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ChE 553 Lecture 22 Introduction To Catalysis
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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
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 Catalysts
Heterogeneous 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
Some Reactions Commonly Catalyzed By Acids And Bases Continued
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Reaction Example Typical ApplicationEsterfication Soap Production
(Attaching an acid to a base eliminating water)
Fragrance Production
2 CH3CH2CH2CHO Fine Chemicals
+ H2O Pharmaceutical production
Alcohol Dehydration(removing a hydrogen and an OH from an alcohol,
producing a double bond)
Cationic Polymerization Propylene
polypropylenePolymer Production
Aldol Condensation Reactions (combining two aldehydes by eliminating
water)
CH3CH2OH +CH3COOH CH3COOCH2CH3 +
H2O
CH3CH2OH CH2=CH2
+ H2O
Alternative fuels
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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Metal Atoms: As Catalysts
H2 2S 2H ad (12.6)
C2H4 + S C2H4 ad
(12.7)
C2H4 (ad) + H(ad) C2H5 (ad) + S
(12.8)
C2H5 ad H ad C2H6 2S
(12.9) 14
Examples Of Reactions Catalyzed By Homogeneous Transition Metal Catalysts
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Reaction Catalyst Olefin Polymerization [TiCl2(C5H5)2]
2+ or TiCl2/Al(C2H5)3
(Ziegler-Natta Catalyst) Olefin Hydrogenation Rh(P(C6H5)3)3Cl
Wilkinson Catalyst C2H4+H2O Acetaldehyde
(Wacker Process) PdCl2(OH)2
C2H4+H2 +CO propylaldehyde (Hydroformylation)
HCo(CO)4
CH3OH + CO CH3COOH (Monsanto Carbonylation
Process)
[Rh(CO)2I2]1-
H2O2 +CH3CH2OH CH3CHO+2 H2O
Fe2+
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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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-D-Glucan
glucanohydrolase
(also called ALPHA-Amylase)
Hydrolysis of 1,4-ALPHA-glucosidic
linkages in oligosaccharides
and polyasaccharides.
Citrate dehydratase
Citrate CIS-aconitate + H2O
Interleukin 1-beta converting enzyme
Release of interleukin 1-beta
by specific hydrolysis at 116-ASP-|-ALA-117 and 27-ASP-|-GLY-28
bonds
Pyruvate decarboxylase
A 2-OXO acid an aldehyde + CO2
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Enzymes Continued
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Maleate isomerase (promotes cis-trans
isomerization of Maleate)
Maleate
Fumarate
Leucine--trna ligase
ATP + L-leucine + t-RNA(leu)
AMP + diphosphate + L-
leucyl-t-RNA(leu).
Cholestenol DELTA-isomerase
5-Alpha-cholest-7-en-3-
beta-ol 5-Alpha-cholest-8-en-3-beta-ol
Pyruvate carboxylase
ATP + pyruvate +
(HCO3) ADP +
phosphate + oxaloacetate
Mannose isomerase D-Mannose
D-fructose
Aspartate--ammonia
ligase
ATP + L-aspartate
+ NH3 AMP +
diphosphate + L-asparagine
Isomerases (promote isomerization reactions)
Ligases (promotes formation of bonds - generally used to catalyze endothermic reactions requiring
ATP)
Radical Initiators As Catalysts
Example:
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C2H6 C2H4 H2
(12.10)
X + I2 2I + X
(12.11)
Then the iodine can react with ethane to start the reaction:
I CH3CH3 HI CH2CH3 (12.12)
Free Radical Polymerization Catalysts:
R R R O O 2 O
(12.13)
Then the radical reacts with the ethylene to start the polymerization process:
R RO 2CH2 OCH2CH2 CH (12.14)
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Free Radicals Catalysts For Ozone Destruction
Cl O3 ClO O2
(12.15)
The ClO can then react via a number of processes to reduce the ozone layer. One particular reaction is:
ClO O3 Cl 2O2 (12.16)
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Some Examples Of Reactions Initiated Or Catalyzed By Free Radicals And Similar Species
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Reaction Initiator Reaction Catalyst Olefin
Polymerization Peroxides,
(Ph)3CC(Ph)3 2SO2 +O2
SO3 (Lead Chamber
Process)
NO/NO2
Hydrocarbon Dehydrogenatio
n
Iodine, NO2 chlorine atoms
Ozone Depletion
Cl
Hydrocarbon Oxidations
CH CH N I ,3 2 4
CH CH N C H COO3 2 4 6 5
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 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
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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
Pictures Of Some Heterogenous Catalysts
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Pores In Heterogeneous Catalysts
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Figure 14.3 A cross sectional diagram of a typical catalyst support.
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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A Selection Of The Reactions Catalyzed By Supported Metals
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Reaction Catalyst Reaction Catalyst Hydrocarbon
Hydrogenation, Dehydrogenation
Pt, Pd, Ni CO + H2 Hydrocarbons
(Fischer-Tropsch)
Fe, Rh
CO oxidation, total oxidation of
hydrocarbons
Pt, Pd, Cu, Ni, Fe, Rh, Ru
Steam reforming for
production of hydrogen
Ni plus additives
CO + 2H2 CH3OH
Cu/ZnO Reforming (Isomerization of
oil)
Pt/Re/Al2O3
A Selection Of The Reactions Catalyzed By Supported Metals
Reaction Catalyst Reaction Catalyst 2 CO + 2NO
2CO2+ N2 Pt, Rh, Ru (catalytic converter)
2NH3 +O2 N2O5 +3H2O
Pt
N2 + 3 H2 2 NH3
Fe, Ru, Rh Alcohols + O2 Aldehydes +
H2O e.g. 2 CH3OH + O2
2 H2CO +H2O
Ag, Cu
2 C2H4 + O2
2 ethylene oxide Ag, Cu R-R' + H2
RH + HR' (Hydrogenolysis)
Ni, Co, Rh, Ru
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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) 31
Transition Metal Oxides, Nitrides, Sulfides:
Bond transition
Metal to O, N, S to reduce activity and to increase selectivity
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A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides
Reaction Catalyst Reaction Catalyst
2 SO2 + O2 2 SO3 V2O5 CO + H2O CO2+
H2 (Water Gas Shift)
FeO, CuO, ZnO
Hydrodesulfurization CoS, MoS, WS 2(CH3)3COH Þ(CH3)3COC(CH
3)3 + H2O
TiO2
CH3CH=CH2 + O2 (Bi2O3)x(MoO3)y 2 CH3CH=CH2 + 3
O2 + 2NH3
CH2=CHCHO + (Bismuth molybate)
2CH2=CHCN +
H2O Uranium Antimonate
6 H2O
(aminoxidation)
(FeO)x(Sb2O3)y
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A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides
Reaction Catalyst Reaction Catalyst
4 NH3 + 4 NO +O2
4N2 + 6 H2O
benzene+O2
maleic anhydride +
water(Selective catalytic reduction)
naphthylene+O2 phthalic anhydride +
waterCH3CH2(C6H5) +O2
CH2=CH(C6H5) + H2O
NiO, Fe2O3,
V2O5, TiO2
(styrene production) CuO, Co3, O4,
MnO2
Aromatiztione.g. Heptane Tolvene
H2 or H2O
Cr2O3/Al2O3 Hydrodenitrogenation
NiS,MoS
V2O5, TiO2 (V2O5)x(PO4)y
FeO Selective oxidation of hydrocarbons
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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
36Figure 12.4 A diagram of the pore structure in Faugasite.
Leads To Shape Selective Catalysis
C HH
HCH
HH
DiffusionChannel
Cavity
37Figure 12.27 An interconnecting pore structure which is selective for the formation of paraxylene.
Summary
• Two types of catalysts– Homogeneous– Heterogeneous
• Homogeneous more active
• Heterogeneous less expensive, easier to use/control
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