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

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14

REACTIONS OF HIGHER ALKANES WITH HYDROGEN

PREFACE

This final major chapter brings us to a large and complex area of metalcatalysed reactions of hydrocarbons, which has been stimulated by the greatest industrial application of catalysis by metals, namely, the reforming of petroleum to produce fuels of higher quality and feedstocks for the petrochemical industry. It had long been known that treatment of petroleum fractions by acidic solids induced some of the desired changes, but it was not until the introduction in the 1950s of bifunctional catalysts having both metallic and acidic functions overcame the disastrously rapid deactivation caused by ‘carbon’ deposition, and even then the further development of bimetallic bifunctional catalysts was needed to increase catalyst life from months to years.

As a result of intensive academic studies, it slowly came to be appreciated that the metallic function alone was capable of effecting many of the transformations that bifunctional catalysts achieved, and for this the position of platinum has remained unchallenged—for reasons that remain somewhat obscure. This chapter endeavours to analyse and classify the enormous literature that has been generated, but apart from a short section that describes simply the way in which bifunctional catalysts operate, our concern will be purely with those reactions that the metal function can accomplish by itself. To extend this to cover those processes that solid acids can bring about with the aid of a metal would require another book.

591

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14.1.INTRODUCTION: PETROLEUM REFORMING AND REACTIONS OF HIGHER ALKANES WITH HYDROGEN

14.1.1. The Scope of This Chapter

The subject matter of this chapter has been kept to last because it calls to a greater or lesser extent on the material presented in all the earlier chapters, and also because the complexity of the reactions involved requires abbreviated treatment of concepts already introduced (especially in Chapters 5 and 13); without these prior discussions, the contents of this chapter would make even less sense. In the last chapter we were concerned only with hydrogenolysis, and with isomerisation that had very limited scope; extension to alkanes to those containing five to seven or eight carbon atoms increases dramatically the range of possible processes. In hydrogenolysis we shall have to consider selectivity in a much broader context; the relative reactions of various types of C––C bond vary quite widely, and simple treatments such as the Kempling-Anderson method found so useful in Chapter 13 are hard to develop and apply. The range of possible skeletal isomerisations, and the mechanisms that may underlie them, are also greatly extended, but even more importantly it is possible for cyclisation to occur by the formation of new C––C bonds; and this provides a route to the making of aromatic compounds by further dehydrogenation.

It will be interesting to see to what extent the observations and concepts relating to the lower alkanes also apply to larger alkanes. Points of direct comparison will include the relative activities of metals, and the kinetics (i.e. orders of reaction and Arrhenius parameters) of hydrogenolysis and skeletal isomerisation. The additional types of reaction made possible by the greater size vastly complicate any simple-minded contrast, and new methods for defining product selectivities will be needed. Finding an orderly and systematic method for presenting the available information within a limited number of pages has proved a daunting task: that adopted, after much thought and several false starts, is far from perfect, but better than some alternatives. It has been necessary to try to identify a number of themes that the various publications address; the obstacles to doing this, and the nature of those revealed, are the subject of Section 14.1.4.

14.1.2. Bifunctional Catalysis: Principles of Petroleum Reforming1–7

We must first differentiate metal-support interactions that undoubtedly occur when a metal particle is placed on a support that has distinctly either acidic or basic character from true bifunctional catalysis. Such interactions have been shown to introduce delicate but significant alteration to the organisation of the metal’s valence electrons, and these have important catalytic consequences: but reactions that are affected are still metal-catalysed, and should not be regarded as instances of

REACTIONS OF HIGHER ALKANES WITH HYDROGEN

593

bifunctional catalysis. Examples of these effects will be considered in Section 14.5.4. They may be contrasted to the enhancement of catalytic activity shown8 when a metal particle resides on a strongly acidic (‘super acid’) support such as sulfated zirconia; hydrocarbon intermediates are then thought to have ionic character.

The Earth provides abundant but not infinite supplies of hydrocarbons, both as ‘natural gas’ and as crude oil; unfortunately not all the major sources are located in the areas where demand is greatest, and this can create problems. The crude oil is first separated into a fraction that is volatile below about 670 K and a non-volatile ‘residual oil’ or ‘resid’; the volatile part is then further divided by fractional distillation into a number of fractions ranging from C1–C4 hydrocarbons to kerosene and light gas oil. These various fractions find a variety of applications and form the mainstay of present-day civilisation. ‘Natural gas’ (chiefly methane) and the light alkanes (light petroleum gas, LPG) are used domestically: other fractions are needed as fuels for vehicles (internal-combustion or diesel engine powered) and aircraft, and also for domestic heating. The part having highest molar mass range, arising from further treatment of ‘resid’, provides lubricating oils and greases. Less volatile fractions can be subjected to cracking, which lowers the mean molar mass, originally performed thermally, but later catalytically using clays, then synthetic aluminosilicates, and most recently zeolites.

All fractions except the lightest contain molecules having either oxygen or nitrogen or sulfur atoms, the concentration of which increases with the boiling point, and for many uses it is necessary to eliminate them or at least reduce their amounts by processes collectively known as hydrotreating.4,9 Of these perhaps the most important is hydrolesulfurisation (HDS), for which a Co-Mo/Al2O3 catalyst has long been used. The technology of petroleum refining and hydrotreating has been well described in a number of publications.3−5 All in all, little is wasted—although the disposal of the sulfur arising from HDS can be a worry.

While each application has its own set of criteria for acceptability, more or less severe, depending on the intended use, we will focus on what is desired for the efficient operation of the internal-combustion engine (ICE). The energy released in the combustion of the fuel in the engine cylinder depends upon its octane rating (OR), which is based on a comparison with mixtures of n-heptane (OR = 0) and isooctane (OR = 100). In brief, high octane ratings are given by aromatics, by branched alkanes and by alkenes, although the last are not favoured because they produce unwanted gum in the engine. Thus the processes originally desired were (i) dehydrogenation of cyclohexane and its derivatives (‘naphthenes’) (see Chapter 12), (ii) dehydrocyclisation of alkanes (‘paraffins’), and (iii) isomerisation of linear alkanes into branched alkanes. Hydrogenolysis (‘hydrocracking’) is not wanted. Now reactions of types (ii) and (iii) in particular proceed via carbocationic intermediates, which are formed on acidic solids, most

594

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easily by adding a proton to an alkene. The great advance that was made in the 1950s was the realisation that combining metallic and acidic functions into one catalyst speeded up alkane dehydrogenation, creating more alkene and hence faster isomerisation, as well as helping the formation of aromatics. So the bifunctional (or dual-function) catalyst was born. Further benefits were that ‘carbon’ deposit was slight, lifetime was considerable, and in situ regeneration could be performed. After early attempts to use nickel as the metal, platinum came to be adopted, and has remained a vital component, either alone or combined with rhenium or iridium (see Section 14.5), of petroleum reforming catalysts. The acidic component has been provided by amorphous silica-alumina, by chlorided alumina, and by zeolites.

The way in which the two components interact is shown schematically in Scheme 14.1A.7 It was thought at first that they ought to be close together,10 but it was later found that alkenes could move quite effectively through the gas phase from metal to acid and back, even at low concentration, because a physical mixture of, for example, Pt/SiO2 + SiO2-Al2O3, performed as well as Pt/SiO2-Al2O3.7 Scheme 14.1B shows the principal routes by which n-hexane is transformed into isohexanes and benzene, with methylcyclopentane as the key intermediate, on a bifunctional catalyst. As we shall see, most of these transformations can also be brought about by the metallic function alone, most notably by platinum. Early work using film11,12 and black,13,14 and particles on neutral supports, showed that the acidic function was not essential: however it continues to be used in industrial practice, and the contribution of the metal in bifunctional catalysis under industrial conditions (except for hydrogenation-dehydrogenation) is hard to assess.

Scheme 14.1A. Schematic representation of the mechanism of skeletal isomerisation on a bifunctional catalyst.

REACTIONS OF HIGHER ALKANES WITH HYDROGEN

595

Scheme 14.1B. Pathways for the reaction of n-hexane on a bifunctional reforming catalyst.

Aromatic molecules are now not so acceptable as components of ICE fuel, due to the carcinogenic behaviour of benzene, although they are still wanted by the petrochemical industry. Other additives including tetraethyl-lead and oxygenated molecules (alcohols, ethers) have been approved from time-to-time as anti-knock or octane-rating enhancers, but each has its disadvantages or environmental hazards as well, and none is totally free from objection. Palladium has sometimes been used with platinum to control ‘carbon’ formation.

Emphasis is now being placed on improving the quality of mid-distillate diesel fuel by lowering sulfur content and the concentration of multi-ring aromatic compounds; in this way the density is lessened, and the cetane number (which measures the concentration of alkanes) is increased.15 This is achieved by hydrogenating compounds such as naphthalene to the fully saturated analogue (i.e. decalin, see Chapter 10), and further converting it by hydrogenolysis to an alkylcyclohexane and thence to a branched C10 alkane (Section 14.2.5). The ring opening is desirably achieved selectively, i.e. without forming light alkanes, and this is easier if the C6 ring is first isomerised to an alkylcyclopentane. These are all metal-catalysed reactions, but some concomitant hydrocracking of larger alkanes to bring them within the necessary boiling range is also needed, so that the use of acidic supports for the metal is recommended.

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14.1.3. Reactions of the Higher Alkanes with Hydrogen

Three alkanes in particular have been chosen for fundamental studies: (i) n-hexane, (ii) methylcyclopentane, and (iii) neopentane (2,2-dimethylpropane). The first is a significant component of industrial feedstocks and the second a likely important intermediate (Scheme 14.3); both can undergo many different reactions. Metal-catalysed reactions of n-hexanes are depicted in Scheme 14.3; not all possible processes are shown, e.g. there are other routes to ‘carbon’. Methylcyclopentane, shown there as being formed by dehydrocyclisation (DHC) of n-hexane, is attractive as a reactant because of the variety of possible reactions, namely, (i) ring-opening, (ii) multiple hydrogenolysis (to lighter alkanes), (iii) demethanation, (iv) ring-expansion, (v) aromatisation, and (vi) dehydrogenation. The attraction of neopentane lies in its having only one type of C––C bond, and its difficult hydrogenolysis must give methane and isobutane as initial products; also it cannot undergo other reactions without first isomerising to 2-methylbutane. Relative rates in this rich panoply of reactions may be expected to depend on the structure and composition of the catalyst used, as well as on operating conditions, and so it has turned out. The greater the size of the alkane, the greater is the variety of possible reactions, and the greater becomes the difficulty of quantitative study and kinetic modelling. So for example n-heptane can give five dimethylcyclopentanes (including two pairs of Z -E isomers), while n-octane can lead to all three xylene isomers. Fortunately no new principles or kind of mechanism emerge from work on alkanes higher than C6, so we shall need to touch only lightly on these further complications.

Other alkanes have not however been neglected. n-Pentane has been less studied, perhaps because the more limited scope of its reactions, and, although the C5 ring is strain-free, n-pentane is less easily cyclised than n-hexane, and indeed n-heptane cyclises even more effectively16 (see Table 14.1). neoHexane (2,2-dimethylbutane)17 has three types of C––C bond, the reactivities of which differ, while 2,2,3,3-tetramethylbutane18 has only two types of C––C bond, the breaking of which gives either two molecules of isobutane or methane + trimethylbutane. These have been usefully employed to characterise catalysts, and the different reactivities of C––C bond in other branched alkanes have also been examined (Section 14.2.4).

To the study of this great family of reactions a wide variety of techniques has been brought. In addition to the use of UHV equipment for examining singlecrystal surfaces (which as noted before usually provides results that are somewhat scattered), simple flow or recirculatory reactors have generally been used, although the reactor type does sometimes affect the results, especially in the early stages of an experiment. The mechanisms of skeletal isomerisation have been illuminated by experiments designed with high intelligence and performed with consummate skill, using alkanes labelled with either 13C or 14C, the products being analysed by mass-spectrometry,19,20 radiochemical methods21 and magic-angle-spinning

REACTIONS OF HIGHER ALKANES WITH HYDROGEN

597

TABLE 14.1. Product Selectivities for the Reactions of n-Alkanes with Hydrogen over Platinum Catalysts16,52 (T = 603 K, PH ≈16 kPa)

Form

Alkane

TOF/s−1 × 103

S<n

Si

SC5

SC6

Sarom

Pt/SiO2

n-C5 H12

3.6

30

51

19

Pt black

 

60

18

22

Pt/SiO2

n-C6 H14

8.2

13

25

41.5

8

12.5

Pt black

 

35

11

9

45

Pt/SiO2

n-C7 H16

0.8

1

6

79.5

13.5

Pt black

 

59

8

18

14

(1)TOF is for reactant removal.

(2)S<n , selectivity to lower alkanes; Si , to skeletal isomers; SC5 , to C5 cyclic molecules; SC6 to C6 cyclic molecules; Saro m, to benzene or toluene.

(3)The Pt/SiO2 is EUROPT-1.

NMR (MASNMR)22 (Section 14.3). Of course, without gas-chromatography almost nothing could have been achieved.

14.1.4. The Scope and Limitations of the Literature

The difficulty of providing a short but informative account of the extensive literature has already been noted (Section 14.1.1); we must now see wherein the complications lie. (1) Almost every paper deals with two or more different reactants or catalysts, and for each it often explores the effects of either conversion, temperature, hydrogen pressure or some other variable: collecting information on any particular facet therefore requires a large number of papers to be scanned. (2) Graphical presentation of results is most common, but often at such a density and on such a scale that their significance is obscured. Tabular presentation is rare.23 (3) Except in a few cases,24−26 quantitative modelling is not attempted: each product is assumed to derive from an independent reaction with its own site demand and kinetic parameters, and the way in which its formation depends on the experimental variable is expressed either by TOFs or rates or by selectivities. (4) Experimental procedures are sometimes not well described, and it may be unclear whether the results pertain to a catalyst in its initial or stable (i.e. partially deactivated) state. It is useful to know, if the level of a variable has been changed, whether this has led to an irreversible change in the extent of ‘carbon’ deposit, or whether the catalyst has been reactivated before the next experiment.25 (5) Finally, a point that has been frequently made before, comparison of the ‘activities’ of a family of catalysts in some defined state is most usually based on rates obtained under a single set of operating conditions, and one is left to wonder whether using some other set would lead to the same conclusion. Nevertheless, in spite of all these difficulties, we have plenty of material to work with, indeed, one might say an embarras de richesse: or perhaps ‘enough is enough, and plenty is too much’.

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A recurring self-imposed task of many of the publications has been to identify the ‘sites’ responsible for each type of reaction, so that structure-sensitivity has been a dominant theme. What has received much less attention is the possibility that, for example, particle size might determine the strength of hydrogen chemisorption, so that the use of constant operating conditions on a series of catalysts might produce results mainly decided by the surface concentration of hydrogen atoms. The dependence of kinetic parameters on particle size or other catalyst feature has been rarely examined.

14.1.5. The Principal Themes

The following classification of the information available will be used. (1) The activities of metals for the relevant reactions, expressed quantitatively as far as possible: the identity of the metal is the focus, and the exact state of the catalyst (e.g. metal dispersion) may not be known. (Section 14.2.1). (2) The effect of conversion on rate and on product selectivities in representative cases: this points to the extent of ‘carbon’ deposition and its effect on selectivities, and to the difficulty of ascertaining the behaviour of the catalyst in its initial clean state (Section 14.2.2).

(3) The following three sections will concentrate on the effects of operating conditions (chiefly temperature and hydrogen pressure) on rates and selectivities for linear (Section 14.2.5) alkanes: effects of catalyst structure, and of chain-length and molecular complexity of the alkane, will be noted. (4) Studies directed mainly to the relevance of the state of the catalyst surface either as induced by the reaction itself or by the method of preparation (precursor, pre-treatment, calcination etc.) will be the subject of Section 14.2.6. We may note here that the great preponderance of publications describe work on platinum-containing catalysts. Although important work has been done with ruthenium and palladium, platinum is unique in its ability to catalyse transformations other than hydrogenolysis. So if the metal used is not specified every time, it is safe to assume it is platinum; if it is not, its identity will be specified.

In an effort to achieve a clear analysis of the literature, Section 14.3 addresses work directed to the understanding of the mechanisms of skeletal isomerisation (Section 14.3.2) and of dehydrocyclisation (Section 14.3.3), covering superlative work with the use of isotopic labels. Structure-sensitivities of the component reactions have also received much careful attention, and papers addressing this matter specifically are considered in Section 14.4. Finally there has been much work on the modification of platinum catalysts to minimise ‘carbon’ formation and hydrogenolysis: studies using rhenium and similar additives (Sections 14.5.3 and 14.5.4), bimetallic systems (Section 14.5.5), sulfur (Section 14.5.6), and supports liable to give the Strong Metal-Support Interaction (Section 14.5.7) are the subject of the final section. While a modicum of repetition, and anticipation of what is

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599

to come, will be inevitable, every effort will be made to make each section as self-contained as possible.

One further general comment may be in order. With a few notable exceptions, each of the main laboratories has concentrated on the use of one particular technique or operating variable to access the heart of the problem, which is to understand what determines how a catalyst attains its unique properties. Thus Hungarian scientists, ably led by Zolt´an Pa´al, have been emphasised, quite properly, the great importance of the amount of hydrogen on the surface, while the late Fran¸cois Gault and his colleagues at Strasbourg studied intently the mechanisms of reactions without employing extensive variations of operating parameters (except particle size). Some limitation of scope is of course inevitable, because life is short (tragically so in Gault’s case) and so are resources, but the conjunction of separate but related studies to construct a unified picture is thereby made less easy.

14.2.REACTIONS OF HIGHER ALKANES WITH HYDROGEN: RATES AND PRODUCT SELECTIVITIES

14.2.1. Activities of Pure Metals

There are few sets of results available for comparing the activities of metals for reactions of alkane greater than C4 by means of Arrhenius parameters based on specific rates or TOFs. Non-specific rates for hydrogenolysis of cyclopentane led27,28 to Arrhenius parameters, the compensation plot for which divides the metals examined into three groups, as follows:

Ru, Rh, Os, Ir > Co, Ni, Pt > Pd

Metals in each group showed similar activity at 455 K, but the difference between each group was about 104. Activation energies ranged from 54 (Ru) to 192 (Pd) kJ mol−1, and various supports and metal concentrations were used. Japanese workers29 have examined the reactions of n-pentane on silicaand carbonsupported of the metals of Groups 8 to 10 (except Os), and reported orders in alkane that were mainly close to unity, and orders in hydrogen between −1.3 and −1.6 at 533 to 673 K, using pressures notably higher than those normally used (up to 40 atm). Arrhenius parameters were also given, but unfortunately, while the text says they were based on rate constants, the tabulated values were stated in units appropriate to rate. Values of ln A were substantially higher than expected by comparisons with lower (and higher) alkanes, perhaps due to the higher pressures of n-pentane that were used. Nevertheless, iron and palladium emerged as the least active, while the activities of the remainder were generally similar.

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Figure 14.1. Compensation plot of Arrhenius parameters for the reactions of (i) neopentane30 and (ii) n-heptane32 with hydrogen on various supported metals. They are compared with selected values for ethane hydrogenolysis, the lines being those used to classify the activities of metals for that reaction in Chapter 13 (see Figures 13.3–13.8). Ethane hydrogenolysis31 ; n-heptane hydrogenolysis32 ; neopentane hydrogenolysis30 ; neopentane isomerisation30 .

There are two sets of results expressed in specific units that may be used to compare with Sinfelt’s extensive set for ethane hydrogenolysis.3 In Figure 14.1 selected points for this reaction are shown as a compensation plot, and they and the lines are those depicted in Figures 13.3 to 13.8. These are then used as a framework against which the Arrhenius parameters for neopentane30 and n-heptane31,32 hydrogenolysis can be compared. Those for neopentane, both for hydrogenolysis and isomerisation, agree well with those for ethane, although gold appears as having very low activity. In the n-heptane reaction,31,32 only palladium and platinum seem to conform; ruthenium, rhodium and iridium were all more active than expected, due in the two last cases to a lower activation energy.

Product distributions were recorded, at necessarily very different temperatures (Pd, 573 K; Ru, 361 K): the five metals (Os was not studied) all gave some isomerisation, but it was only significant with platinum. With palladium, bond breaking was almost exclusively terminal, and mainly so with rhodium, but with the other three metals it was largely statistical. This behaviour conforms to that found with the butanes (Chapter 13) and with other alkanes, as we shall see.

Reflection on some of these results leads to the conclusion that iridium and rhodium sometimes ally themselves with the most active group of metals (e.g. in