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CHEE 3231.1 Catalytic Reaction Kinetics We define a catalyst as a substance that increases the rate of reaction without being substantially consumed in.

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Presentation on theme: "CHEE 3231.1 Catalytic Reaction Kinetics We define a catalyst as a substance that increases the rate of reaction without being substantially consumed in."— Presentation transcript:

1 CHEE 3231.1 Catalytic Reaction Kinetics We define a catalyst as a substance that increases the rate of reaction without being substantially consumed in the process  note that the equilibrium condition is governed by thermodynamics, and a catalyst does not alter the equilibrium state, but the rate at reactions proceed  note also that catalysis can bring the system to a condition that is not lowest in free energy, as predicted by thermodynamics. An initiator generates a species that supports a reaction, which may participate in a large number of substrate transformations but always has a limited lifetime.

2 CHEE 3231.2 Catalytic Activity The addition of molecular hydrogen to an olefin such as ethylene is a highly favourable reaction from a thermodynamic standpoint.  G f o (kJ/mole) C 2 H 6 -32.9 C 2 H 4 68.1 H 2 0  G o reaction -101.0 K eq = exp(-  G o /RT) = exp(101,000J / (8.314J/molK * 298K)) = 5.1*10 17 In spite of this thermodynamic driving force, the direct reaction of ethylene and hydrogen does not occur at appreciable rates.

3 CHEE 3231.3 Catalytic Activity An examination of the molecular orbitals of ethylene and hydrogen demonstrates the reason for a low kinetic rate of hydrogenation, in spite of the large thermodynamic driving force. LUMO HOMO

4 CHEE 3231.4 Catalytic Activity In addition to  bonds from sp 2 orbital overlap, combination of p- orbitals leads to  -molecular orbitals, both bonding and anti- bonding. LUMO HOMO

5 CHEE 3231.5 Catalytic Activity In-phase orbital overlap results in a lowering of the ground state energy of the system, and hence, leads to bonding. The approach of asymmetric orbitals (+ ve, - ve ) leads to no net positive overlap, and the reaction is symmetry forbidden. Direct addition of H 2 to ethylene through a four-centre transition state is symmetry forbidden, as the bonding  orbital of hydrogen (HOMO) and the antibonding  * orbital of the olefin (LUMO) cannot overlap effectively.  Consequently, the rate of hydrogenation by this mechanism is extremely small, and a catalyst is required. LUMO of olefin HOMO of H 2

6 CHEE 3231.6 Catalytic Activity While direct addition of H 2 to an olefin is symmetry forbidden, the reaction can be facilitated by a transition metal complex such as RhCl(PPh 3 ) 3 1. Oxidative addition of H 2 to the metal centre, 2. Coordination of the olefin 3. Migratory insertion of the olefin into the M-H bond, 4. Reductive elimination of the alkane.

7 CHEE 3231.7 Catalytic Selectivity While olefin hydrogenation by RhCl(PPh 3 ) 3 has remarkable activity, catalytic processes are also developed for unique selectivity.  A leading example is the synthesis of Levodopa, an optically active drug generated from non-chiral starting materials for the treatment of Parkinson’s disease. Phosphine ligand of rhodium catalyst precursor

8 CHEE 3231.8 CHEE 323 - Objectives On completing CHEE 323, students will have:  surveyed a wide range of catalytic reactions that are relevant to industrial practice,  integrated fundamental chemistry with principles of reaction kinetics, transport phenomena and thermodynamics,  applied this knowledge to solve “open-ended” design problems. The resources available to help students meet these objectives are:  Lectures: serve as a guide to the course material, introduce the subject matter and highlight difficult elements of the course  Problem Sets: illustrate the course material and allow students to exercise their knowledge  “Open-ended” Design Problems: challenge students to pose their own questions and find original solutions.

9 CHEE 3231.9 CHEE 323 - Course Outline 1. Catalytic Reaction Kinetics  Relationships between kinetics and thermodynamics  Elementary reactions and the reaction coordinate  Formulating complex kinetic rate expressions  Estimating rate parameters from experimental data 2. Free-radical and Carbocationic Chain Reactions  Process dynamics and the kinetic chain length  Design of polymerization, oxidation, alkylation, and isomerization processes 3. Enzyme Catalyzed Reactions  Structure and reactivity of catalytic proteins  Dynamics of closed-sequence, catalytic processes  Enzyme immoblization and mass transfer effects  Coping with catalyst deactivation

10 CHEE 3231.10 CHEE 323 - Course Outline 4. Homogeneous Catalysis by Organometallic Complexes  Structure and reactivity of organotransition metal complexes  Dynamics of olefin transformations  Interfacial mass transfer in gas-liquid reactions  Heat transfer in polymerization processes 5. Heterogeneous Catalysis  Synthesis and characterization of heterogeneous catalysts  Heterogeneous reaction dynamics  Mass and heat transfer in heterogeneous processes  Catalytic combustion – automotive applications 6. Acid-Catalyzed Reactions  Hydrocarbon conversion chemistry for fuel applications  Oil refinery unit operations – design of reaction processes  Highly-Ordered Solid Catalysts - Zeolites and Clays

11 CHEE 3231.11 Open-Ended Design Problems These exercises allow students to engage in more design-oriented activity. Using instructors only for reference as opposed to direct guidance, groups will attempt to solve two process development problems. A problem will be presented in the first design tutorial session, and groups will prepare a list of questions for each of three areas:  Catalytic chemistry requirements  Overall process flowsheet  Catalytic reactor design Where possible, information relating to these questions will be provided. Each group will submit a report (no longer than 9 pages) that details their design concept and calculations.

12 CHEE 3231.12 Food for Thought… Write a two paragraph article that supports one of the following statements:  The world changed from black & white to colour in 1935.  The hum that is heard upon picking up a phone receiver is the operator saying “ooooooo”  The windmills on farms are used to keep the cows cool.  The blind can read speed bumps like braille, thereby allowing them to drive.

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