5 Key excerpts on "Catalysis"

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  eBook - ePub

  Green Chemistry for Beginners

  • Anju Srivastava, Rakesh K. Sharma, Anju Srivastava, Rakesh K. Sharma(Authors)
  • 2021(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  William H. Stein 1909 Catalysis and investigations on the fundamental principles governing chemical equilibria and rates of reaction Wilhelm Ostwald

  3.2 Role of Catalysis

  Catalysis plays a crucial role in producing environmentally benign chemicals, both new and existing, while saving energy, time, raw materials, and cost related to the overall process [4 , 5 ]. The role of a catalyst has been summarized below.

  • Catalysis decreases the activation energy required for a chemical reaction and thus enhances the rate of reaction.
  • It allows reactions to proceed under milder reaction conditions; therefore, catalytic reactions are considered efficient.
  • The greater activity of catalysts can sometimes lead to the conversion of million times their own weight.
  • Catalysts allow high product selectivity in multifunctional compounds by enabling site-specific transformations and diastereomeric control. This allows the effective utilization of resources and minimization of waste.
  • Catalytic methods also circumvent the need for activation and deactivation of starting materials in the prefunctionalization steps, thereby lessening the number of steps in a reaction.
  • Catalysis prevents pollution by avoiding the formation of unwanted side-products in a reaction.
  • Catalysts are also applied for improving air quality by removing and controlling NOX emissions, decreasing the use of volatile organic solvents, and replacing chlorine-based chemicals by developing alternate catalytic methodology.
  • Catalysis is employed for the production of transportation fuels and bulk and fine chemicals all over the world.
  • Catalysts that can be reused provide economic feasibility and facilitate their industrial application.

  Parameters affecting both commercial usefulness and greenness of a particular catalyst are depicted in Fig. 3.4.

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  eBook - ePub

  Industrial Catalytic Processes for Fine and Specialty Chemicals

  • Sunil S Joshi, Vivek V. Ranade(Authors)
  • 2016(Publication Date)
  • Elsevier

    (Publisher)

  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

  •  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

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  eBook - ePub

  Essentials of Chemical Biology

  Structure and Dynamics of Biological Macromolecules

  • Andrew D. Miller, Julian A. Tanner(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  8

  Kinetics and Catalysis

  8.1 Catalysis in chemical biology

  According to some theories about the origins of life, the key to the creation of organisms is molecular complexity that is sufficient to give self-organisation (see Chapter 10). However, self-organisation alone is insufficient to give life. Instead, self-organisation needs to be partnered with the capacity to accelerate or catalyse chemical inter-conversions as well. This capacity to catalyse chemical inter-conversions is known as Catalysis. Catalysis is frequently performed by a catalyst, which is usually defined as an entity that enhances the rate of a given chemical reaction in both forward and reverse directions without being itself permanently changed in the process. Therefore, a biocatalyst is a biologically relevant catalyst. Typically, biocatalysts accelerate biological chemical reactions with relative rate enhancements of between 105 and 1010 relative to the non-catalysed reactions. Catalysis is universal to all cells of all organisms and the range and diversity of known biocatalysts is simply staggering! Biocatalysts are clearly an absolute fundamental for both the origin of life and the promulgation of life.

  Biocatalysts are overwhelmingly proteins (enzymes) and sometimes RNA nucleic acids (ribozymes). They catalyse an amazing diversity of reactions for myriads of different important reasons. Enzymes take centre stage in metabolism, which is the process by which chemical potential is generated and stored by coupling the synthesis of adenosine 5′-triphosphate (ATP) (the preferred ‘form’ of stored chemical energy in all cells) with the stepwise degradation and/or reorganisation of covalent bonds of primary metabolites such as glucose (Chapter 1). For instance, triose phosphate isomerase (TIM) (see Chapter 1) catalyses the seemingly innocuous interconversion between dihydroxyacetone phosphate and glyceraldehyde-3-phosphate in the catabolic pathway known as glycolysis (Figure 8.1 ). Yet surprisingly, TIM is now considered to be a ‘perfect enzyme’ (see Section 8.4.8), which makes the interconversion possible at a rate that is literally as fast as substrate reaches the enzyme active site. Indeed, without TIM the glycolysis pathway would be unable to deliver on a net gain of two ATP molecules for each glucose molecule consumed (see Figure 8.1 ). In a similar vein, the dimeric enzyme malate dehydrogenase (MDH) catalyses the mere reduction of a carbonyl functional group in oxalic acid to give malic acid, making use of the cofactor nicotinamide adenine dinucleotide hydride, reduced form (NADH) (Figure 8.2 ), yet this interconversion establishes closure of the tricarboxylic acid (TCA) cycle, which takes metabolites from glycolysis and delivers on a net gain of reducing cofactor molecules for each complete rotation through the TCA cycle. There are other types of catabolic enzyme, such as chymotrypsin, which within the gut digests polypeptides into oligopeptides (Figure 8.3 ), for absorption across the gut wall into the blood stream. Alternatively, ribonuclease A (RNAse A) does for RNA polynucleotide what chymotrypsin does for polypeptides, albeit by a very different mechanism (Figure 8.4 ). Enzymes are not only involved in catabolism (i.e. breaking down), but also play a role in anabolism (i.e. building up) of monomeric building blocks required for biological macromolecular assembly. In this respect, examples include alanine α-racemase (AlaR), glutamate α-decarboxylase and aspartate transaminase (aspAT) enzymes, which all make use of the cofactor pyridoxal phosphate (PLP) (Figure 8.5

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  eBook - ePub

  Introduction to Catalysis and Industrial Catalytic Processes

  • Robert J. Farrauto, Lucas Dorazio, C. H. Bartholomew(Authors)
  • 2020(Publication Date)
  • Wiley-AIChE

    (Publisher)

  1

  CATALYST FUNDAMENTALS OF INDUSTRIAL Catalysis

  1.1 INTRODUCTION

  Chemical reactions occur by breaking the bonds of reactants and forming new bonds and new compounds. Breaking stable bonds requires the absorption of energy, while making new bonds results in the liberation of energy. The combination of these energies results in either an exothermic reaction in which the conversion of reactants to products liberates energy or an endothermic process in which the conversion process requires energy. In the former case, the energy of the product is lower than that of the reactants with the difference being the heat liberated. In the latter case, the product energy is greater by the amount that must be added to conserve the total energy of the system. Under the same reaction conditions, the heat of reaction (ΔH) being a thermodynamic function does not depend on the path or rate by which reactants are converted to products. Similarly, ΔG of the reaction is not dependent on the reaction path since it too is a thermodynamic state function. This will be emphasized once we discuss catalytic reactions. The rate of reaction is determined by the slowest step in a conversion process independent of the energy content of the reactants or products.

  1.2 CATALYZED VERSUS NONCATALYZED REACTIONS

  In the most basic sense, the purpose of the catalyst is to provide a reaction pathway or mechanism that has a lower activation barrier compared to the noncatalyzed (Enc ) pathway, as illustrated in Figure 1.1 . Also shown is the catalyzed barrier (EMn ). In any reaction, catalyzed or noncatalyzed, the reaction sequence occurs through a series of elementary steps. In a noncatalyzed reaction, the species that participate in the reaction sequence are derived solely from the reactants. In a catalyzed reaction, the catalyst is simply an additional species that participates in the reaction sequence by lowering the activation energy and hence enhances the kinetics of the reaction. Finally, during the catalyzed reaction sequence, the catalyst species returns to its original state. It is the regeneration of the catalyst species to its original state that makes a catalyst a “catalyst” and not a “reactant.” Thus, a catalyst is a species that participates in the reaction sequence—it interacts with the “reactants” to form an intermediate species that undergoes further reaction to form the “product” with the catalyst returning to its original state. This basic sequence of events is illustrated in Figure 1.2

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  eBook - ePub

  Adiabatic Fixed-Bed Reactors

  Practical Guides in Chemical Engineering

  • Jonathan Worstell(Author)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 1

  Introduction

  This chapter discusses the concept of Catalysis and the reasons for designing, building, and operating fixed-bed reactors. It also presents the types of fixed-bed reactors available for use and relates reactor type to the heat transfer requirements of the chemical process. This discussion occurs via examples from the chemical processing industry. It also introduces the classification of fixed-bed reactors as adiabatic or nonadiabatic. This chapter also introduces the concepts of catalyst productivity and selectivity.

  Keywords

  Catalyst; fixed-bed reactor; adiabatic; nonadiabatic; catalyst productivity; selectivity

  1.1 Slow Reactions and Catalysts

  Many reactions are too slow to be commercially viable, no matter how much we desire the product. Over the centuries, we have learned that adding a particular chemical, such reactions proceed at commercially viable rates. We call that particular chemical a “catalyst.” A catalyst is a substance that accelerates a specific reaction toward its equilibrium but remains unchanged after the reaction achieves equilibrium.1 Many times the catalyst and product can be in the same phase at the end of the reaction. In other words, the catalyst contaminates the product and must be removed before the product is sold. Achieving this separation can be expensive, which increases the asking price for the product. Separation costs can actually make the product commercially unviable.

  Two options exist for alleviating the problem of catalyst separation: (1) increase catalyst productivity to the point that only parts per million of it remain in the product and (2) make the catalyst stationary relative to product formation.

  The first option may provide the simplest process, provided we can find a catalyst with sufficient productivity. Catalyst productivity is the rate at which reactant disappears or product appears

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