Catalytic Reaction Kinetics

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. Catalytic Activity • The addition of molecular hydrogen to an olefin such as ethylene is a highly favourable reaction from a thermodynamic standpoint. • DGfo (kJ/mole) • C2H6 -32.9 • C2H4 68.1 • H2 0 • DGoreaction -101.0 • Keq= exp(-DGo/RT) • = exp(101,000J / (8.314J/molK * 298K)) • = 5.1*1017 • In spite of this thermodynamic driving force, the direct reaction of ethylene and hydrogen does not occur at appreciable rates.

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. Catalytic Activity • In addition to s bonds from sp2 orbital overlap, combination of p-orbitals leads to p-molecular orbitals, both bonding and anti-bonding. LUMO HOMO

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 H2 to ethylene through a four-centre transition state is symmetry forbidden, as the bonding s orbital of hydrogen (HOMO) and the antibonding p* 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 H2

6. Catalytic Activity • While direct addition of H2 to an olefin is symmetry forbidden, the reaction can be facilitated • by a transition metal • complex such as RhCl(PPh3)3 • 1. Oxidative addition of H2 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. Catalytic Selectivity • While olefin hydrogenation by RhCl(PPh3)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 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 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 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. 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. 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.