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

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Figure 7.9. Semi-logarithmic plots of deuteroethane and deuteropropane distributions obtained on various platinum catalysts under the following conditions.

	Alkane	Support	Wt.% Pt	PD / PE	T/K	% Conv.	σ	References
A	C2 H4	SiO2	1	1.1	273	45	0.49	52
B	C2 H4	SiO2	0.04	2	273	Low	0.58	28
C	C2 H4	Al2 O3	1	1.67	255	39	0.45	103
D	C2 H4	Al2 O3	1	125	273	100	0.21	103
E	C3 H6	Pumice	5	0.03	291	100	0.46	54
F	C3 H6	Pumice	5	1.23	291	100	0.61	54
G	C3 H6	Pumice	5	42.2	291	100	0.43	54

Scheme 7.1. Parameters describing the reaction of ethane with deuterium (after Kemball).104

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Figure 7.10. Histograms of deuteroethene and -ethane distributions obtained under the following conditions,125 with best-fit calculated values by Kemball’s method104 (see Table 7.4 and text) shown as ◦.

	Metal	Support	PD / PE	T/K	% conv.	P	q	r	s
(i)	Pd	Al2 O3	1	237	5	75	33	90	20
(ii)	Pt	Al2 O3	1	298	10	98	33	99	50

returned to the gas phase, and quantifies all positional isomers of the deuteroethanes where these exist. It may also give the ratio of C2X4 /C2X5 , or some quantity proportional to it. Experimental results were matched to calculated ones by trial-and- error, aided by intuition and experience. A few examples are shown in Figure 7.10; agreement is often good but never perfect, as the selected parameter values are not the optima, because minimisation routines of the type now commonly used were not then available. The method104 does however serve to identify which parameter is most in play when some alteration to experimental conditions is made, and for this reason is more informative than the logarithmic approach described above. A weakness of this analysis, however, is its tendency to underestimate amounts of the more heavily deuterated products; this is partially overcome by a modification105 that allows the C––X bond-breaking parameter r to have a much larger value for X = H than X = D. The complexity of the analysis rises quickly as the number of carbon atoms in the molecule increases. Thus there are fifty-six distinguishable propyl radicals and twenty-four propenes, and although the solution of the eighty simultaneous equations would now present no difficulty one should logically include allylic species as well; however, with the two additional parameters involved, unique matches between theory and experiment would almost certainly become impossible.

We are now in a position to consider how the reactant ratio and temperature affect product distributions. In the simplest manifestation of the Horiuti-Polanyi scheme,8 deuterium is adsorbed dissociatively, and ethene non-dissociatively: their

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interaction will lead in the steady state to surface coverages by ethene, ethyl and deuterium or hydrogen atoms that will vary with reactant ratio in the manner shown in Figure 7.4. The curves drawn relate to some specific but arbitrary value for the equilibrium constant of the alkene-alkyl-H,D atom system, and this will depend upon temperature; the value of the reactant ratio that provides a chosen surface composition is likely to depend on the metal, support and other features of the catalyst. The effect of temperature is harder to predict as it depends on the energetics of all the participating processes; concentrations of adsorbed species will of course tend to decrease with increasing temperature.

When alkene is in excess, the reaction in a constant volume system stops when the deuterium is used up, and the deuterium: alkene ratio decreases continuously as the reaction proceeds. Unreacted but partially exchanged alkene remains at the end, and the deuterium number of the alkanes M is less than two, the alkane-do becoming a major product (see Figure 7.9 for the propene-deuterium reaction on Pt/pumice54). Contrarily when deuterium is in excess, the final alkane deuterium number is greater than two, the alkene-d0 falls to near zero, and the alkane-d2 becomes the major product (see also Figure 7.9). Similar but less complete results were seen with propene103 and ethene28,52,103 on a variety of supported platinum catalysts.

These trends are readily understandable in terms of Figure 7.4 and Scheme 7.1. The rate of alkene exchange will be proportional to the concentration of adsorbed alkene, and will therefore increase as the ratio PA/ PD (A = alkene) increases; M decreases because deuterium atoms are entering the alkene rather than going to make alkane. As PD/ PA increases, hydrogen exchange becomes more important because its rate depends on θx2 or θxθethyl, and alkene exchange diminishes: the alkane distribution sharpens, alkane-d2 becomes the major product, and M increases to a limiting value of about 2.5 (Figure 7.11). The value of σ decreases (compare C and D, and F and G, in Figure 7.9) because addition to alkyl is favoured over alkyl reversal to alkene, there being plenty of adsorbed atoms available, but few empty sites (or ethene molecules) to act as atom acceptors. Higher values of p and lower values or r are found at high PD/ PA. Lower values of σ are also found at low PD/ PA ratios, because here alkene desorption removes deuterium atoms from the surface and progressive substitution in alkyl radicals is inhibited (compare E and F in Figure 7.9). Under these conditions, equilibration of propene-do and -d6 proceeds in parallel with alkane formation.19

On nickel catalysts with equimolar reactant mixtures, ethene exchange increases in importance with rising temperature, being undetected at 195 K and 223 K, but very significant at 403 K;106 however, with a ten-fold excess of deuterium it is absent up to 323 K.19 On platinum catalysts the effects are comparatively minor:28,52,103 in general the yield of alkane-d2 and values of M decrease with rising temperature (Figure 7.12), this signifying that here too alkene exchange is becoming relatively more important. These effects may be in part because alkyl

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Figure 7.11. Dependence of propane-d0 (O)and -d2 ( ) yields, and of M, on ratios of reactant pressures (T = 298 K).54

reversal becomes easier as more free sites are created by thermal desorption of the reactants; this is confirmed by the tendency of r to increase and p to decrease.6

Although it is commonly thought28,48 that alkene hydrogenation is structureinsensitive, as we have seen (Section 7.2.2), this is not quite true: moreover the belief is based on the rate of hydrogenation, but there is ample evidence to show that rates of 52,102alkene exchange depend on the form of the catalyst (in the case of platinum), and this inevitably affects the shape of the deuteroalkane profile. It is therefore incorrect to think that results obtained with one particular platinum

Figure 7.12. Dependence of ethane-d2 yield and of M on temperature O 1% Pt/SiO2 , PD / PE 1.1520.04% Pt/SiO2 , PD / PE = 228

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catalyst epitomise all such catalysts. An early study52 of the behaviour of this metal unsupported and on a number of supports showed significant differences in the relative rate of alkene exchange: it was quite high on platinum foil and on Pt/Al2O3, and low on Pt/SiO2, results which have subsequently been confirmed by other workers28,44,48,102 (see Figure 7.9). Particle size is unlikely to be the course of the differences, as in one case52 the catalysts were made by depositing a platinum sol (size 2 nm) onto the various supports; also sintering of 6.3% Pt/SiO2 (EUROPT-1) made little change to the product distribution.102

The variability of the rate of alkene exchange relative to that of hydrogenation to ethane is also clearly seen with results for Pt/TiO2 and Pt/MgO, where the exchange rate is more prominent than for Pt/Al2O3 and Pt/SiO2.102 Application of Kemball’s modified scheme showed105 this to be due to enhanced chances of ethane desorption (respectively 40 and 20% compared to 10%) rather than to greater alkyl reversal. The Pt/TiO2 may have been partially in the SMSI condition, and similar effects have been shown elsewhere by magnesia, although the cause is unlikely to be the same (Section 3.3.4). This study102 emphasises the importance of the theoretical analysis of the component steps105 in identifying the effects of variables; from the similarity between the values of the s and q parameters it appeared that both ethene and ethyl species drew on the same source of hydrogen and deuterium atoms.

The discussion thus far has been mainly about platinum, but extensive results are also available for nickel wire,54,101,107 for supported ruthenium, rhodium, palladium, osmium and iridium,2,103,108,109 with a few for nickel, iron and tungsten films,104 and for Au/SiO2.110 Each metal is unique in the values it shows for the relative rates of the component processes, and their dependence on operating variables: their behaviour is most economically described by reference to the four parameters p, q, r and s that give approximate fits to the experimental results; a selection of these is given in Table 7.5. Because these parameters are defined as ratios (see Scheme 7.1), differences in their values over-emphasise changes in relative rates; they are therefore re-defined as

i = i /(1 + i )

(7.1)

(i = p, q, r, s). Inspection of this table at once shows that platinum and iridium distinguish themselves by showing very high values of p and r (all over 90%), this accounting for their small propensity for alkene exchange; for all the other metals they are substantially lower, indicating that this process is much more favoured. This is particularly so with Ru/Al2O3 and Os/Al2O3. Thus for example with palladium, with both ethene and propene, the extensive alkene exchange required the mean number of deuterium atoms in the alkene (M ) to be often less than unity, and alkane-d0 the major product (Figure 7.10); hydrogen exchange was minimal.2,109 Alkene and hydrogen exchange was also very marked in the reaction

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TABLE 7.5. Values of Parameters Describing the Ethene-Deuterium Reaction

Metal	Form	PD / PE	T/K	p	q	r	s	Notes	References
Ni	Film	—	173	75	67	92	50	a	104
Ru	/Al2 O3	0.54	326	17	90	71	80	b	6
Rh	/Al2 O3	1	255	75	50	80	80		6
Rh	/Al2 O3	1	349	37	67	90	80		6
Pd	/Al2 O3	1	340	67	33	90	10		6
Os	/Al2 O3	0.56	317	44	90	50	80		6
Ir	/Al2 O3	1	257	99	33	90	90		103
Pt	/Al2 O3	1	327	98	33	95	33	c	103

The parameters i give the percentage of reaction in the i direction (Scheme 7.1) : values are selected to give approximate fits to results obtained under the conditions given in columns 3 and 4.

a An iron film under these conditions is fitted by the same parameters.

b Re/SiO2 gives products similar to those of ruthenium and osmium although its activity is very low. c These parameters also fit the product description given by Au/SiO2 at 459 K.

of propene on Rh/Al2O3 at 363 K, so that lightly deuterated propanes were major products, but these processes were somewhat less evident with ethene, especially at low temperature (255 K); the main effect of temperature in this case was on the ethene desorption parameter p (Table 7.4).111 Gold showed results similar to those for platinum, but of course rates were much lower;110 tungsten film at 193 K afforded mainly ethane-d2 with little ethene exchange.104

We must now consider the sources of the atoms that are added to adsorbed alkenes and alkyls. While the simplest form of the Horiuti-Polanyi scheme would suggest that only deuterium atoms should be available, it is quite clear that hydrogen atoms can also be used; this is at once apparent from the appearance of alkane-d0 and -d1, and is confirmed by the values of the parameters q and s (Table 7.4). Thus for example with all the quoted metals (except Ru and Os) q is only in the range 33–67%, and where q and s take similar values there may be a common process of atom addition;102 but with rhodium and iridium s is much greater than q (80–90%, against 33–67%), suggesting that a molecule of deuterium may be involved in alkane formation. The most obvious sources of hydrogen atoms are adsorbed atoms released in alkyl reversal, and hydrogen-rich alkyl radicals; the two are almost equivalent and are kinetically indistinguishable.

There is evidence with nickel112 and platinum52 that molecular addition is unimportant: virtually the same alkane profile is given by non-equilibrated and equilibrated mixtures of hydrogen and deuterium.2 Unfortunately the measurements with platinum were made with propene where ‘direct addition’ as shown by semi-logarithmic plots is absent (it appears with ethene but not propene, Figure 7.9). In addition to the evidence provided by the high values of q and s for ruthenium and osmium, which incidentally overcome the extensive alkene exchange to make ethane-d2 the major product,6 the orders in deuterium are often somewhat

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above unity (Table 7.2), which is hard to explain without some contribution from molecular addition.

Iwasawa and his associates have made a number of studies of the changes to metal particles brought about by high temperature reduction, using supports that have been claimed to induce the Strong Metal-Support Interaction (SMSI, see Section 3.3.5). Some of these have employed XPS48 or XANES50 to characterise the effects, and the ethene-deuterium and ethene-hydrogen + deuterium reactions have been used as tests. With Rh/Nb2O5 and Ir/Nb2O5, activation energies for these reactions between 210 and 321 K were not affected by reduction temperature, but values were low (16–23 kJ mol−1) and exceedingly low for Ir/Nb2O5 after HTR.113 Both catalysts after LTR gave ethane-d2 as the main product of the ethenedeuterium reaction, but little or no more fully deuterated products were found, so that values of M were very low: this implies the substantial occurrence of alkene exchange (a fact not remarked on), but unfortunately there was no analysis of the ethenes to confirm this. After HTR, both catalysts showed peculiar distributions in which ethane-d0 and -d2 were major products, but ethane-d1 was smaller; again no product contained more than two deuterium atoms, and hydrogen deuteride formation was small. These results were interpreted by a model involving metal sites, peripheral sites influenced by the support, and sites on the support itself.

In a further study, catalysts comprising platinum on yttria, zirconia, titania, niobia and vanadium sesquioxide were examined similarly:50 changes in the rates of addition and of equilibration caused by HTR, and activation energies for the former, are listed in Table 7.6. The addition rate at 233 K was unaffected by HTR in the case of Pt/Y2O3 and actually raised with Pt/ZrO2: with both Pt/V2O3 and Pt/TiO2 the rates as expected were severely depressed. Deuteroethane profiles were not much affected by HTR, except in the case of Pt/V2O3, where deactivation gave products containing 86% ethene-d2. In all other cases, values of M were well below two, and ethanes-d5 and -d6 were never seen, but analysis of the ethenes receives only a passing mention: conversions at which the analyses were made are also not

TABLE 7.6. Effect of High-Temperature Reduction on the Rate of Ethane Hydrogenation and on the Rate of the Simultaneous H2 + D2 Reaction over Various Platinum Catalysts.

			Hydrogenation	Equilibration
Support		r	E /kJmol−1	r
Y2 O3	0		40	0.22
ZrO2	2.2		119	0.86
V2 O3	0.02		—	0.10
TiO2	0.04		37	0.09

r is the factor by which rate changes on altering reduction temperature from low to high: this has little effect on E .

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mentioned. Although the PD/ PE ratio was changed 50-fold, its effect on product distributions was not stated. Changes in the electronic structure of the metal brought about by HTR, and then by the presence of the reactions, were revealed by XANES measurements, which assessed the density of unoccupied 5d states. This was generally decreased by HTR, except with V2O3, which has metallic character, so that easy exchange of electrons between metal and support is possible; this change occurred even with yttria and zirconia, which do not exhibit the SMSI. The sense of the electron shift between metal and ethene was also support-dependent. The multiplesite model was again used to explain the deuteroethane distributions: it is otherwise very hard to account for the highly unusual distributions shown by platinum in the SMSI state; it is possible for one site to show massive ethene exchange, so that ethane-d0 > −d1 > −d2, and for another to give only direct addition, but identification of the sites responsible is speculative. The high activation energies shown by Pt/ZrO2 await explanation. Effects on ethene hydrogenation of adding niobia, vanadia, molybdena and manganese oxide to Rh/SiO2 have also been discussed.114

Addition of sodium ion to Pt/SiO2 increased the rate of ethene deuteration but not that of the simultaneous hydrogen-deuterium equilibration;115 activation energy of the former increased from 33 to 40 kJ mol−1 and the Pt 4 f1/2 binding energy increased. This implies the acquisition of negative charge, although it is hard to see how sodium cations can provide electrons. Deuteroethane profiles on the sodium containing catalysts showed M values well over two, showing that hydrogen exchange must have been important: there were sharp cut-offs after ethene-d4. The multiple site model was again invoked.

Lest it be thought that the Horiuti-Polanyi scheme, or some modification of it, provides all the answers to the problems of alkene hydrogenation and exchange, it must be said that there is considerable evidence that the latter process may involve a quite different mechanism, involving the dissociative chemisorption of the alkene,16 as for example

C2H4 + 2 → C2H3 + H
C2H3 + H + D2 → C2H3D + HD + 2	(7.F)

Rapid exchange of ethene and propene on iron and nickel films appears to proceed in this way.36 Very detailed studies by Japanese scientists using microwave spectroscopy have identified the structure of propene-d1 formed in reaction of propene with deuterium over metals of Groups 10 and 11, either supported on silica2,23,36 or as powders.82 Interpretation of the results is somewhat difficult because although addition and exchange show very similar kinetics, and are therefore thought to have the same intermediates, the locations of the deuterium atom in the propene-d1 are not entirely as expected by the alkyl reversal mechanism. Except on palladium and platinum, the major initial product was propene-2-d1:1,116 this could arise if

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addition of the first deuterium atom gave chiefly the n-propyl radical. On platinum however E -propene-1-d1, was initially preferred, while on palladium exchange occurred at all possible positions. Further intra-molecular processes suffered by the primary products made for additional complications in the interpretation.23,82 On Au/SiO2 and Ag/SiO2,23 exchange took place faster than addition. Over nickel, palladium and rhodium powders, intermolecular exchange of propene-3-d1 occurred in the absence of hydrogen or deuterium; π -propenyl species were first formed, and the liberated atoms formed propyls from which the exchanged propenes were formed.1,82 If these processes were also to occur during the normal course of alkene hydrogenation, the methodology devised by Kemball would be seriously compromised.

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-d1;117 it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals82 (33– 43 kJ mol−1) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-d1 clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2, as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover.82

Exchange of alkenes with tritium (as hydrogen tritide, HT) is more sensitive bur less informative than exchange with deuterium.118

7.2.5. Reactions on Single Crystal Surfaces119

It is convenient to collect information on the interactions of alkenes with hydrogen or deuterium on single crystal surfaces into a separate sub-section because the application of methods unsuited to supported metals and powders isolates and identifies species and elementary steps, the existence of which can only be surmised by conventional kinetic studies: this makes an invaluable contribution to a final discussion of mechanism.

Although most of the work so far reported has used the Pt(111) surface,120−122 a recurring theme of all the more recent publications is the pivotal role played by the weakly-adsorbed π -alkene in hydrogenation. Sum-frequency generation123−128 (SFG) (Section 4.3) clearly shows this to be the case with both ethene62,120,127,129 and propene31 at room temperature, this form being more reactive than the more strongly held σ -form “by several orders of magnitude”. This had long been

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suspected, but positive confirmation had to await the development of SFG. Theoretical work using DFT on the Pd(111) surface gas however concluded that only the di-σ form is able to react with hydrogen atoms.130 The surface coverage of the π -form under reaction conditions may be quite low ( 0.04; it cannot be detected by IRAS122), so that turnover frequencies based on the whole surface could be 25 times too small.62 A dynamic molecular beam study of the ethene-hydrogen reaction also implicates this form as the key intermediate, the rate of ethane formation being proportional to its coverage and that of hydrogen atoms.131

The ethene-deuterium reaction has been studied over Pt(111) between 300 and 370 K;44,46,121 ethane-d1 was the chief product and the mean deuterium number of the ethanes at 333 K was only 1.57 (Figure 7.8): this increased with time, so ethane exchange was obviously occurring, but as so often happens the deuteroethene composition was not reported. The kinetic parameters obtained are included in Table 7.2. An important feature of this work was the observation that after reaction the surface was coated by ‘carbonaceous deposits’ which later work identified as ethylidyne groups. The reaction did not however exhibit self-poisoning, a second reaction proceeding at the same rate as the first: it was therefore concluded that reactions went above the ethylidynes, which therefore acted as a bridge for transfer of hydrogen atoms from metal to π -ethene.43 This notion, previously suggested by Thomson and Webb to explain the apparent structure insensitivity of this reaction, has not however been applied quantitatively to account for either the differences between reactions on various metals as outlined in the last section, or the kinetic parameters. The idea that ethylidyne radicals were themselves intermediates in ethene hydrogenation has not been sustained by further investigations, and they are now relegated to a spectator status. However, the possibility that they may act as bridges in the transfer of hydrogen atoms to the reactant alkane has not been entirely eliminated. The reaction of ethene with a mixture of hydrogen + deuterium has been followed on Pt(100), (110) and (111) between 318 and 423 K; rate dependences on temperature and on ‘hydrogen’ pressure were determined, but no effects due to ‘carbon’ formation were seen or discussed.

The propene-deuterium reaction has been examined more recently on the same surface using TPD spectroscopy; the extensive results obtained are described in a lengthy paper.132 Above 230 K, all deuteropropenes and propanes were found, yields of the latter decreasing logarithmically with increasing number of deuterium atoms. An unexpectedly small amount of propane-d7 was explained by the reluctance of the hydrogen on the central carbon atom to exchange. The traditional mechanism was confirmed, and adsorption of propene as a vinylic or π -allylic species was definitely ruled out.

Finally, as concerns Pt(111), the reaction of co-adsorbed ethene-d4 and hydrogen has been studied using a combination of laser-induced thermal desorption, mass-spectrometry and RAIRS:133 exchange occurred above 215 K with an activation energy of 46 kJ mol−1, this being below the point at which conversion to