Nanostructured Catalysts: Selective Oxidations: Volume 19 (Nanoscience & Nanotechnology Series) - Hardcover

Hardcover

ISBN 10: 0854041869 ISBN 13: 9780854041862

Publisher: Royal Society of Chemistry, 2011

Synopsis

A comprehensive up-to-date summary of the existing information on the structural/electronic properties, chemistry and catalytic properties of vanadium and molybdenum containing catalysts.

About this title

About the Author

Professor Schlögl is Director at the Fritz Haber Institute of the Max Planck Society since 1994

From the Back Cover

The book gives a comprehensive summary of the existing information on the structural/electronic properties, chemistry and catalytic properties of vanadium and molybdenum containing catalysts. As at least one of these two elements is essential to most partial oxidation catalysts the book is of great interest to advanced students of catalysis and researchers both in academia and industry. Despite major developments in the field, the last book with a similar scope was published in 1989 and therefore a new book is overdue. The book benefits largely from the fact that the subject is a major research focus at the Department of Inorganic Chemistry amongst other departments at the Fritz Haber Institute, a world leading institution for catalysis research, ensuring an up-to-date treatise. Such an integrated approach including the relation of in situ spectroscopic results from real catalysts to those of model catalytic systems has not been accomplished before. The introductory chapter discusses the importance of nanoscience for the controlled synthesis of catalysts with functional properties. The following two chapters introduce the necessary background regarding surface properties and preparation techniques, leading from a textbook level to the current state of knowledge. The following chapter starts with an extensive survey and analysis of the existing open and patent literature, which is an essential knowledge source for the development of the new generation of partial oxidation catalysts and will be of particular value to those developing new partial oxidation catalysts. In the remaining subchapters, important examples from current research on partial oxidation reactions are reviewed from experts in the field. The next chapter discusses the importance of 2- and 3-dimensional model systems for a fundamental understanding of the structure of transition metal oxide catalysts and its correlation to reactivity. These results are related to those of the real catalytic systems. The last chapter gives an outlook on research opportunities within the area of partial oxidation reactions.

Excerpt. © Reprinted by permission. All rights reserved.

Nanostructured Catalysts

Selective Oxidations

By Christian Hess, Robert Schlögl

The Royal Society of Chemistry

Copyright © 2011 Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-186-2

Contents

Chapter 1 Introduction Christian Hess, 1, 
Chapter 2 C–H Activation of Alkanes in Selective Oxidation Reactions on Solid Oxide Catalysts Johannes A. Lercher and Frederik N. Naraschewski, 5, 
Section I Oxide-Catalyzed Selective Oxidations of Light Hydrocarbons (Chapters 3–7), 
Chapter 3 Catalysis of Oxidative Methane Conversions Evgenii V. Kondratenko and Manfred Baerns, 35, 
Chapter 4 Propane-Selective Oxidation to Acrylic Acid Annette Trunschke, 56, 
Chapter 5 Ammoxidation of Propylene and Propane to Acrylonitrile Robert K. Grasselli, 96, 
Chapter 6 Selective Oxidation of n-Butane over Vanadium–Phosphorous Oxide Moises A. Carreon and Vadim V. Guliants, 141, 
Chapter 7 Routes to Methacrylic Acid via Partial Oxidation Stephan A. Schunk and Nadine Brem, 169, 
Section II Other Selective Oxidations (Chapters 8–11), 
Chapter 8 Gold as a Selective Oxidation Catalyst Graham J. Hutchings, 193, 
Chapter 9 Ethylene Epoxidation over Silver Catalysts Valerii I. Bukhtiyarov and Axel Knop-Gericke, 214, 
Chapter 10 Ruthenium Active Catalytic States: Oxidation States and Methanol Oxidation Reactions Raoul Blume, Michael Hävecker, Spiros Zafeiratos, Detre Techner, Axel Knop-Gericke, Robert Schlögl, Luca Gregoratti, Alexei Barinov and Maya Kiskinova, 
248, 
Chapter 11 Styrene Synthesis: In-Situ Model Catalysis Studies on Ethylbenzene Dehydrogenation Wolfgang Ranke, 266, 
Section III Supported Vanadium-Oxide Systems and Mechanistic Studies (Chapters 12–14), 
Chapter 12 Highly Dispersed Vanadium-Oxide Catalysts Christian Hess, 299, 
Chapter 13 Surface-Science Models with Chemical Complexity Christian Hess, 326, 
Chapter 14 Mechanistic Aspects of Short Contact Time Oxidative Functionalization of Propane and Ethane from Temporal Analysis of Products Evgenii V. Kondratenko, 340, 
Chapter 15 Characteristics of Selective Oxidation Reactions Robert Schlögl and Christian Hess, 355, 
Chapter 16 Reaction Engineering of Oxidation Reactions Arne Dinse and Reinhard Schomäcker, 398, 
Chapter 17 Outlook Christian Hess, 427, 
Subject Index, 430, 

CHAPTER 1

Introduction

CHRISTIAN HESS

Technische Universität Darmstadt, Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Petersenstr. 20, 64287 Darmstadt, Germany

The production of organic chemicals via heterogeneously catalyzed selective oxidations is one of the most important segments in modern chemical industry. Important products include acrylic acid, acrylonitrile, ethylene oxide, formaldehyde, maleic anhydride, methacrylic acid, and phthalic anhydride. Table 1.1 lists major heterogeneously catalyzed selective oxidation processes discussed within the scope of this book. Due to the high level of empirical development of these processes, further improvements represent a tremendous challenge, which will largely benefit from a mechanistic understanding of selective oxidation catalysts. However, despite extensive research activities over the last decades, still very little is known about the mode of operation of selective oxidation reactions on an atomistic level.

A working catalyst requires an interplay of processes over multiple length- and timescales. With respect to time these range from elementary steps such as the breaking of bonds in the substrate and active site (~100 fs) to transport phenomena as well as solid-state transformations of the catalyst (up to years). Simultaneously, in the course of these processes lengthscales from subnanometers up to meters are covered.

An important aspect of the rational development of more efficient selective oxidation processes is the ability to control the catalyst structure and particle size on the nanometer scale, strongly linking research in heterogeneous catalysis with material science. Such nanostructured catalysts are naturally divided into supported and bulk systems. In general, supported catalysts consist of an oxide support such as Al2O3, SiO2 or TiO2 onto which either metal nanoparticles are deposited or metal-oxide aggregates are grafted forming monolayer-type systems. If reduced to a small number of atoms, such systems may be designed as "single-site" catalysts, which allow for molecular control of the active site and its surrounding environment. Besides, with recent progress in the development of nanostructured (mesoporous) materials, designed regular-pore systems are now available that can serve as support for the anchoring of active sites. The ultimate goal in rational catalyst synthesis is the preparation of catalysts on the basis of identified active-site structures. The synthesis of bulk systems can then be envisioned as assembly of these sites into nanostructured inorganic solids with high surface area, similar to the synthesis of polymers starting from basic building blocks. A special case of bulk systems are heteropoly compounds, which are built on nanoclusters of a central heteroatom caged by oxygen-linked MO6 octahedrons.

While methane and ethylbenzene can be considered as limiting cases of low and high reactivity, respectively, the C2–C4 substrates ethane, propane, propene, butane, isobutene and isobutene (see Table 1.1) due to their similar reactivity behavior in oxidative dehydrogenation and oxidative functionalization form a suitable platform for a discussion of general principles. The type of catalysts used for these reactions are in general vanadium and/or molybdenum containing bulk oxide materials including vanadium phosphorus oxides (VPO), heteropoly compounds (HPC) or mixed-metal oxides (MMO) such as MoV-TeNb oxide. Many supported systems also constitute efficient catalysts for the above processes. However, with the exception of titania supported vanadium oxide (commercially used for benzene/naphthalene to phthalic anhydride conversion) bulk systems give higher yields as compared to supported systems and are therefore the focus of industrial research. Nevertheless, due to their "simplicity" supported systems can give valuable insights into the operation of selective oxidation reactions, as will be shown in detail below.

There exist various reviews and books covering heterogeneously catalyzed selective oxidation reactions. However, the high level of empirical development of many of the above processes strongly contrasts our current level of scientific understanding. It is probably fair to say that the current development of selective oxidation catalysts is largely based on phenomenological concepts (among which the principle of site isolation and the principle of phase cooperation are fundamental) rather than a profound understanding of their mode of operation. To this end, the purpose of this book is to bring together the current state of knowledge on selective oxidation reactions and, by combination with previous findings, to develop a consistent picture of the working principle of selective oxidation catalysts.

Commercially important classes of selective oxidation reactions are the oxidative dehydrogenation of methanol and the epoxidation of ethylene. The epoxidation of propylene has the potential to be commercialized. For these reactions mainly catalysts based on coin metals (Cu, Ag, Au) are used. Ag is a particularly interesting material as it can serve as a catalyst for two completely different processes, methanol oxidation to formaldehyde and epoxidation of ethylene to ethylene oxide. For formaldehyde production besides silver, iron molybdate catalysts are used.

A detailed discussion of all aspects related to selective oxidation catalysts is outside the scope of this book. For example, an important aspect of selective oxidation reactions that has barely been addressed in the literature is the influence of steam on the catalyst structure and dispersion. In general, water is a product of selective oxidation reactions. In addition, water vapor is often added to the feed to improve the catalyst performance. It should be mentioned that under the conditions of operation hydrothermal reactions involving oxolation and olation processes may lead to polymerization/depolymerization of an initial Mx Oy condensate, which sets high standards for catalyst stability towards sintering. Another example is the role of carbon deposits on catalytic performance in selective oxidation reactions, which represents a largely unexplored research area.

CHAPTER 2

C–H Activation of Alkanes in Selective Oxidation Reactions on Solid Oxide Catalysts

JOHANNES A. LERCHER AND FREDERIK N. NARASCHEWSKI

Technische Universität München, Department of Chemistry and Catalysis Research Center, Lichtenbergstraße 4, 85747 Garching, Germany

2.1 Introduction

Activation of C–H bonds coupled to the functionalization of the carbon atom is one of the most important and challenging elementary reaction steps in organic synthesis. The challenge does not only lie in the homolytic or heterolytic cleavage of the bond itself, in most cases it has to occur under as mild as possible conditions to allow the subsequent reactions to proceed under very controlled circumstances. As a result, highly active catalysts are needed and the elementary steps in these reactions are dominated by single-electron processes and homolytic C–C bond cleavage, when oxygen is involved, while heterolytic C–H bond breaking is observed only in the minority of cases.

The C–H bond activation and conversion of alkanes in refining processes are somewhat more facile to realize, as the more robust target molecules allow for higher reaction temperatures. Two activation principles dominate, i.e., homolytic cleavage of C–H bonds on metals leading to elimination of hydrogen, a reaction, which is mostly equilibrated under reaction conditions and the acid-catalyzed addition of a proton to an alkane leading to a carbonium ion, which decomposes spontaneously to smaller fragments and a carbenium ion, as well as the abstraction of a hydride leading directly to the formation of a carbenium ion. Thus, the kinetically dominating C–H activation steps in refining are dominated by processes in which the C–H activation occurs via an ionic bond separation.

While it is hardly used for the synthesis of energy carriers, selective oxidation is one of the key reactions in chemical industry. Roughly estimated, the worth of chemicals produced by catalytic oxidation processes lies between $20 and $40 billion in 1991. Especially for the synthesis of intermediates and fine chemicals, the pressure to change the feedstock in the chemical industry over the last decade arising from a combination of the limited availability of conventional starting molecules and the pressure to shift to less-expensive ones forced the use of alkanes rather than alkenes in many of the selective oxidation routes to functionalized chemicals. This has led in turn to an intense interest in selective oxidation, but has only materialized in a few heterogeneously catalyzed reactions among which butane to maleic anhydride (see Chapter 6) or the oxidation of propane to acrylic acid (see Chapter 4) and the ammoxidation of propane to acrylonitrile (see Chapter 5) are the most prominent ones.

Understanding the complexity of the catalysts and the multistep multielectron processes during selective oxidation at an atomistic and molecular level poses a formidable challenge. More than with any other type of catalytic reaction, it requires that the catalysts are characterized chemically and structurally under realistic reaction conditions in order to be able to draw meaningful conclusions with respect to the surface chemistry (operando investigations). This is related to the fact that the sites active for catalysis are only present in small concentrations and that the catalysts change their oxidation state and surface structure in dependence of exogenic influences such as atmosphere, pressure or temperature. In addition, especially alkanes are rather inactive interacting mostly through dispersion forces with oxide surfaces.

The reason for the inertness of alkanes is related to the situation that for carbon and hydrogen the number of valence electrons is equal to the number of valence orbitals and only σ-bonds are present. Reactive modification of these energetically low-lying highest occupied molecular orbitals or energetically high-lying lowest unoccupied molecular orbitals requires very reactive moieties such as radicals or high temperatures. Additionally, the tetrahedral coordination of the carbons in sp3-hybridization efficiently shields the carbon atoms and complicates possible attacks together with the low polarity of the C–H bond. The primary products after activation contain functional groups or heteroatoms, which are by far more reactive and susceptible for further chemical reactions.

In consequence, the activation of the first C–H bond is the rate-determining step and after addition of the more electronegative substituent to the desired extent, the reaction needs to be kinetically stabilized to prevent total oxidation. Thus, achievable yields in these reactions are strictly depending on the ratio of the rate constants for activation to (unwanted) further oxidation (k1:k2). Maximum yields in a selective oxidation reaction for a model of two consecutive first-order reactions are shown as an example in Figure 2.1. To reach yields above 70% k1 needs to be at least one magnitude larger than k2, posing a major challenge for the design of catalysts and the process environment.

For this chapter we will limit ourselves to the discussion of the oxidative activation of the C–H bond in alkanes. Model experiments, kinetic analysis of complex reactions and theoretical studies will be combined to provide an overview on the current state of insight into the processes as well as their potential and limitations. We will first discuss processes on oxide clusters, followed by the influence of anions and cations on the activation mechanism and finally cover bifunctional catalysts used to achieve oxidative functionalization.

2.2 Models of C–H Bond Activation over Supported and Unsupported Oxide Clusters

We will first discuss the chemistry of small alkanes such as ethane, propane and butane, because of the easier C–H activation in these molecules, and will treat methane activation as a special case. This is done so, as with all these molecules the activated alkanes are able to eliminate hydrogen and may desorb, while this is not possible with methane. It should be emphasized that except for the higher bond strength of the C–H bond of primary carbon atoms, the other principal chemistry will be the same in the initial C–H bond breaking or polarizing step.

2.2.1 Vanadium-Based Clusters

C–H activation on catalysts containing vanadium-oxide species is certainly the most widely studied elementary reaction. The catalytic properties of the vanadium moieties are affiliated with the redox properties of vanadium that changes its oxidation states between + III, + IV and + V (see Chapter 15). The initial step of the C–H activation is so difficult to assess, because the precursor to the homolytic cleavage does not necessarily involve polarization of the C–H or the V–O bonds. Thus, only the final states of the first reaction would be accessible to the spectroscopic characterization.

Most of the experimental studies initially addressed were focused on supported monolayers of vanadia clusters on oxide surfaces (see Chapter 12). Pioneering systematic studies came from the groups of Wachs and Iglesia and Bell. The surface geometry of these clusters was determined with a multitude of indirect spectroscopic characterizations. Overall, it was suggested that monomers, dimers and oligomers are present on the surface (see Figure 2.2) and that the catalytic reactivity and the oxidation state depends critically on the size of the surface cluster. With increasing cluster size of the surface bound vanadia the reactivity towards alkanes increased. The same trend was observed with decreasing electronegativity of the support (according to Sanderson) indicating that a lower charge at the terminal oxygen facilitates C–H bond breaking in the activation of propane. It is interesting to note that the selective oxidation of methanol exhibits the reverse influence of the support oxide, i.e., the catalysts were more active as the charge at the lattice oxygen increased.

One of the central issues in these experimental studies concerned questions with respect to the nature of the active oxygen species or more specifically, the functional group involved in breaking the first C–H bond. In principle, three V–O bonds can be distinguished, the terminal V=O bond, the bridging V–O– V bond and the bridging V–O-support bond. For activation of ethane and propane the V–O–V bond was concluded to be catalytically irrelevant as the turnover frequency did not change with the coverage of the support by vanadia species, which should increase the relative concentration of this species. Also, the concentration of the V=O groups did not influence the observed rate of alkane activation leaving the V–Osupport bond to be the only remaining oxygen, which was then also concluded to be catalytically active.

However, as this rationale is based on an indirect elimination following more plausibility arguments than rigorous proof, let us turn to model studies using well-defined clusters in the gas phase combined with modeling of the structures and barriers between the intermediates. These clusters may be charged in order to increase their reactivity without changing the relative reactivity of the oxygen atoms in the cluster. The first well-defined case of such an approach was a study of the oxidation of propane on a [V3O7]+ cluster by the groups of Schwarz and Sauer. The cluster was chosen, as it is the smallest polynuclear entity that formally contains only V in +5 oxidation state. The oxidative dehydrogenation of propane involves the reduction of the metal center, brought about by the addition of two hydrogen atoms.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2011
• Language: English
• ISBN 10: 0854041869
• ISBN 13: 9780854041862
• Binding: Hardcover
• Number of pages: 452
• Editor: Christian Hess