Platinum- silica catalyst activity

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... 

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). 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support may be catalyticaliy inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts ( 1.2.8) constitute an extreme example of this. In naming such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or platinum/silica-alumina. The solidus is sometimes replaced by the word on, for example, platinum on alumina. 

The catalytic activation of CO2 and its reaction with C2H4 and H2O was studied over several silica-supported platinum-tin catalysts under different reaction conditions. The lactic acid production is related to the content of the PtSn alloy in the catalyst. 

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... 

Platinum-iron on alumina catalysts were characterized by Mbssbauer spectroscopy (Section 4) and their activity tested. Iron in clusters with high Pt Fe ratios, about 5, and fully combined with platinum, was catalytically inert for the CO-H2 synthesis reaction, attributed to a decrease in the electron density of the iron as indicated by the Mbssbauer isomer shift. The direction of electron transfer was opposite to that proposed for alkali-metal promoted iron catalysts. At low Pt Fe ratio, 0.1, ferromagnetic iron as well as Fe " ions and PtFe clusters were produced and dominated the activity/selectivity pattern. Rhodium on silica catalysts produced C2-compounds containing oxygen, specifically acetic acid, acetaldehyde and ethanol, with methane as the other major product. The addition of iron moved the C2-product formation sharply in favour of ethanol and now methanol was also formed. ... 

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. 

Figure 4 shows the total conversion of ethanol as a function of temperature as measured by gas chromatography. Except for the silica catalysts, the platinum catalysts exhibit equal or lower light-ofif temperatures than the supported catalysts with palladium as active material (compare with Figure 7). The platinum on alumina and platinum on titania catalysts are more active than the other catalyst combinations. The conversion curves for the Pd and Pt on ceria catalysts practically coincide, which implies that ceria would be a more suitable support material for a palladium catalyst than for a platinum catalyst. The activities of the silica catalysts are low. This observation is consistent with recent results in another research project using the same type of silica sol (Zwinkels et al, 1994). According to these experiments, it is crucial to reduce the alkali content to a very low level in the support, since sodium increases the mobility of silica, which poisons the active platinum and palladium sites. Platinum is apparently more sensitive to this phenomenon than palladium.

The palladium catalysts do not exhibit the pronounced difference in ethanol oxidation activity between the different support materials as is observed for the platinum catalysts (see Figure 7). Palladium on titania is the most active catalyst below 200°C. As in the case of the platinum catalysts, the activity of the silica catalyst differs from the other three. The activity of Pd/Si02 levels off at a higher conversion, though. 

By this time we were doing about as well as had been done previously with a molybdenum oxide alumina catalyst, but with considerably less carbon formation. So now things became more serious, but not serious enough to get people very excited about it. After all, we had been using a 3% platinum on silica catalyst, and even in those days 3% platinum was pretty expensive. Platinum on silica-alumina did much better with respect to octane number but we could not control the hydrocracking very well, so we switched to alumina which had an intermediate activity. The results looked pretty good, particularly because we could run for days without much loss in activity. 

Metal and Supported Metal Catalysts. Another group of catalysts active in the addition of the Si—H bond to unsaturated compounds are metals supported on inorganic materials or carbon. Initially, only a platinum catalyst supported on carbon, silicates, and silica appeared to be effective in the reactions of trichlorosi-lane with ethylene, acetylene, butadiene, allyl chloride, and vinylidene fluoride. However, it was soon established that other metals could also be used to catalyze hydrosilylation reactions (viz. Rh, Ru, Pd, Ni, and Ir). These metals are usually supported on active carbon, y-Al203, Si02, or CaCOs. Platinum supported on carbon (usually in 5 wt% concentration) is the most common and most efficient metal catalyst for the polyaddition and hydrosilylation of carbon—carbon multiple bonds (3) (for recent review, see (125)). 

When minute particles of an active material are dispersed on a less active substance to produce a catalytic effect, such catalysts are called supported catalysts. The active material is usually a pure metal or a metal alloy. Examples of supported catalysts are the platinum on-alumina catalyst used in petroleum reforming and vanadium pentoxide on silica catalyst used in oxidation of sulphur dioxide. 

One study reports on the use of a silica-supported platinum catalyst bound through an isocyanide link. A tethered PtCl2(CNR)2 catalyst was used to study the hydrogenation of cyclohexanone under the mild conditions of 40 °C and 1 atm H2. Activity was shown to be higher than that of the separate homogeneous complex even after extended use, catalyst activity remained constant, with no evidence of catalyst leaching. 

Catalysts for reforming typically contain platinum or a mixture of platinum and other metal promoters on a silica-alumina support. Only a small concentration of platinum is used, averaging about 0.4 wt %. The need to sustain catalyst activity and the expense of the platinum make it common practice to pretreat the reformer s feedstock to remove catalyst poisons. 

L-zeolite, potassium exchanged and also containing some platinum, is an active catalyst for the conversion of n-hexane to aromatic compounds. L-zeohte is a wide-pore zeolite, with a silica/alumina ratio of six and a unidimensional pore structure. 

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