Zeolites as supports

Certainly, water-gas shift has in the past been carried out using zeolites as supports for Rh (e.g., Rh/Y Zeolite and Rh/NaY Zeolite539), ZnO (Na/mordenite540), and Fe oxide (Na/mordenite540) in high temperature shift catalyst studies. More recent investigations are aimed at applying zeolites and related materials for use in low temperature shift catalysts. 

The catalytic combustion is an efficient way for VOCs abatment when present in dilute emissions and complex matrix. Both noble metals and transition metals (TM) based catalysts can be used (17, 18), and zeolites as support are receiving more and more attention. 

Fig. 5 illustrates that the catalysts Pt/NaY, Pt/mordenite and Pt/erionite give rise to defined peaks between 370 K and 570 K. This is due to an appropriate range of Pt particle sizes, which are induced by the regular structures of the supports during the catalyst preparation. Pt/support interaction is negligible in this case (commercial zeolites as supports). 

With y-alumina, silica and Na-Y zeolite as supports, catalysts derived from [PPN][Ru3Co(CO)i3][ I and other Ru-Co clustersl were studied in the hydrogenation of CO to methane. The formation of CO2 suggested the occurrence of the WGSR and the possibility, at low CO/H2 ratios (1 4), of there being some CO2 methanation. Under these conditions the best yields were obtained with alumina as support. 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

Many zeolites have been evaluated as supports for Cu in SCR catalysis. Almost all the earlier work, however, is limited to medium- and large-pore zeolites, such as ZSM-5 (MFI, 10-ring), ferrierite (FER, 10-ring), mordenite (MOR, 12-ring), Y (FAU, 12-ring), and beta (BEA, 12-ring). Among them, Cu/ZSM-5 and Cu/beta are the two most studied systems. Of the two, Cu/beta catalysts show better hydrothermal stability and were favored by industry, while Cu/ZSM-5 catalysts were primarily studied by academia. It was not until recently that both industry and academia shifted their interests to small-pore zeolites as supports of Cu SCR catalysts. 

In the case of the Diels-Alder reaction [68] (Scheme 12), several soUds (AlSBA-15, MCM-41, MSU-2 and zeolite HY) were tested as supports for the bis(oxazoline)-copper complexes. The best enantioselectivity results were obtained with the zeolite HY, although the yield was the poorest (16% yield, 41% ee). As happened with the aziridination reaction, the enantioselectivity changed with time. Short reaction times led to the same major enantiomer as observed in homogeneous reactions. However, at higher conversions, i.e., longer reaction times, the opposite major enantiomer was obtained. 

Although hollow fibers are thought to be an excellent candidate to be used as support-they are cheap and have a very high surface area to volume (>1000 m m ) - very few reports on hollow-fiber-supported zeolite membranes exist in the open literature. For zeohte membranes, ceramic hollow fibers are preferred because of their mechanical and thermal stability. Recently, Alshebani... 

Commercial porous ceramic tubes (SCT /US Filter Membralox Tl-70 [7]) were used in this study as support for the zeolite material. They are made (Figure 1) of three consecutive layers of tnacroporous a-Al203 with average pore sizes decreasing from the external to the internal layer. A thin toplayer made of mesoporous y-Al203 was also present in some samples. For gas permeability, gas separation and catalytic measurements the tubes were first sealed at both ends with an enamel layer before zeolite synthesis. Tubes with porous lengths up to 20 cm were used in this study. 

Oxides are widely used as supports. The major examples are silica (porous Si02), alumina (porous AI2O3), and zeolites. Oxides are also u.sed as catalysts. In bulk chemistry they are... 

The effectiveness of zeolites in catalysis and separation can often be improved by the textural and chemical properties of the matrices in which they are imbedded. Chitosan gels issued from renewable resources are already used as supports for the preparation of heterogeneous catalysts in the form of colloids, flakes or gel beads [1, 2], In this study we present several methods for the incorporation of zeolites in chitosan matrices and characterize the synergic effect of the components on the properties of the composite. 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

Table 10.6 shows the catalytic performances of the selective benzene oxidation on the zeolite-supported Re catalysts under steady-state reaction conditions [107]. Catalyhc activity and selectivity largely depended on the types of zeolites and the preparation methods. The Re catalysts prepared by CVD of MTO exhibited higher catalyhc achvity and phenol selechvity than those prepared by the convenhonal impregnation method as supports (Table 10.6). Physical mixing of MTO with the supports provided poor phenol synthesis.

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