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Catalysis and Catalytic Processes
CHAPTER 1
Catalysis and Catalytic Processes V.V. Ranade, S.S. Joshi CSIR-National Chemical Laboratory, Pune, India
1.1 Introduction Chemical and allied industries manufacture products that are essential for creating and sustaining modern societies. The chemical (and biological) transformations necessary to make these essential products often involve the use of catalysts. The catalyst (which can be either homogeneous or heterogeneous) provides a reduced activation energy barrier to transformations and facilitates better control on selectivity. The development and selection of the right catalyst, therefore, can make a substantial impact on process viability and economics. Besides the right catalyst, it is essential to develop the right reactor type and process intensification strategies for effective translation of the laboratory process to practice. With strict environmental regulations, rising raw material prices, depleting feedstocks, and a call for green chemistry as driving forces, the chemical industry faces a larger challenge with both opportunities and risks. Catalysis is of paramount importance in the chemical industry due to its direct involvement in the production of 80% of industrially important chemicals. Catalysts are involved in more than $10 trillion in goods and services of the global gross domestic product (GDP) annually. It is estimated that the global demand on catalysts is more than $30 billion, and a very robust growth is projected in the future. There is an urgent need to develop cost-effective and environmentally benign methods of converting natural resources into fine and specialty chemicals using highly efficient catalysts and employing cleaner methodologies. The advancements in catalysis and applications to the chemical industry are very significant and are responsible for cleaner processes. Replacement of the stoichiometric reactions by catalytic reactions and application of new catalyst systems and technologies to make the processes environmentally friendly, energy efficient, and globally competitive are current needs. A catalyst is a substance that provides an alternative route of reaction where the activation energy is lowered. Catalysts don’t affect the chemical equilibrium associated with a reaction; they merely change the rates of reactions. Catalysts are classified in a variety of different ways. The commonly used classification by reaction engineers is based on number of phases, such as Industrial Catalytic Processes for Fine and Specialty Chemicals. http://dx.doi.org/10.1016/B978-0-12-801457-8.00001-X # 2016 Elsevier Inc. All rights reserved.
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• •
homogenous catalysis (catalyst and substrate in same phase) or heterogeneous catalysis (solid catalyst and substrate is a gas and/or liquid)
Basic concepts of catalysis are briefly introduced in the following section. It is important to combine the understanding of catalysis with key reaction engineering expertise to translate the potential of a catalyst in the form of a practically implemented catalytic process or plant. Any catalytic reactor has to carry out several functions like bringing reactants into intimate contact with the active sites on a catalyst (to allow chemical reactions to occur), providing an appropriate environment (temperature and concentration fields) for adequate time, and allowing for removal of products. A reactor engineer has to ensure that the evolved reactor hardware and operating protocol satisfy various process demands without compromising safety, the environment, and economics. Naturally, successful reactor engineering requires bringing together better chemistry [thermodynamics, catalysis (replace reagent-based processes), improved solvents (supercritical media, ionic liquids), improved atom efficiency, waste prevention — leave no waste to treat] and better engineering (fluid dynamics, mixing and heat and mass transfer, new ways of process intensification, computational models, and real-time process monitoring and control). Some of these aspects are briefly discussed in Section 1.3. Organization of this book is outlined in the last section of the chapter.
1.2 Catalysts and Catalytic Reactions The word catalyst was first used in 1835. Over the years, it has been established that a catalyst influences kinetics of a process without undergoing any change itself. A catalyst does not alter the thermodynamics of a reaction. In simple words, a catalyst alters the route without altering the destination (see Fig. 1.1). Here the route is the metaphor for activation energy — minimum energy input for a chemical system to undergo a chemical reaction and the transition state of a chemical reaction. Reaction without catalyst
Energy
Reaction with catalyst
Ea (Y → X)
Ea (X → Y)
Y ΔH
X
Reaction path
Fig. 1.1 A catalyst allows reaction to proceed through an alternative path.
Catalysis and Catalytic Processes 3 The terms that are often used in the context of catalytic activity are turnover number (TON), to define the productivity of a catalyst, and turnover frequency (TOF), to define the catalyst activity or TON per unit time. The TOF is defined in terms of active catalytic centers, such as TOF ¼
volumetric rate of reaction moles volume ¼ ¼ time1 number of centers=volume volume time moles
TOF may be in a range of 102 to 102 for industrial applications. The TON is defined as a measure of capacity of the catalyst for accelerating the reaction such as TON ¼ TOF Lifetime of a catalyst Typically TON is in the range of 106 to 107 for industrial applications. The role of a catalyst becomes even more important when multiple reactions are thermodynamically feasible. In such cases, an appropriate catalyst manipulates the reaction rates in such a way that selectivity toward a desired product increases. Several factors and parameters influence the overall performance of a catalyst. The selection of a catalyst for an industrial process therefore depends on the role it is supposed to play. The effect of a catalyst on kinetics of the reaction needs to be understood in detail to get an insight about the surface chemistry involved that would help in the design of a specific catalysis. It is therefore important to understand the elementary steps through which a catalyst influences overall performance. Homogeneous catalysts typically form a complex with one of the reactants, which eventually transforms it into the product after interacting with other reactants. The process is essentially similar to homogeneous reactions in the absence of a catalyst and is often controlled by mixing reactants and a catalyst species on a molecular level. In contrast to this, in a heterogeneous catalyst, several additional steps are involved along with reaction occurring on the catalyst surface, such as • • • • • • •
external diffusion toward a catalyst pellet internal diffusion toward a catalyst surface molecular adsorption on a catalyst surface surface reaction desorption from a catalyst surface internal diffusion away from the catalyst surface external diffusion away from the catalyst pellet
These steps need to be understood to select an appropriate reactor and operating strategy. This procedure will be discussed later in this book. Heterogeneous catalysis allows easy separation and reuse of a catalyst. An example of heterogeneous catalysis is Haber’s process, where iron powder is used as a catalyst to enable the
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Chapter 1
conversion of nitrogen and hydrogen gas to ammonia. A heterogeneous catalyst has “active sites,” which are the centers of reaction. Once adsorbed (either physically or chemically), the substrate undergoes a reaction, and the product is desorbed subsequently, rendering the surface of the catalyst free for further activity. Homogeneous catalysis has the inherent disadvantage of lack of ease of separation of product and catalyst after the reaction. Esterification of acetic acid with methanol to give methyl acetate in the presence of an H+ ion is a very common example of homogeneous catalysis. Significant work is also being actively pursued to develop a “heterogenized” homogeneous catalyst that uses solids as supports for anchoring of the homogeneous catalysts. This would make it technically heterogeneous, but it would retain the characteristic reactivity pattern of a homogeneous catalyst. One example of such heterogenized homogeneous catalyst is silica-supported sulfuric acid. Besides the classical catalysts mentioned earlier, several other catalytic processes have been developed, which include biocatalysis (using enzymes), photocatalysis (acceleration of photoreaction using a catalyst), and electrocatalysis (acceleration of electrochemical (half) reactions). Enzymes are being increasingly used as catalysts for a variety of chemical transformations, including conversion of organic wastes to useful chemicals. Significant efforts are being made to develop the next generation of electrocatalysts for fuel cell applications or for converting carbon dioxide into a variety of useful chemicals. Without getting into the details of catalysts and catalytic processes, it will be useful to discuss the key properties of a catalyst here.
1.2.1 Characteristics of Catalysts Catalytic substances have a tendency to form complexes. A large number of substances that have been observed to show catalytic properties are from the VIII Group and IB Group of the periodic table (which have unpaired d electrons). Another interesting property of catalytic materials is the small energy differences between valence shells, which lead to a number of oxidation states. The characteristics of a catalyst may be defined by activity, selectivity, stability, and accessibility. All four terms refer to the favorability of a catalyst to form a product. Activity is generally found to increase with temperature. All the other three are trade-offs with activity and depend on a specific reaction. Ideally a catalyst should undergo the same catalytic cycle multiple times without a reduction in its ability to influence the reaction. The number of times a catalyst converts a substrate to product is measured in terms of TON. Selectivity of the catalyst is characterized in following different ways: •
•
Chemo selectivity is when a catalyst favors reaction with one substrate in a reaction mixture over another. For example, an oxidizing agent may favor the oxidation of an aldehyde group over a hydroxyl group present on the same moiety. Regioselectivity is when a catalyst favors the synthesis of a product based on the position it acquires in the substrate. For example, a formyl group can be attached to either the primary,
Catalysis and Catalytic Processes 5
• •
terminal carbon atom or the secondary, internal carbon atom, leading, respectively, to the linear and the branched product in hydroformylation. Diastereoselectivity is a phenomena wherein a catalyst may direct a substrate selectively favoring formation of one stereomer over another if a substrate has stereogenic centers. Enantioselectivity refers to the catalyst favoring synthesis of one enantiomer of product over another, even when the substrate itself is achiral.
Morphology and material strength are important characteristics for heterogeneous catalysts. They are manufactured in a variety of morphology like pellets, trilobes, and extrudates. The overall pressure drop and effectiveness of the catalyst depends on size and shape of the heterogeneous catalyst. Resistance to crushing, attrition, and abrasion are also important characteristics that need to be understood in case of solid or solid supported catalysts. Thermal characteristics and the range of temperature for which the activity would be the highest without compromising on the selectivity is an important factor in the selection of a catalyst for the desired process. The catalyst characteristics need to be appropriately accounted for (including activation as well as deactivation of catalyst) while designing a suitable reactor for carrying out catalytic reactions in practice. Before discussing some aspects of practical reaction and reactor engineering, key aspects of homogeneous and heterogeneous catalysts are briefly discussed in the following sections.
1.2.2 Homogeneous Catalysts Many industrial processes have been developed using homogeneous catalysts. It is being employed in oxidation, carbonylation, hydroformylation, oligomerization, polymerization, hydrocyanation, and synthesis of fine chemicals, among other processes (Hagen [23]). Some other homogenously catalyzed reactions include ester hydrolysis, Diels-Alder reaction, Cannizzaro reaction, and enzymatic processes. As mentioned earlier, catalytic processes that occur in the same phase as the reaction medium are termed homogeneous catalytic processes. The applicability of a homogeneous reaction mixture has been known for several centuries, such as for fermentation process. Charles Bernard Desormes and Nicolas Clement were arguably the first researchers to make an attempt to postulate a rational theory for catalysis or the intermediate compound theory [1] to explain the homogeneous catalytic effect of nitrogen oxides for the manufacture of sulfuric acid using the lead chamber process. These catalysts may be metal complexes or common reagents such as mineral acids, and they can be uniformly distributed in the bulk reaction mixture. Because in a homogenous catalyst system each molecule of the catalyst is distributed in the reaction mixture, it would mean more active sites are available to interact with the substrate. Hence, these reactions proceed at milder conditions and lower catalyst concentration. Another advantage of homogeneous catalysts is the ease of understanding the catalytic chemistry because the mechanism of the reaction is only dependent on the kinetics and not on the diffusion rates.
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Homogeneous catalysts used in industrial chemistry are generally from organometallic compounds (compounds with a metal-carbon bond). The central metal atom is bound to organic and inorganic ligands. The catalyst environment can be easily modified to alter the catalytic properties by manipulating ligands. Transition metals play a major role in the development of these organometallic complexes. This is because of the availability of d-orbitals of transition metals, which allow ligands to bond in such a way that they are available for further reaction. Rhodium phosphine-based metal complexes such as [RhCl(PPh3)3] have been found to be an effective catalyst for the hydrogenation of olefins. On account of the stability of transition metal complexes, the process temperatures are generally limited to 200°C, and this becomes a limitation of homogeneous catalysis. Because the catalyst is completely dispersed in the reaction media, these systems face difficulties in separation or recovery of catalysts. Significant efforts have been and are being spent on deciphering mechanisms of homogeneous catalysis to facilitate further development of new catalyst systems. Tolman [2] proposed a mechanism with which a reaction is catalyzed by homogenous organometallic complexes, which was referred to as the 16 or 18 electron rule (see Fig. 1.2). It postulates the role of the oxidation state and coordination number of the metal center of the transition metal complex. The organometallic complexes referred to are the transition metal complexes with CO, N2, CN, RNC, PR3, π-aryl, π-allyl, –SiR3, and π-acyl ligands, which have high ligand field strength and covalent bonding. The two major postulates of the rule are as follows [2]: • •
Diamagnetic organometallic complexes of transition metals exist in any measurable quantity only if the valence shell of central metal contains 16 or 18 electrons. The intermediates that are formed during the course of the reaction should also contain 16 or 18 valence shell electrons. Saturated 18e complex
Product Unsaturated 16e complex Saturated 18e complex
Substrate
Saturated π complex, 18e Unsaturated 16e complexes
Fig. 1.2 Cycle explaining the 16/18 electron rule.
Catalysis and Catalytic Processes 7 To understand the catalytic cycle in homogeneous catalysis, a stoichiometric reaction with well-defined transition metal complexes can be used to elucidate the steps involved. Labeled compounds can also be used to validate the postulated reaction mechanism by employing spectroscopic identification techniques. Various in situ spectroscopy techniques such as infrared spectroscopy (IR), nuclear magnetic resonance (NMR), electron spin resonance (ESR), and Raman are very helpful in developing a better understanding of homogeneous catalysis. It has been observed that Infrared spectroscopy has been very useful in studying carbonyl complexes.
1.2.3 Heterogeneous Catalysts The use of heterogeneous catalysts in the chemical industry began in the early 1800s with Faraday being among the pioneers of heterogeneous catalysis and discovering the use of platinum for oxidation. These systems were in use during the Second World War for reactions such as dehydrogenation of methyl cyclohexane to form toluene in the presence of Pt-Al2O3 or in alkane isomerization using Cr2O3-Al2O3. After the war, with diversification in chemicals synthesized and advancement of technology, heterogeneous catalysts were used for the hydrocracking of high-boiling petroleum using Ni-aluminosilicate to form fuels. This revolutionized the automobile industry. Another application of solid catalysts was in the synthesis of polyethylene from ethylene by polymerization in the presence of Ziegler-Natta (TiCl4-Al(C2H5)3) catalysts. Heterogeneous catalysts are used for innumerable reactions such as oxidation, nitration, coupling, condensation, and hydrogenation. Heterogeneous catalysis facilitates a large number of chemical reactions. The use of heterogeneous catalysts in fine chemicals is gaining importance because of the following reasons [3]: •
•
•
•
Because the catalyst is not in the same phase as the reacting molecules, it allows for a higher possibility of catalyst recovery and recyclability. Chemical bonds are formed with the catalyst either through physisorption or chemisorption during the reaction and broken thereafter to regenerate the catalyst, albeit with loss of activity in some cases [4]. Solid acid catalysts are easier to handle in comparison with conventional mineral acids such as H2SO4 and hydrofluoric acid (HF). They reduce capital cost and also ensure material safety because they have less corrosivity. Heterogeneous catalysts for bulk chemicals have been used since the beginning of chemical industries, hence the processes and their roles in the mechanism of the organic synthesis are well understood in most cases. Therefore they can be downscaled for their applications in fine chemicals to some extent. Myriad catalysts with acidic or basic properties exist or have been designed to synthesize particular species, which ensures product maximization. Mixed metal oxide, clays, zeolites, silica, alumina, zirconia, and heteropolyacids are a few classes of catalysts
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•
used predominantly. They can be modified to a large extent through impregnation of homogeneous catalysts or metals and structural changes. Microporous and mesoporous structures or sieves and honeycomb-like structures allow heterogeneous catalysts to be highly shape and stereo selective. These designs give enzyme-like efficiency to the catalyst.
A new stage of development in heterogeneous catalysts came with the objective of using renewable feedstocks and environmentally benign processes and techniques for downstream waste reduction. Catalysts that have high efficiency and better surface properties are being developed for process intensification [5]. A catalyst facilitates reaction through the formation of complexes with reacting species. The product formed doesn’t have the tendency to bond with the catalyst, which implies the catalyst surface is regenerated. However, this is only partially correct. The surface and structure of the catalyst are modified with each reaction. For instance, in the case of a pure metal catalyst, surface roughness and crystallinity change, whereas in the case of metal oxides there is a change in the ratio of metal and oxygen. Commercial catalysts are generally available in various physical forms such as powder, pellets, granules, and extrudates. Pore size plays a major role in structure and therefore in catalytic performance (conversion, selectivity, yield, TOF, and TON). Porous catalysts offer a large surface area, the ability to support varied chemical functionalities, and the ability to form different networks according to the applications. Broadly catalysts are classified into three kinds of porous materials: •
•
•
Microporous: Pore diameter is less than 2 nm. A typical example of a microporous catalyst is zeolite. It has a crystalline and well-defined structure. It has a silicon, aluminum, and oxygen framework, and water or another cation may be present in the pores. Activated carbon is also microporous adsorbent and has varying origins, thermal resistance, and porosity, depending on the method of synthesis. Mesoporous: Pore diameter is between 2 and 50 nm. Mesoporous solids are synthesized through a templating approach, wherein surfactants are used for directing the structure. Subsequently, the surfactant is removed, and a mesoporous system is obtained that replicates the surfactant assembly [6]. Macroporous: Pore diameter is greater than 50 nm. Macroporous material can be synthesized by a sol-gel method such as porous silica, alumina, and zirconia gels. In the case of zirconia gels, a metal salt precursor is used for the epoxide mediated sol-gel method followed by phase separation. Morphology of the catalyst would be governed by temperature and amount of solvents or reactants used [7].
Activity is the rate at which a reaction proceeds in presence of a catalyst. The activity of the heterogeneous catalyst depends on the reaction conditions of temperature, pressure, and catalyst loading with respect to reactants and on reactor conditions such as flow rate and surface area of reactor. Another characteristic of a catalyst is selectivity, which is the extent
Catalysis and Catalytic Processes 9 to which a catalyst promotes synthesis of the desired product over all the possible products, including those with lower free energy. (Note: in a reaction without the selective catalyst, only products with lowest free energy would have been formed.) Selectivity is dependent on time, temperature, and other reaction parameters. Among the most commonly used metals for heterogeneous catalysts are transition metals like platinum, rhodium, nickel, ruthenium, palladium, cobalt, magnesium, vanadium, and iron, among others [8]. Another important aspect of the catalyst is possible catalyst deactivation. The most typical causes of deactivation of heterogeneous catalyst are the following: • • • •
Aging/thermal degradation: deactivation resulting from changes in structure. Sintering: an increase in the average size of the crystallites due to coalescence of small particles on continued usage of catalyst. Fouling/coking: deposition of high-molecular-weight “carbon-hydrogen” compounds or primary carbons on the catalyst surface. Poisoning: inhibitory substances bind strongly to the active catalytic sites on the surface.
Catalytically active complex in homogeneous catalysis may similarly be deactivated due to structural changes in the active complex as well as poisoning because of binding with inhibitory substances. The reasons for possible deactivation need to be understood to develop appropriate regeneration strategies. To prevent deactivation, “promoters” may be added such as in the case of ammonia synthesis, where aluminum oxide is added along with iron to prevent fusion of the particles. Catalyst characterization plays an important role in understanding and improving overall performance of catalytic processes. A substrate may be either physically adsorbed (physisorption) or chemically adsorbed (chemisorption) on the surface of a heterogeneous catalyst. The difference between the two phenomena needs to be explicitly understood. Although there are no bonds formed in physisorption, only weak van der Waals forces are responsible for keeping the substrate on the catalyst surface. In chemisorption, there is electron transfer and formation of strong bonds between catalyst and substrate. This renders chemisorption to be a more selective process and leads to the formation of a single layer on the surface unlike in physisorption, where multiple layers may be formed with each adsorbed molecule of the first layer acting as a site for the next [9]. The commonly available techniques of characterization of catalysts with most researchers are TEM (transmission electron microscopy), XRD (X-ray diffraction), and EXAFS (extended X-ray absorption fine structure). More details of characterization are discussed later in the book. With the characterization and properties of the catalyst known, specific catalysts may be designed for the required process. It is important to remember that test conditions are not the same as the reaction conditions, thus there might be some variation in the properties of the catalysts. Besides this,
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several other factors related to reaction and reactor engineering need to be taken into account while translating the catalytic process into practice. These aspects are briefly outlined in the following section (and are discussed later in Chapter 7).
1.3 Reaction and Reactor Engineering Reaction and reactor engineering involves establishing a relationship between reactor hardware and operating protocols with various performance issues as listed in Table 1.1. Table 1.1 Reaction and reactor engineering Reactor Performance
Hardware and Operating Protocol
Conversion and selectivity
Reactor configuration: size and shape, feed and exit nozzles Mode of operation: batch, semibatch, continuous Start-up and shutdown protocols Operating conditions: flow rate, pressure, temperature, flow regimen, RTD Reactor internals: baffles, heat transfer coils, distributors
Product quality Catalyst activity and life Stability and operability Safety Environmental impact
A process engineer is faced with a host of questions while establishing a relationship between reactor hardware, operating protocol, and reactor performance. In this section, some of these questions and the relevant tasks of a reactor engineer are discussed briefly. The major questions being faced by a reactor engineer can be grouped into three classes: • • •
What chemical transformations are expected to occur? How fast will these changes occur? What is the best way to carry out these transformations?
The first question is in the realm of thermodynamics and chemistry. Knowledge of chemistry and reaction mechanism is helpful to identify various possible chemical reactions. Thermodynamics provides models and tools to estimate free energies and the heat of formations of chemical compounds from which the energetics of all the possible chemical reactions can be examined. These tools help a reactor engineer to identify thermodynamically more favorable operating conditions. More information on these topics can be found in chemical engineering thermodynamics textbooks [10,11]. The second question of estimating how fast the thermodynamically possible chemical transformations will occur involves a knowledge of chemistry, reaction kinetics, and various transport processes like mixing, heat, and mass transfer. Analysis of the transport processes and their interaction with chemical reactions can be quite difficult and is intimately connected to the underlying fluid dynamics. Such a combined analysis of chemical and physical processes constitutes the core of chemical reaction engineering.
Catalysis and Catalytic Processes 11 The first step in any reaction engineering analysis is formulating a mathematical framework to describe the rate (and mechanism) by which one chemical species is converted into another in the absence of any transport limitations (chemical kinetics). The rate is the mass, in moles of a species, transformed per unit time, whereas the mechanism is the sequence of individual chemical events, whose overall result produces the observed transformation. Although the knowledge of mechanism is not necessary for reaction engineering, it is of great value in generalizing and systematizing the reaction kinetics. The knowledge of rate of transformation, however, is essential for any reaction engineering activity. The rate of transforming one chemical species into another cannot be predicted with accuracy. It is a specific quantity that must be determined from experimental measurements. Measuring the rate of chemical reactions in the laboratory is itself a specialized branch of science and engineering. The rate is formally defined as the change in moles of a component per unit time and per unit volume of reaction mixture. It is important that this rate be an intrinsic property of a given chemical system and not a function of any physical process such as mixing or heat and mass transfer. Thus, the rate must be a local or point value referring to a differential volume of reaction mixture around that point. It is, therefore, essential to separate the effects of physical processes from the measured experimental data to extract the information about the intrinsic reaction kinetics. It is a difficult task. More information about chemical kinetics and laboratory reactors used for obtaining intrinsic kinetics can be found in textbooks like Smith [12], Levenspiel [13], and Doraiswamy and Sharma [14]. Assuming that such intrinsic rate data is available, chemical kineticists have developed a number of valuable generalizations for formulating rate expressions, including those for catalytic reactions. Various textbooks cover aspects of chemical kinetics in detail [12,13,15]. Once the intrinsic kinetics is available, the production rate and composition of the products can be related, in principle, to the reactor volume, reactor configuration, and mode of operation. This is the central task of a reaction and reactor engineering activity. The first step of reactor engineering is to select a suitable reactor type. In catalytic reactors, multiple phases are almost always involved (see examples cited in Refs. [14,16–19]). Several types of reactors are used for such catalytic and multiphase applications. Broadly, these reactors may be classified based on presence of phases, such as • •
•
gas-liquid reactors: stirred reactors, bubble column reactors, packed columns, and loop reactors; gas-liquid-solids reactors: stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, and loop reactors; or gas-solid reactors: fluidized bed reactors, fixed bed reactors, and moving bed reactors.
Existence of multiple phases opens up a variety of choices in bringing these phases together to react. Krishna and Sie [20] have discussed a three-level approach for reactor design and selection:
12 •
•
•
Chapter 1 Strategy level I: catalyst design strategy gas-solid systems: catalyst particle size, shape, porous structure, and distribution of active material gas-liquid systems: choice of gas-dispersed or liquid-dispersed systems, ratio between liquid-phase bulk volume and liquid-phase diffusion layer volume Strategy level II: injection and dispersion strategies reactant and energy injection: batch, continuous, pulsed, staged state of mixedness of concentrations and temperature separation of product or energy in situ contacting flow pattern: co-, counter-, or cross-current Strategy level III: choice of hydrodynamic flow regimen packed bed, bubbly flow, churn-turbulent regimen, dense-phase, or dilute-phase riser transport
Besides these considerations for selecting an appropriate reactor and mode of operation, several other factors need to be considered while designing a catalytic reactor. Some of the key issues are the following: •
•
•
•
Understanding gas-liquid and liquid-solid transport processes: mass and heat transfer across multiple phases play a crucial role in determining the performance of multiphase catalytic reactors. Ramchandran and Chaudhari [18] have elucidated these points very well in their classic book on three-phase catalytic reactors, and interested readers should consult the original book. Understanding intraparticle transport processes: mass and heat transfer effects are important even on a catalyst particle scale. Most of the catalysts are porous, and therefore species and heat transport within the pores of catalyst particles control concentration and temperature profiles within the catalyst particle (and therefore conversion and selectivity). There are several ways by which effective Thiele modulus is defined to account for different shapes of catalyst pellets and different reaction orders. Interested readers may consult Levenspiel [21]. Compensating inhibition/deactivation of catalyst: various possible reasons for catalyst deactivation were mentioned earlier. Catalyst activity may be reduced due to deposition of inhibitors on active sites. Inhibitors may be consumed in reactions unlike catalysts. The most commonly used strategies with which one may compensate for reduced activity of catalyst are by reducing flow rate or increasing temperature to maintain conversion at the design level. Manipulate selectivity of desired product: several strategies for enhancing selectivity of desired products have been proposed by the classical chemical reaction engineering (CRE) approach. These include manipulation of operating temperature or temperature profile across the reactor according to difference in activation energies of competing reactions (use high temperature if activation energy of reaction producing desired product is higher than
Catalysis and Catalytic Processes 13 reactions producing by-products). Several possible ways of enhancing selectivity by manipulating pore sizes of catalyst are discussed by Worstell [22] and may be followed. For translating this understanding into practice, more often than not, key obstacles are lack of knowledge on how flow-patterns and contacting influence process performance and how these change with the reactor scale. It is impossible to provide detailed quantitative treatment to issues discussed earlier in this chapter. More detailed treatment of reaction and reactor engineering is provided in Chapter 7.
1.4 Organization of This Book The book is aimed at providing a comprehensive methodology and state-of-the-art tools for industrial catalysis. The intended audience of the book is chemical engineers, process development chemists, and technologists working in chemical industries and industrial research laboratories as well as research students working in the area of industrial catalysis and catalytic processes. This book will be an important source for researchers and scientists working in the chemical industry involved in developing improved catalysts and catalytic processes. This introductory chapter introduces readers to the interesting, challenging, and important field of catalysis and catalytic processes. Part I covers fundamentals of catalysis and catalytic reaction engineering. Part II covers important industrial applications of catalysis and catalytic processes. The epilog recaptures the key points and the lessons learned from our experience of applying the material discussed in this book for addressing practical process engineering problems. The potential benefits of catalytic processes and the probable pitfalls are reemphasized. Some comments on future trends in catalysis and catalytic processes are included.
References [1] A.J.B. Robertson, The early history of catalysis, Platin. Met. Rev. 19 (2) (1975) 64–69. [2] C. Tolman, The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis, Chem. Soc. Rev. 1 (3) (1972) 337–353. [3] R.A. Sheldon, H. van Bekkum, Fine Chemicals Through Heterogeneous Catalysis, Wiley, Weinheim, 2008. Retrieved from: https://books.google.co.in/books?id¼RW8griumzqcC. [4] S.M. George, Introduction: heterogeneous catalysis, Chem. Rev. 95 (3) (1995) 476–477. [5] M.J. Climent, A. Corma, S. Iborra, Heterogeneous catalysts for the one-pot synthesis of chemicals and fine chemicals, Chem. Rev. 111 (2011) 1072–1133. [6] N. Linares, A.M. Silvestre-Albero, E. Serrano, J. Silvestre-Albero, J. Garcia-Martinez, Mesoporous materials for clean energy technologies, Chem. Soc. Rev. 43 (22) (2014) 7681–7717. http://doi.org/10.1039/ C3CS60435G. [7] X. Guo, J. Song, Y. Lvlin, K. Nakanishi, K. Kanamori, H. Yang, Preparation of macroporous zirconia monoliths from ionic precursors via an epoxide-mediated sol-gel process accompanied by phase separation, Sci. Technol. Adv. Mater. 16 (2) (2015) 25003. Retrieved from: http://stacks.iop.org/1468-6996/16/i¼2/ a¼025003.
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[8] C.G. Hill, An Introduction to Chemical Engineering Kinetics and Reactor Design, John Wiley & Sons Inc., New York, NY, 1977 [9] J. Haber, Manual on catalyst characterization, Pure Appl. Chem. 63 (9) (1991) 1227–1246. [10] S.I. Sandler, Chemical and Engineering Thermodynamics, third ed., John Wiley & Sons, New York, NY, 1998. [11] J.M. Smith, H.S. Van Ness, An Introduction to Chemical Engineering Thermodynamics, second ed., McGrawHill, New York, NY, 1959. [12] J.M. Smith, Chemical Engineering Kinetics, second ed., McGraw-Hill, New York, NY, 1970. [13] O. Levenspiel, Chemical Reaction Engineering, second ed., John Wiley & Sons, New York, NY, 1972. [14] L.K. Doraiswamy, M.M. Sharma, Heterogeneous Reactions — Analysis Examples and Reactor Design, vol. 2, John Wiley & Sons, New York, NY, 1984. [15] G.F. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design, John Wiley & Sons, New York, NY, 1984. [16] M.P. Dudukovic, F. Larachi, P.L. Mills, Multiphase reactors — revisited, Chem. Eng. Sci. 54 (1999) 1975–1996. [17] D. Kunni, O. Levenspiel, Fluidization Engineering, John Wiley & Sons, New York, NY, 1991. [18] P.A. Ramchandran, R.V. Chaudhari, Three Phase Catalytic Reactors, Gordon and Breach, New York, NY, 1983. [19] Y.T. Shah, Design Parameters for Mechanically Agitated Reactors, Adv. Chem. Eng. 17 (1991) 1–206. [20] R. Krishna, S.T. Sie, Strategies for multiphase reactor selection, Chem. Eng. Sci. 49 (1994) 4029–4065. [21] O. Levenspiel, Chemical Reaction Engineering, third ed., Wiley, New York, NY, 1999. [22] J.H. Worstell, Don’t act like a novice about reaction engineering, Chem. Eng. Prog. (March) (2001) 68–72. [23] J. Hagen, Industrial Catalysis: A Practical Approach, second ed. (2006).
Catalysis and Catalytic Processes V.V. Ranade, S.S. Joshi CSIR-National Chemical Laboratory, Pune, India
1.1 Introduction Chemical and allied industries manufacture products that are essential for creating and sustaining modern societies. The chemical (and biological) transformations necessary to make these essential products often involve the use of catalysts. The catalyst (which can be either homogeneous or heterogeneous) provides a reduced activation energy barrier to transformations and facilitates better control on selectivity. The development and selection of the right catalyst, therefore, can make a substantial impact on process viability and economics. Besides the right catalyst, it is essential to develop the right reactor type and process intensification strategies for effective translation of the laboratory process to practice. With strict environmental regulations, rising raw material prices, depleting feedstocks, and a call for green chemistry as driving forces, the chemical industry faces a larger challenge with both opportunities and risks. Catalysis is of paramount importance in the chemical industry due to its direct involvement in the production of 80% of industrially important chemicals. Catalysts are involved in more than $10 trillion in goods and services of the global gross domestic product (GDP) annually. It is estimated that the global demand on catalysts is more than $30 billion, and a very robust growth is projected in the future. There is an urgent need to develop cost-effective and environmentally benign methods of converting natural resources into fine and specialty chemicals using highly efficient catalysts and employing cleaner methodologies. The advancements in catalysis and applications to the chemical industry are very significant and are responsible for cleaner processes. Replacement of the stoichiometric reactions by catalytic reactions and application of new catalyst systems and technologies to make the processes environmentally friendly, energy efficient, and globally competitive are current needs. A catalyst is a substance that provides an alternative route of reaction where the activation energy is lowered. Catalysts don’t affect the chemical equilibrium associated with a reaction; they merely change the rates of reactions. Catalysts are classified in a variety of different ways. The commonly used classification by reaction engineers is based on number of phases, such as Industrial Catalytic Processes for Fine and Specialty Chemicals. http://dx.doi.org/10.1016/B978-0-12-801457-8.00001-X # 2016 Elsevier Inc. All rights reserved.
1
2
Chapter 1
• •
homogenous catalysis (catalyst and substrate in same phase) or heterogeneous catalysis (solid catalyst and substrate is a gas and/or liquid)
Basic concepts of catalysis are briefly introduced in the following section. It is important to combine the understanding of catalysis with key reaction engineering expertise to translate the potential of a catalyst in the form of a practically implemented catalytic process or plant. Any catalytic reactor has to carry out several functions like bringing reactants into intimate contact with the active sites on a catalyst (to allow chemical reactions to occur), providing an appropriate environment (temperature and concentration fields) for adequate time, and allowing for removal of products. A reactor engineer has to ensure that the evolved reactor hardware and operating protocol satisfy various process demands without compromising safety, the environment, and economics. Naturally, successful reactor engineering requires bringing together better chemistry [thermodynamics, catalysis (replace reagent-based processes), improved solvents (supercritical media, ionic liquids), improved atom efficiency, waste prevention — leave no waste to treat] and better engineering (fluid dynamics, mixing and heat and mass transfer, new ways of process intensification, computational models, and real-time process monitoring and control). Some of these aspects are briefly discussed in Section 1.3. Organization of this book is outlined in the last section of the chapter.
1.2 Catalysts and Catalytic Reactions The word catalyst was first used in 1835. Over the years, it has been established that a catalyst influences kinetics of a process without undergoing any change itself. A catalyst does not alter the thermodynamics of a reaction. In simple words, a catalyst alters the route without altering the destination (see Fig. 1.1). Here the route is the metaphor for activation energy — minimum energy input for a chemical system to undergo a chemical reaction and the transition state of a chemical reaction. Reaction without catalyst
Energy
Reaction with catalyst
Ea (Y → X)
Ea (X → Y)
Y ΔH
X
Reaction path
Fig. 1.1 A catalyst allows reaction to proceed through an alternative path.
Catalysis and Catalytic Processes 3 The terms that are often used in the context of catalytic activity are turnover number (TON), to define the productivity of a catalyst, and turnover frequency (TOF), to define the catalyst activity or TON per unit time. The TOF is defined in terms of active catalytic centers, such as TOF ¼
volumetric rate of reaction moles volume ¼ ¼ time1 number of centers=volume volume time moles
TOF may be in a range of 102 to 102 for industrial applications. The TON is defined as a measure of capacity of the catalyst for accelerating the reaction such as TON ¼ TOF Lifetime of a catalyst Typically TON is in the range of 106 to 107 for industrial applications. The role of a catalyst becomes even more important when multiple reactions are thermodynamically feasible. In such cases, an appropriate catalyst manipulates the reaction rates in such a way that selectivity toward a desired product increases. Several factors and parameters influence the overall performance of a catalyst. The selection of a catalyst for an industrial process therefore depends on the role it is supposed to play. The effect of a catalyst on kinetics of the reaction needs to be understood in detail to get an insight about the surface chemistry involved that would help in the design of a specific catalysis. It is therefore important to understand the elementary steps through which a catalyst influences overall performance. Homogeneous catalysts typically form a complex with one of the reactants, which eventually transforms it into the product after interacting with other reactants. The process is essentially similar to homogeneous reactions in the absence of a catalyst and is often controlled by mixing reactants and a catalyst species on a molecular level. In contrast to this, in a heterogeneous catalyst, several additional steps are involved along with reaction occurring on the catalyst surface, such as • • • • • • •
external diffusion toward a catalyst pellet internal diffusion toward a catalyst surface molecular adsorption on a catalyst surface surface reaction desorption from a catalyst surface internal diffusion away from the catalyst surface external diffusion away from the catalyst pellet
These steps need to be understood to select an appropriate reactor and operating strategy. This procedure will be discussed later in this book. Heterogeneous catalysis allows easy separation and reuse of a catalyst. An example of heterogeneous catalysis is Haber’s process, where iron powder is used as a catalyst to enable the
4
Chapter 1
conversion of nitrogen and hydrogen gas to ammonia. A heterogeneous catalyst has “active sites,” which are the centers of reaction. Once adsorbed (either physically or chemically), the substrate undergoes a reaction, and the product is desorbed subsequently, rendering the surface of the catalyst free for further activity. Homogeneous catalysis has the inherent disadvantage of lack of ease of separation of product and catalyst after the reaction. Esterification of acetic acid with methanol to give methyl acetate in the presence of an H+ ion is a very common example of homogeneous catalysis. Significant work is also being actively pursued to develop a “heterogenized” homogeneous catalyst that uses solids as supports for anchoring of the homogeneous catalysts. This would make it technically heterogeneous, but it would retain the characteristic reactivity pattern of a homogeneous catalyst. One example of such heterogenized homogeneous catalyst is silica-supported sulfuric acid. Besides the classical catalysts mentioned earlier, several other catalytic processes have been developed, which include biocatalysis (using enzymes), photocatalysis (acceleration of photoreaction using a catalyst), and electrocatalysis (acceleration of electrochemical (half) reactions). Enzymes are being increasingly used as catalysts for a variety of chemical transformations, including conversion of organic wastes to useful chemicals. Significant efforts are being made to develop the next generation of electrocatalysts for fuel cell applications or for converting carbon dioxide into a variety of useful chemicals. Without getting into the details of catalysts and catalytic processes, it will be useful to discuss the key properties of a catalyst here.
1.2.1 Characteristics of Catalysts Catalytic substances have a tendency to form complexes. A large number of substances that have been observed to show catalytic properties are from the VIII Group and IB Group of the periodic table (which have unpaired d electrons). Another interesting property of catalytic materials is the small energy differences between valence shells, which lead to a number of oxidation states. The characteristics of a catalyst may be defined by activity, selectivity, stability, and accessibility. All four terms refer to the favorability of a catalyst to form a product. Activity is generally found to increase with temperature. All the other three are trade-offs with activity and depend on a specific reaction. Ideally a catalyst should undergo the same catalytic cycle multiple times without a reduction in its ability to influence the reaction. The number of times a catalyst converts a substrate to product is measured in terms of TON. Selectivity of the catalyst is characterized in following different ways: •
•
Chemo selectivity is when a catalyst favors reaction with one substrate in a reaction mixture over another. For example, an oxidizing agent may favor the oxidation of an aldehyde group over a hydroxyl group present on the same moiety. Regioselectivity is when a catalyst favors the synthesis of a product based on the position it acquires in the substrate. For example, a formyl group can be attached to either the primary,
Catalysis and Catalytic Processes 5
• •
terminal carbon atom or the secondary, internal carbon atom, leading, respectively, to the linear and the branched product in hydroformylation. Diastereoselectivity is a phenomena wherein a catalyst may direct a substrate selectively favoring formation of one stereomer over another if a substrate has stereogenic centers. Enantioselectivity refers to the catalyst favoring synthesis of one enantiomer of product over another, even when the substrate itself is achiral.
Morphology and material strength are important characteristics for heterogeneous catalysts. They are manufactured in a variety of morphology like pellets, trilobes, and extrudates. The overall pressure drop and effectiveness of the catalyst depends on size and shape of the heterogeneous catalyst. Resistance to crushing, attrition, and abrasion are also important characteristics that need to be understood in case of solid or solid supported catalysts. Thermal characteristics and the range of temperature for which the activity would be the highest without compromising on the selectivity is an important factor in the selection of a catalyst for the desired process. The catalyst characteristics need to be appropriately accounted for (including activation as well as deactivation of catalyst) while designing a suitable reactor for carrying out catalytic reactions in practice. Before discussing some aspects of practical reaction and reactor engineering, key aspects of homogeneous and heterogeneous catalysts are briefly discussed in the following sections.
1.2.2 Homogeneous Catalysts Many industrial processes have been developed using homogeneous catalysts. It is being employed in oxidation, carbonylation, hydroformylation, oligomerization, polymerization, hydrocyanation, and synthesis of fine chemicals, among other processes (Hagen [23]). Some other homogenously catalyzed reactions include ester hydrolysis, Diels-Alder reaction, Cannizzaro reaction, and enzymatic processes. As mentioned earlier, catalytic processes that occur in the same phase as the reaction medium are termed homogeneous catalytic processes. The applicability of a homogeneous reaction mixture has been known for several centuries, such as for fermentation process. Charles Bernard Desormes and Nicolas Clement were arguably the first researchers to make an attempt to postulate a rational theory for catalysis or the intermediate compound theory [1] to explain the homogeneous catalytic effect of nitrogen oxides for the manufacture of sulfuric acid using the lead chamber process. These catalysts may be metal complexes or common reagents such as mineral acids, and they can be uniformly distributed in the bulk reaction mixture. Because in a homogenous catalyst system each molecule of the catalyst is distributed in the reaction mixture, it would mean more active sites are available to interact with the substrate. Hence, these reactions proceed at milder conditions and lower catalyst concentration. Another advantage of homogeneous catalysts is the ease of understanding the catalytic chemistry because the mechanism of the reaction is only dependent on the kinetics and not on the diffusion rates.
6
Chapter 1
Homogeneous catalysts used in industrial chemistry are generally from organometallic compounds (compounds with a metal-carbon bond). The central metal atom is bound to organic and inorganic ligands. The catalyst environment can be easily modified to alter the catalytic properties by manipulating ligands. Transition metals play a major role in the development of these organometallic complexes. This is because of the availability of d-orbitals of transition metals, which allow ligands to bond in such a way that they are available for further reaction. Rhodium phosphine-based metal complexes such as [RhCl(PPh3)3] have been found to be an effective catalyst for the hydrogenation of olefins. On account of the stability of transition metal complexes, the process temperatures are generally limited to 200°C, and this becomes a limitation of homogeneous catalysis. Because the catalyst is completely dispersed in the reaction media, these systems face difficulties in separation or recovery of catalysts. Significant efforts have been and are being spent on deciphering mechanisms of homogeneous catalysis to facilitate further development of new catalyst systems. Tolman [2] proposed a mechanism with which a reaction is catalyzed by homogenous organometallic complexes, which was referred to as the 16 or 18 electron rule (see Fig. 1.2). It postulates the role of the oxidation state and coordination number of the metal center of the transition metal complex. The organometallic complexes referred to are the transition metal complexes with CO, N2, CN, RNC, PR3, π-aryl, π-allyl, –SiR3, and π-acyl ligands, which have high ligand field strength and covalent bonding. The two major postulates of the rule are as follows [2]: • •
Diamagnetic organometallic complexes of transition metals exist in any measurable quantity only if the valence shell of central metal contains 16 or 18 electrons. The intermediates that are formed during the course of the reaction should also contain 16 or 18 valence shell electrons. Saturated 18e complex
Product Unsaturated 16e complex Saturated 18e complex
Substrate
Saturated π complex, 18e Unsaturated 16e complexes
Fig. 1.2 Cycle explaining the 16/18 electron rule.
Catalysis and Catalytic Processes 7 To understand the catalytic cycle in homogeneous catalysis, a stoichiometric reaction with well-defined transition metal complexes can be used to elucidate the steps involved. Labeled compounds can also be used to validate the postulated reaction mechanism by employing spectroscopic identification techniques. Various in situ spectroscopy techniques such as infrared spectroscopy (IR), nuclear magnetic resonance (NMR), electron spin resonance (ESR), and Raman are very helpful in developing a better understanding of homogeneous catalysis. It has been observed that Infrared spectroscopy has been very useful in studying carbonyl complexes.
1.2.3 Heterogeneous Catalysts The use of heterogeneous catalysts in the chemical industry began in the early 1800s with Faraday being among the pioneers of heterogeneous catalysis and discovering the use of platinum for oxidation. These systems were in use during the Second World War for reactions such as dehydrogenation of methyl cyclohexane to form toluene in the presence of Pt-Al2O3 or in alkane isomerization using Cr2O3-Al2O3. After the war, with diversification in chemicals synthesized and advancement of technology, heterogeneous catalysts were used for the hydrocracking of high-boiling petroleum using Ni-aluminosilicate to form fuels. This revolutionized the automobile industry. Another application of solid catalysts was in the synthesis of polyethylene from ethylene by polymerization in the presence of Ziegler-Natta (TiCl4-Al(C2H5)3) catalysts. Heterogeneous catalysts are used for innumerable reactions such as oxidation, nitration, coupling, condensation, and hydrogenation. Heterogeneous catalysis facilitates a large number of chemical reactions. The use of heterogeneous catalysts in fine chemicals is gaining importance because of the following reasons [3]: •
•
•
•
Because the catalyst is not in the same phase as the reacting molecules, it allows for a higher possibility of catalyst recovery and recyclability. Chemical bonds are formed with the catalyst either through physisorption or chemisorption during the reaction and broken thereafter to regenerate the catalyst, albeit with loss of activity in some cases [4]. Solid acid catalysts are easier to handle in comparison with conventional mineral acids such as H2SO4 and hydrofluoric acid (HF). They reduce capital cost and also ensure material safety because they have less corrosivity. Heterogeneous catalysts for bulk chemicals have been used since the beginning of chemical industries, hence the processes and their roles in the mechanism of the organic synthesis are well understood in most cases. Therefore they can be downscaled for their applications in fine chemicals to some extent. Myriad catalysts with acidic or basic properties exist or have been designed to synthesize particular species, which ensures product maximization. Mixed metal oxide, clays, zeolites, silica, alumina, zirconia, and heteropolyacids are a few classes of catalysts
8
Chapter 1
•
used predominantly. They can be modified to a large extent through impregnation of homogeneous catalysts or metals and structural changes. Microporous and mesoporous structures or sieves and honeycomb-like structures allow heterogeneous catalysts to be highly shape and stereo selective. These designs give enzyme-like efficiency to the catalyst.
A new stage of development in heterogeneous catalysts came with the objective of using renewable feedstocks and environmentally benign processes and techniques for downstream waste reduction. Catalysts that have high efficiency and better surface properties are being developed for process intensification [5]. A catalyst facilitates reaction through the formation of complexes with reacting species. The product formed doesn’t have the tendency to bond with the catalyst, which implies the catalyst surface is regenerated. However, this is only partially correct. The surface and structure of the catalyst are modified with each reaction. For instance, in the case of a pure metal catalyst, surface roughness and crystallinity change, whereas in the case of metal oxides there is a change in the ratio of metal and oxygen. Commercial catalysts are generally available in various physical forms such as powder, pellets, granules, and extrudates. Pore size plays a major role in structure and therefore in catalytic performance (conversion, selectivity, yield, TOF, and TON). Porous catalysts offer a large surface area, the ability to support varied chemical functionalities, and the ability to form different networks according to the applications. Broadly catalysts are classified into three kinds of porous materials: •
•
•
Microporous: Pore diameter is less than 2 nm. A typical example of a microporous catalyst is zeolite. It has a crystalline and well-defined structure. It has a silicon, aluminum, and oxygen framework, and water or another cation may be present in the pores. Activated carbon is also microporous adsorbent and has varying origins, thermal resistance, and porosity, depending on the method of synthesis. Mesoporous: Pore diameter is between 2 and 50 nm. Mesoporous solids are synthesized through a templating approach, wherein surfactants are used for directing the structure. Subsequently, the surfactant is removed, and a mesoporous system is obtained that replicates the surfactant assembly [6]. Macroporous: Pore diameter is greater than 50 nm. Macroporous material can be synthesized by a sol-gel method such as porous silica, alumina, and zirconia gels. In the case of zirconia gels, a metal salt precursor is used for the epoxide mediated sol-gel method followed by phase separation. Morphology of the catalyst would be governed by temperature and amount of solvents or reactants used [7].
Activity is the rate at which a reaction proceeds in presence of a catalyst. The activity of the heterogeneous catalyst depends on the reaction conditions of temperature, pressure, and catalyst loading with respect to reactants and on reactor conditions such as flow rate and surface area of reactor. Another characteristic of a catalyst is selectivity, which is the extent
Catalysis and Catalytic Processes 9 to which a catalyst promotes synthesis of the desired product over all the possible products, including those with lower free energy. (Note: in a reaction without the selective catalyst, only products with lowest free energy would have been formed.) Selectivity is dependent on time, temperature, and other reaction parameters. Among the most commonly used metals for heterogeneous catalysts are transition metals like platinum, rhodium, nickel, ruthenium, palladium, cobalt, magnesium, vanadium, and iron, among others [8]. Another important aspect of the catalyst is possible catalyst deactivation. The most typical causes of deactivation of heterogeneous catalyst are the following: • • • •
Aging/thermal degradation: deactivation resulting from changes in structure. Sintering: an increase in the average size of the crystallites due to coalescence of small particles on continued usage of catalyst. Fouling/coking: deposition of high-molecular-weight “carbon-hydrogen” compounds or primary carbons on the catalyst surface. Poisoning: inhibitory substances bind strongly to the active catalytic sites on the surface.
Catalytically active complex in homogeneous catalysis may similarly be deactivated due to structural changes in the active complex as well as poisoning because of binding with inhibitory substances. The reasons for possible deactivation need to be understood to develop appropriate regeneration strategies. To prevent deactivation, “promoters” may be added such as in the case of ammonia synthesis, where aluminum oxide is added along with iron to prevent fusion of the particles. Catalyst characterization plays an important role in understanding and improving overall performance of catalytic processes. A substrate may be either physically adsorbed (physisorption) or chemically adsorbed (chemisorption) on the surface of a heterogeneous catalyst. The difference between the two phenomena needs to be explicitly understood. Although there are no bonds formed in physisorption, only weak van der Waals forces are responsible for keeping the substrate on the catalyst surface. In chemisorption, there is electron transfer and formation of strong bonds between catalyst and substrate. This renders chemisorption to be a more selective process and leads to the formation of a single layer on the surface unlike in physisorption, where multiple layers may be formed with each adsorbed molecule of the first layer acting as a site for the next [9]. The commonly available techniques of characterization of catalysts with most researchers are TEM (transmission electron microscopy), XRD (X-ray diffraction), and EXAFS (extended X-ray absorption fine structure). More details of characterization are discussed later in the book. With the characterization and properties of the catalyst known, specific catalysts may be designed for the required process. It is important to remember that test conditions are not the same as the reaction conditions, thus there might be some variation in the properties of the catalysts. Besides this,
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Chapter 1
several other factors related to reaction and reactor engineering need to be taken into account while translating the catalytic process into practice. These aspects are briefly outlined in the following section (and are discussed later in Chapter 7).
1.3 Reaction and Reactor Engineering Reaction and reactor engineering involves establishing a relationship between reactor hardware and operating protocols with various performance issues as listed in Table 1.1. Table 1.1 Reaction and reactor engineering Reactor Performance
Hardware and Operating Protocol
Conversion and selectivity
Reactor configuration: size and shape, feed and exit nozzles Mode of operation: batch, semibatch, continuous Start-up and shutdown protocols Operating conditions: flow rate, pressure, temperature, flow regimen, RTD Reactor internals: baffles, heat transfer coils, distributors
Product quality Catalyst activity and life Stability and operability Safety Environmental impact
A process engineer is faced with a host of questions while establishing a relationship between reactor hardware, operating protocol, and reactor performance. In this section, some of these questions and the relevant tasks of a reactor engineer are discussed briefly. The major questions being faced by a reactor engineer can be grouped into three classes: • • •
What chemical transformations are expected to occur? How fast will these changes occur? What is the best way to carry out these transformations?
The first question is in the realm of thermodynamics and chemistry. Knowledge of chemistry and reaction mechanism is helpful to identify various possible chemical reactions. Thermodynamics provides models and tools to estimate free energies and the heat of formations of chemical compounds from which the energetics of all the possible chemical reactions can be examined. These tools help a reactor engineer to identify thermodynamically more favorable operating conditions. More information on these topics can be found in chemical engineering thermodynamics textbooks [10,11]. The second question of estimating how fast the thermodynamically possible chemical transformations will occur involves a knowledge of chemistry, reaction kinetics, and various transport processes like mixing, heat, and mass transfer. Analysis of the transport processes and their interaction with chemical reactions can be quite difficult and is intimately connected to the underlying fluid dynamics. Such a combined analysis of chemical and physical processes constitutes the core of chemical reaction engineering.
Catalysis and Catalytic Processes 11 The first step in any reaction engineering analysis is formulating a mathematical framework to describe the rate (and mechanism) by which one chemical species is converted into another in the absence of any transport limitations (chemical kinetics). The rate is the mass, in moles of a species, transformed per unit time, whereas the mechanism is the sequence of individual chemical events, whose overall result produces the observed transformation. Although the knowledge of mechanism is not necessary for reaction engineering, it is of great value in generalizing and systematizing the reaction kinetics. The knowledge of rate of transformation, however, is essential for any reaction engineering activity. The rate of transforming one chemical species into another cannot be predicted with accuracy. It is a specific quantity that must be determined from experimental measurements. Measuring the rate of chemical reactions in the laboratory is itself a specialized branch of science and engineering. The rate is formally defined as the change in moles of a component per unit time and per unit volume of reaction mixture. It is important that this rate be an intrinsic property of a given chemical system and not a function of any physical process such as mixing or heat and mass transfer. Thus, the rate must be a local or point value referring to a differential volume of reaction mixture around that point. It is, therefore, essential to separate the effects of physical processes from the measured experimental data to extract the information about the intrinsic reaction kinetics. It is a difficult task. More information about chemical kinetics and laboratory reactors used for obtaining intrinsic kinetics can be found in textbooks like Smith [12], Levenspiel [13], and Doraiswamy and Sharma [14]. Assuming that such intrinsic rate data is available, chemical kineticists have developed a number of valuable generalizations for formulating rate expressions, including those for catalytic reactions. Various textbooks cover aspects of chemical kinetics in detail [12,13,15]. Once the intrinsic kinetics is available, the production rate and composition of the products can be related, in principle, to the reactor volume, reactor configuration, and mode of operation. This is the central task of a reaction and reactor engineering activity. The first step of reactor engineering is to select a suitable reactor type. In catalytic reactors, multiple phases are almost always involved (see examples cited in Refs. [14,16–19]). Several types of reactors are used for such catalytic and multiphase applications. Broadly, these reactors may be classified based on presence of phases, such as • •
•
gas-liquid reactors: stirred reactors, bubble column reactors, packed columns, and loop reactors; gas-liquid-solids reactors: stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, and loop reactors; or gas-solid reactors: fluidized bed reactors, fixed bed reactors, and moving bed reactors.
Existence of multiple phases opens up a variety of choices in bringing these phases together to react. Krishna and Sie [20] have discussed a three-level approach for reactor design and selection:
12 •
•
•
Chapter 1 Strategy level I: catalyst design strategy gas-solid systems: catalyst particle size, shape, porous structure, and distribution of active material gas-liquid systems: choice of gas-dispersed or liquid-dispersed systems, ratio between liquid-phase bulk volume and liquid-phase diffusion layer volume Strategy level II: injection and dispersion strategies reactant and energy injection: batch, continuous, pulsed, staged state of mixedness of concentrations and temperature separation of product or energy in situ contacting flow pattern: co-, counter-, or cross-current Strategy level III: choice of hydrodynamic flow regimen packed bed, bubbly flow, churn-turbulent regimen, dense-phase, or dilute-phase riser transport
Besides these considerations for selecting an appropriate reactor and mode of operation, several other factors need to be considered while designing a catalytic reactor. Some of the key issues are the following: •
•
•
•
Understanding gas-liquid and liquid-solid transport processes: mass and heat transfer across multiple phases play a crucial role in determining the performance of multiphase catalytic reactors. Ramchandran and Chaudhari [18] have elucidated these points very well in their classic book on three-phase catalytic reactors, and interested readers should consult the original book. Understanding intraparticle transport processes: mass and heat transfer effects are important even on a catalyst particle scale. Most of the catalysts are porous, and therefore species and heat transport within the pores of catalyst particles control concentration and temperature profiles within the catalyst particle (and therefore conversion and selectivity). There are several ways by which effective Thiele modulus is defined to account for different shapes of catalyst pellets and different reaction orders. Interested readers may consult Levenspiel [21]. Compensating inhibition/deactivation of catalyst: various possible reasons for catalyst deactivation were mentioned earlier. Catalyst activity may be reduced due to deposition of inhibitors on active sites. Inhibitors may be consumed in reactions unlike catalysts. The most commonly used strategies with which one may compensate for reduced activity of catalyst are by reducing flow rate or increasing temperature to maintain conversion at the design level. Manipulate selectivity of desired product: several strategies for enhancing selectivity of desired products have been proposed by the classical chemical reaction engineering (CRE) approach. These include manipulation of operating temperature or temperature profile across the reactor according to difference in activation energies of competing reactions (use high temperature if activation energy of reaction producing desired product is higher than
Catalysis and Catalytic Processes 13 reactions producing by-products). Several possible ways of enhancing selectivity by manipulating pore sizes of catalyst are discussed by Worstell [22] and may be followed. For translating this understanding into practice, more often than not, key obstacles are lack of knowledge on how flow-patterns and contacting influence process performance and how these change with the reactor scale. It is impossible to provide detailed quantitative treatment to issues discussed earlier in this chapter. More detailed treatment of reaction and reactor engineering is provided in Chapter 7.
1.4 Organization of This Book The book is aimed at providing a comprehensive methodology and state-of-the-art tools for industrial catalysis. The intended audience of the book is chemical engineers, process development chemists, and technologists working in chemical industries and industrial research laboratories as well as research students working in the area of industrial catalysis and catalytic processes. This book will be an important source for researchers and scientists working in the chemical industry involved in developing improved catalysts and catalytic processes. This introductory chapter introduces readers to the interesting, challenging, and important field of catalysis and catalytic processes. Part I covers fundamentals of catalysis and catalytic reaction engineering. Part II covers important industrial applications of catalysis and catalytic processes. The epilog recaptures the key points and the lessons learned from our experience of applying the material discussed in this book for addressing practical process engineering problems. The potential benefits of catalytic processes and the probable pitfalls are reemphasized. Some comments on future trends in catalysis and catalytic processes are included.
References [1] A.J.B. Robertson, The early history of catalysis, Platin. Met. Rev. 19 (2) (1975) 64–69. [2] C. Tolman, The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis, Chem. Soc. Rev. 1 (3) (1972) 337–353. [3] R.A. Sheldon, H. van Bekkum, Fine Chemicals Through Heterogeneous Catalysis, Wiley, Weinheim, 2008. Retrieved from: https://books.google.co.in/books?id¼RW8griumzqcC. [4] S.M. George, Introduction: heterogeneous catalysis, Chem. Rev. 95 (3) (1995) 476–477. [5] M.J. Climent, A. Corma, S. Iborra, Heterogeneous catalysts for the one-pot synthesis of chemicals and fine chemicals, Chem. Rev. 111 (2011) 1072–1133. [6] N. Linares, A.M. Silvestre-Albero, E. Serrano, J. Silvestre-Albero, J. Garcia-Martinez, Mesoporous materials for clean energy technologies, Chem. Soc. Rev. 43 (22) (2014) 7681–7717. http://doi.org/10.1039/ C3CS60435G. [7] X. Guo, J. Song, Y. Lvlin, K. Nakanishi, K. Kanamori, H. Yang, Preparation of macroporous zirconia monoliths from ionic precursors via an epoxide-mediated sol-gel process accompanied by phase separation, Sci. Technol. Adv. Mater. 16 (2) (2015) 25003. Retrieved from: http://stacks.iop.org/1468-6996/16/i¼2/ a¼025003.
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