Hydrocarbon over platinum

Another problem in mechanistic steps is posed by the oxidation of hydrocarbons over platinum metals, where the small amounts of aldehydes or acids observed appear not to be intermediates from the main sequence of steps. For example, Kemball and Patterson (II), in oxidizing C2H4 over palladium at 50° to 140°C., found acetic acid in amounts up to 3% accompanying the main product, C02. It might be thought that the... 

Fig.9 Reaction network shape-selective reforming of C7-hydrocarbons over platinum-erionite/alumina catalysts...

Schmitt, Jr., J. L., and P. L. Walker, Jr. 1972. Carbon molecular sieve supports for metal catalysts-n. Selective hydrogenation of hydrocarbons over platinum supported on polyfiirfuryl alcohol carbon. Carbon 10 87-92. 

Patterson, M., Angove, D. and Cant, N. (2000). The effect of carbon monoxide on the oxidation of four Cg to Cg hydrocarbons over platinum, palladium and rhodium, Appl. Catal. B Environmental, 26, pp. 47-57. 

Veser, G. and Schmidt, L. (1996). Ignition and extinction in the catalytic oxidation of hydrocarbons over platinum, AIChE Journal, 42, pp. 1077-1087. 

T. R. Hughes, W.C. Buss, P.W. Tamm, R.L. Jacobson et al., Aromatization of hydrocarbons over platinum alkaline earth zeolites. Stud. Surf. Sci. Catal. 28,725-732 (1986)... 

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Anderson and Avery s bond shift mechanism has the consequence of predicting that a quaternary carbon atom cannot be generated in the hydrocarbon product. In fact, Anderson and Avery (24) showed that in the isomerization of isopentane over platinum films, only a very small amount (<1%) of neopentane was produced (although the equilibrium constant for isopentane <= neopentane is 0.16 at 278°C). Furthermore,... 

C5 Cyclization of various alkanes 38, 38a) over platinum on carbon was first observed in 1954. Barron et al. 15a) postulated the formation of a surface C5 cyclic intermediate, which may desorb as a cyclopentanic hydrocarbon or may produce skeletal isomers without desorption. 

With gem-dimethyl hydrocarbons, isomerization and demethylation compete with each other. Isomerization predominates in the presence of acidic centers. Over platinum supported by acidic alumina 1,1-dimethylcyclo-hexane gave 18% toluene and 60% o-xylene (percentages of the total aro-... 

Hydrogen undergoes catalytic hydrogenation adding to unsaturated hydrocarbons, such as alkenes and alkynes forming alkanes. The reaction is catalyzed by nickel, platinum or palladium catalysts at ambient temperature. Hydrogenation of benzene over platinum catalyst yields cyclohexane, C6H12. 

Effect of Oxygen Concentration on Catalytic Oxidation and Ionization. A number of the hydrocarbons studied were oxidized over platinum by the usual procedure using a mixture of 60% oxygen and 40% nitrogen... 

As a result of the studies discussed above, a reasonably consistent picture of the kinetics and mechanism of the dehydrocyclization reaction over oxide catalysts has evolved. However, as pointed out earlier in this section, relatively few kinetic data have been reported for dehydrocyclization over platinum-alumina reforming-type catalysts. The data which have been reported include those of Hettinger and co-workers (H7), who studied the dehydrocyclization of re-heptane over platinum catalysts. These investigators found that the rate of dehydrocyclization decreased with increasing total pressure at a constant hydrogen-to-hydrocarbon ratio (Fig. 9). They also reported that the extent of dehydrocyclization was substantially greater for re-nonane than for re-heptane, which is consistent with the results obtained on oxide catalysts. In a later study of the kinetics... 

C5- and C6- cyclizations are parallel reactions. Csicsery has shown that isomerization of tetralin to methylindan over platinum-alumina at 371°C is extremely slow (22). Davis and Venuto provided further evidence by showing that methylindan is also not converted to tetralin or naphthalene over platinum on silica-alumina (23). This behavior is similar to that observed in the cyclization of aliphatic hydrocarbons. Davis and Venuto also reported that the major aromatic products obtained from ten C8-C9 paraffins and olefins at 482°C are only formed by direct six-membered ring... 

From examination of data for the oxidation of olefins over the noble metals, it was concluded that the observations were most satisfactorily represented by two compensation relations that refer to reactions with oxygen over platinum and palladium (204- 209a), Table III, H, and the same rate processes over silver, gold, ruthenium, and rhodium (103, 207, 209a), Table III, I. Arrhenius parameters for the oxidation of hydrocarbons on platinum and palladium, reported by Moro-oka et al. (42,230), defined points that were between the compensation lines Table III, H and I. When these latter observations were included with other results for reactions on the same two metals, the new line, Table III, J, was found. Values of B and e were appreciably changed so that lines of different slope (i.e., Table III, H and J) intersected near the region of maximum density of points on the compensation plot. Again, the calculated values of B and e were sensitive to the selection of results for quantitative consideration. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

The sulfur compounds contained as impurities in a substrate or solvent may have a profound effect on hydrogenation, particularly over platinum metals where the amounts of catalyst used are usually much smaller than in the case of base metals. An excellent way to remove such impurities is to treat the sample with Raney Ni at slightly elevated temperatures22 (usually 50-80°C). The impurities in benzene or cyclohexane can thus be removed simply by refluxing with Raney Ni for 0.5 h (see Section 13.3). Granatelli applied this desulfurization with Raney Ni to determine quantitatively as little as 0.1 ppm of sulfur contained in 50 g of nonolefinic hydrocarbons.23... 

Hydrogenation of ketones over platinum metals in alcoholic solvents, especially in methanol and ethanol, may be accompanied by the formation of acetals (and also hemiacetals and enol ethers) in the presence of a mineral acid and may lead to the formation of ethers, together with the formation of alcohols and hydrocarbons 98-100 The reactions involved under these conditions are shown in Scheme 5.4 for cyclohexanone. At an equilibrium in acidic methanol, acetals are present predominantly over hemiacetals for most ketones.101... 

The hydrogenations over platinum may be accompanied by hydrogenolysis to give hydrocarbons, especially with unhindered ketones in acidic media. 

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. 

The stereochemistry of hydrogenolysis of optically active 2-methyl-2-phenylaziridine (57, X = H) has been studied by Mitsui and Sugi.124,125 The products were mostly 2-phenyl-l-propylamine (58, X = H)) (92-100%), along with small amounts of 2-phenyl-2-propylamine or 2-cyclohexypropylamine (0-7%) and hydrocarbons (2-phenylpropene or -propane) (-0-5%). Over Pd(OH)2 in ethanol, the configuration of 58 (X = H) was predominantly of inversion, while over platinum oxide it was largely retained. With addition of NaOH to palladium, however, the retained product... 

Several derivatives of phenols have been found to be especially suited for hydrogenolysis by catalytic hydrogenation. Phenol ethers prepared by the reaction of phenols with l-phenyl-5-chlorotetrazole or with 2-chlorobenzoxazole are hydrogenolyzed over 5% palladium on carbon or over platinum oxide, but not over Raney nickel. The hydrogenations are run at 35 °C in benzene, ethanol or tetrahydrofuran, and give 35-89% yields of the corresponding hydrocarbons. The reaction sequence is exemplified by conversion of phenylphenols to biphenyls (equation 77). 

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