Download Epoxidation of α-pinene mediated by cobalt(III) catalysts

Epoxidation of α-pinene
mediated by cobalt(III)
catalysts
Birinchi K Das
DEPARTMENT OF CHEMISTRY
GAUHATI UNIVERSITY
GUWAHATI INDIA
RRB2
RRB2 Conference,
Conference, York
York (2006)
(2006)
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
Atom Economy
H
H
C
H
H
H2, Ni
C
H C
H
H
+
% Yield =
Actual yield of product
Theoretical yield of product
X 100
Molecular weight of atoms utilized
% Atom economy =
Molecular weight of all reactants used
X 100
H
C H
H
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
Catalysis for Green Chemistry
• important
chemistry
tool
for
implementing
green
• increasing selectivity
• decreasing processing and separation agents
• heterogeneous catalysts to make processes
greener
• catalysts can significantly lower the energy
demands of many manufacturing processes
PRODUCING CHEMICALS FROM
RENEWABLE RESOURCES
Advantages
• Conservation of fossil resources
• CO2 neutrality
• Non-toxicity of raw materials
• Biodegradable substances
Disadvantages
• Expensive raw materials
• New technolgy requirements
• Business logistics and infrastructure require
development
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.
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
COBALT CATALYSTS FOR OXIDATION
‰ Haber-Weiss cycle
?
ROOH + Co(II)
ROOH + Co(III)
RO· + Co(III) + HO–
RO2· + Co(II) + H+
ROOH + Co(III)
RO· + Co(IV) + HO–
ROOH + Co(IV)
RO2· + Co(III) + H+
Feasibility of the first set of twin reactions is determined by
the reduction potential for the Co3+/Co2+ couple.
Suitable reduction potential for the Co4+/Co3+ couple may
also permit the use of Co(III) compounds as catalysts.
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 Co3+/Co2+ redox couple has a suitable reduction
potential of ~ +1.8 V to activate triplet O2 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.
3Σ
g
1∆
g
Metal ions activate O2 by accommodating
dioxygen, at least momentarily, as either O2(superoxo) or O22- (peroxo) ligands.
O22- has no unpaired electrons.
O2- 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!
Oxidation of olefins
TWO PATHWAYS
Epoxidation
H2C
Allylic oxidation
C
H
CH2
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.
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].
Reaction scheme of α-pinene conversion
O
verbenone
OH
verbenol
O
OH
α-pinene
α-pinene oxide
+
+
OH
isopinocamphenone
Lewis acid
H2O/ H
H
OH
isopinocamphenol
O
OH
OH
O
1,2-pinanediol
OH
trans carveol
trans sobrerol
campholenic aldehyde
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, O2, TBHP, NaOCl, H2O2 or peroxy acids
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 O2
•
Mn(II) complex on montmorillonite clay. NaOCl
•
Co(OAc)2/bromide. O2
•
Polystyrene supported Co(II) acetylacetonate complex
(epoxidation). O2/sacrificial aldehyde
•
CoCl2, CoBr2, Co(OAc)2 and Co(NO3)2. Air
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
Cubane-like cobalt(III) complexes
.
Co(NO3)2 6H2O + NaO2CR
+L
H2O2 (30%, v/v)
MeOH
Pink Solution
Olive Green Complexes
[Co4(µ3-O)4(µ-O2CR)4L4]
L = pyridine or its derivatives
such as 4-Mepy, 4-CNpy
Co(III) Cubane Cluster
[Co4(µ3-O)4(µ-O2CMe)4(NC5H5)4]
Cyclic voltammogram
E½ = +0.73 V
In MeCN
HOMOGENEOUS CATALYSIS
Oxidation of α-pinene by compressed air under homogeneous
conditions at atmospheric pressure has been investigated using
[Co4III(µ3- O)4(µ-O2CC6H5)4(4-CNpy)4] as the catalyst.
•
Both epoxidation and allylic oxidation products are found to form
O
Catalyst I, ∆
Compressed Air
α-pinene
•
+
+
OH
α-pinene oxide
Verbenol
O
Verbenone
Effects of reaction temperature and catalyst concentration are studied
Effect of Reaction Temperature on
α-pinene Epoxidation
35
30
α-pinene oxide
verbenol
verbenone
Reaction Conditions:
α-pinene = 3.97mL (25mmol)
25
1,4 dioxane = 40mL
Yield (%)
20
Amount of catalyst = 25mg
15
Oxidant, O2 = 15mL/min
10
T = 60, 80 & 100ºC
Reaction time = 24h
5
0
60
80
100
0
Reaction temperature ( C)
Effect of reaction temperature on α-pinene
oxidation by [Co4III(µ3- O)4(µ-O2CC6H5)4(4-CNpy)4]
Reaction
Conversion
temperature
(%)
(°C)
Composition of product yield (%)
α-Pinene
oxide
Verbenol
Verbenone
Other
Products
60
6.97
3.5
1.28
2.19
0
80
30.62
9.73
5.07
7.9
7.92
100
66.84
31.43
4.76
15.19
15.46
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%
81.4
2520
48.0 (68)
4.53
16.97
11.89
25mg,
0.08mol%
66.8
288
31.43
4.76
15.19
15.46
150mg,
0.5mol%
63.2
48
33.07
4.83
13.98
11.33
100
Amount of Catalyst = 0.01mol%
Yield (%)
80
α-pinene
pineneoxide
verbenol
verbenone
60
40
20
0
0
5
10
15
20
25
Time (h)
Effect of catalyst concentration on air oxidation of α-pinene
Amount of catalyst = 0.08mol%
100
α -pinene
α -pinene oxide
verbenol
verbenone
Yield (%)
80
60
40
20
0
0
5
10
15
20
25
Time (h)
Effect of catalyst concentration on air oxidation of α-pinene
Am ount of catalyst = 0.5m ol%
100
α -pinene
α -pinene oxide
verbenol
verbenone
Yield (%)
80
60
40
20
0
0
5
10
15
20
25
Time (h)
Effect of catalyst concentration on air oxidation of α-pinene
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
Immobilisation of Co(III) cubanes on
Hexagonal Mesoporous Silica
O
O
Si O
O
O
O
Si O
O
CN
n-DDA
EtOH/H2O, RT
HMS
H2SO4(aq.)
Catalyst A
Co(III) Cubane Cluster
∆, H2O (MeCN)
HMS
COOH
Immobilisation via ligand exchange
CN
Immobilisation of Co(III) cubanes on
Hexagonal Mesoporous Silica
TEOS CTES
n-dodecylamine
EtOH + H2O
HMS-(CH2)2-CN
H2SO4 (aq)
CATALYST B
Co(OAc)2 + 4-CNpy
H2O
HMS-(CH2)2-COOH
In-situ immobilisation
Scanning electron micrograph of the supported
reagent Co4O4(O2CCH3)4(4-CNpy)4 on HMS
CATALYST B
Scanning electron micrograph of the supported
reagent Co4O4(O2CCH3)4(4-CNpy)4 on HMS
CATALYST B
Scanning electron micrographs of the supported reagent
Co4O4(O2CCH3)4(py)4 on HMS
N2 Adsorption Data and Metal Loading of Cobalt(III)
Cubane Complexes Supported on HMS
BET Surface
Area, m2/g
Pore Volume
(ml/g)
% of Pores
between
3.2-6.0 nm
AAS Cobalt
Loading
mM/g
Co4O4(O2CCH3)4(4-CNpy)4
620
0.57
91
0.99
Co4O4(O2CPh)4(4-CNpy)4
714
0.53
85
1.07
Co4O4(O2CCH3)4(py)4
426
0.51
36
0.86
Co4O4(O2CCH3)4(4-NH2py)4
542
0.71
40
1.7
Co4O4(O2CCH3)4(4-tBupy)4
376
0.28
25
1.23
Co4O4(O2CCH3)4(4-CNpy)4
(in situ prepared)
622
0.60
87
1.08
Immobilised
Complex
Ps/Po (Adsorption) = 0.9814
Adsorption isotherms for (a) HMS-(CH2) 2COOH and HMS-supported
cobalt(III) catalysts prepared by immobilizing
(b) Co4O4(O2CCH3)4(4-CNpy)4 and (c) Co4O4(O2CPh)4(4-CNpy)4
Type IV isotherms
1.1
Transmittance (%)
1.0
0.9
0.8
ν(CN) = ~ 2243 cm
-1
0.7
0.6
0.5
HMS-COOH
Co4O4(O2CCH3)4(4-CNpy)4
Immobilised catalyst
0.4
(in-situ)
0
2500
2000
1500
1000
-1
Wavenumber (cm )
500
1.0
Transmittance (%)
0.9
0.8
0.7
0.6
0.5
0.4
2500
in-situ
ligand exchanged
0
2000
1500
1000
500
-1
Wavenumber (cm )
Immobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared in-situ
Immobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared via ligand exchange
Effect of temperature on α-pinene epoxidation
Catalyst B:
Immobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared in-situ
50
α-pinene oxide
Reaction Conditions:
verbenol
verbenone
α-pinene = 3.97mL (25mmol)
40
Yield (%)
1,4 dioxane = 40mL
30
Amount of catalyst = 0.09mol%
Oxidant, O2 = 15mL/min
20
T = 60, 80 & 100ºC
Reaction time = 24h
10
0
60
80
100
0
Reaction Temperature ( C)
0
100
α-pinene
α-pinene oxide
90
verbenol
verbenone
80
Yield (%)
70
60
50
Amount of Catalyst = 0.005 mol%
40
T = 100ºC
30
20
10
0
0
0
5
10
15
Time (h)
20
25
100
α-pinene
α-pinene oxide
90
verbenol
verbenone
80
Yield (%)
70
60
50
Amount of Catalyst = 0.02 mol%
40
T = 100ºC
30
20
10
0
0
0
5
10
15
Time (h)
20
25
100
Amount of Catalyst = 0.09 mol%
α-pinene
α-pinene oxide
verbenol
verbenone
Yield (%)
80
60
T = 100ºC
40
20
0
0
0
5
10
15
TIme (h)
20
25
Homogeneous and Heterogeneous
α-pinene epoxidation
Catalyst
Co/benz/4-CNpy
Co/benz/4-CNpy
Co/benz/4-CNpy
Catalyst B
Catalyst B
Catalyst B
Amount of
catalyst
(Co)
3 mg
(0.01mol%)
TOF
α-pinene
% GC Yield
2,3verbenol
epoxypinane
verbenone
105
18.87
48.01
4.53
16.97
25 mg
(0.08mol%)
12
33.16
31.43
4.76
15.19
150 mg
(0.5mol%)
2
36.79
33.07
4.83
13.98
24.22
24.21
8.51
16.33
5 mg
(0.005mmol) 157
25 mg
(0.02mmol)
38
27
41.08
8.05
14.36
100 mg
(0.09mmol)
9
18.54
50.07
2.9
16.94
Reaction condition: α-pinene(25mmol), 1,4-dioxan(40mL), reaction time(24h),
temperature(100ºC),
ACKNOWLEDGEMENTS
Rajesh Chakrabarty
Green Chemistry Network
Department of Science & Technology
Government of India
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.