Catalysis

Catalysis

An Integrated Textbook for Students

Edited by Ulf Hanefeld and Leon Lefferts

Editors

Prof Ulf HanefeldTechnische Universiteit DelftVan der Maasweg 92628 HZ DelftNetherlands

Prof Leon LeffertsUniversity of TwenteFaculty of Science and TechnologyPO Box 2177500 AE EnschedeNetherlands

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v

Contents

Preface xiii

1 Introduction 1Leon Lefferts, Ulf Hanefeld, and Harry Bitter

1.1 A Few Words at the Beginning 11.2 Catalysis in a Nutshell 11.3 History of Catalysis 31.3.1 Industrial Catalysis 41.3.2 Environmental Catalysis 51.4 Integration Homo–Hetero-Biocatalysis 51.5 Research in Catalysis 101.5.1 S-Curve, Old Processes Improvement Is Knowledge Intensive 101.5.2 Interdependence with Other Fields 111.5.3 Recent and Future Issues 121.5.3.1 Biomass 121.5.3.2 CO2 as a Feedstock 131.6 Catalysis and Integrated Approach or How to Use this Book 14

References 14

2 Heterogeneous Catalysis 15Leon Lefferts, Emiel Hensen, and Hans Niemantsverdriet

2.1 Introduction 152.1.1 Concept of Heterogeneous Catalysis 152.1.2 Applications of Heterogeneous Catalysis 162.1.2.1 Transportation Fuels 172.1.2.2 Chemicals 202.1.2.3 Environmental Pollution Control 212.1.3 Catalytic Cycle 232.2 Adsorption on Surfaces 232.2.1 Physisorption and Chemisorption 242.2.2 Adsorption Isotherms 262.2.3 Chemisorption and Chemical Bonding 282.2.4 Connecting Kinetic and Thermodynamic Formulations 332.3 Surface Reactions 352.3.1 Reaction Mechanism and Kinetics 35

vi Contents

2.4 Types of Heterogeneous Catalysts 412.4.1 Supported Metals 412.4.1.1 Understanding Trends in Reactivity 412.4.1.2 Structure Sensitivity 422.4.1.3 Support Effects 472.4.2 Oxides and Sulfides 512.4.2.1 Molecular Aspects 512.4.2.2 Processes 522.4.2.3 Transition Metal Sulfides 592.4.3 Solid Acid Catalysts 62

Question 1 69Question 2 69References 70

3 Homogeneous Catalysis 73Elisabeth Bouwman, Martin C. Feiters, and Robertus J. M. Klein Gebbink

3.1 Framework and Outline 733.1.1 Outline of this Chapter 733.1.2 Definitions and Terminology 743.2 Coordination and Organometallic Chemistry 753.2.1 Coordination Chemistry: d Orbitals, Geometries, Crystal Field

Theory 753.2.2 σ and π donors and back-donation: CO, alkene, phosphane, H2 773.2.3 Organometallics: Hapticity, Metal–Alkyl/Allyl, Agostic Interaction,

Carbenes 803.2.4 Electron Counting: Ionogenic or Donor-Pair versus Covalent or

Neutral-Ligand 813.2.5 Effect of Binding on Ligands and Metal Ions, Stabilization of Oxidation

States 833.3 Elementary Steps in Homogeneous Catalysis 843.3.1 Formation of the Active Catalyst Species 843.3.2 Oxidative Addition and Reductive Elimination 853.3.2.1 Concerted Addition 853.3.2.2 SN2 Mechanism 863.3.2.3 Ionic Mechanism 863.3.2.4 Radical Mechanism 873.3.2.5 Reductive Elimination 873.3.3 Migration and Elimination 873.3.4 Oxidative Coupling and Reductive Cleavage 903.3.5 Alkene or Alkyne Metathesis and σ-Bond Metathesis 903.3.6 Nucleophilic and Electrophilic Attack 923.4 Homogeneous Hydrogenation 953.4.1 Background and Scope 953.4.2 H2 Dihydride Mechanism: Wilkinson’s Catalyst 963.4.3 H2 Monohydride Mechanism and Heterolytic Cleavage 973.4.4 Asymmetric Homogeneous Hydrogenation 983.4.5 Transfer Hydrogenation with 2-Propanol 100

Contents vii

3.4.6 Other Alkene Addition Reactions 1023.5 Hydroformylation 1043.5.1 Scope and Importance of the Reaction and Its Products 1043.5.2 Cobalt-Catalyzed Hydroformylation 1053.5.3 Rhodium-Catalyzed Hydroformylation 1073.5.4 Asymmetric Hydroformylation 1103.6 Oligomerization and Polymerization of Alkenes 1123.6.1 Scope and Importance of Oligomerization and Polymerization 1123.6.2 Oligomerization of Ethene (Ni, Cr) 1133.6.3 Stereochemistry and Mechanism of Propene Polymerization 1153.6.4 Metallocene Catalysis 1173.6.5 Polymerization with Non-Metallocenes (Pd, Ni, Fe, Co) 1183.7 Miscellaneous Homogeneously Catalyzed Reactions 1183.7.1 Cross-Coupling Reactions: Pd-Catalyzed C–C Bond Formation 1183.7.2 Metathesis Reactions 120

Question 1 (total 20 points) 122Question 2 (total 20 points) 122References 123Further Reading 124

4 Biocatalysis 127Guzman Torrelo, Frank Hollmann, and Ulf Hanefeld

4.1 Introduction 1274.2 Why Are Enzymes So Huge? 1294.3 Classification of Enzymes 1374.3.1 Oxidoreductases (EC 1) 1394.3.1.1 Flavomonooxygenases 1444.3.1.2 P450 Monooxygenases 1444.3.1.3 Diiron-Dependent Monooxygenases 1454.3.1.4 Peroxidases (EC 1.11.1) and Peroxygenases (EC 1.11.2) 1464.3.2 Transferases (EC 2) 1474.3.3 Hydrolases (EC 3) 1474.3.4 Lyases (EC 4) 1574.4 Concepts and Methods 1574.4.1 Cofactor Regeneration Systems 1584.4.2 Methods to Shift Unfavorable Equilibria 1594.4.2.1 Kinetic versus Thermodynamic Control 1594.4.2.2 Working in Organic Solvents 1614.4.3 Two-Liquid-Phase Systems (and Related) 1644.4.4 (Dynamic) Kinetic Resolutions and Desymmetrization 1644.4.5 Enantiomeric Ratio E 1684.5 Applications and Case Studies 1694.5.1 Oxidoreductases (E.C. 1) 1694.5.1.1 Dehydrogenases 1694.5.1.2 Oxidases 1734.5.1.3 Old Yellow Enzymes 1744.5.1.4 Monooxygenases (EC 1.14.13) 175

viii Contents

4.5.1.5 Peroxidases/Peroxygenases 1774.5.1.6 Dioxygenases 1774.5.2 Transferases (EC 2) 1774.5.3 Hydrolases (EC 3) 1794.5.3.1 Lipases and Esterases (EC 3.1.1) 1794.5.4 Lyases (EC 4) 1814.5.4.1 Nitrile hydratase (EC 4.2.1) 181

Question 1 186Question 2 186Question 3 187Question 4 188Further Reading 188

5 Chemical Kinetics of Catalyzed Reactions 191Freek Kapteijn, Jorge Gascon, and T. Alexander Nijhuis

5.1 Introduction 1915.2 Rate Expressions – Quasi-Steady-State Approximation and

Quasi-Equilibrium Assumption 1935.3 Adsorption Isotherms 1985.3.1 One-Component Adsorption 1985.3.2 Multicomponent Adsorption 1995.3.3 Dissociative Adsorption 2005.4 Rate Expressions – Other Models and Generalizations 2005.5 Limiting Cases – Reactant and Product Concentrations 2025.6 Temperature and Pressure Dependence 2065.6.1 Transition-State Theory 2075.6.2 Forward Reaction – Temperature and Pressure Dependence 2085.6.3 Forward Reaction – Limiting Cases 2095.6.3.1 Strong Adsorption of A 2095.6.3.2 Weak Adsorption of A and B 2095.6.3.3 Strong Adsorption of B 2105.6.3.4 Intermediate Adsorption of A and B 2105.7 Sabatier Principle – Volcano Plot 2135.8 Concluding Remarks 214

Notation 216Greek 217Subscripts 217Superscripts 217Question 1 217Question 2 218Question 3 218References 219

6 Catalytic Reaction Engineering 221Freek Kapteijn, Jorge Gascon, and T. Alexander Nijhuis

6.1 Introduction 2216.2 Chemical Reactors 222

Contents ix

6.2.1 Balance and Definitions 2226.2.2 Batch Reactor 2246.2.2.1 Multiple Reactions 2266.2.3 Continuous Flow Stirred Tank Reactor (CSTR) 2286.2.4 Plug-Flow Reactor (PFR) 2316.2.5 Comparison between Plug-flow and CSTR Reactor 2336.2.5.1 Reactor Size 2336.2.5.2 Reactor Selectivity 2356.3 Reaction and Mass Transport 2366.3.1 External Mass Transfer 2376.3.2 Internal Mass Transport 2426.3.2.1 Effectiveness Factor for Internal Mass Transfer 2446.3.3 Gas–Liquid Mass Transfer 2486.3.3.1 Gas–Liquid Mass Ttransfer Followed by Reaction (Heterogeneously

Catalyzed) 2496.3.3.2 Gas–Liquid Mass Transfer Simultaneously with a Reaction

(Homogeneously Catalyzed) 2506.3.4 Heat Transfer 2546.3.4.1 External Heat Transfer 2556.3.4.2 Internal Heat Transport 2566.4 Criteria to Check for Transport Limitations 2576.4.1 Numerical Checks 2576.4.1.1 External Mass Transfer; Carberry Number 2576.4.1.2 Internal Mass Transfer; Wheeler–Weisz Modulus 2576.4.1.3 External Heat Transfer 2586.4.1.4 Internal Heat Transfer 2586.4.1.5 Radial Profiles and Distributions in Concentration and

Temperature 2596.4.2 Experimental Checks 260

Notation 264Greek symbols 265Subscripts 265Question 1 265Question 2 266Question 3 267References 269

7 Characterization of Catalysts 271Guido Mul, Frank de Groot, Barbara Mojet-Mol, and Moniek Tromp

7.1 Introduction 2717.1.1 Importance of Characterization of Catalysts 2717.1.2 Overview of the Various Techniques 2717.2 Techniques Based on Probe Molecules 2737.2.1 Temperature-Programmed Techniques 2737.2.2 Physisorption and Chemisorption 2757.2.2.1 Physisorption 2767.2.2.2 Chemisorption 279

x Contents

7.3 Electron Microscopy Techniques 2807.4 Techniques from Ultraviolet up to Infrared Radiation 2837.4.1 UV/vis Spectroscopy 2837.4.2 Infrared Spectroscopy 2867.4.2.1 Probe Molecules 2877.4.2.2 In Situ Experiments 2877.4.2.3 Liquid-Phase Analysis 2887.4.3 Raman Spectroscopy 2897.5 Techniques Based on X-Rays 2917.5.1 Introduction 2917.5.2 Interaction of X-Rays with Matter 2937.5.3 X-Ray Photoelectron Spectroscopy (XPS) 2947.5.4 X-ray Absorption Spectroscopy (XAS) 2957.5.4.1 XANES 2957.5.4.2 EXAFS 2987.5.5 X-Ray Scattering 2997.5.5.1 WAXS/XRD 2997.5.5.2 SAXS 3007.5.6 X-Ray Microscopy 3027.6 Ion Spectroscopies 3037.7 Magnetic Resonance Spectroscopy Techniques 3047.7.1 NMR 3047.7.2 EPR 3077.7.2.1 Metal-Centered Radicals 3087.7.2.2 Ligand-Centered Radicals 3087.8 Summary 310

Question 1 310Question 2 311Question 3 312References 313

8 Synthesis of Solid Supports and Catalysts 315Petra de Jongh and Krijn de Jong

8.1 Introduction 3158.2 Support Materials 3178.2.1 Mesoporous Metal Oxides 3188.2.1.1 Fumed Oxides 3198.2.1.2 Silica Gel and Other Hydrothermally Prepared Oxides 3208.2.1.3 Alumina 3228.2.1.4 Ordered Mesoporous Materials 3248.2.1.5 Extending the Ordered Mesoporous Materials Family 3258.2.2 Ordered Microporous Materials 3268.2.2.1 Zeolites 3268.2.2.2 Metal Organic Frameworks 3308.2.2.3 Zeolitic Amidizolate Frameworks 3318.2.3 Carbon Materials 3318.2.4 Shaping 333

Contents xi

8.3 Synthesis of Supported Catalysts 3338.3.1 Colloidal Synthesis Routes 3348.3.2 Chemical Vapor Deposition 3378.3.3 Ion Adsorption 3388.3.4 Deposition Precipitation 3418.3.5 Co-Precipitation 3458.3.6 Impregnation and Drying 3498.3.6.1 Impregnation 3508.3.6.2 Drying 3528.3.6.3 Calcination/Thermal Treatment 3548.3.6.4 Activation of the Catalyst 356

Question 1 357Question 2 357Question 3 358References 358

Index 361

xiii

Preface

This books aims to be a concise yet thorough introduction to catalysis for theinterested reader, be it a student or a research scientist who quickly wants torefresh essential knowledge. The book Catalysis An Integrated Textbook is basedon the CAIA course (Catalysis, An Integrated Approach) organized by NIOK(Nederlands Instituut voor Onderzoek in de Katalyse) for more than quarter ofa century. Since 2001, this course is taught in one week on the North Sea islandSchiermonnikoog. It has been the first post-doctoral course on catalysis in thebroadest sense and this book intends to capture this spirit. The unifying conceptsof bio-catalysis, homogeneous and heterogeneous catalysis are being highlighted,decorated with catalyst characterization, preparation, kinetics, and catalytic engi-neering. The course is a true crown-jewel of NIOK but after almost 20 years, thebook used at this course was in need of an update. It was our pleasure to lead theconception of this entirely new book based on the course as it is given today. Wehope that students and colleagues will find it helpful also for studying or teachingcatalysis courses all around the globe. We thank especially all authors, teachers atthe CAIA course, who contributed to the different chapters, as well as The Asso-ciation of the Industrial Advisory Board of NIOK, for their financial support. Wealso gratefully acknowledge the help of the NIOK office, in particular Dr IreneHamelers and Dr Erica Ording for their help in managing the team of authors.

Leon Lefferts and Ulf HanefeldEnschede and DelftSpring 2017

1

1

IntroductionLeon Lefferts1, Ulf Hanefeld2, and Harry Bitter3

1Science and Technology, Universiteit Twente, Langezijds Bldg., 7500 AE Enschede, The Netherlands2Technische Universiteit Delft, Gebouw 58, Van der Maasweg 9, 2629 HZ Delft, The Netherlands3Biobased Chemistry and Technology, Wageningen University & Research, Bornse Weilanden 9, 6708 WGWageningen, The Netherlands

1.1 A Few Words at the Beginning

Catalysis is at the very basis of life. The fact that this sentence could be read isdue to the catalytic processes proceeding in our body, in this case biocatalyticprocesses. At the same time a heterogeneous catalytic process, the Haber–Boschsynthesis of ammonia, made it possible that the world has its current size of popu-lation. Without it insufficient fertilizer would exist and only half as many humanscould live and they would have significantly less meat to eat. Catalysis is thus anessential science that finds its way into every aspect of our lives. It is commonlydivided into three different disciplines, namely, homogeneous catalysis, hetero-geneous catalysis, and biocatalysis. All these disciplines work according to thesame underlying principles. This book therefore aims at explaining these princi-ples while at the same time teaching each of the three disciplines as well as theengineering that is necessary to bring catalysis into industrial action.

1.2 Catalysis in a Nutshell

Catalysis is purely a kinetic and not a thermodynamic phenomenon. It is aboutspeeding up reactions and lowering activation barriers, not about changing equi-libria. Catalysis is a cycle in which reacting molecules bind to the catalyst, wherethey react to a product that subsequently desorbs and leaves the catalyst availablefor the next reaction sequence. Catalysis is thus a cycle of stoichiometric elemen-tary reactions. Neither of these steps can be called catalytic in itself. It is the cycliccombination of events from which the catalyst emerges unchanged that makesthe sequence catalytic. When different types of catalysts are used, these steps canbe very different even when the starting material and the product are the same.Indeed, some processes have been industrialized with heterogeneous catalysisand then switched to biocatalysis or homogeneous catalysis and vice versa.

Catalysis: An Integrated Textbook for Students, First Edition. Edited by Ulf Hanefeld and Leon Lefferts.© 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Introduction

E

CO

O

O

O

O C O

O

OC

C

O

OC

Adsorption

Desorption

Eact

Eact

gas-phase

reaction

Surface

reaction

Reaction coordinate

Figure 1.1 Potential energy scheme of a heterogeneously catalyzed reaction: the COoxidation.

A reaction in heterogeneous catalysis starts with the adsorption of the reac-tants onto the surface. For example, let us consider the catalytic oxidation ofCO on a noble metal such as platinum (Pt) ( Figure 1.1). Homogeneous catal-ysis as well as biocatalysis operates in the same manner; however, nomenclaturecan be rather different as will be discussed in Section 1.4. Carbon monoxideadsorbs molecularly, while O2 dissociates. Adsorption is an exothermic processthat decreases the potential energy. Next, the adsorbed CO and O react on thesurface to produce CO2, which is weakly bound and desorbs into the gas phase,leaving the surface free for the next reaction event.

Figure 1.1 shows how the catalyst offers an alternative pathway for the reac-tion, with a mechanism that is more complex than the direct gas-phase reaction,but which leads to a lower overall activation energy for the reaction. Generallyaccording to the Arrhenius Law, the rate constant, k, of an elementary reactiondepends exponentially on the activation energy

k = v e−Eact∕RT (1.1)

where v is the pre-exponential factor, R is the gas constant, and T is the tempera-ture. Hence, from Figure 1.1, one realizes immediately that a decreased activationenergy accelerates the reaction tremendously.

Note that the overall change in free energy is determined by the reactants andproducts and in no way by the catalyst. Hence, if the conversion of a gas-phasereaction under certain conditions of temperature and pressure is limited to theequilibrium concentrations of products and reactants, then a catalyst cannot alterthis. A catalyst affects the kinetics of a reaction, but NOT the thermodynamics.

Another important point to note is that the catalyst offers an energeticallyfavorable pathway not only for the forward reaction but also for the reverseone. Hence an effective catalyst used for the formation of CO2 from CO and O2

1.3 History of Catalysis 3

would also be a good choice for the reverse reaction. For example,

CH4 + H2O ⇋ CO + 3H2 (1.2)

The forward reaction is known as the steam reforming of methane to producesynthesis gas (CO + H2), which is an endothermic reaction carried out withnickel catalysts at high temperature. The reverse reaction is the (exothermic)methanation reaction that is applied to purify H2 from traces of CO or even toproduce substitute natural gas from coal or biomass. This reaction also utilizesnickel catalysts but at temperatures much lower than the steam reforming.

Expertise in catalysis is extremely important in the chemical industry for twomain reasons. By definition, catalysts enable reactions to proceed faster so thatsmaller reactors as well as milder conditions, that is, temperatures and pressures,can be employed. Thus, costs to achieve the chemical conversion can be reduced.However, even more important is the fact that catalysts allow enhancement of thedesired conversion, without increasing the rate of formation of undesired prod-ucts. In other words, selectivity of the conversion can be improved. Therefore,more useful product can be produced per amount of feedstock, limiting the pro-duction of waste as well as the need to separate targeted products from wasteproduced by unselective reactions. The result is that costs can be reduced, interms of both investment and operating costs.

1.3 History of Catalysis

Without catalysis, there would be no life. All organisms in nature, includingourselves, exist by the grace of enzymes, biocatalysts, steering the chemicalprocesses in the organisms that allow them to act at all, including self-repair andreproduction. Obviously, mankind using catalysis to make a product is muchmore recent than the origin of life. Still it is a long time ago and occurred withoutany notion of the concept of catalysis. Enzymes in yeast have been used for 8000years or so to convert sugars, producing products such as wine, beer, and bread.

Catalysis, more specifically biocatalysis, thus has allowed us to form societiesand live as civilization. Once humans started to live closely together in largegroups, safe supply of water and food and the storage thereof became very impor-tant. Usage of yeast to produce alcoholic beverages and at the same time sup-pressing all other pathogenic microorganisms ensured relatively safe drinkingsupplies. Equally, bread made with yeast allowed the production of storable pro-duce. Lactobacillus and similar organisms that outgrow pathogens while creat-ing acidic media and thus suppressing all other microorganisms were and areanother common application of catalysis. Yoghurt, cheese, and countless otherdairy products to preserve milk and sauerkraut, salami, and many other forms offood rely on this type of catalytic formation of lactic acid. Meat was made tenderby wrapping it into papaya leaves, in this case proteases from the leaves digestedthe meat a little making it more suitable for human consumption. All of these wereperformed without any knowledge of the term and the meaning of catalysis.

The concept of catalysis started to develop in the nineteenth century [1]. Itwas observed that ethanol could either be decomposed to acetaldehyde with a

4 1 Introduction

pungent smell or to ethylene, depending on which solid was added. We nowknow that metals catalyze dehydrogenation, whereas oxides with acid function-ality induce dehydration. A first human-designed application, again withoutknowing the concept at that time, was actually a sensor that is able to detectexplosive gases. In 1817 Davy, assisted by a young Michael Faraday, discoveredthat a heated Pt wire would detect H2, CO, and CH4 in air as oxidation of thesegases would produce so much heat that the wire would light up. This provideda way to detect these gases in coal mines before they would increase to levelswithin the explosion limits. Based on this the Miner’s lamp was developed, animportant safety device in coal mines at that stage. It was JJ Berzelius in 1835who for the first time suggested a definition for the underlying mechanism andproposed the term catalysis, inspired by the Greek word for “loosen.” It wasproposed that the catalyst was able to induce decomposition of bodies like thecomponents in mine gas. The concept was not essentially different from theconcept of dissociative adsorption in surfaces as we know it now.

1.3.1 Industrial Catalysis

Despite the lack of understanding, the first industrial application, albeit on a verysmall scale as compared to today’s standards, was developed in the early nine-teenth century. In 1831, the production of sulfuric acid was patented, in whichoxidation of SO2 to SO3 using finely divided Pt was a slow critical step. Much later,Pt was replaced by the much cheaper vanadium. In 1838, it was discovered thatammonia could be oxidized over Pt to produce nitric acid, which is essentiallythe foundation of the current process for production of nitric acid via ammoniaoxidation over Pt–Rh gauzes. At that time, natural nitrate in the form of Chilesaltpeter was much cheaper.

By the turn of the nineteenth century those natural nitrate resources soon ranout and N-containing salts were in high demand as fertilizers. A lack of N wouldhave caused a major famine. Several eminent scientists have addressed this prob-lem in the early twentieth century and Os and U were found to be active butinsufficiently available. The founding father of the industrial production of NH3was Fritz Haber (University of Karlsruhe) and he collaborated with Baden Anilineand Soda Factory (BASF) where Mittasch led a crash program to find a cheapercatalytic material for ammonia production. They found that Fe3O4 from a specificmine in Sweden was very active. Impurities in that specific ore (like Na and K)were found to be responsible for this advantageous effect. Finally, Bosch was ableto build an industrial process based on these findings, mastering the high pres-sure that is required on a large scale. The first plant was started in 1910 in Oppau,near Ludwigshafen. The process is described in somewhat more detail in Section2.1.2.2. These developments around ammonia and nitrate chemistry were notonly essential for increasing the capacity of agriculture in order to feed mankind,but also for the production of explosives, for example, in the First World War.Today more than 85% of all ammonia produced via the Haber–Bosch process isutilized to produce fertilizers.

Political and military issues have been influencing the development of specificcatalytic processes over the time. Catalytic cracking (see Section 2.1.2.) of the

1.4 Integration Homo–Hetero-Biocatalysis 5

heavy fraction of mineral oil was developed to industrial scale in the United Statesin order to make available sufficient kerosene for the US Air Force in WW II.On the other side, Nazi Germany had no access to oil and produced liquid fuelsfrom synthesis gas via coal gasification. The Fischer–Tropsch process was devel-oped for that reason and more details can be found in Section 2.1.2.1. Later in1970s and 1980s, South Africa developed this technology further due to the lackof access to mineral oil, which was caused by an international embargo because oftheir Apartheid politics. In recent times, very large investments have been madeat remote oil fields, in order to convert associated stranded gas to liquid fuels,for example, in the Middle East. In China, Fischer–Tropsch technology is usedin combination for syngas production via coal gasification.

The ever-increasing capacity of fuel production also induced the increasingavailability of cheap feedstocks for chemicals such as ethylene, propylene, andaromatics (Benzene, Toluene, Xylene, or short BTX). Based on these platformmolecules, numerous catalytic processes were developed to produce polyethy-lene, polypropylene, polystyrene, polyamides, and polyesters. As costs of chemi-cals are structurally higher than that of fuels, the added value of chemical prod-ucts, or precursor of those like naphtha, is essential for the overall profitability ofa refinery. Processes are therefore highly integrated and interdependent.

1.3.2 Environmental Catalysis

Environmental catalysis became important in 1970s and 1980s, with increas-ing concern about the environment in leading industrial countries at that time.The result was the development of new and improved processes to remove Sand N from fuels (hydrodesulfurization, HDS and hydrodenitrogenation, HDN);as discussed in Chapter 2. In addition, catalytic technology was employed toclean exhaust gases of both large-scale installations like power stations (selectivecatalytic reduction (SCR) and DeNOx), industrial boilers, and incinerators andsmall-scale mobile sources like combustion engines in cars and heavy-duty trucks(three-way catalyst). More information can be found in Chapter 2. Today, thesetechnologies are being implemented in the vast-growing metropolitan areas, fac-ing enormous challenges to ensure air quality, in countries with rapidly growingeconomies such as Asia and South America.

1.4 Integration Homo–Hetero-Biocatalysis

In the early days of catalysis, Ostwald included as catalytic phenomena [2] thefollowing:

• release of super-saturation,• catalysis in homogeneous mixtures,• catalysis in heterogeneous systems, and• enzyme action

Except for supersaturation, this is exactly the scope of this book. Remarkably,the three subareas, that is, homogeneous, heterogeneous, and biocatalysis, have

6 1 Introduction

been developed along different lines in the last century. Biocatalysis has beendominated by biology and biochemistry, whereas heterogeneous catalysis wasinspired strongly by solid state and surface chemistry in combination with engi-neering and practical application. Homogeneous catalysis was developed muchmore along the lines of (metal-) organic chemistry. As a result of this, the subfieldsbecame detached to an important extent thereby even leading to differences invocabulary and technical terms, see Table 1.1. We should never though forget thatthe underlying principles are the same and luckily the scientific community in theNetherlands decided in 1999 to invest in reconstructing the interaction amongour subdisciplines. The Netherlands’ Catalysis and Chemistry Conference andthe Catalysis: An Integrated Approach (CAIA) course are important instrumentsto strengthen the understanding and cooperation between these subdisciplines.The terms used in Table 1.1 will be extensively described in the following chaptersof this book and the reader is referred to this table for understanding the termi-nology. To point out just a few examples:

• A substrate in homogeneous catalysis and biocatalysis is identical to a reac-tant in heterogeneous catalysis; in the surface science subdiscipline, the wordsubstrate is sometime used with a completely different meaning, that is, thesupporting material for a model catalyst.

• With the term metal in homogeneous catalysis, normally a single metal ion isaddressed, which is a typical active site in a homogeneous catalyst. A metalnanoparticle would be termed metal-black in homogeneous catalysis. In het-erogeneous catalysis, the term metal is normally used to describe a metallicactive phase, which could be, for example, metal nanoparticles on a supportmaterial or even bulk metals such as Pt–Rh gauzes. In biocatalysis the metal isthe metal in the active site of the enzyme, catalyzing the reaction, very similarto homogeneous catalysis.

Table 1.1 Technical terms in different fields occasionally differ although they describe thesame thing.

Homogeneous catalysis Biocatalysis Heterogeneous catalysis

Substrate Substrate ReactantDecomposition Leaves the active site DesorptionHougen–Watson Michaelis–Menten Langmuir–HinshelwoodMetal black – MetalMetal Metal Metal· · ·ion, oxide, compoundOxidative addition Oxidative addition Dissociative adsorptionAssociation Docking AdsorptionTON TTN TONTOF TON TOF

TON: turnover number; TTN: total turnover number; TOF: turnover frequency.

1.4 Integration Homo–Hetero-Biocatalysis 7

• Remarkably, the first interaction of a catalyst with a reactant is called adsorp-tion in heterogeneous catalysis: this is termed from the perspective of thereactant (or substrate). In contrast, in homogeneous catalysis, the complexcatalyst is chosen as perspective and the substrate (reactant) thereforeassociates with the catalyst. In biocatalysis, the substrate docks into theactive site.

Catalytic processes based on each of the three subdisciplines have strengthsand weaknesses (see Table 1.2). Heterogeneous catalysis involves relatively robustmaterials that can be handled easily, can be easily separated from products, andcan withstand high temperatures so that the reaction rates can be very high byoperating at high temperature. However, the control on the active site is nor-mally not very good because the structure of sites at the surface is hard to con-trol; as a result, activity (at mild conditions) and selectivity are generally inferiorwhen compared to homogeneous catalysis and even more when compared to

Table 1.2 General strengths and weaknesses of the different types of catalysts.

Property Homogeneous catalysis Biocatalysis Heterogeneous catalysis

Catalyst Molecule,complex

Enzyme, molecule Solid

Medium Generally liquid Generally liquid Gas or liquidActivity High Very high VariableSelectivity High Very high VariableReactionconditions

Mild Mild Harsh

Service life ofcatalyst

Variable Variable Long (most cases)

Diffusionproblems

None None High

Product separation May beproblematic

May be problematic Easy

Catalyst recovery Expensive Variable Not necessaryTunability High (via ligands) High (via genetics) Low (via promotors)Mechanisticunderstanding

High High Less

Scale ofapplication

Many smallprocesses

Small and mediumscale

Large processes

Application Fine chemicals,specialties

Fine chemicals,food

Bulk chemicals, fuels,environmental cleanup

Type of process Batch Batch ContinuousScientificdiscipline

Chemistry (Bio)chemistry Mainly physicalchemistry andchemical engineering

8 1 Introduction

biocatalysis. Table 1.2 provides a very generalized description. The actual choicefor a process using a specific catalyst is made in practice based on all factors influ-encing the costs, including both investments and operation cost. It is clear thatsuch an evaluation is very specific for each process, but in general will be influ-enced by the following:

• the size of reactors, heat exchangers, and separation units,• costs of construction material in view of corrosion, temperature, and pressure,• complexity of any separation required,• cost of the catalyst itself,• integration with facilities onsite,

and many, many more.Table 1.2 shows that all types of catalysis have advantages and disadvantages.

Which catalyst is applied is often dependent on the current level of knowledge.Historically acetic acid (vinegar) was always produced via fermentation and bio-catalysis. With an increase of demand approaches via ethylene with heteroge-neous catalysis were investigated but did not reach the market. Instead, carbony-lation of methanol via homogeneous catalysis turned out to be the best currentapproach. Three different metals are commonly used, Co in the BASF process, Rhin the Monsanto process (Figure 1.2), and Ir in the Cativa process. With a changein feedstocks that takes place with biomass as starting material these processesmight, however, again be replaced [3].

Ethanol is also historically a product of fermentation and still is. However,ethanol and other small alcohols are today produced via heterogeneous catalysis,typically with heterogeneous acids. Indeed, the hydration of isopropene viaheterogeneous catalysis is one of the oldest bulk petrochemical processes thathad already started in 1920s. Ethanol was also produced like this since the

O

O

O–

I

I CO

CORh

I

CO

COCO

ORh

Rh

COCH3

CH3OH + HI CH3I + H2O

CH3OH + CO CH3COOH

CH3I

CH3CO

CORh

II

I

II

I

II

I

CH3

O

OH+ HI

+ H2OI

–

–

–

Figure 1.2 The Monsanto acetic acid process.

1.4 Integration Homo–Hetero-Biocatalysis 9

1950s, in addition to the classical fermentation for consumption. More recentlywith the advent of biomass as a feedstock, a movement back to fermentation istaking place.

Acrylamide, the monomer of polyacrylamide, is another molecule that demon-strates that the choice of catalysts is not always obvious. Initially, the hydrolysis ofacrylonitrile to acrylamide was done with a homogeneous catalyst (sulfuric acid).This was then substituted with a heterogeneous catalyst, since this allowed con-tinuous processes and catalyst recycling. However, a selectivity issue remained,as acrylic acid was also produced next to acrylamide leading to a difficult prod-uct separation. Today, this process is catalyzed by an enzyme. The enzyme nitrilehydratase catalyzes only the acrylamide formation but not the next step to acrylicacid. With a selectivity >99.99%, conversions >99% are obtained. Since the cata-lyst is immobilized the separation and reuse is straightforward [3].

A good knowledge of all the types of catalysis is thus essential to make theright choice and to find the catalyst that is optimal for the process under inves-tigation. All the types of catalysis are used industrially and all have its own dis-tinct advantages. A beautiful example where all the types of catalysis were usedto their full strength is the synthesis of the antiflu drug Tamiflu (developed byresearchers at F. Hoffmann–La Roche, Figure 1.3). Starting from a very cheap

OMe

O

AcHN

COOEt

NH3 H2PO4

Tamiflu

HO

MeO

OMe

OMe

CO, Pd(OAc)2,

dppp, KOAc

H2, Ru/Al2O3

EtOH, 110 °C, 20 h

MeO

Br

OMe OMe

OMe

OMe

MeO

MeO

O

O

MeO

O

COOEt

COOEt

COOEt

COOEt

COOEt

COOH

MeO

MeO

O

O

COOEt

COOEt Lipase fromAspergillus

oryzae

COOEt

COOEt

BrO

Figure 1.3 Hoffmann–La Roche approach for the synthesis of Tamiflu.

10 1 Introduction

achiral aromatic compound, homogeneous catalysis was utilized to introducetwo carbonyl groups and provide the essential ester group of Tamiflu. Then avery clean catalytic hydrogenation with a heterogeneous catalyst led to a prochi-ral molecule, abolishing aromaticity on the way. Chirality was then introducedwith a highly selective enzyme, generating five stereocenters in one single step.All these were only possible by applying each type of catalyst for its particularstrengths [4].

1.5 Research in Catalysis

1.5.1 S-Curve, Old Processes Improvement Is Knowledge Intensive

Since catalysis started to develop in the nineteenth century (Section 1.3), it devel-oped in different directions and with different time frames. First mainly (thoughnot exclusively) focusing on fossil resources followed by environmental catalysisand more recently on the use of renewable resources and CO2 as feedstock. As aresult, the development of catalysts and catalytic processes is in different phases.These phases are often described by an S-curve (Figure 1.4).

For new processes and catalysts, initially a significant research effort is neededto explore and understand the new field. When successful, the research makesa leap and improvements are achieved at a higher rate. In the consecutive step,the field reaches maturity and significant efforts are again needed to make smallimprovements. The profitability of a process follows more or less an inverse trend.In the beginning large investments in research are needed to advance a field, whenmore maturity is reached the process becomes more economically viable andtherefore profit is made. In the middle stage, however, there is a period that is alsocalled “the valley of death.” In the initial stage, at the beginning of the S-curve,large research efforts are needed; however, the need for investment is relativelymoderate. At the end of the S-curve, the process is proven to be profitable andtherefore return on investment (ROI) can be well calculated. In the middle stage,

Fossil

Biomass

Time

Matu

rity

CO2

Figure 1.4 S-curve forresearch in catalysis.

1.5 Research in Catalysis 11

large investments are needed in pilot and demo plants, yet the process is notmature enough to secure the ROI. This is the phase that is difficult to overcome.

Reasoning from a feedstock point of view, that is, fossil, biomass, and CO2, itis reasonable to state that they are in different phases on the S-curve, the conver-sions of fossil sources are in general mature and at the end of the curve. Kindlynote that large research efforts are still ongoing in that field, but improvementsare small though economically very relevant. The use of biomass is at the bot-tom of the S-curve. New insights have emerged in recent years though the majorbreakthrough has not yet been made. The use of CO2 is at the moment at its dawn.It is regarded as a potential source for carbon in fuel and chemicals in the future,but its potential is not fully clear yet.

1.5.2 Interdependence with Other Fields

To go all the way from feedstock to products a number of steps are needed ofwhich the catalytic conversion step is a crucial one, though it is not the onlyone. The feedstock has to be collected, transported to the conversion plant(by pipeline, boat, truck, etc.), converted to the desired products, and finallythe product has to be delivered to the customer/consumer. The logistic partof the production falls outside the scope of this book, though it shows thatcatalysis blossoms when interacting with other fields. Here, we will focus onthe conversion part, that is, convert the feedstock to the desired product. Inaddition that is not a single, isolated, field. To be able to convert the feedstock ithas to be loaded into a reactor, the feedstock, optionally after cleaning, has to beconverted, and the product has to be separated from byproducts or unconvertedfeedstock. Therefore, reactor technology, catalysis, and separation are the fieldsthat need to collaborate closely in order to find the most efficient way to thedesired product. A good process design is crucial in that respect.

Conversely, a process design can also identify the bottlenecks for the otherfields of science to solve. For example, as discussed above, HDN and HDS areimportant catalytic processes to clean a feedstock before it can be converted in acatalytic process. The HDN and HDS processes are needed to protect the catalystfrom poisoning. Thus, when new N and S tolerant catalysts can be developed onehas to rethink the whole process to make it more efficient.

The issue of effective process design is even more relevant for biomass con-version. Since biomass conversion is still at the bottom of the S-curve, it is stilla question as in which technology is most efficient. For example, when workingwith aqueous solutions, not uncommon in biomass conversion, it is beneficial tomake a product that is apolar and separates out easily from the reaction mixtureto lower separation costs. In addition, when looking at the molecular structure ofbiomass (cellulose, lignin, etc.), which is highly functionalized, the conversion tofunctionalized products should be possible with only limited energy input. Thiswould then allow conversions in small-scale plants, which need rethinking of thewhole processing chain, as now we are using large-scale plants. The latter is dueto the fact that often economy of scale applies due to the need of heat exchangersthat are more effective on larger scales.

12 1 Introduction

1.5.3 Recent and Future Issues

Different scenarios on the development of the human population exist(Figure 1.5). It is realistic to expect that about 10 billion people will live on theearth at the end of the twenty-first century with ever-increasing wealth. To keepour planet inhabitable, it is therefore crucial that we use renewable resources ina sustainable manner. Catalysis is one of the key elements to achieve this.

Currently, our infrastructure and needs are aligned with the use of fossilresources. Oil refineries are energy efficient and dedicated to make the fuels andchemicals we currently need. However, the feedstock is not sustainable; thus onthe long term, the desirability of the use of fossil resources is questionable.

1.5.3.1 BiomassCurrently, the biobased economy is often discussed as a driver of research. Whenswitching from fossil resources to biobased renewable resources (in a properway), less amount of CO2 is emitted to the atmosphere thus reducing globalwarming. Moreover, as the resources of choice in that case are renewable we donot deplete the scarce resources of our planet. Wubbo Ockels (the first Dutchastronaut) stated in one of his last letters “The problem is that Humanity needsto find a sustainable balance with its environment, with our home the planetEarth and its Nature.”

Biomass on the other hand is renewable but has a completely different chem-ical composition and structure when compared to the fossil resources. Fossilresources mainly consist of C and H, while biomass also contains significantamounts of O (Table 1.3). The latter is a challenge when trying to make themolecules present in the biomass compatible with the current infrastructureand need for fuels. In that case, the biomass needs to be defunctionalized, thatis, the oxygen which increases the energy content of the remaining molecules

15

10

5

Bill

ions o

f people

01800

Estimated UN high UN medium UN low Actual

1850 1900 1950

Year

2000 2050 2100

Figure 1.5 Potentialdevelopments of number ofinhabitants on the earth.Adapted from http://upload.wikimedia.org/wikipedia/commons/thumb/7/77/World-Population-1800-2100.png/587px-World-Population-1800-2100.png.

1.5 Research in Catalysis 13

Table 1.3 Comparison of oil and biomass composition.

Oil Biomass

Element Percentage (%wt) Element Percentage (%wt)

C 83–87 C 50H 10–14 H 11N 0.1–2 N 0.5O 0.1–1.5 O 35S 0.5–0.6 S 0.1Metals <0.1 Ash 2

Dry biomass 50% moisture(http://www.treepower.org)

needs to be removed. However, energy has to be invested to remove the oxygen.When that energy originates from renewable resources (wind or solar) this canbe viable from a renewable point of view. Conversely, one can ask the questionwhy do we not use the renewable resources directly?

On the other hand, when talking about chemicals often functionality is desiredin the feedstock molecules. For example, carboxylic acid functionality andamino groups are needed to make nylon from adipic acid and hexanediamine.From that point of view, biomass is a more viable resource as it already containsfunctionalities. One should also ask the question whether the currently utilizedmolecules are the most desired ones or whether new molecules should be madefrom biomass. This, however, needs a change of mindset about which moleculeswe need and which process we need to make them. This might conflict withthe currently existing infrastructure, which were installed with high investmentcost, thus changes to biobased feedstocks/products might be slow for financialreasons. When installing new capacity, however, we should think about the mosteffective way from a renewable and financial point of view.

1.5.3.2 CO2 as a FeedstockUsing CO2 as a feedstock is an interesting approach to address the major envi-ronmental issues that we have today while producing chemicals. When CO2 isconverted to fuels and chemicals, the increase in CO2 levels in the atmosphereis slowed down or the levels can even be decreased. To make that possible, chal-lenging hurdles have to be overcome. For example, when we want to decrease theCO2 levels in the atmosphere effective capture methods are needed. In addition,a grand challenge is how to effectively convert CO2 to other products. Carbon-dioxide is thermodynamically a very stable molecule, thus energy is needed tovalorize it. In fact, C–O bonds have to be converted to C–H bonds and C–Cbonds, that is, reductions are needed.

For these reason, electrochemistry and photochemistry are revived again.Recent developments in the fields of solar energy (photovoltaic) and wind energymay indicate future availability of cheap electrical power, in contrast to the

14 1 Introduction

normal perception that electrical power is relatively expensive. The consequencewould be that conversion of electrical power to fuels and chemicals might see animportant revival, for example, via electrochemistry. In photochemistry, light isused to generate electrons that can be used to reduce CO2. Thus, photocatalysisis again, certainly at academic level, becoming an important issue. The energyneeded for these processes needs to come from renewable resources such as tomake the processes sustainable.

Another option to use CO2 as feedstock is to use it as building block inchemical synthesis. Examples are the reactions of CO2 with alcohols and polyols,amines, olefins, dienes, and alkynes, to form carboxylates, carbonates, andcarbamates. However, also in that case energy is needed to convert the CO2,maybe not directly to drive the specific reaction but the energy is then alreadyintroduced in the reactant by the introduction of reactive bonds.

The challenges we have sketched here are in no way complete, and the futuremay take humanity in yet another direction. One thing, however, is obvious:Catalysis will help us to face those challenges.

1.6 Catalysis and Integrated Approach or How to Usethis Book

This book evolved out of the annual course Catalysis: an Integrated Approach(CAIA). The chapters are written by the teachers of the course, and each chaptercan be read on its own to get up to speed on its topic. This is also the common wayof presenting catalysis, separated and not integrated. The real value of this book isthat, as we hope, all the chapters also show the link to the other fields of catalysisand that all topics are presented side by side. This eases cross reading. The aimis that all the chapters can be read also by non-experts in the field allowing toquickly gain insight into the opportunities of each type of catalysis as to allow forthe choice of the best catalyst and the matching reactor.

References

1 Ross, J.R.H. (2012) Heterogeneous Catalysis, Fundamentals and Applications,Elsevier.

2 Ostwald, W. (1902) Nature, 65, 522–526.3 Sheldon, R.A., Arends, I., and Hanefeld, U. (2007) Green Chemistry and Catal-

ysis, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.4 Zutter, U., Iding, H., Spurr, P., and Wirz, B. (2008) J. Org. Chem., 73,

4895–4902.