Benzene, hydrogenation

The benzene hydrogenation was selected as a model reaction for determining the metallic surface sites, because it is a structure-sensitive reaction. The sole product was cyclohexane, as shown below. 

The catalysts are palladium supported on different carbons. Palladium oxide was reduced in metallic Pd°. Table 3.3 shows the dispersions, total surface areas, metallic areas, and particle diameters of the Pd° on two supports [16]. 

The rate per site or turnover frequency was calculated based on the rate of benzene formation at 373 K and the number of surface sites after CO chemisorption, as presented in Table 3.4. Results show that the turnover frequency (TOF) changed with the different carbon supports. 

The turnover frequency (TOF) results confirm that the reaction is structure sensitive (RSS). However, the catalyst with lower dispersion presented higher frequency factor ko. Higher frequency factors indicate high effective collisions of molecules on active sites. Since the turnover frequency is a mean value and has 

The activation energy was calculated for different catalysts, as shown in Fig. 3.13. It was approximately equal 12 1 kcal/mol. However, the frequency factor changed [16]. 

Large quantities of benzene are required throughout the world for a wide range of applications. A high proportion is hydrogenated to provide cyclohexane, an intermediate in the production of nylon fibers and resins. 

The reaction involved is very simple and has been well known since Sabatier and Sendeiens reported on their experiments in 1901. They passed hydrogen saturated with benzene vapor at ambient temperature over a nickel catalyst at 180-200°C. At this temperature an almost complete conversion of benzene to cyclohexane was achieved. They made two important observations  

Although Sabatier and Senderens claimed that other metals did not hydrogenate benzene, later work by Zelinsky showed that benzene was easily hydrogenated by platinum metals. 

During the late 1800s it was reahzed that cyclohexane was identical with Caucasian petroleum. Subsequently, natural gas liquids became an important source of up to 85% pure cyclohexane. Even as late as 1%8 some 20% of the cyclohexane used in the United States was obtained in this way, although the cyclohexane content was increased to about 98% by the isomerization of methylcyclopentane during fractional distillation. 

In an alternative vapor phase process, a platinum catalyst is used in a tubular reactor at 30 atm and about 400 C to give almost 100% selectivity. The catalyst used in this process is substantially more expensive than nickel oxide. 

The (3/a ratio was rather constant in the benzene conversion range between 0.1 and 8%. Its value increased sharply at higher conversions. This increase was attributed to the consumption of inactive cyclohexene, representing the primary product of diene hydrogenation. The results could 

Cyclohexadiene was converted totally on a Ni catalyst when the benzene conversion was - 5-6%. From this point onwards, the concentration of cH dropped dramatically with a simultaneous increase in its specific radioactivity exceeding that of cH, when the Bz conversion reached 10% as shown in Fig. 3. The formation of radioactive cyclohexene from [ C]-benzene supplied further evidence of the existence of stepwise hydrogenation, even if it is not the exclusive route. 

Experiments were performed using palladium Pd supported on carbon and were conducted isothermally, without interference of mass or heat transfer. Table 13.3 presents the results of Pd dispersion and the specific activities in terms of TOR 

The activity or TOF values were calculated from the rate consumption of benzene at 373 K and from CO chemisorption measurements. TOF values increase with decreasing dispersions. 

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High purity cyclohexane is manufactured by hydrogenating benzene at 400-500°F and 500 psig. Some cyclohexane was earlier produced by fractionating naphtha but its purity of 85-90% was too low to compete with 99-t- percent purity from benzene hydrogenation. A number of cyclohexane processes based on benzene hydrogenation are available. 

Similar to the alkylation and the chlorination of benzene, the nitration reaction is an electrophilic substitution of a benzene hydrogen (a proton) with a nitronium ion (NO ). The liquid-phase reaction occurs in presence of both concentrated nitric and sulfuric acids at approximately 50°C. Concentrated sulfuric acid has two functions it reacts with nitric acid to form the nitronium ion, and it absorbs the water formed during the reaction, which shifts the equilibrium to the formation of nitrobenzene ... 

It is obvious that Figure 3-6 serves as a calibration curve with the help of which the chlorine content of unknown, similar materials could be readily determined. It will now be shown that the solid line agrees closely with kf values calculated from the mass absorption coefficients of benzene, hydrogen, and chlorine small variations in effective wavelength will be disregarded. 

The nickel supported catalysts formed in this way have some specific features (144)- The catalysts containing about 3% of Ni are paramagnetic. When varying the nickel content from 0.1 to 20%, all the nickel the reduced catalyst (the exposed surface area of nickel was about 600 m2/g Ni) is oxidized by oxygen. The activity in benzene hydrogenation is very high and increases in proportional to the nickel content in the catalyst. 

C.A. Cavalca, and G.L. Haller, Solid Electrolytes as Active Catalyst Supports Electrochemical Modification of Benzene Hydrogenation Activity on Pt/p"(Na)Al203, /. Catal. Ill, 389-395(1998). 

Figure 9.19. Effect of catalyst potential, Na coverage and benzene partial pressure on the rate of benzene hydrogenation on Pt/p"-Al203 27 28 T=I30°C, pH2=33.35 kPa, flow rate=81 cm3(STP)/min.

Benzene hydrogenation, electrochemically promoted, 288, 452 Bipolar cells... 

Replacement of an aromatic hydrogen by an aliphatic group is called alkylation and the attached group is called an alkyl group. Industrially, benzene is alkylated by reaction with an olehnic hydrocarbon such as ethylene to make ethylbenzene, or with propylene to produce isopropylbenzene. The replaced benzene hydrogen becomes part of the attached group. 

Substitution of four benzene hydrogens by the same group (tetrasubsti-tution) also results in three positional isomers 1,2,3,4-, 1,2,3,5-, and 1,2,4,5. 

Benzene hydrogenation was used to probe metal site activity. A 12/1 H2/benzene feed was passed over the catalysts at 700 kPa with a weight hourly space velocity of 25. The temperature was set to 100°C and the conversion of benzene to cyclohexane was measured after 2 hours at temperature. The temperature was then increased at 10°C increments and after two hours, the conversion remeasured. 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

Fig. 9. Benzene hydrogenation on Nd203-modified Si02-Al203 (100°C, 12/1 H2/C6H5, 700 kPa 25 WHSV).

Similarly to Iridium and rhodium nanoparticle studies, Dupont describes benzene hydrogenation in various media by platinum(O) nanoparticles prepared by simple decomposition of Pt2(dba)3 in BMI PFe at 75 °C and under 4 bar H2 [68]. The Pt nanoparticles were isolated by centrifugation and char-... 

The isolated Ru(0) nanoparticles were used as solids (heterogeneous catalyst) or re-dispersed in BMI PP6 (biphasic liquid-liquid system) for benzene hydrogenation studies at 75 °C and under 4 bar H2. As previously described for rhodium or iridium nanoparticles, these nanoparticles (heterogeneous catalysts) are efficient for the complete hydrogenation of benzene (TOP = 125 h ) under solventless conditions. Moreover, steric substituent effects of the arene influenced the reaction time and the decrease in the catalytic TOP 45, 39 and 18h for the toluene, iPr-benzene, tBu-benzene hydrogenation, respectively, finally. The hydrogenation was not total in BMI PPg, a poor TOE of 20 h at 73% of conversion is obtained in the benzene hydrogenation. 

GP 16][R4] An Ru-Zn catalyst was used for benzene hydrogenation, as a Pd-coatedmicro-channelreactorcouldnotbeappliedsuccessfully( Pbenzene=H P ... 

This review covers the personal view of the authors deduced from the literature starting in the middle of the Nineties with special emphasis on the very last years former examples of structure-sensitive reactions up to this date comprise, for example, the Pd-catalyzed hydrogenation of butyne, butadiene, isoprene [11], aromatic nitro compounds [12], and of acetylene to ethylene [13], In contrast, benzene hydrogenation over Pt catalysts is considered to be structure insensitive [14] the same holds true for acetonitrile hydrogenation over Fe/MgO [15], CO hydrogenation over Pd [16], and benzene hydrogenation over Ni [17]. For earlier reviews on this field we refer to Coq [18], Che and Bennett [9], Bond [7], as well as Ponec and Bond [20]. 

Study the effect of varying space time on the fractional conversions Xj and X2 and evaluate the compositions of benzene, hydrogen, diphenyl and triphenyl. 

Benzene, Raney nickel catalyst See Benzene Hydrogen, Raney nickel... 

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