Platinum-silica catalysts effects

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). 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

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. 

Other molecules present in the gas mixtures like alcohols or ketones may have a moderate effect on aromatic oxidation but the reverse (inhibition of alcohol oxidation by aromatics) is most often observed. Different supports of Pt were used for toluene oxidation Al203, Al203/Al, Zn0/Al203, Ti02, mesoporous fibrous silica or monoliths. Zeolites, generally promoted by platinum, were shown to give excellent catalysts for aromatic oxidation. Basic zeolites showed excellent performances in oxidation of m-xylene even in the absence of platinum. Palladium catalysts, either supported on alumina or ceria-alumina, were also investigated for oxidation of benzene and several alkylbenzenes. ... 

This paper identifies alumina, rare earths, platinum, and magnesia as important SOx capture materials. Alumina is either incorporated directly into the matrix of a cracking catalyst or added as a separate particle. Cerium is shown to promote the capture of SO2 on high alumina cracking catalyst, alumina, and magnesia. Other rare earths are ranked by their effectiveness. The promotional effect of platinum is shown between 1200 and 1400 F for SO2 capture on alumina. Silica, from free silica or silica-alumina in the matrix of cracking catalyst, acts as a poison by migrating to the additive. 

We found a solution with new, doubly promoted platinum catalysts on silica and reported their scope and limitations for the synthesis of various aniline derivatives (ref. 5). In the following communication we describe the development of the most effective catalyst using as model reaction the N-alkylation of 2-methyl-6-ethylaniline with methoxy-2-propanol. 

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... 

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