Preparation, characterization, and catalytic testing of different Me–chitosan complexes for triglycerides transesterification

Article abstract of DOI:10.1007/s11164-015-2330-9

A number of homogeneous and heterogenized chitosan complexes with different metals (Zn, Co, Ca, Ni, Sn, Pb) were synthesized and tested as catalysts in the reaction of transesterification of tributyrin simulating the process of biodiesel production from renewable natural feedstock (triglycerides, vegetable oils). It is found that chitosan complexes are effective catalysts for transformation of tributyrin into the corresponding esters. The comparative analysis of Me–chitosan (homogeneous), Me–chitosan (heterogeneous) and Me–chitosan/SiO₂ catalysts testify that the efficiency of three-component egg-shell systems is close to that of homogeneous catalysts.

Full text of DOI:10.1007/s11164-015-2330-9

Res Chem Intermed
DOI 10.1007/s11164-015-2330-9
Preparation, characterization, and catalytic testing
of different Me–chitosan complexes for triglycerides
transesterification
E. D. Finashina1 N. V. Kramareva1 A. V. Kucherov1
•
•
•
O. P. Tkachenko1 L. M. Kustov1
•
Received: 10 July 2015 / Accepted: 20 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A number of homogeneous and heterogenized chitosan complexes with
different metals (Zn, Co, Ca, Ni, Sn, Pb) were synthesized and tested as catalysts in
the reaction of transesterification of tributyrin simulating the process of biodiesel
production from renewable natural feedstock (triglycerides, vegetable oils). It is
found that chitosan complexes are effective catalysts for transformation of tributyrin
into the corresponding esters. The comparative analysis of Me–chitosan (homoge-
neous), Me–chitosan (heterogeneous) and Me–chitosan/SiO2 catalysts testify that
the efficiency of three-component egg-shell systems is close to that of homogeneous
catalysts.
Keywords Chitosan Á Transesterification Á Triglyceride
Introduction
Production of esters of long-chain acids from triglycerides by transesterification
became a mature technology. Such esters are well known as biodiesel fuel [1, 2].
Usually methanol is used for transesterification under quite mild conditions
(50–80 °C) using alkali (including NaOH or ROH) [3–6] or acid catalysts (H2SO4,
HC1, H3PO4) [7, 8].The major part of biodiesel produced world wide is based on the
use of homogeneous catalysts for transesterification [9].
However, the severe requirements to esters used in biodiesel (neutral pH, low
content of alkaline metals, surfactants and other undesirable side products) stimulate
&
E. D. Finashina
finesta@mail.ru
1
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47,
Moscow, Russia
123

E. D. Finashina et al.
the development of heterogeneous catalysts for the transesterification of
vegetable oil.
Since the mechanisms of transesterification on acid and basic centers are
different, the acid and basic heterogeneous catalysts are typically considered
separately. A number of basic catalysts are described in the literature, some of them
demonstrating a high activity in transesterification under mild conditions. Diverse
oxides like hydrotalcites [10], MgO, MgO/Al2O3 [11], CaO [12, 13], catalysts based
on zinc oxide, mixed oxides M–Al–O (M = Sr, Ba), M–Mg–O (M = Y, La) and
M–La–O (M = Sr, Ba) [14] are known in the art.
The mechanism of transesterification on Broensted or Lewis acid sites includes
the stage of formation of alcoholate species. The acid-type catalysts are widely used
in biodiesel production and include basically homogeneous systems, such as
Broensted acids (H2SO4, p-toluenesulfonic acid) [8] and Lewis acids (metal
acetates, other metal complexes) [15, 16]. In studying transesterification of palm
and coconut oil, it was shown that the solid oxide acid catalysts can be arranged in
the following order with respect to their activity: sulfated ZrO2 [ sulfated
SnO2 [ ZnO [ ZrO2 [17].
The strength of acid sites should be optimal for the process to occur, because very
strong Lewis acid sites do not demonstrate activity in transesterification as a result
of very strong adsorption of products [18–20].
One of the most important problems is the development of a heterogeneous
catalyst demonstrating a reasonably high activity under milder conditions compared
to the conventional conditions used for heterogeneous catalysts (220–240 °C,
4.0–6.0 MPa), with the stability in aqueous solutions being another important
factor.
Recently the interest is focused on the use of polymer matrices for immobiliza-
tion of the active component (metal ions) for the transesterification reaction.
It was also reported that metal compounds of the type Me(3-hydroxy-2-methyl-4-
pyrone)2(H2O)2, where M = Sn, Zn, Pb and Hg, are active for soybean oil
methanolysis [21, 22]. The activity diminishes in the order Sn2? ꢀ Zn2?
[ Pb2? & Hg2?. It is worth to note that the complexes of Sn and Zn showed
significant activities in this reaction, with the yield of the product approaching 90 %
in 3 h at a molar ratio 400:100:1 (methanol:oil:catalyst). Thus, the complexes of this
type, especially immobilized in a polymer matrix may show some promising results
in the transesterification reaction.
Now, organic polymers including natural abundant and ‘‘green’’ materials (such
as cellulose, chitin, and especially chitosan) are widely employed as carriers for
immobilizing metal complexes that serve as catalysts in diverse processes. Chitosan
(poly(1-4)-N-acetyl-b-D-glucosamine) is characterized by a high nitrogen content,
which explains, in turn, its ability to concentrate metal ions and even neutral atoms
of different metals via different mechanisms such as ion exchange or chelate
formation, depending on the metal nature and pH of the solution.
Metal–chitosan complexes are now widely used as catalysts in diverse reactions
[23]: oxidation (Cu, Fe, Co) [24–27], hydrogenation (Pd, Pt, Ni, Cr, Zn) [28], allylic
substitution [Trost–Tsuji reaction, Pd(II), Pd(0)], Suzuki and Heck reactions (Pd),
polymerization (Cu, Co, Y, La, Pr, Nd, Er), olefins cyclopropanation (Cu),
123

Preparation, characterization, and catalytic testing of…
carbonylation (Pd/Ni, Rh), asymmetric dihydroxylation of olefins (Os, Co),
dechlorination (Pd/Fe) [29], alkynes hydroamination [30].
Thus, the goals of this study were to design heterogenized chitosan complexes
with a representative set of non-noble metals, including basic (Ca) and acidic (Zn,
Co, Ni, Sn, Pb) transesterification catalysts and to characterize their structure and
catalytic performance.
Experimental
Materials
As-received chitosan powder (made from crab shells, molecular weight
100,000–150,000) was used without further purification. The deacetylation degree
of amino groups was 70 %, the moisture content in the chitosan powder was 3 wt%.
NiCl2, Pb(OAc)2, CaCl2, ZnCl2, SnCl2, CoCl2 were purchased from Fluka AG
(Switzerland) as an analytical grade reagent, HCl and NaOH were also of the
analytical grade purity. Glyceryl tributirate (tributyrine), 98 %, Aldrich product,
was used without purification.
Complex preparation
Chitosan was modified with a number of metals [Ni(II), Pb(II), Ca(II), Zn(II),
Sn(II), Co(II)] by different methods, described early in our previous works [6–9],
such as adsorption of metal ions from water solutions on the surface of chitosan or
coprecipitation of a soluble form of chitosan with divalent metal precursors. The
metal content varied from 5 to 9 wt%.
Homogeneous metal–chitosan complex (starting material for the heterogenized
complexes)
A calculated amount of metal chloride (or acetate in the case of Pb) was added to a
1.5 wt% solution of chitosan in 0.1 M HCl at room temperature, and the mixture
was stirred until the formation of a transparent clear solution.
Coprecipitation method
A transparent clear solution of the homogeneous metal–chitosan complex was
prepared as described above and added dropwise to a glass beaker with a 0.5 M
NaOH solution, which led to immediate coagulation of the drops into spherical
globules. The spherical particles formed (diameter, 2–3 mm) were filtered off after
10 min and repeatedly washed with distilled water until neutral pH. After this
procedure, the globules of chitosan containing immobilized Lewis acid were filtered
off and air-dried at room temperature for 48 h.
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E. D. Finashina et al.
Impregnation (adsorption) method
A 1.5 wt% chitosan solution was added dropwise to a glass beaker with a 0.5 NaOH
solution, with resulting immediate formation of spherical globules of pure chitosan.
The particles were filtered off after 10 min, repeatedly washed with distilled water
until neutral pH, filtered again and air-dried at room temperature for 48 h. A
weighed amount of a metal chloride (or acetate in the case of Pb) was dissolved in
20 ml of distilled water and stirred with 1.0 g of dried chitosan particles for 1–3 h.
Then the particles were removed by filtration, washed with distilled water and air-
dried for 24 h.
Immobilization of the metal–chitosan complex on the surface of porous silica gel
One gram of amorphous SiO2 (aerosil; fraction 0.25–1 mm; S(BET) = 330 m2/g;
water adsorption capacity 1.2 ml/g) was impregnated with 1.2 ml of a solution of
the homogeneous metal–chitosan complex prepared as mentioned above. Then
complex-loaded silica was placed in a 0.5 M solution of NaOH for 15 min, filtered
off and repeatedly washed by distilled water until neutral pH. The particles obtained
were dried at room temperature in air for 24 h and then in a vacuum for 10 h.
Characterization
IR spectroscopy
Transmission FTIR spectra were recorded at 20 °C using a Nicolet Protege 460
spectrometer in the range of 4000–400 cm-1 at a resolution of 8 cm-1 and also with
a Matteson Galaxy Series FTIR 5000 spectrometer in the range of 4000–600 cm-1
at a resolution of 4 cm-1. For the study of the homogeneous palladium–chitosan
complex, some droplets of the solution were deposited on quartz plates and air-dried
for 48 h. Dry globules of heterogeneous chitosan samples (adsorption and
coprecipitation complexes) were ground in a mortar, the fine powder was mixed
with KBr, pressed into a thin pellet, and placed in the sample holder.
X-ray absorption spectroscopy
XAS measurements were carried out at HASYLAB (DESY in Hamburg, Germany)
on the beamline X1 (Pb L3-edge, 13035 eV) using a double-crystal Si(311)
monochromator, which was detuned to 50 % of the maximum intensity to exclude
higher harmonics in the X-ray beam. Ni K edge X-ray absorption spectra at 8333 eV
were measured at HASYLAB on the beamline E4 using a Si(111) double crystal
monochromator, which was detuned to 30 % of the maximum intensity to exclude
higher harmonics. Samples were pressed into wafers (with polyethylene as a binding
agent) in a standard 13-mm die, using the optimum weight to maximize the signal-
to-noise ratio. The spectra were recorded in the transmission mode at a low
temperature (T = 80 K) in order to decrease the Debye–Waller factors. All spectra
were measured simultaneously with the reference spectrum of Pb or Ni foil placed
123

Preparation, characterization, and catalytic testing of…
between the 2nd and 3nd ionisation chambers for energy calibration. The spectra of
Pb or Ni foils, PbO and Pb(OAc)2, NiO, Ni(NO3)2, Ni2(OH)2CO3 and NiCl2 were
used as references and were collected under the same conditions.
Analysis of the EXAFS spectra was performed with the software VIPER for
Windows [31, 32]. In the spectra of the absorption coefficient l, a Victorian
polynomial was fitted to the pre-edge region for background subtraction. A smooth
atomic background l0 was evaluated using a smoothing cubic spline. The Fourier
analysis of the k2-weighted experimental function v = (l - l0)/l0 was performed
with a Kaiser window. The required scattering amplitudes and phase shifts were
calculated by the ab initio FEFF8.10 code for PbO and NiO. The fitting was done in
the k- and r-spaces. The shell radius r, coordination number N, Debye–Waller factor
r2 and adjustable ‘‘muffin-tin zero’’ DE were determined as fitting parameters. The
errors of the fitting parameters were found by decomposition of the statistical v2
function near its minimum, taking into account maximal pair correlations.
SEM
Micrographs of sample surfaces were taken by scanning electron microscopy (JSM-
5300LV JEOL). Before measurement samples surface was decorated by fine Au
layer using JFC-1100E set-up.
Catalytic experiments
Catalytic experiments on tributyrine transesterification were performed in a batch
reactor. Ttributyrine—1,3-di(butanoyloxy)propan-2-yl butanoate—is a triglyceride
naturally present in butter. It is an ester composed of butyric acid and glycerol. The
reaction temperature was t = 65 °C, the reaction time, 3–6 h, catalyst/tribu-
tyrine = 1/20, a threefold excess of CH3OH, the stirring rate—750 rpm. The
reaction was carried out without any solvents. Ester formation was controlled by the
GC method (capillary column PONA Dimethylpolysiloxane, Length: 50 m, Inner
Diameter: 0.20 mm, Film Thickness: 0.50 mm).
Results and discussion
Catalytic tests
Tributyrine was chosen as a model substrate to estimate the catalytic activity of
metal–chitosan complexes in the transesterification reaction (Fig. 1).
Homogeneous complexes of chitosan with different metals demonstrated the
activity as indicated in Table 1 and Fig. 2. The most active were the Pb- and Ni-
containing samples.
When we switched from homogeneous to heterogenized systems, we found that
their activity as catalysts of the tributyrine transesterification reaction depended on
the method of preparation. The Me–chitosan complexes prepared by adsorption
were virtually inactive in the reaction. When these complexes were tested, the
123

E. D. Finashina et al.
O
O
O
t, cat
3
O
O
O
+ 3MeOH
O
+
O
CH CH2
CH2
OH
OH
OH
Fig. 1 Transesterification of tributyrin. Reaction conditions: t = 65 °C, catalyst/substrate = 1/20, a
threefold excess of CH3OH, the reaction time—3 h
Table 1 Catalytic performance
of homogeneous complexes
Me–chitosan
s (h)
Ester concentration (g/ml)
Pb
Ni
Sn
Zn
Ca
Co
1.00
2.00
3.00
0.34
0.50
0.53
0.07
0.40
0.48
0.12
0.17
0.23
0.12
0.17
0.30
0.04
0.08
0.21
0.05
0.06
0.09
Conversion [%]
80
60
40
20
0
Co
Zn
Sn
Ni
Pb
Ca
Fig. 2 Conversion of tributyrin on homogeneous metal(II)–chitosan complexes
reaction solutions were colored, which is an evidence for the leaching of the metal
ions.
Catalytic testing of heterogenized Me–chitosan complexes prepared by the
coprecipitation method revealed a good performance of all the catalysts for the
reaction of transesterification. The Pb- and Ni–chitosan complexes demonstrated the
best catalytic activity, Sn and Ca-complexes exhibited the activity comparable with
that of the leading metals (Pb, Ni). The tributyrin conversion reached 65 % for the
Pb-containing catalysts and 50 % for the Ni-sample, with the selectivity of 98 %.
Consequently, the catalytic activity order found for the homogeneous chitosan
complexes is reproduced for the heterogenized catalysts obtained by the coprecip-
itation method.
As we found earlier, metal–chitosan complexes immobilized on the surface of
macroporous silica to produce the egg-shell catalysts possess a more developed
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Preparation, characterization, and catalytic testing of…
0.6
0.5
0.4
0.3
0.2
0.1
0
Pb-chitosan
coprecipitation
complex
Pb-
chitosan/SiO2
Pb-chitosan
homogeneous
complex
0
1
2
3
4
Time (h)
Fig. 3 Activity of Pb-containing catalysts
surface area (*300 m2/g) and a higher mechanical stability as compared with bulky
heterogenized chitosan complexes [25, 26].
The efficiency of the heterogenized egg-shell Me–chitosan/SiO2 catalysts was
compared with the activity of heterogenized Me–chitosan complexes and homo-
geneous samples. It was shown that the activity of the egg-shell systems is close to
the activity of the homogeneous catalysts.
The results obtained for the most active system, Pb–chitosan, are displayed in
Fig. 3.
Catalyst stability was confirmed by SEM and XAS methods. SEM data testify
that the polymer complex film covering the surface of Pb-containing catalyst is not
changing during the catalytic operations (Table 1). All the catalysts are stable during
four cycles of operations. SEM images and XAS results for catalysts after catalysis
are given after the 4-th cycle of operations (Fig. 4).
IR spectroscopy experiments as well provide information about the polymer
complex structure being saved during the catalysis (Fig. 5).
The IR spectra of all samples studied are characterized by the absorption bands of
chitosan and its derivatives (Fig. 5, lower curve): (1) a broad unstructured
absorption between 3700 and 3100 cm-1 related to the superposition of OH and NH
group stretching vibrations; (2) narrow bands of CH group stretching vibrations at
2800–3000 cm-1, assigned to CH2 (2860 and 2930 cm-1) and CH (*2900 cm-1
)
fragments; and (3) bands of bending NH group vibrations in the 1660–1530 cm-1
range. Additionally, the IR spectra of Pb–chitosan complexes as prepared and after
catalysis exhibit an intense set of bands in the low-frequency region
(800–400 cm-1). Presumably, these bands are ascribed to stretching vibrations of
Pb–N, Pb–O bonds.
This set of bands is observed as a broad feature peaking at 657 cm-1
.
Furthermore, there is also a change in relative intensities in the region of bending
NH group vibrations (1500–1680 cm-1). The spectra of Pb-complexes before and
after catalysis are similar.
Figure 6 shows Pb L3 XANES spectra of the samples with the reference
compounds: Pb foil, PbO and Pb(OAc)2. The spectra of Pb samples are quite
different from those of Pb foil and PbO and resemble the spectrum of Pb(OAc)2.
123

E. D. Finashina et al.
(A)
10µm
1mm
(B)
1mm
20µm
(C)
1mm
20µm
Fig. 4 SEM images of catalyst/carrier particles and catalyst/carrier surface. a SiO2 (SBET = 330 m2/g);
b egg-shell system Pb–chitosan/SiO2 before catalysis (SBET = 275 m2/g); c egg-shell system Pb–
chitosan/SiO2 after catalysis (SBET = 270 m2/g)
The Fourier transformations of EXAFS oscillations of Pb samples and references
are presented in Fig. 7. In the spectrum of Pb foil, two main peaks correspond to the
˚
first coordination shell containing 12 Pb atoms at 3.500 A real distances from the
central Pb atom. In the spectrum of PbO, the first peak corresponds to the first
˚
coordination shell containing 4 O atoms at 2.308 A real distances from the central
Pb atom.
The FT EXAFS spectra of Pb–chitosan samples are different from those of three
˚
references. Two peaks between 1.5 and 2.5 A uncorrected distances are developed
in these spectra.
123

Preparation, characterization, and catalytic testing of…
Pb-chitosan
complex after
catalysis
Pb-chitosan
complex before
catalysis
Chitosan
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumbers (cm-1)
Fig. 5 IR-spectra of initial chitosan and chitosan modified with lead (coprecipitation complex) before
and after catalysis
1.8
5
4
3
1.6
Pb L3 edge
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
1
13000
13025
13050
13075
13100
Photon energy (eV)
Fig. 6 Pb L3 XANES spectra of Pb–chitozan samples and references: 1 Pb foil, 2 PbO, 3 Pb(OAc)2, 4
Pb–chitosan before catalysis, 5 Pb–chitosan after catalysis
The Pb L3 EXAFS spectra of the samples may be readily fitted in both r- and
k-spaces with a double-shell model—first: an oxygen shell around the central
absorbing lead atom and second: an oxygen and/or nitrogen shell around the central
absorbing lead atom. Because the atoms (O and N) are similar in the atomic weight
and atomic size, the X-ray scattering amplitudes and phases for these atoms are
similar, so, they are practically indistinguishable as scattering atoms in EXAFS
spectroscopy. The results of the model fits of the EXAFS spectra are given in
Table 2.
123

E. D. Finashina et al.
0.5
0.4
0.3
0.2
0.1
0.0
5
4
3
1
2
0
2
4
6
8
Uncorrected distance (Å)
Fig. 7 Fourier transforms of Pb L3 EXAFS spectra of Pb–chitozan samples and references: 1 Pb foil, 2
PbO, 3 Pb(OAc)2, 4 Pb–chitosan before catalysis, 5 Pb–chitosan after catalysis
Table 2 EXAFS results for Pb–
chitosan samples
˚
r (A)
Sample
Path
Coordination number
Before catalysis Pb–O
Pb–O/N 3.116 0.006 2.2 0.1
Pb–O 2.267 0.012 0.7 0.1
Pb–O/N 3.116 0.012 1.9 0.2
2.267 0.009 0.6 0.1
After catalysis
XAS (XANES and EXAFS) spectra show that Pb in the samples under study
exists as Pb2? and nearest-neighbors are oxygen and nitrogen atoms. Both the
electronic and coordination states of lead species anchored in the chitosan matrix
did not change during the catalytic reaction.
Because the Pb-polymer complex could not be considered as a good candidate
for ‘‘green catalysis’’, we have investigated the Ni-polymer complex as well.
The Ni K-edge XANES spectra of the catalysts and the reference samples are
depicted in Fig. 8. The positions of the Ni K-edge energy in the spectra of samples
are different from that of Ni-foil and close to those of NiO, Ni(NO3)2, Ni2(OH)2CO3
and NiCl2Á6 H2O, thus indicating that the Ni species in Ni–chitosan samples
prepared by different methods are Ni2? ions. The intensity of the white line in the
spectra of the Ni–chitosan samples are the same for both samples, and after catalysis
it does not change. Noteworthy that the intensity of the white line in the spectra of
catalysts is different from those observed for references.
The Fourier transformation of EXAFS oscillations of the Ni–chitosan samples
and Ni references are presented in Fig. 9. In the spectrum of NiO, the first two peaks
correspond to the first and second coordination shells containing 6 O atoms at
˚
˚
2.084 A and 12 Ni atoms at 2.947 A real distances from the central Ni atom. In the
spectrum of Ni(NO3)2 a single peak observed corresponds to the first coordination
123

Preparation, characterization, and catalytic testing of…
4
9
8
Ni K-edge
7
6
5
3
4
2
1
0
3
2
1
8320
8340
8360
8380
8400
Photon energy (eV)
Fig. 8 Normalized XANES spectra of Ni–chitosan catalysts and reference compounds: 1 Ni foil, 2 NiO,
3 Ni(NO3)2, 4 Ni2(OH)2CO3, 5 NiCl2, 6 Ni–chitosan, co-precipitated, before catalysis, 7 Ni–chitosan, co-
precipitated, after catalysis, 8 Ni–chitosan, impregnated, before catalysis, 9 Ni–chitosan, impregnated,
after catalysis
12
9
8
7
10
8
6
5
6
4
3
2
4
1
2
0
0
2
4
6
8
Uncorrected distance (Å)
Fig. 9 Fourier transforms of Ni K-edge EXAFS spectra of Ni–chitosan catalysts and references: 1 Ni
foil, 2 NiO, 3 Ni(NO3)2, 4 Ni2(OH)2CO3, 5 NiCl2, 6 Ni–chitosan, co-precipitated, before catalysis, 7 Ni–
chitosan, co-precipitated, after catalysis, 8 Ni–chitosan, impregnated, before catalysis, 9 Ni–chitosan,
impregnated, after catalysis
˚
shell containing 6 O atoms at 2.419 A. In the spectrum of NiCl2Á6 H2O, one can
observe one peak corresponding to the first and second coordination shells
˚
containing 2 O atoms at 1.783 A and 2 O atoms at 2.273 A.
˚
The FT EXAFS spectra of Ni–chitosan samples are different from those of NiO.
˚
Two peaks near 1.5 and 2.5 A uncorrected distances are developed in the spectra of
the Ni–chitosan samples prepared by the co-precipitated method. The FT EXAFS
123

E. D. Finashina et al.
Table 3 EXAFS data for 5 % Ni/chitosan catalysts and references
2
r2 (10-3 A ) DE (eV)
˚
Atomic pair r (A)
˚
Sample
Treatment
CN
Co-precipitated Before catalysis Ni–Cl
Ni–O
2.170 0.009
2.057 0.006
2.170 0.009
2.059 0.006
2.087 0.005
2.089 0.005
5.6 0.5 12
6.2 0.3
6.0 0.5 13
6.4 0.3
1
-3
14
-3
15
16
16
1
1
1
1
1
1
9
1
After catalysis
Ni–Cl
Ni–O
1
9
1
Impregnated
Before catalysis Ni–O
6.7 0.3 12
6.6 0.2 12
1
After catalysis
Ni–O
Ni–Ni
Ni–O
Ni–Ni
Ni–O
Ni–O
Ni–Ni
Ni–Cl
1
Ni-foil
NiO
2.485 0.001 11.6 0.2
2.092 0.008 5.9 0.4 11
2.944 0.003 11.9 0.4
4
0.1
2
10 0.2
0.8
3
7
6
8
9
0.3
0.5
0.7
0.5
1
0.1 0.4
-1 0.4
0.1 0.4
0.4 0.5
Ni(NO3)2
2.043 0.003
2.046 0.004
3.112 0.004
2.251 0.006
4.9 0.2
4.9 0.2
6.1 0.3
Ni2(OH)2CO3
NiCl2
6.0 0.3 16
2
1
spectra of the Ni–chitosan samples prepared by impregnation are different. A single
˚
peak near 1.5 A uncorrected distance is observed.
Table 3 presents apparent coordination number (CN) and distance obtained by
fitting coordination shells with the amplitude and phase functions extracted from the
data for NiO and NiCl2.
The fitting revealed that the Ni–O coordination number is around 6 for the
samples prepared by the co-precipitation method and about 7 for the samples
˚
prepared by impregnation. The Ni–O real distance is about 2.06 A in the co-
˚
precipitated samples and about 2.09 A in the impregnated samples The first
coordination shell of Ni in the samples prepared by the co-precipitation method
˚
contains about 6 Cl atoms at the 2.17 A real distance. No changes were found
between the Ni–chitosan samples before and after catalysis.
Earlier we have found for the Pd–chitosan complexes that the method of
preparation of the complex (co-precipitation or adsorption) determines the way of
the metal coordination [24]. The monodentate coordination predominates in the
complexes prepared by adsorption, whereas chelated coordination complexes are
formed in the case of the complexes prepared by co-precipitation. The data obtained
by the EXAFS method for the Ni–chitosan complexes prepared by different
methods are consistent with the results found for Pd–chitosan complexes.
Conclusions
The proposed chitosan-supported metal complexes with different metals (Pb, Sn, Ni,
Ca, Zn, and Co) belong to the hybrid nanomaterials [33], especially the
heterogenized systems on an oxide support. They were synthesized and character-
ized by IR and XAS methods. All the complexes catalyze the transesterification of
triglycerides. It was found that the most active catalysts are Pb-, and Ni-containing
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Preparation, characterization, and catalytic testing of…
polymer complexes. With the example of Pb-containing complexes, it was shown
that the efficiency of egg-shell systems Me–chitosan/SiO2 is close to that of
homogeneous Me–chitosan catalysts.
Acknowledgments Authors thank HASYLAB (DESY, Germany) for X-ray beam time (Projects
I-20060226 EC and I-20060224 EC) and the Russian Science Foundation (Grant No. 14-50-00126).
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