Catalysis Introduction

ChE 511 - Catalysis Prof. Dr. Işık Önal

1

CATALYSTSUsed in Chemical and Allied Industries. Approximately 1/3 of Material

GNP of U.S. involves a catalytic process somewhere between raw materials and finished products.

TYPES OF CATALYTIC PROCESSESHomogeneous : Catalyst is in a solution with at least one of the

reactants.Heterogeneous : Involves more than one phase where usually

catalyst is in solid phase and reactants and products are gases or liquids.

MOST IMPORTANT FUNCTIONS OF A CATALYSTActivity

SelectivityLong-term stability


2

ChemicalReactor

ChemicalKinetics

DesignMathematicsFluid FlowThermo.

Mass Transfer

Heat Transfer

Materials Process Control

Economics

ProductMarketing

ChemicalProcessDesign of a

chemicalreactor

Bench-scaleBatch Reactor

Bench-scaleContinuousReactor

Pilot Plant Demonstration Plant----Operating Plant

LECTURE-1

ChE311


3


4

LECTURE-1

ChE311


5

Category Typical PlantCapacity Price Waste Products

Tons/yr $/1b lb/lb___________________ _______

Petroleum Refining 106 - 108 0.1 0.1Commodity Chemicals 104 – 106 0.1-2 1 - 3Fine Chemicals 102 – 104 2 – 10 2 – 10Foods 1 – 50Materials 0 - Pharmaceuticals 10 - 103

Very different tasks in each category for a Ch.E.

ImportantIn petroleum + commodity Low-costFine chemicals Patent

protectionFoods & pharmaceuticals Patents, trademarks

Marketing, advertising

LECTURE-1

ChE311


6

DEFINE PROBLEM/OBJECTIVESActivitySelectivity ChemicalDeactivation DiffusionalRegeneration MechanicalCostAvailability

CATALYST RESEARCHCatalyst Prop. Catalytic Mech.Catalyst Rate EquationSupport Reaction StepsPromoters EnergeticsPreparation AdsorptionPretreatment Mass & Heat Pellet Form Transfer

PATTERNS OF BEHAVIORTHEORIES OF

CATALYSIS

CATALYTIC CHEMISTRY

Surface Chemistry and Physics Physical Chemistry Solid State Physics Patents Literature Experimental Programs

CHEMICAL ENGINEERING

Process Analysis Reaction Engineering

Economics

PROCESS ENGINEERING

Comparison with Similar Processes

PROCESS NEED

New Process Modification of Old

Process


7

CATALYST DESIGN

New Experimental Commercial

CATALYST TESTINGExploratoryBench ReactorsModel Reactions

CATALYST PREPARATIONLaboratory MethodsSmall ScalePreparation Parameters

COLLOID CHEM.TechniquesDescription

COMMERCIAL MANUFACTUREScale-up of ProceduresPlant FormulationsLarge Batches

UNIT OPERATIONSPrecipitationWashingDryingCalcining Solid Handling

REACTOR ENG.Unit Design and OperationData AnalysisModeling

PILOT UNIT TESTINGReal FeedsProcess ConditionsProcess VariablesLife-time StudiesScale-up Data

PROCESS DESIGNProcess and Economic OptimizationPlant Design

CHEMICAL AND MECHANICAL ENG.Plant DesignMaterialsSpecifications


8

TARGET REACTION

STOICHIOMETRIC ANALYSIS

THERMODYNAMIC ANALYSIS

MOLECULAR MECHANISM

SURFACE MECHANISM

REACTION PATH

CATALYST PROPERTIES

CATALYTIC MATERIALS

PROPOSED CATALYSTSteps in catalyst design


9

TARGET REACTION

LIST STOICHIOMETRIC REACTIONS

GENERATE THERMODYNAMIC PARAMETERS

PROPOSE MOLECULAR MECHANISMS

PROPOSE SURFACE MECHANISMS

DEFINE CATALYST PROPERTIES

PROPOSED CATALYST

CHEMISTRYDATA BASE

THERMODYNAMICDATA BASE

REACTIONDATA FILE

SURFACE CHEMISTRY FILE

CATALYST PROPERTY FILE

OperatorInput

OperatorInput

OperatorInput


10

Propylene Oxide Formation on Cu2O (001) Surface

(by utilizing, DFT, VASP code)


11

11

Relative Energy Profile of Propylene Oxide, Propionaldehyde and Allyl Radical Formation

-4,5

-4

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

0

0,5

1R

elat

ive

Ene

rgy

(eV

)

Surface Intermediate Type Mechanism and Acrolein Formation Pathway on Cu2O(001)

C3H6 (ads)

OMMP2(ads)

PO(ads)

PO (g)

Allyl-radical(ads)

Allyl-radical(g)

Acetone (ads)

Acetone (g)TS(Acetone)

TS(Acrolein) Acrolein (g)

Acrolein(ads)

TS (PO)


12

R1

R2

R3A

B

C

CH2=CH2 + O2

Ag

Pt

PdCl2, CuCl2, HCl

Homogeneous

OCH2

CH2

2 CO2 + H2O

CH3CHO


13

REACTION MECHANISM •KINETICS (MACROSCOPIC LEVEL)

•DYNAMICS (MICROSCOPIC LEVEL)

HETEROGENEOUS CATALYSIS BY

METALS

SYNTHESIS OF CATALYSTS CHARACTERIZATION OF CATALYSTSSOLID STATE CHEMISTRY

SURFACE AND COLLOID CHEMISTRY

ORGANOMETALLIC CHEMISTRY

STRUCTURE, TEXTURE, SPECTROSCOPY, AND DIFFRACTION


14

HOMOGENEOUS CATALYSIS

CH3-CH=CH2 + CO + H2

CH3-CH2

CHO

CH3isobutylaldehyde

CH3-CH2-CH2-CHOn-butylaldehyde

CoComplexLiq. Phase Catalyst

Propylene


15

HETEROGENEOUS CATALYSIS

+ H2C=CH2

Solid Phase Catalyst

Acid Catalyst

C2H5

Benzene Ethylene Ethylbenzene

C2H5

Ethylbenzene

Dehydrogenation

CatalystC2H5-CH=CH2

[Fe3O4-type] Styrene

(Phosphoric

Acid Type

or

ZSM-5 Type)


16

C + H2O

Coal or biomass

CO+ H2

“Syn-gas”

CH4 + O2

Natural Gas

CH3OH

Methanol

+ COAcetic Acid

Cellulose Acetate

(base for photographic film)

+Cellulose

Ag HCHO Formaldehyde

Polymers

(fabrics and building materials)

(CH2)n

Petrol (fuel)

ZeoliteCH3

Toluene (solvent)

Zeolite

Cu/ZnO

CH4 Methane Substitute

Natural GasNi/Al2O3

(CH2)n

Polyethylene (polyethene)

Ru

Alkanes, Alcohols,

Alkenes (fuels, aviation or

diesel), solvents,

detergents

Fe, Co

CH2OH

CH2OH

And related glycols (antifreeze agents)


17

Pote

ntia

l Ene

rgy

Reaction Coordinate

Reactants

Ehet

Ehom

ΔH

Products


18

1 2 3 4 5 6 70

1

2

3

4

5

6Lo

g 10

(rat

e co

nsta

nt),

arbi

trar

y un

its

103/T

Ahet

Ahom

Homogeneous Reaction Heterogeneous

Reaction

Measurable Range

YX


19


20

SURFACE ASSISTED (CATALYTIC) REACTIONS

Example: Catalytic Hydrogenation of EthyleneC2H4 + H2

ΔG C2H6

Postulated Mechanisms:For Homogeneous Reaction:

C2H4 + H2ΔG* C2H4 . H2 Activated Complex

Where ΔG* = Free Energy of Activation

C2H6

For Heterogeneous Reaction:

C2H4 + [S]1ΔG1 C2H4[S]1

H2 + C2H4[S]1ΔG2 C2H4[S]1 H2

C2H4[S]1 H2ΔG3 C2H6 + [S]1


21

Published Data from Boudart:

CuO-MgO

Catalyst

At 600K;

A difficult homogeneous reaction is replaced by a more easily executed heterogeneous surface reaction involving adsorbed C2H4 .

(Eley-Rideal Mechanism)These are simply phenomenological models, not mechanisms. The actual mechanism is quite complex.


22

HETEROGENEOUS CATALYSIS MUST ALWAYS BE PRECEDED BY ADSORPTION

desorptionAB

Cadsorption

A

B

C

Surface Reaction

Surface Migration

Two possible ways in which heterogeneous catalysis proceeds at a surface; the Langmuir-Hinshelwood Mechanism (left) and the Eley-Rideal Mechanism (right)

A


23

C-C=C-C

HMethyl Cyclopropane

Butene ButaneC-C-C-C

Metal

Support

A bifunctional catalyst such as platinum on Al2O3 facilitates the isomerization of methylcyclopropane to 2-butene as well as the

hydrogenation of 2-butene to butane (after Boudart)


24

Physical Adsorption•Weak Forces of Molecular Interaction (Van der Waals)

•Small Heats of Adsorption (10-40 kj/mol)

•Occurs only at ~Tb.p.

•Equilibrium rapidly attained, low E

•Highly reversible

•Multilayer coverage possible

•Nonspecific, almost indistinguishable from condensation

•In general, not responsible for catalysis; however may help to form chemisorption precursors

•Important for determination of total surface area and pore size of catalyst support

Chemisorption•Formation and Rupture of Chemical Bonds

•Large Heats of Adsorption (80-400 kj/mol)

•No such restriction at high T

•Nonactivated: Low E Activated: High

• Often Irreversible

•Less than a monolayer

•Highly specific (a function of a particular crystalline face)

•Necessary for catalysis

•Important for determination of active-center surface area and elucidation of surface reaction kinetics


25


26


27

Oxidation of CO to CO2 Pd-Ce0.75Zr0.25O2 Catalyst

Surface

Quantum Mechanical DFT Calculations


28


29


30


31


32


33


34


35


36


37


38

DFT COMPUTATIONSCO conversion energetics

CO‐gp

CO‐ads

CO2‐des O2‐gp

O2‐ads

2nd CO‐gp

2nd CO22 CO2 gp

‐200,0

‐180,0

‐160,0

‐140,0

‐120,0

‐100,0

‐80,0

‐60,0

‐40,0

‐20,0

0,01 3 5 7 9 11

Relativ

e En

ergy (kcal/mol)

Pd‐CeZrO2


39

The Langmuir Treatment of Adsorption

Ideal Surface Requirements

1. Energetically homogeneous or uniform surface;

2. Energy of interaction with the adsorbate is not affected by the presence or absence of adsorbate on adjoining sites;

3. Each site can accommodate only one adsorbate molecule or atom;

4. Adsorption would occur when an adsorbate molecule or atom with the required energy strikes an unoccupied site;

5. The energy contours would be unaffected by the extent of adsorption;

6. All adsorption has the same mechanism; each has the same structure;

7. The extent of adsorption is less than one complete monolayer


40

If ra=Rate of adsorption,

rs= No. of molecules striking the surf./ time-area (bare surf.)

Then ra= s . rs

ra=kaPA(1- θ)

where; ka=Ads. Rate Const.

θ=Fraction occupied by already adsorbed molecules

Also if rd= Rate of desorptionrd=kd θ

At Equilibrium: ra = rd


41

If A + S ka

kd

A S (Elementary Step)

with microscopic reversibility, then

and

Introduce Where V=Vol. Adsorbed

Vm= Vol. Adsorbed by all the active sites covered

NOTE: This treatment is applicable to both physical ads. & chemisorption

Catalysis Introduction

Documents

Transcript of Catalysis Introduction