Hydrogenolysis unsupported

Another example is the hydrogenation of the homoallylic eompound 4-methyl-3-cyclohexenyl ethyl ether to a mixture of 4-methylcyclohexyl ethyl ether and methylcyclohexane. The extent of hydrogenolysis depends on both the isomerizing and the hydrogenolyzing tendencies of the catalysts. With unsupported metals in ethanol, the percent hydrogenolysis decreased in the order palladium (62.6%), rhodium (23 6%), platinum (7.1%), iridium (3.9%), ruthenium (3.0%) (S3). 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

Menon and Froment (755c) studied the activity of Pt-Al203 and Pt-black pretreated in hydrogen at various temperatures. The overall activity had a very sharp minimum as a function of the pre-reduction temperature (550°C for supported, 500°C for unsupported platinum). This was due mainly to the almost complete ceasing of hydrogenolysis, whereas the amount of Cg products altered to a lesser extent. 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

In order to study the hydrogenolysis in phenyl ether and its relationship to the formation of intermediates, Fukuchi and Nishimura hydrogenated phenyl ether and related compounds over unsupported ruthenium, rhodium, osmium, iridium, and platinum metals in f-butyl alcohol at 50°C and the atmospheric hydrogen pressure.151 The results are shown in Tables 11.11 and 11.12. In general, the greater part of the initial products as determined by an extrapolation method has been found to be cyclohexyl phenyl ether, phenol, and cyclohexane (Table 11.11). Over ruthenium, however, cyclohexanol was found in a greater amount than phenol even in the initial products. Small amounts of cyclohexyl ether, 1-cyclohexenyl cyclohexyl ether, cyclohexanol, cyclohexanone, and benzene were also formed simultaneously. 

Similar effects on reduction/oxidation have been observed in other studies but not necessarily explained in the same way. One obvious area of explanation involves sintering and then re-dispersion of the platinum particles under oxidizing conditions possibly by molecular migration. It has also been suggested that apparent loss of dispersion results from decomposition of platinum particles or clusters into an atomic form, which becomes incorporated into the alumina support.High-temperature reduction also decreased hydrogenolysis activity (restored by oxidation at 773 K, followed by reduction at 673 K) but the effect appeared to be specific to platinum itself because similar results could be obtained with unsupported platinum black. ... 

Unsupported and SiC/Al203-supported W2C catalysts are multifimctional. n-Alcanes are transformed by aromatization, isomerization, and hydrogenolysis. Aromatization can be increased by the addition of O2 traces. 

This review is followed by a consideration of some of the features characteristic of hydrocarbon reactions on catalysts comprising individual metals from Groups VIII and IB of the periodic table. Finally, the activities of a series of unsupported nickel-copper alloys for hydrogenolysis and dehydrogenation reactions are discussed. These latter studies were made to obtain information on the selectivity phenomenon with bimetallic catalysts of known structure. The nickel-copper alloys were characterized by a variety of chemical and physical probes. 

Direct experimental verification of very highly dispersed bimetallic clusters is complicated by limitations in the ability of physical methods to obtain structural information on such systems. In such a system, however, a catalytic reaction can serve as a sensitive probe to obtain evidence of interaction between the atoms of the two metallic components. For supported bimetallic combinations of a Group VIII and a Group IB metal, the hydrogenolysis of ethane to methane is a useful reaction for this purpose. In the case of unsupported bimetallic systems of this type, as discussed previously, the interaction between the Group VIII metal and the Group IB metal results in a marked suppression of the hydrogenolysis activity of the former. 

The reactions include hydrogenolysis or hydrocracking (rupture of C—C bonds), isomerization, dehydrocyclization and aromatization of the C U2n + 2 molecules " . They are observed with varying selectivities, which depend on the experimental conditions, e.g. temperature, pressure and the type of catalyst. A multitude of supported (Al203,Si02) or unsupported catalysts (e.g. monometallic transition metals inter-metallic compounds , bimetallic alloys, etc.) are applied to induce skeletal rearrangements of hydrocarbons. The latter, however, are the most important catalysts used in industrial practice, which allow reforming processes at low pressure and at low temperatures. 

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