Transcript che 377 lectures - Classnotes For Professor Masel's Classes

ChE 553 Lecture 22
Introduction To Catalysis
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+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
6
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
7
Types Of Catalysts:
Homogeneous Catalysts
Heterogeneous Catalysts
8
Homogeneous Catalysts:
•
•
•
•
Acids or Bases
Metal salts
Enzymes
Radical initiators
9
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
10
Some Reactions Commonly Catalyzed By
Acids And Bases Continued
Reaction
Example
Esterfication
CH3CH2OH +CH3COOH
(Attaching an acid to a base  CH COOCH CH +
3
2
3
eliminating water)
H2O
Aldol Condensation
Reactions (combining two
aldehydes by eliminating
water)
2 CH3CH2CH2CHO 
+ H2O
Alcohol Dehydration
CH3CH2OH  CH2=CH2
(removing a hydrogen and
+ H2O
an OH from an alcohol,
producing a double bond)
Cationic Polymerization
Propylene 
polypropylene
Typical Application
Soap Production
Fragrance Production
Fine Chemicals
Pharmaceutical
production
Alternative fuels
Polymer Production
11
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)
12
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)
13
Metal Atoms: As Catalysts
H 2  2S  2H ad 
(12.6)
C2 H 4 + S  C2 H 4 ad 
(12.7)
C2 H 4 (ad) + H(ad)  C2 H5 (ad) + S
(12.8)
C 2 H 5 ad   H ad   C 2 H 6  2S
(12.9)
14
Examples Of Reactions Catalyzed By
Homogeneous Transition Metal Catalysts
Reaction
Olefin Polymerization
Olefin Hydrogenation
C2H4+H2OAcetaldehyde
(Wacker Process)
C2H4+H2 +CO propylaldehyde
(Hydroformylation)
CH3OH + CO  CH3COOH
(Monsanto Carbonylation
Process)
H2O2 +CH3CH2OH 
CH3CHO+2 H2O
Catalyst
[TiCl2(C5H5)2]2+ or
TiCl2/Al(C2H5)3
(Ziegler-Natta Catalyst)
Rh(P(C6H5)3)3Cl
Wilkinson Catalyst
PdCl2(OH)2
HCo(CO)4
[Rh(CO)2I2]1Fe2+
15
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
16
Enzymes Continued
Hydrolases
(Promote hydrolysis/cleavage reactions)
Lyases
(promote addition of CO2, H2O and NH3 to
double bonds or formations of double bonds
via elimination of CO2, H2O or NH3)
Carboxylesterase
(Promotes
hydrolysis of ester
linkages)
A carboxylic ester +
H2O  an alcohol + a
carboxylic anion
Carbonate
dehydratase
(Dehydrates
carbonates)
H2CO3 CO2 + H2O
1,4-ALPHA-DGlucan
glucanohydrolase
(also called ALPHAAmylase)
Hydrolysis of 1,4ALPHA-glucosidic
linkages in
oligosaccharides and
polyasaccharides.
Citrate dehydratase
Citrate  CISaconitate + H2O
Interleukin 1-beta
converting enzyme
Release of interleukin
1-beta by specific
hydrolysis at 116-ASP|-ALA-117 and 27ASP-|-GLY-28 bonds
Pyruvate
decarboxylase
A 2-OXO acid an
aldehyde + CO2
17
Enzymes Continued
Isomerases (promote isomerization
reactions)
Maleate isomerase
(promotes cis-trans
isomerization of Maleate)
Cholestenol DELTAisomerase
Mannose isomerase
Maleate 
Fumarate
Ligases (promotes formation of
bonds - generally used to catalyze
endothermic reactions requiring
ATP)
Leucine--trna
ligase
5-AlphaPyruvate
cholest-7-en-3- carboxylase
beta-ol 5Alpha-cholest8-en-3-beta-ol
D-Mannose  Aspartate-ammonia
D-fructose
ligase
ATP + L-leucine
+ t-RNA(leu) 
AMP +
diphosphate + Lleucyl-tRNA(leu).
ATP + pyruvate +
(HCO3)  ADP +
phosphate +
oxaloacetate
ATP + L-aspartate
+ NH3  AMP +
diphosphate + Lasparagine
18
Radical Initiators As Catalysts
Example:
C2 H 6  C2 H 4  H 2
(12.10)
X + I 2  2I + X
(12.11)
Then the iodine can react with ethane to start
the reaction:
I  CH 3CH 3  HI  CH 2 CH 3
(12.12)
19
Free Radical Polymerization
Catalysts:
R  O  O  R  2R O 
(12.13)
Then the radical reacts with the ethylene to
start the polymerization process:
RO   CH 2 CH 2  ROCH 2 CH 2 
(12.14)
20
Free Radicals Catalysts For
Ozone Destruction
Cl  O 3  ClO  O 2
(12.15)
The ClO can then react via a number of
processes to reduce the ozone layer. One
particular reaction is:
ClO  O 3  Cl  2O 2
(12.16)
21
Some Examples Of Reactions Initiated Or Catalyzed By
Free Radicals And Similar Species
Reaction
Olefin
Polymerization
Initiator
Peroxides,
(Ph)3CC(Ph)3
Hydrocarbon
Dehydrogenatio
n
Hydrocarbon
Oxidations
Iodine, NO2
chlorine atoms
 CH CH
 CH CH
3
3
2
Reaction
2SO2 +O2 
SO3
(Lead Chamber
Process)
Ozone
Depletion
Catalyst
NO/NO2
Cl
 N I,
 N C H COO
4
2
4
6
5
22
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
23
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
24
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.
25
Pictures Of Some
Heterogenous Catalysts
26
Pores In Heterogeneous
Catalysts
Figure 14.3 A cross sectional diagram of a typical
catalyst support.
27
Advantage Of Heterogeneous Catalysts
Compared To Homogeneous:
• Cheaper separation
• More selective
• Generally cheaper
Disadvantage
• Not quite as active or a per metal atom
basis
28
A Selection Of The Reactions
Catalyzed By Supported Metals
Reaction
Hydrocarbon
Hydrogenation,
Dehydrogenation
Catalyst
Pt, Pd, Ni
Reaction
Catalyst
Fe, Rh
CO + H2 
Hydrocarbons
(FischerTropsch)
CO oxidation, Pt, Pd, Cu, Ni, Steam reforming Ni plus additives
total oxidation of
Fe, Rh, Ru
for
hydrocarbons
production of
hydrogen
Cu/ZnO
Reforming
Pt/Re/Al2O3
CO + 2H2 
(Isomerization of
CH3OH
oil)
29
A Selection Of The Reactions
Catalyzed By Supported Metals
Reaction
2 CO + 2NO 
2CO2+ N2
N2 + 3 H2  2
NH3
2 C2H4 + O2
2 ethylene oxide
Catalyst
Pt, Rh, Ru
(catalytic
converter)
Fe, Ru, Rh
Ag, Cu
Reaction
2NH3 +O2 
N2O5 +3H2O
Catalyst
Pt
Alcohols + O2 
Aldehydes +
H2O e.g.
2 CH3OH + O2

2 H2CO +H2O
R-R' + H2 
RH + HR'
(Hydrogenolysis)
Ag, Cu
Ni, Co, Rh, Ru
30
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)
31
Transition Metal Oxides,
Nitrides, Sulfides:
Bond transition
Metal to O, N, S to reduce activity and to
increase selectivity
32
A Selection Of The Reactions Catalyzed By Transition
Metal Oxides, Nitrides, And Sulfides
Reaction
2 SO2 + O2  2 SO3
Catalyst
V2O5
Reaction
CO + H2O CO2+
H2
Hydrodesulfurization
CoS, MoS, WS
Catalyst
FeO, CuO, ZnO
(Water Gas Shift)
2(CH3)3COH
Þ(CH3)3COC(CH
3)3 + H2O
CH3CH=CH2 + O2  (Bi2O3)x(MoO3)y 2 CH3CH=CH2 + 3
O2 + 2NH3
(Bismuth
CH2=CHCHO +
2CH2=CHCN +
molybate)
Uranium
H2O
6 H2O
Antimonate
(aminoxidation)
TiO2
(FeO)x(Sb2O3)y
33
A Selection Of The Reactions Catalyzed By Transition
Metal Oxides, Nitrides, And Sulfides
Reaction
4 NH3 + 4 NO +O2 
4N2 + 6 H2O
Catalyst
V2O5, TiO2
(Selective catalytic
reduction)
CH3CH2(C6H5) +O2 
CH2=CH(C6H5) + H2O
(styrene production)
FeO
Aromatiztion
e.g. HeptaneTolvene
H2 or H2O
Cr2O3/Al2O3
Reaction
Catalyst
benzene+O2  (V2O5)x(PO4)y
maleic
anhydride +
water
naphthylene+O
2
 phthalic
anhydride +
water
Selective
oxidation of
hydrocarbons
Hydrodenitroge
nation
NiO, Fe2O3,
V2O5, TiO2
CuO, Co3, O4,
MnO2
NiS,MoS
34
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
35
Very Complex Pore Structure
Figure 12.4 A diagram of the pore structure in Faugasite.
36
Leads To Shape Selective
Catalysis
Cavity
C
H
H
H
H
H
H
C
Diffusion
Channel
Figure 12.27 An interconnecting pore structure which is
selective for the formation of paraxylene.
37
Summary
• Two types of catalysts
– Homogeneous
– Heterogeneous
• Homogeneous more active
• Heterogeneous less expensive, easier
to use/control
38