Hydrogenation of aromatic hydrocarbons

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

Dissociation of phosphite from the dihydride (57), prior to reaction with the arene, is thought to give the nondetected intermediate r)1-C3H5CoH2L—tj4-C6H6 (58), and then the sequence shown accounts for the stereoselectivity. NMR evidence was presented for 57, which also decomposes mainly according to reaction (78) in the absence of arene, 

Of a number of Tj6-arene complexes subsequently tested for reactivity toward H2, i76-C6H5CH3—Ru6C(CO)14 was converted stoichiometrically at 150°C to methylcyclohexane, and the t76-C6(CH3)6Ru-i74-C6(CH3)6 complex (cf. 58) was found to be a long-lived homogeneous catalyst for arene hydrogenation (444) in contrast to the cobalt system, extensive H- D exchange occurred in the aromatic ring and in substituent methyls of xylene substrates. 

The i76-C6H6M(C6F5)2 complexes (M = Ni, Co), obtained via metal atom syntheses are reported to be short-lived catalysts for arene hydrogenation (445). 

The rhodium cyclopentadienyl complex [T75-C5(CH3)5RhCl2]2, in the presence of base at 50 atm H2, also effects stereoselective catalytic hydrogenation of benzenes. Substrates with unprotected —OH or —C02H groups were not effectively hydrogenated, but aryl ethers, esters, and ketones and N,iV -dimethylaniline were all reduced, some- 

Ruthenium has a marked tendency to interaction with phenyl groups in PPha by c rr/ic -metallation. Thus it is not surprising that [RuH2(PPh3)3] is an effective catalyst for the hydrogenation of naphthalene to tetrahydronaphthalene/  

---

Rate Constants k (mmole min g ) of Isolated Reactions, and Relative Reactivities S from Competitive Reactions Obtained in the Hydrogenation of Aromatic Hydrocarbons... 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

Table 16.1 Homogeneous catalysts, tethered single-site catalysts, and biphase catalysts for the hydrogenation of aromatic hydrocarbons.

Table 16.2 Hydrogenation of aromatic hydrocarbons with the IFP process.a)...

The dimer of chloro(l,5-hexadiene)rhodium is an excellent catalyst for the room temperature hydrogenation of aromatic hydrocarbons at atmospheric pressure. The reaction is selective for the arene ring in the presence of ester, amide, ether and ketone functionalities (except acetophenone). The most useful phase transfer agents are tetrabutylammonium hydrogen sulfate and cetyltrimethylammonium bromide. The aqueous phase is a buffer of pH 7.6 (the constituents of the buffer are not critical). In all but one case the reaction is stereospecific giving cis products... 

C. The Detection of Cyclohexene Intermediates The postulate that olefins are released from the surface during the hydrogenation of aromatic hydrocarbons has gained considerable support. Madden and Kemball (89) observed cyclohexene during the early stages of the vapor phase hydrogenation (flow system) of benzene over nickel films at 0° to 50°. The ratio of cyclohexene to cyclohexane diminished with time, and little or none of the alkene was detected if the films were annealed at 50° in a stream of hydrogen. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

In 1948 Maxted and Walker studied the detoxification of catalyst poisons in the hydrogenation of aromatic hydrocarbons and found that the isomeric thienothiophenes 1 and 2 could be converted into the sul-fones of fully hydrogenated thienothiophenes 1 and 2, which do not poison the catalysts. This conversion is performed by brief preliminary hydrogenation and subsequent oxidation by hydrogen peroxide or per-molybdic acid. However, no data on the isolation or foe properties of these disulfones are available. It has been reported that direct oxidation of thienothiophenes 1 and 2 does not produce sulfones. 

The table shows that the selectivity for 4,6-DMDBT reactions was not severely changed by naphthalene inhibition all rates were lowered by about the same amount. The activity for hydrogenation of aromatic hydrocarbons was inhibited more than for the hydrogenation of sulfur-containing compounds. Interestingly, the desulfurization of the hydrogenated derivatives of 4,6-DMDBT was inhibited less than that of the fully aromatic parent. 

Excessive hydrogen consumption due to nonselective hydrogenation of aromatic hydrocarbon components in the fuels being hydrotreated... 

TABLE 11.3 Rates of Hydrogenation of Aromatic Hydrocarbons over Nickel and Cobalt Catalysts 1 ... 

P-1 Ni16 and P-1 Co17 boride catalysts have also proved to be good catalysts for the hydrogenation of aromatic hydrocarbons. Table 11.3 compares the activities of these nickel and cobalt catalysts in the hydrogenation of some aromatic hydrocarbons in hydrocarbon solvent at 80°C and the initial hydrogen pressure of 7.8 MPa.18 It is noted that, as in the cases of Raney Ni and Raney Co, P-1 Co boride is generally more active than P-1 Ni boride, except for o-xylene. 

S. Siegel and N. Garti, The effect of pressure on the catalytic hydrogenation of aromatic hydrocarbons on rhodium, in Catalysis in Organic Syntheses 1977 (G.V Smith, ed.). Academic Press, New York, 1977, p. 9. 

Lin SD, Vannice MA (1993) Hydrogenation of aromatic hydrocarbons over supported platinum catalysts. I. Benzene hydrogenation. J Catal 143 539... 

Many mechanisms have been proposed for the adsorption and hydrogenation of aromatic hydrocarbons over supported Group VIII metal catalysts (2,3). Studies with particular reference to the role of benzene as a 7t-complex on the catalyst surface have been reviewed by Garnett (4). 

Source A. V. Sapre and B. C. Gates, Hydrogenation of Aromatic Hydrocarbons Catalyzed by Sulfided C0O-M0O3 y-Al203. Reactivities and Reaction Networks, Industrial and Engineering Chemistry Process Design and Development 20 68-73 (1981). With permission. 

Rowan [45] investigated the applicability of hydrogenation and dehydrogenation reactions in gas chromatography for analytical purposes. He showed that it is not a complicated problem to carry out selective hydrogenation of aromatic hydrocarbons and olefins as a group reaction. It was shown that a 1.4% platinum on alumina catalyst... 

Eisen and Ivanov [54] converted hydrocarbons of various types in a stream of hydrogen on a 0.5% palladium on silica gel catalyst. For carrying out reaction chromatographic analysis of hydrocarbon mixtures, optimum temperatures for the hydrogenation of aromatic hydrocarbons are 315—325°C. Simultaneously at the specified temperatures hydrogenation of olefins, diolefins, cyclopentenes, etc., occurs. Conversions of the hydrocarbons are presented in Table 4.1. 

---