CHBE 452 Lecture 27 Catalysis I 1. Importance Of Catalysis 90% of all chemical processes use...
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Transcript of CHBE 452 Lecture 27 Catalysis I 1. Importance Of Catalysis 90% of all chemical processes use...
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
5
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
8
Common Examples Of Catalysts
HomogeneousEnzymes (detergents, digestion, tears)
HeterogeneousCatalytic 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 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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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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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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