Yttrium-grafted mesostructured SBA-3 catalyst for the transesterification of glycerol with methyl acetate to synthesize fuel oxygenates

Article abstract of DOI:10.1016/j.apcata.2013.03.045

The synthesis of di- and triacetyl glycerol (DAG and TAG) via the transesterification of glycerol with methyl acetate was investigated over series of solid acid catalysts prepared by grafting yttrium, 1-3.5 wt.%, into the framework of SBA-3 support. The p

Full text of DOI:10.1016/j.apcata.2013.03.045

Accepted Manuscript
Title: Yttrium-grafted mesostructured SBA-3 catalyst for the
transesterification of glycerol with methyl acetate to
synthesize fuel oxygenates
Author: M.S. Khayoon B.H. Hameed
PII:
S0926-860X(13)00192-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.03.045
APCATA 14172
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-12-2012
18-3-2013
30-3-2013
Please cite this article as: M.S. Khayoon, B.H. Hameed, Yttrium-grafted
mesostructured SBA-3 catalyst for the transesterification of glycerol with methyl
acetate to synthesize fuel oxygenates, Applied Catalysis A, General (2013),
http://dx.doi.org/10.1016/j.apcata.2013.03.045
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Yttrium-grafted mesostructured SBA-3 catalyst for the transesterification of
glycerol with methyl acetate to synthesize fuel oxygenates
M.S. Khayoon, B.H. Hameed*
School of Chemical Engineering, Engineering Campus,
Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
*
Corresponding author Fax: +6045941013
E-mail address: chbassim@eng.usm.my (B.H. Hameed)
1
Page 1 of 41

Abstract
The synthesis of di- and triacetyl glycerol (DAG and TAG) via the transesterification
of glycerol with methyl acetate was investigated over series of solid acid catalysts prepared
by grafting Yttrium, 1 to 3.5 wt%, into the framework of SBA-3 support. The prepared
catalysts were characterized for their intrinsic physicochemical and textural properties using
BET surface area, surface acidity, XRD, SAXS, FT-IR, TEM, SEM and EDX. The catalyst
3%Y/SBA-3 exhibited superior catalytic performance achieving complete glycerol
conversion with corresponding selectivity of 32% and 64% toward DAG and TAG. The best
reaction conditions are: molar ratio of glycerol to methyl acetate of 1:12, reaction temperature
o
of 110 C and reaction time of 3 h. The effect of different operational parameters such as
reaction temperature, the molar ratio of glycerol to methyl acetate, catalyst weight and
reaction time on the transesterification reaction was investigated. The stability of the catalytic
activity was examined via leaching and reusability tests through four consecutive batch runs.
Keywords: SBA-3; Yttrium; Glycerol; Transesterification; Triacetin.
2
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1. Introduction
The increased demand for global energy consumption as a result of rapid industrial
growth, soaring world population, escalating in crude oil prices and the consequences of
petroleum fuel combustion on the environment have motivated the biocrude refinery industry
[1, 2]. Recently, bio-based fuels (bioethanol, biogasoline and biodiesel) which are built of
renewable biohybrocarbons have come to the forefront of the green fuel industry [3].
Biodiesel, also known as fatty acid methyl esters (FAME), has been categorized as "future
fuel" due to its high BTUs per gallon and high cetane number, which translated into higher
combustion efficiency in internal combustion engines [4, 5]. It can be obtained via the
transesterification reaction of vegetable oils with short chain alcohol (mostly methanol).
Inevitably, the biodiesel synthesis process furnishes glycerol as undesirable by-
product with about 10 wt.% of the total biodiesel produced [6]. Therefore, it seems
imperative to find more economical destinations for this glycerol by converting it to valuable
commodity chemicals; otherwise the economic feasibility of biodiesel is imperiled. Several
studies have been reported for the transference of glycerol to diversified chemicals via
numerous routes such as oxidation [7], acetalization [8], esterification [9], hydrogenolysis
[10], etherification [11], dehydration [12] and carbonylation [13]. Moreover, glycerol can be
transformed into value added products of mono-, di- and triacetyl glycerol (namely MAG,
DAG and TAG, respectively), which have found colossal industrial applications [14]. They
can be obtained through the acetylation and/or esterification of glycerol with acetic acid
and/or acetic anhydride [6, 15].
Recently, the transesterification of glycerol with methyl acetate has been introduced
as an alternative pathway for the synthesis of glycerol acetates (MAG, DAG and TAG) [16].
Methyl acetate (C
3
H
6
O
2
) with its low boiling point of 56.9 °C and non-toxic nature can
3
Page 3 of 41

effectively lead to the new strategy for the synthesis of glycerol acetates (especially DAG and
TAG) through the acid catalyzed transesterification reaction. It's worth mentioning that the
ability of methyl acetate as acyl group donor has been tested and described in the literature
for the transesterification of vegetable oils to synthesize FAME and TAG simultaneously by
using either enzymatic [17] or alkali catalyzed processes [18].
Compared to the acetylation reaction that generates water as a byproduct which may
harm the catalyst and affect the product quality, the transesterification reaction produces
methanol as a byproduct, which can be recycled to the biodiesel production process. In
addition, employing methyl acetate in such a reaction could contribute toward: (i) providing
an environmental reaction pathway for the synthesis of glycerol acetates (fuel additives), (ii)
reducing the technical problems associated with the use of acetic acid such as corrosion,
water by-production, and the difficulty to separate the produced acetates due to their
miscibility with acetic acid, (iii) increasing the effectiveness of the catalysts used in the
reaction when water-free process is applied, (iv) switching of the reaction pathways that
results in an improved selectivity.
In 2011, the first and the only research paper for the stated reaction has been reported
[16]. The authors studied the reaction using sulfonic acid-functionalized mesostructured
SBA-15 catalyst, with harsh reaction conditions of methyl acetate to glycerol molar ratio
o
(
50:1), reaction temperature of 170 C, and 4 h of reaction time. The process utilized the
conversion of 99% of glycerol with corresponding selectivity of 74.2% toward both DAG and
TAG with no more than 6% of TAG formed. Thus, it seems encouraging for the research
community to explore this interesting reaction toward more economized process with higher
productivity of the favored TAG product. Therefore, this research aims to investigate the
synthesis of highly active heterogeneous catalyst that can effectively bring down the
operational conditions and thereby the cost of the produced additives. The herein investigated
4
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the reaction of glycerol with methyl acetate represents the breakthrough for the synthesis of
triacetyl glycerol (TAG) from glycerol.
On the other hand, catalyst preparation by grafting of metal species may result in
hindering of migration, agglomeration and sintering during the subsequent thermal treatment
and thus, furnishes highly dispersed metal ions within the catalyst structure. According to
IUPAC, grafting has been defined as the formation of strong (covalent) bonding between
support and metal complexes (in the early step of immobilization). Contrary to the
impregnation which proceed through adsorption (chemi- and/or physisorption) of metal
clusters onto the support surface, grafting technique proceeds through surface reaction
−
(
condensation, protolysis) of the supporters' functional groups (e.g. OH ) and the metal
precursor. Therefore, the present work emphasized on the grafting method to prepare the
yttrium containing SBA-3 catalysts.
2. Materials and methods
2.1 Materials
Anhydrous glycerol of high purity (>99%) was obtained from Sigma, Germany.
Methyl acetate (≥99.5%) and HPLC grade ethanol and methanol (99.7%) were supplied by
Merck, Malaysia. Tetraethylorthosilicate (TEOS), Yttrium nitrate hexahydrate
3 3 2
(Y(NO ) .6H O) of 98% purity and Cetyltrimethyl ammonium bromide (CTMABr) were
purchased from Sigma-Aldrich, Malaysia. Hydrochloric acid (HCl 37%) was obtained from
Mallinckrodt, USA. All reagents were used without further purification. Deionized water was
used throughout this work.
2.2 Synthesis of support (SBA-3)
5
Page 5 of 41

The Mesoporous SBA-3 material was synthesized according to the method reported in
literature [19]. The synthesis was performed using cetyltrimethyl-ammonium bromide
(CTMABr) and TEOS as a template and Si source, respectively. Typically, 2.0 g of CTMABr
and 40 ml of HCl (37%) were dissolved in 100 ml of water and stirred at room temperature
for 15 min. TEOS (10 ml) was added dropping to the acidic solution of CTMABr with
o
vigorous stirring at 30 C for 2 h . The pH of the reaction system was controlled by adding
drops of an aqueous solution of HCl (37%). Later on, the synthesis gel was aged at room
o
temperature for 12 h. The solid was removed by filtration and then dried overnight at 100 C.
The literature method used extraction technique with ethanol to remove the surfactant
followed by the calcination of SBA-3 material, which is costly and lengthy. In this work,
o
o
SBA-3 was calcined at 560 C in air for 8 h with a heating rate of 2 C/min.
2.3 Synthesis of Yttrium-grafted into SBA-3 catalysts
A series of catalysts with varying amounts of yttrium (Y, 1–3.5 wt%) loaded onto the
SBA-3 support were synthesized either by grafting Yttrium into the framework of SBA-3
material or by wet impregnation technique. Catalysts synthesized by grafting technique
followed the foregoing procedure for the synthesis of pristine SBA-3 material except that
3 3 2
Y(NO ) .6H O was introduced during the synthesis process. Typically, a calculated amount
of Yttrium nitrate was added to the CTAMBr solution and stirred for 30 min. TEOS was then
added and the same procedure was continued. For comparison, Y/SBA-3(Im) catalysts were
prepared by impregnation method [20]. Solution A: 1 g of the calcined SBA-3 material was
dispersed in ethanol. Solution B: the appropriate amount of Yttrium nitrate with the preferred
Y loading was dissolved in deionized water. Solution B was added into solution A and the
mixture was stirred for 30 min and then aged in a sealed flask at ambient temperature for 24
h. The solvents were removed by evaporation and the solid was collected, washed with
6
Page 6 of 41

o
deionized water and air dried for 12 h. The catalysts were calcined at 560 C in air for 8 h
o
with a heating rate of 2 C/min. The prepared catalysts (by grafting and by impregnation)
were employed for glycerol transesterification with methyl acetate.
2.4 Characterization of the catalysts
The determination of surface areas and textural characteristics of the prepared
catalysts was carried out using Brunauer Emmett Teller (BET) method over Micromeritics
ASAP 2020 surface area and porosity analyzer (Micromeritics instruments corporation,
USA). X-ray diffraction (XRD) patterns of the pristine SBA-3 support and the Y containing
SBA-3 catalysts were acquired on Bruker D8 Focus X-ray diffractometer (Bruker, Germany)
o
o
over 10 ≤ 2θ ≤ 90 using Cu k
α
radiation (λ=1.5406˚). Small angle X-ray scattering (SAXS)
o
o
diffractograms over 0.5 ≤ 2θ ≤ 5 were used to determine the structural arrangement of the
synthesized materials. FTIR spectra of the support and catalyst samples were acquired on
Perkin–Elmer System 2000 spectrometer by using the standard KBr technique over the range
–1
of 1500–400 cm . Scanning electron microscopy (SEM) was used to study the morphology
of the synthesized materials 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.
Energy-dispersive X-ray spectroscopy (EDX) was performed in conjunction with SEM to
determine the metallic composition of the catalyst. The acidity of prepared catalysts was
determined by a neutralization titration procedure.
2.5 Catalytic reactions
The transesterification reaction of glycerol with methyl acetate was carried out in 50
ml round bottom glass flasks immersed in an oil bath and mounted on hot plate equipped with
a magnetic stirrer. In a typical run, 4 g of anhydrous glycerol and 27.5–41.3 ml of methyl
7
Page 7 of 41

acetate at variant molar ratios of glycerol to methyl acetate were fed to the reactor, and 0.15
g of solid catalyst was added. The reaction mixture was heated to the desired reaction
o
temperature (80–120 C) at constant stirring of 530 rpm for reaction duration of 30–180 min.
After completion of the reaction, the used catalyst was separated by centrifugation and
o
o
reactivated by washing it with methanol at 45 C and then air dried at 80 C for 4 h to
recuperate its catalytic activity.
To study the reaction profile, reaction products were analyzed by a 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 adding 20 μL of ethanol as an internal standard to 0.20 ml of the
sample. About 0.20 μL of the final sample was then injected into the column. The column
o
o
o
temperature was initially set at 60 C (2 min) followed by ramping of 10 C/min to 260 C
o
o
and then heated at 30 C/min to 300 C. The FID and injection temperatures were fixed at 350
o
o
C and 250 C, respectively. Helium at 1.3 ml/min was used as carrier gas. The injection split
ratio was 10.
3
3
3
. Results and discussion
.1 Catalysts Characterization
.1.1 Textural properties and surface acidity
p
Table 1 presents the characterization results (SBET, V , and the pore size) calculated
o
from the N
2
adsorption–desorption isotherms at −197 C. All the prepared catalysts
2
−1
possessed high surface areas ranged from 1462–1622 m g . It was observed that catalyst
preparation by grafting of Yttrium species resulted in samples (Y/SBA-3) with higher surface
area and pore volume than the pristine SBA-3 material. This increment in the surface
properties after the grafting of yttrium ions into the lattice of SBA-3 material is caused by an
8
Page 8 of 41

inhibitory effect to the sintering of nano-sized yttrium components [21]. The observed effect
of yttrium toward increasing the surface area of the catalyst is corroborating with that
reported by Fernandez and coworkers [22]. On the contrary, the catalyst samples prepared by
impregnation method showed decreased surface area and pore volume, which might be
explained due to the occupation of guest species on the surface of the pores as well as the in
equable contribution of the additional mass of Y(NO
results fit well with the observations reported in literature for the increased surface area [21,
3].
3 3
) in the synthesis mixture. These
2
Fig.1a and b show the N
SBA-3 and the so modified 3%Y/SBA-3 materials. It can be seen that the pristine SBA-3
material exhibits the type IV N isotherm with a desorption hysteresis loop of type H2, which
2
adsorption/desorption isotherms obtained for the pristine
2
is characteristic of mesoporous materials [24]. The pristine SBA-3 material presented three
distinct regions of physisorption (Fig.1a). The first shows a high “knee”, followed by a
slower growth in the adsorbed volume at low relative pressures (P/P
attributable to a monolayer-multilayer adsorption on the pore walls. The as-shown high
knee" may indicate the presence of micropores in the pristine SBA-3 material. The second
region assigned at a relative pressure P/P 0.15 to 0.3 shows a sharp step characteristic of
o
< 0.1), which might be
"
o
capillary condensation within the mesopores [25]. It was also observed that the hysteresis
loop size increases after the grafting of yttrium ions into SBA-3, being the big hysteresis are
attributable to the existence of pore cavities larger than the openings (throats), indicating the
presence of ink-bottle shaped pores in this material [26]. The isotherm shows a very slow
o
linear increase at higher values of the relative pressure (P/P >0.3), explaining the adsorption
of N
2
on the outer surface of SBA-3.
Fig.1b shows the characteristic N
2
physisorption isotherm for the 3% Y/SBA-3
material, which is of the type IV with H4 hysteresis loop. This is characteristic of
9
Page 9 of 41

mesoporous materials, and could evidently explain that introducing Y species to the SBA-3
structure did not influence the mesoporosity of the virgin surface to a great extent. The
change of the hysteresis loop type, compared to the pristine material isotherm, and shifting it
o
toward higher values of the relative pressure (0.65< P/P >0.95) indicate the formation of
plate-like particles giving rise to slit-shaped pores. This case is often associated with narrow
slit-like pores and for the case of 3% Y/SBA-3 material, it may be ascribed to the
intercalation of yttrium species into the support structure.
The superior textural properties of SBA-3 material are believed to introduce a
catalytic material (3%Y/SBA-3) that could efficiently facilitate the transesterification of
glycerol with methyl acetate. The herein developed catalyst presented extremely high
catalytic activity during the acetylation reaction, achieving complete glycerol conversion with
favorable results of selectivity during the preliminary test of its catalytic activity. This has
motivated us to embark on further investigation of its catalytic activity.
The acidity of all the synthesized catalysts is also illustrated in Table 1. The acidity of
the solid materials was determined by the standard neutralization titration technique [6]. The
surface acidity of this catalyst (3%Y/SBA-3) was 1342 μmol/g, which is mostly attributable
to the presence of the acidic yttrium ions within the its structure. Catalyst surface acidity was
found to be strongly dependant on the amount of Y grafted to SBA-3 material i.e., the higher
Y loading the higher surface acidity (Table 1). This could be explained by the enhanced
−
protonation of the surface hydroxyl groups (OH ) with the increase of Y loading, as the
pristine silica surface is highly anionic.
Also, it was observed that the impregnation method provides higher acidity than
grafting method at the same Y loading, which might be explained by the mechanism of each
method. In impregnation, an electrostatic adsorption action draws the metal precursor into the
−
pores and result in the protonation of the surface hydroxyl groups (OH ). This mechanism
1
0
Page 10 of 41

leads to the formation of water coordinated metal ions that possess stronger Br nsted acidity
Ö
than free water. For these chemisorbed moieties, the following acid-base equilibria are
occurring:
+
+
≡
≡
Y−OH
Y−OH
2
≡Y−OH + H
−
+
≡Y−O + H
Whereas the acidity obtained by grafting method is most likely attributable to the
ionic pairing of the catalyst components, which leads (in most cases) to strong bonding
between the metal complexes and surface groups. Further thermal treatments could result in
decomposition or desorption of surface protonated groups due to the transformation of the
precursor into catalytically active phase, but do not affect the surface acidity to a great extent.
This feature is then believed to be one of the reasons for the superior activity of this catalyst
in the acid catalyzed reactions, but not the main cause of the selective behavior.
3.1.2 Small angle X-ray scattering (SAXS)
This analysis, at very low angles (typically 0.1–10°), provides information about the
shape and size of macromolecules, characteristic distances of partially ordered materials, pore
sizes, and other data. SAXS is capable of delivering structural information of
macromolecules between 5 and 25 nm, of repeat distances in partially ordered systems of up
to 150 nm. In this work, SAXS analysis was performed for the SBA-3 and 3%Y/SBA-3
materials, as shown in Fig.2. The pattern shows three distinguished Bragg angles in
hexagonal lattice symmetry, indicating the ordered structure of SBA-3 material before and
o
o
after the modification with yttrium species [19]. The well resolved peaks at 2θ =2.62 , 4.74
o
and 5.50 were noticed for SBA-3 and Y/SBA-3 materials, which explains that the obtained
1
1
Page 11 of 41

mesoporous material has a highly ordered pore system with a high porosity [27]. The sturdy
peak as well as the two weaker peaks are corresponding to typical unit cell indices of hkl =
[
100], [110] and [200], respectively, which is characteristic of one-dimensional hexagonal
P6m) mesostructure.
(
The recorded patterns for the yttrium modified sample showed signals of lesser
intensities, when compared to the pristine material. Moreover, the patterns showed that the
grafting of yttrium ions did not disturb the crystallization of mesoporous structure of SBA-3
material. It is worth mentioning that the attempts to introduce a higher dose of the yttrium
ions (> 5 wt.%) into the SBA-3 structure were unsuccessful and resulted in an amorphous
material. The unit cell parameters were calculated and their values are shown in Table 1. At
higher angles, no distinct peaks are observed, implying the amorphous wall nature of the
SBA-3 material and suggesting the presence of intra-wall mounted micropores [21]. The
presence of such pores is coinciding with the P6m mesostructure in higher amounts than that
of P6mm structure, which might be attributed to their wall structures as also proved by XRD
analysis. Indeed, this can give a logical explanation for the structural stability of this catalyst
under the transesterification reaction conditions.
3.1.3 X-ray diffraction (XRD)
The obtained XRD patterns for the pristine SBA-3 and the so-modified 3%Y/SBA-3
2
materials are shown in Fig.3. A typical X-ray diffractogram for the SiO based crystalline
o
materials with wide peak assigned at 2θ of 23.6 was observed for both materials, suggesting
the amorphous nature of the SBA-3 silicate wall [20, 28]. The modification of the silicate
support with yttrium species resulted in decreasing the intensity of that peak and
o
straightening the base line of the diffractogram after the value of 2θ of 23.6 . This might be
ascribed to the decrease in crystallinity caused by the interaction of yttrium ions with the
1
2
Page 12 of 41

silicate substrate. The distinct distortion to the base line of the modified catalyst (3%Y/SBA-
2
3) could be attributed to the oscillatory diffractions of the yttrium grafted SiO material.
3.1.4 Fourier transform infrared analysis (FTIR)
In order to gain more insights into the catalytic activity of the synthesized catalysts,
FTIR spectra for those materials were collected and accordingly analyzed. Fig. 4 presents the
FT-IR spectra of the pristine and the yttrium grafted catalyst samples in the same fingerprint
–1
region. For both samples, a broad band is seen at about 1075 cm which can be attributed to
the asymmetric stretching vibrations of the structural siloxane bond (Si–O–Si) tetrahedron
4
SiO regardless of the grafting of yttrium ions [19, 29]. Whereas the IR absorptions at about
–
1
–1
4
60 cm and 860 cm indicate the symmetric stretching vibrations of the Si–O–Si in the
–1
structural siloxane bond and the band at 1350 cm is assigned to free silica [19, 25]. The
–1
grafting of Yttrium components resulted in masking the shoulder at about 960 cm in the
spectrum of pristine SBA-3 and extending this shoulder to a well-resolved peak. The shoulder
was assigned to the stretching modes of Si–O bonding in the spectrum of SBA-3 material.
–1
The appearance of the dentate band at about 960 cm in the case of 3%Y/SBA-3 spectrum is
due to the new bonding with the substrate material, and this band could be ascribed to the
stretching vibration modes of Y–O–Si [21, 23]. The presence of Y–O bonding within SBA-3
–1
material is probably masked by the broad peak assigned at 460 cm which was defined as
(Si–O–Si) bonding for the pristine support (SBA-3), but thereafter interacted with that of
–1
Y–O bond to generate a broader band [24, 25]. The band at about 570 cm has been reported
to be associated with the presence of Y–O within the structure [23]. It is worth mentioning
–1
that no precise definition to the shoulder located at 1245 cm was found in the reported
literature. In conclusion, the structure of the as-synthesized 3%Y/SBA-3 catalyst has
contained the active yttrium species as a building component that could possibly explain the
1
3
Page 13 of 41

stability of the catalytic activity of this catalyst during the transesterification of glycerol with
methyl acetate.
3.1.5 Scanning electron microscopy (SEM)
The as-synthesized (SBA-3 and 3%Y/SBA-3) materials were microscopically scanned
to investigate their particle morphology, as shown in Fig.5a and b. Fig.5a shows that SBA-3
material consists of globules with a diameter of 2–3 µm agglomerated in decorated
aggregates. It is important to point out that, in spite of the few reports available on controlling
the morphology of SBA-3 [19, 27], the observed textural structure of the crystals was of good
morphology that efficiently supported the yttrium clusters. The 3%Y/SBA-3 catalyst was also
scanned to evaluate the morphology change after the grafting of yttrium species into the
lattice of SBA-3 (Fig.5b). It can be seen that the morphology of the support was not affected
after the modification with yttrium species, which prefer to allocate in the form of small
spheres on the circumference of the support particles. According to Fig.5b, small fibers are
appearing on the support globules with unrecognized boundaries from the globule side,
indicating the strong interaction of yttrium units with the silica support.
3.1.6 Elemental analysis (EDX)
The mean metallic content in the 3%Y/SBA-3 catalyst was determined by EDX for
the fresh and spent catalysts. The results revealed that the catalyst preparation procedure was
highly accurate, since the actual loading of yttrium was close to its theoretical value (Table
2). The obtained values from this analysis depend mainly on the mass fraction of each
contributing component. The results of this analysis revealed that the percentage of yttrium in
1
4
Page 14 of 41

the prepared catalyst was found to be 2.96 wt.%, which is close to the theoretical value of
3
wt.%.
3.1.7 Transmission electron microscopy (TEM)
Fig.6a–c show the obtained TEM images of the as-synthesized SBA-3 material and
%Y/SBA-3 catalyst. The images confirmed the presence of well ordered cubic 3D
3
mesoporous channels and confirmed that 3%Y/SBA-3 catalyst possesses a P6m cubic
structure as does SBA-3. The pore size (~28 Å) is clearly obvious from the TEM image.
Fig.6a shows the beam direction TEM image of the pristine SBA-3 material, where it can be
clearly seen the highly ordered surface channels. The presence of hexagonal pores arranged
in parallel lines over SBA-3 surface is elucidated in the sectional TEM image (Fig.6b). The
grafting of yttrium ions to the lattice of SBA-3 material did not affect the primary structure of
the support as shown in the bidirectional image in Fig.6c. The size of particles determined
from TEM analysis were correlated with those calculated using Debye–Scherrer formula, i.e.,
8.4 and 12 ± 2 nm for SBA-3 and 3%Y/SBA-3, respectively.
3.2 Catalytic activity: transesterification of glycerol with methyl acetate
In this study, the catalytic activity was identified by means of glycerol conversion to
glycerol esters namely: mono-, di-, and triacetyl glycerol (MAG, DAG and TAG,
respectively) and the selectivity toward these products. The path of such reaction involves
the conversion of glycerol to MAG, whereas DAG and TAG products will be formed
1
5
Page 15 of 41

through consecutive esterification reactions [30, 31]. This mechanism suggests that low
MAG content product will be of high DAG and/or TAG content (based on high glycerol
conversion) and vice-versa.
It is reported that the presence of MAG in the product mixture is inevitable, which is
undesired due to its high solubility in water [9, 32]. Thus, minimizing its amount in the final
product represents the challenge to overcome the solubility problem and to furnish a product
with high DAG and TAG content to be directly utilized as neat fuel additives. This could be
achieved through different synthesis route, where the performance of the esterification agent
(acyl group donor) should be considered. As an alternative, the transesterification of glycerol
with methyl acetate over a 3%Y / SBA-3 catalyst was explored in this work. The reaction
O
was investigated over a temperature range of 80–120 C and using a low molar ratio of
reactants.
According to Table 3, catalyst preparation by impregnation presented lower catalytic
activity than the grafted ones, being the 3%Y grafted catalyst showed the highest activity.
The catalyst was notably active, achieving 97% of glycerol conversion with corresponding
selectivity of 4%, 32%, and 64% toward MAG, DAG, and TAG, respectively, using the
o
molar ratio of glycerol/methyl acetate of 1:12 at 110 C. For comparison, uncatalyzed
o
experiment was performed using a molar ratio of reactants of 1:14 at 110 C for 3 h. Only
19% of glycerol conversion was obtained with corresponding selectivity of 83% and 17% to
MAG and DAG with no TAG formation.
The superior catalytic performance of 3%Y/SBA-3 catalyst might be attributable to its
high surface area and its high acidity, as earlier presented in Table 1, and also to its uniform
surface structure, as confirmed by TEM results. In addition, FTIR analysis unveiled the
presence of yttrium oxide within the structure (Y–O bonding), which is believed to play a key
role in enhancing the catalytic performance. In fact, the 3.5%Y/SBA-3 catalyst possessed the
1
6
Page 16 of 41

highest surface acidity of 1365 µmol/g but did not exhibit the best catalytic performance.
This may suggest that the catalytic activity is not only depending on the surface acidity, but
the well dispersion of metallic species as well. In this regard, it is well established that (in
some cases) the higher metallic loading may result in the agglomeration and/or sintering of
metallic clusters and thus, the poor dispersion of the metallic species. It is therefore believed
in this study that the good activity of 3%Y/SBA-3 catalyst is partially attributable to the
mode of yttrium species distribution over that of 3.5%Y/SBA-3 catalyst. Compared to the
literature, only one research paper has been published on this reaction with the claimed
results of 99.5% of glycerol conversion with combined selectivity toward DAG and TAG of
7
1
4% [16]. The reaction was performed using a high molar ratio of glycerol/methyl acetate of
o
:50 at 170 C. Therefore, the herein obtained results are claimed to be breakthrough to those
recently reported in the literature.
3
3
.3 Effect of reaction conditions
.3.1 Effect of glycerol/methyl acetate molar ratio
The influence of glycerol/methyl acetate on glycerol conversion and product
selectivity was equally investigated in this work. The excess methyl acetate amount was
required to shift forward the equilibrium of the transesterification reaction. As the formed
MAG during the first reaction step is water-soluble, thus its fast transformation to higher
glycerol acetates (DAG and TAG) is a necessity. Therefore, there is a need to shift the
reaction towards completion as much as possible by using an excess of methyl acetate. In
spite of these merits, this option may result in increasing the cost of raw materials and also
the cost of products separation and purification. Moreover, increasing this ratio will possibly
result in diluting the system mixture and thereby depreciating the quality of the products.
1
7
Page 17 of 41

It is imperative to find the optimum molar ratio of glycerol/methyl acetate to diminish
the observed disadvantages of increasing that molar ratio. Table 3 presents the values of
glycerol conversion and product selectivity obtained at different molar ratios of
glycerol/methyl acetate. The obtained results strongly confirm the above debate and clearly
explain that the formation of higher glycerol acetates (DAG and TAG) is more favored at a
higher molar ratio of 1:12. Whereas the selectivity to MAG increased at the molar ratio of
1:14, which logically support the above argument for the solubility of the produced esters at
higher amounts of methyl acetate. Since the target of this work was to minimize the amount
of MAG in the product mixture and thereby maximizing that of DAG and TAG, the molar
ratio of 1:12 was chosen as the best for this reaction.
3.3.3 Effect of catalyst amount
The effect of catalyst amounts on glycerol conversion was examined using variable
amount of catalyst (0.10–0.50 g), as shown in Fig. 7. It was observed that the conversion of
glycerol increases with increasing the amount of the catalyst. As such, the maximum
conversion obtained was 97% at 0.40 g of catalyst. The selectivity to DAG and TAG was also
varied with the catalyst amount. An increase in the catalyst amount enhanced the
transformation of glycerol to MAG and DAG, which consequently converted to TAG (as
evidenced by TAG selectivity of 15% at 0.10 g of catalyst and over 64% at 0.40 g of
catalyst). At 0.50 g of catalyst, the conversion was slightly increased to 98% associated with
increasing DAG selectivity to 34% and decreasing that of TAG to 62%. This could be
explained by the accelerated conversion of MAG to DAG. Therefore, 0.40 g of 3%Y/SBA-3
catalyst (corresponding to 10 % of glycerol mass) represents the best catalyst weight which
resulted in 97% of glycerol conversion with corresponding selectivity of 4%, 32%, and 64%
1
8
Page 18 of 41

toward MAG, DAG, and TAG, respectively. Thus, this amount of catalyst was selected for
further investigation of the reaction conditions.
3.3.4 Effect of reaction time
The influence of reaction time on glycerol conversion and product selectivity was
investigated and the results revealed that there was a steady increase in glycerol conversion
with the reaction duration. The study was performed at varying reaction time in the range of
0.5–4 h with fixing the other reaction parameters. According to the presented results in Fig. 8,
it was found that reaction time of 3 h is sufficient for the majority of glycerol to convert to
glycerol acetates. It was also observed that increasing reaction time beyond 3 h resulted in
slightly increase the conversion of glycerol. From the selectivity point of view, the
transesterification reaction was strongly time dependent, i.e. glycerol was mainly converted
to MAG with traces of DAG within the first hour. After 1.5 h, the selectivity to DAG was
33% and the formation of TAG accompanied by the expense of MAG has embarked, leading
to the conclusion that the transformation of DAG to TAG was started at 64% of glycerol was
consumed. Afterwards, the selectivity to TAG kept increasing until its ultimate value of 64%
was achieved after 3 h. To this end, the time factor was kept constant for all the
transesterification studies. In addition to the reaction time, the process cost could be also
minimized by reducing the reaction temperature.
3.3.5 Effect of reaction temperature
The effect of reaction temperature on the transesterification of glycerol with methyl
o
acetate was investigated in the range of 80 to 120 C, as shown in Fig. 9. The conversion of
1
9
Page 19 of 41

o
glycerol increased linearly as the temperature raised from 80 to 110 C, leading to nearly
complete conversion. This can be explained by the endothermic energetic pathway for the
−
interaction between the acylium moiety of methyl acetate with the OH anion of glycerol
o
molecule, which increases as the temperature increased. At 110 C, the formation of higher
glycerol acetates (DAG and TAG) was enhanced due to the protonation of the remaining
hydroxyl groups of the glycerol molecule by steric factors and/or interaction with the active
sites on the catalyst surface [6]. The higher temperature utilized the endothermic subsequent
esterification reactions toward the formation of DAG and TAG. Beyond this temperature, no
considerable change in glycerol conversion was observed.
The reaction temperature was found to be a key factor affecting the selectivity to
o
DAG and TAG, being the higher reaction temperature the higher TAG formation. At 80 C,
o
MAG was the main product with traces of DAG. As the temperature increase to 90 C, the
formation of DAG and TAG was notably increased. The formation of mainly MAG at lower
temperatures might be explained by the formation of an acylium ion intermediate via the
ionic attack of a hydroxyl group of glycerol on the carbonyl group of methyl acetate [33].
The rate of acylium ion transformation to MAG on the catalyst surface represents the fastest
and determining step during the esterification reaction [9,34]. In general, the formation of
higher glycerol acetates (DAG and TAG) usually follows a consecutive endothermic
esterification reactions starting with the formation of MAG, MAG to DAG, and DAG to
TAG through endothermic energetic reactions [9]. In this study, the best results of glycerol
OC
transesterification with methyl acetate were obtained at110 , where 97% of glycerol were
converted to final product containing 4%, 32%, and 64% of MAG, DAG, and TAG,
respectively.
o
Morales and co-workers have investigated the same reaction at 170 C, a temperature
at which three phases are existed in the reactor with possibility to build up an autogenous
2
0
Page 20 of 41

vapor pressure [16]. These phases are the methyl acetate vapor phase, the liquid glycerol and
the solid catalyst. As the evaporated amount of methyl acetate increases with temperature,
this will lead to serious difficulty to bring it in contact with glycerol. In order to ascertain the
liquid phase of both reactants, nitrogen was used to pressurize the vapor methyl acetate in
their study. Therefore, the present work emphasized on reducing the reaction temperature not
only to avoid the technical issues, but also to minimize the overall process cost.
3.4 Catalyst stability tests
The stability of the catalytic activity of 3%Y/SBA-3 catalyst has been examined in
this work via the leaching and reusability tests. The tests were performed to ascertain that the
catalyst can maintain the same productivity of TAG in this process. The leaching test was
performed according to the hot filtration method, whereas the reusability of the catalyst was
tested by recycling it through four consecutive batch runs. For the leaching test, the catalyst
was first immersed in methyl acetate and the mixture then heated to the reaction temperature
o
(
110 C) and kept under constant stirring rate of 530 rpm for 3 h. Afterward, the catalyst was
removed from the mixture and 4 g of glycerol was added to the in-flask methyl acetate. The
methyl acetate-glycerol mixture was subjected to the same conditions described above,
allowing them to react in the absence of the heterogeneous catalyst. Thereafter, the reaction
sample was analyzed to determine the conversion of glycerol, which was found to be 18.3%
and close to that obtained from the blank test 16.1% (blank test means the reaction of
glycerol with acetone without any catalytic material). It seems that there was no appreciable
leaching of the yttrium species from the catalyst structure to the liquid phase, which strongly
validate the claim for the stability of the prepared catalyst.
After each catalytic experiment, the catalyst (3%Y/SBA-3) was separated by
centrifugation, and regenerated by thoroughly washing it with methanol followed by drying it
2
1
Page 21 of 41

o
in an oven at 75 C for 4 h. Thereafter, the regenerated catalyst was reused in another
catalytic experiment. This procedure was repeated three times to achieve four consecutive
recycles. According to Fig. 10, the catalyst was reliably stable with no appreciable change in
glycerol conversion and TAG selectivity. The observed stability of this catalyst could be
assigned to its highly stable crystalline structure, as earlier confirmed by SAXS and XRD
analysis. After each transesterification recycle, the catalyst was separated, washed with
methanol, dried and reused. Despite the repeated wash, clogs of glycerol molecules with
yellowish spots could be seen covering the catalyst surface. After the fourth recycle, the
catalyst was re-characterized by FTIR and BET analysis to inspect the possible changes in its
surface properties. The results revealed that the fresh and spent catalyst exhibited the same
FTIR pattern, explaining the unchanged chemical bonding within the catalyst structure. A
minute reduce in the surface properties (BET surface area, pore volume and pore diameter)
was observed in the four times recycled catalyst, which strongly assert the mechanical
stability of the catalyst particles under the stirring conditions.
4. Conclusions
The transesterification reaction of glycerol with methyl acetate was carried out over a
series of heterogeneous catalysts (1 to 3.5 wt% of yttrium grafted into SBA-3 support). Under
the best transesterification conditions (molar ratio of glycerol/methyl acetate of 1:12, reaction
o
temperature of 110 C and reaction time of 3 h), glycerol was completely converted to a final
product containing 4%, 32%, and 64% of MAG, DAG, and TAG, respectively. The superior
performance of the 3%Y/SBA-3 catalyst during the transesterification of glycerol with
methyl acetate is partly due to the increase in surface properties obtained when it was grafted
with yttrium species, as earlier presented in Table 1. Moreover, the catalyst was reliably
2
2
Page 22 of 41

stable and the leaching was less than 3%. The acidity of this catalyst was relatively high
1342 μmol/g), which might be obtained by the ionic pairing of the catalyst components, is
(
believed to be a key factor to enhance its activity in this reaction.
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Universiti
Sains Malaysia, under the Postgraduate Research Grant Scheme (PRGS,
1001/PJKIMIA/8044025).
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2
5
Page 25 of 41

Figures captions:
Fig. 1:
Nitrogen adsorption–desorption isotherms for the pristine SBA-3 material (a);
the 3%Y/SBA-3 catalyst (b).
Fig. 2:
Fig. 3:
Fig. 4:
SAXS patterns for the pristine SBA-3 and 3%Y/SBA-3 materials.
XRD patterns of the pristine SBA-3, and 3%Y/SBA-3 materials.
FTIR spectra of the pristine SBA-3 material and the so modified 3%Y/SBA-3
catalyst.
Fig. 5:
Scanning electron micrographs of mesoporous silica morphologies; (a) SBA-3
and (b) 3%Y/ SBA-3.
Fig. 6:
Fig. 7:
TEM images of (a) SBA-3; (b) SBA-3; (c) 3%Y/SBA-3.
Effect of catalyst weight on glycerol transesterification with methyl acetate.
o
Reaction conditions: reaction time: 3 h; reaction temperature: 110 C; molar
ratio of glycerol to methyl acetate: 1:12.
Fig. 8:
Fig. 9:
Fig. 10:
Effect of reaction time on the transesterification of glycerol with methyl
acetate. Reaction conditions: catalyst weight: 0.40 g; reaction temperature: 110
o
C; molar ratio of glycerol/methyl acetate: 1:12.
Effect of reaction temperature during glycerol transesterification. Reaction
conditions: catalyst weight: 0.40 g; molar ratio of glycerol to methyl acetate:
1
:12; reaction time: 3 h.
Reusability test performed for 3%Y/SBA-3 catalyst. Reaction conditions for
each run: catalyst loading: 0.40 g; reaction time: 3 h; reaction temperature: 110
o
C; molar ratio of glycerol/methyl acetate: 1:12. MAG: monoacetyl glyceride;
DAG: diacetyl glyceride; TAG: triacetyl glyceride; Xgly.: glycerol conversion
(%).
2
6
Page 26 of 41

List of Table:
Table 1:
Textural properties and surface acidity of the pristine and the Y modified
SBA-3 materials.
Table 2:
Table 3:
EDX elemental analysis results for the fresh and spent 3%Y/SBA-3 catalyst.
Comparison of the catalytic activity of variously grafted SBA-3 catalysts with
different yttrium amounts during the transesterification of glycerol using variant
o
glycerol to methyl acetate molar ratio (R1=1:10; R2=1:12; R3=1:14) at 110 C
after 3 h.
2
7
Page 27 of 41

Table 1
Materials
BET surface
area
Total pore
volume
Average pore
d
100
a
o
Acidity
a
diameter (Å)
(
Å)
(Å)
(±10 μmol/g)
2
–1
3
–1
(
m g )
(cm g )
0.73
0.78
0.79
0.80
0.81
0.79
0.83
0.58
0.53
SBA-3
1462
1470
1513
1536
1568
1523
1622
815
28.6
27.8
27.1
26.7
25.4
24.6
26.2
27.1
25.7
71.4
81.5
82.6
90.8
97.2
97.0
–
86
89
94
102
110
110
–
429
765
1
2
2
3
3
3
1
3
% Y/SBA-3
%Y/SBA-3
.5%Y/SBA-3
%Y/SBA-3
913
1071
1342
1328
1365
1023
1644
b
%Y/SBA-3R
.5%Y/SBA-3
%Y/SBA-3(Im)
–
–
%Y/SBA-3(Im)
586
–
–
a
Pore diameter calculated from the desorption isotherm by the BJH method.
Catalyst after forth recycle.
b
2
8
Page 28 of 41

Table 2
Catalyst
Elemental content (%)
Theoretical
By EDX
Si
Y
Si
63
–
O
Y
O
3
3
%YSBA-3
3
34
–
2.96
2.93
65.4
31.64
31.67
%YSBA-3R
–
65.12
2
9
Page 29 of 41