MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous Base Catalyst for Synthesis of Dimethyl Carbonate by Transesterification

Article abstract of DOI:10.1007/s10562-017-2146-x

Abstract: A series of Mg₃:Fe_x + Ce_1?x LDHs (3:1) were synthesized by co-precipitation method by varying molar ratio of Fe:Ce between 1:0 to 0:1 (LDH-1 to LDH-6). All synthesized LDHs were characterized by XRD, FT-IR, TEM, N₂ sorption, benzoic acid titration and XPS in detail and evaluated for selective synthesis of dimethyl carbonate by transesterification of ethylene carbonate with methanol. It was demonstrated that the structural and basic properties of synthesized LDHs were strongly dependent on the Fe:Ce molar ratio (Ce concentration). The correlation between their physicochemical properties and catalytic performance was studied in detail. Among all synthesized LDHs the best result was obtained with LDH-3 (Fe:Ce = 0.85:0.15) where LDH structure remained intact, and showed high number of strong basic sites on LDH surface. LDH-3 was recycled 7 times while maintaining high catalyst activity and selectivity towards DMC. The obtained results elucidate the important role of Ce in modifying the basic properties of LDH in enhancing the catalytic activity for DMC synthesis. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2146-x

Catal Lett
DOI 10.1007/s10562-017-2146-x
MgFeCe Ternary Layered Double Hydroxide as Highly Efficient
and Recyclable Heterogeneous Base Catalyst for Synthesis
of Dimethyl Carbonate by Transesterification
Nayana T. Nivangune1 · Vivek V. Ranade2 · Ashutosh A. Kelkar1
Received: 14 February 2017 / Accepted: 9 June 2017
© Springer Science+Business Media, LLC 2017
Abstract A series of Mg3:Fex +Ce1−x LDHs (3:1) were
synthesized by co-precipitation method by varying molar
ratio of Fe:Ce between 1:0 to 0:1 (LDH-1 to LDH-6). All
synthesized LDHs were characterized by XRD, FT-IR,
TEM, N2 sorption, benzoic acid titration and XPS in detail
and evaluated for selective synthesis of dimethyl carbonate
by transesterification of ethylene carbonate with methanol.
It was demonstrated that the structural and basic properties
of synthesized LDHs were strongly dependent on the Fe:Ce
molar ratio (Ce concentration). The correlation between
their physicochemical properties and catalytic performance
was studied in detail. Among all synthesized LDHs the best
result was obtained with LDH-3 (Fe:Ce=0.85:0.15) where
LDH structure remained intact, and showed high number of
strong basic sites on LDH surface. LDH-3 was recycled 7
times while maintaining high catalyst activity and selectiv-
ity towards DMC. The obtained results elucidate the impor-
tant role of Ce in modifying the basic properties of LDH in
enhancing the catalytic activity for DMC synthesis.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2146-x) contains supplementary
material, which is available to authorized users.
* Vivek V. Ranade
V.Ranade@qub.ac.uk
* Ashutosh A. Kelkar
aa.kelkar@ncl.res.in
1
Chemical Engineering and Process Development (CEPD)
Division, National Chemical Laboratory, Pune 411008, India
2
School of Chemistry and Chemical Engineering, Queen’s
University Belfast, Belfast BT9 5AG, Northern Ireland, UK
Vol.:(0123456789)
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N. T. Nivangune et al.
Graphical Abstract
of methanol there is a risk associated with the explosion
hazard of CO/O2 mixture. One clean and sustainable route
for the synthesis of DMC is the transesterification of cyclic
carbonate [ethylene carbonate (EC)/propylene carbonate
(PC)] with methanol [8]. This is a safe and atom efficient
process and ethylene glycol is formed as the by-product.
Ethylene glycol is important chemical and also can be con-
verted back to ethylene carbonate by reaction with urea [9].
Significant work is being carried out on the development
of improved catalysts for the synthesis of DMC from EC
and methanol. Simple inorganic, organic bases and ionic
liquids (ILs) were found to be active catalysts for DMC
synthesis; still applications are limited because of diffi-
culties in catalyst-product separation due to its homoge-
neous nature [10–12]. Further Immobilization of the IL
on different supports have been explored but high cost of
catalyst has limited their application [13, 14]. Considerable
amount of work has been carried out on the development
of heterogeneous catalysts for the synthesis of DMC. Het-
erogeneous catalysts based on mixed metal oxides [15–19],
Keywords Dimethyl carbonate · Ethylene carbonate ·
Mg–Fe–Ce ternary hydrotalcite · Transesterification
1 Introduction
Dimethyl carbonate (DMC) is an important chemical,
which gained intense interest from scientific and industrial
fields, due to its low toxicity and good biodegradability.
DMC is being looked as a green chemical for the produc-
tion of polycarbonate and other chemicals [1]. It is also
used as a safe replacement for toxic phosgene in carbon-
ylation and methylation reactions [2]. In addition DMC has
potential applications as electrolyte in lithium batteries as a
polar aprotic solvent and as an octane booster in gasoline to
meet oxygenate specifications [3–5]. Conventionally DMC
was synthesized via phosgenation [6] and presently manu-
factured by oxidative carbonylation of methanol [7]; both
of these routes require corrosive and poisonous gases like
chlorine, COCl2 and CO. In case of oxidative carbonylation
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MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous…
smectite [20], anion-exchange resin [21], Na-dawsonite
[22], mesoporous graphitic carbon nitride [23] and Binary
hydrotalcites (i.e. LDHs) [24, 25] etc. have been studied
for transesterification reaction. Recently Xu et al. reported
78% DMC yield at 160 °C in 6 h with meso structured gra-
phitic carbon nitride (CN-MCF) catalyst [23]. The catalyst
was recycled five times without significant loss in catalyst
activity; however, harsh reaction conditions (160 °C and
0.6 MPa CO2 pressure) were necessary to achieve observed
results. Among all heterogeneous catalysts investigated
mixed metal oxides were found to be active catalysts for
this reaction, however, they require either high reaction
temperature (100–160 °C) or high catalyst loading (10–25
wt%) [15–18]. The catalyst was recycled four to five times,
but catalyst activation at high temperature was required
in most of the cases during each recycle experiment. This
potentially may cause problem in continuous or large
scale operation. In this context it is still challenging task
to develop heterogeneous catalysts exhibiting high catalyst
activity and high stability for the synthesis of DMC from
EC and methanol under mild reaction conditions. Hydrotal-
cites (HTs) or Layered double hydroxides (LDHs) consti-
tute a class of layered compounds complementary to classic
clays, as they contain positively charged layers and anions
in the interlamellar space [26]. LDHs are widely studied
and successfully used as basic catalysts for several reac-
tions such as condensation, Michael addition, transesterifi-
cation, and alkylation etc [27]. From the literature it was
observed that there are very limited reports on the use of
binary LDHs/HTs as catalysts for transesterification of EC
with methanol. Bajaj et al. reported that the catalyst activ-
ity was influenced by Mg–Al ratio and Mg–Al LDH with
molar ratio of 5:1 showed maximum activity [25]. Unfor-
tunately, significant decrease in activity was observed after
fourth recycle due to structural changes in catalyst indicat-
ing lower stability of the catalyst. According to Watanabe
et al. basicity (–OH sites) of LDH plays an important role
in this transestrification reaction [24]. Mg–Al LDH with
excess amount of –OH anions was found to be an effective
catalyst for DMC synthesis. It is well known that basicity
of LDH can be tuned by proper choice of M(II) and M(III)
metal cations or by varying the molar ratio of M(II)/M(III)
[25, 28, 29]. Recently incorporation of third metal cation
in parent LDH has attracted much more attention with the
aim of modifying basicity of the catalyst. Recently Zhang
et al. [30] have reported the synthesis of Mg/Fe+Ce–CO3
ternary LDH for the first time, however, many important
details on characterization are lacking and its catalytic
applications have not been investigated.
In this work we have synthesized
a series of
Mg3FexCe1−x ternary LDHs, by varying molar ratio of
Fe:Ce. All synthesized LDHs were characterized in detail
by various spectroscopic techniques. Influence of Ce and
its concentration on the properties and catalytic activity of
Mg3: Fe layered double hydroxide (Mg3FexCe1−x) for DMC
synthesis (Scheme 1) was investigated in detail. Ceria has
lower electronegativity compared to Fe and incorporation
of Ce in the Mg3-Fe LDH could result in increase in the
basicity of Mg3FexCe1−x LDHs and better activity towards
DMC synthesis. With this idea in mind the present work on
the synthesis of Mg3FexCe1−x ternary LDHs was initiated.
To the best of our knowledge, this is the first report on the
use of Ternary LDHs (Mg3FexCe1−x) as a catalyst for the
synthesis of DMC from EC and methanol.
2 Experimental
2.1 Catalyst Preparation
A series of Mg3FexCe1−x LDHs were synthesized by co-
precipitation method followed by hydrothermal treat-
ment. The Mg:Fe+Ce molar ratio was kept constant at
3:1; while Fe:Ce molar ratio was varied (1:0 to 0:1). In a
typical procedure solution A was prepared by dissolving
desired amount of Mg(NO3)2·6H2O, Fe(NO3)3·9H2O and
Ce(NO3)3·6H2O in deionized water and solution B was pre-
pared by dissolving NaOH and Na2CO3 (2 M) in deionized
water. Solutions A and B were added simultaneously while
pH of the resultant solution was maintained at 10–11 with
constant stirring at room temperature. The gel obtained was
hydrothermally treated at 90 °C for 15 h. Then the precipi-
tate was filtered, washed several times with deionized water
till filtrate became neutral. Finally, the synthesized ternary
LDHs were dried at 100 °C for 12 h in air. LDHs prepared
with different Fe:Ce molar ratios 1:0, 0.95:0.05, 0.85:0.15,
0.75:0.25, 0.55:0.45, and 0:1 were named as LDH-1, LDH-
2, LDH-3, LDH-4, LDH-5 and LDH-6 respectively.
Scheme 1 Schematic diagram
of synthesis of dimethyl carbon-
ate from ethylene carbonate and
methanol over layered double
hydroxide (LDHs)
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N. T. Nivangune et al.
Elemental chemical compositions of Mg3FexCe1−x
LDHs were determined with the help of ICP-OES method
(Table S1). Metal composition in all “neat” LDH samples,
was found to be in good agreement with the values based
on compositions used for the preparation.
stirrer and reflux condenser. Typically, the reactor was
charged with 23 mmol of ethylene carbonate, 230 mmol
of methanol and 2.5 wt% of LDH catalyst (relative to EC).
The reaction was carried out at 70 °C for 3 h reaction time
under vigorous stirring. After 3 h of the reaction, the glass
reactor was cooled to room temperature; the solid catalyst
was separated from the solution by filtration. Sample was
analyzed by gas chromatography to monitor the progress
of the reaction. The gas chromatograph (Agilent 6890N)
was equipped with an FID detector and an innowax cap-
illary column (30 m length×0.53 mm×ID 0.25 μm film
thickness). Formation of HEMC (2-hydroxy ethyl methyl
carbonate, intermediate product), DMC and EG as prod-
ucts were confirmed by GC-MS. Activity of the catalyst
was based on conversion of limiting reagent under standard
reaction conditions. Stability and leaching of LDHs was
investigated under optimized reaction conditions (Page S4).
Various ternary Mg3Fe0.85M0.15 LDHs were prepared by
same method by varying M(III) [where M(III)=La, Sm, Y
and Cr]. See supporting information for the details of syn-
thesis and characterization of Mg3Fe0.85M0.15 LDHs (Page
S3).
2.2 Catalyst Characterization
X-ray diffraction (XRD) patterns were recorded on a P
Analytical PXRD system (Model X-Pert PRO-1712), using
Ni filtered Cu Kα radiation (λ=0.154 nm) as an X-ray
source (current intensity, 30 mA; voltage, 40 kV) and an
X-accelerator detector. The samples were scanned in a 2θ
range of 10°–80°.
FT-IR spectra were recorded on a Shimadzu 8201 spec-
trophotometer in 400–4000 cm−1 region. The samples were
diluted prior to measurement with KBr in a 2/98 mixture
ratio.
3 Results and Discussion
3.1 Powder X‑ray Diffraction
Transmission electron microscopy (TEM) analysis was
performed on a Jeol Model JEM 1200 electron microscope
operated at an accelerating voltage of 120 kV. A small
amount of specimen was prepared by ultrasonically sus-
pending the powder sample in IPA, and drops of the sus-
pension were deposited on a carbon coated copper grid
dried at room temperature before analysis.
The XRD patterns of all synthesized ternary Mg3FexCe1−x
LDHs with different Fe:Ce molar ratios are shown in
Fig. 1. LDH-1 to LDH-3 showed typical features of
highly crystalline LDH materials with an R3m rhombohe-
dral space group symmetry [33, 34]. No other phase was
detected in these samples indicating that cerium species are
successfully incorporated into LDHs. Nevertheless, with
increase in Ce concentration for LDH-4 to LDH-6 decrease
in crystallinity (better ordering of brucite sheets) which
is directly proportional to the peak intensity and sharp-
ness of (003) and (006) planes; was observed. This is due
The N2 adsorption–desorption isotherms at −196 °C
were obtained using a Thermo surfer BET instrument and
the surface areas were deduced using the BET equation.
Before analysis, the samples were out gassed at 100 °C for
6 h.
The basic properties were determined by titration with
0.01 M benzoic acid solution in toluene using 0.15 g of
vacuum dried solid sample suspended in 2 mL of indicator
solution. Indicator solution for the determination of weak
basic sites (pKa=7.1) contained 0.01 g of bromothymol
blue in 100 mL toluene. The amount of strong basic sites
(pKa=9.3) were determined in the presence of an indicator
solution containing 0.01 g of Phenolphthalein in 100 mL
toluene [31, 32].
XPS spectra were recorded on a VG Microtech Multilab
ESCA3000 spectrometer equipped with non-monochroma-
tised Mg-Kα radiations (hv=1253.6 eV).
2.3 Catalytic Tests
The transesterification of ethylene carbonate with metha-
nol over the Ce incorporated ternary LDHs was performed
in a 50 ml jacketed glass reactor equipped with magnetic
Fig. 1 XRD patterns of all synthesized Mg3FexCe1−x LDHs (LDH-1
to LDH-6)
1 3

MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous…
to the substitution of Fe (0.65 Å) by larger ionic radii of
Ce (1.01 Å), which inhibits the intercalation of Ce in LDH
structure and also leads to significant distortion in the lay-
ers [35]. At higher Ce concentration (LDH-6, Fe:Ce=0:1)
the layered structure collapsed completely and formation
of phases like Mg(OH)2 (PCPDF-86-0441), Ce(OH)4 [36,
37] and Ce2(CO3)2·(OH)2·H2O (PCPDF-46-0369) was
observed. The lower electronegativities of rare earth ele-
ments (REEs), could favour formation of these species at
the initial stage of precipitation [38].
The lattice parameters ‘a’ and ‘c’ were calculated and
are summarized in Table 1.
Parameter ‘a’ (a=2d110) depends mainly on the aver-
age radius of the metal cation and reflected the density of
metal ions in 110 plane. [35] The observed results may be
due to the isomorphous substitution of Fe by Ce having
larger ionic radii [39]. Thus more Ce insertion leads to an
increase in average radius of cations in the layers and this
leads to increase in ‘a’ parameter.
Fig. 2 FT-IR spectra of synthesized Mg3FexCe1−x LDHs
band centered around 3500 cm−1 due to the stretching
mode of hydroxyl groups present in the brucite-like lay-
ers and from the interlayer water molecules of the hydro-
talcite structure. The band at 3681 cm−1 was observed in
LDH-4 << LDH-6; which is characteristic of non-hydro-
gen bound –OH group [41]. A weaker band at 1630 cm−1
is owing to the bending mode of water molecules. IR
absorption bands at 1510 cm−1, 1382 cm−1 (υ3), 852 cm−1
(υ2), and 1056 cm−1 (υ1) are attributed to the carbon-
ate anion environment [42, 43]. The bands recorded in
the low-frequency region of the spectrum (<700 cm−1)
are assigned to the translational mode of M–O–H and
M–OH–M vibrations. Significant difference in these
bands was observed with an increase in Ce concentration
of synthesized LDHs. The FT-IR spectra of the LDH-1 to
LDH-3 showed the typical bands of the hydrotalcite-like
compounds [42]. In case of LDH-4 to LDH-6 a new band
was observed at 3681 cm−1, which corresponds to non
bonded –OH group vibrations for M(OH)x.
Parameter ‘c’ (c=3d003) depends on the thickness of
octahedral sheets, the anion size and their orientation
within the interlayer space and the electrostatic attraction
between different layers [40]. Lower polarizing ability of
the Ce results in weak electrostatic interaction between the
layer and interlayer anions and leads to increase in the layer
spacing resulting in increase of the ‘c’ parameter value.
Thus increase in values of parameters ‘a’ and ‘c’ with
increase in Ce concentration (Table 1, LDH-1<<LDH-5)
may be attributed to larger ionic radii and lower polarizing
ability of Ce [39, 40].
3.2 FT‑IR Spectroscopy
The FT-IR spectral information can be used to determine
the actual bonding type in the expected compounds and
to determine the substitution of Ce in the brucite layer.
FT-IR spectra of all LDH samples (Fig. 2) showed broad
Table 1 Characteristics of the
Sample Lattice parameters (Å)
d(003) d(110)
Surface
area
Pore
Pore diam- Basic sites (×10−4
eter (nm) mol/g)
prepared samples
volume
(m2/g)
(cm3/g)
a
c
Weak
basic
sites
Strong
basic
sites
LDH-1 7.78
1.566
1.564
1.572
1.576
1.577
nd
3.112 23.34
67
0.51
0.8
0.81
0.78
0.76
0.73
16.8
23
24.3
22.7
21.6
20.2
1.8
2.4
2.8
3.0
3.5
4.5
1
LDH-2 7.85
LDH-3 7.88
LDH-4 7.92
LDH-5 7.93
LDH-6 nd
3.128 23.55 128
3.144 23.64 136
3.152 23.76 134
3.154 23.79 133
1.2
1.3
1.0
0.8
0.5
nd
nd
129
nd not detected
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N. T. Nivangune et al.
3.3 Transmission Electron Microscopy
to LDH-3. LDH-3 showed nearly two times more surface
area (136 m2/g) than parent LDH-1 (67 m2/g). With fur-
ther increase in Ce content from LDH-4 to LDH-6; slight
decrease in surface area and pore volume was observed.
This may be due to (1) distortion in layer structure because
of incorporation of excess amount of Ce, (2) deposition
of the agglomerated particles formed during precipitation
conditions or their incorporation into the pore system of
LDH respectively [45].
The influence of Ce concentration on the crystal shape and
size was clearly observed from the morphology of all syn-
thesized materials (Fig. 3). The TEM images (Fig. 3A–C)
showed that the LDHs (LDH-1 to LDH-3) are composed of
crystallites with the typical plate like morphology and often
hexagonal shaped [39]. However, in LDH-4 and LDH-5
(Fig. 3D, E) particles of irregular shape with agglomer-
ated small particles of [Ce(OH)4] on surface and edges of
crystallites were observed. In case of LDH-6 (Fig. 3F) the
layered structure was absent with significant increase in
agglomerated particles. The crystalline nature of the indi-
vidual materials (LDH-1 to LDH-6) was further checked by
selected area electron diffraction (SAED) patterns shown in
the Fig. 3(A–F). LDH-1 to LDH-3 samples showed good
crystalline nature which decreased with increase in Ce con-
centration (LDH-4 to LDH-6) [44].
3.5 Basicity Measurement
The nature of the active species present on the surface is
important for establishing the properties of the catalyst.
Consequently, basic properties of the catalysts were deter-
mined by benzoic acid titration of the basic sites in the
presence of pH indicators and the results are presented in
Table 1. The strong basic sites correspond to –OH sites
present in brucite like structure and weak basic sites cor-
respond to carbonate species and simple metal bonded
–OH group [M(OH)x] present in synthesized materials
[46, 47]. From the Table 1 it can be clearly observed that
the weak basic sites increased consistently with increase
in Ce concentration LDH-1 to LDH-6. However strong
basic sites increased with increase in Ce concentration
from LDH-1 to LDH-3 and decreased with further increase
3.4 Surface Area Measurements
BET surface areas and textural properties of catalysts
were determined by nitrogen adsorption–desorption
isotherms and the values obtained are summarized in
Table 1. The BET surface area, pore volume and pore size
increased with increase in Ce concentration from LDH-1
Fig. 3 TEM images and SAED patterns of synthesized Mg3FexCe1−x LDHs A LDH-1, B LDH-2, C LDH-3, D LDH-4, E LDH-5 and F LDH-6
1 3

MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous…
in Ce concentration for LDH-4 to LDH-6. Parent LDH-1
has less number of strong and weak basic sites than Ce
incorporated LDHs (LDH-2 and LDH-3). The increase in
basic properties and also the increase in surface area for Ce
incorporated LDHs (LDH-2 and LDH-3) can be correlated
to exchange of Fe with Ce having lower electronegetivity
[39].
and Oβ (Table S2). Parent compound (LDH-1) has surface
Oα concentration of 78.3% and increased with Ce load-
ing for materials having LDH structure intact (86.9% for
LDH-2 and 87.2% for LDH-3). Further increase in ceria
concentration (LDH-4 to LDH-6) led to marginal drop in
the concentration of surface Oα (85.2–81.6%) with rise in
Oβ (14.3–18.4%). Probably with increase in Ce concentra-
tion (LDH-4 to LDH-6) the distortion in the layer structure
occurred; which led to marginal decrease in surface Oα
group and relatively Oβ species are exposed to the sample
surface.
3.6 X‑ray Photoelectron Spectroscopy
The XPS spectra of Ce 3d and O1s have been analyzed
in detail (see Supporting Information pages S5–S7), in
order to understand the effect of Ce concentration on sur-
face active sites (–OH) of synthesized LDHs. Figure S2
shows the XPS spectra of Ce 3d for the synthesized LDHs
(LDH-2 to LDH-5). XPS spectra of the Ce 3d core level
can be resolved into 10 groups; and the representative
example of the fitting procedure for the Ce3d of LDH-6 is
presented in Fig. 4 [48]. The Ce 3d spectrum of all sam-
ples (LDH-2–LDH-6) indicated presence of mixed valence
states of Ce3+ and Ce4+. The peaks attributed to O 1s are
observed at 530.6 and 532.3 eV (Fig. S3). The first one,
with a very low intensity is characteristic of O2− (attributed
to carbonate species) designated as ‘‘Oβ”, whereas the high
intensity second peak corresponds to the oxygen species in
hydroxide form (–OH) designated as ‘‘Oα” [49]. The quan-
titative analysis of the Ce 3d and O1s XPS peaks for the
samples is summarized in Table S2 and includes percentage
concentrations of the Ce4+, Ce3+, Oα and Oβ species present
on the material surface. From the Table S2 it was clearly
observed that the gradual increase in Ce4+ concentration
has taken place from LDH-4 to LDH-6 (60.2–69.1%) with
increase in Ce concentration. This indicates that increase
in Ce concentration has led to increase in Ce4+ present on
the surface in the form of Ce(OH)4. The amount of Ce in
LDH structure also affects the concentration of surface Oα
The detailed characterization results demonstrate that
the structural, textural and chemical properties of the LDHs
can be fine-tuned by doping with appropriate amount
of another trivalent metal such as Ce. XRD, FT-IR and
TEM analysis (Figs. 1, 2, 3) clearly showed that LDH-1 to
LDH-3 formed well defined LDH structure with high crys-
tallinity (better ordering of brucite sheets). No other phase
was detected, implying that Ce was well incorporated in
the prepared LDHs. Among which LDH-3 showed high
surface area and pore volume with high amount of strong
basic sites present on LDH surface (Table 1). Further with
increase in Ce concentration (LDH-4 to LDH-6) distor-
tion in layer structure was observed from XRD, TEM and
XPS analysis. This may be due to the isomorphic substitu-
tion of Fe by large ionic radii cation Ce. In case of LDH-4
and LDH-5 precipitation of Ce(OH)4 on surface was clearly
observed and confirmed by XRD, TEM and FT-IR analysis.
LDH structure was totally absent for LDH-6 (Fe:Ce=0:1)
and mixed hydroxide, carbonate phases were observed.
These observations are found to be consistent with sur-
face area, pore volume and strong basic site densities
(hydroxyl groups present in the brucite-like structure) of
LDHs, which marginally decreased from LDH-4 to LDH-6.
Distortion in LDH structure results in the decrease in sur-
face area, pore volume and structure bonded –OH groups
(strong basic sites). According to literature reports, the
structure bonded –OH group are strong basic in nature than
simple metal bonded –OH groups (Mg(OH)2 or Ce(OH)4)
[46, 47]. The decrease in the intensity of FT-IR band at
3500 cm−1 (–OH) for LDH-4 to LDH-6 (Fig. 2); also sup-
ports this observation. Characterization of the catalysts
indicated that layered structure was intact till LDH-3 and
LDH-3 was found to be material with higher surface area,
pore volume and higher amount of strong basic sites.
3.7 Catalyst Activity
Ce promoted Mg3FexCe1−x LDHs were investigated
for the transesterification reaction and the results are
presented in Fig. 5. From the results, it was clearly
observed that catalyst activity trend is in the order of
LDH-6 < LDH-5 < LDH-1 < LDH-4 < LDH-2 < LDH-3.
Fig. 4 Representative example of the fitting procedure for the Ce 3d
peak of LDH-6
1 3

N. T. Nivangune et al.
Fig. 6 Effect of EC:MeOH molar ratio (A) and reaction temperature
(B) on the catalytic transesterification. Reaction conditions: A EC:
23 mmol, EC: MeOH: 1:5–1:20, catalyst: 2.5 wt% relative to EC,
reaction time: 3 h, temperature: 70 °C. B EC: 23 mmol, EC: MeOH:
1:10, catalyst: 2.5 wt% relative to EC, reaction time: 3 h, temperature:
30–70 °C
Fig. 5 Effect of Ce concentration in Mg3FexCe1−x LDHs towards the
transesterification reaction. Reaction conditions: EC: 23 mmol, EC:
MeOH molar ratio: 1:10, catalyst: 2.5 wt% relative to EC, reaction
time: 3 h, temperature: 70 ˚C
Catalyst activity increased with increase in Ce concentra-
tion up to a level and decreased with further increase in
Ce concentration. Among all synthesized catalysts LDH-3
showed maximum EC conversion (87%) and DMC selec-
tivity (100%). It should be noted that both binary systems
LDH-1 (Mg3:Fe) and LDH-6 (Mg3:Ce) were found to be
less active as compared to LDH-3 (EC conversion 62 and
26% with DMC selectivity 93 and 48% respectively).
Thus the activity observed was strongly dependent on the
Ce concentration in the synthesized LDHs. The observed
activity trend was found to be in good agreement with
physicochemical properties of synthesized LDHs. Thus,
EC conversion was lower with LDH-1 and increased with
Ce concentration (LDH-2<LDH-3). Best results were
obtained with LDH-3 (Mg3Fe0.85Ce0.15) having high-
est amount of strong base sites present on LDH surface
with high surface area and pore volume as compared to
other LDHs. With further increase in Ce concentration
(LDH-4–LDH-6) strong basic sites, surface area and pore
volume decreased and could be attributed to the forma-
tion and deposition of Ce(OH)4 phase and distorted nature
of layered structure of LDHs; which reduced the catalyst
activity. This is in accordance with the results obtained by
Kannan et al. for hydroxylation of phenol using CoNiAl
ternary LDH as catalyst [50]. They concluded that strong
basic sites (hydroxyl groups) play an important role in
catalytic activity and the appropriate geometry and con-
centration of both the metal cations in a well ordered two
dimensional lattice could be responsible for the activ-
ity observed. The most active catalyst (LDH-3) was taken
for further study. Effect of EC/CH3OH molar ratio on the
transesterification reaction was investigated using LDH-3
catalyst (Fig. 6A). From Fig. 6A it was clearly observed
that the EC/CH3OH feed molar ratio had a significant
impact on the transesterification reaction. Transesterifica-
tion of EC and methanol (EC + MeOH ⇌ DMC + EG) is
an equilibrium controlled reaction and hence excess meth-
anol is required to shift the equilibrium towards right and
achieve high selectivity to DMC as the product [16–19].
EC conversion (63–87%) and DMC selectivity (92–100%)
increased gradually with increase in the feed molar ratio
of EC /CH3OH from 1:5 to 1:10. Further increase in EC/
CH3OH ratio from 1:15 to 1:20 had marginal effect on
EC conversion (92–94%) with 100% selectivity to DMC.
The effect of reaction temperature (30–70 °C) also has a
significant impact on the activity and selectivity of the
reaction. Thus at 30 °C EC conversion of 18% with 43%
selectivity to DMC was observed. EC conversion and DMC
selectivity increased with increase in temperature and
51% conversion of EC with 66% selectivity to DMC was
observed at 50 °C. Low selectivity to DMC was because
of the formation of HEMC as major product. Results indi-
cate that higher temperature as well as higher EC:methanol
mole ratio are necessary for the conversion of intermedi-
ate HEMC to DMC. Zhang et al. [12] also have observed
similar results in the transesterification of EC and metha-
nol using carboxylic functionalized imidazolium salt as the
catalyst. Best results (87% EC conversion and 100% DMC
selectivity) were obtained at 70 °C. The effect of catalyst
loading is presented in Fig. 7. Activity was low at a catalyst
loading of 0.6 wt% and 72% conversion of EC with 93%
selectivity to DMC was observed. Activity and selectivity
to DMC increased with catalyst loading and 87% conver-
sion of EC with 100% selectivity to DMC was observed at
a catalyst loading of 2.5 wt%. The improved performance
1 3

MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous…
Fig. 7 Effect of catalyst loading on transesterification of EC with
methanol using LDH-3. Reaction conditions: EC: 23 mmol, EC:
Fig. 8 Effect of reaction time on EC conversion and DMC selectiv-
MeOH: 1:10, catalyst: 0.6–5 wt% relative to EC, reaction time: 3 h,
temperature: 70 °C
ity. Reaction conditions: EC: 23 mmol, EC: MeOH: 1:10, catalyst:
2.5 wt% relative to EC, reaction time: 0.5–5 h, temperature: 70 °C
observed with increase in catalyst loading can be attributed
to increase in basic sites available for the reaction. Fur-
ther increase in catalyst loading (2.5–5 wt%) had marginal
effect on EC conversion (87–89%) showing high selectivity
to DMC (100%). Catalyst loading of 2.5 wt% was taken as
optimum for this reaction and used for further study.
All the experiments in this study were carried out with
sampling in a time range of 1–5 h. However, since the
data is exhaustive; the detailed parametric study data with
results at different time intervals is presented in supporting
information (Figs. S4–S6).
selectivity followed in the order of La≈Ce>Sm>Y>Cr.
Among all LDHs Mg3Fe0.85Ce0.15 (LDH-3) showed best
activity and selectivity for this reaction. The results were
found to be consistent with the electro negativities of the
metal cations (1.1≈1.12<1.17<1.22<1.66 respectively).
Incorporation of third cation which has low electronega-
tivity results in the increase in basic property of LDH and
leads to better activity [37]. The difference in the elec-
tronegativities of La and Ce is very small and expectedly
the catalyst activities observed were comparable. Thus Ce
incorporated Mg3:Fe LDH was found to be best catalyst for
transesterification reaction.
Finally typical conversion/selectivity vs time plot under
optimized reaction conditions using LDH-3 catalyst is
presented in Fig. 8. EC conversion and DMC selectivity
increased, while HEMC selectivity decreased with increase
in reaction time. Maximum EC conversion (87%) and
DMC selectivity (100%) was observed at 3 h reaction time.
Further prolonging the reaction time had no positive effect
on EC conversion indicating that the reaction has reached
the equilibrium [17]. Lower selectivity of DMC observed
at intermediate time intervals is due to the formation of
HEMC as intermediate product.
3.8 Catalyst Stability
To check the stability of the catalyst recycle experiments
were carried out and the results are presented in Fig. 9. It
was observed that the catalyst retained its original activity
Table 2 Effect of different metal cation Mg3:Fe0.85:M0.15 for transes-
terification reaction of EC with MeOH
Catalyst
EC conversion (%)
DMC
After optimization of reaction conditions with
Mg3Fe0.85Ce0.15 (LDH-3) it was decided to investigate the
effect of incorporation of other trivalent metals (instead of
Ce) on the performance of the catalyst. For this purpose
LDHs were prepared by varying M(III) [where M(III)=La,
Sm, Y and Cr] by keeping the molar ratio of 3:0.85:0.15
constant and tested for transesterification of EC with meth-
anol under identical reaction conditions. From the Table 2
it was clearly observed that doping of different trivalent
metal cations in parent LDH-1 (Mg3:Fe) affected the EC
conversion and DMC selectivity significantly. Activity and
selectivity
(%)
Mg3:Fe1
62
85
87
81
67
56
94
99
100
95
82
78
Mg3:Fe0.85:La0.15
Mg3:Fe0.85:Ce0.15
Mg3:Fe0.85:Sm0.15
Mg3:Fe0.85:Y0.15
Mg3:Fe0.85:Cr0.15
Reaction conditions: EC: MeOH molar ratio: 1:10, catalyst: 2.5 wt%
relative to EC, reaction time: 3 h, temperature: 70 °C.
1 3

N. T. Nivangune et al.
layered double hydroxide catalyst occurs on the basal planes
of the outer crystal surface whereas the hydrolysis reaction
takes place on the crystal edges [51]. Additionally, Green-
well et al. have presented evidence form density functional
theory (DFT) calculations that surface bonded –OH anion
may directly participate in the catalytic reaction and act
more than the simple metal hydroxyl group [52]. In the
present work, significant improvement in EC conversion
was observed with incorporation of Ce in the Mg3-Fe LDH
(LDH-3). Detailed characterization of the catalyst indicated
the presence of more amount of active sites (-OH group)
on LDH surface with high surface area compared to parent
Mg3-Fe LDH (LDH-1). Taking into account the literature
reports and results observed in the present work a probable
mechanism has been proposed (scheme 2); where the –OH
species and metal cations present above the LDH surface
act as strong base and Lewis acid sites respectively.
Fig. 9 Catalyst
recycles
study,
reaction
conditions:
EC: 23 mmol, EC: MeOH: 1:10, catalyst: 2.5 wt% relative to EC,
reaction time: 3 h, temperature: 70 °C
The reaction begins with the abstraction of proton from
methanol on –OH sites of LDH to generate methoxy anion
and activation of carbonyl carbon of EC on Lewis acid sites
of LDHs to generate relatively charged carbonyl carbon
(step I). Generated methoxy anion attacks on the activated
carbonyl carbon of EC to form HEMC as the intermediate
product (step II). In next step (III) another methanol mol-
ecule will attack in similar fashion to form DMC and EG as
the final products and the catalyst is regenerated back (step
IV). The formation of HEMC as an intermediate product
was observed at low EC: CH3OH ratios (Fig. 6A) and at
low reaction temperature (Fig. 6B) leading to poor selectiv-
ity to the target DMC.
and selectivity for seven recycle experiments indicating
excellent stability. Further to check whether the catalyst
leached out in reaction mixture; reaction was carried out by
hot filtration and the results are summarized in the Fig. S7.
It was found that in the absence of the catalyst, there was
no further increase in the EC conversion, which indicated
that there was no leaching and transesterification is purely
a heterogeneously catalyzed reaction. At the end of the 7
recycle experiments the XRD analysis of the used catalyst
was carried out (Fig. S8). No change in the XRD pattern
was observed for used catalyst, indicating high stability of
the catalyst (LDH-3) towards transeserification reaction.
From the literature it was observed that mixed metal oxides
were active catalysts for DMC synthesis and high reaction
temperature (100–180 °C) and catalyst loading (10–25 wt%
of EC) was required in these cases [15–17]. The catalysts
were stable up to 4–5 recycle experiments; however acti-
vation of the recovered catalyst at higher temperature was
necessary before each reuse. Ternary Mg3Fe0.85Ce0.15
(LDH-3) catalyst developed in the present work does not
require any pre-treatment and could be recycled up to seven
times without loss in EC conversion and DMC selectivity
under mild reaction conditions.
4 Conclusions
Transesterification of EC with methanol to DMC was
investigated in detail using Mg3FexCe1−x ternary LDH as
the catalyst. The LDHs were synthesized by varying Fe:Ce
molar ratio in a range of 1:0–0:1. Both the end members
of this series Mg3Fe (LDH-1) and Mg3Ce (LDH-6) showed
lower catalytic activity and selectivity to DMC. The sig-
nificant increase in EC conversion and DMC selectivity
was observed with appropriate concentration of Ce pre-
sent in LDH structure. The activity varied in the order
LDH-6 < LDH-5 < LDH-1 < LDH-4 < LDH-2 < LDH-3.
Among the synthesized catalysts LDH-3 (Fe:Ce molar
ratio: 0.85:0.15) showed best catalytic performance (87%
EC conversion with 100% DMC selectivity) under mild
reaction conditions. LDH-3 was found truly heterogeneous
catalyst and was recycled seven times without loss in cata-
lytic activity and selectivity to DMC. The activity trend was
found to be in good agreement with structural and surface
basic properties of the synthesized LDHs. Various trivalent
metals were also used to modify the LDH with composition
3.9 Catalytic Mechanism
Mechanism of the transeterification of EC with methanol
has been discussed in several reports [11, 23, 25]. Hydroxy
groups (strong basic sites) present on LDH surface play
an important role in transesterification reaction which was
studied by many researchers using various spectroscopic
techniques. According to Roeffaers et al. transesterifica-
tion of 5-carboxyfluorescein with 1-butanol over [Li–Al]
1 3

MgFeCe Ternary Layered Double Hydroxide as Highly Efficient and Recyclable Heterogeneous…
Scheme 2 A possible mecha-
nism for the transesterification
of ethylene carbonate with
methanol catalyzed by LDH-3
8. Fukuoka S, Tojo M, Hachiya H, Aminaka M, Hasegawa K
Mg3Fe0.85M0.15 [where M(III)=La, Sm, Y and Cr] and the
activity trend followed in order of La≈Ce>Sm>Y>Cr.
The best results were obtained with Ce modified Mg3:Fe
LDH (Mg3Fe0.85Ce0.15) as the catalyst. The results were
found to be in good agreement with the electronegativi-
ties of incorporated third metal cations [M(III)]. To the
best of our knowledge this is the first report on the use of
Mg3Fe0.85Ce0.15 ternary LDH as a catalyst for this reaction.
(2007) Polym J 39:91
9. Fakhrnasova D, Chimento RJ, Medina F, Urakawa A (2015)
ACS Catal 5:6284
10. Du GF, Guo H, Wang Y, Li WJ, Shi WJ, Dai B (2015) JSaudi
Chem Soc 19:112
11. Yang ZZ, Dou XY, Chanfreau SB (2010) Tetrahedron Lett
51:2931
12. Zhang SJ, Wang JQ, Sun J, Cheng WG, Shi CY, Dong K, Zhang
XP (2012) Catal Sci Technol 2(5):600
13. Xu J, Wu HT, Ma CM, Xue B, Li YX, Cao Y (2013) Appl Catal
A 464:357
Acknowledgements Authors would like to acknowledge partial
financial support to the work from the project CSC0123 funded by
CSIR-Delhi, India. NTN would like to acknowledge CSIR-Delhi,
India for fellowship grant.
14. Kim DW, Lim DO, Cho DH, Koh JC, Park DW (2011) Catal
Today 164:556
15. Bhanage BM, Fujita SI, Ikushima Y, Arai M (2001) Appl Catal
A 219:259
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19. Sankar M, Satav S, Manikandan P (2010) ChemSusChem 3:575
20. Bhanage BM, Fujita SI, Ikushima Y, Torii K, Arai M (2003)
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1 3