1 Homogeneous Catalysis HMC-5- 2013Dr. K.R.Krishnamurthy National Centre for Catalysis Research Indian Institute of Technology,Madras Chennai 1

2 Homogeneous Catalysis- 5Homogeneous Oxidation Oxidation reactions Types of oxidation Wacker process Epoxidation Oxidation of cyclohexane Oxidation of p-Xylene

3 Saturated hydrocarbons Paraffins Isoparaffins Alicyclic (cyclohexane) Aromatics Alkyl aromatics Unsaturated hydrocarbons Olefins Alkynes Oxidants: (Triplet /singlet) Nitric acid Hypochlorites (NaOCl, CaOCl2) PhOI Peracids, Peroxides (H2O2, t-Butyl hydroperoxide, etc.) N2O Dioxygen (O2)(air) Objectives Selectivity Atom efficiency Eco-friedlyness Clean solvents/No solvents Use of dioxygen

4 Homogeneous OxidationObjectives Introduction of oxygen- Paraffins, Olefins, Aromatics, Naphthenes Conventional- Inorganic oxidising agents

5 Oxidizing agents Chemicals- StoichiometricMolecular oxygen, ozone, hydrogen peroxide, 40-60% HNO3, Permanganates, dichromates, chromium trioxide, transition metal oxides. Catalysts Metals: Cu, Ag, Pt, Pd; Oxides: CuO+Cu2O, V2O5, Co2O3 Mixed oxides: Bi2O3.MoO3, CoO.WO3, Molybdates

6 Oxidation of hydrocarbonsReaction mechanisms Reduction-Oxidation- Insertion of lattice oxygen Reduction of metal oxide Epoxidation- Oxygen insertion through activation Free radical based- Initiation-Propagation-Termination

7 Homogeneous Oxidation-Reaction MechanismsCH2=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

8 Motivation for research in selective oxidation processes

9 Major catalytic processes for PetrochemicalsRK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980

10 Large scale Oxidation processesEthylene (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)

11 1.Wacker Oxidation: Based on organometallic ChemistryOxidation 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+ → Pd Cu+ c) Oxidation of Cu+ by O2 2Cu H+ + ½O2 → 2Cu H2O Other Examples of Wacker oxidation Ethylene + acetic acid + ½O2 → Vinyl acetate + H2O Ethylene + R-OH + ½O2 → vinyl ether + H2O R R1 + ½O2 → R2 Ketones R O Oxidation of internal olefin Note: The reaction media are highly corrosive due to free acids, Cl- ion and dioxygen

12 The Wacker-Hoechst ProcessCH2=CH ½ O2 → CH3CHO ∆H = -244 kJ mol-1 Pd H2O → Pd(0) + 2H+ + ‘O’ CH2=CH2 + Pd H2O → Pd(0) + 2H+ + CH3CHO Catalytic cycle

13 Wacker-Hoechst process: Oxidation of alkenesPd(0) by Cu(II) Alkene coordination Reductive elimination To generate aldehyde Nucleophilic (OH-) attack On ethylene Hydride shift Reductive elimination

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15 Wacker oxidation –Reaction stepsNucleophilic attack by water on coordinated ethylene -Hydride abstraction and coordination by vinyl alcohol Intra molecular hydride attack to the coordinated vinyl group 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: 2Cu Pd(0) → 2Cu+ + Pd2+ 2Cu ½ O H+ → 2Cu H2O

16 The Wacker reaction in D2O (at 5o C)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

17 Wacker oxidation of etheneWacker products Reactants Product H2O CH3CHO H2O / HCl CH2Cl-CH2OH H2O / HNO3 O2NO-CH2-CH2-ONO2 HOAc CH2=CHOAc

18 Wacker Process- Flow scheme

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20 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 <0.1 Bulk Chemicals <1 – 5 Fine Chemicals >50 Pharmaceuticals >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.

21 Epoxidation of ethylene to EO - Fact fileFirst 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 °C; Pressure - ~ bar Organic chlorides (ppm level) as moderators Reactions C2 H4 + 1/2O C2H4 O C2H4O + 2 1/2O CO2 + 2H2 O C2H4 + 3O CO2 + 2H2O Per pass conversion % EO Selectivity % Global production -19 Mill.MTA (SRI Report- 2008) Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)

22 Epoxidation of ethylene - Reaction SchemeSelective Epoxidation – 100 % atom efficient reaction

23 Epoxidation The simplest example and one of the most important epoxide intermediates is ethylene oxide CH2=CH2 + ½ O2 → Ag Catalyst→ CH2 CH2 ∆H = kJ mol-1 O 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: oC; 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

24 Epoxidation of ethylene - EO selectivityAssumptions O2- Selective oxidation O- - Non selective oxidation - No recombination Cl- - Retards O- formation Alkali/Alkaline earth - Form Peroxy linkages - Retard Ag sintering Selective oxidation EO selectivity > 86 % realized in lab & commercial scale !!! Non- selective oxidation WMH Sachtler et. al., Catal. Rev. Sci. Eng, 10,1,(1974)& 23,127(1981); Proc. Int. Congr Catal.5 th, 929 (1973) 6 C2H4 + 6O → 6 C2H4O + 6 O- C2H4 + 6O → 2 CO2 + 2H2O Maximum theoretical selectivity- 6/7 = 85.7 % Molecular Vs Atomic adsorbed Oxygen for selectivity

25 Epoxidation of ethylene - Reaction pathwaysStrength & 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

26 Epoxidation of ethylene - Transition stateEthylene 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

27 Epoxidation of EthyleneAlkali 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

28 Trends in EO selectivityImprovements in selectivity brought out by Changes in catalyst formulation Process optimization Understanding reaction mechanism

29 Epoxidation of EthyleneWhy 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

30 Epoxidation of PropyleneChlorohydrin process Hydro peroxide process

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32 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

33 Styrene monomer & Propylene oxide process- SMPO

34 Oxidation of CyclohexaneCaprolactum Monomer for Nylon-6 Adipic acid Monomer for Nylon-66

35 3.Cyclohexane to Adipic acid & Caprolactum

36 Synthesis of Nylon -6 ROP Nylon 6 Caprolactum

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38 Metal-catalyzed liquid Phase OxidationExample: Co and Mn catalyzed oxidation of cyclohexane Cyclohexane cylohexanol + cyclohexanone (K-A oil) Conversion of cyclohexane in the first step is limited to about 5-6 % The OL to ONE ratio varies in different processes. K-A-Oil (the mixture of cyclohexanol and cylohexanone) is subjected to dehydrogenation over Cu/ZnO catalyst to give cyclohexanone 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. DuPont’s process for reduction of N2O to N2 Possibility of using N2O as an oxidant being explored

39 Production of adipic acidTwo step process STEP.1 Oxidation of Cyclohexane to Cyclohexanol + Cyclohexanone Cobalt Aectate\ Naphthenate\ Octanoate K, PSIG 10 % conversion, % selectivity for KA-Oil STEP.2 Oxidation of KA-Oil to Adipic acid 50-60% HNO3 / Cu2+ & V5+ 1-3 Atmos, K 80-90% yield of AA

40 Free radical catalyzed OxidationAuto oxidation

41 Oxidation of Cyclohexane- Reaction intermediatesGeneration of peroxy radical Conversion of peroxy radical

42 KA Oil to Adipic acid

43 Catalytic roles of V & Cu ions

44 The oxidation of cyclohexanol with nitric acid to adipic acid proceeds via two stableintermediates known from the literature viz. 6-hydroxyimino-6-nitro hexanoic acid and the hemihydrate of 1,2-cyclohexanedione, Two substances forming beside each other in a given ratio. This ratio can be calculated as a function, of the temperature and of the nitric acid and nitrous acid concentrations. The nitrous acid plays a very important part in the oxidation process. The oxidation of cyclohexanone proceeds in the same way as that of cyclohexanol, provided sufficient nitrous acid is present. In the absence of HNO2 the oxidation does not proceed at all at low temperatures. The catalysts used - ammonium vanadate and copper nitrate - have very different functions. Under the influence of the vanadate the hemihydrate of cyclohexanedione is rapidly converted to adipic acid, whereas in the absence of vanadate this substance is slowly broken down to glutaric acid, succinic acid and oxalic acid. Copper is effective only at higher temperatures where it prevents the further break- down of unstable intermediates. 6-hydroxyimino-6-nitro hexanoic acid

45 Production of adipic acid: N2O issueNitric 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!

46 N2o abatement technologyGlobal warming potential many times more than CO2 High temperature ( oc) 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

47 Production of KA- oil (cyclohexanol + cyclohexanone) from cyclohexaneLIQ.PHASE BORIC ACID HYDRATION SOLVENT-FREE OXIDATION MODIFIED CYCLOHEXENE CLEAN TECH. CONDITIONS 180OC; 1-2MPa OC NOT KNOWN 100OC;1.5MPa CATALYST SOLUBLE Co SOLUBLE Co SOLUBLE SOLID FeAlPO-5 SALTS SALTS Ti,Cu,Cr CoAlPO-36 INITIATOR/ CrIII META-BORIC H2SO4,HNO3 NONE SOLVENT ACID TUNGSTIC CONVERSION < 6% NOT KNOWN 10-12% 8-12% MAIN PERBORATE CYC-OL CYC-OL & PRODUCT CHHP ESTER CYC-ONE BY- MANY NONE NONE ADIPIC ACID PRODUCTS ACIDS,ETC VALERIC ACID DOWN- CAUSTIC HYDROLY- SEPARA- NONE STREAM 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.TIME TAGES CAT.RECOV. MENT COSTS PROCESS HIGH OL/ONE PROCESS/ DuPont/BASF/ HALCON ASAHI J.Am.Chem.Soc. LICENSOR DSM ,121,11926

48 Production of adipic acid1. Nitric acid oxidation of KA oil Conditions: oC; MPa; 60% HNO3 Catalyst: V5+, Cu metal Initiator/solvent: None Yield: 90% Main products: Adipic acid, glutaric acid and succinic acid By-products: N2O and other oxides of nitrogen, CO2, lower members of dicarboxylic acids Down-stream; Bleacher to remove NO2 and absorber to recover HNO3 Advantages: High yield of adipic acid Disadvantages: 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

49 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

50 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 acid By-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

51 Production of adipic acid4. Aerial oxidation of cyclohexane (solvent-free clean technology route) Conditions: One-step process, oC, 1.5 MPa, air Catalyst: Solid FeAlPO-31 Initiator/solvent: None Yield: 65% Main products: Adipic acid and cyclohexanone By-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 promoters Heterogeneous catalyst, ease of catalyst recycle and recovery Low processing costs Disadvantages: Long reaction time (24 h)

52 Cyclohexane to adipic acidCo2+/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)

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54 Alternative routes to adipic acidMethyl 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

55 Adipic acid

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57 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 Mn3O O M The sequence of oxidation:

58 Oxidation of p-Xylene to PTAAmoco MC Process ºC 15-30 bar Co-Mn-Br / Co-Mn-Br-Zr 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 Intermediates Witten Process Oxidative esterification of p-Xylene to DMT

59 Choice of Co (III)- Redox potentialReaction e(ev) Reduction H2O2 Decomp. Co Co fast fast V V moderate moderate Fe Fe moderate moderate Ti Ti difficult difficult As As moderate moderate Sn Sn moderate moderate Activation of side chain alkyl group

60 Radical Mechanism-Elementary stepsIn2- Organic radical initiator 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 important Oxidation potential of the metal ion: Mn+1 ⇋ Mn+ Eo Co3+ ⇋ Co ev

61 p-Xylene oxidation- Catalyst systemCo/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+ Co e- ↔ Co2+ (E = 1.92 V) Mn2+ ↔ Mn e-(E = 1.2 V) Br- ↔ Br. + e- (E = 1.06 V) Cl- ↔ Cl. + e- (E = 1.36 V)

62 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 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

63 p-Xylene Oxidation- Elementary stepsHydrogen abstraction by Br Re-oxidation of Co( II) to Co (III) Oxidation of other methyl group follows similar steps

64 Bromine cycle GW Parashall, Homogeneous Catalysis, Wiley,NY,1980

65 p-Xylene to PTA- Reaction path & kineticsp-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 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.

66 Purification of PTA ~ 2500 ppm Pd/Carbon 275ºC, 70 Kg/cm2< 15 ppm in product

67 Production of terephthalic acid1. Amoco- Mid Century Process Conditions: oC; kPa Catalyst: Soluble Cobalt/Manganese/bromine system Initiator/solvent: 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: Recovery of TPA by solid-liquid separation; solvent recovery; refluxing the condensate Advantages: Excellent yield Disadvantages: 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

68 Production of terephthalic acid2. Oxidation with an activator and/or bromine in acetic acid (Eastman Chemical; Mobil Chemicals) Conditions: oC; kPa Catalyst: Soluble Cobalt/Manganese Initiator/solvent: Acetaldehyde, 2-butanone, bromine, acetic acid Main product(s): formylbenzoic acid and terephthalic acid By-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

69 Production 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-tolualdehyde Catalyst: Manganese/bromine system Initiator/solvent: None Main product(s): p-Tolualdehyde and terephthalic acid By-products: None Down stream process: The complex has to be decomposed before p- tolualdehyde can be oxidized in water with a manganese/bromine catalyst Advantages: Toluene as a potential feedstock is cheaper than p-xylene; acetic acid is not required Disadvantages: Complexities of handling HF-BF3 and need for CO Catalyst recovery and recycle Process is rather expensive

70 Production of terephthalic acid4. Liquid phase oxidation of p-xylene in air (Solvent-free clean technology route) Conditions: oC; 2.5 MPa Catalyst: Solid CoAlPO-36 Initiator/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