Synthesis And Catalytic Properties Of PW₁₁M@Cu₃（BTC）₂ Hybrid Materials

Because of their strong Lewis acidity,higher proton numbers（than non-substituted）,and variable metal valence,transition metal substituted Keggin-type polyoxometalates have drawn a lot of interest in the field of catalysis.In this thesis,the transition metal mono-substituted Keggin-type phosphotungstates(PW₁₁M,M=Ti⁴⁺,Cr³⁺,Mn²⁺,Fe³⁺,Co²⁺,Ni²⁺,Cu²⁺and Zn²⁺)was loaded into the metal-organic framework Cu₃（BTC）₂ to obtain a series of transition metal mono-substituted polyoxometalate-based MOFs hybrids PW₁₁M@Cu₃（BTC）₂,codenamed NENU-n（n=30-37）.And the nanosized PW₁₁M@Cu₃（BTC）₂ was studied to obtain a series of nano-micron-sized hybrid material,codenamed NENU-na（n=30-37）.The catalytic activity of sulfide oxidation of the above hybrids has been studied,and the main achievements are as follows:1.The transition metal mono-substituted Keggin-type phosphotungstates(PW₁₁M)were synthesized and loaded in the main framework Cu₃（BTC）₂ by one-step hydrothermal method to obtain a series of polyoxometalate-based MOFs hybrids.The series of hybrids were determined to be isostructural by single crystal X-ray diffraction（SXRD）analysis with the chemical formula is[Cu₂（BTC）_4/3（H₂O）₂]₆[Hx PW₁₁M（H₂O）O₃₉](C₄H₁₂N)·m H₂O,abbreviated as PW₁₁M@Cu₃（BTC）₂.The main framework has three kinds of pores,one of which is occupied by polyoxometalates,and the other two are occupied by small molecules.The nitrogen adsorption experiment showed that the porosity of the main framework remained after loading the polyoxometalates.The synthetic hybrids were used to catalyze the oxidative desulfurization of model oil.It was found that the aromatic sulfide dibenzothiophene（DBT）in fuel oil could be oxidized to dibenzothiophene sulfone（DBTO₂）in the presence of oxygen,and conversion rates were all above 93%with selectivity up to 100%.2.Nanosized of PW₁₁M@Cu₃（BTC）₂ series polyoxometalate-based MOFs were studied.The nanometer scale PW₁₁M@Cu₃（BTC）₂ series hybrids were obtained by conventional liquid phase method,mechanical grinding method and liquid assisted grinding method respectively.Scanning electron microscopy（SEM）observed that the particle sizes of hybrids prepared by different methods were significantly different.The particle sizes of hybrids prepared by conventional liquid-phase method were about 1μm,while the particle sizes of hybrids prepared by mechanical grinding method and liquid assisted grinding method were about 150 nm and 100 nm,respectively.The nanometer hybrids were characterized by infrared spectroscopy（FT-IR）,powder X-ray diffraction（PXRD）and thermogravimetric（TG）analysis.The nitrogen adsorption study showed that the surface area of nano-sized crystals increased by more than 30%over the corresponding crystals.3.The catalytic activity of nano-sized PW₁₁M@Cu₃（BTC）₂ was studied using the oxidation of 2-hydroxy-1-ethyl mercaptan（RSH）as a model reaction.It was found that Fe^Ⅲ-substituted phosphotungstate-based MOFs hybrid material[PW₁₁Fe@Cu₃（BTC）₂,particle size about 100 nm]obtained by liquid assisted grinding method under the condition of oxygen as oxidant showed the best catalytic activity.The reaction time was 2.5 h,and the selectivity and conversion rate reached 100%.However,the conversion rate of other transition metal-substituted phosphotungstate-based MOFs as catalyst were all lower than 20%.We also used PW₁₁Fe@Cu₃（BTC）₂ to catalyze the oxidation of hydrogen sulfide（H₂S）,and found that H₂S can be rapidly oxidized to elemental sulfur at room temperature and in the presence of oxygen.4.The catalytic oxidation mechanism of PW₁₁M@Cu₃（BTC）₂ series polyoxometalate-based MOFs were investigated for different sulfides with oxygen as oxidant.Because the oxidation potential of polyoxometalate is much lower than that of aromatic sulfide DBT（1.3V）,it is difficult for polyoxometalate to oxidize DBT.When isobutyric aldehyde（sacrificial reagent）was added,polyoxometalates catalyzed its oxidation to generate isobutyl peroxic acid and then oxidized DBT to DBTO₂.The reduced polyoxometalates were reoxidized to polyoxometalates by oxygen to complete the catalytic reaction.The oxidation potential of Fe^Ⅲ（0.771 V）in PW₁₁Fe@Cu₃（BTC）₂ is much higher than that of RSH（0.15V）and H₂S（0.142V）.Therefore,without the addition of any sacrificial reagents,it can undergo redox reaction with it to produce the corresponding disulfide（RSSR）,elemental S and Fe^II.And Fe^Ⅱis reoxidized to Fe^Ⅲby oxygen to achieve catalytic cycle.