Dehydrogenation of cyclohexane

Yamamoto et al. [27,28] revealed the advantages of applying a microreactor for the dehydrogenation of cyclohexane by using a Pd membrane as a heterogeneous catalyst These authors demonstrated that the yield of this reaction can be doubled by inserting a stainless steel rod with a proper size into a tubular Pd membrane reactor supported by an a-alumina porous tube to form microchannels. 

This reaction is structure insensitive (RIS), being the rate proportional to the number of surface active sites, and thus, the intrinsic activity (TOF) is independent of the particle size. It is also true for the reverse reaction, namely, the hydrogenation of benzene. The dehydrogenation of cyclohexane forms simple benzene and hydrogen as products, when performed at atmospheric pressure and temperatures varying between 250 and 300 °C [15]. 

Results of cyclohexane dehydrogenation, expressed as turnover frequency (TOF), for the Pt/Al203 and the promoted (Pt + Sn)/Al203 catalysts, are presented in Table 3.1. Note that the dispersion of Pt on alumina is almost 100 %, but with the addition of a second metal or promoter Sn, the dispersion decreases significantly. 

Benzene is the sole product and no deactivation was observed. Both the dispersion and the rate decreased in the presence of a second metal however, the intrinsic activity (TOP) remained constant around 1 (s ), indicating that the rate is proportional to the surface active sites, and thus it is a structure-insensitive reaction. 

The activation energy was constant equal to 24 kcal/mol. The activity (TOP) was almost constant, and therefore the promoters Sn and In do not affect the structural and surface atoms. 

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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). 

Aromatization. The two reactions directly responsible for enriching naphtha with aromatics are the dehydrogenation of naphthenes and the dehydrocyclization of paraffins. The first reaction can he represented hy the dehydrogenation of cyclohexane to benzene. 

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. 

Cyclohexane is a colorless liquid, insoluble in water but soluble in hydrocarbon solvents, alcohol, and acetone. As a cyclic paraffin, it can be easily dehydrogenated to benzene. The dehydrogenation of cyclohexane... 

A technique used to overcome the unfavorable thermodynamics of one reaction is to couple that reaction with another process that is thermodynamically favored. For instance, the dehydrogenation of cyclohexane to form benzene and hydrogen gas is not spontaneous. Show that, if another molecule such as ethene is present to act as a hydrogen acceptor (that is, the ethene reacts with the hydrogen produced to form ethane), then the process can be made spontaneous. 

Giu, T, Eang, j.. Maxwell, J., Gardner, J., Besser, R., Elmore, B., Micromachining of microreactor for dehydrogenation of cyclohexane to benzene, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, p. 488... 

The dehydrogenation of cyclohexane to benzene is an endothermic process (206kjmol ),e.g.,performedat200°C.Theequilibriumconversionamountsto 18.9%[127]. 

FIGURE 1.3 Data for dehydrogenation of cyclohexane to benzene over several Pt single crystals from reference 19. 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

Fig. 6. Activities of copper-nickel alloy catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm., respectively. Cyclohexane dehydrogenation activities were obtained at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively (74).

Dehydrogenation of Cyclohexane to Benzene Porous AljOj membranes Porous Vycor glass membranes Nonporous Pd/Ag membranes Fleming (1987) Shinji et al. (1982), Itoh (1987, Itoh et al. 1988) Sun and Khang (1988) Wood (1968), Itoh (1987), Gryaznov (1970)... 

Dehydrogenation of cyclohexane (1) Pt catalyst deposited within the pores of the membrane (Pt 34 wt.%) (2) Pt/Vycor glass pellets packed on tube side (Pt 34wt.%). 

Dehydrogenation of cyclohexane Pt/Al20j catalytic pellets packed on tube side. 

Dehydrogenation of cyclohexane Pt/Al203 catalytic pellets (0.5 wt.% Pt) packed on lube side. 

Dehydrogenation of cyclohexane. Pd catalyst deposited within the pores of the membrane. 

Shinji, O., M. Misono and Y. Yoneda. 1982. The dehydrogenation of cyclohexane by the use of a porous-glass reactor. Bull. Chem. Soc. Japan 55(9) 2760-2764. 

Wood, B. J. 1968. Dehydrogenation of cyclohexane on a hydrogen-porous membrane. J. Catal. 11 30-34. 

Hishida, Uchijima, and Yoneda (25) have measured the rates of the dehydrogenation of cyclohexane, mono-, di-, and trimethylcyclohexanes to... 

In bimetallic catalysts, Cu-Ru is an important system. Combinations of the Group Ib metal (Cu) and Group VIII metal (Ru)-based catalysts are, for example, used for the dehydrogenation of cyclohexane to aromatic compounds and in ethane hydrogenolysis involving the rupture of C-C bonds and the formation of C-H bonds (Sinfelt 1985). Here we elucidate the structural characteristics of supported model Cu-Ru systems by EM methods, including in situ ETEM. 

The fact that the catalyst can promote dehydrogenation as well as isomerization is shown in Figure 5, where the weight per cent benzene in the products of experiments on dehydrogenation of cyclohexane is shown plotted against the reaction temperature. These results are compared to Von Muffling s (7) calculated equilibrium line. The... 

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