Selective oxidation catalysis

Centi, G. and Perathoner, S. Encyclopedia of Catalysis, Selective Oxidation — Section E (ed. I.T. Horvath), John Wiley Sons, Inc., New York... 

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. 

Konigsmann M, Donati N, Stein D, Schonberg H, Harmer J, Sreekanth A, Griitzmacher H (2007) Metalloenzyme-inspired catalysis selective oxidation of primary alcohols with an iridium-aminyl-radical complex. Angew Chem Int Ed 46 3567-3570... 

Stahl SS (2004) Palladium oxidase catalysis selective oxidation of organic chemicals by direct dioxygen-coupled turnover. Angew Chem Int Ed 43(26) 3400 420... 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

Volume 72 New Developments in Selective Oxidation by Heterogeneous Catalysis. 

Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la Neuve, Belgium, April8-10,1991... 

Figure 9.19. Secondary-ion mass spectrum of a promoted Fe-Sb oxide catalyst used for the selective oxidation of propylene and ammonia to acrylonitrile, showing the presence of Si, Cu, and Mo along with traces of alkali in the catalyst. [Reproduced from J.W. Niemantsverdriet, Spectres-200 gpy jfj Catalysis, 2" Edn.

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

The resulting microgel-stabilized metal nanoclusters are easily isolated, stored and further manipulated. Their remarkable catal5dic activity in technologically relevant reactions, such as C-C couplings [13a- ] and selective oxidations with molecular oxygen [13e] has been demonstrated. Extension of the applications of these nanoparticles to other areas of catalysis and materials science is currently underway. 

G. Centi, F. Cavani and F. Trifiro, Selective Oxidation by Heterogeneous Catalysis, Kluwer Academic/Plenum Press, New York, 2001. 

Desrosiers, P., Guram, A., Hagemeyer, A. et al. (2001) Selective oxidation of alcohols by combinatorial catalysis. Catal. Today, 67, 397. 

The structure of the review is organized as follows. In Section 6.2, we will address experimental aspects concerning apparatus developments and oxide nanolayer preparation methods, and briefly comment on the interplay between experimental and theoretical results. Section 6.3 constitutes the main body of this chapter, where we present case studies of selected oxide-metal systems. They have been chosen according to their prototypical oxide nanosystem behavior and because of their importance in catalysis. We conclude with a synopsis and a brief outlook speculating on future developments. 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

Shi, F., Tse, M.K., Pohl, M.M., Bruckner, A., Zhang, S.M. and Beller, M. (2007) Tuning catalytic activity between homogeneous and heterogeneous catalysis improved activity and selectivity of free nano-Fe203 in selective oxidations. Angewandte Chemie International Edition, 46 (46), 8866-8868. 

Pillai, U.R. and Sahle-Demessie, E. (2002) Selective oxidation of alcohols in gas phase using light-activated titanium dioxide. Journal of Catalysis, 211 (2), 434—444. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

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