Platinum-alumina selectivity

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

With the introduction of Pt/Re catalysts, it is possible to achieve the ensemble control with much smaller sulfur addition. The su1fur-free Pt/Re catalyst by itself has a higher relative activity for hydrogenolysis than a platinum catalyst. However, this is changed when sulfur Is present In the feed. Kughes has described the first observations in a pilot plant. The catalyst produced more methane than any other that had been tested, and the run would probably have been aborted if it had not been an ordinary catalyst screening test. However, after the first and second weeks on stream, the selectivity improved and finally became similar to that of a fresh platinum/ alumina catalyst and as the run continued, the catalyst proved to be more stable than any previous catalyst tested. These results were ascribed to the presence of sulfur in the feed and could be obtained even with very low sulfur contents, l.S ppm. ... 

Nafta reforming and alkane dehydrogenation processes are directly connected on platinum alumina based catalysts. The stability and selectivity requirements for industrial purposes induced the addition of a second metal like Sn, Ge or Re. In fact, these promoters coupled to other acidity controllers added on the support enhanced the catalytic efficiency [1-3]... 

The label has been located in the isomerization products obtained from 2-methylpentane-2- C, 2-methylpentane-4- C and 3-methylpentane-3- C on 10% platinum-alumina and single crystals under 20 Torr hydrogen pressure (54, 60). Under these conditions, the scrambling of the label was found to be extremely limited less than 10% of abnormal varieties are obtained. However, for these alkanes, on such catalysts and in these conditions, the selective cyclic mechanism is widely predominant and yields... 

As developed in the introduction, a number of important features in hydrogenolysis of cycloalkanes on platinum-charcoal catalysts emerges from the work of the Soviet school of catalysis. In a different approach, the hydrogenolysis of methyl- and 1,3-dimethylcyclopentanes was investigated on a series of platinum-alumina catalysts with various metal loadings (0.2-20%) (84, 85). It was found that the product distribution changed substantially with the percentage of platinum on the carrier. An almost selective... 

While the present study has not examined the performance of zeolite based catalysts, Table V summarizes patent literature data(23b) for a mordenite and platinum/alumina mixture. Data for the Pt and SAPO-11 mixture obtained under similar conditions are presented for comparison. The mordenite catalyst is significantly less selective than SAPO-11, giving 25.6% ethylbenzene conversion with only 0.5% net xylene production. 

Several platinum-alumina catalysts were prepared on low acidity aluminas such as A, B, and D listed in Table II. Practically all of the data obtained with such catalysts fell on a single selectivity curve correlating % conversion to MO with % selectivity to MO formation. Selectivity was found to be quite high, as illustrated by the following results for platinum on low acidity alumina ... 

A catalyst was prepared on the strongly acidic alumina E, using exactly the same preparation procedure used to make the platinum-low acidity alumina compositions. This catalyst was evaluated under the standard conditions and gave results very similar to those obtained with the commercial platinum-alumina reforming catalysts. At a conversion to MO of 8%, the selectivity to MO formation was 31%. 

After a preliminary study of the properties of a number of platinum and nickel catalysts deposited on various carriers (activated carbon, chromia, alumina, molybdena, etc.) a platinum-alumina catalyst was selected for the present investigation with a 0.5 % Pt content. Cyclopentane (238.2 g) was passed over the catalyst at 0.43 hr. i space velocity, 20 atmospheres hydrogen pressure, and a temperature of 460 . As a result 184.3 grams of liquid product were obtained containing 9 % by volume of aromatics including benzene (81.9%), toluene, and p-xylene, and also w-pentane,... 

The considerable amoimts of aromatic hydrocarbons and methylcyclopentane that were found in the products are especially noteworthy. This new fact shows that under the conditions selected cyclopentane imdergoes a number of interesting reactions, an important part in which is played by methylene radicals arising from disintegration of the pentamethylene ring under the influence of the platinum-alumina catalyst ... 

C. Leclcrq, L. Leclerq, and R. Maurcl. Hydrogenolysis of Saturated Hydrocarbons. III. Selectivity in Hydrogenolysis of Various Aliphatic Hydrocarbons on Platinum/Alumina. J. Catal. 50 87 (1977). 

Some metallic compounds present in trace level (ppm) in petroleum feed, adsorbed to the active site of the catalyst, act and change the selectivity of the reaction by producing more and more unwanted products. When water vapour is present in the sulphur dioxide-air mixture supplied to a platinum-alumina catalyst, a decrease in oxidation activity occurs. This type of poisoning is due to the effect of wafer on fhe sfrucfure of the alumina carrier and is known as stability poisoning. The resulting increase in diffusional resistance may dramatically increase the Thiele modulus, and reduce the effectiveness factor for the reaction. In extreme cases, the pressure drop through a catalyst bed may also increase dramatically. 

Selective oxidation is able to decrease CO in a methanol reformed gas (anode fuel supply stream) from 1% to approximately 10 ppm using a platinum/alumina catalyst. The resulting performance of the anode catalyst, though satisfactory, is impacted even by this low amount of CO. Research at Los Alamos National Laboratory has demonstrated an approach to remedy this problem by bleeding a small amount of air or oxygen into the anode compartment. 

Early attempts by Mobil to minimize after-burning in TCC imits led to the addition of chromium oxide to their Durabead catalyst to oxidize carbon monoxide, but this uirfortunately also decreased cracking selectivity. Mobil then introduced a platinum/alumina additive in 1976 to control carbon monoxide combustion in the regenerator. Platinum was added either as a component of the cracking catalyst or in separate particles. Complete combustion of carbon monoxide was achieved by adding the equivalent of 0.5 ppm of platinum to the catalyst inventory. 

Solvents influence rate as well as selectivity. The effect on rate can be very great, and a number of factors contribute to it. In closely related solvents, the rate may be directly proportional to the solubility of hydrogen in the solvent, as was shown to be the case for the hydrogenation of cyclohexene over platinum-on-alumina in cyclohexane, methylcyclohexane, and octane 48). Solvents can compete for catalyst sites with the reacting substrates, change viscosity and surface tension (108), and alter hydrogen availability at the catalyst surface. 

The actual selectivity depends on the nature of the catalyst. For example, the following data were reported for n-hexane transformed over platinum and palladium supported on the same alumina 44) (pulse system, hydrogen carrier gas, T = 520°C) ... 

When platinum is alloyed with iridium (on an a-alumina support), which is a C5 cyclizing metal, the C5 cyelization selectivity of alloys containing 30-70% iridium remains nearly the same (about 0.15) and falls between that of platinum (about 0.6) and that of iridium (0.025). The aromatization (and... 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

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