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K.M. Parida et al. / Journal of Catalysis 276 (2010) 161–169
complexes or indirectly by forming metal complexes between the
host layers following intercalation of the ligands [24].
In the present work, we describe a new protocol for immobili-
zation of Ti(IV)-complex in the interlayer of LDH by replacing the
interlayer gallery anions in an ion-exchange method. The heteroge-
neous Ti-complex so formed is found to be an excellent catalyst for
epoxidation of cyclohexene in a solvent-free condition using H2O2
as oxidant.
dried LDH and stirred for 1 h. To an ethanolic suspension of
LDH, 0.5 mmol (0.274 g) of metal complex was transferred and re-
fluxed at 60 °C for 24 h with constant stirring. The final product
was isolated by filtration, washed with ethanol and kept
overnight in vacuum at 70 °C. The schematic representation of
the total synthesis pathway is depicted in Scheme 1. Anal.
Found/calcd(%). For [Zn2Al(OH)6NOꢁ](Ti-complex)0.33 0.63H2O(Zn,
3
Al–LDH/Ti-complex): Zn, 24.76/25.15; Al, 4.91/5.19; C, 16.48/
16.75; N, 6.92/7.10; H, 2.08/2.28. IR (KBr) 3450, 3300, 1725,
1640, 1384, 675, 445 cmꢁ1
.
2. Experimental
Elemental analysis of LDH/Ti(IV)-complex gave 9.2 wt.% Ti.
Hence, the uptake of metal complex is in the same proportion as
the amount of Ti present. The ratio of Ti/Zn + Al in the final product
is found to be 0.3.
2.1. Materials
3-Amino-2-pyrazine carboxylic acid, 2-pyridine carboxalde-
hyde, titanium tetra-isopropoxide (Aldrich), Zn(NO3)2ꢀ6H2O and
Al(NO3)3ꢀ9H2O (CDS) were used without further purification. Etha-
nol was dried using molecular sieve 5A prior to its use in the
reaction.
2.3. Physico-chemical characterization of the catalyst
Powder XRD measurements were performed on a Rigaku D/
MAX2500 diffractometer, using Cu K
a radiation at 40 kV, 30 mA,
a scanning rate of 5 °/min and a 2h angle ranging from 3° to 80°.
The FT-IR spectra of the samples were recorded using a Varian
800-FT-IR in KBr matrix in the range of 4000–400 cmꢁ1. The co-
ordination environments of the samples were examined by diffuse
reflectance UV–Vis spectroscopy. The spectra were recorded in a
Varian-100 spectrophotometer in the wavelength range of 200–
800 nm in BaSO4 phase. Surface area was determined by N2
adsorption–desorption at liquid nitrogen temperature (77 K) using
ASAP 2020 (Micromeritics). Prior to adsorption–desorption mea-
surement, the samples were equilibrated by degassing at 120 °C
for 6 h. SEM images were obtained using HITACHI 3400N micro-
scope. Thermogravimetric/differential thermal analysis (TG/DTA)
was performed under air with a Shimadzu TGA-50 system at a
heating rate of 5 °C minꢁ1. The chemical composition of the prod-
ucts was confirmed quantitatively and qualitatively by energy-
dispersive X-ray (EDX) using a HITACHI 3400N microscope. The
Ti loading in the catalyst and in the leaching solution was deter-
mined by using atomic absorption spectroscopy (Perkin-Elmer
AAS 300 with acetylene (C2H2) flame). 1H and 13C CP MAS NMR
spectra were recorded on 200 and 100.62 MHz, respectively, using
a Bruker Avance 400 MHz spectrometer.
2.2. Preparation of the catalysts
2.2.1. Preparation of LDH
The layered double hydroxide containing Zn:Al molar ratio 2:1
was prepared by the co-precipitation method at a constant pH of
11 [25]. The synthesis was carried out by dropwise addition of
mixed metal nitrate solution [Zn(NO3)2ꢀ6H2O (0.133 M) and
Al(NO3)3ꢀ9H2O (0.066 M)] to 2 M NaOH solution taken in a flask
containing 100 ml of deionized water under magnetic stirring,
and nitrogen atmosphere was maintained throughout the addition.
The resulting slurry was kept stirring for 1 h at room temperature.
Then, it was filtered, washed thoroughly with deionized water till
the washings were neutral to litmus and dried at 100 °C overnight
[26,27]. Elemental analysis showed the following composition.
Found/calcd (%). For Zn0.68ꢀAl0.32 (OH)2 (NOꢁ)0.32 0.38 H2O (Zn,
3
Al–LDH–NO3): Zn, 38.87/39.07; Al, 7.53/7.63; N, 3.86/3.93; H,
2.19/2.26. IR (KBr) 3450, 1620, 1384, 675, 445 cmꢁ1
.
2.2.2. Synthesis of the metal complex
3-Amino-2-pyrazine carboxylic acid treated with Na2CO3 in
ethanolic medium produces the sodium salt of 3-amino-2-pyrazine
carboxylic acid. In 30 ml of ethanol, 10 mmol of sodium salt of
3-amino-2-pyrazine carboxylic acid was refluxed with 10 mmol
of 2-pyridine carboxaldehyde. The whole mixture was kept on
water bath at 60 °C for 2 h to produce the Schiff base ligand.
Ti-complex was formed by refluxing 0.456 g (1.8 mmol) of sodium
salt of Schiff base ligand and 0.29 ml (1 mmol) of titanium tetra-
isopropoxide (C12H28O4Ti) in ethanolic medium at 60 °C for 2 h.
The final product was filtered, washed several times with ethanol
to get rid of nonreacted ligand and recrystalized from diethyl ether.
Finally, the metal complex was dried in vacuum and kept in a des-
iccator. Anal. Found/calcd (%). For ligand: C, 52.43/52.8; H, 2.57/
2.8; N, 21.9/22.4. For Ti-complex: C, 47.98/48.17; H, 2.32/2.55;
N, 20.02/20.43. IR (KBr) 3300, 1725, 1640 cmꢁ1. For Schiff base
ligand: 1H NMR (CDCl3, room temp., 200 MHz): d = (7.15, t),
(7.63, t), (7.7, d), (8.41, d), (8.59, s), (8.86, d), (8.71, d). 13C CP
MAS NMR (100.62 MHz, CDCl3): d = 130.1, 130.9, 131.4, 133.9,
134.1, 134.7, 138.7, 139.0, 148.2, 167.0, 167.7. For Ti-complex:
1H NMR (CDCl3, room temp., 200 MHz): d = (7.18, t), (7.67, t),
(7.9, d), (8.23, d), (8.34, s), (8.89, d), (8.76, d). 13C CP MAS NMR
(100.62 MHz, CDCl3): d = 131.1, 131.4, 132.2, 132.8, 135.1, 135.4,
137.4, 138.2, 147.2, 166.1, 166.5.
2.4. Catalytic reaction
Catalytic test of the prepared catalyst was carried out in a 100-
ml two-necked round-bottom flask fitted with a reflux condenser.
In a solvent-free condition, 10 mmol of cyclohexene and 0.05 g of
catalyst were taken. To this, 30% H2O2 (30 mmol) was added drop-
wise. Reaction was carried out at 70 °C for 6 h (Scheme 2). The
reaction products were analyzed by offline GC (Shimadzu
GC-2010) equipped with a capillary column (ZB-1, 30 m length,
0.53 mm I.D. and 3.0
l film thickness) using a flame ionization
detector (FID). The selectivity of the epoxide (cyclohexene oxide)
is a measure of the reactivity of the catalyst. The percentage of con-
version of the substrate and the percentage of selectivity of the
products in the epoxidation reaction are calculated as:
Substrate conversion ð%Þ
¼ ½substrate converted ðmolesÞ=substrate used ðmolesÞꢂ ꢃ 100
Product selectivity ð%Þ
¼ ½product formed ðmolesÞ=substrate converted ðmolesÞꢂ ꢃ 100
2.2.3. LDH/Ti(IV)-complex
One gram of Zn–Al/LDH was used after drying under reduced
pressure at 100 °C. Thirty milliliter of ethanol was added to 1 g
The decomposition of H2O2 was followed by measuring the
volume of oxygen liberated at atmospheric pressure by conven-
tional gasometric method [28].