Epoxidation, heterogeneous catalysis

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Heterogeneous catalysis is widely used in technology for gas-phase oxidation of hydrocarbons to alcohols, aldehydes, epoxides, anhydrides, etc. Homogeneous catalysis predominates in the liquid-phase oxidation technology. Nevertheless, a series of experimental studies was devoted in the 1970s to 1990s to heterogeneous catalysis. The main objects of study were metal oxides and metals as catalysts. 

From the chemical point of view, in diasteieoselective syntheses, several kinds of reactions like hydrogenation [273,277-286], hydrogenolysis [287-293], isomerization [294], and epoxidation [295-300] are involved. Hydrogenation is the most important application of heterogeneous catalysis because of its potential to produce a wide variety of chiral functional groups. 

The research reviewed here reflects the intense activity of the preceding 30 years in the field of oxidation catalyst immobilization. Obviously, the literature contains many erroneous and unreliable results, particularly with respect to leaching of active metal components. Examples are the reactions with silica- or alumina-supported Mo, W, or Cr and organic peroxides as the oxidants. The majority of these reports simply deal with homogeneous catalysis. Nevertheless, many concepts have been proposed in which truly heterogeneous catalysis has been obtained. Examples include the Mo-polybenzimidazole epoxidation catalyst (243), the Os-tetrasubstituted dio-late catalyst for dx-dihydroxylation (391), the heteronuclear P-W epoxidation catalysts (359, 377), and the dioxirane systems (406, 407). 

In heterogeneous catalysis, transition metal nanoparticles are supported on different substrates and are utilized as catalysts for different reactions [57], such as hydrogenations and enantioselective synthesis of organic compounds [58], oxidations and epoxidations [59], and reduction and decomposition [57],... 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

General G. Centi, F. Cavani, F. Trifiro, Selective Oxidation by Heterogeneous Catalysis, Kluwer Academic/Plenum Publishers, New York, 2001 G. Ertl, H. Knotzinger, J. Weitkamp, (Eds.), Handbook of Heterogeneous Catalysis, Volume 5, Wiley-VCH, Weinheim 1997 R.A. Sheldon, J.K. Kochi, Metal Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981 G. Strukul (Ed.), Catalytic Oxidations with Hydrogen Peroxide as Oxidant, Kluwer, Dordrecht, 1992 K. Weissermel, H.-J. Arpe, Industrial Organic Chemistry, 4th Edition, Wiley-VCH, Weinheim 2003. Epoxidations J.C. Zomerdijk, M.W. Hall, Catal. Rev. Sci. Eng., 1981, 23, 163 ... 

This section describes in detail three topics in heterogeneous catalysis to which DFT calculations have recently been applied with great effect, the prediction of CO oxidation rates over RuO2(110), the prediction of ammonia synthesis rates by supported nanoparticles of Ru, and the DFT-based design of new selective catalysts for ethylene epoxidation. All three examples involve the careful application of DFT calculations and other appropriate theoretical methods to make quantitative predictions about the performance of heterogeneous catalysts under realistic operating conditions. 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

The examples chosen show that heterogeneous catalysis can be tuned to the versatile reactivity of epoxides, affording successful catalysis in the synthesis of the chemical products desired. Because of the manifold methods available for epoxi-dation, epoxide rearrangement is the most straightforward, and, therefore, cheapest, route to valuable aldehydes and ketones. 

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