| Polyoxometalates(POMs)is a kind of metal-oxygen cluster compound formed by the high polymerization of O and transition metal elements such as W and Mo,which has a spatial network structure.Because of the unique structure and excellent physical and chemical properties,POMs is widely concerned in the fields of medicine,materials science and surface science,especially the catalytic chemistry.However,it is not easy to be separated and recycled from the reaction system when the POMs is used as homogeneous catalyst.On the other hand,the POMs exhibits poor catalytic activity as a result of the small specific surface area when it is used as heterogeneous catalyst.To achieve high efficient catalytic activity,we can construct POMs catalyst supported on appropriate supporters such as oxides,increasing the specific surface area.Layered Double Hydroxide(LDH)is a kind of layered compound composed of positively charged metal hydroxide layers and negatively charged interlayer anions.LDH with adjustable pore structure,as a catalyst or catalyst carrier,plays an important role in the catalysis field.Based on the structural characteristics of the LDH,we replace the interlayer inorganic anions by POM anions to prepare novel composite layered material.As the paper reported,we designed and synthesized a series of new composite materials composed of LDH and POMs with three vacancies and transition metals replaced,via the method of ion exchange and hydrothermal synthesis.Specific research work is as follows:1.Study on the synthesis technology of LDH with small sizeIn order to design and synthesize LDH composites with high catalytic activity,we combined the co-precipitation and hydrothermal method.The LDH with small size and large specific surface area was fabricated successfully by changing the reaction rate,pressure and temperature conditions.The structure of the LDH samples were detected by XRD and FT-IR.The results show that the particle size of prepared LDH is about 50-100 nanometers,with uniform hexagonal lamellar structure.2.Design,synthesis and characterization of silicontungstic acid complex anion-pillared Hydrotalcites(POMs-LDHs)We synthesized a series of POMs-LDHs inorganic composite functional materials via an ion exchange method(10 hours under 100?),with the Mg-A1 LDHs of small size,Keggin-structural ?-SiW9,?-SiW9Mm,?-SiW9Fe3 and ?-SiW9Cu3 as precursors.The structure and morphology of the target products were characterized by the XRD,FT-IR,TG-DTA and SEM and the results show that POMs-pillared LDHs were constructed successfully.The Keggin framework structure was kept perfectly and the POMs were connected with LDHs by strong chemical bonding through out the new composite.3.Study on the electrochemical properties of silicontungstic acid complex anion-pillared Hydrotalcites(POMs-LDHs)The electrochemical properties of the synthesized composite were investigated by the cyclic voltammetry in a traditional three-electrode system with the glass carbon electrode coated with the composite as the working electrode,the platinum foil as the counter electrode and the SCE as the reference electrode.The results indicate that the electrochemical performance of this novel composite is similar to the matrix POMs,showing obvious redox characteristics over the cyclic voltammetry curves.4.Catalytic performance of silicontungstic acid complex anion-pillared Hydrotalcites(POMs-LDHs)The catalytic activity of this new POMs-based solid catalyst to epoxidation reaction of alkenes was investigated with styrene as the reactant.The results show that this new catalyst synthesized above represent high catalytic activity and selectivity to the epoxidation of styrene.The catalytic effect of the composite was not obviously attenuated after the catalyst has been used circularly for five times.It is also proved that there is a strong chemical interaction between the LDHs and POMs and the POMs anion is difficult to dissolve. |
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