Metal-Catalysed Reactions of Hydrocarbons / 07-Hydrogenation of Alkenes and Related Processes

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ethylidyne was seen. This latter reaction had the higher activation energy of about 70 kJ mol−1. As is often found,134 pre-adsorbed hydrogen favoured the formation of the π -state of ethene; both forms reacted to give an ethyl radical, through which exchange took place, but the π -form reacted faster.

Less, but no less interesting, work has been done with single crystal surfaces of the other metals of Groups 8 to 10. With films of iron and cobalt grown epitaxically on Ni(100), and having fcc structure, ethene reacted with pre-adsorbed hydrogen to give (once again) the π -state, the binding energies being respectively 5 and 8 kJ mol−1 greater than for nickel.135 Platinum deposited on Ni(100) has also been examined.136 On Rh(111) as on Pt(111), hydrogenation of ethylidyne to ethane and exchange in its methyl group were some 106 times slower than hydrogenation of π - ethene.45 A HREELS study of ethene’s reaction with adsorbed hydrogen atoms on Rh(100) at 110 K also gave the π -state with a π σ factor of 0.39: ethyl radicals were also formed, and were converted to ethane below 200 K.137 TPD measurements on Rh(111) led to much the same conclusions.138 Similar work on Pd(100)(1 × 1) with ethene and propene showed that exchange with adsorbed deuterium atoms started at 120 K, and that alkanes were formed at 250–300 K.139 On Pd(100) the initial adsorption of ethene on an ordered overlayer of hydrogen or deuterium atoms at 90 K gave the π -state plus another even more weakly-held form; they inhibited further adsorption of hydrogen, but ethene-d4 exchanged via ethyl radicals.140 On Ir(111) and Ir(110)(2 × 1) above 400 K, the reaction of propene with hydrogen afforded lower hydrocarbons in small amounts that grew as temperature was raised; coverage by strongly-held carbon species followed suit.141 The earliest work on single-crystal faces was performed using nickel.142 Work on Ni(111) led to the curious conclusion that dissolved hydrogen atoms emerging from within were more effective than adsorbed atoms because they were more energetic.143

Model catalysts (Section 2.3) permit the use of the same techniques of examination as single crystals. Palladium particles formed on alumina-coated NiAl(100) adsorbed ethene in both the π - and σ -forms, but the latter was favoured with increase in particle size: this could indeed be the basis for the weak size dependence noted previously (Section 7.2.2) in alkene hydrogenation. Hydrogen adsorbed more strongly on small particles, and in a now familiar way shifted the adsorbed states of ethene towards the π -form: this reacted with weakly-held hydrogen atoms to form ethane.62

7.2.6.The Reaction Mechanism: Microkinetic Analysis, Monte Carlo Simulation, and Multiple Steady States

The nature of the mechanism by which ethene is hydrogenated and the related reactions take places has excited the interest (and sometimes the emotions) of scientists for three-quarters of a century. It is humbling—not to say humiliating— to find that the questions being discussed in detail some 50 years ago have not

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yet been definitively answered; new questions have also arisen to muddy the water further. It is typical of heterogeneous catalysis that reactions of great formal simplicity admit of so many and varied interpretations. In addressing the First International Congress on Catalysis, the late Sir Eric Rideal remarked, with characteristic modesty as follows. A great number of workers in the field of catalysis from Sabatier onward have given explanations of the mechanism of the reaction; I myself have advanced three. At least two must be erroneous and judging by the fact that no fewer than three communications are to be made on this subject during this week, it is quite likely that all three of them are wrong.144 As we shall shortly see, the number of candidate mechanisms has grown considerably since 1956.

An attempt was made to list the items of knowledge required in making a statement of mechanism (Section 5.3), but this list was minimal, and is capable of extension. There are several suggestions in the previous sections that the simple form of the Horiuti-Polanyi mechanism8 as expressed in process 7.B and Scheme 7.1 is not entirely adequate; this has long been suspected, and the evidence for a vinylic exchange mechanism needing the dissociative adsorption of the alkene,36 and the recognition of the likely role of π -alkenylic species on some metals, has only added to the feeling of inadequacy. We may focus on two particular worries. First, there are a number of other possible and plausible reaction steps that can lead to the same range of products as would the Horiuti-Polanyi mechanism; these include for example the reaction of a hydrogen or deuterium molecule with either ethene or ethyl, and the disproportionation of two ethyl radicals to give either two ethenes plus a hydrogen or deuterium molecule, or ethene + ethane, without the intervention of hydrogen or deuterium atoms. This by no means exhausts the possibilities. Second, it has several times been proposed that hydrogen or deuterium molecules can chemisorb dissociatively either in competition with sites acceptable to the alkene, or on sites at which the alkene cannot itself adsorb. This is a distinction that did not occur to Horiuti and Polanyi, but has troubled many since their day. It is most convenient to review the current state of mechanistic theory by outlining three quite different approaches, and summarising their conclusions. Those desiring an historical perspective are urged to read the relevant papers and discussional sections in Discussions of the Faraday Society, Vol. 8, (1950); they will find much of interest.

The term microkinetic analysis has been applied1,27,144 to attempts to synthesise information from a variety of sources into a coherent reaction model for the hydrogenation of ethene. The input includes steady-state kinetics (most importantly the temperature-dependence of reaction orders38), isotopic tracing,28 vibrational spectroscopy and TPD; it uses deterministic methods, i.e. the solution of ordinary differential equations, for estimating kinetic parameters. It selects a somewhat eclectic set of elementary reactions, and in particular the model

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provides for both competitive and non-competitive adsorption of hydrogen, as well as further ‘activation’ of both sorts of atom before they can react. It ignores minor products of the ethene-deuterium reaction such as ethanes-d3 to -d6, deuterated ethenes and hydrogen deuteride. It coalesces results obtained on Pt(111) with those for small platinum particles; and so by assuming structure-insensitivity it represents its conclusions as applicable to all platinum catalysts.

A major conceptual difficulty with this model is the lack of definition of the different sorts of adsorption sites proposed. The results presented in Chapters 3 and 4 indicate that there are indeed several possible locations for hydrogen atoms, one of which perhaps could be involved with alkene adsorption and thus be ‘competitive’; but the possibility of interconversion of the two types is not considered, nor is the physical nature of ‘activation’ discussed. Site requirements for the hydrocarbon species clearly allow for site-blocking, as ethyl radicals are taken to use two sites rather than one; the π - and σ -forms of adsorbed ethene are not differentiated. These models shown in Chapter 4 reveal how hard it is to relate observed structures to specific locations on the metal atom lattice that may appear to be logical adsorption ‘sites’. Kinetic parameters for the seven forward and seven reverse reactions have been evaluated, and the model with these values is claimed to describe the results adequately. Orders in hydrogen greater than first were not however reproduced. It is important that work of this kind should be undertaken, but we must guard against thinking that the model is wholly valid and its conclusions totally reliable.

Monte Carlo simulations145−150 claim to avoid some of the above problems by using a stochastic model. Duca and colleagues considered145,146 a square array of sites (i.e. the fcc (100) plane), a set of adsorption and reaction steps, and the probabilities of their occurrence. This set contains some unusual (and unnecessary?) features; ethane is formed in a physically adsorbed state using two ‘sites’ and is in equilibrium with gaseous ethane, while ethyl radicals take up three sites, i.e. the two on which ethene was adsorbed and one for a hydrogen atom. The ethyl radical is regarded as a kind of virtual species, and again π - and σ -forms of ethene are not distinguished: the reaction set is somewhat simpler than that used in microkinetic analysis. A ‘steric hindrance parameter’ defining the number of carbon atoms allowed to be adjacent to a given atom is included; if its value is set low, non-competitive hydrogen adsorption is possible. Hydrogen is ‘activated’ if it is on a site next to a carbon atom, and ethane is formed when two hydrogen atoms are adjacent to an ethene molecule.

The model was operated146 by starting with a bare surface, and evaluating the probabilities of all possible events in a ‘time-slice’: this was repeated about 30 million times after which steady-state behaviour was found. The probabilities were related to real-time by collision numbers based on kinetic theory, and experimental sticking coefficients values of variable parameters were fixed to give best

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agreement with the experimental dependence of TOF on hydrogen pressure at 298 K; these values then reproduced the order in ethene very satisfactorily, including the change from negative to zero order as its pressure is increased. This followed from the necessarily assumed low value of the steric hindrance parameter, which allows non-competitive hydrogen adsorption (and hence reaction) to occur even when the surface is as fully covered by ethene as possible. While achieving striking success, an unfortunate limitation of this model is its failure to consider the results of isotopic tracer experiments. It is able however to ‘study’ the reaction under conditions not yet examined experimentally, for example, where the ethene coverage becomes so low that the rate is proportional to its pressure.

A further application151 of the Monte Carlo procedure has been directed to accounting for sudden transitions from a low to a high activity state in ethene hydrogenation as either temperature or ethene pressure is changed; they are capricious, in that they are sometimes very large and sometimes quite small, and frequently do not appear at all. They have been observed with a fresh Pt/SiO2 catalyst,39 but not with Pt/MoO3 or EUROPT-1; they were more noticeable with small platinum particles (5–6 nm) than with large ones (16–19 nm), but particle size dependence does not seem to be a uniformly satisfactory explanation. The Monte Carlo simulation used only the basic Horiuti-Polanyi reaction set, and assumed the π -state of ethene, and competitive adsorption of reactants: the transition was observed because, unlike the earlier model, adjacent adsorbed ethenes were allowed, and coverage by ethyls was specifically included. In the low activity state, hydrogen atoms were isolated in a sea of mainly ethene molecules whereas in the high activity state most were present as adjacent pairs. The slow step was therefore thought to change from ethyl formation to ethane formation, thus explaining the change in activation energy. This would require a change in reaction orders, but this has not been tested experimentally.39 The procedure predicted the occurrence of rate discontinuities as ethene pressure was changed, but the calculated size of the rate change was very large, and much greater than that found in practice in the same study. It also attempted to account for the products of the reaction of ethene with deuterium, and the variation of activity with composition of palladium-gold bimetallic catalysts; the effect of particle geometry has also been addressed.

The group originating this methodology has continued to expand and refine its procedures with thermodynamic and quantum mechanical inputs.152 The same reaction set and steric hindrance parameter were retained, and the activity transitions referred to in the last paragraph were again targeted;39 a quantitative treatment of this concept was developed. The rate transitions do however require there to be an order-disorder transition of the adsorbed hydrogen atoms. The arguments deployed are both complex and subtle, and are not easily summarised: the papers cited in this section deserve careful reading as showing the depth of theoretical reasoning that can be applied to this simple reaction.

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The third theoretical paradigm that should perhaps be mentioned is the use of advanced deficiency theory,153 (a sub-set of chemical reaction network theory) to interpret the multiple steady states,154which have been observed in ethene hydrogenation over rhodium film in a flow system.53 These alternative states of high and low activity were apparently achieved by alteration of the direction of temperature or hydrogen pressure change, and although they may be related to those described by Jackson et al.39 it reads more as if the plots of rate vs. variable showed hysteresis; in the absence of actual results one cannot be sure. The theoretical armoury of chemical engineers was then deployed53 to consider thirteen basic sets of unit reactions, including three variants of the Horiuti-Polanyi mechanism, and some that are only available in American doctoral theses. If this were not enough, these thirteen schemes were then combined in every possible way to allow for duplicate pathways for ethene formation, to give 80 candidate mechanisms, of which 71 were rejected. Unfortunately the limited intelligence of the author prevents any attempt at describing the process by which this conclusion was reached, but one must suppose that it is an advance of some kind if experiments based only on a simple measurement of rate can be shown to be consistent with only nine reaction mechanisms.

DFT has been applied to unravel the means by which hydrogen atoms add to ethene molecules chemisorbed on the Pd(111) surface in the opening step of the hydrogenation sequence.130 Binding energies of the π and σ forms were similar at low coverage, but the former had to transform into the latter before it could react; computed activation energies were similar to those found experimentally, and references to earlier theoretical work were given.

7.2.7.Catalysis by Hydrogen Spillover and the Reactivity of Hydrogen Bronzes

It has been blithely assumed in what has gone before that the reaction occurs solely on the metal in the case of supported metal catalysts. The phenomenon of hydrogen spillover was introduced in Section 3.34, and the idea of spillover catalysis, i.e. reaction on the support induced in some way by the presence of the metal, has been noted (Section 5.4) as a possible complicating factor when studying particle size effects. The possibility that spillover catalysis contributes to alkene hydrogenation was considered some years ago as a means of accounting for different rates shown by platinum on various supports,36,59 but this is now thought very unlikely: one of the strongest contrary arguments is the general similarity, not only of TOF but also of internal kinetic parameters (alkene exchange etc.), shown by single crystals, wires and supported metals. There are however some somewhat special and limited conditions under which alkene hydrogenation can occur in a spillover mode, and these will now be briefly examined, not so much because reaction in this way can qualify as being ‘metal-catalysed’ as because,

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being metal-induced, it is an ever-present threat to the rational interpretation of results. So one must always be on one’s guard against it, and indeed in Chapter 10 it will emerge as being more than likely.

The various ways in which hydrogen spillover catalysis can emerge may be classified as follows. (1) Hydrogen atoms may migrate from metal particles to a ceramic oxide support where they may react with the alkene; the possibility that the observed reaction is taking place on the metal is overcome by physically removing or isolating the metal-containing catalyst. (2) In the presence of both reactants, continuing reaction can be found on the support after removal of the metal catalyst.

(3) Hydrogen atoms may dissolve into the lattice of certain transition metal oxides (V2O5, MoO3, WO3, see Section 3.3.4), and these may react with alkene. (4) The hydrogen bronze so formed may catalyse hydrogenation in the absence of the metal. Numbers 1 and 3 are examples of spillover catalysis; numbers 2 and 4 might be termed induced catalysis because, although the metal played a role, it was (probably) not itself the catalyst. Catalysis by reverse spillover occurs when the metal remains but the hydrogen gas is removed: spillover hydrogen (hydrogen spillage) returns to the metal by diffusion, where it reacts with the alkene. This process is recognised when the amount of product vastly exceeds what might be formed by reactants retained on the metal. Short summaries of experimental findings follow.

In work with ceramic oxides, the oxide to be activated is initially in contact with a supported metal catalyst, and the two are subjected to defined treatments before the activated oxide is separately examined. One standard procedure involved heating in 1 atm hydrogen at 573 K for 8 h, lowering the temperature to 383 K and leaving for a further 8 h.34,155 Two methods have been used for eliminating the catalyst from the subsequent examination. In the first, the catalyst (Ni/Al2O3 or Pt/Al2O3) was placed in a bucket immersed in the oxide; it was then raised up by a windlass, and isolated.155 In the second, the catalyst as pellets rested at the foot of the reactor containing the oxide through which hydrogen flowed; it was then replaced by nitrogen and after purging, ethene was introduced above the pellets under conditions such that its back-diffusion to the metal was unlikely.156 These experiments led to the following conclusions.

Hydrogen spillover to γ - or δ-Al2O3 affords atoms that can react with ethene, and it generates sites at which continuous but slow hydrogenation can occur. The activation is either activated or endothermic, because hydrogen spillage increases with temperature. A chain reaction such as

H2	+ H	(7.G)
H + C2H4 −→ C2H5 −→ C2H6

may describe what happens, as reaction is inhibited by nitric oxide, which may either react with the hydrogen atoms or displace them.36 No such effect was shown

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by silica, and silica-alumina improved with repeated reductions.156 Rates were slow and the number of reacting centres small (<1 per 10−18 m2). The location of the hydrogen atoms has never been firmly established: they may have been attached to oxide (O2−) or hydroxyl ions,158 but ‘carbon’ deposition has been observed after reaction, so this may in fact be where reaction occurs.

Little interest has been shown in the phenomenon in recent years, although the role of hydrogen spillover in controlling carbon deposition during petroleum reforming,159 or even in the reforming process itself,160,161 has been canvassed. This early work may therefore not have been entirely in vain.

Hydrogen bronze formation with vanadia,162,163 molybdena164,165 and tungsta166 is achieved by depositing a small amount of an activating metal (usually Pd or Pt) and exposing the material to hydrogen; alternatively the oxides can be admixed with a catalyst (e.g. Pt/Al2O3) before hydrogen treatment.164 In the latter case, use of a bucket reactor allows study of the reactivity of the bronze by itself; in the former case the effects of metal and bronze cannot be distinguished. With 0.1% Pt (or Pd)/MoO3, successively longer exposures (up to three days) and higher temperatures (up to 333 K) gave Hx MoO3, where x is 0.34, 0.9, 1.6 and 2: a mixture of molybdena with Pt/Al2O3 at 433 K gave only H1.6MoO3. The bronze phases were stable in ambient air, but were oxidised at 333 K, regenerated molybdena from which the bronze could again be formed; H1.6MoO3 reacted with ethene above 373 K, giving in sequence the lower bronzes. This phase in the absence of metal began to react with ethene above 353 K; at 453 K as the ethene pressure was raised, the order was initially positive but became zero, and the activation energy was 54 kJ mol−1. This is about that for proton migration in the bulk. The H1.6MoO3 itself acted catalytically for ethene hydrogenation: at 453 K at low ethene pressure (<10 kPa) the order in ethene was first, becoming zero at higher pressures however the time-dependent rate was always zero order, so the ethene pressure in some way determined the state of the surface. The initial rate varied only slightly with hydrogen pressure; the activation energy was 38 kJ mol−1. Pt/MoO3 has sometimes been used for catalytic reductions without the possibility of bronze formation being recognised.

A similar detailed study has been reported162,167 of the reactivity of vanadium hydrogen bronze Hx VO2.5 (x = 1.7–1.9)168 formed by depositing platinum on vanadia and exposure to hydrogen at 335 K. This phase has interesting physical properties,168 and its structure has been examined by inelastic neutron scattering,169 which shows the hydrogen atoms to have reacted with the VO bond to give H2O that remains bonded to the vanadium atom; no V––H bonds were detected, although its IR spectrum confusingly showed no bands due to hydroxyl ions either.162 NMR spectroscopy provides evidence both for162 and against169 the presence of V––H bonds, so the structure is still a matter for discussion. Proton mobility is however at least 102 slower than in H1.6MoO3. Its reactivity towards ethene is greater than that of H1.6MoO3 and the kinetics are similar, but it retains some oxidising

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character, and more readily loses water on outgassing. Reactivity is clearly not simply determined by proton mobility, and unlike H1.6M0O3, which has metallic character because of electron transfer from the hydrogen to the conduction band, HxVO2.5 is merely a semi-conductor. This difference was thought to explain its greater reactivity.162 The sesquioxide V2O3 oxidises on storage,170 but is reduced back by hydrogen spillover on application of platinum.

Detailed studies have also been performed165 on the formation of the tungsten hydrogen bronze Hx WO3 (x ≤ 0.6) and of reverse spillover with it and with HxMoO3.

7.3.REACTIONS OF THE BUTENES WITH HYDROGEN AND WITH DEUTERIUM6,36

7.3.1. The n-Butenes

The further reactions that are readily observed when the alkene contains four or more carbon atoms were introduced in Section 7.2, so that now it is only necessary to recall the main features that characterise each metal before entering into a summary of the results and a discussion of mechanism. The three observable processes are (i) addition (hydrogenation) leading to n-butane, (ii) isomerisation (double-bond migration or Z-E isomerisation), and (iii) exchange in the reactant alkene if deuterium is used. The number of deuterium atoms entering isomerised butenes, and the Z /E ratio in 2-butene formed from 1-butene, are also matters of interest. These reactions, using each of the three n-butenes as reactant, have been widely studied for the extra insights they provide into the form of the adsorbed intermediates and the ways in which they react. All of the metals of Groups 8 to 10 have been examined, those of Group 10 having had the lion’s share of attention. The Group 11 metals have been largely neglected, although gold is reported to isomerise 1-butene at 573–673 K in the absence of hydrogen;171 it will hydrogenate 1-butene if hydrogen atoms are supplied to the gold surface.172

We may start by considering the results obtained with platinum catalysts: the reactions of the n-butenes with hydrogen and or deuterium have been studied using Pt/Al2O3,103 on catalysts prepared from reverse micelles173,174 (Section 2.32), and on platinum foil and various single crystal surfaces.175 There are a number of common features: (i) orders of reaction, where measured, are either accurately or close to first in hydrogen and zero in the butene; (ii) activation energies for the macroscopic forms are between 33 and 43 kJ mol−1 for both 1-butene and Z -2-butene,175 but lower for the latter on Pt/Al2O3 (21 kJ mol−1);103 (iii) rates of reactant removal for each butene appear to be structure-insensitive173,175 and all three isomers react at about the same rate.103 A notable characteristic of platinum catalysts prepared conventionally or in macroscopic form is the slow rate of

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isomerisation ri compared to the hydrogenation rate rh ; this is in line with the small amount of ethene exchange seen with this metal (Section 7.3.1), and is again explained by the reluctance of the alkene once adsorbed to desorb (Table 7.4). There is however evidence for some variability of ri/rh because isomerisation is relatively more important on the unconventional catalysts, although the cause of this was not identified.173

Relative isomerisation rates ri/rh naturally vary with the structure of the reactant butene, depending on the variance of the initial condition from that of equilibrium between the butene isomers (Figure 7.3) and on the stabilities σ the adsorbed intermediates. Values of ri/rh at room temperature over Pt/Al2O3 were estimated as103

Z -2-butene:E -2-butene:1–butene = 15:1:5

Their interpretation rests on conformational analysis of the adsorbed 2-butyl radical; this can adopt three staggered configurations (Figure 7.2): loss of a hydrogen atom from I will give either the Z - or the E -isomer, depending on which goes, II will give only E and III only Z . Their relative stabilities (discounting interaction

of the methyl group with the surface) are II > I = III, so that conversion of Z to E should be easier than E to Z , as observed. The medium ri/rh shown by 1-butene follows from the fact that it can form both 1- and 2-butyl radicals, the former only reverting to 1-butene or forming butane. The conformational factors also explain why the Z /E ratio is far from the equilibrium value for the free molecules ( 40 at 298 K);6 in fact it is about 0.6, rising to 1.4 at 379 K as thermal motion overcomes the repulsive interactions between the methyl groups.

When the n-butenes interact with deuterium on platinum catalysts,103,173,174 there is little exchange of the reactant alkene although the Z - and E -2-butenes formed from 1-butene are substantially and about equally exchanged. The 2-butene-d0 is however the major product, but, especially on very small particles, products up to 2-butene-d8 have been observed.173 Unless the exchangeisomerisation is followed as a function of conversion,103 and particularly if products are only analysed at a single high conversion,173 it is difficult to assess the importance of the sequential reaction of the primary products, but the following picture seems feasible. There is a rapid interchange of hydrogen and deuterium atoms (X) between X , C4X8 and C4X9 , so that each species will contain about the same fraction of each kind. In this way the butene and butyl species acquire deuterium atoms in diminishing numbers up to the possible total, but addition of the second atom to the butyls is faster than butene desorption: those molecules that do desorb are mainly isomerised, but may be quite extensively exchanged. All possible deuterobutanes are formed, butane-d2 being the major product, but in consequence of the low relative rates of both hydrogen and alkene exchange the mean number of deuterium atoms is close to two.

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Microwave analysis of the mono-exchanged 1-butene and Z-2-butene formed by double-bond migration adds some detail to this scenario:173 in the 1-butene- d1, the two terminal methylenic hydrogen atoms, taken together, are slightly or significantly less exchanged than that on C2, while in Z -2-butene-d1 it is one of the C1 atoms that is twice as likely to be exchanged as that on C2. These observations suggest that 1-butyl radicals are somewhat stabler than 2-butyl radicals. The Horiuti-Polanyi mechanism, supplemented by conformational analysis of the intermediate species, therefore provides a reasonably satisfactory explanation of the results, although minor contributions from other mechanisms are not ruled out.173

The behaviour of Ir/Al2O3 resembled that of Pt/Al2O3, but relative isomerisation and exchange rates were even lower at 253–293 K.103 No isomerisation of 1-butene was detected, and only 2.5% of 1-butene-d1 was detected: mean deuterium numbers of the butane were close to two.

Other metals of Groups 8 to 10 have very different characteristics in respect of reactions of the butenes with hydrogen and deuterium:6 as might be expected from the way they behave in the ethene- (and propene-) deuterium reactions, nickel, palladium, ruthenium, rhodium and osmium are able under some conditions to exhibit much higher values of ri/rh , so that the butenes are able to achieve their equilibrium concentrations before their hydrogenation is finished.

The reactions have been followed on supported nickel catalysts,2,6,18,19,176,177 on nickel film176 and wire,6,18,178 and on ‘solvated metal atom dispersed catalysts’.179 Orders of reaction and activation energies for the possible reactions on all three butenes have been reported, but values of the latter are sometimes very low (8–15 kJ mol−1), this suggesting diffusion limitation. Reactions of 1-butene and Z-2-butene with deuterium have been extensively studied on Ni/Al2O3,6,19 Ni/SiO2,177 Ni/pumice,176 and nickel wire.6,18,20,178 The work performed by Taylor and Dibeler18 is especially commendable, as it was undertaken before the advent of gas chromatography; analysis of the butene isomers had to be made by IR spectroscopy. Exchange and isomerisation have higher activation energies than addition, so they become more significant as temperature increases.6,176 E/Z ratios in 2-butene formed from 1-butene lie between 1.5 and 2, without any marked effect of temperature.176 With 1-butene, extensive stepwise exchange means that butene- d0 and -d1 are major initial products,18 and the change of product concentrations with conversion recalls that found with the ethene-deuterium reaction (Figures 7.5 and 7.6). Z - and E -2 butenes are equally and substantially exchanged; microwave analysis shows that at low temperature (<273 K) the mono-exchanged 1-butene is mainly 1-butene-2d1, although exchange at C1 increases with temperature.176 In the reaction of Z -2-butene, the isomerised 1-butene is more heavily exchanged than the E -2-butene; this is understandable if adsorbed 1-butene and 1-butyl radicals are stabler than the 2-butenes and 2-butyls because of the smaller unfavourable repulsive interactions between methyl groups, allowing more interconversion to