Metal-Catalysed Reactions of Hydrocarbons / 09-Hydrogenation of Alkynes

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Figure 9.5. Hydrogenation of ethyne over Al2 O3 -supported metals: effect of temperature on selectivity (PE = 50 Torr; PH = 200 Torr).27

10 (and copper) it is frequently the major process, but on the noble metals the oligomer yield is 10–30% (Table 9.1). The composition of the oligomers will be considered later (Section 9.3.3).

There has been much discussion in the literature as to whether there is only one type of active site on palladium catalysts, on which ethyne is very much more strongly adsorbed than ethene (a factor of 2200 has been derived from one study), or whether on the other hand there is also another type which is specific to ethene chemisorption and hydrogenation, which is poisoned by carbon monoxide, and which may possibly develop as the surface becomes partially coated with carbonaceous residues. This latter possibility is envisaged from work in which a large excess of ethene has been used in order to simulate industrial conditions, and which frequently employed labelled ethene or a double-labelling procedure (e.g. 14C2H4 + C2H2 + D2). This work will be discussed in the next Section; for the moment we may simply note that in static systems in the absence of added ethene the selectivity to ethene remains remarkably constant on palladium (and most other metals) until the ethyne has almost disappeared, so that under these conditions the second type of site either does not exist or is in very low concentration: it may of course arise or develop under the simulated industrial conditions. However the fact that in static systems the ethyne pressure at which acceleration begins is increased by raising the pressure of hydrogen or of added ethene, or by temperature,17 shows that competitive adsorption occurs, as is indeed shown by the negative orders in ethyne that are sometimes seen42 (Table 9.1). When the initial PH/PC ratio reaches 10, the reaction is non-selective throughout its course,17 an observation that has also occasioned discussion.17,47,48

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Most metal catalysts lose activity as they are used for ethyne hydrogenation (platinum may be an exception,16 and palladium sometimes); this is caused by the formation of strongly-adsorbed derived species, which could include ethylidyne, and which are usually called simply carbonaceous residues or deposits.22−24,26,42,49−51 They are not formed on supported metals from ethyne itself,50 but only during hydrogenation. It has also been shown by simultaneous X-ray diffraction that a carbide phase is also created in palladium,27,52 and by the same method the relevance of the dissolved hydrogen content has been established.53−55 The carbide phase PdC0.13 is not however capable of forming the hydride phase.56 The unique ability of palladium in selective hydrogenations has often been ascribed to its marked propensity to dissolve hydrogen;42,57,58 hydrogen dissolved in nickel is also reactive towards ethyne. It has been firmly established that (1) prolonged exposure to hydrogen poisons its activity for ethene hydrogenation, thus improving its selectivity,16 and (2) the α-phase (low H content) is more selective than the hydrogen-rich β-phase19 (see however reference 59). Thus the better selectivity shown by small particles26,60 (see also Section 9.3.1), the effect of including an inactive metal (Section 9.3.5), and the effects of reaction conditions, may all find their cause in the level of the dissolved hydrogen,55,61,62 which if high may create a high concentration of surface hydrogen atoms (or atoms in sites readily accessible to the surface), thus causing non-selective behaviour. We may recall that the low selectivity shown by iridium in butadiene hydrogenation (Table 8.6) was attributed to occluded hydrogen,44 which participated in the reaction: similar low selectivity is seen in ethyne hydrogenation26,44,45,63 (Table 9.1), and may have the same cause. Although the phenomenon is not the same as that shown by palladium, the effect on the availability of hydrogen at the surface is the same, and that without doubt determines alkene selectivity.

A kind of transmutation of elements has been achieved by electrochemicallypumping sodium64 or potassium65 into a thin platinum film from the electro-active support Aβ -alumina (A = Na or K). Selectivity for ethene rose from a very modest <20% to 90% in the case of potassium; although the rates of hydrogenation of both ethyne and ethene individually were suppressed by increasing alkali metal concentration on the platinum, the latter was the more affected, so selectivity increased. The effects were reversible. By reference to work on Pt(111), it was though that the alkali metal tended to convert strongly-held di-σ ethene to the weaker π form, while at the same time increasing the strength of the hydrogen chemisorption.65

Ag/SiO2 and Ag/TiO2 after activation by oxidation and reduction were active for ethyne hydrogenation at 353–443 K66 (Table 9.1); both showed 100% selectivity to ethene and no oligomer formation at the lower temperatures, but selectivity fell and more oligomers were made as temperature increased. Rates were slower than for butadiene; the activation energy on Ag/SiO2 was 39 kJ mol−1.

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Au/Al2O3 was active between 313 and 523 K, and also formed ethene with 100% selectivity;67 ethyne reacted 2000 times faster than ethene, and the rate was maximal for gold particles of 3 nm size (Table 9.1).

9.2.3. The Formation of Benzene from Ethyne

We have noted that the formation of oligomers usually occurs in parallel with the hydrogenation of ethyne; we shall return to this later, but under certain conditions the main or only product is benzene. This reaction has been intensively studied using single crystal surfaces of palladium,68,69 chiefly Pd(111),70−73 although benzene has been detected as a product of the interaction of ethyne with the surfaces of palladium foil74 and film, Pd/Al2O3 and Pd/C, as well as on nickel and copper single crystal surfaces, and Ni/SiO2.69 Since much of this work was carried out in static UHV reactors, it is convenient to mention it briefly at this point.

The reaction is clearly structure-sensitive because it occurs most extensively on Pd(111).68,69 This would have gladdened the heart of A.A. Balandin had he lived to learn of it, but although the hexagonal structure of this face is clearly relevant, the reaction proceeds through an adsorbed C4 species73 which is tilted with respect to the surface.70 On Pd(100) the process is inhibited by adsorbed hydrogen,43 although on foil it is accelerated. Electron donors assist it and electron acceptors inhibit it.75 Trimerisation of propyne only occurs on the Pd(111) surface.68 It is suggested that the surface is first covered with a layer of ethylidyne, on which the reaction actually proceeds,74 although the C4 species may be formed by reaction of ethyne with ethylidene.43,70 Further insights into the mechanism of trimerisation are obtained by the study of bimetallic systems in which the characteristic properties of palladium are modified by tin76 (PdSn/SiO2) or gold77 (Au/Pd(111) and Pd/Au(111)) or by deposition on a single crystal of another metal having a different crystal habit (Ru).78 With PdSn/SiO2, benzene selectivity was highest at low PH/PC ratios, n-hexane becoming the major product when this ratio exceeded two; no ethane was apparently observed.76 Deposition of gold onto Pd(111) greatly enhanced benzene formation, Pd6Au and Pd7 ensembles being responsible; depositing palladium onto Au(111) to give the (√3 × √3)R30◦ structure (equivalent to Pd2Au) was far more effective than unmodified Pd(111).77 Reactive desorption from Pd/Ru(001) revealed three states of the adsorbed precursor, corresponding to different modes of bonding.78

9.2.4. The Reaction of Ethyne with Deuterium27,79

As was the case with ethene and other alkenes, replacing hydrogen by deuterium and analysis of reaction products reveals hitherto hidden aspects of reaction mechanism. Early work with platinum40 and nickel2,80 indicated that neither

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TABLE 9.2. Initial Distributions of Deuteroethenes from the Reaction of Ethyne with Deuterium (PD /PC = 2)

Metal	Form	T/K	-d0	-d1	-d2	-d3	-d4	M	References
Nia	/Pumice	368	2	20	67	10	1	1.88	81
Pd	/Al2 O3	288	2	22	65	9	2	1.87	35
Pt	/Al2 O3	362	1	19	66	11	3	1.96	35
Rh	/Al2 O3	408	1	12	36	28	23	2.60	35
	/Al2 O3	393	2	13	38	27	20	2.50	35
Ru	/Al2 O3	443	6	7	30	36	21	2.59	41
Os	/Al2 O3	458	3	13	33	35	16	2.48	41

a The reaction on nickel was performed with C2 D2 + H2 (for convenience).

exchanged ethyne nor hydrogen deuteride was returned to the gas phase in significant amounts, and this has been confirmed at least for the metals of Groups 10 in later extensive studies. Nevertheless all possible deuterated ethenes were found35,41,81 (Table 9.2), so that redistribution reactions must have occurred within the adsorbed layer. Analysis of the reaction scheme (Scheme 9.2) proceeds as for ethene, but is simpler because no provision of ethyne desorption is needed: the important parameter is that for vinyl reversal (1- p), as this determines the breadth of the ethene distribution. The parameters s and q fix the chance of adding a deuterium atom to respectively an ethyne or a vinyl (or ethenyl) radical, their values and that of p being taken as independent of deuterium content. C––H and C––D bonds are assumed to be equally strong, so that probabilities of loss of these atoms is determined by simple statistics. Solution of the set of simultaneous equations for formation of the ethenes then allows optimum values of the parameters to be found.35,81 The ranges within which the observed values fall are shown in Table 9.3; although reaction conditions affect the precise values (see below), these ranges are sufficiently distinct to merit discussion. In most cases the values of s and q may be taken as equal, so this permits a further simplification.

Inspection of Table 9.3 shows that the values of s, q are all between 60 and 90%, suggesting (as noted above) that variation in the vinyl reversal term p is chiefly responsible for differences in the ethene distributions. This is indeed so; its values are high in Groups 8 and 9, accounting for the high deuterium numbers

Scheme 9.2. Analysis of deutero-ethenes formed from ethyne and deuterium.35,81 Species in square brackets are in the adsorbed state.

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TABLE 9.3. Reaction of Ethyne with Deuterium: Range of Values of s and (1- p)

Parameter	Ni	Pd	Pt	Ru	Ir	Ru	Os
s	80	79–86	85–92	75–88	78–88	70–90a	60–90a
(1- p)	40	20–38	21–37	79–89	60–81	85–96	85–96

a s ≥ q (for convenience).

M in the ethenes (Table 9.2), but much lower in Group 10, where M is much closer to two. These trends are similar to those of ethyl reversal found in the ethene-deuterium reaction (Section 7.2.4). The procedure allows the estimation of the composition of the alkynes (Table 9.4); the effect of the different extents of vinyl reversal is evident. This is also reflected in the extents to which hydrogen deuteride is returned to the gas phase, since when M exceeds two it must take place, there being little alkyne exchange. In Group 10 it is zero or very slight; in Group 9, moderate; and in Group 8, very marked.7 To some extent this may reflect the temperature range in which convenient activity is found; this varies in the sequence 10 > 9 > 8.

Similar trends in product distributions were found with each metal; increasing temperature led to less ethene-d2 and a broader distribution, and this was also the case when the ratio PH/PC was decreased.82 These two effects are linked, because the parameter p is lowered in each case, this being due to a decrease in the concentration of adsorbed hydrogen atoms and/or vinyl radicals.

Ethene-d2 exists in three forms, namely, the E, the Z and the asymmetric (a); these are distinguishable by infrared spectroscopy.80 The results obtained show that the reaction is more complex than simple Z-addition, as significant amounts of the E-isomer were always seen, and especially with the metals of Groups 8 and 9 the a-isomer was also clearly apparent (Figure 9.6). Evidently the process of vinyl reversal allows the formation of the E- and a-isomers; this is made apparent by the systematic decreases in their yields as the total amount of ethene-d2 increases, this being achieved by an increase in p (Figure 9.6). The points for the noble metals of Groups 8 to 10 lie about the same curves, while those for nickel lie slightly above them. The suggested mechanism by which the E- and a-isomers are formed is shown in Scheme 9.3; this would however predict equal amounts of the two, which is clearly not the case, if as is assumed the atoms are always added

TABLE 9.4. Reaction of Ethyne with Deuterium:

Calculated Surface Concentrations (%) of Ethynes

Metal	-d0	-d1	-d2
Pd, Pt	90	9	1
Rh, Ir	70	25	5
Ru, Os	40	40	20

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Figure 9.6. Yields of E-C2 H2 D2 and of a-C2 H2 D2 as a function of total yield of C2 H2 D2 on various Al2 O3 -supported metals.27

from below. The difficulty may be resolved if there is another way in which the E-isomer can be formed at the expense of the Z-isomer; for example, the vinyl radical may isomerise to a free-radical or a tri-σ form (Scheme 9.4) in which there is a planar carbon atom, to which a hydrogen or deuterium atom can add from either side, giving equal amounts of the two isomers. This free radical is also thought to initiate the process of oligomerisation by attacking other ethyne or vinyl species, or simply dimerising. Analysis of the butadiene formed by reaction of deuterium on Pd/Al2O3 showed the d2 molecule to be the major one (61%), the composition

Scheme 9.3. Routes to the formation of Z-, E- and a-ethene-d2 .

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Scheme 9.4. Isomerisation of the ethenyl radical into the free-radical form.

of the hydrogen + deuterium atom pool from which it arose being defined by a Burwell N parameter of 0.85, similar to that for the ethene.48 Butadiene therefore was made mainly by recombination of two C2HD species. Similar results were obtained with Pd/TiO2, but the N parameter was smaller ( 0.7). A C4 free-radical intermediate has been detected on palladium by a radical trapping technique,83 although it may not have been an adsorbed vinylic species. Scheme 9.3 suggests that if p were to become unity and ethene-d2 the only product, some 10–12% would be in the E -form, so that 20–34% of the vinyls would have passed through the free-radical form; this figure would be larger if some had entered the process of oligomerisation. Calculated relative concentrations of the ethynes on various metals are given in Table 9.4.

Ethyne-d2 was found to react with ethyne-d0 over nickel/pumice above 333 K, forming ethyne-d1 and nothing else;84 at 392 K equilibrium was reached in 2h, the equilibrium constant (3.4) being close to that calculated by statistical mechanics. The reaction followed a first-order course, the order in total pressure being 0.65 and the activation energy of 45 kJ mol−1. Reproducible rates were found with freshly reduced catalyst, but performing a hydrogenation experiment led to some subsequent loss of exchange activity. Ethyne-d2 exchanged only with the acidic hydrogen of propyne, with an order in total pressure of 0.47 and an activation energy of 53 kJ mol−1. It seemed unlikely that the ethynes dissociated on chemisorption, and an intermolecular mechanism was suggested. This interesting reaction also occurred at and below room temperature on Ni/kieselguhr,80 but it has received no further attention since its discovery in 1951; it did however take place on Pd(111) at 298 K.68

9.3.HYDROGENATION OF ETHYNE: 2, IN DYNAMIC SYSTEMS WITH ADDED ETHENE85

9.3.1. Kinetics and Selectivity

It is no easy task to summarise the extensive work that has been undertaken to understand how ethyne can be selectively removed by hydrogenation in the presence of a concentration of ethene that can by up to at least 400 times greater. Extensive studies have been performed in the laboratories of Guczi (Hungary) and Weiss (U.S.A.),42,57,82,86,87 of Borodzinski´ (Poland),8,9,49,52,88 of

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Duca (Italy)4,23,24,89,90 and of Gigola (Argentina):91,92 many others have made shorter but important contributions.2,10,15,22,38,93−99 For the most part, standard analytical methods have been used, but the singleand double-labelling techniques (using respectively either 13C-94 or 14C-labelled reactants,96 or one of these plus deuterium82,94) have provided vital information. Reactant concentrations have been chosen to imitate either front-end or tail-end conditions,4 although they have also been varied over wide ranges. Palladium has been employed as black,82 but chiefly supported, on alumina, silica8 and pumice.4 Industrial catalysts usually have α-Al2O3 as support, but pumice-supported catalysts show very good stability because their macroporous structure is not easily blocked by oligomers.4 The particle size of the palladium is by no means always measured, but it plays a determining role. The following paragraphs attempt to draw together results that bear in particular on (i) the effect of particle size on rates and selectivity, (ii) kinetic and isotopic-labelling results, leading to ideas on site multiplicity and identity, and (iii) the use of mathematical models to describe the system.

The form of the reactor has a bearing on the significance of the results obtained, especially in the measurement of selectivity at high ethyne conversion. The use of a gradientless reactor52 is strongly recommended, where the catalyst is in the form of a very thin bed, so that there is no concentration gradient through the bed.

Table 9.5 provides information on the effects of varying metal dispersion. Although the temperature in every case was close to 300 K, the supports differ,42 and only in two cases are the reactant concentrations about the same. It is therefore perhaps not surprising that the results are not in agreement: in three of the cases, the highest turnover frequencies were found at the lowest dispersions, although their values differ very considerably. This point is not discussed in the literature, and there is no obvious explanation. The selectivity expressed as the absence of ethane formation either improved as dispersion increased,57,60,91 or was independent of it.4,8,89 This small exercise illustrates vividly the problem so often encountered in heterogeneous catalysis, namely that of perceiving some general framework on which analysis of mechanisms and mathematical modelling can be performed. Each group has perforce to construct a model to fit its own results, and it is rare for any one group to pause to wonder why its findings do not agree with those of others. This is not a good way of achieving progress in science.

We may turn now to consider work in which the composition of the reaction mixture has been changed in order to identify the source of the non-selective product ethane. In the light of the foregoing paragraph, it is worth pointing out that most of these studies have used only one catalyst: fortunately, many have used industrially manufactured catalysts containing 0.04% Pd/α-Al2O3. The ICI 38-3 has been reported as having a dispersion of only 18%,91 which seems low for such a low loading; however, the surface area of the support is also small. The dispersion of the Polish C-31-1A was 28%.9 However we must first note work performed in static systems using catalysts of quite high metal loading (5%) and relatively low

TABLE 9.5. Particle-Size Sensitivity of Ethyne Hydrogenation in the Presence of Excess Ethene, and Estimations of Turnover-Frequencies, on Palladium Catalysts

Support	[Pd]/%	Varianta	D/%	[C2 H2 ]	[H2 ]	[C2 H4 ]	Ptot /k Pa	T/K	TOF/s−1	Sb	References
SiO2	2.6	Tcalc	29	0.3	0.5	39	101	353	0.11	| D	8
	3.1		5						0.70	| D
Pumice	0.05	Tred	65	0.3	0.55	97	101	300	2 × 10−3		4,89
	0.05		12						34 × 10−3	↑
α-A12 O3	0.06	Tred	58	0.84	16	72	1520	288	0.28		91
	0.04		5						7.3	↑
A12 O3	0.07	—	60	0.15	0.35	99.5	101	300	26 × 10−3		57
	8.7		6						8 × 10−3

a Procedure used to alter the dispersion : Tcalc = calcination temperature; Tred = temperature of reduction or pretreatment in hydrogen. b Selectivity to ethene or decrease in ethane formation : | D = independent of dispersion; ↑, increases as dispersion increases.

Inlet concentrations of reactants are in %; balance to 100% made up by inert gas (He, Ar, CH4 ).

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TABLE 9.6. Classification of Active Centres for Ethyne Hydrogenation in the Presence of Excess Ethene, and Their Functions

Type				Notes	References
A1	C2 H2	and C2 H4	adsorbed competitively	[A1 + A2 ] > [E]	8,9,49,88
A2	C2 H2	and C2 H4	adsorbed non-competitively

EC2 H2 , C2 H4 and H2 adsorbed competitively

I	C2 H2	→ C2 H4		15,51
II	C2 H2	→ C2 H6
III	C2 H4	→ C2 H6
X	C2 H2	and C2 H4 adsorbed competitively	[X] > [Y]	38,94

YC2 H4 → C2 H6

α-PdH	C2 H2	→ C2 H4	↑→ C2 H6	α-PdH on large particles 19,29,104
β-PdH	C2 H2	→ C2 H6	: C2 H4 −×→	β-PdH on small particles

ethene/ethyne ratios. Under these conditions with Pd/Al2O317 (and PdAg/Al2O316) progressive increase in the amount of ethene caused the acceleration in rate to start even earlier, suggesting that it competed with the ethyne for the available surface. Use of 14CCH4 showed15 that most of the ethane (85%) came from the ethyne, but the amount of labelled ethane increased with the 14CCH4 pressure, again indicating a contribution from a competitive mechanism. Thermodynamic selectivity was even greater with rhodium (92%) and iridium (97%). Surfaces were however substantially covered by carbonaceous overlayers, and the same amount of 14CCH4 was adsorbed in the presence and absence of ethyne. These observations pointed to the existence of three types of site15 (see Table 9.6), but independent evidence for them is lacking.

Experiments with 13CCH4 and an ethene/ethyne ratio of 49 carried out with an ICI catalyst showed38,94 that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200; Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2. When the pressure of ethyne was varied in the presence of excess ethene,9,88 its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation fell to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski´ and colleagues led8,9,49 to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (Es) that may play a role on certain supports.100 Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>CCH2),