Introduction To Catalysis [PDF]

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CATALYSIS Theory and Applications

Paul C.J. Kamer, Gadi Rothenberg, Ron Wever University of Amsterdam

Definition of Catalysis and Catalyst A catalyst is a substance that increases the rate of a reaction remaining itself unchanged

A

B

CAT

Importance of Catalysis • increasingly important in synthesis • selectivity in production of fine chemicals • clean processes, high atom economy (bulk processes) • production of high-tech products / materials • mild conditions (low energy consumption) • environmentally friendly Example: methyl metacrylate (MMA) Old process: 2.5 kg waste / kg product O

+ HCN

HO

CN 1. H2SO4

2. CH3OH

COOCH3

New catalytic process: 50 g waste / kg product CO, Methanol catalyst

COOCH3

+ NH4SO4

Catalyst Activation and Regeneration A + catalyst

B + catalyst

a catalytic reaction wherein the catalyst is unchanged

By-product D Reagent C

active catalyst

Substrate A

inactive Product B catalyst catalyst precursor

a catalytic reaction wherein the catalyst is deactivated and needs to be regenerated (re-activated) or requires an incubation time

How Does a Catalyst Work? •Lowering activation energy •Stabilization of a reactive transition state •Bringing reactants together •proximity effect •orientation effect •Enabling otherwise inaccessible reaction paths

Energy profile of a reaction

Example: Lewis Acid Catalyzed Diels-Alder Reaction −

O +

−

δ

δ

AlCl3

δ

O

AlCl3

Frontier orbitals overlap

HOMO of butadiene (Ψ2) LUMO of ethylene (π*)

LUMO of butadiene (Ψ3*) HOMO of ethylene (π)

Secondary overlap stabilizes endo transition state O

fast

O O

O O

O

HOMO O O

O O

O

slow

O O

O

O

LUMO

Frontier orbital interactions for DA reactions energy

LUMO

LUMO

LUMO LUMO

LUMO LUMO

HOMO HOMO

HOMO

HOMO HOMO

HOMO

X

Z

dienophile with a low energy LUMO

dienophile with neither a low energy LUMO nor a high energy HOMO

dienophile with a high energy LUMO

Influence Lewis Acids on endo-selectivity +

H

CO2Me

O H

+ AlCl3

CO2Me

CO2Me

without AlCl3 at 0° with AlCl3 at 0° with AlCl3 at -80°

O

+ H

88% 96% 99%

AlCl3 H

12% 4% 1%

O

H H

Frontier orbital energies and coefficients acrolein LUMO

2.5

O

.51 -.48

-.39

H

O

.59

-.70

-.09

O HOMO -14.5

-.58

H

.37

H

-7

LUMO

.60

-.30 .48

H

.58

H

O

-.54

-.10

acrolein

.65

H

.53

protonated acrolein

-23.5 HOMO

Increased regioselectivity acid catalyzed DA

X

O -.39

X H

.59

HOMO LUMO without catalysis

O -.09

H H

.60

HOMO LUMO with catalysis

Increased endo selectivity acid catalyzed DA

-.48

O

without catalysis

-.7

HO

with catalysis

Catalysts • general acid and base catalysis (ester hydrolysis), • Lewis acids as catalysts (Diels-Alder reactions), • organic catalysts (thiazolium ions in Cannizzarro reactions), • porphyrin complexes (epoxidations, hydroxylations), • enzymatic processes, • co-ordination complexes (polyester condensations), • catalytic antibodies

world market for catalysts: 9 billion USD

Catalysts affect both rate and selectivity Reaction

A+B

uncatalyzed

d[P]/dt = k1[A][B]

catalyzed

d[P]/dt = k2[Cat][A][B]

Atom economy E factor

P

number of atoms ended up in product number of atoms from the reagents

mass units of waste mass units of product

Diels Alder

+ atom economy 100%

Hydroformylation

cat R R H2/CO

CHO

+R

CHO

atom economy 100%

Examples atom economy "Rh" H 2 /CO

100%

CO

H2

R

R

100%

R "Ru"

R +

100%

"Pd" br

-HBR

26/28 (69 mass percent)

E factor

Industry

Production ton/year

waste/product ratio

Oil Bulk Finechemicals Pharmaceuticals

106-108 104-106 102-104 10-103

0.1 100

Homogeneous catalysis

Definition: Catalyst components and substrates of the reaction are in the same phase, most often the liquid phase (some catalytic reactions in the gas phase are also known). Generally homogeneous catalysis refers to the use of organometallic complexes as the catalysts.

Advantages / disadvantages Advantages: - understandable kinetics - reproducibility - relatively mild reaction conditions - high selectivity - easy modification of catalyst properties - efficiency; all molecules are accessible - mechanistic studies are relatively easy to perform (complex identification by IR, NMR, UV etc.) Disadvantages: - water and oxygen sensitivity - separation of catalyst and products often difficult

Development of Transition Metal Catalysts Choice of metal Use of ligands with proper donor atoms Ligand effects on activity and selectivity Steric effects Electronic effects Bite Angle Secondary interactions Substrate-metal Substrate-ligand Medium effects level of sophistication

Rational catalyst design alkene metathesis R1

R2

R1

3

4

R

R

catalyst: LnM

+

+ 4

R2 3

R

R

R R'

alkyne metathesis R2

R1 R4

+

R3

R1

R2

+ 4

R

3

R

catalyst: Mo(CO)6 + HO rate: 2 mol / mol / h Mortreux (1974)

O t-Bu

"designed catalyst": t-Bu

W

O t-Bu

O t-Bu

Cl

rate: 300,000 mol / mol / h Schrock (1984)

Selectivity in nickel catalyzed butadiene reactions n

n

n

''Ni''

Selectivity to desired product steered by choice of ligands

Chemoselectivity

OH O

H2, kat

O OH

H2

Regioselectivity

CHO CO / H2

CHO

Enantioselectivity

COOR' R

H2 + * + NHCOR" [HRh L 2] R

*

COOR' NHCOR"

99 % ee

Diastereoselectivity

R R

*

OH

*

OH

H2

R

*

OH

Heterogeneous catalysis Advantages: - easy separation of catalyst and products - low sensitivity to water and oxygen - high temperature stability - shape selectivity (zeolites) Disadvantages: - kinetics can be complex - often high pressures and temperatures required - catalyst poisoning by sulfur compounds - sintering - heat transfer - surface chemistry; using a few atoms only

Immobilization of transition metal complexes Supported Homogeneous Catalysts - Organic Polymers - Dendrimers - Inorganic Metal Oxides Supported Aqueous Phase Catalysis Two-phase catalysis

- organic solvents (SHOP) - fluorous phase - aqueous phase

Sol-Gel catalysis

disadvantages:

Extraction (acid / base)

- degradation of polymer - leaching of metal - oxidation of anchored ligands

Ultrafiltration

Product shape-selectivity selective diffusion of para-xylene out of the pores of silicalite

Product shape-selectivity computer simulation of para-xylene in a silicalite pore

Dendrimers as support Dendrimer particle that serves as a catalyst support

Catalyst immobilisation on dendrimers Si

Si Si

Si

Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Si

Si P

Si

Si

Fe

Si Si

P Si

Si Si Si Si

Si

Si

Si Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si Si Si

G.E. Oosterom et al. Chem. Commun.1999, 1119

Si

Ph2P Ph2P Ph2P Ph2P Me Si PPh2 Ph2P 2 Me2Si SiMe2 Me2Si SiMe PPh2 2PPh Ph2P PPh2 SiMe2 2 SiMe2 Me Si Si 2 Si Ph2P Si SiMe2 PPh2 Me2 Si Si SiMe2 PPh2 Ph2P Me2 Si Si Me 2SiMe Ph2P 2 PPh SiMe2 2 Si SiMe2 Si Ph2P Si SiMe2 Me2Si Si Ph2P Si Si PPh SiMe2 2 Me2Si Si PPh2 Si Me2Si Si Me Si Ph2P Me2 2 PPh2 Si SiMe2 Si Me2PPh2 Ph2P Me2Si Me2 Si Si Si Ph2P Me2Si Si Si SiMe2 Si PPh2 Me2 PPh2 Me Si Ph2P Ph2PMe2Si 2 PPh2 SiMe 2 PPh2 SiMe2SiMeSiMe 2 Ph2P 2 PPh2 PPh2 PPh2 PPh2

D. de Groot et al. Chem. Commun.2000, 711

Dendritic transition metal catalysts: covalent approach Core functionalization

Periphery functionalization CAT CAT CAT CAT CAT CAT CAT CAT

CAT

CAT CAT CAT CAT CAT CAT CAT

CAT

CAT

CAT

CAT CAT

CAT CAT CAT CAT CAT CAT

• site isolation • micro-environment

CAT CAT CAT CAT CAT

CAT

• multiple active sites • potential cooperativity • easy accessibility

Direct comparison core and periphery functionalized dendrimers Ph2P Ph2P

[Pd]

OAc

PPh2

Me2N

Fe

Ph2P

N

N

N

N

Fe

Me2N

PPh2

SiMe2

Fe

SiMe2

Ph2P

PPh2

Me2N

SiMe2

NMe2 PPh2

Fe Ph2P

90

linear, trans

linear, cis

branched

80

Fe

Si SiMe2

Me2N Ph2P

NMe2 PPh2

Me2 Si

Fe

Si

Si

Si

Si Me2

Ph2P Me2N

70

PPh2

SiMe2

Fe PPh2

Ph2P

Fe

Me2Si

NMe2

Me2Si Si

PPh2

conversion / %

Fe

60

Ph2P Me N 2

50

Me2Si

Fe Ph2P

40

PPh2

Fe

NMe2

NMe2

Ph2P

30

PPh2

PPh2

SiMe3

Me3Si

Me3Si

10

Me3Si

2

4

6

8

reactor volumes

Room temperature, solvent: CH2Cl2, P/Pd=2.

10

SiMe3 SiMe3

Si

Si

SiMe3

Si

Si

Si

Si

SiMe3 SiMe3

Me3Si

0

SiMe3

SiMe3 Me3Si

Me3Si

0

SiMe3

Me3Si

NMe2

Fe

Ph2P

20

PPh2

Me2Si

Me2Si

Si P

Si Me3Si

SiMe3

Fe P

Me3Si

Si

Si Me3Si

Si

Me3Si Me3Si

Si

Me3Si Me3Si Me3Si

SiMe3 SiMe3

Si

Si SiMe3

Si

Si SiMe3 3 SiMeSiMe 3

Me3Si

SiMe3 Me3SiMe3Si

SiMe3

Biocatalysis: the use of enzymes or cells Enzyme optimization: • protein “engineering” • molecular evolution • enzyme “engineering” Enzymes can be applied in: • Water • Organic solvents • Supercritical CO2 • Ionic liquids

Energy profile of an enzyme catalyzed reaction

Enzymes give great rate enhancements Michaelis-Menten parameters for selected enzymes

Enzyme specificity

Examples of classes of enzymes

Coenzymes, vitamins and metals Some important coenzymes and related vitamins

Active site of carboxypeptidase A

Ideal catalyst characteristics

• • • •

High activity (mild reaction conditions) High selectivity (chemo, regio, enantio) Stable catalyst Easy separation and reuse of catalyst

New “green” processes:

• Atom economic • No or as little as possible waste • decrease in solvent use • Mild reaction conditions • Catalytic use of reagents instead of stochiometrisch • Non-toxic reagents, • New sustainable starting materials