Platinum on silica

A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BET surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 

A procedure similar to that used in the investigation of the hydro-demethylation of xylenes was also employed in a study of the consecutive hydrogenation of phenol via cyclohexanone to cyclohexanol in gaseous phase on a platinum on silica gel catalyst (p. 27) at 150°C [scheme (VI)] at this temperature both reactions were irreversible under the excess hydrogen used. 

Supported platinum. The STEM and TPD data for platinum supported on alumina and silica are summarized In Table I. The platlnum-slllca samples show a high degree of variability In size and mass. This variability Is Indicative of the mobility of platinum on silica at elevated temperatures, l.e., 500°C. These samples were of little... 

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... 

The first metallic catalyst used for dehydrocyclization of alkanes (/) was platinum on carbon (10-40 w/w% metal). It is typically used around atmospheric pressure and temperatures not exceeding 300°C. Such catalysts are inadequate for praetical purposes. This is the reason for commercial dual-function catalysts—typically platinum on silica-alumina—having been developed 32). 

Bursian et al. (66a) suggested metallic platinum sites for dehydrogenation and Pt " sites for ring closure. They studied the effect of several elements added to platinum-on-silica catalyst on the aromatizing activity of n-hexane. Benzene yield increased parallel to the amount of soluble platinum (66b) at the same time, the crystallinity of platinum decreased in the presence of additives promoting aromatization. These are elements (e.g., Ce, Sc, Zr) which do not form an intermetallic compound with platinum (66c). 

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. 

The high degree of dispersion of platinum on alumina has also been confirmed by the hydrogen chemisorption measurements of Keavney and Adler (Kl), and by the chemisorption studies of Gruber (G2). In addition, high dispersion of platinum on silica-alumina has been observed by Hughes and associates, using a carbon monoxide chemisorption technique (H10). 

The rate of dehydrocyclization is probably often inhibited by (bicyclic) product desorption. Apparent first-order rate constants in the dehydrocyclization of -butylbenzene over platinum-on-silica-gel decreased with increasing levels of bicyclic aromatic product (Fig. 4). Sinfelt and co-workers also found that product desorption is rate-controlling in methylcyclohexane aromatization (20). 

Fig. 4. Dehydrocyclization of n-butylbenzene over 2% platinum on silica gel catalyst. (—) Cyclization to methylindan and methylindenes and (---) cyclization to naphthalene. According to Csicsery (13).

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... 

Over platinum-on-carbon catalyst at relatively low temperature (310°C), C5-cyclization of alkylbenzenes probably proceeds by direct closure of the ring between the carbon atoms of the side-chain and the benzene ring, bypassing dehydrogenation to olefins (25-27). However, at higher temperatures and on platinum-alumina or platinum-on-silica C5-dehydro-cyclization could involve olefinic intermediates (7,13, 28). 

The methyl group in the y-position of the side-chain interferes with cyclization. The rate of C5-cyclization of n-butylbenzene at 371°C over platinum-on-silica gel is 3.5 times higher than that of 2-phenylpentane (Table IV) (14). The difference in C6-cyclization is even larger. Now, the side-chain carbon atoms involved in the cyclizations of n-butylbenzene and 2-phenylpentane have identical natures (i.e., secondary in five-membered ring closure and primary in six-membered ring closure). The difference between the two molecules (the extra methyl group) is far removed from the two carbon atoms involved in the formation of the new bond ... 

Over platinum-on-silica catalysts, different alkylindans are at equilibrium with the corresponding alkylindenes. Similarly, I-methylindene is at equilibrium with 3-methylindene (13, 14). 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

Cyclization selectivities are very different over platinum on silica-alumina than over platinum on silica gel (Table IV). In the case of n-butylbenzene, for example, methylindan/naphthalene ratios differ by about an order of... 

These rate differences show that, over platinum on silica-alumina, two cyclization reactions occur simultaneously. One is catalyzed by the platinum metal it is the mechanism observed over platinum on silica. Acid catalyzed self-alkylation is the second reaction. The following steps are involved. 

About half of the 1-methylnaphthalene formed from n-pentylbenzene and 2-phenylpentane isomerizes to 2-methylnaphthalene over platinum on silica-alumina (while over platinum on silica less than 3% of the methylnaphthalene isomerizes to 2-methylnaphthalene). Alkylindan (and alkyl-indene) isomerization is also considerable over platinum on silica-alumina (13, 14). 

Platinum on alumina has properties between those of platinum on silica gel and platinum on silica-alumina. Only about one-fifth of the methylindan is produced by the acid-catalyzed route (13). Also, the isomerization of... 

Cyclization of phenylbutenes over the platinum-on-silica gel catalyst almost exactly parallels that of -butylbenzene. Since rapid hydrogenation of the phenylbutenes results in about the same n-butylbenzene/phenylbutene ratio as observed with the n-butylbenzene feed, this is hardly unexpected. 

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. 

The relationship between the two catalytic components is quite complex. Interactions between the support and the hydrogenation component can alter this relationship. For example, Larson et- al. (6) showed that, with platinum on silica-alumina, a selective adsorption of platinum by acid sites causes a reduction in catalyst acidity. Similarly, nickel reacts with the acid sites on silica-alumina forming nickel salts of the silica-alumina acid sites. It has been suggested (J) that one of the effects of sulfiding a nickel on... 

An example is washcoating of platinum and silica using a sol containing both a silica and a platinum precursor [58,59]. After calcination a catalyst containing platinum on silica-washcoated monolith is obtained. Since the platinum precursor is homogeneously mixed with silica, encapsulation of platinum may occur, which reduces the effectiveness of platinum as compared to that of a two-step preparation. In this respect, the method may be more suitable for less expensive active phases. 

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. 

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... 

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