CHBE 452 Lecture 27
Catalysis I
1
Importance Of Catalysis
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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
2
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
3
Catalytic Reaction Occurs Via A
Catalytic Cycle:
reactants + catalyst  complex
complex  products + catalyst
4
Example: Rhodium Catalyzed
CH3OH+COCH3COOH
CH3COOH
CH3OH
HI
H2O
CH3COI
C
O
I
I
O
C
O
I
C
H3C
I
C
O
Rh
CH3I
[Rh(CO)2I2]-
CH3
Rh
C
O
I
I
CO
Figure 12.1 A schematic of the catalytic cycle for Acetic acid production via the
Monsanto process.
Printing press analogy
5
The Rate Enhancement Of A Number Of
Reactions In The Presence Of A Catalyst
Reaction
Catalyst
Rate
Temperature
Enhancement
Ortho H2  Para H2
Pt (solid)
1040
300K
2NH3N2 + 3H2
Mo (solid)
1020
600K
C2 H4 + H2  C2 H6
Pt (solid)
1042
300K
H2 +Br2  2HBr
Pt (solid)
1  108
300K
2NO + 2H2 N2 + 2H2 O Ru (solid)
3  1016
500K
CH3COH  CH4 + CO
I2 (gas)
4  106
500K
CH3CH3  C2H4 +H2
NO2 (gas)
1  109
750K
(CH3)3 COH 
(CH3)2CH2CH2+H2O
HBr (gas)
3  108
750K
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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
8
Common Examples Of Catalysts
Homogeneous
Enzymes (detergents,
digestion, tears)
Heterogeneous
Catalytic 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 reaction
1)
Run the reaction on the wire
2)
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
RA
TN =
NS
RA = Rate per unit area (molecules/cm2-sec)
NS = Number of exposed metal atoms /
unit area (Atoms/cm2)
13
2
10
Turnover Number, sec
-1
Turnover Numbers For Some Typical
Reactions
Dehydrogenation
Hydrogenation
Silicon
Deposition
0
10
GaAs
Deposition
-2
10
-4
10
-6
10
Olefin
Isomerization
Alkane
Hydrogenolysis
Cyclization
200
400
600
800
1000
1200
1400
Reaction Temperature, K
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Typical Catalytic Kinetics
13
450 K
450 K
2
Rate, Molecules/cm -sec
10
10
12
440 K
440 K
425 K
410 K
415 K
390 K
10
11
10
-8
-7
10
CO pressure, torr
10
-6
10
-8
-7
10
O 2 pressure, torr
10
-6
Figure 2.15 The influence of the CO pressure on the rate of CO
oxidation on Rh(111). Data of Schwartz, Schmidt, and Fisher
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Typical Catalyst Kinetics
R CO 
k1PCO PO 2
1  K 2PCO 
2
Called a Langmuir-Hinshelwood rate
law. Also called Monod rate law.
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Rate, Molecules/cm 2-sec
Temperature Dependence
PO2=2.5E-8 torr
PCO=2.E-7 torr
1E+13
C
1E+12
F
E
D
B
1E+11
A
400
600
Temperature, K
800
400
600
800
Temperature, K
Figure 2.18 The rate of the reaction CO
+ 2 O2  CO2 on Rh(111). Data of
Schwartz, Schmidt and Fisher[1986].
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Catalysts Do Not Work Over A Broad Temp
Range
1000
Rate, Moles/lit sec
1
Gas Phase -- No
Wall Reactions
Catalyst Alone
0.001
1E-6
Approximate
Effect Of Walls
1E-9
1E-12
1E-15
0
500
1000
1500
2000
2500
Temperature, K
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 Catalysts
Heterogeneous Catalysts
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Homogeneous Catalysts:
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Acids or Bases
Metal salts
Enzymes
Radical initiators
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Table 12.2-Some Reactions Commonly
Catalyzed By Acids And Bases
Reaction
Example
Typical Application
Isomerization
(Rearranging the
structure of a molecule)
CH2=CHCH2CH3 
CH3CH=CHCH3
Octane Enhancement
Monomer Production
Paraxylene Production
Alkylation
(Making too little
molecules into a bigger
one)
CH3CH=CHCH3 +
CH3CH2CH2CH3 
(CH3CH2)CH(CH3)(C4H9)
Pharmaceutical
Production
Monomer Production
Fine Chemicals
Butane + olefin
octane
Cracking
(Taking a big molecule
and making it into two
littler ones).
C12H24 C7H14 + C5H10
Crude Oil Conversion
Digestion
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Acids And Bases As Catalysts
Benzene  ethylene  ethylbenzene
(12.2)
a proton reacts with the ethylene to form an
ethyl ion:
H   CH 2CH 2   CH 3CH 2 
(12.3)
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Acids As Catalysts Continued
The ethyl ion reacts with benzene to yield
and ethylbenzene ion:
  C H  CH CH C H 
CH
CH
 3 2
 3 2 6 6
6 6
(12.4)
Then the ethylbenzene ion loses a proton:
CH 3CH 2C6H 6   CH 3CH 2C6H5  H 
(12.5)
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Solid Acids And Bases As
Catalysts
Table 12.9 Some common solid acids and bases
Material
silica/alumina
alumina
Y-zeolite
Faugasite
Sodalite
HF-SbF5
H2[Ti6O4(SO4)4(
OEt)10]
MgO
Type
solid acid
solid acid
zeolite
Material
Mordenite
ZSM-5
VFI
zeolite
superacid
superacid
Offretite
HSO3F
Sulfated
Zirconia
Na2O
solid base
Type
zeolite
zeolite
large pore
zeolite
zeolite
superacid
superacid
base
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Very Complex Pore Structure
Figure 12.4 A diagram of the pore structure in Faugasite.
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Enzymes As Catalysts
Oxidoreductases (promote
oxidation reduction reactions)
Transferases
(promote transfer of functional groups)
NADH
peroxidase
(Oxidizes
NADH with
peroxides
NADH + H2O2
NAD(+)+2
H2O.
Dimethylallylcistransferase (Transfer
dimethylallyl groups)
Dimethylallyl diphosphate +
isopentenyl
disphosphatediphosphate +
dimethylallylcisisopentenyldiphosphate
Ferroxidase
(oxidizes
Iron)
4 Fe2+ + 4 H+ +
O2  4 Fe3+ + 2
H2 O
Glycoaldehyde
transferase
(Transfer’s
Glucoaldeydes)
Also called
Transketolase
Sedoheptulose 7-phosphate + Dglyceraldehyde 3-phosphate  Dribose 5-phosphate + D-xylulose
5-phosphate
Glucose
oxidase
(oxidizes
Glucose)
-D-Glucose +
O2  Dglucono-1,5lactone + H2O2
Alanine
aminotransferase
(Transfer amino
groups from alanine)
L-Alanine + 2-oxoglutarate 
pyruvate + L-glutamate
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Solvents: As Catalysts
CH 3I  NaCl  CH 3Cl + NaI
(12.17)
Table.12.6 The rate of reaction (12.17) in
several solvents. All measurements have
been extrapolated to 25 C
Solvent
Gas Phase
Water
Methol
Methyl
Cyanide
DMF
Rate const,
lit/mole sec
about 10-45
3.5  10-5
3  10-6
0.13
2.5
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Next: Heterogeneous Catalysis
Examples of heterogeneous catalysts
include:
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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
Use support because platinum
very expensive and only the
surface is active.
Spread platinum out on cheap
support.
Support also provides strength
Figure.12.3 A picture of a
supported metal catalyst.
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Advantage Of Heterogeneous Catalysts
Compared To Homogeneous:
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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+C2H4C2H6)
H 2 + 2S  2 H(ad)
(12.18)
C 2 H 4  S  C 2 H 4 ad 
(12.19)
C2 H 4 ad   H ad   C2 H5ad   S
(12.20)
C2 H5ad   H ad   C2 H 6  2S
(12.21)
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Summary
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Catalysts make a tremendous difference to
rates
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Two types of catalysts
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1013 enhancement average 1040 possible
Kinetics change – rarely linear
Optimum conditions often at max rate
 Zero order kinetics
Homogeneous
Heterogeneous
Homogeneous more active
Heterogeneous less expensive to use/control
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Query
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What did you learn new in this lecture?
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