Platinum- silica catalyst isomerization

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. 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

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

Polyaromatics react under hydrocracking conditions to undergo partial or complete saturation of the aromatic rings, together with isomerization and cracking of intermediate and perhydro products. Phenanthrene, for example, can lead to tetralin and methyl cyclohexane as the principal products. With a platinum/ silica-alumina catalyst,67 the major products are isomerized perhydrophenan-threnes, which includes some adamantanes and cracked products, the total number of products exceeding 100. 

Isomerization of alkylcyclopentanes may also occur on the platinum catalyst surface or on the silica/alumina. For example, methylcyclopen-tane isomerizes to cyclohexane ... 

Pentafining A process for isomerizing pentane in a hydrogen atmosphere, using a platinum catalyst supported on silica-alumina. Developed by the Atlantic Richfield Company. 

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... 

Licensors offer a variety of catalysts to promote the isomerization— silica alumina by itself or enhanced with a noble metal like platinum or a non-noble metal like chromium. Another uses hydrofluoric acid with boron trifluoride In the case of the noble metal catalytic process, the feed enters a vessel with a fixed catalyst bed at 850°F and 14.5 psi. As is often the case, a small amount of hydrogen is present to reduce the amount of coke laying down on the catalyst. The effluent is processed in a standard fashion to separate the hydrogen, the para- and ortho-xylene, and any unreacted or miscellaneous compounds. Yields of para-xylene are in the 70% range. 

Several years ago, one of the authors found that nickel, platinum, and some other hydrogenating agents, when deposited on fresh synthetic silica-alumina cracking catalyst, made a new catalyst that would isomerize paraffin and naphthene hydrocarbons in the presence of hydrogen at elevated pressures and nominal temperatures. Table I shows some early typical results calculated from mass spectrometer analyses of the products obtained by passing methyl cyclopentane, cyclohexane, and n-hexane over a catalyst composed of 5% nickel in silica-alumina at the indicated reaction conditions. Isomerization of a number of other hydrocarbons has also been studied and reported elsewhere (2). 

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. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

In the presence of hydrogen the isomerization of paraffins of five or more carbon atoms over dual function catalysts, such as amorphous silica-alumina supported platinum, can be described by the following scheme ... 

Fig. 2. Isomerization of n-heptane over mixtures of particles of silica-alumina and particles of inert-supported platinum (W5). The dashed lines represent conversions over platinum-impregnated silica-alumina. Conditions of runs 25 atm., Hj/nC = 4/1, space velocity = 0.7 g. nC7 per hour per gram of catalyst.

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). 

Isomerization over the neutral platinum-on-silica gel catalyst proceeds by two different mechanisms. 

All side-chain isomers are formed in acid-catalyzed isomerization. Carbonium ions are the intermediates here. Over dual-function catalysts, such as platinum-on-alumina and platinum-on-silica-alumina, platinum increases the rate of isomerization by dehydrogenating alkanes to olefins. This facilitates the formation of carbonium ions. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Pentafining a pentane isomerization process using a regenerable platinum catalyst on a silica-alumina support and requiring outside hydrogen. 

Commercial zeolite based hydroisomerization catalysts comprise alumina bound and platinum impregnated dealuminated mordenite. The activity and selectivity of the hydroisomerization of n-paraffins is strongly influenced by acid leaching. The influence of silica to alumina ratio has been studied for pentane isomerization over platinum mordenite many times since one of the first papers published (6). 

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. 

A number of hydrocarbon transformations have been shown by Haensel and Donaldson 10), Heinemann et al. 11), and by Ciapetta and Hunter 12) to be catalyzed by solid catalysts in which a transition metal, notably platinum or nickel, has been combined with an acidic oxide carrier substance such as, for example, silica-alumina, or halogen containing alumina. These include the isomerization of paraffins, the hydroisomerization of naphthene rings, and the hydrogenative cracking of paraflBns. 

Figure 11 shows conversion to iso-heptanes to be negligible for (0.5 wt. %) platinum supported on activated carbon (Pt/C) as the only catalyst, and also for (0.4 wt. %) platinum on silica-gel (Pt/Si02). No detectable conversion was obtained with silica-alumina. A mechanical mixture of either of the Pt-bearing particles with silica-alumina of about 150 m.Vg-surface area, both in millimeter diameter particle size (1000m), immediately resulted in appreciable isomerization ( SiAl with Pt/C SiAl with Pt/Si02). Isomerization increases rapidly for smaller component particle sizes, of 70/i and S i diameters. It approaches the performance of a silica-alumina that has been directly impregnated with platinum, and which has... 

Foger and Anderson (739) measured the selectivity and turnover number for isomerization and hydrogenolysis of neopentane on a number of catalysts where platinum was deposited on silica or Y zeolites. The selectivity did not vary widely with metal dispersion down to a limiting value of 10 A, below which isomerization disappears completely. The same trend toward increased selectivity as that observed by Boudart was noted when calcining the catalysts... 

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. 

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