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prepared by scraping and sonicating the strips to dislodge the
particles from the surface and collecting them by centrifugation.
The AuNPs samples were prepared by dropping an ethanolic
suspension of scratched AuNPs on a copper grid and allowing it to
dry at room temperature. The amount of nanogold in the catalyst
was determined by inductively coupled plasma optical emission
spectrometry (ICP-OES; PlasmaQuant PO 9000 - Analytik Jena). The
samples were first digested in a dilute mixture of HNO3 and HCl.
Calibration curves were prepared for gold using standard solutions
(ICP Element Standard solutions, Merck). Specimens for FESEM (Lyra
3, Tescan) imaging were prepared from a chunk of a decorated jute
stem on alumina stubs coated with gold in an automatic gold
coater (Quorum, Q150T E). The surface composition and oxidation
states were determined using an X-ray photoelectron spectroscope
(XPS) equipped with an AlÀ Kα micro-focusing X-ray monochroma-
tor (ESCALAB 250Xi XPS Microprobe, Thermo Scientific, USA). The
binding energy scale was calibrated, and base pressure was used.
The chamber pressure was 2×10À 9 torr. Catalytic products were
identified using a Shimadzu 2010 Plus gas chromatograph coupled
to a mass spectrometer (GC-MS, Japan). The disappearance of the
reactant and sequential appearance of the product was recorded in
real time. The species were identified in terms of their molecular
ion (M+) by comparing and matching them with those in the Wiley
Registry Mass Spectral Library, in addition to the identification of
mass fragmentation. Catalytic reactions were performed in a 10-
place parallel reactor, from Bibby Scientific, UK (model# Electro-
thermal Omni Series 1025).
strated gold nanoparticles (AuNPs) supported on TiO2 can act as
a highly chemoselective catalyst for nitroarene reduction in the
presence of molecular hydrogen.[28] Researchers have reported
that nanogold supported on functionalized graphene oxide,[29]
polyaniline fibers,[30] and calcium alginate[31] are effective
catalysts. Furthermore, Esumi and co-workers prepared a nano-
gold-encapsulated dendrimer nanocomposite by laser irradi-
ation and successfully applied it for nitrophenol reduction using
NaBH4 as the hydrogen source.[32] Pradhan et al. have demon-
strated that nanoparticles of coinage metals (copper, silver, and
gold) can be used to catalyze the reduction of nitroarenes using
NaBH4 in an aqueous or micellar medium.[33] Researchers have
also found that the heterogeneous catalytic system with tannin-
stabilized AuNPs supported on porous alumina can be used to
convert 4-nitrophenol to 4-aminophenol.[34] However, recovery
of these supported nano-sized metal catalysts for subsequent
use by filtration, centrifugation, or precipitation is a challenge.
Application of metal nanoparticles is also hindered by their
inherent tendency to agglomerate, which is favored by their
high surface energy.[35] Hence, additives such as surfactants[36]
and polymers[37] have been used to inhibit agglomeration in the
catalytic system, which always enhance the possibility of
interference with the reactivity of the catalyst.
Biogenic surfaces with unique functionalities have the
potential to mimic a support surface and possess better
properties than their counterparts used as supports. Cellulose in
plant stems, a natural polymer, can very well withstand most of
the reaction conditions used for organic catalytic
transformations.[38] Jute plant sticks (JPS), which are a rich
source of cellulose and an agricultural residue, are produced in
large quantities in the state of West Bengal, India, and
Bangladesh as a byproduct of the jute fiber industry. This
wasted residue can be isolated without much effort and can be
utilized as a catalyst support. Biodegradability, natural abun-
dance, low cost, sustainability, and green nature are superior
features that make JPS much better than conventional synthetic
supports.[39]
Synthesis of AuNPs/JPS: AuNPs-decorated JPS was prepared using
the self-reducing and metal binding properties of JPS. After
harvesting fully grown jute plants, the fibers were removed by
retting and the remaining sticks were bundled and dried in open
sunlight for 5 days. The sticks were then cut into thin slices with
dimensions of~ 2 cm×0.5 cm×0.1 cm and dried overnight in an
°
oven at 100 C. The oven dried thin strips were immersed in a
HAuCl4 (40 mg) solution in DI water (25 mL). The vials containing
°
the strips were heated at 70 C, and the color of the solution
changed to purple. Heating was continued for 4 h to complete the
reduction and settling down of the floating AuNPs on the surface
of the strips. The purple decorated strips with AuNPs were removed
from the solution and dried in air for 24 h. The supernatant gold
solution as well as the strips were prepared for characterization.
Procedure for the reduction of nitroarenes using AuNPs/JPS
catalyst: 0.5 mmol of nitroarene were dissolved in a solution of
25 mmol of NaBH4 in 5 mL of DI water. The resulting solution was
As a part of a continuous effort,[40–43] the use of nanogold
(AuNPs) supported on a thin slice of jute plant stem (JPS) as an
efficient ‘dip-catalyst’, AuNPs/JPS, for the reduction of a wide
variety of nitroarenes in water under mild reaction conditions is
reported here. This ‘dip-catalyst’ has an enhanced selectivity
towards a range of products formed using sodium borohydride
as the hydrogen source. The kinetics and reusability of the
catalyst are also reported.
°
stirred for 5 min. at 60 C and 1 cm of AuNPs/JPS catalyst
(containing 1.2×10À 5 mmol of gold) was added with continued
stirring. The progress of the reactions was monitored using UV-
visible absorption spectroscopy with stirring for 3.5 h. The crude
product was extracted with ethylacetate (2×5 mL) and the organic
layer was dried with anhydrous Na2SO4. The dried organic layer
obtained by evaporating the solvent under vacuum was dissolved
in ethylacetate and analyzed using GC-MS to determine the
percentage conversion and selectivity towards the product (amino-
arenes, azoarenes, and/or azoxyarenes).
Experimental
Procedure for the reduction of quinoline using AuNPs/JPS catalyst:
Quinoline hydrogenation was performed in a Teflon-lined reactor in
an autoclave fitted with a pressure gauge, mechanical stirring, and
an automatic temperature controller. Quinoline (0.5 mmol), anhy-
drous dichloromethane (DCM) (5 mL), and the catalyst, AuNPs/JPS
(1 cm strip) were added to the reactor. The reactor was first flushed
with H2 repeatedly and finally filled with 30 bar of H2. The
hydrogenation reaction was initiated and allowed to complete by
Materials and methods: All chemicals were purchased from Sigma-
Aldrich and were used as-received unless stated otherwise.
Standard procedures were followed to produce dry and deoxy-
genated solvents. Deionized (DI) water (specific conductivity:
18.2 MΩ) was used in all the experiments. UV-Vis absorption
spectroscopic data were obtained on a UV-Vis-NIR system, model
Cary 500 from Agilent, USA, within the wavelength window of 200–
800 nm. TEM images were acquired on a JEOL JEM2100F trans-
mission electron microscope. The Specimens for TEM imagery were
°
heating the autoclave to 100 C with continuous stirring (300 rpm)
and continuing heating for 20 h. Subsequently, the reactor was
cooled to room temperature and depressurized. The degree of
Chem Asian J. 2021, 16, 1–12
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