligand and Bi as the metal node.The existence of exciton effect in Bi-MOFs was proved by luminescence spectra at room temperature and low temperature.In other words,the existence of Bi in metal nodes can effectively promote the intersystem transition of organic ligands from singlet state to triplet state,thus producing more triplet excitons.The triplet excitons then transferred to the ground state oxygen through the energy transfer process to form singlet oxygen(1O2)with moderate oxidation capacity.1O2 can be used as the active species for photocatalytic oxidation of benzyl alcohol,so both Bi-MOFs show high selectivity in this reaction.In addition,by comparing the performance of the two Bi-MOFs,the influence of the material structure on the photocatalytic effect was obtained,that is,the improvement of the organic ligands containing triazinyl structure units in the light absorption performance and the long life of the triplet state MOFs.In chapter tree,Zr based MOFs,UiO-66 were synthesized by solverthermal method with Zr4Z+as metal node and terephthalic acid as organic ligand.The natural antioxidant riboflavin(VB2)and organic synthetic oxidant 2,3-dichloro-5,6-dicyandiamoquinone(DDQ)were introduced into the crystal structure of MOFs by direct synthesis,and it was proved that both VB2 and DDQ were doped in single molecule state.(1)XRD and thermogravimetric spectra show that the doping of VB2 does not affect the original crystal structure of UiO-66,and VB2 is connected with UiO-66 by chemical bond.Infrared spectrum,Raman spectrum and XPS spectrum further proved that VB2 was chemically combined with Zr by carbonyl group on isoprazine ring,and the comparison with DRS spectrum of VB2 monomer showed that VB2 was doped into UiO-66 in single molecular state.The photophysical properties of doped UiO-66 have undergone great changes.In terms of thermodynamics,VB2 could originally produce a variety of reactive oxygen species such as O2·-,H2O2 and 102,but the species of reactive oxygen products were inhibited after complexation with UiO-66.Through TMB oxidation experiment and ESR spectrum,it was proved that 1O2 became the only active oxygen species.Therefore,UiO-66 doped with VB2 exhibited excellent stability and oxidation selectivity of benzyl alcohol,and the original VB2 homogeneous catalyst was transformed into a recyclable heterogeneous catalyst.At the same time,this is the first time to solve the three problems of poor stability,unrecyclability and low selectivity of VB2 in the meanwhile,and provide a reference for UiO-66 anchoring the foreign organic molecules.(2)Based on the above studies,we selected the oxidizing agent DDQ as the foreign organic molecule to be anchored again,and proved by IR spectrum,XPS spectrum that DDQ was the chemical complexation by N atoms in-C≡N with Zr to achieve single molecule doping.The doped UiO-66 blocked the original electron transmission path of DDQ,leaving only the energy transfer path,and singlet oxygen became the only reactive oxygen species.Therefore,in the process of visible photocatalytic oxidation of benzyl sulfide to benzyl sulfoxide,it is not affected by the presence or absence of methanol as a sacrificial agent,and the selectivity is always high.At the same time,the original DDQ used as homogeneous catalyst was transformed into a recyclable heterogeneous catalyst,which has a great inspiration for the study of the photoexcitation process of DDQ.In chapter four,we designed and prepared two kinds of porous materials(MOFs and CTFs)with triazinyl groups.Since N atoms on triazine ring have lone pair electrons which can provide coordination site,metal ions or metal complexes can be introduced through post-modification method as photocatalytic reactive sites,which can significantly improve the photocatalytic hydrogen production capacity of materials.(1)Triazinyl MOFs was synthesized by hydrothermal method with H3TATB as the organic ligand and Co2+ as the metal node,denoted as PCN-9(Co).Using the adjustability of structure and function of MOFs,the Pt(Ⅳ)metal complex was introduced by simple post-modification to obtain PCN-9-Pt.By means of XRD,XPS and IR spectrum,it was proved that Pt is bound to N atoms in PCN-9(Co)in molecular state.Photophysical and photochemical results showed that PCN-9-Pt has better carrier separation capability than PCN-9(Co).The results of photocatalytic experiments confirmed this hypothesis.Under the same conditions,the photocatalytic hydrogen production capacity of PCN-9-Pt with highly dispersed Pt(Ⅳ)metal complex was 6.8 times than that of PCN-9(Co).(2)TTPT is a triazinyl covalent porous compound obtained by Friedel-Crafts reaction,being synthesized by 2,4,6-trichloro-1,3,5-triazine and 2,3,4,5-tetraphenylthiophene with AICl3 as catalysts.The Ni(Ⅱ)metal ions were bond to the N atoms in TTPT structure by stirring in Ni(Ⅱ)aqueous solutions at room temperature.As expected,the original TTPT did not have any photocatalytic hydrogen production property,while the TTPT-Ni with the introduction of Ni(Ⅱ)ions displayed photocatalytic hydrogen production capacity under UV-Visible light,and the hydrogen production amount was related to the introduction amount of Ni(Ⅱ).In chapter five,we obtained Bi-MOFs material,CAU-7-TATB,by hydrothermal method,and proved that it has excellent long-term water stability and can be used as an effective adsorbent for the treatment of heavy metal pollution in water.After comparing the adsorption capacity of CAU-7-TATB on different metal ions,we found that CAU-7-TATB has the best adsorption capacity and preferential adsorption selectivity for Pb2+.Therefore,the thermodynamic and kinetic model of Pb2+adsorption and the influence of different pH and interfering ions on the adsorption behavior on CAU-7-TATB were studied,and the optimal adsorption pH value was obtained.CAU-7-TATB was successfully applied to the treatment of heavy metal ion pollution in realistic water.By means of XRD and IR spectrum,we verified that heavy metal ions coordinate with N on triazine ring by chemisorption method,and explained the reason for the different adsorption of CAU-7-TATB to different metal ions by the theory of hard and soft acid base theory(HSAB).In chapter six,we summarized the work of this paper and pointed out the innovation points of this paper.Then we also put forward the shortcomings of this paper and proposed the prospects of subsequent work.