T. Chakraborty et al.
MolecularCatalysis462(2019)104–113
Hence, in our present work we sought to address these challenges by
developing a new approach for heterogenizing molecular transition-
metal Schiff base complexes as catalysts onto Alumina decorated with
silver nanoparticles. Although synthesis and applicability of various
alumina supported metal/metal oxide nanoparticles have been reported
in literature, including Co/Al2O3 [14a], Ru/Al2O3 [14b], Cu/Al2O3
[14c], Ag/Al2O3[14d], MnO-Al2O3, FeO-Al2O3[14e], Al2O3@Mn,
Al2O3@Fe complex [13], there are no such reports on Ag NPs decorated
Al2O3 supported metal Schiff base complexes. Present approach in-
volves (1) Synthesis of two complexes as homogeneous catalysts
namely CoL and CuL(2) Immobilization of CoL and CuL onto Al2O3
support to generate heterogeneous catalysts HT1 and HT2 and finally
(3)In-situ addition of Ag NPs to HT1 and HT2 to generate our new
catalysts namely AgHT1and AgHT2. Firstly both the complexes wer-
echaracterized by single-crystal X-ray structure analysis, FT-IR, UV–vis,
and conductometric analyses. Then both the complexes were im-
mobilized on Al2O3 to generate Al2O3@CoL (HT1) and Al2O3@CuL
(HT2). Finally, Silver was dispersed onto both the heterogeneous
complexes to generate our desired heterogeneous catalyst Ag NPs-
Al2O3@CoL (AgHT1) and Ag NPs- Al2O3@CuL (AgHT2).This approach
is expected to counteract the problem of surface area reduction but is
also complimented by high catalytic activity as compared to conven-
tional immobilized catalysts [15]. AgHT1 and AgHT2 have been thor-
oughly characterized by BET, FESEM, HRTEM, FT-IR, DLS and PXRD
analyses. From BET analyses of HT1 and AgHT1(taken as re-
presentative) we concluded that Ag NPs definitely has a role in in-
creasing surface area, the effect of which is reflected in their catalytic
activity such as alcohol oxidation, alkene epoxidation and Alkane oxi-
dation. At the end of this work, we have also tried to put forward a
plausible mechanism of various catalytic reactions concerned with
AgHT1 and AgHT2, thereby highlighting the role of (a) metal com-
plexes CoL and CuL(b) role of Al2O3 as a support and ultimately(c) role
of Ag species dispersed on Al2O3@Metal complex. It is noteworthy to
mention that this new catalyst system i.eAgHT1 and AgHT2 are fairly-
dispersed in the reaction medium, easily recoverable from reaction
mixture, and can be reused up to six times without significant loss in
catalytic performance.
2.2. Synthesis of homogeneouscatalysts
2.2.1. Synthesis of [Co(L2)]·NO3·3H2O (CoL)
A methanolic solution (5 ml) of 2-(2-Pyridyl)methylamine (0.212 g,
2 mmol) was added dropwise to a methanolic solution (10 ml) of 5-
Bromo-2-hydroxy-3-methoxy benzaldehyde (0.462 g, 2 mmol) and the
resulting mixture was refluxed for 2 h. Then a methanolic solution of Co
(NO3)2.6H2O (0.925 g, 2.5 mmol) was added and the resulting mixture
was stirred for 2 h. The clear deep brown solution was kept in a CaCl2
desiccator in dark condition. After few days later a rectangular shaped
brown crystal of complex 1 was formed which was suitable for X-ray
data collection. Anal. Calcd for C28H24Br2Co1N5O10: C (48.09%); H
(3.46%); N (8.01%); Found: C (48.30%); H (3.02%); N (8.25%); FT-IR
data (KBr pellet): I.R: ν(C = N) 1639 cm−1; ν(skeletal vibration)
1538 cm−1;ν(NO3-)
λ
1387 cm-1;
UV/Vis
(H2O,nm):
max(ε) = 385.32(14 L mol−1 cm−1), 247.29(74 L mol−1 cm−1).
2.2.2. Synthesis of [Cu(L)]·CH3OH.ClO4·2H2O (CuL)
CuL was synthesized following the same procedure as for CoL using
Cu(ClO4)2.6H2O (0.727 g, 2.5 mmol) instead of Co(NO3)2.6H2O. After a
few days, deep green rectangular shaped crystals suitable for X-ray data
collection were obtained.Anal.Calcd for
C15H18Br1Cl1Cu1N2O9: C
(41.86%); H (3.51%); N (6.97%); Found: C (41.50%); H (3.89%); N
(6.40%); FT-IR data (KBr pellet): I.R: ν(C = N) 1629 cm−1; ν(skeletal
vibration) 1538 cm−1
λ
;
ν(ClO4
)
1117 cm−1;UV/Vis (H2O,nm):
−
max(ε) = 380.71(28 L mol−1 cm−1),239.77(196 L mol−1 cm−1).
2.3. Synthesis of colloidal Ag nanoparticles (Ag-NPs)
Preparation of colloidal Silver nanoparticles has been done fol-
lowing identical procedure as reported earlier [16].
2.4. Synthesis of heterogeneous catalysts
2.4.1. Synthesis of [Al2O3@CoL] complex; (HT1)
Al2O3@CoL was prepared by impregnating 1 g of Al2O3 with a so-
lution of CoL (0.20 g in 10 ml CH3CN) with continuous stirring for
30 h.The solid mass obtained was collected by filtration and the residue
was washed with acetonitrile for several times until the filtrate became
colourless. The resultant solid was dried at 100⁰C for 10 h to generate
Al2O3@CoL (HT1).
2. Experimental section
2.1. Materials and methods
2.4.2. Synthesis of [Al2O3@CuL] complex; (HT2)
All chemicals were obtained from commercial sources and used as
received. Solvents were dried according to standard procedure and
distilled prior to use.
Al2O3@CuL was prepared by mixing 1 g of Al2O3 with a solution of
CuL (0.20 g in 10 ml CH3CN) with continuous stirring for 30 h.The solid
mass obtained was collected by filtration and the residue was washed
with acetonitrile for several times until the filtrate became colourless.
The resultant solid was dried at 100⁰C for 10 h to generate Al2O3@CuL
(HT2).
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin-Elmer 240C elemental analyzer. Infrared spectra
(4000-500 cm−1) were recorded at 27 °C using a Perkin-Elmer RXI FT-
IR spectrophotometer with KBr pellets. Electronic spectra
(800–200 nm) were obtained at 27 °C using a Shimadzu UV-3101PC
with acetonitrile as solvent and reference. Thermal analyses (TG-DTA)
were carried out on a Mettler Toledo (TGA/SDTA851) thermal analyzer
in flowing dinitrogen (flow rate: 30 cm3 min−1). Conductance of the
methanolic solution of complexes was measured using a SYSTRONICS
306 conductivity meter. Field Emission Scanning Electron Microscope
(FESEM) measurement was carried out with JEOL JSM-6700 F.
Transmission Electron Microscope (TEM) measurement was carried out
with a JEOL (Japan) JEM2100 high-resolution transmission electron
microscope. X-Ray powder diffraction (XRPD) was performed on a
XPERT-PRO Diffractometer monochromated Cu-Kα radiation (40.0 kV,
30.0 mA) at room temperature. A vibrating sample magnetometer (EV-
9, Microsenseand ADE) was utilized for obtaining the magnetization
curves. Electrospray mass spectra have been recorded on a WATERS
Xevo G2-S Q Tof mass spectrometer using HRMS grade acetonitrile as
solvent.
2.4.3. Synthesis of AgNPs-Al2O3@CoL;[AgHT1]
AgHT1 with 2.4% Ag loading was prepared by the following pro-
cedure: 250 mg of HT1 was dispersed in 15 ml of Ag NPs colloidal so-
lution and the resultant mixture was stirred for 5 h at room tempera-
ture. After stirring for 5 h the resultant mixture was centrifuged to
obtain black coloured suspension of Ag NPs-Al2O3@CoL. The black
coloured precipitate suggested the loading of AgNPs onto
Al2O3@CoLThe resultant solid was washed with water for several times
and dried overnight at room temperature. Amount of silver content was
determined by ICP-OES technique.
2.4.4. Synthesis of AgNPs-Al2O3@CuL; [AgHT2]
AgHT2with 2.4% Ag loading was synthesized following the same
procedure as that of [AgHT1].250 mg of HT2 was dispersed in 15 ml of
Ag-NPs colloidal solution and was stirred for 5 h at room temperature.
After stirring for 5 h the resultant mixture was centrifuged to obtain
105