5-Intro 2 Catalyst - Dicky Dermawan

Report
ITK-329 Kinetika & Katalisis
Chapter 5
Introduction to Catalyst &
Catalysis
Dicky Dermawan
www.dickydermawan.net78.net
[email protected]
88
Most chemical processes use catalyst at some
stage in production process
Catalyst (Ostwald):
A substance one adds to a chemical reaction to
tremendously speed up the reaction without the
catalyst undergoing a chemical change itself
Actually catalysts do undergo chemical changes during the course of reaction. It’s just
that the changes are reversible.
Catalyst:
• Selectively enhances the rate of a reaction
• Changes the reaction pathway
• Neither consumed nor formed
• Does not affect equilibrium
89
Types of Catalysts

Homogeneous Catalyst
• Acids or bases
• Metal salt
• Enzymes
• Radical initiators
• Solvents
Reaction
C2H4  Polyethylene
C2H4  Polyethylene,
C6H5CH=CH2  Polystyrene
C2H4 + ½ O2  CH3CHO
(Wacker process)
Olefins + CO +H2  aldehydes
(hydroformylation)
CH3OH + CO  CH3COOH
SO2 + ½ O2  SO3
(lead chamber process)
CH3COOH + CH3OH 
CH3COOCH3 + H2O
Sucrose  glucose + fructose
Catalyst
TiCl4/Al(C2H5)3
(Ziegler – Natta
catalyst)
Peroxides
PdEt3
Co(CO)6
RhCl3
NO/NO2
Acids or bases
Invertase
90
Types of Catalysts (cont’)

Heterogeneous Catalyst
Catalyst
Pt on alumina,Ni on alumina
Pt/Sn on acidic alumina
Solid acids (zeolites)
Ag
V2O5
Platinum gauze
Reaction
Hydrogenation/dehydrogenation
Reforming
Hydrocarbon isomerization,
cracking
C2H4 + ½ O2  ethylene oxide
SO2 + ½ O2  SO3
2 NH3 + 4 O2  N2O5 + 3 H2O
High surface area
91
How Catalysts Work
92
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
NO CAT ALIST
WIT HCAT ALIST
Init iat ion:
Init iation:
k
1
C 2 H 6 
2CH 3 
1
NO 2  C 2 H 6 
C 2 H 5   HNO2
P r opagat ion:
P r opagat ion:
k
k
2
CH 3   C 2 H 6 
CH 4  C 2 H 5 
k3
C 2 H5   C 2 H 4  H 
k4
H  C 2 H 6  C 2 H5   H 2
T ermin at ion:
k5
2C 2 H5   C 4 H10
k
3
C 2 H 5  
C2H 4  H 
k
4
H  C 2 H 6 
C 2 H 5  H 2
T ermin ation:
k
5
2C 2 H 5  
C 4 H10
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
Acid catalysts:
CH3CH2HC=CH2
+
H
CH3CH2HC=CH2 + H+
CH2
+
H
CH3CH2HC
CH3HC=CHCH3 + H+
CH2
CH3CH2HC
CH2
+
H
CH3CH2HC
CH3CH2HC
CH3HC=CHCH3
+
H
CH3CH2HC=CH2 + H+
H


CH2
CH2 =CHCH2CH3 + H+
94
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:
HI,[ Rh ( CO ) 2 I 2 ]
CH 3 OH  CO     
 CH 3COOH
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How Catalysts Work (cont’):
Intermediate Stabilization
Reactant + Catalyst  Stable Complex  Products + Catalyst
Gas phase, no catalyst:
On solid Pt(111) catalysts:
108 enhancement factor
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How Catalysts Work (cont’)
Example in Heterogeneous Catalysis
Ethylene hydrogenation to ethane on Ni catalyst
Heterogeneous Catalysis = adsorption – (surface) reaction - desorption
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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.
Tn []
molecules/ sec R A

[] sec 1
surface_ atom N s
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How Catalysts Work (cont’):
Configuration Dependent
Pd
3C 2 H 2  C 6 H 6
Zeolite
CH 3 C 6 H 5  CH 3 OH  
 CH 3 C 6 H 4 CH 3  H 2 O
Molecule
Platinum
surface
structure
Linear alkane
Isoalkane
Benzene
Paraxylene
Ortoxylene
2-Methyl alkenes
Naphtalene
Zeolite
Chabazite
Zeolite A
Erondite
Ferrierite
ZSM-5
Offretite
Mordenite
Faugasite
VFI
Size of Diffusion
Channel, ?
3.6 x 3.7
4.1 x 4.1
3.6 x 5.2
4.3 x 5.5
5.5 x 5.6
6.4 x 6.4
6.7 x 7.0
7.4 x 7.4
13 x 13
Min.
Diameter, ?
4
5.5
5.1
5.1
5.7
5.1
7.3
Size of
Cavity, ?
5
6.5
11.6
6.5
10.5
6.5
10.5
11.9
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How Catalysts Work (cont’)
Catalyst lowers the activation barrier for the reaction,
Reaction
Catalyst
Ea,
Ea’,
kcal/mol kcal/mol
44
14
500K Rate
enhancement
1013
H2 + I2  2 HI
Pt
2 N2O  2 N2 + O2
Au
58
29
1013
(C2H5)2O  2 C2H4 + H2O
I2
53
34
108
Reaction
Catalyst
Rate
Temperature
enhancement
1040
300
ortho H2  para H2
Pt (solid)
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
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
2 NO + 2 H2  N2 + 2 H2O
typically by 19
– 30 kcal/mol,
thus…
lower the
temperature
where a
reaction takes
place
101
Catalytic Kinetics

Various & significantly different from those of uncatalyzed kinetics
Effect of
Concentration/
Pressure
Rh(111)
CO  21 O 2   CO 2
Pt wire
NH 3   21 N 2  32 H 2
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Catalytic Kinetics (cont’)

Temperature effect: does not obey Arrhenius law
Rh(111)
CO  21 O 2   CO 2
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
Pt wire
NH 3   21 N 2  32 H 2
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
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