General Types of Catalysis

7/23/2019 General Types of Catalysis

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Some Of General Types Of Catalyst

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Mahmoud Galal ZidanUniversity of Tanta , faculty of science

PRENTICE HALLUpper Saddle River , NJ 07458

SlideShare

Reviewed by Prof.

Paula Bruice


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Brief Contents

Chapter 1 History of Catalysis1.1 Humphry Davys Classic Paper 51.2

Dobereiners Experiment 6

1.3 Faradays Review 6

1.4 Fusinieris Theory 7

1.5 The Beginnings of Industrial Catalysis 7

Chapter 2 Introduction to Catalysis

2.1.

Whats catalyst 9

2.2.

Catalysts Can Be Atoms, Molecules, Enzymes and Solid

Surfaces 10

2.3.

Why is Catalysis Important? 102.3.1. Catalysis and Green Chemistry

2.3.2. Atom Efficiency, E Factors and Environmental

Friendliness

2.4.

Catalysis as a Multidisciplinary Science 122.4.1. The Many Length Scales of a Catalyst

2.4.2. Time Scales in Catalysis

2.5.

The Scope of This Book 14

2.6. Catalysis in Journals 14

Chapter 3 Types of Catalysis

3.1.

Types of catalytic reactions 16

3.2.

What is a phase? 16

3.3.

Heterogeneous catalysis 17

3.3.1.

How the heterogeneous catalyst works (in general

terms)

3.3.2. Examples of heterogeneous catalysis

3.3.2.1. The hydrogenation of a carbon-carbon

double bond

3.3.2.2. Catalytic converters

3.3.2.2.1.

The use of vanadium(V) oxide in the

Contact Process3.4.

Homogeneous catalysis 21

3.4.1. Examples of homogeneous catalysis

3.4.1.1. The reaction between persulphate ions and

iodide ions

3.4.1.2. The destruction of atmospheric ozone

3.5.

Autocatalysis 23

3.5.1.

The oxidation of ethanedioic acid by manganate (VII)

ions

Chapter 4 Heterogeneous catalysis

4.1.

Principles of heterogeneous catalysis 254.2.

CLASSIFICATION OF REACTION MECHANISMS 29


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4.3.

Coadsorption 33

4.4.

Poisons and Promoters 31

4.5.

Electropositive impurities 31

4.6. Determination of the Reaction Rate 32

Chapter 5 Industrial Uses of Heterogeneous catalysis5.1.

Heterogeneous industrial catalysis Examples 33

5.2. General requirements for a heterogeneous catalyst 36

5.3. Aluminium oxide, silicon dioxide, aluminosilicates and zeolites

37

5.3.1. Zeolite catalysts

Chapter 6 Homogeneous catalysis

6.1.

Whats Homogeneous catalysis 42

6.2. Acid catalysis 42

6.3. Organometallic chemistry 42

6.4.

Other forms of homogeneous catalysis 436.4.1. Contrast with heterogeneous catalysis

Chapter 7 Industrial Uses of Homogeneous catalysis

7.1.

Homogeneous industrial catalysis Examples 44

Chapter 8 Bio-catalysis8.1.

Whats Biocatalysis 47

8.2.

Definition 48

8.3.

BIOTECHNOLOGY 53

8.3.1. What is biotechnology

8.3.2. Liquid fuels

8.3.3.

The Starch industry

8.3.4. BIOTECHNOLOGY TODAY

8.4.

Enzyme Technology 54

8.5. Genetic Engineering 56

Reference Page 57
https://en.wikipedia.org/wiki/Homogeneous_catalysis#Acid_catalysishttps://en.wikipedia.org/wiki/Homogeneous_catalysis#Organometallic_chemistryhttps://en.wikipedia.org/wiki/Homogeneous_catalysis#Other_forms_of_homogeneous_catalysishttps://en.wikipedia.org/wiki/Homogeneous_catalysis#Other_forms_of_homogeneous_catalysishttps://en.wikipedia.org/wiki/Homogeneous_catalysis#Organometallic_chemistryhttps://en.wikipedia.org/wiki/Homogeneous_catalysis#Acid_catalysis


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ne hundred and forty years ago it was possible for one man to prepare an annual report on the

progress of the whole of chemistry, and for many years this task was undertaken by the noted Swedishchemist J. J. Berzelius for the Stockholm Academy of Sciences. In his report submitted in 1835 andpublished in 1836 Berzelius reviewed a number of earlier findings on chemical change in bothhomogeneous and heterogeneous systems, and showed that these findings could be rationally co-ordinated by the introduction of the concept of catalysis. In a short paper summarizing his ideas oncatalysis as a new force, he wrote (I):It is, then, proved that several simple or compound bodies, soluble and insoluble, have the propertyof exercising on other bodies an action very different from chemical affinity.By means of this action they produce, in these bodies, decompositions of their elements and differentrecombination of these same elements to which they remain indifferent.

Berzelius proceeded to propose the existence of a new force which he called the catalytic force and

he called catalysis the decomposition of bodies by this force. This is probably the first recognition

of catalysis as a wide-ranging natural phenomenon.

Metallic catalysts had in fact been used in the laboratory before 1800 by Joseph Priestley, thediscoverer of oxygen, and by the Dutch chemist Martinus van Marum, both of whom madeobservations on the dehydrogenation of alcohol on metal catalysts. However, it seems likely that theseinvestigators regarded the metal merely as a source of heat. In 1813, Louis Jacques Thenarddiscovered that ammonia is decomposed into nitrogen and hydrogen when passed over various red-hotmetals, and ten years later, with Pierre Dulong, he found that the activity of iron, copper, silver, gold,and platinum for decomposingammonia decreased in the order given. This is one of the earliest recorded examples of a pattern ofcatalytic activity.Thenard, the son of a peasant, after experience with Vauquelin in Paris as a laboratory boy, becameassistant and later Professor at the lkole Polytechnique. In 1857, then a peer, he became Chancellor ofthe University of Paris. His collaborator Dulong was Professor of Physics in the &ole Polytechnique,

later becoming Director. He is, of course, famed for the law of Dulong and Petit. He also proposed ahydrogen theory of acids independently of Davy. Homogeneous catalytic processes have been used bymankind for some thousands of years, for example in fermentation. Perhaps the first attempt at arational theory of catalysis is the intermediate compound theory proposed by Charles BernardDesormes and NicolasClement (2) for the homogeneous catalytic effect of nitrogen oxides in the lead chamber process forthe manufacture of sulphuric acid.This particular process was later discussed more fully by Humphry Davy, and has received a hugeamount of attention and discussion since these early times.Clement studied science in Pans and became assistant at the Ecole Polytechnique.Later he became Professor at the Conservatoire des A r t s et Mtiers, and after winning a lottery,married the daughter of Dtsormes. That a chemical substance can speed up a chemical reactionwithout itself being chemically changed became clear in a research on the decomposition of hydrogenperoxide, carried out by L. J. Thenard, who announced his discovery of this substance in 1818.


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Thenard had become interested in barium peroxide, probably because of the notable discovery byDavy in 1807 of sodium and potassium, which he prepared by the electrolysis of their moisthydroxides. Davys first Bakerian lecture in 1806 On some Chemical Agencies of Electricitydescribed work of such importance that the 3000 francs prize given by Napoleon I was awarded tohim. Napoleon seems to have been somewhat peeved that the prize went to an Englishman when astate of war existed between England and France, and, noting the vital part the large Royal Institutionvoltaic pile had played in the research, he ordered two large voltaic piles to be built. Perhaps headopted the type of argument we note even now from some political circles, that two voltaic piles willdo twice as much useful work as one, irrespective of who originates the ideas to be tried out with thepiles. Thtnard probably investigated the alkaline earth oxides with the initial aim of electrolyzingthem. The work he did on the reaction of barium peroxide with nitric or hydrochloricacid led to the discovery of hydrogen peroxide. This he found to decompose when in contact withmany solids, some of which were not chemically changed. He also found that the action of metals inbringing about decomposition became more vigorous as the metal was reduced to a finer state ofsubdivision.

1.1.

Humphry Davys Classic Paper

The first clear realization that chemicalreaction between two gaseous reactantscan occur on a metal surface withoutthe metal being chemically changed isfound in a paper by Humphry Davy (3)published by the Royal Society in 1817.This describes the discovery of a newand curious series of phenomena.During the researches which led to theminers safety-lamp, Davy fixed a fineplatinum wire above a coal-gas flame in

a safety-lamp. When additional coal gaswas introduced into the lamp, the flamewent outbut the platinum wire remained hot formany minutes. Davy immediatelydeduced that the oxygen and coal gascombined without flame when incontact with the hot wire, therebyproducing enough heat to keep the wireincandescent.A hot platinum wire introduced into amixture of coal gas and air immediatelybecame incandescent. Furthermoremany combustible vapors mixed withair were found by Davy to produce thesame effect.Davy had discovered the phenomenonof heterogeneous catalytic oxidation.Only platinum and palladium wires were effective; wires of copper, silver, gold and iron wereineffective. This is one of the earliest recorded patterns of catalytic activity.In these researches Davy was assisted by Faraday but it is unclear whether Faraday contributed to thedevelopment of the ideas or acted mainly by carrying out Davys instructions. The two-man teamwhich made

these discoveries about heterogeneous catalysis must have been one of the strongest in the wholehistory of chemistry.


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In order to initiate catalytic oxidation the wires used by Davy needed to be hot, but in 1820 EdmundDavy , Professor of Chemistry at Cork in Ireland, formerly Assistant: at the Royal Institution, andcousin of Humphry Davy, prepared a finely-divided platinum catalyst of such high activity that itacted at room temperature. When this catalyst was dropped on to any porous substance moistenedwith alcohol, oxidation occurred so rapidly that the catalyst became red hot.

1.2.Dobereiners Experiment

J. W. Dobereiner , Professor of Chemistry at Jena and formulator of the law of triads, reduced acatalyst prepared by the procedure Edmund Davy had described and obtained a spongy platinumwhich brought about the combination of hydrogen and oxygen at room temperature and quicklybecame red hot as a result. This remarkable discovery was rapidly followed up by Dulong andThenard in Paris, who discovered that palladium and iridium could also act at ordinary temperatureswhereas cobalt, nickel,rhodium, silver and gold acted catalytically only at higher temperatures. Their noteactually appeared in the Annales de Chimie et de Physique before Dobereiners paper because

his discovery was communicated privately by Liebig to Dulong and Thenard.Dobereiners work had, however, been communicated from Paris to Faraday in a letter of September16th, 1823, from J. N. P. Hachette and within a few days Faraday had proceeded to repeat theexperiment and confirm the findings. In a brief note signed merely M. F. he wrote it wascommunicated to me by M. Hachette and having verified it I think every chemist will be glad to hearits nature.

1.3.

Faradays Review

Dulong and Thenard in a later paper reported that the ability to bring about gaseous combination is ageneral property of sufficiently- heated solids, while Faraday , in a very good review of early work onheterogeneous catalysis, drew attention to the merits of their experiments. He wrote (paragraph 611):

In the two excellent papers by MM. DULONG and Thenard ,philosophers show that elevation oftemperature favors the action, but does not alter its character, Sir HUMPHRY DAVYSin candescentplatina wire being the same phenomenon with DBEREINWS spongy platina. They show that allmetals have this power in a greater or smaller degree, and that it is even possessed by such bodies ascharcoal, pumice, porcelain, glass, rock crystal, etc., when their temperatures are raised; and thatanother of DAVYs effects, in which oxygen and hydrogen had combined slowly together at a heatbelow ignition, was really dependent upon the property of the heated glass, which it has in commonwith the bodies named above.Later in the paper Faraday wrote (paragraph 618): The effect is evidently produced by most, if notall, solid bodies, weakly perhaps by many of them, but rising to a high degree in platina. DULONG dThenard philosophically extended our knowledge of the property to its possession by all the metals,and by earths, glass, stones, etc. (611); and every idea of its being a known and recognized electricaction is in this way removed.This last sentence in effect rejects a somewhat vague suggestion madeby Dulong andThenard that the power of solids to bring about reactions of gases is electrical in origin, although, asFaraday noted, Dulong and Thenard expressed themselves with great caution on the theory of theeffect.Additional significant experimental discoveries concerning heterogeneous catalytic oxidation weremade by William Henry (IO), who used as a catalyst either platinum sponge or molded balls made ofchina clay and spongy platinum. These catalytic balls were first described by Dobereiner, to whomHenry refers. The two papers by Dobereiner and Henry were the first to describe the use of asupported platinum catalyst. Dobereinerscontributions to catalysis have been reviewed in more detailby McDonald .

Henry found that ethylene prevented the surface combination of a mixture of hydrogen and carbonmonoxide with oxygen, and carbon monoxide slightly slowed down the surface reaction between


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hydrogen and oxygen. He also found, when working with mixtures of hydrogen, carbon monoxideand methane, that selective catalytic oxidation on platinum is possible.Henry had studied under Joseph Black in Edinburgh and had also worked with the noted Scottishchemist Thomas Thomson. Later he directed his fathers chemical works. He is famed for Henryslaw concerning gaseous solubilities.

1.4.

Fusinieris Theory

The first attempt to give a detailed theoretical discussion of heterogeneous catalytic oxidation onplatinum was made by the Italian physician Ambrogio Fusinieri in a paper published in 1824. Hediscussed the effects discovered by Dobereiner and others. He also explained in more detail his ownexperimental observations (first expounded in a note) and remarked that these would dispel allconfusion and doubt, writing : Ora espongo in pih dettaglio le mie osservazioneche dissipano ogniconfusione ed ogni dubbio; ed altre ancora ve ne soggiungo di pih recenti.Fusinieri contended, inopposition to Day, that in fact the combustion of ether on platinum occurs with flame, which may beobscured by light from the platinum, or may be invisible. He further contended that during theoxidation of ether concrete laminae ofthe combustible substance could be seen with the naked eye,

the laminae running over the platinum surface and then disappearing by burning. Fusinieri thoughtthat the ether forming the laminae was solidified, although he noted a relation between the appearanceand disappearance of the laminae and capillary action of liquids. According to Fusinieri, the platinumcatalyst acted like a candle wick with the laminae burning like candle wax. As an explanation of theformation and burning of the concrete laminae, which Fusinieri thought he saw, he proposed theconcept of nativecaloric (Faradays translation).

Faraday in his momentous paper of 1834 , which proposed the idea (increasingly supported since theresearches of Langmuir) of simultaneous adsorption of both reactants on a platinum surface, reviewedFusinieris viewsat some length, but he confessed that he could not form a distinct idea of the conceptof native caloric, and remarked that his knowledge of the language in which the Fusinieri memoir iswritten was imperfect. Faraday during his European tour with Davy and Lady Davy, in which he

acted as scientific assistant and unofficial valet, had learnt Italian, but the Fusinieri paper of 1824 usesItalian which is as out-dated as Chaucerian English, and sentences in which caloric nativo appearshave been said to be totally incomprehensible . It is not surprising that Faraday was unable to form adistinct idea of what Fusinieri meant by caloric nativo.

1.5.

The Beginnings of Industrial Catalysis

The industrial possibilities of heterogeneous catalytic oxidation were appreciated as early as 1831 byPeregrine Phillips, Junior, who in that year took out a British Patent (No. 6096) for CertainImprovements in Manufacturing Sulphuric Acid commonly called Oil of Vitriol. The specificationstates :The first improvement then, namely, the instantaneous union of sulphurous acid with the

oxygen of the atmosphere, I effect by drawing them in proper proportions by the action of an air pumpor other mechanical means thro an ignited tube or tubes of platina, porcelain, or any other materialnot acted on when heated by the sulphurous acid gas. In the said tube or tubes I place fine platina wireor platina in any finely-divided state, and I heat them to a strong yellow heat, and by preference in thechamber of a reverberatory furnace; and I do affirm that sulphurous acid gas being made to pass witha sufficient supply of atmospheric air through tubes as described, properly heated and managed, willbe instantly converted into sulphuric acid gas, which will be rapidly absorbed as soon as it comes intocontact withwater.This far-sighted patent has made Phillips one of the inventors who have devised a really new processof manufacture of an important chemical substance. The process was first worked to make fumingsulphuric acid by Messel in 1875. Very little is known about the inventor, despite a search of local

records made by E. Cook except that he was the son of a tailor and was born in Milk Street, Bristol.


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The first to use platinum as a catalyst for ammonia oxidation was C. F. Kuhlmann who reported hisresults to the Scientific Society of Lille, France, in 1838. An account of early developments withplatinum catalysts for the ammonia oxidation process for nitric acid manufacture has been given by L.B. Hunt From these small beginnings vast industries based on catalysis have arisen and a hugequantity of scientific information has been accumulated. Despite numerous theoretical discussionscatalytic action is still to some extent a mystery-indeed it is notable that large-scale processes arebased on catalysts which from a theoretical point of view can hardly be described. Catalysis a centuryand a half after Davys discovery still remains achallenge to the chemist.


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sk the average person in the street what a catalyst is, and he or she will probably tell you that a

catalyst is what one has under the car to clean up the exhaust. Indeed, the automotive exhaustconverter represents a very successful application of catalysis; it does a great job in removing most ofthe pollutants from the exhaust leaving the engines of cars. However, catalysis has a much widerscope of application than abating pollution.

For example, living matter relies on enzymes, which are the most specific catalysts one can think of.Also, the chemical industry cannot exist without catalysis, which is an indispensable tool in theproduction of bulk chemicals, fine chemicals and fuels.For scientists and engineers catalysis is a tremendously challenging, highly multidisciplinary field.Let us first see what catalysis is, and then why it is so important for mankind.

2.1.

Whats catalyst ?

A catalyst accelerates a chemical reaction. It doesso by forming bonds with the reacting molecules,and by allowing these to react to a product, whichdetaches from the catalyst, and leaves it unaltered

such that it is available for the next reaction. Infact, we can describe the catalytic reaction as acyclic event in which the catalyst participates andis recovered in its original form at the end of thecycle.Let us consider the catalytic reaction betweentwo molecules A and B to give a product P, seeFig. 1.1. The cycle starts with the bonding ofmolecules A and B to the catalyst. A and B thenreact within this complex to give a product P,which is also bound to the catalyst. In the finalstep, P separates from the catalyst, thus leaving

the reaction cycle in its original state.

To see how the catalyst accelerates the reaction,we need to look at the potential energy diagramin Fig. 1.2, which compares the non-catalytic and the catalytic reaction.For the non-catalytic reaction, the figure is simply the familiar way to visualize the Arrheniusequation: the reaction proceeds when A and B collide with sufficient energy to overcome theactivation barrier in Fig. 1.2. The change in Gibbs free energy between the reactants, A + B, and theproduct P is G.The catalytic reaction starts by bonding of the reactants A and B to the catalyst, in a spontaneousreaction. Hence, the formation of this complex is exothermic, and the free energy is lowered. Therethen follows the reaction between A and B while they are bound to the catalyst. This step is associatedwith an activation energy; however, it is significantly lower than that for the uncatalyzed reaction.Finally, the product P separates from the catalyst in an endothermic step.

2


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The energy diagram of Fig. 1.2 illustrates severalimportant points:

The catalyst offers an alternative path forthe reaction, which is obviously morecomplex, but energetically much morefavorable.

The activation energy of the catalyticreaction is significantly smaller than that ofthe uncatalyzed reaction; hence, the rate ofthe catalytic reaction is much larger

The overall change in free energy for thecatalytic reaction equals that of theuncatalyzed reaction. Hence, the catalystdoes not affect the equilibrium constant forthe overall reaction of A + B to P. Thus, if areaction is thermodynamically unfavorable,a catalyst cannot change this situation. A

catalyst changes the kinetics but not thethermodynamics.

The catalyst accelerates both the forward and the reverse reaction to the same extent. In otherwords, if a catalyst accelerates the formation of the product P from A and B, it will do thesame for the decomposition of P into A and B. Thus far it is immediately evident that thereare also cases in which the combination of catalyst with reactants or products will not besuccessful.

If the bonding between reactants and catalyst is too weak, there will be hardly any conversionof A and B into products. Conversely if the bond between the catalyst and one of thereactants, say A, is too strong, the catalyst will be mostly occupied with species A, and B isnot available to form the product. If A and B both form strong bonds with the catalyst, theintermediate situation with A or B on the catalyst may be so stable that reaction becomes

unlikely. In terms of Fig. 1.2, the second level lies so deep that the activation energy to formP on the catalyst becomes too high. The catalyst is said to be poisoned by (one of) thereactants.

In the same way, the product P may be too strongly bound to the catalyst for separation tooccur. In this case the product poisons the catalyst. Hence, we intuitively feel that thesuccessful combination of catalyst and reaction is that in which the interaction betweencatalyst and reacting species is not too weak, but also not too strong. This is a looselyformulated version of Sabatiers Principle, the catalyst has been an unspecified, abstract body,so let us first look at what kind of catalysts exist.

2.2.

Catalysts Can Be Atoms, Molecules, Enzymes and Solid Surfaces

Catalysts come in a multitude of forms, varying from atoms and molecules to large structures such aszeolites or enzymes. In addition they may be employed in various surroundings: in liquids, gases or atthe surface of solids. Preparing a catalyst in the optimum form and studying its precise compositionand shape are an important specialism, which we describe in later chapters.It is customary to distinguish the following three sub disciplines in catalysis:

Homogeneous .

Heterogeneous .

Bio catalysis .

2.3.

Why is Catalysis Important?

The chemical industry of the 20thcentury could not have developed to its present status on the basis ofnon-catalytic, stoichiometric reactions alone. Reactions can in general be controlled on the basis of


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temperature, concentration, pressure and contact time. Raising the temperature and pressure willenable stoichiometric reactions to proceed at a reasonable rate of production, but the reactors in whichsuch conditions can be safely maintained become progressively more expensive and difficult to make.In addition, there are thermodynamic limitations to the conditions under which products can beformed, e.g. the conversion of N2 and H2 into ammonia is practically impossible above 600 C.Nevertheless, higher temperatures are needed to break the very strong N_N bond in N2. Withoutcatalysts, many reactions that are common in the chemical industry would not be possible, and manyother processes would not be economical.Catalysts accelerate reactions by orders of magnitude, enabling them to be carried out under the mostfavorable thermodynamic regime, and at much lower temperatures and pressures. In this way efficientcatalysts, in combination with optimized reactor and total plant design, are the key factor in reducingboth the investment and operation costs of a chemical processes. But that is not all.

2.3.1. Catalysis and Green Chemistry

Technology is called green if it uses raw materials efficiently, such that the use of toxic andhazardous reagents and solvents can be avoided while formation of waste or undesirable byproducts is

minimized. Catalytic routes often satisfythese criteria.A good example is provided by theselective oxidation of ethylene toethylene epoxide, an importantintermediate towards ethylene glycol(antifreeze) and various poly-ethers andpolyurethanes (Fig. 1.6).

The old, non-catalytic route (called the epichlorohydrine process) follows a three step synthesis:

Cl2+ NaOH HOCl + NaCl (1)

C2H4+ HOCl CH2ClCH2OH (epichlorohydrine) (2)

CH2ClCH2OH + 1/2Ca(OH)2 1/2CaCl2+ C2H4O +H2O (3)or in total:

Cl2+ NaOH + 1/2Ca(OH)2+ C2H4 C2H4O + 1/2CaCl2+ NaCl + H2OHence, for every molecule of ethylene oxide, 1 molecule of salt is formed, creating a waste problemthat was traditionally solved by dumping it in a river. Such practice is of course now totallyunacceptable. The catalytic route, however, is simple and clean, although it does produce a small

amount of CO2. Using silver, promoted by small amounts of chlorine, as the catalyst, ethylene oxideis formed directly from C2H4 and O2 at a selectivity of around 90 %, with about 10% of the ethyleneending up as CO2. Nowadays all production facilities for ethylene oxide use catalysts.

2.3.2.

Atom Efficiency, E Factors and Environmental FriendlinessNumerous organic syntheses are based on stoichiometric oxidations of hydrocarbons with sodiumdichromate and potassium permanganate, or on hydrogenations with alkali metals, borohydrides ormetallic zinc. In addition, there are reactions such as aromatic nitrations with H2SO4 and HNO3, oracylations with AlCl3 that generate significant amounts of inorganic salts as byproducts.Fine chemicals are predominantly (but not exclusively!) the domain of homogeneous catalysis, wheresolvents present another issue of environmental concern.According to Sheldon, the best solvent is no solvent, but if a solvent is unavoidable, then water is a

good candidate.


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Sheldon has introduced several indicators to measure the efficiency and environmental impact of areaction. The atom efficiency is the molecular weight of the desired product divided by the totalmolecular weight of all products. For example the conventional oxidation of a secondary alcohol

3C6H5CHOHCH3+ 2CrO3+ 3H2SO4 3C6H5COCH3+ Cr2(SO4)3+ 6H2Ohas an atom efficiency of 360/860 = 42%. By contrast, the catalytic route

C6H5CHOHCH3+ 1/2O2 C6H5COCH3+ H2Ooffers an atom efficiency of 120/138 = 87%, with water as the only byproduct. The reverse step, acatalytic hydrogenation, proceeds with 100% atom efficiency:

C6H5COCH3+ H2 C6H5CHOHCH3as does the catalytic carbonylation of this molecule:

C6H5CHOHCH3+ CO C6H5CH(CH3)COOHAnother useful indicator of environmental acceptability is the E factor the weight of waste orundesirable byproduct divided by the weight of the desired product.

As Table 1.1 shows, theproduction of fine chemicalsand pharmaceuticals generate

the highest amounts of wasteper unit weight of product.Atom efficiencies andE factorscan be calculated from eachother, but in practiceE factorscan be higher due to yieldsbeing less than optimum andreagents that are used in excess. Also, loss of solvents should be included, and perhaps even theenergy consumption with the associated generation of waste CO2.To express that it is not just the amount of waste but rather its environmental impact, Sheldonintroduced the environmental quotient EQ as the E factor multiplied by an unfriendliness quotient, Q,which can be assigned a value to indicate how undesirable a byproduct is. For example, Q = 0 for

clean water, 1 for a benign salt, NaCl, and 1001000 for toxic compounds. Evidently, catalytic routesthat avoid waste formation are highly desirable, and the more economic value that is placed on, forexample, the unfriendliness quotient, the higher the motivation to work on catalytic alternatives.Waste prevention is much to be preferred over waste remediation.

2.4.Catalysis as a Multidisciplinary Science

2.4.1. The Many Length Scales of a Catalyst

Catalysis is a very broad field of study that isclosely interwoven with numerous other scientificdisciplines. This becomes immediately evident if

we realize that catalysis as phenomenonencompasses many length scales. Figure 1.8illustrates this for the case of heterogeneouscatalysis. For the examples given above, thebonding of reactant molecules to a catalyst to givea reactive complex and the final separation ofcatalyst and product is catalysis on the molecularlevel. For heterogeneous, homogeneous orenzymatic catalysis, this is the level on which thechemistry takes place. Understanding reactions atthe elementary level of the rupture of bonds inreactants and the formation of bonds in products

is at the heart of the matter, and requires the mostadvanced experimental techniques and theoretical


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descriptions available. This is the domain of spectroscopy, computational chemistry and kinetics andmechanism on the level of elementary reaction steps. The length scales of interest are in thesubnanometre region. Publications on research at the molecular scale are found predominantly ingeneral journals in chemistry, physical chemistry and physics.

The next level is that of small catalytically active particles, with typical dimensions of between 1 and10 nm, and inside the pores of support particles (_m range). The questions of interest are the size,shape, structure and composition of the active particles, in particular of their surfaces, and how theseproperties relate to catalytic reactivity. Although we will deal with heterogeneous catalysis, theanchoring of catalytic molecules or even enzymes to supporting structures is also of great interest inhomogeneous and biocatalysis. This is the domain of catalyst preparation, characterization, testing onthe laboratory scale, and mechanistic investigations. Transport phenomena such as the diffusion ofmolecules inside pores may affect the rate at which products form and become an importantconsideration on this level. Much academic research as well as exploratory work in industry occurs onthis scale. Journals dedicated to catalysis deal largely with catalysis on this mesoscopic length scale.The next level is that of shaped catalysts, in the form of extrudates, spheres, or monoliths on lengthscales varying from millimeters to centimeters, and occasionally even larger. Such matters are to a

large extent the province of materials science. Typical issues of interest are porosity, strength, andattrition resistance such that catalysts are able to survive the conditions inside industrial reactors. Thisarea of catalysis is mainly (though not exclusively) dealt with by industry, in particular by catalystmanufacturers. Consequently, much of the knowledge is covered by patents. The macroscopic level isthat of reactors, be it the 25 cm test reactor in the laboratory or the 10 m high reactor vessel in anindustrial plant. The catalyst forms the heart of the reactor. Nevertheless catalysis as a discipline isonly one of many other aspects of reaction engineering, together with, for example, the design ofefficient reactors that are capable of handling high pressure, offer precise control of temperature,enable optimized contact between reactants and catalyst and removal of products, are resistant tocorrosion, make optimum use of energy resources, and are safe during operation. In describing thekinetics of catalytic reactions on the scale of reactors, extrinsic factors dealing with the mass and heattransport properties of reactants and products through the catalyst bed are as important as the intrinsic

reactivity of the molecules reacting at the catalytic site. The catalysts mechanical stability, sensitivityto trace impurities in the reactant feed, and degradation of particles, e.g. due to exposure to hightemperatures, are important in addition to its intrinsic properties such as activity and selectivity.Literature on these aspects of catalysis is largely found in chemical engineering journals and patents.

2.4.2.

Time Scales in CatalysisThe characteristic times on which catalytic events occur vary more or less in parallel with thedifferent length scales discussed above. The activation and breaking of a chemical bond inside amolecule occurs in the picosecond regime, completion of an entire reaction cycle from complexationbetween catalyst and reactants through separation from the product may take anywhere betweenmicroseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. Onthe mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take

between seconds and minutes, and the residence times of molecules inside entire reactors may befrom seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke,which stay on the catalyst.


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2.5.

The Scope of This Book

The emphasis of this book is on the fundamental level of catalytic reactions on the molecular andthemesoscopic scale. Dealing with the acceleration of reactions, catalysis is a kinetic phenomenon.Hence, we start with a chapter on kinetics to describe how the rate of the catalytic reaction cycle

depends on the main external process variables by which reactions can be influenced: concentration,pressure and temperature. Next is a chapter on the theory of reaction rates, which aims to make theconnection between the properties of reactant molecules and their reactivity. To make the connectionbetween fundamental catalysis and real life, we also describe what catalysts look like onthemesoscopic level, how they are made and how they are characterized. Owing to the spectacularadvances in theoretical chemistry and computational facilities over the last decade it is now possibleto predict reaction rates from first principles, albeit for idealized cases only. We intend to provide thereader with the necessary background to have at least a feeling for what fundamental catalysiscurrently means. Consequently, we describe phenomena such as adsorption and reaction on surfaces,along with the tools needed to investigate this. Finally, we describe a number of industrially importantcatalytic processes, from both applied and fundamental points of view. The emphasis is on conceptsand general trends rather than on specific details, and the aim has been to provide students with the

necessary background to appreciate the more specialized literature on fundamental catalysis. Theliterature section gives references to a number of general books in catalysis and related disciplines.

2.6.

Catalysis in Journals

The results of research in catalysis are published in a wide range of general and more specializedjournals. This reflects the highly multidisciplinary nature of the field. Referring to the different lengthscales in Fig. 1.8, research in the microscopic domain, dealing with the fundamentals of adsorbedmolecules and elementary reaction steps is often reported in general journals, such as theJournal ofChemical Physics, theJournal of Physical Chemistry,Physical Chemistry-Chemical Physics, SurfaceScience, Langmuir and Physical Review, and sometimes even Science and Nature. Also, thespecializedJournal of Catalysis and Catalysis Letters publish articles in this area. The mesoscopicdomain of real catalysts is mostly covered by the typical catalysis periodicals, such asAppliedCatalysis, the Journal of Catalysis, Catalysis Letters, Topics in Catalysis, Catalysis Today,Microporous Materials andZeolites, although occasionally articles also appear inJournal of PhysicalChemistry and Physical Chemistry-Chemical Physics, and many others. The macroscopic domain islargely covered by journals dedicated to chemical engineering: Chemical Engineering Science,Industrial & Engineering Chemistry Research, and theJournal of the American Institute of ChemicalEngineers (AIChE Journal) are some of the best known in this field.Exciting, new results that may not yet be fully understood but for which it is important that thecatalysis community learns about them, are published in the form of Letters, Notes and RapidCommunications. Specialized Letter Journals are Chemical Communications, Catalysis Letters,Chemical Physics Letters and Physical Review Letters, while several regular journals have sections

for letters, such as the Priority Communications in theJournal of Catalysis.In addition, there are the highly appreciated review journals, which publish overviews of the status ofcertain topics of interest in the field. Advances in Catalysis, Catalysis Reviews: Science &Engineering and Catalysis, Specialist Periodical Reports publish the most exhaustive reviews.CaTTech is a colorful magazine that publishes shorter reviews along with news from the catalysiscommunity. Also the News Brief section of Applied Catalysis fulfills this important role. As in allscientific fields, conferences are frequently followed up by a book of proceedings, containing shortaccounts of the presentations. However, proceedings are becoming less popular among scientists. Asrefereeing procedures tend to be less strict, and the amount of information that can be included isusually limited, the quality of these proceedings is not always what it should be. Often, workpublished in expensive proceedings is also published in regular papers, rendering proceedings toooften a waste of effort and money.

Midway between conference proceedings, reviews and regular research papers are the topical issuespublished by Catalysis Today and Topics in Catalysis. Rather than publishing the proceedings of an


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entire conference, the editor makes a selection of particularly interesting contributions and invites thepresenters to write a not too short article about their work. Such topical issues often give a valuableoverview of the status of a certain field, while the quality of the publications is much higher than thatof the average conference proceedings.To get an impression of which journals are the most read and cited, the Institute of ScientificInformation (Philadelphia, PA, USA) provides two interesting indicators (Journal Citation Reports,Science Edition). The impact factor for a given journal is the number of citations in that year toarticles published by the journal in the two previous years. Thus, if each article published in, forexample, either 1998 or 1999 is cited exactly once in 2000, the journal will have an impact factor of 1in that year.Authors use impact factors to decide where to submit their publications. For specialized journals, e.g.those focusing on catalysis, impact factors of 3 and higher are considered high. Review journalsusually have higher impact factors, whereas letter journals usually score lower.A much less appreciated but in fact highly informative parameter and indicator of quality is thecitation half-life of a journal. This is the period in years (going back from the current year) duringwhich cited papers were published, from which it received half of all the citations in the current year.A long citation half-life indicates that the journal has published quality articles that have long kept

their value to the scientific community. Care should be exercised with these numbers, as relativelyyoung journals need years before they can reach an appreciable citation half-life to prove that theyindeed published papers of long lasting value. Table 1.5 lists the impact factor and citation half-lifefor several relevant journals.


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3.1 Types of catalytic reactions

Catalysts can be divided into two main types - heterogeneous and homogeneous. In aheterogeneous reaction, the catalyst is in a different phase from the reactants. In ahomogeneous reaction, the catalyst is in the same phase as the reactants.

3.2 What is a phase?

If you look at a mixture and can see a boundary between two of the components, thosesubstances are in different phases. A mixture containing a solid and a liquid consists of two

phases. A mixture of various chemicals in a single solution consists of only one phase,because you can't see any boundary between them.

You might wonder why phase differs from the term physical state (solid, liquid or gas). Itincludes solids, liquids and gases, but is actually a bit more general. It can also apply to twoliquids (oil and water, for example) which don't dissolve in each other. You could see the

boundary between the two liquids.

3


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If you want to be fussy about things, the diagrams actually show more phases than arelabelled. Each, for example, also has the glass beaker as a solid phase. All probably have agas above the liquid - that's another phase. We don't count these extra phases because theyaren't a part of the reaction.

3.3

Heterogeneous catalysis

This involves the use of a catalyst in a different phase from the reactants. Typical examplesinvolve a solidcatalyst with the reactants as either l iquids or gases.

Note: It is important that you remember the difference between the two terms heterogeneousandhomogeneous.

heteroimplies different(as in heterosexual). Heterogeneous catalysis has the catalyst in a different phase fromthe reactants.

homoimplies the same(as in homosexual). Homogeneous catalysis has the catalyst in the same phase as thereactants.

3.3.1 How the heterogeneous catalyst works (in general terms)

Most examples of heterogeneous catalysis go through the same stages:

One or more of the reactants are adsorbedon to the surface of the catalyst at active sites.

Adsorption is where something sticks to a surface. It isn't the same as a bsorptionwhere one substance is taken up within the structure of another. Be careful!

An active site is a part of the surface which is particularly good at adsorbing thingsand helping them to react.

There is some sort of interaction between the surface of the catalyst and the reactantmolecules which makes them more reactive.

This might involve an actual reaction with the surface, or some weakening of thebonds in the attached molecules.


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The reaction happens.

At this stage, both of the reactant molecules might be attached to the surface, or onemight be attached and hit by the other one moving freely in the gas or liquid.

The product molecules are desorbed.

Desorption simply means that the product molecules break away. This leaves theactive site available for a new set of molecules to attach to and react.

A good catalyst needs to adsorb the reactant molecules strongly enough for them to react, butnot so strongly that the product molecules stick more or less permanently to the surface.

Silver, for example, isn't a good catalyst because it doesn't form strong enough attachmentswith reactant molecules. Tungsten, on the other hand, isn't a good catalyst because it adsorbstoo strongly.

Metals like platinum and nickel make good catalysts because they adsorb strongly enough tohold and activate the reactants, but not so strongly that the products can't break away.

3.3.2 Examples of heterogeneous catalysis3.3.2.1 The hydrogenation of a carbon-carbon double bond

The simplest example of this is the reaction between ethene and hydrogen in the presence of anickel catalyst.

In practice, this is a pointless reaction, because you are converting the extremely usefulethene into the relatively useless ethane. However, the same reaction will happen with anycompound containing a carbon-carbon double bond.

One important industrial use is in the hydrogenation of vegetable oils to make margarine,which also involves reacting a carbon-carbon double bond in the vegetable oil with hydrogenin the presence of a nickel catalyst.

Ethene molecules are adsorbed on the surface of the nickel. The double bond between thecarbon atoms breaks and the electrons are used to bond it to the nickel surface.

Hydrogen molecules are also adsorbed on to the surface of the nickel. When this happens, the

hydrogen molecules are broken into atoms. These can move around on the surface of thenickel.


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If a hydrogen atom diffuses close to one of the bonded carbons, the bond between the carbonand the nickel is replaced by one between the carbon and hydrogen.

That end of the original ethene now breaks free of the surface, and eventually the same thingwill happen at the other end.

As before, one of the hydrogen atoms forms a bond with the carbon, and that end also breaksfree. There is now space on the surface of the nickel for new reactant molecules to go throughthe whole process again.

3.3.2.2 Catalytic converters

Catalytic converters change poisonous molecules like carbon monoxide and various nitrogenoxides in car exhausts into more harmless molecules like carbon dioxide and nitrogen. Theyuse expensive metals like platinum, palladium and rhodium as the heterogeneous catalyst.

The metals are deposited as thin layers onto a ceramic honeycomb. This maximises thesurface area and keeps the amount of metal used to a minimum.


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Taking the reaction between carbon monoxide and nitrogen monoxide as typical:

Catalytic converters can be affected by catalyst poisoning. This happens when somethingwhich isn't a part of the reaction gets very strongly adsorbed onto the surface of the catalyst,

preventing the normal reactants from reaching it.

Lead is a familiar catalyst poison for catalytic converters. It coats the honeycomb ofexpensive metals and stops it working.

In the past, lead compounds were added to petrol (gasoline) to make it burn more smoothly inthe engine. But you can't use a catalytic converter if you are using leaded fuel. So catalyticconverters have not only helped remove poisonous gases like carbon monoxide and nitrogenoxides, but have also forced the removal of poisonous lead compounds from petrol.

3.3.2.2.1 The use of vanadium(V) oxide in the Contact Process

During the Contact Process for manufacturing sulphuric acid, sulphur dioxide has to beconverted into sulphur trioxide. This is done by passing sulphur dioxide and oxygen over asolid vanadium(V) oxide catalyst.

This example is slightly different from the previous ones because the gases actually react

with the surface of the catalyst, temporarily changing it. It is a good example of the ability oftransition metals and their compounds to act as catalysts because of their ability to changetheir oxidation state.

The sulphur dioxide is oxidised to sulphur trioxide by the vanadium(V) oxide. In the process,the vanadium(V) oxide is reduced to vanadium(IV) oxide.

The vanadium(IV) oxide is then re-oxidised by the oxygen.

This is a good example of the way that a catalyst can be changed during the course of areaction. At the end of the reaction, though, it will be chemically the same as it started.

3.4 Homogeneous catalysis

This has the catalyst in the same phase as the reactants. Typically everything will be presentas a gas or contained in a single liquid phase. The examples contain one of each of these . . .


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3.4.1 Examples of homogeneous catalysis

3.4.1.1 The reaction between Persulphate ions and iodide ions

This is a solution reaction that you may well only meet in the context of catalysis, but it is alovely example!

Persulphate ions (peroxodisulphate ions), S2O82-, are very powerful oxidizing agents. Iodide

ions are very easily oxidized to iodine. And yet the reaction between them in solution inwater is very slow.

If you look at the equation, it is easy to see why that is:

The reaction needs a collision between two negative ions. Repulsion is going to get seriouslyin the way of that!

The catalyzed reaction avoids that problem completely. The catalyst can be either iron(II) oriron(III) ions which are added to the same solution. This is another good example of the useof transition metal compounds as catalysts because of their ability to change oxidation state.

For the sake of argument, we'll take the catalyst to be iron(II) ions. As you will see shortly, itdoesn't actually matter whether you use iron(II) or iron(III) ions.

The persulphate ions oxidize the iron(II) ions to iron(III) ions. In the process the persulphateions are reduced to sulphate ions.

The iron(III) ions are strong enough oxidizing agents to oxidize iodide ions to iodine. In theprocess, they are reduced back to iron(II) ions again.

Both of these individual stages in the overall reaction involve collision between positive andnegative ions. This will be much more likely to be successful than collision between twonegative ions in the uncatalyzed reaction.

What happens if you use iron(III) ions as the catalyst instead of iron(II) ions? The reactionssimply happen in a different order.

3.4.1.2 The destruction of atmospheric ozone

This is a good example of homogeneous catalysis where everything is present as a gas.

Ozone, O3, is constantly being formed and broken up again in the high atmosphere by theaction of ultraviolet light. Ordinary oxygen molecules absorb ultraviolet light and break intoindividual oxygen atoms. These have unpaired electrons, and are known as fr ee radicals.They are very reactive.


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The oxygen radicals can then combine with ordinary oxygen molecules to make ozone.

Ozone can also be split up again into ordinary oxygen and an oxygen radical by absorbingultraviolet light.

This formation and breaking up of ozone is going on all the time. Taken together, these

reactions stop a lot of harmful ultraviolet radiation penetrating the atmosphere to reach thesurface of the Earth.

The catalytic reaction we are interested in destroys the ozone and so stops it absorbing UV inthis way.

Chlorofluorocarbons (CFCs) like CF2Cl2, for example, were used extensively in aerosols andas refrigerants. Their slow breakdown in the atmosphere produces chlorine atoms - chlorinefree radicals. These catalyse the destruction of the ozone.

This happens in two stages. In the first, the ozone is broken up and a new free radical is

produced.

The chlorine radical catalyst is regenerated by a second reaction. This can happen in twoways depending on whether the ClO radical hits an ozone molecule or an oxygen radical.

If it hits an oxygen radical (produced from one of the reactions we've looked at previously):

Or if it hits an ozone molecule:

Because the chlorine radical keeps on being regenerated, each one can destroy thousands ofozone molecules.


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3.5 Autocatalysis3.5.1 The oxidation of ethanedioic acid by manganate(VII) ions

In autocatalysis, the reaction is catalysed by one of its products. One of the simplest examplesof this is in the oxidation of a solution of ethanedioic acid (oxalic acid) by an acidified

solution of potassium manganate(VII) (potassium permanganate).

The reaction is very slow at room temperature. It is used as a titration to find theconcentration of potassium manganate(VII) solution and is usually carried out at atemperature of about 60C. Even so, it is quite slow to start with.

The reaction is catalysed by manganese(II) ions. There obviously aren't any of those presentbefore the reaction starts, and so it starts off extremely slowly at room temperature. However,if you look at the equation, you will find manganese(II) ions amongst the products. More andmore catalyst is produced as the reaction proceeds and so the reaction speeds up.

You can measure this effect by plotting the concentration of one of the reactants as time goeson. You get a graph quite unlike the normal rate curve for a reaction.

Most reactions give a rate curve which looks l ike this:

Concentrations are high at the beginning and so the reaction is fast - shown by a rapid fall in

the reactant concentration. As things get used up, the reaction slows down and eventuallystops as one or more of the reactants are completely used up.


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An example of autocatalysis gives a curve like this:

You can see the slow (uncatalyzed) reaction at the beginning. As catalyst begins to be formedin the mixture, the reaction speeds up - getting faster and faster as more and more catalyst isformed. Eventually, of course, the rate falls again as things get used up.


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4.1 Principles of heterogeneous catalysis

About 90% of all industrial processes involve heterogeneous catalysis, the catalyst beingtypically a solid while the reactants are in gas or in the liquid phase. The advantage comparedto homogeneous catalysts is that they are easier to prepare, handle and separate from thereaction mixture. The catalytic process is run inside a reactor typically operated withcontinuous flow under steady-state conditions. The rate is determined, apart from the natureof the catalytically active surface, by external parameters like temperature, partial pressuresand flow rate.

Usually heterogeneous catalysts consist of fine, often Nano sized, powders supported onnominally inert oxide substrates (Alumina, MgO,...) which exhibit all possiblecrystallographic faces. The catalyst is often a metal to which chemical and structural

promoters or poisons are added to enhance the efficiency and /or the selectivity.

A catalytic reaction is said to be homogeneous if the catalyst is in the same phase as thereactants Examples:

1. the molecule chlorine-tris(triphenylphosphine)-rodium(I) [RhCl(PPh3)3] is added toalkene solutions for hydrogenation reactions (Wilkinson catalyst)

2. Nickel(IV) acetylacetonate (benzene synthesis)3. Dicarbonyl diiodine-iridium(III) (acetic acid synthesis, Cativa process)4. di-cobalt(II) Octacarbonyl (hydroformylation of akenes to aldehydes)

A classic example are the iron powders used in the Haber process to enable the reaction of Hand N to NH . The H2N2NH3. triple bond between the nitrogen atoms in the N2molecules is

then broken by dissociative chemisorption.A catalytic reaction is said to be heterogeneous if the catalyst is in a different phase than thereactants, e.g. when the reactants chemisorb on a solid surface. The weakening of the internal

bonds makes the formation of new bonds with other molecules easier. The product must havea lower affinity with the catalyst in order to be released into the gas phase (desorption).

Other Examples:1) Pt/Rh (Ostwald process for the production of nitric acid)2) Titanium tetrachloride and an organometallic composite of Al (Ziegler- Natta process

for polymerization)3) Chromium oxide for the Phillips process for the polymerization of ethylene to

polyethylene.4) Zeolite ZSM-5 for hydrocarbon conversion.

4


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An example of heterogeneous catalysis from everyday life is the automobiles

catalytic converter transforming poisonous gases like carbon monoxide and nitrogenoxides, generated from the combustion of gasoline, into much lessdangerous molecules like carbon dioxide and nitrogen.

:

The catalyst is composed by a thin metal film of Pt, Pd and Rh deposited on a ceramicsurface. In this way the amount of expensive metal is minimized and the activesurface maximized allowing to keep down costs at an affordablelevel.

The catalyst causes thedissociative adsorption of

NO. The N atoms recombineto N2 which desorbs. The Oatoms react with adsorbedCO yielding CO2which

desorbs, too.


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Industrial catalysis is still governed by empirical experience. The single crystal surfacesstudied under ultra-high vacuum conditions, necessarily differ from the real catalysisconditions. One speaks in particular of material and pressure gaps. The material gap can be

bridged by studying the reaction for different single crystal surfaces including surfaces withwell-defined defects such as monoatomic steps or using particles with uniform particle size.Analysis of the catalyst composition enables the deliberate modification of the chemicalcomposition studying the way in which the reaction digs its own bed.

The reaction rate is determined by the surface concentration of the reactants and by barriersto the adsorption which determine the pressure gap. It can be bridged using lowertemperatures to increase the surface coverage and supersonic molecular beams to study thehigh energy tail of the Boltzmann distribution of the reactants


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4.2 CLASSIFICATION OF REACTION MECHANISMS

A(gas)+B(gas)C(gas)+(D(ad) or D(gas))Langmuir-Hinshelwood:both reactants are adsorbed at the surface and the reaction occurs

between surface species

A(gas)+B(gas)A(ad)+B(ad)A(ad)+B(ad) Cg+ D(ad) (or D)Four processes are involved: adsorption of A and B at the surface diffusion of A(ad) andB(ad) reaction between A(ad) and B(ad) desorption of C Example: Haber-Bosch synthesis of

NH3;Eley-Rideal: only one reactant has to be adsorbed at the

surfaceA(gas)A(ad)B(gas)+A(ad) C +D (ad) (or D)Example: H(g) +D/Cu(111) HDH+Dad

HD energy balance: E(HD)=Ediss(HD)+Ekin(H) + Eads(D)=2.3 eV on Cu(111) This energyis carried away by the products of the reaction which keep memory of the velocity of theimpinging H atoms On Ni(110), however, also D2 is formed indicating that part of theimpinging H ends up in a translationally hot precursorwhich may transfer its energy to the Datoms which then react by collision with other Dadatoms.


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4.4 Poisons and Promoters

Typically electronegative species act as poisons, electropositive species aspromotersCO methanation on Ni (powders as well as supported clusters). Sulphur attenuates thecatalytic activity of Ni, 10 sites deactivated per S atom Two possibilities:

1) S blocks 10 sites and the reaction sequence needs for a critical step of the reaction2) S has an electronic effect which extends on nearest neighbor sites

a. If holds changing the poison has little effect,b. if holds a less electronegative adsorbate has a smaller effect

phosphorus is less poisoning than S electronic effect

4.5 Electropositive impurities:

Potassium may accelerate certain steps in a reaction. CO methanation over Ni occurs by CO

dissociation and subsequent reaction of the carbidic carbon produced with H. The carbidic Ccoverage saturates at 50% of a ML. Large H2 pressure needed to find sites where todissociate.The presence of K does not modify the reaction barrier, but 0.1% of K increase the carboncoverage in working conditions from 0.1 ML to 0.3 ML on Ni(100).The reaction is poisoned above T=650 K since carbidic carbon coalesces into unreactivegraphite.Structural Promoter:Adsorbate which modifiesthe surface structure tomake it more or less

reactive .


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4.6 Determination of the Reaction Ratea. Power laws

Partial pressure of reactants pi and productspjr= k pAapB

buseful approach but contains still

no information about the progress of the reaction

b.

Attempt to describe the elementary steps: Langmuir approachThe various steps of the reaction are described in terms of rate equations for adsorption ,desorption and surface reaction. If adsorption and desorption processes are fast then the

partial coverages of the reactants are related to the relative partial pressures throughadsorption isotherms.This approach may look hopeless given the various structural elements of a real catalyst.However, it works, and the reason why it works is the same of why catalysts are robust andcan stand a wide range of operative conditions

For constant T the reaction passes through a maximum since A blocks

the sites for the adsorption of Bc.

c) Micro kinetics parameters obtained by DFT calculations of the elementarysteps involved

d.

d) Kinetic Monte Carlo: takes into account the actual neighborhood of theadsorbed particle detailed insight but heavy computational effort.

SelectivityA catalyst may lead to different products which then need to be separated. The selectivitytowards one particular reaction channel is thus often more valuable than the overall reactivity.

the desired product it must then be interrupted at such point The reaction has two branchescharacterized by reaction rates r1 and r2.The selectivity is then s1= r1 / (r1 + r2 ) and can be affected by poisoning one of the pathwaysOstwald process: Ammonia oxidation for the production of Nitric acid In industry the catalystis Pt based and used at T>1000 K Surface science demostrated that the reaction may work atT=500 K on RuO2 .


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5.1. Heterogeneous industrial catalysis Examples

The most common examples of heterogeneous catalysis in industry involve the reactions ofgases being passed over the surface of a solid, often a metal, a metal oxide or a zeolite (Table1).

Process Catalyst EquationMakingammonia IronMaking synthesis gas

(carbon monoxide and

hydrogen)

Nickel

Catalytic cracking of gas

oil

Zeolite Produces:a gas (e.g. ethene, propene)a liquid (e.g.petrol)a residue (e.g. fuel oil)

Reforming of naphtha Platinum andrhenium onalumina

Makingepoxyethane Silver on alumina

Makingsulfuric acid Vanadium(V)oxide on silica

Makingnitric acid Platinum andrhodium

Table 1 Examples of industrial processes using heterogeneous catalysis.

The gas molecules interact with atoms or ions on the surface of the solid. The first processusually involves the formation of very weak intermolecular bonds, a process known asphysisorption, followed by chemical bonds being formed, a process known as chemisorption.

Physisorption can be likened to a physical process such as liquefaction. Indeed the enthalpychanges that occur in physisorption are ca -20 to -50 kJ mol-1, similar to those of enthalpychanges when a gas condenses to form a liquid. The enthalpies of chemisorption are similarto the values found for enthalpies of reaction. They have a very wide range, just like therange for non-catalytic chemical reactions.

An example of the stepwise processes that occur in heterogeneous catalysis is the oxidation

of carbon monoxide to carbon dioxide over palladium. This is a very important process ineveryday life. Motor vehicles are fitted with catalytic converters. These consist of a metal
http://www.essentialchemicalindustry.org/chemicals/ammonia.html#haber_processhttp://www.essentialchemicalindustry.org/chemicals/ammonia.html#syn_gashttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#cat_crackinghttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#reforminghttp://www.essentialchemicalindustry.org/chemicals/epoxyethane.html#epoxyethanehttp://www.essentialchemicalindustry.org/chemicals/epoxyethane.html#epoxyethanehttp://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html#contact_processhttp://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html#contact_processhttp://www.essentialchemicalindustry.org/chemicals/nitric-acid.html#nitric_acidhttp://www.essentialchemicalindustry.org/chemicals/nitric-acid.html#nitric_acidhttp://www.essentialchemicalindustry.org/chemicals/nitric-acid.html#nitric_acidhttp://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html#contact_processhttp://www.essentialchemicalindustry.org/chemicals/epoxyethane.html#epoxyethanehttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#reforminghttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#cat_crackinghttp://www.essentialchemicalindustry.org/chemicals/ammonia.html#syn_gashttp://www.essentialchemicalindustry.org/chemicals/ammonia.html#haber_process


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casing in which there are two metals, palladium and rhodium, dispersed very finely on thesurface of a ceramic support that resembles a honeycomb of holes. The converter is placed

between the engine and the outlet of the exhaust pipe.

The exhaust gases contain carbon monoxide and unburned hydrocarbons that react with the

excess oxygen to form carbon dioxide and water vapour, the reaction being catalyzedprincipally by the palladium:

The exhaust gases also contain nitrogen(II) oxide (nitric oxide, NO), and this is removed byreactions catalyzed principally by the rhodium:

The accepted mechanism for the oxidation of carbon monoxide to carbon dioxide involvesthe chemisorption of both carbon monoxide molecules and oxygen molecules on the surfaceof the metals. The adsorbed oxygen molecules dissociate into separate atoms of oxygen. Eachof these oxygen atoms can combine with a chemisorbed carbon monoxide molecule to form acarbon dioxide molecule. The carbon dioxide molecules are then desorbed from the surfaceof the catalyst. A representation of these steps is shown in Figure 1.

Figure 1 A mechanism for the oxidation of carbon monoxide.

Each of these steps has a much lower activation energy than the homogeneous reactionbetween the carbon monoxide and oxygen.

The removal of carbon monoxide, unburned hydrocarbons and nitrogen(II) oxide from carand lorry exhausts is very important for this mixture leads to photochemical smogs whichaggravate respiratory diseases such as asthma.


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Platinum, palladium and rhodium are all used but are very expensive metals and indeed eachis more expensive than gold. Recently, much work has been devoted to making catalysts withvery tiny particles of the metals, an example of the advances being made bynanotechnology.

It is not simply the ability of the heterogeneous catalyst's surface to interact with the reactant

molecules, chemisorption, that makes it a good catalyst. If the adsorption is too exothermic,i.e. the enthalpy of chemisorption is too high, further reaction is likely to be too endothermicto proceed. The enthalpy of chemisorption has to be sufficiently exothermic forchemisorption to take place, but not so high that it does not allow further reaction to proceed.For example, in the oxidation of carbon monoxide, molybdenum might at first sight befavoured as a choice, as oxygen is readily chemisorbed by the metal. However, the resultingoxygen atoms do not react further as they are too strongly adsorbed on the surface. Platinumand palladium, on the other hand, have lower enthalpies of chemisorption with oxygen, andthe oxygen atoms can then react further with adsorbed carbon monoxide.

Another point to consider in choosing a catalyst is that the product must not be able to adsorb

too strongly to its surface. Carbon dioxide does not adsorb strongly on platinum andpalladium and so it is rapidly desorbed into the gas phase.

A testimony to the importance of catalysis today is the award of the Nobel Prize in Chemistryin 2007 to Gerhard Ertl for his work in elucidating, amongst other processes, the mechanismfor the synthesis of ammonia (the Haber Process):

Ertl obtained crucial evidence on how iron catalyses the dissociation of the nitrogenmolecules and hydrogen molecules leading to the formation of ammonia(Figure 2):

Figure 2 A mechanism for the catalytic synthesis of ammonia.

Figure 3 shows how the activation energy barriers are much lower than the estimatedactivation energy barrier (which would be at least 200 kJ mol1) for the uncatalysed synthesis

of ammonia.
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Figure 3 The activation energy barriers for the reactions occurring during the catalyticsynthesis of ammonia.

5.2. General requirements for a heterogeneous catalyst

To be successful the catalyst must allow the reaction to proceed at a suitable rate underconditions that are economically desirable, at as low a temperature and pressure as possible.It must also be long lasting. Some reactions lead to undesirable side products. For example inthecracking of gas oil,carbon is formed which is deposited on the surface of the catalyst, azeolite, and leads to a rapid deterioration of its effectiveness. Many catalysts are prone to

poisoning which occurs when an impurity attaches itself to the surface of the catalyst andprevents adsorption of the reactants. Minute traces of such a substance can ruin the process,One example is sulfur dioxide, which poisons the surface of platinum and palladium. Thus all

traces of sulfur compounds must be removed from the petrol used in cars fitted with catalyticconverters.

Further, solid catalysts are much more effective if they are finely divided as this increases thesurface area.
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Figures 4 and 5 Two ways by which the surface area of a catalyst can be increased.

In Figure 4, the platinum-rhodium alloy (used in themanufacture of nitric acid)is in the form of very finewire that has been woven to construct a gauze.By kind permission of Johnson Matthey.

In Figure 5, vanadium(V) oxide (used in the manufacture of sulfuric acid) has been produced in a'daisy'shape.By kind permission of Haldor Topse A/S.

At high temperatures, the particles of a finely dividedcatalyst tend to fuse together and the powder may 'cake',a process known as sintering. This reduces the activityof the catalyst and steps must be taken to avoid this. Oneway is to add another substance, known as a promoter.When iron is used as the catalyst in the Haber Process,aluminum oxide is added and acts as a barrier to the

fusion of the metal particles. A second promoter isadded, potassium oxide, that appears to cause thenitrogen atoms to be chemisorbed more readily, thusaccelerating the slowest step in the reactionscheme. Figure 6 A platinum-rhodium gauze is used asa catalyst in the reaction between ammonia and methaneto produce hydrogen cyanide, an intermediate in theproduction of methyl 2-methylpropenoate. The gauzeoperates at 1270 K and is thus glowing. The photographwas taken though a sight glass located on the reactor.

5.3. Aluminium oxide, silicon dioxide, aluminosilicates and zeolites

One of the most important reactions in which aluminium oxide, Al2O3, (often referred to asalumina) takes part in an industrial reaction is in platforming, in which naphtha is reformedover aluminina impregnated with platinum or rhenium. Both the oxide and the metals havecatalytic roles to play, an example ofbifunctional catalysis.There are hydroxyl groups on thesurface of alumina which are, in effect, sites which are negatively charged to which ahydrogen ion is attached that can act as an acid catalyst.
http://www.essentialchemicalindustry.org/chemicals/nitric-acid.html#nitric_acidhttp://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html#contact_processhttp://www.essentialchemicalindustry.org/polymers/polymethyl.html#monomerhttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#platforminghttp://www.essentialchemicalindustry.org/processes/catalysis-in-industry.html#platforminghttp://www.essentialchemicalindustry.org/processes/catalysis-in-industry.html#platforminghttp://www.essentialchemicalindustry.org/processes/cracking-isomerisation-and-reforming.html#platforminghttp://www.essentialchemicalindustry.org/polymers/polymethyl.html#monomerhttp://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html#contact_processhttp://www.essentialchemicalindustry.org/chemicals/nitric-acid.html#nitric_acid


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Silicon dioxide(silica) is another acidic oxide used in industry. It becomes particularly activeif it has been coated with an acid (such as phosphoric acid), thereby increasing the number ofactive acidic sites. For example, the manufacture of ethanol is achieved by the hydration ofethene using silica, coated with phosphoric acid:

The mechanism involves the formation of a carbocation (Figure 7):

Figure 7 A mechanism for the hydration of ethene to ethanol.

Aluminosilicatesare also used as catalysts when an acid site is required. These are made fromsilicon dioxide (silica) and aluminium oxide. They contain silicate ions, SiO4

4- that have atetrahedral structure which can be linked together in several ways. When some of the Siatoms are replaced with Al atoms, the result is an aluminosilicate. Hydrogen ions are againassociated with the aluminium atoms:


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5.3.1. Zeolite catalysts

A particular class of aluminosilicates that has excited huge interest in recent years is thezeolites. There are many different zeolites because of the different ways in which the atomscan be arranged. Their structure of silicate and aluminate ions can have large vacant spaces in

three dimensional structures that give room for cations such as sodium and calcium andmolecules such as water. The spaces are interconnected and form long channels and poreswhich are of different sizes in different zeolites.

Figure 8 The structure of a zeolite (example figure)

A zeolite which is commonly used in many catalytic reactions is ZSM-5 which is preparedfrom sodium aluminate (a solution of aluminium oxide in aqueous sodium hydroxide) and acolloidal solution of silica, sodium hydroxide, sulfuric acid and tetrapropylammonium

bromide.

It is, for example, a very effective catalyst for the conversion of methylbenzene (toluene) tothe three dimethylbenzenes (xylenes). Alas, the mixture produced only contains about 25%1,4-dimethylbenzene, (p-xylene) the isomer needed for the manufacture of thepolyesters andthe rest, 1,2- (o-xylene) and 1,3-dimethylbenzenes (m-xylene), is not wanted in such largequantities.
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However, if the zeolite is washed with phosphoric acid and heated strongly, minute particlesof phosphorus(V) oxide are deposited on the surface making the pores slightly smaller. Thisrestricts the diffusion of the 1,2- and 1,3-isomers and they are held in the pores until they areconverted into the 1,4-isomer and can escape (Figure 9).

This remarkable selectivity enables the yield of the 1,4-isomer to be increased from 25% to97%.

Figure 9 A zeolite acting as a molecular sieve and a catalyst during

the formation of 1,4-dimethylbenzene from methylbenzene.


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The ability of the zeolite to adsorb some molecules and to reject others gives it the ability toact as a molecular sieve. For example, in the manufacture of ethanol from ethene or from

biomass, an aqueous solution of ethanol is produced, in which there is 4% water still presenteven after repeated distillations. Further purification of ethanol requires the use of a zeolitewhich absorbs the water preferentially. Table 2 gives examples of industrial processes

involving zeolites.

Process Catalyst Equation

Catalytic cracking of gas oil Zeolite Produces:

a gas (e.g. ethene, propene)

a liquid (e.g.petrol)

a residue (e.g. fuel oil)

Reforming of naphtha Platinum andrhenium onzeolite

Disproportionation of

methylbenzeneZeolite

Dealkylation of

methylbenzeneZeolites

Making cumene (1-

methylethyl)benzene


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6.1. Whats Homogeneous catalysis ?

In chemistry, homogeneous catalysisiscatalysis in a solution by a soluble catalyst. Strictlyspeaking, homogeneous catalysis are catalytic reactions where the catalyst is in the same

phase as the reactants, so homogeneous catalysis applies to reactions in the gas phase and

even in a solid. Heterogeneous catalysis is the alternative to homogeneous catalysis, wherethe catalysis occurs at the interface of two phases, typically gas-solid. The term is usedalmost exclusively to describe solutions and it is often implies catalysis by organometalliccompounds. The area is one of intense research and many practical applications, e.g., the

production ofacetic acid.Enzymes are examples of homogeneous catalysts.

6.2. Acid catalysis

The proton is the most pervasive homogeneous catalyst because water is the most commonsolvent. Water forms protons by the process ofself-ionization of water.In an illustrative case,

acids accelerate (catalyze) thehydrolysis ofesters:

CH3CO2CH3+ H2O CH3CO2H + CH3OH

In the absence of acids, aqueous solutions of most esters do not hydrolyze at practical rates.

6.3. Organometallic chemistry

Processes that utilize soluble organometallic compounds as catalysts fall under the categoryof homogeneous catalysis, as opposed to processes that use bulk metal or metal on a solidsupport, which are examples of heterogeneous catalysis. Some well-known examples of

homogeneous catalysis include hydroformylation and transfer hydrogenation, as well ascertain kinds ofZiegler-Nattapolymerization andhydrogenation.Homogeneous catalysts hasalso been used in a variety of industrial processes such as theWacker process Acetaldehyde(conversion of ethylene to acetaldehyde) as well as the Monsanto process and the Cativa

process for the conversion ofMeOH andCO toacetic acid.

Many non-organometallic complexes are also widely used in catalysis, e.g. for the productionofterephthalic acid fromxylene.
https://en.wikipedia.org/wiki/Catalysishttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Acetic_acidhttps://en.wikipedia.org/wiki/Self-ionization_of_waterhttps://en.wikipedia.org/wiki/Hydrolysishttps://en.wikipedia.org/wiki/Esterhttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Hydroformylationhttps://en.wikipedia.org/wiki/Transfer_hydrogenationhttps://en.wikipedia.org/wiki/Ziegler-Nattahttps://en.wikipedia.org/wiki/Hydrogenationhttps://en.wikipedia.org/wiki/Wacker_processhttps://en.wikipedia.org/wiki/Ethylenehttps://en.wikipedia.org/wiki/Acetaldehydehttps://en.wikipedia.org/wiki/Monsanto_processhttps://en.wikipedia.org/wiki/Cativa_processhttps://en.wikipedia.org/wiki/Cativa_processhttps://en.wikipedia.org/wiki/MeOHhttps://en.wikipedia.org/wiki/Carbon_monoxidehttps://en.wikipedia.org/wiki/Acetic_acidhttps://en.wikipedia.org/wiki/Terephthalic_acidhttps://en.wikipedia.org/wiki/Xylenehttps://en.wikipedia.org/wiki/Xylenehttps://en.wikipedia.org/wiki/Terephthalic_acidhttps://en.wikipedia.org/wiki/Acetic_acidhttps://en.wikipedia.org/wiki/Carbon_monoxidehttps://en.wikipedia.org/wiki/MeOHhttps://en.wikipedia.org/wiki/Cativa_processhttps://en.wikipedia.org/wiki/Cativa_processhttps://en.wikipedia.org/wiki/Monsanto_processhttps://en.wikipedia.org/wiki/Acetaldehydehttps://en.wikipedia.org/wiki/Ethylenehttps://en.wikipedia.org/wiki/Wacker_processhttps://en.wikipedia.org/wiki/Hydrogenationhttps://en.wikipedia.org/wiki/Ziegler-Nattahttps://en.wikipedia.org/wiki/Transfer_hydrogenationhttps://en.wikipedia.org/wiki/Hydroformylationhttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Esterhttps://en.wikipedia.org/wiki/Hydrolysishttps://en.wikipedia.org/wiki/Self-ionization_of_waterhttps://en.wikipedia.org/wiki/Acetic_acidhttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Organometallic_compoundhttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Catalysis


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6.4. Other forms of homogeneous catalysis

Enzymes are homogeneous catalysts that are essential for life but are also harnessed forindustrial processes. A well-studied examplecarbonic anhydrase,which catalyzes the releaseof CO2into the lungs from the blood stream.

6.4.1. Contrast with heterogeneous catalysis

Homogeneous catalysis differs from heterogeneous catalysis in that the catalyst is in adifferent phase than the reactants. One example of heterogeneous catalysis is the

petrochemical alkylation process, where the liquid reactants are immiscible with a solutioncontaining the catalyst. Heterogeneous catalysis offers the advantage that products are readilyseparated from the catalyst, and heterogeneous catalysts are often more stable and degrademuch slower than homogeneous catalysts. However, heterogeneous catalysts are difficult tostudy, so their reaction mechanisms are often unknown.

Enzymespossess properties of both homogeneous and heterogeneous catalysts. As such, theyare usually regarded as a third, separate category of catalyst.
https://en.wikipedia.org/wiki/Enzymehttps://en.wikipedia.org/wiki/Carbonic_anhydrasehttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Alkylationhttps://en.wikipedia.org/wiki/Enzymehttps://en.wikipedia.org/wiki/Enzymehttps://en.wikipedia.org/wiki/Alkylationhttps://en.wikipedia.org/wiki/Heterogeneous_catalysishttps://en.wikipedia.org/wiki/Carbonic_anhydrasehttps://en.wikipedia.org/wiki/Enzyme


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7.1. Homogeneous industrial catalysis Examples

Homogeneous catalysts are less frequently used in industry than heterogeneous catalysts as,on completion of the reaction, they have to be separated from the products, a process that can

be very expensive.

Manufacture Catalyst EquationEthane-1,2-diol Sulfuric acid

2,2,4-Trimethylpentane(iso-octane)

Hydrogen fluoride

Phenol and propanone Sulfuric acid

Bisphenol A Sulfuric acid

Table 3 Examples of industrial processes using homogeneous catalysis.

However, there are several important industrial processes that are catalyzed homogeneously,often using an acid or base (Table 3).

One example is in the manufacture of ethane-1,2-diol from epoxyethane where the catalyst isa trace of acid:
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General Types of Catalysis

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