Catalysis Fundamentals
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Facts At Your Fingertips CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2015 47 C atalysis is among the most im- portant chemical phenomena in industrial chemistry, as well as in many biological and research fields. This one-page reference pro- vides a review of several of the funda- mental concepts underlying catalysis. Catalyst basics A catalyst interacts with chemical re- actants to increase the reaction rate. Catalysts form fleeting intermediate chemical complexes with reactants, allowing the reaction to follow a differ- ent mechanistic pathway that requires lower activation energy (E a ) than the corresponding uncatalyzed reaction. E a is often thought of as an energy barrier over which the reactants must pass to form products. Activation energies are often shown on graphs that plot reaction coordinate against thermodynamic free energy (Figure). Reaction coordinates are one-dimen- sional representations of the progress of a chemical reaction. Catalysts are broadly categorized as homogeneous or heterogeneous. Homogeneous refers to those cata- lysts that are dissolved in the reac- tion medium, forming a single phase with the reactants. Heterogeneous catalysts exist as a distinct phase from the reaction mixture and are of- ten porous solid particles. Both categories are important for industrial chemistry. Examples of liq- uid-phase, acid-base-catalyzed reac- tions include hydrolysis of esters and amides, enolization of aldehydes and ketones, esterification of alcohols, halogenation of acetone and others. Heterogeneous catalysts play a key role in the production of petrochemi- cals, including cracking, alkylation, polymerization, isomerization, dehy- drogenation and many others. Mechanism of action Most chemical reactions involve simul- taneous (rather than sequential) bond breaking and bond forming. Along the pathway of reactants to products, the molecules adopt a configuration that represents the highest potential energy state, known as the transition state. The transition state is characterized by bonds that are both partially formed and partially broken. Catalysts form an intermediate species with one of the reactants and stabilize the transition state, allowing the reaction to pro- ceed with a mechanism that requires lower energy. As products form, the catalyst is regenerated. Catalyst features The following summarizes key catalyst characteristics: Reversible reactions. In the case of reversible reactions, the catalyst acts on both the forward and reverse reactions. The catalyst does not affect the position of the equilib- rium, but it does accelerate the rate at which equilibrium is reached. Energy. The presence of catalyst does not affect the potential energy of the reactants or products. It affects only the activation energy. Selectivity. In reactions with multiple feasible mechanisms, catalysts can often exhibit selectivity by binding to the transition state in a way that fa- vors one reaction pathway over oth- ers. Catalyzed reactions often show a different product distribution than the same uncatalyzed reaction. Reaction rate. In homogeneous-cat- alyzed reactions, the reaction rate is generally proportional to the concen- tration of the catalyst, while in hetero- geneous catalysis, the reaction rate is proportional to the surface area of the solid catalyst and the concentration of active centers (catalytic sites). Solid-catalyzed reaction steps Solid-catalyzed, fluid-phase chemical reactions generally undergo the fol- lowing steps: Diffusion. 1. Reactants in the fluid phase diffuse to the exterior sur- face of a catalyst particle and into the catalyst pores Adsorption. 2. Reactants adsorb to the active centers in catalyst pores Reaction. 3. The surface-adsorbed reactants form products Desorption. 4. The product mol- ecules desorb from the exterior surface of the catalyst pores Diffusion. 5. Product molecules dif- fuse back into the bulk fluid In most cases, one of these steps contributes most significantly to the overall reaction rate, and often the others steps can be ignored or com- bined when determining reaction rates. The significance of each step depends heavily on the reactants and the reaction conditions. Influencing factors The following are factors that can play a large role in determining which step is more or less significant: Fluid-dynamic factors • Catalyst properties (such as par- • ticle size, porosity, pore geometry and surface characteristics) Diffusion characteristics of fluid re- • actants and products Activation energy requirements for • adsorption and desorption of re- actants and products to and from solid surfaces Overall • E a of the catalyzed reaction Thermal factors (temperature and • heat-transport characteristics) References 1) Perry, R.H. and Green, D.W., “Perry’s Chemical Engi- neering Handbook,” 7th ed., McGraw Hill Professional, Section 4, Chapter 12. 1997. 2) Wijngaarden, R.J. and others, “Industrial Catalysis: Opti- mizing Catalysts and Processes,” Wiley-VCH, Weinheim, Germany, 1999. 3) University of Texas, Chemistry 302. Course material on chemical kinetics. Accessed from ch302.cm.utexas. edu, April 2015. Catalysis Fundamentals Department Editor: Scott Jenkins Reaction coordinate Increasing energy E a for uncatalysed reaction Activation energies for the steps of a catalyzed reaction Reactants Products E a1 E a2 E a3 H for both catalyzed and uncatalyzed reactions Energy, kJ
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Catalysis Fundamentals - Chemical Engineering
Transcript of Catalysis Fundamentals
-
Facts At Your Fingertips
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 201547
Catalysis is among the most im-portant chemical phenomena in industrial chemistry, as well
as in many biological and research fields. This one-page reference pro-vides a review of several of the funda-mental concepts underlying catalysis.
Catalyst basicsA catalyst interacts with chemical re-actants to increase the reaction rate. Catalysts form fleeting intermediate chemical complexes with reactants, allowing the reaction to follow a differ-ent mechanistic pathway that requires lower activation energy (Ea) than the corresponding uncatalyzed reaction.
Ea is often thought of as an energy barrier over which the reactants must pass to form products. Activation energies are often shown on graphs that plot reaction coordinate against thermodynamic free energy (Figure). Reaction coordinates are one-dimen-sional representations of the progress of a chemical reaction.
Catalysts are broadly categorized as homogeneous or heterogeneous. Homogeneous refers to those cata-lysts that are dissolved in the reac-tion medium, forming a single phase with the reactants. Heterogeneous catalysts exist as a distinct phase from the reaction mixture and are of-ten porous solid particles.
Both categories are important for industrial chemistry. Examples of liq-uid-phase, acid-base-catalyzed reac-tions include hydrolysis of esters and amides, enolization of aldehydes and ketones, esterification of alcohols, halogenation of acetone and others. Heterogeneous catalysts play a key role in the production of petrochemi-cals, including cracking, alkylation, polymerization, isomerization, dehy-drogenation and many others.
Mechanism of actionMost chemical reactions involve simul-taneous (rather than sequential) bond breaking and bond forming. Along the pathway of reactants to products, the molecules adopt a configuration that represents the highest potential energy state, known as the transition
state. The transition state is characterized by bonds that are both partially formed and partially broken. Catalysts form an intermediate species with one of the reactants and stabilize the transition state, allowing the reaction to pro-ceed with a mechanism that requires lower energy. As products form, the catalyst is regenerated.
Catalyst featuresThe following summarizes key catalyst characteristics:Reversible reactions. In the case of reversible reactions, the catalyst acts on both the forward and reverse reactions. The catalyst does not affect the position of the equilib-rium, but it does accelerate the rate at which equilibrium is reached.Energy. The presence of catalyst does not affect the potential energy of the reactants or products. It affects only the activation energy. Selectivity. In reactions with multiple feasible mechanisms, catalysts can often exhibit selectivity by binding to the transition state in a way that fa-vors one reaction pathway over oth-ers. Catalyzed reactions often show a different product distribution than the same uncatalyzed reaction.Reaction rate. In homogeneous-cat-alyzed reactions, the reaction rate is generally proportional to the concen-tration of the catalyst, while in hetero-geneous catalysis, the reaction rate is proportional to the surface area of the solid catalyst and the concentration of active centers (catalytic sites).
Solid-catalyzed reaction stepsSolid-catalyzed, fluid-phase chemical reactions generally undergo the fol-lowing steps:
Diffusion.1. Reactants in the fluid phase diffuse to the exterior sur-face of a catalyst particle and into the catalyst pores Adsorption.2. Reactants adsorb to the active centers in catalyst pores Reaction.3. The surface-adsorbed reactants form products
Desorption.4. The product mol-ecules desorb from the exterior surface of the catalyst pores Diffusion.5. Product molecules dif-fuse back into the bulk fluidIn most cases, one of these steps
contributes most significantly to the overall reaction rate, and often the others steps can be ignored or com-bined when determining reaction rates. The significance of each step depends heavily on the reactants and the reaction conditions.
Influencing factors The following are factors that can play a large role in determining which step is more or less significant:
Fluid-dynamic factors Catalyst properties (such as par-ticle size, porosity, pore geometry and surface characteristics)Diffusion characteristics of fluid re-actants and productsActivation energy requirements for adsorption and desorption of re-actants and products to and from solid surfacesOverall Ea of the catalyzed reactionThermal factors (temperature and heat-transport characteristics)
References1) Perry, R.H. and Green, D.W., Perrys Chemical Engi-
neering Handbook, 7th ed., McGraw Hill Professional, Section 4, Chapter 12. 1997.
2) Wijngaarden, R.J. and others, Industrial Catalysis: Opti-mizing Catalysts and Processes, Wiley-VCH, Weinheim, Germany, 1999.
3) University of Texas, Chemistry 302. Course material on chemical kinetics. Accessed from ch302.cm.utexas.edu, April 2015.
Catalysis FundamentalsDepartment Editor: Scott Jenkins
Reaction coordinate
Increasing energy
Ea for uncatalysed reaction
Activation energies for the steps of a catalyzed reaction
Reactants
Products
Ea1
Ea2
Ea3
H for both catalyzed and uncatalyzed reactions
Ener
gy,
kJ
