A.V. Tran, et al.
AppliedCatalysisA,General587(2019)117245
urethanes very efficiently in the CuSe2-catalyzed reductive carbonyla-
tion. Unlike the Pd-based catalyst, the CuSe2 is a unique heterogeneous
catalyst that provides easy catalyst separation as well as extremely high
selectivity to carbamates. Moreover, it can be synthesized from 1st
transition metal and main group element, leading to a good way to
escape from using precious noble metals in the reductive carbonylation
nitroarenes.
Herein, we report the synthesis and characterization of CuSe2 and
supported system, CuSe2/CeO2, as well as its catalytic activity for the
reductive carbonylation of nitroarenes for generating urethanes (car-
bamates) (Eq. 1). Furthermore, a plausible reaction mechanism is
proposed using this heterogeneous catalyst system, specifically in-
voking the importance of CuSe2(μ-CO) species.
mortar to make fine powder before use.
2.3.2. Synthesis of 5 wt% CuSe2 supported on CeO2 (CuSe2/CeO2)
The CeO2 was dried in the oven at 110 °C for 5 h prior to use. The
synthesis of CuSe2/CeO2 was carried out using a similar procedure for
preparing CuSe2 except for adding 3.1 g of CeO2 into the water solution
containing CuCl2, SeO2, and CTAB. The liquid mixture was stirred for
30 min before transferring into the glass liner of the stainless steel au-
toclave reactor.
2.4. Carbonylation reaction
All experiments were conducted in a 100 mL stainless steel auto-
clave reactor equipped with a magnetic stirrer, a thermocouple, and an
electric heater. The reactor was loaded with NB and a catalyst. The
reactor was pressurized to 2.5 MPa of CO at room temperature and then
heated to the desired temperature with vigorous stirring. The reactor
was pressurized to 8.3 MPa at 160 °C and then maintained constant
throughout the reaction using a gas reservoir equipped with a high-
pressure regulator and a pressure transducer. After 4 h reaction, the
reactor was cool down to room temperature. Liquid products obtained
from the carbonylation reaction were quantified using an Agilent
6890 N gas chromatograph (GC) equipped with a HP-5 capillary
column and FID detector and heptane was used as an external standard.
To ensure the identification of products, GC–MS (Agilent 6890 N-5975
MSD GC-Mass spectrometer) and 1H NMR, 13C NMR (300 MHz Bruker
NMR spectrometer) analyses were used.
T,P,t
ArNO2+MeOH+3CO⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ArNHC(=O)OMe+2CO2
(1)
5wt%CuSe /CeO
2
2
2. Experimental
2.1. Chemicals
All chemicals including copper(II) chloride (CuCl2, reagent grade
97%), selenium dioxide (SeO2, reagent grade powder 98.0%), ni-
trobenzene (ACS reagent ≥ 99.0%), cetyltrimethylammonium bromide
(CTAB, > 99.0%), cerium(IV) oxide (CeO2, powder, < 5 μm, 99.9%), γ-
Al2O3 (99.97% metals basis, 110 μm powder, S.A. 60 m2/g), and
charcoal (activated, untreated, granular, 8–20 mesh) were purchased
from Sigma-Aldrich. Nitrobenzene (NB) and methanol were freshly
distilled using appropriate drying agent under N2 atmosphere before
use. Carbon monoxide (CO) with 99.95% purity was purchased from
Uniongas in Korea and used as received.
2.5. FT-IR studies of carbonyl selenide
2.5.1. Using O2 as an oxidant
CO chemisorption experiment was carried out in the presence of
CO/O2 gas mixture (v/v = 80:20) as an oxidant. The experiment was
conducted by reacting 0.1 g of CuSe2 with 3.4 MPa of CO/O2 gas mix-
ture (v/v = 80/20) in the presence of 30 mL of methanol in a stainless
steel reactor at 100 °C for 4 h. After completion of the reaction, the solid
was collected by filtration and dried under reduced pressure for 12 h.
The obtained solid was pelletized with KBr and analyzed by FT-IR
spectroscopy. The same procedure was also applied to CuSe2/CeO2 and
Se(0) metal for CO chemisorption.
2.2. Instrumentation
The morphology of 5 wt% CuSe2/CeO2 was investigated using
scanning electron microscope (SEM, JEOL JSM-6701 F) and transmis-
sion electron microscope [TEM, JEM-2100 F (UHR)]. The associated
EDX/EDS system (JEOL-Model INCA) was used for analyzing elemental
compositions. X-ray photoelectron spectroscopy (XPS) was studied to
characterize the surface composition and oxidation states of elements at
the surface of catalysts using Thermo Fisher instrument equipped with
Al K-α radiation source (energy range from 100 eV to 3k eV). In order to
study the crystal structure and purity of the CuSe2 and its loading on
support, X-ray diffraction (XRD) analysis was carried out using Bruker
D8 Advance diffractometer with Ni-filtered Cu Kα radiation. FT-IR
spectra were recorded using Nicolet FT-IR spectrometer.
2.5.2. Using nitrobenzene as an oxidant
CO chemisorption experiment was conducted with the presence of
nitrobenzene as an oxidant. 0.1 g of CuSe2 was reacted with 8.3 MPa of
CO pressure in a stainless steel autoclave reactor with the presence of 1
equivalent NB to CuSe2 in 30 mL of methanol at 100 °C for 4 h. After
completion of the reaction, the solid was collected by filtration and
dried under reduced pressure for 12 h. The obtained solid was pelle-
tized with KBr and analyzed by FT-IR spectroscopy.
2.3. Catalysts preparation
2.3.1. Synthesis of cupric diselenide (CuSe2)
A mixture of CuCl2 (1.0 g, 7.4 mmol), SeO2 (3.3 g, 29.7 mmol), and
CTAB (1.0 g, 2.7 mmol) was dissolved in 30 mL of H2O. The mixture
was then transferred into the glass lined stainless steel autoclave reactor
equipped with a magnetic stirrer, a thermocouple, and an electric
heater. Before the reaction, the reactor was purged 3 times with CO to
remove remaining air. The reactor was filled with 2.5 MPa of CO at
room temperature and then heated to the desired temperature with
vigorous stirring. The reactor was further pressurized to 8.3 MPa at the
160 °C and then maintained constant throughout the reaction using a
gas reservoir equipped with a high-pressure regulator and a pressure
transducer. After a reaction period of 1 h, the suspension was neu-
tralized using NaOH solution (0.05 M, 5 mL) and the liquid was re-
moved by filtration using cannula filter, and the remaining solid pre-
cipitates were washed with distilled water (20 mL) 5 times. The
resulting solid (CuSe2) were soxhleted in methanol for 6 h and dried
under reduced pressure for 12 h. The prepared CuSe2 was ground in the
3. Results and discussion
3.1. Activities of various catalyst systems
The synthesis of the methyl-N-phenyl carbamate (MPC) from re-
ductive carbonylation of nitrobenzene (NB) was investigated in the
presence of a catalyst system comprising three-component, CuCl2, SeO2,
and cetyltrimethylammonium bromide (CTAB) in methanol as reactant
and solvent. As shown in Table 1, this catalyst system selectively pro-
duces MPC in an excellent yield of 97.5% at 100% conversion of NB. It
is interesting to note that the reaction is only effective when all three of
CuCl2, SeO2, and CTAB are employed simultaneously. Removing one of
the components results in reduced activities (entry 2∼4). As expected,
the single component system shows even worse results (entry 5∼7).
However, SeO2 as a sole catalyst system affords 25.8% (entry 6), [SeO2
+ CTAB] delivers 27.4% (entry 3), and [CuCl2 + SeO2] produces
2