Metal-Catalysed Reactions of Hydrocarbons / 06-Exchange of Alkanes with Deuterium

was not observed. Further attention to processes dependent on the dehydrogenation of methane will be given in Chapter 12.

6.2.2. Ethane and Higher Linear Alkanes4,8,15,36,38,39

The exchange of ethane and higher alkanes with deuterium has been the subject of fewer investigations than that of methane, with which there are some similarities but many differences; these may be summarised as follows. (i) The reaction occurs at somewhat lower temperatures, due to the easier activation of ethane. (ii) Tungsten, molybdenum and tantalum films are highly active (at 193– 273 K) but give only stepwise exchange: in this respect their behaviour is like that of tungsten in methane exchange. (iii) On other metals of Groups 4 to 10, but especially on cobalt, rhodium and palladium, multiple exchange giving mainly ethane-d6 accompanies stepwise exchange, initial deuterium numbers M being 2.3 to 5.4;2,4 their values usually increase with temperature17,22 (not however on Pt(111)40), but separate kinetic expressions for the two processes have not been derived. (iv) Unlike methane exchange, where satisfactory (and satisfying) compensation plots are shown (Figure 6.2), the Arrhenius parameters for ethane exchange when plotted in this way are somewhat scattered8 (Figure 6.3).

Arrhenius parameters and selected values of M are given in Table 6.2: in conjunction with Figure 6.3, the following comments can be made. (i) Sintered films of palladium and platinum again have low activities and show high activation energies: this appears to be the only bimetallic system looked at. (ii) Certain

Figure 6.3. Compensation plot of Arrhenius parameters for ethane total exchange with deuterium:2,19 (see Table 6.2): O foils, films, Pt(111); blacks, powders; supported Pt. The enclosed points are for Pt and Pd (see text).

268 CHAPTER 6

a Sintered film. See also footnotes to Table 6.1.

surfaces appear abnormally active, at least in comparison with other physical forms (W (see preceding text; Rh film;38 Pt(111)41); with these exceptions, activities do not vary greatly (Table 6.2). Rates on films of other Transition Series metals have been reported,10 but only on the basis of unit weight of metal.1,8,38 Orders of reaction were in the range: for ethane, 0.8 to 1.2, and for deuterium, −0.55 to −0.8 (excluding tungsten).

There is a readier means of achieving multiple exchange with ethane than the methane; this involves the repeated inter-conversion of ethyl radicals with ethane, thus:

Once again it is possible that ethanes-d4 and -d5 result from incomplete multiple exchange caused by intervention of hydrogen atoms not efficiently removed from the surface during the single residence of the ethyl radical.15 On rhodium film, ethane-d6 was the main product at low deuterium pressure, but all ethanes appeared as primary products at higher pressures. Stepwise or single exchange might as with methane occur either by reaction of an ethyl radical at a site, or by a mechanism that does not allow formation of ethene: the high activity and low multiplicity shown by tungsten, molybdenum and tantalum suggest that the mechanism proposed by Frennet1 for single exchange of methane may apply here also. While with platinum the temperature ranges in which exchange and hydrogenolysis occur are well separated, with rhodium the two ranges overlap1 because of

Figure 6.4. Hegarty and Rooney scheme for the exchange of ethane with deuterium.42

this metal’s much higher activity for C C bond breaking; ethane exchange for this case will therefore be re-visited in Chapter 14.

Deuteroethane distributions have been interpreted in terms of a parameter P, which is the quotient of the rate constants for ethyl to ethene and ethyl reverting to ethane.2,4,38 For molybdenum, tantalum, rhodium and palladium films, a single value of P (respectively 0.25, 0.25, 18 and 28) sufficed to reproduce the observed distribution, assuming that a further deuterium atom is acquired at every opportunity. With other metals, however, two simultaneous values of P appeared to operate, one contributing 30 to 50% of the reaction having a high P value (13.5–18) and another having a much lower P value (0.36–2). This analysis has not however been accorded an interpretation in terms of the metals’ physical properties or of ensemble sizes and structures responsible for each participant.

An alternative way to explain product distributions has been suggested42 (Figure 6.4). This involves two sites denoted A and B, on the first of which the ethyl radical is formed; di-σ ethene adsorption needs both. This then transforms to π-ethene (there is independent evidence that removal of hydrogen from ethyl gives the di-σ and not the π-ethene41), thence to ethyl on the B site and finally to ethane. Assigning numerical values to the three rate constant quotients produces calculated distributions in fair agreement with those observed. There are supposed to be few type A sites and many type B. This scheme also necessitates three independent parameters, and one wonders whether all the niceties of Microscopic Reversibility are accommodated by it.

Frennet6,15 has discussed his observations concerning ethane exchange on rhodium film by analogy with methane exchange, namely, a dual mechanism in which an ethyl radical is heavily exchanged before desorbing, and an ethane molecule is monosubstituted during a brief encounter with the surface. This analysis was again supported by detailed analyses of hydrocarbon coverage.

It has long been a goal of research on alkane exchange to identify isomers of the same mass, e.g. ethane-12-d2 (CH3-CHD2) and ethane-1,2-d2 (CH2D-CH2D); this has at last been accomplished by NMR of 13C-containing ethane,40 and the

to symbolise adsorption sites in processes, figures and in the text ought not to cause confusion; the linear form is used in the text for simplicity). The generally observed increase in the amount of multiple exchange with temperature is then explained by the decrease in the surface concentration of deuterium atoms, which favours the dehydrogenation of ethene into ethylidyne. Dehydrogenation beyond ethene has been rejected in other mechanisms, but Zaera’s proposal merits serious consideration.

There remain a number of observations that do not admit of easy explanation by the mechanisms so far presented. The deposition of nickel films in UHV under various conditions produced very varied product distributions: thus films created on a hot substrate (573 K) and orientated so that either (111) or (100) faces were preferentially exposed both gave only ethane-d2, -d6 and -d4 decreasing in that sequence (M = 3.8), while a randomly oriented film deposited at 273 K but sintered at 613 K gave mainly ethane-d1 and -d6(M = 3.3).8,10 Temperatures were 443–463 K, but at 273 K, on an unsintered film, ethene-d1 was almost the only product. Very large amounts of ethane-d2 and -d6 were also formed on nickel powder ( PD/ PE = 9, T = 420–540 K).43 The means whereby ethane-d2 is formed, sometimes to the exclusion of the -d1 and -d3 species, has not been much discussed, but it would appear that on certain sites loss of two hydrogen atoms gives ethene (either π - or di-σ ) and reversal with two deuterium atoms occurs without the intrusion of stepwise exchange.

A synthesis of mechanistic opinion is made difficult by the profound studies by Frennet,15 Zaera40 and others having been made on different surfaces. The invocation of ethylidyne as the means of achieving complete exchange is as perfectly reasonable for rhodium as for platinum, but unfortunately the scheme of equation 6.K and Figure 6.5 has not received a quantitative treatment, so its validity is not yet assured. What is very noticeable however is that palladium behaves quite ‘normally’ in ethane exchange, unlike its failure to give multiple exchange in the reaction of methane. Nothing could point more clearly to the differences in the mechanisms by which these alkanes exchange with deuterium.

6.2.3. Higher Linear Alkanes

With the introduction of the third and further carbon atoms into the linear alkane chain, the number of conceivable monoand di-adsorbed structures increases rapidly (see Tables 4.3–4.5), and quantitative modelling of the distribution of exchanged molecules becomes progressively more difficult. Thus exchange may proceed not only by the easy αβ mechanism so favoured by ethane (process 6.J), but also (e.g. on rhodium) through reversible formation of αγ species (i.e. 1-propyl ↔ 1,3-diadsorbed propane (Table 4.5)):44 further possibilities exist when there are four or more carbon atoms present (e.g. an αδ process26). In the case of propane, early work indicated4,45 that on nickel film the hydrogen atoms on the secondary

carbon exchanged faster than those on the primary carbons, as might be expected on the basis of the relative bond strengths, but more recent work44 with Rh/SiO2 using deuterium NMR as the analytical tool showed that exchange of both secondary hydrogens was somewhat hindred. Apart from this work, there has been no attempt to identify the numerous isotopomers formed in higher alkane exchange with deuterium.

An additional feature of the reaction of the higher alkanes is that, depending on the temperature and the metal used, it is sometimes difficult to separate exchange from hydrogenolysis, as the relevant temperature ranges overlap:26,46,47 this is particularly so with nickel, ruthenium and rhodium, all of which have high activities for hydrogenolysis, but some workers have turned this to advantage, using exchange to illuminate the process of C C bond breaking. Exchange reactions on platinum are however usually untroubled by hydrogenolysis.48

For each metal the general features of the distributions of exchanged products for all linear alkanes resemble that shown by ethane. Multiple exchange occurs widely, and is perhaps even more significant than with ethane: it increases in important with rising temperature,4,26,49,50 and with decreasing PD/ PC ratio, but is inhibited by carbon deposition and by adsorbed oxygen atoms.4,20 Comparison of metals’ tendencies in stepwise and multiple exchange is complicated by their different activities and the consequent need to use different temperature ranges; thus, rhodium film at 250 K showed much less multiple exchange (M = 5.7) than palladium film at 320 K (M = 7.6),4,51 but rhodium catalysts have given very complete exchange of n-hexane and n-heptane at moderate temperatures.52,53 Various forms of nickel catalyst all show some degree of multiple exchange; with Ni/SiO2, increase in particle size by sintering gave more multiple exchange. These results, and those concerning the effect of poisoning (mentioned above), suggest4,50 that the course of alkane exchange is both particle-size-sensitive and ensemble- size-sensitive, and that a larger active centre is needed for multiple than for stepwise exchange. There is usually some stepwise exchange, but with the larger alkanes such as n-pentane and n-hexane it is difficult to estimate amounts in the centre of the distribution with high accuracy.48 Hexane-d2 was however a significant product of the exchange of n-hexane on rhodium film.4,49

With pumice-supported metals, there was apparently a third process causing a peak in the centre of the distribution,4 but an interpretation needing five disposable parameters to account for eight quantities was rightly criticised as having too many variables. There was no significant difference between the products formed from n-hexane on Pt(111) and the kinked Pt(10,8,7), showing the reaction is not facesensitive; active centres for multiple exchange must be freely available on both. On these surfaces however multiple exchange did not rise with temperature.48

Table 6.4 shows Arrhenius parameters, rates and multiplicity factors for higher alkane exchanges with deuterium, mainly on metal films: rates are generally somewhat higher than for ethane, and the temperature ranges used correspondingly

Notes: a Exchange of secondary hydrogen atoms only.

See also footnotes to Table 6.1.

lower. There are insufficient data to create a meaningful compensation plot. Orders of reaction are usually positive in alkane (0.5–1) and negative in deuterium,4,54 becoming positive at high temperatures as desorption sets in.

Variously structured nickel films also produced some very peculiar product distributions,4,10 as they did with ethane: at 393–433 K on (111) and (100) oriented films gave propane-d8 and (usually) -d2 as the major products, but randomly orientated but sintered film at 400–435 K yielded propane-d1 and -d8, and an unsintered film at 273 K gave only the -d1 isomer. These results closely resemble those obtained with ethane, and lend stress to the importance of surface structure in deciding what intermediates are formed and how they react. The absence of results for single crystal surfaces of nickel is sorely felt.

6.2.4. Branched Alkanes55

Provided a branched alkane has only primary, secondary and tertiary carbon atoms, all hydrogen atoms can be exchanged by the αβ mechanism; the carbon skeletons (devoid of hydrogen atoms) of some such molecules are shown in the first row of Table 6.5. The presence of a quaternary carbon atom as in neopentane (2,2-dimethylpropane) prevents the formation of an αα-diadsorbed species, so that multiple exchange, if it occurs, must of necessity proceed through either αα- or αγ -diasorbed structures, or where possible through an αδ structure.26,56−58 Carbon skeletons of molecules of this type are shown in the lower part of Table 6.5, but

TABLE 6.5. Structure of Branched Alkanes

in some cases the αβ mechanism can operate in part of the molecule,54,59,60 and where it can the number of exchangeable hydrogen atoms is shown. Where no secondary or tertiary carbon atom is present, stepwise exchange predominates at low temperatures,26,61 but multiple exchange by the αα or αγ process becomes more evident with rising temperature, clearly with rhodium44 but even more so with ruthenium.26 Prohibition of the passage of exchange past a quaternary atom is in itself evidence of the difficulty of forming αγ -diadsorbed structures, the ease of occurrence of exchange therefore being summarised as αβ >αα>αγ . It is however difficult to find quantitative expressions for the relative energetics of these processes. There are similarities between neopentane and methane, in for example the non-appearance of multiple exchange in both.2,20,62,623 Variation of the extents of exchange by the αβ and αγ processes on Pd/SiO2 and Pt/SiO2 having various particle sizes did not however accord with the expectation that the latter should become easier as size increased.64 Some rates and Arrhenius parameters are given in Table 6.6; palladium’s very high activation energy stands out.

There is one further interesting and useful feature shown by branched alkanes.1 3-Methylhexane (see Table 6.5) has a centre of optical activity, and the (+) form is observed to racemise during exchange.50,52 The process over

Notes: Exchange of tertiary hydrogen atom only; see also footnotes to Table 6.1.

a nickel/kieselguhr catalyst had similar kinetics to exchange, and almost every molecule suffering exchange at its tertiary C H bond also inverted its configuration. The mechanism by which this occurs has not been fully established.1 At some stage the conformation about the tertiary carbon atom must become planar, and the deuterium atom must add at the opposite side from that at which the hydrogen atom was removed. One possibility1 is that adsorption of the alkane occurs by Frennet’s mechanism,2 and its Eley-Rideal reversal may allow inversion to occur, thus:

The existence of exchange in such a way prompts the speculation that it may also occur with other molecules, but remain undetected because no optically active centre is present.

6.3. EQUILIBRATION OF CYCLOALKANES WITH DEUTERIUM8,65

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes: indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the αβ exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism41 for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g.

−H

The similarity of this scheme with that for multiple exchange is quite evident, and indeed the αβ exchange mechanism is sometimes called the Horiuti-Polanyi mechanism:66 the only difference (which turns out to be a point of some importance) is that in exchange the adsorbed alkene is formed from an alkyl radical, whereas in hydrogenation it comes from the gaseous alkene. It may also be wondered whether it is logical to speak of adsorbed alkene being formed under exchange conditions that in many cases are very different (e.g. much higher in temperature) from those in which hydrogenation occurs. It is true that the alkylalkene equilibrium set up during exchange will be heavily on the alkyl side, and the alkene surface concentration very low, but it is sufficient in most cases to sustain multiple exchange, and no other proposal has been made to account for it.

The original discovery by Kemball, since frequently confirmed, was that, while five hydrogen atoms on one side of the cyclopentane ring are easily exchanged, there is a considerable energy barrier to accessing the five on the other side of the ring: this is manifested by a higher activation energy, the importance of complete exchange always increasing with temperature. Thus in a typical case at lower temperature there will be maxima at cyclopentane-d1 and -d5 (sometimes at -d2 as well) with no more extensively exchanged molecules, but rising temperature will cause the maximum at -d5 to diminish and finally disappear, with cyclopentane-d10 becoming the dominant product. This apparently simple observation has however created a crop of problems, and efforts to resolve them have revealed hitherto unsuspected depths and subtleties in the mechanisms of metal-catalysed reactions of hydrocarbons. Central to the ensuing discussion has been the way in which the molecule inverts to enable the second set of hydrogen atoms to exchange. Careful consideration of the results obtained under a variety of conditions has led to the conclusion that there are four (or perhaps five) distinct mechanisms at work, each proceeding on a site appropriate to it, but the research has focussed more on the nature of the intermediates than on the form of sites required. It was perhaps unfortunate that this work was undertaken at a time (mainly in the 1960s and 1970s; little since 1980) when single crystal surfaces were not commonly used. The important observation that stepwise and multiple exchange of ethane proceed side by side on the highly uniform Pt(111) surface showed that radically different types of site were not needed, and attention in that case therefore moved to the nature of the intermediate species. It is surprising to say the least that the problems posed by the complex nature of cyclopentane exchange have not been addressed by studies using single-crystal surfaces, which would surely have removed many of the still existing uncertainties.

Before attempting to summarise the debates on mechanisms, a little further background is needed. Cycloalkane exchange is faster than for linear and branched alkanes, and with metal films2,4,67 and powders68 it is frequently possible to work at subambient temperatures, even as low as 77 K. Deactivation by ‘carbon deposition’ is however often a problem; its removed by treatment with hydrogen shows