che 551 lectures - Classnotes For Professor Masel's Classes
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CHBE 452 Lecture 27
Catalysis I
1
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
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+COCH3COOH
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
300K
2NH3N2 + 3H2
Mo (solid)
1020
600K
C2 H4 + H2 C2 H6
Pt (solid)
1042
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
6
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
______________________________________________________________
7
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
9
Rate Laws Different Than With
Gas Phase Reactions
Rate proportional to the surface area not
the volume.
10
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!
11
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.
12
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
14
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
15
Typical Catalyst Kinetics
R CO
k1PCO PO 2
1 K 2PCO
2
Called a Langmuir-Hinshelwood rate
law. Also called Monod rate law.
16
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].
17
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
18
Types Of Catalysts:
Homogeneous Catalysts
Heterogeneous Catalysts
19
Homogeneous Catalysts:
Acids or Bases
Metal salts
Enzymes
Radical initiators
20
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
21
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)
22
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)
23
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
24
Very Complex Pore Structure
Figure 12.4 A diagram of the pore structure in Faugasite.
25
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
disphosphatediphosphate +
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
26
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:
Supported Metals
Transition Metal Oxides and Sulfides
Solid Acids and Bases
Immobilized Enzymes and Other
Polymer Bound Species
28
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.
29
Advantage Of Heterogeneous Catalysts
Compared To Homogeneous:
Cheaper separation
More selective
Generally cheaper
Disadvantage
Not quite as active or a per metal atom
basis
30
Typical Mechanism Of Heterogeneous
Catalysis (H2+C2H4C2H6)
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 H5ad S
(12.20)
C2 H5ad H ad C2 H 6 2S
(12.21)
31
Summary
Catalysts make a tremendous difference to
rates
Two types of catalysts
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
32
Query
What did you learn new in this lecture?
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