S. Hasannia, B. Yadollahi / Polyhedron 99 (2015) 260–265
261
ion exchange pathway are reported. These compounds were pre-
of nitrate salt of appropriate transition metal in 200 mL of water
and adjusting the pH to 4.8 using 1 M solution of HNO
Zn–Al LDH suspension was added slowly to above synthesized
POM solution along with fixing the pH at 4.8 using HNO . The
resulting mixture was then kept at 60 °C for 4 h under nitrogen
atmosphere to complete the anion exchange reaction. The final
product was obtained after being washed two times by deionized
water and drying at 80 °C for 12 h.
nÀ
pared from sodium salts of Keggin type POMs, [PW11MO39
]
3
.
(
M = Cr, Mn, Fe, Co, Ni, Cu, Zn), and previously prepared Zn–Al
LDH. The resulting LDH-PWM composites were assessed as cata-
lyst in the green oxidation of alcohols with H as oxidant
Scheme 1).
3
2
O
2
(
2
. Experimental
.1. Materials and methods
Zn–Al LDH and transition metal substituted Keggin type POMs
2.3. Typical procedure for catalytic oxidation of alcohols in the
2
presence of LDH-PWFe
In a typical experiment 0.05 g of LDH-PWFe, which contains
about 0.01 mmol of PWFe, was dispersed in an aqueous solution
of benzyl alcohol (1 mmol) and 30% H O (5 mmol) in 3 mL of
2 2
water. The mixture was stirred for 2.5 h at reflux and progress
of the reaction was monitored by GC. At the end of oxidation
reaction ethyl acetate (2 Â 3 mL) was added and the organic
phase was extracted. Pure products were obtained by flash chro-
matography over a short column of silica gel (eluent: n-hexane/
ethyl acetate).
were synthesized according to literature [9,34]. The synthesis of
Zn–Al LDH pillared transition metal substituted Keggin type POMs
was performed by ion exchange pathway. Characterization of the
synthesized compounds was performed by different techniques
2
such as FTIR, N adsorption–desorption, X-ray fluorescence, ther-
mogravimetric analysis (TG–DTG), X-ray diffraction (XRD) and ele-
mental analysis. FTIR spectra were obtained on a JASCO 6300 FTIR
spectrometer. XRF was performed using Bruker S4 X-ray fluores-
cence analyzer. The TG–DTG analysis of LDH-PWM was evaluated
using a Perkin Elmer STA 6000 TG/DTGA analyzer from 25 °C to
8
00 °C with temperature rate 10 °C/min in air. XRD patterns were
recorded using Bruker D8 advance powder diffraction system (Co
, 40 kV, 40 mA). The surface area and pore size of the synthe-
sized catalyst were calculated using BET and BJH methods at
7 K on Belsorp-max system. The oxidation products were ana-
3
. Results and discussion
K
a
3.1. Synthesis and characterization of LDH-PWM
7
LDH-PWM composites were prepared through a synthetic route
lyzed by gas chromatography (GC) experiments on a Shimadzu
GC-16A instrument with a flame-ionization detector using silicon
DC-200 column.
which is shown in Scheme 2. In this method, pH controlling is
essential in the period of anion exchange pathway. This is because
POMs and LDH are shown some sensitivity to pH tolerance.
The FTIR spectra of Zn–Al LDH and LDH-PWFe (Fig. 1) have been
shown that LDH-PWFe composite exhibits the characteristic peaks
2
2
.2. Preparation of LDH-PWM composite
À1
of Keggin type structure at 802, 889, 964, and 1056 cm , which
are related to the WAO AW, WAO AW, W@O and PAO vibrations
c b
.2.1. Preparation of Zn–Al LDH
Zn–Al LDH was synthesized by a coprecipitation method [34].
(Fig. 1a). In comparison with pure Fe substituted POM [8], these
vibration bands revealed that the structure of Fe substituted Keg-
gin type POM is remained intact after ion exchange between LDH
layers. On the other hand, the characteristic peaks of LDH after pil-
For this purpose 30 mmol of Zn(NO
3
)
2
Á6H
2
O and 10 mmol of Al
(
NO O were dissolved in 50 mL deionized water (solution
3
3
) Á9H
2
I). In the following, a solution of NaOH (1 M) was added dropwise
to the vigorously stirred solution I under nitrogen atmosphere.
Addition of NaOH solution continued until the pH was fixed at 7.
The resulting suspension was kept at 80 °C for 12 h under vigor-
ously stirring. One portion of the resulting LDH precursor was
dried overnight at 110 °C followed by centrifuge and washing with
deionized water and used for characterization.
À1
laring is appeared at 611, 1363, 1626, and 3438 cm . A vibration
À1
À
band at 1363 cm , which is attributed to the NO
3
anion, was
shown that the anion exchange reaction is not completed and
some of the nitrate anions remained intact.
The thermal behavior of LDH-PWFe composite was tested
using TGA (Fig. 2). As it could be seen in Fig. 2, the TG–DTG
curves indicated two-step weight loss. The first weight loss,
between 50 and 150 °C, is related to the removal of adsorbed
water molecules and the second is due to the removal of inner
structural waters from compound. At high temperatures, between
2
.2.2. Preparation of LDH-PWM composites
Transition metal substituted Keggin type POMs were synthe-
sized according to previously reported procedure [9]. A solution
of transition metal substituted POM was prepared by mixing
3
50 and 800 °C, a little weight loss could be seen. These observa-
tions indicated the stability of this LDH-PWFe composite at high
temperatures.
9
.1 mmol of Na
2
HPO
4
, 100 mmol of Na
2
4
WO Á2H
2
O and 12 mmol
The elemental analysis results have been shown the molar
ratios for P:Fe:W is about 1:1:11, which is matched with Keggin
structure of Fe substituted POM. The Zn:Al molar ratio was also
about 2:1, that suggests [Zn
LDH host. Based on the calculated formula from elemental analysis
data, for the synthesized LDH-PWFe compound, [Zn Al(OH)
NO O, about 40% of nitrate anions have
2
6
Al(OH) ]NO
3 2
ÁmH O formula for the
2
6
]
(
3
)
0.6(PFeW11
O
39
)
0.1ÁmH
2
been exchanged by POM anions. According to the results from FTIR
and elemental analysis, it could be concluded that the structure of
POM is preserved after ion exchange process between LDH layers.
The pillared structure of LDH-PWFe was also confirmed by XRD
patterns (Fig. S1). This amorphous pillared structure exhibits the
characteristic peaks of a Keggin type POM at 2h = 9°, 10–11° and
19–22°. The characteristic peaks of Zn–Al LDH, which are appeared
2 2
Scheme 1. Catalytic oxidation of alcohols with H O in the presence of LDH-PWM
catalysts.