Gold catalysts selective oxidation

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

Over highly dispersed gold catalysts, CO oxidation can take place even at -77° C, and propylene oxide can be selectively produced at temperatures around 100° C. 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

CU/AI2O3, and AU/AI2O3 catalysts and the effects of Ii20 and CeO addition [69]. However, the additives caused a decrease in the N2 selectivity but remarkably improved the catalytic activity, in particular, a decrease in Tso over 200°C in the case of gold. Gold catalysts have a potential for NH3 oxidation at lower temperature if a proper kind of support metal oxides is selected. 

The selectivity of the catalyst is of major importance in the case of chlorinated VOCs the oxidation products should not contain even more harmful compounds than the parent-molecule, for example, formation of dioxins should be avoided. In addition, the minimization of CI2 and maximization of HCl in a product gas should be achieved [61]. These are just a few examples of why researchers are continuing the search for VOC oxidation catalysts as well as new reactor concepts. The new possibilities include, for example, utilization of nanosized gold catalysts in the oxidation of sulfur-containing VOCs and microwave-assisted processes where combination of adsorption and oxidation is used in low-concentration VOC oxidation [62, 63]. 

Selective Oxidation with Molecular Oxygen on Gold-based Catalysts 237... 

B. Qiao, and Y. Deng, Highly effective ferric hydroxide supported gold catalyst for selective oxidation of CO in the presence of H2, Chem. Commun. 17, 2192-2193 (2003). 

C. Bianchi, F. Porta, L. Prati, and M. Rossi, Selective liquid phase oxidation using gold catalysts. Top. Catal. 13(3), 231-236 (2000). 

S. Carrettin, P. McMom, P. Johnston, K. Griffin, and G. J. Hutchings, Selective oxidation of glycerol to glyceric acid using a gold catalyst in aqueous sodium hydroxide, Chem. Commun. 7, 696-697 (2002). 

Rossignol, C. and Arrii, S. and Morfin, F. and Piccolo, L. and Caps, V. and Roussel, J. (2005). Selective oxidation of CO over model gold-based catalysts in the presenee of H2. Journal of Catalysis, 230,476-483. 

In a recent study, Xu et al. managed to lower temperatures to 100 °C by using gold catalyst and oxygen as oxidant. The authors compared the Au/C catalyst and supported Pd or Pt catalysts, and concluded that these systems offered similar performance and that selectivity generally depended on cyclohexene conversion [226]. 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

Gold has emerged as an effective catalyst for the selective oxidation of methane to methanol. Various possible pathways for the oxidation are discussed.29 Suitably substituted furans are transformed into phenols by the use of gold catalyst (1). It has been suggested, on the basis of kinetic isotope effect and trapping studies, that the key intermediate is an arene oxide. The postulation is also supported by DFT calculations.30... 

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