Epoxidation of α-pinene mediated by cobalt(iii) catalysts

Transcription

1 Epoxidation of α-pinene mediated by cobalt(iii) catalysts Birinchi K Das DEPARTMENT OF CHEMISTRY GAUHATI UNIVERSITY GUWAHATI INDIA RRB2 Conference, York (2006)

2 Green Chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It also refers to the discovery of new chemistry and/or technology leading to prevention and/or reduction of environmental, health and safety impact at source. JH Clark

3 Atom Economy H H C C H H H 2, Ni H H H C C H H H + % Yield = Actual yield of product Theoretical yield of product X 100 % Atom economy = Molecular weight of atoms utilized Molecular weight of all reactants used X 100

4 Some ways of practicing Green Chemistry Catalysis Use of good solvents or no solvents Use of non-hazardous substances Production of chemicals from renewable resources Others

5 Catalysis for Green Chemistry important tool for implementing green chemistry increasing selectivity decreasing processing and separation agents heterogeneous catalysts to make processes greener catalysts can significantly lower the energy demands of many manufacturing processes

6 PRODUCING CHEMICALS FROM RENEWABLE RESOURCES Advantages Conservation of fossil resources CO 2 neutrality Non-toxicity of raw materials Biodegradable substances Disadvantages Expensive raw materials New technolgy requirements Business logistics and infrastructure require development

7 α-pinene Pinene is a bicyclic monoterpene. α-pinene is one of the two structural isomers found in nature, the other one being β-pinene. Both forms are important constituents of pine resin, they also occur in other plants. They are mostly obtained from turpentine, which is a clear liquid produced as a by-product of pulping of pine wood. Turpentine contains 58%-65% α-pinene and about 30% of β-pinene. α and β pinenes are fractionated by distillation. α and β pinenes are key components in the synthesis of flavour and fragrance chemicals used in tooth paste, detergents, shampoos Selective oxidation of pinene gives important pharmaceutical and flavouring materials.

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9 OXIDATION CHEMISTRY..one of the most essential and one of the most polluting chemical technologies... Anastas& Warner Oxidation of hydrocarbon compounds Use of air as the oxidant rather than peroxides Catalyst stability Heterogeneous catalysts Metal leaching Selectivity to products Coordination Chemistry at the heart of Transition Metal Catalysed Oxidation

10 COBALT CATALYSTS FOR OXIDATION Haber-Weiss cycle ROOH + Co(II) RO + Co(III) + HO ROOH + Co(III) RO 2 + Co(II) + H +? ROOH + Co(III) RO + Co(IV) + HO ROOH + Co(IV) RO 2 + Co(III) + H + Feasibility of the first set of twin reactions is determined by the reduction potential for the Co 3+ /Co 2+ couple. Suitable reduction potential for the Co 4+ /Co 3+ couple may also permit the use of Co(III) compounds as catalysts.

11 More on redox activity of cobalt ions Reversibility of electrochemical processes means retention of molecular structure during and after electron transfer. It is essential for redox catalysts that are expected to have a long lifetime. The Co 3+ /Co 2+ redox couple has a suitable reduction potential of ~ +1.8 V to activate triplet O 2 which is a powerful oxidant from a thermodynamic point of view, but is kinetically inert due to the necessity of overcoming the kinetic barrier involved in the triplet to singlet conversion. 3 Σ g 1 g

12 Metal ions activate O 2 by accommodating dioxygen, at least momentarily, as either O - 2 (superoxo) or O 2-2 (peroxo) ligands. O 2-2 has no unpaired electrons. O 2- has one unpaired electron. But in its formation, triplet oxygen, which has two unpaired electrons, has already reacted to the presence of the metal ion in its proximity!

13 Oxidation of olefins TWO PATHWAYS Epoxidation Allylic oxidation H 2 C C H CH 2 Allylic position (saturated) Allyl groups have weaker C-H bonds (360 kj/mol) compared to alkenes and alkanes (bond energies ~400 kj/mol). This is why an allyl radical is more stable than an alkyl radical.

14 Oxidation of α-pinene Uncatalysed autoxidation at 100 C in the dark leads to the formation of 9% verbenone, 16% verbenol, 13% α-pinene epoxide, 8% transpinocarveol, 2% trans-carveol and 1% myrtenal [1956].

15 Reaction scheme of α-pinene conversion verbenol OH O verbenone α-pinene O OH O α-pinene oxide isopinocamphenol isopinocamphenone H + H H + 2 O/ Lewis acid OH OH OH OH OH 1,2-pinanediol O campholenic aldehyde trans carveol trans sobrerol

16 Rearrangement of α-pinene epoxide is known to produce over 100 products under various reaction conditions, particularly in presence of acid sites in the catalysts. Selectivity thus appears to be highly important in catalytic oxidation of α-pinene. Common Oxidants: Air, O 2, TBHP, NaOCl, H 2 O 2 or peroxy acids

17 Catalysts reported Both homogeneous and heterogeneous Zeolite encapsulated ruthenium and cobalt schiff base complexes (allylic oxidation); higher Ru activity. Air Ti-HMS catalyst (Campholenic aldehyde). TBHP (Zn-Al) Layered double hydroxide-hosted chiral sulfonatosalen manganese(iii) complex catalysts (stereoselective epoxidation). Air or O 2 Mn(II) complex on montmorillonite clay. NaOCl Co(OAc) 2 /bromide. O 2 Polystyrene supported Co(II) acetylacetonate complex (epoxidation). O 2 /sacrificial aldehyde CoCl 2, CoBr 2, Co(OAc) 2 and Co(NO 3 ) 2. Air

18 Cobalt(III) oxidation catalysts Co(II) compounds are important as (even industrial) catalysts. They are still good catalysts if environmental acceptability is not an issue. Under ordinary circumstances Co(II) is very stable. Once oxidized in presence of suitable ligands, Co(III) also may remain stable because cobaltic complexes are kinetically inert. Thermal activation may be necessary in case of cobalt(iii)-based oxidation catalysts. B. K. Das and J.H. Clark, Chem. Commun., 2000, 605 Investigation on the applicability of Co(III) systems for α-pinene (an olefin) oxidation

19 Cubane-like cobalt(iii) complexes. MeOH Co(NO 3 ) 2 6H 2 O + NaO 2 CR + L Pink Solution H 2 O 2 (30%, v/v) Olive Green Complexes [Co 4 (µ 3 -O) 4 (µ-o 2 CR) 4 L 4 ] L = pyridine or its derivatives such as 4-Mepy, 4-CNpy Co(III) Cubane Cluster

20 [Co 4 (µ 3 -O) 4 (µ-o 2 CMe) 4 (NC 5 H 5 ) 4 ]

21 Cyclic voltammogram E ½ = V In MeCN

22 HOMOGENEOUS CATALYSIS Oxidation of α-pinene by compressed air under homogeneous conditions at atmospheric pressure has been investigated using [Co III 4 (µ 3 -O) 4 (µ-o 2 CC 6 H 5 ) 4 (4-CNpy) 4 ] as the catalyst. Both epoxidation and allylic oxidation products are found to form Catalyst I, Compressed Air O + + OH O α-pinene α-pinene oxide Verbenol Verbenone Effects of reaction temperature and catalyst concentration are studied

23 Effect of Reaction Temperature on α-pinene Epoxidation 35 Yield (%) α-pinene oxide verbenol verbenone Reaction Conditions: α-pinene = 3.97mL (25mmol) 1,4 dioxane = 40mL Amount of catalyst = 25mg Oxidant, O 2 = 15mL/min T = 60, 80 & 100ºC Reaction time = 24h Reaction temperature ( 0 C)

24 Effect of reaction temperature on α-pinene oxidation by [Co 4 III (µ 3 -O) 4 (µ-o 2 CC 6 H 5 ) 4 (4-CNpy) 4 ] Reaction temperature ( C) Conversion (%) α-pinene oxide Composition of product yield (%) Verbenol Verbenone Other Products

25 Effect of Catalyst Concentration The catalyst concentration was varied between 0.01mol% to 0.5mol%. The reaction with the lowest amount of catalyst shows the highest conversion of 81.4% with a turnover frequency (TOF) of 105. High selectivity of 62-68% for α-pinene oxide has been observed with 0.01mol% of catalyst at 100 C. Product Yield (selectivity) (%) Amount of catalyst Conversion (%) TON α-pinene oxide (%) Verbenol (%) Verbenone (%) Other products (%) 3mg, 0.01mol% (68) mg, 0.08mol% mg, 0.5mol%

26 100 Amount of Catalyst = 0.01mol% 80 α-pinene pineneoxide verbenol verbenone Yield (%) Time (h) Effect of catalyst concentration on air oxidation of α-pinene

27 100 Amount of catalyst = 0.08mol% α-pinene α-pinene oxide verbenol verbenone 80 Yield (%) Time (h) Effect of catalyst concentration on air oxidation of α-pinene

28 Amount of catalyst = 0.5mol% α-pinene α-pinene oxide verbenol verbenone Yield (%) Time (h) Effect of catalyst concentration on air oxidation of α-pinene

29 HETEROGENEOUS CATALYSIS Immobilization of known homogeneous catalysts on porous supports provides a way towards eco-friendliness of chemical synthesis Work-up procedures become simpler Efficiency sometimes rises Strategies for Immobilisation Ion exchange Physisorption Covalent binding Encapsulation Incorporation into support framework

30 Immobilisation of Co(III) cubanes on Hexagonal Mesoporous Silica O Si O O O O O Si O O CN n-dda EtOH/H 2 O, RT HMS CN H 2 SO 4 (aq.) Catalyst A Co(III) Cubane Cluster, H 2 O (MeCN) HMS COOH Immobilisation via ligand exchange

31 Immobilisation of Co(III) cubanes on Hexagonal Mesoporous Silica TEOS CTES n-dodecylamine EtOH + H 2 O HMS-(CH 2 ) 2 -CN H 2 SO 4 (aq) CATALYST B Co(OAc) CNpy H 2 O HMS-(CH 2 ) 2 -COOH In-situ immobilisation

32 Scanning electron micrograph of the supported reagent Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 on HMS CATALYST B

33 Scanning electron micrograph of the supported reagent Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 on HMS CATALYST B

34 Scanning electron micrographs of the supported reagent Co 4 O 4 (O 2 CCH 3 ) 4 (py) 4 on HMS

35 N 2 Adsorption Data and Metal Loading of Cobalt(III) Cubane Complexes Supported on HMS Immobilised Complex BET Surface Area, m 2 /g Pore Volume (ml/g) % of Pores between nm AAS Cobalt Loading mm/g Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) Co 4 O 4 (O 2 CPh) 4 (4-CNpy) Co 4 O 4 (O 2 CCH 3 ) 4 (py) Co 4 O 4 (O 2 CCH 3 ) 4 (4-NH 2 py) Co 4 O 4 (O 2 CCH 3 ) 4 (4- t Bupy) Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 (in situ prepared) P s /P o (Adsorption) =

36 Adsorption isotherms for (a) HMS-(CH 2 ) 2 COOH and HMS-supported cobalt(iii) catalysts prepared by immobilizing (b) Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 and (c) Co 4 O 4 (O 2 CPh) 4 (4-CNpy) 4 Type IV isotherms

37 Transmittance (%) ν(cn) = ~ 2243 cm -1 HMS-COOH Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 Immobilised catalyst (in-situ) Wavenumber (cm -1 )

38 1.0 Transmittance (%) in-situ ligand exchanged Wavenumber (cm -1 ) Immobilised Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 prepared in-situ Immobilised Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 prepared via ligand exchange

39 Effect of temperature on α-pinene epoxidation Catalyst B: Immobilised Co 4 O 4 (O 2 CCH 3 ) 4 (4-CNpy) 4 prepared in-situ Yield (%) α-pinene oxide verbenol verbenone Reaction Conditions: α-pinene = 3.97mL (25mmol) 1,4 dioxane = 40mL Amount of catalyst = 0.09mol% Oxidant, O 2 = 15mL/min T = 60, 80 & 100ºC Reaction time = 24h Reaction Temperature ( 0 C)

40 α-pinene α-pinene oxide verbenol verbenone Yield (%) Amount of Catalyst = mol% T = 100ºC Time (h) 0

41 α-pinene α-pinene oxide verbenol verbenone Yield (%) Amount of Catalyst = 0.02 mol% T = 100ºC Time (h) 0

42 Amount of Catalyst = 0.09 mol% α-pinene α-pinene oxide verbenol verbenone Yield (%) T = 100ºC TIme (h) 0

43 Catalyst Co/benz/4-CNpy Co/benz/4-CNpy Co/benz/4-CNpy Catalyst B Catalyst B Catalyst B Homogeneous and Heterogeneous α-pinene epoxidation Amount of catalyst (Co) TOF α-pinene 2,3- epoxypinane % GC Yield verbenol verbenone 3 mg (0.01mol%) mg (0.08mol%) mg (0.5mol%) mg (0.005mmol) mg (0.02mmol) mg (0.09mmol) Reaction condition: α-pinene(25mmol), 1,4-dioxan(40mL), reaction time(24h), temperature(100ºc),

44 ACKNOWLEDGEMENTS Rajesh Chakrabarty Green Chemistry Network Department of Science & Technology Government of India

45 Ironwood Tree Mesua ferrea Nahar (Assamese) Nageswar (Sanskrit) The sweetly fragrant flowers do not smell of diesel in spite of the fact that this plant is gaining importance as a source of biodiesel. Nahar comes into full bloom around the middle of April when people celebrate a festival called Bihu in Assam. Boys and girls dance, make merry and sometimes they marry one another.