V. V. Phatake, B. M. Bhanage
require high temperature, high pressure and catalyst is
non-recyclable (Scheme 1).
DMAB as a reducing agent at 100 °C in propylene carbon-
ate/water as a green solvent medium.
The selection of solvents plays very crucial role in the
fixation of carbon dioxide into valuable chemicals. Tra-
ditionally organic solvents such as DMF, DCM, THF,
DMSO etc have been utilized. However, toxicity, flamma-
bility, environmentally hazardous nature of these solvents
imposes their applicability. From a green and sustainable
chemistry point of view, replacement of these solvents by
greener one will be a great achievement. The use of pro-
pylene carbonate as a solvent is an example which can
overcome the limitations associated with the traditional
organic solvents. The employment of propylene carbon-
ate as a solvent has a number of advantages such as non-
toxicity, aprotic, odorless, high boiling point, biodegrad-
able [33]. Based on these benefits propylene carbonate
is considered as green, environmental friendly and safe
solvent. Moreover, the propylene carbonate score good to
excellent in the GlaxoSmithKline (GSK) solvent selec-
tion guide [34, 35]. This showing that there is no concern
of using propylene carbonate as a solvent at all criteria.
Furthermore, on combustion propylene carbonate does not
produce oxides of nitrogen and sulfur as it having only
carbon, hydrogen and oxygen. Therefore propylene car-
bonate is a sustainable alternative solvent to DMF, DMSO
and DCM.
2 Experimental
2.1 Materials and Methods
All chemicals were purchased from Sigma Aldrich, Alfa
Aesar and used without further purification. The progress
of the reaction was monitored by the thin layer chroma-
tography (silica gel 60 F-254 plates) and the yields for the
products were confirmed by gas chromatography (GC) of
PerkinElmer Clarus 400 with a flame ionization detector
and a capillary column (30 m × 0.25 mm × 0.25 µm) and.
All of the products were identified by GC-MS (Shimadzu
QP 2010 ultra, Rtx-17, 30 m × 25 mm ID), column flow
3 mL min−1, 70 °C to 220 °C at 10 °C min−1 rise. Prod-
ucts were purified by column chromatography on 100–200
mesh silica gel. NMR spectra were recorded with an Agi-
lent Technologies (1H NMR at 400 MHz, 13C NMR at
100 MHz) spectrometer. The U-g-C3N4 was prepared by
pyrolysis of easily available urea [41] and Cu@U-g-C3N4
was prepared by precipitation method with slight modifi-
cation in previously reported work [42]. We prepared three
copper catalyst on the basis of copper loading (Cu@U-
g-C3N4-1 = 3% Cu, Cu@U-g-C3N4-2 = 6% Cu, Cu@U-g-
C3N4-3 = 10% Cu). The SEM images of these catalysts are
shown in supporting information (Fig. S4). The detailed
procedure of catalyst preparation is shown in supporting
information.
Recently, g-C3N4 attracted much attention in the last
few years due to its high thermal stability, basic property,
easy recyclability, low cost, biocompatibility etc. Further-
more, the g-C3N4 is commonly used as a photocatalyst and
active support for various metal catalysts. The advantage
of these materials (simple recovery and recyclability) as
compared to homogeneous catalyst can lead to environ-
mental friendly chemical process. The advantageously, the
g-C3N4 has van-der-waals interaction between the single
layer of g-C3N4 like graphite which makes it insoluble in
THF etc [38–40]. Therefore g-C3N4 has wide potential
benign catalyst. Therefore, our approach is to support less
precious metal copper on the U-g-C3N4 to increase its
activity towards the synthesis of benzimidazole. As cop-
per nanoparticles (Cu NPs) have advantages such as easy
preparation, inexpensive, heterogeneous, high catalytic
activity etc, it has enormous application in catalysis as
well as in synthesis.
2.2 Experimental Procedure for the Synthesis
of Benzimidazoles
Synthesis of benzimidazole from o-phenylenediamine by
CO2 and DMAB in presence of Cu@U-g-C3N4 was car-
ried out in high pressure reactor equipped with an over-
head stirrer. In a general experiment for the synthesis of
benzimidazole, o-phenylenediamine (1.00 mmol), DMAB
(3 mmol), PC:H2O (3 mL:1.5 mL), Cu@U-g-C3N4 (20 mg)
were loaded into the reactor at room temperature, reactor
was sealed, flushed three times with CO2 and 2.5 MPa CO2
pressure was loaded in to reactor, heated to required tem-
perature with stirring (600 rpm). After completion of the
reaction, the reactor was cooled to room temperature and
the pressure was slowly released. The catalyst was sepa-
rated by filtration, washed with ethyl acetate and water.
The combined mixture was concentrated in vacuo and the
products were purified by the column chromatography
with silica gel of 100–200 mesh size and petroleum ether-
ethyl acetate used as an eluent to afford pure products
Notwithstanding, previous reports have one or more
disadvantages such as the use of expensive catalysts, non-
recyclability of catalysts, the need for high reaction tempera-
ture and difficult catalyst preparation methods etc. Herein,
we report the synthesis, characterization and application of
Cu@U-g-C3N4 as an efficient and recyclable catalyst for
the synthesis of benzimidazoles by using of OPD, CO2 and
1 3