Catalysis - UFSCar

General principles 25.1 The language of catalysis 25.2 Homogeneous and heterogeneous

catalysts

Homogeneous catalysis

25.3 Alkene metathesis 25.4 Hydrogenation of alkenes 25.5 Hydroformylation 25.6 Wacker oxidation of alkenes 25.7 Asymmetric oxidations 25.8 Palladium-catalysed CeC bond-

forming reactions 25.9 Methanol carbonylation: ethanoic

acid synthesis

Heterogeneous catalysis

25.10 The nature of heterogeneous catalysts

25.11 Hydrogenation catalysts 25.12 Ammonia synthesis25.13 Sulfur dioxide oxidation 25.14 Catalytic cracking and the inter-

conversion of aromatics by zeolites25.15 Fischer–Tropsch synthesis25.16 Electrocatalysis and photocatalysis25.17 New directions in heterogeneous

catalysis

Heterogenized homogeneous and hybrid catalysis

25.18 Oligomerization and polymerization25.19 Tethered catalysts25.20 Biphasic systems

Further reading

Exercises

Tutorial problems

25 In this chapter we apply the concepts of organometallic chemistry, coordination chemistry, and materials chemistry to catalysis. We emphasize general principles, such as the nature of catalytic cycles, in which a catalytic species or surface is regenerated in a reaction, and the delicate bal-ance of reactions required for a successful cycle. We see that there are numerous requirements for a successful catalytic process: the reaction being catalysed must be thermodynamically favour-able and fast enough when catalysed; the catalyst must have an appropriate selectivity towards the desired product and a lifetime long enough to be economical. We then survey homogeneously catalysed reactions and show how proposals about mechanisms are invoked. The fi nal part of the chapter develops a similar theme in heterogeneous catalysis, and we shall see that many parallels exist between homogeneous and heterogeneous catalysis. In neither type of catalysis are mecha-nisms necessarily fi nally settled, and there is still considerable scope for making new discoveries.

A catalyst is a substance that increases the rate of a reaction but is not itself consumed. Catalysts are widely used in nature, in industry, and in the laboratory, and it is estimated that they contribute to one-sixth of the value of all manufactured goods in industrialized coun-tries. As shown in Table 25.1 , 16 of the top 20 synthetic chemicals in the US are produced directly or indirectly by catalysis. For example, a key step in the production of a dominant industrial chemical, sulfuric acid, is the catalytic oxidation of SO 2 to SO 3 . Ammonia, another chemical essential for industry and agriculture, is produced by the catalytic reduction of N 2 by H 2 . Inorganic catalysts are also used for the production of the major organic chemicals and petroleum products, such as fuels, petrochemicals, and polyalkene plastics. Catalysts play a steadily increasing role in achieving a cleaner environment, through, for example, the destruction of pollutants (as with the catalytic converters found on the exhaust systems of vehicles), the development of better industrial processes that are more effi cient with higher product yields and fewer unwanted by-products, and in clean-energy generation in fuel cells. Industrially important catalysts are almost invariably inorganic (which justifi es their discus-sion in this book). Enzymes, a class of biochemical catalysts, often with a metal ion at the centre of a complex molecule, are discussed in Chapter 26.

In addition to their economic importance and contribution to the quality of life, cata-lysts are interesting in their own right: the subtle infl uence a catalyst has on reagents can completely change the outcome of a reaction. The understanding of the mechanisms of catalytic reactions has improved considerably in recent years with the greater availabil-ity of isotopically labelled molecules, improved methods for determining reaction rates, improved spectroscopic and diffraction techniques, and much more reliable molecular orbital calculations.

Catalysis

Those fi gures with an asterisk (*) in the caption can be found online as interactive 3D structures. Type the following URL into your browser, adding the relevant fi gure number: www.chemtube3d.com/weller/[chapter number]F[fi gure number]. For example, for Figure 4 in chapter 7, type www.chemtube3d.com/weller/7F04.

Many of the numbered structures can also be found online as interactive 3D structures: visit www.chemtube3d.com/weller/[chapter number] for all 3D resources organized by chapter.

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http://www.chemtube3d.com/weller/

http://www.chemtube3d.com/weller/7F04

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729General principles

General principles

A catalysed reaction is faster than an uncatalysed version of the same reaction because the catalyst provides a different reaction pathway with a lower activation energy. The term negative catalyst is sometimes applied to substances that retard reactions. Substances that block one or more elementary steps in a catalytic reaction are called catalyst poisons .

25.1 The language of catalysis Before we discuss the mechanism of catalytic reactions, we need to introduce some of the terminology used to describe the rate of a catalytic reaction and its mechanism.

(a) Energetics

Key points: A catalyst increases the rates of processes by introducing new pathways with lower Gibbs energies of activation; the reaction profi le contains no high peaks and no deep troughs.

A catalyst increases the rates of processes by introducing new pathways with lower Gibbs energies of activation, Δ ‡ G . We need to focus on the Gibbs energy profi le of a catalytic reaction, not just the enthalpy or energy profi le, because the new elementary steps that occur in the catalysed process are likely to have quite different entropies of activation. A catalyst does not affect the Gibbs energy of the overall reaction, Δ r G < , because G is a state function. 1 The difference is illustrated in Fig. 25.1 , where the overall reaction Gibbs energy is the same in both energy profi les. Reactions that are thermodynamically unfa-vourable cannot be made favourable by a catalyst.

Figure 25.1 also shows that the Gibbs energy profi le of a catalysed reaction contains no high peaks and no deep troughs. The new pathway introduced by the catalyst changes the mechanism of the reaction to one with a very different shape and with lower maxima. However, an equally important point is that stable or nonlabile catalytic intermediates do not occur in the cycle. Similarly, the product must be released in a thermodynamically favourable step. If, as shown by the blue line in Fig. 25.1 , a stable complex were formed with the catalyst, it would turn out to be the product of the reaction and the cycle would terminate. Similarly, impurities may suppress catalysis, by coordinating strongly to cata-lytically active sites, and act as catalyst poisons.

Table 25.1 The top 20 synthetic chemicals in the US in 2008 (based on mass)

Rank Chemical Catalytic process Rank Chemical Catalytic process

1 Sulfuric acid SO 2 oxidation, heterogeneous 12 Ammonium nitrate Precursors catalytic

2 Ethene Hydrocarbon cracking, heterogeneous 13 Urea NH 3 precursor catalytic

3 Propene Hydrocarbon cracking, heterogeneous 14 Ethylbenzene Alkylation of benzene, homogeneous

4 Polyethene Polymerization, heterogeneous 15 Styrene Dehydrogenation of ethylbenzene,

5 Chlorine Electrolysis, not catalytic 16 HCl Heterogeneous

6 Ammonia N 2 + H 2 , heterogeneous 17 Cumene Alkylation of benzene

7 Phosphoric acid Not catalytic

8 1,2-Dichloroethane Ethene + Cl 2 , heterogeneous 18 Ethylene oxide Heterogeneous

9 Polypropene Polymerization, heterogeneous 19 Ammonium sulfate Ethene + O 2 , heterogeneous

10 Nitric acid NH 3 + O 2 , heterogeneous 20 Sodium carbonate Precursors catalytic

11 Sodium hydroxide Electrolysis, not catalytic

Source: Facts & Figures for the Chemical Industry, Chem. Eng. News , 2009, 87 , 33.

Gib

bs e

nerg

y

Extent of reaction

Δ‡G

ΔrG<

(a)

(b)

(c)

Figure 25.1 Schematic representation of the energetics of a catalytic cycle. The uncatalysed reaction (a) has a higher Δ ‡ G than a step in the catalysed reaction (b). The Gibbs energy of the overall reaction, Δ r G < , is the same for routes (a) and (b). The curve (c) shows the profi le for a reaction mechanism with an intermediate that is more stable than the product. 1 That is, G depends only on the current state of the system and not on the path that led to the state.


730 25 Catalysis

(b) Catalytic cycles

Key point: A catalytic cycle is a sequence of reactions that consumes the reactants and forms products, with the catalytic species being regenerated after the cycle.

The essence of catalysis is a cycle of reactions in which the reactants are consumed, the products are formed, and the catalytic species is regenerated. A simple example of a catalytic cycle involving a homogeneous catalyst is the isomerization of prop-2-en-1-ol (allyl alcohol, CH 2 aCHCH 2 OH) to prop-1-en-1-ol (CH 3 CHaCHOH) with the catalyst [Co(CO) 3 H]. The fi rst step is the coordination of the reactant to the catalyst. That complex isomerizes in the coordination sphere of the catalyst and goes on to release the product and reform the catalyst ( Fig. 25.2 ). Once released, the prop-1-en-1-ol tautomerizes to propanal (CH 3 CH 2 CHO). As with all mechanisms, this cycle has been proposed on the basis of a range of information like that summarized in Fig. 25.3 . Many of the components shown in the diagram were encountered in Chapter 21 in connection with the determination of mechanisms of substitution reactions. However, the elucidation of catalytic mechanisms is complicated by the occurrence of several delicately balanced reactions, which often cannot be studied in isolation.

Two stringent tests of any proposed mechanism are the determination of rate laws and the elucidation of stereochemistry. If intermediates are postulated, their detection by NMR, IR, or UV–visible spectroscopy also provides support (Chapter 8). If specifi c atom-transfer steps are proposed, then isotopic tracer studies may serve as a test. The infl uences of different ligands and different substrates are also sometimes informative. Although rate data and the corresponding laws have been determined for many overall catalytic cycles, it is also necessary to determine rate laws for the individual steps in order to have reasonable confi dence in the mechanism. However, because of experimental complications, it is rare that catalytic cycles are studied in this detail.

(c) Catalytic effi ciency and lifetime

Key points: A highly active catalyst—one that results in a fast reaction even in low concentrations—has a large turnover frequency. A catalyst must be able to survive a large number of catalytic cycles if it is to be of use.

The turnover frequency , f , is often used to express the effi ciency of a catalyst. For the con-version of A to B catalysed by Q and with a rate v ,

A B

d[B]d

Q⎯ →⎯ =�t

(25.1)

provided the rate of the uncatalysed reaction is negligible, the turnover frequency is

f = �

[ ]Q (25.2)

H

Co

COOC

OCOH

OH

H

OH

Co

COOC

OC

H

Co

COOC

OC

OH

H

Co

COOC

OC

OH

Figure 25.2 The catalytic cycle for the isomerization of prop-2-en-1-ol to prop-1-en-1-ol.


731General principles

A highly active catalyst—one that results in a fast reaction even in low concentrations—has a high turnover frequency.

In heterogeneous catalysis, the reaction rate is expressed in terms of the rate of change in the amount of product (in place of concentration), and the concentration of catalyst is replaced by the amount present. The determination of the number of active sites in a het-erogeneous catalyst is particularly challenging, and often the denominator [Q] in eqn 25.2 is replaced by the surface area of the catalyst.

The turnover number is the number of cycles for which a catalyst survives. If it is to be economically viable, a catalyst must have a large turnover number. However, it may be destroyed by side reactions to the main catalytic cycle or by the presence of small amounts of impurities in the starting materials (the feedstock ). For example, many alkene polymeri-zation catalysts are destroyed by O 2 , so in the synthesis of polyethene (polyethylene) and polypropene (polypropylene) the concentration of O 2 in the ethene or propene feedstock should be no more than a few parts per billion.

Some catalysts can be regenerated quite readily. For example, the supported metal cata-lysts used in the reforming reactions that convert hydrocarbons to high-octane gasoline become covered with carbon because the catalytic reaction is accompanied by a small amount of dehydrogenation. These supported metal particles can be cleaned by interrupt-ing the catalytic process periodically and burning off the accumulated carbon.

(d) Selectivity

Key point: A selective catalyst yields a high proportion of the desired product with minimum amounts of side products.

In industry, there is considerable economic incentive to develop selective catalysts , which yield a high proportion of the desired product with minimum amounts of side products ( Box 25.1 ). For example, when metallic silver is used to catalyse the oxidation of ethene with oxygen to produce oxirane (ethylene oxide, 1 ), the reaction is accompanied by the more thermodynamically favoured but undesirable formation of CO 2 and H 2 O. This lack of selectivity increases the consumption of ethene, so chemists are constantly trying to devise a more selective catalyst for oxirane synthesis. Selectivity can be ignored in only a very few simple inorganic reactions, where there is essentially only one thermodynami-cally favourable product, as in the formation of NH 3 from H 2 and N 2 . One area where selectivity is of considerable and growing importance is asymmetric synthesis, where only one enantiomer of a particular compound is required and catalysts may be designed to produce one chiral form in preference to any others.

Determine

adsorption

isotherms

Identify

surface species;

note analogies

with organometallic

compounds

Determine overall rate

law and selectivity as

a function of concentration

Determine

rate laws of

individual steps

Determine formation

of metal complex

Identify reaction

intermediates; note

analogies with

known reactions

Note

support

effect

Note

differential

poisoning

Note

isotope

effect

Note

stereochemistryNote

solvent and

ligand effect

Infer best

mechanism

Heterogeneous General Homogeneous

Postulate

mechanism

1 Oxirane (ethylene oxide)

Figure 25.3 The determination of catalytic mechanisms.


732 25 Catalysis

25.2 Homogeneous and heterogeneous catalysts Key points: Homogeneous catalysts are present in the same phase as the reagents, and are often well defi ned; heterogeneous catalysts are present in a different phase from the reagents.

Catalysts are classifi ed as homogeneous if they are present in the same phase as the rea-gents; this normally means that they are present as solutes in liquid reaction mixtures. Catalysts are heterogeneous if they are present in a different phase from that of the reac-tants; this normally means that they are present as solids with the reactants present either as gases or in solution. Both types of catalysis are discussed in this chapter and will be seen to be fundamentally similar.

From a practical standpoint, homogeneous catalysis is attractive because it is often highly selective towards the formation of a desired product. In large-scale industrial pro-cesses, homogeneous catalysts are preferred for exothermic reactions because it is easier to dissipate heat from a solution than from the solid bed of a heterogeneous catalyst. In principle, every homogeneous catalyst molecule in solution is accessible to reagents, poten-tially leading to very high activities. It should also be borne in mind that the mechanism of homogeneous catalysis is more accessible to detailed investigation than that of heterogene-ous catalysis, as species in solution are often easier to characterize than those on a surface and because the interpretation of rate data is frequently easier. The major disadvantage of homogeneous catalysts is that a separation step is required.

Heterogeneous catalysts are used very extensively in industry and have a much greater economic impact than homogeneous catalysts. One attractive feature is that many of these solid catalysts are robust at high temperatures and therefore tolerate a wide range of oper-ating conditions. Reactions are faster at high temperatures, so at high temperatures solid catalysts generally produce higher outputs for a given amount of catalyst and reaction time than homogeneous catalysts operating at lower temperatures in solutions. Another reason for their widespread use is that extra steps are not needed to separate the prod-uct from the catalyst, resulting in effi cient and more environmentally friendly processes. Typically, gaseous or liquid reactants enter a tubular reactor at one end and pass over a bed of the catalyst, and products are collected at the other end. This same simplicity of design applies to the catalytic converter used to oxidize CO and hydrocarbons and reduce nitrogen oxides in automobile exhausts ( Fig. 25.4 ; see also Box 25.2 in Section 25.10c).

Homogeneous catalysis

Here we concentrate on some important homogeneous catalytic reactions based on orga-nometallic compounds and coordination complexes. We describe their currently favoured mechanisms, but it should be noted that, as with nearly all mechanistic proposals, catalytic mechanisms are subject to refi nement or change as more detailed experimental informa-tion becomes available. Unlike simple reactions, a catalytic process frequently contains many steps over which the experimentalist has little control. Moreover, highly reactive

BOX 25.1 Atom economy

‘Atom economy’, one of the most important concepts in green chemistry, gives an indication of the effi ciency of a chemical reaction in terms of con-version of starting materials into products. In an ideal reaction, all atoms in the starting materials are present in the product. Atom economy is given by the equation

Atom economy (%) =

Molecular mass of desired productMolecullar mass of all reactants

×100

Effi cient reactions have high atom economies, produce little waste, and are viewed as environmentally sustainable. Atom economy should not be confused with yield, as percentage yield gives no indication of the quantity of waste or by-products produced. Atom economy can be poor even when

the yield is high. For example, if the desired product is an enantiomer, the atom economy may be low with respect to that enantiomer, even though the yield of the reaction could be close to 100 per cent.

Catalysts play a crucial role in increasing the atom economies of reac-tions. Catalytic routes typically involve fewer reaction steps, greater selec-tivity, and regeneration of the catalyst. For example, the noncatalytic route to the industrial production of ethylene oxide ( 1 ), of which in excess of 15 million tonnes is produced worldwide each year, had an atom economy of 26 per cent and produced 3.5 kg of CaCl 2 waste for every 1 kg of eth-ylene oxide produced. The catalytic route is now used exclusively. It uses a heterogeneous silver catalyst in a one-step addition reaction of oxygen to ethene with an atom economy of 100 per cent.

CO2, H

2O,

N2, O

2,

H2S

CO,

hydrocarbons,

NOx, N

2, O

2,

SOx

Catalyst

Support

Figure 25.4 A heterogeneous catalyst in action. The automobile catalytic converter oxidizes CO and hydrocarbons, and reduces nitrogen and sulfur oxides. The particles of a metal catalyst are supported on a robust ceramic honeycomb.


733Homogeneous catalysis

intermediates are often present in concentrations too low to be detected spectroscopically. The best attitude to adopt towards these catalytic mechanisms is to learn the pattern of transformations and appreciate their implications but be prepared to accept new mecha-nisms that might be indicated by future work.

The scope of homogeneous catalysis ranges across hydrogenation, oxidation, and a host of other processes. Often the complexes of all metal atoms in a group will exhibit catalytic activity in a particular reaction, but the 4d-metal complexes are often superior as catalysts to the complexes of their lighter and heavier congeners. In some cases the difference may be associated with the greater substitutional lability of 4d organometallic compounds in comparison with their 3d and 5d analogues. It is often the case that the complexes of costly metals must be used on account of their superior performance compared with the complexes of cheaper metals.

Chapters 21 and 22 described reactions that take place at metal centres: these reactions lie at the heart of the catalytic processes we now consider. In general, we need to invoke a variety of the processes described in those chapters, and it will be useful to review the following sections:

• Ligand substitution reactions: Sections 21.5–21.9 and Section 22.21

• Redox reactions: Sections 21.10–21.12

• Oxidative addition and reductive elimination reactions: Section 22.22

• Migratory insertion reactions: Section 22.24

• 1,2-Insertions and β-hydride eliminations: Section 22.25

Together with the direct attack on coordinated ligands, these reaction types (and in some cases their reverse), often in combination, account for the mechanisms of most of the homogeneous catalytic cycles that have been proposed for organic transformations. Other reactions yet to be fully investigated will undoubtedly use other reaction steps.

25.3 Alkene metathesis Key points: Alkene metathesis reactions are catalysed by homogeneous organometallic complexes that allow considerable control over product distribution; a key step in the reaction mechanism is the dissociation of a ligand from a metal centre to allow an alkene to coordinate.

In an alkene metathesis reaction, carbon–carbon double bonds are redistributed, as in the cross-metathesis reaction:

R1 R2

R1

R2

++

Alkene metathesis was fi rst reported in the 1950s with poorly defi ned mixtures of reagents, such as WCl 6 /Bu 4 Sn and MoO 3 /SiO 2 , being used to bring about a number of different reac-tions ( Table 25.2 ). In recent years, a number of newer catalysts have been introduced, and the development of the well-defi ned ruthenium alkylidene compound ( 2 ) by Grubbs in 1992 and the molybdenum imido-alkylidene compound by Schrock in 1990 ( 3 ) were of seminal importance. 2

Alkene metathesis reactions proceed through a metallacyclobutane intermediate:

[M]

R1

R3R2

[M]

R1

R3R2

[M]

R1

R3R2

In the case of Grubbs’ catalyst, it is known that the dissociation of a PCy 3 (Cy = cyclohexyl) ligand from the Ru metal centre is crucial in allowing the alkene molecule to coordinate prior to metallacyclobutane formation.

PCy3

Ru

PCy3

Cl

Cl

Ph

2

NAr

MoCRO

RO

CMe2Ph

H

3

2 The 2005 Nobel prize was awarded to Robert Grubbs, Yves Chauvin, and Richard Schrock for their work on developing metathesis catalysts.


734 25 Catalysis

The identifi cation of this mechanism led Grubbs to replace one of the PCy 3 ligands with a bis(mesityl) N-heterocyclic carbene (NHC) ligand, reasoning that the stronger σ-donor and poorer π-acceptor ability of the NHC ligand would both encourage PCy 3 dissociation and stabilize the alkene complex. In a triumph of rational design, the so-called second-generation Grubbs’ catalyst ( 4 ) proved to be more active than the original bisphosphine complex. The second-generation Grubbs’ catalyst is active in the presence of a large num-ber of different functional groups on substrates and can be used in many solvent systems. It is commercially available and has been widely used, including in the total synthesis of a number of natural products. In Grubbs’ third-generation catalyst the phosphine ligand is replaced by a heterocycle, such as pyridine, and has a reduced initiation time.

Schrock developed tungsten and molybdenum alkylidene complexes that were commer-cialized by 1990, and developed the fi rst chiral metathesis catalysts in 1993. These chiral catalysts were molybdenum-based complexes ( 5 ) and were synthesized to address stereo-chemical control, known in this context as tacticity (Section 25.18), in ROMP processes (see Table 25.2). The catalysts were quickly applied to the enantioselective (see Section 25.4) synthesis of small organic molecules.

The driving force for alkene metathesis reactions varies. For ROM and ROMP (see Table 25.2 ) it is the release of ring strain from a strained starting material that provides the energy to drive the reaction. For metathesis reactions that result in the generation of ethene (such as RCM or CM) it is the removal of the liberated ethene that can be used to encour-age the formation of the desired products. Where there is no clearly identifi able thermo-dynamically favourable product possible, mixtures of alkenes result, with their relative proportions being determined by the statistical likelihood of their formation.

25.4 Hydrogenation of alkenes Key points: Wilkinson’s catalyst, [RhCl(PPh 3 ) 3 ], and related complexes are used for the hydrogenation of a wide variety of alkenes at pressures of hydrogen close to 1 atm or less; suitable chiral ligands can lead to enantioselective hydrogenations.

The addition of hydrogen to an alkene to form an alkane is favoured thermodynamically (Δ r G < = −101 kJ mol −1 for the conversion of ethene to ethane). However, the reaction

Ru

PCy3

Cl

Cl

Ph

NN

4

N

Mo

Me

MePh

iPriPr

O

O

O

O

PhPh

PhPh

5

Table 25.2 The scope of the alkene metathesis reaction

X X

n

n

Ring-opening metathesis polymerization (ROMP)

X X

n

– nn

Acyclic diene metathesis polymerization (ADMET)

X–

X

Diene ring-closing metathesis (RCM)

XX

Enyne ring-closing metathesis (RCM)

X

R

R

X

Ring-opening metathesis (ROM)

R1

R2

R1 R2

–

Cross-metathesis (CM or XMET)



rate is negligible at ordinary conditions in the absence of a catalyst. Effi cient homogene-ous and heterogeneous catalysts are known for the hydrogenation of alkenes and are used in such diverse areas as the manufacture of nondairy spreads, pharmaceuticals, and petrochemicals.

One of the most studied catalytic systems is the Rh(I) complex [RhCl(PPh 3 ) 3 ], which is often referred to as Wilkinson’s catalyst . This useful catalyst hydrogenates a wide variety of alkenes and alkynes at pressures of hydrogen close to 1 atm or less at room temperature. The dominant cycle for the hydrogenation of terminal alkenes by Wilkinson’s catalyst is shown in Fig. 25.5 . It involves the oxidative addition of H 2 to the 16-electron complex [RhCl(PPh 3 ) 3 ] (A), to form the 18-electron dihydrido complex (B). The dissociation of a phosphine ligand from (B) results in the formation of the coordi-natively unsaturated complex (C), which then forms the alkene complex (D). Hydrogen transfer from the Rh atom in (D) to the coordinated alkene yields a transient 16-elec-tron alkyl complex (E). This complex takes on a phosphine ligand to produce (F), and hydrogen migration to carbon results in the reductive elimination of the alkane and the reformation of (A), which is set to repeat the cycle. A parallel but slower cycle (which is not shown) is known in which the order of H 2 and alkene addition is reversed. Another cycle is known, based around the 14-electron intermediate [RhCl(PPh 3 ) 2 ]. Even though there is very little of this species present, it reacts much faster with hydrogen than [RhCl(PPh 3 ) 3 ] does, and makes a signifi cant contribution to the catalytic cycle. In this cycle, (E) would eliminate alkane directly, regenerating [RhCl(PPh 3 ) 2 ], which rapidly adds H 2 to give (C).

Wilkinson’s catalyst is highly sensitive to the nature of the phosphine ligand and the alk-ene substrate. Analogous complexes with alkylphosphine ligands are inactive, presumably because they are more strongly bound to the metal atom and do not readily dissociate. Similarly, the alkene must be just the right size: highly hindered alkenes or the sterically unencumbered ethene are not hydrogenated by the catalyst, presumably because the steri-cally crowded alkenes do not coordinate and ethene forms a strong complex that does not react further. These observations emphasize the point made earlier that a catalytic cycle is usually a delicately poised sequence of reactions, and anything that upsets its fl ow may block catalysis or alter the mechanism.

Wilkinson’s catalyst is used in laboratory-scale organic synthesis and in the produc-tion of fi ne chemicals. Related Rh(I) phosphine catalysts that contain a chiral phosphine ligand have been developed to synthesize optically active products in enantioselective reac-tions (reactions that produce a particular chiral product). The alkene to be hydrogenated must be prochiral , which means that it must have a structure that leads to R or S chiral-ity when complexed to the metal. The resulting complex will have two diastereomeric

H

Rh

PPh3

PPh3Cl

HPh3P

RhPPh3Cl

PPh3Ph3P

H

RhPPh3Cl

Ph3P

H

RhPPh3Cl

HPh3P

H

RhPPh3Cl

HPh3P

R

R

R

MeHR

PPh3

RhPPh3Cl

HPh3P

MeHR

A

B

C

D

F

E

PPh3H2

PPh3

Figure 25.5 The catalytic cycle for the hydrogenation of terminal alkenes by Wilkinson’s catalyst.


736 25 Catalysis

forms, depending on which face of the alkene coordinates to the metal atom. In general, diastereomers have different stabilities and labilities, and in favourable cases one or the other of these effects leads to product enantioselectivity. Enantioselectivities are normally measured in terms of the enantiomeric excess (ee), which is defi ned as the percentage yield of the major enantiomeric product minus the percentage yield of the minor enantiomeric product.

A brief illustration A reaction that gives 51 per cent of one enantiomer and 49 per cent of another would be described as having an enantiomeric excess of 2 per cent; a reaction that gave 99 per cent of one enantiomer and 1 per cent of another would have ee = 98 per cent.

An enantioselective hydrogenation catalyst containing a chiral phosphine ligand referred to as DiPAMP ( 6 ) is used to synthesize l-dopa ( 7 ), a chiral amino acid used to treat Parkinson’s disease. An interesting detail of the process is that the minor diastereomer in solution leads to the major product. The explanation of the greater turnover frequency of the minor isomer lies in the difference in activation Gibbs energies ( Fig. 25.6 ). Spurred on by clever ligand design and using a variety of metals, this fi eld has grown rapidly and provides many clinically useful compounds; of particular note are systems derived from ruthenium(II) BINAP ( 8 ). 3

25.5 Hydroformylation Key point: The mechanism of hydrocarbonylation is thought to involve a pre-equilibrium in which oct-acarbonyldicobalt(0) combines with hydrogen at high pressure to give a monometallic species that brings about the actual hydrocarbonylation reaction.

In a hydroformylation reaction , an alkene, CO, and H 2 react to form an aldehyde contain-ing one more C atom than in the original alkene:

RCH CH CO H RCH CH CHO2 2 2 2= + + → The term ‘hydroformylation’ derived from the idea that the product resulted from the addition of methanal (formaldehyde, HCHO) to the alkene, and the name has stuck even though experimental data indicate a different mechanism. A less common but more appro-priate name is hydrocarbonylation . Both cobalt and rhodium complexes are used as cata-lysts. Aldehydes produced by hydroformylation are normally reduced to alcohols that are used as solvents and plasticizers, and in the synthesis of detergents. The scale of production is enormous, amounting to millions of tonnes annually.

The general mechanism of cobalt-carbonyl-catalysed hydroformylation was proposed in 1961 by Heck and Breslow by analogy with reactions familiar from organometallic chemistry ( Fig. 25.7 ). Their general mechanism is still invoked, but has proved diffi cult to verify in detail. In the proposed mechanism, a pre-equilibrium is established in which octacarbonyldicobalt(0) combines with hydrogen at high pressure to yield the known tet-racarbonylhydridocobalt complex (A) in Fig. 25.7 :

[ ( ) ] [ ( ) ]Co CO H 2 Co CO H2 8 2 4+ → This complex, it is proposed, loses CO to produce the coordinatively unsaturated complex [Co(CO) 3 H] (B):

[ ( ) ] [ ( ) ]Co CO H Co CO H CO4 3→ + It is thought that [Co(CO) 3 H] then coordinates an alkene, producing (C), whereupon the coordinated hydrido ligand migrates onto the alkene, and CO recoordinates. The product at this stage is a normal alkyl complex (D). In the presence of CO at high pressure, (D) undergoes migratory insertion and coordinates another CO, yielding the acyl complex (E), which has been observed by IR spectroscopy under catalytic reaction conditions. The for-mation of the aldehyde product is thought to occur by attack of either H 2 (as depicted in Fig. 25.7 ) or the strongly acidic complex [Co(CO) 4 H] to yield an aldehyde and generate

P*

P*

OMe

OMe

6 DiPAMP

HO

HONH2

COOH

7 L-dopa

X

XRuBr2

8 [Ru(BINAP)Br 2 ]. X = PPh 2

3 Ryoji Noyori and William Knowles were jointly awarded the 2001 Nobel Prize in Chemistry for their work on asymmetric hydrogenation. The prize was shared with Barry Sharpless for his work on asymmetric oxidations (Section 25.7).

Major

isomer (R)

Minor

isomer (S)

Δ‡G

(R)

Δ‡G

(S)

Gib

bs e

ne

rgy

Extent of reaction

Figure 25.6 Kinetically controlled stereoselectivity. Note that Δ ‡ G( S) < Δ ‡ G (R) , so the minor isomer reacts faster than the major isomer.



[Co(CO) 4 H] or [Co 2 (CO) 8 ], respectively. Either of these complexes will regenerate the coor-dinatively unsaturated [Co(CO) 3 H].

A signifi cant portion of branched aldehyde is also formed in the cobalt-catalysed hydro-formylation. This product may result from a 2-alkylcobalt intermediate formed when reaction of (C) leads to an isomer of (D), with hydrogenation then yielding a branched aldehyde as set out in Fig. 25.8 . When the linear aldehyde is required, such as for the synthesis of biodegradable detergents, the isomerization can be suppressed by the addi-tion of an alkylphosphine to the reaction mixture. One plausible explanation is that the replacement of CO by a bulky ligand disfavours the formation of complexes of sterically crowded 2-alkenes:

CoPBu3OC

OCCo

PBu3OCOC

K << 1

Here again we see an example of the powerful infl uence of ancillary ligands on catalysis. Another effective hydroformylation catalyst precursor is [Rh(CO)H(PPh 3 ) 3 ] ( 9 ), which

loses a phosphine ligand to form the coordinatively unsaturated 16-electron complex

Rh

H

PPh3

C

O

9 [Rh(CO)H(PPh 3 ) 3 ]

H

Co

COOC

OC

H

CoCO

OC

OC

CO

CO

CoCO

OCOC

CO

CO

CoCO

OCOC

H

CoCO

OCOC

R

R

CO

CO

CO

A

B

C

D′E′

R

R

CHO

R

H2

H

Co

COOC

OC

H

CoCO

OC

OC

CO

CO

CoC

OCOC

CO

R

CO

CoCO

OCOC

R

H

CoCO

OCOC

R

RR

CHO

CO

CO

CO

A

B

C

DE

H2

O Figure 25.7 The catalytic cycle for the hydroformylation of alkenes by a cobalt carbonyl catalyst.

Figure 25.8 The formation of branched aldehydes in hydroformylation reactions occurs when the alkyl group is not terminally bound.


738 25 Catalysis

[Rh(CO)H(PPh 3 ) 2 ], which promotes hydroformylation at moderate temperatures and 1 atm. This behaviour contrasts with the cobalt carbonyl catalyst, which typically requires 150°C and 250 atm. The rhodium catalyst is useful in the laboratory as it is effective under convenient conditions. Because it favours linear aldehyde products, it competes with the phosphine-modifi ed cobalt catalyst in industry. The cobalt catalyst is used for synthesis of medium- and long-chain aldehydes and the rhodium catalyst is used for hydroformylation of prop-1-ene.

E X AMPLE 25.2 Interpreting the infl uence of chemical variables on a catalytic cycle

An increase in CO partial pressure above a certain threshold decreases the rate of the cobalt-catalysed hydroformylation of 1-pentene. Suggest an interpretation of this observation.

Answer The decrease in rate with increasing partial pressure suggests that CO suppresses the concentration of one of the catalytic species. An increase in CO pressure will lower the concentration of [Co(CO) 3 H] in the equilibrium

[ ( ) ] [ ( ) ]Co CO H Co CO H CO4 3 +

This type of evidence was used as the basis for postulating the existence of [Co(CO) 3 H] as an important intermediate, even though it is not detected spectroscopically in the reaction mixture.

Self-test 25.2 Predict the infl uence of added triphenylphosphine on the rate of hydroformylation catalysed by [Rh(CO)H(PPh 3 ) 3 ].

E X AMPLE 25.1 Predicting the products from a hydroformylation reaction

Predict the products formed when pent-1-ene reacts with CO and H 2 in the presence of [Co 2 (CO) 8 ]. Comment on the effect of adding PMe 3 or PPh 3 to the reaction mixture. How would increasing the CO partial pressure affect the ratio of any linear and branched products?

Answer By analogy with the cycles in Figs 25.7 and 25.8 we would expect two possible intermediates to be formed following coordination of the alkene and hydrido migration:

CoCOOC

OCCo

COOCOC

and

Completion of the catalytic cycles will yield the linear and the branched products, CH 3 CH 2 CH 2 CH 2 CH 2 CHO and CH 3 CH 2 CH 2 CH(CHO)CH 3 respectively. The added phosphine would coordinate to the catalysts and the increased steric crowding would inhibit production of the branched product. This effect would be greater for PPh 3 than for PMe 3 . Increasing the CO pressure would reduce the concentration of the coordi-nation ively unsaturated [Co(CO) 3 H] species. This species allows the coordinated alkene to isomerize via β-hydride elimination. Thus, increasing the CO pressure will favour the linear alkene.

Self-test 25.1 Predict the product or products from the hydroformylation of cyclohexene.

25.6 Wacker oxidation of alkenes Key points: The Wacker process is used to produce ethanal from ethene and oxygen; the most success-ful system uses a palladium catalyst to oxidize the alkene, with the palladium being reoxidized via a secondary copper catalyst.

The Wacker process is used primarily to produce ethanal (acetaldehyde) from ethene and oxygen:

C H O CH CHO 197 kJ mol2 4 2 3 r1+ → = − −Δ G<

Its invention at the Wacker Consortium für Elektrochemische Industrie in the late 1950s marked the beginning of an era of production of chemicals from petroleum feedstock. Although the Wacker process is no longer of major industrial concern, it has some interest-ing mechanistic features that are worth noting.



The actual oxidation of ethene is known to be caused by a palladium(II) salt:

C H PdCl H O CH CHO Pd 2 HCl2 4 2 2 3+ + → + +( )0

The exact nature of the Pd(0) species is unknown, but it probably is present as a mixture of compounds. The slow oxidation of Pd(0) back to Pd(II) by oxygen is catalysed by the addition of Cu(II), which shuttles back and forth to Cu(I):

Pd 2 CuCl Pd 2 CuCl 4 Cl

2 CuCl O 2 4

2 22

2 2

( ) [ ] [ ]

[ ]

012

+ → + ++ +

− + − −

− HH 4 Cl 2 CuCl H O42

2+ − −+ → +[ ]

The overall catalytic cycle is shown in Fig. 25.9 . Detailed stereochemical studies on related systems indicate that the hydration of the alkene/Pd(II) complex (B) occurs by the attack of H 2 O from the solution on the coordinated ethene rather than the insertion of coordinated OH. Hydration, to form (C), is followed by two steps that isomerize the coordinated alco-hol. First, β-hydrogen elimination occurs with the formation of (D), and then migration of a hydride results in the formation of (E). Elimination of the ethanal and an H + ion then leaves Pd(0), which is converted back to Pd(II) by the auxiliary copper(II)-catalysed air oxidation cycle.

One important observation that the mechanism must account for is that, when the reaction is carried out in the presence of D 2 O, no deuterium is incorporated into the fi nal product. This observation suggests that either intermediate (D) is very short-lived and does not exchange the Pd–H for a Pd–D, or that intermediate (C) rearranges directly to (E).

Alkene ligands coordinated to Pt(II) are also susceptible to nucleophilic attack, but only palladium leads to a successful catalytic system. The principal reason for palladium’s unique behaviour appears to be the greater lability of the 4d Pd(II) complexes in compari-son with their 5d Pt(II) counterparts. Furthermore, the potential for the oxidation of Pd(0) to Pd(II) is more favourable than for the corresponding Pt couple.

25.7 Asymmetric oxidations Key point: Appropriate chiral ligands can be used in conjunction with d-metal catalysts to induce chi-rality into oxidation products of organic substrates.

In addition to catalysing reductions, d-metal complexes are also active in oxidations. For example, in the Sharpless epoxidation , prop-2-en-1-ol (allyl alcohol) or a derivative is oxi-dized with tert -butylhydroperoxide, in the presence of a Ti catalyst with diethyl tartrate as a chiral ligand, producing an epoxide:

Pd(II)

Pd(0)

H2C CH2

Pd(II)

Pd(II)

OH

Pd(II)

OH

O

Pd(II)H

OH

2 Cu(I)

2 Cu(II)

O2 + 2 H+

H2O

H2O

H+

+ H+

A

B

C

D

E

F

Figure 25.9 The catalytic cycle for the palladium-catalysed oxidation of alkenes to aldehydes.


740 25 Catalysis

OH

O

OH

HO

EtO2C CO2Et

OH

Ti(OR)4

+ tBuOOH + tBuOH

The reaction is thought to go through a transition state in which both the peroxide and the allyl alcohol are coordinated to a Ti atom through their O atoms. Each Ti atom is known to have one diethyl tartrate attached to it, and the chiral environment that the diethyl tartrate produces around the Ti atom is suffi cient to differentiate the two prochiral faces of the allyl alcohol. Additional experimental evidence points to a dimeric intermediate ( 10 ). Enantiomeric excesses of greater than 98 per cent have been reported for Sharpless epoxidations.

A Jacobsen oxidation is a reaction in which the catalyst is a Mn complex of the mixed 2N,2O donor ligand known as salen ( Fig. 25.10 ). Hypochlorite ions (ClO − ) are used to oxidize the Mn(III) complex to a Mn(V) oxide, which is then able to deliver its O atoms to an alkene to generate an epoxide. This oxidation has been used with a wide variety of substrates and routinely delivers enantiomeric excesses greater than 95 per cent. The mechanism of the reaction has not been defi ned precisely, but proposals include the exist-ence of a dimeric form of the catalyst or a radical oxygen transfer step.

25.8 Palladium-catalysed CeC bond-forming reactions Key points: A number of palladium-catalysed coupling reactions are known; they all proceed through oxidative addition of reagents at the metal centre followed by the reductive elimination of the two fragments.

A large number of palladium-catalysed carbon–carbon bond-forming (‘coupling’) reac-tions are known. They include the coupling of a Grignard reagent with an aryl halide and the Heck, Stille, and Suzuki coupling reactions: 4

XR′

R′R R

X

R

E

R′ R R′

catalyst

base

+

Suzuki: E = B(OH)2

Stille: E = SnR3

catalyst

base+Heck

Normally, either a Pd(II) complex, such as [PdCl 2 (PPh 3 ) 2 ], in the presence of additional phosphine or a Pd(0) compound, such as [Pd(PPh 3 ) 4 ], is used as the catalyst, although

O

EtO2C CO2Et

O

O

EtO2C CO2Et

O

Ti Ti

O

OtBu

O

O

tBu

O

O

10

4 The 2010 Nobel Prize in Chemistry was awarded to Richard Heck, Akira Suzuki, and Ei-ichi Negishi for palladium-catalysed coupling reactions.

N

O

N

O

tBu

tBu

tBu

tBu

Mn

Cl

ON

O

N

O

tBu

tBu

tBu

tBu

Mn

Cl

ClO–

R

O

R

+ Cl–

Figure 25.10 The Jacobsen epoxidation relies on a manganese complex of a salen-based ligand and chlorate(I).



many other Pd/ligand combinations are active. The precise reaction pathway is unclear (and probably differs with each Pd/ligand/substrate combination) but it is apparent that all these reactions follow the same general sequence. Figure 25.11 shows an idealized catalytic cycle for the coupling of an ethenyl group with an aryl halide. An initial oxida-tive addition of an aryl–halogen bond to an unsaturated Pd(0) complex (A) results in a Pd(II) species (B). Coordination of an alkene results in complex (C); 1,2-insertion results in an alkyl complex (D), which can be deprotonated with the loss of the halide to give the organic product attached to the palladium atom (E).

In other palladium-catalysed coupling reactions, such as that of a Grignard reagent with an aryl halide, initial oxidative addition proceeds as in Fig. 25.11 . The second organic group is thought to be introduced with the Grignard reagent behaving as the nucleophilic R − group displacing the halide at the metal centre in (B), to give two organic fragments attached to the Pd atom, as indicated in Fig. 25.12 . These two adjacent fragments can then couple and reductively eliminate to regenerate the starting Pd(0) species (A).

In all palladium-catalysed coupling reactions, it is necessary for the two fragments that are coupling to be cis to each other at the metal centre before insertion or reductive elimi-nation can take place; this requirement has led to the use of chelating diphosphines such as dppe ( 11 ) and the ferrocene derivative ( 12 ).

Palladium-catalysed coupling reactions are tolerant to a wide range of substitution on both fragments, and a versatile reaction that takes place at room temperature and in aque-ous solution is the Sonogashira coupling:

HC CR R X R C CR HXPd(0)/Cu(I) catalystbase≡ + ′ ⎯ →⎯⎯⎯⎯⎯⎯ ′ ≡ +

The two catalysts are a Pd(0) complex, such as [PdCl 2 (PPh 3 ) 2 ], and a Cu(I) halide. This reaction is used in the synthesis of many pharmaceuticals, including treatments for psoria-sis, Parkinson’s disease, Tourette’s syndrome, and Alzheimer’s disease. The Heck coupling is used in the synthesis of steroids, strychnine, and the herbicide Prosulfuron ® , which is produced industrially on a large scale. The CeC coupling step in the synthesis is shown below:

SO3–

N2+ CF3

+

SO3–

CF3

Pd catalyst

PPh2Ph2P

11 dppe

FePPh2

PPh2

12

Ph

Pd

PPh3

BrPh3P

Ph3P

Ph

Pd

PPh3

RPh3P

Ph3P

R– Br–

Figure 25.12 The exchange of a halide for an organic fragment at a Pd centre can be thought of as nucleophilic displacement.

Ph

Pd

PPh3

BrPh3P

Ph3P

Pd PPh3Ph3P

Ph3P

PdPh3P

Ph3P

Ph

PdBr

Ph3P

Ph3P

R

R

Ph

R

PdBr

Ph3P

Ph3P

R

R

Br

PPh3

HBr

PPh3

A

B

C

D

E

Figure 25.11 An idealized catalytic cycle for the coupling of a substituted prop-1-ene to an aryl halide in the Heck reaction.


742 25 Catalysis

25.9 Methanol carbonylation: ethanoic acid synthesis Key point: Rhodium and iridium complexes are highly active and selective in the carbonylation of methanol to form acetic acid.

The time-honoured method for synthesizing ethanoic (acetic) acid is by aerobic bacte-rial action on dilute aqueous ethanol, which produces vinegar. However, this process is uneconomical as a source of concentrated ethanoic acid for industry. A highly successful commercial process is based on the carbonylation of methanol:

CH OH CO CH COOH3 3+ → The reaction is catalysed by all three members of Group 9 (Co, Rh, and Ir). Originally a Co complex was used, but then a Rh catalyst developed at Monsanto greatly reduced the cost of the process by allowing lower pressures to be used. As a result, the rho-dium-based Monsanto process was used throughout the world. Subsequently, British Petroleum (now BP) developed the Cativa process , which uses a promoted Ir catalyst. Both processes are highly selective and generate ethanoic acid of suffi cient purity that it can be used in human food.

The Monsanto and Cativa processes follow essentially the same reaction sequence, so the rhodium-based cycle described here captures the principal features of the iridium-based process too ( Fig. 25.13 ). Under the conditions used, iodide ions react with methanol to set up an appreciable concentration of iodomethane in the fi rst step of the reaction. Starting with the four-coordinate, 16-electron complex [Rh(CO) 2 I 2 ] − (A), the next step is the oxidative addition of iodomethane to produce the six-coordinate, 18-electron complex [Rh(Me)(CO) 2 I 3 ] − (B). This step is followed by methyl migration, yielding a 16-electron acyl complex (C). Coordination of CO restores an 18-electron complex (D), which is then set to undergo reductive elimination of acetyl iodide with the regeneration of [Rh(CO) 2 I 2 ] − . Water then hydrolyses the acetyl iodide to acetic acid and regenerates HI. Under normal operating conditions, the rate-determining step for the rhodium-based system is the oxida-tive addition of iodomethane, whereas for the iridium-based system it is the migration of the methyl group. An important feature is that methyl migration on iridium is favoured by formation of a neutral intermediate, and iodide-accepting promoters help facilitate substi-tution of an iodide ligand by CO in the Ir analogue of complex (B).

Heterogeneous catalysis

Numerous industrial processes are facilitated by heterogeneous catalysis. Practical hetero-geneous catalysts are high-surface-area materials that may contain several different phases and operate at pressures of 1 atm and higher. In some cases the bulk of a high-surface-area material serves as the catalyst, and such a material is called a uniform catalyst. A simple

Me

Rh

I

COI

COI

CO

Rh

I

COI

CI

Rh

I

COI

CI

Rh

COI

COI

Me

Me

A

B

C

D

O

I

HI

CH3I

H2O

MeOH

H2O

CH3COOH

CO

O

O Figure 25.13 The catalytic cycle for the formation of ethanoic (acetic) acid with a rhodium-based catalyst. The oxidative addition step (A → B) is rate-determining.


743Heterogeneous catalysis

example is a very fi nely divided metal, as in skeletal nickel. Another example is the cata-lytic zeolite ZSM-5, which contains channels or pores through which molecules diffuse, producing a high internal surface area for the reaction. More often, multiphasic catalysts are used, which consist of a high-surface-area material that serves as a support on to which an active catalyst is deposited ( Fig. 25.14 ). Heterogeneous catalysts generally fall into two categories in terms of the location of the active surfaces. Many heterogeneous catalysts are fi nely divided solids where the active sites lie on the particle surfaces; others, particularly the microporous zeolite family and mesoporous materials, have pore-like structures, and the active sites are the internal surfaces, such as pores and cavities, within the individual crystallites. We shall discuss examples that illustrate some of the range of heterogene-ous catalysts, but fi rst we need to describe some of the unique mechanistic features they exhibit. We concentrate on the inorganic chemistry involved in reactions on surfaces, not the physical chemical aspects of adsorption and reaction.

25.10 The nature of heterogeneous catalysts There are many parallels between the individual reaction steps encountered in heterogene-ous and homogeneous catalysis, but we need to consider some additional points.

(a) Surface area and porosity

Key point: Heterogeneous catalysts are high-surface-area materials formed as either fi nely divided substrates or crystallites with accessible internal pores.

An ordinary dense solid is unsuitable as a catalyst because its surface area is quite low. Thus α-alumina, which is a dense material with a low specifi c surface area, is used much less as a catalyst support than the microcrystalline solid γ-alumina, which can be prepared with small particle size and therefore a high specifi c surface area (the surface area divided by the mass of the sample). The high surface area results from the many small but con-nected particles like those shown in Fig. 25.14 , and a gram or so of a typical catalyst sup-port has a surface area equal to that of a tennis court. Similarly, polycrystalline quartz is not used as a catalyst support but the high-surface-area versions of SiO 2 are widely used. In a typical heterogeneous catalyst this substrate surface is coated with active sites or par-ticles, such as metals or metal oxides, producing a large number of active sites.

Both γ-alumina and high-surface-area silica are metastable materials, but under ordi-nary conditions they do not convert to their more stable phases (α-alumina and polycrys-talline quartz, respectively). The preparation of γ-alumina involves the dehydration of an aluminium oxide hydroxide:

2 AlO(OH) -Al O H O2 3 2Δ⎯ →⎯ γ +

Similarly, high-surface-area silica is prepared from the acidifi cation of silicates to produce Si(OH) 4 , which rapidly forms a hydrated silica gel from which much of the adsorbed water can be removed by heating (Section 24.1a). When viewed with an electron microscope, the texture of the silica or alumina appears to be that of a rough gravel bed with irregularly shaped voids between the interconnecting particles (as in Fig. 25.14 ). Other high-surface-area materials used as supports in heterogeneous catalysts include TiO 2 , Cr 2 O 3 , ZnO, MgO, and carbon.

Zeolites (Section 24.11) are examples of uniform catalysts. They are prepared as very fi ne crystals that contain large regular channels and cages defi ned by the crystal structure ( Fig. 25.15 ). The openings in these channels vary from one crystalline form of the zeolite to the next, but are typically between 0.3 and 2 nm. The zeolite absorbs molecules small enough to enter the channels and excludes larger molecules. This selectivity, in combina-tion with catalytic sites inside the cages, provides a degree of control over catalytic reac-tions that is unattainable with silica gel or γ-alumina. The synthesis of new zeolites and similar shape-selective solids and the introduction of catalytic sites into them is a vigorous area of research (Section 24.12).

(b) Surface acidic and basic sites

Key point: Surface acids and bases are highly active for catalytic reactions such as the dehydration of alcohols and isomerization of alkenes.

Pt particleSilica gel

60 nm

Figure 25.14 Schematic diagram of metal particles supported on a fi nely divided silica such as silica gel.

H C

O

Al,Si

Figure 25.15* A view into the channels of zeolite theta-1 with an absorbed benzene molecule in the large central channel. (Based on A. Dyer, An introduction to molecular sieves . John Wiley & Sons (1988).)


744 25 Catalysis

When exposed to atmospheric moisture, the surface of γ-alumina is covered with adsorbed water molecules. Dehydration at 100 to 150°C leads to the desorption of water, but sur-face OH groups remain and act as weak Brønsted acids:

Al Al Al

OH OH OH

Al Al Al

OHO

+ H2O

At even higher temperatures, adjacent OH groups condense to liberate more H 2 O and generate exposed Al 3+ Lewis acid sites as well as O 2− Lewis base sites ( 13 ). The rigidity of the surface permits the coexistence of these strong Lewis acid and base sites, which would otherwise immediately combine to form Lewis acid–base complexes. Surface acids and bases are highly active for catalytic reactions such as the dehydration of alcohols and isomerization of alkenes. Similar Brønsted and Lewis acid sites exist on the interior of certain zeolites. Different oxides and their mixtures show variations in surface acidity; thus a SiO 2 /TiO 2 mixture is more acidic than SiO 2 /Al 2 O 3 , and promotes different catalytic reactions.

E X AMPLE 25.3 Using IR spectra to probe molecular interaction with surfaces

Infrared spectra of hydrogen-bonded pyridine complexes (X–H...py) show bands near 1540 cm −1 , and Lewis acid complexes of pyridine (py), such as Cl 3 Al(py), display bands near 1465 cm −1 , due to Al − py Lewis acid–base interactions. A sample of γ-alumina that had been pretreated by heating to 200°C and then cooled and exposed to pyridine vapour had absorption bands near 1540 cm −1 and none near 1465 cm −1 . Another sample that was heated to 500°C, cooled, and then exposed to pyridine had bands near 1540 and 1465 cm −1 . Correlate these results with the statements made in the text concerning the effect of heating γ-alumina. (Much of the evidence for the chemical nature of the γ-alumina surface comes from experiments like these.)

Answer The positions of the absorption bands seen in the IR spectrum are characteristic of the various functional groups of the molecules present. The types of species present at each stage of the reaction can be surmised by assigning the bands observed in the spectrum to these groups. In the text it is stated that when heated above 150°C the surface H 2 O is lost but OH − bound to Al 3+ remains. These groups, which appear to be mildly acidic as judged by colour indicators, interact with pyridine to produce absorption bands at about 1540 cm −1 that indicate the presence of hydrogen-bonded pyridine generated in the reaction

Al

OH + py

Al

O H py

When heated to 500°C, much, but not all, of the OH is lost as H 2 O, leaving behind O 2− and exposed Al 3+ . The evidence is the appearance of the 1465 cm −1 absorption bands, which are indicative of Al 3+ −NC 5 H 5 , as well as the 1540 cm −1 band from the residual O–H…py interactions.

Self-test 25.3 What intensities for the diagnostic IR bands would be expected in the spectrum obtained from a sample of γ -alumina heated to 900°C, cooled in the absence of water, and then exposed to pyridine vapour?

Highly active acidic and basic surfaces act as useful substrates for depositing other catalytic centres, particularly metal particles. Treatment of γ-alumina with H 2 PtCl 6 fol-lowed by heating in a reducing environment produces Pt particles of dimensions 1–50 nm distributed over the alumina surface.

(c) Surface metal sites

Key points: Very small metal particles on ceramic oxide substrates are very active catalysts for a range of reactions.

Metal particles are often deposited on supports to provide a catalyst. For example, fi nely divided Pt/Re alloys distributed on the surface of γ-alumina particles are used to inter-convert hydrocarbons, and fi nely divided Pt/Rh rhodium alloy particles supported on γ-alumina are used in the catalytic converters of vehicles to promote the combination of O 2 with CO and hydrocarbons to form CO 2 and the reduction of nitrogen oxides to

Al

O

HB

:

13



nitrogen ( Box 25.2 ). A supported metal particle about 2.5 nm in diameter has about 40 per cent of its atoms on the surface, and the particles are protected from fusing together into bulk metal by their separation. The high proportion of exposed atoms is a great advantage for these small supported particles, particularly for metals such as platinum and the even more expensive rhodium.

The metal atoms on the surface of metal clusters are capable of forming bonds such as MeCO, MeCH 2 R, MeH, and MeO ( Table 25.3 ). Often the nature of surface ligands is inferred by comparison of IR spectra with those of organometallic or inorganic com-plexes. Thus, both terminal and bridging CO groups can be identifi ed on surfaces by IR spectroscopy, and the IR spectra of many hydrocarbon ligands on surfaces are similar to those of discrete organometallic complexes. The case of the N 2 ligand is an interesting

BOX 25.2 Catalytic converters

Catalytic convertors are used to reduce toxic emissions from internal com-bustion engines, which include nitrogen oxides (NO x ), carbon monoxide, and unburnt hydrocarbons (HC). By 2009, when the Euro V emission stand-ards came into use in Europe, the levels of these compounds in exhaust gases were restricted to 0.50 (CO), 0.23 (NO x + HC), and 0.18 (HC) g km −1 , respectively; these values are similar to those demanded in California. The catalytic converter consists of a honeycomb stainless steel or ceramic struc-ture on to which silica and alumina are deposited, followed by a mixture of platinum, rhodium, and palladium as nanoparticles, with diameters typi-cally between 10 and 50 nm. A three-way converter, used with petrol (gaso-line) engines, catalyses the following three reactions:

2 NO g O g N g

2 CO g O g 2 CO g

C H g 2 O

2 2

2 2

2 2 2

x

x x

x

x

( ) ( ) ( )

( ) ( ) ( )

( )

→ ++ →

++ (( ) ( ) ( )g CO g 2 H O g2 2→ +x x

The fi rst stage of the catalytic converter involves reduction of the NO x on a reduction catalyst, which consists of a mixture of platinum and rhodium;

rhodium is highly reactive towards NO. The second stage involves catalytic oxidation, which removes the unburnt hydrocarbons and carbon monoxide by oxidizing them on the mixed metal platinum/palladium catalyst. A two-way catalytic converter used in conjunction with most diesel engines under-takes only the oxidation reactions (the second and third of those above).

In order to facilitate the near-complete conversion of the gases emerg-ing from the engine, the correct initial air : fuel mixture is used. The ideal stoichiometric air : fuel mixture entering the engine is 14.7 : 1 so that after combustion the gases entering the catalytic converter are about 0.5 per cent oxygen. If richer or leaner air : fuel mixtures are used (that is, with lower or higher air contents, respectively), then the level of oxygen enter-ing the exhaust stream may be too high or low for effective operation of the catalytic converter. For these reasons, various metal oxides, particularly Ce 2 O 3 and CeO 2 , are incorporated into the catalytic coating to store and release oxygen as the oxygen content of the gases emerging from the engine changes.

Table 25.3 Chemisorbed ligands on surfaces

NH3

Al3+ Pt

OO

Pt Pt

H

Pt

H

Pt Pt

CH2

CH3

H

Pt

N

Fe Fe

N

FeFeFe

a b c d e f

H–

Zn2+

H+

O2–

g

Pt

h

O2–

M2+

O2–

M2+

k

O

M

O–

j

Pt Pt

i

a Ammonia adsorbed on the Lewis and Al 3+ sites of γ-alumina.

b,c CO coordinated to platinum metal.

d Hydrogen dissociatively chemisorbed on platinum metal.

e Ethane dissociatively chemisorbed on platinum metal.

f Nitrogen dissociatively chemisorbed on iron metal.

g Hydrogen dissociatively chemisorbed on ZnO.

h Ethene η 2 coordinated to a Pt atom.

i Ethene bonded to two Pt atoms.

j Oxygen bound as a superoxide to a metal surface.

k Oxygen dissociatively chemisorbed on a metal surface.

Adapted from R.L. Burwell, Jr., Heterogeneous catalysis. Surv. Prog. Chem. , 1977, 8 , 2.


746 25 Catalysis

contrast because coordinated N 2 was identifi ed by IR spectroscopy on metal surfaces before di nitrogen complexes had been prepared.

The development of new techniques for studying single-crystal surfaces has greatly expanded our knowledge of the surface species that may be present in catalysis. For exam-ple, desorption of molecules from surfaces (thermally or by ion or atom impact) combined with mass spectrometric analysis of the desorbed substance provides insight into the chemi-cal identity of surface species. Similarly, Auger and X-ray photoelectron spectroscopy (XPS; Section 8.9) provide information on the elemental composition of surfaces. Low-energy electron diffraction (LEED) provides information about the structure of single-crystal sur-faces and, when adsorbate molecules are present, their arrangement on the surface. One important fi nding from LEED is that the adsorption of small molecules on a surface may bring about a structural modifi cation of the surface. This surface reconstruction is often observed to reverse when desorption occurs. Scanning tunnelling microscopy (STM; Section 8.16) provides an unrivalled method for locating adsorbates on surfaces. This striking tech-nique provides a contour map of single-crystal surfaces at, or close to, atomic resolution.

Although most of these modern surface techniques cannot be applied to the study of supported multiphasic catalysts, they are very helpful for revealing the range of probable surface species and circumscribing the structures that may plausibly be invoked in a mech-anism of heterogeneous catalysis. The application of these techniques to heterogeneous catalysis is similar to the use of X-ray diffraction and spectroscopy for the characterization of organometallic homogeneous catalyst precursors and model compounds.

(d) Chemisorption and desorption

Key point: Adsorption is essential for heterogeneous catalysis to occur but must not be so strong that it blocks the catalytic sites and prevents further reaction.

The adsorption of molecules on surfaces often activates molecules just as coordination activates molecules in complexes. The desorption of product molecules that is necessary to refresh the active sites in heterogeneous catalysis is analogous to the dissociation of a complex in homogeneous catalysis.

Before a heterogeneous catalyst is used it is usually ‘activated’. Activation is a catch-all term. In some instances it refers to the desorption of adsorbed molecules such as water from the surface, as in the dehydration of γ-alumina. In other cases it refers to the prepara-tion of the active site by a chemical reaction, such as by reduction of metal oxide particles to produce active metal particles.

An activated surface can be characterized by the adsorption of various inert and reac-tive gases. The adsorption may be either physisorption , when no new chemical bond is formed, or chemisorption , when surface−adsorbate bonds are formed ( Fig. 25.16 ). Low-temperature physisorption of a gas such as nitrogen is useful for the determination of the total surface area of a solid, whereas chemisorption is used to determine the number of exposed reactive sites. For example, the dissociative chemisorption of H 2 on supported platinum particles reveals the number of exposed surface Pt atoms.

The interaction of small molecules with metal surfaces is similar to their interaction with low-oxidation-state metal complexes. Table 25.4 shows that a wide range of metals

Ni

H2

Ni

H H

(a)

(b)

Figure 25.16 Schematic representation of (a) physisorption and (b) chemisorption of hydrogen on a nickel metal surface.

Table 25.4 The abilities of metals to chemisorb simple gas molecules*

O 2 C 2 H 2 C 2 H 4 CO H 2 CO 2 N 2

Ti, Zr, Hf, V, Ta, Cr, Mo, W, Fe, Ru, Os + + + + + + +

Ni, Co + + + + + + +

Rh, Pd, Pt, Ir + + + + + + +

Mn, Cu + + + + ± + +

Al, Au + + + + − − −

Na, K + + − − − − −

Ag, Zn, Cd, In, Si, Ge, Sn, Pb, As, Sb, Bi + − − − − − −

* Chemisorption is strong (+), weak (±), unobservable (−). Adapted from G.C. Bond, Heterogeneous catalysis , Oxford University Press (1987).



chemisorb CO, and that fewer are capable of chemisorbing N 2 , just as there is a much wider variety of metals that form carbonyls than form dinitrogen complexes. Furthermore, just as with metal carbonyl complexes, both bridging and terminal CO surface species have been identifi ed by IR spectroscopy. The dissociative chemisorption of H 2 is analogous to the oxidative addition of H 2 to metal complexes (Sections 10.5 and 22.22).

Although adsorption is essential for catalysis to occur, it must not be so strong as to block the active sites and prevent further reaction. This factor is in part responsible for the limited number of metals that are effective catalysts. The catalytic decomposition of methanoic (formic) acid on metal surfaces,

HCOOH CO H OM2⎯ →⎯ +

provides a good example of this balance between adsorption and catalytic activity. It is observed that the catalysis is most effective using metals for which the metal methanoate is of intermediate stability ( Fig. 25.17 ). The plot in Fig. 25.17 is an example of a ‘volcano dia-gram’, and is typical of many catalytic reactions. The implication is that the earlier d-block metals form very stable surface compounds whereas the later noble metals such as silver and gold form very weak surface compounds, both of which are detrimental to a catalytic process. Between these extremes the metals in Groups 8 to 10 have high catalytic activity, especially the platinum metals (Group 10). In Section 25.4 we saw a similar high activity of these metal complexes in the homogeneous catalysis of hydrocarbon transformations.

The active sites of heterogeneous catalysts are not uniform, and many diverse sites are exposed on the surface of a poorly crystalline solid such as γ-alumina or a noncrystalline solid such as silica gel. However, even highly crystalline metal particles are not uniform. A crystalline solid has typically more than one type of exposed plane, each with its char-acteristic pattern of surface atoms ( Fig. 25.18 ). In addition, single-crystal metal surfaces have irregularities such as steps that expose metal atoms with low coordination numbers ( Fig. 25.19 ). These highly exposed, coordinatively unsaturated sites appear to be particu-larly reactive. As a result, the different sites on the surface may serve different functions in catalytic reactions. The variety of sites also accounts for the lower selectivity of many heterogeneous catalysts in comparison with their homogeneous analogues.

(e) Surface migration

Key point: Adsorbed atoms and molecules migrate over metal surfaces.

The surface analogue of fl uxional mobility in clusters is diffusion, and there is abundant evidence for the diffusion of chemisorbed molecules or atoms on metal surfaces. For example, adsorbed H atoms and CO molecules are known to move over the surface of a metal particle. These diffusion pathways generally involve the adsorbed molecules moving through a variety of different coordination sites on the metal surface. So, for example, CO migration can result from a molecule moving between sites interacting with one (terminal CO) and between two and four (bridging CO) metal atoms on the surface. The energy bar-rier to this process is relatively low—a few tens of kilojoules per mole—and thus migration rates are very high under typical catalytic reaction conditions. This mobility is important in catalytic reactions as it allows atoms or molecules to fi nd and approach one another rapidly.

25.11 Hydrogenation catalysts Key points: Alkenes are hydrogenated on supported metal particles by a process that involves H 2 dis-sociation and migration of H⋅ to an adsorbed ethene molecule. Skeletal nickels can be used to reduce alkanals to alkanols.

A milestone in heterogeneous catalysis was Paul Sabatier’s observation in 1890 that nickel catalyses the hydrogenation of alkenes. He was in fact attempting to synthesize [Ni(C 2 H 4 ) 4 ] in response to Mond, Langer, and Quinke’s synthesis of [Ni(CO) 4 ] (Section 22.18). However, when he passed ethene over heated nickel he detected ethane, so he repeated the experiment but included hydrogen with the ethene, whereupon he observed a good yield of ethane.

The hydrogenation of alkenes on supported metal particles is thought to proceed in a manner very similar to that in metal complexes. As pictured in Fig. 25.20 , H 2 , which is

600

500

400

300

T / K

– 2

50

– 3

50

– 4

50

ΔrH< / kJ mol–1

Au

Ag

Rh

Pd

Pt Ir

Ru

Cu

Co

Ni

Fe

W

Figure 25.17 A ‘volcano diagram’, in this case the reaction temperature for a set rate of methanoic (formic) acid decomposition plotted against the stability of the corresponding metal methanoate as judged by its enthalpy of formation. (Based on W.J.M. Rootsaert and W.M.H. Sachtler, Z. Phyzik. Chem . 1960, 26 , 16.).

(100)

(001)

(111)

(010)

(111)–

–

(111)–

Figure 25.18 Some possible metal crystal planes that might be exposed on a metal surface to a reactive gas. The planes labelled [I 1 1], [1 I 1], etc. are hexagonally close- packed and the planes represented by [100], [010], etc. have square arrays of atoms.

Kink

Step

Figure 25.19 Schematic representation of surface irregularities, steps, and kinks.


748 25 Catalysis

dissociatively chemisorbed on the surface, is thought to migrate to an adsorbed ethene molecule, giving fi rst a surface alkyl and then the saturated hydrocarbon. When ethene is hydrogenated with D 2 over platinum, the simple mechanism depicted in Fig. 25.20 indi-cates that CH 2 DCH 2 D should be the product. In fact, a complete range of C 2 H n D 6− n ethane isotopologues is observed. It is for this reason that the central step is written as reversible; the rate of the reverse reaction must be greater than the rate at which the ethane molecule is formed and desorbed in the fi nal step.

One of the most important classes of heterogeneous hydrogenation catalysts is the ‘skel-etal nickels’, sometimes referred to as ‘Raney nickel’, which are used for various processes such as conversion of alkanals (aldehydes) to alkanols (alcohols), as in

CH CH CH CHO H CH CH CH CH OH3 2 2 2 3 2 2 2+ →

and reduction of alkylchloronitroanilines to the corresponding amines. Skeletal nickels and similar catalytically active metal alloys are produced by preparing a metal alloy such as NiAl at high temperature and then selectively dissolving most of the aluminium by treatment with sodium hydroxide. Other metals, such as molybdenum and chromium, can be added to the original alloy and may act as promoters that affect the catalyst’s reactivity and selectivity for certain reactions. The resulting spongy or porous metals are rich in nickel (>90 per cent) and their high surface areas lead to very high catalytic activities. One further application of these catalysts is to the conversion of naturally occurring polyunsaturated fats, which are liquids, to solid polyhydrogenated fats, as in nondairy spreads.

25.12 Ammonia synthesis Key point: Catalysts based on iron metal are used for the synthesis of ammonia from nitrogen and hydrogen.

The synthesis of ammonia has already been discussed from several different viewpoints (Section 15.6). Here we concentrate on details of the catalytic steps. The formation of ammonia is exergonic and exothermic at 25°C, the relevant thermodynamic data being Δ r G < = −116.5 kJ mol −1 , Δ r H < = −146.1 kJ mol −1 , and Δ r S < = −199.4 J K −1 mol −1 . The nega-tive entropy of formation refl ects the fact that two NH 3 molecules form in place of four reactant molecules.

The great inertness of N 2 (and to a lesser extent H 2 ) requires that a catalyst be used for the reaction. Iron metal, together with small quantities of alumina and potassium salts and other promoters, is used as the catalyst. Extensive studies on the mechanism of ammonia synthesis indicate that the rate-determining step under normal operating conditions is the dissociation of N 2 coordinated to the catalyst surface. The other reactant, H 2 , undergoes much more facile dissociation on the metal surface and a series of insertion reactions between adsorbed species leads to the production of NH 3 :

N2(g) N2(g) N N

H2(g) H2(g) H H

N + H NH NH2 NH3 NH3(g)

H H

Because of the slowness of the N 2 dissociation, it is necessary to run the ammonia synthesis at high temperatures, typically 400°C. However, because the reaction is exothermic, high temperature reduces the equilibrium constant of the reaction. To recover some of this reduced yield, pressures in the order of 100 atm are used to favour the formation products. A catalyst operating at room temperature that could give good equilibrium yields of NH 3 , such as the enzyme nitrogenase (Section 26.13), has long been sought.

In the course of developing the original ammonia synthesis process, Haber, Bosch, and their co-workers investigated the catalytic activity of most of the metals in the periodic table and found that the best are Fe, Ru, and U, promoted by small amounts of alumina and potassium salts. Cost and toxicity considerations led to the choice of iron as the basis

Adsorb Adsorb

CH2=CH

2 D–D

CH2=CH

2D D

CH 2–CDH 2

CDH 2–CDH 2

D

Desorb

Figure 25.20 Schematic diagram of the stages involved in the hydrogenation of ethene by deuterium on a metal surface.



of the commercial catalyst. The role of the various promoters, particularly K, in the Fe metal catalyst had been the subject of much scientifi c research. G. Ertl 5 found that, in the presence of potassium, N 2 molecules adsorb more readily on the metal surface and the adsorption enthalpy is made more exothermic by about 12 kJ mol −1 , probably as a result of the increased electron-donating abilities of the Fe/K surface. The more strongly adsorbed N 2 molecule is then cleaved more easily in the rate-determining step in the process.

25.13 Sulfur dioxide oxidation Key point: The most widely used catalyst for the oxidation of SO 2 to SO 3 is molten potassium vanadate supported on a high-surface-area silica.

The oxidation of SO 2 to SO 3 is a key step in the production of sulfuric acid (Section 16.13). The reaction of sulfur with oxygen to produce SO 3 gas is exergonic (Δ r G < = −371 kJ mol −1 ) but very slow, and the principal product of combustion is SO 2 :

S s O g SO g2 2( ) ( ) ( )+ → The combustion is followed by the catalytic oxidation of SO 2 :

SO g O g SO g2 2 3( ) ( ) ( )+ →12

This step is also exothermic and thus, as with ammonia synthesis, has a less favourable equilibrium constant at elevated temperatures. The process is therefore generally run in stages. In the fi rst stage, the combustion of sulfur raises the temperature to about 600°C, but by cooling and pressurizing before the catalytic stage the equilibrium is driven to the right and high conversion of SO 2 to SO 3 is achieved.

Several quite different catalytic systems have been used to catalyse the combination of SO 2 with O 2 . The most widely used catalyst at present is potassium or caesium vanadate molten salt covering a high-surface-area silica. The current view of the mechanism of the reaction is that the rate-determining step is the oxidation of V(IV) to V(V) by O 2 ( Fig. 25.21 ). In the melt, the vanadium and oxide ions are part of a polyvanadate complex (Box 19.1), but little is known about the evolution of the oxo species.

25.14 Catalytic cracking and the interconversion of aromatics by zeolites Key points: Zeolite catalysts have strongly acidic sites that promote reactions such as isomerization via carbonium ions; shape selectivity may arise at various stages of the reaction because of the relative dimensions of the zeolite channels and the reactant, intermediate, and product molecules.

The zeolite-based (Sections 14.15 and 24.12) heterogeneous catalysts play an important role in the interconversion of hydrocarbons and the alkylation of aromatics as well as in oxidation and reduction. Two important zeolites used for such reactions are faujasite ( Fig. 25.22 ), also known as zeolite X or zeolite Y (the X or Y terminology is defi ned by the Si:Al

5 Gerhard Ertl was awarded the 2007 Nobel Prize for Chemistry for his work on chemical processes at solid surfaces.

O2–+ 2V(V)

2 V(IV)

SO2

SO31/2 O2

Figure 25.21 Cycle showing the key elements involved in the oxidation of SO 2 by V(V) compounds.

Figure 25.22 * The zeolite faujasite (also known as zeolite X or Y) framework structure, showing the large pores in which catalytic cracking occurs. Tetrahedra are SiO 4 or AlO 4 .


750 25 Catalysis

ratio of the material: X has a higher Al content) and zeolite ZSM-5, an aluminosilicate zeolite with a high Si content. 6 The channels of ZSM-5 consist of a three-dimensional maze of intersecting tunnels ( Fig. 25.23 ). As with other aluminosilicate catalysts, the Al sites are strongly acidic. The charge imbalance of Al 3+ in place of tetrahedrally coordinated Si(IV) requires the presence of an added positive ion. When this ion is H + ( Fig. 25.24 ) the Brønsted acidity of the aluminosilicate can be higher than that of concentrated H 2 SO 4 and is termed a superacid (Section 4.14); the turnover frequency for hydrocarbon reactions at these sites can be very high.

Natural petroleum consists of only about 20 per cent of alkanes suitable for use in petrol (gasoline) and diesel, with chain lengths ranging from C 5 H 12 (pentane) to C 12 H 26 . Conversion of the higher-molar-mass hydrocarbons to the valuable lighter ones involves breaking the CeC bonds but also structural rearrangement of the hydrocarbons through dehydrogenation, isomerization, and aromatization reactions. All these processes are cata-lysed by a solid acid catalyst based on alumina, silica, and zeolites. Acidic clays and mixed Al 2 O 3 /SiO 2 were originally used for this process in the 1940s but since the 1960s these catalysts have been largely superseded by zeolites. The principal zeolite used for catalytic cracking is zeolite Y, in which the extra-framework cations have been replaced with lan-thanoid ions, typically a mixture of La, Ce, and Nd. The mechanism of the catalytic crack-ing initially involves protonation of the alkane or alkene chain by the Brønsted acid sites in the zeolite pores, followed by cleavage of the CeC bond in the β-position to the C atom carrying the positive charge. For example,

RCH C HCH CH R RCH CH CH CH CH R2 2 2 2 2 2 2 2+ +′ → = + ′( )

Acidic zeolite catalysts also promote rearrangement reactions via carbonium ions. For example, the isomerization of 1,3-dimethylbenzene to 1,4-dimethylbenzene probably occurs by the following steps:

CH3

CH3

CH3

CH3

H

H CH3

CH3 CH3

CH3H

H

H+

H+

–H+

–H+

Reactions such as dimethylbenzene (xylene) isomerization and methylbenzene (toluene) disproportionation illustrate the selectivity that can be achieved with acidic zeolite catal-ysis. The shape selectivity of these zeolite catalysts has been attributed to a variety of processes.

In reactant selectivity the molecular sieving abilities of zeolites are important, as only molecules of appropriate size and shape can enter the zeolite pores and undergo a reaction.

6 The catalyst was developed in the research laboratories of Mobil Oil; the initials stand for Zeolite Socony-Mobil.

Figure 25.23 * The zeolite ZSM-5 structure, highlighting the channels along which small molecules may diffuse. Tetrahedra are SiO 4 or AlO 4 .

OSi

OAl

OSi

O

O O O O O O

OSi

OAl

OSi

O

O O O O O O

H

H

OSi

OAl

OSi

O

O O O O O O

+ BH+

B

Figure 25.24 The Brønsted acid site in H-ZSM5 and its interaction with a base, typically an organic molecule. (Based on W.O. Haag, R.M. Lago, and P.B. Weisz, Nature , 1984, 309 , 589.).



In product selectivity, a reaction-product molecule that has dimensions compatible with the channels will diffuse faster, allowing it to escape; molecules that do not fi t the channels diffuse slowly and, on account of their long residence in the zeolite, have ample oppor-tunity to be converted to the more mobile isomers that can escape rapidly. A currently more favoured view of zeolite selectivity is based on transition-state selectivity, where the orientation of reactive intermediates within the zeolite channels favours specifi c products. In the case of dimethylbenzene (xylene) isomerization, the narrower intermediates formed during the generation of 1,4-dialkylbenzene molecules fi t better within the pores. Another common reaction in zeolites is the alkylation of aromatics with alkenes.

E X AMPLE 25.4 Proposing a mechanism for the alkylation of benzene

In its protonated form, ZSM-5 catalyses the reaction of ethene with benzene to produce ethylbenzene. Write a plausible mechanism for the reaction.

Answer We should recall that protonated forms of zeolite catalysts are very strong acids. Therefore, a mechanism involving protonation of the organic species present in the system is the likely pathway. The acidic form of ZSM-5 is strong enough to generate carbocations from aliphatic hydrocarbons, so the initial stage would be

CH CH H CH CH2 2 2 3+ →+ +

As we saw in Section 4.10, the carbocation can attack benzene as a strong electrophile. Subsequent depro-tonation of the intermediate yields ethylbenzene:

CH CH C H C H CH CH H2 6 6 6 5 2 3+ ++ → +

Self-test 25.4 A pure silica analogue of ZSM-5 can be prepared. Would you expect this compound to be an active catalyst for benzene alkylation? Explain your reasoning.

Mesoporous silicates discovered in the 1990s (Section 24.29) have large, ordered arrays of pores in the range 12–20 nm and generate very high specifi c surface areas (of over 1000 m 2 g −1 ). The large pores allow larger molecules to undergo catalytic processes, although their acidity is weak compared with the zeolites. More importantly, other cata-lytic centres, such as nanoparticles of metals, alloys, and metal oxides such as platinum or Pt/Sn, may be deposited within the mesostructured channels. As one example, Co depos-ited on the mesoporous silica support MCM-41 promotes the cycloaddition of alkynes with alkenes and carbon monoxide to produce cyclopentenones. 7 This is an example of the Pauson–Khand reaction in which an alkene, an alkyne, and CO react together to form an unsaturated, fi ve-membered cyclic ketone:

O

EtO2C

EtO2CEtO2C

EtO2C20atm CO

130°C

Other porous inorganic framework materials (Section 24.14) are also being investigated as potential catalysts, either in their own right or as hosts for active species. For example, the d-metal constituents of metal–organic frameworks can take part in redox reactions and also have very large pores that can incorporate nanoparticles of metals, such as Pt.

25.15 Fischer–Tropsch synthesis Key point: Hydrogen and carbon monoxide can be converted to hydrocarbons and water by reaction over iron or cobalt catalysts.

The conversion of syngas , a mixture of H 2 and CO, to hydrocarbons over metal catalysts was fi rst discovered by Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for

7 MCM-41 stands for Mobil Crystalline Material type 41.


752 25 Catalysis

Coal Research in Mülheim in 1923. In the Fischer–Tropsch reaction, CO reacts with H 2 to produce hydrocarbons, which can be written symbolically as the formation of the chain extender (−CH 2 −), and water:

CO 2 H CH H O2 2 2+ → − − + The process is exothermic, with Δ r H < = −165 kJ mol −1 . The product range consists of ali-phatic straight-chain hydrocarbons that include methane (CH 4 ) and ethane, LPG (C 3 to C 4 ), gasoline (C 5 to C 12 ), diesel (C 13 to C 22 ), and light and heavy waxes (C 23 to C 32 and >C 33 , respectively). Side reactions include the formation of alcohols and other oxygenated products. The distribution of the products depends on the catalyst and the temperature, pressure, and residence time. Typical conditions for the Fischer–Tropsch synthesis are a temperature range of 200–350°C and pressures of 15–40 atm.

There is general agreement that the fi rst stages of the hydrocarbon synthesis involve the adsorption of CO on the metal, followed by its cleavage to give a surface carbide (and water), and the successive hydrogenation of such species to surface methyne (CH), methylene (CH 2 ), and methyl (CH 3 ) species, but there is still debate on what happens next and how chain growth occurs. One proposal suggests a polymerization of bridging sur-face −CH 2 − groups initiated by a surface −CH 3 group. However, the fact that many such species have been isolated and are stable as metal complexes suggests that the mechanism is probably not so simple. Other mechanistic investigations of these processes have sug-gested an alternative possibility for chain growth in the hydrocarbon synthesis, namely that it proceeds by combination of surface bridging −CH 2 − groups and alkenyl chains (MeCH = CHR), rather than by the combination of alkyl chains (MeCH 2 CH 2 R) with methylene groups in the surface.

Several catalysts have been used for the Fischer–Tropsch synthesis; the most important are based on Fe and Co. Cobalt catalysts have the advantage of a higher conversion rate and a longer life (of over fi ve years). The cobalt catalysts are in general more reactive for hydrogenation and produce fewer unsaturated hydrocarbons and alcohols than iron catalysts. Iron catalysts have a higher tolerance for sulfur, are cheaper, and produce more alkene products and alcohols. The lifetime of the iron catalysts is short, however, and in commercial installations generally limited to about eight weeks.

25.16 Electrocatalysis and photocatalysis Key points: Overpotentials represent the kinetic barrier for electrochemical reactions, and electrocata-lysts may be used to increase current densities in such processes. A photocatalyst enhances the rate of a photoreaction such as the splitting of water into hydrogen and oxygen.

Kinetic barriers are quite common for electrochemical reactions at the interface between a solution and an electrode and, as mentioned in Section 5.18, it is common to express these barriers as overpotentials η (eta), the potential in addition to the zero-current cell potential (the emf) that must be applied to bring about an otherwise slow reaction within the cell. The overpotential is related to the current density, j (the current divided by the area of the electrode), that passes through the cell by 8

j j ea= 0η

(25.3) where j 0 and a are best regarded for our purposes as empirical constants. The constant j 0 , the exchange current density , is a measure of the rates of the forward and reverse electrode reactions at dynamic equilibrium. For systems obeying these relations, the reaction rate (as measured by the current density) increases rapidly with increasing applied potential difference when aη > 1. If the exchange current density is high, an appreciable reaction rate may be achieved with only a small overpotential. If the exchange current density is low, a high overpotential is necessary. There is therefore considerable interest in increasing the exchange current density. In an industrial process an overpotential in a synthetic step is very costly because it represents wasted energy.

8 The exponential relation between the current and the overpotential is accounted for by the Butler–Volmer equation, which is derived by applying transition-state theory to dynamical processes at electrodes. See P. Atkins and J. de Paula, Physical chemistry , Oxford University Press and W. H. Freeman & Co (2009).



A catalytic electrode surface can increase the exchange current density and hence dra-matically decrease the overpotential required for sluggish electrochemical reactions, such as H 2 , O 2 , or Cl 2 evolution and consumption. For example, ‘platinum black’, a fi nely divided form of platinum, is very effective at increasing the exchange current density and hence decreasing the overpotential of reactions involving the consumption or evolution of H 2 . The role of platinum is to dissociate the strong HeH bond and thereby reduce the large barrier that this bond strength imposes on reactions involving H 2 . Palladium also has a high exchange current density (and hence requires only a low overpotential) for H 2 evolution or consumption.

The effectiveness of metals can be judged from Fig. 25.25 , which also gives insight into the process. The volcano-like plot of exchange current density against MeH bond enthalpy suggests that MeH bond formation and cleavage are both important in the cata-lytic process. It appears that an intermediate MeH bond energy leads to the proper bal-ance for the existence of a catalytic cycle, and the most effective metals for electrocatalysis are clustered around Group 10.

Ruthenium dioxide is an effective catalyst for both O 2 and Cl 2 evolution and it also is a good electrical conductor. It turns out that at high current densities RuO 2 is more effective for the catalysis of Cl 2 evolution than for O 2 evolution. RuO 2 is therefore extensively used as an electrode material in the commercial production of chorine. The electrode processes that contribute to this subtle catalytic effect do not appear to be well understood.

There is great interest in devising new catalytic electrodes, particularly ones that decrease the O 2 overpotential on surfaces such as graphite. Thus, tetrakis(4- N -methylpyri-dyl)porphyriniron(II), [Fe(TMPyP)] 4+ ( 14 ), has been deposited on the exposed edges of graphite electrodes (on which O 2 reduction requires a high overpotential) and the resulting electrode surface was found to catalyse the electrochemical reduction of O 2 . A plausible explanation for this catalysis is that [Fe(III)(TMPyP)] attached to the electrode (indicated below by an asterisk) is fi rst reduced electrochemically:

[ ( )( )]* [ ( )( )]*Fe III TMPyP e Fe II TMPyP+ − → The resulting [Fe(II)(TMPyP)] forms an O 2 complex:

[ ( )( )]* [ ( )( )( )]*Fe II TMPyP O Fe II O TMPyP2 2+ → This iron(II) porphyrin oxygen complex then undergoes reduction to water and hydrogen peroxide:

[ ( )( )]* [ ( )( )]* ( , )Fe II O TMPyP e H Fe II TMPyP H O H O2 2 2 2+ + → +− +n n n Although details of the mechanism are still elusive, this general set of reactions is in har-mony with electrochemical measurements and the known properties of iron porphyrins. The investigation of porphyrin iron complexes as catalysts was motivated by Nature’s use of metalloporphyrins for oxygen activation (Section 26.10).

One application where cheap and effi cient electrocatalysts are much needed is in PEM fuel cells (PEM stands for ‘proton exchange membrane’ and sometimes ‘polymer electrolyte membrane’; Box 5.1). As these systems operate at quite low temperatures (50–100°C), they are suitable for applications that involve transport and mobile power, such as mobile telephones (cellphones). The electrolyte, a polymer that conducts pro-tons but not electrons, separates an anode in contact with hydrogen gas and a cathode over which fl ows oxygen gas. As noted above, hydrogen gas is readily dissociated at the anode using a platinum-metal catalyst, but the oxygen reduction reaction at the cathode presents more of a problem. There is a substantial overpotential associated with this reduction, which considerably reduces the effi ciency of the cell, giving operating volt-ages near 0.7 V as compared with the theoretical value of 1.23 V. Platinum may again be used to catalyse the reaction but, even so, the reaction is ineffi cient and use of large quantities of platinum costly. Considerable research effort is being applied to fi nding better electrocatalysts, and recently the alloy Pt 3 Ni (111 surface) has proved to have greatly enhanced properties.

Light has the ability to drive reactions in inorganic chemistry—for example, CO sub-stitution reactions (Section 22.21), where UV light promotes the initial dissociation of the CO molecule from a metal centre. However, many reactions that should be driven by light are not, as the light energy is either weakly absorbed, re-emitted, or converted to heat.

N

N

N

N Fe

NN

N N

4(ClO4–)

14 [Fe(TMPyP)] 4+

9

7

5

3

2

10

–lo

g j

0

0 100 200 300 400

Tl In

CdGa

Ag

Bi

Pb

Zn

Fe

Co Ni

Au

Pt

Rh

Re

Ir

W

Mo

Nb

Ti

Ta

Sn

B(M–H) / kJ mol–1

Figure 25.25 The rate of H 2 evolution expressed as the logarithm of the exchange current density plotted against MeH bond energy.


754 25 Catalysis

A heterogeneous photocatalytic material will often absorb specifi c wavelengths of light strongly by electron promotion from a valence band into a conduction band. Provided the excited electron and residual hole in the valence band separate quickly (rather than recombining) they can travel to the surface of the photocatalyst and react with molecular species such as H 2 O, O 2 , or organic compounds to produce radicals. TiO 2 ( Box 25.3 ) is the most widely studied heterogeneous photocatalyst, though other oxides and complex oxides are also being researched.

25.17 New directions in heterogeneous catalysis Key point: Continuing developments in heterogeneous catalysis include research on new compositions for the controlled partial oxidation of hydrocarbons.

The development of solid-phase catalysts is very much a frontier subject for inorganic chemistry with the continuing discovery of compositions for promoting reactions, par-ticularly for petrochemicals. One very active area is the investigation of selective heter-ogeneous oxidation catalysts, which allow partial oxidation of hydrocarbons to useful intermediates in, for example, the polymer and pharmaceutical industries. Examples of such reactions include alkene epoxidation, aromatic hydroxylation, and ammoxidation (an oxidation in the presence of ammonia that generates nitriles) of alkanes, alkenes, and alkyl aromatics. In all these cases it is desirable to produce the products without complete oxidation of the hydrocarbon to carbon dioxide.

One example where new catalysts are being investigated is in the partial oxidation of benzene to phenol. At present, the three-step cumene process produces about 95 per cent of the phenol used in the world and gives propanone (acetone) as a by-product, although the market for acetone is oversupplied from other industrial processes. The cumene pro-cess involves three stages, namely alkylation of benzene with propene to form cumene (a process catalysed by phosphoric acid or aluminium chloride), direct oxidation of cumene to cumene hydroperoxide using molecular oxygen, and fi nally cleavage of cumene hydrop-eroxide to phenol and acetone, which is catalysed by sulfuric acid. Far better would be a single-stage process that accomplishes the reaction

C H O C H OH6 6 2 6 5+ →12

BOX 25.3 Titanium dioxide photocatalysts

Photocatalysis is the catalysis of a photoreaction. Absorption of UV radia-tion generates free radicals on the catalyst surface, which then take part in the reaction. The best known photocatalyst is TiO 2 . TiO 2 has been used for centuries as a white pigment (Section 24.16) and the result of its pho-tocatalytic activity (Section 24.17) can be observed in the fl aking of white paint and the dicolouration of uPVC in window frames. TiO 2 occurs natu-rally in two crystal forms, rutile and anatase, and anatase is much more photoactive than rutile. Both forms have a wide band gap (anatase 3.2 eV, rutile 3.1 eV) and absorb radiation in the UV region below 390 nm. This absorption leads to the formation of pairs of electrons and holes. The holes diffuse to the TiO 2 surface and react with adsorbed water molecules, forming hydroxyl radicals, • OH. The electrons typically react with molecular oxygen to produce superoxide radical anions O2

i− These very reactive spe-cies then take part in the catalysed reaction.

TiO 2 attracted much interest as a photocatalyst when electrolysis of water to produce H 2 was observed using Pt deposited on TiO 2 . Any organic compounds present were also oxidized and this oxidizing power led to the development of TiO 2 for the treatment of pollutants and the purifi cation of water. In these applications, a slurry of powdered TiO 2 in water can be used, although it is often immobilized on a solid support for ease of handling and

separation. The oxidation reactions take place only at the surface of the TiO 2 , so thin fi lms are more effective and more effi cient than bulk powders. Thin fi lms of anatase are used as photocatalytic self-cleaning coatings ( Box 24.4 ) on window glass and blinds. The cleaning process is only effective when the number of incident photons is much greater than the number of organic molecules arriving on the surface, so the degree of self-cleaning is limited in some climates.

The photocatalytic decomposition process can also be applied to micro-organisms, and E. coli cells are destroyed after one hour of exposure to sunlight on a TiO 2 surface. Light generated from incandescent and fl uores-cent bulbs or sunlight entering buildings is much less intense than direct sunlight, so until recently this property of TiO 2 had limited applications indoors. However, if TiO 2 is doped with Cu or Ag the photocatalytic anti-bacterial activity is enhanced. The reactive species generated on the TiO 2 surface attack the cell membrane as before but this is followed by migration of the copper or silver ions into the cell, killing it.

Other applications of TiO 2 photocatalysts include sterilization of surgical instruments, removal of unwanted fi ngerprints from sensitive optical com-ponents, antifouling marine paints, cleanup of crude oil, decontamination of water, and decomposition of polyaromatic hydrocarbon pollutants.


755Heterogenized homogeneous and hybrid catalysis

Examples of catalysts investigated for this and similar processes include iron-containing zeolites with the silicalite framework type (Section 24.12), mixtures of FeCl 3 /SiO 2 , photo-catalysts based on Pt/H 2 SO 4 /TiO 2 , and various vanadium salts.

Heterogenized homogeneous and hybrid catalysis

Chemists have begun to look at catalytic systems that cannot simply be defi ned as either homogeneous or heterogeneous. Research has centred on trying to achieve the best of both systems: the high selectivity of homogeneous catalysts coupled with the ease of separa-tion of heterogeneous catalysts. This approach is sometimes referred to as ‘heterogenizing’ homogeneous catalysts.

25.18 Oligomerization and polymerization Key points: Ethene can be oligomerized to linear alkenes by homogeneous catalysis with a nickel catalyst. Heterogeneous Ziegler–Natta catalysts are used in alkene polymerization; the Cossee–Arlman mechanism describes their function; low-molar-mass homogeneous catalysts also catalyse the alkene polymerization reaction; considerable control over polymer tacticity is possible with judi-cious ligand design.

The development of alkene polymerization catalysts in the second half of the twentieth century, producing polymers such as polypropene and polystyrene, ushered in a revolution in construction materials, fabrics, and packaging. Polyalkenes are most often prepared by the use of organometallic catalysts. The catalysts used can be homogeneous, heterogenized homogeneous, or heterogeneous, and polymerization provides a good example of how homogeneous catalysis has infl uenced the design of industrially important heterogeneous catalysts. All three types of catalyst are discussed in this section.

Ethene is readily available from natural gas and petroleum by steam cracking of heavier hydrocarbons. It can be converted to the much more valuable long-chain alkenes, some-times still referred to as olefi ns, by processes such as the Shell Higher Olefi n Process (SHOP). SHOP was developed for the conversion of ethene to C10–C14 internal alkenes—that is, alkenes in which the double bond is not at the end of the carbon chain. The product alk-enes are mainly converted into linear primary alcohols for use in detergents, but they can be modifi ed to obtain linear alkenes of just about any range. SHOP is a three-step process. The fi rst step is homogeneously catalysed alkene oligomerization to form short chains of up to 10 monomer units. The catalyst is generated in situ from bis(cyclooctadiene)nickel(0) and a bidentate phosphine carboxylate ligand. A nickel hydride complex is generated by displacement of the cycooctadiene by an incoming ethene molecule. An initial hydride shift (1,2-insertion reaction; Section 22.25) is followed by successive alkene migrations to build the oligomer:

O

Ni

H

PPh2

O O

Ni

PPh2

CH2CH3

O O

Ni

PPh2

CH2CH3

O

O

Ni

CH2CH2CH2CH3

PPh2

OCH2=CH2

CH2=CH2

The hydrocarbon chain fi nally terminates by β-elimination to produce a terminal alkene:

O

Ni

CH2CH2(CH2CH2)nCH2CH3

PPh2

O O

Ni

PPh2

O

+ CH2=CH(CH2CH2)nCH2CH3H


756 25 Catalysis

PN

P

O

O O

O

15

The products are linear α-alkenes (with the double bond between the fi rst and second carbon atoms) with between 4 and 20 carbon atoms, and are separated by fractional distillation.

The products may then undergo isomerisation or metathesis to internal alkenes (Section 25.3) or be further processed into aldehydes and alcohols by hydroformylation (Section 25.5). Whereas the SHOP system is not very selective (the products have to be separated), some other catalysts are very effective for selective oligomerization. For instance, a homo-geneous chromium catalyst generated in situ by mixing a Cr(II) or Cr(III) halide with a so-called PNP phosphine ( 15 ), followed by activation with methyaluminumoxane (MAO) under ethene, gives a system that is very active for trimerization of ethene, resulting in 99.9 per cent 1-hexene. This system generates no polymer by-product, which is important in industry where reactors need to be kept free from solid materials. Changing the PNP ligand can alter the products; Ph 2 PN(iPr)PPh 2 generates 1-octene and 1-hexene as the major products.

In the 1950s J.P. Hogan and R.L. Banks discovered that chromium oxides supported on silica, a so-called Philips catalyst , polymerized alkenes to long-chain polyenes. Also in the 1950s, K. Ziegler, working in Germany, developed a catalyst for ethene polymerization based on a catalyst formed from TiCl 4 and Al(C 2 H 5 ) 3 , and soon thereafter G. Natta in Italy used this type of catalyst for the stereospecifi c polymerization of propene. Both the Ziegler–Natta catalysts and the chromium-based heterogeneous polymerization catalysts are widely used today.

The full details of the mechanism of Ziegler–Natta catalysts are still uncertain, but the Cossee–Arlman mechanism is regarded as highly plausible ( Fig. 25.26 ). The catalyst is prepared from TiCl 4 and Al(C 2 H 5 ) 3 , which react to give polymeric TiCl 3 mixed with AlCl 3 in the form of a fi ne powder. The alkylaluminium alkylates a Ti atom on the sur-face of the solid and an ethene molecule coordinates to the neighbouring vacant site. In the propagation steps for the polymerization, the coordinated alkene undergoes a migra-tory insertion reaction. This migration opens up another neighbouring vacancy, and so the reaction can continue and the polymer chain can grow. The release of the polymer from the metal atom occurs by β-hydrogen elimination, and the chain is terminated. Some catalyst remains in the polymer, but the process is so effi cient that the amount is negligible.

The proposed mechanism of alkene polymerization on a Philips catalyst involves the initial coordination of one or more alkene molecules to a surface Cr(II) site followed by rearrangement to metallocycloalkanes on a formally Cr(IV) site. Unlike Ziegler–Natta catalysts, the solid-phase catalyst does not need an alkylating agent to initiate the polym-erization reaction; instead, this species is thought to be generated by the metallocycloal-kane directly or by formation of an ethenylhydride by cleavage of a CeH bond at the chromium site.

Homogeneous catalysts related to the Philips and Ziegler–Natta catalysts provide addi-tional insight into the course of the reaction and are of considerable industrial signifi -cance in their own right, being used commercially for the synthesis of specialized polymers.

TiCl4

TiEtCl

ClCl

TiEtCl

ClCl

TiCl

ClCl

Et

TiCl

ClCl

Et

etc.

AlEt3

Figure 25.26 The Cossee–Arlman mechanism for the catalytic polymerization of ethene. Note that the Ti atoms are not discrete but are part of an extended structure containing bridging chlorides.



ZrL

Me

+

16 [Zr(η 5—Cp) 2 (CH 3 )L] +

Kaminsky catalysts utilize metals from Group 4 (Ti, Zr, Hf) and are based on a bis(cyclopentadienyl) metal system: the tilted-ring complex [Zr(η 5 -Cp) 2 (CH 3 )L] + ( 16 ) is a good example. These Group 4 metallocene complexes catalyse alkene polymerization by successive insertion steps that involve prior coordination of the alkene to the elec-trophilic metal centre. Catalysts of this type are used in the presence of a co-catalyst, the so-called methyl aluminoxane (MAO), a poorly defi ned compound of approximate formula (MeAlO) n , which, among other functions, serves to methylate a starting chloride complex. Kaminsky catalysts can also be supported on silica and are used industrially for the polymerization of α-alkenes and styrene. These are thus examples of heterogenized homogeneous catalysts .

Additional complications arise with alkenes other than ethene. We shall discuss only terminal alkenes such as propene and styrene, as these are relatively simple. The fi rst com-plication to consider arises because the two ends of the alkene molecule are different. In principle, it is possible for the polymer to form with the different ends head-to-head ( 17 ), head-to-tail ( 18 ), or randomly. Studies on catalysts such as ( 16 ) show that the growing chain migrates preferentially to the more highly substituted C atom of the alkene, thus giving a polymer chain that contains only head-to-tail orientations:

ZrR

+

Zr

+R

Zr

+R

Zr

+R

If we consider propene, we can see that the coordinated alkene induces less steric strain if its smaller CH 2 end is pointing into the cleft of the (Cp) 2 Zr catalyst ( 19 ), rather than the larger methyl substituted end, ( 20 ). The migrating polymer chain is thus adjacent to the methyl-substituted end of the propene molecule, and it is to this methyl-substituted C atom that the chain attaches, giving a head-to-tail sequence to the whole polymer chain.

The second structural modifi cation of polypropene is its tacticity , the relative orienta-tions of neighbouring groups in the polymer. In a regular isotactic polypropene, all the methyl groups are on the same side of the polymer backbone ( 21 ). In regular syndiotactic polypropene, the orientation of the methyl groups alternates along the polymer chain ( 22 ). In an atactic polypropene, the orientation of neighbouring methyl groups is random ( 23 ). Control of the tacticity of a polymer is equivalent to controlling the stereospecifi city of the reaction steps. The orientation of neighbouring groups is not simply of academic interest, because the orientation has a signifi cant effect on the properties of the bulk polymer. For example, the melting points of isotactic, syndiotactic, and atactic polypropene are 165°C, 130°C, and below 0°C, respectively.

It is not possible to control the tacticity of polypropene with a Zr catalyst such as ( 19 ), and an atactic polymer results. However, with other catalysts it is possible to con-trol the tacticity. The type of catalyst normally used to control the tacticity has a metal atom bonded to two indenyl groups that are linked by a CH 2 CH 2 bridge. Reaction of the bis(indenyl) fragment with a metal salt gives rise to three compounds: two enanti-omers, ( 24 ) and ( 25 ), which have C 2 symmetry, and a nonchiral compound, ( 26 ). These compounds are called ansa -metallocenes (the name is derived from the Latin for ‘handle’, and is used to indicate a bridge). It is possible to separate the two enantiomers from the nonchiral compounds, and both enantiomers of these ansa -metallocenes can catalyse the stereoregular polymerization of propene.

If we now consider the coordination of propene to one of the enantiomeric compounds ( 24 ) or ( 25 ), there is a second constraint in addition to the steric factor mentioned above (the CH 2 group pointing towards the cleft). Of the two potential arrangements of the methyl group shown in ( 27 ) and ( 28 ), the latter is disfavoured by a steric interaction with the phenyl ring of the indenyl group. During the polymerization reaction, the R group migrates preferentially to one side of the propene molecule; coordination of another

17

18

ZrMe

+

19

+

ZrMe

20

21

22

23

ZrCl

Cl

Zr ClCl

24

ZrCl

Cl

Zr ClCl

25

ZrCl

Cl

Zr ClCl

26


758 25 Catalysis

alkene is then followed by migration, and so on. Figure 25.27 shows how an isotactic polypropene then results.

ZrR

Zr R

+

27

ZrR

Zr R

+

28

V

Cl

Cl

N

O

30

NN

M

RRR′R′

X X

31

E X AMPLE 25.5 Controlling the tacticity of polypropene

Show that polymerization of propene with a catalyst containing CH 2 CH 2 -linked fl uorenyl and cyclopenta-dienyl groups ( 29 ) should result in syndiotactic polypropene.

Answer We need to consider how the propene reactant will coordinate to the catalyst: in complex ( 29 ) a coordinated propene will always coordinate with the methyl group pointing away from the fl uorenyl and towards the cyclopentadienyl ring. A series of sequential alkene insertions, as outlined in Fig. 25.28 , will therefore lead to a product that should be syndiotactic.

Self-test 25.5 Demonstrate that polymerization of propene with a simple [Zr(Cp) 2 Cl 2 ] catalyst would give rise to atactic polypropene.

ZrCl

R

Zr RCl

29

Although the Group 4 metal catalysts are by far the most common for alkene poly-merization, active catalysts based on other transition metals and lanthanoids have been reported. Polymerization with Group 5 metal complexes has been limited because of their thermal instability, but careful ligand design has enabled synthesis of catalysts ( 30 ) that are thermally stable and highly active for ethene polymerization in the presence of organo-aluminium chloride co-catalysts. Catalytic activity of the Pd- and Ni-diimine complexes ( 31 ) is affected by the N-aryl groups of the ligand, and electron-donating groups on the aryl group stabilize the cationic metal centre to give a high-molecular-weight polymer. A variety of neodymium-based catalysts are used in industry for the polymerization of 1,3-butadiene.

ZrR

Zr

Zr

Zr

Zr

ZrR

HR

R

H

R

HR

Zr R

ZrR

Figure 25.27 When propene is polymerized with an indenyl metallocene catalyst, isotactic polypropene results. The zirconium species all have a single positive charge; this has been omitted for clarity.



25.19 Tethered catalysts Key point: The tethering of a catalyst to a solid support allows easy separation of the catalyst with little loss in catalyst activity.

One popular technique has been the tethering of a homogeneous catalyst to a solid sup-port. Thus, a hydrogenation catalyst such as Wilkinson’s catalyst can be attached to a silica surface by means of a long hydrocarbon chain:

Si

PPh2

Rh ClPh3P

PPh3

Si

PPh2

(PPh3)3RhCl

When the silica monolith is immersed in a solvent, the rhodium-based catalytic site behaves as though it is in solution, and reactivity is largely unaffected. Separation of the products from the catalyst simply requires the decanting of the solvent. Commercially available functionalized silica precursors include amino-, acrylate-, allyl-, benzyl-, bromo-, chloro-, cyano-, hydroxy-, iodo-, phenyl-, styryl-, and vinyl-substituted reagents. Relatively sim-ple reactions can result in the synthesis of a whole host of further reagents (such as the phosphine compound in the scheme above). In addition to silica, polystyrene, polyethene, polypropene, and various clays have been used as the solid support and have led to reports of the successful heterogenization of most reactions that rely on soluble metal complexes.

In a recent novel application of tethered catalysis a dicationic ionic liquid was tethered to superparamagnetic iron oxide nanoparticles as a catalyst for the effi cient synthesis of Betti bases ( 32 ). which are attractive chiral ligands in enantioselective reactions as well as being precursors for the synthesis of molecules being evaluated for their effects on bradycardia and hypotension in humans. Because of the magnetic properties of the catalyst, it can be successfully recovered by a simple external magnet, removing the need for fi ltration.

NH2

OH

32

R

H

R

HR

ZrR

ZrR

Zr

R

H

Zr

R

H

Zr

Zr

Zr

Zr R

Figure 25.28* When a propene is polymerized with a fl uorenyl metallocene catalyst, syndiotactic polypropene results. The zirconium species all have a single positive charge; this charge has been omitted for clarity.


760 25 Catalysis

In some cases the activity of a supported catalyst is greater than that of its unsupported analogue. This improvement normally takes the form of enhanced selectivity brought about by the steric demands of approaching a catalyst constrained to a surface, or an increase in catalyst turnover frequencies brought about by protection from the support. Often, however, supported catalysts suffer from catalyst leaching and reduced activity.

25.20 Biphasic systems Key point: Biphasic systems offer another way of combining the selectivity of homogeneous catalysts with the ease of separation of heterogeneous catalysts.

Another popular method of combining the best of both homogeneous and heterogeneous catalysts has been the use of two liquid phases that are not miscible at room tempera-ture but are miscible at higher temperatures. The existence of immiscible aqueous/organic phases together with a compound that facilitates the transfer of reagents between the two phases will be familiar; two other systems worthy of note are ionic liquids and the ‘fl uorous biphase’ system.

Ionic liquids are typically derived from 1,3-dialkylimidazolium cations ( 33 ) with coun-ter-ions such as PF BF6 4

− −, , and CF SO3 3−. These systems have melting points of less than (and

often much less than) 100°C, a very high viscosity, and an effectively zero vapour pressure. If it can be arranged that the catalysts are preferentially soluble in the ionic phase (Section 4.13g) (for instance, by making them ionic), immiscible organic solvents can be used to extract organic products. As an example, consider the hydroformylation of alkenes by a Rh catalyst with phosphine ligands. When the ligand used is triphenylphosphine, the cata-lyst is extracted from the ionic liquid together with the products. However, when an ionic sulfonated triphenylphosphine is used as the ligand, the catalyst remains in the ionic liquid and product separation from the catalyst is complete. It should be borne in mind that the ionic liquid phase is not always unreactive, and may induce alternative reactions.

The fl uorous biphase system, which typically consists of a fl uorinated hydrocarbon and a ‘normal’ organic solvent such as methylbenzene, offers two principal advantages over aqueous/organic phases: the fl uorous phase is unreactive, so sensitive groups (such as hydrolytically unstable groups) are stable in it, and polyfl uorinated and hydrocarbon solvents, which are not miscible at room temperatures, become so on heating, giving rise to a genuinely homogeneous system. A catalyst that contains polyfl uorinated groups is preferentially retained in the fl uorous solvent, with the reactants (and products) preferen-tially soluble in the hydrocarbon phase. Figure 25.29 indicates the type of sequence that is used. Separation of the products from the catalyst becomes a trivial matter of decanting one liquid from another. A number of ligand systems that confer fl uorous solubility on a catalyst have been developed; typically they are phosphine based such as ( 34 ), ( 35 ), and ( 36 ). Rhodium- and Pd-based catalysts of these ligands have then been prepared. With a fl uorous solvent such as perfl uoro-1,3-dimethylcyclohexane ( 37 ), miscibility with an organic phase can be achieved at 70°C, and catalysts have been used in hydrogenation (Section 25.4), hydroformylation (Section 25.5), and hydroboration reactions.

N NR'R

+

33

OC6F13 PPh3-x

x

34

C6F13 C2H4 O PPh3-xx

35 PP O C6F13OC6F13

2 2

36

FC

F2CCF2

CF2

FC

F2

CF3C CF3

37

FURTHER READING

R. Whyman, Applied organometallic chemistry and catalysis . Oxford University Press (2001).

G.W. Parshall and S.D. Ittle, Homogeneous catalysis . John Wiley & Sons (1992).

See H.H. Brintzinger, D. Fischer , R. Mülhaupt, B. Rieger , and R.M. Waymouth, Angew. Chem., Int. Ed. Engl ., 1995, 34 , 1143 for a review of the area of control of polymer tacticity.

See P. Espinet and A.M. Echavarren, Angew. Chem., Int. Ed. Engl. , 2004, 43 , 4704 for a good review of the Stille reaction that touches on the mechanism of all palladium-catalysed coupling reactions.

See Chem. Rev ., 2002, 102 , 3215 for an issue of a major journal devoted to the subject of supported homogeneous catalysts.

E.G. Hope and A. M Stuart , J. Fluorine Chem ., 1999, 100 , 75. Fluorous biphase systems are discussed in great detail.

Hydrocarbon

phase

(contains

reactants)

Hydrocarbon

phase

(contains

products)

Fluorous

phase

(contains

catalyst)

Fluorous

phase

(contains

catalyst)

Homo-

geneous

mixture

Heat Cool

Figure 25.29 The sequence of processes that occurs during the use of a fl uorous biphase catalyst.


761Exercises

T.M. Trnka and R.H. Grubbs, Acc. Chem. Res ., 2001, 34 , 18. A review of the development of alkene metathesis catalysts.

V. Ponec and G.C. Bond, Catalysis by metals and alloys . Elsevier (1995). Comprehensive discussion of the basis of chemisorption and catalysis by metals.

R.D. Srivtava, Heterogeneous catalytic science . CRC Press (1988). A survey of experimental methods and several major heterogeneous catalytic processes.

M. Bowker, The basis and applications of heterogeneous catalysis . Oxford Chemistry Primers vol. 53. Oxford University Press (1998). Concise coverage of heterogeneous catalysis.

J.M. Thomas and W.J. Thomas, Principles and practice of heterogene-ous catalysis . VCH (1997). Readable introduction to the fundamen-tal principles of heterogeneous catalysis, written by world-renowned experts.

K.M. Neyman and F. Illas, Theoretical aspects of heterogeneous cataly-sis: applications of density functional methods. Catalysis Today , 2005, 105 , 15. Modelling methods applied to heterogeneous catalysis.

M.A. Keane, Ceramics for catalysis. J. Mater. Sci ., 2003, 38 , 4661. Overview of heterogeneous catalysis illustrated with three estab-lished methods: (i) catalysis using zeolites, (ii) catalytic converters, and (iii) solid oxide fuel cells.

F.S. Stone, Research perspectives during 40 years of the Journal of Catalysis. J. Catal ., 2003, 216 , 2. A historical perspective on devel-opments in catalysis.

G. Rothenberg, Catalysis: concepts and green applications . Wiley-VCH (2008). Catalysis and sustainability.

D.K. Chakrabarty and B. Viswanathan, Heterogeneous catalysis . New Age Science Ltd (2008).

D. Takeichi, Recent progress in olefi n polymerization catalysed by tran-sition metal complexes: new catalysts and new reactions , Dalton Trans. , 2010, 39 , 311–328.

D. Astruc, The metathesis reactions: from a historical perspective to recent developments, New J. Chem. , 2005, 29 , 42–56. A readable account of the development of the area.

EXERCISES

25.1 Which of the following constitute genuine examples of catalysis and which do not? Present your reasoning. (a) The addition of H 2 to C 2 H 4 when the mixture is brought into contact with fi nely divided platinum. (b) The reaction of an H 2 /O 2 gas mixture when an electrical arc is struck. (c) The combination of N 2 gas with lithium metal to produce Li 3 N, which then reacts with H 2 O to produce NH 3 and LiOH.

25.2 Defi ne the terms (a) turnover frequency, (b) selectivity, (c) catalyst, (d) catalytic cycle, (e) catalyst support.

25.3 Classify the following as homogeneous or heterogeneous catalysis and present your reasoning. (a) The increased rate in the presence of NO(g) of SO 2 (g) oxidation by O 2 (g) to SO 3 (g). (b) The hydrogenation of liquid vegetable oil using a fi nely divided nickel catalyst. (c) The conversion of an aqueous solution of d -glucose to a d , l mixture catalysed by HCl(aq).

25.4 You are approached by an industrialist with the proposition that you develop catalysts for the following processes at 80°C with no input of electrical energy or electromagnetic radiation:

(a) The splitting of water into H 2 and O 2 .

(b) The decomposition of CO 2 into C and O 2 .

(c) The combination of N 2 with H 2 to produce NH 3 .

(d) The hydrogenation of the double bonds in vegetable oil.

The industrialist’s company will build the plant to carry out the process and the two of you will share equally in the profi ts. Which of these would be easy to do, which are plausible candidates for investigation, and which are unreasonable? Describe the chemical basis for the decision in each case.

25.5 Addition of PPh 3 to a solution of Wilkinson’s catalyst, [RhCl(PPh 3 ) 3 ], reduces the turnover frequency for the hydrogenation of propene. Give a plausible mechanistic explanation for this observation.

25.6 The rates of H 2 gas absorption (in dm 3 mol −1 s −1 ) by alkenes catalysed by [RhCl(PPh 3 ) 3 ] in benzene at 25°C are: hexene, 2910; cis -4-methyl-2-pentene, 990; cyclohexene, 3160; 1-methylcyclo-hexene, 60. Suggest the origin of the trend and identify the affected reaction step in the proposed mechanism ( Fig. 25.5 ).

25.7 Sketch the catalytic cycle for the production of butanal from prop-1-ene. Identify the step at which selectivity to the n or iso isomer occurs.

25.8 Draw the catalytic cycle for the Ziegler–Natta polymerization of propene. Explain each of the steps involved and predict the physical properties of the polymer produced.

25.9 Infrared spectroscopic investigation of a mixture of CO, H 2 , and 1-butene under conditions that bring about hydroformylation indicate the presence in the reaction mixture of compound (E) in Fig. 25.7 . The same reacting mixture in the presence of added tributylphosphine was studied by infrared spectroscopy and neither (E) nor an analogous phosphine-substituted complex was observed. What does the fi rst observation suggest as the rate-limiting reaction in the absence of phosphine? Assuming the sequence of reactions remains unchanged, what are the possible rate-limiting reactions in the presence of tributylphosphine?

25.10 Show how reaction of MeCOOMe with CO under conditions of the Monsanto ethanoic acid process can lead to ethanoic anhydride.

25.11 Suggest reasons why (a) ring-opening alkene metathesis polymerization and (b) ring-closing metathesis reactions proceed.

25.12 (a) Starting with the alkene complex shown in Fig. 25.9 , with trans -DHC = CHD in place of C 2 H 4 , assume dissolved OH − attacks from the side opposite the metal. Give a stereochemical drawing of the resulting compound. (b) Assume attack on the coordinated trans -DHC = CHD by an OH − ligand coordinated to Pd, and draw the stereochemistry of the resulting compound. (c) Does the stereochemistry differentiate these proposed steps in the Wacker process?

25.13 Aluminosilicate surfaces in zeolites act as strong Brønsted acids, whereas silica gel is a very weak acid. (a) Give an explanation for the enhancement of acidity by the presence of Al 3+ in a silica lattice. (b) Name three other ions that might enhance the acidity of silica.

25.14 Why is the platinum/rhodium catalyst in automobile catalytic converters dispersed on the surface of a ceramic rather than used in the form of a thin metal foil?


762 25 Catalysis

25.15 Alkanes are observed to exchange hydrogen atoms with deuterium gas over some platinum metal catalysts. When 3,3-dimethylpentane in the presence of D 2 is exposed to a platinum catalyst and the gases are observed before the reaction has proceeded very far, the main product is CH 3 CH 2 C(CH 3 ) 2 CD 2 CD 3 plus unreacted 3,3-dimethylpentane. Devise a plausible mechanism to explain this observation.

25.16 The effectiveness of platinum in catalysing the reaction 2 H + (aq) + 2 e − → H 2 (g) is greatly decreased in the presence of CO. Suggest an explanation.

25.17 Describe the role of electrocatalysts in reducing the overpotential in the oxygen reduction reaction in fuel cells.

25.18 Consider the validity of each of the following statements and provide corrections where required:

(a) A catalyst introduces a new reaction pathway with lower enthalpy of activation.

(b) As the Gibbs energy is more favourable for a catalytic reaction, yields of the product are increased by catalysis.

(c) An example of a homogeneous catalyst is the Ziegler−Natta catalyst made from TiCl 4 (l) and Al(C 2 H 5 ) 3 (l).

(d) Highly favourable Gibbs energies for the attachment of reactants and products to a homogeneous or heterogeneous catalyst are the key to high catalytic activity.

TUTORIAL PROBLEMS

25.1 A catalyst might not just lower the enthalpy of activation, but might make a signifi cant change to the entropy of activation. Discuss this phenomenon. (See A. Haim, J. Chem., Educ. , 1989, 66 , 935.)

25.2 The addition of promoters can further enhance the rate of a catalysed reaction. Describe how the promoters allowed the iridium-based Cativa process to compete with the rhodium-based process in the carbonylation of methanol. (See A. Haynes, P.M. Maitlis, G.E. Morris, G.J. Sunley, H. Adams, and P.W. Badger, J. Am. Chem. Soc. , 2004, 126 , 2847.)

25.3 When direct evidence for a mechanism is not available, chemists frequently invoke analogies with similar systems. Describe how J.E. Bäckvall, B. Åkermark, and S.O. Ljunggren ( J. Am. Chem. Soc ., 1979, 101 , 2411) inferred the attack of uncoordinated water on η 2 -C 2 H 4 in the Wacker process.

25.4 Whereas many enantioselective catalysts require the precoordination of a substrate, this is not always the case. Use the example of asymmetric epoxidation to demonstrate the validity of this statement, and indicate the advantages that such catalysts might have over catalysts that require precoordination of a substrate. (See M. Palucki, N.S. Finney, P.J. Pospisil, M.L. Güler, T. Ishida, and E.N. Jacobsen, J. Am. Chem. Soc. , 1998, 120 , 948.)

25.5 Discuss shape selectivity with respect to catalytic processes involving zeolites.

25.6 Summarize the potential impact of heterogeneous oxidation catalysts in chemistry. (See J.M. Thomas and R. Raja, Innovations in oxidation catalysis leading to a sustainable society. Catalysis Today , 2006, 117 , 22.)

25.7 Discuss the applications and mechanisms of oxidation and ammoxidation catalysts such as bismuth molybdate. (See, for example, R.K. Grasselli, J. Chem. Educ. , 1986, 63 , 216.)

25.8 Discuss the advantages of a solid support in catalysis by reference to the use of [Ni(POEt) 3 ] 4 in alkene isomerization.

(See A.J. Seen, J. Chem. Educ. , 2004, 81 , 383; and K.R. Birdwhistell and J. Lanza, J. Chem. Educ. , 1997, 74 , 579.)

25.9 J.A. Botas et al discuss the catalytic conversion of vegetable oils into hydrocarbons suitable for use as biofuels ( Catalysis Today , 2012, 195 , 1, 59). What are the most important features of catalysts that are used for these reactions? How was the incorporation of transition metals expected to modify catalyst properties? Outline how the modifi ed catalysts were prepared and characterized. What reactions occurred in the reactor in addition to catalytic cracking? Which reactions lead to aromatic products? Which of the modifi ed catalysts produced most coke build-up? Explain why this did not deactivate the catalyst.

25.10 α-Pinene is a major component of natural turpentine, and its polymer is nontoxic and used in a wide variety of industrial applications such as adhesives, varnishes, food packaging, and chewing gum. Novel catalysts for the polymerization of α-pinene have been produced based on niobium and tantalum pentahalides (M. Hayatifar, et al. , Catalysis Today , 2012, 192 , 1, 177). Which catalyst was the most effective for the polymerization and why are the reaction conditions so attractive? Describe the nature of the catalyst − α-pinene interaction.

25.11 A. Arbaoui and C. Redshaw ( Polym. Chem. , 2010, 1 , 801) review catalysts for the synthesis of biodegradable polymers via ring-opening metathesis polymerization. Summarize the need for biodegradable polymers and why new catalysts are required. From the details given, identify which groups of metals give the most active catalysts and with which types of ligands. Illustrate with examples.

25.12 The Schrock and Grubbs catalysts are widely used for alkene metathesis reactions. Write a review of how these homogeneous catalysts have been heterogenized.


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