Group VIII Metal Catalysts

We consider next perhaps the bet understood catalyzed reaction the oxidation of CO over group VIII metal catalysts. The reaction is an important environmental one since it involves the conversion of CO to CO2 in automobile catalytic converters. The mechanism is straightforward ... 

Raney Type and Supported Group VIII Metal Catalysts. Effect of Metal on Selectivity... 

Only one report mentions the cyclopropanation with diazodiphenylmethane in the presence of a group VIII metal catalyst. Remarkably enough, the selectivity of the reaction with 5-methylene-bicyclo[2.2.1]hept-2-ene (8) can be reversed completely. With Rh2(OAc)4 as catalyst, the exocyclie double bond is cyclopropanated exclusively (>100 1), whereas in the presence of bis(benzonitrile) palladium(II) chloride the endocyclic C=C bond is attacked with very high selectivity (>50 1)47). 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

Nolan, P. E., Lynch, D. C., and Cutler, A. H. 1998. Carbon deposition and hydrocarbon formation on Group VIII metal catalysts. J. Phys. Chem. B 102 4165-75. 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Catalytic Features of Carbon-Supported Group VIII Metal Catalysts for Methanol Carbonylation... 

Activities of Group VIII Metal Catalysts. Methanol conversions to methyl acetate and acetic acid on group VIII metals supported upon activated carbon are illustrated in Figure 1, The yield was calculated as methanol conversion to acetyl group. For each catalyst, acetic acid formation is predominant at high temperature while methyl acetate has a point of maximum yield. 

Fig. 26. Forms of pressure fall against time curves observed in the hydrogenation of acetylene over noble Group VIII metal catalysts.

Metal catalysed decomposition of diazocarbonyl compounds in the presence of alkenes provides a facile and powerful means of constructing electrophilic cyclopropanes. The cyclopropanation process can proceed intermolecularly or intramolecularly. Early work on the topic of intramolecular cyclopropanation (mainly using diazoketones as precursors) has been surveyed31. With the discovery of powerful group VIII metal catalysts, in particular the rhodium(II) derivatives, metal catalysed cyclopropanation of diazocarbonyls is currently the most fertile area in cyclopropyl chemistry. In this section, we will review the efficiency and versatility of the various catalysts employed in the cyclopropanation of diazocarbonyls. Cyclopropanations have been organized according to the types of diazocarbonyl precursors. Emphasis is placed on recent examples. 

Ko El, Garten RL. Ethane hydrogenolysis studies of TiC>2-supported group VIII metal catalysts. J Catal. 1981 68 233-6. 

The hydrogenolysis of acylfurans in the gas phase at 200-300°, with group VIII metal catalysts on carbon carriers, can lead to... 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

COMPARATIVE STUDY OF THE DEACTIVATION OF GROUP VIII METAL CATALYSTS BY THIOPHENE POISONING IN ETHYLBENZENE HYDROGENATION... 

The deactivation of Group VIII metal catalysts by thiophene poisoning In ethylbenzene hydrogenation has been studied. With the exception of Pt, the sequence of sulfur resistance found, Pt < Pd < N1 < Rh < Ru, correlates with the decreasing order of the density of states at the Fermi level. This behavior is explained by a competitive adsorption of both ethylbenzene and thiophene on the metal sites, which is related to the basicity of the organic compounds and the electronic structure of the metals. This hypothesis is supported by XPS analysis of both the fresh and poisoned catalysts. 

The hydrodechlorination of CFC-115 to HFC-125 (eq 24) has been carried out using a variety of Group VIII metal catalysts [88],... 

The search for new catalysts which could effect hydroformylation under milder conditions and with higher yields of the desired aldehyde resulted in new processes which utilize Rh as the group VIII metal catalyst. Therefore, the great majority of both journal and patent literature in the last decade has been devoted to Rh catalysis. Smaller efforts have involved Ru and, more recently, Pt. 

Of the group VIII metal catalysts, Co has received the most attention. With CojfCOlg, at 180-185 C and CO H2 pressures of 35 MPa, CH3OH gives a product distribution of 38.8% ethanol, 4.7% n-propanol, 9.0% methyl acetate, 6.3% ethyl acetate, 8.5% methane and traces of acetaldehyde, methyl formate, propyl acetate and butanol. At 160-180° and 21 MPa of CO H2, tert-butanol gives a 63% yield of iso-amyl alcohol, and iso-propanol gives 11% of a mixture of n- and iso-butanol. n-Propanol reacts slowly at 180°C. 

The literature is filled with various processes and catalyst compositions and systems for these transformations. Promoted platinum and sulfided platinum are the most selective group VIII metal catalysts but depending on reaction conditions and the nature of the halogenonitrobenzene, some undesirable halo-azo and azoxy compounds are left in the product (refs. 3, 11). 

Extensive studies have been carried out on the hydrogenation of benzene on Group VIII metal catalysts. In the hydrogenation of benzene on Group VIII metals it is generally observed that cyclohexane is the sole product at moderate reaction temperatures, <300 °C whereas hydrogenolysis occurs at temperatures exceeding 300 °C (1). 

Many mechanisms have been proposed for the adsorption and hydrogenation of aromatic hydrocarbons over supported Group VIII metal catalysts (2,3). Studies with particular reference to the role of benzene as a 7t-complex on the catalyst surface have been reviewed by Garnett (4). 

It is noteworthy that electropositive promoters, e.g. iron and zinc, exhibit significant effects modifying both the activity and the selectivity of ethanol formation in the hydrogenolysis of ethyl acetate on supported Group VIII metal catalysts, such as Pd-Zn/AbOs and Co-Rh-Fe/Ti02. ... 

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