CHBE 553 Lecture 27 Continue Mechanisms Of

Catalyst Action

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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.

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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.

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Other Important Concepts• One needs a catalytic cycle to get

reactions to happen.• Mass transfer limitations are more

important when a catalyst is present.

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Main Effect: Stabilizing Intermediates

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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

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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.

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Holding Reactants In The Right Configuration

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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

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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

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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

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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

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Consider

R R RRHC C H C CHH (12.96)

Possible Mechanism: Ion

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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

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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

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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

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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

Pocket

CHH

H

C CC

BindingSite

Blocking Group

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Figure 12.24 A diagram of propylene polymerization in a Ziegler-Natta catalyst.

Catalysts Can Be Designed To Donate And Accept Electrons

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Cu2O 1 / 2O2 2CuO

2CuO CO Cu2O CO2

(12.106)

(12.107)

A Diagram Of A Polymer Fuel Cell

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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.

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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.

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