Dehydrogenation of cyclohexene

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 ... 

Fig. 59. Catalytic oxidative dehydrogenation of cyclohexene (O, surface catalysis) and oxidation of acetaldehyde ( , bulk-type II) the catalyst was HjPMonO supported on Si02. Masses catalyst 0.2 g for cyclohexene and 0.1 g for acetaldehyde. (From Ref. 327.)...

Fig. 60. Correlations between catalytic activity and oxidizing ability for (a) oxidation of acetaldehyde (surface reaction) and (b) oxidative dehydrogenation of cyclohexene (bulk-type 11 reaction). (From Ref. 327.) r(aldehyde) and r(hexene) show the rates of catalytic oxidation of acetaldehyde and oxidative dehydrogenation of cyclohexene, respectively. (From Ref. 337.) r( CO) is the rate of reduction of catalysts by CO r(H2) is the rate of reduction of catalysts by H2. M, denotes M,H3-,PMO 2O40. Na2-1, 2, 3, and 4 are Na2HPMoi2O40 of different lots, of which the surface areas are 2.8, 2.2, 1.7, and 1.2 m2 g, respectively.

Bulk type 11 Oxidation of H2, oxidative dehydrogenation of cyclohexene, isobutyric acid. 

Figure 6. Correlations between catalytic activity and oxidizing ability (a) oxidation of acetaldehyde (surface type) and surface oxidizing ability (b) oxidative dehydrogenation of cyclohexene (bulk type) and bulk oxidizing ability [4, 38] (/(aldehyde) and /(hexene) are the rates of catalytic oxidation of acetaldehyde and cyclohexene, respectively).

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. 

VARIOUS CHARACTERISTICS OF SUPPORTED CoPc ON AI2O3, Si02 AND Si02-Al203 AS SELECTIVE CATALYSTS IN THE OXIDATIVE DEHYDROGENATION OF CYCLOHEXENE. 

In the catalytic oxidative dehydrogenation of cyclohexene (OXD), both the flow of molecular oxygen in the system [33] and the presence of peroxides [34] in cyclohexene feed stock were found to be of prime importance as operational conditions. 

The oxidative dehydrogenation of cyclohexene was carried out at temperatures ranging between 280 and 350 C the range being limited by the probable change in CoPc structure (including fi agmentation) at t > 350 °C and by maximum adsorptivity of cyclohexene at t < 280 OC. 

A similar iridium complex, [IrH2(acetone)2PPh3]SbF6 [73], catalyzes the selective dehydrogenation of cyclohexenes to arenes. In this case, the cyclohexenes work as the substrate and also as the hydrogen acceptor. 

Fundamental correlations between redox properties and catalytic activity have successfully been established for the hydrogen form and alkali salts of 12-molybdophosphoric acid [1]. Provided that the contributions of surface- and bulk-type catalysis are properly taken into account, good monotonic relationships are obtained between the catalytic activity for oxidation and the reducibihty (or the oxidizing power) of the catalyst. The rate of oxidation of aldehydes, a surface-type reaction, correlates linearly with the surface reducibility of the catalyst, and the rate of oxidative dehydrogenation of cyclohexene, a bulk-type reaction, with the bulk reducibility [2]. 

Much higher yields are found for the oxidation of alkylarenes to aldehydes, ketones and carbon acids by oxygen in the presence of transition metal compounds (MnBr2, CoBr2, CuBr) as catalysts [57]. For example, a yield of 30 % benzaldehyde and only 10 % benzoic acid was found for the oxidation of toluene. Also the dehydrogenation of cyclohexene and cyclohexane to benzene with Pt02 as catalyst was investigated at 375 °C [34]. 

Here Wi is the rate of doublet dehydrogenation of cyclohexane into cyclohexene W[, that of the reverse reaction Ws, the rate of dehydrogenation of cyclohexene into benzene and W2, the rate of the direct dehydrogenation of cyclohexane into benzene. 

The doublet dehydrogenation of cyclohexene into cyclohexadiene and that of the latter into benzene are not considered separately, being much faster reactions. 

Ethylene is more rapidly oxidized than propylene. Furthermore, the substituted ethylenes do not display the dependency on reactivity of allyl C—H bonds shown over bismuth molybdate (Tables XI and XII). It is clear that the C02-producing reaction is favored by unsaturation, but not by allyl hydrogens. In fact, over Pt ter<-butylethylene, without any allyl hydrogen, was oxidized about as fast as the methylethylenes. Dienes and acids were found to inhibit the oxidation of olefins over the metals. Acetone, like acetic acid from ethylene over Pd, is considered a side reaction product rather than an intermediate. The only selective oxidation observed was an oxidative dehydrogenation of cyclohexene to benzene over Pd at —20 to +30° here no CO2 was produced. 

Terry, P. A. (1999). Catalytic dehydrogenation of cyclohexene using coated silica oxide ceramic membrane. Journal of Porous Material, 6, 267—274. 

Next, in order to learn more about the rates of dehydrogenation of cyclohexenes resulting from Diels-Alder reactions between butadiene and olefins, VCH, HCH and MCH were earlier subjected to thermal reactions at 530- 665 C ( ). The main reactions in these cases were reverse Diels-Alder reactions and dehydrogenations. Dehydrogenations which are related to the productions of cyclohexa-diene and benzene homologues were 1 10 in selectivity as compared to that of the reverse Diels-Alder reaction. An interesting observation related to cyclic compound formation is that, in the case of MCH pyrolysis, cyclohexadiene and cyclopentene are formed at almost the same rates as butadiene and propylene. So that, in this case, about 60% of MCH is employed in the formation of cyclic compounds. 

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