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ITK-329 Kinetika & Katalisis

Introduction to Catalyst & Catalysis

Dicky Dermawanwww.dickydermawan.net78.net

dickydermawan@gmail.com

Chapter 5

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Catalyst:• Selectively enhances the rate of a reaction• Changes the reaction pathway• Neither consumed nor formed • Does not affect equilibrium

Catalyst (Ostwald):A substance one adds to a chemical reaction to tremendously speed up the reaction without the catalyst undergoing a chemical change itselfActually catalysts do undergo chemical changes during the course of reaction. It’s just that the changes are reversible.

Most chemical processes use catalyst at some stage in production process

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Types of Catalysts Homogeneous Catalyst

Reaction Catalyst C2H4 Polyethylene TiCl4/Al(C2H5)3

(Ziegler – Natta catalyst)

C2H4 Polyethylene, C6H5CH=CH2 Polystyrene

Peroxides

C2H4 + ½ O2 CH3CHO (Wacker process)

PdEt3

Olefins + CO +H2 aldehydes (hydroformylation)

Co(CO)6

CH3OH + CO CH3COOH RhCl3

SO2 + ½ O2 SO3 (lead chamber process)

NO/NO2

CH3COOH + CH3OH CH3COOCH3 + H2O

Acids or bases

Sucrose glucose + fructose Invertase

• Acids or bases• Metal salt• Enzymes• Radical initiators• Solvents

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Types of Catalysts (cont’) Heterogeneous Catalyst

Catalyst Reaction Pt on alumina,Ni on alumina Hydrogenation/dehydrogenation Pt/Sn on acidic alumina Reforming Solid acids (zeolites) Hydrocarbon isomerization,

cracking Ag C2H4 + ½ O2 ethylene oxide V2O5 SO2 + ½ O2 SO3 Platinum gauze 2 NH3 + 4 O2 N2O5 + 3 H2O

High surface area

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How Catalysts Work

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How Catalysts Work (cont’)Catalyst can initiate reactions. The mechanisms are similar to the mechanism without a catalyst, but the initiation process is much faster with the catalyst

104k

52

252k

62

42k

52

524k

623

3k

62

HCHC2

:ationminTer

HHCHCH

HHCHC

HCCHHCCH

:opagationPr

CH2HC

:Initiation

CATALIST NO

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4

3

2

1

104k

52

252k

62

42k

52

252k

622

HCHC2

:ationminTer

HHCHCH

HHCHC

:opagationPr

HNOHCHCNO

:Initiation

CATALIST WITH

5

4

3

1

Radical Initiator in polymerization:peroxides ROOR, AIBN [(CH3)2C(CN)N]2

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How Catalysts Work (cont’):Intermediate Stabilization

Reactant + Catalyst Stable Complex Products + Catalyst

CH3CH2HC CH2

H +CH3HC=CHCH3 + H+

CH3CH2HC CH2

H +CH3CH2HC=CH2 + H+

CH3CH2HC CH2

H +CH2 =CHCH2CH3 + H+

CH3CH2HC=CH2 + H+ CH3CH2HC CH2

H +

CH3CH2HC=CH2 CH3HC=CHCH3 H

Acid catalysts:

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How Catalysts Work (cont’):Intermediate Stabilization

Reactant + Catalyst Stable Complex Products + Catalyst

Enzymatic Reactions:

O

NH2CNH2 + HOH 2NH3 + CO2

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How Catalysts Work (cont’):Intermediate Stabilization

Reactant + Catalyst Stable Complex Products + Catalyst

Metalic cluster catalyst:

COOHCHCOOHCH 3]I)CO(Rh[,HI

322

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How Catalysts Work (cont’):Intermediate Stabilization

Reactant + Catalyst Stable Complex Products + Catalyst

On solid Pt(111) catalysts:108 enhancement factor

Gas phase, no catalyst:

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Ethylene hydrogenation to ethane on Ni catalyst

Heterogeneous Catalysis = adsorption – (surface) reaction - desorptionHeterogeneous Catalysis = adsorption – (surface) reaction - desorption

How Catalysts Work (cont’)Example in Heterogeneous Catalysis

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Heterogeneous Catalyst Activity:Turnover Number/Turnover Frequency

Tn = the rate that molecules are converted per active site in the surface of the catalyst per second.

1

s

An sec][

N

R

atom_surface

sec/molecules][T

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How Catalysts Work (cont’):Configuration Dependent

66Pd

22 HCHC3 OHCHHCCHOHCHHCCH 23463

Zeolite3563

Platinum surface structure

Zeolite Size of Diffusion Channel, ?

Size of Cavity, ?

Chabazite 3.6 x 3.7 5 Zeolite A 4.1 x 4.1 6.5 Erondite 3.6 x 5.2 11.6 Ferrierite 4.3 x 5.5 6.5 ZSM-5 5.5 x 5.6 10.5 Offretite 6.4 x 6.4 6.5 Mordenite 6.7 x 7.0 10.5 Faugasite 7.4 x 7.4 11.9 VFI 13 x 13

Molecule Min. Diameter, ?

Linear alkane 4 Isoalkane 5.5 Benzene 5.1 Paraxylene 5.1 Ortoxylene 5.7 2-Methyl alkenes 5.1 Naphtalene 7.3

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Reaction Catalyst Ea, kcal/mol

Ea’, kcal/mol

500K Rate enhancement

H2 + I2 2 HI Pt 44 14 1013

2 N2O 2 N2 + O2 Au 58 29 1013

(C2H5)2O 2 C2H4 + H2O I2 53 34 108

Reaction Catalyst Rate

enhancement Temperature

ortho H2 para H2 Pt (solid) 1040 300

2 NH3 N2 + 3 H2 Mo (solid) 1020 600

C2H4 + H2 C2H6 Pt (solid) 1042 300

H2 + Br2 2 HBr Pt (solid) 1 x 108 300

2 NO + 2 H2 N2 + 2 H2O Ru (solid) 3 x 1016 500

CH3CHO CH4 + CO I2 (gas) 4 x 106 500

CH3CH3 C2H4 + H2 NO2 (gas) 1 x 109 750

(CH3)3CHO (CH3)2C=CH2+ H2O HBr (gas) 3 x 108 750

typically by 19 – 30 kcal/mol, thus…lower the temperature where a reaction takes place

How Catalysts Work (cont’)

Catalyst lowers the activation barrier for the reaction,

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Catalytic Kinetics Various & significantly different from those of uncatalyzed kinetics

2)111(Rh

221 COOCO

223

221 wirePt

3 HNNH

Effect of Concentration/ Pressure

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Catalytic Kinetics (cont’) Temperature effect: does not obey Arrhenius law

2)111(Rh

221 COOCO A: PCO = 2,5 x 10-8 torr

B: PCO = 1,0 x 10-7 torr C: PCO = 8,0 x 10-7 torr D: PO2 = 4,0 x 10-7 torr E: PO2 = 2,5 x 10-8 torr F: PCO = 2,5 x 10-8 torr, PO2 = 2,5 x 10-8 torr

223

221 wirePt

3 HNNH

A: PNH3 = 0,3 ; PH2 = 0,15 B: PNH3 = 0,3 ; PH2 = 0,44 C: PNH3 = 0,05; PH2 = 0,15 D: PNH3 = 0,05; PH2 = 0,45