Heterogeneous Catalysis in Organic Transformations

Varun Rawat, Anirban Das, Chandra Mohan Srivastava

• Publisher: CRC Press
• Year: 2022
• Print ISBN: 9780367647872
• eBook ISBN: 9781000555530
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

As the broad challenges around energy and the environment have become the focus of much research, scientists and experts have dedicated their efforts to developing more active and selective catalytic systems for key chemical transformations. For many decades environmentally viable protocols for the synthesis of fine chemicals have been the crux of academic and industrial research. Heterogeneous Catalysis in Organic Transformations serves as an overview of this work, providing a complete description of role of heterogeneous catalysis in organic transformations and offering a review of the current and near future technologies and applications.

• Discusses the fundamentals of catalysis and compares the advantages and disadvantages of different types of catalyst systems
• Examines oxide nanoparticles and noble metal nanoparticles
• Consider organometallic compounds, solid-supported catalysts, and mesoporous materials
• Describes recent advances in metal-based heterogeneous catalysts and new reactions with possible mechanistic pathways

Providing a comprehensive review of heterogeneous catalysis from the basics through recent advances, this book will be of keen interest to undergraduates, graduates, and researchers in chemistry, chemical engineering, and associated fields.

1 Introduction to Heterogeneous Catalysis in Organic Transformation

Garima Sachdeva, Gyandshwar Kumar Rao, and Varun Rawat

Amity University Haryana, Gurugram (Haryana), India

Ved Prakash Verma and Kaur Navjeet

Banasthali Vidyapith, Newai (Rajasthan), India

DOI: 10.1201/9781003126270-1

Contents

1. 1.1 Introduction
2. 1.2 Types of Catalysts
   • 1.2.1 Homogeneous Catalyst
   • 1.2.2 Heterogeneous Catalyst
   • 1.2.3 Heterogenized Homogeneous Catalysts
   • 1.2.4 Biocatalysts
3. 1.3 Origin of Heterogeneous Catalysis
4. 1.4 Comparison between Homogeneous and Heterogeneous Catalysis
5. 1.5 Contribution of Heterogeneous Catalysis
6. 1.6 Mechanism of Heterogeneous Catalysis
   • 1.6.1 Langmuir–Hinshelwood Mechanism
   • 1.6.2 Eley–Rideal Mechanism
   • 1.6.3 Harris–Kasemo Mechanism
7. 1.7 Categorization of Heterogeneous Catalysts
   • 1.7.1 Catalysts Having Basic Sites
   • 1.7.2 Catalysts Having Acidic Sites
8. 1.8 Characterization Techniques Needed for Heterogeneous Catalysts
   • 1.8.1 X-Ray Diffraction
   • 1.8.2 X-Ray Absorption Spectroscopy
   • 1.8.3 Electron Microscopy
   • 1.8.4 Nuclear Magnetic Resonance
   • 1.8.5 Electron Spin Resonance Microscopy
9. 1.9 Significance of Heterogeneous Catalysis
10. References

1.1 Introduction

The concept of the term “Catalysis” was first described by Berzelius in 1835 [1] and later defined scientifically by Ostwald in 1894 [2]. However, catalysts have been used for thousands of years and the oldest example that we still encounter today is the fermentation process which the Egyptians first discovered to produce wine [3]. Catalysis has been the backbone of industrial applications and is used extensively in manufacturing agro- and petrochemicals, cosmetics, pharmaceuticals and medicines, polymers, aliments, and many more [4]. By definition, a catalyst is a substance that can increase a chemical reaction’s speed without undergoing any change itself. Figure 1.1 gives a very simplistic view of the energetics involved in a catalyzed reaction. A catalyst can change the reaction pathways that lower its activation energy and increase the rate of reaction. However, a catalyst does not alter the equilibrium of a reaction, which means that the product formation is achieved at a faster rate, whereas the yield of the reaction remains unaffected. A catalyst undergoes a reversible chemical change, and it regenerates its previous form at the end of a chemical cycle.

Figure 1.1 Energy profile diagram of a reaction with and without catalyst.

In many organic transformations without catalysts, either the reactions do not occur or they proceed very slowly. For example, the addition of water to an alkene proceeds to completion if an acid is added in a catalytic amount, leading to the formation of an alcohol. Without the catalyst, the reaction does not proceed at all. This reaction involves breaking a pi-bond in the alkene and an O–H bond in water and the formation of C–H and C–OH bonds. By the addition of an acid, the concentration of hydronium ions (H₃O⁺) will increase in the solution, which acts as a catalyst for the reaction. The H₃O⁺ ions then react with the alkene molecule to form an alkyl cation according to Markovnikov’s rule (Figure 1.2). The alkyl cation has a strong tendency for water molecules, and even though water molecules are weak nucleophiles, they rapidly attack the alkyl cation forming alcohol and leaving protons to regenerate H₃O⁺ molecules. Without an acid catalyst, the neutral water molecule would not be electrophilic enough for the pi-bond to attack it, and thus, the reaction would not proceed [5].

Figure 1.2 Hydration of alkenes in the presence of an acid.

1.2 Types of Catalysts

In general, on the basis of their phase in a reaction, catalysts are divided into two broad categories – homogeneous and heterogeneous. However, the other classifications also include heterogenized homogeneous catalysts and biocatalysts (Figure 1.3). Their brief accounts are summarized as follows:

Figure 1.3 Classification, properties, and usage of catalysts.

1.2.1 Homogeneous Catalyst

A homogeneous catalyst has the same phase in the reaction mixture as that of reactants. The high homogeneity of the same phase of reactants and catalyst results in their high interactions, leading to high reactivity and selectivity under mild reaction conditions. Some important instances of homogeneous catalysts include Brønsted and Lewis acids, transition metal complexes, organometallic complexes, and organocatalysts. Some notable examples of chemical reactions that use homogeneous catalysts are carbonylation, oxidation, hydrocyanation, metathesis, hydrogenation, and C–H and C–C activation/functionalization [6].

1.2.2 Heterogeneous Catalyst

As the name suggests, the catalysts exist in a different phase than reactants in the reaction mixture [7]. For example, heterogeneous catalysts are used in the Haber–Bosch process for the ammonia synthesis (iron as a catalyst) and Fischer–Tropsch process to produce hydrocarbons (transition metal catalysts used such as iron, cobalt, ruthenium, and nickel). The recovery, reusability, and easy separation of products from the reaction mixture make heterogeneous catalysts the first choice for industrial use. These catalysts (primarily metals) can be used as fine particles, powders, granules, deposited on the solid surface (supported catalysts), or used in bulk form.

1.2.3 Heterogenized Homogeneous Catalysts

The complex nature of heterogeneous catalysts prevents their analysis and characterization at a molecular level, making their development difficult through structure–reactivity relationships. Additionally, the traditional heterogeneous catalysts are less reactive and show poor selectivity in a chemical reaction. In order to overcome these issues, homogeneous catalysts are often embedded on solid surfaces to add heterogeneity in their nature. This approach brings features of both homogeneous (selectivity and reactivity) and heterogeneous (reproducibility) catalysts together in one catalyst, which greatly enhance the outcome of a reaction. Heterogeneity can be obtained by immobilizing catalysts on the solid surface via surface processes like physisorption or chemisorption [8].

1.2.4 Biocatalysts

The reactions in our biological systems are carried out by natural biocatalysts, which are primarily enzymes. Enzymes are natural proteins and can be used to catalyze very specific chemical reactions in a laboratory. They are isolated from animal or plant tissues and microbes such as yeast, bacteria, or fungi. Biocatalysts are notable for their great selectivity and efficiency, as well as for their environmental friendliness and gentle reaction conditions. Nowadays, biocatalysts are an alternative to conventional industrial catalysts. Immobilizing these enzymes on solid supports turns enzymes into heterogeneous solid catalysts, enhancing their activity and stability. It also increases their lifetime, and they can be recycled for many usages [9]. The last two categories of catalysts classification are used arbitrarily and can be merged into other types depending upon the usage and nature of the catalyst.

1.3 Origin of Heterogeneous Catalysis

The evolution of heterogeneous catalysts took place by hit and trial methods. For instance, the progress in ammonia synthesis in the early 1900s demonstrated how empirical screening could yield an excellent catalytic process [10]. Although the worldwide ammonia production was enhanced with this discovery, its mechanism was unknown for an extended period until Gerhard Ertt discovered and improved the process by introducing promoters [11]. Moreover, his work also concentrated on many other heterogeneous catalytic procedures like catalytic oxidation of carbon monoxide (CO) over platinum. For his seminal work on examining chemical processes on a solid surface, he was awarded the Nobel Prize in chemistry in 2007. The complexity of a catalyst can range from well-defined supported metal nanoparticles to millimeter-sized multicomponent catalyst bodies with various often distinct activities. Heterogeneous catalysts are still extensively utilized in the production of bulk chemicals, but now they are also used for the selective synthesis of intermediates and fine chemicals. This has been made feasible because of significant advances in catalyst designing at the molecular level, like combining the benefits of nanostructured solids and grafted organometallic complexes.

1.4 Comparison between Homogeneous and Heterogeneous Catalysis

It is well known that depending on the chemical phase of the catalyst and reactants, different forms of catalysis can be distinguished. In homogeneous catalysis, the catalysts work in the same phase as the reactants and are often found in the liquid phase. It has been seen that mechanistic studies of homogeneous catalysts are comparatively easier than he...

Table of contents

• Cover Page
• Half Title Page
• Series Page
• Title Page
• Copyright Page
• Contents
• Preface
• Editors
• Contributors
• 1 Introduction to Heterogeneous Catalysis in Organic Transformation
• 2 Oxide Nanoparticles in Heterogeneous Catalysis
• 3 Noble Metal Nanoparticles in Organic Catalysis
• 4 Organometallic Compounds as Heterogeneous Catalysts
• 5 Solid-supported Catalyst in Heterogeneous Catalysis
• 6 Mesoporous Materials in Heterogeneous Catalysis
• 7 Role of Metal-heterogeneous Catalysts in Organic Synthesis
• Index