New poly(organophosphazene) - Supported cobalt or copper (II) catalysts for the oxidation of alkenes

Article abstract of DOI:10.1080/15421400801904740

Polymer-supported heterogeneous catalysts in form of complexes of poly[bis(pyridine-2-oxy)phosphazene] with cobalt(II) or copper(II) acetate were synthesized. The catalysts were prepared by a three-step reaction procedure. First, poly(dichlorophosphazene) was synthesized by thermal polymerization of cyclic trimer of hexachlorocyclotriphosphazene. In the second stage, the poly(dichlorophosphazene) structure was converted into the 2-hydroxypyridine by a nucleophilic substitution reaction. The last stage of the catalyst synthesis was the reaction of poly[bis(pyridine-2-oxy)phosphazene] with cobalt acetate or copper acetate. FTIR, 31P NMR, AAS, BET, SEM, SEM-EDS and TG were employed for characterization of the obtained catalysts. The complexes were applied as heterogeneous catalysts in oxidation of dodecene, indene, limonene in the presence of molecular oxygen in acetonitrile (as solvent) at 60C, using isobutyraldehyde as an oxygen transfer agent. Oxidation reactions of aliphatic and aromatic hydrocarbons let to formation of epoxides or ketones with good yield and high selectivity.

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New Poly(organophosphazene)
- Supported Cobalt or Copper
(II) Catalysts for the Oxidation
of Alkenes
a
E. Błaż & J. Pielichowski a
a Department of Polymer Chemistry and Technology ,
Cracow University of Technology , Poland
Published online: 22 Sep 2010.
To cite this article: E. Błaż & J. Pielichowski (2008) New Poly(organophosphazene)
- Supported Cobalt or Copper (II) Catalysts for the Oxidation of Alkenes,
Molecular Crystals and Liquid Crystals, 484:1, 345/[711]-355/[721], DOI:
10.1080/15421400801904740
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Mol. Cryst. Liq. Cryst., Vol. 484, pp. 345=[711]–355=[721], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400801904740
New Poly(organophosphazene) - Supported Cobalt or
Copper (II) Catalysts for the Oxidation of Alkenes
E. Błaz˙ and J. Pielichowski
Department of Polymer Chemistry and Technology, Cracow University
of Technology, Poland
Polymer-supported heterogeneous catalysts in form of complexes of poly[bis(pyri-
dine-2-oxy)phosphazene] with cobalt(II) or copper(II) acetate were synthesized.
The catalysts were prepared by a three-step reaction procedure. First, poly(dichloro-
phosphazene) was synthesized by thermal polymerization of cyclic trimer of hexa-
chlorocyclotriphosphazene. In the second stage, the poly(dichlorophosphazene)
structure was converted into the 2-hydroxypyridine by a nucleophilic substitution
reaction. The last stage of the catalyst synthesis was the reaction of poly[bis(pyri-
dine-2-oxy)phosphazene] with cobalt acetate or copper acetate. FTIR, 31P NMR,
AAS, BET, SEM, SEM-EDS and TG were employed for characterization of the
obtained catalysts. The complexes were applied as heterogeneous catalysts in
oxidation of dodecene, indene, limonene in the presence of molecular oxygen in
acetonitrile (as solvent) at 60ꢀC, using isobutyraldehyde as an ‘‘oxygen transfer’’
agent. Oxidation reactions of aliphatic and aromatic hydrocarbons let to formation
of epoxides or ketones with good yield and high selectivity.
Keywords: alkenes; oxidation; polyphosphazene-based catalysts
INTRODUCTION
Catalytic oxidation is an area of particular significance for homo-
geneous catalysis as most of industrial processes involving catalytic
oxidation are practiced in the homogeneous phase. However, the pro-
blems related to corrosion, decomposition on the reactor walls, recov-
ery of the catalyst from the reaction mixture and its reuse are
limitations in homogeneous catalytic processes. The present day strin-
gent ecological standards also require attention to the environment
friendly methods. Recovery of the catalysts and their reusability can
Address correspondence to J. Pielichowski, Department of Polymer Chemistry and
ꢀ
Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krakow,
Poland. E-mail: pielich@usk.pk.edu.pl
345=[711]

346=[712]
E. Błaz˙ and J. Pielichowski
be simplified if a homogeneous complex is heterogenized by supporting
on an insoluble support. Polymers used as support have gained atten-
tion as they are inert, non-toxic, non-volatile, insoluble and often
recyclable [1].
Thus, search for viable polymer-supported catalysts has gained
attention in industry as well as in chemical laboratories [2,3].
The aerobic epoxidation of alkenes with transition metal catalysts
has been widely studied over the past decade. One of the well-known
methods of alkene epoxidation in homogeneous system is the
Mukaiyama procedure, where substrate is epoxidized using a tran-
sition metal complex as a catalyst, molecular oxygen as an oxidant
and an aliphatic aldehyde as a co-reactant [4]. Transition metal com-
plexes, such as Schiff’s base complexes, are also suitable for the aero-
bic epoxidation of alkenes with a co-reacting aldehyde. For example,
cobalt (II) Schiff’s bases give good results; however, these catalysts
are rather not selective for epoxidation [5,6].
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. It offers several advantages in preparation
procedures. The polymer is very stable (in the meaning of thermal
properties) even in the oxidative atmosphere [7].
For example, the epoxidation of propene, octane and other alkenes
using a polymer-bound MoVIO2(acac)2 catalyst was shown to be poss-
ible with yields up to 100% [8–10]. A Jacobsen-type manganese cata-
lyst has been anchored to various polymer supports and has been used
with different oxidants to make epoxides with 95% yield [11].
A polyaniline-supported cobalt(II) catalyst was prepared first by
Pielichowski [12] and Iqbal [13] and was used for the aerobic epoxida-
tion of alkenes. For example, a polyaniline-supported catalyst has
been used for epoxidation of trans-stilbene with very high yield and
selectivity [14,15].
More recently, polystyrene-bound nickel complexes have been
synthesized by Han and Lei [16]. These catalytic systems showed high
activity in the aerobic epoxidation of cyclohexene.
In recent years molecular oxygen has received much attention as a
cheap, clean and readily available oxidant. For instance, Mukaiyama
and co-workers have reported that molecular oxygen can be used as
the terminal oxidant in the epoxidation of alkenes with an aldehyde
or primary alcohol as coreactant and a metal b-diketonate as a catalyst
[17,18]. The oxidation scheme is shown below (Scheme 1).
In our current studies, we have focused on polyphosphazenes that
form one of the largest and most diverse classes of inorganic backbone
polymer systems. Both linear polymers and cyclic phosphazene

New Poly(organophosphazene) for Alkenes Oxidation
347=[713]
SCHEME 1 Oxidation reaction of alkene.
monomers can be functionalized with a broad range of organic substi-
tuents. The most widely studied phosphazene polymer systems have a
linear backbone of alternating phosphorus and nitrogen atoms with
two organic groups linked to each phosphorus atom. Several hundred
different polymers of this type are known, with properties that range
from tough hydrophobic elastomers to water soluble and biodegrad-
able polymers [19–21].
The modified polyphosphazenes are good candidates as catalysts for
the oxygenation of organic compounds. Thus, in this paper, we report
novel polyphosphazenes-supported cobalt (II) and copper (II) catalysts
that were then tested in dodecene, indene and limonene oxidation
reactions.
EXPERIMENTAL
Materials
The following materials were used in this work:
¨
. hexachlorocyclotriphosphazene (HCPT) (pure, Riedel-de-Haen,
USA)
. 2-hydroxypyridine (>99%), sodium hydride 60% oil dispersion,
cobalt(II) or copper(II) acetate tetrahydrate (99,9%) (Aldrich Chemical
Co., USA)
. tetrahydrofuran (max. 0,005% H2O, puriss, stabilized, Riedel-de-
¨
Haen, USA)
PREPARATION OF CATALYSTS AND THEIR USE
In the current work we present synthesis of novel polymeric catalysts
and oxidation reactions that represent their catalytic properties.
There are three stages of the catalysts synthesis:
(1) Polymerization of phosphonitrilic chloride trimer
Hexachlorotriphosphazene (2 g) was placed in a sealed ampoule and
heated in a furnace at 240ꢀC until the clear melted mixture became

348=[714]
E. Błaz˙ and J. Pielichowski
highly viscous but still slightly mobile. The ampoule was opened and
only linear polymer (NPCl2)n was dissolved in dry toluene.
(2) Substitution reaction, precipitation, and purification of the crude
product
2-Hydroxypyridine (0,017 mol) and sodium hydride (0,9 g) were dis-
solved in 50 ml of dry THF. Next, 1 g of poly[di(chloro)phosphazene]
in 10 ml of dry toluene was added and a small amount of catalyst – tet-
rabutylammonium bromide (TBAB), was added. The reaction mixture
was refluxed for 48 hours. The polymer was isolated by precipitation
with water. The reaction yield ranged from 70–80%.
The obtained product was identified by FT-IR and 31P NMR:
IR (KBr), cmꢁ1: 3064 (ring OC5H4N); 1587(CC), 1470(CN) i 1431
(OC5H4N);
1215, 1185 (nPN); 1280 (nCꢁOP); 1046 (nPꢁOC); 900 i 951
(dPOC);743 i 785 (dPNP)
31P NMR (DMSO): 7.72 ppm ([NP(OC5H4N)2] units)
(3) Preparation of poly[bis(pyridine-2-oxy)phosphazene] complexes
with metal salts – cobalt or copper acetate
Poly[bis(pyridine-2-oxy)phosphazene] (0,5 g) and cobalt or copper
acetate (II)(1g) were added into 50 cm3 of acetonitrile. The mixture
was stirred for 48 h at room temperature. Next, the catalysts were fil-
tered off from reaction mixture and washed several times with aceto-
nitrile. The catalysts were dried and kept in a desiccator; they were
denoted as follows:
. poly[bis(pyridine-2-oxy)phosphazene] þ cobalt acetate–Poly-2-POP-Co
. poly[bis(pyridine-2-oxy)phosphazene] þ copper acetate–Poly-2-POP-Cu
(4) The oxidation of hydrocarbons in the presence of the catalysts
In order to check activity of the prepared catalysts, several reac-
tions were carried out. In a typical procedure catalyst (30 mg), acetoni-
trile (30 mL) and substrate (2 mmol) were placed in a reactor. The
mixture was heated up to 60ꢀC, stirred with magnetic stirrer and
bubbled with molecular oxygen. 2-methylpropanal was added (6 mmol)
to reaction mixture. It worked as a reducing agent and as a co-catalyst.
The reactions were carried out under atmospheric pressure. During
the reaction, samples were drawn and dissolved in acetone. Then
the conversion ratio of hydrocarbon into products was examined by
GC-MS [22–26]. 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 techniques.
Dodecene, indene and limonene were subjected to the oxidation
reactions.

New Poly(organophosphazene) for Alkenes Oxidation
349=[715]
RESULTS AND DISCUSSION
The complexes were characterized by AAS method. AAS measure-
ments were done on apparatus Perkin Elmer Analyst 300. The sam-
ples of the catalysts were demineralized in 65% nitric acid and
ionized using oxy-acetylene blow pipe. Next, the samples were ana-
lyzed using standard cobaltic or cupric cathode. The complexes were
characterized by specific surface area using multilayer BET theory.
The results are shown in Table 1.
The results show, that the catalyst poly-2-POP-Cu has large content
of metal and better developed specific surface area in comparison to
poly-2-POP-Co.
The thermal properties of the catalysts were investigated by TG
method. Thermogravimetric analysis of the catalysts was performed
on Netzsch TG-209 analyzer. The heating rate was 10 K=min from
30 to 600ꢀC. Measurements were done in argon atmosphere (flow rate:
20 cm3=min). The TG results are shown in Figure 1.
Both complexes decomposed in two steps. The first step of mass loss
of about 10% for poly-2POP-Co and poly-2POP-Cu was at 181 and
167ꢀC, respectively, and the mass loss of about 30% for poly-2POP-
Co was at 592ꢀC and 255ꢀC for poly-2-POP-Cu. The amount of solid
residue was 69,8 and 57,7%, respectively (Fig. 1). Generally, poly-2-
POP-Co was more stable than poly-2-POP-Cu.
The decomposition degree at T30% for poly-2-POP-Co catalyst and
T47% for poly-2-POP-Cu is decreased if the cobalt or copper acetate
content is increasing (AAS data). This suggests, that incorporation
of metals ions into the polymer matrix decreases its decomposition
temperature. This is connected with the fact that the bonds in the com-
plexes are weaker due to the coordination between the metal ion and
the nitrogen atoms.
The specimen analyses were carried out using a HITACHI S-4700
field emission scanning electron microscope (SEM) equipped with a
NORAN Vantage energy dispersive X-ray spectrometer (EDS). A
focused electron beam was restored across the sample surface. The
secondary or backscattered electrons produced were detected and used
TABLE 1 Total Co (II) and Cu (II) Content in Catalysts and Specific Surface
Content metal [mg metal=
Specific surface–
BET [m2=g]
Catalysts
1 g sample]
Poly-2-POP-Co
Poly-2-POP-Cu
65,2
80,7
85,8
92,3

350=[716]
E. Błaz˙ and J. Pielichowski
FIGURE 1 TG profiles of poly[bis(pyridine-2-oxy)phosphazene, poly-2-POP-Co
and poly-2-POP-Cu.
to map the surface topography and compositional contrast based on
the density differences. The X-rays emitted when the electron beam
strikes a sample yielded information about the chemical composition
of the sample surface (EDS). The analysis depth of EDS varied
between 0.3 and 4.5 mm depending on the material analyzed and the
primary beam energy. Two catalysts – poly-2-POP-Co and poly-2-
POP-Cu – were examined by SEM=EDXS. Prior to analysis the cata-
lyst samples were coated with conductive carbon films to a thickness
of 20–30 nm.
Figure 2 shows surface images of poly[bis(pyridine-2-oxy)phospha-
zene]. Regular crystal forms can be observed.
The surface images of the catalysts are shown Figure 3. It can be
seen that these heterogeneous catalysts created agglomerates with
the size of the order of 20–30 mm and smaller about 5–10 mm. These
agglomerates consist of many smaller lamellar crystal grains in case
of poly-2-POP-Co and flaky-lamellar for poly-2-POP-Cu.
EDS spectra of the catalysts gave information about the relative
contents of cobalt or copper on the sample surface and confirmed
the efficiency of the doping process of poly[bis(pyridine-2-oxy)-
phosphazene] with cobalt or copper acetate. The Co content was
4,9% for poly-2-POP-Co and Cu content was 8,1% for poly-2-POP-
Cu (Fig. 4).

New Poly(organophosphazene) for Alkenes Oxidation
351=[717]
FIGURE 2 SEM images of poly[bis(pyridine-2-oxy)phosphazene.
Application of EDS spectroscopy coupled with SEM let us testing
the surface distribution of chemical elements, for example metals, in
the form of a surface map – Figure 5.
We observed that metal was spaced exactly on the catalyst surface.
We can suggest that a chemical reaction take place between polymer
and cobalt or copper (II) salt and only one compound was obtained.
FIGURE 3 SEM images of a) Poly-2-POP-Co, b) Poly-2-POP-Cu.

352=[718]
E. Błaz˙ and J. Pielichowski
FIGURE 4 EDS spectrums of a) Poly-2-POP-Co, b) Poly-2-POP-Cu.
Catalytic Oxidation
Using these catalysts the aerobic oxidations of different organic
compounds – dodecene, indene and limonene – have been carried
out. The oxidation reactions are shown in Scheme 2. and the results
are collected in Table 2.

New Poly(organophosphazene) for Alkenes Oxidation
353=[719]
FIGURE 5 Metals distribution on poly[bis(pyridine-2-oxy)phosphazene;
kobalt (left) and copper (right) respectively.
Limonene gives two epoxides and indene give ketone (1,3-dihydro-
2H-inden-2-on) with very high selectivity and relatively high yield.
The oxidation reaction of indene needs shorter time than oxidation
SCHEME 2 Oxidations of alkenes.

354=[720]
E. Błaz˙ and J. Pielichowski
TABLE 2 Results of the Polymer – Supported Cobalt and Copper Catalysts
Oxidation with Molecular Oxygen
Products Yield [%] (time [h])
Catalysts
2
4
6=7
Poly-2-POP-Co
Poly-2-POP-Cu
30 (21)
37 (20)
97 (1)
95 (1)
75=20 (2)
67=28 (1,5)
of limonene. The best results were obtained for poly-2-POP-Cu.
Alkenes with terminal double bond show low reactivity therefore they
need longer reaction time. Under these conditions 1-dodecene gives
epoxide with very high selectivity. No changes in yield were observed
when the same reactions were carried out longer.
Oxidation of double bonds proceeds easier when the double bond is
activated by the presence of another group. In comparison, polymers-
supported manganese porphyrin gave similar results as a catalyst in
the oxidation reactions, but longer reaction time was necessary [27].
CONCLUSIONS
In conclusion, we have described the synthesis of polyphosphazene –
based catalysts, which are very effective in oxidizing of alkenes under
mild conditions. The main advantages of these catalysts are their
efficiency, selectivity and simplicity of separation of the heterogeneous
catalysts from the reaction medium by filtration. The oxidation reac-
tions occur in relatively short time. Generally, poly(organo)phospha-
zenes can be considered as a new group of macromolecular oxidations
catalysts, with advantageous properties.
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