N. Kulal, et al.
AppliedCatalysisA,General598(2020)117550
motivated to design a catalyst for the synthesis of 2-imidazolidinone
from EDA and CO2. Among the oxides tested, ZnO was reported to give
40 % conversion and 63 % selectivity and happened to be the second in
the order of activity. This prompted us to check ZnO as an active in-
gredient in the catalyst system and further improve its activity by sui-
table modification. It is well known that CO2 is activated by basic sites,
whereas amines are activated by acidic sites. At the same time, stronger
acidity may lead to deactivation of the catalyst by formation of poly-
meric products which also decreases selectivity for cyclic urea. By
considering these aspects, the combination of ZnO and basic KF/Al2O3
was conceptualized for designing a catalyst for this reaction. There is
only one report found in the literature on a catalyst KF/Zn(Al)O which
was prepared by loading KF on calcined Zn-Al hydrotalcite and used as
catalyst for transesterification of vegetable oil [21]. This material is
different from ZnO/KF/Al2O3 taken in this study in terms of method of
preparation, optimization of catalyst recipe as well as the reaction
studied for this catalyst. Hence, there is a lot of scope to investigate
ZnO/KF/Al2O3 as a material as such to investigate its physico-chemical
properties and catalytic activity as a bifunctional catalyst. In the present
study, the activity of ZnO was enhanced by supporting on KF/Al2O3 by
wet impregnation method. KF/Al2O3 is a well-known solid base catalyst
which has been applied as a catalyst for many organic reactions
[22–24]. The loading of KF on Al2O3 creates new basic sites by the
formation of potassium hydroxide and potassium hexafluoroaluminate
(K3AlF6) and F− species during preparation [22,25]. ZnO loading on
and basicity on its surface. It is shown in this study that the nature and
number of active sites can be tuned by varying both ZnO and KF loading
on γ-Al2O3 as well as calcination temperature for the synthesis of 2-
imidazolidinone from EDA and CO2. The catalyst has been character-
ized using various techniques including XRD, ICP-OES, BET, TPD, TEM,
XPS and FT-IR. The reaction parameters have been optimized and sol-
vent study has been conducted to achieve better yields for 2-imidazo-
lidinone.
ray diffractometer (Bruker D2 phaser) with Cu Kα radiation source and
high-resolution Lynxeye detector. N2 sorption experiments were per-
formed at liquid nitrogen temperature 77 K after degassing the samples
at 200 °C for 4 h under 10−2kPa using Belsorb Mini (II) instrument (BEL
Japan). The surface area, pore size, pore volume and N2 adsorption-
desorption isotherms of the catalyst were determined in this study.
Temperature programmed desorption (TPD) studies were conducted on
a Belcat-II (BEL Japan) instrument using NH3 and CO2 as probe mole-
cules for acidity and basicity respectively. In a typical procedure, 0.1 g
of sample was pretreated at 550 °C under He for 1 h in a quartz U- tube.
The temperature was then decreased to 50 °C and 10 % NH3 (CO2) in He
as adsorption gas was passed through the sample for 30 min with a flow
rate of 30 mL/min. The physisorbed NH3 (or CO2) gas was then re-
moved by purging with He gas for 15 min. TPD test was carried out by
heating sample up to 600 °C at a rate of 10 °C/min under constant flow
of He (30 mL/min) using TCD detector. The elemental composition of
the catalysts was measured by ICP-OES (Perkin Elmer). Transmission
electron microscopy (TEM) and high-resolution transmission electron
microscopy (HRTEM) techniques were used to determine the mor-
phology of the catalysts using JEM-2100 (JEOL) instrument at an ac-
celerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS)
analysis was performed on Kratos Axis Ultra DLD using Al Kα radiation
dual anode source (Energy, hυ = 1486.6 eV). The carbon (C 1s) peak
(284.8 eV) was used as the reference to calibrate the binding energy of
all XPS peaks. Fourier transform infrared (FT-IR) spectra were recorded
using the Bruker Alpha T in the 400 – 4000 cm-1 range by using KBr
pellets. Elemental analysis for Zn and K was conducted for leaching
study from Perkin Elmer AAnalyst 200 atomic absorption spectrometer
(AAS) using zinc nitrate and potassium nitrate standard solutions as
references respectively.
2.4. Catalytic activity studies
The carbonylation reaction of EDA with CO2 to make 2-imidazoli-
dinone was performed in a mechanically stirred 100 mL high-pressure
stainless steel reactor (Amar Equipments Pvt. Ltd., India). In a typical
procedure, reactant EDA was taken in a methanol solvent in the reactor
and the catalyst was added to it. The reactor was then tightened and
pressurized with CO2 from the cylinder connected to the gas inlet valve
of the reactor. The reactor was equipped with pressure gauge, furnace
and thermocouple to monitor the reaction temperature at a given time.
The reaction mixture was stirred during the reaction with a speed of
700 rpm. After completion of the reaction, the stirring and heating were
stopped and the reactor was immersed in an ice bath to condense the
vapors completely. The excess CO2 gas was vented out (after the gas
analysis) and the liquid product was centrifuged to separate the catalyst
from the rest. The liquid sample thus obtained was analyzed by an
Agilent 7890B GC with HP-5 column and FID detector. The gas sample
was obtained from the outlet of the reactor and analyzed by Thermo
Scientific Trace GC-700 equipped with a packed column (Porapak Q)
and TCD detector.
2. Experimental
2.1. Materials
Zinc nitrate hexahydrate, methanol and cerium (IV) oxide were
purchased by Merck India Ltd. Potassium fluoride and EDA were pro-
cured from Loba Chemie Pvt. Ltd. Plural SB (pseudoboehmite) was
procured from Sasol Germany GmbH.
2.2. Catalyst preparation
Pseudoboehmite was calcined at 550 °C for 4 h to get γ-Al2O3.
Different weight percentages of ZnO and KF were loaded on γ-Al2O3 by
wet impregnation method. In a typical procedure, 100 mL of an aqu-
eous solution containing the required amount of KF was mixed with
20 g of γ-Al2O3 under constant rotation of 150 RPM in a rotary eva-
porator at 60 °C for 4 h and followed by evaporation of water. Then the
sample was dried in an oven at 150 °C for 6 h and calcined at 550 °C for
4 h. After calcination, the catalyst was placed in 70 mL distilled water,
stirred well and filtered to remove any physisorbed KF present on the
catalyst, and dried at 200 °C. In a second step, a known amount of ZnO
(from zinc nitrate precursor) was loaded on KF/Al2O3 by wet impreg-
nation similar to the above procedure. Different amounts of ZnO and KF
on Al2O3 catalysts are designated as (X)/ZnO/(Y) KF/Al2O3, where X
and Y are mmol/g loading of ZnO and KF respectively (from measured
values of Zn and K by ICP-OES).
XEDA(i)
XEDA(f )
x 100
XEDA(i)
X2
imidazolidone
Y
2
x 100
XEDA(i)
Where XEDA and
Y
are EDA conversion and 2-imidazolidi-
none yield respect2ively. XEDA(i) and XEDA(f ) correspond to initial and
imidazolidone
final molar concentrations of EDA respectively. X2
is the
imidazolidone
molar concentration of 2-imidazolidinone formed in the reaction.
In order to understand if any active species of the catalyst were
leached into the reaction mixture, a leaching test was performed for the
synthesis of 2-imidazolidinone from EDA and CO2. The reaction was
stopped after 1 h and the reaction mixture was cooled down and de-
pressurized by releasing the gases slowly. The catalyst was separated
2.3. Catalyst characterization
Powder X-ray diffraction (XRD) experiments were conducted over
the 2θ range of 10–80° with steps of 0.02 and an interval of 0.5 s on X-
2