Solvent free transesterification of glycerol into glycerol carbonate over nanostructured CaAl hydrotalcite catalyst

Article abstract of DOI:10.1166/jnn.2018.15265

Drastic increase in green house gases due to fossil fuels usage urges the mankind to look for alternative fuel resources. Biodiesel is one of the alternative fuels which attracted the attention of many researchers. In recent years, bio-diesel drags much attention as an alternative clean fuel. Glycerol is an unavoidable byproduct in the transesterification process of vegetable oils into bio diesel and therefore market is flooded with glycerol. So it is high time to find ways of utilizing the abundant glycerol into value added products. Herein we report the catalytic transesterification of glycerol using dimethyl carbonate over MgAl-hydrotalcite (MgAl-HT), CaAl-hydrotalcite (CaAl-HT) and nano structured CaAl-HT catalysts. All the catalysts were characterized by XRD, FT-IR, TPDCO₂, BET, SEM and HR-TEM techniques. Among them Ca4Al-HT was found to be best in terms of conversion of glycerol (82.4%) and selectivity (95.9%) towards glycerol carbonate. The effect of CTAB template concentration in the nano synthesis of Ca₄Al-HT on conversion and selectivity was studied and Ca₄Al-HT synthesized with 0.4 moles of CTAB showed the best conversion of glycerol (98.7%) and the highest selectivity towards glycerol carbonate (97.9%). The recyclability test performed with the best catalyst showed that the catalyst was recyclable even after 5 cycles. Valorization of glycerol yields glycerol carbonate (GC) which is a very good polar solvent with high boiling point, building block in several organic syntheses and used in the production of surfactants, poly urethanes etc.

Full text of DOI:10.1166/jnn.2018.15265

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 4588–4599, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Solvent Free Transesterification of Glycerol Into
Glycerol Carbonate Over Nanostructured CaAl
Hydrotalcite Catalyst
Arulselvan Devarajan, Sivakumar Thiripuranthagan∗,
Ramakrishnan Radhakrishnan, and Sakthivel Kumaravel
Catalysis Laboratory, Department of Applied Science and Technology, Alagappa College of Technology,
Anna University, Chennai 600025, Tamilnadu, India
Drastic increase in green house gases due to fossil fuels usage urges the mankind to look for
alternative fuel resources. Biodiesel is one of the alternative fuels which attracted the attention of
many researchers. In recent years, bio-diesel drags much attention as an alternative clean fuel.
Glycerol is an unavoidable byproduct in the transesterification process of vegetable oils into bio
diesel and therefore market is flooded with glycerol. So it is high time to find ways of utilizing the
abundant glycerol into value added products. Herein we report the catalytic transesterification of
glycerol using dimethyl carbonate over MgAl-hydrotalcite (MgAl-HT), CaAl-hydrotalcite (CaAl-HT)
and nano structured CaAl-HT catalysts. All the catalysts were characterized by XRD, FT-IR, TPD-
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
CO2, BET, SEM and HR-TEM techniques. Among them Ca4Al-HT was found to be best in terms
Copyright: American Scientific Publishers
of conversion of glycerol (82.4%) and selectivity (95.9%) towards glycerol carbonate. The effect
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of CTAB template concentration in the nano synthesis of Ca4Al-HT on conversion and selectivity
was studied and Ca4Al-HT synthesized with 0.4 moles of CTAB showed the best conversion of
glycerol (98.7%) and the highest selectivity towards glycerol carbonate (97.9%). The recyclability
test performed with the best catalyst showed that the catalyst was recyclable even after 5 cycles.
Valorization of glycerol yields glycerol carbonate (GC) which is a very good polar solvent with high
boiling point, building block in several organic syntheses and used in the production of surfactants,
poly urethanes etc.
Keywords: Glycerol, Transesterification, Glycerol Carbonate, Solid Base Catalyst.
1. INTRODUCTION
oils and animal fat by transesterification process leads to
the formation of glycerol as an unavoidable by-product.
37 billion gallons of biodiesel was produced world wide
by 2016 which implies the production of approximately
4 billion gallons of glycerol.7 Glycerol is also obtained
by alcoholysis, hydrolytic cleavage or saponification with
alkalies of several glycerides.8–10 Henceforth, there is a
surplus availability of glycerol.11–13 The excess availabil-
ity of glycerol makes it as an affordable and adaptable
building block chemical. The accumulation of crude glyc-
erol creates economic and environmental problems.14ꢀ15
Glycerol has a broad spectrum of usage in various
fields such as food, cosmetic, detergent, pharmaceuti-
cal, explosive etc., and can also be converted into value
added chemicals such as esters, ethers, acetals, ketals,
diols, epoxides, polyethers, polyols, triacetin, alkyd resins,
The growing demand of energy due to increase in popu-
lation, industrial developments and motorization have led
to a steep depletion of conventional fossil fuels such as
petroleum oil, coal and natural gas. The burning of these
fossil fuels also affects the environment badly and is
the key factor for the present global warming crises.1–3
Hence the field of alternate fuels has gained much interest
over the last two decades and there has been much research
on the production of new, environmentally friendly fuels.
Bio-diesel, an exceptional alternative fuel can replace
the primary energy sources like fossil fuels and in turn
will reduce the green house gas emissions.4–6 Methyl
esters of fatty acids (biodiesel) produced from vegetable
∗Author to whom correspondence should be addressed.
4588
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doi:10.1166/jnn.2018.15265

Arulselvan et al.
Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
etc.,16–18 Despite the multi-dimensional usage of glycerol
its surplus production urges the scientific researchers to
find new applications for glycerol.
ZnO–La2O3 and ionic liquids.40–45 Despite the advantages
of heterogeneous catalysts the above catalysts have certain
limitations such as poor reusability, requiring high reaction
temperature, prolonged reaction time and high molar ratio
of reactants.
Hence in this article we report the transesterification
process of glycerol to GC in presence of DMC under
solvent free conditions using solid base catalysts such
as MgxAl-HT and CaxAl-HT (x = 2, 3 and 4). Between
these catalysts, Ca4Al-HT was found to be better in terms
of conversion and selectivity. Ca4Al-HT nano catalyst
were synthesized with different molar concentration of
the surfactant (CTAB) and among the catalysts Ca4Al-HT
0.4CTAB was found to be the best as it converts 98.7%
of glycerol with high selectivity (97.9%) towards glycerol
carbonate.
Among the various technologies for glycerol conver-
sion, conversion through catalytic route drawn much inter-
est and attention.19ꢀ20 Catalytic conversion of glycerol to
value-added products has been done using either homo-
geneous catalysts or heterogeneous catalysts using vari-
ous reactions such as oxidation, reduction, esterification,
hydrogenolysis, etherification, polymerization, halogena-
tion, dehydration and reforming etc.,21–24 In this context,
catalytic transesterification of glycerol to glycerol carbon-
ate (GC) is also considered as an industrially important
reaction. GC has received much attention in the last two
decades due to versatile reactivity.25 The advantageous
properties such as low toxicity, excellent biodegradabil-
ity and high boiling point make this compound as a very
good intermediate in several organic syntheses, precursor
in biomedical applications, important component in mem-
brane used for gas separation and also in the production
of poly urethanes, surfactants, high boiling polar solvents
etc.8 The –OH and 2-oxo-1,3-dioxolane groups present
in GC makes this molecule as a highly reactive one and
therefore finds application in different domains such as
solvents, polymers etc. GC readily reacts with many nucle-
ophilic reagents such amines etc. The reaction with amino
silanes yield highly branched siloxy polymers.26
2. EXPERIMENTAL SECTION
2.1. Materials
Mg(NO3ꢂ2 · 6H2O and Al(NO3ꢂ3 · 9H2O were purchased
from Sigma and used as received for the synthesis of Mg-
Al-HT, whereas Ca(NO3ꢂ2 ·5H2O (Merck) and Al(NO3ꢂ3 ·
9H2O (Sigma) were used as received for the synthesis of
CaAl-HT catalysts. NaOH and Na2CO3 purchased from
SRL and were used as received. The surfactant Cetyl
trimethylammonium bromide (CTAB) was purchased from
Sigma aldrich and used as received.
Hydrotalcite is the well known layered material and
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
Copyright: American Scientific Publishers
finds applications as absorbents, ion exchangers, catalyst,
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2.1.1. Synthesis of MgAl-HT and CaAl-HT Catalysts
fire retardant additives, polymer composites, controlled
drug delivery, cement additives, etc.27 Owing to its peculiar
tunable properties (ion exchange, intercalation), structural
stability at harsh conditions and cheaper availability, it has
been used as a commercial solid base catalyst to carryout
various industrially important reactions.28–30 Hydrotalcite
Magnesium–Aluminum hydrotalcite (MgAl-HT) with dif-
ferent Mg/Al molar ratios (2, 3 and 4) were synthesized
by direct precipitation method. Solution A was prepared
by dissolving 25.6 gm of Mg(NO3ꢂ2 ·6·H2O and 18.6 gm
of Al(NO3ꢂ3 ·9H2O in 70 ml deionised water (Mg/Al = 2).
Solution B was prepared by dissolving sodium hydroxide
and sodium carbonate of appropriate concentration. Then
solution B was added slowly into solution A and stirred
while maintaining the pH between 11 and 13. After 6 h,
the resulting gel was stirred and transferred into an auto-
clave and crystallized at 393 K for 18 h. Then the resultant
mass was filtered, washed several times with hot water
until the washings showed neutral pH. The samples were
dried in an air oven at 393 K and calcined at 823 K for
8 h to get Mg2Al-HT catalyst. Adopting similar procedure
MgxAl-HT (x = 3 and 4) catalysts were also prepared. For
the synthesis of Calcium–Aluminum hydrotalcite (CaAl-
HT) catalysts Ca(NO3ꢂ2 ·6·H2O and Al(NO3ꢂ3 ·9H2O were
used. The same procedure described above was used to
prepare CaxAl-HT (x = 2, 3 and 4) catalysts.
belongs to the class of anionic clay type with general for-
m+
mula of [M2+ M3+(OH)2ꢁn+mꢂ
]
[Am/n]
x−. YH2O, where
M2+(Mg, Ca, Zn, Ni, Cu, Co), M3+(Al, Cr, Ga, Fe, Mn,
etc.) and Ax− is a charge compensating anion (CO23−, Cl−,
SO34−, etc.). The anions in the interlayer compensate the
positive charges arise due to the divalent and trivalent
ions. In the present study transesterification of glycerol has
been carried out using MgxAl-HT and CaxAl-HT (x = 2,
3 and 4).
GC from glycerol can be obtained by various methods
such as carbonylation with phosgene or CO,31ꢀ32 Glycerol-
ysis of urea with Lewis acid catalyst,33ꢀ34 reaction under
35ꢀ36
super-critical conditions in presence of CO2
and using
homogeneous catalysts such as NaOH, K2CO3.37ꢀ38 How-
ever these reactions have serious drawbacks like toxicity,
low product yield and tedious catalyst–product separation.
All these drawbacks will be overcome through hetero-
geneous catalytic route.39 Transesterification of glycerol
to GC in presence of dimethyl carbonate (DMC) has
also been reported by using heterogeneous catalysts such
as CaO, CaO–La2O3, KF-Hydroxyapatite, Ca2CO3–MgO,
2.1.2. Synthesis of Nano Ca4Al-HT Catalysts
In order to improve the catalytic activity of Ca4Al-HT cat-
alyst, nano sized Ca4Al-HT catalysts were prepared by
using CTAB surfactant. Calculated amount of CTAB in
100 ml of distilled water were taken and added to the
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Arulselvan et al.
white sol containing precursors of Ca and Al to prepare
Ca4Al-HT yCTAB catalysts where y is the molar concen-
tration of CTAB = 0.1, 0.2, 0.4 and 0.6. Same procedure as
described in Section 2.1.1 was followed for the synthesis
of nano hydrotalcites.
The total basicity of the hydrotalcite catalysts was deter-
mined by Temperature programmed desorption of CO2
(CO2-TPD) experiments. Prior to the analysis, 100 mg of
catalyst was pre-treated at 773 K for 1 h under He stream
(20 mL/min), cooled to 363 K and CO2 (10 vol.%) was
introduced for adsorption at this temperature for 1 h. Then
the catalyst was swept with He (20 ml/min) for 60 min to
remove the physisorbed CO2 from the catalyst surface, the
temperature was increased linearly with rate of 10 K/min
in He (10 ml/min) and the signal of CO2 (M/e = 44) was
recorded by online mass spectrometry. The amount of CO2
was quantified by a calibration curve constructed by using
known amounts of NaHCO3.
2.2. Characterization of the Catalysts
X-ray diffraction patterns of the MgAl-HT and CaAl-HT
catalysts were recorded on a Philips X-pert diffractometer
with CuKꢃ radiation (ꢄ = 1.54056 Å) at operating voltage
of 30 kv, 10 mA. The data were collected in the 2ꢅ range
from 5ꢀ to 80ꢀ at the scan rate of 0.04ꢀ/min. The peak
processing was performed using origin 8 software and was
indexed in the hexagonal system on the rhombohedral lat-
tice with the space group of R-3m (166). The standards
JCPDS card No. 89-0460 was used for comparison. The
interplanar spacing (d300ꢂ value was used for the identifi-
cation of the basal plane. The lattice parameters a, c and
V were determined by using the following equation:
2.3. Catalytic Experiments and Analysis Method
The liquid phase of transesterification of glycerol
(Sigma, 99%) with dimethyl carbonate (DMC, SRL,
99.5%) was investigated at atmospheric pressure under sol-
vent free conditions. For the catalytic reaction, glycerol
(0.04 mol) and dimethyl carbonate (0.16 mol) were taken
in 100 ml round bottom flask fitted with stirrer and reflux
condenser. The temperature of the reaction was maintained
by the heating coil immersed in silicon oil integrated with
the thermocouple and control unit. When the reaction tem-
perature was reached, catalyst of 100 mg was added and
stirred. After 2 h of reaction time, sample aliquots were
withdrawn centrifuged and the catalysts were separated
1
d2
4 h2+hk+k2 l2
=
+
c2
3
a2
Where d—interplanar spacing, hkl are the corresponding
miller planes, a and c are the lattice parameters. The unit
cell volume V (Å3) was calculated using,
V = ꢁa2cꢂsin 60ꢀ
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to obtain a homogeneous transparent solution. The liquid
Fourier Infrared spectra (FT-IR) were recorded on a
Copyright: American Scientific Publishers
products namely glycerol carbonate with unreacted DMC
and glycerol in the reaction medium was analyzed by gas
chromatograph (SHIMADZU 17A) equipped with flame
ionization detector (FID) using a Zebron-FFAP capillary
column (30 m × 0ꢆ32 mm × 0ꢆ25 ꢇm). Conversion and
selectivity values of the transesterification catalytic tests
were calculated according to the following equations:
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Thermo-Nicolet IR 2000 spectrophotometer in order to
determine the various functional groups. The spectra were
recorded in the wavenumber range of 4000–400 cm−1
.
Nitrogen adsorption–desorption isotherms for the calcined
catalysts were constructed at 77 K using a constant vol-
ume adsorption apparatus (Micrometrics ASAP 2020) for
the determination of BET surface area and other texture
characteristics. Prior to the analysis, hydrotalcite samples
were degassed and pretreated at 583 K for 6 h. BJH pore
size distribution was also calculated from the desorption
branch of the isotherms.
moles of glycerol converted
Molar Conversion ꢁ%ꢂ =
moles of glycerol taken
×100
Quantitative determination of Ca, Mg and Al ions was
done by inductively coupled plasma (ICP) technique using
Perkin Elmer Optima 5300 DV. 200 mg of samples were
digested in a mixture of concentrated acids for 6 h, diluted
and filtered through syringe filter prior to the analysis.
Catalyst microstructure observations were performed on
a Quanta 200 FEG scanning electron microscope (HR-
SEM). Prior to the analysis, powdered samples were dried
at 373 K for 6 h, air dispersed over the carbon tape
and mounted for analysis. The high resolution transmis-
sion electron micrographs (HR-TEM) were recorded using
JEOL JEM-2100JF working at the operating voltage of
200 keV. Powder samples were dispersed in absolute
ethanol, ultrasonicated for 10 minutes, placed over holey
carbon coated Cu grid, dried at ambient conditions and
mounted for the analysis.
Glycerol carbonate selectivity ꢁ%ꢂ
moles of glycerol converted to glycerol carbonate
=
moles of converted glycerol
×100
3. RESULTS AND DISCUSSION
3.1. Powder X-ray Diffraction
The phase formation and crystallinity of the synthe-
sized hydrotalcites were determined by the X-ray pow-
der diffraction patterns. Figures 1 and 2 show the X-ray
diffraction patterns of MgAl-HT and CaAl-HT with Ca/Al
and Mg/Al molar ratios 2, 3 and 4.
Both the MgAl-HT and CaAl-HT catalysts show char-
acteristic peaks in the 2ꢅ range of 10–80. The peaks and
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Arulselvan et al.
Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
and V ) calculated from the diffraction patterns are given
in Table I.
The composition considered for the synthesis is retained
in the final products on both the MgAl-HT and CaAl-
HT catalysts. Both the MgAl-HT and CaAl-HT catalysts
show increased basal spacing (d300ꢂ with the increase in
the Mg/Al and Ca/Al ratios. This is in good agreement
with previous reports.48 The reason is apparent as the ionic
radii of Ca2+ (0.99 Å) and Mg2+ (0.66 Å) are different.
With increases in either Mg or Ca in the hydrotalcite the
intensity of 2ꢅ peak corresponding to (003) plan decreases
and shifts towards lower angle. The crystallite size of both
MgAl-HT and CaAl-HT catalysts were also increased with
the increasing d300
.
The effect of CTAB as template on the nano synthesis
of hydrotalcite was studied using powder X-ray diffrac-
tion. Figure 3 shows the X-ray diffraction of Ca4Al-HT
prepared with different molar ratios (0.1, 0.2, 0.4 and 0.6)
of CTAB.
Figure 1. High angle X-ray diffraction patterns of MgxAl-HT hydro-
talcites (a) Mg2Al-HT, (b) Mg3Al-HT and (c) Mg4Al-HT catalysts.
With the increase of CTAB from 0.1 to 0.6, the d300
value increased indicating the increase in the interpla-
nar spacing. This is further confirmed by the increased
lattice parameters. The lattice parameters a and c were
also increased. Particularly the increase along the c-axis
was found to be high i.e., from 23.457 to 25.497 Å.
Moreover the peaks corresponding to 003 and 006 planes
showed slight shifting towards the lower angle. However
the increase of lattice parameter along the a-axis was
their corresponding Miller planes at the corresponding
2ꢅ at 11.64(003), 23.420(006), 34.885(012), 35.449(009),
39.444(015), 46.922(018), 53.086(1010), 60.766(110),
60.928(0015) and 62.110(113) were indexed to a hexag-
onal lattice with rhombohedral symmetry with the space
group of R-3m (166). These peaks are in good in compar-
ison with the standards JCPDS card No. 89-0460.46 The
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Copyright: American Scientific Publishers
found to be minimum i.e., from 3.10 to 3.22 Å. Since both
peaks are sharp and intense indicating high degree of crys-
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the lattice parameter a and c increase the unit cell vol-
tallinity and are also in good agreement with the reported
literatures.47 The peaks with the Miller planes of (003),
(006), (009) and (0015) correspond to the basal planes
while the other are assigned to the non-basal planes. The
molar ratios, composition, calculated values of inter planar
spacing (d300ꢂ, crystallite size and lattice parameters (a, c
ume also increases to 223.29 Å (Table II). The template
CTAB was responsible for the increased interlayer spac-
ing in the formation of hydrotalcites. It is also to be noted
that increase in the molar concentration of CTAB up to
0.6 there was no change in the diffraction patterns, indicat-
ing the retaining of hydrotalcite structure. The results were
further authenticated by SEM and HR-TEM analyses.
3.2. FT-IR Spectral Analysis
The FT-IR spectra of the MgAl-HT and CaAl-HT catalysts
with molar ratios of 2, 3 and 4 are shown in Figures 4
and 5.
The broad peak observed in the region from 3800 to
3200 cm−1 is mainly due to the stretching mode of –OH
(hydroxyls) groups of water molecules. Thus the bands
at 3464 cm−1 in MgAl-HT and 3482 cm−1 in CaAl-HT
are due to the stretching mode of the water molecules.49
The band at 1636 cm−1 arises due to the bending mode
of the –OH group. It is evident that, with the increase in
the molar ratios in MgAl-HT and CaAl-HT catalysts, the
–OH stretching frequency also increase but with decreased
intensity. This may be due to the weaker interaction
between the hydroxyl layer and the carbonate anions. The
carbonate anion gives band at 1384 cm−1, 863 cm−1 and
668 cm−1 due to the stretching mode of the CO23− 50
.
Figure 2. High angle X-ray diffraction patterns of Cax Al-HT hydrotal-
cites (a) Ca2Al-HT, (b) Ca3Al-HT and (c) Ca4Al-HT catalysts.
Thus the main peaks at 1384 cm−1 for MgAl-HT and
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Arulselvan et al.
Table I. Physico-chemical properties of MgAl-HT and CaAl-HT catalysts.
Molar ratioa
x
db
Xsc
Lattice parametersd
SBET
V P
dP
Adsorbed CO2
(mmol/g)
Catalysts
Mt
Ma
Al/Al+M2+
(Å)
(nm)
a (Å)
c (Å)
V (Å3)
(m2/g)
(cm3/g)
(nm)
Mg2Al-HT
Mg3Al-HT
Mg4Al-HT
Ca2Al-HT
Ca3Al-HT
Ca4Al-HT
2
3
4
2
3
4
1.97
2.98
3.94
1.92
2.97
3.90
0.34
0.25
0.22
0.35
0.25
0.21
7.72
7.80
7.85
7.89
7.96
8.06
23.1
30.7
34.7
28.4
39.9
43.2
3.04
3.04
3.06
3.10
3.12
3.13
23.16
23.40
23.55
23.67
23.88
24.18
185.4
187.3
190.9
196.9
201.3
205.2
166
184
192
180
195
234
0.24
0.33
0.41
0.27
0.38
0.42
7ꢆ6
9ꢆ6
12ꢆ2
8ꢆ8
10ꢆ1
12ꢆ6
0.24
0.28
0.34
0.30
0.45
0.57
Notes: aM2+/Al ratio as determined by ICP. bInter planar spacing calculated from XRD pattern of (003) plane. cCrystallite size calculated from Scherrer equation. dLattice
parameter calculated from XRD powder pattern of (003) plane and (110) plane.
1378 cm−1 for CaAl-HT are assigned to the carbonate
anion within the interlayer of the hydrotalcite. A weak
band at 834 cm−1 may be due to the nitrate anion within
the interlayer. The peaks below 1000 cm−1 arise mainly
to 3499 cm−1 indicating the increase in the interlayer
distance.
3.3. N2 Adsorption Isotherm
due to the crystal lattice vibrations. Thus peak at 458 cm−1
,
The N2 adsorption and desorption isotherms of the MgAl-
HT and CaAl-HT catalysts with molar ratios of 2, 3 and 4
are shown in Figure 7.
All the catalysts exhibit type IV isotherm with H3 hys-
teresis loop at relatively high relative pressure (0.6–0.9)
which indicates, that the hydrotalcite prefers the layered
and plate like formations.55 On the basis of the IUPAC
classification, N2 adsorption–desorption isotherm which is
typical to mesoporous materials was obtained. Hence the
563 cm−1, 663 cm−1 and 1050 cm−1 are assigned to
the Mg–O–Al bending, Mg–O–H stretching, Mg/Al–O–H
stretching and Al–O stretching vibration respectively.51ꢀ52
The peak around 2930 cm−1 arises due to the interactions
between the –OH and CO23− within the interlayer.53
Figure 6 Shows the FT-IR spectra of the Ca4Al-HT cata-
lysts prepared with different molar concentration of CTAB.
The FTIR peaks of Ca4Al-HT prepared by using CTAB
surfactants getting sharper with increase in the molar
N2 uptake forms a hysteresis loop at relatively high pres-
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sure from 0.6, which indicates the formation of easily
concentration of CTAB. Sharpness of these peaks indi-
Copyright: American Scientific Publishers
54
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accessible mesopores.
cates the formation of nanosized hydrotalcite sheets.
The broadness of the –OH stretching peaks in the region
of 3200 cm−1 to 3400 cm−1 is not only decreased but
also shifts to the higher frequency from 3432 cm−1
The surface area, pore volume and pore diameter of the
MgAl-HT and CaAl-HT catalysts are given in Table I.
It is evident that with the increase in Ca and Mg molar
ratios, the surface area was found to increase which may
be due to the incorporation of Ca and Mg within the inter-
layer. The pore volume and the pore diameter were also
increased with the increase in the Mg and Ca molar ratios.
These results are in good agreement with the reported
literatures.56
Figure 8 shows the N2 adsorption–desorption isotherm
of Ca4Al-HT catalysts synthesized using CTAB template
with different molar concentrations.
The CTAB modified Ca4Al-HT catalysts shows simi-
lar hysteresis loop at higher partial pressure indicating the
formation of layered structure with porosity. It is evident
that with increase in CTAB concentration, the surface area
was found to increase upto the CTAB molar concentra-
tion of 0.4 and further increase in CTAB concentration,
it decreased. The increase in surface area may be due to
the CTAB incorporation which exfoliates laterally form-
ing sheet like morphology whereas higher CTAB molar
concentration leads to the formation of nano sized porous
hydrotalcite with porous formation. As higher CTAB con-
centration of 0.6 decreases the surface, 0.4 was found to
be the optimum CTAB concentration for the formation of
nano structured hydrotalcite. Similarly the pore volume
Figure 3. High angle X-ray diffraction patterns of Ca4Al-HT hydrotal-
cite prepared with various CTAB concentrations (a) Ca4Al-HT 0.1CTAB,
(b) Ca4Al-HT 0.2CTAB, (c) Ca4Al-HT 0.4CTAB and (d) Ca4Al-HT
0.6CTAB catalysts.
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Table II. Texture parameters of Ca4Al-HT with different molar concentration of CTAB.
da
Xsb
Lattice parametersc
SBET
V P
dP
Adsorbed CO2
(mmol/gm)
Catalyst
(Å)
(nm)
a (Å)
c (Å)
V (Å3)
(m2/g)
(cm3/g)
(nm)
Ca4Al-HT-0.1CTAB
Ca4Al-HT-0.2CTAB
Ca4Al-HT-0.4CTAB
Ca4Al-HT-0.6CTAB
7.819
7.995
8.499
8.222
38.7
42.5
45.8
48.1
3.10
3.15
3.18
3.22
23.457
23.985
25.497
24.666
195.22
206.10
223.29
221.48
214
253
312
276
0.37
0.42
0.48
0.45
10.3
13.2
14.1
14.2
0.55
0.72
0.86
0.67
Notes: aInter planar spacing calculated from XRD pattern of (003) plane. bCrystallite size calculated from Scherrer equation. cLattice parameter calculated from XRD
powder pattern of (003) plane and (110) plane.
and pore diameter was found to increase with the incorpo-
ration of CTAB (Table II).
comparison was made between CaxAl-HT (x = 2, 3 and
4) and CaxAl-HT yCTAB (y = 0.1, 0.2, 0.4 and 0.6)
catalysts, desorption of CO2 took place at higher tem-
perature of the hydrotalcites prepared with surfactants
3.4. Temperature Programmed Desorption of
CO2(TPD-CO2)
The basic sites were estimated from the adsorbed state of
CO2 with the basic sites of hydrotalcites. The CO2-TPD
profile of the CaAl-HT and MgAl-HT catalysts are shown
in Figures 9 and 10. Desorption of CO2 was monitored
over a temperature range of 373 K to 773 K.
Table I shows the total amount of desorbed CO2 with
increase in temperature. With the increase of Ca and Mg
molar ratios, the CO2 desorption temperature was found
to increase. In the case of CaAl-HT catalysts, desorption
temperature from 463 K to 470 K. whereas for Mg4Al-
HT it increases from 452 K to 473 K. Higher desorption
temperature coupled with increaIsPe:i9n3i.n1t7e9n.s9it0y.2in4d8icOante:sTue, 20 Mar 2018 09:58:09
Copyright: American Scientific Publishers
stronger and higher basicity. Ca4Al-HT and Mg4Al-HT
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catalysts show the highest basicity among the correspond-
ing hydrotalcites. The order of basicity as determined by
CO2 desorption is as follows: Ca4Al-HT > Ca3Al-HT >
Mg4Al-HT > Ca2Al-HT > Mg3Al-HT > Mg2Al-HT.
Figure 11 shows the CO2-TPD of Ca4Al-HT catalysts
synthesized using different molar concentration of CTAB.
Here also the desorption temperature increased from
465 K to 479 K indicating stronger basicity. When
Figure 5. FT-IR spectra of CaxAl-HT hydrotalcites (a) Ca2Al-HT,
(b) Ca3Al-HT and (c) Ca4Al-HT catalysts.
Figure 6. FT-IR spectra of Ca4Al-HT hydrotalcite prepared with var-
ious CTAB concentrations (a) Ca4Al-HT 0.1CTAB, (b) Ca4Al-HT
0.2CTAB, (c) Ca4Al-HT 0.4CTAB and (d) Ca4Al-HT 0.6CTAB catalysts.
Figure 4. FT-IR spectra of MgxAl-HT hydrotalcites (a) Mg2Al-HT,
(b) Mg3Al-HT and (c) Mg4Al-HT catalysts.
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Arulselvan et al.
Figure 9. CO2-TPD profile of MgxAl-HT hydrotalcites (a) Mg2Al-HT,
(b) Mg3Al-HT and (c) Mg4Al-HT catalysts.
Figure 7. Adsorption–desorption isotherm of MgxAl-HT and CaxAl-
HT hydrotalcites (a) Mg2Al-HT, (b) Mg3Al-HT (c) Mg4Al-HT (d) Ca2Al-
HT, (e) Ca3Al-HT and (f) Ca4Al-HT catalysts.
(CTAB). Moreover the basicity of CaxAl-HT yCTAB cat-
alysts was also found to be on the higher side i.e., Ca4Al-
HT adsorbed 0.57 mmol of CO2/g whereas Ca4Al-HT
yCTAB (y = 0.1, 0.2, 0.4 and 0.6) catalysts adsorbed 0.55
to 0.86 mmol of CO2/g. The higher basicity of Ca4Al-HT
yCTAB (y = 0.1, 0.2, 0.4 and 0.6) catalysts is due to the
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
higher access of more basic sites created by CTAB tem-
Copyright: American Scientific Publishers
plate as shown in Table II. This result is further authen-
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ticated by the increase in lattice parameters observed in
the XRD powder patterns and increase in surface area
observed in BET adsorption–desorption isotherms.
Figure 10. CO2-TPD profile of CaxAl-HT hydrotalcites (a) Ca2Al-HT,
(b) Ca3Al-HT and (c) Ca4Al-HT catalysts.
3.5. Inductive Coupled Plasma (ICP) Analysis
Table I also contains composition and Mg/Al and Ca/Al
molar ratios of the MgAl-HT and CaAl-HT hydrotalcites.
The composition and molar ratio considered for the
synthesis of MgAl-HT and CaAl-HT were retained in the
solid products which are comparable to the theoretical
values.
Figure 8. Adsorption–desorption isotherm of Ca4Al-HT hydrotalcite
prepared with various CTAB concentrations (a) Ca4Al-HT (b) Ca4Al-HT
0.1CTAB, (c) Ca4Al-HT 0.2CTAB, (d) Ca4Al-HT 0.4CTAB and
(e) Ca4Al-HT 0.6CTAB catalysts.
Figure 11. CO2-TPD profile of Ca4Al-HT hydrotalcites prepared with
various CTAB concentrations (a) Ca4Al-HT 0.1CTAB, (b) Ca4Al-HT
0.2CTAB, (c) Ca4Al-HT 0.4CTAB and (d) Ca4Al-HT 0.6CTAB catalysts.
4594
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Arulselvan et al.
Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Figure 12. SEM micrographs of (a) Mg4Al-HT, low magnification (b) Ca4Al-HT, low magnification (c) Mg4Al-HT, high magnification and (d) Ca4Al-
HT, high magnification.
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
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Figure 13. SEM micrographs of Ca4Al-HT, prepared by using different CTAB molar concentration (a) 0.1, (b) 0.2 (c) 0.4 and (d) 0.6.
J. Nanosci. Nanotechnol. 18, 4588–4599, 2018
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Arulselvan et al.
3.6. Scanning Electron Microscope (HR-SEM)
The morphology of the Mg4Al-HT and Ca4Al-HT are
shown through SEM images (Fig. 12).
and much finer particles in nano size were formed. The
individual nano particles of the Ca4Al-HT-0.4CTAB roll
together with the formation of bundle like nano struc-
tures (Fig. 14(c)). At higher magnification of 20 nm the
Ca4Al-HT-0.4CTAB catalyst clearly shows the particles
with 10–20 nm size. These results further authenticate that
Ca4Al-HT-0.4CTAB are nano sized and confirms with the
XRD powder data and SEM results.
Both Mg4Al-HT (Fig. 12(a)) and Ca4Al-HT (Fig. 12(b))
have fine plate morphology which is further confirmed
at higher magnification (Figs. 12(c and d)) and both are
agglomerated. Similarly the Figure 13 shows SEM images
of Ca4Al-HT prepared using CTAB as template. It is evi-
dent that CTAB is responsible for the formation of nano
structures in the hydrotalcite catalysts. The particle sizes
became smaller and smaller when the CTAB concentration
was increased. (Figs. 13(c and d)).
3.8. Catalytic Studies on Transesterification of
Glycerol with DMC
3.8.1. Effect of Reaction Temperature—Optimization
Studies
3.7. High Resolution Transmission Electron
The effect of reaction temperature on the catalytic transes-
terification of glycerol with dimethyl carbonate was inves-
tigated using batch reactor as shown in Scheme 1.
All the catalysts viz., Mg2Al-HT, Mg3Al-HT, Mg4Al-
HT, Ca2Al-HT, Ca3Al-HT and Ca4Al-HT were evaluated
for their catalytic activity in the temperature range of
333–373 K. The reaction was conducted for 2 h using 1:4
molar ratio of glycerol: DMC and the results are given in
Table III.
Microscope (HR-TEM)
The HR TEM images of Ca4Al-HT hydrotalcites prepared
with and without using CTAB template (0.4 molar con-
centrations) are shown in Figure 14.
The catalyst synthesized using CTAB with 0.4 molar
concentration is designated as Ca4Al-HT-0.4CTAB. The
Ca4Al-HT hydrotalcite synthesized without CTAB is
shown in Figure 14(a). The particles have plate like mor-
phology. However the HR TEM image of the same cata-
lyst prepared using CTAB (Fig. 14(b)) indicate that porous
As tabulated, the conversion of glycerol increased with
increase in reaction temperature for all the catalysts.
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
Copyright: American Scientific Publishers
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Figure 14. HR-TEM micrographs of Ca4Al-HT (a) and Ca4Al-HT 0.4CTAB (b–d) catalysts.
4596
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Arulselvan et al.
Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Table IV. Effect of CTAB concentration in Ca4Al-HT on glycerol con-
O
version and glycerol carbonate selectivity.
O
OH
Hydrotalcite
catalysts
O
O
HO
OH
+
H3C
CH3
Dimethyl carbonate
Selectivity (%)
Reaction
temperature Conversion Glycerol
OH
Glycerol Carbonate
Glycerol
Catalyst
(K)
(%)
carbonate Glycidol
Scheme 1. Transesterification of glycerol with dimethyl carbonate
(DMC).
Ca4Al-HT
353
353
353
353
353
82.4
87.3
92.1
98.7
98.9
95ꢆ9
98ꢆ5
98ꢆ0
97ꢆ9
95ꢆ1
4.1
1.5
2.0
2.1
4.9
Ca4Al-HT-0.1CTAB
Ca4Al-HT-0.2CTAB
Ca4Al-HT-0.4CTAB
Ca4Al-HT-0.6CTAB
Among the MgxAl-HT catalysts, Mg4Al-HT was found
to give the best conversion of of glycerol (63.4%) with
95.7% selectivity towards glycerol carbonate. Although all
the catalysts show increase in conversion, the selectivity
towards GC decreases slightly with increase in reaction
temperature. As a as the catalytic activity is concerned,
CaxAl-HT catalysts show higher catalytic activity than
MgxAl-HT catalysts. In all the cases only glycidol was
found to the other minor product. Among the hydrotalcites
Ca4Al-HT showed the highest glycerol conversion (82.6%)
and the highest selectivity (97.9%) towards glycerol car-
bonate at 353 K. Hence Ca4Al-HT hydrotalcite was chosen
for further optimization studies and 353 K was found to
be the optimum reaction temperature.
Notes: Reaction conditions: Temperature: 353 K, catalyst weight: 100 mg, glyc-
erol/DMC molar ratio: 1:4, reaction time: 2 h.
selectivity over 95% towards GC. Among Ca4Al-HT
yCTAB catalysts, Ca4Al-HT 0.4CTAB converts maximum
glycerol with maximum selectivity. On increasing the
CTAB concentration further to 0.6 molar a slight decrease
in selectivity was observed.
3.8.3. Effect of Time on Stream on
Conversion and Selectivity
In order to investigate the effect of reaction time on
the glycerol conversion and selectivity towards glycerol
carbonate, 100 mg of the best catalyst viz., Ca4Al-HT
0.4CTAB was taken and the reaction was conducted at the
optimized reaction temperature of 353 K for 4 h and the
results are summarized in Table V.
3.8.2. Effect of CTAB Concentration in Ca4Al-HT
Hydrotalcites on Conversion and Selectivity
The effect of CTAB concentration in the synthesis of
Ca4Al-HT catalyst on the conversion of glycerol was
IP: 93.179.90.248 On: Tue, 20 Mar 2018 09:58:09
investigated and the results are given in Table IV.
Although the conversion increase with reaction time
Copyright: American Scientific Publishers
It is evident that the increase in CTAB molar concen-
tration in the preparation of Ca4Al-HT catalysts increased
the conversion from 87.3% to 98.9% with very good
Delivered by Ingenta
the selectivity decreases slightly. As maximum conversion
with highest selectivity could be obtained at 2 h of reaction
time, the reaction time of 2 h was optimized.
Table III. Effect of reaction temperature on the glycerol conversion and
glycerol carbonate selectivity.
3.8.4. Effect of Glycerol: DMC Molar Ratio
In order to check the effect of molar ratio of glycerol
to dimethyl carbonate (DMC) on conversion and selectiv-
ity the reaction was performed with different molar ratios
from 1:1 to 1:4 under the optimized conditions: Catalyst =
Ca4Al-HT 0.4CTAB, temperature = 353 K, time = 2 h and
the results are given in Table VI.
The results clearly indicate that the molar ratio of 1:4
was found to be the best in terms of conversion and
selectivity.
Selectivity (%)
Reaction
Glycerol
Catalyst
temperature (K) Conversion (%) carbonate Glycidol
Mg2Al-HT
333
353
373
333
353
373
333
353
373
333
353
373
333
353
373
333
353
373
24.6
32.5
35.0
27.5
39.0
38.6
32.9
51.6
63.4
54.6
71.3
73.2
62.0
78.3
80.7
67.0
82.4
82.6
100
100
–
–
98ꢆ8
100
98ꢆ8
98ꢆ1
100
98ꢆ8
95ꢆ7
100
99ꢆ7
98ꢆ0
99ꢆ4
98ꢆ6
93ꢆ8
98ꢆ9
95ꢆ9
91ꢆ8
1.2
–
0.2
1.9
–
0.2
4.3
–
0.3
2.0
0.6
1.4
6.2
1.1
4.1
8.2
Mg3Al-HT
Mg4Al-HT
Ca2Al-HT
Ca3Al-HT
Ca4Al-HT
Table V. Effect of reaction time on the glycerol conversion and glycerol
carbonate selectivity.
Selectivity (%)
Reaction
Glycerol
Catalyst
time (h) Conversion (%) carbonate Glycidol
Ca4Al-HT-0.4CTAB
Ca4Al-HT-0.4CTAB
Ca4Al-HT-0.4CTAB
Ca4Al-HT-0.4CTAB
1
2
3
4
72.6
98.7
99.1
99.5
97.8
97.9
96.8
94.2
2.2
2.1
3.2
5.8
Notes: Reaction conditions: Temperature: 333, 353 and 373 K, catalyst weight:
Notes: Reaction conditions: Temperature: 353 K, catalyst weight: 100 mg, glyc-
100 mg, glycerol/DMC molar ratio: 1:4, reaction time: 2 h.
erol/DMC molar ratio: 1:4, reaction time: 1, 2, 3 and 4 h.
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Solvent Free Transesterification of Glycerol Into GC Over Nanostructured CaAl Hydrotalcite Catalyst
Arulselvan et al.
Table VI. Effect of glycerol to DMC molar ratio on glycerol conversion
and glycerol carbonate selectivity.
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Ca4Al-HT-0.4CTAB
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3.8.5. Recyclability of Ca4Al-HT 0.4CTAB Catalyst
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CaAl-HT and MgAl-HT with molar ratio of 2, 3 and 4
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Received: 5 July 2017. Accepted: 21 July 2017.
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