CHBE 551 Lecture 24 Catalysis I

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Importance Of Catalysis

90% of all chemical processes use catalysts

Changes in catalysts have a giant influence on rates and selectivity’s of reactions. More than anything else

Most real reactor design associated with optimizing performance of catalyst

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Catalysis Definition

Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process

Not being consumed catalyst does change

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Catalytic Reaction Occurs Via A Catalytic Cycle:

reactants + catalyst complex 

complex products + catalyst

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Example: Rhodium Catalyzed CH3OH+COCH3COOH

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HI

H O2

CH OH3

CH I3

CH COOH3

CH COI3[Rh(CO) I ]2 2

-

RhI

H C3C I

I

O

RhI

CH3

CI

I

O

CO

CO

CO

Figure 12.1 A schematic of the catalytic cycle for Acetic acid production via the Monsanto process.

Printing press analogy

The Rate Enhancement Of A Number Of Reactions In The Presence Of A

Catalyst

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Reaction Catalyst Rate Enhancement

Temperature

Ortho H2 Para H2 Pt (solid) 300K

2NH3 N2 + 3H2 Mo (solid) 600K

C2 H4 + H2 C2 H6 Pt (solid) 300K

H2 +Br2 2HBr Pt (solid) 1 108 300K

2NO + 2H2 N2 + 2H2 O Ru (solid) 3 1016 500K

CH3COH CH4 + CO I2 (gas) 4 106 500K

CH3CH3 C2H4 +H2 NO2 (gas) 1 109 750K

(CH3)3 COH

(CH3)2CH2CH2+H2O

HBr (gas) 3 108 750K

1040

1020

1042

Examples Of Effects Of Solvent

Table 13.2 The rate of the SN2 reaction NaCl + CH3I NaI + CH3Cl

at 350 K______________________________________________________________

Rate Constant, Rate Constant,

Solvent liter/(mol-second) Solvent liter/(mol-second)

______________________________________________________________Gas phase ~10-45 ______ _______

Water 3.5 x 10-6 Methylcyanide 0.13

Methanol 3.1 x 10-6 Dimethylformamide 2.5______________________________________________________________

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Types Of Catalysts

• Homogeneous catalysts:Soluble compounds that go in

solutions

• Heterogeneous catalysts:Solids that sit in reactors

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Common Examples Of Catalysts

HomogeneousEnzymes (detergents, digestion, tears)

HeterogeneousCatalytic converter

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Rate Laws Different Than With Gas Phase Reactions

Rate proportional to the surface area not the volume.

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Effects Of Surface Area

Consider a platinum catalyzed reaction.

You can run the reaction1) Run the reaction on the wire2) Take the wire and smash it with a

hammer and then run the reaction.

The rate will be higher on the wire you smashed with a hammer!

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Why Does Smashing A Wire Change The Rate?

• When you squashed the platinum you created more surface area.

• You also changed the shape of the surface which can affect the rate.

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Turnover Numbers

Rates of catalytic reactions often expressed as turnover number

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SNA

N

R = T

RA = Rate per unit area (molecules/cm2-sec)NS = Number of exposed metal atoms / unit area (Atoms/cm2)

Turnover Numbers For Some Typical Reactions

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200 400 600 800 1000 1200

Reaction Temperature, K

10-6

10-4

10-2

100

102

Tur

nove

r N

um

be

r, s

ec-1 Hydrogenation

OlefinIsomerization

Dehydrogenation

AlkaneHydrogenolysis

Cyclization

SiliconDeposition

GaAsDeposition

1400

Typical Catalytic Kinetics

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1012

1013

10-6

Ra

te, M

ole

cule

s/cm

-se

c2

1011

10-7

10-8

10-6

CO pressure, torr10

-710

-8

O pressure, torr2

390 K

410 K415 K

450 K450 K

440 K440 K

425 K

Figure 2.15 The influence of the CO pressure on the rate of CO oxidation on Rh(111). Data of Schwartz,

Schmidt, and Fisher

Typical Catalyst Kinetics

2CO2

2OCO1CO

PK1

PPkR

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Called a Langmuir-Hinshelwood rate law. Also called Monod rate

law.

Temperature Dependence

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Rat

e, M

olec

ules

/cm

-se

c2

Temperature, KTemperature, K

1E+13

1E+12

1E+11

600400 800600 800400

P =2.E-7 torrCO

A

B

C

2P =2.5E-8 torrO

DEF

Figure 2.18 The rate of the reaction CO + 2 O2 CO2 on

Rh(111). Data of Schwartz, Schmidt and Fisher[1986].

Catalysts Do Not Work Over A Broad Temp Range

Gas Phase -- NoWall Reactions

Catalyst Alone

Approximate Effect Of Walls

0 500 1000 1500 2000 2500

Temperature, K

1E-15

1E-12

1E-9

1E-6

0.001

1

1000R

ate

, M

ole

s/lit

se

c

Figure 12.2 The rate of hydrogen oxidation on a platinum coated pore calculated with a) only heterogeneous (catalytic) reactions, b) only radical reactions, and c) combined radical, homogeneous reactions

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Types Of Catalysts:

Homogeneous CatalystsHeterogeneous Catalysts

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Homogeneous Catalysts:

Acids or Bases Metal salts Enzymes Radical initiators

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Table 12.2-Some Reactions Commonly Catalyzed By Acids And

Bases

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Reaction Example Typical Application

Isomerization(Rearranging the

structure of a molecule)

CH2=CHCH2CH3

CH3CH=CHCH3

Octane EnhancementMonomer Production

Paraxylene Production

Alkylation(Making too little molecules into a

bigger one)

CH3CH=CHCH3 + CH3CH2CH2CH3

(CH3CH2)CH(CH3)(C4H9)

Pharmaceutical Production

Monomer ProductionFine Chemicals

Butane + olefin octane

Cracking(Taking a big molecule and making it into two

littler ones).

C12H24 C7H14 + C5H10 Crude Oil ConversionDigestion

Acids And Bases As Catalysts

Benzene ethylene ethylbenzene

(12.2)

a proton reacts with the ethylene to form an ethyl ion: H CH2CH2 CH3CH2

(12.3)

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Acids As Catalysts Continued

The ethyl ion reacts with benzene to yield and ethylbenzene ion: CH3CH2 C6H6 CH3CH2C6H6

(12.4)

Then the ethylbenzene ion loses a proton: CH3CH2C6H6 CH3CH2C6H5 H

(12.5)

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Solid Acids And Bases As Catalysts

Table 12.9 Some common solid acids and bases

Material Type Material Type silica/alumina solid acid Mordenite zeolite

alumina solid acid ZSM-5 zeolite Y-zeolite Faugasite

zeolite VFI large pore zeolite

Sodalite zeolite Offretite zeolite HF-SbF5 superacid HSO3F superacid

H2[Ti6O4(SO4)4(OEt)10]

superacid Sulfated Zirconia

superacid

MgO solid base Na2O base

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Very Complex Pore Structure

25Figure 12.4 A diagram of the pore structure in Faugasite.

Enzymes As Catalysts

Oxidoreductases (promote oxidation reduction

reactions)

Transferases (promote transfer of functional groups)

NADH peroxidas

e (Oxidizes

NADH with peroxides

NADH + H2O2 NAD(+)+2

H2O.

Dimethylallylcis-transferase (Transfer

dimethylallyl groups)

Dimethylallyl diphosphate + isopentenyl

disphosphatediphosphate + dimethylallylcis-

isopentenyldiphosphate

Ferroxidase (oxidizes

Iron)

4 Fe2+ + 4 H+ + O2 4 Fe3+

+ 2 H2O

Glycoaldehyde transferase(Transfer’s

Glucoaldeydes)Also called

Transketolase

Sedoheptulose 7-phosphate + D-glyceraldehyde 3-

phosphate D-ribose 5-phosphate + D-xylulose 5-

phosphate

Glucose oxidase(oxidizes Glucose)

-D-Glucose + O2 D-

glucono-1,5-lactone + H2O2

Alanine aminotransferase(Transfer amino

groups from alanine)

L-Alanine + 2-oxoglutarate pyruvate + L-glutamate

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Solvents: As Catalysts

CH3I NaCl CH3Cl + NaI

(12.17)

Table.12.6 The rate of reaction (12.17) in several solvents. All measurements have been extrapolated to 25 C

Solvent Rate const, lit/mole sec

Gas Phase about 10-45 Water 3.5 10-5 Methol 3 10-6 Methyl Cyanide

0.13

DMF 2.5

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Next: Heterogeneous Catalysis

Examples of heterogeneous catalysts include:

  Supported Metals Transition Metal Oxides and Sulfides Solid Acids and Bases Immobilized Enzymes and Other

Polymer Bound Species

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Supported Metal Catalysts

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Figure.12.3 A picture of a

supported metal catalyst.

Use support because platinum very expensive and only the surface is active. Spread platinum out on cheap support. Support also provides strength

Advantage Of Heterogeneous Catalysts Compared To

Homogeneous: Cheaper separation More selective Generally cheaper Disadvantage Not quite as active or a per metal

atom basis

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Typical Mechanism Of Heterogeneous Catalysis (H2+C2H4C2H6)

H2 + 2S 2 H(ad)

(12.18)

C2H4 S C2H4 ad

(12.19)

C2H4 ad H ad C2H5 ad S

(12.20)

C2H5 ad H ad C2H6 2S

(12.21)

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Summary

Catalysts make a tremendous difference to rates 1013 enhancement average 1040 possible Kinetics change – rarely linear Optimum conditions often at max rate

Zero order kinetics Two types of catalysts

Homogeneous Heterogeneous

Homogeneous more active Heterogeneous less expensive to use/control

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Query

What did you learn new in this lecture?

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