Controllable Preparation And Catalytic Properties Of Transition Metal Oxide Mesoporous Molecular Sieves

Porous materials are frequently used as catalyst, catalyst support, and adsorbent as well, for their high surface areas or high active phase dispersions as well as fast mass transfer of the reactants and products to and from the catalytic sites, which are required from any active catalyst. Among them the mesoporous materials discovered by Mobil Company in 1992 received great attention for their very high surface area, monodispersed pore diameters in the range 2-50 nm, and a stereoregular arrangement of these channels which mimics the liquid crystals formed by these surfactants used in their preparation. The large amounts of Si-OH inside the mesopores also facilitate the modification of the surface with metals and organic groups to form heterogenerous catalyst. The nano-scale mesopores of these materials make it possible to grow nano-scale materials within them, thus developed the "ship in bottle" synthesis of nano-scale materials.The research of this work developed the preparation of isolated vanadium oxides by grafting method and formation of nano-scale transition metal oxide within the mesopores of mesoporous materials. The activity of isolated vanadium oxide upon modified mesoporous materials were tested, and compared with conventional impregnated catalysts in the catalytic oxidative dehydrogenation of propane. Through grafting of 3-aminopropyltrimethoxysilane upon the surface Si-OH, amino- groups can be immobilized on the surface of mesoporous materials, subsequent interaction of polyacid anions with amino- groups can fix these metal centers within the mesoporous channels, hydrolysis of polyacid within these channels can produce nano-scale transition metal oxides. The grafted amino- groups can also immobilize ions through the formation of coordination bond between, which provide a route for the heterogenization of homogenerous catalysis process.1. A route for the preparation of isolate vanadium species upon mesoporous materials is developed, Bis-(cycloPentayl) vanadium was used as precursor was dissolved in chloroform in anhydrous glove box, after diffusing for 2 hr, Et3N was added to activate the surface silanols, calcinations after the grafting process formed isolated vanadium species with C3V symmetry, UV-Raman and 51V MAS NMR unambiguously proved that only isolated species exist in grafted samples, however, impregnated samples consist of mixedvanadium species. In the catalytic oxidative dehydrogenation of propane, the conversion of propane and yield of propene of samples prepared by two different methods are similar, the surface acidity of these samples influence selectivity to propene dramatically, weak acidity are beneficial for propene yield, intermediate acidity decrease the selectivity to propene, and strong acidity will form coal and poison the catalytic activity. Thus, we conclude that it is acidity that effect the selectivity to propene.2. Transition metal oxides are widely used as functional materials and catalysts in many areas, nano-scale metal oxides exhibit extraordinary properties due to quantum size effect, thus draw great attention in recently years. The conventional nano- materials are nano- particles, which grow along one dimension will produce nano- wires, or along two dimension to form nano- thin films, three dimensional nano- material will be difficulty to form because growing along three dimension fully will form bulk maters. Since many of these transition metals can form poly acids, which is easily attached to protoned amino- groups upon modified mesoporous silica by static electric interaction, by this way metal centers can be immobilized within mesopores. Hydrolysis of this material by heating in air can produce nano- scale transition metal oxide within channels of mesoporous silica, after the removal of the silica wall, pure nano- scale materials of wires and 3-dimensional nanowire arrays can produce. We successfully synthesized WO3, Cr2O3, V2O5 nano- materials. Coprecipitation of silica and H3PW12O40 can produce nano- particles of WO3. Cr2O3, V2O5 nano- mat...