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

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415

Figure 9.7. Rates of ethyne removal and of ethane and C4 products formation as a function of PE over Pd/Al2 O3 at 343 K in the presence of excess ethene (outlet pressures, ethene = 40 kPa, hydrogen = 0.6 kPa).88

while Type E was larger and could adsorb both ethyne and ethene (competitively), as well as hydrogen (Table 9.6). The model has also been found to describe results obtained at 373–498 K with higher reactant concentrations but lower TOFs caused by extensive deactivation.101 Double-labelling work with 14CCH2 or 14CCH4 and deuterium confirmed that ethene was not hydrogenated above a critical pressure of ethyne.87 Relative rates of hydrogenation of the two hydrocarbons were however structure-sensitive, ethane formation from ethene being much more marked on large palladium particles.91

Marked oscillations have been observed102 in the production of hydrogen deuteride during the reaction of ethyne with a mixture of hydrogen + deuterium on Pd/Al2O3 in the presence of carbon monoxide at 363 K, the amplitude being greatest when its concentration was about 100 ppm; this signifies periodic change to surface coverage by the reactants. No other reaction parameters were however recorded.

9.3.2. Mechanisms and Modelling85

We now approach the unenviable task of divining what common elements are to be found in the diverse studies surveyed above; what significant features of the materials and procedures used affect the results obtained; what extensions to the simple scheme (Scheme 9.1) are necessary or desirable; and what reaction models have been used to give quantitative description of the results.

It must be said at once that no single mechanism or model can be sufficiently flexible to apply to all systems. Certain themes do however emerge regularly; these are (i) the probable occurrence of vinylidene (>CCH2) and ethylidyne

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(C––CH3) as alternatives to symmetrically-bonded species, either in the predominant route to ethene43 and ethane,88 or just in conditions of low selectivity;17,48 (ii) the probable operation of two or three separate types of site during ethyne hydrogenation with excess ethene (Table 9.6); (iii) the likely importance of carbonaceous deposits in determining selectivity or in creating sites at which selective reaction can occur;8,23,49,51,88,90,101 and (iv) face sensitivity.103 Other imponderables already noted include the possible formation of carbide and hydride phases in palladium. To add to the misery, we have seen that even the sense of the particle size effect on TOF and selectivity cannot be agreed (Table 9.6), and supports appear to exert an important but poorly understood influence.

As to (i) there appears to be little direct evidence, although the balance of argument favours some role for them;15,43,48,51 as to (ii), although they are sometimes2,49 (but not always4) employed in mechanistic schemes with apparent success, it is disappointing to see the lack of agreement on their functions (Table 9.6), so it must be concluded that this question is still sub judice. It would seem to be a case of Quot homines, tot sententiae. In two cases at least it was recognised that different sites and different modes of chemisorption constitute equivalent descriptors of events.8,49,94 A further possibility, first mentioned many years ago104 but never fully evaluated, is that there may exist a range of particle sizes, such that some features of the reaction may occur on small particles, and others on large particles. Thus the small-size fraction, being in the α-hydride phase, might catalyse the sequential process of ethyne to ethene to ethane, but with ethene being the major product, while the larger-size fraction in the β-hydride phase might chemisorb ethyne but not ethene, and convert it wholly to ethane (Table 9.6). Unfortunately the very low metal contents of the industrial catalysts makes detailed characterisation (e.g. by TEM) difficult.

Concerning (iii) we are on firmer ground, because there is direct evidence15,51 for the existence of ‘carbonaceous deposits’, which are probably adsorbed oligomers, although in the region of ambient temperature they may simply be a dehydrogenated or re-structured form of ethyne (e.g. HCC–– or C––CH3). At higher temperatures carbon ‘whiskers’ are seen growing away from the surface, and on Ni/NiAl2O4 they appear to take the form of nanotubes.105 In static reactors it is possible that they form quickly when ethyne is admitted first, and that they then affect the rate and the form of the pressure-time curve; but whether they or the hydrogen atoms they contain actually assist the continuous reaction remains uncertain. With respect to (iv), evidence for face sensitivity lies in the claim that palladium particles exposing only (100) and (110) facets are very selective for producing ethene and are not prone to ‘carbon’ deposition.103

Notwithstanding these difficulties, a number of detailed mechanistic schemes have been proposed and evaluated quantitively:2,4,10,49,98,101 they contain up to eleven unit steps, and provide varying degrees of harmony with experiment. Alternatively, rate expressions based purely on Langmuir-Hinshelwood formalism

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have been examined.99 A time-dependant Monte Carlo algorithm has also been used23,43,90 to study the effects of carbonaceous residues for frontand tail-end mixtures; the involvement of steric hindrance by surface species was said to be essential for successful simulation.

9.3.3. Oligomerisation

A besetting problem with the industrial process to remove traces of alkynes alkadienes from alkene streams using palladium catalysts has been the formation of higher hydrocarbons by oligomerisation.5 Although in this respect palladium is better than base metals such as nickel28 (which presumably explains why this cheaper metal is not used), and while the fraction of ethyne that reacts in this manner is small, nevertheless in a continuous operation these higher products accumulate, and cause problems. The carbonaceous deposits, so often mentioned, may be partly C2 species such as ethylidyne, but they also comprise adsorbed forms of oligomers: in the steady state their formation is followed by release into the fluid phase.

The literature mentions numerous analyses of their composition. On palladium, C4 products predominate; they contain mainly butenes (chiefly 1-butene) and butadiene:39 but C6 products including benzene are also found (Section 9.2.3), but molecules covering as wide range of molar masses have also been found.2,38 On Ni/pumice the range extends to greater than C31; even-number carbon atom product are favoured.36,39 It is not however clear what conditions allow the apparently exclusive formation of benzene,9,69,71,73,75 as discussed in Section 9.2.3. The yield decreases with increasing chain length, rather in the style of Fischer-Tropsch synthesis, suggesting a radical-chain mechanism, initiated by the free-radical form of the vinyl radical (see Scheme 9.1). The formation of oligomers and their strongly bonded precursors only occurs during hydrogenation and not when catalysts are exposed to ethyne alone; and only ethyne, and species derived from it, participate in making them. Thus hydrogenation of ethyne in the presence of excess propene gives C4 but not C5 products.38 Temperature is not an important variable, but oligomerisation is usually made slower by raising the hydrogen/ethyne ratio.2 As we have seen, the partial blocking of the surface by strongly held oligomers is held to be necessary in order to lower the mean size of the active centres, and thus to prevent further hydrogenation to ethane.

9.3.4. Gaseous Promoters

It has long been recognised that the continuous injection of low levels of carbon monoxide into flow reactors large or small has a beneficial effect on selectivity in the sense of inhibiting ethane formation when ethene is present in large excess.2,106 Levels of about 1500 ppm are sufficient to stop ethane being

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produced.94 It appears that it is chemisorbed on palladium with a strength intermediate between those of ethyne and ethene, preventing the latter but not noticeably interfering with the former;11 indeed ethyne can displace part of the chemisorbed carbon monoxide. It is likely to occupy sites too small to accept the hydrocarbons, and its main role may be to discourage hydrogen chemisorption;15 a lower concentration of hydrogen on (or in) the metal should also improve mechanistic selectivity. Carbon monoxide is a notorious poison for the diffusion of hydrogen through palladium devices intended to purify it. Continuous feeding is necessary because it appears as a component of the liquid oligomers, named green oil, the production of which it inhibits.94 Sulfur and phosphorus compounds are also effective both in improving selectivity and suppressing oligomerisation.107 Surface sulfur also improves the selectivity shown by Pt/kieselguhr catalyst.38

9.4.USE OF BIMETALLIC CATALYSTS FOR ETHYNE HYDROGENATION85,108–110

The extensive work that has been performed on ethyne hydrogenation using bimetallic catalysts based on palladium has had as its objective (i) the further improvement of the selectivity with which it can be hydrogenated in the presence of excess ethene, and (ii) lowering the production of ‘green oil’. Work of this type is not novel; Sheridan briefly examined NiAg/pumice,35 and some relevant patents were granted more than 50 years ago.28 It was natural to believe that diluting palladium with an inert metal might be beneficial. As we have seen (Section 5.5) there have been two schools of thought as to how such benefits might arise: (1) by creating isolated atoms (or small groups of atoms) of the active component, processes requiring large ensembles might be minimised, and (2) by some alteration in the electron concentration111 or the orbital energies of the active elements, a favourable moderation of adsorption energies might arise. Since many bimetallic catalysts show improvement over palladium in both the targets, it is possible that both causes are at work. Reduction in the size of the palladium ensembles might well be expected to inhibit oligomerisation, while weakening the adsorption of the ethene might well improve selectivity.112 Furthermore the second component will certainly affect the solubility of hydrogen and the range of stability of the hydride phases.

Industrial practice will clearly favour low metal loadings, and it is to be expected that it will be difficult to achieve conjunction of two (or more) components when their surface concentrations are small;5 nevertheless there are strong indications that they are successfully made and used, at least in some installations. Methods for preparing bimetallic catalysts have been reviewed in depth,111 and the merits of employing organometallic complexes containing two elements have been emphasised. The cost of large-scale manufacture by such a method may however

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be considerable, and bimetallic particles containing palladium and silver can be made at a total loading of 0.45% by sequential or simultaneous impregnation of precursor solutions.59 Adequate description of the method used is not however always provided.111

An early study using a static system showed16 that PdAg/α-Al2O3 catalysts containing 10–30% silver gave better mechanistic selectivity (98–100%) than Pd/α-Al2O3; orders in hydrogen (1.7–1.8) were raised, and the activation energy was greater (84 kJ mol−1). Added ethene however competed with the ethyne. No methods of surface analysis were available at that time, but surface enrichment by silver is to be expected, and is indeed found.111 Silver decreases hydrogen solubility, but raises the temperature at which the β-hydride phase decomposes.111 However, diffusivity is improved, and a palladium-silver alloy is used for purifying hydrogen by selective diffusion; tendency to failure through hydrogen embrittlement is also reduced (Section 3.1).

The palladium-silver system continues to attract attention.59,112−115 Highresolution TEM has been applied to PdAg/SiO2 catalysts, the selectivity of which (with excess ethene) depends critically on the type of pretreatment. A sequence of precursor calcination, reduction, re-oxidation and reduction at 773 K gave excellent selectivity and less oligomerisation; this was attributed to a surface reconstruction that lowered the mean size of the palladium ensembles.59 The addition of nitrous oxide to a palladium-silver catalyst operating under excess ethene gave a further improvement in selectivity,112 and metal aluminates have been claimed as supports for catalysts containing palladium plus silver or a base metal.113 The design of the support is important to minimise deactivation due to pore blocking by oligomers; a recent patent114 recommends the use of ‘a moulding of trilobal cross-section with holes through the lobes’ as a support for palladium-silver. Copper,45,112,116 gold112,117,118 and tin112 have also been explored as moderators of palladium catalysts.

Other additives or promoters have been advocated. The oxides of cerium, titanium and niobium added to Pd/SiO2 improved selectivity with excess ethene at high conversion of ethyne,119 and the sodium content of pumice or of synthetic supports also had a beneficial effect,120−123 due it was thought to some induced change in the electron concentration in the palladium weakening the chemisorption of ethene. Silicon deposited on Pd/SiO2 by CVD of silane, and then oxidised to silica, has the effect of increasing selectivity in the presence of excess ethene by

(i) inhibiting chain-growth in oligomerisation, (ii) weakening ethene adsorption and (iii) lowering the amount of hydrogen that can be adsorbed.124

The best-known metallic promoter is lead. This has long been known in the shape of the Lindlar catalyst,11,125,126 which is PdPb/CaCO3 together with a nitrogen base (quinoline). It has been widely used in organic synthesis in the liquid phase, where it has excellent selectivity for carbon triple bond reduction, giving Z-alkenes in high yields. The use of PdPb/α-Al2O3 also appears to have

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been helpful in ethyne hydrogenation,127 although there is no indication of its being much used in this connection. In the Lindlar catalyst the lead is not in solid solution, but rather it appears to decorate the surface of palladium particles,11,128 as might be expected from its method of preparation, which involves treating Pd/CaCO3 with lead acetate. This, with Gigola’s procedure which used PbnBu4 as the lead source, places this type of catalyst in the class of those developed by Figu´eras, Coq and their colleagues,129 who applied alkyls of Group 14 elements to various supported metals to create two-dimensional surface ‘alloys’ (Section 1.3.1; see also 9.4.5). It has been suggested130 that in the Lindlar catalyst the reacting hydrogen atoms carry a fractional negative charge, thus making the attack on the triple bond nucleophilic, whereas on normal palladium it is positively charged, and the process is electrophilic.

The selective performance of platinum is raised by the addition of rhenium,131 and admixing with copper or gold helps even that of iridium.128

Arrhenius parameters for ethyne hydrogenation on nickel-copper powders are shown as a compensation plot in Figure 9.8. While at 323 K rates decreased progressively as the copper content rose, those at 473 K were maximal at 40 and 60% copper, as indicated by their positions on the compensation plot. As discussed in Section 5.5, this result makes protracted discussion of the relevance of solidstate parameters to catalytic activity of doubtful value, if activity measurements are confined to a single temperature. Results are also available for the nickelcobalt system. Intermetallic compounds based on cobalt (CoGe, CoGe2, CoAl) were much more selective than cobalt itself.132

Figure 9.8. Compensation plot for ethyne hydrogenation on nickel-copper powders (A in units of rate of pressure fall m−2 ).32

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The poisoning of Pd/Al2O3 by mercury vapour is a complex phenomenon that has been fully investigated;33 activity for ethene hydrogenation was suppressed while that of ethyne remained, so that improved selectivity resulted. Other metals examined in less detail showed similar behaviour.

9.5. HYDROGENATION OF HIGHER ALKYNES

9.5.1. Propyne133–140

The hydrogenation of propyne has been much less fully investigated than that of ethyne, with which it shares broadly similar characteristics; orders in hydrogen are close to first, and in propyne zero or slightly negative or positive, tending to become more negative as temperature is raised.135,139 Activation energies lie in the range 30–70 kJ mol−1, being on average a little greater than comparable values for ethyne (Table 9.1). Addition of the methyl group has no profound consequences, and there is no suggestion that access of hydrogen to the surface is any easier than with ethyne. Alkene selectivities are just as high if not higher, but the striking difference is the much smaller amount of oligomerisation (Table 9.7). This must be due to the greater difficulty of forming carbon-carbon bonds, as the methyl group causes the adsorbed species not to have the necessary propinquity. Propene selectivities and oligomer yields generally decrease with increasing PH/ PP and decreasing temperature, being largely governed (as in the case of ethyne) by the concentration of adsorbed hydrogen atoms. This may be determined by the size of the active centre remaining after an almost complete layer of strongly adsorbed (perhaps partially dehydrogenated) reactant molecules has been formed; various palladium catalysts retained di-σ propene, σ π propyne and propylidyne on the surface after hydrogenation at 273 K.140 A scheme analogous to the second in

TABLE 9.7. Kinetic Parameters for the Hydrogenation of Propyne

Metal	Form	E /kJ mol−1	T/K	S3,ia	Sob	References
Fe	powder	34	452	98	—	136
Co	powder	31	318	83	—	136
Ni	powder	51	340	83	—	136
Ni	/pumice	59	364	86	13	137
Ni	powder	38	358	86	3	134
Cu	powder	89	446	100	47	134
Pd	/pumice	69	409	94	7	136
Pt	/pumice	72	348	95	6	136
Pt	/SiO2	—	333	100	—	139

In all cases the order in H2 was about 1, and in propyne either zero or slightly negative, becoming more negative with increasing temperature.

a Initial propene selectivity b Selectivity to oligomers

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Table 9.6 was proposed. Addition of copper to nickel in powder form improved selectivity, but oligomerisation also increased;134 it was also especially marked with cobalt.63 Simultaneous hydrogenation of ethyne and propyne shows that the former must be somewhat more strongly adsorbed on nickel and platinum; this is understandable, since the C––C single bond in propyne is shorter (146 pm) than that in ethane (154 pm) due to hyperconjugation with the triple bond, which is thereby weakened.28 The relief of strain upon adsorption is therefore less.

The reaction has been studied on copper catalysts:133,134,141 on Cu/SiO2 prepared by ion exchange, reaction with deuterium produces propyne-1-d1 and propenes containing deuterium only in the vinylic positions (CH3––CXCX2, where X may be H or D).133

9.5.2. The Butynes27,142

Extending the alkyl substituent from methyl to ethyl makes little difference to the characteristics of the hydrogenation; the only new feature introduced with 1-butyne is the possibility of Z- and E-2-butenes being formed in addition to the expected 1-butene; in fact this constitutes the sole or major product in most cases. On Pd/Al2O3,103,144 and Pd/BaSO4 in ethanol solution,27 it is 98%, and with other palladium catalysts7 (including a number of rare-earth intermetallics145) the 2-butenes were formed only in traces. The exception seems to be Cu/SiO2,146 where 1-butene was only 72% of the products. Total selectivity to butenes was very high on palladium7,143,144 and copper146 catalysts, but as expected was lower with platinum. On Pt/Al2O3 it was 85% (independent of dispersion, pressure and temperature147) or 90%148 or rising to 80% as deactivation proceeded with a threefold excess of hydrogen: a much larger ratio gave much lower selectivities.149 It would appear that the larger alkyl group does not create steric interference between adjacently adsorbed molecules, creating sites for non-competitive adsorption of hydrogen, although with these metals the extent of oligomerisation is slight. With palladium7,149 and rhodium147 catalysts, turnover frequencies increased with decreasing dispersion, but there is very little kinetic information to illuminate the cause. The selectivity shown by palladium and platinum has however been ascribed150 to the existence of a strongly adsorbed form of 1-butyne, which is in equilibrium with the reactive form, but blocks low coordination number sites, thus acting as a selective poison.

Once again the most informative technique for elaborating the mechanism is the use of deuterium as an isotopic tracer. On 0.03% Pd/Al2O3 the 1-butene was 72% 1-butene-1,2-d2 and no molecules contained more than three deuterium atoms;143 thus only the terminal hydrogen could have exchanged. A small amount of 1-butyne-1-d1 was observed, and this may have been the precursor to 1-butene-d3. In another study,149 1-butene-d2 was 70–80% of the product, the rest having been formed by a ‘different route’. The amounts of the 2-butenes formed

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Scheme 9.5A. Reaction of 1-butyne with deuterium.

have been too small for their isotopic composition to be determined, but it is thought they may be formed through isomerisation of adsorbed 1-butyne to 1,2-butadiene (see Scheme 9.5a).

The hydrogenation of 2-butyne presents an even simpler picture. On the base metals (Fe, Co, Ni, Cu)27,151,152 and palladium12,153 the total selectivity was unity, and on all other metals examined it was above 90%.27 Even with iridium with an unfavourable 10/1 hydrogen:butyne ratio it was 96%. Z -2-Butene was the exclusive product on palladium and copper,27,153 and on the former the reaction with deuterium gave 99% Z-2-butene-2,3-d2 at 287 K: the yield of other products however rose with temperature.

The cleanliness of the reactions of propyne, butynes and higher alkynes on certain metals, especially palladium and copper, and of ethyne on silver and

Scheme 9.5B. Reaction of 2-butyne with deuterium. Note: in Schemes 9.5A and 9.5B, all species are shown as only bonded by σ bonds for simplicity; butenyl radicals can be written in free radical forms, and final products assume X = D.

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TABLE 9.8. Product Distributions from the Hydrogenation of 2-Butyne over Alumina-Supported Metals, and Certain Activation Energies

Metal		T /K	Z -2-B	E -2-B	1-B	Stot /%	E /kJmol−1	References
Fe		473	76	4	20	700	—	27
Ru		363	79	5	16	97	42	155
Os		393	74	4	22	90	46	155
Co		413	88	7	5	100	28b	152
Rha		427	85	8	7	99	−	152
a
Ir		433	87	8	5	96	−b	152
Ni		425	95	5	1	100	38	152
Pd		298	100	0	0	100	−	152
a
Pt		431	87	8	5	97	− b	152
Cu		397	100	0	0	100	140	152
a	:2-butyne = 4 (Rh); 10(Ir); 7(Pt); :2 in all other cases
b H2
Unsupported powders

gold, requires some consideration. With the Group 11 metals, a low coverage by hydrogen atoms may discourage further reaction of the alkene, which doubtless desorbs readily once formed. In the case of palladium, the difference between ethyne and 2-butyne is quite marked; the former as we have seen can readily succumb to non-selective reaction when the conditions are right, and the apparent failure of 2-butyne to follow suit may be due to its inability to form the analogue of the αα-diadsorbed vinylidene. The expectation that the larger molecules would fit less well on the surface, and hence leave more gaps for the non-competitive adsorption of hydrogen, does not seem to be realised in practice. The possibility that molecular hydrogen participates in the rate-determining step finds little support, but the occurrence of hydrogen orders greater than unity still requires explanation.

The reaction on palladium-gold wires also gave results of great simplicity.123 The activation energy was constant within error across the series (0–94% Au), the rate being determined solely by the pre-exponential factor, which fell smoothly with increasing gold content (Figure 9.9A). The manner of the activity change suggested (reasonably) that an ensemble of four palladium atoms made the active centre. This assumed the absence of surface segregation or island formation. The marked decrease in rate as the gold content was raised necessitated the use of progressively higher temperature ranges, so that it appears that product distributions were more reflecting temperature than surface composition. Below about 400 K the rise in the small amounts of E-2-butene and 1-butene correlated better with gold content than with temperature, but thereafter they correlated well with temperature (Figure 9.9B). Unfortunately the dependence of product yields on temperature for each catalyst was not reported, so the two variables cannot be reliably separated.

A supported palladium-lead intermetallic compound, represented as Pd3Pb/CaCO3, was better than the Lindlar catalyst for the semi-hydrogenation