Selective Oxidation of hydrocarbons Part-1 Dr.K.R.Krishnamurthy National Centre for Catalysis Research (NCCR) Indian Institute of Technology Chennai INDIA 10 th Orientation Course in Catalysis for Research Scholars 28 th November to 16 th December,2009

Selective oxidation of Hydrocarbons- Part-1 Oxidation /ammoxidation of Propylene Epoxidation of Ethylene Oxychlorination of Ethylene

Chemical Industry- Products pattern Chemicals- Intricately woven with our day to day life Petrochemicals-37%

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

Important heterogeneous oxidation processes RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980

Oxidation & ammoxidation of Propylene

Scenario in feedstock for petrochemicals RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980 Current scenario reflects the predictions

Processes for manufacture of Acrylonitrile JL.Callahan, RK.Grasselli, EC.Millberger & HA Strecker. Ind.Eng.Chem.,Proc.Res & Dev.9, 134 (1970)

Acrylonitrile- Fact file Global production & Consumption MMT Growth rate - 3% /yr Versatile chemical SOHIO’s Ammoxidation process Significant Landmark in History of chemical industry

Allylic Oxidation processes Oxidation/ Ammoxidation of Propylene – Key Process RK Grasselli &JD. Burrington, Adv. Catalysis, 30, 133,1980

Selective oxidation /Ammoxidation of Propylene Proceed through Mars- Krevelen mechanism Cyclic reduction- re-oxidation of the catalyst Catalyst systems contain binary/multi compoent metal oxides Bismuth molybdates ( α-β-γ- phases ) most active & selective Facile reduction- re-oxidation capability Hydrocarbon gets activated and not oxygen Redox Cycle for the catalyst Surface reactions in selective oxidation/ Ammoxidation of propylene

Mechanism of Oxidation/Ammoxidation of Propylene Experiments labeled with 14 C Labeling in 1-or 3- position results in acrolein with 14 C scrambled in both positions Oxidation with C Propylene did not lead to scrambling Formation of allylic species from adsorbed propylene proposed as the first step Sachtler WH & de Boer, NH, Proc.Inetrn Congr.Catal.3 rd 1964,252(1965)

Mechanism of oxidation/ammoxidation of Propylene α -Hydrogen abstraction leading to allylic species- rate determining step CR Adams & JT Jennings,J.Catal.3,549,1964 HH.Voge, CD.Wagner & DP.Stevenson,J.Catalysis, 2, 58,1963

Role of Bi & Mo Bi 2 O 3 - Highly active but not selective MoO 3 - Highly selective but not that active Bismuth molybdates- Active & Selective On Bi 2 O 3 propylene forms 1,5 Hexadiene / Benzene via allyl radical On MoO 3 Allyl iodide gets converted to acrolein Bi-O sites – Abstraction of alpha Hydrogen & formation of allyl radical Mo-O sites- Selective insertion of oxygen/nitrogen in allylic moiety * Grzybowska B & Haber J & Janas J., J.catalysis, 49, 150 (1977 )

Role of gas phase/lattice oxygen Oxidation of propylene in the absence of gas phase oxygen Participation of lattice oxygen in oxidation/ ammoxidation Oxidation with 18 O 2 in gas phase & on 18 O 2 exchanged Bi-Mo - Lattice oxygen gets incorporated in the product [ CR.Adams, Proc.Intern Congr.Catal.3 rd 1964,1,240 (1965) WH.Sachtler & NH deBoer, Proc.Itern Congr.Catal.3 rd 1964,1,252 (1965)] Lattice oxygen vacancies replenished by gas phase oxygen Facile internal diffusion of oxygen leads to oxygen insertion / replenishment [GW.Kelks J.Cat.19, 232,(1970); T.Otsubo et.al J.Catal.36,240,1975] Terminal Mo-O bond with double bond character responsible for selective oxidation- IR absorption band at cm -1 [F.Trifiro et.al J Catal.19,21(1970)] Two types of lattice oxygen in Bi-Mo-O- Selective & Non selective [RK.Grasselli & DD.Suresh, J Catal.25, 273,(1972)] Loss of selectivity related to disappearance of terminal Mo-O bond- IR study (TSR Prasada Rao,KR Krishnamurthy & PG.Menon, Proc.Intrn Conf “ Chemistry & uses of Molybdenum, Michigan, p.132,1979)

Crystal structure of Bismuth Molybdate Layered structure helps in facile Oxygen diffusion

Shear structure of Bismuth molybdate Mo-O- Corner shared Oh On loss of oxygen edge shared Oh formed Shear structure imparts Structural stability Amenable to redox cycles Partial reduction tempers M-O bond strength - Criterion for selectivity

Features of selective oxidation catalysts Selection of appropriate redox-couple- redox potential Suitable electronic configuration - Partially filled orbitals - Alpha H abstraction - Full orbitals - Olefin adsn., O/N insertion Typical commercial catalyst formulations

Desirable catalyst characteristics Hydrogen abstraction Labile lattice oxygen O/N insertion Redox stability Layered structure/Shear structure Matrix stabilization Typical redox process – Phase stability is the key

RK.Grasselli, Appl.Catal.15, 127,1985

TSR.Prasada Rao & KR.Krishnamurthy, J.Catalysis,95,209,1985

Model for multi-component molybdate catalysts Role of different phases Bi-Mo - Activity & Selectivity Fe-Mo - Facilitate re-oxidation of Bi & Mo Co,Ni-Mo - Hold excess MoO 3 in bulk molybdate phase - Ensure structural stability K,Cs - Moderate Mo-O bond strength, acidity, Fe 3+ phase Fe 2+ phase

Seven principles/Seven pillars for selective oxidation Lattice oxygen, Metal–oxygen bond strength, Host structure, Redox characteristics Multi-functionality of active sites, Site isolation, Phase co-operation RK Grasselli, Topics in Catalysis, 21,79,2002

Burrington, JD, Kartisek,CT,& Grasselli,RK J.Catalysis, 63, 235,(1980)

Selective oxidation / ammoxidation of Propylene Surface transformations

Selective oxidation of Propylene- Mechanism

Selective ammoxidation of Propylene -Mechanism

Selective Oxidation/ammoxidation of Propylene

Epoxidation of Ethylene

Epoxidation of ethylene - 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 °C; Pressure - ~ bar Organic chlorides (ppm level) as moderators Reactions C2H4 + 1/2O2 -> C2H4O C2H4O + 2 1/2O2 -> 2CO2 + 2H2O C2H4 + 3O2 -> 2CO2 + 2H2O Per pass conversion % EO Selectivity % Global production -19 Mill.MTA ( SRI Report ) Best example of Specificity - catalyst (Ag) & reactant ( Ethylene)

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

Epoxidation of ethylene - EO selectivity 6 C 2 H 4 + 6O 2 - → 6 C 2 H 4 O + 6 O - C 2 H 4 + 6O - → 2 CO 2 + 2H 2 O Maximum theoretical selectivity- 6/7 = 85.7 % Assumptions O 2 - Selective oxidation O - - Non selective oxidation - No recombination Cl - - Retards O - formation Alkali/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. Congr Catal.5 th, 929 (1973) EO selectivity > 86 % realized in lab & commercial scale !!! Molecular Vs Atomic adsorbed Oxygen – Key for selectivity

Continuous improvements in selectivity

Epoxidation of ethylene- Surface species & reactivity No adsorption of ethylene on clean Ag surface Ethylene adsorbs on Ag surface with pre-sorbed Oxygen O 2 - unstable beyond 170 K EO formed with atomic O - - in-situ IR & TPRS studies ( EL Force & AT Bell, J.Catal,44,175, (1976) Sub-surface O ss oxygen essential for EO formation O ss influences the nature of O ads Cl - decreases O ads but weakens its binding to Ag Alkali facilitates adsorption of O 2 & ethylene [ RA.van Santen et.al, J.Catal. 98, 530,(1986); AW.Czanderna, J. Vac.Sci.Technolgy, 14,408,(1977)] Surface species identified Comprehensive picture of surface species

Epoxidation of ethylene - Reaction pathways Strength & nature of adsorbed oxygen holds the key 2 different O ads 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 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 O ads on Ethylene leads to EO ( Case a) Cl - weakens Ag-O bond & helps in Formation of EO (Case c) Strongly bound bridged O ads 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 CO 2 & H 2 O

Epoxidation of ethylene- Surface transformations J.Greeley & M Mavrikakis, J.Pys.Chem. C, 111, 7992,2007 S.Linic & MA.Barteau, JACS,124,310,2002; 125,4034,2003 S.Linic, H.Piao,K.Adib & MA.Barteau, Angew.Chem.Intl.Ed.,43,2918,2004 Based on DFT, TPD & HREELS studies Similar intermediates in epoxidation of butadiene A new approach to surface transformations

Ethyene epoxidation- Reactivity of Surface species Reactivity of oxametallacycle governs selectivity

Epoxidation of Ethylene- Why only Silver & Ethylene? Bond strength & nature of adsorbed oxygen Governed by O ss & Cl ads No stable oxide under reaction conditions Inability to activate C-H bond Other noble metals activate C-H bond Oxametallacycles on other metals 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

Oxychlorination of Ethylene

Ethylene Oxychlorination Production of Ethylene Di Chloride (EDC) for VCM

Ethylene Oxychlorination- VCM production EDC- Precursor for VCM

Ethylene Oxychlorination- Source for EDC

Ethylene Oxychlorination

Ethylene Oxychlorination- Major route for VCM Alternative routes for VCM Global VCM capacity MMTA (2008) ( Nexant Report)

C2H4 + Cl2 → C2H4Cl2 C2H4Cl2 → C2H3Cl + HCl C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2O C2H4 + Cl2 → C2H4Cl2 2 C2H4Cl2 → 2 C2H3Cl + 2 HCl C2H4 + 2HCl + ½O2 → C2H4Cl2 + H2O overall, 2 C2H4 + Cl2 + ½O2 → 2 C2H3Cl + H2O Ethylene Oxychlorination –Relevance to VCM Process steps for VCM Direct chlorination to EDC Thermal cracking of EDC Oxychlorination of ethylene Overall process for VCM Oxychlorination ensures Complete utilization of Chlorine

Ethylene Oxychlorination- Reaction mechanism Follows redox pathway – CuCl 2 / Cu 2 Cl 2 Elementary steps C 2 H 4 + 2CuCl 2 C 2 H 4 Cl 2 + 2CuCl 2CuCl + ½ O 2 Cu 2 OCl 2 Cu 2 OCl 2 + 2HCl 2CuCl 2 + H 2 O Unique role of CuCl 2 lattice & redox character

Ethylene oxychlorination- Catalyst characteristics CuCl 2 - KCl/ Alumina- + Rare earth oxide promoters Active phases identified – CuCl 2, K CuCl 3, Cu (OH) Cl, Cu aluminate Cu hydroxy chlorides bound to alumina R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987 G.Lambert,et.al., J.Catalysis,189, 91 & KR.Krishnamurthy et.al, Ind J,Chem.,35A,331,1996 Phase transformations in Catalyst during oxychlorination

GC.Pandey, KV.Rao, SK.Mehtha, K.R.Krishnamurthy,DT.Goakak &PK.Bhattacharya, Ind.J.Chemistry, 35A, 331, 1996 Characterization of Ethylene Oxychlorination catalysts

SampleFrom DRS (x 103cm-1) Wt / Wt, %Cu/K Ratio Phases identified CuK CB CuCl 2 [3Cu(OH) 2 ], CuOHCI CB CuCl 2 [3Cu(OH) 2 ] CB CuCl 2 [3Cu(OH) 2 ], KCI CB CuCl 2 [3Cu(OH) 2 ] CuOHCI CB CuCl 2 [3Cu(OH) 2 ] Crystalline phase identified in oxychlorination catalysts of different compositions by X-ray powder diffractometry

Ethylene Oxychlorination catalyst- XPS study Fresh catalyst contains Cu 2 + and Cu + states Spent catalyst shell has Cu in both oxidation states Spent catalyst core shows only Cu + state Structural & electronic changes across catalyst geometry R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987 No Potassium in the core

XPS data on Oxychlorination catalysts

Ethylene oxychlorination catalyst- TPR study TPR profiles indicate presence of Cu 2+ & Cu + states in fresh & spent shell Catalyst & only Cu + in spent core section- Confirms XPS data R.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery, Canada, 1766,1988 XPS & TPR indicate slow re-oxidation of Cu + in core part

Ethylene oxychlorination catalyst- TPO study TPO profiles indicate the presence of Cu + in fresh catalyst R.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery,Canada,1766,1988

Ethylene oxychlorination catalyst- TPO study Difference in re-oxidation rates- Core-Sphere & Core-Powder R.Vetrivel, KV.rao, K.Seshan,KR Krishnamurthy & TSR.Prasada Rao,Proc.9 th Intern. Congr. Catal. Calgery, Canada,1766,1988 Spherical shape detrimental – Retards re-oxidation of Cu

Ethylene oxychlorination catalyst – Further developments Studies indicate that re-oxidation of Cu + to Cu 2+ is the limiting step Observations supported by G.Lamberti et.al (J.Catalysis, 189,91 & 105 (2000), 202,279(2001) 205,375 (2002) Angew.Chem.Intl Ed., 41,2341(2002) All further commercial formulations changed the shape- -Spherical to Annular ring – Racsig ring Developments are towards increasing catalyst life