Platinum cycloalkanes

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

Volter (66) reported that a part of Pt(IV) (which is present as a non-stoichiometric oxychlorinated complex over his alumina support after calcination) is reduced reversibly below 550°C to a Pt complex related to soluble platinum. Above this temperature, reduction to the metallic state is complete. He attributed direct alkane- cycloalkane cyclization to this platinum complex (24, 66). 

Formation of cycloalkane-1,2-dicarboxylate esters by reduction of dibromoalkanedicarboxylate esters at a platinum cathode in tetrahy-drofuran. Data from ref. [109]... 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

Nickel has been reported to show behaviour similar to platinum [236], Further, in the reactions of cycloalkenes with deuterium, the product cycloalkanes are much more extensively exchanged over palladium than over nickel or platinum [236]. Such behaviour is not unexpected by comparison with the results obtained in the hydrogenation of linear alkenes (Sect. 3, p. 25). 

As developed in the introduction, a number of important features in hydrogenolysis of cycloalkanes on platinum-charcoal catalysts emerges from the work of the Soviet school of catalysis. In a different approach, the hydrogenolysis of methyl- and 1,3-dimethylcyclopentanes was investigated on a series of platinum-alumina catalysts with various metal loadings (0.2-20%) (84, 85). It was found that the product distribution changed substantially with the percentage of platinum on the carrier. An almost selective... 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

The objective of the process is to convert saturated hydrocarbons (alkanes and cycloalkanes) in petroleum naphtha fractions to aromatic hydrocarbons as selectively as possible, since the latter have excellent antiknock ratings (1,2). Naphtha fractions are composed of hydrocarbons with boiling points in the approximate range of 50-200°C. Reaction temperatures of 425-525°C and pressures of 10-35 atm are employed in the process. Reforming catalysts commonly contain platinum (3-5) or a combination of platinum and a second metallic element such as rhenium (6) or iridium (2,7). 

Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

In Figures 5.5 and 5.6, data on the platinum-iridium and platinum-rhenium catalysts are shown for the reforming of a 70-190 C boiling range Persian Gulf naphtha to produce 98 research octane number product at a pressure of 28.2 atm and a temperature of 490 C (33). The naphtha contained (on a liquid volume percentage basis) 69.7% alkanes, 18.5% cycloalkanes, and 11.8% aromatic hydrocarbons. The density of the naphtha was 0.7414 g/cm3. The data in Figure 5.5 show that the platinum-iridium catalyst is almost twice as active as the platinum-rhenium catalyst. 

The attractive features of platinum-rhenium and platinum-iridium catalysts can be combined in a reforming operation. The data for the reactions of selected hydrocarbons considered earlier for platinum-rhenium and platinum-iridium catalysts indicate that the former catalyst is more selective for the conversion of cycloalkanes to aromatics, while the latter is more selective for the dehydrocyclization of alkanes. Since cycloalkane conversion occurs primarily in the initial part of a reforming system while dehydrocyclization is the predominant reaction after the cycloalkanes have reacted, it is reasonable to use a platinum-rhenium catalyst in the front of the system and to follow it with a platinum-iridium catalyst (32). 

Dehydration of cyclopentanemethanol over aluminum oxide at 320° tgave an olefin mixture whence hydrogenation afforded a mixture (94%) of [14C]methylcyclopentane with about 34% of [14C]cyclohexane. The mixed cycloalkanes were transformed into [14C]cyclo-hexane by Nenitzescu and Cantuniari s method,154 and dehydrogenation on a platinum-charcoal catalyst at 360° then yielded [14C1]benzene. 

In both problems (a) and (b) ether is the solvent in which the reaction is carried out. This yields an unstable intermediate that is treated with an acid to obtain an alcohol. In problem (c) the reducing process is changed where hydrogen attacks the carbonyl moiety with the facility of a metal catalyst, platinum. The double bond is also reduced by the addition of hydrogen to yield a cycloalkane. The solvent used to carry out the catalytic hydrogenation is ethanol. 

Cycloalkanes and cycloalkanes containing one or more six-memberea rings can usually be dehydrogenated to the corresponding benzoid derivatives by heating to a high temperature with a platinum or palladium catalyst. 

The mononuclear catalyst [Ru(CO)3(dppe)] is of lower activity than monophosphine ruthenium complexes, but of higher selectivity. Platinum complexes promoted with SnCl2 are also of low activity if the phosphine ligand is dppe, but are much more active with dppb, or a related 6/5(phosphinomethyl)cycloalkane ligand, which are known to form bridged structures. ... 

For hydrocarbon reactions, metals (particularly platinum and its alloys) are frequently applied to acidic supports to catalyse hydrogen transfers. Thus platinum on a chlorinated alumina support accelerates the acid catalysed isomerization of n-alkanes (at about 150°C). In hydrocracking, the metal catalyses hydrogenation of heavy aromatic and polyaromatic components the resulting cycloparaffins (cycloalkanes) undergo zeolitic cracking, with... 

When 1,2-dimethylcyclohexene (below) is allowed to react with hydrogen in the presence of a platinum catalyst, the product of the reaction is a cycloalkane that has a melting point of—50 °C and a boiling point of 130 °C (at 760 torr). (a) What is the stmcture of the product of this reaction (b) Consult an appropriate resource (such as the web or a CRC handbook) and tell which stereoisomer it is. 

Methane undergoes activation with the greatest difficulty, followed by other alkanes, cycloalkanes, aromatic hydrocarbons, and hydrocarbon derivatives. Methane is activated by means of platinum(II) complexes in acetic acid solution. 

A good number of monometallic and bimetallic catalysts have been presented in the literature for dehydrogenation of the cycloalkane reaction. Platinum supported on alumina (Pt/Al203) is a common catalyst for this reaction. Bimetallic catalysts containing a small amount of Pt, in which the second metal enhances the activity and selectivity of the catalyst, have been investigated extensively. For instance, Pt-W, Pt-Re, Pt-Rh, and Pt-Ir have been suggested for the dehydrogenation of cyclohexane. 

---