Metal Oxides olefin metathesis

Since tin-containing co-catalysts are essential for the metathesis of functionalized olefins [26], it was soon discovered that 1 supported on acidic metal oxides forms metathesis catalysts that are active without additives even for functionalized olefins [26]. Standard supports are Al203-Si02, or Nb205 and the activity is related to the surface acidity [2, 3, 26]. A high metathesis activity is observed when MTO is chemisorbed on the surface. No evidence for a surface carbene species was obtained, but there appears to be a correlation between the catalytic activity and the presence of an alkyl fragment on the surface [26a-c]. 

The second part of the book compiles some practical aspects of metal oxides, with emphasis in catalytic applications. Metal oxides represent an expanding class of compounds with a wide range applications in several areas such as materials science and catalysis, chemical sensing, microelectronics, nanotechnology, environmental decontamination, analytical chemistry, solid-state chemistry, and fuel cells. Our basic knowledge on the metal oxide chemistry is relatively far from that for metals, and as yet, little is known about fundamental relationships between reactivity of oxide compounds and their chemical compositions, crystal structures, and electronic properties at the surface. When examining the importance of metal oxides, and specifically TMOs, in several reactions such as dehydration, selective oxidations, olefin metathesis, VOCs removal, photocatalysis, water splitting, and electrocatalysis, attempts will be made in order to connect properties of the oxides and their reactivity. Since the catalytic phenomenon is confined to the external surface of the solids where molecules or atoms interact, the study of this interaction... 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

Olefin metathesis has been discovered on heterogeneous catalysts, which are typically transition-metal oxides supported on oxide supports, for example... 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Enyne metathesis is caused by transition metals. There are two types of enyne metathesis one is caused by a carbene complex, as is olefin metathesis, via [2+2] cocyclization and the other type is a reaction that proceeds via oxidative cycli-zation by a low-valent transition metal complex (Scheme 2). 

MTO supported on acidic metal oxides was quickly discovered to form metathesis catalysts that are active without the need for additives, even for functionalised olefins [70]. Standard supports are zeolites and niobia (Nb205), and the activity was reported to be related to the surface acidity [79]. 

The breakdown of metal-metal bonds without changes in the oxidation state. The processes of this kind have been first discovered by Schrock et al. in the course of investigation of the mechanisms of the catalytical action of low-valent tungsten alkoxocomplexes in the reaction of olefine metathesis [349,1017] ... 

T. J. Katz, Metal Carbenes in Low Oxidation States as Initiators for Olefin Metathesis and Related Reactions, Angew. Chem. Int. Ed. 44, 3010-3019 (2005). 

Some of these reactions use finely divided metals (often supported on an inexpensive inert material, often an oxide or a clay mineral) while others use metal compounds e.g., molybdenum sulfide for hydrodesulfurization (Section 3.2.3), tunsten oxide (for metathesis. Section 6.3), or titanium or chromium salts (for olefin polymerization. Section 7.3). 

Olefin metathesis began as an industrial process involving ill-defined heterogeneous catalysts comprising high oxidation state metal salts and various activating metal oxides [3]. Due to low concentrations of the active species, no spectroscopic evidence could be obtained and little mechanistic data was avail-... 

As discussed in a previous section, metal oxides represent an important class of materials exhibiting a broad range of properties from insulators to semiconductors and conductors and have found applications as diverse as electronics, cosmetics and catalysts. Metal oxides have been widely used in many valuable heterogeneous catalytic reactions. Typical metal oxide-catalyzed reactions, including alkane oxidation, biodiesel production, methanol adsorption and decomposition, destructive adsorption of chlorocarbons and warfare agents, olefin metathesis and the Claisen-Schmidt condensation will be briefly discussed as examples of metal oxide-catalyzed reactions. 

Katz TJ. Olefin metathesis and related reactions initiated by 128. carbene derivatves of metals in low oxidation states. Angew. 

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