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Homogeneous Catalysis for Clean Oxidation Reactions: A Comprehensive Study

This study delves into homogeneous oxidation catalysis, covering types of oxidation, oxidants used, catalysts involved, reaction mechanisms, and major catalytic processes for petrochemicals. The focus is on selectivity, atom efficiency, and eco-friendliness, using various oxidizing agents to achieve clean oxidation of hydrocarbons. The text explores processes like Wacker oxidation and epoxidation, detailing the reaction steps and mechanisms involved. The objective is to understand and optimize these processes for a sustainable and efficient approach to catalyst-assisted oxidation.

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Homogeneous Catalysis for Clean Oxidation Reactions: A Comprehensive Study

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  1. Homogeneous CatalysisHMC-5- 2013 Dr. K.R.Krishnamurthy National Centre for Catalysis Research Indian Institute of Technology,Madras Chennai-600036

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

  3. Hydrocarbons: 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 Oxidation Objectives Introduction of oxygen- Paraffins, Olefins, Aromatics, Naphthenes Conventional- Inorganic oxidising agents

  5. Oxidizing agents Chemicals- Stoichiometric Molecular 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 hydrocarbons Reaction 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 Mechanisms • 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

  8. Motivation for research in selective oxidation processes

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

  10. 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

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

  12. 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

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

  14. Wacker oxidation –Reaction steps • Nucleophilic 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: 2Cu2+ + Pd(0) → 2Cu+ + Pd2+ 2Cu+ + ½ O2 + 2H+ → 2Cu2+ + H2O

  15. 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

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

  17. Wacker Process- Flow scheme

  18. Table 2.2. Concepts that define the enviro-soundness of processes [4]

  19. 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 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)

  20. Epoxidation of ethylene - Reaction Scheme Selective Epoxidation – 100 % atom efficient reaction

  21. Epoxidation The 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 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

  22. Assumptions O2- Selective oxidation O- - Non selective oxidation - No recombination Cl- - Retards O- formation Alkali/Alkaline earth - Form Peroxy linkages - Retard Ag sintering Epoxidation of ethylene - EO selectivity 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 + 6O2- → 6 C2H4O + 6 O- C2H4 + 6O-→ 2 CO2 + 2H2O Maximum theoretical selectivity- 6/7 = 85.7 % Molecular Vs Atomic adsorbed Oxygen for selectivity

  23. 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

  24. 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

  25. 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

  26. Trends in EO selectivity • Improvements in selectivity brought out by • Changes in catalyst formulation • Process optimization • Understanding reaction mechanism

  27. Epoxidation of Ethylene Reactivity of oxametallacycles 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 S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003

  28. Epoxidation of Propylene Chlorohydrin process Hydro peroxide process

  29. Epoxidation of Propene CH3-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

  30. Styrene monomer & Propylene oxide process- SMPO

  31. Oxidation of Cyclohexane • Cyclohexane Monomer for Nylon-6 Caprolactum Adipic acid Monomer for Nylon-66

  32. 3.Cyclohexane to Adipic acid & Caprolactum

  33. Synthesis of Nylon -6 ROP Nylon 6 Caprolactum

  34. Metal-catalyzed liquid Phase Oxidation Example: 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

  35. 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, 70-90-% selectivity for KA-Oil STEP.2 Oxidation of KA-Oil to Adipic acid 50-60% HNO3 / Cu2+ & V5+ 1-3 Atmos, 233-253 K 80-90% yield of AA

  36. Free radical catalyzed Oxidation Auto oxidation

  37. Oxidation of Cyclohexane- Reaction intermediates Generation of peroxy radical Conversion of peroxy radical

  38. KA Oil to Adipic acid

  39. Catalytic roles of V & Cu ions

  40. The oxidation of cyclohexanol with nitric acid to adipic acid proceeds via two stable • intermediates 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

  41. 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!

  42. 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

  43. 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;1.5MPa 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 1999,121,11926

  44. Production of adipic acid 1. Nitric acid oxidation of KA oil Conditions: 60-120oC; 0.1-0.4 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

  45. 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

  46. Production of adipic acid 3.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

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