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:
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.
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.
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
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
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
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.
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
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.
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
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
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.
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.