Transcript MS PowerPoint - Catalysis Eprints database

Homogeneous Catalysis
HMC-3- 2010
Dr. K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology,Madras
Chennai-600036
Homogeneous Catalysis- 3
Carbonylation
Carbonylation of
Methanol
Methyl acetate
Methyl acetylene
Acetylene
CARBONYLATION
1. Methanol to acetic acid
CH3OH + CO → CH3COOH
∆H = -136 6 kJ mol-1
(BASF [Co(CO) 4]-, BP-Monsanto[RhI2 (CO)2]-, BP (The Cativa Process, Ir + Ru)
2. Methyl acetate to acetic anhydride
CH3COOCH3 + CO → (CH3CO)2O ∆H = - 94.8 kJ mol-1
(Eastman Kodak, [RhI2 (CO)2] 3. Propyne to methylmethacrylate
CH3C=CH + CO + CH3OH → CH3C(=CH2)-COOCH3
(Shell, a Pd complex)
4. Benzyl chloride to phenylacetic acid
PhCH2Cl + CO → PhCH2COOH
(Montedison, Phase transfer catalyst and a Co catalyst)
5. Carbonylation of appropriate secondary alcohol in the synthesis of Ibuprofen
(Hoechst, Pd catalyst)
Related catalytic reactions that involve CO
a) Water-gas shift reaction: CO + H2O → CO2 + H2
b) Fischer-Tropsch reaction: CO + H2 → Hydrocarbons + oxygenated products
Methanol carbonylation -Reactions
Methanol to acetic acid
1. BASF Process based on Co(CO)4 complex
1. Monsanto-BP Process based Rh carbonyl complex
2. BP-Cativa process based on Ir carbonyl complex
More than 60% of the world acetic acid production
employs the Methanol Carbonylation route
Acetic acid Processes
Options
Catalyst
Reaction
conditions
Yield
Methanol
Carbonylation
Rh complex
180-220oC
30-40 atm
MeOH:99%
CO:85%
Acetaldehyde
Oxidation
Mn acetate or
Co acetate
50-60oC
atm.press
Direct oxidation
Of Ethylene
Pd/heteropoly
Hydrocarbon
Oxidation
(n-butane,
Naphtha)
Co acetate or
Mn acetate
150-160oC
acid/metal80 atm
CH3CHO:
95%
By-product
none
none
ethylene:
87%
CH3CHO
CO2
150-230oC
nC4: 50%
Formic acid
50-60 atm
naphtha:
40%
propionic
acid, etc.
BASF Process- Formation of active Co catalyst
2CoI2 + 2H2O + 10CO
⇋
Co2(CO)8 + 4HI + 2CO2
Co2(CO)8 + H2O + CO
⇋
2HCo(CO4) + CO2
3 Co2(CO)8 + 2nMeOH
⇋
2[Co(MeOH)n]2+ + 4[Co(CO)4]- + 8 CO
HCo(CO)4 produced in these reactions catalyze FT type reactions and
lead to the formation of by products
The rate of Co catalyzed carbonylation is strongly dependent on both CO
and MeOH concentrations and pressure.
The complex Co(CO)4- is an 18 e- nucleophile.
The attack on CH3I is a comparatively slow step.
High temperatures are therefore required with the Co catalyst.
This in turn necessitates high pressure of CO to stabilize the Co(CO)4- at
high temperatures.
The BASF Process: The Catalytic & Organic cycles
CO insertion
Catalytic cycle
Organic cycle
1. Nucleophilic attack by Co(CO)4- on CH3I
The organic chemical cycle:
2. Carbonyl insertion into a metal-alkyl bond
CH3OH + HI
⇋ CH3I + H2O
3. Another CO group adds to the 16 e- species
CH3COI + H2O →CH3COOH + HI
4. Reaction with I- to eliminate acetyl iodide
Side products by FT Synthesis
H2 + CO → Hydrocarbons, oxygenated products, etc.
In Co-catalyzed BASF process, under high temperature and pressure, the
side-Products are formed by FT reaction with soluble Co catalyst.
Compare SASOL process:
K and Cu promoted heterogeneous Fe catalysts
(Carbide intermediates)
Rh catalyzed carboxylation of ethanol, n-propanol and iso-propanol have been
known. Central to the mechanism is the reaction of alkene with hydrido
complex.
Hydrocarboxylation reaction:
R-CH=CH2 + CO + H2O
→
R-CH2CH2COOH +
CH3CH(R)-COOH
Methanol to Acetic acid by Carbonylation- Process
BASF(1955)
Metal concentration
Temperature, oC
Pressure, bar
Selectivity (%) based on
a) methanol
b) CO
10-1mole per liter of
Co
230
500 – 700
90
70
BP-Monsanto (1970)
10-3 mole per liter of
Rh
180 – 190
30 – 40
> 99
90
By-Products
CH4, glycil acetate
other oxygenated HCs
CO2, H2
Effect of H2
Amount of by-products
increases
Essential
No effect
Promoter, CH3I
Essential
Carbonylation of methanol with Rh complex
BP-Monsanto Process with Rh- Methanol to acetic acid
Methyl migratory insertion
In M-CO bond
Methanol carbonylation- Reactions
While water is essential for steps 1 & 5 higher water content would mean loss of CO – (6)
HI also can form by- products as shown below
In equations 7 & 8 Rh(I) is getting oxidised to Rh(III) which is to be reduced back to
Rh(I) by water & CO as per Eqn.6 Rh(III) Iodide may precipitate in the absence of water
Methanol Carbonylation- Reactions
BP-Monsanto Process with Rh – Key features
1. The organometallic and the organic catalytic steps can be combined.
There is an extra catalytic intermediate, which is involved in additional
product forming pathway.
2. There is oxidative addition (of CH3I), insertion reaction (methyl) and a
reductive elimination (CH3COI).
3. 16 and 18 e- complexes
4. The stoichiometry remains as: CH3OH + CO → CH3COOH
The only reactants that irreversibly enter the loop.
5. CH3COI is the product of primary cycle. Water, though required for
hydrolysis is generated in the reaction of methanol and HI. It is, therefore,
not involved in the overall stoichiometry. To make the cycle operational,
small amounts of CH3I and water are added in the beginning.
Mechanistic studies
The rate: Zero order dependence on [CH3OH], [CO] and [CH3COOH]
It is first order with respect to: rate = k [Rh] [CH3I]
The rate determining step in the catalytic cycle is the oxidative addition of
CH3I to the [I2RhCO2]- (the 16 e- catalyst complex)
through an SN2 type of mechanism
Reaction Intermediates
 At actual catalytic process conditions only [I2Rh(CO)2]- is seen by in situ IR
spectroscopy. Other intermediates are not seen.
 At ambient temperature and in neat CH3I solution, IR and NMR signals for complexes:
I
CH3 CO I
C=O A
OC
Rh
Rh CO
B
I
CO
I
I
are seen. A undergoes facile conversion, insertion and reductive elimination reactions.
 React [I2Rh(CO)2]- with CH3I under non catalytic conditions (absence of CO), a solid
has been isolated:
I
C=O I
OC
I
I
C=O 2Rh
dimerizes to
Rh
Rh
I
I CO
C
I
I
CO I
The latter reacts with CO under IR and NMR observations to give [I2Rh(CO)2]- and
CH3COI
 Oxidative addition of CH3I (or HI) to: ( -60oC; 13C NMR)
I
CO
OC
I
CH3
Rh
⇋
Rh
I
CO
I
I
CO
Two eq.C atoms with JRh-C= 60Hz and methyl group with JRh-C =14 Hz at  = -0.6 ppm
Water gas shift reaction and Rh catalyzed carbonylation
The acidic pH is responsible for CO2 and H2 formation
Two roles of WG shift reaction:
1. Stabilizing the Rh Catalyst.
In the absence of CH3I, the acetic acid forming catalytic cycle
ceases to exist.
However, since water-gas shift cycle continues to be
operational, Rh remains solution and does not precipitate out.
2. Metals other than Rh: Useful mechanistic insights into the
heterogeneous water-gas shift reaction
For example, at high pH, Fe(CO)5 as pre catalyst and alkali
metal hydroxide as promoter
BP-Monsanto process
High water concentration in the reactor (14-15%) leads to:
Separation of water from the acetic acid product
major energy consumer and can limit the unit capacity.
Carbon monoxide yield loss due to water-gas shift reaction.
Increases the formation of by-products such as propionic acid and
thus lowers the acetic acid quality.
Operation with Lower water concentration
- Addition of Li or Na iodide promoter
(Celanese and Daicel, 1980s)
- Overcome the above limitations
Other metal complexes: Ni and Ir with other metal additives
- Ir-based process at lower water levels (BP Chemicals, 1996)
Low water Process (Celanese and Daicel)
Addition of Li iodide to methyl iodide increases the stability of Rh catalyst at
Low reactor water concentrations (4-5%) and decreases the liquid by-product
formation
The addition of a significant quantity of Gr I metal causes the Rh complex
to be more coordinated by CH3COO- and increases the rate of oxidative
insertion of Methyl iodide (the rate determining step), thus promoting the
primary carbonylation reaction
+ L (I-,OAc-)
[RhI2(CO)2]-
[RhI2(CO)2L]2- (strong nucleophilic
five-coordinate dianionic
intermediate)
+ CH3I
-L
-L
+ CH3I
[CH3RhI2(CO)2I]fast
[CH3CORhI2(CO)2I
Low water operation with alkali-iodide promoter
-Improvements, overall economics
-Many earlier Monsanto plants had been revamped with
Celanese additions
Limitations:
-Higher iodide environment
-Higher residual iodide in the final product
-Catalyst poisoning problems in the down stream
processes (like VAM)
Solutions
-Treatment with active carbon, additional distillation, etc
-Silver guard process to remove traces of I- in the product
-Reduce Iodide level to below 1 ppb
Methanol carbonylation-The CATIVA process
BP introduced an Ir based catalyst (Ir + Ru) in 1996
Process with higher rates at low water content
Oxidative addition to Ir is much faster than that to the Rh complex,
i.e., the reaction 2 is much faster
Equilibrium is on the trivalent state
Overall, the new catalyst system shows high rate.
Migration is now the slowest step.
The metal to C (-bonds) are stronger, more localised
Ru Rh
and more covalent for the third row metals than those
Os
Ir
in the 2nd row metal complexes.
For Pt complexes the migration reactions are slower than those for Pd
For Ir complexes, the migration reactions are slower than those for Rh
A relativistic stabilization of Ir-CH3 bond
Quantum mechanical calculations based on DFT for
The free energies of activation for the migration reaction are:
116.3 kJ mol-1 for Ir and 72.2 kJ mol-1 for Rh
Experimental:
128.5 kJ mol-1 for Ir and 81.1 kJ mol-1 for Rh
Two classes of promoters:
Simple iodide complexes of Zn, Cd, Hg, In, Ga and
Carbonyl complexes of W, Re, Ru, Os
Pd
Pt
The Cativa (Ir) Process
Operate at reduced water levels ( < 8 wt%)
Price of Rh (US$ 500 per oz) vs Ir (US$ 60 per oz) was the motivation when
research started, now Ir price is at US$ 450 per oz!
Mechanism:
Oxidative additon of MeI to the Ir center is about 150 times faster
than the equivalent reaction with Rh
MeI addition is therefore not the rate-determining step
The slowest step is the insertion of CO to form Ir-acetyl species,
involves the elimination of ionic iodide and coordination of
additional CO ligand.
Hence,
rate  [catalyst] [CO] [I-]
High rates should be achieved by operating at low iodide concentration.
Inclusion of species capable of assisting in the abstraction of iodide should
promote the rate-determining step.
The patent suggests that Ru or Re are the preferred promoters
A proprietary blend of promoters has been found to increase the reaction rate
No addition of Li iodide!
Methanol carbonylation on Ir complex
Rate is about 25 % faster than the Monsanto Rh catalysts.
Acetic acid selectivity of >99% based on CH3OH.
The oxidative addition is no longer rate-determining and migration of the methyl
group to the coordinated carbon monoxide is rate-determining.
Methanol Me oAc carbonylation Processes
Carbonylation of Methanol/ MeOAc
Heterogeneous Rh Catalyzed carbonylation of methanol
Recent research on immobilization of Rh complex on a support
-Active carbon
-Inorganic oxides, silica,alumina, zeolites, etc.
-Ion exchange resins based on cross linked
polystyrene or
vinyl pyridine resin
-Much lower reaction rates
-Metal (Rh) leaching from
the matrix/resins
-Decomposition of resins
themselves
Chiyoda introduced a novel pyridine resin-based catalyst (1994-1996) and
claimed
- high activity, long catalyst life and
- no significant Rh loss
Chiyoda – UOP: introduced “the acetica process” (1999)
Rh complex on a novel poly-vinyl pyridine resin (tolerant to elevated
temperatures and pressures) (160-200oC; 30-60 atm)
No additives and low H2O (3-7 wt%) content
Demonstration of the process: Bubble column reactor
(or gas lift reactor) (not limited by gas solution rate)
Rhodium Immobilization
I-
+
N
[Rh(CO) 2I2]-
CH3
+
N
CH3
+ [Rh(CO) 2I 2]-
Poly vinylpyridine
Active form of
Rh at high CO
partial pressure
Rh-Complexed Resin
The strong ionic association between pyridine nitrogen groups and the Rh
complex causes the immobilization.
The concentration of Rh on the solid phase is determined by the ionexchange equilibrium.
Because equilibrium strongly favors the solid phase, virtually all the Rh in the
reaction mixture is immobilized.
Methane carbonylation
Production of acetic acid directly from methane: How nice!
→
→ CH3COOH
in presence of
Pd(OCOCH3)2/Cu(OCOCH3)2 and K2S2O8/CF3COOH (recently CaCl2)
Conditions: 85oC; CH4-2 atm; CO- 30 atm; Yield, 93.8% (CH4) after 140 h.
The reaction seems to follow a radical mechanism through CH3* and trapped by
CO to form CH3CO radical and finally converted to acetic acid.
(M. Asadullah et al., Angew.Chem., Int.Ed., 39 (2000) 2475)
1. CH4 + CO
2. CH4 + CO + ½ O2
→
CH3COOH (in high yield)
with RhCl3 as catalyst in water at 100oC
The reaction rate was enhanced by Pd/C or I-. Pd/C might catalyze the activation
of CO. The addition of I- formed [Rh(CO)2I2]- during the reaction (similar to
Monsanto catalyst)
The reaction rates are reported to be too slow for an economically viable industrial
Process (low turn over rates)
But has great potential to reduce the cost of acetic acid production
(M.Lin and A. Sen, Nature, 368 (1994) 613; J.Am.Chem.Soc.,118 (1996) 4574)
Manufacture of Acetic anhydride
Manufacture of Acetic anhydride
Conventional method: Ketene (H2C=C=O) with acetic acid
Eastman Chemical Co.
500 million lbs per yr acetic anhydride
165 million lbs per yr acetic acid
Coal (gasification) → H2 + CO
→ Methanol (heterogeneous catalyst) →
(acetic acid) → Methyl acetate → CO carbonylation
→ Acetic anhydride
[RhI2(CO)2]- as the catalyst with CH3I and H2O
Compare:
CH3OH + CO
→
CH3COOH
CH3COOCH3 + CO
→
CH3CO(O)COCH3
∆G = -75 kJ mol-1
∆G = -10.5 kJ mol-1
1. Thermodynamics less favourable for Acetic anhydride.
2. The reaction is operated closer to equilibrium
Process for Acetic anhydride
Salt effect
AcI + MOAc ↔ Ac2 +MI
(12)
MeOAc + MI ↔ MeI +MOAc (13)
Two more differences:
1. In the Eastman process (since 1983, several 100,000 tons/y and
10-20% HOAc) 5% H2 is added to the CO.
In presence of H2, the induction period is absent. The Eact measured is
higher at 114 kJ mol-1 in the absence of H2 and 63 kJ mol-1 in presence
of H2 (which is the same as Monsanto process, ∆H = 60.5 kJ mol-1 and
∆S = -27 eu).
2. The addition of cations such as, Li+ or Na+ is necessary (salt effect)
Carbonylation of MeOAc
Carbonylation of MeOAc-Reactions
Alkaline salts LiI stabilize [Rh(CO)2I2 ]- and increase reaction rate
Side reaction-formation of acetone
Polyester poly ketones
Carbonylation of alkynes
The Reppe Chemistry
The Shell process for the manufacture of
Methylmethacrylate using Pd complex
Carbonylation of alkynes: Methylmethacrylate (MMA)
The conventional method: A large amount of solid wastes
O
O
HO
+
O
CN
HCN
OH
OMe
Pd catalyzed homogeneous reaction by Shell
A Pd complex catalyzes the reaction between propyne, methanol and CO
O
+
CO
+
H2O
Regio selectivity as high as 99.95%
Based on Reppe Chemistry (1930, Reppe,Germany) (Hydrocarboxylation)
C2H2 + CO + H2O → (NiBr2/CuI) → CH2=CH-COOH (acrylic acid)
BASF, Rohm-Hass: 100 bar/220oC: Mechanism not fully understood.
OH
Shell Process for MMA
Milder conditions, 60ºC & 10-60 bar pressure.
Methanol as a solvent as well as a reactant
The pre catalyst is Pd(OAc)2 mixed with an excess of phosphine ligand to
generate the active catalytic intermediate in situ.
HX as a co-catalyst. A novel ligand
Ph
Ph
P
P
Pd
N
+
Ph
Ph
P
Pd
X
NH
Pd can chelate with P and N. The fourth coordination may be a solvent molecule.
In the protonated form, the ligand acts as a labile, weakly coordinating ligand and
Easily displaced by reactants, such as CO, methlacetylene, etc.
Carbonylation of propyne in methanol to MMAShell process
Pd(OAc)2
+2
HX
+2
P
N
+
+
MeOH
2 HOAc
MeOH
P
P
NH
X2
Pd
P
N
P
Pd
CO
OMe
n
X2
N
P
NH X
N
CO
P
O
Pd
N
OMe
X
OMe
P
P
P
NH
NH X
X2
Pd
OMe
N
O
O
P
O
P
NH X
P
N
OMe
Pd
X
N
P
NH X
OMe
Pd
P
NH
X2
Carbonylation of Propyene
Rate is first order with respect to [methyl acetylene] and zero order with
respect to [acid]
The rate of MMA formation is three orders of magnitude higher with new
ligand than with a PPh2 group in 3 or 4 position of the pyridine ring.
They cannot act as bidentate ligands.
Ph
Ph
P
P
Pd
N
+
Ph
Ph
P
Pd
NH
X
Ibuprofen synthesis - Hoechst
Carbonylation of appropriate secondary alcohol with
a Pd catalyst
O
(Ac)2O
H2
Pd/C
OH
OH
CO
O
Cl
*
Pd
PdCl2(PPh3)2
O
O
*
Cl
L
Pd
Pd
*
L
*
Cl
CO
[ organic solvent + HCl : 50 bar : 130 oc]