Platinum dehydrogenation reactions

Trevor D J, Whetten R L, Cox D M and Kaldor A 1985 Gas-phase platinum cluster reactions with benzene and several hexanes evidence of extensive dehydrogenation and size-dependent chemisorption J. Am. Chem. Soc. 107 518... 

The dehydrogenation reaction is an extremely rapid endothermic reaction which converts alkylcyclohexanes to aromatics almost quantitatively. It is promoted by the catalyst platinum function and is so rapid that it is normally limited by diffusion into the catalyst particle. 

We finish this section with an example of a dehydrogenation reaction on a metal, which is instructive as it does not rely on equilibrium adsorption, unlike most other examples given in this book. The dehydrogenation of methylcydohexane, CfiHiiCHs, to toluene, QH5CH3, on platinum is well described by a reaction mechanism consisting of consecutive steps in the forward direction only ... 

In conclusion, hydrogenolysis processes and coke formation occur on large ensembles of surface platinum atoms [160], while dehydrogenation reactions would proceed on single (isolated) Pt atoms [169]. The presence of tin atoms regularly distributed on the metal surface diminishes the size of the ensemble [130,170-173], the same is observed for copper atoms on nickel surfaces [174] or tin atoms on rhodium and nickel surfaces [137,175-177], leading to site isolation and therefore to selectivity. 

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

A stoichiometric dehydrogenation reaction has been utilized for the synthesis of the natural product Rhazinilam in which the key step was a platinum-mediated C-H activation, followed by P-elimination (213,214). This step is shown in Scheme 50. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

In a series of studies, the variation of the turnover number for the dehydrogenation reaction (the number of product molecules/platinum surface atoms/second) with the hydrogen to hydrocarbon ratio at a constant hydrocarbon pressure of 4 x 10"8 Torr was determined. The results are shown in Fig. 20 for the several stepped surfaces studied. The reaction rates increase with increasing hydrogen to hydrocarbon ratio. If no hydrogen is introduced into the reaction chamber, the catalyst behaves very differently. No benzene... 

More important, the carbonaceous deposit on the platinum catalyst surfaces was often ordered, and ordering imparted to it unique properties. The presence of an ordered overlayer eliminated the poisoning of dehydrogenation reactions (C6H10 to C6H6). The dehydrocyclization of -heptane to... 

The platinum-catalyzed reaction of alkanes with chlorine leads to alkyl chlorides and alcohols (Table 6, entry 46) with modest rates and conversions [50], Cydooctane can be easily dehydrogenated (Table 6, entry 47) in the presence of a stabilized vinylalkane by use of the neutral rhenium compound ReH7(PR3)2 [51]. By employing an iridium-based catalyst, the photochemical dehydrogenation of methylcydohexane to methylenecyclohexane is performed at room temperature... 

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [1] reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. 

After the Pt-Sn catalyst was exposed to the reaction mixture for two minutes, the HD formation rate fell to about a third compared to the activity for the fresh catalyst. For the platinum catalyst, the activity fell to l/30th after only 15 seconds dehydrogenation reaction,... 

Whilst the ability of platinum-based catalysts to effect the dehydrogenation of alkanes to the corresponding alkenes is well established [1-4], carbon laydown and consequential deactivation of the catalyst during the dehydrogenation reactions is a well known phenomenon... 

Shuikin (370) passed methyl and dimethyl cyclohexanes over nickel at 330-350°. In addition to the usual demethylation and dehydrogenation reactions, he found evidence of methyl transfer methylcyclohexane gave some p-xylene, while dimethylcyclohexane gave some trimethylbenzene. Platinum at these temperatures did not cause this methyl transfer. Plate and O. A. Golovina (306) reported that appreciable demethylation of 2,2,4-trimethylpentane took place over molybdena-alumina at 150-250°C. and was accompanied by the formation of small amounts of aromatics. 

A detailed study of over 45 catalysts, primarily from Group VIII metal salts and complexes, showed palladium(II) compounds to be the most effective in the dehydrogenation of a variety of aldehydes and ketones. Soluble palladium(II) salts and complexes such as dichloro(triphenylphosphine)palladium(II) and palladium(II) acetylacetonate have been shown to be optimal, with the salts of rhodium, osmium, iridium and platinum having reduced efficacy. Since the dehydrogenation reaction is accompanied by reduction of the palladium(II) catalyst to palladium(O), oxygen and a cooxidant are required to effect reoxidation. Copper(II) salts are favored cooxidants, but quinones, and especially p-benzoquinone, are also effective (Scheme... 

Metals that are known to be highly catalytic for hydrogenation and dehydrogenation reactions are similar to platinum with respect to the h.e.r. These include palladium, iridium, rhenium, nickel, and cobalt. In the case of palladium the situation is further complicated by the tendency of atomic hydrogen, which is formed on the surface as an intermediate, to diffuse into the metal and dissolve in it. This effect... 

The conversion of cyclohexanes to aromatics is a classical dehydrogenation reaction which will readily take place on many transition metals and metal oxides. On chromia-alumina Herington and Eideal (S) have demonstrated the occurrence of cyclo-olefin intermediate products. Weisz and Swegler 25) have demonstrated the effect on benzene yield of allowing early diffusional escape of cyclo-olefin from the porous catalyst particle. Prater et al. 26) have developed evidence that cyclohexene occurs as a quasi-intermediate in aromatization catalysis over platinum catalyst also, although at a smaller concentration, because of a larger ratio of effective rate constants fe/Zci in the scheme... 

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