Platinum dehydrogenation reactions cyclohexane

Another synthesis of pyrogaHol is hydrolysis of cyclohexane-l,2,3-trione-l,3-dioxime derived from cyclohexanone and sodium nitrite (16). The dehydrogenation of cyclohexane-1,2,3-triol over platinum-group metal catalysts has been reported (17) (see Platinum-GROUP metals). Other catalysts, such as nickel, rhenium, and silver, have also been claimed for this reaction (18). 

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

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. 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

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. 

The oxidative dehydrogenation of cyclohexane to benzene has been studied more extensively. Transition metal ion-exchanged forms of zeolite Y have been shown (34-39) to be particularly active catalysts for this reaction. Although the platinum metal ions exhibit the highest activity, CuY was found to be the most selective for benzene formation (38, 39). 

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... 

Primary structure sensitivity resulting from the effect of changing particle size on step and kink density appears therefore to be present here at short reaction times. Secondary structure sensitivity (including the effect of carbonaceous poisoning on the reaction rate) appears not to be present here. Thus Somorjai has reported that the dehydrogenation reaction of cyclohexane to cyclohexane is insensitive to both structural featureSt whereas the dehydrogenation of cyclohexene to benzene la very sensitive to the densities of atomic steps and kinks and the order of the carbonaceous overlayer on the platinum crystal surface. 

Naturally, site density calculations are difficult or impossible when the reaction is very complex. Even then an attempted calculation may indicate that the reaction is complex such is the case with the dehydrogenation of cyclohexane over platinum (66) and palladium supported on alumina. Thus, even after 40... 

A good number of monometallic and bimetallic catalysts have been presented in the literature for dehydrogenation of the cycloalkane reaction. Platinum supported on alumina (Pt/Al203) is a common catalyst for this reaction. Bimetallic catalysts containing a small amount of Pt, in which the second metal enhances the activity and selectivity of the catalyst, have been investigated extensively. For instance, Pt-W, Pt-Re, Pt-Rh, and Pt-Ir have been suggested for the dehydrogenation of cyclohexane. 

Dehydrogenation of Cyclohexanes to Aromatics and Hydrogen. Dehydrogenation is a metal-catalyzed reaction, typically because of the platinum on the reforming catalyst. 

Platinum was deposited by impregnation into the framework of y-alumina membrane tubes with an asymmetric configuration, using ammoniac-hexachloroplatinic solutions at different pH values and dipping times. Metallic platinum was obtained after calcination and reduction. The microstructure of the membranes was studied by SEM and BET their gas permeabilities were measured as well. The heat delivered during the formation of PtO on membranes prepared in different conditions were measured in order to compare their activities. Cyclohexane dehydrogenation reaction was carried out on these membranes. Tlie effect of the preparation conditions on the catalytic activities is discussed. 

Three platinum containing membrane tubes with the same size as mentioned above were prepared using impregnating solutions of different pH values (pH = 4, 7, 12 respectively). Cyclohexane dehydrogenation reaction was carried out on these membranes in the catalytic membrane reactor. In order to compare their reaction activities, the relative conversion rates of cyclohexane over these membranes at different flow rates are shown in Figure 5. The relations obtained are similar to that of the DTA measurements. The best result is obtained when the platinum containing catalytic membrane is prepared by impregnating in an ammoniac hexachloroplatinic solution of pH 12. 

Quantitative estimation of cyclohexane in the presence of benzene and aUphatic hydrocarbons may be accompHshed by a nitration-dehydrogenation method described in Reference 61. The mixture is nitrated with mixed acid and under conditions that induce formation of the soluble mononitroaromatic derivative. The original mixture of hydrocarbons then is dehydrogenated over a platinum catalyst and is nitrated again. The mononitro compounds of the original benzene and the benzene formed by dehydrogenation of the cyclohexane dissolve in the mixed acid. The aUphatic compound remains unattacked and undissolved. This reaction may be carried out on a micro scale. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

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