Platinum-rhodium oxide, hydrogenation

Rylander and Kilroy studied the formation of cyclohexyl phenyl ether intermediate in the hydrogenation of phenyl ether over binary platinum-rhodium oxide catalysts in cyclohexane at room temperature and atmospheric hydrogen pressure. The yield of the intermediate varied greatly with the catalyst composition. The highest yield (48%) was obtained over the catalyst consisting of 30% Pt-70% Rh.149... 

Reductions with platinum rhodium oxide Ring hydrogenation... 

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

Many examples of this type of reaction are known the decomposition of arsine the decomposition of phosphine on surfaces of glass, f porcelain, J silica the decomposition of formic acid vapour on a variety of different surfaces— glass, platinum, rhodium, titanium oxide, and others the decomposition of nitrous oxide on the surface of gold Tf the decomposition of sulphuryl chloride on the surface of glass the decomposition of hydrogen iodide on the surface of platinum ff the decomposition of hydrogen selenide on the surface of selenium. J J A general discussion... 

Ruthenium is commonly used with other platinum metals as a catalyst for oxidations, hydrogenations, isomerizations, and reforming reactions. The synergetic effect of mixing ruthenium with catalysts of platinum, palladium, and rhodium lias been found for the hydrogenations of aromatic and aliphatic nitro compounds, ketones, pyndine, and nitriles. 

Rylander and Rakoncza compared the rates of hydrogenation of pyridine V-oxide over 5% palladium-, platinum-, rhodium-, and ruthenium-on-carbon in methanol, water, and acetic acid.224 Rhodium was always the most active, although the pyridine ring was hydrogenated concomitantly with the reduction of the V-oxide group. 

Over platinum metals such as platinum, rhodium, and ruthenium, aromatic compounds can be hydrogenated even at room temperature and a low hydrogen pressure. Adams and Marshall hydrogenated various aromatic hydrocarbons with use of the Adams platinum oxide in acetic acid at 25-30°C and 0.2-0.3 MPa H2, although often a long reaction time was required.6 Examples are shown in eqs. 11.5 and 11.6. 

Nakahara and Nishimura studied the selectivities of copper-chromium oxides, nickel, palladium, rhodium, and ruthenium catalysts in the hydrogenation of phenan-threne, 9,10-dihydrophenanthrene (DHP), and 1,2,3,4-tetrahydrophenanthrene (THP), usually in cyclohexane at 80°C (150°C for copper-chromium oxide) and an initial hydrogen pressure of 11 MPa (5 MPa for platinum metals). The hydrogenations over Os-C, Ir-C, and Pt-C were very slow and not investigated further. The varying compositions of the reaction mixture versus reaction time have been analyzed on the basis of the reaction sequences shown in Scheme 11.20 by means of a computer simulation, assuming the Langmuir-Hinshelwood mechanism.262 The results are summarized in Table 11.23. 

The platinum catalyzed oxidation of alcohols with molecular oxygen has been known for a long time.52,53 Palladium and iridium are also effective for this reaction but not rhodium or ruthenium.54 The reaction proceeds by the initial dehydrogenation of the alcohol to produce the aldehyde or ketone with the adsorbed hydrogen then reacting with the oxygen to give water.53>55-58 These... 

Table III shows the rate constants and activation energies for the hydrogenations over platinum and rhodium catalysts. For each compound the activation energy was greater for the hydrogenation with the supported catalyst. Activation energies were calculated from the least-squares , slopes. It is of interest to note that the values of fci.o for the supported rhodium catalyst are, at worst, only a factor of 2 to 3 less than for the corresponding values for pure platinum oxide. This indicates that the supported catalyst is far more active per unit weight of catalytic metal. Hydrogenation was attempted with pure rhodium oxide, but the reaction did not go at all under the conditions used for the other hydrogenations. It is likely that the conditions were not sufficient to reduce the oxide to the metal form.

Platinum black, finely divided metal, is a well-established catalyst for hydrogenation and dehydrogenation reactions. Platinum-rhodium gauzes are used as catalysts for the large-scale oxidation of ammonia to nitric oxide. 

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