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

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6

EXCHANGE OF ALKANES

WITH DEUTERIUM

PREFACE

The replacement of hydrogen atoms by deuterium (or tritium) atoms is the simplest catalytic change that can happen to a hydrocarbon; it occurs with surprising ease on a number of metal surfaces, and it focuses on the reactivity of C H bonds, ignoring the C C bonds. It has been a popular field of study, and has revealed (as all isotopic labelling studies have) an unsuspected wealth of detail, and a number of mechanistic questions have arisen that have required the study of carefully selected large hydrocarbon molecules whose structures allow only certain pathways to operate. It is one of the paradoxes of heterogeneous catalysis that reactions of larger molecules sometimes give more direct and unambiguous information about reaction mechanisms than smaller molecules are capable of. Studies of such molecules do however illuminate the stereochemical principles governing the chemisorption and reactions of hydrocarbon molecules of all sizes; and the reactions singled out for examination here will be found to occur as part of, or in parallel with, the more profound processes to be met in the following chapters. They therefore deserve careful attention.

6.1. INTRODUCTION1,2

The process whereby hydrogen atoms in an alkane are replaced by deuterium or tritium atoms in the presence of a metal catalyst is the simplest way of making recognisably different products, but ones that nevertheless are almost chemically identical to the reactants. The simplicity of these reactions, which proceed at conveniently modest temperatures on many metals, and which are generally (but not

257

258

CHAPTER 6

necessarily3) separable from those such as dehydrogenation and hydrogenolysis, has made them an attractive field of study since the earliest work on the lower linear alkanes using a nickel/kieselguhr catalyst in the 1930s.4 Apart from its intrinsic interest, the behaviour of adsorbed alkyl radicals revealed by its study is relevant to other reactions of hydrocarbons such as hydrogenation and hydrogenolysis in which they are also intermediates: and recently the discovery of homologation of adsorbed carbon species with its potential for utilisation of the lower alkanes has added a further use for the information that exchange reactions provide. Although the reaction may be performed with amounts of alkane and deuterium containing equal numbers of hydrogen and deuterium atoms,5 it has been usual to employ a large excess of deuterium (up to alkane/deuterium = 10) in order to minimise reverse reactions and so to identify the true initial products: under such conditions, deactivation due to ‘carbon’ deposition is rarely a problem with methane (although time-dependent changes have sometimes been seen1,6), although the larger the alkane the more noticeable it usually becomes, to an extent depending on the type of catalyst being used, and on the plane exposed.7–9

The term ‘exchange’ has been widely used, although strictly speaking it is only the hydrogen atoms that are exchanged or substituted, and ‘equilibration’ has much to commend it to describe what occurs. Plainly in the absence of any side reactions a position of equilibrium will be reached, the composition of which can be calculated at least approximately by the binomial theorem, knowing the numbers of hydrogen and deuterium atoms present. So, for example, when methane reacts with an excess of deuterium, the final product will contain mainly methane-d4 and -d3 (CD4 and CHD3). Exchange reactions are almost but not quite thermoneutral because of zero-point energy effects, and so equilibrium compositions are not exactly the statistical ones, although these are approached more closely with rising temperature. Kinetic isotope effects are also observed: thus the reaction of methane- d4 with hydrogen is slower than that of methane with deuterium, and the activation energy for n-hexane-d14 plus hydrogen on palladium film was some 4 kJ mol−1 greater than for n-hexane plus deuterium.8

A common feature of the reactions of alkanes with deuterium is the occurrence of two simultaneous but distinguishable processes: in one of these, termed single or stepwise exchange, only one atom is substituted during a single residence of the molecule on the surface, while in the other, termed multiple exchange, all or almost all are substituted. The rate of each can be measured by analysis of the products formed, and each can be assigned its characteristic kinetic parameters:2 multiple exchange usually shows the more negative order in deuterium and the higher activation energy. In the case of methane the stepwise mechanism predominates at low temperatures on many metals (Section 6.2.1). The separate character of the two processes is shown most clearly by (i) a minimum in the centre of the deuterated product distribution, and (ii) changes in product concentration with conversion when equal numbers of hydrogen and deuterium atoms are used: thus for example with methane + deuterium, where multiple exchange predominates, methane-d3

0 = CH4 ;

EXCHANGE OF ALKANES WITH DEUTERIUM

259

Figure 6.1. Formation of exchanged methanes over (A) rhodium powder at 429 K; (B) Rh25 Pd25 powder at 418 K: multiple exchange predominates in (A) and stepwise exchange in (B).23

1 = CH3 D; 2 = CH2 D2 ; 3 = CHD3 ; 4 = CD4 .

and -d4 pass through maxima (Figure 6.1a), while when stepwise exchange is the main process methane-d1 is initially formed in excess of its equilibrium proportion5 (Figure 6.1b). In a flow-reactor, the two processes are completely separated on rhodium film, no methane-d2 or -d3 being formed at low conversion;1 when natural convection is relied on, re-adsorption of initial products (CH3D, H2, HD, CD4) before they can diffuse away from the surface can obscure the initial product mix. Methane-d1 and -d4 are also the sole products formed on (111) and (100) orientated nickel films at 520–540 K.8,10

The total process is therefore characterised by (i) the rate of removal of the light (reactant) alkane, (ii) the separate rates of stepwise and multiple exchange (but only for methane, (iii) the total rate of introduction of deuterium atoms into the molecule, and (iv) the mean number M of such atoms initially entering a molecule.

260

CHAPTER 6

Assigning orders of reaction and activation energises to the formation of individual products is not especially helpful.11 A first-order kinetic equation for a reversible process is often observed;12 mechanisms proposed for the types of exchange will be considered in due course.

A question that must receive attention is the way in which alkanes chemisorb and desorb in the presence of hydrogen or deuterium; their atoms will cover at least part of the surface, and a possible mechanism is therefore1

CH4 + D ↔ CH3 + HD

(6.A)

The forward process also requires a number of ‘free’ sites because of the methane molecule’s size, and these constitute the ‘landing site’ or ‘free potential site’ mentioned in Sections 4.8 and 5.4. The quantitative development of this concept1,13 is however rendered difficult by uncertainty as whether the sites for the atoms are in fact the same as those for the methyl radicals.

A further problem that has received much attention is the mechanism by which cycloalkanes undergo exchange. With cyclopentane there is an energy barrier to transferring the exchange process from one side of the ring to the other, so that the -d5 molecule is dominant at low temperatures and the -d10 only at high temperatures. The barrier is smaller with cyclohexane and cycloheptane, and absent from larger rings. Several mechanisms for this switch have been suggested (Section 6.3) and a number of molecules having quite complex cyclic structures have been devised or selected, and their exchange characteristics studied, with the intention of discriminating between the possibilities. There have also been a number of studies using bimetallic catalysts, especially with methane, to see whether there is an ensemble size effect on the relative rates of single and multiple exchange. Strangely enough there have been few studies of particle size effects on exchange reactions.

Activities of metals for alkane exchange increase with metal-metal bond strength (i.e. with sublimation energy), presumably because of stronger bonding of the radicals formed when the alkane dissociates, and their consequent higher coverages.8

Where substitution of deuterium atoms into a molecule occurs in parallel with their hydrogenation (e.g. alkenes (Chapter 7), aromatics (Chapter 10)) the matter is discussed in the relevant chapter.

6.2.EQUILIBRATION OF LINEAR AND BRANCHED ALKANES WITH DEUTERIUM14

6.2.1. Methane1,2,8,15,16

Most studies of the methane-deuterium reaction have been performed in static systems, which do not clearly reveal changes of activity with time: in a flow-system, however, films of metals in Groups 5 and 6, and nickel, increase in activity with

EXCHANGE OF ALKANES WITH DEUTERIUM

261

time-on-stream, due it is thought to the slow build-up of reaction intermediates.1 A flow-system also enables ready measurement of the effects of varying conversion, and hence the use of very low conversions at which the true initial product distribution is revealed. Frennet1 has argued convincingly that this is hard to find in a static system, where sufficient hydrogen is released by chemisorption of methane to confuse the picture, even at conversions below 1%. He believed certainly with rhodium, and probably with all the metals studied,8 that only methane-d1 and -d4 were formed initially, and that the appearance of the others resulted from an incomplete multiple exchange caused by the unwanted presence of some hydrogen atoms. Nevertheless the procedures used for separating the kinetics of stepwise and multiple exchange provide conclusions that are probably not much in error.

The principal features of the kinetics are easily summarised.2,4 (i) Orders in methane are similar for stepwise and multiple exchange, ranging from +0.1 to 1.0 depending on the metal and the coverage of the surface with hydrocarbon radicals. (ii) Orders in deuterium are more negative for multiple exchange (−0.6 to −1.4) than for stepwise exchange (−0.1 to −0.9); multiple exchange is therefore suppressed by raising deuterium pressure. (iii) Multiple exchange always has the higher activation energy, so that its importance increases with rising temperature.1,2,17,18 (iv) Stepwise exchange predominates on tantalum, molybdenum, tungsten1 and (most notably) palladium,2,19,20 but multiple exchange is substantial on nickel,10 ruthenium, rhodium, iridium and platinum.1,2,17,18 (v) There are no major differences in activity between the metals when the same forms are compared; thus, of the metals examined by Kemball, tungsten was most active, and palladium least, but very considerable differences in specific activity have been shown by a given metal in various physical forms (powders, blacks, films: nothing on single crystals) (Table 6.1). Powders of palladium and of platinum showed higher rates than films (rates calculated at 423 K using measured Arrhenius parameters) and films sintered at 773 K were especially low in activity.19 Powders and blacks have highly defective surfaces, so it is difficult to avoid the conclusion that the reaction is structure-sensitive, although its degree depends upon the temperature selected for the comparison, since activation energies differ enormously (Table 6.1). As the footnote to this table states, rates were measured with greatly different reactant pressures, and so structural effects cannot be clearly separated from kinetic effects. The effect of particle size has not been widely studied, but with Pt/Al2O3 the TOF for stepwise exchange decreased with size above 1.5 nm.21

Compensation plots (Section 5.6) represent a convenient way of comparing activities measured under a variety of conditions where Arrhenius parameters are available. A suitable starting point is the work of McKee on ruthenium-palladium bimetallic powders22 (Figure 6.2A); data points for stepwise and multiple exchange, and for methane removal, lie mainly close to a single line, the range of apparent activation energies being from about 40 to190 kJ mol−1. Points along the line have almost the same rates at 423 K, differing from one end of the line to the other only about two-fold; small differences from the line do however result in

262 CHAPTER 6

TABLE 6.1. Rates of Methane Exchange with Deuterium at 423 K (r 423 ) and Apparent Arrhenius Parameters (E , ln A) for Metals in Various Physical Forms

Metal

Form

D2 /CH4

E /kJ mol−1

ln A

ln r 423

References

Ru

Black

10

97

62.9

35.32

17

 

Powder

0.5

113

67.94

35.81

22

 

/SiO2

8

85

60.69

36.62

26

Rh

Black

10

94

63.77

37.05

17

 

Powder

0.5

46

50.4

37.22

5, 23

 

Film

1

40.49

2

 

Film

1

36.64

1

 

/TiO2

10

97

63.08

35.50

25

 

/SiO2

10

103

62.80

33.51

23

Pd

Black

10

111

63.56

32.00

20

 

Powder

0.5

90.4

63.35

37.65

12

 

Powder

0.5

119

70.01

36.17

22

 

Powder

0.5

131

70.9

33.54

5, 23

 

Film

1

98

62.64

34.69

2

 

Thick Film

7

157

72.4

27.76

19

Pt

Black

10

104.5

66.2

36.4

18

 

Powder

0.5

86

63.6

39.21

23

 

Film

1

36.64

2

 

Thick Film

7

113

56.9

24.77

19

Ir

Black

10

111

68.19

36.63

20

 

 

 

 

 

 

 

Notes:

1.The rates are those for methane removal and have been measured at various reactant pressures, so that they and the apparent Arrhenius parameters on which they are based have no absolute significance.

2.A and r in molecules m−2 s1 .

3.Activation energies are generally rounded to two or three significant figures, but rate may have been calculated from more accurate values.

significant rate changes. Figure 6.2B has results for rhodium-platinum-palladium bimetallic powders,23 and 6.2C shows values for palladium-gold powders:12,24 values for single metal films2,19 and blacks17 are contained in 6.2D. There are few values for supported metals.25,26 Lines of identical slope are used in all parts of this Figure. The positions of the points for palladium are highlighted; they are sometimes (but not always) well below the line, signifying low activity: it may be that the possibility of poisoning by dissolved deuterium atoms has not always been thought of; this might also determine this metal’s reluctance to catalyse multiple exchange.

The chief purpose of studies with bimetallic systems, either supported11,27–29 or unsupported, has been to establish the size of the ensemble or ‘landing site’ necessary for the reactions and to see whether there was variation in the multiplicity of the exchange. We must therefore look at effects of composition on rate that are not visible in the compensation plots. Addition of a Group 11 metal to a Group 10 metal11,12,28,30 has led generally to a progressive loss of activity, but in the palladium-gold system12,30 there was sometimes a small increase in rate at low gold concentrations, as is seen on other reactions catalysed by this combination.

EXCHANGE OF ALKANES WITH DEUTERIUM

263

Figure 6.2. Compensation plots of Arrhenius parameters for methane exchange with deuterium: O stepwise exchange; multiple exchange; total exchange based on reactant removal (see Table 6.1). (A) Ru-Pd powders;22 (B) Rh-Pd powders;23 (C) Pd-Au powders;12 (D) films (open, ref. 2; shaded, ref. 1) and blacks (half-filled) of single metals. Identical lines are used in each part: Ti = 461 K; points for palladium are filled.

Unfortunately much of this work was done before surface analysis became commonplace, so interpretation of activity changes in terms of ensemble size has not been possible. Use of bimetallics containing metals drawn from Groups 8 to 10 does not contribute much of interest: in most cases a continuous range of solid solutions is formed (Pd-Pt;19,23 Pd-Rh;23 Pt-Rh;5 Pt-Ir;29 Pd-Ni31), and the low

264

CHAPTER 6

activity of palladium has been manifested in systems containing it, as rates rose rapidly as the amount of the more active partner increased, before a constant rate was obtained. The likely surface composition could thereby be inferred. A distinct activity maximum was seen in the platinum-rhodium system5 at about Pt60Rh40, the cause of which is not clear. The ruthenium-palladium system22 is complicated by their limited mutual solubility; compositions from pure palladium to Ru21Pd79 were fcc, while those containing 40 and 62% ruthenium had mixed fcc and cph phases, each with the maximum amount of the metal of opposite structure. There was a marked maximum in rate at 473 K and in M at Ru40Pd60, but again the reason is not evident: this composition showed very low activation energies, not fully compensated by the ln A terms. It is worth stressing however that all the rate variations are accommodated by motion along the compensation line, with only minor divergences: the sole exception is with high concentrations of gold in palladium12 (Figure 6.2C). This observation will contribute usefully to the discussion of reaction mechanisms, to which we must now turn.

This is a matter of some importance, as the release of adsorbed alkyl radicals to the gas phase by reaction with hydrogen is the final step in both hydrogenation and hydrogenolysis, and of course alkane adsorption initiates the latter process. A discussion of mechanism is complicated by the existence of two quite radically different proposals, which may be outlined as follows. Kemball2 proposed that methane is first dissociatively adsorbed as

CH4 + 2 → CH3 + H

(6.B)

and, with the hydrogen atom being quickly replaced by a deuterium atom by one of the mechanisms set down in Chapter 3, the reverse reaction afforded methane-d1. Then on certain sites or under certain conditions the methyl radicals becomes methylene,

CH3 + 2 → CH2 + H

(6.C)

and so reiteration of this process and its reverse, the hydrogen atom each time being changed to deuterium, leads to methyl-d3 and hence in all likelihood to methane-d4. If the process does not proceed to completion, the other intermediate methanes would be formed. Further dehydrogenation to methylidyne (HC ) to speed the process has been sometimes suggested.32 This mechanism provides a logical explanation for the more negative order in deuterium for multiple exchange, as more free sites are needed, and the higher apparent activation energy possibly follows from the creation of more free sites as temperature rises, thus facilitating the further exchange.

A different mechanism suggested by Frennet1,6,13,15,33,34 merits serious consideration, as it is based on extensive studies of the chemisorption both of methane

EXCHANGE OF ALKANES WITH DEUTERIUM

265

and of hydrogen, and of the exchange kinetics, performed mainly in rhodium films. Attention is focused first on multiple exchange, because (as noted above) he believes with good evidence that methane-d1 and -d4 are the only proper initial products. Methane chemisorption is formulated as

CH4 + H + Z → CH3 + H2 + Z

(6.D)

(see Section 4.8), where Z is the number of uncovered metal atoms constituting the ‘landing site’ (Z 7), although their role after adsorption is accomplished is not made clear: furthermore, the films being polycrystalline, the exact geometry of this ensemble cannot be determined. All sites are assumed to be energetically equivalent, and no distinction is drawn between those occupied by hydrogen atoms and those on which methane adsorbs. Sites represented by the asterisk seem to be equated to metal atoms (i.e. atop sites), notwithstanding the preference of hydrogen atoms for threeor four-fold sites (Section 3.2). The rate of methane-d4 formation rm is given by

rm = c PD

(6.1)

where θc is the coverage by carbon-containing species (in fact methyl radicals) and PD is the deuterium pressure. Methane-d4 therefore results from the complete exchange of the hydrogens in a methyl radical, presumably by repetition of a process such as

CH3 + 2 → CH2 + H

(6.E)

CH2 + D → CH2D + 2

(6.F)

The methyl radical is finally released to the gas phase as

 

CD3 + D2 → CD4 + D

(6.G)

which is an Eley-Rideal step conforming to the kinetic equation. The strongly negative dependence of rate upon deuterium pressure2,4 is explained by the need for a large landing site for the methane molecules. Frennet noted1 that methane-d4 formation occurred best on those metals from which hydrocarbon radicals are most easily displaced by hydrogen.

Methane-d1 was thought to be formed by a totally different process, which involves only a fleeting interaction of methane with the surface, viz.

CH4 + D → [CH4D] → CH3D + H

(6.H)

266

CHAPTER 6

Evidence for the formation of CH5+ (e.g. in a mass-spectrometer) was adduced in support; methane-d1 is formed chiefly on metals on which it is difficult to displace hydrocarbon species with hydrogen.

Although clearly based on sound experimental evidence, and an unusually thorough independent assessment of surface coverages by reactants, this mechanistic scheme leaves several questions unanswered. (1) The rate of methane-d1 formation should depend on θD and not on a negative power of deuterium pressure, unless the adsorption of the methane has a smaller landing site requirement than for multiple exchange. (2) Rates are not correlated with any physical properties of the metals, nor are the underlying characteristics of methane chemisorption, and the apparent structure sensitivity of the process is not discussed. (3) The conclusions are based on studies of considerable depth on only a small number of metals, and palladium, noted for its limited ability for multiple exchange, was not examined. (4) Changes in rate and reaction character with bimetallic systems have not been analysed. (5) Finally, for what it is worth, Arrhenius parameters for methane-d1 formation and for multiple exchange both lie generally close to the same compensation line (Figure 6.1), which can be attributed to there being common features in the mechanisms of the two processes. Thus although the classical mechanism originally proposed by Kemball,2,4 and since adopted by others, does not fully harmonise with Frennet’s data, there remain a number of issues which neither scheme wholly resolves.

While there has been no study of methane exchange on any single crystal surface because of insuperable practical difficulties, the reaction of methyl radicals formed by thermal decomposition of methyl iodide with co-adsorbed deuterium has been followed on Pt (111) using TPD and RAIRS:32,35 this technique which has been widely used to observe the reactions of adsorbed alkyl radicals,36 avoids the difficult dissociative chemisorption of the alkyl radical. In TPD, all possible methanes from -d1 to -d4 vacated the surface below 300 K, and perdeuterated methyl and methylene radicals were observed by RAIRS; interconversion of methylene and methylidyne was suggested to account for the multiply exchanged products. The importance of this works lies in the recognition that the same set of sites can accommodate both single and multiple exchange, thus answering one of the questions posed by both the conflicting theories discussed above.

Lest it be thought that these results and their interpretation are archaic and of little current interest, we must note that the catalytic activation of methane and its consequent use to make hydrogen, oligomers, or oxygenated products such as methanol, has been and continues to be a field of great practical interest, and studies of methane exchange throw light on the molecule’s adsorption and decomposition. On 5%Ni/SiO2 the kinetic isotope effect mentioned previously22 has been confirmed,37 as has the formation of methane-d4 as the sole product when deuterium is in large excess; the self-exchange process (see also Section 6.4)

CH4 + CD4 → CH3D + CHD3

(6.I)