New catalysts based on poly(aspartic acid) for oxidation reaction

Article abstract of DOI:10.1080/15421401003719746

In the current work we describe synthesis of novel poly(D, L-aspartic acid) catalysts for oxidation reactions. The catalysts were prepared by a three-step reaction procedure. First, poly(D, L-aspartic acid) (PAA) was synthesized by polycondensation reaction D, L-aspartic acid with propylene carbonate under microwave irradiation. In the second stage, poly(aspartic acid) sodium salt (PAA-Na) was obtained during the reaction of hydrolysis of cyclic forms of PAA in water solutions of sodium. The last stage of catalysts synthesis were reaction of poly(aspartic acid) sodium salt with cobalt or copper acetate. FTIR, ¹H NMR, SEM, SEM-EDS, and SIMS were employed for characterization of the obtained catalysts were employed for characterization of the obtained catalysts. Oxidation reactions of some compounds were carried out on synthesized catalysts at atmospheric pressure in the presence of molecular oxygen. As the main products epoxides and ketone were obtained with very high yield and selectivity. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421401003719746

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New Catalysts Based on Poly(aspartic
acid) for Oxidation Reaction
a
b
a
E. Hebda , R. Orlicki & J. Pielichowski
a
Department of Chemistry and Technology of Polymers , Cracow
University of Technology , Krakow, Poland
b
Higher School of Dental Enerineering in Ustron , Ustroń, Poland
Published online: 28 May 2010.
To cite this article: E. Hebda , R. Orlicki & J. Pielichowski (2010) New Catalysts Based on
Poly(aspartic acid) for Oxidation Reaction, Molecular Crystals and Liquid Crystals, 522:1, 75/[375]-81/
[381], DOI: 10.1080/15421401003719746
To link to this article: http://dx.doi.org/10.1080/15421401003719746
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Mol. Cryst. Liq. Cryst., Vol. 522: pp. 75=[375]–81=[381], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003719746
New Catalysts Based on Poly(aspartic acid)
for Oxidation Reaction
1
E. HEBDA, R. ORLICKI, AND J. PIELICHOWSKI
2
1
1
Department of Chemistry and Technology of Polymers, Cracow
University of Technology, Krakow, Poland
2
Higher School of Dental Enerineering in Ustron, Ustro n´ , Poland
In the current work we describe synthesis of novel poly(D, L-aspartic acid) catalysts
for oxidation reactions. The catalysts were prepared by a three-step reaction pro-
cedure. First, poly(D, L-aspartic acid) (PAA) was synthesized by polycondensation
reaction D, L-aspartic acid with propylene carbonate under microwave irradiation.
In the second stage, poly(aspartic acid) sodium salt (PAA-Na) was obtained during
the reaction of hydrolysis of cyclic forms of PAA in water solutions of sodium. The last
stage of catalysts synthesis were reaction of poly(aspartic acid) sodium salt with cobalt
1
or copper acetate. FTIR, H NMR, SEM, SEM-EDS, and SIMS were employed for
characterization of the obtained catalysts were employed for characterization of the
obtained catalysts. Oxidation reactions of some compounds were carried out on synthe-
sized catalysts at atmospheric pressure in the presence of molecular oxygen. As the
main products epoxides and ketone were obtained with very high yield and selectivity.
Keywords Alkenes; oxidation reactions; poly(aspartic acid); polymer catalysts
Introduction
The aerobic epoxidation of alkenes with a transition metal catalyst 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 transition metal complex as catalyst, molecular oxygen as oxidant and an
aliphatic aldehyde as co-reactant. Transition metal complexes, such as Schiff’s base
complex, are also suitable for the aerobic epoxidation of alkenes with a co-reacing
aldehyde. For example, cobalt(II) Schiff’s bases give good results; however, these
catalysts are rather not selective for epoxidation.
It has been reported in 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 the preparation procedures. The polymer is very
stable (in the meaning of thermal properties) even in the oxidative atmosphere. As
an example, a polyaniline-supported cobalt(II) catalysts were prepared first by
Pielichowski and Iqbal and used for the aerobic epoxidation of alkenes.
Address correspondence to Jan Pielichowski, Department of Polymer Chemistry and
Technology, Cracow University of Technology, Warszawska 24, Krakow 31-155, Poland.
E-mail: pielich@usk.pk.edu.pl
75=[375]

7
6=[376]
E. Hebda et al.
For example, a polyaniline-supported catalyst has been used for epoxidation of
trans-stilbene with very high yield and selectivity [1–5].
In the last decade biodegradable polymers have deserved a special attention as
their decomposition products are non-toxic. To this group of polymers – due to
its advantages – belongs e.g., poly(aspartic acid) (here: PAA), which is a biologically
inert polyamino acid. Poly(aspartic acid) has been recently very intensely examined
over the possibility of the utilization of PAA and its derivatives in medicine and agri-
culture [6–8]. Because poly(D, L-aspartic acid) have functional groups to coordinate,
thus, in this paper, we report on novel poly(aspartic acid) supported cobalt(II) or
copper(II) catalysts, which were tested in the oxidation processes of hydrocarbons
– it is hoped that the catalysts chemically linked with polymeric support will be stable
and able to be separated easily after the reaction.
These catalysts, based on poly(aspartic acid), have been obtained and tested in
trans-stilbene, indene and 1-decene oxidation reactions.
Experimental
In the Department of Chemistry and Technology of Polymers, Cracow University of
Technology has used new, original method synthesis of poly(aspartic acid) under
microwave irradiation. The application of the microwave irradiation has permitted
shortening of polymerisation time to several minutes, increase in efficiency and elim-
ination of the catalyst – this fact is very important in synthesis compounds, which are
used for the preparation of catalysts.
The general method for poly(aspartic acid) sodium salt (PAA-Na) synthesis is
illustrated in Figure 1.
In our experiments the catalyst were prepared by a three-step method:
(
1) Polycondensation of aspartic acid
The synthesis of cyclic PAA was carried out in a microwave reactor ‘‘Milestone’’
of 1000 W capacity. Polycondensation of cyclic PAA was realized at the tem-
ꢀ
ꢀ
perature range from 176 C to 230 C, carbonate propylene was used as solution.
Water was removed from the system by an azeotropic distillation under normal
pressure. Cyclic PAA was precipitated with methanol, washed in water and
dried. The reaction efficiency was above 93%.
Figure 1. Synthesis of PAA sodium salt.

Catalysts for Oxidation Reaction
2) Hydrolysis of cyclic forms of PAA
77=[377]
(
PAA sodium salt has been obtained during the reaction of hydrolysis of cyclic
forms of PAA, water solutions of sodium. The process of hydrolysis was carried
out in the solution with pH ¼ 8.5 to 13.0 [4].
1
The obtained product was identified by FT-IR and H NMR:
FT-IR (KBr): characteristic absorption band of stretching vibrations of C=O
ꢁ
1
group is about 1610 and 1660 cm , strong absorption band of OH stretch-
ꢁ
1
ing vibrations is in the range of 3400–3500 cm
HNMR (D O): the methane proton is seen as two resonances at about 4.7 and
1
2
4
2
.5 ppm, the methylene protons are observed as three resonances at about
.8, 2.7 and 2.55 ppm.
(
3) Synthesis of complex of PAA sodium salt with cobalt(II) or copper(II) acetate
The complex of PAA sodium salt with cobalt(II) or copper(II) acetate was
obtained by mixing of 1.00 g of PAA-Na with the corresponding amount of
metal acetate.
In this way were obtained two catalysts:
–
–
PAA-Na þ cobalt acetate – PAA-Co
PAA-Na þ copper acetate – PAA-Cu
(4) The oxidation of hydrocarbons in the presence of the catalysts
In order to check activity of the prepared catalysts several reactions were carried
out. In a typical procedure catalyst (30 mg), acetonitrile (30 mL), and substrate
ꢀ
(
2 mmol) were placed in a reactor. The mixture was heated up to 60 C, stirred with
magnetic stirrer and bubbled with molecular oxygen. 2-methylpropanal was added
6 mmol) to the 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 hydro-
carbon 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.
Trans-stilbene, 1-decene and indene were subjected to the oxidation reactions.
Results and Discussion
The two catalysts – PAA-Co and PAA-Cu – were examined by SEM=EDXS. 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
to map the surface topography and compositional contrast based on the density
differences. The X-rays emitted when the electron beam struck 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. Prior to the analysis the catalyst samples
were coated with conductive carbon films to a thickness of 20–30 nm. The surface
images of the catalysts are shown in Figure 2. It can be seen that these heterogeneous
catalysts created agglomerates with the size of the order of 20–40 mm and smaller, of

7
8=[378]
E. Hebda et al.
Figure 2. SEM images of (a) PAA-Co, (b) PAA-Cu and metals distribution on PAA cobalt (a)
and copper (b) respectively.
about 5–10 mm. These agglomerates consist of many smaller spherical grains in case
of PAA-Co and flaky-lamellar ones for PAA-Cu.
The application of EDS spectroscopy coupled with SEM let us test the surface
distribution of chemical elements, for example metals, in the form of a surface
map – Figure 2. We observed that metal was spaced exactly on the catalyst surface.
We can suggest that a chemical reaction took place between the polymer and cobalt
or copper(II) salt, and only one compound was obtained.
The received catalysts were tested for the presence of metals using time-of-flight
secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS measurements were
performed using a PHI TRIFT 2100 time-of-flight secondary ion mass spectrometer
equipped with gallium liquid metal ion gun (LMIG). This instrument allows
spectroscopy, for the characterization of surface chemical composition. The system
uses a pulsed primary ion beam to desorbs and ionize species from a sample surface
[
9–13].
The results for PAA-Na and its cobalt and copper derivatives, assigned as
PAA-Co and PAA-Cu respectively, are reported here. Positive mass spectrum of
PAA-Na is dominated by sodium (Fig. 3), which reflects low ionization potential
of this metal under the measurement conditions. The secondary fragment ions,
despite their low intensities, support the structure of PAA-Na. The characteristic
fragments of PAA-Na ions present in positive and negative mass spectra are shown

Catalysts for Oxidation Reaction
79=[379]
Figure 3. Positive secondary ion mass spectrum (a) and negative secondary ion mass spectrum
b) of PAA-Na.
(
in Table 1. The lines are of low intensities and 93% of the total positive ion emission
originates from sodium.
The process of ion exchange resulted in the replacement of mobile sodium ions
with cobalt ion, which is shown in Figure 4. The spectra clearly indicate that most of
the sodium was substituted during the process. The spectrum for PAA-Cu is similar
for PAA-Co.
þ
Figure 4 show the presence of the [COCo] fragment, which suggests incorpor-
ation of cobalt into the precursor’s structure (not just a copper oxide).
The characteristic fragments of PAA-Co and PAA-Cu ions present in positive
mass spectra are shown in Table 2.
Table 1. Characteristic fragments of PAA-Na ions present in positive and negative
mass spectra
Characteristic Fragments of Ions
Positive
Negative
Formula
m=z, amu
Formula
m=z, amu
þ
þ
ꢁ
[
[
[
[
[
CH N]
2
CH N]
4
28.02
30.03
44.01
44.05
46.01
[CN]
[CNO]
26.00
42.00
45.00
55.03
58.01
71.01
72.03
ꢁ
þ
ꢁ
ꢁ
ꢁ
ꢁ
CONH2]
C H N]
[COOH]
[C H O]
þ
2
6
3
3
þ
CH O ]
2
[C H O ]
2 2 2
C H O ]
3 3 2
C H NO ]
2
2
[
[
ꢁ
2
2

8
0=[380]
E. Hebda et al.
Table 2. Characteristic fragments of ions present in positive mass spectra of
PAA-Co (cobalt modification) and PAA-Cu (copper modification)
PAA-Co
Formula
PAA-Cu
m=z, amu
Formula
m=z, amu
þ
þ
Co
58.93
74.93
75.94
76.95
86.92
87.93
88.94
Cu
[CuO]
62.93
78.92
79.93
80.94
90.92
91.93
92.94
þ
þ
[
[
[
[
[
[
CoO]
CoOH]
CoOH2]
COCo]
CHOCo]
þ
þ
þ
þ
[CuOH]
þ
[CuOH2]
þ
[COCu]
þ
þ
þ
[CHOCu]
þ
[CH OCu]
2
CH OCo]
2
þ
þ
The presence of [COCo] and [COCu] fragment ions provides evidence for the
chemical interaction between the catalyst precursor (PAA-Na) and both the cobalt
and copper cations. It means that these metals are not simply supported as their
oxides on PAA-Na but they are chemically incorporated into it.
Catalytic Oxidation
Using these catalysts the aerobic oxidations of different organic compounds – trans-
stilbene, 1-decene and indene – have been carried out. The results of catalytic
oxidation of organic compounds are collected in Table 3. Selectivity of the main
products was given and the reaction time was shown in brackets.
Trans-stilbene and 1-decene give epoxides, but indene gives ketone (1,3-dihydro-
2
H-inden-2-on) with very high selectivity. The oxidation reaction of 1-decene needs
longer time than the oxidation of trans-stilbene and indene. Under these conditions
-decene gives epoxide with very high selectivity. No changes in yield were
1
observed when the same reactions were carried out longer. The 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 [14].
Table 3. Results of the polymer – supported cobalt and copper catalysts oxidation
with molecular oxygen
Products
Selectivity [%] (time [h])
Catalysts
Epoxystilbene
Epoxydecene
Indene-2-one
PAA-Co
PAA-Cu
85 (0.5)
91 (0.5)
62 (30)
56 (32)
98 (1)
97 (1)

Catalysts for Oxidation Reaction
81=[381]
Figure 4. Positive secondary ion mass spectrum of PAA-Co.
Conclusion
In conclusion, we have described the synthesis of poly(aspartic acid) – 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 reactions occur in a relatively short time. Generally, poly(aspartic
acid) can be considered as a new group of macromolecular oxidation catalysts, with
advantageous properties.
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