Platinum alumina catalyst

Fig. 5. Electron micrograph of 2.5% (w/w) platinum/alumina catalyst. Prepared by impregnation with chloroplatinic acid, reduced in hydrogen at 210°C. Micrograph obtained by thin sectioning. The black dots are platinum particles. (X 100,000). Reproduced with permission from R. L. Moss, Platinum Metals Rev. 11 (4), 1 (1967) and British Crown Copyright.

Fig. 6. Platinum crystallite size distribution for 2.5% (w/w) platinum/alumina catalyst. Full line, number distribution broken line, surface area distribution. After G. R. Wilson and W. K. Hall, J. Catal. 17, 190 (1970).

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

Shabaker (SI), for the hydrogenation of propylene over a platinum alumina catalyst, selected a Hougen-Watson model of the form... 

As an illustration, consider the hydrogenation of propylene over a platinum alumina catalyst discussed in Section II. These date were taken from 0 to 35°C, 1 to 4 atm total pressure, and 0 to 45 % propylene. Equations (7) and (8) were obtained after considerable sifting and winnowing of rate equations. Both fit the observed data reasonably well. 

There are two methods of manufacture of the xylenes. The major one is from petroleum by catalytic reforming with a platinum-alumina catalyst. The second method (which has been developed recently) is by processes involving the disproportionation of toluene or the transalkylation of toluene... 

The foregoing paragraphs have described platinum on alumina catalysts in a very general way and have also attempted to give a brief discussion of the reason why platinum is so commonly used as a reforming catalyst. In the following sections, the physical and chemical properties of platinum-alumina catalysts will be discussed in more detail, since this information will be of considerable help in understanding the way in which these catalysts function. 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

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.

Fig. 9. Effect of total pressure on dehydrocyclization of n-heptane over platinum-alumina catalyst (H7). Conditions 496°C., H2/HC = 5/1.

Il in and Usov have shown that over acidic platinum-alumina catalyst there are at least two consecutive pathways from n-nonane to indan (57) either through alkylaromatic intermediates, by first closing the six-membered ring or through alkylcyclopentane intermediates, by first closing a five-membered ring (Table VII and Fig. 6). 

This reaction was first observed by Plate, Erivanskaya, and Khalima-Mansur over platinum-on-carbon and platinum-on-alumina catalysts (43-48). Platinum-on-carbon catalyzes this reaction between 310°C and 390°C (above which the catalyst is poisoned) (44). Over an acidic platinum-alumina catalyst containing 0.5 wt% platinum and 0.1 wt% sodium, 16.7% acenaphthenes and 1.5% acenaphthylene are obtained at 460°C and at 0.4 liquid hourly space velocity in hydrogen diluent. Conversions are considerably lower in helium. 

Fig. 18. Conversion of n-hexane over CCU chlorinated platinum-alumina catalyst effect of temperature on reaction products 86). (Reprinted with permission of North-Holland Publishing Company.)...

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 paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

L. L. Hegedus, and K. Baron (1975) Effects of poisoning and sintering on the pore structure and diffusive behavior of platinum/alumina catalysts in automotive converters, Journal of Catalysis 37(1) 127-132... 

Further reports on asymmetric synthesis in the presence of Cinchona alkaloids have been made.142 " For example, hydrogenation of methyl pyruvate with a platinum-alumina catalyst containing quinine gives (+)-(/ )-methyl lactate in 87% optical yield.1426 Asymmetric induction with optical yields up to 36 and 26% has been observed in the Michael addition of thiols and nitro-alkanes to ct/ -unsaturated ketones in the presence of quaternary salts derived from the Cinchona alkaloids.142"... 

Van Nordstrand, R. A., Lincoln, A. J., and Carnevale, A., "Determination of Metallic Platinum in Platinum-Alumina Catalysts by X-ray Diffraction", Anal. Chem. 1964, 36, 819. 

Shabaker, R. H., Kinetics and effectiveness factors for the hydrogenation of propylene on a platinum-alumina catalyst, Ph. D. thesis. University of Wisconsin-Madison (1965). 

Feng, Kostrov and Stewart (1974) reported multicomponent diffusion data for gaseous mixtures of helium (He), nitrogen (N2) and methane (CH4) through an extruded platinum-alumina catalyst as functions of pressure (1 to 70 atm), temperature (300 to 390 K), and terminal compositions. The experiments were designed to test several models of diffusion in porous media over the range between Knudsen and continuum diffusion in a commercial catalyst (Sinclair-Engelhard RD-150) with a wide pore-size distribution. 

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