Homogeneous Catalysis
HMC-5- 2010
Dr. K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology,Madras
Chennai-600036
Homogeneous Catalysis- 5
Homogeneous Oxidation
Oxidation reactions
Types of oxidation
Wacker process
Epoxidation
Oxidation of cyclohexane
Oxidation of p-Xylene
Hydrocarbons:
Saturated hydrocarbons
Paraffins
Isoparaffins
Alicyclic (cyclohexane)
Aromatics
Alkyl aromatics
Unsaturated hydrocarbons
Olefins
Oxidants: (Triplet /singlet)
Alkynes
Nitric acid
Hypochlorites (NaOCl, CaOCl2)
PhOI
Objectives
Peracids,
Selectivity
Peroxides (H2O2, t-Butyl
Atom efficiency
hydroperoxide, etc.)
Eco-friedlyness
Clean solvents/No solvents
N2O
Use of dioxygen
Dioxygen (O2)(air)
Homogeneous Oxidation-Reaction Mechanisms
1. CH2=CH2
→
CH3CHO
Organometallic and Redox chemistry of Pd
Nucleophilic attack by water on coordinated ethylene is the
key step
2. Cyclohexane and p-xylene oxidation by air:
Chain reaction of organic radicals
Soluble Co and Mn ions catalyze the initiation step
Auto-oxidation reaction involving dioxygen
3. Propylene to Propylene oxide (Epoxidation)
Selective oxygen atom transfer chemistry;
Oxygen source is organic hydroperoxide, e.g., tert-butyl hydroperoxide
Homogeneous Oxidation
Objectives
Introduction of oxygen- Paraffins, Olefins, Aromatics, Naphthenes
Conventional- Inorganic oxidising agents
Large scale Oxidation processes
Ethylene (CH2=CH2)
→
Acetaldehyde (CH3CHO)
→
Ethylene oxide
O
(CH2 – CH2)
Cyclohexane (C6H12) → Adipic acid (HOOC-(CH2)4-COOH)
p-Xylene
(H3C-C6H4-CH3)
Propylene (CH3-CH=CH2)
→
terephthalic acid
(HOOC-C6H4-COOH)
→
propylene oxide
(CH3-CH – CH2)
O
1.Wacker Oxidation: Based on organometallic Chemistry
a) Oxidation of ethylene by Pd2+ in H2O
Pd2+ + H2O + CH2=CH2
→
H
CH3-C=O + Pdo + 2H+
→
Pd2+ + 2Cu+
→
2Cu2+ + H2O
→
Vinyl acetate + H2O
→
vinyl ether + H2O
b) Oxidation of Pdo to Pd2+ by Cu2+
Pdo + 2Cu2+
c) Oxidation of Cu+ by O2
2Cu+ + 2H+ + ½O2
Other Examples of Wacker oxidation
i)
Ethylene + acetic acid + ½O2
ii) Ethylene + R-OH
+ ½O2
iii) R1
+ ½O2
→
R2
R1
R2
Ketones
O
Oxidation of internal olefin
Note: The reaction media are highly corrosive due to free acids, Cl- ion and dioxygen
The Wacker-Hoechst Process
→
CH2=CH2 + ½ O2
Pd2+ + H2O
→
CH3CHO
∆H = -244 kJ mol-1
Pd(0) + 2H+ + ‘O’
CH2=CH2 + Pd2+ + H2O
→
Catalytic cycle
Pd(0) + 2H+ + CH3CHO
Wacker-Hoechstprocess:
process: Oxidation
ofof
alkenes
Wacker-Hoechst
Oxidation
alkenes
O
Pd(II) + Cu(II)
RHC CH2 + O2
H3C
Oxidation of
Pd(0) by Cu(II)
Reductive elimination
To generate aldehyde
R
R = H, aldehyde
R = CnHn+2, ketone
Alkene coordination
Nucleophilic (OH-) attack
On ethylene
Hydride
shift
Reductive elimination
Wacker oxidation –Reaction steps
1. Nucleophilic attack by water on coordinated ethylene
2. -Hydride abstraction and coordination by vinyl alcohol
3. Intra molecular hydride attack to the coordinated vinyl group
4. Formation of Pd in zero oxidation state
Direct re-oxidation of Pd by oxygen is extremely slow, so Cu2+ is
used as the Co-catalyst:
2Cu2+ + Pd(0)
2Cu+ + ½ O2 + 2H+
→
→
2Cu+ + Pd2+
2Cu2+ + H2O
The nucleophilic attack of water or hydroxide takes place in an “anti” fashion.
i.e., The reaction is not an insertion of ethene to the Pd-O bond., O attacks
from the outside of Pd complex
Rate = k [PdCl4]2- [C2H4] / [H3O+] [Cl-]2
Inter or intra molecular reaction between coordinated ethylene and H2O ?
The Wacker reaction in D2O (at 5o C)
Hydroxyl proton does not end up in the ethanal formed. The decomposition
of the 2-hydroxyethyl is not a simple -elimination to Pd-hydride and vinyl alcohol,
which then isomerizes to ethanal. Instead the four protons stemming from
ethene are all present in the final ethanal product.
“Intra molecular hydride shift” as the key step of the mechanism
Wacker oxidation of ethene
Wacker products
Reactants
Product
H2O
H2O / HCl
H2O / HNO3
HOAc
CH3CHO
CH2Cl-CH2OH
O2NO-CH2-CH2-ONO2
CH2=CHOAc
PdII
R
Nu-
[Pd(0)] + Nu
R
Wacker Process- Flow scheme
Table 2.2. Concepts that define the enviro-soundness of processes [4]
1. The E-factor
Industry
Petroleum
Bulk Chemicals
Fine Chemicals
Pharmaceuticals
Product tonnage
106-108
104-106
102-104
10-103
Kg byproduct / Kg product
(E-factor)
<0.1
<1 – 5
5 - >50
25 - >100
2. Environmental Quotient (EQ) = (E-factor x unfriendliness quotient, Q).
Q can be 1 for NaCl and 100 – 1000 for heavy metal salts etc.
3. Atom Efficiency = Weight of desired product / weight of all products.
Epoxidation of ethylene to EO - Fact file
First patented in 1931
Process developed by Union Carbide in1938
Currently 3 major processes - DOW, SHELL & Scientific Design
Catalyst- Ag/α-alumina with alkali promoters
Temperature 200-280°C; Pressure - ~ 15- 20 bar
Organic chlorides (ppm level) as moderators
Reactions
C2 H4 + 1/2O2
C2H4 O
Utilization of Ethylene Oxide
C2H4O + 2 1/2O2
2CO2 + 2H2 O
C2H4 + 3O2
2CO2 + 2H2O
8%
5%
Per pass conversion -10-20 %
9%
EO Selectivity 80- 90 %
7%
Global production -19 Mill.MTA
(SRI Report- 2008)
71%
Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)
MEG
Higher glycols
Ethoxylates
Ethanolamine
Others
Epoxidation of ethylene - Reaction Scheme
Selective Epoxidation – 100 % atom efficient reaction
Epoxidation
The simplest example and one of the most important epoxide intermediates
is ethylene oxide
CH2=CH2 + ½ O2
→ Ag Catalyst→ CH2 CH2
O
∆H = -1300 kJ mol-1
The reaction is highly exothermic.
The oxidation by dioxygen also leads to formaldehyde, acetaldehyde and
some CO2 and H2O
Ethylene does not have a great affinity to clean Ag surface, but when O2 is
preadosrbed on Ag, ethylene adsorbs rapidly.
O2 adsorbs on Ag diatomically and dissociatively and is relatively weekly
adsorbed.
Electrophilic attack of mono oxygen on the  electrons of ethene
Suppression of further oxidation is important.
Conditions: 230-270oC; 20 bar and ethylene, oxygen, CO2 & ballast gas
nitrogen/methane- explosion limits consideration
Organic chloride in ppm levels introduced to moderate activity and
maximize selectivity towards EO
Epoxidation of ethylene - EO selectivity
Selective oxidation
Assumptions
O2- Selective oxidation
O- - Non selective oxidation
- No recombination
Cl- - Retards O- formation
Alkali/Alkaline earth
- Form Peroxy linkages
- Retard Ag sintering
EO selectivity > 86 % realized
in lab & commercial scale !!!
Non- selective oxidation
6 C2H4 + 6O2- → 6 C2H4O + 6 OC2H4 + 6O→ 2 CO2 + 2H2O
Maximum theoretical selectivity- 6/7 = 85.7 %
WMH Sachtler et. al.,
Catal. Rev. Sci. Eng, 10,1,(1974)&
23,127(1981); Proc. Int. Congr
Catal.5 th, 929 (1973)
Molecular Vs Atomic adsorbed Oxygen for selectivity
Epoxidation of ethylene - Reaction pathways
Strength & nature of adsorbed oxygen holds the key
2 different Oads species besides subsurface oxygen
Reactivity of oxygen species governs the selectivity
Elelctrophillic attack /insertion of Oxygen → Selective oxidation
RA.van Santen &
PCE Kuipers, Adv.
Catal. 35, 265,1987
Nucleophillic attack of Oxygen → Non selective oxidation
Reaction paths in line with observed higher selectivity
Epoxidation of ethylene - Transition state
Ethylene adsorbed on oxygenated
Ag surface
Electrophillic attack by Oads on
Ethylene leads to EO ( Case a)
Cl- weakens Ag-O bond & helps in
Formation of EO (Case c)
Strongly bound bridged Oads attacks
C-H bond leading to non-selective
Oxidation ( Case b)
Non-selective oxidation proceeds via
isomerization of EO to acetaldehyde
which further undergoes oxidation to
CO2 & H2O
RA. Van Santen & HPCE Kuipers, Adv.Catalysis, 35,265,1987
Epoxidation of Ethylene
Alkali metal Cs & Re are known to be promoters , besides chloride
Amongst halogens chloride is most effective; directly related to their
electron affinity
Nitrate facilitates transfer of selectively to ethylene , directly or indirectly
Trends in EO selectivity
Improvements in EO selectivity
EO Selectivity(%)
100
90
80
70
60
1960
1970
1980
1990
2000
2010
Year
Improvements in selectivity brought out by
Changes in catalyst formulation
Process optimization
Understanding reaction mechanism
2020
Epoxidation of Ethylene
Why only Silver & Ethylene?Reactivity of oxametallacycles
Bond strength & nature of adsorbed oxygen
Governed by Oss & Clads
No stable oxide under reaction conditions
Inability to activate C-H bond
Other noble metals activate C-H bond
Reactivity of Oxametallacycles governs EO selectivity
On other metals Oxametallacycles are more stable
Butadiene forms epoxide- 3,4 epoxy 1-butene
Propylene does not form epoxide due to
- facile formation of allylic species
- its high reactivity for further oxidation with active Oads
S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003
Epoxidation of Propene
CH3-CH=CH2 + ROOH → CH3-CH CH2 + ROH
O
High valent Ti or Mo complex as Lewis acid
CH2
CH
CH3
tBu
+
O – tBu
H2C
→
O
O – Ti
CH
→
O
CH3
Ti
CH2
HC
CH3
tBu
O
+
O
Ti
Ti = Ti4+(OR-)3
Isobutane + O2 → tBuOOH
Ph-CH2-CH3 + O2 → Ph-CH – CH3
OOH
→
Ph-CH-CH3 + CH3-CH-CH2
OH
O
Ti(iPrO)4 (immobilised: Shell) or Mo complex as catalyst Homogeneous medium
SMPO process: ARCO-Atlantic Richfield
Styrene monomer & Propylene oxide process- SMPO
Ethyl benzene + TBHP → C6H5-CHOO-CH3
C3H6
C6H5-CHOO-CH3 + H2 C-CH- CH3 Propylene Oxide
O
Dehydrogenation
C6H5-CH=CH2 Styrene
Oxidation of Cyclohexane
Caprolactum
Monomer for Nylon-6
Adipic acid
Monomer for Nylon-66
• Cyclohexane
3.Cyclohexane to Adipic acid & Caprolactum
OH
[O]
H2
O
+
Nylon 6, 6
O
O
HO
OH
O
CO + MeOH
NOH
O
Nylon 6
NH
Caprolactum
Synthesis of Nylon -6
ROP
Caprolactum
Nylon 6
Metal-catalyzed liquid Phase Oxidation
Example: Co and Mn catalyzed oxidation of cyclohexane
Cyclohexane
cylohexanol + cyclohexanone
(K-A oil)
1. Conversion of cyclohexane in the first step is limited to about 5-6 %
2. The OL to ONE ratio varies in different processes.
3. K-A-Oil (the mixture of cyclohexanol and cylohexanone) is subjected to
dehydrogenation over Cu/ZnO catalyst to give cyclohexanone
4. The oxidation of cyclohexanone by nitric acid leads to the generation of
NO2, NO, and N2O. The first two gases can be recycled for the synthesis
of nitric acid, but N2O is a ozone depleter and cannot be recycled.
5. DuPont’s process for reduction of N2O to N2
6. Possibility of using N2O as an oxidant being explored
Production of adipic acid
Two step process
STEP.1
Oxidation of Cyclohexane to Cyclohexanol + Cyclohexanone
Cobalt Aectate\ Naphthenate\ Octanoate
423-473 K,115-175 PSIG
10 % conversion, 70-09-% selectivity for K-Oil
STEP.2
Oxidation of K-Oil to Adipic acid
50-60% HNO3 / Cu2+ & V5+
1-3 Atmos, 233-253 K
80-90% yield of AA
Free radical catalyzed Oxidation
Auto oxidation
Oxidation of Cyclohexane- Reaction intermediates
Generation of peroxy radical
Conversion of peroxy radical
KA Oil to Adipic acid
Catalytic roles of V & Cu ions
Production of adipic acid: N2O issue
Nitric acid oxidation of KA (cyclohexanone)
Oxidation chemistry controlled by nitrous acid in equilibrium with NO, NO2,
HNO3 and H2O in reaction mixture;
Reaction pathway through Nitrolic acid (Nitro-6-hydroxyimino hexanoic
acid), which is hydrolyzed (slow step) and N2O is formed by further
reactions of N-containing products of hydrolysis;
NO and NO2 are adsorbed and converted back to nitric acid, but N2O
cannot be recovered in this manner;
0.15 to 0.3 tons of N2O per ton of adipic acid!
N2o abatement technology
Global warming potential many times more than CO2
High temperature (1200-1500oc) thermal reaction:
Natural gas + N2O reduces to N2+ CO2 + H2O (>99% efficiency for N2O)
abatement)
Catalytic: N2 O → NO (1000o C)-which can be oxidized to NO2
(Dupont, Rhodia)
Low temp. Catalytic process: destroy N2 O without the formation of NOx
Production of KA- oil (cyclohexanol + cyclohexanone)
from cyclohexane
LIQ.PHASE
OXIDATION
CONDITIONS
CATALYST
INITIATOR/
SOLVENT
CONVERSION
MAIN
PRODUCT
BYPRODUCTS
DOWNSTREAM
ADVANTAGES
DISADVANTAGES
PROCESS/
LICENSOR
180OC; 1-2MPa
SOLUBLE Co
SALTS
CrIII
< 6%
CHHP
MANY
ACIDS,ETC
CAUSTIC
PHASE
LOW –OL/
ONE RATIO
Cr DISPOSAL
CAT.RECOV.
DuPont/BASF/
DSM
BORIC ACID
MODIFIED
HYDRATION
SOLVENT-FREE
CYCLOHEXENE CLEAN TECH.
140-160OC
SOLUBLE Co
SALTS
META-BORIC
ACID
NOT KNOWN
PERBORATE
ESTER
NONE
NOT KNOWN
SOLUBLE
Ti,Cu,Cr
H2SO4,HNO3
TUNGSTIC
10-12%
CYC-OL
HYDROLYSE ESTER
RING
PROTECTION
HIGH INVESTMENT COSTS
HALCON
SEPARATION/DISTIL.
HIGH YIELD
OF –OL
THREE-STEP
PROCESS
ASAHI
NONE
100OC;1.5MPa
SOLID FeAlPO-5
CoAlPO-36
NONE
8-12%
CYC-OL &
CYC-ONE
ADIPIC ACID
VALERIC ACID
NONE
ONE STEP,
HETEROGEN.
HIGH RES.TIME
HIGH OL/ONE
J.Am.Chem.Soc.
1999,121,11926
Production of adipic acid
1. Nitric acid oxidation of KA oil
Conditions:
Catalyst:
Initiator/solvent:
Yield:
Main products:
By-products:
Down-stream;
Advantages:
Disadvantages:
60-120oC; 0.1-0.4 MPa; 60% HNO3
V5+, Cu metal
None
90%
Adipic acid, glutaric acid and succinic acid
N2O and other oxides of nitrogen, CO2, lower members of
dicarboxylic acids
Bleacher to remove NO2 and absorber to recover HNO3
High yield of adipic acid
2.0 mol of N2O per mole of adipic acid
Corrosive nature → Ti or stainless steel material of
construction
Reaction is very exothermic (6280 kJ kg-1)
Catalyst recovery and recycle very expensive
Production of adipic acid
2. Butadiene-based route (BASF)
Conditions:
Two-step carbomethoxylation of butadiene with CO
and MeOH
Catalyst: Homogeneous Co catalyst
Initiator/solvent:
Excess pyridine
Yield:
70%
Main products:
Dimethyl adipate and 3-pentenoate
By-products:
None
Down-stream;
Hydrolysis of diester to adipic acid and methanol
Advantages:
Suppression of lower carboxylic acids
Disadvantages:
Catalyst recovery and recycle ;
recovery of excess pyridine; very high pressures
Production of adipic acid
3. Butadiene based route (DuPont)
Conditions:
Two-step dihydrocarboxylation of butadiene
Catalyst:
Pd, Rh, Ir
Initiator/solvent:
Yield:
Halide promoter such as HI and saturated carboxylic acid
(e.g.,pentanoic acid) used as solvent
Not known
Main products:
By-products:
3-pentanoic acid and adipic acid
2-Methyl glutaric acid and 2-ethyl succinic acid
Down-stream;
Recycle 3-pentanoic acid produced by the first hydrocarboxylation step
Advantages:
2-methyl glutaric acid and 2-ethyl succinic acid could be
isomerized to adipic acid by the same catalyst system
Recovery and recycle of solvent;
transport and disposal of promoter;
costly extraction procedure
Disadvantages:
Production of adipic acid
4. Aerial oxidation of cyclohexane (solvent-free clean technology route)
Conditions:
One-step process, 100-130oC, 1.5 MPa, air
Catalyst:
Solid FeAlPO-31
Initiator/solvent:
None
Yield:
65%
Main products:
By-products:
Adipic acid and cyclohexanone
Glutaric and succinic acid
Down-stream;
Hydrolysis of diester to adipic acid
Advantages:
Molecular O2 (air) as oxidant; no green house gas (N2O)
No corrosive solvents or promoters
Heterogeneous catalyst, ease of catalyst recycle and recovery
Low processing costs
Long reaction time (24 h)
Disadvantages:
Cyclohexane to adipic acid
Co2+/Mn2+ catalyzed oxidation of CYCLOHEXANE,
Liquid phase reaction; the free radical intermediate is more active than
cyclohexane ,
Hence conversion is restricted to 3-8 mol%
Alternative technologies for production of KA oil:
H3BO3 as catalyst, borate ester (Halcon Process);
CH= by selective partial hydrogenation of benzene by aqueous Ru
catalyst,followed by hydration of CH= using ZSM-5 catalyst
(Asahi Chemicals);
Vapour or liquid phase hydrogenation of phenol using Pd/Al2O3 catalyst
Benzene to phenol using N2O (Fe-ZSM-5, one-step, vapour phase)
(Solutia/Monsanto)
Alternative routes to adipic acid
Methyl acrylate → dimerized to dimethyl adipate
Dimerization of acrylonitrile to adiponitrile (propylene as source)
Air/oxygen oxidation of cyclohexane, cyclohexanol or n-hexane
Oxidation of cyclohexane and/or cyclohexanol using H2 O 2
“Green” route
Renewable glucose to adipic acid via the formation of muconic acid
Adipic acid
H2
Homo
Fe
AlPO-31
OH
O
+
[O]
V 5+ / Cu
HNO3
[O]
2 NOX
O
[O]
OH
HO
CoAPO
O
Two
Step
Carboxymethylation or
Hydrodicarboxylation
CO + MeOH
Homogenous Catalysis
Co (BASF) or Pd, Rh-Ir (Dupont)
Oxidation of p-xylene
Terephthalic acid is produced by the oxidation of p-xylene in homogneous
Acetic acid medium, catalyst being a combination of Co and Mn salts with
Bromide ion promoter
The formation of 3-oxo bridged heteronuclear Co/Mn cluster complex is
postulated to be the active species.
Heteronuclear CoMn2O is more active
M
M
than mono nuclear Co3O4 and Mn3O4
O
M
The sequence of oxidation:
CHO
COOH
COOH
COOH
CHO
COOH
Oxidation of p-Xylene to PTA
Amoco MC Process
190-205ºC
Co-Mn-Br / Co-Mn-Br-Zr
15-30 bar
Co & Mn salts as catalysts in homogeneous Acetic acid medium, with Br - ion as promoter
One of the largest industrial scale applications of homogeneous catalysis
Reaction sequence
CHO
COOH
COOH
COOH
CHO
COOH
Intermediates
Witten Process
Oxidative
esterification of
p-Xylene to DMT
Choice of Co (III)- Redox potential
Reaction
e(ev)
Co3+
Co2+
V5+
V4+
Fe3+
Fe2+
Ti4+
Ti3+
1.82
Reduction
H2O2 Decomp.
fast
fast
1.00
moderate
moderate
0.77
moderate
moderate
0.06
difficult
difficult
As5+
As3+
0.56
moderate
moderate
Sn4+
Sn2+
0.15
moderate
moderate
Activation of side chain alkyl group
Radical Mechanism-Elementary steps
In2- Organic radical initiator
Initiation:
In2
→
In* + RH →
2In*
(Metal ion)
InH + R*
Propagation:
R* + O2 →
RO2* + RH →
RO2*
RO2H + R*
Termination:
2RO2*
→
Oxygenated precursors
Metal ions and organic hydroperoxides
RO2H + Mn+
RO2H + M(n+1)+
RH +
M(n+1)+
Additional propagation:
RO* + RH
Note:
→
→
→
RO* + HO- + M(n+1)+
RO2* + H+ + Mn+
R* + H+ + Mn+
→
ROH + R*
RH bond strength is important
Oxidation potential of the metal ion: Mn+1 ⇋ Mn+ Eo
Co3+ ⇋ Co2+ 1.82 ev
Bromine cycle
GW Parashall, Homogeneous Catalysis, Wiley,NY,1980
p-Xylene oxidation- Catalyst system
Co/Mn/Br - Co & Mn as acetates & Br as HBr, NH4Br, Tetrabromoethane
Improved catalyst system- Co/Mn/Br/Zr
Active species- MIIIMII[Br-(OOCR)1.2]
Co3+ when bound to RCOO- is a powerful oxidizing agent
Mn2+ less active than Co3+- Synergistic effect of Co & Mn
Co-Mn pair facilitates formation of Br.
Reaction of Co2+ peracid to give Co3+
Co3+ oxidizes Mn2+ to Mn3+
Co(III) + Mn(II)
Co(II)+ Mn(III)
Mn3+ oxidizes Br- to Br.
Co3+ + e- ↔
.
Mn(III) + Br
Mn(II) + Br Mn2+
↔
Br
↔
Br. generates another HC radical
Cl↔
R-H +Br.
R. + HBr
Dimeric Co2+-Co3+ pairs, once formed are inactive
Zr retards formation of dimers by complexation with Co3+
Co2+
(E
3+
Mn + e (E
Br. + e- (E
Cl. + e- (E
= 1.92 V)
= 1.2 V)
= 1.06 V)
= 1.36 V)
Selectivity control
1. All organic substances will probably be destroyed → CO2 + acetic acid (inert)
2. RCH2OO* and Br abstract weakly bound hydrogen
C-H bond strength in CH3 group = 85 kcal mol-1
in benzene = 104 kcal mol-1
3. Benzylic carbon is stabilized by resonance
Compare activity of :
p-NO2toluene is 31 times
less active than p-oMe
touene
+
NO 2
OMe
COOH
Since there are twice
as many oxidizable
H aoms in p-xylene
than in p-toluic acid,
p-xylene, in effect, is
2 x 4.9 = 9.8 times
more reactive than p-toluic acid
k1
COOH
k2
COOH
k1 = 4.9 x k
2
p-Xylene Oxidation- Elementary steps
Hydrogen abstraction by Br
Re-oxidation of Co( II) to Co (III)
Oxidation of other methyl group follows similar steps
p-Xylene to PTA- Reaction path & kinetics
p-Xylene to p-toluic acid is an easier oxidation
Even mononuclear Co and Mn complexes will be active
p-Toluic acid to terephthalic acid is difficult
H abstraction from CH3 group of p-toluic acid is  4.9 times more difficult
t than from p-xylene- Reduction in ring e- density due to -COOH group
Only Co/Mn/Br- in HOAc at high temperatures and pressures could achieve
100% conversion of p-xylene.
+
NO 2
OMe
COOH
k1
COOH
k2
COOH
k1 = 4.9 x k
2
Purification of PTA
Pd/Carbon
275ºC, 70 Kg/cm2
~ 2500 ppm
< 15 ppm in product
Production of terephthalic acid
1. Amoco- Mid Century Process
Conditions:
Catalyst:
Initiator/solvent:
175-225oC; 1500-3000 kPa
Soluble Cobalt/Manganese/bromine system
acetic acid
Main product(s): Toluic acid, 4-formylbenzoic acid and terephthalic acid
By-products:
vapours of acetic acid, nitrogen, carbon oxides
Down stream process:
Advantages:
Disadvantages:
Recovery of TPA by solid-liquid separation; solvent
recovery; refluxing the condensate
Excellent yield
Highly corrosive environment→ Ti lined equipment;
highly exothermic reaction (2 x108 J kg-1)
disposal of bromine salts; solvent/catalyst recovery &
recycle; high solvent loss;
purification step to remove 4-formylbenzoic acid
impurity
Production of terephthalic acid
2. Oxidation with an activator and/or bromine in acetic acid
(Eastman Chemical; Mobil Chemicals)
Conditions:
Catalyst:
Initiator/solvent:
120-140oC; 1500-3000 kPa
Soluble Cobalt/Manganese
Acetaldehyde, 2-butanone, bromine, acetic acid
Main product(s):
By-products:
4-formylbenzoic acid and terephthalic acid
Vapours of acetic acid
Down stream process:
Crude TPA leached using excess acetic acid
followed by sublimation and centrifugation
Advantages:
Ti-lined vessels are not needed
Disadvantages:
Costly activators; catalyst recovery and recycle;
purification step, solvent recovery, recycle and
disposal
Production of terephthalic acid
3. From Toluene- without solvent (acetic acid) (Mitsubishi )
Conditions:
Catalyst:
Initiator/solvent:
Complex between toluene and HF-BF3 is first
formed, which is subsequently carbonylated with
CO to p-tolualdehyde
Manganese/bromine system
None
Main product(s):
By-products:
p-Tolualdehyde and terephthalic acid
None
Down stream process:
The complex has to be decomposed before ptolualdehyde can be oxidized in water with a
manganese/bromine catalyst
Toluene as a potential feedstock is cheaper than
p-xylene; acetic acid is not required
Complexities of handling HF-BF3 and need for CO
Catalyst recovery and recycle
Process is rather expensive
Advantages:
Disadvantages:
Production of terephthalic acid
4. Liquid phase oxidation of p-xylene in air
(Solvent-free clean technology route)
Conditions:
130-150oC; 2.5 MPa
Catalyst:
Initiator/solvent:
Solid CoAlPO-36
None
Main product(s):
Toluic acid, 4-formylbenzoic acid and terephthalic acid
By-products:
None
Down stream process:
Esterification of terephthalic acid
Advantages:
No need for corrosive solvents, activators and
bromine; heterogeneous catalyst,
ease of separation and recycle
Low yield: high residence times.
purification step to remove 4-formylbenzoic acid
Disadvantages: