Platinum-alumina catalyst isomerization

Proportions of Isotopically Labeled Products from Isomerization of Hexanes over 0.2% Platinum /Alumina Catalyst at 273°C ... 

Pig. 4. Effect of nCs/Hs ratio on nCt isomerization at 372°C. over platinum-alumina catalyst (S4). 

Fig. 5. Isomerization rate versus pentene partial pressure (S4). Comparison of n-pentane isomerization rate over platinum-alumina catalyst with the rate of skeletal isomerization of 1-pentene over the platinum-free catalyst 372°C.

Fig. 6. Effect of hydrogen pressure on rate of isomerization of n-heptane over platinum-alumina catalyst (R3). The rate n is relative to the rate of isomerization at 471 °C., ph = 5.8 atm.

Goble and Lawrence attributed the high isomerization activity of chlorinated platinum-alumina catalyst to the creation of a localized dual site comprising a Lewis acid site and an adjacent platinum site. However, as has since been pointed out by Asselin et al. (88), carbonium ion intermediates over low-temperature isomerization catalysts are probably created by the same process as that observed for Friedel-CrEifts catalyst abstraction of hydride ion from the paraffin by a strong Brdnsted acid according to the equation... 

The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (52, 55). 

Platinum-alumina catalysts have been of profound importance in petroleum naphtha reforming and paraffin isomerization processes. Much information has been established concerning platinum crystallite size and interaction with the acidic support in terms of... 

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Isomar [Isomerization of aromatics] A catalytic process for isomerizing xylene isomers and ethylbenzene into equilibrium isomer ratios. Usually combined with an isomer separation process such as Parex (1). The catalyst is a zeolite-containing alumina catalyst with platinum. Developed by UOP and widely licensed by them. It was first commercialized in 1967 by 1992, 32 plants had been commissioned and 8 others were in design or construction. See also Isolene II. 

In hydrocarbon reforming processes the vapour of an alkane is passed over a supported metal catalyst such as platinum on silica or alumina. Dehydrocyclization, isomerization and cracking reactions all take place to... 

Davis (94b) aromatized several Cg and C9 hydrocarbons with a quaternary carbon atom over chromia- and platinum-on-alumina catalysts. Here the reactions of 1,1-dimethylcyclohexane, and 2,2- and 3,3-dimethylhexanes will be compared (Table V). 1,1-Dimethylhexane suffered demethylation predominantly over chromia and alkaline platinum however, with less alkaline platinum, isomerization to xylenes occurred. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

From this beginning, an extensive study of the isomerization of n-heptane was made with platinum on silica-alumina catalysts. Figure 2 shows curves plotted from the data obtained illustrating the total isomer yield versus conversion and the temperatures that produced these conversions. The conversion-isomer yield curve follows closely the 45° theoretical yield line, goes through a maximum at about 65% isomer yield, and then drops sharply because of cracking. The temperature at which the maximum yield of isomers was obtained was about 660° F. 

A study of the kinetics of isomerization of n-pentane at 372°C. over a platinum on alumina catalyst (0.3% platinum) has been reported by Sinfelt et al. (S4). The rate measurements were made in a flow system at low conversion levels (4-18%). The n-pentane was passed over the catalyst in the presence of hydrogen at total pressures ranging from 7.7 to 27.7 atm. and at hydrogen to n-pentane ratios varying from 1.4 to 18. Over this range of conditions the rate was found to be independent of total pressure and to increase with increasing n-pentane to hydrogen ratio (Fig. 4). The rate data were correlated by an expression of the form... 

Thus, at 372°C. and over the range of pressures and hydrogen to n-pentane ratios covered in the investigation, it appears that the proposed mechanism can account in large part for the observed kinetic data. However, Starnes and Zabor (S8) have proposed an alternative mechanism, based on their studies of n-pentane isomerization over platinum-alumina-halogen catalysts. They postulate that the paraffin is adsorbed on platinum sites with dissociation of a hydrogen atom, followed by polarization of the adsorbed species. 

Isomerization catalysts were developed along two paths—by Friedel-Crafts halide systems or by dual site heterogeneous catalysts, originating with the commercial introduction of platinum-aluminas for catalytic reforming in the 1940,s. The Friedel-Crafts systems (aluminum chloride-hydrocarbon complexes) were used exclusively during the early stages of... 

World War II when the first commercial isomerization processes were introduced to manufacture isobutane as a feedstock for aviation alkylate. The Friedel-Crafts catalysts are highly active at 100°-200°F whereas the conventional platinum-alumina reforming catalysts give reasonable reaction rates only above 850°F. 

This mechanistic interpretation is based on the assumption that, once formed, five- or six-membered products of dehydrocyclization do not undergo interconversion. As discussed above, isomerizations are extremely slow at 317°C for tetralin to methylindan and methylindan to tetralin over alumina, silica-alumina, platinum-on-alumina, and platinum-on-silica-alumina catalysts (22, 23). 

As discussed in Section IV, Barron et al. (55, 61) found the cyclic mechanism of isomerization to be predominant, perhaps the sole route, on a highly dispersed platinum-alumina (0.2% w/w Pt). The cyclic mechanism was shown to be important also over platinum films and supported platinum of moderate dispersion (>100 A). Here, although the product distributions were very different from that found over the dispersed catalyst, the initial product distributions at 300°C were practically identical in the isomerization and in methylcyclopentane hydrogenolysis. At lower temperatures they were somewhat different as they also were at all temperatures on platinum films. It was suggested that, especially on platinum films, a bond-shift isomerization could accompany the cyclic... 

In contrast, we shall see that in a paraflhi isomerization system a platinum on silica-alumina catalyst is a multifunctional, specifically, a hifunc-tional catalyst the platinum sites catalyze distinctly different reactions and reaction steps than do the silica-alumina sites neither catalyze the reactions of the other component furthermore, both tj ies of reactions are relevant to accomplish the over-all reactions of the desired conversion system. 

We obtained initial rates for the reaction of neopentane on supported platinum and platinum powder catalysts at 300°, 1 atm total pressure, and a hydrogen-to-neopentane ratio equal to 10. As before, surface platinum atoms were titrated by selective chemisorption of hydrogen (27). Before discussing the results, it is important to stress the reproducibility of the results on samples of different origin but nearly identical dispersion and pretreatment. Thus, the same value of the selectivity to isomerization was found on two catalysts an experimental catalyst containing 2% platinum on t -alumina and a commercial sample with 0.6% platinum on y-alumina. Percentage dispersion of the metal was 64 and 73, respectively, and the selectivity was 1.5. Both samples were reduced at 500° under identical standard conditions. 

Fio. 3. Isomerization of 2-methyIbioyclo[2.2.1]heptane (12) at 250 , vapor phase, in the presence of platinum-silica-alumina catalyst. 

The label has been located in the isomerization products obtained from 2-methylpentane-2- C, 2-methylpentane-4- C and 3-methylpentane-3- C on 10% platinum-alumina and single crystals under 20 Torr hydrogen pressure (54, 60). Under these conditions, the scrambling of the label was found to be extremely limited less than 10% of abnormal varieties are obtained. However, for these alkanes, on such catalysts and in these conditions, the selective cyclic mechanism is widely predominant and yields... 

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