Metal-Catalysed Reactions of Hydrocarbons / 12-Dehydrogenation of Alkanes

12

DEHYDROGENATION OF ALKANES

PREFACE

We have now concluded our survey of the processes in which hydrogen is added to the various types of carbon-carbon unsaturation, the final products of which are alkanes of some type or other. We must now look at the reverse processes, since in many cases the unsaturated products that can be derived by removing hydrogen from an alkane are more useful and therefore more valuable than the reactant; and hydrogen if it can be recovered is also a desirable product. Dehydrogenation is however usually endothermic and needs high temperatures to secure reasonable yields of products; the exceptions are cyclohexane and related molecules, which convert to aromatic molecules at much lower temperatures, due to the pull of the resonance stabilisation. A further attractive process that has been much studied is the conversion of methane by dehydrogenation to higher alkanes.

The process of dehydrogenation does not enjoy the plethora of mechanistic subtleties that have engaged our attention in earlier chapters. The principal concern has been to discover catalysts that are stable at the necessarily high temperatures, and that are selective towards dehydrogenation, rather than the competing processes of hydrogenolysis and of carbon deposition that bedevil attempts to isolate the target reaction.

12.1. INTRODUCTION

The dehydrogenation of acyclic alkanes is at one and the same time both simpler and more complex than any of the reactions so far considered: it is simpler in the sense that the mechanism for the formation of alkenes involves only adsorbed alkyl radicals, alkenes and hydrogen atoms, and with rare exceptions lacks any of the subtlety so common with hydrogenations; but it is more complex

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in the sense that it is markedly endothermic (to the extent that the corresponding hydrogenations are exothermic), and therefore requires high temperatures to achieve significant progress towards equilibrium. At such temperatures, parasitic reactions involving excessive dehydrogenation, and formation of carbonaceous deposits, lead to rapid loss of activity, and so the large-scale realisation of the economically attractive production of alkenes from alkanes demands the design of catalysts able to withstand this type of insult. Much of what follows in this Chapter concerns the ways in which this is done.

The success of the petrochemical industry has depended in no small measure on the conversation of C2 to C5 alkenes to more useful and valuable products, by processes that include polymerisation, oligomerisation, selective oxidation and epoxidation:1 these feedstocks were formerly available as by-products of refinery operations such as naphtha cracking, but the supply did not match demand, and so alkane dehydrogenation became economically viable. The processes are however equilibrium-limited, that is, complete conversion cannot be attained at any realistic temperature; so for example the maximum conversions at 800 K and 1 atm pressure are for ethane, 10%; for propane, 28%; and for isobutane, 43%.2 Another way of expressing the problem is to note the temperatures needed for 50% conversion; these are respectively 983, 863 and 813 K. This limitation can of course be overcome by using oxidative dehydrogenation, where the hydrogen removed is converted to water; this has also led to extensive research to discover oxide catalysts that are selective for this process, but this work is beyond the scope of this book. Another way of defeating equilibrium is to combine dehydrogenation with continuous reaction of the alkene with methanol; this avoids a costly separation step, and produces an ether. Much attention has been given to the sequence of reactions: isobutane → isobutene → methyl-tert-butylether (MTBE), which has value as a fuel additive.2 The equilibrium limitation can also be overcome by continuously removing the hydrogen produced by diffusion through a palladium-based permselective membrane.2,3 With cyclic alkanes, however, the thermodynamics are (as we shall see, Section 12.3) very much more favourable, so that much lower temperatures will suffice.

The higher the temperature used, the more likely are the adsorbed hydrocarbon species to lose more than the two desired hydrogen atoms, and thus to form more than two C––M bonds. These species will polymerise, and form the carbonaceous deposits that have been termed4 the beteˆ noire of all hydrocarbon reactions, and which we have met continually in the preceding five chapters (see Section 12.5). Now one way of inhibiting this unwanted happening would be to add hydrogen to suppress the tendency to over-dehydrogenation, by providing a greater surface coverage by hydrogen atoms (Section 12.2.6). However the process of dehydrogenation results in an increase in pressure at constant volume, so by Le Chatelier’s Principle the equilibrium conversion attainable will fall as pressure is increased: theoretically 100% conversion could be obtained at zero pressure.

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The need to suppress carbon formation by adding hydrogen is thus in conflict with the desirability of operating at reduced pressure, but to do this entails additional power consumption and construction costs. Each industrial process resolves these conflicting factors in its own way.2

There is however one beneficial way of using the tendency of metal surfaces to dehydrogenate alkanes and to form polymeric structures. Methane is abundantly and cheaply available, and it has been found that on certain metals it will do just this, and by alteration of the reaction conditions the hydrogen released can sometimes crack the deposits into mainly C2 and C3 fragments that appear as ethane and propane. There will be some hydrogen left over, which is a bonus on top of the formation of the more useful alkanes. The considerable literature on this process is summarised in Section 12.3.

Most of the metallic catalysts used for alkane dehydrogenation are based on platinum, because of its low activity for hydrogenolysis, but by itself it cannot prevent its deactivation by carbon deposits, and so much thought has been given to its promotion, chiefly by combining it with one or more other inert metals. The thinking was that the steps leading to the carbon deposit required a larger ensemble of the active atoms than those that led just to alkene, so that diluting platinum with an inert partner would lead on average to smaller ensembles, and hence to better performance. As well as lessening carbon formation, the promoter ought to increase the rate of dissociative chemisorption of the alkane, improve the tolerance of the catalyst to carbon, and help to gasify the carbon that is formed. Progress in this direction is reviewed in Sections 12.2.2 and 12.2.3. Control of particle size might also be relevant in limiting carbon deposition, and this has also been examined (Section 12.23). Polymerisation of the alkene, once formed, by acid centres on the support is another possible route to carbon deposition, since larger alkanes (and alkenes) dehydrogenate more easily than smaller ones. Supports must therefore be non-acidic and the importance of correct choice is stressed in Section 12.2.2.

The process of converting an acyclic alkane into a cyclic one is also formally a dehydrogenation, and is termed dehydrocyclisation (DHC).5 At one time the superior octane rating of aromatics (see Section 14.1.1) encouraged research into this process, and it was established that n-hexane could cyclise into methylcyclopentane (MCP), which then underwent ring-enlargement to cyclohexane; dehydrogenation gave benzene. It is convenient to separate this last stage, which has been deeply researched, from the earlier steps, which occur alongside the multifarious reactions of C5 and higher alkanes that constitute the business of petroleum reforming. Dehydrogenation of cyclohexane (and cyclohexene) is discussed in Section 12.3; the other reactions are reserved to Chapter 14.

We may conclude this Introduction by referring to Scheme 12.I, which lays out the main transformations that alkanes can undergo on catalysts having only a metallic function. This portrayal is somewhat simplified, as it omits some of

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Scheme 12.1. Formal display of the reactions of hydrocarbons.

•Horizontal processes involve addition or removal of hydrogen

•Vertical processes involve isomerisation

•Not all possible processes are included (e.g. isotopic exchange, epimerisation, and some routes to ‘carbon’), but all that are shown are possible in practice.

the possible routes for carbon deposition and hydrogenolysis as well as one or two reactions of interest but of minor importance, such as epimerisation and isotopic exchange. As already noted, dehydrocyclisation may proceed in a way not depicted here: this and skeletal isomerisation will occupy us in Chapter 14, and hydrogenolysis in the next chapter.

12.2. DEHYDROGENATION OF ACYCLIC ALKANES

12.2.1. Introduction: Alkane Chemisorption

Most of the publications to be noted in this Section refer to the dehydrogenation of propane or of isobutane, and almost all have employed platinum as the active ingredient, for the reason mentioned above: most describe the behaviour of this metal modified by an inert partner, which is usually tin. Other metals have been examined only rarely,6 although early work highlighted the usefulness of the noble metals of Groups 8 to 10,4 but the rapid deactivation of metals in the pure state has made them hard to study in a quantitative way. Other added metals besides tin have been tried,6−8 and occasionally it appears that they may contribute to activity, perhaps through bimetallic sites.7

Some reference has already been made to the chemisorption of alkanes on metal surfaces (Sections 4.8 and 6.1); it is a difficult process requiring a significant activation energy, and probably two ‘sites’, although as we have seen they may not

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be identical, as the hydrogen atom may be content with a place that would not suit an alkyl radical. In the steady state, the rates of desorption of hydrogen and of alkene, and adsorption of alkane, will determine the concentrations of alkyl, alkene and hydrogen atoms: since these are thought to be low, the rate-determining step is usually reckoned to be the chemisorption of the alkane, the reaction then proceeding by a reversal of the Horiuti-Polanyi mechanism for alkene hydrogenation9 (Section 7.26). Although alkane chemisorption is faster on rough single-crystal surfaces (e.g. fcc(110)-(1 × 2) than on smooth surfaces (e.g. fcc(111), dehydrogenation has long been recognised as being particle-size insensitive,2 but for the reasons discussed earlier (Section 5.4) face-sensitivity and particle-size sensitivity are not necessarily equivalent: moreover, particle-size insensitivity can be induced by carbon deposition.10 Alkane chemisorption is inhibited by pre-adsorbed hydrogen,2 so the rate of dehydrogenation is expected to show a negative order in hydrogen.

12.2.2. Supported Platinum and Platinum-Tin Catalysts

In more recent work, such information as is available concerning platinum itself has appeared in conjunction with work on a bimetallic system, and it is therefore best not to try to separate it. Very much attention has been paid to the method of preparation of platinum-tin catalysts, to their characterisation (by methods that include EXAFS,7,11 Mossbauer¨ spectroscopy12,13 and more conventional techniques), and to the choice of support. It is clear that the way in which a catalyst is prepared, and in particular the conditions used for calcination and reduction, determine the structure of the finished article.14,15 Although the most usual procedure starts with chloroplatinic acid and stannous chloride, many other compounds of each metal have been tried (e.g. PtCl2P2;16 Z -PtCl(SnCl3)P2 (P=PPh3);17 SnnBu4 added to Pt/support;18 aqueous Sn(OH)nBu3):11 co-impregnation14 and successive impregnation16 have had their adherents. Curiously, however, little use seems to have been made of the PtCl2(SnCl3)2 complex which is formed by interaction of chloroplatinic acid with stannous chloride, some of the SnII being used to reduce PtIV to PtII: this complex has an intense blue colour, and was formerly much used for the colorimetric estimation of platinum in dilute solution.

What is very clear indeed is that the degree of interaction between the two metals depends very much on the nature of the support. Both alumina and silica have often been used, and their comparison is instructive. With silica, reduction leads easily to the formation of bimetallic particles19 (PtSn, PtSn2), as well as platinum and tin, depending on the ratio in which the two metals are present.14 At low Pt/Sn ratios, tin segregates at the surface; hydrogenolysis and fast deactivation then result. At Pt/Sn = 1, where PtSn predominates, the catalyst has been shown to be more stable, for n-hexane dehydrogenation giving high selectivity to alkene, with little benzene or hydrogenolysis. The bimetallic particles were however extremely large (20–30 nm) compared to the platinum particles ( 2 nm). Other reports

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contradict this, but there is general agreement that smaller activity is the price that has to be paid for improved selectivity.13,19

The amount of hydrogen that can be chemisorbed decreased with increasing tin content, and, where particles of different composition are present, temperatureprogrammed desorption cannot be used to estimate dispersion. Analysis of products formed from isobutane extends the conclusions reached with n-hexane. A 6/1 Pt/Sn ratio hardly altered the character of 1.2% Pt/SiO2, which gave 33% isomerisation to n-butane and only 26% selectivity to isobutene: smaller alkanes were the other products. With Pt/Sn = 1:1 or 1:3, these almost disappeared and there was no isomerisation: isobutene selectivity was 99% at 673 K. Most significantly, a 0.04% Pt/SiO2 showed almost the same behaviour: these results point very clearly to the need for larger platinum ensembles for the parasitic reactions, and the sufficiency of a small ensemble for dehydrogenation. Decomposition of SnnBu4 on the surface of Pt/SiO2 gave tin atoms on the platinum, and at 823 K a performance similar to that just described.18 Pt/SiO2 catalysts prepared by the reverse micelle method or by the sol-gel route are reported to dehydrogenate propane with only modest carbon deposition.20

The story with alumina is quite different. The usual pre-treatment leaves much of the tin in an oxidised state (probably SnII) associated with the support, and only a small part in contact with the platinum; nevertheless, there are many beneficial effects. Dispersion and stability in propane dehydrogenation were improved, although the initial activity was the same as in the absence of tin, the early high production of ethane and methane decreasing to 10% of the products with continued use.21 With n-heptane at 773 K, tin suppressed hydrogenolysis and aromatisation somewhat, but isomerisation still occurred: n-heptene selectivity was 63%.22 This system has been very thoroughly studied. The tin either increases or decreases the amount of hydrogen chemisorbed, the very high H/Pt ratios sometimes found being attributed to hydrogen spillover (Section 3.3.4). This seems to be assisted by the tin aluminate layer which forms, it is thought, on the surface of the support, and which modifies the properties of the largely platinum particles that reside on it. The tin is said to promote the mobility of ‘carbon’ residues from metal to support, although it is not obvious how this is accomplished. It is certain, however, that tin does not diminish the amount of ‘carbon’ formed, indeed, quite the opposite; but the ‘carbon’ that is created is less inimical to activity, because much of it is located on the support. Temperature-programmed oxidation of the ‘carbon’ on PtSn/Al2O3 revealed three distinct types19 – on the metal (A), on the support (B), and graphitic carbon on the support (C). Types A and B increased with temperature and propane pressure, and were inhibited by hydrogen: Type A reached a limit, and Type B increased continually with time-on-stream.

The theory and practice of preparing bimetallic catalysts has been comprehensively reviewed.25

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A limited number of other supports have been examined. The need to incorporate basicity makes magnesia an obvious choice,12,17 but increasing tin concentrations cause a marked growth in particle size, and increased breadth of the size distribution: quite severe sintering also occurred during use. With isobutane at 753 K, a catalyst containing 2.7% platinum (Pt/Sn = 1:1) gave only 50% isobutane, and considerable isomerisation.17 Aluminates (MgAl2O4) are useful supports: the latter is used in the Phillips Petroleum Company STAR process, while alumina promoted with zinc and cobalt (also presumably as (Zn,Co)Al2O4) is used in the UOP Oleflex process.2 MgAl2O4 formed by decomposition of the corresponding hydrotalcite when supporting platinum + tin gave a stable catalyst for propane dehydrogenation.26 Pt/Nb2O5 gave much less aromatisation with n-heptane than Pt/Al2O3, and Pt1Sn1/Nb2O5 gave 78% selectivity to n-heptene, with little hydrogenolysis or isomerisation. Platinum (1%) with 10 or 20% niobia on alumina gave similar results.22

High selectivity to isobutene does not depend upon small particle size or the presence of a support. The Pt(111) surface covered to 25% by tin atoms was slightly more active for isobutane dehydrogenation at 673 K than Pt(111) itself, and the selectivity, already high (99%), scarcely increased.27

12.2.3. Other Metals and Modifiers

Much less work has been done with metals other than platinum or modifiers other than tin. Palladium, rhodium and iridium (as well as platinum) have been examined in conjunction with tin, indium, antimony, titanium, vanadium, molybdenum, manganese and iron for dehydrogenating the butanes.6 Many of the combinations gave useful selectivities to the butenes, but the best were PtSn/Al2O3, PtIn/Al2O3 (S = 99%) and PtFe/SiO2 (S = 97%). The kinetics of isobutane dehydrogenation on PtIn/Al2O3 have been fully explored,28 this system closely resembling PtSn/Al2O3.

An especially useful catalyst for dehydrogenating the lower alkanes has been prepared by depositing the complex [PtMo6O24]8− onto magnesia;7 after calcination at 773 K, Pt4+ ions replaced Mg2+ ions in the surface, and Mo6+ ions in distorted octahedral coordination also resided on the surface but not in contact with the platinum. Hydrogen reduction at 773 K led to small ( 1 nm) platinum particles and some reduction of the Mo6+ ions, which then formed dimers. Such a catalyst was much more active and (below 773 K) stabler for dehydrogenating the butanes than Pt/Al2O3 or a PtMo/MgO prepared conventionally; the improvement in the case of propane was somewhat less, but selectivities of 96–97% were obtained. The cause of the better performance was discussed, but, notwithstanding the excellent and extensive use of EXAFS, no firm conclusion was reached. The large-scale manufacture of this catalyst might prove to be very expensive.

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It is somewhat strange that a number of potentially useful supports have not been mentioned in the open literature: no use seems to have been made of titania or zirconia, or of CaAl2O4 or CaSiO3. There may of course be good reasons; perhaps they have been tried and found wanting.

12.2.4. Kinetics and Mechanism

Quantitative studies of the kinetics of dehydrogenation of lower alkanes are few and far between, but those few have considerable significance.

In terms of the value of the conclusion reached, the examination of propane dehydrogenation on platinum-gold powders has perhaps been the most useful.2 In a deservedly much-quoted paper8 it was shown that at low platinum content (0–15%) the rate at 633 K was a linear function of it; this clearly suggested that a single platinum atom constituted the active centre.29 The addition of gold lowered the activation energy only slightly, but increased the order in propane from −1.1 to −0.49 without changing the order in hydrogen. This observation helps to explain the apparent structure-insensitivity of alkane dehydrogenations, although other factors may sometimes contribute (see Section 12.32).

Two independent studies9,23 of isobutane dehydrogenation have reached the same conclusions (an occurrence almost unique in the history of metal catalysis). The mechanism is well described by an inverse Horiuti-Polanyi scheme, for which the constants for the four forward and four backward reactions have been evaluated.9 Adsorption of isobutane is the rate-limiting step, and reaction in the presence of deuterium showed extensive exchange in the isobutane, but little into the unreacted isobutane. At or above 773 K, the alkyl-alkene interconversion was clearly much faster than alkyl to alkane, but the exchanged isobutane had about the same deuterium content as the isobutane,23 showing that both arose from the same alkyl + alkene ‘pool’. Rate dependences on reactant pressures have been shown graphically,9 but precise values of orders (negative in hydrogen, positive in isobutane) were not given. The activity of PtSn/SiO2 was increased somewhat by the addition of potassium (Pt:Sn:K = 1:1:3), but a much larger effect (× 50) was produced by using potassium-loaded L zeolite (KL) as support. Kinetic parameters were unaltered, but the cause of the effect was not established with certainty. The role played by hydrogen in dehydrogenations, and its unique character on platinum, will be considered below (Section 12.4).

Although alkane dehydrogenations appear to be structure-insensitive, there is one study30 that reveals an interesting aspect of particle-size effects. Catalysts containing metal particles with mean sizes of from 2 to 5 nm were prepared by a combination of altering metal content and sintering temperature. The areal rate of dehydrogenation of 2,3-dimethylbutane decreased about four-fold as particle size was increased, but the relative amounts of the two possible products

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(i.e. 2,3-dimethyl-1- and -2-butenes) also changed, the fraction of the former being formed initially rising from about 0.58 to 0.70, and on platinum wire it was 0.90. Equilibration between the isomers continued as the reaction proceeded. It was thought that the alkene existed in the α,β-diadsorbed form on large particles, the C=C bond then preferring to be in the central position, while on small particles it adopted a π -allylic form, from which both isomers could arise.

The composition of the butene isomers formed in the dehydrogenation of n-butane over Pt/Al2O3 has been followed as a function of temperature, space velocity and time-on-stream in the continuous flow mode, and also at 673 K in the pulse mode.31 In the former case, the initial distribution at 673 K was: 1-butene, 23%; E -2-butene, 42%; and Z -2-butene, 35%, which approximates to the equilibrium proportions (respectively 21, 42 and 36%), but with increasing time, and loss of activity due to ‘carbon’ deposition, the 1-butene proportion fell, but the Z /E ratio remained constant. This change may have been a consequence of the increased ‘crowding’ of the active centres as the coverage by ‘carbon’ increased. At higher temperatures (773 and 873 K), the initial concentration of 1-butene rose (52% at 873 K), apparently exceeding the equilibrium amount, the decrease with time-on- stream continuing. In the pulse mode, product composition was independent of pulse number, but the amount of 1-butene exceeded its equilibrium amount. Some isobutane was also formed by isomerisation.

12.3. DEHYDROGENATION OF CYCLOALKANES

12.3.1. Overview

A number of strands converge in this section, because several different kinds of process occur simultaneously, and it is not easy to separate them. Most of the work that concerns us has been carried out with cyclohexane, because not only is the progress of its dehydrogenation easily followed (as with benzene hydrogenation) but also because of its relevance to the catalytic chemistry of petroleum reforming (see Chapter 14). The rich variety of the processes that take place is due entirely to the ease with which, at typical reaction temperatures, hydrogen atoms migrate from one hydrocarbon species to another, so that their surface concentrations depend in a statistical manner on reactant/product concentrations in the gas phase, and on temperature. To illustrate this point, it hardly matters whether one starts with cyclohexane or cyclohexene, because the former will very soon transform to the latter on the surface, and a position of quasi-equilibrium comprising cyclohexyl and cyclohexene, in amounts that depend on the number of available hydrogen atoms, will quickly be set up. As their further dehydrogenation towards benzene proceeds, transitory diene species may be made on the surface, but they do not desorb.32 Just

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as benzene is not easily hydrogenated to cyclohexene, so the dehydrogenation of cyclohexane does not stop at it; benzene is usually the sole product (on Pt(111) even at 235 K), although the conditions that favour its selective hydrogenation do not appear to have been tried for selective dehydrogenation.

The thermochemistry for the dehydrogenation of cycloalkanes is more favourable than that for acyclic molecules; the free energy change becomes zero at 560 K, and the reaction becomes detectable in the hydrogenation of aromatics (i.e. reactions do not go to completion) above about 473 K. Lower temperatures can therefore be used, and one advantage of this is that carbon deposition, although not negligible, is less serious, so it is easier to work with pure metals, and the use of bimetallic systems has been less common. However, hydrogenolysis (chiefly to methane) also occurs at the same time with the more active metals (Ni,33 Ru34). A further reaction dimension is provided by the fact that at these moderate temperatures the hydrogen atoms released from a chemisorbed molecule of cyclohexene can add to another molecule, the nett reaction being

3 C6H10 → C6H6 + 2C6H12

(12.A)

Cyclohexene has sometimes been used for hydrogenation of organic molecules in place of molecular hydrogen. The situation on the surface in the presence of hydrogen is therefore one of considerable complexity;35,36 in what follows we focus on the dehydrogenation of cyclohexane.

12.3.2. Reaction on Pure Metals

By far the largest amount of work has been performed with platinum catalysts. There have been a number of studies employing single crystals,32,37–41 and these showed that stepped and kinked platinum surfaces were more active than smooth faces. A recent detailed study32,40 using Pt(111) and sum-frequency generation started with cyclohexene, which at 200 K was chemisorbed as a π -species, which at 217 K transformed to the di-σ -state, and at 283 K to a three-centre π -allylic C6H9 form, and thence to benzene at 383 K. However, experiments with bismuth postdoping after cyclohexane adsorption showed that benzene existed on this surface as low as 235 K, and that no intermediate form could be displaced.42,43 Dehydrogenation of cyclohexene was faster on Pt(100) than on Pt(111) because only the 1,3-cyclohexadiene (and not the 1,4-isomer) was formed, this being thought the necessary intermediate in benzene formation.40 Various C6 cyclic molecules have also been examined on Ni(100).44

Little attention has been paid to the kinetics of dehydrogenations. Sinfelt45 has re-examined early work on methylcyclohexane,46 and has concluded that at 588 K it reacts eight times more slowly than cyclohexane, the activation energy E ← k being 138 kJ mol−1. On platinum black, cyclohexane exchanged with deuterium