Reaction Engineering, Catalyst Preparation, and Kinetics

Jorge Marchetti

• Publisher: CRC Press
• Year: 2021
• Print ISBN: 9781138605985
• eBook ISBN: 9780429882845
• Topic: Biological Sciences
• Subtopic: Biotechnology
• Index: Biological Sciences

This book serves as an introduction to the subject, giving readers the tools to solve real-world chemical reaction engineering problems. It features a section of fully solved examples as well as end of chapter problems. It includes coverage of catalyst characterization and its impact on kinetics and reactor modeling. Each chapter presents simple ideas and concepts which build towards more complex and realistic cases and situations.

• Introduces an in-depth kinetics analysis
• Features well developed sections on the major topics of catalysts, kinetics, reactor design, and modeling
• Includes a chapter that showcases a fully worked out example detailing a typical problem that is faced when performing laboratory work
• Offers end of chapter problems and a solutions manual for adopting professors

Aimed at advanced chemical engineering undergraduates and graduate students taking chemical reaction engineering courses as well as chemical engineering professionals, this textbook provides the knowledge to tackle real problems within the industry.

1 Catalysis Preparation and Characterization

DOI: 10.1201/9780429466847-1

1.1 Introduction: Basic Concept and Origin of Catalysis

The word catalysis is an important constituent of our daily life and contributes substantially to societal welfare. In today’s circumstances, approximately 80–90% of the world’s total chemical production processes use catalytic systems to accelerate various chemical reactions. A distinctive characteristic of a catalytic system is to change the rate of a chemical reaction, accelerate the formation of intermediate compounds, and generate desired products, while strongly controlling the selectivity and increasing the yield.

A catalyst is a substance that increases the rate of a chemical reaction without participating in it. The rate of a chemical reaction is mainly determined by the energetic barriers that need to be overcome to transform reactants to products. This energy, called activation energy, can be considerably decreased by using catalysts. A schematic representation can be seen in Figure 1.1.

Figure 1.1 Energy between reactants and products without catalyst (E₁) and with a catalyst (E₂).

An effective catalytic system provides high efficiency to convert raw materials into a single or multiple product through a series of elementary steps (or repeated cycles). It is regenerated to its original form at the end of each cycle during its lifetime. A catalyst only modifies reaction kinetics, and not the thermodynamics of a reaction.

The phrase catalytic process is believed to have been coined by Berzelius (1836). However, it is also claimed that some of the characteristics of catalysts were first presented by Fulhame in 1794, who demonstrated that the presence of water (H₂O) was required for the oxidation of carbon monoxide (CO) and that H₂O was unaffected by this chemical transformation. In 1812, Kirchhoff presented a similar interpretation where starch was hydrolyzed to sugars by using dilute acids. In 1817, Sir Humphry Davy stated that combustible gases explode in the presence of air after exposure to heated platinum (Pt), at temperatures below the ignition threshold. In 1834, Faraday proposed that reactants adsorb on the surface of the catalyst. In 1877, Lemoine explained that catalysts do not affect the position of reaction equilibrium, but only alter the rate at which a reaction will progress. Thus, the proposed definition of a catalyst is that it accelerates a chemical reaction without affecting the position of the equilibrium.

1.2 Cataloging Catalytic Material

Depending on the nature of the material, we can separate the different catalysts into homogeneous and heterogeneous types and heterogeneous can be categorized into supported and non-supported catalysts. In addition, we make a new distinction based on the size, introducing nanocatalysts. This section will discuss different types of catalysts while in Sections 1.3 and 1.4, their preparation and characterization techniques, respectively, will be provided.

1.2.1 Homogeneous Catalysis

A homogeneous catalyst is a catalytic material that is in the same phase as the reactants and products. For instance, for the transformation of vegetable oil with sodium methoxide to produce biodiesel, all chemicals are in liquid phase. Homogeneous catalysts can also be categorized as acidic and basic, as well as organic and inorganic. Enzymatic homogeneous catalysts have been associated here but it has been room for some debate, so they are placed into a category of their own. However, as long as a catalyst is not supported, i.e., it is in liquid form, with the reaction also in liquid phase, it fits within this definition and category as well.

The advantages of homogeneous catalyst are high selectivity due to a large and defined number of active sites, and the mild reaction conditions required, in terms of temperature and time. However, by being in the same phase as the reactants, the separation of the catalyst from the rest of the reaction medium is regularly a very complicated and time consuming task, leading to a very expensive process which is less viable from an industrial perspective. Another drawback of these catalysts is that their thermal stability is lower than that of heterogeneous catalysts.

1.2.2 Heterogeneous Catalysis

Heterogeneous catalyst implies that the reactants and the catalytic material are not in the same phase, like liquid–solid reactions or gas–solid systems. Heterogeneous catalysts are used in over 80% of the industrial chemical processes due to several benefits in comparison with homogeneous alternatives. Even though they could be less selective than homogeneous alternatives and the operational conditions are usually harsher, due to the heterogeneity of the materials, they are easily separated from the reaction medium (typically, a simple filtration is enough). Furthermore, they can be produced with different techniques and have different properties such as porosity, porous size, porous distribution, and surface area, allowing the catalysts to be tailor-made depending on the application and the reactants involved. As these materials are custom-made, their reproducibility is high, and their performance is quite stable. The major drawback of heterogeneous catalysts is that they may suffer from deactivation, i.e., that after some time, the catalyst will no longer perform desirably.

A chemical reaction catalyzed by heterogeneous catalytic materials generally takes place at the interface between two phases. This demands in-depth investigation of the reaction pathway to understand the actual behavior of the material from a scientific research perspective. Figure 1.2 shows a schematic representation of a gas reaction over a catalytic surface and shows the three steps of adsorption of reactants, surface reaction, and desorption of the product.

Figure 1.2 Scheme of a gas–solid reaction.

The heart of heterogeneous catalyst lies in the surface-active sites. It is highly recommended to investigate the physicochemical properties of heterogeneous catalytic systems before its optimization for a particular catalytic process. In other words, determining an optimal number of active sites per reactor volume is very important from a reaction stoichiometry and process economics viewpoint.

In the context of development of heterogeneous catalysts, the characterization of materials is highly essential as it provides insights into the relationship between the activity of the catalyst and physicochemical properties of the material. If the structure and composition of catalysts can be correlated with its activity and selectivity, the performance of the catalyst can be understood, thus, improving reproducibility. Moreover, the field of heterogeneous catalysis is wide and highly interdisciplinary in nature, which demands the cooperation between chemists, physicists, surface scientists, material scientists, reaction engineers, and theorists and experimentalists. Additionally, based on the industry, like food, pharmaceuticals, automobiles, petrochemical, and biochemical industries, other experts are involved. With an approximation of around 80–90% of all the modern-day chemical processes using heterogeneous catalysts, consistent research efforts are under scientific attention for exploring different materials and upgrading their technical performance. The type of characterization performed on solid materials is dependent on the preparation methods and the estimated physicochemical properties. According to the International Union of Pure and Applied Chemistry (IUPAC), materials are categorized according to their average pore diameters as microporous (less than 2 nm), mesoporous (2–50 nm), and macroporous (more than 50 nm). The following sections provide a few examples of different types of solid materials that are used as catalysts in a variety of chemical reactions.

1.2.2.1 Nanoscale Catalytic Materials

Nanoscale catalytic materials are catalysts with particles size between 1 and 100 nm, which are seen as the future of the catalysis world because they exhibit excellent electronic, physical, mechanical, thermal, and catalytic properties. Nanomaterials can possess different structural shapes such as rods, tubes, fibers, wires, cubes, colloids, and quantum dots. Nanoscale catalysts are a subarea of the heterogeneous catalysts and are discussed in a subsection since they are used and produced differently. The main interest when developing nanoscale catalysts is to augment the number of active catalytic sites by specifically designing the structure and downsizing the catalyst particle size, based on the premise that heterogeneous materials assisted catalytic reactions are governed by the adsorption–reaction–desorption pathway. Significant success has been achieved in controlling the structural features of solid materials, even at the atomic level. The development of nanocatalysts has also been important due to the need to increase the specific surface area by minimizing its size. This, in return, has a positive impact on deciding the rate of ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Contents
• Preface
• Acknowledgements
• Author
• 1 Catalysis Preparation and Characterization
• 2 Reactor Design Mole and Energy Balance
• 3 Reaction Kinetics
• 4 Completely Solved Example
• Index