Part I Chemical Basics

j1 Part I Chemical Basics Applied Homogeneous Catalysis, First Edition. Arno Behr and Peter Neubert. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA. j3 1 Definition, Options, and Examples What Actually Is Catalysis? 1.1 be obtained in quantitative amounts. In Definition of Catalysis this case, the sulfuric acid serves as a catalyst. Let us consider a reversible chemical reac- Another example is the highly exother- tion between the educts A and B which leads mic reaction of the two gases hydrogen and to the products C and D: oxygen. In a sealed vessel, they can be mixed to any ratio without reaction, but if a small A þ B K C þ D ð1:1Þ amount of a platinum sponge is added to the mixture it reacts spontaneously. The cata- A typical example is the formation of an lyst, with its large surface area, initiates the ester from a carboxylic acid and an alcohol, so-called hydrogen–oxygen reaction: which involves the elimination of water, þ : K ð : Þ while the reverse reaction involves the H2 0 5O2 H2O 1 3 hydrolysis of an ester to a carboxylic acid and an alcohol: The reaction heat indicates a thermody- namically permitted reaction that will pro- 0 0 RÀCOOHþR ÀOHKRÀCOOR þH2O ceed even at room temperature and at low ð1:2Þ pressure. This reaction also requires a catalyst – the transition metal, platinum – to get The steady state of this reaction – it started: only the catalyst is able to run the depending on pressure and temperature – reaction. is controlled by Nature and determined The adjuvant effect of the platinum by the thermodynamics (cf. Chapter 10). sponge was first discovered by Johann Therateatwhichequilibriumisachieved Wolfgang D€obereiner (Figure 1.1) who, on is described by kinetics (cf. Chapter 11). the 27th of July 1823, used a platinum As is well known from organic chemistry, sponge to construct a gas lighter (Fig- the reaction requires a small amount of ure 1.2). In the lighter, the crank, e, is used sulfuric acid to get it started. Then, if the to dip a piece of zinc into a tank, c, which water produced is removed constantly contains sulfuric acid; this leads to the from the reaction vessel, the ester can formation of hydrogen (Eq. (1.4)): Applied Homogeneous Catalysis, First Edition. Arno Behr and Peter Neubert. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA. 4j 1 Definition, Options, and Examples namesake, Goethe, who initially had problems using it. In 1828, Goethe wrote to Dobereiner:€ Ew. Hochwohlgeboren haben die Gefalligkeit€ gehabt, einen dem Anblick und Zweck nach sehr anmutigen Apparat zuzusenden, aber ich will nur gestehen, daß Figure 1.1 Johann ohngeachtet der genauen beigefugten€ Wolfgang Dobereiner.€ Beschreibung ich doch das Experiment nicht zu unternehmen getraue. þ 2 þ Zn þ 2H ! Zn þ H2 ð1:4Þ (Approximate English translation: Your honor has been so kind to send me the very The hydrogen is first collected in an charming apparatus; however, I have to upright pipe and then released from the admit that I dont feel able to carry out the pipes upper end through a simultaneously experiment.) opened valve. The hydrogen is mixed with However, some time later Goethe is said oxygen from the ambient air and ignited by to have used the lighter on a regular basis. a piece of platinum sponge, f, attached The first definition of a catalyst was pro- immediately above the valve. The first auto- vided in 1836 by the Swedish chemist Jons€ matic gas lighter had been invented! Jakob Berzelius (Figure 1.3) of the Univer- Dobereiner€ gave one of these lighters to his sity of Stockholm, who noted that: The substances that cause the decomposition of H2O2 do not achieve this goal by being incorporated into the new compounds (H2O and O2); in each case they remain unchanged and hence act by means of an inherent force whose nature is still unknown... (Berzelius meant that, in the case of the hydrogen–oxygen reaction, the substance – the platinum sponge – would waken the dozing gases hydrogen and oxygen. Figure 1.3 Jons€ Jakob Figure 1.2 Dobereiner€ s gas lighter. Berzelius. 1.2 The Different Varieties of Catalysis j5 Figure 1.4 The Chinese character for catalysis and matchmaker (tschu mei). Figure 1.5 A general catalysis cycle. Moreover, this action can be repeated many However, he was unclear about the details of thousands or even millions of times. The the processes driven by the catalyst.) most commonly used method used to rep- Berzelius used the term katalysis, which resent this behavior graphically is a circle; is derived from the Greek, meaning to this is often referred to as catalytic cycle. untie, and on that basis he considered the A generalized type of cycle incorporating bond-breaking stage to be the crucial step. In the addition reaction A þ B ! A À B, cat- contrast, the Chinese took a somewhat dif- alyzed by C, is shown in Figure 1.5. ferentapproachtocatalysis;theChinesechar- Here, the catalyst C first adds reactant acter tschu mei (Figure 1.4) means both A and, subsequently, also reactant B. The two catalysis and marriage broker, with the reactantsare unitedinthe immediate vicinity emphasis being placed much more on the of the catalyst (the marriage broker), such linking properties. that a new product, A–B, is formed. The fi When considering todaysde nition of a catalyst remains in the condition it was in catalyst: at the start of the cycle and, therefore, is available for another cycle. A catalyst is a substance that increases the rate at which a chemical system 1.2 reaches its equilibrium. The Different Varieties of Catalysis . A catalyst participates in a chemical reaction, but is not itself consumed. The reaction in Eq. (1.2) contains liquid reactants – carboxylic acid and alcohol – Although, in theory, a catalyst should be which are reacted to produce a liquid ester active for ever, there is of course no ideal in the presence of liquid sulfuric acid. All of philosophers stone. Whilst each catalyst the components are completely soluble and, has a certain lifetime, during which it will as there is only one homogeneous liquid catalyze a reaction in an economical fashion, phase in the vessel, this is referred to as its efficiency will decrease steadily until, homogeneous catalysis. ultimately, it is dead. The lifetime of a In contrast, in Eq. (1.3) two gases react catalystmaybeveryshort,oritmaybeseveral in the presence of a solid catalyst, the years (this point is described in detail in platinum sponge. In this type of catalysis Chapter 4). there are at least two phases, and some- One factor that all catalysts have in com- times even three (gaseous, liquid, and mon is that, immediately after they have solid). Based on such heterogeneity, this successfully catalyzed a first reaction, they variant is referred to as heterogeneous are available to catalyze another reaction. catalysis. 6j 1 Definition, Options, and Examples Table 1.1 Comparison of homogeneous and heterogeneous catalysis. Criterion Heterogeneous catalysis Homogeneous catalysis Catalyst stoichiometry Often undefined Common Catalyst structure Often undefined Common Catalyst variability Little Very variable Catalyst reproducibility Often difficult Very high Mechanism knowledge Often very little Available Number of active centers Only surface atoms All metal atoms Catalyst activity Different High Catalyst selectivity Usually poor Usually high Diffusion problems Present Barely present Conditions Often harsh Usually mild Catalyst lifetime Different Different Deactivation through poisoning Common Rarely Separation and recycling Usually easy Difficult There are, however, different subtypes of heterogeneous catalysis (see Table 1.1). the two variants, depending on the type of Unfortunately, however, homogeneous catalyst being used. transition metal catalysis has one inherent disadvantagethatderivesfromitsdefinition; Homogeneous catalysts These can be: that is, as all of the reactants, catalysts and . soluble acids or bases ) homogeneous products are in the same homogeneous acid/base catalysis phase during reaction, any subsequent sep- . organic compounds ) homogeneous aration of the catalyst for its reuse can bevery organocatalysis difficult. Catalyst separation is, nonetheless, . soluble enzymes ) homogeneous essential as the typically very expensive tran- enzyme or biocatalysis sition metal catalyst must be recycled . soluble transition metal salts or soluble uncompromisingly for economic reasons, transition metal complexes ) and not discharged with the product. homogeneous transition metal catalysis. Likewise, for ecological reasons, neither the transition metal compounds nor any Heterogeneous catalysts These can be: associated organic compounds must be . insoluble acids or bases ) released into the environment. For exam- heterogeneous acid/base catalysis ple, although nickel is an important catalyst . solid immobilized enzymes ) metal, many people will suffer an allergic heterogeneous enzyme catalysis response if they come into contact with it. solid metals or metal oxides ) Because of these problems, several chapters heterogeneous metal catalysis. of this book relate to separation concepts associated with homogeneous catalysis (see The main subject of this book is homoge- Chapters 14 to 18). neous transition metal catalysis, which pro- Homogeneous transition metal catalysts are vides excellent opportunities for both gentle usually well-defined compounds; typically, andselectivereactions.Infact,suchcatalysis their chemical composition is well known offers a range of benefits compared to and their structure can be determined easily 1.2 The Different Varieties of Catalysis j7 Figure 1.7 Fritz Haber. Figure 1.6 Geoffrey Wilkinson. surface and in the pores during reaction are not exactly known. The production of hetero- by using various spectroscopic methods. geneous catalysts is largely the domain of a Such catalysts can be synthesized according few manufacturers that have an extensive, to definite, reproducible synthetic protocols. mostly empirically acquired, know-how of A typical example of a homogeneous tran- catalyst synthesis.

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