Magnesium oxide nanosheets as effective catalysts for the synthesis of diethyl carbonate from ethyl carbamate and ethanol

Article abstract of DOI:10.1039/c4cy01109k

A series of MgO catalysts were prepared by thermal decomposition and precipitation methods. Their catalytic performance was evaluated in the synthesis of diethyl carbonate (DEC) from ethyl carbamate (EC) and ethanol. Among them, MgO prepared using sodium carbonate as the precipitant and calcined at 450°C (MgO-SC-450) exhibited much higher catalytic activity. An excellent DEC yield of 58.0% with a high DEC selectivity of 92.1% could be achieved over the MgO-SC-450 catalyst under optimized reaction conditions: 210°C, ethanol/EC molar ratio of 10, and 3 h. Moreover, the catalytic activity could be essentially retained during recycling experiments. The structure and surface basicity of the MgO catalysts were characterized by X-ray diffraction (XRD), Mastersizer 2000, N₂ adsorption, field-emission scanning electron microscopy (FE-SEM), and temperature-programmed desorption of CO₂ (CO₂-TPD). It was found that MgO-SC-450 possessed nanosheet morphology. Furthermore, a larger amount of appropriate medium basic β sites of MgO-SC-450 with the peak located between 225°C and 275°C was favourable for obtaining much superior catalytic activity. Quasi in situ FT-IR experiments were carried out to elucidate the adsorption behaviours of reactants. It was found that EC could be effectively activated and ethanol could be dissociated to a strong nucleophilic ethoxy group by MgO. In addition, theoretical calculation proved the co-adsorption of EC and ethanol on the MgO surface. Based on the results of quasi in situ FT-IR experiments and theoretical calculation, a plausible reaction mechanism has been proposed for the catalytic reaction.

Full text of DOI:10.1039/c4cy01109k

Catalysis
Science &
Technology
View Article Online
View Journal | View Issue
PAPER
Magnesium oxide nanosheets as effective
catalysts for the synthesis of diethyl carbonate
from ethyl carbamate and ethanol†
Cite this: Catal. Sci. Technol., 2015,
5, 1021
Fengjiao Li,ab Huiquan Li, a Liguo Wang,a Peng Hea and Yan Caoa
*
A series of MgO catalysts were prepared by thermal decomposition and precipitation methods. Their
catalytic performance was evaluated in the synthesis of diethyl carbonate (DEC) from ethyl carbamate (EC)
and ethanol. Among them, MgO prepared using sodium carbonate as the precipitant and calcined at
450 °C (MgO-SC-450) exhibited much higher catalytic activity. An excellent DEC yield of 58.0% with a high
DEC selectivity of 92.1% could be achieved over the MgO-SC-450 catalyst under optimized reaction
conditions: 210 °C, ethanol/EC molar ratio of 10, and 3 h. Moreover, the catalytic activity could be essen-
tially retained during recycling experiments. The structure and surface basicity of the MgO catalysts were
characterized by X-ray diffraction (XRD), Mastersizer 2000, N2 adsorption, field-emission scanning electron
microscopy (FE-SEM), and temperature-programmed desorption of CO2 (CO2-TPD). It was found that
MgO-SC-450 possessed nanosheet morphology. Furthermore, a larger amount of appropriate medium
basic β sites of MgO-SC-450 with the peak located between 225 °C and 275 °C was favourable for
obtaining much superior catalytic activity. Quasi in situ FT-IR experiments were carried out to elucidate
the adsorption behaviours of reactants. It was found that EC could be effectively activated and ethanol
could be dissociated to a strong nucleophilic ethoxy group by MgO. In addition, theoretical calculation
proved the co-adsorption of EC and ethanol on the MgO surface. Based on the results of quasi in situ
FT-IR experiments and theoretical calculation, a plausible reaction mechanism has been proposed for the
catalytic reaction.
Received 27th August 2014,
Accepted 3rd October 2014
DOI: 10.1039/c4cy01109k
www.rsc.org/catalysis
promising oxygen-containing fuel additive because of its
higher oxygen content than methyl tert-butyl ether (MTBE),
1. Introduction
Recently, great efforts have been put on the development of
new synthesis methods based on CO2 as a feedstock for
organic carbonates due to their excellent properties and
apparent commercial applications.1–3 As an important homo-
logue of the dialkyl carbonate family, diethyl carbonate (DEC)
is gaining increasing popularity. A broad variety of applica-
tions of DEC have been demonstrated in the fields of syn-
thetics, solvents, electrolytes, additives, etc. For instance, DEC
is an environmentally friendly substitute for ethyl halides and
phosgene in ethylation and carbonylation processes, respec-
tively.4 DEC is also used as a raw material, a reactive interme-
diate and a solvent in the synthesis of organic compounds.5,6
Moreover, DEC is extensively applied as an electrolyte in
lithium ion batteries.7 Importantly, DEC is considered to be a
more favourable gasoline/water distribution coefficient than
dimethyl carbonate (DMC) and ethanol, and negligible envi-
ronmental pollution from its biodegradation products (CO2
and ethanol).8–10
DEC was first synthesized from ethanol and phosgene in
1941,11 which involved the extremely hazardous phosgene.
Hence, several environmentally compatible methods have
been developed for producing DEC, including oxidative car-
bonylation of ethanol,9,10,12–14 carbonylation of ethyl nitrite,15
ethanolysis of CO2,16 coupling reaction from carbon dioxide,
epoxides and ethanol,17 transesterification of organic carbonates
such as dimethyl carbonate18,19 and propylene carbonate,20
and ethanolysis of urea.21–24 Among these methods, ethanolysis
of urea is particularly promising owing to the merits of
cheaply and abundantly available urea and ethanol and
simple product separation. In addition, the co-produced NH3
could be easily separated, recycled and further utilized to
synthesize raw material of urea by reaction with CO2. In the
process of ethanolysis of urea, the intermediate ethyl carba-
mate (EC) is first formed from urea and ethanol (eqn (1)),
and subsequently converted to DEC by further reaction with
a Key Laboratory for Green Process and Engineering, National Engineering
Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of
Process and Engineering, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: hqli@ipe.ac.cn; Fax: +86 10 62621355; Tel: +86 10 82544825
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy01109k
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1021

View Article Online
Paper
Catalysis Science & Technology
another molecule of ethanol (eqn (2)). However, in a previous
report, low DEC yields were obtained directly from
ethanolysis of urea due to the severe decomposition of urea
under the reaction conditions, which were usually conducted
at high temperatures, e.g. 210 °C.24 It is generally accepted
that the intermediate EC can be produced by the reaction of
urea with ethanol at lower temperature in high yield and
selectivity even in the absence of catalysts. Therefore, the
employment of EC as raw material could avoid the severe
decomposition of urea in the aforementioned process. It
should also be noted that the reaction of intermediate EC
with ethanol to DEC is usually considered to be the rate-
determining step.21–23 Therefore, the challenge in the effec-
tive synthesis of DEC via ethanolysis of urea is now focused
on developing effective catalysts towards the reaction of EC
and ethanol.
A few catalysts were reported for DEC synthesis from EC
and ethanol. Guo et al. reported that lead oxide has good
catalytic activity for this reaction, and the yield of DEC was
16.2%.22 An et al. prepared various ZnO-PbO catalysts, and
the highest DEC yield obtained was 20.6%.21 In our recent
work, a solid base catalyst generated from waste slag was
developed for the DEC synthesis via ethanolysis of EC. It was
found that a satisfactory DEC yield of 43.6% could be
obtained over the fresh slag-derived solid base catalyst.25
However, the catalytic activity of the catalyst without any
post-treatment gradually decreased along with the recycling
runs. Re-calcination was necessary to regenerate the catalytic
activity of the used slag-derived solid base catalyst, and DEC
yield still decreased to the value of around 27% after several
recycle experiments. Thus, the development of more efficient
and stable catalyst systems for DEC synthesis via ethanolysis
of EC is highly desirable.
(1)
(2)
2. Experimental section
2.1 Materials
Ethyl carbamate (EC), ethanol, magnesium oxide (MgO),
magnesium hydrate IJMgIJOH)2), magnesium carbonate hydroxide
pentahydrate (4MgCO3·Mg(OH)2·5H2O) and magnesium nitrate
hexahydrate (Mg(NO3)3·6H2O) were purchased from Sinopharm
Chemical Reagent Co., Ltd. Na2CO3, NaOH, and ammonia
liquor were purchased from Xilong Chemical Industry Incorpo-
rated Co., Ltd. Cyclohexanol was purchased from J&K Scientific
Ltd. KBr (spectral purity) was purchased from PIKE Technolo-
gies Spectroscopic Creativity. All of the chemicals were of
analytical grade and used without further purification unless
noted otherwise.
2.2 Catalyst preparation
A series of MgO catalysts were prepared by thermal decompo-
sition and precipitation methods.
In the thermal decomposition method, commercial magne-
sium carbonate hydroxide pentahydrate IJ4MgCO3·MgIJOH)2·5H2O,
denoted as MgO-MCBP-Pre) was calcined at 450 °C for 3 h in
air to obtain the MgO catalyst (denoted as MgO-MCBP-450).
Commercial magnesium hydrate (denoted as MgO-MH-Pre)
was calcined at 450 °C for 3 h in air to obtain the MgO catalyst
(denoted as MgO-MH-450).
In the precipitation method, MgIJNO3)3·6H2O was used as
raw material; sodium carbonate (SC), sodium hydroxide (SH),
and ammonia (NH) were selected as precipitants. Two solu-
tions, A and B, were prepared. In order to obtain solution A,
Mg(NO3)3·6H2O was dissolved in distilled water to form
1.5 mol L−1 Mg2+ solution. Solution B contains 2 mol L−1
sodium carbonate, sodium hydroxide, or ammonia. Both solu-
tions were co-added to 50 mL of distilled water under continu-
ous magnetic stirring at room temperature. The pH value
during the precipitation process was maintained constant
at 10. The resulting precipitate was kept under continuous
stirring at 65 °C for 3 h and then statically aged at 100 °C for
15 h. After filtration, washing with distilled water, drying
and grinding, the obtained powder (denoted as MgO-SC-Pre,
MgO-SH-Pre, and MgO-NH-Pre) was calcined at different
temperatures for 3 h in air. The calcined MgO samples were
designated as MgO-SC-T, MgO-SH-T, and MgO-NH-T, where T
represents the calcination temperature. For comparison, com-
mercial MgO (denoted as MgO-Com) was used as received.
Magnesium oxide has received a great deal of attention in
various reactions as a catalyst and catalyst support. It has
been well established that the shape and size of MgO can sig-
nificantly influence the catalytic activities.26–28 Great efforts
were devoted to synthesizing highly efficient MgO catalysts
with specific morphologies in recent years. MgO with differ-
ent morphologies, e.g. nanosheet,29,30 nanocube,31 and nano-
flake,27 has exposed varied crystal facets on the surface.
Therefore, it could lead to catalysts with varied basicities
associated with the coordination environment, e.g. terraces,
step edges, corners, and lattice defects, which is an impor-
tant factor influencing the catalytic performance.
In this work, a series of MgO samples were prepared
and their catalytic performances were evaluated in the DEC
synthesis from EC and ethanol. The structure and surface
basicity of MgO samples were characterized by X-ray diffrac-
tion (XRD), Mastersizer 2000, N2 adsorption, field-emission
scanning electron microscopy (FE-SEM), and temperature-
programmed desorption of CO2 (CO2-TPD). The catalytic
activities and recyclability of the catalysts were systematically
studied. Moreover, quasi in situ FT-IR experiments and theo-
retical calculations were conducted to disclose the possible
reaction mechanism.
2.3 Catalyst characterization
X-ray diffraction (XRD) patterns of the powder samples were
recorded with a PANalytical Empyrean diffractometer using
1022 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Cu Kα radiation at 40 kV and 40 mA. Scans were performed
at a step size of 0.0130° (2θ) over the 2θ range of 5°–90°. The
crystallite size of the samples was calculated using Scherrer's
equation.32
was separated by centrifugation. For recycling studies, the used
catalysts were separated by simple centrifugation, washed
several times with ethanol and directly reused for the next
recycle experiment. The compositions of the resulting liquid
mixture were measured by GC-MS (Agilent 6890N/5975B).
The resulting liquid mixture was quantitatively analyzed by
gas chromatography with a GC-2014 gas chromatograph
(Shimadzu, Japan) equipped with a Rtx-5 capillary column
(30 m × 0.25 mm × 0.25 μm), an autosampler and a flame ioniza-
tion detector (FID). Cyclohexanol was used as the internal stan-
dard. The relative standard deviation (RSD) for the same sample
through six sampling analyses by GC was less than 0.2%.
The conversion of EC, yield of DEC and yield of N-ethyl
ethyl carbamate (NEEC) were calculated using the following
formulas:
N2 adsorption measurement was performed using a
Quantachrome Autosorb-1 at 77 K. Each sample was degassed
under vacuum at 573 K for 3 h before measurement. The
specific surface areas were calculated from N2 adsorption
data ranging from P/P0 = 0.05 to 0.30 according to the Brunauer–
Emmett–Teller (BET) method. The total pore volumes were
determined from the amounts adsorbed at relative pressures
of 0.99. Pore size distributions were obtained by analysing the
desorption branch according to the Barrett–Joyner–Halenda
(BJH) method.
Field-emission scanning electron microscopy (FE-SEM)
was conducted using an FEI SEM Quanta 250 microscope.
CO2 temperature-programmed desorption (CO2-TPD) was
carried out to determine the basicity of samples using an
AutoChem II 2920 equipped with a TCD. Samples were
preheated in situ in 25 mL min−1 helium gas at 120 °C for
0.5 h, cooled to 50 °C and then saturated with a 25 mL min−1
gas mixture of 10 vol.% CO2/He for 1 h. Weakly adsorbed
CO2 was eliminated by flushing with 25 mL min−1 helium gas
at 50 °C for 0.5 h. Finally, the temperature was increased to
600 °C at a ramp rate of 10 °C min−1.
n0 (EC)  n(EC)
Conv. of EC 
100%
n0 (EC)
n(DEC)
n0 (EC)
Yieldof DEC 
100%
n(NEEC) 2
n0 (EC)
Yieldof NEEC 
100%
The quasi in situ FT-IR spectroscopic experiments were
performed in a closed stainless steel cell equipped with ZnSe
windows by applying the KBr pellet technique for scanning at
suitable temperatures, and the spectra were recorded with a
Tensor 27 Fourier-transform infrared spectrometer in the
region of 4000–650 cm−1. The spectra reported here were the
accumulation of 16 scans at 4 cm−1 resolution and plotted
in transmittance mode. Three types of experiments were
performed, namely, (a) 2 mg of EC and 100 mg of KBr were
ground finely and pressed into a thin disk, and then the disk
was put into the closed cell; (b) the sample was prepared by
mixing 30 mg of MgO-SC-450 and 30 mg of EC, and then
4 mg of mixing samples and 100 mg of KBr were ground
finely and pressed into a thin disk, and then the disk was
put into the closed cell; and (c) 2 mg of MgO-SC-450 and
100 mg of KBr were ground finely, pressed into a thin disk
and the disk was put into the closed cell, and 10 μL of etha-
nol was then injected to the closed cell. In all experiments,
the cell was heated from 30 °C to 210 °C, and spectra were
collected once every 2 min.
2.5 Computational details
All calculations were carried out using a periodic DFT
method as implemented in the CASTEP code using the
GIPAW algorithm.33 The generalized gradient approximation
(GGA) and PBE functionals34,35 were used, while the core–
valence interactions were described by the ultrasoft pseudo-
potentials.36 The Monkhorst–Pack scheme37 κ-point grid
sampling was set to 2 × 2 × 1 for the irreducible Brillouin
zone. The convergence criteria for the structural optimization
and energy calculation were 2 × 10−6 eV per atom for a self-
consistent field (SCF), 2 × 10−5 eV per atom for energy,
0.05 eV Å−1 for the maximum force, 0.1 GPa for the maximum
stress, and 2 × 10−3 Å for the maximum displacement. The
bulk crystal structure of MgO spinel was first calculated,
which yielded the following lattice parameters: a = b = 5.86 Å,
c = 18.42 Å; α = β = γ = 90°.
For simulation of the (100) surface of MgO, a (2 × 2)
slab model was used. The number of layers was six
(84 atoms). The slabs were all separated by 10 Å thick
vacuums in the convergence tests. The lengths of the three
directions of the models were larger than 10 Å, which was
sufficient to screen the self-interaction effects of the periodic
boundary conditions. The atomic positions of the top three
layers were allowed to relax, whereas the other layers were
fixed in space to simulate the bulk effects. For the equilib-
rium structure, the Mulliken populations38 were investigated
using a projection of the plane wave states onto a linear
combination of atomic orbital basis sets, which is a widely
used approach to perform charge transfers and population
analysis.
2.4 Reaction procedure of ethanolysis of EC
All reactions were carried out in a 100 mL stainless steel
autoclave reactor equipped with a magnetic stirrer and a tem-
perature controller. Typically, a mixture of EC (34 mmol),
ethanol (0.34 mol) and 5 wt.% catalyst (with respect to EC)
was added into the reactor, sealed, heated to 200 °C and
maintained at 200 °C for 3 h under vigorous magnetic stir-
ring (500 rpm). At the end of the reaction, the reactor was
cooled to room temperature and depressurized. The catalyst
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1023

View Article Online
Paper
Catalysis Science & Technology
The binding energy of the adsorbed EC is defined as39
in Fig. 1a, all precursors of MgO-MH-Pre, MgO-SH-Pre, and
MgO-NH-Pre are indexed to hexagonal Mg(OH)2. After calci-
nation at 450 °C, the resulting MgO-MH-450, MgO-SH-450,
and MgO-NH-450 samples presented five diffraction peaks
at 36.8°, 42.8°, 62.0°, 74.4°, and 78.4° corresponding to
(111), (200), (220), (311) and (222) crystal planes of the
face-centered cubic phase of MgO (JCPDS 01-075-0447),
respectively.40 Fig. 1b shows that the single crystalline
cubic-phase MgO (JCPDS 01-075-0447) is also obtained by cal-
cining commercial magnesium carbonate hydroxide pentahydrate
(MgO-MCBP-Pre) at 450 °C. Fig. 1c represents the XRD pat-
terns of the synthesized MgO-SC-Pre precursor using sodium
carbonate (SC) as the precipitant; the calcined samples at dif-
ferent temperatures (MgO-SC-T) and MgO-SC-450 catalyst
were recovered after recycling five times (MgO-SC-450-fifth).
It can be seen that the MgO-SC-Pre precursor can be indexed
Eb = Esur + EEC − Eadsorb
where Esur is the total energy of the bare slab of the surface,
EEC is the total energy of free EC, and Eadsorb is the total
energy of the slab with the adsorbed EC molecule on the sur-
face. Therefore, a positive Eb value corresponds to exothermic
adsorption.
3. Results and discussion
3.1 Catalyst characterization
XRD patterns of the catalyst precursors and catalysts calcined
at various temperatures are depicted in Fig. 1a–d. As shown
Fig. 1 XRD patterns of the catalyst precursors and catalysts after calcination at various temperatures. (a) MgIJOH)2 precursors (MgO-MH-Pre,
MgO-SH-Pre and MgO-NH-Pre) and their corresponding MgO products (MgO-MH-450, MgO-SH-450, and MgO-NH-450). (b) MgO-MCBP-Pre
precursor and its corresponding MgO product (MgO-MCBP-450). (c) MgO-SC-Pre precursor and its corresponding products (MgO-SC-300,
MgO-SC-450, MgO-SC-600, and MgO-SC-750). (d) Commercial MgO (MgO-Com).
1024 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
to the hydromagnesite phase (4MgCO3·Mg(OH)2·4H2O). The
XRD pattern of MgO-SC-300 was quite different from that of
MgO-SC-T (T ≥ 450 °C). MgO-SC-300 shows no evident peaks
of MgO, which is probably attributed to the incomplete
decomposition of the MgO-SC-Pre precursor. However, all
MgO-SC-T (T ≥ 450 °C) samples can be assigned to cubic
MgO phases (JCPDS 01-075-0447). Furthermore, with the
increasing calcination temperature of MgO-SC-T (T ≥ 450 °C),
the intensity of MgO peaks becomes stronger. Obviously,
MgO-Com (Fig. 1d) is assigned to the cubic structure of MgO
(JCPDS 01-075-0447).
The average crystallite sizes of the MgO samples calcined
at 450 °C were calculated from X-ray line broadening of the
most intense (200) diffractions using Scherrer's equation
(Table 1). Except for MgO-Com (58.9 nm) and MgO-SC-750
(32.6 nm) with relatively larger crystallite sizes, other MgO sam-
ples calcined at 450 °C have crystallite sizes in the range of
4–11 nm. For instance, the crystalline sizes of MgO-MH-450,
MgO-SH-450, and MgO-NH-450 samples were determined to be
9.0 nm, 7.5 nm, and 8.8 nm, respectively. The crystalline size
of MgO-MCBP-450 (4.8 nm) was close to that of MgO-SC-450
(5.1 nm). It could be noted that MgO-SC-450-fifth possessed a
crystalline size of 5.5 nm, suggesting that MgO-SC-450 has
good structural properties. Based on the above analyses, the
crystalline sizes of all MgO samples were less than 60 nm
especially that of the MgO-SC-450 sample calcined at low
temperature, which was much smaller than those calcined at
high temperatures.
(MgO-SC-600) and 750 °C (MgO-SC-750), the BET surface area
decreased sharply to 121 m2 g−1 and 32 m2 g−1, respectively.
MgO-SC-300 possessed a smaller BET surface area compared
with MgO-SC-450, which possibly resulted from incomplete
decomposition of the MgO-SC-Pre precursor, as evidenced by
the XRD result in Fig. 1c.
As shown in Fig. 2, large differences in morphologies
were observed among MgO samples prepared by different
methods. For example, MgO-Com (Fig. 2a) consisted of irreg-
ular aggregates. MgO-MH-450 (Fig. 2b) showed morphologies
of hexagonal nanoplates. MgO-MCBP-450 (Fig. 2c), MgO-SC-450
(Fig. 2d) and MgO-SC-600 (Fig. 2e) possessed sheet-like crys-
tals with a thickness of 20–50 nm, indicating that the MgO
nanosheet could be obtained by simple precipitation using
sodium carbonate as the precipitant. In addition, the high
calcination temperature has great influence on the morphol-
ogy of MgO-SC-T, e.g. calcination at 750 °C (Fig. 2f) resulted
in aggregates with spherical morphology. MgO-SH-450 (Fig. 2g)
exhibited smaller nanoplates than MgO-MH-450 with some
aggregates. The morphology of MgO-NH-450 (Fig. 2h) exhibits
dense aggregates with irregular shapes. In association with the
N2 adsorption results, the MgO-SC-450 sample with nanosheet
morphology and larger surface area could contribute to the
exposure of more accessible active basic sites, which was
beneficial for obtaining the high catalytic activity of catalysts.
The CO2-TPD profiles of the various MgO samples pre-
pared by different methods are shown in Fig. 3a and b. It can
be observed that the TPD profile of these MgO samples con-
tain several CO2 desorption peaks, indicating that a variety of
basic sites with different strengths are present on each sur-
face. The strength of basic sites increases as the temperatures
of the peaks appear in the CO2-TPD profiles. According to the
description of reported studies,41–43 the chemical nature of
the basic sites on MgO is associated with the surface atomic
arrangements: low coordination O2− anions > oxygen in
Mg2+–O2− pairs > hydroxyl groups. Here, depending on the
temperature of desorption, these desorption peaks have been
divided into three groups exhibiting weak (CO2 desorption
below 160 °C), medium (CO2 desorption between 160 °C and
400 °C) and strong basicity (CO2 desorption higher than
400 °C).30,43 The peaks below 160 °C can be ascribed to the
Textural and structural properties of various MgO catalysts
are summarized in Table 1. Notably, catalyst preparation
methods have a noticeable impact on the textural and struc-
tural properties. MgO-Com possessed a smaller BET surface
area of 24 m2 g−1 and a smaller pore volume of 0.17 cm3 g−1
.
For the two MgO catalysts obtained by thermal decomposi-
tion, the BET surface area of MgO-MCBP-450 (260 m2 g−1
)
was much higher than that of MgO-MH-450 (194 m2 g−1). The
BET surface areas of the three MgO catalysts obtained by pre-
cipitation, i.e., MgO-SC-450, MgO-SH-450 and MgO-NH-450,
were distributed in a narrow region of 245–256 m2 g−1. Further-
more, when the calcination temperature of the MgO-SC-Pre
precursor was elevated to higher temperatures, e.g. 600 °C
Table 1 Textural and structural properties of MgO samples prepared by different methods
Sample
BET surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Average pore diameter (nm)
Crystalline sizea (nm)
MgO-Com
24
0.17
0.80
0.75
0.57
1.10
1.03
0.30
1.22
0.82
0.95
28.4
16.5
11.6
29.4
18.0
34.1
37.1
19.9
12.7
15.5
58.9
9.0
4.8
—
5.1
10.5
32.6
7.5
8.8
5.5
MgO-MH-450
MgO-MCBP-450
MgO-SC-300
MgO-SC-450
MgO-SC-600
MgO-SC-750
MgO-SH-450
MgO-NH-450
MgO-SC-450-fifthb
194
260
77
245
121
32
246
256
246
a
b
Crystallite size estimated using Scherrer's equation from XRD peak broadening at (200) diffraction. MgO-SC-450 catalyst was reused five
times, washed with ethanol and dried.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1025

View Article Online
Paper
Catalysis Science & Technology
Fig. 2 SEM images of (a) MgO-Com, (b) MgO-MH-450, (c) MgO-MCBP-450, (d) MgO-SC-450, (e) MgO-SC-600, (f) MgO-SC-750, (g) MgO-SH-450
and (h) MgO-NH-450.
Fig. 3 CO2-TPD profiles of various catalysts (a) MgO-Com, MgO-SC-450, MgO-SC-600, MgO-SC-750, and (b) MgO-MCBP-450, MgO-MH-450,
MgO-SH-450, MgO-NH-450.
interaction of CO2 with the weak basic sites that correspond
to hydroxyl groups on the surface. The second group between
160 °C and 400 °C can be associated with adsorption of CO2
on the Mg2+–O2− sites. Finally, those higher than 400 °C
could be attributed to the presence of strong basic sites,
between 380 °C and 445 °C). It should be noted that two
γ peaks, γ1 and γ2, needed to be deconvoluted based on the
peak shapes in the aforementioned region of the γ peak for
MgO-MH-450 and MgO-NH-450 samples. In addition, differ-
ent values were exhibited in the Y-axis for clarity of profiles.
It is worth noting that the β peak is located at lower desorp-
tion temperatures in the medium region.
Table 2 summarizes the basicity and the distribution
of basic sites on various MgO samples. It was found that
the total amount of the basic sites followed the order
MgO-MH-450 > MgO-MCBP-450 > MgO-NH-450 > MgO-SC-450 >
MgO-SH-450 > MgO-SC-600 > MgO-Com > MgO-SC-750.
probably corresponding to isolated O2− 43,44
In addition, on
.
the basis of the peak shapes of MgO samples, all profiles
were further deconvoluted into several peaks, including α1
peak (the peak located below 100 °C), α2 peak (the peak
located between 150 °C and 200 °C), β peak (the peak located
between 225 °C and 275 °C), γ peak (the peak located
between 280 °C and 325 °C) and δ peak (the peak located
1026 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Table 2 The basicity and the distribution of basic sites over various MgO samples
Number of basic sites (mmol g−1) and contributionsa
Number of total
Sample
basic sites (mmol g−1
)
α1 Site
α2 Site
β Site
γ Site
δ Site
MgO-Com
1.038
5.623
5.574
4.638
2.126
0.771
3.641
5.250
0.038 (3.7)
0.116 (2.1)
0.215 (3.9)
0.096 (2.1)
0.094 (4.4)
0.043 (5.6)
0.182 (5.0)
0.142 (2.7)
0.142 (13.7)
0.243 (4.3)
0.488 (8.8)
0.662 (14.3)
0.466 (21.9)
0.109 (14.1)
0.413 (11.3)
0.277 (5.3)
0.227 (21.9)
0.505 (9.0)
0.662 (11.9)
1.317 (28.4)
0.882 (41.5)
0.303 (39.3)
0.694 (19.0)
0.760 (14.5)
0.481 (46.4)
4.092b (72.8)
2.886 (51.8)
2.067 (44.6)
0.449 (21.1)
0.279 (36.2)
2.025 (55.6)
3.565b (67.9)
0.150 (14.3)
0.667 (11.8)
1.323 (23.6)
0.496 (10.6)
0.235 (11.1)
0.037 (4.8)
0.327 (9.1)
0.506 (9.6)
MgO-MH-450
MgO-MCBP-450
MgO-SC-450
MgO-SC-600
MgO-SC-750
MgO-SH-450
MgO-NH-450
a
Values in parentheses are the contributions of the single basic site to the number of total basic sites. b The total number of basic sites γ1 and γ2.
Notably, the total number of basic sites on MgO-SC-T
(T ≥ 450 °C) samples decreased with increasing calcination
temperature, which may be because the decrease in the specific
surface area could decrease the exposed basic sites. It is worth
mentioning that the number of basic β sites of MgO-x samples
followed the order MgO-SC-450 > MgO-SC-600 > MgO-NH-450 >
MgO-SH-450 > MgO-MCBP-450 > MgO-MH-450 > MgO-SC-750 >
MgO-Com. It could be concluded that MgO-SC-450 possessed
the largest number of basic β sites.
in a narrow region of 245–256 m2 g−1, their catalytic activities
(entries 5, 8–9) showed a large difference and followed
the order MgO-SC-450 > MgO-NH-450 > MgO-SH-450, indi-
cating that a large BET surface area was not the determin-
ing factor influencing the catalytic activities of catalysts.
MgO calcined directly from commercial magnesium hydrate
(MgO-MH-450, entry 3) or commercial magnesium carbonate
hydroxide pentahydrate (MgO-MCBP-450, entry 4) exhibited
inferior activities than the aforementioned three MgO cata-
lysts obtained by precipitation.
It should be noted that, among all of the MgO catalysts
tested in this study, MgO-SC-450 presented the activity with a
high DEC yield of 40.7% and a good DEC selectivity of
3.2 Catalytic performance
The catalytic performance of various MgO catalysts was evalu-
ated in the ethanolysis of EC to DEC. The results are summa-
rized in Table 3. A very low DEC yield of 4.4% was obtained
in the absence of catalysts (entry 1). Nevertheless, it was
found that all of the MgO catalysts prepared by different
methods presented different catalytic activities and yielded
DEC in the range of 8.1–40.7%, indicating that the prepara-
tion methods have a significant impact on the catalytic prop-
erties of MgO catalysts.
When commercial MgO (MgO-Com, entry 2) was used in
this study, only 8.1% DEC yield was obtained, which is just a
bit higher than that of the blank experiment (entry 1). Com-
pared to commercial MgO, other MgO catalysts showed
higher EC conversions and DEC yields. As could be seen,
although the BET surface areas of MgO-SC-450, MgO-SH-450
and MgO-NH-450 obtained by precipitation were distributed
80.4%. The corresponding TOF of 3522 mgDEC gcat h−1 was
−1
achieved over MgO-SC-450, which has been the best result
documented so far.21,22,25 Therefore, the influence of calcina-
tion temperature on the catalytic activity of MgO-SC-T was
also investigated (entries 5–7). As shown in Table 3, with
increasing calcination temperature, EC conversion decreased
to 38.0% for MgO-SC-600 and 28.3% for MgO-SC-750, while
the DEC yield decreased to 35.5% for MgO-SC-600 and 16.5%
for MgO-SC-750, suggesting that calcination temperature has
a great impact on the catalytic performance of MgO catalysts.
In addition, the relationship between the TOF (mgDEC gcat−1 h−1)
and the number of β sites of various MgO samples is depicted
in Fig. 4. Under the same reaction conditions, the TOF
value increased in the series of MgO-Com < MgO-SC-750 <
MgO-MH-450 < MgO-MCBP-450 < MgO-SH-450 < MgO-NH-450 <
Table 3 The catalytic performance of catalysts on the synthesis of DEC from EC and ethanol
Entry
Catalyst
Conv. of EC (%)
Yield of DEC (%)
Yield of NEEC (%)
Sel. of DEC (%)
TOFa (mgDEC gcat−1 h−1
)
1
2
3
4
5
6
7
8
9
None
MgO-Com
6.1
15.1
37.7
36.9
50.6
38.0
28.3
31.2
38.7
4.4
8.1
Trace
1.7
1.4
1.7
1.9
2.3
1.9
1.6
1.5
72.1
53.9
48.6
58.4
80.4
93.4
58.4
74.9
77.9
—
703
MgO-MH-450
MgO-MCBP-450
MgO-SC-450
MgO-SC-600
MgO-SC-750
MgO-SH-450
MgO-NH-450
18.3
21.5
40.7
35.5
16.5
23.3
30.1
1589
1864
3522
3056
1431
2022
2608
Reaction conditions: molar ratio of EtOH/EC = 10 : 1, 5 wt.% catalyst dosage (based on the weight of EC), 200 °C and 3 h.a TOF was calculated
based on the DEC amount formed (mg) per gram of catalyst per hour.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1027

View Article Online
Paper
Catalysis Science & Technology
3.3.2. Effect of EtOH/EC molar ratio. The effect of
EtOH/EC molar ratio on DEC is shown in Fig. 5b. With the
increase in EtOH/EC molar ratio, the DEC yield and
selectivity increase first in the range of 6–10 and then
decrease in the range of 10–22. It can be clearly observed that
when the EtOH/EC molar ratio was 10, the DEC yield and
DEC selectivity reach the maximum 58.0% and 92.1%,
respectively. When the molar ratio of EtOH/EC is decreased
to 6, EC concentration is higher and the side reaction of
N-ethylation of EC will take place easily, resulting in lower
DEC yield and selectivity. Theoretically, a higher molar ratio
of EtOH/EC will shift the reaction equilibrium towards the
side of DEC formation. However, a too high molar ratio of
EtOH/EC will lead to the reduction of EC concentration.
Meanwhile, since the following quasi in situ FT-IR experi-
ments and theoretical calculation found that EC and ethanol
were co-adsorbed on the MgO surface, some active sites of
EC will also be hidden by a larger amount of ethanol. Conse-
quently, the reaction rate decreases and a lower DEC yield
will be obtained when the molar ratio of EtOH/EC is too
high. From the practical viewpoint, the suitable molar ratio
of EtOH/EC is 10.
−1
Fig. 4 Relationship between TOF (mgDEC gcat h−1) and number of
β sites of various MgO samples. Reaction conditions: molar ratio of
EtOH/EC = 10 : 1, 5 wt.% catalyst dosage (based on the weight of EC),
200 °C and 3 h.
MgO-SC-600 < MgO-SC-450, which was in accordance with the
trend of the number of β sites. Clearly, the MgO-SC-450 catalyst
with a larger amount of appropriately medium β sites exhibited
better catalytic performance. Interestingly, an approximately
linear relationship between the TOF value and the number of
basic β sites could be observed. This indicated that appropri-
ately medium basic β sites of MgO catalysts played a crucial
role in the synthesis of DEC via the ethanolysis of EC. Based
on the above results, MgO-SC-450 was selected as the most
effective catalyst for the following experiments.
3.3.3. Effect of catalyst dosage. The effect of catalyst dosage
(based on the weight of EC) on the DEC synthesis is presented
in Fig. 5c. With increasing catalyst dosage, the conversion of
EC increases gradually, while the DEC yield and selectivity
reach their maximum at the catalyst dosage of 5 wt.%.
Nevertheless, the DEC yield decreases very slightly when the
catalyst dosage is further increased to 11 wt.%, which could be
attributed to the fact that the reaction is close to equilibrium.
Therefore, the preferable catalyst dosage is 5 wt.%.
3.3.4. Effect of reaction time. The effect of reaction time
on the DEC synthesis is demonstrated in Fig. 5d. As the reaction
proceeds, the conversion of EC increases. Nevertheless, DEC
yield and DEC selectivity reach their maximal values at 3 h
and then decrease gradually. Interestingly, at the initial
reaction time of 1 h, the conversion of EC and DEC yield are
16.4% and 9.1%, respectively. However, when the reaction
proceeds for 2 h, the conversion of EC and DEC yield
increase sharply to 51.1% and 40.9%, respectively. That
means that the reaction proceeds much faster in the subsequent
duration time than that of the initial 1 h. It indicates that the
induction period may exist at the initial reaction time of 1 h.
In addition, an undetectable intermediate probably exists in
the reaction of EC and ethanol over MgO-SC-450 and requires
a necessary time to convert to DEC. Therefore, the DEC yield
and selectivity are further increased to 58.0% and 92.1%
when the reaction time is increased to 3 h, respectively.
However, too long reaction time is disadvantageous to DEC
synthesis, e.g., DEC yield decreases to 41.0% at 5 h. As a
result, the suitable reaction time is chosen as 3 h.
3.3 Effect of reaction parameters
3.3.1. Effect of reaction temperature. The effect of reaction
temperature on the DEC synthesis is plotted in Fig. 5a.
Obviously, the reaction temperature has a significant impact
on the DEC synthesis. The EC conversion increases with the
increase in reaction temperature. Nevertheless, the DEC yield
and selectivity increase sharply in the temperature range of
190–210 °C and decrease gradually when the reaction
temperature exceeds 210 °C. Specifically, a low DEC yield of
6.8% with a DEC selectivity of 24.6% was obtained at 190 °C,
whereas the maximum DEC yield of 58.0% with the highest
DEC selectivity of 92.1% was achieved when the reaction
temperature was further increased to 210 °C. As declared
in the literature,21,23 a too high reaction temperature will
accelerate the side reactions including N-ethylation of EC, as
shown in eqn (3). It can be clearly seen that the DEC yield
decreased sharply to 44.4% and the N-ethyl ethyl carbamate
(NEEC) yield was up to 6.0% at 230 °C. Hence, 210 °C was
selected as the suitable reaction temperature.
Based on the above results, it could be concluded that
the optimized reaction conditions for DEC synthesis over
(3)
1028 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Fig. 5 Effect of reaction parameters on the DEC synthesis over MgO-SC-450. (a) Effect of reaction temperature. Reaction conditions: molar ratio
of EtOH/EC = 10 : 1, 5 wt.% catalyst dosage (based on the weight of EC), and 3 h. (b) Effect of molar ratio of EtOH/EC. Reaction conditions: 210 °C,
5 wt.% catalyst dosage (based on the weight of EC), and 3 h. (c) Effect of catalyst dosage (based on the weight of EC). Reaction conditions: 210 °C,
molar ratio of EtOH/EC = 10 : 1, and 3 h. (d) Effect of reaction time. Reaction conditions: 210 °C, molar ratio of EtOH/EC = 10 : 1, and 5 wt.%
catalyst dosage (based on the weight of EC).
MgO-SC-450 are as follows: 210 °C, EtOH/EC molar ratio of
10 : 1, catalyst dosage of 5 wt.%, and 3 h. Under the above
reaction conditions, the DEC yield and selectivity are 58.0%
and 92.1%, respectively.
and then remained almost constant. Importantly, a good DEC
yield of >48.3% could be retained even after five recycling experi-
ments, which was still higher than the reported results.21,22,25
To give insight into the structure change of the MgO-SC-450
catalyst, XRD and N2 adsorption were used to characterize
the catalyst. As shown in Fig. 1c, all of the diffraction peaks
in the pattern of the MgO-SC-450 catalyst reused 5 times
(denoted as MgO-SC-450-fifth) correspond to the cubic-phase
MgO. The crystallite size of the MgO-SC-450-fifth catalyst was
estimated to be 5.5 nm at the (200) crystal plane (seen in
Table 1), which was not found to increase obviously during
recycling experiments. Moreover, as listed in Table 1, the
BET surface area of MgO-SC-450-fifth (246 m2 g−1) was found
to change little compared with that of the fresh catalyst
(245 m2 g−1). Based on the above results, it could be inferred
3.4. Reusability of the MgO catalyst
Reusability of catalysts plays a crucial role in industrial pro-
duction. Hence, on the basis of screening out a highly effec-
tive MgO-SC-450 catalyst, its reusability was also investigated.
The used MgO-SC-450 was recovered by centrifugation, washing
with ethanol, and centrifugation and then directly used for the
next run. Their catalytic performance was evaluated and the
results are shown in Fig. 6. With the reuse of MgO-SC-450, the
conversion of EC and DEC yield decreased at the second run
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1029

View Article Online
Paper
Catalysis Science & Technology
stretching vibration of NCO bound to the metal oxide sur-
face (OCN–M stretching).46 Compared to the results of
the blank test without adding the catalyst, the characteristic
peak of NCO was clearly observed at a lower temperature
of 120 °C for MgO. Moreover, when the temperature was ele-
vated to 210 °C, the characteristic peak of NCO still
existed with a decreased amount of NCO. This result dem-
onstrated that EC could be effectively activated by MgO. The
value gradually shifted towards lower frequency regions with
increasing temperature. Meanwhile, the intensity decreased
with increasing temperature, suggesting the enhanced desorp-
tion of isocyanate-containing species. It should be noted that
the decomposition of EC to isocyanate-containing species was
observed in the absence of ethanol, which may be different
from the real reaction pathway with ethanol as the reaction
medium. Nevertheless, the FT-IR spectra evolution along with
the temperature has provided evidence for the obvious adsorp-
tion and activation effect of EC by MgO.
3.5.2. Adsorption of ethanol. The FT-IR spectra evolution
of ethanol adsorbed onto MgO was performed during step-
wise temperature-programmed desorption in the closed cell.
It can be observed in Fig. 7c that ethanol adsorbed on MgO
displays absorption regions at 1121 cm−1 in ethanol adsorp-
tion test. This band could be attributed to deprotonated
ethanol, or ethoxide species, coordinated to a cationic Mg
surface atom. This peak assignment was in accordance with
a previously reported study of adsorbed ethanol on MgO.47
This band became more obvious at higher temperatures,
indicating that ethanol was prone to dissociating to ethoxy
groups on the MgO surface at higher temperature.
Fig. 6 Reusability of the MgO-SC-450 catalyst. Reaction conditions:
210 °C, molar ratio of EtOH/EC = 10 : 1, 5 wt.% catalyst dosage (based
on the weight of EC), and 3 h.
that MgO-SC-450 has an excellent structure stability and reus-
ability for DEC synthesis from EC and ethanol.
3.5. Proposed reaction mechanism
The adsorption behaviours of reactants on the heterogeneous
catalyst surface are the important factors influencing the
reaction pathways. In order to explore the mechanism of the
reaction, quasi in situ FT-IR under varied temperatures and
theoretical calculation was conducted.
3.5.3 Theoretical calculation. Theoretical calculation was
conducted for the adsorption behaviours of ethanol and EC
on the (100) plane of MgO.
3.5.3.1 The reactivity of single molecules. The Fukui
function48 is one of the widely used local density functional
descriptors to model chemical reactivity and selectivity. The
Fukui function is a local reactivity descriptor that indicates
the preferred regions where a chemical species will change its
3.5.1. Adsorption of EC. Fig. 7a showed the FT-IR spectra
evolution of EC with increasing temperature. It could be
observed that a new band appeared at 2168 cm−1, which was
attributed to the formation of isocyanic acid (OCN
stretching) through the decomposition of EC when the tem-
perature is 160 °C or higher.45 Then, the adsorption of EC
over MgO was studied at given temperature regions, as shown
in Fig. 7b. The band around 2200 cm−1 could be assigned to
Fig. 7 FT-IR spectra evolution of (a) EC, (b) EC + MgO, and (c) ethanol + MgO.
1030 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
density when the number of electrons is modified. Therefore,
it indicates the propensity of the electronic density to deform
at a given position upon accepting or donating electrons. The
main atoms of the adsorbed molecules are labelled as shown
in Fig. S1,† and the oxygen atom of MgO is labelled as OM.
Fukui functions for selected atomic sites are listed in
Fig. 8 and 9. The blue fields represent a high chemical reac-
tivity. For ethanol molecule, the maximum values of the local
electrophilic reactivity occurred at the O atom of the hydroxyl
group. In the same way, the maximum values of the nucleo-
philic reactivity at the H1 atom of the hydroxyl group indicate
that this site is prone to nucleophilic attack. For the EC mole-
cule, the O atom of carbonyl is prone to electrophilic attack,
while the N atom is prone to nucleophilic attack.
3.5.3.2 The adsorption of EC and ethanol on the MgO surface.
Based on the reactivity of molecules, molecular adsorption
processes were elucidated. The EC adsorption on the clean
(100) surface was first explored and is shown in Fig. 10a. One
stable structure was obtained, with the binding modes of the
EC molecule perpendicular to the surface, the EC molecule
acted as a Lewis acid and the H1–OM bond would be the main
driving force for EC adsorption. Another thing is that the
O1–Mg length was 2.223 Å, which was close enough to each
other to generate bonds that could significantly enhance the
stability of the state. Furthermore, the generation of O1–Mg
bonds could allow the two atoms of the EC molecule (O1 and
H1) and the two atoms on the surface (Mg and OM) to form a
four-membered ring, which was also an important reason for
the higher stability of the EC adsorption state.
After elucidating the molecular adsorption process,
we turned to the adsorption of ethanol on the MgO (100)
surface. From the equilibrium configuration in Fig. 10b, it
can be observed that the adsorbed H1 atom of the ethanol
molecule could be attached to the OM atom to form a H1–OM
bond with a bond length of 1.721 Å. Simultaneously, the
O atom approached an Mg atom near the surface to form a
four-membered ring.
It is observed that when ethanol is adsorbed on the
MgO surface, dissociative adsorption takes place. That is, a
surface hydroxyl is generated and a strong O–Mg bond is
formed between the O atom of the ethoxy group and a vici-
nal Mg cation of the substrate. The Mulliken charges of H
and O atoms of ethanol with free molecules and activated
state, respectively, are shown in Table 5. It can be noted
that the important modifications correspond to the O and
H atoms: on these sites the O atom of ethanol receives a
negative charge, while the H atom loses a negative charge.
The charge transfer is large enough for dissociative adsorp-
tion to occur.
Having discussed the single molecule adsorption on the
(100) surface, we turned our efforts to the co-adsorption
mechanism. A comparison of the adsorption energies of all
of the states (Table 4) showed that the co-adsorption type
was more stable than other types of adsorption. Thus, the
co-adsorption on the MgO (100) surface would be thermo-
dynamically more favourable. The optimized structure of
co-adsorption (Fig. 10c) showed that the co-adsorption type
has the shortest bond length. The high bond energy could
Fig. 8 Fukui field of the ethanol molecule. (a) The Fukui field of electrophilic attack. (b) The Fukui field of nucleophilic attack.
Fig. 9 Fukui field of the EC molecule. (a) The Fukui field of electrophilic attack. (b) The Fukui field of nucleophilic attack.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1031

View Article Online
Paper
Catalysis Science & Technology
Fig. 10 Optimized geometrical structure of (a) single EC adsorbed, (b) single ethanol adsorbed, and (c) EC and ethanol molecules co-adsorbed on
the (100) surface of MgO. N: blue spheres, H: white spheres, C: gray spheres, O: red spheres, and Mg: green spheres.
Table 4 Structural parameters and adsorption energies of molecules
adsorbed in different configurations on the MgO (100) surface
than single EC or ethanol adsorption. Therefore, based
on the experimental results and theoretical calculations,
the reaction of EC and ethanol possibly took place over
Adsorption
MgO-SC-450, as shown in Scheme 1. CH3CH2OH was acti-
vated through the abstraction of Hδ+ by the basic sites O2−
of MgO, giving a strong nucleophilic ethoxy group, which
formed a complex with the unsaturated Mg2+ site. Mean-
while, ethyl carbamate (EC) was coordinated with a cationic
Mg surface atom through the O atom of the carbonyl group.
Subsequently, a lone pair of electrons of the oxygen atom in
the ethoxy group could form a bond with the electrophilic
carbonyl carbon of activated EC to form DEC, while the
hydrogen proton breaking away from the ethanol molecule
combines with the amino group leaving from the activated
EC molecule to form NH3.
Adsorption type
O–Mg (Å)
H–OM (Å)
energy (eV)
Single EC
Single ethanol
Co-adsorption
2.223
2.310
2.209
2.276
1.742
1.721
1.648
1.727
0.525
0.451
1.052
EC
Ethanol
Table 5 The Mulliken charges of H and O atoms of ethanol with free
molecules and activated state, respectively
Atoms
The free state
The activated state
H1
O
0.33
−0.67
0.52
−0.87
increase the stability of the co-adsorption structure. There-
fore, it could be concluded that MgO could effectively activate
EC and ethanol.
3.5.4 Possible reaction mechanism. In the early
documents, An et al.21 proved that both ZnIJNCO)2IJNH3)2 and
metal Pb were the active components for the ZnO-PbO cata-
lyst and proposed a possible reaction mechanism for DEC
synthesis from EC and ethanol over the ZnO-PbO catalyst.
In their study, EC was activated by Zn(NCO)2(NH3)2, while
ethanol was activated by PbO. In our study, we found that
MgO-SC-450 possessed excellent structure stability after the
fifth recycle experiment, as evidenced by XRD characteriza-
tion (Fig. 1c). Moreover, quasi in situ FT-IR experiments
indicated that EC was effectively activated and ethanol was
dissociated to a strong nucleophilic ethoxy group on the MgO
surface. Theoretical calculation found that the co-adsorption
of EC and ethanol was more thermodynamically favourable
Scheme
1 Possible reaction mechanism for DEC synthesis via
ethanolysis of ethyl carbamate over the MgO catalyst.
1032 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
8 M. A. Pacheco and C. L. Marshall, Energy Fuels, 1997, 11, 2–29.
9 B. C. Dunn, C. Guenneau, S. A. Hilton, J. Pahnke,
E. M. Eyring, J. Dworzanski, H. L. C. Meuzelaar, J. Z. Hu,
M. S. Solum and R. J. Pugmire, Energy Fuels, 2001, 16,
177–181.
10 N.-S. Roh, B. C. Dunn, E. M. Eyring, R. J. Pugmire and
H. L. C. Meuzelaar, Fuel Process. Technol., 2003, 83, 27–38.
11 I. E. Muskat and F. Strain, US Pat., 2,379,250, 1941.
12 P. Zhang, S. Huang, Y. Yang, Q. Meng, S. Wang and X. Ma,
Catal. Today, 2010, 149, 202–206.
13 P. Zhang, S. Huang, S. Wang and X. Ma, Chem. Eng. J., 2011,
172, 526–530.
14 S. Huang, J. Zhang, Y. Wang, P. Chen, S. Wang and X. Ma,
Ind. Eng. Chem. Res., 2014, 53, 5838–5845.
15 M. Fan, P. Zhang and X. Ma, Fuel, 2007, 86, 902–905.
16 F. Gasc, S. Thiebaud-Roux and Z. Mouloungui, J. Supercrit.
Fluids, 2009, 50, 46–53.
17 L. Wang, H. Li, S. Xin, P. He, Y. Cao, F. Li and X. Hou, Appl.
Catal., A, 2014, 471, 19–27.
18 H.-P. Luo and W.-D. Xiao, Chem. Eng. Sci., 2001, 56, 403–410.
19 I. Zielinska-Nadolska, K. Warmuzinski and J. Richter, Catal.
Today, 2006, 114, 226–230.
20 S.-J. Wang, S.-H. Cheng, P.-H. Chiu and K. Huang, Ind. Eng.
Chem. Res., 2014, 53, 5982–5995.
21 H. An, X. Zhao, L. Guo, C. Jia, B. Yuan and Y. Wang, Appl.
Catal., A, 2012, 433, 229–235.
22 L. Guo, X. Zhao, H. An and Y. Wang, Chin. J. Catal., 2012,
33, 595–600.
23 D. Wang, B. Yang, X. Zhai and L. Zhou, Fuel Process.
Technol., 2007, 88, 807–812.
24 S. Xin, L. Wang, H. Li, K. Huang and F. Li, Fuel Process.
Technol., 2014, 126, 453–459.
4. Conclusions
In conclusion, an effective catalyst system was developed
for diethyl carbonate synthesis via ethanolysis of ethyl carba-
mate. The MgO catalyst prepared using sodium carbonate
as the precipitant and calcined at 450 °C (MgO-SC-450)
possessed nanosheet morphology and a large surface area of
245 m2 g−1. The corresponding turnover frequency (TOF)
−1
obtained at 200 °C was 3522 mgDEC gcat h−1, which was
much higher than the results previously reported.21,22,25 The
reaction parameters, e.g. reaction temperature, ethanol/EC
molar ratio, catalyst dosage, and reaction time, were investi-
gated in detail. Under the optimized reaction conditions of a
molar ratio of ethanol/EC of 10, catalyst dosage of 5 wt.%,
210 °C and 3 h, a higher DEC yield of 58.0% with 92.1%
excellent selectivity could be achieved. In addition, it was
found that the phenomenon of an induction period existed
at the initial stage of the reaction. Moreover, the DEC yield
slightly decreased to 46.8% at the second run and the cata-
lytic activity remained almost constant in the subsequent
recycle experiments. Importantly, the higher catalytic activity
could be attributed to the larger amount of appropriately
medium basic β sites. The quasi in situ FT-IR experiments
and theoretical calculation indicated that EC and ethanol
were co-adsorbed on the MgO surface, and ethanol was disso-
ciated to a strong nucleophilic ethoxy group. A plausible reac-
tion mechanism was also proposed for the catalytic reaction.
This work has provided an effective and stable catalyst for
the economical synthesis of diethyl carbonate via ethanolysis
of ethyl carbamate derived from urea, which shows promis-
ing application in synthesis of diethyl carbonate on the
industrial scale.
25 L. Wang, H. Li, S. Xin and F. Li, Catal. Commun., 2014, 50,
49–53.
26 F. Wang, N. Ta and W. Shen, Appl. Catal., A, 2014, 475, 76–81.
27 N. Sutradhar, A. Sinhamahapatra, S. K. Pahari, P. Pal,
H. C. Bajaj, I. Mukhopadhyay and A. B. Panda, J. Phys.
Chem. C, 2011, 115, 12308–12316.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (no. 21206180, 21406245 and
21476244) and the National Key Technology R&D Program
(no. 2013BAC11B03).
28 Y. Zhang, M. Ma, X. Zhang, B. Wang and R. Liu, J. Alloys
Compd., 2014, 590, 373–379.
29 K. Zhu, J. Hu, C. Kübel and R. Richards, Angew. Chem., 2006,
118, 7435–7439.
Notes and references
1 T. Sakakura and K. Kohno, Chem. Commun., 2009,
1312–1330.
30 J. Hu, K. Zhu, L. Chen, C. Kübel and R. Richards, J. Phys.
Chem. C, 2007, 111, 12038–12044.
2 Q. Su, J. Sun, J. Wang, Z. Yang, W. Cheng and S. Zhang,
Catal. Sci. Technol., 2014, 4, 1556–1562.
3 J. Xu, K.-Z. Long, T. Chen, B. Xue, Y.-X. Li and Y. Cao, Catal.
Sci. Technol., 2013, 3, 3192–3199.
4 E. Leino, P. Mäki-Arvela, V. Eta, D. Y. Murzin, T. Salmi and
J. P. Mikkola, Appl. Catal., A, 2010, 383, 1–13.
5 S. Matsumura, S. Harai and K. Toshima, Macromol. Chem.
Phys., 2000, 201, 1632–1639.
6 P. Arockiam, V. Poirier, C. Fischmeister, C. Bruneau and
P. H. Dixneuf, Green Chem., 2009, 11, 1871–1875.
7 D. Becker, G. Cherkashinin, R. Hausbrand and
W. Jaegermann, J. Phys. Chem. C, 2014, 118, 962–967.
31 G. Glaspell, H. M. A. Hassan, A. Elzatahry, L. Fuoco,
N. R. E. Radwan and M. S. El-Shall, J. Phys. Chem. B,
2006, 110, 21387–21393.
32 N. F. Henry, W. A. Wooster and H. Lipson, The interpretation
of X-ray diffraction photographs, 1951.
33 Accelrys, Inc., Materials Studio, 3.0 V, Accelrys, Inc., San Diego,
CA, 2003.
34 A. D. Becke, J. Chem. Phys., 1988, 88, 2547–2553.
35 J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter
Mater. Phys., 1992, 45, 13244–13249.
36 D. R. Hamann, M. Schlüter and C. Chiang, Phys. Rev. Lett.,
1979, 43, 1494–1497.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 5, 1021–1034 | 1033

View Article Online
Paper
Catalysis Science & Technology
37 H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Solid State,
1976, 13, 5188–5192.
38 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1833–1840.
39 M. M. Branda, P. G. Belelli, R. M. Ferullo and N. J. Castellani,
Catal. Today, 2003, 85, 153–165.
43 M. Verziu, B. Cojocaru, J. Hu, R. Richards, C. Ciuculescu,
P. Filip and V. I. Parvulescu, Green Chem., 2008, 10,
373–381.
44 Z. Liu, J. A. Cortés-Concepción, M. Mustian and M. D. Amiridis,
Appl. Catal., A, 2006, 302, 232–236.
40 S.-W. Bian, J. Baltrusaitis, P. Galhotra and V. H. Grassian,
J. Mater. Chem., 2010, 20, 8705–8710.
45 J. Shang, X. Guo, F. Shi, Y. Ma, F. Zhou and Y. Deng,
J. Catal., 2011, 279, 328–336.
41 Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang and Y. Qian,
Chem. Mater., 2001, 13, 435–440.
46 Q. Li, N. Zhao, W. Wei and Y. Sun, J. Mol. Catal. A: Chem.,
2007, 270, 44–49.
42 J. I. Di Cosimo, V. K. Diez, C. Ferretti and C. R. Apesteguia,
in Catalysis: Volume26, The Royal Society of Chemistry, 2014,
vol. 26, pp. 1–28.
47 T. W. Birky, J. T. Kozlowski and R. J. Davis, J. Catal., 2013,
298, 130–137.
48 K. Fukui, Science, 1987, 218, 747–754.
1034 | Catal. Sci. Technol., 2015, 5, 1021–1034
This journal is © The Royal Society of Chemistry 2015