Catalytic Kinetics

479 Pages·2005·9.532 MB·English

by  Dmitry Murzin and Tapio Salmi (Auth.)

Preface
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.
vi
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
Chapter 1. Setting the scene
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.
J6ns Jakob Berzelius Wilhelm Ostwald and Svante Arrhenius
This new concept was immediately critized by Liebig, as this notion was putting catalysis
somewhat outside other chemical disciplines. A catalyst was later defined by Ostwald as a
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.
iiiiiiiiiiiiiiiiiiiiii
~iiiiiiiiiiiiiii .................. iiiiiiiiiiiiiii~i:iiiiiiiiiiiiiii
erugiF .1.1 citylataC cycle
Figure 1.1 is an example of a catalytic reaction between two molecules A and B with the
involvment of a catalyst.
In order to understand how a catalyst can accelerate a reaction a potential energy diagram
should be considered.
x~
0
P÷Q
Re.on etanidreo¢
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.
*X
"1 I \ eht( noitcuder
/F~', in ~GA bythe
If \ ~ catalyst)
G ~/ dezylataC
A+B
A + B . " P + Q ~
noitcaeR etanidrooc
Figure 1.3. Potential energy diagram for catalytic reactions
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 Hoff (1893)
proposed an equation for the temperature dependence of reaction rates
k = AT" e -E~T (1.1)
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
4
k = ok e KG (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
erugiF .4.1 noitartnecnoC vs time secnedneped for a elbisrever noitcaer
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.
_ kc,,/ PALl, ,,, = x (1.3)
It also implies that if a catalyst is active in enhancing a rate of the forward reaction, it will do
the same with a reverse reaction.
Figure 1.3 is somewhat simplified as it does not take into account possible bonding of the
catalyst and reactant. In order for a catalyst to be effective, the energy barrier between the
catalyst -substrate and activated complex must be less than between substrate and activated
complex in the uncatalyzed reaction. The binding of substrate to an enzyme lowers the free
energy of the catalyst substrate complex relative to the substrate (Figure 1.5). This is a general
feature of catalysis and is relevant for heterogeneous, homogeneous and enzymatic catalysis.
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.
0
bmulb~g reactlott sq~aration
read:ion coordinate
Figure 1.5. Potential energy margaid of a heterogeneous catalytic reaction .1( Chorkendorfl, J.W.
,teirdrevstnameiN Concepts of nredoM sisylataC dna ,sciteniK Wiley, .)3002
Chemical kinetics as a discipline adresses how the reaction rates depend on reactant
concentration, temperature, nature of catalysts, pH, solvent, to name a few- reaction
parameters.
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.
The immense importance of catalysis in chemical industry is manisfested by the tact that
roughly 85-90% of all chemical products have seen a catalyst during the course of production.
1997 Chemical 2003 ...................Chemical
olymer
Polymer
Refinerl Refiner
Environmental Environmental
Billion US$ 7.4 9.0
* toll manufacturing fees only
The Catalyst Group: The Intelligence Report: Global Shifts in the Catalyst ¢2ttsudnI
Figure 1.6. Worldwide catalyst market
Figure 1.6 demonstrates applications of catalysis in industry. In the last years there is an
increase of catalytic applications also for non-chemical industries: treatment of exhaust gases
from cars and other mobile sources, as well as power plants (Figure 1.7).
Figure 1.7. Catalytic treatment of NOx ni a) mobile )b stationary sources
A comparison between homogeneous and heterogeneous catalysts from the viewpoint of a
homogeneous catalysis expert is presented below
Homogeneous Heterogeneous
Activity high variable
Selectivity high variable
Conditions of reaction dlim harsh
Life time of catalyst variable long
Sensitivity to deactivation low high
Problems due to diffusion none difficult to solve
Recycling of catalyst usually difficult nac easily be done
Steric dna electronic properties easily changed no vm'iation possible
Mechanism realistic models exist not obvious
The topics adressed above will be dicussed in more detail in the subsequent chapters.
A great variety of homogeneous catalysts are known: metal complexes and ions, Bronsted
and Lewis acid, enzymes. Homogeneous transition metals are used in several industrial
processes, a few of them are given below
ssecorP dlroW yticapac Catalyst Temperature erusserP
noillim( t/a) )K( )rab(
edyhedlatecA 5.2 uC/dP 504-573 8-3
citecA acid 0.4 hR 574-524 06-03
slohocla-oxO 7 oC or hR 074-533 03/002
lyhtemiD etalahthperet 3.3 oC 544-514 8-4
cilahthpereT acid 4.9 oC 505-054 03-51
Metal complexes can have a very sophisticated structure with a variety of ligands. An
example of such ligands for Rh catalysed hydroformylation is given below (Fig.l.8) along
with some images of heterogeneous catalysts (Fig. 1.9)
erugiF .8.1 A dnagil rof hR catalysed noitalymrofordyh
erugiF .9.1 Images of suoenegoreteh stsylatac
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 601 to 601 at mild reaction
conditions and displaying very high specificity and capability of regulation.
erugiF .I .01 A citamehcs view on na enzyme erutcurts
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.
erugiF .11.1 Specificity of emyzne sisylatac
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. Formal kinetics
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