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DAS ET AL.
O2,[13] VOSO4/t-BuBpy/O2,[14] Ru (II)-NNN complex/
KOtBu,[15] and K2OsO4.2H2O/K3[Fe (CN)6][16] with good
catalytic activity and selectivity. Major disadvantages asso-
ciated with homogeneous catalysts were their poor recycla-
bility, stability, and difficulty in handling, especially,
during industrial operations. Moreover, these processes
usually employed a large amount of environmentally ill-
disposed heavy-metal waste that is undesirable from the
standpoint of green and sustainable chemistry. Therefore,
organic reactions using heterogeneous catalysts[17,18] have
received substantial interest due to their operational
simplicity, better selectivity, cost-effectiveness as well as
recyclability compared with the traditional homogeneous
counterparts. Heterogeneous systems involving complexes
of various transition metals, like Fe3O4@SiO2-APTES-FeL/
TBHP/O2,[19] Ni (II)riboflavin complex/H2O2,[20] Pd-
PdO@rGO/O2,[21] Pt@Ca-ZSM-5/aerial oxygen,[22] RuO2-
Supported Mn3O4/air,[23] binuclear Rh (II)complex/O2,[24]
and used as received without further purification. 1H and
13C NMR spectra were obtained on a Bruker-300 spec-
trometer (300 MHz) and JEOL Spectrometer (400 MHz)
in CDCl3 and DMSO-d6 solutions with TMS as an
internal reference. Field emission scanning electron
microscopy (FE-SEM) images were recorded with a Zeiss
(Zemini) scanning electron microscope. Energy-
dispersive X-ray spectroscopy (EDX) experiment was
carried out by using a Hitachi S3400N SEM–EDX instru-
ment. Transmission electron microscopic images were
collected on a JEOL 2010 transmission electron micros-
copy (TEM) operated at 200 kV. XRD data of the powder
sample were obtained in transmission mode using a
Bruker D2 Phaser X-ray diffractometer (30 kV, 10 mA)
using Cu-Kα (λ = 1.5406 Å) radiation. Chemical states of
the heterogeneous material were determined by X-ray
photoelectron spectroscopy (XPS) using a PHI 5000 (versa
Probe II, FEI Inc). The particle size distribution and zeta
potential were evaluated by the dynamic light scattering
(DLS) system (Malvern Zetasizer Nano). The specific
surface area and porosity of the catalyst were character-
ized by the Brunauer–Emmett–Teller (BET) method with
N2 adsorption–desorption isotherms, measured at 77 K
using Quantachrome Autosorb® iQ-MP/iQ-XR. GC
analysis was performed using Perkein Elmer Clarus
600 Gas Chromatograph. Column chromatography was
performed on silica gel (60–120 mesh) from SRL, India.
Thin layer chromatographic separations were performed
on precoated silica gel plates using silica gel G for TLC
(E. Merck).
[25]
[26]
Au@CeO2/O2,
V2O5@GO/O2,
and Mo (VI)@Merri-
field resin/H2O2,[27] have been also documented in recent
years. Recently, Mahmoudi et al[28] developed an aerobic
oxidative protocol for the oxidation of benzylic
alcohols and alkylbenzenes to carbonyls using
Fe3O4@SiO2@(TEMPO)-co-(Chlorophyll-CoIII) as an effi-
cient magnetically recyclable nanocatalyst in an aqueous
medium under ambient conditions. Moreover, recent
developments[29] on the application of supported metal
nanoparticles for the oxidation of alcohols have been
reported. Serious shortcomings of the existing oxidative
protocols are associated with the involvement of harmful
and expensive metal catalysts, employing high oxygen
pressure, high concentration of oxidants, use of expensive
ligands, generation of over oxidized product, lack of
chemoselectivity, and relatively long reaction time. There-
fore, the development of a cost-effective, operationally sim-
ple, easily accessible, and efficient catalytic system for the
oxidation of alcohols using environmentally benign aerial
oxygen as an eco-friendly oxidant is of great demand in
the present environmental perspective. We report herein
our new findings on the excellent catalytic attributes of
alumina-supported Ni nanoparticles[30,31] for oxidative
transformation during the highly chemoselective and
ligand-free conversion of benzylic alcohols to the
corresponding carbonyl compounds in the presence of
aerial oxygen as an eco-friendly oxidant.
2.2 | Preparation of alumina supported
Ni nanoparticles
To a solution of nickel chloride hexahydrate (4.75 g,
20 mmol) in distilled water (15 ml), neutral alumina
(20.0 g) was added under the stirring condition to get the
slurry of nickel chloride on neutral alumina. It was dried
thoroughly in the air when nickel chloride adsorbed on
neutral alumina was obtained as a greenish-white easy
flowing powder. To the magnetically stirred suspension of
this material in methanol (20 ml), sodium borohydride
(1.512 g, 40 mmol) was added in small portions, and
stirring was continued for another 1 h under ambient
atmosphere. By this time, the greenish-white mass turned
grey. Then, it was filtered and washed successively with
methanol (3 Â 5 ml), water (3 Â 10 ml), and methanol
(2 Â 5 ml). The residue was dried at 130ꢀC for 2 h to afford
alumina-supported nickel nanoparticles (21.10 g, referred
to as Ni-Alumina) as a grey free-flowing powder. It can be
stored under the ambient condition for months without
appreciable deterioration of its catalytic activity.
2 | EXPERIMENTAL
2.1 | Materials and instrumentation
All reactants were purchased from SRL, AVRA
Chemicals, Alfa-aesar, Spectrochem, and Sigma Aldrich