Transcript catalysis lecture

Lecture Topic 5:
Catalytic Chemistry in Industry
Premise:
Catalysts are extremely important in industrial
organic synthesis.
Goal:
Students should be able to
1) describe the nature of a catalyst
2) describe the important reaction types
involved in homogeneous catalysis
3) describe the important considerations
involved in heterogeneous catalyst design
What is a Catalyst?
A catalyst is a substance that increases the rate at which a
chemical reaction approaches equilibrium, but is not consumed in
the process.
Thus a catalyst affects the
but does not affect the
,
of a reaction.
Most catalysts used in industrial and research laboratories are
inorganic (often organometallic) compounds.
Biochemical catalysts are known as
Why are Catalysts Important?
It is estimated that catalysts contribute 1/6th of the value of ALL
manufactured goods in industrialized countries!
Catalysts can:
• make a reaction possible under achievable conditions
• reduce the necessity of expensive & dangerous conditions
• generate high yields and high product purity
• reduce the amount of side-product and waste created
• generate non-racemic mixtures of enantiomers
• make a chemical process “greener”
Synthetic chemical
Rank
Catalytic Process
Sulfuric acid (H2SO4)
Ethylene (CH2CH2)
Lime (CaO)
Ammonia (NH3)
Sodium hydroxide (NaOH)
Chlorine (Cl2)
Phosphoric acid (H3PO4)
Propylene (CH3CHCH2)
Sodium carbonate (Na2CO3)
1,2-Dichloroethane (ClCH2CH2Cl)
Nitric acid (HNO3)
Urea (CO(NH2)2)
Ammonium nitrate (NH4NO3)
Benzene (C6H6)
Ethylbenzene (C6H5CH2CH3)
Carbon dioxide (CO2)
Vinyl chloride (CH2CHCl)
Styrene (C6H5CHCH2)
Terephthalic acid (1,4-(COOH)C6H4)
Methanol (CH3OH)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
SO2 oxidation to SO3 ; heterogeneous
N2 + H2 ; heterogeneous
Electrocatalysis; heterogeneous
C2H4 + Cl2 ; homogeneous
NH3 + O2 ; heterogeneous
Petroleum refining; heterogeneous
Alkylation of benzene; homogeneous
Chlorination of C2H4; heterogeneous
Dehydrogenation of ethylbenzene; heterog.
Oxidation of p-xylene; homogeneous
CO + H2; heterogeneous
How Does a Catalyst Work?
A catalyst provides an alternative mechanism for the
chemical reaction, with a lower activation energy.
General Principles of Catalysis
a) Homogeneous vs. Heterogeneous
•
•
catalysts: in the same physical phase as reagents
catalysts: present in a different phase (solid state)
b) Catalytic efficiency
• Homogeneous catalysts: efficiency expressed as turnover freq., N
• Heterogeneous catalysts: rate of change of reagents is important
c) Selectivity
• a “selective” catalyst yields a high proportion of desired product
with a minimal amount of side-products.
Homogeneous Catalysis
Homogeneous catalysts are more attractive for study in research labs
because:
a) the mechanism is accessible to detailed study
(i.e., the rate data is easier to interpret)
b) the species in solution are more easily characterized
Advantages of homogeneous catalysis on an industrial scale:
1. High selectivity
2. Ease of heat dissipation from exothermic reactions
Disadvantages of homogeneous catalysis on an industrial scale:
1. Scale-up can be costly, difficult, and dangerous
2. Separation is required
Important Homogeneous Catalytic Processes
1. Oxo process: Hydroformylation of alkenes
O
RHC CH2 + CO + H2
Co(I) or Rh(I)
R
H
Important Homogeneous Catalytic Processes
2. Wacker-Hoechst process: Oxidation of alkenes
O
RHC CH2 + O2
Pd(II) + Cu(II)
H3C
R
R = H, aldehyde
R = CnHn+2, ketone
Important Homogeneous Catalytic Processes
3. Monsanto process: Carbonylation of methanol to acetic acid
O
CH 3OH + CO
[RhI2(CO)2]
H3C
OH
4. Hydrocyanation of butadiene to adiponitrile
+ 2 HCN
Ni(P(OR)3)4
N
N
5. Oligomerization of ethylene
n H2C CH2
NiHL
H2C CH(CH2CH2)n-2CH2CH3
Important Homogeneous Catalytic Processes
6. Olefin metathesis: Alkene dismutation
2 H2C CHCH3
WOCl4/AlCl2Et
H2C CH2 +
H3CHC CHCH3
7. Asymmetric hydrogenation of prochiral alkenes
H
COOR
+ H2
R
[Rh(DiPAMP)2]
NHCOR
8. Cyclotrimerization of acetylene
3 HC CH
Ni(acac)2
COOR
RH2CC* H
NHCOR
Homogeneous Catalysis:
5 Important Reaction Steps
There are 5 types of reactions (and their reverse) which, in
combination, account for most homogeneous catalytic cycles
involving hydrocarbons.
1. Ligand Coordination and Dissociation
2. Insertion and Elimination
3. Nucleophilic attack on coordinated ligands
4. Oxidation and Reduction
5. Oxidative addition and Reductive elimination
1. Ligand Coordination and Dissociation
Homogeneous catalysis requires that:
• reagents be easily coordinated to the metal center
• products be easily lost from the coordination sphere
THEREFORE, a metal catalyst must be:
• a highly labile metal complex
• coordinatively unsaturated
Many square-planar 16e- complexes
meet these criteria.
E.g., Wilkinson’s catalyst
Thus, ML4 complexes of Pd(II),
Pt(II) and Rh(I) are commonly used
as catalysts.
Rh
Ph3P
Ph3P
Cl
PPh3
2. Insertion and Elimination
Insertion : the migration of alkyl (R) or hydride (H) ligands
from the metal center to an unsaturated ligand
L
R
L
+
M
C O
M
H
O
C R
M
CH 2
CH 2
M
CH 2CH3
Elimination: the migration of alkyl (R) or hydride (H) ligands
from a ligand to the metal center
e.g., β-hydride elimination
H
M
CH2
CH3
M
CH2
CH2
H
M
H
CH2
CH2
-C2H4
M
+Sol
Sol
3. Nucleophilic Attack on
Coordinated Ligands
A (+)ve charge on a metal-ligand complex tends to activate the
coordinated C atom toward attack by a nucleophile.
Pd
L
L
H H 2+
OH2
L C
H
C
R
L
L
H H
L
Pd
C C OH
H R
+
+ H+
4. Oxidation and Reduction
During a catalytic cycle, metal atoms frequently alternate between
two oxidation states:
Cu2+/Cu+
Co3+/Co2+
Mn3+/Mn2+
Pd2+/Pd
Catalytic Oxidation: generating alcohols and carboxylic acids
The metal atom 1) initiates the formation of the radical R•
2) contributes to the formation of R-O-O• radical
R H + Co(III)
R
R O O H + Co(II)
+ O2
R + H + Co(II)
R O O
R O + Co(III)OH
R H
AND
R O O H + R
R O O H + Co(III)
R O O + H + Co(II)
5. Oxidative Addition
& Reductive Elimination
Oxidative Addition of a molecule AX to a complex:
• dissociation of the A—X bond
• coordination of the two fragments to the metal center
A
L
L
M
L
L
+ AX
L
L
M
L
X
L
Reductive Elimination is the reverse process:
• formation of a A—X bond
• dissociation of the AX molecule from the coordination sphere
Heterogeneous Catalysis
Heterogeneous catalysts are more widely used in industry because:
1) Solid catalysts are robust at
high T, P.
2) No solvents are required.
3) No separation of the product
from the catalyst is required.
Classes of Heterogeneous Catalyst
There are 2 classes of Heterogeneous catalyst:
1. Uniform catalyst is a high surface area material.
e.g., ZSM-5
Na3(AlSi31O64)3•16H2O
2. Multiphasic catalyst is deposited on a high surface area material.
Typical inert supports
Silica gel, SiO2
microcrystalline γ-alumina, Al2O3
Heterogeneous Catalysis: Considerations
A. Surface area and porosity
Special measures must be taken to ensure that reagent molecules
achieve contact with catalytic sites.
B. Surface acidic and basic sites
Surface OH groups act as weak Brønsted acids.
Exposed Al3+ acts as a Lewis acid site.
Exposed O2- acts as a Lewis base site.
Heterogeneous Catalysis: Considerations
C. Surface metal sites
A supported metal particle (25  diameter) has ~40% of its atoms
exposed on the surface.
The metal atoms are capable of forming a variety of bonds.
O
O
N
C
C
Al
Pt
H
H
H
Pt
CH 3
Pt
H
H
H2C
H
Pt
Pt
Pt
Pt
N
N
Fe Fe Fe Fe Fe
Question: Why is a support used instead of just finely divided
metal powder?
D. Chemisorption and desorption
“Activation”: desorption of H2O, reduction of M with H2(g), etc.
Heterogeneous Catalysts: Active Sites
The active sites of heterogeneous catalysts are NOT uniform.
Many types of sites are exposed on the surface of a material.
Each type of site may serve a unique function in the catalysis.
e.g., crystalline solids
Irregularities (e.g., steps)
Many exposed planes