Polymer - Supported cobalt (II) catalysts for the oxidation of alkenes

Article abstract of DOI:10.3390/11010115

Polymer-supported heterogeneous catalysts in a form of complexes of 8-hydroxyquinoline with cobalt acetate were synthesized. Conjugated polymers - polyaniline (PANI), poly-o-toluidine (POT), poly-o-anisidine (POA) - were used as supports. Oxidation reactions of aliphatic and aromatic hydrocarbons were carried out in the presence of molecular oxygen at atmospheric pressure and epoxides or ketones were obtained as the main products with high selectivity.

Full text of DOI:10.3390/11010115

Molecules 2006, 11, 115-120
molecules
ISSN 1420-3049
http://www.mdpi.org
Polymer – Supported Cobalt (II) Catalysts for the Oxidation of
Alkenes
Edyta Błaż and Jan Pielichowski *
Department of Polymer Science and Technology, Krakow University of Technology, ul. Warszawska
24, 31-155 Krakow, Poland. Fax: +48 12 6282038
* Author to whom correspondence should be addressed; e-mail: pielich@usk.pk.edu.pl
Received: 16 February 2005; in revised form: 5 December 2005 / Accepted: 5 December 2005 /
Published: 31 January 2006
Abstract: Polymer-supported heterogeneous catalysts in a form of complexes of 8-hydroxy-
quinoline with cobalt acetate were synthesized. Conjugated polymers – polyaniline (PANI),
poly-o-toluidine (POT), poly-o-anisidine (POA) – were used as supports. Oxidation
reactions of aliphatic and aromatic hydrocarbons were carried out in the presence of
molecular oxygen at atmospheric pressure and epoxides or ketones were obtained as the
main products with high selectivity.
Keywords: Epoxidation, cobalt complexes, polyaniline based-catalysts, conjugated-
polymers, catalysts.
Introduction
Catalytic liquid phase oxidation of compounds is a problem that has been evoking interest for many
years. Heterogeneous catalysis allows running of various processes in a selective way and under mild
conditions. Oxidation reactions constitute an important group of processes in organic chemistry, but
the toxicity of many widely used oxidants and need for a solvent are the main obstacles in further
development of the technology. Low selectivity obtained in many processes also makes them more
difficult to run. Due to that, new methods of oxidation in organic chemistry and also applications of
“friendly” solvents for the natural environment were examined. Molecular oxygen is one of the most
interesting oxidizing agents. Immobilized metal complexes with diimines are used as catalysts in
oxidation processes of hydrocarbons. A catalyst chemically bound to a support has increased stability
and makes it easier to separate after the reaction [1-3].

Molecules 2006, 11
116
It has been reported in the literature that the epoxidation of alkenes by various oxidants can proceed
very efficiently when a polymer-supported catalyst is used. The use of polymer-supported catalysts
offers several advantages in preparation procedures. The polymer is typically very stable (with regards
to thermal properties) even in an oxidative atmosphere [4]. Polyaniline (PANI) is one of the polymers
which can be used as a matrix for catalysts. It is cheap, easy to synthesize and insoluble in a large
majority of commonly used solvents, which is the main advantage of supported catalysts [5].
Moreover, PANI is thermally stable to 300°C and chemical resistant to oxidation [6]. A polyaniline-
supported cobalt (II) catalyst was prepared first by Pielichowski and Iqbal [7] and used for the aerobic
epoxidation of alkenes. For example, a polyaniline-supported catalyst has been used for epoxidation of
trans-stilbene with very high yield and selectivity [8]. In this paper, we report a specific series of
reactions leading to novel catalysts based on a cobalt (II) 8-hydroxyquinoline complex supported by
polyaniline or polyaniline derived polymers. These catalysts have been designed and tested in
oxidation reactions of α-pinene, limonene and 1-decene.
Results and Discussion
In our experiments the catalysts were prepared via a three-step sequence and were then used in a
fourth one for the oxidation reactions:
1. Synthesis of 8-hydroxyquinoline complex with cobalt (II) acetate.
2. PANI, POT and POA were obtained via oxidative polymerization.
3. Cobalt-based catalysts were supported on the polymers (see Scheme 1).
4. The oxidation of hydrocarbons in the presence of the obtained catalysts.
Scheme 1
O
N
Co
N
O
R
R
R
R
N
H
N
N
N
H
n
bis(8-hydroxyquinoline)cobalt (II) - polymer
R =
CH3,
H,
OCH3
Alkene oxidations reactions (Scheme 2) were carried out by refluxing and stirring the reagents in
acetonitrile in a glass reactor thermostated at 65˚C. Molecular oxygen which was bubbled through the
reaction medium was used as the oxidizing agent and isobutyraldehyde was used as reductant and co-

Molecules 2006, 11
117
catalyst. During the reactions samples were periodically drawn for analysis and the conversion ratios
of hydrocarbon into the main product(s) was examined. The results are collected in Table 2.
Scheme 2
O
O ,
catalyst,
CHO
2
+
o
MeCN,
C
65
O
1
2
3
O
O ,
catalyst,
CHO
2
+
o
MeCN,
C
65
O
4
5
6
O
O ,
catalyst,
CHO
2
C8H17
C8H17
o
MeCN,
C
65
7
8
Table 2. Results of the Polymer-Supported Cobalt Catalysts Oxidation of
α-Pinene, Limonene and 1-Decene with Molecular Oxygen a
Substrates
α-pinene
limonene
1-decene
Catalysts
Time
Yield
Time
Yield
Time
Yield
[%]
[h]
[%]
[h]
[%]
[h]
PANI + Co (II) 8-
hydroxyquinoline
POT + Co (II) 8-
hydroxyquinoline
POA + Co (II) 8-
hydroxyquinoline
2 - 50
3 - 36
2 - 55
3 - 41
2 - 59
3 - 32
5 - 69
6 - 17
5 - 58
6 - 28
5 - 58
6 - 28
1
1
16
63
70
65
1
1
2
2
16
16
aAll the products were characterized by analytical and spectroscopic data
All the catalysts appeared to be active in the tested reactions, although their relative activity and
selectivity depended on the nature of the hydrocarbon. Catalysts were selective in the oxidation of
1-decene, which gave only one product, the epoxide derivative 8, in high yield, but aliphatic
hydrocarbons are less reactive. In these reactions α-pinene was transformed into two derivatives – 2
and 3. The conversions and molar ratios of the α-pinene derivatives were similar for all the catalysts.
The oxidation of limonene involved longer reaction times.
The main advantage of the prepared catalysts is the simplicity of separating off the heterogeneous
catalysts from the reaction medium by simple filtration. Oxidation reactions of alkenes were carried
out using synthesized the PANI, POT and POA catalysts at atmospheric pressure in the presence of

Molecules 2006, 11
118
molecular oxygen. These polymers can be used as a new group of effective oxidations catalysts, as
shown by the research conducted. We also studied the total Co(II) content in all tested polymers before
and after the oxidation reaction. The results summarized in Table 2 show that the Co(II) content varies
on the different supports and decreased during the oxidation reactions.
Table 2. Total Co(II) content in catalysts before and after oxidation reaction.
Co(II) content [wt.%]
Catalyst
before
10.04
9.30
after
9.96
8.89
PANI+Co(II)8-hydroxyquinoline
POT+ Co(II) 8-hydroxyquinoline
POA+
Co(II)
8-
9.28
9.19
hydroxyquinoline
Taking into consideration the Co(II) content in the polymers, which ranged between 9.28 and
10.04 wt.%, and the fact that only insignificant changes in product yields were observed, it could be
supposed that only some of the Co species are active in the oxidation reaction. This is probably due to
the heterogeneous character of the catalyst that makes contact between the active catalytic sites and
substrate taking part in oxidation reaction difficult. It was also observed that only a small part of the
initial amount of catalyst is deposited into the polymer matrix.
Conclusions
We have described the synthesis of conjugated polymer-supported cobalt based catalysts, which are
very effective in oxidizing alkenes under mild conditions. Molecular oxygen used as oxidant is a
cheap, effective and environmentally friendly (“Green Chemistry”) material. The main advantage of
our catalysts is the simplicity of separating heterogeneous catalysts from the reaction medium by
filtration.
Experimental
General
The synthesised substances were identified by means FT-IR, GC-MS and atomic absorption
spectroscopy (AAS). All chemicals were of reagent grade (Aldrich, Fluka), and were used without
purification.
Synthesis of the 8-hydroxyquinoline complex with cobalt (II)acetate:
N,N-Dimethylformamide (50 mL) and 8-hydroxyquinoline (7.3 g, 50 mmol) were mixed and
heated to achieve dissolution of the 8-hydroxyquinoline. Cobalt (II) acetate (6.2 g, 25 mmol) was then

Molecules 2006, 11
119
added to the flask and the mixture was heated to boiling for 10 hours. After cooling the crystals of
Co(II)8-hydroxyquinoline complex were filtered and dried at 60°C under vacuum overnight.
General procedure for monomer polymerization: preparation of polyaniline (PANI):
Before polymerization aniline was distilled under vacuum in the presence of zinc powder. In a
typical procedure, freshly distilled aniline (0.1 mol, 9.3 g) was dissolved in 1.5 M aqueous HCl
solution (150 mL). The solution was then cooled to about –5°C in a salt-ice mixture. Ammonium
persulfate (0.125 mol, 28.5 g), dissolved in water was added dropwise to the vigorously stirred
mixture. The rate of addition of the oxidizing agent was controlled by the temperature of the reaction
mixture, which varied between -5°C and -3°C. The reaction was stirred for 4 h. The greenish-black
precipitate was filtered off and washed with water and then with methanol until the filtrate became
colorless. Oligomers were extracted from polyaniline with tetrahydrofuran in a Soxhlet apparatus until
the extract became colorless. Poly-o-toluidine (POT) and poly-o-anisidine (POA) were washed with
water and methanol only. The polymer was then dried at 60°C for 24 h and deprotonated with 12%
aqueous ammonia solution (250 mL) for 6h. Deprotonated polymer was dried at 60°C for 24h and
washed again with water and methanol. All these purification steps are important for the entire removal
of all oligomeric species accompanying the polymer.
Preparation of polymer supported Co (II) 8-hydroxyquinoline:
A mixture of polymer (500 mg) and Co(II) 8-hydroxyquinoline (500 mg) was stirred in acetonitrile
(50 mL) for 48 h at room temperature. The reaction mixture was filtered and the solid filtrate was
washed with acetonitrile (5 x 5 mL). The catalyst was dried at 110°C for 24 h.
The oxidation of α-pinene, limonene and 1-decene:
The products were obtained by refluxing and stirring the reagents in a glass thermostatic reactor at
65˚C in the presence of oxygen to give the products. Substrates, i.e. α-pinene, limonene and 1-decene
(5 mmoles) were dissolved in acetonitrile (20 mL) and then isobutyraldehyde (10 mmoles) and catalyst
(20 mg) were added to the mixture. After completion of the oxidation of alkenes the solvent was
evaporated to yield a residue which was dissolved in ethyl acetate. Then the mixture was extracted
successively with aqueous NaHCO3 and water. Finally, the organic layer was dried and the solvent
was evaporated to give the desired product. All the products were characterized by analytical and
spectroscopic data [9].
References and Notes
1. Hirao, T.; Higuchi, M.; Ohshiro, Y.; Ikeda, I. A novel synthetic metal catalytic system for
dehydrogenative oxidation based on redox of polyaniline. J. Chem. Soc., Chem. Commun. 1993,
194.

Molecules 2006, 11
120
2. Hirao, T.; Higuchi, M.; Hatano, B.; Ikeda, I. A novel redox system for the palladium (II)-
catalyzed oxidation based on redox of polyanilines. Tetrahedron Lett. 1995, 36, 5925.
3. Higuchi, M.; Imoda, D.; Hirao, T. Redox behavior of polyaniline-transition-metal complexes in
solution. Macromolecules 1996, 29, 8277.
4. Wentzel, B.; Leinonen, S.; Thompson, S.; Sherrington, D.; Feiters, M.; Nolte, R. Aerobic
epoxidation of alkenes using polymer-bound Mukaiyama catalysts. J. Chem. Soc. Perkin Trans.
2000, 3428.
5. Punniyamurthy, T.; Iqbal, J. Polyaniline supported cobalt(II) salen catalysed synthesis of
pyrrolidine containing α-hydroxyamides core structures as inhibitors for HIV proteases.
Tetrahedron Lett. 1997, 38, 4463.
6. Pielichowski, K.; Pielichowski, J.; Iqbal, J.; Gurtat, P. Polyaniline-based catalysts characterized
by dynamic DSC. Appl. Catal. A 1997, 161, L25
7. Pielichowski, J.; Iqbal, J.; Polaczek, J. Pol. Appl. PL 323292, 1997.
8. Kowalski, G.; Pielichowski, J; Jasieniak, M. Polymer supported cobalt(II) catalysts for alkene
epoxidation. Appl. Catal. A: General 2003, 247, 295.
9. Błaż, E.; Tylek, E.; Polaczek, J.; Pielichowski, J. Poly(aspartic acid) sodium salt-based novel
catalyst and its application in oxidation of organic compounds. Mod. Polym. Mat. Environ. Appl.
2004, 1, 17.
Sample Availability: Available from the authors.
© 2006 by MDPI (http:www.mdpi.org). Reproduction is permitted for noncommercial purposes.