Homogeneous CatalysisHMC-5- 2010

Dr. K.R.KrishnamurthyNational Centre for Catalysis ResearchIndian Institute of Technology,Madras

Chennai-600036

Homogeneous Catalysis- 5

Homogeneous OxidationOxidation reactions

Types of oxidation

Wacker process

Epoxidation

Oxidation of cyclohexane

Oxidation of p-Xylene

Hydrocarbons:Saturated hydrocarbons

ParaffinsIsoparaffinsAlicyclic (cyclohexane)AromaticsAlkyl aromatics

Unsaturated hydrocarbonsOlefinsAlkynes

Oxidants: (Triplet /singlet)Nitric acidHypochlorites (NaOCl, CaOCl2)PhOIPeracids, Peroxides (H2O2, t-Butyl hydroperoxide, etc.) N2ODioxygen (O2)(air)

ObjectivesSelectivityAtom efficiencyEco-friedlynessClean solvents/No solventsUse of dioxygen

Homogeneous Oxidation-Reaction Mechanisms

1. CH2=CH2 → CH3CHO

Organometallic and Redox chemistry of PdNucleophilic attack by water on coordinated ethylene is the

key step

2. Cyclohexane and p-xylene oxidation by air:

Chain reaction of organic radicalsSoluble Co and Mn ions catalyze the initiation stepAuto-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 OxidationObjectivesIntroduction of oxygen- Paraffins, Olefins, Aromatics, NaphthenesConventional- Inorganic oxidising agents

Large scale Oxidation processes

Ethylene (CH2=CH2) → Acetaldehyde (CH3CHO) O

→ Ethylene oxide (CH2 – CH2)

Cyclohexane (C6H12) → Adipic acid (HOOC-(CH2)4-COOH)

p-Xylene (H3C-C6H4-CH3) → terephthalic acid (HOOC-C6H4-COOH)

Propylene (CH3-CH=CH2) → propylene oxide

(CH3-CH – CH2) O

1.Wacker Oxidation: Based on organometallic Chemistry

a) Oxidation of ethylene by Pd2+ in H2O H

Pd2+ + H2O + CH2=CH2 → CH3-C=O + Pdo + 2H+

b) Oxidation of Pdo to Pd2+ by Cu2+

Pdo + 2Cu2+ → Pd2+ + 2Cu+

c) Oxidation of Cu+ by O2

2Cu+ + 2H+ + ½O2 → 2Cu2+ + H2O

Other Examples of Wacker oxidationi) Ethylene + acetic acid + ½O2 → Vinyl acetate + H2O

ii) Ethylene + R-OH + ½O2 → vinyl ether + H2O

iii) R1 R1

+ ½O2 → R2 Ketones R2 O

Oxidation of internal olefinNote: The reaction media are highly corrosive due to free acids, Cl- ion and dioxygen

The Wacker-Hoechst Process

CH2=CH2 + ½ O2→ CH3CHO∆H = -244 kJ mol-1

Pd2+ + H2O → Pd(0) + 2H+ + ‘O’

CH2=CH2 + Pd2+ + H2O → Pd(0) + 2H+ + CH3CHO

Catalytic cycle

Wacker-Hoechst process: Oxidation of alkenes

RHC CH2 + O2Pd(II) + Cu(II)

O

H3C RR = H, aldehydeR = CnHn+2, ketone

Alkene coordination

Nucleophilic (OH-) attackOn ethylene

Reductive eliminationHydrideshift

Reductive eliminationTo generate aldehyde

Oxidation ofPd(0) by Cu(II)

Wacker-Hoechst process: Oxidation of alkenes

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+ + Pd2+

2Cu+ + ½ O2 + 2H+ → 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 attacksfrom 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 decompositionof the 2-hydroxyethyl is not a simple -elimination to Pd-hydride and vinyl alcohol,which then isomerizes to ethanal. Instead the four protons stemming fromethene 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 ProductH2O CH3CHO

H2O / HCl CH2Cl-CH2OH

H2O / HNO3 O2NO-CH2-CH2-ONO2

HOAc CH2=CHOAc

PdII RNu-

[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 Product tonnage Kg byproduct / Kg product (E-factor)Petroleum 106-108 <0.1Bulk Chemicals 104-106 <1 – 5Fine Chemicals 102-104 5 - >50Pharmaceuticals 10-103 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 fileEpoxidation 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

C2H4O + 2 1/2O2 2CO2 + 2H2 O

C2H4 + 3O2 2CO2 + 2H2O Per pass conversion -10-20 % EO Selectivity 80- 90 % Global production -19 Mill.MTA

(SRI Report- 2008)

Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)

Utilization of Ethylene Oxide

71%

7%

9%

5%8%

MEG

Higher glycols

Ethoxylates

Ethanolamine

Others

Epoxidation of ethylene - Reaction SchemeEpoxidation of ethylene - Reaction Scheme

Selective Epoxidation – 100 % atom efficient reaction

EpoxidationThe simplest example and one of the most important epoxide intermediates is ethylene oxide

CH2=CH2 + ½ O2→ Ag Catalyst→ CH2 CH2 ∆H = -1300 kJ mol-1

O

The reaction is highly exothermic.The oxidation by dioxygen also leads to formaldehyde, acetaldehyde and some CO2 and H2OEthylene 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 etheneSuppression of further oxidation is important.Conditions: 230-270oC; 20 bar and ethylene, oxygen, CO2 & ballast gas nitrogen/methane- explosion limits considerationOrganic chloride in ppm levels introduced to moderate activity and maximize selectivity towards EO

Epoxidation of ethylene - EO selectivity

6 C2H4 + 6O2- → 6 C2H4O + 6 O-

C2H4 + 6O- → 2 CO2 + 2H2OMaximum theoretical selectivity- 6/7 = 85.7 %

AssumptionsO2

- Selective oxidationO- - Non selective oxidation - No recombinationCl- - Retards O- formationAlkali/Alkaline earth - Form Peroxy linkages - Retard Ag sintering Selective oxidation

Non- selective oxidation

WMH Sachtler et. al.,Catal. Rev. Sci. Eng, 10,1,(1974)&23,127(1981); Proc. Int. CongrCatal.5 th, 929 (1973)

EO selectivity > 86 % realizedin lab & commercial scale !!!

Molecular Vs Atomic adsorbed Oxygen for selectivity Molecular Vs Atomic adsorbed Oxygen for selectivity

Epoxidation of ethylene - Reaction pathwaysEpoxidation 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

Nucleophillic attack of Oxygen → Non selective oxidation

RA.van Santen &PCE Kuipers, Adv.Catal. 35, 265,1987

Reaction paths in line with observed higher selectivityReaction paths in line with observed higher selectivity

Epoxidation of ethylene - Transition state

RA. Van Santen & HPCE Kuipers, Adv.Catalysis, 35,265,1987

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

Epoxidation of EthyleneEpoxidation 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

60

70

80

90

100

1960 1970 1980 1990 2000 2010 2020

Year

EO

Sel

ectiv

ity(%

)

Improvements in selectivity brought out by Changes in catalyst formulation Process optimization Understanding reaction mechanism

Epoxidation of Ethylene

Why only Silver & Ethylene?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

Reactivity of oxametallacycles

S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003

Epoxidation of PropeneCH3-CH=CH2 + ROOH → CH3-CH CH2 + ROH

O

High valent Ti or Mo complex as Lewis acid

CH2 tBu H2C O – tBu CH2 tBu

CH + O → CH O → HC O + O

CH3 O – Ti CH3 Ti CH3 Ti

Ti = Ti4+(OR-)3

Isobutane + O2 → tBuOOH

Ph-CH2-CH3 + O2 → Ph-CH – CH3 → Ph-CH-CH3 + CH3-CH-CH2

OOH 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

• Cyclohexane

Caprolactum

Adipic acid

Monomer for Nylon-6

Monomer for Nylon-66

3.Cyclohexane to Adipic acid & Caprolactum

Synthesis of Nylon -6

Caprolactum

ROP Nylon 6

Metal-catalyzed liquid Phase Oxidation

Example: Co and Mn catalyzed oxidation of cyclohexane

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 todehydrogenation over Cu/ZnO catalyst to give cyclohexanone

4. The oxidation of cyclohexanone by nitric acid leads to the generation ofNO2, 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

Cyclohexane cylohexanol + cyclohexanone(K-A oil)

Production of adipic acid

Two step process

STEP.1

Oxidation of Cyclohexane to Cyclohexanol + CyclohexanoneCobalt 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 acid50-60% HNO3 / Cu2+ & V5+

1-3 Atmos, 233-253 K

80-90% yield of AA

Free radical catalyzed OxidationAuto 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 BORIC ACID HYDRATION SOLVENT-FREE OXIDATION MODIFIED CYCLOHEXENE CLEAN TECH.

CONDITIONS 180OC; 1-2MPa 140-160OC NOT KNOWN 100OC;1.5MPaCATALYST SOLUBLE Co SOLUBLE Co SOLUBLE SOLID FeAlPO-5

SALTS SALTS Ti,Cu,Cr CoAlPO-36INITIATOR/ CrIII META-BORIC H2SO4,HNO3 NONESOLVENT ACID TUNGSTICCONVERSION < 6% NOT KNOWN 10-12% 8-12%MAIN PERBORATE CYC-OL CYC-OL &PRODUCT CHHP ESTER CYC-ONEBY- MANY NONE NONE ADIPIC ACIDPRODUCTS ACIDS,ETC VALERIC ACIDDOWN- CAUSTIC HYDROLY- SEPARA- NONESTREAM PHASE SE ESTER TION/DISTIL.ADVAN- LOW –OL/ RING HIGH YIELD ONE STEP,TAGES ONE RATIO PROTECTION OF –OL HETEROGEN.DISADVAN- Cr DISPOSAL HIGH INVEST- THREE-STEP HIGH RES.TIMETAGES CAT.RECOV. MENT COSTS PROCESS HIGH OL/ONEPROCESS/ DuPont/BASF/ HALCON ASAHI J.Am.Chem.Soc.LICENSOR DSM 1999,121,11926

Production of adipic acid

1. Nitric acid oxidation of KA oil

Conditions: 60-120oC; 0.1-0.4 MPa; 60% HNO3

Catalyst: V5+, Cu metalInitiator/solvent: NoneYield: 90%Main products: Adipic acid, glutaric acid and succinic acidBy-products: N2O and other oxides of nitrogen, CO2, lower members of

dicarboxylic acidsDown-stream; Bleacher to remove NO2 and absorber to recover HNO3

Advantages: High yield of adipic acidDisadvantages: 2.0 mol of N2O per mole of adipic acid

Corrosive nature → Ti or stainless steel material of constructionReaction is very exothermic (6280 kJ kg-1)Catalyst recovery and recycle very expensive

Production of adipic acid2. 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 acid3. Butadiene based route (DuPont)

Conditions: Two-step dihydrocarboxylation of butadiene

Catalyst: Pd, Rh, Ir

Initiator/solvent: Halide promoter such as HI and saturated carboxylic acid (e.g.,pentanoic acid) used as solvent

Yield: Not known

Main products: 3-pentanoic acid and adipic acidBy-products: 2-Methyl glutaric acid and 2-ethyl succinic acid

Down-stream; Recycle 3-pentanoic acid produced by the first hydro-carboxylation step

Advantages: 2-methyl glutaric acid and 2-ethyl succinic acid could be isomerized to adipic acid by the same catalyst system

Disadvantages: Recovery and recycle of solvent; transport and disposal of promoter;costly extraction procedure

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: Adipic acid and cyclohexanoneBy-products: 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 promotersHeterogeneous catalyst, ease of catalyst recycle and recoveryLow processing costs

Disadvantages: Long reaction time (24 h)

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

O+

OH

OHOH

O

O

CO + MeOH

Carboxymethylation or Hydrodicarboxylation

Homogenous Catalysis

Co (BASF) or Pd, Rh-Ir (Dupont)

TwoStep

[O]

CoAPO

[O]FeAlPO-31

V 5+ / CuHNO3

2 NOX

[O]

Homo

H2

Oxidation of p-xylene

Terephthalic acid is produced by the oxidation of p-xylene in homogneousAcetic 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 Mthan mono nuclear Co3O4 and Mn3O4 O

M

The sequence of oxidation:

CHO COOH COOH

CHO

COOH

COOH

Oxidation of p-Xylene to PTAOxidation of p-Xylene to PTA

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

CHO COOH COOH

CHO

COOH

COOH

Reaction sequenceReaction sequence

IntermediatesIntermediates

190-205190-205ºCºC

15-30 bar15-30 bar

Witten ProcessWitten ProcessOxidative esterification of p-Xylene to DMT

Amoco MC Process

Co-Mn-Br / Co-Mn-Br-Zr

Choice of Co (III)- Redox potentialChoice of Co (III)- Redox potential

Reaction e(ev) Reduction H2O2 Decomp.

Co3+ Co2+ 1.82 fast fast

V5+ V4+ 1.00 moderate moderate

Fe3+ Fe2+ 0.77 moderate moderate

Ti4+ Ti3+ 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

Initiation:In2 → 2In* (Metal ion)In* + RH → InH + R*

Propagation: R* + O2 → RO2*RO2* + RH → RO2H + R*

Termination: 2RO2* → Oxygenated precursors

Metal ions and organic hydroperoxides

RO2H + Mn+ → RO* + HO- + M(n+1)+

RO2H + M(n+1)+ → RO2* + H+ + Mn+

RH + M(n+1)+ → R* + H+ + Mn+

Additional propagation:RO* + RH → ROH + R*

Note: RH bond strength is importantOxidation potential of the metal ion: Mn+1 ⇋ Mn+ Eo

Co3+ ⇋ Co2+ 1.82 ev

In2- Organic radical initiator

Bromine cycle

GW Parashall, Homogeneous Catalysis, Wiley,NY,1980

p-Xylene oxidation- Catalyst systemp-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..

Mn(III) + Br- Mn(II) + Br.

Br. generates another HC radical

R-H +Br. R. + HBr

Dimeric Co2+-Co3+ pairs, once formed are inactive

Zr retards formation of dimers by complexation with Co3+

Co3+ + e- ↔ Co2+ (E = 1.92 V)Mn2+ ↔ Mn3+ + e-(E = 1.2 V)Br- ↔ Br. + e- (E = 1.06 V)Cl- ↔ Cl. + e- (E = 1.36 V)

NO2 OMe

COOH COOH

COOH

+

k1 k2

k1 = 4.9 x k2

Selectivity control

1. All organic substances will probably be destroyed → CO2 + acetic acid (inert)

2. RCH2OO* and Br abstract weakly bound hydrogenC-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 timesless active than p-oMetouene

Since there are twiceas many oxidizableH aoms in p-xylene than in p-toluic acid,p-xylene, in effect, is2 x 4.9 = 9.8 timesmore reactive than p-toluic acid

p-Xylene Oxidation- Elementary stepsp-Xylene Oxidation- Elementary steps

Oxidation of other methyl group follows similar steps

Hydrogen abstraction by Br

Re-oxidation of Co( II) to Co (III)

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.

NO2 OMe

COOH COOH

COOH

+

k1 k2

k1 = 4.9 x k2

p-Xylene to PTA- Reaction path & kineticsp-Xylene to PTA- Reaction path & kinetics

Purification of PTAPurification of PTA

Pd/Carbon275ºC, 70 Kg/cm2

~ 2500 ppm < 15 ppm in product

Production of terephthalic acidProduction of terephthalic acid

1. Amoco- Mid Century Process

Conditions: 175-225oC; 1500-3000 kPaCatalyst: Soluble Cobalt/Manganese/bromine systemInitiator/solvent: acetic acid

Main product(s): Toluic acid, 4-formylbenzoic acid and terephthalic acidBy-products: vapours of acetic acid, nitrogen, carbon oxides

Down stream process: Recovery of TPA by solid-liquid separation; solventrecovery; refluxing the condensate

Advantages: Excellent yieldDisadvantages: 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 acidProduction of terephthalic acid2. Oxidation with an activator and/or bromine in acetic acid

(Eastman Chemical; Mobil Chemicals)

Conditions: 120-140oC; 1500-3000 kPaCatalyst: Soluble Cobalt/ManganeseInitiator/solvent: Acetaldehyde, 2-butanone, bromine, acetic acid

Main product(s): 4-formylbenzoic acid and terephthalic acidBy-products: 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 acidProduction of terephthalic acid3. From Toluene- without solvent (acetic acid) (Mitsubishi )

Conditions: Complex between toluene and HF-BF3 is first formed, which is subsequently carbonylated with

CO to p-tolualdehydeCatalyst: Manganese/bromine systemInitiator/solvent: None

Main product(s): p-Tolualdehyde and terephthalic acidBy-products: None

Down stream process: The complex has to be decomposed before p-tolualdehyde can be oxidized in water with a

manganese/bromine catalystAdvantages: Toluene as a potential feedstock is cheaper than

p-xylene; acetic acid is not requiredDisadvantages: Complexities of handling HF-BF3 and need for CO

Catalyst recovery and recycleProcess is rather expensive

Production of terephthalic acidProduction of terephthalic acid4. Liquid phase oxidation of p-xylene in air

(Solvent-free clean technology route)

Conditions: 130-150oC; 2.5 MPa

Catalyst: Solid CoAlPO-36Initiator/solvent: 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

Disadvantages: Low yield: high residence times.purification step to remove 4-formylbenzoic acid