Molecular Heterogeneous Catalysis, Wiley (2006), 352729662X

8Self Organization and Self Assembly of Catalytic Systems

9Heterogeneous Catalysis and the Origin of Life, Biomineralization

XII Preface

In order to bridge the knowledge between elementary reaction steps and their kinetics and dynamics under more realistic operational conditions requires one a range of di erent methods which enable one seamlessly to span time and length scales. This hierarchical coupling is critical to describing complex multiscale catalytic systems but is still in its infancy in modeling catalysis. Herein we describe some of the earliest e orts which, for the most part, have decoupled the di erent time and length scales and pass on appropriate knowledge between the them.

Ab initio quantum mechanical methods can be used to establish the electronic structure for model surfaces and clusters, and to predict chemical bonding and reactivity of di erent molecular reactants, intermediates and products on and within di erent model systems. Elementary reaction steps that are activated require time scales that are typically on the order of 10−4 sec or longer. Non-activated processes such as di usion are typically faster. Transition-state reaction rate theory can, therefore, be used to establish the kinetics for these systems. For systems where the di usion is on the order of the reaction time scale or slower, molecular dynamics methods have proven to be very useful provided one knows or can develop the interatomic interaction potentials between the intermediates and the surface. The full simulation of dynamics, however, can become intractable if the time scales of interest are significantly greater than the time scales for di usion. Systems that contain many degrees of freedom and consist of many interacting molecules, therefore, typically require more coarse-grained methods to advance to longer time and length scales. Monte Carlo statistical methods are typically used to determine the lowest free energy states and the chemical potential of complex systems. Dynamic Monte Carlo techniques, on the other hand, are used to simulate complex kinetic behavior. We review the application of each of these techniques to many catalytic systems in this book. An introduction to each of these methods is provided in the Appendix.

Classical heterogeneous catalysts are the subject of four major chapters in the book. We elaborate in detail on our current understanding of the molecular events that underline their catalytic phenomena and attempt to deduce from these results important catalytic reactivity concepts. This detailed understanding provides a basis for the comparison of the mechanistic principles between heterogeneous, enzyme, and homogeneous organometallic cluster catalysis.

We review the governing mechanisms for many of these systems thus having in mind the leading scientific question: “What are the fundamental similarities and di erences between these systems?” We explore the use of these insights towards the design of new catalytic systems. Of particular interest is the comparison of traditional heterogeneous systems with biochemical systems for specific reactions. This leads to an understanding of the fundamental di erences between enzymes and chemo-systems, which tend to relate to the di erences in the adaptability of the enzyme to di erent stereochemical requirements as the reaction proceeds to that of the typically non-adaptable chemical-based systems. In addition, the biochemical systems are of great interest due to their internal complexity, which appears to be responsible for formation, replication and metabolism of living cellular systems.

Insights on chemo-evolutionary theories of proto-cellular systems are given in the final chapters of the book, with the purpose of defining criteria or discover conditions by which a catalytically active protocell can be designed. The biochemical catalyst design exploits immunoresponse or evolutionary recombinatorial cell growth techniques. It has been discovered that the processes fundamental to the formation of the microporous

Preface XIII

zeolites with their well defined cavities and channel systems have a number of similarities to such biochemical processes.

The comparison of these interesting features between biochemical and chemical catalysts, along with the theories on the origin of the metabolic protocellular systems, we hope will be an inspiration for endeavors to design new catalytic systems that are able to self assemble themselves for use in a particular catalytic application.

The book is targeted at the readership at the graduate student level. We hope that the book will help to convince the research community of the importance of molecular level research and the fruitfulness of theoretical approaches.

The stimulating environment of the Schuit Institute of Catalysis and the University of Virginia and the collaboration with many colleagues and coworkers over the past 15 years have been invaluable to us. We are grateful for the input this has provided to our work and have used the opportunity when writing this book to use as examples many of the results produced in our laboratories. We have placed these results in the context of the most relevant advances in catalysis research by the international research community. Of necessity a selection had to be made for which, we take full responsibility.

This book could have never been realized without the invaluable assistance of Joop van Grondelle, who did most of the editing of the book. MN would like to acknowledge the valuable input from past and present students and academic as well as industrial colleagues. In particular, he o ers special thanks to Professor Robert Davis, Michael Janik, Dr. Randall Meyer, Chris Taylor and Dr. Sally Wasileski for their valuable input to di erent sections in the book. RAvS appreciates the elucidating comments from past and present TUE colleagues, especially Dr. A.P.J. Jansen, Professor M. Koper, Professor J.W. Niemantsverdriet, Dr. X. Rozanska, Dr. N. Sommerdijk and Dr. E. Hensen. MN would also like to thank his wife Dory and daughters Nicole and Sabrina for their unending love and support and their understanding of why Dad was not able to play. RAvS thanks his wife Edith especially for her patience and companionship on the many working visits on behalf of the book.

Most importantly, without the love and encouragement of our spouses Edith and Dory, we could not have embarked on or finished this endeavor.

CHAPTER 1

Introduction

1.1 Importance of Catalysis

Catalysis is ubiquitous to life as well as to society. Catalysts are used in the production of the foods that we eat, the clothes that we wear, the energy necessary to heat and cool our homes, the enzymatic transformations that occur throughout our body to provide function to nearly every organ, the purification of the air that we breathe, the fuels used in our cars, and the fabrication of the materials used in and around our homes and o ces. Catalysts are at the heart of nearly all biological as well as many chemical transformations of molecules and mixtures into useful products. Enzymes in our body, for example, carry out nearly all of the biological conversions necessary for us to live. They are critical in fighting o infection, building DNA, digesting foods, moving muscles, stimulating nerves and aiding breathing. In terms of chemical conversions, catalysts are responsible for the production of over 60% of all chemicals that are made and are used in over 90% of all chemical processes worldwide[1−2]. This accounts for 20% of the Gross Domestic Products in the USA. Catalyst manufacturing alone accounts for over $10 billion in sales worldwide and is spread out across four major sectors: refining, chemicals, polymerization, and exhaust emission catalysts[1−2]. Refining is the largest sector with the production of catalysts for alkylation, cracking, hydrodesulfurization, fluid catalytic cracking, hydrocracking, isomerization, and reforming chemistry. The value derived from catalyst sales, however, is really only a very small fraction of the total value derived from catalysis overall, which includes the value of the products that are produced, i.e. chemical intermediates, polymers, pesticides, pharmaceuticals, and fuels. The overall impact of catalysis is estimated to be $10 trillion per year[1] . The intermediates made by catalysis are used in the production of materials, chemicals, and control devices that cross many di erent manufacturing industries including petroleum, chemicals, pharmaceuticals, automotives, electronic materials, food and energy[1−3] .

As we look to the future, catalysis holds the promise of eliminating, or at least substantially reducing, pollution from chemical and petroleum processes, electronics manufacturing, pharmaceutical synthesis, and stationary and vehicular emission sources. Heterogeneous catalysis is at the heart of many of the proposed green chemical processes targeted to reduce emissions dramatically. A catalyst, by definition, is a material that is used to convert reactants to products without itself being consumed. The goal then is to tailor atomically the structure of an active catalyst so as to convert reactants directly to products without the production of by-products along the way which typically go on to become waste. Catalysts then by nature would help eliminate the production of side products, thus eliminating most waste.

Fossil fuels currently make up the backbone of the US energy economy. The processing of these fuels leads to considerable levels of CO2 production. An estimated 1.5 billion tons of carbon in the form of CO2 is emitted each year. About 40% is produced in the conversion of fuel into electricity. Ine cient chemical processes can also be added to the list of major energy consumers. For example, petroleum reforming and ammonia synthesis both consume considerable amounts of resources in order to provide the heat necessary to drive their respective reactions. In addition, they operate at high temperatures, which tends to lead to the greater production of combustion products and thus lower overall selectivities. The design of catalysts which are more active would lower the temperature of

Molecular Heterogeneous Catalysis. Rutger Anthony van Santen and Matthew Neurock

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-29662-X

2Chapter 1

operation thus dramatically reducing the energy demands. There is currently a worldwide research e ort aimed at the development of new catalytic materials to reduce energy consumption.

A third environmental issue concerns the generation of toxic waste solvents used to carry out various liquid acid catalytic conversions. Many of today’s petroleum refining processes carry out acid-catalyzed isomerization and alkylation reactions using corrosive or toxic liquid acids such sulfuric acid and hydrofluoric acid. These solvents pose significant environmental concerns. Various solid acid materials have been targeted to replace these corrosive liquids. While there are at least three new patents on processes that can use solid acid catalysts, nearly all alkylation processes are still carried out using liquid acids.

Perhaps one of the greatest challenges facing society over the next quarter of a century will the production of energy resources necessary to sustain the 1010 people on the planet. This situation has been clearly outlined by Professor Richard Smalley of Rice University[4] . This will require a minimum of 10 terawatts of power from cheap, clean and potentially renewable energy sources. Smalley indicates that this problem will likely transcend many other societal issues such as water and food shortages since a solution to the energy problem could be used in solution strategies to these others. While there is no current solution to this major challenge to energy, catalysis is likely play a pivotal role. In particular, novel catalytic materials will be required for the advancement of three major areas. The first involves the development of catalysts for the photocatalytic reduction of CO2. Any solution strategy that uses combustion or oxidation of hydrocarbons to provide hydrogen still face the great challenge of dealing with CO2 emissions. New catalytic methods that can reduce CO2 will clearly be necessary. The second area involves the development of novel photocatalysts that could activate water leading to the direct production of hydrogen and oxygen. The third area requires the development of inexpensive, highly reactive electrocatalysts that are resistant to poisoning in order to advance significantly the deployment of fuel cells. The biggest issues are associate with fuel cells are the high overpotentials that exist at the cathode as well as the anode. This is directly tied to their sluggish catalytic activity and the inherent dependence on Pt-based catalysts which are rather expensive[5−6] . Meeting the energy demands of the 21st century is clearly an unsolved dilemma where catalysis will play an important role.

As we look further into the future, it may become clear that the current practice of the production of chemicals may be antiquated. We currently use severe temperatures in order refine petroleum feedstocks into a range of hydrocarbon chemical intermediates. We then subsequently attempt to add selectively oxygen or nitrogen functionality back into the molecule by selective oxidation or amination processes, respectively, in order to produce valuable functionalized intermediates. Nature, however, already starts with many of these functional intermediates trapped inside the structures of carbohydrates and other natural occurring feedstocks. In the production of specialty chemicals, it might make more sense to try to carve out selectively the chemical and stereochemical functionality needed directly from nature. Indeed, there is currently a strong and growing e ort in what some call the Bioindustrial Revolution[7]. The current goal is to design chemical and biological catalysts that can convert bio-based feedstocks into chemicals and fuels. These feedstocks are renewable and, therefore, very attractive from an environmental standpoint. In addition, they contain a wide range of intermediates that could lead to new products provided that we can design inexpensive catalytic materials and processes to carry out these conversions. Some would argue though that traditional petrochemical processes will always be cheaper and that the production of crude oil will last for many

Introduction 3

years into the future. Despite these arguments there are a number of processes and plants built around the concepts of bio-renewable chemicals and energy.

It is clear that catalysis plays an important role in society today and will be a critical technology for advancing our future.

The inception of industrial heterogeneous catalysis started early in the 20th century with the invention of a continuous process to produce ammonia from nitrogen and hydrogen[8] . This provided a very low cost route to produce ammonia, a major ingredient of dynamite and agricultural fertilizers. Ammonia is currently one of the largest commodity chemicals produced worldwide. The Born–Haber process to produce ammonia was a technological breakthrough since nitrogen together with hydrogen gas could simply be passed through a tube that contained an inorganic solid, thus promoting the two to react to generate ammonia, which was collected at the end of the tube in a continuous fashion. Such continuous heterogeneous catalytic processes were revolutionary since they could be readily scaled up in order to produce desired product yields. This was in sharp contrast to the conventional batch processes, which up to that point in time were the main modes of industrial production.

The key to the Born–Haber process was the inorganic packing material inside the tube, for reaction would not occur in the absence of this material. The preferred catalytic material for this process was a fused iron doped with potassium. The nitrogen and hydrogen reactant gases were converted to ammonia without a change in the macroscopic performance of the catalyst material over the course of the reaction. This agrees with the classic Berzelius definition of a catalyst: a material which will increase or decrease the rate of a particular reaction without itself being consumed in the process.

1.1.1 Additional Suggested Textbooks on Heterogeneous Catalysis

The specific aim of this book is to provide a molecular basis and in-depth understanding of the mechanisms involved in heterogeneous catalysis. The presentation is at an advanced level. There are many important books on catalysis that, in general, provide either an introductory and explanatory view of catalysis or that are focused on specific aspects of catalysis such as kinetics or synthesis or related to industrial catalysis. Here we provide a short list of selected relevant books for the interested reader.

These texts cover di erent catalytic principles and disciplines along with their application to industrial practice. Collectively they span a wide range of material including basic concepts in heterogeneous catalyst synthesis, characterization, kinetics, reaction engineering and their application to industrial catalytic systems.

1.J.M. Thomas, W.J. Thomas, Priciples and Practice of Heterogeneous Catalysis,

Wiley-VCH, First Edition (1967), Second Edition (1997).

2.B.C. Gates, J.R. Katzer G.C.A. Schuit, Chemistry of Catalytic Processes, McGrawHill (1979), integrating chemical understanding of catalytic processes.

3.C. N. Satterfield, Heterogeneous Catalysis in Industrial Practice, (1991).

4.B.C. Gates, Catalytic Chemistry, Wiley (1992).

5.R.J. Farrauto, C.H. Bartholomew, Fundamentals of Industrial Catalytic Processes., Blackie-Chapman and Hall (1997).

6.M. Bowker, The Basis and Application of Heterogeneous Catalysis, Oxford Science Publishers (1998).

4Chapter 1

7.R.A. van Santen, P.W.N.M. van Leeuwen, J.A. Moulijn, B.A. Averill, Catalysis, an Intergrated Approach, Elsevier (1999).

8.J. Hagen Industrial Catalysis, Wiley-VCH (1999).

9.I. Chorkendor , J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics,

Wiley-VCH (2003).

There are various books which cover in detail the fundamental principles established from surface science and their application to heterogeneous catalysis:

10.R.I. Masel, Principles of Adsorption and Reaction on Solid Surfaces, Wiley (1996).

11.G.A. Somorjai, Surface Chemistry and Catalysis, Wiley (1994).

12.K. W. Kolasinski Surface Science: Foundations of Catalysis and Nanoscience, Wiley (2001).

The following two texts describe the fundamental kinetics and modeling of heterogeneous catalytic systems:

13.M. Boudart, Kinetics of Chemical Processes, Prentice-Hill (1968).

14.J.A. Dumesic, D.F. Rudd, L. M. Aparicio, J.E. Rekoske, A.A. Trevino, Micro Kinetics of Heterogenous Catalysis, American Chemical Society (1992).

There are two modern compilations on heterogeneous catalysis that are comprised of a series of volumes which cover many aspects of catalysis. Individual sections are written by leading experts. Both series are highly recommended as general references:

15.G. Ertl, H. Kn¨ozinger, J. Weitkamp, Handbook of Heterogeneous Catalysis WileyVCH (1997).

16.I.T. Horvath, Encyclopedia of Catalysis, Wiley International (2003).

1.2Molecular Description of Heterogeneous Catalysis

The ability to predict catalyst performance as a function of chemical composition, molecular structure and morphology is the foundation for the science and technology of catalysis. We aim to describe the use of currently available theoretical and computational methods for both qualitative and quantitative predictions on the molecular events on which the catalytic reaction is based. This relates to the prediction of catalyst structure and morphology as well as the simulation of dynamic changes that occur on the catalyst surface as the result of reaction.

We will provide the reader with an introduction to fundamental concepts in catalytic reactivity and catalyst synthesis derived from the results of computational analysis along with physical and chemical experimental studies. The tremendous advances in nanoscale materials characterization, in-itu spectroscopy to provide atomic and molecular level resolution of surfaces and adsorbed intermediates under reaction conditions, predictive ab initio quantum mechanical methods and molecular simulations that have occurred over the past two decades have helped to make catalysis much more of a predictive science. This has significantly enhanced the technology of catalysis well beyond the historical ammonia synthesis and petrochemical processes.

Herein we attempt to highlight advances in the molecular science of heterogeneous catalysis. We will focus on the mechanistic phenomena that make catalysis possible. This enables one to begin to answer the chemist’s questions: What are the fundamental processes that occur at the catalyst surface and how do they act to control its remarkable behavior? What are the molecular system parameters that control rate and selectivity for

Introduction 5

a specific catalytic process?. The ultimate goal would be the prediction of the catalytic behavior of an arbitrary material along with an arbitrary catalytic reaction system. We introduce the concepts along with a methodology that is fundamental to catalyst design, based on mechanistic analyses. This cross-cuts a range of di erent sub-fields of chemistry including physical, synthetic, organometallic, inorganic, coordination, theoretical, biochemical and solid state and biochemistry as well as chemical engineering.

While it is important to devise strategies that may help to predict material features that could improve catalyst performance, let us not forget that the ability to synthesize materials that contain these features presents yet an even greater challenge. Synthesis is still somewhat of an art that requires marrying the knowledge and skills of the inorganic chemist with those of the solid state chemist. The increased molecular understanding along with its application will require increased precision in the molecular design of catalysts and their specific features. This moves catalyst synthesis from solid state colloidal chemistry into molecular inorganic chemistry as ideas and techniques from coordination chemistry and organometallic chemistry now play much more important roles. The link between well-defined molecular complexes used as homogeneous catalysts in the liquid phase and heterogeneous catalysts applicable to gas phase catalytic reactions can be made when synthetic approaches are developed to immobilize the organometallic complexes. This is an important field of current research.

Significant progress has been made in terms of the design of organometallic and inorganic complexes that provide molecular models of the reactive centers as identified by spectroscopy. These models can therefore be manipulated at the atomic scale to help establish the necessary structural and chemical features required for the molecular design of these systems. One of the early pioneering inventions in heterogeneous catalysis was the discovery that increases in the catalyst surface area lead to significantly improved catalytic e ciency. Solid heterogeneous catalysts are active because much of their surface is exposed to the reacting molecules. Hence, the rate of the reaction increases with increases in the surface area of an inorganic material. The high temperatures often applied in the catalytic process make the application of powders di cult, since they sinter and, hence, will rapidly lose surface area. Well-dispersed nanoscopic inorganic particles, however, can be stabilized on high surface area porous inorganic supports which can yield substantially improved catalytic performance. The activity of these materials may be significantly higher, due to the high surface areas a orded by these supported nanoparticles.

In the middle of the last century, synthesis techniques were developed that enabled the fabrication of well-defined and highly regular microporous silica materials, such as zeolites, with pore sizes comparable to the size of the molecules one would like to catalyze[9] . The appropriate matching of size and shape of these micropores with shape and size of reactant, intermediate or product molecules has been demonstrated to be an important factor in the control of catalytic performance. This is analogous to the lock and key reactivity principle which was developed in the early part of the last century as a way to describe the activity of enzymes, the biochemical proteins in living systems. This process involves matching the shape and size of the catalytic cavity with that of the reactant molecules.

The mechanisms which control zeolite catalytic systems show features similar to those known in biochemistry and lead to the formulation of a more general question: What are the basic di erences between chemocatalysis for reactions carried out in man-made catalytic systems and biocatalysis for reactions as they occur in biochemical systems?

Over the past century we have witnessed an impressive increase in our understanding