Ref 7 catalytic kinetics

This book is partially based on several courses, which the authors have taught at Abo Akademi University over the recent years, namely "Heterogeneous Catalysis", "Chemical Kinetics", Che

by Dmitry Murzin , Tapio Salmi

Chemistry and chemical technology have been at the heart of the revolutionary developments of the 20th century The chemical industry has a long history of combining theory (science) and practice (engineering) to create new and useful products Worldwide, the process industry (which includes chemicals, petrochemicals, petroleum refining, and pharmaceuticals) is a huge, complex, and interconnected global business with an annual production value exceeding $4 trillion dollars

The performance of a majority of chemical reactors (and hence the processes) is significantly influenced by the performance of the catalysts Catalyst research has been devoted to increase the catalyst activity and selectivity to improve process economics and reduce environmental impact through better feedstock utilization Catalysis-based chemical synthesis accounts for 60% of today's chemical products and 90% of current chemical processes Catalysis development and understanding thus is essential to the majority of chemical synthesis advances Because the topic of chemical synthesis is so broad and catalysis

is so crucial to chemical synthesis, catalysis should be specifically addressed Although in industry special focus is in heterogeneous catalysis; homogeneous, enzymatic, photochemical and electrochemical catalysis should not be overlooked, as the major aim is to produce certain chemicals in the best possible way, applying those types of catalysis, which suit a particular process in the most optimal way

For instance bioprocesses have become widely used in several fields of commercial biotechnology, such as production of enzymes (used, tbr example, in tbod processing and waste management) and antibiotics As techniques and instrumentation are refined, bioprocesses may have applications in other areas where chemical processes are now used Advantages of bioprocesses over conventional chemical methods of production are lower temperature, pressure, and pH and application of renewable resources as raw materials with less energy consumption

Catalyst development in industry is inseparable from understanding of catalysis on microscopic (elementary reactions) and macroscopic levels (transport phenomena)

This book presents an attempt to unify the main sub disciplines forming the cornerstone of practical catalysis Catalysis according to the very definition of it deals with enhancement of reaction rates, i.e with catalytic kinetics Diversity of catalysts, e.g catalysis by acids, organometallic complexes, solid inorganic materials, enzymes resulted in the fact, that these topics are usually treated separately in textbooks, despite the fact, that there are very many analogues in the kinetic treatment of homogeneous, heterogeneous and enzymatic catalysis Catalytic engineering includes as an essential part also macroscopic considerations, more specifically transport phenomena

Such an integrated approach to kinetics and transport phenomena in catalysis, still recognizing the fundamental differences between different types of catalysis, could be seldom found in the literature, where quite often artificial borders are build, preventing free exchange

of useful ideas and concepts Cross-disciplinary approach can be only beneficial for the advancement of catalytic reaction engineering

it should be mentioned, that it is not the aim of the authors to provide exhaustive bibliography Contrary, as we are trying to cover a variety of topics, we would like to limit ourselves to the main monographs, review articles and key references The hope of the authors

is that the book could be also used as a textbook in catalytic kinetics and catalytic reaction engineering

This book is partially based on several courses, which the authors have taught at Abo Akademi University over the recent years, namely "Heterogeneous Catalysis", "Chemical Kinetics", Chemical Reaction Engineering", "Chemical Reactors", "Chemical Technology",

"Bioreaction Engineering", where topics covered in the present textbook were touched in one way or another

Chapters 1-8, 9.4, 9.6-9.11, 10.1-10.2, 10.7-10.9 were written by D.Yu Murzin, material for chapters 9.1-9.3, 9.5 and 10.3-10.6 was prepared by T Salmi

The authors are very grateful to many colleagues from academia and industry who shared their knowledge and expertise in kinetics and mass transfer In particular the late Professor M.I Temkin introduced one of the authors into the field of heterogeneous catalysis and chemical reaction engineering in the broader context of physical chemistry and practical industrial needs and was a role model as a scientist and a person

Special thanks go to Dr Nikolai DeMartini, who carethlly read the manuscript and corrected the language, also giving several advices regarding the presentation of material Finally help ofElena Murzina in making the corrections is appreciated, as well as her patience during the many weekends and evenings when I was working on the book

The authors understand that it is very difficult to cover the whole field in one book, therefore the selection of topics and examples and especially allocated space to particular topics might be considered biased We will be delighted to receive critics and comments, which will help to improve the text

Dmitry Murzin

June, 2005, Turku/Abo

Ch 1 Setting the scene 1

1.1 History

All processes occur over a time ranging from femtosecond to billions of years The same holds for chemical and biochemical transformations Kinetics (derived from the Greek word KtvrlxtZo ¢ meaning dissolution) is a science which investigates fine rates of processes Chemical kinetics is the study of reaction rates

However complex a process is, it can be in principle divided into a number of elementary processes which can be studied separately

Chemical kinetics emerged as a branch of physical chemistry in the 1880-s with seminal works of Harcourt and Esson demonstrating the dependence of reaction rates on the concentrations of reactants It was a German scientist K Wenzel who stated that the affinity

of solid materials towards a solvent is inversely proportional to dissolution time and 100 years before Guldberg and Waage (Norway) formulated a law, which was later coined the "law of mass action," meaning that the reaction "forces" are proportional to the product of the concentrations of the reactants

When the rate of a certain process is measured, especially if it is of practical importance, a curious mind is always eager to know if it is possible to accelerate its velocity Moreover, one could even imagine a situation that for a system demonstrating complete inertness introduction of a foreign substance could enhance the rate dramatically Conversion of startch

to sugars in the presence of acids, combustion of hydrogen over platinum, decomposition of hydrogen peroxide in alkaline and water solutions in the presence of metals, etc were critically summarized by a Swedish scientist J J Berzelius in 1836, who proposed the existance of a certain body, which "effectiing the (chemical) changes does not take part in the reaction and remains unaltered through the reaction" He called this unknown tbrce, catalytic force, and defined catalysis as decomposition of bodies by this force

compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction This definition allows for the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly lost

1.2 Catalysis

Already from these definitions it is clear that there is a direct link between chemical kinetics and catalysis, as according to the very definition of catalysis it is a kinetic process There are different views, however, on the interrelation between kinetics and catalysis While some authors state that catalysis is a part of kinetics, others treat kinetics as a part of a broader phenomenon of catalysis

Despite the fact that catalysis is a kinetic phenomenon, there are quite many issues in catalysis which are not related to kinetics Mechanisms of catalytic reactions, elementary reactions, surface reactivity, adsorption of reactants on the solid surfaces, synthesis and structure of solid materials, enzymes, or organometallic complexes, not to mention engineering aspects of catalysis are obviously outside the scope of chemical kinetics

Some discrepancy exists whether chemical kinetics includes also the mechanisms of reactions In fact if reaction mechanisms are included in the definition of catalytic kinetics it will be an unnecessary generalization, as catalysis should cover mechanisms

Catalysis is of crucial importance for the chemical industry, the number of catalysts applied

in industry is very large and catalysts come in many different forms, from heterogeneous catalysts in the form of porous solids to homogeneous catalysts dissolved in the liquid reaction mixture to biological catalysts in the form of enzymes Catalysis is a multidisciplinary field requiring efforts of specialists in different fields of chemistry, physics and biology to work together to achive the goals set by the mankind Knowledge of inorganic, organometallic, organic chemistry, materials and surface science, solid state physics, spectroscopy, reaction engineering, and enzymology is required for the advancements of the discipline of catalysis

Despite the fundamental differences between elementary steps in catalytic process on surfaces, with enzymes or homogeneous organometalics there are stricking similarities also in terms of chemical kinetics Although superficially it is difficult to find something in common between the reaction of nitrogen and hydrogen forming ammonia on a surface of iron, D- fructose 6-phosphate with ATP involving an enzyme phosphofructokinase, or ozone decomposition in the atmosphere in the presence of NOx, all these trasnformations require that bonds are formed with the reacting molecules Such a complex then reacts to products leaving the catalyst unaltered and ready for taking part in a next catalytic cycle

In order to understand how a catalyst can accelerate a reaction a potential energy diagram should be considered

0

x~

P÷Q

R e o n ¢oerdinate

Figure 1.2 Potential energy diagram

Figure 1.2 represents a concept for a non-catalytic reaction of An'henius, who suggested that reactions should overcome a certain barrier before a reaction can proceed

The change in the Gibbs free energy between the reactants and the products AG does not change in case of a catalytic reaction, however the catalyst provides an alternative path for the reaction (Figure 1.3)

In general reaction rates increase with increasing temperature Kooij and van't H o f f (1893) proposed an equation for the temperature dependence of reaction rates

where A is pre-exponential factor and activation energy, Ea, is related to the potential energy barrier This equation, which could be derived on the basis of transition sate theory, in a slightly simplified tbrm

k = ko e K G (1.2)

was applied by Arrhenius and is reffered to as the Arrhenius law It is immediately clear from equation (1.2) that a decrease in activation energy will lead to an increase of the rate constant and thus the reaction rate (a discussion on the relationship between the rate and rate constant will be given below)

At the same time the catalyst (heterogeneous, homogeneous or enzymatic) affects only the rate of the reaction, it changes neither the thermodynamics of the reaction (Gibbs energy) nor the equilibrium composition An important conclusion is thus that a catalyst can change kinetics but not thermodynamics of a reaction and if a process is thermodynamically unfavorable, there is no need to apply any modern and fancy methods (high throughput screening and alike) to find such a catalyst

Concentration

Time

Figure 1.4 Concentration vs time dependences for a reversible reaction

The dashed line in Figure 1.4 demonstrates the equlibrium that cannot be ovecome for a given set of parameters

Furthermore the ratio of rate constants in the forward and reverse direction for catalytic and noncatalytic reactions is the same

If the energy is lowered too much, without a greater lowering of the activation energy then catalysis would not take place, meaning that bonding between a catalyst and a reactant should not be too strong Alternatively if it is too weak, then the catalytic cycle could not proceed

bmulb~g reactlott sq~aration

Chemical kinetics together with other means of studying catalytic reactions, like spectroscopy of catalysts and catalyst models, quantum-chemical calculations for reactants, intermediates and products, calculation of the thermodynamics of reactants, intermediates and products from measured spectra and quantum-chemical calculations form the modern basis for understanding catalysis

Kinetic investigations are one of the ways to reveal reaction mechanisms The following problems can be solved using the kinetic model:

• choosing the catalyst and comparing the selectivity and activity of catalysts and their performance under optimum conditions for each catalyst;

• the determination of the optimum sizes and structure of catalyst grains and the necessary amount of the catalyst to achieve the specified values of the selectivity of the process and conversion of the starting products;

• the determination of the composition of all byproducts formed during the process;

• the determination of the stability of steady states and parametric sensitivity; that is, the influence of deviations of all parameters on the steady-state regime and the behavior of the reactor under unsteady state conditions;

• the study of the dynamics of the process and deciding if the process should be carried out under unsteady-state conditions;

• the study of the influence of mass and heat transfer processes on the chemical reaction rate and the determination of the kinetic region of the process;

• choosing the type of a reactor and structure of the contact unit that provide the best approximations to the optimum conditions

Very often the rates of chemical transformations are affected by the rates of other processes, such as heat and mass transfer The process should be treated as a part of kinetics The gas/liquid mass transfer in multiphase heterogeneous and homogeneous catalytic reactions could be treated in a similar way The mathematical framework for modelling diffusion inside solid catalyst particles of supported metal catalysts or immolisided enzymes does not differ that much, but proper care should be taken of the reaction kinetics

Chemical Chemical

olymer Polymer

Figure 1.6 Worldwide catalyst market

Figure 1.6 demonstrates applications o f catalysis in industry In the last years there is an increase o f catalytic applications also for non-chemical industries: treatment o f exhaust gases from cars and other mobile sources, as well as power plants (Figure 1.7)

Figure 1.7 Catalytic treatment of NOx in a) mobile b) stationary sources

A comparison between h o m o g e n e o u s and heterogeneous catalysts from the viewpoint o f a

h o m o g e n e o u s catalysis expert is presented below

and Lewis acid, enzymes Homogeneous transition metals are used in several industrial processes, a few of them are given below

Figure 1.8 A ligand for Rh catalysed hydroformylation

Figure 1.9 Images of heterogeneous catalysts

Enzymes represent a special type of homogeneous catalyst They are large proteins (Figure 1.10) capable of increaing the reaction rates by a factor of 106 to 106 at mild reaction conditions and displaying very high specificity and capability of regulation

Specificity (Figure 1.11) is controlled by the enzyme structure, more precisely a unique fit

of substrate with the enzyme controls the selectivity for the substrate and the product yield

Figure 1.11 Specificity of enzyme catalysis

Superficially there is not that much in common between a large protein and a Pt/A1203 heterogeneous catalyst At the same time the chemical reactions which occur with both types

of catalysts involve certain active sites, e.g regions where catalysis occurs Whatever the specific reaction, these active sites can be represented by Figure 1.5, which is a schematic representation of a catalytic reaction This in turn means that the kinetics of either heterogeneous or homogeneous catalytic reactions can be very similar and in fact they are

1.3 F o r m a l k i n e t i c s

Chemical kinetics as a dispipline concerns the rates (the velocities) of chemical reactions and deals with experimental measurements of the velocities in batch, semibatch or continuous reactors Interpretation of the experimental data is currently done using the laws of physical chemistry

One of the fathers of chemical kinetics, Louis Jacques Th6nard, discovered hydrogen peroxide and measured its decomposition rates He demonstrated for the first time, that rates

of chemical reactions varied with the concentrations of the reactants In later study Ludwig Ferdinand Wilhelmy investigated the inversion of cane sugar in the presence of acids and

developed a rate equation, which was the first attempt to interpret the temperature dependence

o f the rate constant Unfortunately this work remained in oblivion until 1884 In 1865 rate laws combined with mass balances for a batch reactor were proposed by Augustus George Vernon Harcourt and William Esson, giving a mathematical expression for concentration vs t for first order, second order and consecutive reactions, representing a major breakthrough for modern chemical kinetics

reaction rates for a chemical reaction described by the following equation

coefficients An equation for a chemical reaction is written in such a way that all the molecules particpating in the reaction are balanced

Very often in chemical reaction egineering the stoichiometric coefficient v, is defined as the amount o f product produced after one run o f the reaction It implies that the stoichiometric coefficient is positive for a product and negative for a reactant

Thus for the reaction

o f any reaction entity i (reactant or product) and ni is the corresponding amount in moles Thus d ~/dt is an extensive property, which is measured in moles and cannot be considered

a reaction rate, as it is proportional to the size o f the reactor

In general, for a homogeneous reaction for which the reaction rate changes with time and also it is not unitbrm over a volume o f a reactor the reaction rate is

A + B ~ C (1.9) the rate o f consumption o f reactant A is then

In reallity the chemical equation (1.4) does not tell us how reactants become products - it is

a summary o f the overall process In fact it is molecularity, e.g the number o f species that must collide to produce the reaction which determines the form o f a rate equation Reactions whose rate law can be written from its molecularity are called elementary The kinetics o f the elementary step depends only on the number o f reactant molecules in that step

For the reaction

the rate expression based on the formal kinetics is

with the overall order defined as the sum of orders to each reactant being equal to 3 However the reaction mechanism is more complicated and consists of several elementary steps

and the overall order is just two

For elementary reactions the reaction orders have orders that are integers which are usually equal to one or two (Figure 1.12), and occasionally three for trimolecular reactions

I r a t e / / / S ~ r First order

~ A ] or 0 th o r d e r

Figure 1.12 Representation of reaction kinetics of different orders

In practice, reaction orders can be fractional, indicating a complex reaction mechanism The majority of this book is devoted to such cases, as catalytic reaction mechanisms, which follow from the general considerations above, are typical examples of complex reactions

Reaction orders for a reaction A ~ P described by a following equation for the rate

1.4 Acquisition of kinetic data

Kinetic data for a chemical reaction is gathered in different type of reactors and we will briefly mention some requirements for chemical reactors from the viewpoint of kinetic analysis A high precision of the data is needed as large deviations in the values of the experimentally measured rates will be a serious obstacle for quantitative considerations Reproducibility of rate measurements over a broad range of parameters is also of importance Another necessary feature is the possibility to reach a goal of obtaining the maximum amount

of kinetic information in minimum time Analysis of products as well as reactor lay-out should preferbably be as easy as possible

Essential features for catalytic reactions is the readiness in reduction/activation of heterogeneous catalysts and a possibility to utilize them in the needed geometrical form Despite the strict definition of catalysis, which states, that the catalyst does not change during the catalytic reactions, some activity deterioration takes place and therefore measurements of catalytic kinetics should always monitor the catalyst activity

Different types of reactors are applied in practice (Figure 1.14) Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and nmltiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)

formly dxed

mixed

Figure 1.14 Different types of sth'red tank reactors

Alternatively, tubular reactors with plug flow (piston flow) (PFR) are used and operated in continuous mode (Figure 1.15)

W /

.~ [ ~ e ~

Hs~e ~ae £6e

e rg~/pot'~ ~S

k a a

Figure 1.16 Approach to kinetic analysis in batch reactors

The high precision and wide range of parameters afforded by this operation mode made batch reactors very popular for kinetic studies especially in the field of fine and pharmaceutical chemicals• Another advantage is the possibility to utilize heterogeneous catalysts of different geometrical shape•

Such reactors can be made either of glass or stainless steel to sustain high pressures (Figure 1.17) and can be applied in a parallel mode

Figure 1.17 Batch reactors : a) glass, b) high pressure, c) in parallel mode

At the same time, for heterogeneous catalytic reactions, activity control presents a challenge and will be discussed further in Chapter 8 Moreover, catalyst pretreatment (reduction) and regeneration are not straightforward• Quantitative treatment is not easy and will be briefly discussed below•

flow

no A

flow

Figure 1.18 A volume element

For an infinitesimal volume element AV in Figure 1.18 the mass balance could be written in

a form

1N + G E N E R A T I O N = O U T + A C C U M U L A T I O N leading to a ~bllowing equation in terms of moles

assuming that the catalyst effectiviness factor r/A is equal to 1

boundary conditions (t-0, c~-0) we arrive at

and taking into account

In fact treatment of heterogeneous, homogeneous and enzymatic reactions is basically the same with the only difference in the expressions of reaction rates, which reflect different reaction mechanisms Some specific cases will be discussed in Chapters 5-7 Here we present few examples

Inserting the expression of reaction rate in eq (1.25) for a reaction A ~ P , which occurs over a catalyst

which after integration gives an expression for reaction time

1.4.2 CSTR

Examples of continuous stirred tank reactors are presented in Figure 1.14 Such a system can

be applied for both homogeneous and heterogeneous systems Figure 1.19 illustrates the differences between batch and CSTR reactors

Figure 1.19 Stirred tank reactors in a) batch, b) continuous mode

For a perfectly mixed CSTR at steady state it holds that there is no accumulation

dn• = 0

therefore

An e x a m p l e o f a tubular reactor is presented in Fig 1.20

Figure 1.20 A tubular reactor

For heterogeneous catalysis, the catalyst is packed in such reactors, which are easy to design and control, as the gases or liquids pass through the reactor and are analyzed Such reactors are efficient for catalyst screening, especially when they are arranged in a parallel mode (Figure 1.21)

Figure 1.21 Multitubular reactor

The apparent drawback is that one experiment leads to only one data point On the other hand catalyst deactivation with time on stream could be easily seen (Figure 1.22)

Figure 1.22 Catalyst deactivation with time-on-stream at different conditions

Quantitative treatment of plug flow- reactors is somewhat cumbersome, therefore several assumptions are usually made The fluid composition is considered to be unform along the reactor cross section (i.e there is no radial dispersion) This is valid only when

In general, nonisothermicity of a tubular reactor should also be taken into account, since so- called isothermal reactors are seldom isothermal (Figure 1.23), however for the sake of simplicity it will not be considered below

The mass balance for a plug flow reactor (Figire 1.23) is then given by

d n A

d t

Figure 1.23 Temperature profile in a tubular reactor

Replacing the amount in moles through concentration and volume and considering an infinitely small volume

Figure 1.24 Molar flows in a packed bed reactor

the balance is exactly the same as for homogeneous PFR except that the rate in eq (1.48) is replaced by rAp~, where p~ is the catalyst bulk density, giving

dc A dc A

For the steady state de" A / dt = 0 the design equation is

1.4 4 Gradient-free recycle reactors

Such reactors (applied for gas and liquid systems) are stirred flow or recirculation reactors, characterized ideally by very small concentration and temperature gradients within the

catalyst region

For a recirculation reactor with an external mixing device (Figure 1.25) the pressure in the system is constant and gas (liquid) composition does not change with time or in space and each single pass of reactants through the bed results in very small conversion as the circulation rate is very high The mixture of gases are fed through pipe 1 to the reactor 4 and then the outlet 2 for further analysis (GC) Pump 3 provided the necessary circulation of the mixture

Figure 1.25 A recirculation reactor with an external mixing device

The system is then free from concentration gradients along the catalyst bed, concentration gradients due to axial dispersion and temperature gradients The treatment of data is simplified as the rate is extracted directly in the differential tbrm

St

Efficient circulation pumps are needed and the inventory of reactants should be sufficient enough The relaxation time (e.g time to reach steady-state) can be significant These disadvantages are however compensated by the easy mathematical treatment of experimental data

This reactor system can operate in a closed mode also Then the entire kinetic curve can be measured in one experiment At the same time, by-products can accumulate in the cycle The concept of a circulating flow reactor was further developed in the Buss reactor technology (Figure 1.26) Large quantities of reaction gas are introduced via a mixer to create

a well dispersed mixture This mixture is rapidly circulated by a special pump at high gas/liquid ratios throughout the volume of the loop and permits the maximum possible mass transfer rates A heat exchanger in the external loop allows for independent optimisation of heat transfer For continuous operation, the product is separated by an in-line cross-flow filter which retains the suspended solid catalyst within the loop Such a system can operate in batch, semi-continuous and continuous mode

Some other types of gradientless reactors with stirred flow are the Berty reactor which circulates gas past a stationary catalyst bed (Figire 1.27)

Figure 1.27 Berty gradientless reactors

and a spinning basket reactor (Figure 1.28) In the latter case the solid catalyst is retained in a spinning, woven wire mesh basket to allow gas-liquid circulation with low pressure drop The large catalyst particles required for this operation mode often lead to conducting the reaction

in the region of external diffusion

Figure 1.28 Rotating basket

By utilizing structured catalysts these problems can be avoided Active carbon cloths for instance could be used in combination with a propeller stirrer, which serves at the same time

as a catalyst holder (Fig 1.29) and is coupled to a Rushton turbine tbr effective gas distribution

Figure 1.29 Integrated gadget of a combined stirrer and catalyst holder

1.5 K i n e t i c s a n d t h e r m o d y n a m i c s

The rates of forward and reverse reactions are related through thermodynamics While thermodynamics tells us the final product distribution for a given set of reactants concentrations, pressures and temperature, kinetics describes how fast the products will be generated Expressing the rates of the forward reaction

where v is the stoichiometric coefficient for species and /l is the chemical potential of this species According to De Donder formulation the relationship between the rates of forward and reverse reactions is expressed by the following equation

More rigorous treatment of the temperature dependence will be presented in a chapter on transition state theory Taking the natural log of both sides of the equation (1.68)

For example, the hydrogenation of o-xylene (Figure 1.31) is thermodynamically preferred up

Figure 1.31 Reaction network for o-xylene hydrogenation

- 1 0 3

523

At the same time the rate clearly passes through a maximum (Figure 1.33)

400 4D

The apparent activation energy defined as

E a = - R 81nk _ R T 2 81n C

is negative, demonstrating an interplay between adsorption of xylene

(diminishing with T) and reaction (increase with temperature)

o n

(1.71)

the surface

Chapter 2 Catalysis

2 Homogeneous catalysis

In case of homogeneous catalysis a catalyst is in the same phase as the reactants Three types of homogeneous catalysis are usually considered: gas phase catalysis, acid-base and by transition metals

2.1 Gas-phase catalysis

The most common example of gas phase catalysis is the decomposition of ozone (03) into oxygen (02) This is catalysed by CFC's (chloroflourocarbons), VOC or nitric oxide (NO) These both catalyse the decomposition of trioxygen (03 or ozone)

Let us consider the mechanism of the following gas-phase reaction in which hydrogen is oxydized in the atmosphere with an aid of NO2 catalyst

the rate follows the expressions

where for water solutiions

as [OH-] ~ Kw/[H +] with Kw= l0 -14 mol2dm -6 at 25°C)

It follows from eq (2.4) that at high acid concentrations, catalysis by hydroxide ions is minor, while at high base concentrations, catalysis by hydrogen ions is minor

pH

lgk'-lgkoH_+lgKw+PH

Figure 2.1 Dependence of the rate constant on pH

Acid catalysed reactions involve formation of rc complexes

and carbonium ions

which react further and the proton is fully recovered

2.3 Catalysis by transition metals

Catalysis by organometallic compounds is based on activation of the substrates by coordinating it to the metal, which lowers the activation energy of the reaction between substrates As in other types of catalysis the use of a homogeneous catalyst in a reaction provides a new pathway, because the reactants interact with the metallic complex first Homogeneous transition metal catalysts are increasingly being applied in industrial processes

to obtain bulk chemicals, fine chemicals and polymers

Examples of metal complex catalysts are: RhCI(PPh3)3 for olefins hydrogenation, C02(CO)s for carbonylation and metallocenes for polymerization

Industrial applications are toluene and xylene oxidation to acids, oxidation of ethene to aldehyde, carbonylation of methanol and methyl acetate, polymerization over metallocenes (Figure 2.2), hydroformylation of alkenes, etc

M(CH3)2

Figure 2.2 Metallocene catalysts

A facinating application of homogeneous catalysis is asymmetric catalysis The 2001 Nobel Prize in Chemistry was given for research in the field of chiral transition metal catalysts for stereoselective hydrogenations and oxidations

Many of the compounds associated with living organisms are chiral (not superposable on its mirror image), for example DNA, enzymes, antibodies and hormones

Figure 2.3 Chiral enantiomers

Therefore enantiomers (Figure 2.3) of compounds (e.g pairs of optical isomers) may have distinctly different biological activity For many drugs, only one of these enantiomers has a beneficial effect, and the other enantiomer can be inactive or even toxic

In 1968 Knowles at Monsanto Company, St Louis showed that a chiral transition metal based catalyst could transfer chirality to a nonchiral substrate resulting in a chiral product with one

of the enantiomers in excess Knowles's aim was to develop an industrial synthesis process for the rare amino acid LDOPA which had proved useful in the treatment of Parkinson's

disease Knowles and co-workers at Monsanto discovered that a cationic rhodium complex containing DiPAMP (Figure 2.4), a chelating diphosphine with two chiral phophorus atoms, catalyzes highly enantioselective hydrogenations of enamides (Figure 2.5)

Figure 2.4 a) L-DOPA, b) DiPAMP

The pathway to L-DOPA including asymmetric hydrogenation is depicted in Figure 2.5

Noyori discovered a chiral diphosphine complex, BINAP Rh(I) complexes of the enantiomers of BINAP are remarkably effective in various kinds of hydrogenation reactions

Organometallic catalysts also include specific ligands besides the atom or group of metal atoms They can be easily modified by ligand exchange A very large number of different types of ligands can coordinate to transition metal ions Once the ligands are coordinated, the reactivity of the metals may change dramatically The rate and selectivity of a given process can be optimized to the desired level by controlling the ligand enviro~nent

Figure 2.7 Sharpless epoxidation

Transition metals have partially occupied d-orbitals, the symmetry of which is suitable for formation of chemical bonds with neutral molecules These metals also have several stable oxidation states and can have different coordination numbers as a result of the changes in the number of d-electrons (Figure 2.8)

Figure 2.8 Formation of chemical bonds during catalysis

Different types of elementary steps are possible with organometallic catalysts (Figure 2.9)

13 _elimination

H

r

M _ C H 2 / C H - R M ~H + CH2=CHR

Catalytic cylcles in homogeneous catalysis involve changes in the state of the central ion during the reaction At the same time the initial state of the central ion could be the same or different from the final state

Examples for the mechanisms of double bond migration or hydrogenations reactions which occur in the systems forming catalytic cycles are given in Figures 2.10 and 2.11

1 Cl3Pd~3 2

1 Cl4Pd-2 + ~3 2

Figure 2.10 Mechanism for double bond migration over an organometallic catalyst

c u

02H4

Figure 2.11 Mechanism o f a hydrogenation reaction over an organometallic catalyst

which could be presented in a general form for a reaction A ~ P (Figure 2.12)

MC :

Figure 2.12 A general form of a catalytic cycle for a reaction A ~ P

In some other cases, like hydrolysis of ethylene the catalytic cycle is not closed

finally leading to a production of acetaldehyde from ethylene C2H4+ 1/202~CH3CHO and closing the catalytic cycle

m ~ - J " -~ p

In industrial practice the process is organized in such a way that reaction and re-oxidation

of Cu in performed in one reactor

Polymerization reactions can be described by similar types of catalytic cycles

~ /M H CHz=CH 2

Figure 2.13 Catalytic cycles for polymerization reactions M = Ti, Zr, Cr, V; L = PR3

Due to the fact, that organometallic complexes are highly soluble in organic solvents their behavior throughout the catalytic reaction can be studied even in-situ using various spectroscopic techniques, like NMR, IR, Raman These measurements may provide information about the structure of complexes Kinetic studies are much rarer in the study of homogeneous catalysis by transition complexes then for heterogeneous catalysis One of the reasons could be that the generally adopted reaction schemes sometimes look too complicated for non specialists in kinetics, as analytical expressions could be very cumbersome to derive Recent attempts were devoted to heterogenization of metal complexes to inorganic supports

~//C;O2Me ca[ ~ ~/CO2Me

Figure 2.14 Immobilization of organometallic catalysts (C.Li, Chiral synthesis on catalysts immobilized in microporous and mesoporous materials, Catalysis Reviews, 46 (2004), 419-492)

2.4 Biocatalysis (catalysis by enzymes)

Enzymes are proteins that function as biological catalysts They mediate a vast array of biochemical reactions Most reactions in a human body are too slow to sustain life without a catalyst, for instance the digesting (hydrolyze) of food, the oxidation of glucose, making ATP (useable form of energy for cells), the synthesis cholesterol for membranes, coping DNA for cell division to name a few

Biological catalysts have been postulated since the early 1800's The term enzyme was coined in 1878 to describe the component in yeast involved in the fermentation of sugar into alcohol The enzyme jack bean urease, which catalyzes hydrolysis of urea, was crystallized

in 1926 Comparison between chemical and enzymatic catalysis demonstrates the specificity

of enzymatic catalysis

Chemocatalysts

A variety of inorganic, organometallic substances

Increase the rate of chemical reactions

One catalyst can facilitate multiple reactions

Could require high temperature/pressure

Enzymes Mostly proteins, few are RNA Rate increases of 10 6 to 1012 Specificity

Mild reaction conditions The catalytic activity of enzymes is dependent upon the native protein conformation The primary, secondary, tertiary, and quaternary structures are essential for catalytic activity Similarly to homogeneous and heterogeneous catalysis, enzyme catalyzed reactions occur

at a specific active site, which is dependent on the arrangement of functional groups

In terms of elementary reactions, there is no substantial differences between enzymatic catalysis and other types of catalysis as the same type of chemical reactions: breaking, ~brming and rearranging bonds are present At the same time, the high specificity of enzymes is dictated by the enzyme active site, which interacts predominantly with one particular substrate, although some active sites allow for multiple substrates Multi-point contact with the substrate, structural flexibility to undergo collective and rapid changes and the possibility to combine several catalytic features, like acid and base catalysis, hydrophilic/hydrophobic interactions at the same time, make enzymes so distinct from homogeneous transition metal complexes and heterogeneous catalysts

Some enzymes require cofactors, which are amino acids, vitamin derivatives, or metals (minerals) that bound as co-substrates or remain attached through multiple catalytic cycles The specificity of enzymes is associated with their geometrical (special structure), as the substrates have to fit (geometry), and with their affinity provided by formation of hydrogen bonds, electrostatic interactions and hydrophobicity

Enzymes have been named by adding the suffix "-ase" to the name of the substrate or to a word or phrase describing their activity Enzymes are classified according to reaction type There are 6 major classes (with subclasses) Oxidoreductases catalyze oxidation-reduction reactions, the transfer of hydrogen atoms and electrons, for example dehydrogenation of lactate:

Ligases use ATP to catalyze the tbrmation of new covalent bonds, i.e C-C, C-S, C-O, and C-N bonds

Enzymes difl'er from ordinary catalysts, as the rates are typically 106 to 1012 times faster Such higher rates are achieved at milder reaction conditions, e.g body temperature, neutral

pH, atmospheric pressure, and with greater reaction specificity for substrate and product rarely having side reactions or side products Capacity for regulation is associated with a possibility to modify enzymatic reactions by various agents (e g., modifiers, inhibitors)

In addition the activity of enzymes can be regulated by allosteric interactions This term refers to the ability of the enzyme to bind at a remote site, thus inducing changes in the protein structure, which finally influence the active site

The substrate can bind to the enzyme at the active site via noncovalent interactions (van der Waals, electrostatic, hydrogen bonding, hydrophobic), and with a specific geometric complementarity, as the surface of the active site of that enzyme is complementary in shape to the substrate Electronic complementarities are due to the fact that the amino acid residues at the active site are arranged to interact specifically with the substrate Although most enzymes are amino acids with hundreds of acids in the chain, the active site is the size of the substrate Complementarity in the structure and charge between the enzyme and the substrate is illustrated by the so-called lock-and-key concept (Figure 2.15)

Figure 2.15 Lock-and-key concept

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Figure 2.16 induced fit concept

Enzymes are highly stereospecific in binding chiral substrates and in catalyzing reactions This stereospecificity arises because enzymes are made of L-amino acids and form asymmetric active sites Similar to homogeneous and heterogeneous catalysis, chemical reactions proceed on active sites of enzymes, which represent a small part of the total protein