Assembly Of Nano Polyoxometalates (POM)/porous Materials, Characterization And Catalytic Studies

Polyoxometalates(POM) are useful materials as acidic and oxidation catalysts for various reactions and have been practically applied in industry. As solid acid catalysts, especially characterized in providing a unique "pseudoliquid phase" reaction environment, POM catalysts should find more and more applications in green/sustainable and atomic economic reactions. However, bulk POM generally exhibit poor catalytic performance due to their low surface area and low stability, their applications have been limited. The mesoporous materials, which have been developed rapidly in recent years, providing nano-scale space for growing nano-scale POM particles and large amounts of-OH group and large surface area for facilitating the modification and dispersion of POM. The concept to assembly nanometer POM crystals with mesoporous materials using the mesopore space as nanoreactor has not been reported so far in literature. For this purpose, the characteristics of structure, composition, properties of POM and the chosen carrier should be further studied in detail. The assembly methods are also the key factor for the materials finally obtained. It is a challenge for the present paper to prepare useful and efficient nano solid Bronsted acid materials utilizing the mesopores of mesoporous molecular sieves as microreactor, and hence to promote the development of green chemistry.The goal of the present work is to find efficient assembly methods of nanometer POM particles in porous materials including mesoporous molecular sieve SBA15 and silica gel. Novel vacuum impregnation and in-situ crystallization methods have been developed to prepare nano POM in the channels of porous materials. Comparing with common impregnation method, the methods developed by the present work show significant superiority. The sol-gel method was also applied to synthesize nano 12-tungstophosphoric acid in the pores of silica gel. Nano Cs-substituted tungstophosphric acid salt was deposited in the pores of SBA-15 by a improved twostep synthesis method. In addition to the above methods, POM was also immobilized onto the surface of TiO2 particle by grafting to prevent POM from leaching in polar solvent media. Meanwhile, the nano POM/ porous materials were tested in acid-catalyzed benzene alkylation with 1-dodecene, cyclic dimmerision of a-methylstyrene (AMS), and in the selective oxidation of benzene to phenol by molecular oxygen.The main contents are as follows:1. Vacuum impregnation method was developed, in which the molecular sieve SBA-15 with a pore size of 36 ran was selected according to the size of the hydrated form of POM. The support was evacuated under high vacuum to remove impurities in the pores of mesoporous molecular sieve, then aqueous or alcoholic 12-tungstophosphoric acid(PW) solution was added, and the adsorption equilibrium was achieved under vacuum. Large numbers of PW molecules diffused into the clear channels of silica and nano PW crystals was formed into the mesopores. When alcohol was used as media, PW could deeply enter inside of the channels more easily because the lower dielectric constant of solvents provided a higher probability for POM anions to diffuse further in the pores with less restriction due to reduced electrostatic interactions with the silica support. However, when alcohol was used as media, the reduction of heteropoly acid to heteropoly blue was a disadvantage for the property of the products.2. In-situ crystallization method was used for the first time in the pores of SBA-15. Heteropoly acid etherate was used tentatively in the present work, which was an intermediate in the synthesis of POM. After filling water in pores by vacuum impregnation, POM etherate was impelled into the channels under hydrothermal condition, then dissociated and crystallized with the help of water finally. Nanometer polyoxometalate clusters was crystallized in-situ inside the channels of mesopores.3. Sol-gel method was improved. POM etherate was used here again. The mainidea of th...