Preparation And Application In The Environmental Pollutants Degradation Of Composite Intercalated Materials Of Polyoxometalate Layered Double Hydroxides

Polyoxometalates (POMs) are a class of discrete anionic metal-oxides of group V or VI elements, formed by condensation of numerous small polyhedron structure units. The structure of POMs containing heteroatoms is called heteropoly acid and containing homoatoms is called isopolyacid. Layered double hydroxides (LDHs) as a group of 2 dimensional layered materials are formed through the electrostatic interactions between the layers with positive charge and the anions with negative charge. Abundant hydroxy on the layer and in the gallery of LDHs guarantee that LDHs have a good dispersion in water. POMs are a kind of active catalysis materials while LDHs can be served as a good supporter. As a result, the preparation of POMs-intercalated LDHs (POMs-LDHs) materials has become a hot spot in the research nowadays. At present, the traditional methods for synthesis of POMs-LDHs materials including ion exchange, co-precipitation and calcination-rehydration, usually require adjusting the pH of the synthetic solution, due to that the metal ions in the LDHs layer can leach out into the solution, resulting in impurities, when LDHs exist in the acidic solution. On the other hand, POMs are stable only in a certain range of pH, too strong acidity or basicity can both lead to the decomposed of structure of the POMs and decrease the catalytic activity and stability of the materials. And in order to guarantee the balance of charge density and geometrical match between the LDHs layer with the size of POMs, usually the POMs with negative charge less than 4 can’t be intercalated into the gallery of LDHs. Therefore, it is urgent to design a new method for preparation POMs-LDHs, which can resolve the problems above.In this paper, the covalent modification of the LDHs layer by Tris can weaken the interactions between the LDHs layer and the anions in the gallery of the LDHs. With the ion exchange method, we have prepared a series of POMs-LDHs intercalated materials successfully. This method is very facial and without the production of impurities during the preparation process. To study the catalytic properties of the as-prepared POMs-LDHs, we have carried out the following two researches:1. The catalytic material of Tris-LDH-PMo10V2 was synthesized with the method of covalent modification LDHs layer. We studied the application of the material to the degradation of phenol in the waste water. Under mild condition,99% degradation efficiency of phenol can be achieved by using Tris-LDH-PMo10V2. The reaction mechanism and expanded substrates were also investigated. The recycling experiments demonstrated that the catalyst can be recycled more than 10 times without obvious decrease of catalytic activity. As an eco-friendly catalyst, the intercalated materials have a good potential in waste water treatment.2. Rare earth-containing POMs of Na-LnW10 were intercalated into the Tris-LDH through ion exchange, resulting in a series of Tris-LDH-LnW10(Ln =La, Ce, Eu and Gd). Among these POMs-LDHs materials, Tris-LDH-LaW10 was used as a heterogeneous catalyst for simultaneous sulfur and nitrogen removal of mixed model oil. With the extractive and catalytic oxidation system (ECODS),99% deep desulfurization and denitrogenation can be reached by using Tris-LDH-LaW10. The catalyst and the ionic liquids can both be recovered and recycled. Moreover, the structure of the recovered catalyst retain unchanged compared to the fresh catalyst. Therefore, Tris-LDH-LaW10 shows excellent catalytic activity and stability.