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Homogeneous Catalysis
HMC-4- 2010
Dr. K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology, Madras
Chennai-600036
Homogeneous Catalysis- 4
Hydroformylation
Oxo reaction
Reaction of CO and H2 with terminal alkene
Production of n-butyraldehyde, butanols, C6- C9 alcohols
& Long-chain fatty alcohols
Hydroformylation- The beginning
Journey from Fischer- Tropsch to Hydroformylation / Oxo reaction
Discovery of Hydroformylation
Hydroformylation- Discovery
Hydroformylation-History
1938 Otto Roelen found the Hydroformylation
At first cobalt-catalyst
1965 Wilkinson found rhodium catalyst
1980 process had been upgraded
RCH/RP-process- separation of rhodium from the product
Also named oxo-process
Useful to converting terminal alkenes into other organic products
Carbon chain increased by one
Main industrial application is in the production of butanal from propene
Hydroformylation-Applications
Oxo process- For plasticizer alcohols
Hydroformylation
Addition of a H atom and a formyl group to the double bond of an alkene.
Efficient with terminal alkene
R
→
+ CO + H2
R
CHO + R
CHO
“normal”
“iso”
linear product
branched product
Thermodynamics: Hydrogenation Vs. Hydroformylation
H2 + CH3CH=CH2
∆G
∆H
→
63
21
CH3CH2CH3
-25
-105
H2 + CH3CH=CH2 + CO →
∆G
∆H
63
21
-138
-109
= -88 kJ mol-1
= -126 kJ mol-1
CH3CH2CH2CHO
-117(1) = -42 kJ mol-1
-238
= -150 kJ mol-1
Though alkane is the thermodynamically more favourable, the actual product
is the aldehyde, because “kinetic” control occurs.
The reaction is highly exothermic and is conducted at adiabatic conditions.
70 years of Oxo synthesis
Five quantum leaps of development:
1. “Diaden process” with heterogeneous cobalt catalysts;
2. High pressure process with homogeneous Cobalt catalyst;
3. Introduction of Rh as the central atom of complex catalysts;
4. The ligand modified Rh or Co catalysts; and
5. The two-phase catalysis
Plasticizer alcohols via Hydroformylation
Hydroformylation - Features
Preparation of Co complex
Hydroformylation of propene with Cobalt catalyst
HCo(CO)3 (
)
-CO
HCo(CO)4
Co(CO)3(nPr)
HCo(CO)3
Co(CO)3(iPr)
nPrCHO
Co(CO)3(COPrn)
CO
H2
Co(CO)3(COPri)
iPrCHO
H2
is propylene
The inner cycle shows the formation of liner product and the outer cycle the
Branched isomer
Cobalt catalyzed hydroformylation- Mechanism
Co2(CO)8 + H2 ⇋ (high pressure) 2[CoH(CO)4]
1. The complex loses CO to produce the coordinatively unsaturated complex
catalyst.
2. The complex catalyst coordinates to the alkene (Propene).
3. Undergoes insertion reaction → n-alkyl complex (may be branched also).
4. Propene coordination followed by olefin insertion into metal-H bond in a
Markovnikov or anti-Markovnikov fashion gives the branched or the linear
metal-alkyl complex, respectively.
5 At high pressures of CO, the complex undergoes migratory insertion of CO to
yield the acyl complex (IR spectral evidence).
6. Attack by H2, as strongly acidic HCo(CO)4 will yield the aldehyde and
regenerate.
7. The last step is often rate-determining.
Cobalt catalyzed hydroformylation- Mechanism
The catalytic cycle is firmly established.
Both Co(CO)4 (COPrn) and Co(CO)4 (COPriso) have been isolated and
their reactions with H2 as well as HCo(CO)4 have been studied.
There are spectroscopic data and/or strong theoretical arguments in favour
of the existence of all the catalytic intermediates.
Hydroformylation- General features
There are two ligands, CO and L = PPh3.
Hydroformylation with Rh can also be effected in the absence of phosphine.
In such a situation, CO acts as the main Ligand.
The intermediate complexes in the catalytic cycle switch from 18e- to 16 ecomplexes
The catalyst precursor undergoes ligand disssociation to generate
coordinatively unsaturated species.
There are two insertion steps (alkene and CO)
The selectivity to n-butyraldehyde is determined in the first insertion step.
This is then followed by oxidative addition step (of H2), which is the ratedetermining step.
Reductive elimination gives the aldehyde and generates the catalyst
General catalyst composition- Hx My (CO)n Ln
Hydroformylation activity pattern with unmodified ligand (n=0)
Rh >> Co >Ir,Ru > Os >Pt >Pd > Fe >Ni
Rh & Co most commonly used; Pt asymmetric hydroformylation
Reaction cycle for hydroformylation on Co
Mechanism with long chain olefins, > C3
Mechanism on Rh catalysts
Approach of Propylene
leads to isomers
Ligand effects
Steric effects by ligands
Ligand effects
Steric (cone angle bite angle), substituents & electronic factors of ligands
affect the activity & n/i ratio
Product selectivity: Linear Vs. Branched aldehydes: Regio-selectivity
Two different ways of inserting an alkene into a metal-hydrogen bond:
H
R
+
R
→
Rh
→
Rh
Rh
R
AntiMarkovnikov
Markovnikov
It is considered to be primarily an effect of steric crowding around the metal center.
The normal alkyl requires less space and therefore formed more easily than the
branched one in the presence of bulky ligands.
Higher selectivity for the linear aldehyde by the addition of alkylphosphine
Replacement of CO by a bulky ligand disfavours the formation of complexes
of sterically crowded 2-alkane
Co(CO)2PBu3
CHCH2CH2CH3
isomerize→
Co(CO)2PBu3
CH3CH-CH2CH3
k << 1
Phosphine derivatives as ligands for Co catalysts
Phosphine derivatives -PR3 – R = C6 H5, n-C4H9
Variations in Gr V elements
Ph3P >> Ph3 N > Ph3 As > Ph3Sb> Ph3Bi; Phosphine ligands –Best choice
Alkyl phosphines
Strong electron donors and thus dissociation of CO is retarded,
Leads to more stable catalysts, but also much slower catalysts.
Aryl phosphines
Do not seem to be very effective ligands.
They are weaker electron donors and form less stable complexes in the
competition with CO
They quickly decompose at higher temperatures.
The more electron withdrawing the aryl group is, the faster the decomposition.
The reported order of activity (195oC, 36 bar)
Ph2EtP > PhBu2P > Bu3P > Et3P > PhEt2P > Cy3P
The linear .Vs. branched ratio decreases as follows:
Bu3P > Et3P = PhEt2P = Cy3P = PhBu2P > Ph2EtP, ranging from 5.5 to 3
N containing ligands like amines, amides & isonitriles show lower reaction
rates due to stronger bonding with metal
The Rh Catalyst
Rh phosphine complexes are more active catalysts than cobalt complexes
→
[RhH(CO)(PPh3)3
[RhH(CO)(PPh3)2] + PPh3
Lower temperatures and Pressures
Higher selectivity to linear aldehydes
Catalyst separation is made easier
Rate expression:
d[RCHO]/dt
=
k[Rh]a[alkene]b[H2] / [CO]
Value of b depends on the alkene; both a and b may be less than one.
Inverse dependence of rate on the concentration of CO is observed only
at total pressure exceeding 40 atm.
At lower pressures, the rate increases with increase in pressure.
Industrially, a large excess of the phosphine is used, which is necessary
for a good selectivity to n-butyraldehyde.
Hydroformylation of propylene with Rh/PPh3- Catalyst
cycle
L
L
L
L
H
-L
H
Rh
CO
H
Rh
OC
L
L
Rh L
CO
L
OC
Rh
L
CO
O
H
L
OC
H
O
L
Rh
L H
OC
O
L
OC
H2
Rh
L
L = Phosphorous ligand
CO
Rh
L
Mechanistic insights
In-situ IR and multinuclear NMR studies under less severe conditions:
The oxidative addition of dihydrogen is most likely the rate-determining step
Hydroformylation of
3,3,dimethyl1-butene
1-Octene
CH3
C=C–C–C
CH3
O
have been identified by NMR
Rh(COR)(CO)4
intermediate is seen
by IR
Styrene
O
Ph
C
C
RhL2(CO)2
RhL2(CO)2
[ ]
No CO, L = 3
L = PPh3
High selectivity for
n-butyraldehyde
[ ]
[ ]
[ ]
[ ]
Anti-Markovnikov
[ ]
[ ]
[ ]
[ ]
Markovnikov
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
←No L, only CO
HRh(CO)4
Schematic drawing of catalytic cycles coupled by substitution of L and CO in Rh catalyst
In a typical industrial process, the Rh to phosphorus molar ratio is
between1:50 to 1:100, while the partial pressure of CO is about 25 bar.
All the complexes with the general formula, HRh(CO)nL4-n
n = 0 – 4) might be present with variable concentrations.
In fact, even HRh(CO)4 without any phosphorus ligand can be used as
catalyst.
The selectivity towards anti-markovnikov product increases with more
phosphinated intermediates, whereas more carbonylation shifts the
selectivity towards Markovnikov product.
This is to be expected in view of the fact that sterically crowded
environment around the metal center favours anti-Markovnikov addition
and Yields n-aldehyde (linear aldehyde)
Commercial homogeneous Oxo processes
Cobalt catalyzed Hydroformylation is the old Workhorse
The key issue in the hydrofomylation reaction is the ratio of the linear to the
branched products (Regio-selectivity)
Linearity can be influenced and maximized by influencing the kinetics and
changing the ligand characteristics.
Three major process types
The first processes were based on cobalt carbonyl complexes.
The second is the process based on phosphorus modified Co-based
catalyst system developed by Shell.
Third Rh based catalysts - Ruhrchemie/Rhone-Poulenc, MitsubishiKasei, Union Carbide and Celanese
Supported liquid phase & supported aqueous phase catalysts
Oxo processes – The evolution
Process Parameters
Cobalt
Carbonyl
Cobalt
+Phosphine
Rhodium
+Phosphine
Temperature,oC
140-180
160-200
90-110
Pressure, atm.
200-300
50-100
10-20
Alkane formation
Low
Considerable
Low
Main product
Aldehyde
Alcohol
Aldehyde
Selectivity to
n-butyraldehyde
75-80
85-90
92-95
Difficult
HCo(CO)4
is volatile
Less difficult
Isolation of catalyst
Less difficult:
Water soluble
phosphine, a
major advancement
Oxo processes- Variations
Aqueous biphasic catalysis:
Ruhrchemie/Rhone-Poulenc Oxo process
In aqueous, homogeneous two-phase catalysis, the active catalyst for the
reaction is (and remains) dissolved in water.
The reactants and the products, which are ideally organic and relatively
non-polar, can be separated off after the reaction is complete by simply
separating the second phase from the catalyst solution, thus making it easy
to recirculate the latter.
Positive influence of water and
Inherent advantages of homogeneous catalysis.
The new oxo process was completely different from the earlier ones :
a) The catalyst is applied in water soluble form;
b) The process procedure;
c) The reactor with special mixing of the two-phases;
d) The energy balance; and
e) Simple circulation of the aqueous catalyst solution.
For the water soluble ligands, the price and the performance are the key
Water – soluble ligands of Rh
TPPTS: Tri(m-sulfonyl)triphenylphospine, vulgo triphenylphosphine,
trisulfonated
Hydroformylation with TPPTS
New generation oxo process- Features
1. High selectivity, producing virtually exclusive aldehydes with up to 96%
linearity;
2. Simplicity in terms of apparatus and process operation;
3. Net steam supplier, making excellent utilization of the heat of the reaction
by means of the “heat transfer medium” n-butyraldehyde;
4. Very simple recycling of the homogeneous oxo catalyst by immobilization
using the “mobile support” water;
5. Excellent economics owing to minimal losses of the catalyst metal, Rh;
6. Low purity demands on the reactants;
7. Excellent process potential from safety and environmental points of view.
Synthesis of TPPTS
Sulfonation of triphenylphosphine: pH controlled selective extraction and
Re-extraction procedures;
Further Improvement in ligand synthesis
BISBIS: sulfonated bis(diphenylphosphine)biphenyl, varying grades of
sulfonation;
BINAS: sulphonated NAPHOS, binaphthyl structures
Rh/BINAS is the most active and selective water-soluble hydroformylation
catalyst.
Its reactivity is up to 10 times higher than TPPTS with n/iso selectivities of
98/2 compared to 96/4 with TPPTS
Hydroformylation- Process variations
The Shell Process
Higher alkene feed
→
Detergent alcohol
Phosphine-modified Co catalyst for production of linear alcohols with 12-15 C
atoms
Important characteristics:
1. HCo(CO)3(P n-Bu3) is less active for hydroformylation than HCo(CO)4, but
more active for subsequent hydrogenation of the aldehyde.
2. Both hydroformylation and hydrogenation of the aldehyde are catalyzed
by the same catalyst.
3. Phosphorus ligand substituted derivatives are more stable than their
carbonyl analogues at high temperatures and lower pressures.
Linear alcohols - Chemical considerations
The internal alkenes must be isomerized to terminal one from the
thermodynamic mixture, where the terminal alkene will be very small.
The hydrofomylation catalyst must have a very strong preference for the
terminal carbon atom to give 60-80% linear products.
HCo(CO)4 is an effective isomerization catalyst.
The terminal alkenes undergo faster or preferential formation of 1-alkyl
group or a faster migration of the 1-alkyl group.
The activity of 1-alkene is much higher than that of internal alkenes (1000
times faster).
Linear Alcohols by Hydroformylation of internal alkenes
Thermodynamic Vs. Kinetic Pull
H2 + CO → Slow→
O
Very fast ↑ ↓
HCo(CO)4
H2 + CO → Fast→
O
Cobalt, once attached to an alkene, “runs” along the chain until an irreversible
Insertion of CO occurs (Chain running).
Alkene does not dissociate from the cobalt hydride during the isomerization process.
Ligand substitution on the Co carbonyl by tert-alkyl phosphines: Shell process
The reaction is a hundred times slower, the phosphine complex is less active;
Hence higher temperatures, 170oC versus 140oC
The selectivity to linear products increases (75-90% versus 60-70%)
The carbonyl complex formed, HCoL(CO)3 is much more stable; Hence the
process is operated at lower pressure (25-100 bar versus 200-300 bar)
The catalyst acquires activity for hydrogenation
Isomerization
Other Hydroformylation reactions
O
AcO
'Rh'
OAc
H2, CO
O
OAc
H
H
OAc
'Rh-L'
H2, CO
O
Vitamin A
OH
H
H2, CO
OH
HO
1, 4 Butane diol
OH
'Rh-L'
OH
OAc
O
OH
OH
3-methyln1, 5 pentane diol
1. BASF/Hoffmann-LaRoche;Intermediate for Vitamin A;Rh catalyst without
Phosphorus
2. ARCO; An inermeidate for 1,4-butanediol; Rh with Phosphorus ligand
3. Kuraray; An intermediate for 3-Me1,5-pentanediol; Rh with CO and
Phosphorus
4. Mitshubishi Kasei; Isononyl aldehyde; Rh catalyst with PPh3 oxide as a
weak ligand
Homogeneous catalysts