Selective Oxidation of hydrocarbons Part-1 10 th Orientation Course in Catalysis for Research Scholars 28 th November to 16 th December,2009 Dr.K.R.Krishnamurthy National Centre for Catalysis Research (NCCR) Indian Institute of Technology Chennai-600036 INDIA Selective oxidation of Hydrocarbons- Part-1 Oxidation /ammoxidation of Propylene Epoxidation of Ethylene Oxychlorination of Ethylene Chemical Industry- Products pattern Petrochemicals-37% Chemicals- Intricately woven with our day to day life 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 1 2 3 4 5 JL.Callahan, RK.Grasselli, EC.Millberger & HA Strecker. Ind.Eng.Chem.,Proc.Res & Dev.9, 134 (1970) Acrylonitrile- Fact file Global production & Consumption 2008- 5.2 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 Redox Cycle for the catalyst Surface reactions in 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 Mechanism of Oxidation/Ammoxidation of Propylene Experiments labeled with 14C Labeling in 1-or 3- position results in acrolein with 14C scrambled in both positions Oxidation with 2- 14C 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.3rd 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 Bi2O3 - Highly active but not selective MoO3 - Highly selective but not that active Bismuth molybdates- Active & Selective On Bi2O3 propylene forms 1,5 Hexadiene / Benzene via allyl radical On MoO3 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 18O2 in gas phase & on 18O2 exchanged Bi-Mo - Lattice oxygen gets incorporated in the product [CR.Adams, Proc.Intern Congr.Catal.3rd 1964,1,240 (1965) WH.Sachtler & NH deBoer, Proc.Itern Congr.Catal.3rd 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 990-1000 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 Fe3+phase Fe2+ phase Role of different phases Bi-Mo - Activity & Selectivity Fe-Mo - Facilitate re-oxidation of Bi & Mo Co,Ni-Mo - Hold excess MoO3 in bulk molybdate phase - Ensure structural stability K,Cs - Moderate Mo-O bond strength, acidity, 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 200-280°C; Pressure - ~ 15- 20 bar Organic chlorides (ppm level) as moderators Reactions Utilization of Ethylene Oxide C2H4 + 1/2O2 -> C2H4O C2H4O + 2 1/2O2 -> 2CO2 + 2H2O 8% 5% C2H4 + 3O2 -> 2CO2 + 2H2O Per pass conversion -10-20 % EO Selectivity 80- 90 % Global production -19 Mill.MTA (SRI Report- 2008) 9% MEG Higher glycols Ethoxylates 7% Ethanolamine Others 71% Best example of Specificity - catalyst (Ag) & reactant ( Ethylene) Epoxidation of ethylene - Reaction Scheme Selective Epoxidation – 100 % atom efficient reaction 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 – 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 O2- 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 Oss oxygen essential for EO formation Oss influences the nature of Oads Cl- decreases Oads but weakens its binding to Ag Alkali facilitates adsorption of O2 & 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 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- Surface transformations Based on DFT , TPD & HREELS studies Similar intermediates in epoxidation of butadiene 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 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 Oss & Clads 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 VCM Production-Feedstocks 18% 82% Ethylene Acetylene Global VCM capacity- 42.7 MMTA (2008) ( Nexant Report) Alternative routes for VCM Ethylene Oxychlorination –Relevance to VCM Process steps for VCM C2H4 + Cl2 → C2H4Cl2 Direct chlorination to EDC C2H4Cl2 → C2H3Cl + HCl Thermal cracking of EDC 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 Oxychlorination of ethylene Overall process for VCM Oxychlorination ensures Complete utilization of Chlorine Ethylene Oxychlorination- Reaction mechanism Follows redox pathway – CuCl2 / Cu2Cl2 Elementary steps C2H4 + 2CuCl2 2CuCl + ½ O2 Cu2OCl2+ 2HCl C2H4Cl2 + 2CuCl Cu2OCl2 2CuCl2 + H2O Unique role of CuCl2 lattice & redox character Ethylene oxychlorination- Catalyst characteristics CuCl2- KCl/ Alumina- + Rare earth oxide promoters Active phases identified – CuCl2, K CuCl3, 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 &105 2000 KR.Krishnamurthy et.al, Ind J,Chem.,35A,331,1996 Phase transformations in Catalyst during oxychlorination Characterization of Ethylene Oxychlorination catalysts GC.Pandey, KV.Rao, SK.Mehtha, K.R.Krishnamurthy,DT.Goakak &PK.Bhattacharya, Ind.J.Chemistry, 35A, 331, 1996 Characterization of Ethylene Oxychlorination catalysts Crystalline phase identified in oxychlorination catalysts of different compositions by X-ray powder diffractometry Sample From DRS (x 103cm-1) Wt / Wt, % Cu K Cu/K Ratio Phases identified CB-1 19.80 2.74 1.56 2.30 CuCl2 [3Cu(OH)2], CuOHCI CB-2 17.85 6.00 1.56 2.30 CuCl2 [3Cu(OH)2] CB-3 17.54 8.66 0.98 5.45 CuCl2 [3Cu(OH)2], KCI CB-4 18.87 6.13 2.07 1.82 CuCl2 [3Cu(OH)2] CuOHCI CB-5 17.54 8.76 0.90 6.00 CuCl2 [3Cu(OH)2] Ethylene Oxychlorination catalyst- XPS study No Potassium in the core Cu2+ Cu+ states Fresh catalyst contains and Spent catalyst shell has Cu in both oxidation states Spent catalyst core shows only Cu+ state R.Vetrivel, K.Seshan,KR Krishnamurthy & TSR Prasada Rao, Bull.Mat.Sci.,9,75,1987 Structural & electronic changes across catalyst geometry 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 Cu2+ 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