che 553 lecture 22 introduction to catalysis 1. importance of catalysis 90% of all chemical...

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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