Heterogenized transition metal catalysts

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

The hydrogenation reactions occurring on heterogeneous basic catalysts have characteristic features, which distinguish heterogeneous basic catalysts from transition metal catalysts.44 The key features of the base-catalyzed hydrogenation reactions are as follows ... 

From the previous paragraphes it follows that a substantial amount of experimental data exist that illustrate the oxygenation properties of zeolite catalysts. In very general terms zeolites are used to heterogenize transition metal ions in ion exchange or lattice positions, to stabilize transition metal oxide dispersions and to prepare ship-in-bottle complexes. 

The counter-ions of some of the quaternary onium groups were exchanged with an anionic phosphine compound, which was then used to complex palladium. Thus, a polymer material containing phase transfer catalyst and transition-metal catalyst groups was obtained (Fig. 20). The Heck-type vinyla-tion reaction [137] was used to examine the catalytic activity of the heterogeneous system. The polymer-supported catalyst was found to compare favourably with the homogeneous system (Fig. 21). 

A wide range of catalysts have been reported for dehydrocoupling of OH-containing compounds with silanes. These include acids and bases and both homogeneous and heterogeneous transition-metal catalysts. Such reactions can be useful in organic synthesis (protection of OH groups), in... 

The history of the anchoring/grafting preparation may be found in Chapter 1 and Appendix 2 of Ref. 11. It appears that the first works have been performed to heterogenize transition metal complexes used as homogeneous catalysts of polymerization. 

F. Fischer and H. Tropsch, who first described the conversion of synthesis gas into hydrocarbons and oxygen-containing compounds ( oxygenates ) over heterogeneous transition metal catalysts such as iron/zinc oxide. This reaction was developed into a process for the conversion of coal into gasoline. At present such a process is economically feasible only where coal is plentiful and cheap while access to oil products is limited. Currently only South Africa operates plants using the Fischer-Tropsch process. 

A process for the preparation of fluorobcnzencs comprises the heating of fluorobenzaldehydes in the presence of a catalyst. Suitable catalysts are transition metals from the B groups 1, 11. VI. VII and VIII. The best catalytical properties seem to be held by rhodium and the metals of the platinum group, e.g. formation of 1.3-difluorobenzene (5). The reaction maybe carried out in homogeneous solution with soluble rhodium catalysts (Wilkinson s catalyst) or in heterogeneous phase with the catalyst fixed on a carrier. ... 

Alkyne cyclotrimerization occurs at various homogeneous and heterogeneous transition metal and Ziegler-type catalysts [7], Substituted benzenes have been prepared in the presence of iron, cobalt, and nickel carbonyls [8] as well as trialkyl- and triarylchromium compounds [9]. Bis(acrylonitrile)nickel [10] and bis(benzonitrile)palladium chloride [11] catalyze the cyclotrimerization of tolane to hexaphenylbenzene. NiCl2 reduced by NaBH4 has been utilized for the trimer-ization of 3-hexyne to hexaethylbenzene [12]. Ta2Cl6(tetrahydrothiophene)3 and Nb2Cl6(tetrahydrothiophene)3 as well as 7 -Ind-, and 77 -Ru-rhodium... 

Catalysts speed up reactions, and provide selectivity for the desired product. Homogeneous catalysts (often organometallic compounds) act in the same phase as the reactions, heterogeneous catalysts (often transition metals on an oxide support) in another phase. 

Compounds of this type may be related to the heterogeneous transition metal catalyzed reaction of hydrosilane. They may represent a nondissociative activation process, and a possible parallelism can be drawn with heterogeneous activation of metal surface. Reactions of hydrosilanes with nucleophiles in the presence of heterogeneous catalysts were shown to occur with inversion at silicon (cf. Sect. IV-A-2). This was proposed to arise from an adsorption of the hydrosilane to the metal surface, followed by a back-side attack of the nucleophile. It... 

Hydrodesulfurization [HDS, Eq. (1)] is the process by which sulfur is removed from fossil materials upon treatment with a high pressure of H2 (3.5-17 MPa) at high temperature (300-425 °C) in the presence of heterogenexius catalysts, generally transition metal sulfides (Mo, W, Co, Ni) supported on alumina [1]. About 90% of the sulfur in fossil materials is contained in thiophenic molecules, which comprise an enormous variety of substituted thiophenes, and benzo[b]thiophenes, di-benzo[b,d]thiophenes as well as other fused-ring thiophenes, most of which are generally less easily desulfurized over heterogeneous catalysts than any other sulfur compound in petroleum feedstocks (e.g., thiols, sulfide, and disulfides). 

There are of course also parallels between homogeneous and heterogeneous transition metal catalysts. Many reaction mechanisms of homogeneous and heterogeneous catalysts exhibit similarities with regard to the intermediates and the product distribution. 

Compare the key activation steps in the hydrogenation of aUcenes with homogeneous and heterogeneous transition metal catalysts. What are the names of these steps ... 

For example, correlations exist between metalloenzyme and heterogeneous transition metal catalytic processes in the areas of alkane hydroxylation and dehydrogenation, olefin epoxidation, and nitrogen fixation, despite the fact that heterogeneous catalysts typically operate under high temperature and sometimes high pressure conditions, while enzymes catalyze similar transformations under ambient conditions. 

Pis serve as catalysts for transition metal complexes. These metal eom-plexes are suitable for the epoxidation of olefins. The heterogeneous PI-supported transition metal complex catalysts provide superior catal5dic activity, selectivity and stability in the epoxidation of higher olefin. Because of the heterogeneous nature, the catalysts can be easily separated from the reaction product, which eases recycling of the catalysts." ... 

The water gas shift reaction involves the conversion of water and carbon monoxide into hydrogen and carbon dioxide, typically using a heterogeneous transition metal-oxide catalyst (Scheme 12.1). The reaction is reversible, and under certain conditions the reaction can operate in either the forwards (hydrogen production) or reverse (carbon monoxide production) direction. Promoted iron-oxide catalysts are most prevalent in WGS chemistry, although, by comparison, platinum-group metals exhibit a greater catalytic activity. ... 

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