Mg1+xCa1-xO2 as reusable and efficient heterogeneous catalyst for the synthesis of glycerol carbonate via the transesterification of glycerol with dimethyl carbonate

Article abstract of DOI:10.1016/j.apcata.2013.06.044

A reusable and highly active heterogeneous catalyst of the type Mg _1+xCa_1-xO₂ has been developed and thermally treated at 850 C for 6 h. The catalytic activity was investigated during the transesterification of glycerol with dimethyl carbonate to synthesize the valuable glycerol carbonate. A typical Mg_1.2Ca_0.8O ₂ mixed oxides catalyst was characterized by BET surface area, surface base amount, CO₂-TPD, XRD and FTIR, and its microstructure was studied by the aid of scanning and transmission electron microscopy. The investigated characteristics gave a clear insight into the catalytic performance of the catalyst. Over 0.30 g of this catalyst, glycerol conversion was almost complete and the yield of glycerol carbonate was 100% under atmospheric pressure. The best reaction conditions are: molar ratio of glycerol/DMC of 1:2, reaction temperature of 70 C and 90 min of reaction time. The transesterification results showed that the synergism interaction between Ca and Mg species in the catalyst structure was the reason for its high activity.

Full text of DOI:10.1016/j.apcata.2013.06.044

Accepted Manuscript
Title: Mg1 Ca O as reusable and efficient heterogeneous
+x
1−x
2
catalyst for the synthesis of glycerol carbonate via the
transesterification of glycerol with dimethyl carbonate
Author: M.S. Khayoon B.H. Hameed
PII:
S0926-860X(13)00385-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.06.044
APCATA 14304
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-2-2013
22-6-2013
26-6-2013
Please cite this article as: M.S. Khayoon, B.H. Hameed, Mg1 Ca O as reusable
+x
1−x
2
and efficient heterogeneous catalyst for the synthesis of glycerol carbonate via the
transesterification of glycerol with dimethyl carbonate, Applied Catalysis A, General
(2013), http://dx.doi.org/10.1016/j.apcata.2013.06.044
This is a PDF file of an unedited manuscript that has been accepted for publication.
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Mg1+xCa1-xO as reusable and efficient heterogeneous catalyst for the synthesis of
glycerol carbonate via the transesterification of glycerol with dimethyl carbonate
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M.S. Khayoon, B.H. Hameed*
School of Chemical Engineering, Engineering Campus,
Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
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* Corresponding author Fax: +6045941013
E-mail address: chbassim@eng.usm.my (B.H. Hameed)
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Abstract
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A reusable and highly active heterogeneous catalyst of the type Mg1+xCa1-xO has
o
been developed and thermally treated at 850 C for 6 h. The catalytic activity was
investigated during the transesterification of glycerol with dimethyl carbonate to synthesize
the valuable glycerol carbonate. A typical Mg1.2Ca0.8
O
2
mixed oxides catalyst was
characterized by BET surface area, surface base amount, CO
2
-TPD, XRD and FTIR, and its
microstructure was studied by the aid of scanning and transmission electron microscopy. The
investigated characteristics gave a clear insight into the catalytic performance of the catalyst.
Over 0.30 g of this catalyst, glycerol conversion was almost complete and the yield of
glycerol carbonate was 100% under atmospheric pressure. The best reaction conditions are:
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molar ratio of glycerol/DMC of 1:2, reaction temperature of 70 C and 90 min of reaction
time. The transesterification results showed that the synergism interaction between Ca and
Mg species in the catalyst structure was the reason for its high activity.
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Keywords: Glycerol; Glycerol carbonate; Mixed oxide catalyst; Fuel additives.
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1. Introduction
Due to the escalating prices of the petroleum crude in international markets and the
growing concern to reduce the greenhouse gas emission, biodiesel has gained increased
acceptability as an alternative to petrol-diesel fuel [1]. It has several advantages such as the
substantial reduction of unburned hydrocarbon emissions, higher cetane number compared to
petrol-diesel resulting in better engine performance [2]. Moreover, it is non-toxic and
biodegradable fuel which almost completely eliminates life cycle carbon dioxide emission
[3]. Today, there are significant demands in energy consumption worldwide leading to an
increased production of biodiesel. One of the inevitable technical problems accompanying the
biodiesel industry and affecting its economy is the glycerol by-production, which is about 10
wt.% of the total product [4]. Consequently, the market is flooded with the oversupplied
glycerol due to the expansion in biodiesel industry worldwide resulting in plummeting the
price of glycerol and subsequently jeopardizing the biodiesel production economy.
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Recently, the abundance of glycerol at low prices, together with the quest of new
economic synthetic routes for renewable chemicals, have resulted colossal interest in this
renewable polyol as a building block molecule, mainly due to the wide realm of its valuable
derivatives [5]. In this sense, various technologies have emerged for the valorization of
glycerol to valuable chemicals and prominent among these is the synthesis of glycerol
carbonate via transesterification with dimethyl carbonate.
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Glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-one, CAS #931-40-8) is one of
the most promising glycerol derivatives that attracts presently, more scientific and industrial
concern [6]. It is an important green alternative for petrol-based components like ethylene
2
and propylene carbonates for CO sequestration membranes [7, 8] and also a building block
monomer for the synthesis of biodegradable polymers such as polyesters, polycarbonates,
polyurethanes and polyamides [9]. In addition to its importance in textile, plastics,
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pharmaceutical and cosmetics industries, glycerol carbonate has gained increased attention in
lithium ion batteries as eco-friendly electrolyte solvent [6].
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The traditional synthesis of glycerol carbonate from glycerol has been carried out
through different routes: the reaction of glycerol with urea [10], the direct carbonation of
glycerol with phosgene [11] or carbon monoxide and oxygen [12]. However, technical and
environmental drawbacks accompanying the reaction of glycerol with urea include the
o
toxicity of urea and high reaction temperature (150 C). The carbonation reaction has been
categorized as non-environmental process due to the toxicity of phosgene and CO [13]. It was
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also reported that glycerol carbonate can be obtained by carbonylation with CO at harsh
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reaction conditions [14]. In this regard, serious limitations associated with this synthetic route
have been addressed, particularly the low yield of glycerol carbonate and the unfavorable
thermodynamic equilibrium limitations [15].
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In another approach, the synthesis of glycerol carbonate was performed via the
transesterification of glycerol with cyclic carbonate, such as dimethyl carbonate (DMC) in
the presence of catalyst with basic sites (Scheme 1) [16-18]. DMC, which can be obtained
from urea and methanol, is widely utilized as a green carbonylation agent due to its
environmentally benignant nature [19]. In this context, several homogeneous catalysts were
2 3
used to facilitate this reaction such as CaO [20] and K CO [21] with high yield of glycerol
carbonate. However, the possible interactions of such catalysts with the reaction system and
the difficulty to separate them from the product are the demerits of using homogeneous
catalysts. The heterogeneous catalyst of lipase Novozym 435 immobilized onto molecular
sieves also catalyzed the synthesis of glycerol carbonate from glycerol and DMC [22].
Besides the high cost of lipase catalyst and the large amount of molecular sieve used, this
catalyst showed poor activity and 25 h of reaction time was needed for completion. Recently,
various heterogeneous catalysts such as CaO [6], Mg/Al/Zr [18] and KF/HAP [23] have been
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used for the synthesis of glycerol carbonate from glycerol and DMC. However, some studies
were carried out at high temperate with a high molar ratio of DMC/glycerol [6] or in the
presence of toxic solvent [13] to achieve a high yield of glycerol carbonate.
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Therefore, it is of importance to the research community and the industry to develop a
new catalytic process for the synthesis of glycerol carbonate using an economic reaction
conditions with the aid of highly active and reliably reusable heterogeneous catalyst. In this
work, a series of Mg-Ca mixed oxide catalysts were prepared and evaluated for the synthesis
of glycerol carbonate from glycerol and DMC. The variation in the textural and structural
characteristics of the prepared catalysts with the change in atomic composition and
calcination temperature were investigated.
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2. Materials and methods
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2.1 Materials
Anhydrous glycerol of high purity (>99%) and sodium hydroxide (98%) were
obtained from Sigma, Germany. Anhydrous dimethyl carbonate (DMC) of purity ≥99.8%,
HPLC grade methanol of purity ≥99.0% and Calcium nitrate tetrahydrate Ca(NO
(99%) were supplied by Merck, Malaysia. Magnesium nitrate hexahydrate Mg(NO
3
)
2
.4H
2
O
3
)
2
2
·6H O,
synthetic hydrotalcite and 25% Ammonia solution were purchased from Sigma–Aldrich
(Malaysia). Hydrochloric acid (HCl 37%) from Mallinckrodt, USA was used. All reagents
were used without further treatments. Standard glycerol carbonate of 93-95% purity was
freely obtained from Hunstman, USA.
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2.2 Synthesis of the mixed oxide catalyst
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The mixed metal oxide catalysts Mg1+xCa1-x
co-precipitation of the metals hydroxides from their nitrates in 5 M NH
alkaline precipitation media [24]. On the basis of 10 g of the final catalyst, a solution
containing the calculated amounts of metal salts, Ca(NO .4H O and Mg(NO .6H O, was
O
2
(0.1≥ x ≥0.9), were synthesized by the
4
OH solution as
3
)
2
2
3
)
2
2
firstly prepared and magnetically stirred at 550 rpm in 250 mL round bottom flask. The
o
mixture was heated over a water bath to 75 C and thoroughly mixed for 30 min. Then, 5 M
ammonia solution (29 ml of 25% ammonia in 150 ml of distilled water) was added and stirred
for 6 h. Thereafter, a white precipitate was formed signifying the formation of metals
hydroxides. The mixture was then filtered off and the solid was removed and subsequently
o
o
o
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1
1
dried at 80 C for 24 h. Samples were calcined in air at 850 C with a heating rate of 5 C/min
and dwelling for 6 h. The foregoing method describes the synthesized catalyst and in
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particular sample Mg1.2Ca0.8O with Ca: Mg mass ratio of 1: 1.1. Single oxides of MgO and
CaO were separately precipitated from their nitrates adopting the above procedure, except
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CaO which was calcined at 900 C, and directly employed for the transesterification of
glycerol with dimethyl carbonate. It is worth mentioning that the number of oxygen atoms (2)
in the chemical formula was proposed to achieve the zero-valent charge for the catalyst
formula.
2.3 Catalyst characterization
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The total surface area, pore volume and average pore diameter of the developed
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catalysts were measured by nitrogen adsorption/desorption isotherms at –197 C by the
Brunauer-Emmet-Teller (BET) method using ASAP 2020 V3.02 H Micromeritics instrument
(Micromeritics instruments corporation, USA). The base amount was determined by the
neutralization titration method [25]. Catalyst powder 0.10 g was dispersed in 25 mL of 0.02
M HCl solution and the resulting suspension was gently stirred at room temperature for 48 h.
Thereafter, the mixture was filtered off and the alkali filtrate solution was titrated against 0.02
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M NaOH solution. The following formula was used to calculate the concentration of basic
sites (C):
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4
(
Initial moles of HCl – Final moles of HCl)
C (mmol/g) =
(1)
Mass of catalyst (g)
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Also, the base amount of the prepared catalysts was studied with temperature-
programmed desorption of CO as probe molecule (TPD-CO2) using Micromeritics
2
AutoChem 2920 II apparatus equipped with thermal conductivity detector. The analysis was
performed by heating 50 mg of the catalyst sample under a He flow from room temperature
o
o
−1
−1
to 700 C for 1 h (20 C min , 100 mL min ). Then, the temperature was decreased to 100
o
−1
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1
2
2
2
2
2
2
2
C, and a flow of pure CO
2
(50 mL min ) was subsequently introduced into the reactor
o
o
during 1 h. The TPD of CO
2
was carried out between 100 and 700 C under a He flow (10 C
−
1
−1
min , 30 mL min ), and the detection of the desorbed CO
2
was by an on-line gas
chromatograph provided with a TCD.
The mean metallic amounts of Ca and Mg presented within the structure of the
catalyst before and after the transesterification reaction were determined by EDX using Zeiss
Supra TM 35 VP scanning electron microscope (Zeiss, Jena, Germany) coupled with FEI as a
source of electrons and accelerated at 300 kV. The identification of the surface functional
groups was performed by Fourier transform infrared spectroscopy (FTIR). The IR spectra for
the as-synthesized catalysts were obtained on Perkin–Elmer System 2000 spectrometer by
using the standard KBr disc method. The spectra were recorded in the fingerprint region of
–1
2000–400 cm . The structure and the unit cell parameters of the synthesized catalysts have
been investigated by wide angle X-ray diffraction patterns over a diffractometer (Bruker D2
α
Phaser Germany, 2011). The Cu K radiation electrons (λ=1.5406 Å) were accelerated at 30
kV and 10 mA in an evacuated X-ray tube with Ni filter, and the data were interpreted using
the EVA software.
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2.4 Catalytic reaction
The transesterification reaction of glycerol with dimethyl carbonate (DMC) for the
synthesis of glycerol carbonate was investigated using different molar ratios of
glycerol/DMC. Glycerol (10 g, 108.6 mmol), DMC (19.5–34.3 g, 217.2–380 mmol) and 0.30
g of the catalyst were charged into 50 mL round-bottom glass reactor equipped with a
o
magnetic stirrer. Reactions were carried out at 50–90 C, being those conducted at
o
temperatures higher than 75 C were performed in a high pressure autoclave reactor. After the
completion of the reaction, the catalyst was removed by centrifugation and repeatedly washed
o
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with methanol at 45 C and subsequently dried in an oven at 80 C for 12 h. The unreacted
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DMC was separated by evaporation at reduced pressure.
Reaction samples were analyzed by gas chromatograph (GC; Shimadzu 2010 plus,
Japan) equipped with a flame ionization detector (FID) and using a ZB5-HT capillary column
(30 m × 0.25 mm × 0.25 µm). Samples for analysis were prepared by extending 50 mg of the
DMC-free reaction sample in 1 mL of methanol. The column temperature was initially set at
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75 C (2 min) followed by ramping of 25 C/min to 225 C and then heating to 275 C at 10
o
o
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C/min. The FID and injection temperatures were fixed at 250 C and 220 C, respectively.
Helium at 1.3 ml/min was used as carrier gas. The injection split ratio was 20.
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3. Results and discussion
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3.1 Catalysts Characterization
3.1.1 Textural properties and surface basicity (base amount)
Table 1 presents the textural characteristics (BET surface area, pore volume, and
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average pore size) of selected Mg1+xCa1-x
2
O catalyst samples. According to the obtained
results, it revealed that the values of BET surface area of Mg-Ca catalyst samples varied from
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–1
3
–1
9.33 to 31.7 m g and those of pore volumes are close to 0.09 cm g . It was observed that
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the surface area decreases as the amount of Ca in the catalyst increase, which could be
attributed to the increased crystallite size of the catalyst by increasing the Ca content [26].
Indeed, all the tested catalysts exhibited low values of surface area which could
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merely be attributed to the high calcination temperature (850 C) that enhanced the sintering
of fine crystals and lead to the agglomeration of them onto the pore walls [27]. These
agglomerates could contribute to the partial occlusion of the pores and thereby the reduction
of the surface area of the catalyst. Moreover, Taufiq-Yap and coworkers reported that the
excessive presence of CaO phases causes the unfavored filling of the surface pores, which
logically result in the reduction of the total surface area [26].
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On the contrary, the textural properties (BET surface area and pore volume)
marginally increased by increasing the Mg content, which can be ascribed to the ameliorated
crystal phase of the catalyst. This observation is corroborated with the physical elucidation
reported in literature [28]. The catalyst formulation Mg1.2Ca0.8
O
2
exhibited BET surface area
of 31.7 m g and pore volume of 0.091 cm g , that are better than those obtained with the
former catalyst formulation (Mg1-xCa1+x ).
Fig.1a & b are the obtained Nitrogen physisorption isotherms of two different catalyst
) investigated in this work. It is evidently clear that
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–1
O
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2 2
samples (Mg1.2Ca0.8O and Mg0.8Ca1.2O
all the obtained isotherms of type IV with type H2 hysteresis loops, which are the
characteristics of mesoporous materials [29]. This type of hysteresis loops represents the pore
structure, size and shape of materials which have disordered distributions. At a relative
pressure range from 0.45 to 1.0, the isotherms for both catalysts exhibited the typical
hysteresis loops of highly mesoporous materials. A distinguished feature is observed in the
isotherm of the Mg1.2Ca0.8
2 2
O sample, which is the total volume of N adsorbed. That is
apparently shown by the corresponding values of Y-axis in each plot, and explains the higher
N
2
adsorption in larger pore volume than smaller one.
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The base amount of the prepared catalysts was evaluated and the results are also
presented in Table 1. It revealed that the base amount increases with increasing the MgO
content in the catalyst, i.e., it increased in the trend of Mg0.8Ca1.2
O
2
< Mg1.2Ca0.8
O
2
<
4
5
6
7
8
9
0
Mg1.8Ca0.2O . For all the Ca-Mg combinations, the base amount was found to be higher than that of
2
bulk MgO and CaO. The improved base amount might be ascribed to the synergistic effect between
MgO and CaO [30]. The decrease in base amount with increasing CaO content could partly be
attributed to the structural changes caused by the high amount of calcium, which led to partial damage
of the binary oxide structure [26]. These results strongly suggest that the combination of both oxides
has resulted in materials with higher alkalinity, which could be of higher catalytic activity than pure
oxides. Despite the fact that the catalyst formulation Mg Ca O has possessed higher base amount
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.8
0.2
2
1
1
1
1
1
2
3
4
2
than Mg1.2Ca0.8O catalyst, but their catalytic activity during the transesterification reaction has
increased in reverse trend. This suggest that the base amount is not the only factor controlling the
catalytic activity of these materials. These results are corroborating with several scientists [26, 31,
32], as they observed that stronger base amount of Ca–Mg binary system than pure oxides and the
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catalytic activity mainly depends on the nature of the active sites rather than the base amount.
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3.1.2 CO -TPD
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Fig.2 shows the TPD-CO profiles of CaO, MgO and the mixed metal oxide catalyst
samples (Mg0.8Ca1.2
O
2
and Mg1.2Ca0.8
O
2
), and the desorbed amounts CO
2
of are listed in
o
Table 1. Pure magnesium oxide shows weak desorption band between 450 and 600 C,
whereas pure calcium oxide present a very broad desorption band extending from 480 to 660
o
o
C. The mixed oxide catalysts present a broad desorption band between 450 to 700 C, which
was noticed to enlarge as the amount of MgO in the catalyst increased, and another smaller
o
desorption peak at about 735 C. According to the literature [32], the very broad desorption
o
band located between 450 and 700 C might be ascribed to the interaction of CO
2
with basic
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sites of weak and medium strength, mainly associated with Mg –O pairs, for which the
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basic strength is very high. Whereas the well-resolved desorption peak at about 735 C can be
2+
2−
associated to Ca –O pairs. It is observed that the base amount increases with increasing
MgO content in the mixture. The findings of Taufiq and co-workers were considered to
interpret the obtained desorption profiles [26].
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3.1.3 X-ray diffraction (XRD)
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Fig.3a depicts the powder X-ray diffraction patterns of Mg-Ca catalyst samples after
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calcination at 850 C. The patterns were recorded for tow reversed catalyst samples with
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1
2
2
2 2
varied metallic content, Mg1.2Ca0.8O and Mg0.8Ca1.2O , to estimate the effect of metallic
content on the crystallographic phases. The typical peaks of the separate metal oxide
crystalline phases and that of mixed metal oxides were obviously detected in the obtained
patterns [33]. The presence of both oxides has been proven by the characteristic peaks of
o
o
o
o
o
cubic CaO (2θ = 32.3 , 37.2 and 78.9 ) and hexagonal MgO (2θ = 42.7 and 63.0 ). The
catalyst formulation Mg0.8Ca1.2 showed a higher degree of crystallinity of CaO as it
contained higher Ca amount than the catalyst formulation Mg1.2Ca0.8 , indicated by the
increased intensity of the corresponding peaks. As such, the typical MgO peaks were
O
2
+
2
O
2
+
2
decreased as the Ca content increased. It is worthy to mention that the presence of calcium
2
carbonate as calcite in the Mg0.8Ca1.2O sample was detected by the low intensity peak
o
located at 2θ = 29.4 . It is well established that the formation of CaCO
3
is possible as the CO
2
easily to absorb by CaO and form CaCO3, as indicated by the following equation [34];
–1
2
2
CaO + CO
2
CaCO
3
∆H = –178.3 kJ mol
(2)
2
2
3
4
Fig.3b shows the typical XRD patterns of MgO, CaO and that of Mg1.8Ca0.2
O
2
catalyst sample. It can be clearly seen that pure MgO exhibit two typical XRD peaks at 2θ =
1
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42.7 and 62.3 , assigned to the cubic and hexagonal crystalline phases [26], respectively.
o
o
Whereas the XRD pattern of the CaO showed four main peaks located at 2θ = 17.8 , 29.2 ,
o
o
33.8 , and at 46.9 , assigned to different CaO crystalline phases [26, 34]. Other smaller peaks
and shoulders were also observed in the pattern of CaO, possibly ascribed to the presence of
3 2
CaCO . The XRD pattern of the Mg1.8Ca0.2O catalyst formulation showed combination of
MgO and CaO peaks as follows:
o
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1. A sturdy peak at 2θ = 42.8 assigned to the cubic MgO phase.
o
2. A lesser intensity peak at 2θ = 62.1 assigned mainly to hexagonal MgO.
o
o
3. A sharp peak at 2θ =31.2 and a shoulder at 2θ =31.2 assigned to cubic CaO.
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1
1
1
1
0
1
2
3
4
5
Based on the above argument, the presence of MgO and CaO in the Mg-Ca catalyst
samples was observed in the obtained patterns in the form of pure oxides, with no
distinguished crystalline phase attributable to the formation of the mixed MgCa oxide was
2+
2+
observed, mainly due to the different ionic radii of the Ca and Mg ions [28]. Therefore,
further characterization by FTIR could provide more information and evidences on the
formation of mixed metal oxides.
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3.1.4 Fourier transform infrared analysis (FTIR)
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In order to verify the presence of surface species that were not detected by other
techniques, FTIR spectroscopy was used to characterize the as-synthesized Mg-Ca catalyst
2
2
2
2
2
0
1
2
3
4
samples. The spectra were recorded for MgO, CaO, Mg1.2Ca0.8
O
2
, and Mg1.5Ca0.5
O
2
materials
–1
in the range of 4000–400 cm . Since there was no distinct peaks observed in the range of
–1
3000–2000 cm for all the characterized materials, the spectra are presented separately in
two regions.
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Fig.4a presents the obtained IR spectra of the prepared catalyst in the range of 2000–
–1
–1
400 cm , while those in the range of 4000–3000 cm are shown in Fig.4b. The spectra of all
–1
the tested materials revealed the presence of mono and bidentate carbonates at 1600 cm ,
which could be ascribed to the absorption of CO by CaO and MgO to form CaCO and
MgCO , respectively, as also verified by XRD. The spectrum of MgO shows a distinct peak
2
3
3
–1
at 1380 cm , which was not observed in the CaO spectrum. Therefore, this peak was
believed to represent the typical IR response of pure MgO. As such, the pure CaO was
–1
identified by the sharp peak at 880 cm in its spectrum. The intensity of those sharp peaks
–1
(1380 and 880 cm ) was observed to reduce as the metallic content varied in the catalyst
2
formulation Mg1+xCa1-xO for both X=0.2 and X=0.5. Thus, the presence of MgCa combined
1
1
1
1
1
1
1
1
–1
oxides could be assigned by the broad band in the 1400–1600 cm region and by the
–1
appearance of the sharp peaks at 1380 and at 880 cm with reduced intensities. In addition,
–1
the spectrum of the mixed oxide catalysts shows two distinct peaks at 3610 and 3640 cm ,
that can be ascribed to the υOH of surface hydroxyl groups (Mg–OH) and (Ca–OH),
respectively. In the spectra of all catalysts, the presence of the physisorbed water molecules is
–1
noticed by the wide bands at 3450 and 1583 cm . The interpretation of the obtained FTIR
peaks was correlated with the confirmed arguments in the literature [26, 28].
1
1
8
9
3.1.5 Surface morphology
2
2
2
2
2
2
0
1
2
3
4
5
Fig.5a shows the SEM micrograph of the Mg1.2Ca0.8
O
2
catalyst sample calcined at
o
o
850 C, whereas Fig.5b for the same sample calcined at 700 C. The micrographs show the
textural structure of the catalyst with cuboids clusters, which belong to the metallic oxides. It
can be observed that the size of these cuboids reduced as the calcination temperature
increased, which can be explained due to the destruction of bigger crystals into agglomerated
fine crystals and thereby the formation of smaller cuboids. In addition, the obtained images
1
3
Page 13 of 43

1
2
3
4
revealed the porous nature of the catalyst, which is believed to be one the reasons behind its
high performance during the transesterification of glycerol with DMC. The conclusions made
from SEM images are corroborating with the results of BET analysis and are in good
agreement with those for nano-sized MgO and CaO reported in the literature [12].
5
6
7
8
9
0
On the other hand, the porous structure of the catalyst was evidenced by means of
TEM analysis. The obtained TEM images are shown in Fig.6a & b for the same catalyst
o
o
samples calcined at 700 C and 850 C, respectively. It is evidently clear that the porosity of
the catalysts increases with increasing the calcination temperature. This might be explained
by the changes of the crystallite phases accompanied by the modification of the surface pores
due to the thermal treatment.
1
1
1
1
2
3.1.6 Elemental composition of the synthesized catalyst
1
1
3
4
2
The elemental composition of Mg1.2Ca0.8O catalyst was determined by using an
1
1
1
1
1
2
2
5
6
7
8
9
0
1
energy dispersive X-ray detection technique (EDX) conducted in conjunction with SEM
analysis. The knowledge of actual metallic content in the catalyst can provide additional
+
2
+2
verification to the role of Mg and Ca in enhancing the catalytic performance. The results
obtained from EDX analysis unveiled that the catalyst formulation Mg1.2Ca0.8 contained
O
2
33.25 at.% Mg, 24.64 at.% Ca and 42.1 at.% O. The theoretical values for the atomic
percentage (at.%) were 30 at.% Mg, 20.24 at.% Ca and 48.76 at.% O, as calculated using the
atomic ratio formula.
2
2
2
2
3
4
The difference between the theoretical and actual values for the metallic content is
logical and could be attributed to the precipitation conditions of the metal oxides from their
nitrates, as the preparation used ammonia solution to maintain the pH value close to 10. The
1
4
Page 14 of 43

1
2
values obtained from this analysis depend mainly on the proportions by weight of each
contributing component.
3
4
5
6
3.2
Catalytic activity: Transesterification of glycerol with DMC
In this work, the catalytic activity of the synthesized pure and mixed oxide catalysts
7
8
9
0
1
2
3
4
5
6
7
8
9
0
was defined by means of glycerol conversion and glycerol carbonate (GC) yield. Different
Mg-Ca catalysts were prepared based on the various chemical formulae of the possible metals
combinations. The results presented in Table 2 are the conversion of glycerol and the GC
yield obtained after 90 min of reaction for the prepared Mg-Ca catalysts. It is obvious that
MgO has presented higher value of glycerol conversion and lower GC yield than CaO. The
formation of byproducts (methanol and high boiling point diols) was observed and could be
attributable to the selective behavior of CaO species in the tested catalysts. Thus, the
continuous removal of the by-produced methanol resulted in high yield of GC with low molar
ratio of DMC/glycerol, as also observed by Li and Wang [13]. Therefore, two catalyst
formulae were proposed in this study; the first suggests the increase of Ca content; and the
second with increased Mg content. As the Ca content increases in the first catalyst formula,
1
1
1
1
1
1
1
1
1
1
2
2
Mg1-xCa1+xO , the GC yield was found to decrease. This behavior suggests that Ca is
responsible on the selective formation of GC, whereas Mg enhances the conversion of
glycerol.
2
2
2
2
2
1
2
3
4
5
On the contrary, in the second formula, Mg1+xCa1-x
decreased Ca content presented high values of glycerol conversion and GC yield. This was
not true for the case of Mg1.8Ca0.2 catalysts, as they showed decreased
2
O , catalyst samples with
2 2
O and Mg1.9Ca0.1O
values of GC yield as the Ca amount decreased. This might be explained by encapsulation of
the Ca by Mg species due to the higher amount of the latter. The obtained results using the
1
5
Page 15 of 43

1
2
3
4
5
6
7
8
9
0
1
binary oxide catalysts (for both formulae) are stratifying with those obtained using each of
the oxides separately as heterogeneous catalyst. It was also figured out that GC yield
increases with surface area and pore volume increase in the trend of Mg0.8Ca1.2
(as shown in Table 2 and earlier shown in Table 1). This
observation is not correlating with the increase in base amount, as one can see that
possessed the highest base amount of 1519 μmol/g but showed lower value of
catalyst. Therefore, it seems logical not to attribute the catalytic
activity to the catalyst base amount only, but to other characteristics as well. Out of all the
formulae tested, the catalyst formulation Mg1.2Ca0.8 presented complete conversion of
2
O <
Mg1.8Ca0.2
2 2
O < Mg1.2Ca0.8O
Mg1.8Ca0.2
O
2
GC yield than Mg1.2Ca0.8O
2
O
2
1
1
glycerol with almost 100% yield of GC. To this end, this catalyst was selected for further
investigations of its catalytic performance.
1
1
1
1
1
1
1
1
2
2
2
3
4
5
6
7
8
9
0
1
On the other hand, the catalytic activity of the prepared catalysts was compared with
that of conventional hydrotalcite (HT) under the same reaction conditions. It was found that
2
HT possesses lower catalytic activity than most of the tested Mg1+xCa1-xO catalyst series.
Over 0.30 g of HT, glycerol conversion was about 48% and the GC yield was 46.7% after 90
min. As the reaction time was prolonged to 2 h, GC yield has reached to 68%. This could be
explained by the role of Mg-Al interactions in the HT structure. Takagaki et al. have studied
the relationship between the GC yield and the Mg/Al ratio [35]. They observed that synthetic
HT with Mg/Al ratio of 5 (close to that of conventional hydromagnesite) has higher catalytic
activity than other samples with Mg/Al ratio of less than 5. They attributed the catalytic
activity to the HT structure and correlated the activity with the base amount of the catalysts.
2
2
2
2
2
3
4
5
The metallic interaction is evidently responsible for the high GC yield (100%)
2
obtained over the mixed oxide catalyst (Mg1.2Ca0.8O ). On the contrary, the pure metallic
oxides showed a poor GC yield, as earlier presented in Table 2. This could also be confirmed
by the varied values of glycerol conversion and GC yield obtained over a series of catalysts
1
6
Page 16 of 43

1
2
3
4
5
6
7
8
9
with varied Mg and Ca content. As the Mg/Ca ratio was changed, their synergistic interaction
led to catalysts with varied porous structure as would be noticed by the results of BET
+
2
analysis. The porous structure is thought to be enhanced by the Mg that form MgO clusters
masking the backbone of the catalyst structure. In fact, this could physically be explained by
the fact that the ionic radius of Mg atom (150 pm) is slightly bigger than that of Ca atom (114
pm) and thereby the crystallite sizes of their oxides will be accordingly ordered.
After the preliminary evaluation of the catalytic performance, further investigations
were performed to study the effect of different reaction parameters on the conversion of
glycerol and the corresponding GC yield.
1
1
0
1
3.3 Effect of reaction parameters
3.3.1 Effect of calcination temperature
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
4
5
The investigations on the catalytic performance of the Mg1.2Ca0.8
2
O catalyst were
moved to examine the effect of calcination temperature on the conversion of glycerol and GC
2
yield. In this context, series of Mg1.2Ca0.8O catalyst samples were calcined at different
o
temperatures (700–900 C). Fig.7 shows the results of glycerol conversion and GC yield
obtained using these catalysts calcined at varying temperatures and dwelling time (4 and 6 h).
The results showed that calcination temperature is a key factor affecting the performance of
the catalyst. The conversion of glycerol increased from 41% to 77% as the calcination
o
o
temperature changed from 700 C to 850 C for catalyst sample calcined for 4 h dwelling.
Increasing calcination dwelling time to 6 h resulted in a catalyst sample with improved
performance, as evidenced by the results of catalytic performance for catalyst samples
calcined at different temperatures but dwelled for 6 h. An increase in GC yield was observed
with further increase in the calcination temperature and time, indicating the role that these
o
parameters played in GC production. Further increasing the calcination temperature to 900 C
resulted in a catalyst sample with decreased catalytic activity, which can be explained due to
1
7
Page 17 of 43

1
2
3
4
5
6
7
8
9
0
1
the probable constriction of surface pores occurred by the effect of the gaseous diffusion out
of the pores during the thermal treatment. Another possible reason for the reduced activity is
the partial removal of the binding water molecules from the catalyst structure, that could
possibly lead to the destruction of the binary oxide system and thereby the formation of
unknown species of those metals (Mg and Ca).
It was also found that the calcination time and temperature applied during the
preparation of the Mg1.2Ca0.8
O
2
catalyst have affected the catalytic activity dramatically, as
o
this catalyst was shown to be more efficient when calcined at 850 C for 6 h. In the following
catalytic studies, the influence of different reaction parameters such as catalyst amount, the
molar ratio of glycerol/DMC, reaction temperature, and reaction time was also studied in
response to the conversion of glycerol and GC yield.
1
1
1
1
2
3
3.3.2 Effect of catalyst amount
1
1
1
1
1
1
2
2
2
2
2
2
2
4
5
6
7
8
9
0
1
2
3
4
5
6
In this study, the effect of catalyst amount on the transesterification profile (glycerol
conversion and GC yield) was investigated. The catalyst amount varied between 0.1–0.50 g
(0–5 wt.% with respect to glycerol amount), whereas the other reaction were fixed, i.e. molar
o
ratio of 1:2, reaction temperature of 70 C, and reaction time of 90 min. According to Fig.8, it
is clear that the addition of catalyst enhanced the conversion of glycerol and ameliorated the
yield of GC. A linear increase in glycerol conversion and the GC yield was observed by the
effect of the incremental amounts of catalyst added, due to the increase in the number of
accessible active sites over the catalyst surface that participate in the catalysis process. Over
2
0.40 g of Mg1.2Ca0.8O catalyst, glycerol was completely converted and the yield of GC was
100%. It was noticed that further increase in the catalyst amount beyond 0.40 g did not
influence the conversion of glycerol and GC yield, which explain the stability of this catalyst
against the possible emulsification of the solid-liquid mixture. The obtained results are in
good agreement with those reported in literature [20]. The observed stability of Mg1.2Ca0.8O
2
1
8
Page 18 of 43

1
2
3
catalyst structure against the emulsification by the co-produced methanol is not surprising,
since it has found by XRD analysis that this catalyst has a highly crystalline cohered
structure.
4
5
3.3.3 Effect of reaction temperature and the molar ratio of glycerol/DMC
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
The effect of temperature on the transesterification of glycerol with DMC was
o
investigated by carrying out the reaction at varying temperatures (50 to 80 C). Whereas the
effect of molar ratios of glycerol/DMC was studied in conjunction with that of temperature
by using two molar ratios; 1:1 and 1:2. Fig.9 shows the obtained results of glycerol
conversion and GC yield at different temperatures and molar ratios. Since the
transesterification of glycerol with DMC is a reversible reaction, excess DMC was required
to shift the equilibrium towards the GC formation. It can be seen that the obtained values of
conversion and GC yield using 1:1 molar ratio were less than those obtained using a molar
ratio of 1:2 at the same reaction temperatures, which explain the role of the excess DMC in
shifting the forward reaction.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
Generally, the conversion was observed to steadily increase as the temperature
increases at both of the tested molar ratios. This behavior could be explained by the
dependence of the reaction rate on temperature. In such a case, the reaction can perform
faster as the temperature increased and this situation not only increases the contacting
properties but also the GC yield as well. In the case of using a molar ratio of 1:2, the
o
conversion of glycerol attained its maximum value of 100% at 70 C, which can be
considered as the least among the reported studies [16-18, 23, 35, 36].
3.3.3 Effect of reaction time
In accordance with the investigations made and discussed earlier, the
2
transesterification of glycerol with DMC over Mg1.2Ca0.8O catalyst was studied at varying
1
9
Page 19 of 43

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
reaction duration and the obtained results and shown in Fig.10. The figure revealed the effect
of time when it is increased to achieve higher glycerol conversion and GC yield. It was
observed that the conversion of glycerol substantially increased from 37% to 81% as the
reaction time elongated from 30 to 60 min. The highest conversion close to 100% was
achieved after 90 min, a time at which the reaction was shifted to completion as the obtained
GC yield was 100%. Further increasing the reaction time beyond 90 min did not influence
glycerol conversion, but resulted in a slight decrease of GC yield which is attributed to the
solubility effect of the co-produced methanol.
The activity of the synthesized catalyst was compared to other heterogeneous
catalysts reported in previous studies (pure MgO and CaO) and it was found that the
synergistic interactions of the metal anions Mg and Ca played a key role in the observed
1
1
1
1
1
1
1
1
1
1
2
activity of the mixed oxides formulation Mg1.2Ca0.8O . Ochoa-Gomez and coworkers
reported transesterification of glycerol with DMC over a heterogeneous CaO catalyst [6].
They obtained complete glycerol conversion with a corresponding GC yield of 95% by using
o
a molar ratio of glycerol/DMC of 1:3.5 and performing the reaction at 95 C for 1.5 h. Thus,
+
2
the present work emphasized on improving the leaching of the Ca species from the CaO
catalyst by masking it with MgO crystals and thereby economizing the operational
conditions. In this work, glycerol was completely converted to GC by using a molar ratio of
o
glycerol/DMC of 1:2 and running the reaction at 70 C for 90 min.
2
2
0
1
3.4 Catalyst stability tests
2
2
2
2
2
2
3
4
5
6
2
The stability of the catalytic activity of Mg1.2Ca0.8O catalyst was examined by testing
the leaching and the reusability. In order to justify the use of mixed metallic oxides as a
catalyst rather than using either of them, the leaching test was performed for CaO and
Mg1.2Ca0.8
2
O catalysts to show the improved leaching properties. The leaching of the metallic
2
0
Page 20 of 43

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
species was tested according to the hot filtration method, where a mixture of the solid catalyst
o
(0.30 g) and DMC (18.2 mL) was heated to the reaction temperature of 70 C under constant
stirring for 90 min. Afterwards, the catalyst was removed and 8 mL of glycerol was added to
the residual DMC. The liquid mixture was then subjected to same reaction conditions
described above and the values of glycerol conversion and GC yield were accordingly
determined and compared to those obtained from uncatalyzed experiment. It was found that
2
CaO catalyst suffered of leaching to about 21.8%, whereas Mg1.2Ca0.8O catalyst suffered of
7.4% leaching of its active metallic species. It is therefore clear that the combination of MgO
with CaO resulted not only in increasing the base amount of the latter but also improved the
+
2
1
1
1
1
1
1
1
1
1
1
2
2
stability of the Ca species.
On the other hand, the recyclability of the Mg1.2Ca0.8
2
O catalyst was examined by
utilizing it in four consecutive batch runs. The catalyst was removed after each catalytic
experiment and regenerated by washing it thoroughly with methanol and drying it in an oven
o
at 75 C for 4 h. Fig.11 presents the results of glycerol conversion and GC yield obtained
from four successive transesterification experiments using the regenerated Mg1.2Ca0.8O
2
catalyst. The results revealed that this catalyst is reliably reusable with no appreciable
decrease in its activity. The stability of the catalytic activity of this catalyst during the
transesterification of glycerol is attributed to the well organized crystallite structure of its
+
2
+2
phases which resulted in stabilizing the metallic species of Mg and Ca , as also confirmed
by leaching test. Therefore, it is highly recommended to commercialize this catalyst for
industrial scale production of the valuable glycerol carbonate.
2
2
2
2
2
3
4
5
2
The spent Mg1.2Ca0.8O catalyst was also characterized to inspect the changes in its
textural and structural properties after the fourth recycle. The BET surface area was found to
decrease by 4%, the pore volume by 2% and the base amount by 1.3% (Table 1). This change
is associated with the change of catalyst color from white to beige and might be ascribed to
2
1
Page 21 of 43

1
2
3
4
5
6
7
the presence of some of the unreacted glycerol molecules that were not removed by methanol
washing. In general, the decrease in surface properties is marginal and lead to the conclusion
that this catalyst is stable under the transesterification conditions. Also, FTIR characterization
of the spent catalyst unveiled the preservation of the intrinsic chemical bonding within the
structure. Moreover, XRD analysis confirmed the stability of the crystalline structure of the
2
spent Mg1.2Ca0.8O catalyst. Thus, the reusable behavior of this catalyst (Fig.11) could be
mainly attributed to its structural stability under the reaction conditions.
8
9
4. Conclusions
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
2
It can be concluded that Mg1+xCa1-xO mixed oxides are an efficient catalyst for the
transesterification of glycerol with dimethyl carbonate (DMC) to synthesize glycerol
carbonate. The catalytic textural properties and the synergistic interaction between Calcium
and Magnesium were observed to influence the catalytic performance. The investigations
unveiled that the activities of bulk CaO and MgO were lower than that of bimetallic mixed
oxide catalysts, as they gave lower yields of glycerol carbonate when subjected to the
transesterification process under the same conditions. The characterization results of FTIR
XRD, and EDX indicated the presence of combined metallic oxides at the preferred loading.
The Mg-Ca catalysts' activity depends mainly on the mass ratio of the metals and the
2
calcination temperature, being the catalyst Mg1.2Ca0.8O with Mg:Ca mass ratio of 1:1.1 and
o
calcined at 850 C was the most active due to the presence of strong basic sites. Over 0.30 g
of this catalyst, the complete conversion of glycerol with corresponding 100% yield of
o
glycerol carbonate were achieved using a molar ratio of glycerol/DMC of 1:2 at 70 C and
after 90 min. The yield of glycerol carbonate also depends upon the metallic content within
the catalyst structure and the reaction conditions.
2
5
2
2
Page 22 of 43

1
Acknowledgements
The authors acknowledge the research grant provided by the Universiti Sains
2
3
4
5
Malaysia, under the Postgraduate Research Grant Scheme (PRGS, 1001/PJKIMIA/ 8044025),
that resulted in this work.
6
7
8
9
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[32] M. C. G. Albuquerque, J. Santamaría-González, J.M. Mérida-Robles, R. Moreno-
Tost, E. Rodríguez-Castellón, A. Jiménez-López, D. C. S. Azevedo, C. L. Cavalcante
Jr, P. Maireles-Torres, Appl. Catal. A: Gen.347 (2008) 162-168.
[33] J.T. Kozlowski, M.T. Aronson, R.J. Davis, Appl.Catal. B: Env. 96 (2010) 508-515.
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[34] K. Ismail, M.A. Yarmo, Y.H. Taufiq-Yap, A. Ahmad, Int. J. Hyd. Energ. 37 (2012)
3639-3644.
[35] A. Takagaki, K. Iwatani, S. Nishimura, K. Ebitani, Green Chem. 12 (2010) 578-
581.
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1
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1
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[36] J. R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, C.A. Ramírez-López, J. Nieto-
Mestre, B. Maestro-Madurga, M. Belsué, Chem. Eng. J. 175 (2011) 505-511.
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Scheme caption
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0
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2
Scheme1:
Transesterification of glycerol with dimethyl carbonate.
Figure captions
1
3
Fig.1
Nitrogen physisorption isotherms for mixed Mg-Ca oxide catalysts.
1
1
4
5
Fig.2
TPD-CO
2
of pure and mixed metal oxides catalysts (CaO, MgO, Mg0.8Ca1.2
).
Fig.3 a) XRD patterns of Mg-Ca catalyst samples; (a) Mg1.2Ca0.8
O
2
and
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1
1
2
2
2
2
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0
1
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3
Mg1.2Ca0.8O
2
O
2
; (b) Mg0.8Ca1.2
O
2
; (c)
spent Mg1.2Ca0.8
2
O .
b) XRD patterns of MgO, CaO, Mg1.8Ca0.2
2
O .
Fig.4
Fig.5
FTIR spectra of pure and mixed MgCa oxides.
2
Scanning electron micrograph images of Mg1.2Ca0.8O catalyst calcined at; (a) 850
o
o
C; (b) 700 C.
o
o
Fig.6
Fig.7
2
TEM images of Mg1.2Ca0.8O catalyst samples calcined at 700 C(a); and 850 C (b).
2
2
4
5
Glycerol conversion and GC yield obtained using catalyst samples calcined at
o
different temperatures (700–900 C) and times (4–6 h).
2
2
2
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7
8
Fig.8 Effect of catalyst amount on glycerol transesterification with DMC. Reaction
conditions: catalyst amount: 0.30 g; molar ratio of glycerol/DMC: 1:2; reaction
o
temperature: 70 C; reaction time : 90 min.
2
6
Page 26 of 43

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Fig.9 Effect of reaction temperature on glycerol transesterification with DMC studied at
two molar ratios of glycerol/DMC (1:1 and 1:2). Reaction conditions: catalyst
amount: 0.30 g; reaction time: 90 min.
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Fig.10 Effect of reaction time on the transesterification of glycerol with DMC. Reaction
conditions: catalyst weight: 0.30 g; molar ratio of glycerol/DMC: 1:2; reaction
o
temperature: 70 C.
2
Fig.11 Reusability test of Mg1.2Ca0.8O catalyst during the transesterification of glycerol
with DMC. Reaction conditions: catalyst amount: 0.30 g; molar ratio of
o
glycerol/DMC: 1:2; reaction temperature: 70 C; reaction time: 90 min.
1
0
List of Tables:
1
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7
Table 1.
BET surface area, total pore volume, average pore diameter and base amount
of the synthesized Mg-Ca catalysts.
Table 2.
Catalytic activity of different heterogeneous catalysts employed in glycerol
transesterification with DMC. Reaction conditions: Catalyst weight: 0.3 g;
o
molar ratio of glycerol/DMC:1:2; reaction temperature: 70 C; reaction time: 90
min.
1
8
2
7
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Scheme 1
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Fig. 1
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