Platinum-alumina catalyst activity

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 ... 

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. ... 

Bimetallic platinum-rhenium catalysts can be prepared in aqueous acid medium, under hydrogen flow, by a redox reaction between hydrogen activated on a parent platinum-alumina catalyst and the perrhenate ion ReO4. ... 

STABILITY POISONS When water vapor is present in the sulfur 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 water on the structure of the alumina carrier. Temperature has a pronounced ejffect on... 

The studies of n-heptane and methylcyclopentane conversion provide insight into the advantages of platinum-iridium and platinum-rhenium catalysts over catalysts containing only one of the transition metal components, that is, platinum, iridium, or rhenium. If, for example, we consider an iridium-alumina catalyst for the reforming of a petroleum naphtha fraction, we find that it produces a substantially higher octane number reformate than a platinum on alumina catalyst under normal reforming conditions. The iridium-alumina catalyst will also exhibit a lower rate of formation of carbonaceous residues on the surface, with the result that the maintenance of activity with time will be much superior to that of a platinum-alumina catalyst. 

On the basis of activity and activity maintenance, the use of an iridium-alumina catalyst in reforming appears very reasonable (35,36). However, in our experience, the yields of low molecular weight alkanes (methane and ethane) are higher with an iridium-alumina catalyst than with a platinum-alumina catalyst, resulting in lower yields of C5+ reformate. Because of the higher value of C5+ reformate relative to products such as methane and ethane, the iridium-alumina catalyst is not used, despite its higher activity and better activity maintenance. 

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,... 

An earlier study using this same compound, DMMP, led to a mathematical model of the deactivation process. Graven et a/. studied the oxidation of DMMP vapor in a stream of air, or nitrogen, over platinum-alumina catalysts. A commercial catalyst and a number of laboratory-prepared catalysts were investigated over a range of temperatures from 573-773 K, residence times from 0.15 to 2.7 seconds. The average catalyst particle sizes varied from 0.31 to 2.4 mm. They found that the fresh catalyst showed a very high activity, but after a few hours on stream it deactivated to the point that measurable quantities of DMMP vapor appeared in the effluent.. The reaction products over the deactivated catalyst were methanol and phosphorus acid. 

Felfoldi, K., Balazsik, K., Bartok, M. (2003) Heterogeneous asymmetric catalysis. Part 32. High enantioselectivities in the hydrogenation of activated ketones on cinchona alkaloid modified Platinum-alumina catalysts, J. Mo/. Catal. A. Chem. 202, 163-170. 

As one further example, in the middle 60 s we developed a dehydrogenation activity (DA) test which measured the rate constants of reforming or hydrocracking catalysts for converting cyclohexane to benzene. We found that fresh platinum/alumina catalysts had a DA of about 1000. After only one day in a unit the DA dropped to about 1. And, of course, such catalysts are used for months without regeneration in a reforming unit. Now you know why I refer to industrial catalysis as the science of dirty surfaces. 

From the two different preparation processes the coprecipitation route does not give the best catalytic activity, due to the fact that these samples have alkaline impurities. The sol-gel method is preferable and leads to very adive catalysts. For the CO + O2 reaction, the LaCoOa catalyst prepared via the sol-gel route showed a temperature of half conversion 100 K lower than for a 2% platinum alumina catalyst (8). 

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. 

Roumanie et al. tested catalysts in a chip-like silicon microreactor for methylcy-dohexane dehydrogenation [280]. A platinum/alumina catalyst achieved 88.5% conversion, while a platinum film sputtered onto black silicon showed only 2% conversion. Low activity is frequently observed for non-dispersed noble metal surfaces. 

The ceria/titania based catalyst was also more stable. The catalyst showed 60-h durability under the conditions provided in Figure 4.24. However, the weight hourly space velocity of21.2 L (hgcat) was relatively low. About ten-times higher yields can be achieved with platinum/ceria systems. The activity of the platinum/ceria catalyst is approximately 15-times higher compared with a platinum/alumina catalyst for the... 

Son et al. reported increased activity of their platinum/alumina catalysts by exposure to liquid water and subsequent evaporation (326). This effect could be helpful in practical applications, because catalysts might well be exposed to condensing steam during start-up procedures in fuel processors. However, when the catalyst was operated below 100 °C, deactivation occurred, which was attributed to accumulation of the condensed water on the catalyst. Activity could be re-gained when the temperature was increased above 100 °C, clearly this was because the water was evaporated. 

Ayastuy et al. reported higher activity at a much lower temperature for their platinum/ceria catalysts [330]. While the platinum/alumina catalysts showed fiill conversion at 175 °C, the ceria supported samples showed full conversion by 80 °C. However, it is questionable whether platinum/ceria is a suitable catalyst for preferential oxidation, because it has excellent activity for the water-gas shift. This will impair the performance of the catalyst under partial load in a fuel processor environment due to the reverse water-gas shift taking place, as discussed in Sections 3.10.2 and 5.2.2. 

Figure 4.29 Effect of 21 ppm sulfur dioxide on the activity of a platinum/alumina catalyst for the preferential oxidation of carbon monoxide the catalyst contained 3.1 wt.% platinum on alumina reaction temperature 175°C [338].

Nevertheless, the picture was not uniform. Loc et al. (1996) reported a retarding effect of steam over platinum-alumina catalysts. The different effects of steam on Pt activity could be related to the properties of steam adsorption on different supports. These results also indicate that the effect of steam on the reaction rate is related to the catalysts used for the investigations. 

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

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... 

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. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

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