Hydrocarbons, catalytic activity unsaturated

Hydrocarbons 407 can be used as precursors in hydroxyalkylation reactions of a,p-unsaturated carbonyl compounds 408 in the presence of molecular oxygen (Fig. 95) [438]. The reactions are catalyzed by 0.3-1 mol% Co(acac)3 and 30 mol% 398. Alkylated a-hydroxy carbonyl compounds 409 and a-keto carbonyl compounds 410 were obtained in 42-98% yield in a 1-100 1 ratio. The reactions showed a considerable induction period. Co(acac)2, in contrast, initiated the reaction very quickly, but conversion soon ceased. The success using the unreactive Co(III) complex consists of gradual reduction to catalytically active Co(II). 

In recent publications there are contradictory points of view on the basis of unusual activity of CuH-ZSM-5 in both total oxidation of hydrocarbons and NO decomposition [3, 6, 8 - 11, 19, 20]. In our earlier work the decisive role of the low-coordinated square-planar Cu cations in alkane oxidation was demonstrated [11, 12, 14]. In our opinion, the results obtained agree well with these data the preservation of high catalytic activity of Co/CuH-ZSM-S/ gQO correlates well with conservation of just these the most unsaturated sites in Co/Cu-ZSM-S/ g. The physical mechanism of the stabilizing influence of cobalt additive on the conservation of Cu local topography in ZSM-5 matrix is unclear now. The migration of cobalt ions into zeolitic channels with formation of some kind of bi-cationic structures can not be excluded. 

Just above, we have discussed the catalytic activity changes in terms of the possible modifications of the active sites with respect to the unsaturated hydrocarbon, which is often supposed to be of the main importance (see 2.), but we have neglected the possible influence of hydrogen, the second partner of the reaction. In a recent theoretical paper, Sousa et al. [51] argue that the coordination of the surface Pd atoms to other Pd atoms in the second layer are necessary for hydrogen to dissociate and to be trapped with a low energy cost. This points out the importance of the atomic composition and arrangement not only in the outer layer but also in the sublayers to understand the chemical reactivity of alloy surfaces. Anyhow, Cu has a noticeable electronic influence on... 

Other known chemical poisons for the HTS catalyst are halides, although under normal operating conditions they are not present in the feed at an appreciable concentration. Decay in the catalytic activity was also observed with the feed gas which contained minor amounts of unsaturated hydrocarbons, oxygen, and nitric oxides. Under the HTS conditions these compounds were converted into a heavy carbonaceous residue deposited on to the surface of the catalyst, blocking access of the reactants to the catalytic surface. 

The reaction is sensitive to steric hinderance. Aromatic ketones are reduced to hydrocarbons. Unsaturated ketones are fully reduced and with no selectivity. Complexes of the type Ir(Chel)(CH2=CH2)2Cl, with Chel = 2,2 -bipyridine or phenantholine derivatives, behave as catalyst precursors for hydrogen transfer from isopropanol to ketones and Schiff bases. Potassium hydroxide is required as cocatalyst to convert the isopropanol coordinated to the Ir(I) ion, in the neutral isopropoxy derivative. Enolates that are present would act as inhibitors when coordinated to the cationic derivative. Ethylene complexes are better precursors than the corresponding cyclooctadiene derivatives, because they are activated more easily and more completely, and they show high catalytic activity. The most active complexes is the 3,4,7,8-Me4 phen derivative, which, at 83°C, gives turnovers of up to 2850 cycles/min. Reduction of 4-r-butylcyclohexanone affords 97% of the tra/u-alcohol. 

Table I. Catalytic Activity of Lanthanide Compounds towards Unsaturated Hydrocarbons (according to References I, 6, 7, 8)...

Ichimura et al. (1980, 1981a,b) studied the hydrogenation and hydrogenolysis of C2-C5 hydrocarbons and concluded that exposed Co3+ are the loci of catalytic activity. Although ionic cobalt may be active in this kind of reaction, it has been long recognized that coordinatively unsaturated (partially reduced) cations are at the center of active sites. Furthermore, the question is that when metallic centers are present on the surface, they will exhibit a much higher activity than the ionic cobalt. 

Rare-earth complexes of porphyrins also show catalytic activity in the radical oxidation of unsaturated hydrocarbons by various oxidizing agents such as potassium hydrochlorite and tezt-butyl hydroperoxide (Knchnev et al. 1991). Styrene, for example, undergoes oxidation to give a mixture of the aldehydes PhCHO and PhCH2CHO, and the epoxide PhCHCH20. The oxidation of saturated and aromatic hydrocarbons is also promoted by... 

In 2007, synthesis and complexation of a PSiP-pincer hgand, in which a sihcon atom and two phosphorus atoms are tethered by a phenylene group, was first reported by Turculet and coworkers [11-19]. They reported that the PSiP-ruthenium complex exhibited catalytic activity for transfer hydrogenation of ketones [11], and the PSiP-platinum and -palladium complexes efficiently catalyzed reduction of COj to methane by silanes [18[. Shortly after the Turculefs first report in 2007, we reported the first example of utilization of the phenylene-bridged PSiP-pincer complex in carbon-carbon bond formation reactions of unsaturated hydrocarbons [20[. 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

In order to illustrate the potential of cluster chemistry in catalysis, the catalytic activity of cluster species in a number of selected processes will be described. The processes to be considered-hydrogenation and isomerization of unsaturated hydrocarbons, hydroformylation of alkenes, hydrogen reductions of carbon monoxide, and water-gas shift reaction-are interesting not only from the point of view of fundamental knowledge but also because of their great industrial and economical relevance. 

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