Conversion of cyclohexane to benzene

The large difference in gallery heights for Cr3 53 and Cr1 88-montmorillonites leads to dramatic differences in catalytic reactivity (17). Figure 4 illustrates the conversion of cyclohexane to benzene over both materials at 550°C as a function of reaction time. Both catalysts were pre-reduced under H2 in a continuous flow reactor at 500°C, followed by reaction with cyclohexane (weight hourly space velocity = 3, contact time = 6 sec, He carrier gas.) The clay remains intact at these reaction temperatures as evidenced from the thermal data (17). 

Figure 4. Conversion of cyclohexane to benzene over chromia pillared montmorillonites at 550°C (A)...

This discrepancy is partially attributed to the very short contact time through the membrane layer for any significant additional conversion due to the catalytic membrane layer. It may also be the result of the actual catalyst distribution in the membrane and support layers. It has been pointed out that the reaction conversion of cyclohexane to benzene in a porous catalytic membrane reactor is extiemely sensitive to the distribution of the Pd catalyst through the membrane [Cannon and Hacskaylo, 1992]. The last point is supported by the observation that, when the membrane is not catalytic and the remaining conditions arc the same, experimental data and model prediction agree [Becker etal., 1993]. 

Hydrogenation catalysts,such as platinum, palladium, " and nickel. In this case, the reaction is the reverse of double-bond hydrogenation (15-11 and ISIS), and presumably the mechanism is also the reverse, although not much is known.Cyclohexene has been detected as an intermediate in the conversion of cyclohexane to benzene, using Pt. The substrate is heated with the catalyst at 300-350°C. The reactions can often be carried out under milder... 

As shown in Figure 3.5, addition of copper to ruthenium decreases hydrogenolysis activity markedly but has a much smaller effect on dehydrogenation activity. The presence of copper thus improves the selectivity of conversion of cyclohexane to benzene. The data were obtained at 316°C with cyclohexane and hydrogen partial pressures of 0.17 and 0.83 atm, respectively. The ruthenium-copper aggregates were heated to 400°C in hydrogen in their preparation. 

Figure 2, Methylcyclohexane, cyclohexane, and benzene content in the product gas from reforming isooctane+20%methylcyclohexane, showing conversion of cyclohexanes to benzene.

It follows, therefore, that contrary to the assertion of Sabatier that metallic cobalt is inferior to nickel in regard to hydrogenating properties, highly disperse cobalt deposited on activated carbon behaves similarly to nickel, not being less effective than nickel as a hydrogenating catalyst. But it should be mentioned that the dehydrogenating capacity of this catalyst was rather low the degree of conversion of cyclohexane to benzene at 300° and a space velocity of 0.2 was for the 4, 2, and 1 % samples within the limits of 25-27.6%. 

The dehydrogenation of cyclohexane to benzene [12] and of ethylbenzene to styrene [13, 14] have been studied in MRs using glass or alumina membranes. Even if a conversion increase beyond the equilibrium value has been observed in all the cases, compared with Pd-based membranes, which are much more selective with respect to hydrogen, porous membranes are less efficient in improving the conversion. For example, the conversion of cyclohexane to benzene at 200°C and 1 atm (equilibrium conversion 19%) is 45% in a Vycor glass MR whereas it becomes 99.7% in a Pd-Ag MR. 

Problem 6.2. Calculate AH° from the bond energies in Table 1.1 (Chapter 1) for the conversion of cyclohexane to benzene and hydrogen as shown in Equation 6.4 at 25°C and 1 atm. From this, with AS as -100 JK moF and Equation 4.3 estimate AG° under the same conditions. In order for the reaction to proceed as written (without a catalyst) to what temperature must it be heated ... 

The VEEL spectra of the species formed from cyclohexane on Pt(lll) show that at least two intermediate species occur along the decomposition pathway to benzene. These spectra are discussed in Sections VI.A and VI.C, in the context of spectra of species formed from adsorbed cyclohexene (239) and cyclo-l,3-hexadiene (240) on the same surface. On Pt(100) hex, in contrast to Pt(lll), most of the cyclohexane molecules desorb before conversion to benzene, but the latter was formed after adsorption at 300 K. An intermediate in the conversion of cyclohexane into benzene on Pt(100) (1 X 1), stable between ca. 200 and 300 K, was recognized spectroscopically, but not structurally identified, by RAIRS (230) and by VEELS (224). It seems that there is a smooth transition from the spectrum of adsorbed cyclohexane on Pd(100) to that of benzene at temperatures exceeding 250 K without the detection of intermediate spectra (220). 

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. 

The conversion of cyclohexane to methylcyclopentane is nearly complete at 408°C. Under identical conditions benzene is also converted to methylcyclopentane. This shows that the hydrogenation of benzene to cyclohexane, which can be considered the first step in this reaction, is very rapid. 

The conversion of cyclohexane to a mixture of (i) cyclohexene and (ii) benzene plus hydrogen occurred on gold film between 469 and 612 K, with activation energies of respectively,32 74 and 105 kJ mol-1. Its exchange with deuterium took place stepwise between 423 and 513 K with an activation energy of 27 kJ mol-1. 

Figure 9.2 Calculated total conversion profile of cyclohexane to benzene in a porous shell-and-tube Vycor glass membrane reactor with membrane thickness as a parameter [ltohetal.,1985]...

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... 

These results have profound effects for the selective catalytic dehydrogenation of cyclohexane to benzene, a prototypical hydrocarbon conversion reaction. On Pt(lll), the intermediates, cyclohexene and a species, have been identified and the rate constants for some of the sequential reaction steps measured [56]. Adsorption and reaction studies of cyclohexane [39], cyclohexene [44], 1,3-cyclo-hexadiene [48], and benzene [39] on the two Sn/Pt(lll) alloys provide a rational basis for understanding the role of Sn in promoting higher selectivity for this reaction. One example of structure sensitivity is shown in Fig. 2.7, in which a monolayer of cyclohexyl (C H ) was prepared by electron-induced dissociation (EID) of physi-orbed cyclohexane to overcome the completely reversible adsorption of cyclohexane... 

Facile dehydrogenation is consistent with kinetic models derived from catalytic conversion studies of cyclohexane to benzene. These models predict an ensemble size for the active site of only one atom. On the other hand, surface science studies propose a model where several metal atoms, on the order of seven, are required, and suggest that specific orientation with respect to subsurface metal atoms is needed. Theoretical studies suggest that the key is to bring cyclohexane sufficiently close to the metal such that strong orbital overlap will occur. Small clusters may be even more effective than surfaces. Further experiments are needed to identify the chemical state of the products of the cluster reactions in order to connect the results with the surface science and catalysis results. 

Figure 4.5 Selectivity of conversion of cyclohexane over silica-supported bimetallic clusters of ruthenium-copper and osmium-copper at 316°C, as represented by the ratio D/H (1,12), (D is rate of dehydrogenation of cyclohexane to benzene, and H is rate of hydrogenolysis to alkanes.) (Reprinted with permission from Academic Press, Inc.)...

A large number of hydrogenation and dehydrogenation reactions were tested in the early studies of dense-metal membrane reactors (see listing in Shu et al. [34], Hsieh [35], and Gryaznov and Orekhova [36]). Many works tested the dehydrogenation of cyclohexane to benzene as a model reaction since it can be carried out at low temperature with no side reactions and no deactivation a conversion of 99.5% was achieved with a palladium membrane, compared with 18.7% at equilibrium, at 200°C [31]. 

Membrane MSR for the dehydrogenation of cyclohexane to benzene were designed [67]. This is an endothermic reaction whose equilibrium conversion is 18.9% at 200 °C. The conversion can increase beyond equilibrium up to 99% if the hydrogen is removed from the system. Therefore, a Pd-membrane with microchannels has been used to continuously remove hydrogen out of the reaction zone in order to enhance the conversion. The reactors were made of silicon using photo-etching technique, and Pt was used as a catalyst, which was sputtered onto the reaction chamber [67]. Out of two reactors, one example is shown in Figure 6.14. 

It is smaller than for the direct dehydrogenation of cyclohexane to benzene, but still is very high. The reaction is endothermic, and therefore, an increase in temperature leads to an increase in the equilibrium conversion. Hydrogen has a similar effect to that in the dehydrogenation of cyclohexanes, the higher the hydrogen concentration, the lower the equilibrium composition of aromatics. 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

A route to phenol has been developed starting from cyclohexane, which is first oxidised to a mixture of cyclohexanol and cyclohexanone. In one process the oxidation is carried out in the liquid phase using cobalt naphthenate as catalyst. The cyclohexanone present may be converted to cyclohexanol, in this case the desired intermediate, by catalytic hydrogenation. The cyclohexanol is converted to phenol by a catalytic process using selenium or with palladium on charcoal. The hydrogen produced in this process may be used in the conversion of cyclohexanone to cyclohexanol. It also may be used in the conversion of benzene to cyclohexane in processes where benzene is used as the precursor of the cyclohexane. 

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