Construction Of Water-stable MOFs And Efficient Detection And Removal Of Cr(?) And Dyes In Water

Metal-Organic Framework(MOFs)materials have attracted much attention due to their porous characteristics,functional tunability conferred by modular assembly,and excellent light responsiveness.They have been widely deployed in the fields of adsorption and capture,photochemical removal,and fluorescent sensing of pollutants.At present,water pollution caused by heavy metal ions and dyes emissions is becoming more and more serious,which has attracted widespread attention from all walks of life.In response to this major water pollution problem,combined with the unique physical and chemical properties of MOFs materials,the following three aspects of research work have been carried out:1.Preparation of In-MOF and fluorescence sensing detection of heavy metal ions.Trivalent metal In³⁺ions and aromatic carboxylic acid ligands(H₃L=5'-(5-carboxy-1H-benzo[d]imidazol-2-yl)-[1,1':3',1''-terphenyl]-4,4''dicarboxylc acid)were used as raw materials,to synthesize a two-fold interpenetrated 3D framework{[(CH₃)₂NH₂]₃(In₃L₄)}·(solvent)_x(Compound 1)which based on anionic[In(O₂C-)₄]metal clusters,under the condition of solvothermal reaction.The protonated[(CH₃)₂NH₂]⁺generated by the in-situ decomposition of DMF balances the charges of the framework.The strong?-?stacking and conjugation effect between the two single nets of 1 not only enhances the hydrolysis resistance of the framework,but also enhances the excellent fluorescence emission performance of 1.Fluorescence detection experiments demonstrated that In-MOF exhibits strong fluorescence quenching selectivity,anti-interference ability and high sensitivity towards the cationic Pb²⁺,Cu²⁺and Fe³⁺in water,with ultra low detection limits of 31.4,3.88 and4.20 ppb,respectively.More interestingly,the anionic framework also exhibits strong fluorescence quenching response and quantitative detection ability for Cr₂O₇^2-and CrO₄^2-,with the detection limits of 1.20 and 0.523 ppb,respectively.In addition,1also shows excellent cycle detection ability.After 7 cycles,it still maintained the high sensitivity for detecting Cr₂O₇^2-and CrO₄^2-ions.X-ray photoelectron spectroscopy(XPS)and other characterization methods were adopted to reveal the fluorescence emission and quenching mechanism.Detail studies confirmed that interactions between the framework of 1 and the target detection analytes play crucial roles based on the changes of binding energy.Moreover,the results reveal that competitive absorption of excitation light energy by Cr₂O₇^2-,CrO₄^2-and energy transfer of emission light resulted in fluorescence quenching of 1.While the coordination of Pb²⁺,Cu²⁺and Fe³⁺with carboxyl O atoms and imidazole N atoms of 1 has been determined to be the fundamental reason for showing the recognition and detection ability by highly efficient fluorescence quenching.2.Preparation of Cd-MOF and fluorescence sensing detection and photochemical reduction of heavy metal ions.Using Cd²⁺and tricarboxylic acid ligand H₃L as raw materials,Cd₄(L)₂·x(Solvent)(Compound 2)with a brand-new structure and strong fluorescence emission performance was constructed,and a series of characterizations such as Powder X-ray diffraction(PXRD),Fourier transform infrared spectroscopy(FT-IR),and thermal weight loss analysis(TGA)were carried out.Fluorescence sensing experiments show that the MOF is based on ligand luminescence,and can detect trace Pb²⁺,Cr₂O₇^2-and CrO₄^2-in aqueous solution with high sensitivity and selectivity,the detection limits reached 1.90 ppb,4.83 ppb and 2.48 ppb,respectively.The fluorescence quenching mechanism of 2 was analyzed from the perspective of structure-activity relationship by means of XPS and FT-IR.The high sensitivity to Cr₂O₇^2-and CrO₄^2-comes from the competitive absorption of the excitation light energy of 2 by the analyte ions and the energy transfer of the emission light.The immediate quenching detection of Pb²⁺is due to the coordination reaction between Pb²⁺and framework of 2.In addition,2shows excellent photocatalytic reduction removal ability for Cr(?),and the reduction removal rate reached 100%@40 min.Moreover,the removal efficiency of 91.98%@40 min can still be maintained after 7 consecutive photocatalytic reduction cycles.3.Preparation and modification of Zr MOF and its physical adsorption and photocatalytic degradation of organic dyes.Three dimensional porous[Zr₆(L^2-)₈(HCOO^-)₄(OH)₄]_n(Zr-MOF)based on 8-linked[Zr₆O₈]metal clusters with high water stability was synthesized by using high valence metalionsZr⁴⁺and2,2'-bipyridine-4,4'-dicarboxylic(H₂L=2,2?-Bipyridine-4,4?-dicarboxylic acid)as raw materials.Then,the pyridine N atom of Zr-MOF framework was modified with iodomethane as alkylation reagent,and the positive-Zr-MOF(Compound 3)with cationic properties was obtained,which was characterized by TGA,PXRD and FT-IR etc.From the perspective of size limiting effect and ion selectivity,the capture and removal of methyl orange(MO),methylene blue(MB),reactive brilliant red K-2G(RBR),reactive turquoise blue K-GL(BR14)and mixed dyes in aqueous solution were further investigated.The experimental results showed that the adsorption removal rates of the three dyes were MO(100%)>RBR(68%)>RB14(48.5%),the adsorption of MO+MB mixed dyes fully demonstrated the excellent selective capture ability of 3 for small molecular anionic dyes.The results of photocatalytic degradation showed that the photocatalytic degradation rates of compound 3 for four dyes were MO-97.2%@150 min,MB-71.6%@150 min,RBR-74.3%@300 min and RB14-55.7%@200 min,respectively,indicating that the photocatalytic degradation effect of compound 3 for small molecular dyes was much better than that of reactive dyes.