Metal-Catalysed Reactions of Hydrocarbons / 14-Reactions of Higher Alkanes with Hydrogen

Figure 14.12. Compensation effects for the Arrhenius parameters of neopentane hydrogenolysis on and (B) PdAu/SiO2 catalysts265 (series A, •; series B, ) concentrations

of the active metal (%) are shown.

With rhodium-copper films (10 and 50% copper), n-pentane isomerisation (but little cyclisation) occurred above 510 K,68 while with n-hexane DHC selectivity was greater than with pure rhodium.271 Formation of benzene occurred more readily when MCP was used at 540 K, the ring-enlargement mechanism being thought to differ from that for bond-shift isomerisation.271 With RhCu/SiO2 catalysts, 2,2,3,3-tetramethylbutane reacted at 493 K chiefly by the αγ mode until the Rh25Cu75 composition was reached, and in this range MCP reacted by a mainly selective mechanism.272 It was inferred that copper preferred low CN sites (as indeed it ought), and that no ligand effect was operative.

There is little to report on other systems involving a Group 11 metal. Films comprising iridium and gold gave higher aromatisation selectivities than iridium alone in the reaction of n-hexane above 600 K.76,273 Early studies of the iridium-copper224 and nickel-copper274,275systems have not been followed through.

It remains to consider bimetallic systems formed between metals of Group 8 to 10. While studies of such systems have often been uninformative, two have attracted particular attention: the platinum-ruthenium system, and the platinumiridium system, the latter because of its utility in petroleum reforming, where undesirable ‘carbon’ deposition is moderated by the iridium,276 which is more active for hydrogenolysis than platinum. This binary system is therefore less given to deactivation,218 but sulfidation is necessary to control iridium’s activity; this has been shown to reduce hydrogenolysis of n-hexane and to increase Si .277 Both PtIr/SiO2 and PtIr/A12O3 were used, but it appeared that the alumina contributed to isomerisation by an acid-catalysed process. The reactivity of other alkanes on unsulfided PtIr/A12O3 decreased progressively with iridium content,278 but the effect was much less with cyclopentane than with linear alkanes. The different activities of the two metals have been explained in the following way.276 Their bulk physical properties are quite similar, although iridium has much the higher melting temperature and is therefore less inclined to sinter:218 but in the very highly dispersed state, such considerations are irrelevant and metal atoms will behave more as free atoms. The separation of the 5d and 6 p levels increases from iridium to platinum,279 so promotion of a 5d electron to higher levels is easier with the former, which can thereby more easily accept electrons from the adsorbate into its d-shell. Multiply-bonded (e.g. αα-) species of the kind thought to be essential for hydrogenolysis are therefore the more easily formed. It must be remembered however that the ability of metal atoms to form a CM, that is, to bind an αα- diadsorbed species to a single metal atom has been questioned, although such bonds are frequently met in organometallic complexes.

The platinum-ruthenium combination has been of interest in fuel-cell technology, because ruthenium imparts some resistance to poisoning by carbon monoxide; as with platinum-iridium, it is important to see whether joining metals of very different activities in hydrogenolysis creates binary centres having new properties. The few available studies149,150,280,281 all confirmed ruthenium’s superior activity for hydrogenolysis (except for cyclopentane121) and its inability to do much else under normal circumstances. It does however induce other reactions characteristic of platinum at a temperature (493 K) well below that at which that metal would be active by itself.280 With Pt/Al2O3,X-ray absorption spectroscopy showed PtRu interaction,150 but in other work281 the surface composition depended on the support used. The clearest evidence for the operation of a binary centre was provided by the reaction of 2-methylpentane, where intermediate compositions gave maximal amounts of ethane and isobutane.149

Work on the remaining pairs of metals in Groups 8 to 10 can be dealt with swiftly. With PtCo/NaY, the importance of the cyclic mechanism for isomerisation of 2-methylpentane was enhanced at cobalt concentrations above 22%;92,282,283 PdCo/NaY was more active for isopentane isomerisation than Pd/NaY,284 but as we have already seen Co/NaY was itself very selective for skeletal isomerisation,92

unlike Co/SiO2.284 PtPd/SiO2 catalysts showed higher selectivity for n-pentane cyclisation than either metal alone and better values of Si with n-hexane; extensive results on effects of temperature and hydrogen pressure were reported.140 The platinum-rhodium system is another in which the second metal is much the more active,157,232,285 although rates and product selectivities for PtRh/SiO2 were intermediate and binary PtRh sites may have been active:285 consideration of the Arrhenius parameters for a range of alkanes and compositions suggest however that the mixtures partake more of the character of platinum than of rhodium.

14.5.6. The Role of Sulfur

Reference has already been made at several points to the important role that sulfur has to play in petroleum reforming operations; it is desirable to offer a brief summary of these effects, together with some further information (see also Further Reading section 6).

There have been a number of fundamental studies of the chemisorption of sulfur atoms on platinum singe-crystal surfaces;286 they are usually deposited from hydrogen sulfide or thiophene. The Pt––S bond is essentially covalent, but the sulfur atom (0.184 nm) is larger than the platinum atom (0.138 nm), and so its influence will extend over several adjacent metal atoms (see the discussion on butadiene hydrogenation, Section 8.3). Its interaction with small metal particles is more complex: covalent bond formation is weakened if the particle is small and thus somewhat electron-deficient through a metal-support interaction, although the precise electronic consequences may be more subtle (see Section 2.6 and the following section). Difference in electron density between plane and edge atoms, together with steric considerations, determine that sulfur will preferentially reside in trigonal or octahedral sites where it can form bonds to several metal atoms, leaving edge and corner sites clear unless the sulfur concentration is high. These sites will be the seat of activity in industrial operations using Pt/A12O3 with feedstocks containing up to 20 ppm of sulfur. The Ir––S bond is stronger than the Pt––S, which may be explained along the lines set out above (Section 14.55). The Pd––S bond appears to be weaker,287 as PdPt/A12O3 is more thiotolerant than Pt/Al2O3; palladium atoms should preferentially occupy low CN sites (the latent heat of sublimation is lower), and the integrity of the active area should be better preserved.

The modification of the Pt/A12O3 by sulfur is essential (and in a sense inevitable, as feedstocks cannot be made absolutely sulfur-free) if its admittedly limited tendency to hydrogenolysis is to be suppressed, and if therefore it can be made to perform its intended tasks. It has other consequences as well; for example the preferred product in the reaction of n-hexane becomes MCP rather than benzene,214,215 but sulfur present on platinum black as sulfate allows its dehydrogenation to hexenes.215 The importance of sulfur in removing the activity

PtIr/A12O3290 has already been noted; in the former case, in the sulfided state, the rhenium also encourages acid-catalysed reactions occurring on the support.289

Both sulfate and sulfide have been detected by XPS on sulfided Pt/Al2O3 and platinum black;291 sulfide ions selectively eliminated aromatisation and hydrogenolysis of n-hexane, while sulfate ions stopped all processes except dehydrogenation. At high sulfur loadings, where both forms were present, activity was reduced by 95%.

Sulfiding Pt/A12O3 altered the kinetics of hydrogenolysis of cyclopentane, increasing the order in the hydrocarbon from 0.1 to 0.6 and decreasing the hydrocarbon pressure at which the rate was maximal:290 thus paradoxically it strengthened hydrogen adsorption but weakened the hydrocarbon adsorption. It affected different reactions in various ways; thus covering Pt/A12O3 with 0.39 of a sulfur monolayer decreased the rate for cyclopentane by a factor of 25, while for ethane hydrogenolysis it was 280. Similar effects were obtained with PtRe/A12O3 and PtIr/A12O3.

Sulfur can also moderate the behaviour of ruthenium. Ru/TiO2 treated with thiophene gave a value of Si in the reaction of n-hexane of 66% at 753 K;78 untreated it was active for hydrogenolysis at 413 K, so only a small fraction of the original surface remained free. With Ru/SiO2-A12O3, sulfur emerging from the support by reduction of residual sulfate on produced similar effects.

14.5.7. Metal-Support Interactions

These are of two kinds: (i) those of moderate character, experienced by very small metal particles on ceramic (i.e. irreducible) oxides having various acid/base characters, and (ii) strong interactions (SMSI) where the support is partially reduced, or a reducible component has been added, and the metal is decorated by species of indeterminate type, stemming from the support. These effects have been outlined in Section 2.6; their consequences for the reactions of higher alkanes will now be considered. The last shall be first.

One might expect that the presence of reduced entities such as TiOx partially covering a metal particle would have the effect of decreasing the mean size of the active ensemble, and thus suppressing hydrogenolysis and facilitating other reactions. Results presented in Section 13.7 gave some support to this view, which has been confirmed by a study of the n-hexane reaction on Pt/A12O3 containing titania:292 cyclisation benefited from the loss of activity for hydrogenolysis, while isomerisation (which had a notably high activation energy) was little affected. A comparison of Pt/TiO2 and Pt/CeO2 with Pt/SiO2 for the n-hexane reaction at 613 K showed that after reduction at 773 K all three showed quite similar selectivities.293 The effects of raising the reduction temperature from 623 to 773 K were however different; for Pt /TiO2, Si was decreased and C5 and C6 cyclisation

products increased, while for Pt/CeO2 hydrogenolysis by suppressed and only MCP selectivity rose. A very high activity for Pt/CeO2 was recorded after 623 K reduction, but the activity loss on increasing this to 773 K was greatest for Pt/TiO2.

Unfortunately in this particular field one dare not hope to see any simple picture emerging. Reduction of Pt/TiO2 at the more moderate temperatures of 473 to 573 K gave294 catalysts having very small particles (0.8–1 nm) but H/Pt ratios of only 0.3 to 0.16. They were in a partial SMSI state, and showing exceptionally high hydrogenolysis selectivities for n-pentane and neohexane between these temperatures. MCP suffered some demethylation, the ring-opening reaction being about 50% selective; 2,2,3,3-tetramethylbutane also gave selective demethylation at 468 K, but the αδ mode increased with temperature. In a similar study,129 reduction at 573 K gave particles of 1.4 nm and H/Pt = 0.59, and the MCP reaction showed product selectivities and TOFs, similar to those given by Pt/SiO2 ( 65% selective), with no demethylation; this catalyst was only partially afflicted by the reduction. After 773 K reduction, however, all activity for MCP was removed, although some for n-hexane was kept. Partial restoration by oxidation gave catalysts of similar activity to the initial one, and the Arrhenius parameters for the component processes showed compensation (Figure 14.13). They fall on three lines, and suggest (perhaps for the first time) that the route to 2-methylpentane differs from that to the other isomers. Corresponding parameters for Pt/SiO2 were much more closely spaced, and may well have been within experimental error of each other for each process; their variation with Pt/TiO2 implies that variation of pre-treatment affected the reaction environment, but for Pt/SiO2 it did not. The mechanisms of isomerisation of the methylpentanes on Pt/TiO2 reduced at only 473 K have been analysed using 13C-labelled molecules:92 all showed

Figure 14.13. Compensation effects for the Arrhenius parameters of MCP hydrogenolysis on Pt/TiO2 variously pretreated (see text):129 , 2-methylpentane; , 3-methylpentane; , n-hexane; O, C1 -C5 alkanes.

predominance of the bond-shift mechanism, the importance of which decreased for 87 to 58% as metal loading increased from 0.2 to 10%. This was quite unlike the behaviour of Pt/Al2O3 (Section 14.3) where the cyclic mechanism was preferred on small particles.

All these results have been discussed very fully, without firm conclusions being reached. One possible line of thinking is to suppose that the lower temperature reduction (473 K) only affected the support, and that small particle size allowed an electronic influence that negated non-destructive reactions, and led to a preference for bond-shift over cyclic mechanisms for isomerisation. This may be the electron-transfer process

originally advocated by Horsley, although significant charge-transfer is unlikely and the effect may be more subtle (see the final paragraph). The consequences of this partial or incipient SMSI have been likened to those produced by basic additives and by tin, where similar influences may be at work. At higher reduction temperatures, it may be that small particles are entirely encapsulated, while residual activity is due to larger particles that are not much affected.

Reduction of 0.1% Ru/TiO2 at 758 K led to high values of Si in the n-hexane reaction at 633 K, reacting 94% when the precursor had been made by ion exchange; lower values were found with 0.5% Ru/TiO2, and very low values after reduction at only 433 K. Thus as with the effect of sulfur, even the character of the most active hydrogenolysis catalyst is susceptible to modification.

The behaviour of platinum (and other metals) has also been altered by incorporating vanadium295 and niobium296 additives, and using niobia297 as support. Pt/UO2 catalysts containing either 0.2 or 8% metal reduced to 473 K both had small particles (3 nm), notwithstanding the low area of the support (5 m2g- 1), but activities for the reactions of MCP and of 2-methylpentane were lower than for Pt/TiO2 and much lower than for Pt/A12O3298. There have also been strong indications that SMSI-like effects can be produced by magnesia,299 although it has been suggested that traces of sulfur from the support may be responsible.

Some years ago there were a number of reports that high-temperature reduction of metals supported not only on titania, but also on silica300 and alumina,301 resulted in a drastic loss of activity for alkane hydrogenolysis, as well as loss of hydrogen chemisorption capacity: there were other physical changes as well, and the same effects were shown by platinum black.302 Numerous explanations were offered, including SMSI, hydrogen spillover, and superficial alloy formation, but none of these apply to the unsupported metal. The effect does not seem to be related to that of reduction temperature on hydrogen at the interface or on particle shape (see the next paragraph). The most likely explanation is that atomic hydrogen

penetrates into sub-surface sites where it is quite stable and only removable by oxidation. New vibrational bands assigned to such species have been detected by inelastic neutron scattering spectroscopy.303 The probable role of this kind of hydrogen in hydrogenation of unsaturated hydrocarbons was considered in Section 14.2.6. These observations have not been incorporated into mainstream thinking on hydrocarbon transformations, and the lack of kinetic information and isotopic tracing are again keenly felt.

Milder but still significant metal-support interactions are observed with ceramic oxides, especially alumina; raising the reduction temperature of Pt/Al2O3 from 573 to 723 K was shown to remove a layer of hydrogen atoms from the interface, bringing metal and support into direct contact, and changing the particles’ morphology from hemisphere to pancake.304 This however made very little difference to the TOFs for neopentane or MCP hydrogenolysis; product selectivities for the latter were probably the same within error, but Si for neopentane increased from 45 to 68%. The use of AXAFS spectroscopy (Section 2.6) led305 to an explanation of the toxic effects of base (K+) on the activities of Pt/LTL for hydrogenolysis of neopentane and propane, and for Pt/LTL and Pt/SiO2 for the former, in terms of alteration to a ‘change in the energy position of the metal valence-orbitals’. It is however nothing short of tragic that such highly refined characterisation, which led to an intimate understanding of how metal-support interactions work, was applied to such a limited number of catalytic reactions and to such a small range of operational conditions. The results were confined to one table and ten lines of text. Specifically, it is impossible to correlate changes in the electronic arrangements in the metal atom to any fundamental parameter of the reaction, in the absence of kinetic information. Although this mode of operation of supports and modifiers has been hinted at in earlier sections, much remains to be done to exploit these novel techniques and concepts.

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