Post on 06-Jan-2018
description
transcript
CHBE 553 Lecture 27 Continue Mechanisms Of
Catalyst Action
1
Mechanisms Of Catalyst Action
• Catalysts can be designed to help initiate reactions.
• Catalysts can be designed to stabilize the intermediates of a reaction.
• Catalysts can be designed to hold the reactants in close proximity.
• Catalysts can be designed to hold the reactants in the right configuration to react.
2
Mechanisms Continued• Catalysts can be designed to block side reactions.• Catalysts can be designed to sequentially stretch
bonds and otherwise make bonds easier to break.• Catalysts can be designed to donate and accept
electrons. • Catalysts can be designed to act as efficient
means for energy transfer.
3
Other Important Concepts• One needs a catalytic cycle to get
reactions to happen.• Mass transfer limitations are more
important when a catalyst is present.
4
Main Effect: Stabilizing Intermediates
5
Pt
Pd Ir Rh Ru
Cu
Co
Ni
Fe
W Ta
-300-250
-200-150
-100-50
01E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of metal, Kcal/mol
Pt Ir Rh
Ru
Cu
Co
Ni
Fe
W Ta
-110-100
-90-80
-70-60
-50-40
-30-20
-100
1E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of oxygen, Kcal/mol
Tanaka-TamaruSachtler-Frahrenfort
Pd
Rat
e, M
olec
ules
/cm
/se
c2
Rat
e, M
olec
ules
/cm
/se
c2
Figure 12.13 A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.
Stabilizing Intermediates Not Entire Effect
• Leads to 1020 increases in rates – -need other effects to get to 1040
• Does not lead to selectivity
6
Last Time Started Mentioned The Use Of Catalyst To Affect Selectivity
• Catalysts can be designed to hold the reactants in close proximity.
• Catalysts can be designed to hold the reactants in the right configuration to react.
• Catalysts can be designed to block side reactions.
• Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break.
7
Holding Reactants In The Right Configuration
8
NAD CH3CH2OH NADH CH3CHOH
NAD+
OH
HCC
NH
H
HH
:
Figure 12.16 A cartoon of the reaction of ethanol and NAD+ on the active site of liver alcohol dehydrogenase. Adapted from Oppenheimer and Handlon (1992) (In the Enzyme, vol 20 (1992) 453.
(12.92)
Catalysts Can Be Designed To Hold The Reactants In The Correct Configuration To React, Make Bonds Easier To Break
9
C
C
C
C
CC
3C2H2 C6H6
Figure 12.15 The active site for reaction (12.91) on a palladium catalyst.
(12.91)
Active Site
Figure 12.17 A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]
Catalysts Make Bonds Easier To Break
10
Active Site
Figure 12.17 A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]
Catalytic Antibodies
11
C
C
CC
X
C
C
Y+
C
C
CC
X
C
C
Y
(12.94)
Transition Metals – Weaken Bond By Attaching To Antibonding Orbitals
12Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.
Acid Catalysts: Charges Simplify Reactions
13
Consider
R R RRHC C H C CHH (12.96)
Possible Mechanism: Ion
14
R R R RHC C H H HC C HH (12.97)
R R RRHC C HH HC CHH
(12.98)
RR RRHC CHH C CHH H+
(12.99)
Ion Mechanism
In both cases isomerization is rate determining step
Radical Mechanism
X + HC = CRH + H H C CH2 XR RR
(12.100)
X + H C C H2 H C C H2 XR RR R
(12.101)
X + H C C H2 C CH2 H XRR R
R (12.102)
Orbitals For Radical Case
15
C C
R
I
II IV
IIICC
R
I
CC
R
II CC
R
IV
CC
R
III
Reactants Products
Filled
Empty
CC
R
C C
R
C C
R
Filled
Filled Empty
Empty
Figure 12.22 A rough diagram of the key MO's during reactions (12.101).
Note: Only 1 filled with ions.
Catalysts Can Block Side Reactions
Consider syncs of isotatic polypropylene
16
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
CCCHH
H
(12.104)
(12.105)
Possible Reaction Step In PE Production
17
C CCCHH
H
CCCHH
H
HH
H
CCC
CHH
H
+CC
CHH
H
CCCHH
H
CCCHH
H
CCCHH
H
C CCCHH
H
CCCHH
H
CCC
CHHH
+CC
CHH
H
CCCHH
H
CCCHH
H
CCCHH
H
Methyl on wrong side
Figure 12.23 A rough diagram of one step during the production of isotatic polypropylene.
Catalyst For PE Production: Block Side Reactions
Ti
CC
C HHH
CC
C HHH
CC
C HHH
CHH
H
C CC
BindingSite
Blocking Group
18
Figure 12.24 A diagram of propylene polymerization in a Ziegler-Natta catalyst.
Catalysts Can Be Designed To Donate And Accept Electrons
19
Cu2O 1 / 2O2 2CuO
2CuO CO Cu2O CO2
(12.106)
(12.107)
A Diagram Of A Polymer Fuel Cell
20
Polymer Membrane
Cathode
Anode
-
4H O2
e
+HLoad
4 H+ -+ 4 e
H2 2 H+ -+ 2 e
O +2
(12.25)
Summary• Catalysts can be designed to help initiate
reactions.• Catalysts can be designed to stabilize the
intermediates of a reaction.• Catalysts can be designed to hold the
reactants in close proximity.• Catalysts can be designed to hold the
reactants in the right configuration to react.
21
Summary Continued• Catalysts can be designed to block side reactions.• Catalysts can be designed to sequentially stretch
bonds and otherwise make bonds easier to break.• Catalysts can be designed to donate and accept
electrons.• Catalysts can be designed to act as efficient
means for energy transfer.
22