Metal-Catalysed Reactions of Hydrocarbons / 10-Hydrogenation of the Aromatic Ring

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coverage is interpreted by the Langmuir equation, values for the entropy and enthalpy of adsorption can be derived.4,22,0,34,35 A few choose the molecular route,35,50,51 inaptly termed ‘Rideal-Eley’, and in one case it is specifically claimed that three pairwise additions are involved. Martin and his associates35 believed that an ensemble of about four uncovered nickel atoms were needed for adsorption of benzene, this being followed by a molecular hydrogen collision: the rate dependence on hydrogen pressure thus became (1 − θH)4 PH, and the fractional positive orders (Table 9.1) were seen to result from a competition between these two terms.

There is also disagreement concerning the number of types of site involved and whether adsorption of the reactants is competitive50 or non-competitive30,45,48 (or both50). Where two different metallic sites are invoked, this is because steric constraints to the chemisorption of the aromatic molecule may leave gaps in the adsorbed layer through which hydrogen can enter, to occupy (as atoms) highly coordinated (e.g. trigonal) sites. A recurring theme in discussions of mechanism is the likely multiplicity of states of adsorbed hydrogen, of which only the weakest is reactive. Where the aromatic molecule is supposed to reside on the support, the hydrogen molecule cannot of course compete. The presence of benzene did not inhibit para-hydrogen conversion,52 as happens with alkenes and alkynes.

The two most thorough interpretations of the kinetics are those of Keane and Patterson4,5,53 for Ni/SiO2 and of Vannice and associates for various platinum29,46,54 and palladium30,44,47,48 catalysts. In the first case, the reactants were taken to adsorb reversibly on separate sites, the addition of the first hydrogen atom being rate-determining; but regarding this step as a ‘pre-equilibrium’ seems a doubtful benefit. The experimental reaction orders, both of which increased with temperature (Figure 10.3 and Table 10.1), ‘reveal the temperature-induced changes in the reactive species’. In the second case, provision was made for inhibition by strongly-held dissociated species such as phenyl and tolyl, the composition of which depended upon reaction conditions; with the xylenes, however, the played no role. Reaction orders varied with temperature much as with Ni/SiO2. The extent of the contribution from the spillover catalysts was not explicitly stated, but it presumably depended on the degree to which metal sites were rendered unavailable to strongly held entities. The large variations in activity as the support was changed originated entirely in the pre-exponential term, so that the energy profile of the transition state did not depend on the contribution from spillover catalysis, an observation not easily explained.

It seems somewhat odd that two sets of results similar in respect of kinetic parameters, including their temperature-dependence, should be described by mechanisms that differ so considerably. It is of course possible for the mechanistic framework demanded by every catalyst to be unique, however improbable this appears. This seemingly straightforward class of reactions is in fact very complex, and none of the mechanistic proposals embraces all the potentially available information. If each group produces a scheme that satisfies its results within the

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Figure 10.3. Orders of reaction for benzene hydrogenation as a function of temperature. Pd on various supports (fixed pressures, PH = 600 Torr; PB = 50 Torr).30 Ni/SiO2 (fixed pressures, PH = 714 Torr; PB = 30 Torr).4

limitation of the range of variables covered, it is not surprising that there is a diversity of opinions.

It is interesting to compare the mechanisms proposed for the hydrogenations of ethyne and benzene. With the former, there is no suggestion of a role for spillover catalysis; with the latter there is no role for ‘carbonaceous deposits’ in creating active centres or in acting as vehicles for hydrogen atom transfer.

10.2.4. Temperature-Inversion of Rates

When the temperature used for the hydrogenation of benzene or its alkylsubstituted derivatives is raised above a value that is usually between 423 and 437 K, the rate suddenly stops rising and begins to fall. The inversion temperature Tmax is usually sharply defined and can be estimated accurately, although it is less distinct at low hydrogen pressures than high, perhaps because the effect is obscured by deactivation. The phenomenon has been seen with most of the metals of Groups 8 to 10 (Co, Ni, Ru, Rh, Pd, Pt), as well as with technetium,16 rhenium,16 copper,62 and nickel-copper bimetallics59(see also ‘Further Reading’ list at the end of the chapter). Values of Tmax are rarely below but sometimes above the 423–473 K range, especially with palladium. They decrease with increasing alkyl substitution by about 10 K per methyl group.4 They sometimes depend on the hydrogen pressure used,4,5,60 but not with toluene on Ni/SiO2; with Rh/TiO2 they were higher (485 K) after high temperature reduction than after low temperature reduction (420 K).37,38

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Figure 10.4. Compensation plot for the hydrogenation of aromatics on Ni/SiO2 :4 Arrhenius parameters derived from TOFs, conditions as in Figure 10.3.

Some workers have chosen to ignore this effect, but its analysis can be very informative; in what follows, we examine the methodology of Keane and Patterson.4,5,14,53 They have made a detailed study with a Ni/SiO2 catalyst of the temperaturedependence of the reaction order (Figure 10.3 and Table 10.1), and have determined activation energies using benzene, toluene and the three xylene isomers: the change in rate with temperature above Tmax also conforms to the Arrhenius equation, and affords negative apparent activation energies. All the Arrhenius parameters obtained using TOFs both above and below Tmax lay about a single compensation line (Figure 10.4), as did those derived from rate constant, for which activation energies were, as expected, higher (e.g. for benzene, 49 compared to 29 kJ mol−1). However the temperature-dependence of the reaction orders immediately implies a dependence of activation energy on reactant pressures, and this was indeed established for hydrogen pressure variation in the reaction of toluene; values of activation energy derived from rate constants k were essentially invariable. Care is taken not to apply the labels ‘apparent’ and ‘true’ to these activation energies, as classical theory would suggest. The authors carefully point out that what we might call E←k or Ek is not a true activation energy, as it still contains a dependence upon heats of adsorption, values of which were obtained from adsorption coefficients extracted from the temperature-dependence of the reaction orders through a Langmuir-type relation. Values, independent of pressure, ranged from 74 kJ mol−1 for benzene to 120 kJ mol−1 for o-xylene, but for hydrogen they were the same for each aromatic molecule, but coverage-dependent from 31 kJ mol−1 (θ = 0.97) to 77 kJ mol−1 (θ = 0.02). Using a form of the Temkin equation (eqn.5.29) in which

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the heat terms were not moderated by the orders of reaction, values of Et between 155 and 270 kJ mol−1 were obtained; this may reflect the obstinacy of the aromatic ring to attack.

The analysis is still not entirely satisfactory for several reasons. (i) Values of − Ha (H2) seem somewhat large for ‘weakly’ adsorbed hydrogen,50 and others have obtained substantially different values from their results. (ii) The cause of the inversion is not well established. It is probably not due to deactivation by carbonaceous residues,30 as the effect is reversible.34,53,58 Although there are inflexions in the order versus temperature plots at about Tmax (Figure 10.3),4 the Van’t Hoff isochore plots for hydrogen show no breaks at this point,4,5 and it is illogical to ascribe the negative activation energy to the intrusion of the heat of desorption of the aromatic molecule, as this should operate at all temperatures. It might be thought that the decrease in rate of the forward reaction was a consequence of the growing importance of the reverse reaction, due to the effect of temperature on the position of equilibrium, but at least for benzene and toluene it becomes noticeable only at temperatures above Tmax. A definitive explanation is still awaited. (iii) Comparison with the reactions of ethene and of ethyne is instructive, if a little worrying. For these reactions there are no comparably detailed measurements of the effect of temperature on orders of reaction, although negative orders in the hydrocarbons ought to become less negative as temperature is raised if they are caused by overly strong adsorption, and on palladium the hydrogen order for ethyne hydrogenation becomes more positive at higher temperatures.61,63 We have tended to regard activation energies for these reactions of respectively about 45 and 65 kJ mol−1 as being true in the sense that concentrations of adsorbed reactive intermediates do not change significantly within the range of measurement. Although rate maxima have been detected in the reactions of both ethene1 and ethyne,63 they have not been much studied and their cause is obscure. What is certain, however, is that the heats of adsorption of the reactants in their ‘reactive’ states must be considerable, and the application of the Temkin equation (5.29) without modification would lead to much larger values. We are thus faced with the paradox that estimation of true activation energies only requires correction of the apparent values by addition of the heats of adsorption when adsorbed concentrations change significantly, but not otherwise.

10.2.5. Hydrogenation of Benzene Over Bimetallic Catalysts

Studies of benzene hydrogenation over bimetallic catalysts are of limited value in illuminating reaction mechanism except when conducted with deuterium, when comparison of exchange with addition is informative (Section 10.2.6) otherwise what is found is more relevant to the understanding of how bimetallic systems behave, and how their composition determines their activity.

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Figure 10.5. Hydrogenation of benzene on nickel-copper powders: rate dependence on composition., 435K;65 ,435K;67 , 463K.66

The nickel-copper system has been several times investigated, using powders,64−67 foils,68 films69 and silica-supported catalysts,64 so it is of some interest to compare the results, especially where Arrhenius parameters are available. We may recall that the equilibrium surface concentration remains fixed at about 23% nickel over a wide range of total composition at moderate temperatures due to the occurrence of a miscibility gap (Section 1.3),69 although catalysts reduced at low temperatures may not have equilibrated. The form of dependence of activity on composition may therefore be expected to vary with method of preparation. Thus sintered films show constant rates and activation energies in this region,69 while other forms show distinct maxima at about 30–40% copper65−67 (Figure 10.5): this may be due in part to a lesser tendency to deactivation of nickel-rich catalysts by ‘carbon’ deposition.59 Activation energies are higher for the bimetallics, and approximately constant in the mid-composition range (Figure 10.6: the apparent compensation shown in reference 65 may be simply due to experimental scatter); they tend to decrease at high copper concentrations.

Somewhat different behaviour has been found when Group 11 metals are added to platinum70 and palladium.58,65,71 Activities fall continuously, and activation energy initially,65 but the former is determined subsequently by the decrease in the pre-exponential factor (Figure 10.7). Activities were also increased by adding molybdenum,41 rhenium18,72 or iridium18 to platinum. There are conflicting

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Figure 10.6. Compensation plots for the hydrogenation of benzene on nickel-copper powders: ref. 67; ref. 65; filled points, Ni only. Note: in the upper plot, the points within the box almost certainly show compensation because of experimental scatter.

reports on the effect of adding copper to ruthenium.73,74 The more detailed study73 showed (unusually) how activities change with time, the bimetallics being stabler than ruthenium alone and thus after 24 h being much the more active. Orders for both reactants became more positive with increasing temperature, as was found with nickel4 and platinum,34 but maximum rates occurred at the remarkably low temperature of 300 K, so that most results were obtained above this value. Other reports concern the nickel-platinum,75 cobalt-platinum,76 palladium-rhodium,77 osmium-iridium18 and nickel-cobalt and -iron systems.1

Figure 10.7. Hydrogenation of benzene on palladium-copper powders: activation energy and preexponential factor (ln Asp ) as functions of composition.65

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10.2.6. Exchange of Aromatic Hydrocarbons with Deuterium

Substitution of the hydrogen atoms on the aromatic nucleus for deuterium atoms occurs simultaneously with the process of addition, although it is frequently very much faster. It was one of the set of reactions that was used in the 1930s to explore the applications of the newly-discovered deuterium in the laboratories of Polanyi78 and the Farkas brothers.52 Much thought has been given to its mechanism, and this has led to the belief that it is distinct from that of the addition process. Evidence for this rests on the following observations. (1) The dependence of TOFs on particle size is quite different for each process. (2) Kinetic parameters are also not the same. (3) Exchange has been observed on surfaces that have no activity for addition (Ag film;17 PdAu film containing less than 40% Pd79).

The exchange reaction of benzene has been studied in much less detail than its hydrogenation. All the metals of Groups 8, 9 and 10 have been examined,80 but unfortunately only a very short summary is available; more detailed information can only be found for palladium,17,79 platinum,17,52,78,81,82 nickel,69,78,83 and iridium.83 Rates of exchane on several metal films of Transition Metals ran parallel to those for hydrogenation, and correlated with electrical conductivities.8 Exchange is not a simple stepwise process, as more extensively exchanged molecules (up to benzene- d6) appear as initial products: the mean number of deuterium atoms in exchanged benzene M depended on the metal and on temperature (e.g. on Pd film, 1.8 at 273 K, and 2.7 at 311 K17). On nickel, multiple exchange was thought to be absent because most of the deuterated cyclohexane had only six deuterium atoms.83 On iridium the products of exchange were mainly benzene-d1, with a little benzene-d6. Assuming a dissociative exchange mechanism involving repeated interconversion of phenyl and phenylene species (Scheme 10.1), observed distributions could not be reproduced by a single value of the parameter describing the chance of phenyl → phenylene. The extent of multiple exchange is reported80

Scheme 10.1. Dissociative mechanism for the exchange of benzene.

Note: reiteration of these steps can account for the formation of molecules containing up to six deuterium atoms as initial products.

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to decrease in the following sequence (probably obtained with silica-supported metals):

Ru > Rh, Pd, Os > Ni, Ir > Pt, Fe, Co

but the chemical logic behind it is hard to discern. The deuterium content of the cyclohexane increased as the reaction proceeded, due to the use of progressively more exchanged benzenes. Cyclohexane-d12 constitutes the major product over palladium and palladium-gold film,79 where the rate of exchange must exceed that of addition. Cyclohexanes do not exchange further after being formed.

Measurements of kinetic parameters for exchange are sparse. Activation energies on platinum and palladium films were much higher than for addition,79 and on the latter the order in deuterium was negative (−0.5) for exchange but positive (0.8) for addition. Orders of zero for both reactants have been recorded for a number of metal films.8

The ways in which the rates of the two processes depend on particle size has been followed with Pt/Al2O344 (Figure 10.1) and Ir/Al2O3,84 and with Ni/SiO2 and iridium probably supported on silica.83 In each case addition showed a small dependence on size, while the exchange rate increased markedly as the mean size increased. However, the form of the distribution of the initially formed exchanged benzenes and cyclohexanes was not size-dependent.83 Strongly adsorbed sulfurcontaining molecules may be expected to lower the average size of ensembles of free atoms: molecules such as thiophene and sulfur dioxide affected both reactions equally, but elemental sulfur selectively deactivated exchange.43

The two reactions have been examined on bimetallic films. With the nickelcopper system, rates, activation energies and M values were constant in the region of constant surface composition (23% Ni), but activation energies were higher (105 kJ mol−1) than for pure nickel (50 kJ mol−1).69 With palladium-gold films, activity for addition fell to zero when the gold content exceeded 60%, but the activation energy for exchange stayed in the range 68–90 kJ mol−1 up to 82% gold: values for addition were much lower (18–28 kJ mol−1).79

The introduction of one or more alkyl groups onto the aromatic nucleus divides the remaining hydrogen atoms into sets depending on their proximity to the substituent, and the rate of exchange in each set may thereby be distinguished, either for steric reasons (e.g. the alkyl substituent(s) may cause the ring to tilt rather than lying perfectly flat) or because of differences in C––H dissociation energies induced by the substituent(s).12 Exchange may also occur in the alkyl group, the hydrogen atoms of which can also fall into sets depending on how close they are to the ring. Study of the exchange reactions of alkyl benzenes may therefore be expected to illuminate reaction mechanisms. These factors are well illustrated by the exchange of n-propyl benzene with deuterium over nickel film.12 Its hydrogen atoms may be divided into four sets (Figure 10.8); the hierarchy of exchange rates

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Figure 10.8. Exchange of n-propylbenzene with deuterium: allocation of hydrogen atoms into differentiated groups.

on unsintered and sintered films is given in Table 10.3. What was most remarkable was that the rate for ring set A relative to that for the alkyl hydrogen atoms was lowered by sintering by a factor of 50; it was concluded that the ring exchange mechanism could not therefore be the same as that for the alkyl group, which is well established to proceed by dissociation of a C––H bond.

Exchange of p-xylene with deuterium has been examined on various supported platinum catalysts (Pt/α-Al2O3, Pt/γ -Al2O3, Pt/SiO2) and on platinum film.82 At 360-373 K, the ratio of the rates of exchange and addition on Pt/γ - Al2O3 was almost constant between 28 and 100% dispersion, higher values of M appearing at dispersions (D) greater than 83%. On Pt/α-Al2O3 however this ratio was much higher (D = 8 or 18%), but the value of M was lower (1.1). Ring exchange and addition were much faster on platinum film (T = 273 K), exchange in the side-chain being faster than in the ring. p-Xylene exchange has also been studied on films of palladium and tungsten.

An extensive study has been performed on the exchange reaction between benzene and benzene-d6 at 273 K, using films of no fewer than 16 metals.8,12 The mean number of alter-atoms entering each molecule ranged from three for titanium to one for palladium, but for many metals it was close to two. This suggested that under these conditions (i.e. low concentration of either hydrogen or deuterium atoms) the benzene easily lost two hydrogen (or deuterium) atoms to form adsorbed phenylene, which then collected two new atoms to form an exchanged benzene. In each Transition Series, rates appeared to decrease linearly with increasing atomic radius.

TABLE 10.3. Exchange of n-Propylbenzene with Deuterium on Nickel Film: Reactivity Sequence of Groups of Hydrogen Atoms as shown in Figure 10.8130

Condition	Reactivity Sequence
Unsintered film, 273 K	A = C >> B > D
Sintered film, 303–323 K	C > D > A > B

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Scheme 10.2. Mechanism for the hydrogenation of benzene by successive addition of hydrogen (or deuterium) atoms.

Note: all steps are shown as reversible, so that the mechanism could also describe the exchange: however the loss of resonance energy on addition of the first atom probably makes its reversal very unlikely. The mechanism also describes the formation of cyclohexene (Section 10.27) and the dehydrogenation of cyclohexane.

It remains now to review briefly the various mechanisms that have been proposed to describe the process of ring exchange. There seems to be general agreement, based on the criteria briefly listed in the opening paragraph, and subsequently elaborated, that exchange occurs in a way which is quite separate from the route that leads to addition, for which the first and probably slowest step is the addition of a hydrogen or deuterium atoms to the ring, with loss of the resonance stabilisation. It is therefore unlikely that exchange occurs by reversal of this step, and likely that addition of further hydrogen or deuterium atoms is fast (Scheme 10.2). The dissociative mechanism for exchange, perhaps proceeding through a flat π -adsorbed intermediate, may account for the negative order in deuterium, as two free ‘sites’ are needed to chemisorb benzene as phenyl, and also the preference that the reaction apparently has for large ensembles of atoms, as it goes most easily on large particles. The consistently higher activation energy for exchange than for addition may also be consistent with this mode of reaction. Detailed argument along these lines has not however been offered. We must conclude that the mechanism of the exchange of aromatic molecules has not yet been firmly established, although as we shall see shortly the nature of the products obtained in the exchange of naphthalene provides some pointers. There are also some notable gaps in our knowledge of aromatic exchange: it does not seem to have been established for example whether rates exhibit a maximum as temperature is raised, or whether spillover to the support plays any role in the case of supported metals.