CHEE 323 1.1

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.

CHEE 323 1.2

Catalytic Activity

The addition of molecular hydrogen to an olefin such as ethylene is a highly favourable reaction from a thermodynamic standpoint.

Gfo (kJ/mole)

C2H6 -32.9 C2H4 68.1

H2 0 Go

reaction -101.0

Keq= exp(-Go/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.

33222 CHCHHCHCH

CHEE 323 1.3

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

CHEE 323 1.4

Catalytic Activity

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

HOMO

CHEE 323 1.5

Catalytic Activity

In-phase orbital overlap results in a lowering of the ground stateenergy of the system, andhence, leads to bonding.

The approach of asymmetricorbitals (+ve, -ve) leads to nonet positive overlap, and thereaction is symmetry forbidden.

Direct addition of H2 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.

LUMOof olefin

HOMOof H2

CHEE 323 1.6

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.

CHEE 323 1.7

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

CHEE 323 1.8

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.

CHEE 323 1.9

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

CHEE 323 1.10

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

CHEE 323 1.11

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.

CHEE 323 1.12

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.