2. (hydrolysis of triglycerides) are accelerated. As a result, the
transesterification reaction is greatly affected, and the
conversion of waste cooking oils to biodiesel becomes more
difficult.26,27
The use of a catalyst is crucial for accelerating the
transesterification reaction. Sodium and potassium hydroxide
are commonly used as homogeneous base catalysts,28
but there
are many obstacles to their utilization: the hydroxides produce
soap by neutralizing the free fatty acid in waste oils and by
triglyceride saponification.29
The soap formation is an
undesirable surface reaction because it partially consumes the
catalyst, decreases the biodiesel yield, and complicates the
separation and purification steps.30,31
The removal of these
catalysts is technically difficult and adds extra cost to the final
product. Acidic catalysts are also being used for the
transesterification reaction.32−35
However, despite the increase
in the yield of the biodiesel, the acid-catalyzed reaction is much
slower than the alkali catalyzed reaction and also requires
higher temperatures and pressures.36,21
In contrast, the use of a heterogeneous solid catalyst
facilitates its separation from the liquid products, allows its
reusability, and reduces soap formation, resulting in a more
environmentally friendly process. Moreover, the biodiesel
obtained is purified under mild conditions. In an attempt to
reduce the production cost, a variety of catalysts, namely, anion-
exchange resin,37,38
alkaline earth metal oxides,39−41
mixed
metal oxides of alkaline earth group elements,42
rare earth
metal,43
zeolites,44
spinels,45,46
and perovskites47
are being
tested for the industrial production of biodiesel. The common
goals are always to reduce the catalyst amount and lower the
overall energy requirements of the process. The common
problem associated with the heterogeneous biodiesel produc-
tion process is its slow reaction rate due to poor surface contact
between triglycerides and alcohol during the reaction because
of their reciprocal immiscibility.30
Among the solid catalysts,
alkaline earth metal oxides have higher basicity and lower
solubility in alcohol and produce higher biodiesel yield. The
order of activity among the alkaline earth oxide catalyst is BaO
> SrO > CaO > MgO.48
Refaat49
has discussed the reaction
mechanism in detail. BaO is toxic and soluble in methanol,50
affecting the quality of the biodiesel produced51
and is therefore
not suitable for biodiesel production. SrO, despite its lower
surface area52
and the partial solubility of the metal ion in the
reaction medium,53
exhibits excellent catalytic performance for
the transesterification process, accelerating the transesterifica-
tion reaction from hours to seconds when using microwave
irradiation.54,55
Its high activity is mainly due to its alkalinity
and basic sites.
Microwave irradiation is a well-known method for
accelerating and enhancing chemical reactions because it carries
the energy directly to the reactant.57,58
Thus, microwave
irradiation is a potential route to accelerate the trans-
esterification reaction in the presence of SrO based catalysts.55
Patil et al. demonstrated the advantage of using microwave
irradiation for the transesterification of Camelina sativa oil using
the SrO catalyst. The yield of biodiesel (80 wt %) that could be
achieved using microwave irradiation in a short duration of 4
min requires 180 min using conventional heating (using a hot
plate heater with a magnetic stirrer).56
Even though precise
temperature control and uniformity in temperature distribution
are common problems in a modified domestic microwave oven,
the problems could be surmounted by the use of advanced
microwave systems with precise temperature control which is
currently available.
Even though, SrO powder has been demonstrated to be an
active catalyst in a batch process, for its industrial adoptability
such a catalyst needs to be used as a coating on the inert solid
material. Such a supported catalyst in millimeter dimensions
not only eliminates the mass flow constraints and pressure
drops but also reduces the cost of the catalyst drastically. The
cost reduction is due to the effective utilization of the catalyst
by the homogeneous dispersion of the active sites on the glass
beads. Moreover, such a catalyst contributes to the increase of
mass and heat transfer, improves the contact between the liquid
medium and the catalyst surface, and facilitates its separation
from the products.59
Also, depositing the catalyst on solid
supports can help prevent possible health risks caused by
inhalation of fine powders. In addition, catalysts supported on
spherical beads can offer shape-dependent advantages such as
minimizing the abrasion of the catalyst in the reaction
environment.60
The objective of the current work is to successfully design a
heterogeneous solid base catalyst comprising of SrO deposited
on silica beads (SrO@SiO2) that can be used as a potential
catalyst for the conversion of waste cooking oil to biodiesel
under microwave irradiation conditions. In brief, the current
report is the first step toward constructing a semi-industrial
pilot plant for flowing cooking oil through an MW oven
containing a fixed catalyst. The paper is focused on the
evaluation of preparation, properties, and utilization of the
catalyst (SrO@SiO2) for transesterification reaction.
2. EXPERIMENTAL SECTION
2.1. Materials and Methods. The waste vegetable cooking oil was
obtained without charge from a restaurant near Bar-Ilan University and
filtered through a USA standard testing sieve of mesh size 250 μm to
remove residues and impurities. The acid value of the cooking oil was
determined by the titrimetric method61
and was found to be 3.6 mg
KOH/g. This value is substantially lower than the value (17.41 mg
KOH/g) reported by Patil et al.62
To avoid the saponification of free
fatty acids (FFA) and the hydrolysis during the transesterification
reaction, FFA and water were removed from the cooking oil before the
transesterification reaction. The cooking oil was mixed with a basic
solution of potassium hydroxide (KOH) to remove the FFA in the
form of soap. The soap was separated from the oil content by
centrifugation.54
Then, the cooking oil was heated at 110 °C to
evaporate the water. Even though the acid catalyzed esterification
process is well-known for the removal of FFA, usually, higher acid
concentrations, as well as longer reaction times, are required.63
To
avoid these complications a KOH-based pretreatment method was
adopted for the removal of FFA from waste cooking oil. Moreover,
since the subsequent transesterification reaction is also a base catalyzed
(SrO@SiO2) reaction, a base (KOH) catalyzed pretreatment was used
for the removal of FFA.
Strontium nitrate (Sr(NO3)2) (≥99.0%) was used as the precursor
and was purchased from Sigma-Aldrich. Sodium carbonate (Na2CO3)
and silica gel (particles with sizes ranging from 1 to 3 mm and from 3
to 6 mm) were purchased from Sigma-Aldrich. Methanol and
isopropyl alcohol were purchased from Bio Lab and were used as
received. SrO coated silica beads (SrO@SiO2), used as catalysts, were
synthesized by irradiation in a domestic microwave oven (DMWO).
The transesterification reaction was conducted under DMWO
irradiation. The DMWO was operated at 2.45 GHz in a batch
mode. The output of the domestic microwave reactor was 1100 W.
The microwave oven was operated at 70% power (cycle mode of 21 s
on and 9 s off), a cycle mode function provided by the DMWO’s
manufacturer. The reaction temperature attained as a result of
microwave irradiation was measured using a Pyrometer (Fluke, 65
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3. Infrared thermometer) after the irradiation was completed. The
reaction temperature was found to be 333 K. The microwave oven was
modified, so as to have a provision for the distillation column passing
through the MW oven (for enhanced safety of operation) and with a
stirring facility during the reaction. The modification was performed by
replacing the bottom part of the oven by a rounded aluminum plate.
The plate was carefully attached to the framework in such a way as to
allow for magnetic stirring (Figure S1).
2.2. Preparation of SrO@SiO2. 2.2.1. Synthesis of SrO@SiO2
Using Microwave Irradiation. The deposition of SrO on SiO2 gel
consists of dissolving equal molar amounts of Sr(NO3)2 and Na2CO3
in water under vigorous stirring. Optimized catalyst preparation was
obtained by dissolving 4.23 g of Sr(NO3)2 and 2.11 g of Na2CO3 into
100 mL of water taken in a 250 mL round-bottom (RB) flask at room
temperature. Ten mL of ethylenediamine (C2H8N2, EDA) were then
added. EDA was successfully used elsewhere64
as a chelating and
capping agent for the synthesis of SrCO3 nanoparticles (NPs) under
microwave irradiation. Subsequently, 6 g of SiO2 gel was added to the
solution. The contents were then irradiated in a DMWO for 1, 3, and
5 min. After having been cooled, the residual solid mass was separated
by centrifugation from the supernatant, washed with EtOH three
times, and subjected to drying under vacuum. The material (Sr(CO3)2
deposited on SiO2 beads) was subjected to calcination at different
temperatures for varying calcination times to determine the optimal
conditions for the decomposition of Sr(CO3)2 and in situ deposition
of SrO NPs on SiO2 beads.
Further optimization of the catalyst preparation was done by
varying the ratio of the Sr precursor to the amount of silica beads. To
effectively utilize the strontium precursor Sr(NO3)2, keeping the
Sr(NO3)2 and Na2CO3 amounts constant (4.23 and 2.11 g), the
amount of silica beads (SiO2) is steadily increased by an increment of
2 g to 6, 8, and 10 g.
2.2.2. Characterization of SrO@SiO2. The precise decomposition
temperature of SrCO3, a reaction intermediate for the generation of
SrO from Sr(NO3)2, was deduced from the thermogravimetric analysis
(TGA). The TGA curves were recorded using a Q500 Thermogravi-
metric Analyzer (TGA) in the temperature range of 25−1000 °C in
the air atmosphere at a heating rate of 10 °C/min. Powder X-ray
diffraction (PXRD) analyses were conducted to probe the crystallo-
graphic nature of the catalyst (SiO2@SrO). XRD patterns were
collected using a Bruker AXS Advance powder X-ray diffractometer
(Cu Kα radiation; λ = 0.154178 nm) operating at 40 kV/30 mA with a
0.02 step size in the range of 10−80° (2θ). The phases were identified
using the power diffraction file (PDF) database (JCPDS, International
Centre for Diffraction Data). The crystallite size was estimated from
XRD patterns choosing the most intense signal, finding the full width
at half maxima and substituting the parameters λ and θ (in radians) in
the Scherrer’ equation: L = 0.9λ/B cos θ, where L is the crystallite size,
λ is the X-ray wavelength, B is the line broadening, and θ is the Bragg
angle in radians. FT-IR spectra were recorded in KBr pallet mode on a
Nicolet (Impact 410) FT-IR spectrophotometer under atmospheric
conditions. The samples were scanned in the range of 400 and 4000
cm−1
. The imaging and morphology of SrO@SiO2 were obtained
using a high-resolution scanning electron microscopy (HR-SEM)
having a JEOL-JSM 700F instrument and an LEO Gemini 982 field
emission gun SEM (FEG-SEM) and by using environmental scanning
electron microscope (ESEM) having the FEI QUANTA 200F device.
Elemental analysis was carried out using Energy-dispersive X-ray
analysis (EDAX) in conjunction with the HR-SEM instrument.
Transmission electron microscope (TEM) images of SrO particles
were taken with JEM-1400, JEOL to visualize their morphology.
Samples for TEM were prepared by making a suspension of the
particles in isopropyl alcohol, using water-bath sonication. The crystal
structure of the SrO was determined by selected area electron
diffraction (SAED) crystallographic analysis. To evaluate the exact
amount of SrO deposited on silica beads ICP analysis was used.
Typical methodology for this is comprised of taking a known amount
of SrO@SiO2 in concentrated HNO3 and stirring at 50 °C on a
magnetic stirring base for 1 h so as to dissolve the SrO coated on the
silica beads. Subsequently, the silica beads were separated from the
filtrate using Whatman (150 MM Φ) filter paper. The filtrate was
analyzed for Sr2+
ions using an inductively coupled plasma (ICP)
spectrometer (Ultima 2, Jobin Yvon Horiba). Specific surface area
analysis of the catalyst (SrO@SiO2) prepared under optimal reaction
conditions was carried out using a Nova 3200e Quantachrome
analyzer.
2.3. Evaluation of Catalytic Activity of SrO@SiO2. The
transesterification reactions were performed by a DMWO equipped
with a condenser and carried out in a 50 mL round-bottom flask. A
typical batch process of the transesterification reaction comprises of
taking 15 g of cooking oil, 4 mL of MeOH, and 0.5 g of catalyst, SrO@
SiO2, and irradiating the content in a microwave oven for 10 s at 70%
(cycle mode of 21 s on and 9 s off) power. This means that the actual
irradiation time is only 7 s during the reaction. First, SrO@SiO2 was
dispersed in MeOH with high magnetic stirring to ensure a good
dispersion of the catalyst into the MeOH. The cooking oil was
subsequently added, and the mixture was irradiated for 10 s. At the end
of the reaction, the temperature of the mixture was measured by a
pyrometer and was found to be 60 °C. The mixture was then
centrifuged, and three distinguished layers were observed: the top layer
was composed of Fatty Acid Methyl Esters (FAME) and excess
MeOH, the middle one was SrO@SiO2, and the bottom layer was
glycerol. Then, the top layer was extracted, and the excess MeOH was
removed by a rotary evaporator. The catalyst was then separated in
order to recycle it and study the catalyst activity and stability. To each
sample, cooking oil and MeOH were added in the same amounts used
for the initial reaction.
The FAME product was analyzed by 1
H NMR spectroscopy
(Bruker Avance 300 spectrometer). Chloroform (CDCl3) was used as
a solvent for 1
H NMR sample preparation. The conversion was
calculated directly from the integrated areas of the methoxy group in
the fatty acid methyl esters (FAME) at 3.65 ppm (singlet) and of the
α-carbonyl methylene protons present in the triglyceride derivatives at
2.26 ppm (triplet).54,65
Eq 1 was used to estimate the conversion of
the waste cooking oil to FAME
= ×I IConversion (%) [2 /3 ] 100Me CH2 (1)
The conversion ratio of the oil to the resultant fatty acid methyl
ester was obtained by dividing IMe (the integration value of the protons
of the methyl esters) by ICH2 (the integration value of the methylene
protons). The factors 2 and 3 were derived from the fact that the
methylene carbon possesses two protons and the methyl carbon has
three attached protons.66,67
3. RESULTS AND DISCUSSION
3.1. Strategy for the Deposition of SrO on Silica Beads
(SrO@SiO2). 3.1.1. TGA Analysis for Evaluating the
Appropriate Calcination Temperature for the Conversion
of SrCO3@SiO2 to SrO@SiO2. SrCO3@SiO2 is generated after
the microwave irradiation of Sr(NO3)2 and Na2CO3 taken in
Figure 1. XRD of SiO2@SrCO3 prepared with Sr(NO3)2 as Sr
precursor using microwave irradiation.
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4. water and EDA in the presence of millimetric silica beads for 5
min. Deposition of a pure strontianite SrCO3 phase (JCPDF
file no. 84-1778) on silica beads was confirmed by the XRD
analysis (Figure 1) of powder scratched from the SiO2 beads
using a forceps and a spatula. The SiO2 beads coated with
SrCO3 (SrCO3@SiO2) could not be used as such for XRD
analysis, as the analysis required fine powder and the silica
beads coated with SrCO3 are spheres of millimetric size. As
such the crystal structure of SrCO3 was found to be
orthorhombic (space group, Pmcn) with the lattice parameters,
a = 5.10 Å, b = 8.40 Å, and c = 6.02 Å.
To determine the conversion temperature of SrCO3@SiO2
to SrO@SiO2, the TGA of SrCO3 (by scratching the surface of
the coated silica beads) was measured in air at a heating rate of
10 °C/min (Figure 2). A three stage weight loss is observed.
The first one, in the range of 180−210 °C was attributed to the
evaporation of trapped water molecules. The second weight
loss, at approximately 400 °C, is probably due to the
Figure 2. TGA curve of SrCO3 powder.
Figure 3. XRD pattern of SrCO3 deposited on SiO2 (SrCO3@SiO2) in
a short microwave irradiation of 30 s.
Figure 4. XRD pattern of the material obtained after calcination of
SrCO3@SiO2 at 900 °C for 4 h in air.
Figure 5. FT-IR spectra of SiO2@SrCO3 and SrO@SiO2.
Figure 6. SEM images (A and B), EDS spectrum (inset B), and
elemental mapping (C and D) of SrO@SiO2.
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D
5. decomposition of Sr(OH)2. The Sr(OH)2 phase could be the
transient intermediate formed in situ during the TGA analysis in
the air atmosphere containing moisture as this phase is not
observed in the starting material, SrCO3 (Figure 1). The last
weight loss, occurring at 850 °C, is attributed to the
decomposition of SrCO3 to SrO with the release of CO2.
Beyond 900 °C, the weight remained constant. Strontium
carbonate is usually decomposed upon heating to temperatures
higher than 900 °C.68
Therefore, a calcination temperature of
900 °C was subsequently used for the decomposition of the
SrCO3 phase on SiO2 to produce the SrO@SiO2 catalyst.
3.1.2. Determination of the Optimum Time of Microwave
Irradiation for Coating SiO2 Beads with SrCO3. Microwave
irradiation offers an elegant pathway for the conversion of
Sr(NO3)2 to SrCO3 in the presence of NaCO3 and EDA in an
aqueous medium. The SrCO3 generated in situ during the
reaction was deposited on the millimetric silica beads which
were present in the reaction vessel along with the Sr precursor.
Different irradiation times (30 s, 1, 3, and 5 min) were set for
the deposition process. It was observed that even a short
irradiation time would be sufficient for effectively depositing
SrCO3 on silica beads. The XRD pattern typical of SrCO3
(JCPDF file no. 84-1778) was observed in the case of the
SiO2@SrCO3 catalyst obtained after 30 s of microwave
irradiation (Figure 3). In contrast, similar deposition of the
SrCO3 phase on silica beads could not be achieved by stirring at
room temperature even after 1 h without microwave radiation.
This signifies the potential of microwave volumetric heating
facilitating the strong adhesion of SrCO3 on SiO2 beads. This
could be due to the surface etching of silica beads causing
surface roughness and adsorption sites required for the
deposition of SrCO3 on the SiO2 surface. In addition, in the
presence of microwave irradiation, the collisions between the
SrCO3 particles, and surface binding sites of SiO2 particles
might be strong enough to cause the adhesion of SrCO3 on the
SiO2 surface.
The crystallite size values of SrCO3 deposited on silica beads
after 30 s MW irradiation was 12.7 nm. A calcination time for 2
h at 900 °C in the air could not convert all the SrCO3 on the
silica beads to SrO as can be seen in Figure S2 where intense
signals of unreacted SrCO3 are observed. The SrCO3@SiO2
Figure 7. TEM images of the SrO@SiO2 catalyst.
Figure 8. Pictorial depiction of (A) the HR-SEM image of SrO@SiO2,
(B) the SEM image of the upper leaf side indicating the nanotubules of
wax, and (C) lotus leaves exhibiting extraordinary hydrophobicity.
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6. material obtained after the microwave irradiation for 30 s, 1, 3,
and 5 min was calcined at 900 °C in air for 3 h, and the
crystallite sizes of the resulting SrO particles on the SiO2
surface (JCPDF file no. 06-0520; cubic structure with a lattice
constant value of 5.16 Å; Fm3̅m) were found to be 61.6, 57.5,
47.5, and 68.1 nm, respectively, as deduced from the XRD by
Scherrer analysis (Figure S3).
The relatively large crystallite size of the SrO particles
deposited on the silica surface, in comparison to the size
expected based on the precursor SrCO3 particles on the SiO2
surface, could be due to their agglomeration, caused by the high
calcination temperature (900 °C). In addition to the SrO phase,
minor impurities like the Sr(OH)2 and SrCO3 were also
observed in the material SrO@SiO2 after calcination (Figure
S2). The presence of such impurities could be due to the
reaction of the SrO particles with atmospheric moisture, H2O,
and CO2, implying the hygroscopic nature of SrO. Interestingly,
when the SiO2@SrCO3 samples were calcined for 4 h, the
resulting material showed the XRD pattern with the exclusive
SrO phase (JCPDF file no. 06-0520) (Figure 4). Thus, the
optimum time of calcination is 4 h under air at 900 °C. A
typical XRD pattern of SrO@SiO2 (crystallite size of SrO − 55
nm) shown in Figure 4 was similar to that of commercial SrO
with a crystallite size of 60 nm (Figure S4). Therefore, the
developed methodology is an effective way to coat silica beads
with SrO.
3.2. FT-IR Analysis of the Effectiveness of SrO Coating
on SiO2 Beads. Absorption bands typical of normal modes of
vibration of free planar CO3
2−
ions bound to Sr2+
in SrCO3
were observed at 1473, 1075, 856, and 700 cm−1
for the
SrCO3@SiO2 material obtained by the microwave irradiation
method.69
After calcination of the aforementioned material, the
stretching and deformation peaks of CO bonds of carbonate
were drastically suppressed with a new band appearing at 592
cm−1
denoting the Sr−O bond stretching. This indicates the
effectiveness of the methodology developed for the deposition
of SrCO3 on silica beads and also its subsequent decomposition
to SrO on the surface of the silica beads (see Figure 5).
3.2.1. Morphology and Chemical Composition of the
SrO@SiO2 Catalyst. The SEM images, as well as the EDS
spectrum and the elemental mapping recorded on the SrO@
SiO2 catalyst, are shown in Figure 6. It is interesting to note
that the spherical morphology of the millimetric silica bead is
retained even after microwave irradiation for 30 s followed by
calcination at a high temperature in air for 3 h at 900 °C
(Figure 6 (A)). The deposition of SrO on the SiO2 surface was
confirmed from the EDS spectrum (Figure 6 (B)). Moreover,
the uniform distribution of SrO on the surface of SiO2 can be
envisaged from the presence of the elements Sr and O
throughout the surface of silica beads (Figure 6 (C) and (D)).
Raja et al. observed such homogeneous distribution of SrO on
the mesoporous carbon surface (CMK-3).70
This analysis
reveals the potential of the methodology designed for the
preparation of SrO coated silica beads in an innovative and
green methodology.
In agreement with the XRD analysis, the TEM image of SrO
deposited on SiO2 beads shows agglomerates of SrO nano-
particles (Figure 7 (A)). The TEM image of the SrO
nanoparticles on the SiO2 surface was also taken at higher
magnification as depicted in Figure 7(B). As a fine powder
sample is required for the TEM analysis, SrO particles
scratched from the SiO2 beads of the SrO@SiO2 surface
using a forceps and a spatula were used for recording the TEM
images. For further examination of the morphology of the SrO
particles deposited on the silica surface, the selected area
electron diffraction pattern (SAED) was recorded (inset Figure
7(B)). The average particle size of SrO nanoparticles was 56.8
nm. The SAED obtained from the SrO particles exhibits diffuse
and hollow concentric rings of bright spots. Such a ring pattern
is generated by the diffraction of transmitted electrons through
the nanocrystal with different orientations. The ring pattern
observed corresponds to the polycrystalline nature of the SrO
material with aggregates of SrO particles.
3.3. Parameters Affecting the SrO Loading on SiO2
Beads. 3.3.1. Influence of Ratio of Sr Precursor and Silica
Beads on the Deposition of SrO on SiO2 Beads. The amount
Figure 9. Reusability study of SiO@SiO2 and its effect on the conversion of waste cooking oil.
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7. of silica beads has been varied from 2 to 10 g while keeping the
amounts of Sr(NO3)2 (4.23 g) and Na2CO3 (2.11 g) constant.
The size of the SiO2 beads was 1−3 mm, and the microwave
irradiation time was 1 min. The amount of SrO deposited on
the SiO2 surface was found to be 20.8, 33.5, 5.2, and 1.4 wt %
when the initial amount of SiO2 beads in the reaction medium
was 2, 6, 8, and 10 g, respectively. Thus, 6 g of SiO2 beads was
found to be the optimum amount for the effective utilization of
the Sr precursor resulting in the highest loading of SrO.
Moreover, the amount of SrO loading on the silica beads was
found to have a significant effect on the conversion of waste
cooking oil to FAME. Conversion values of 97.6, 99.2, 95.1, and
71.2 wt % were observed with the SrO@SiO2 catalyst loaded
with 20.8, 33.5, 5.2, and 1.4 wt % SrO.
When the amount of SiO2 beads in the reaction medium is
low (<6 g) the probability of SrCO3 particles adhering to the
beads is lower and most of the SrCO3 particles remain
suspended in the aqueous medium owing to the lower number
of effective collisions between the SrCO3 particles and SiO2
beads. The higher the amount of the silica beads the probability
of deposition of the SrCO3 particles on the silica surface is
higher upon the action of microwave irradiation owing to the
larger surface area offered by the silica surface. So an increase in
SrO loading from 20.8 to 33.5 wt % with an increase in the
bead content from 2 to 6 g is observed; but beyond 6 g of the
SiO2 beads, owing to the presence of excess silica bead content
in the reaction medium, the effective microwave radiation
experienced by individual SrCO3 particles could be lower due
to the shielding effect offered by the excessive presence of the
beads.71
This, in turn, might result in the precipitation of the
SrCO3 particles in the reaction medium rather than being
coated on the surface of the bead. A pictorial representation of
the process of deposition of SrCO3 onto the silica beads under
microwave irradiation is shown in Figure S5.
3.3.2. Influence of Diameter of SiO2 Beads on SrO
Deposition. After examining the optimum ratio (1.42) of
SiO2 beads (6 g) to Sr(NO3)2 (4.23 g) for the effective
deposition of SrO, the size effect of the silica beads was also
studied. When the size of the SiO2 beads was varied from 1 to 3
mm to 3−6 mm with other reaction conditions being constant,
the wt % loading of SrO was found to increase from 33.5 to
41.3 indicating a further enhancement in the utilization of Sr.
Such an improvement in the SrO deposition could be due to
more surface area of the bigger silica beads (3−6 mm) available
as binding sites for SrO. The BET analysis of the catalyst with
optimum SrO loading (41.3 wt % SrO on SiO2) was analyzed
and was observed to be 0.2 m2
/g. The low specific surface area
value is due to the bigger particle size (3−6 mm) of the
nonporous silica beads comprising the catalyst support. The
activity of the catalyst (41.3 wt % SrO deposited on SiO2
beads) thus generated under optimum reaction conditions (1
min microwave irradiation, 6 g beads, 3−6 mm diameter, 4.23 g
Sr(NO3)2, and 2.11 g Na2CO3) was evaluated for biodiesel
production from waste cooking oil.
3.4. Catalytic Activity of SrO@SiO2 (41.3 wt %) for
Transesterification of Waste Cooking Oil to Biodiesel.
The solid base catalyst, SiO2@SrO, thus generated was used for
the transesterification of waste cooking oil. A typical digital
image of the transesterification product is shown in Figure S6.
The transesterification reaction product obtained after the
microwave irradiation was analyzed by 1
H NMR (Figure S7). A
conversion value of 99.4 wt % of waste cooking oil to FAME
was observed using the novel composite catalyst indicating the
successful design of the catalyst.
Of significance is the use of a modest mass (wt %) ratio of
the catalyst, SrO@SiO2 (0.5 g), to cooking oil (15 g) to achieve
almost complete conversion of triglycerides to FAME.
Moreover, since the loading of SrO on SiO2 is 41.3 wt %, 0.5
g of the SrO@SiO2 catalyst corresponds to 0.2065 g of the
active component SrO which is, in fact, a lower amount of the
catalyst used for the reaction compared to previous reports.54,55
Catalysts with lower loadings of the active component (SrO)
on silica beads resulted in lower conversion values.
The reaction continued further even after the irradiation time
of 10 s as the temperature remained stable owing to the use of a
condenser. Therefore, the reaction product was collected from
the reaction vessel in the microwave oven after 15 min. To
prove the efficiency of DMWO and its acceleration of the
reaction rate, the same reaction was done at room temperature
under stirring, and equivalent conversion of the waste cooking
oil was achieved after 45 min. The high conversion of
triglycerides in such conditions is due to the nanoscale SrO
particles that offer high surface area and larger active sites for
the catalytic conversion of waste cooking oil.
3.4.1. Structure−Activity Relationship of the SrO@SiO2
Catalyst. HR-SEM analysis was performed to elucidate the
observed high activity of the SrO@SiO2 catalyst at such modest
loading (41.3 wt %) of SrO coated on silica beads. A typical
HR-SEM image of the SrO@SiO2 prepared under an optimal
reaction (Figure 8 (A)) unraveled a structure−property
correlation. In brief, nanometric tubules of SrO were formed
on the silica bead surface under the microwave irradiation
conditions followed by calcination at high temperature (900
°C) in the air. The average length and width of the nanotubules
of SrO were 139 and 50 nm. Moreover, the structural features
at the nanometric level were analogous to that observed in a
lotus leaf, probably making it hydrophobic, as depicted in
Figure 8 (B and C). The hydrophobic property of the SrO@
SiO2 catalyst is only a hypothesis based on the observed
structural features, and quantitative measurements have not
been carried out.
The nanotubules present on the SrO@SiO2 could be
attributed to the observed high activity of the catalyst even at
a modest loading of 41.3 wt % of SrO. Such nanotubules may
act as a repelling agent for the water molecules if any are
present in the cooking oil and promote transesterification of the
cooking oil more effectively.
3.4.2. Reusability of the SrO@SiO2 Catalyst for FAME
Production. Pretreatment of the waste cooking oil has a
significant effect on the catalyst activity and its reusability.
SrO@SiO2 exhibited sustainable activity for 10 consecutive
cycles of the transesterification reaction of pretreated waste
cooking oil. Even after 10 repeated runs, the catalyst activity
only decreased from 99.4 to 95 wt % (Figure 9).
From an economic viewpoint, the catalyst cost is a major
factor involving biodiesel production relative to waste cooking
oil or methanol, the two principal reactants. The stability and
sustained activity of the catalyst are of great importance for the
industrial application of the catalyst. Therefore, the present
composite catalyst offers an innovative pathway for production
and exploitation of a reusable solid base catalyst (SrO@SiO2)
for biodiesel production.
Without the pretreatment by SrO@SiO2, the conversion
value drastically decreased from 99.4 to 78.6 wt %. Such a loss
in the catalytic activity owing to the presence of FFA was also
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b00256
Energy Fuels XXXX, XXX, XXX−XXX
G
8. observed by Viola et al.72
Pretreatment of the cooking oil allows
the removal of the free fatty acid (FFA) and the water content,
thereby hindering the saponification and hydrolysis of the
triglycerides. This is another reason for the sustainable activity
(Figure 9) observed in the case of SrO@SiO2 for the
transesterification reaction.
4. CONCLUSION
The novel solid base catalyst (SrO@SiO2) was synthesized by
fabrication and deposition of SrO nanoparticles on millimetric
silica beads (SrO@SiO2). The catalyst showed excellent activity
for the conversion of waste cooking oil to FAME in just 10 s.
Moreover, the catalyst could be easily separated and reused for
10 consecutive reaction runs without any significant activity
loss. Its high activity was attributed to the unique morphology
of SrO particles on the silica bead surface at the nanometric
level, resulting in well dispersed active sites for the trans-
esterification reaction. This low cost and reusable catalyst will
be advantageous for the transformation of waste cooking oil.
Further high throughput could be obtained using continuous
flow microwave irradiation. Initial results obtained for
continuous microwave irradiation processing are promising
with a conversion value close to 100% in a single cycle for 1 L
of oil and methanol. Thus, fast and efficient biodiesel
production based on the solid catalyst together with continuous
microwave irradiation could be an important process for waste
cooking oil transformation. Doubtlessly, the scale-up of this
process will involve several scientific and engineering
constraints, a subject of future endeavor.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.energy-
fuels.6b00256.
Image of the modified microwave oven (Figure S1),
XRD pattern of SiO2@SrCO3 catalysts (Figures S2 and
S3) and of SrO commercial (Figure S4), pictorial
representation of the catalyst preparation (SrO@SiO2)
under microwave irradiation (Figure S5) and of the
transesterification product (Figure S6), and a representa-
tive 1
H NMR spectrum of FAME product from waste
cooking oil (Figure S7) (PDF)
■ AUTHOR INFORMATION
Corresponding Author
*Phone: 972-3-5318315. Fax: 972-3-7384053. E-mail:
gedanken@mail.biu.ac.il.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
Gedanken thanks the Israeli Ministry of Science, Technology
and Space for the research grant (206712) for supporting this
work. Grateful thanks are due to India-Israel cooperative
scientific research grant (203768) for supporting this research.
■ REFERENCES
(1) Abbaszaadeh, A.; Ghobadian, B.; Omidkhah, M. R.; Najafi, G.
Current biodiesel production technologies: A comparative review.
Energy Convers. Manage. 2012, 63, 138−148.
(2) Zhang, Y. H. P. What is vital (and not vital) to advance
economically-competitive biofuels production. Process Biochem. 2011,
46 (11), 2091−2110.
(3) Aransiola, E. F.; Ojumu, T. V.; Oyekola, O. O.; Madzimbamuto,
T. F.; Ikhu-Omoregbe, D. I. O. A review of current technology for
biodiesel production: State of the art. Biomass Bioenergy 2014, 61,
276−297.
(4) Lourinho, G.; Brito, P. Advanced biodiesel production
technologies: novel developments. Rev. Environ. Sci. Bio/Technol.
2015, 14, 287−316.
(5) Roy, M. M.; Wang, W.; Alawi, M. Performance and emissions of a
diesel engine fueled by biodiesel-diesel, biodiesel-diesel-additive and
kerosene-biodiesel blends. Energy Convers. Manage. 2014, 84, 164−
173.
(6) Boon, J.; Van Dijk, E.; De Munck, S.; Van Den Brink, R. Steam
reforming of commercial ultra-low sulphur diesel. J. Power Sources
2011, 196, 5928−5935.
(7) Yaakob, Z.; Narayanan, B. N.; Padikkaparambil, S.; Unni, S.;
Akbar, M. A review on the oxidation stability of biodiesel. Renewable
Sustainable Energy Rev. 2014, 35, 136−153.
(8) Ghanei, R.; Moradi, G.; Heydarinasab, A.; Seifkordi, A. A.;
Ardjmand, M. Utilization of constructional lime as heterogeneous
catalyst in biodiesel production from waste frying oil. Int. J. Environ.
Sci. Technol. 2013, 10, 847−854.
(9) Demirbas, A. Progress and recent trends in biodiesel fuels. Energy
Convers. Manage. 2009, 50, 14−34.
(10) Shin, D. Y.; Cho, H. U.; Utomo, J. C.; Choi, Y.-N.; Xu, X.; Park,
J. M. Biodiesel production from Scenedesmus bijuga grown in
anaerobically digested food wastewater effluent. Bioresour. Technol.
2015, 184, 215−221.
(11) Bora, P.; Konwar, L. J.; Phukan, M. M.; Deka, D.; Konwar, B. K.
Microemulsion based hybrid biofuels from Thevetia peruviana seed
oil: Structural and dynamic investigations. Fuel 2015, 157, 208−218.
(12) Li, L.; Quan, K.; Xu, J.; Liu, F.; Liu, S.; Yu, S.; Xie, C.; Zhang, B.;
Ge, X. Liquid Hydrocarbon Fuels from Catalytic Cracking of Waste
Cooking Oils Using Basic Mesoporous Molecular Sieves K 2 O/Ba-
MCM-41 as Catalysts. ACS Sustainable Chem. Eng. 2013, 1, 1412−
1416.
(13) Hengst, K.; Arend, M.; Pfützenreuter, R.; Hoelderich, W. F.
Deoxygenation and cracking of free fatty acids over acidic catalysts by
single step conversion for the production of diesel fuel and fuel blends.
Appl. Catal., B 2015, 174−175, 383−394.
(14) Xie, Q.; Addy, M.; Liu, S.; Zhang, B.; Cheng, Y.; Wan, Y.; Li, Y.;
Liu, Y.; Lin, X.; Chen, P.; et al. Fast microwave-assisted catalytic co-
pyrolysis of microalgae and scum for bio-oil production. Fuel 2015,
160, 577−582.
(15) Li, H.; Niu, S.-L.; Lu, C.-M.; Cheng, S.-Q. The stability
evaluation of lime mud as transesterification catalyst in resisting CO2
and H2O for biodiesel production. Energy Convers. Manage. 2015, 103,
57−65.
(16) Dai, Y.-M.; Wu, J.-S.; Chen, C.-C.; Chen, K.-T. Evaluating the
optimum operating parameters on transesterification reaction for
biodiesel production over a LiAlO2 catalyst. Chem. Eng. J. 2015, 280,
370−376.
(17) Tran-Nguyen, P. L.; Go, A. W.; Ismadji, S.; Ju, Y.-H.
Transesterification of activated sludge in subcritical solvent mixture.
Bioresour. Technol. 2015, 197, 30−36.
(18) Veiga, P. M.; Luna, A. S.; De Figueiredo Portilho, M.; Audia De
Oliveira Veloso, C.; Assumpç, C.; Henriques, A. Zn,Al-catalysts for
heterogeneous biodiesel production: Basicity and process optimiza-
tion. Energy 2014, 75, 453−462.
(19) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Technical aspects of
biodiesel production by transesterificationa review. Renewable
Sustainable Energy Rev. 2006, 10, 248−268.
(20) Gude, V. G.; Grant, G. E.; Patil, P. D.; Deng, S. Biodiesel
production from low cost and renewable feedstock. Cent. Eur. J. Eng.
2013, 3 (4), 595−605.
(21) Pukale, D. D.; Maddikeri, G. L.; Gogate, P. R.; Pandit, A. B.;
Pratap, A. P. Ultrasound assisted transesterification of waste cooking
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b00256
Energy Fuels XXXX, XXX, XXX−XXX
H
9. oil using heterogeneous solid catalyst. Ultrason. Sonochem. 2015, 22,
278−286.
(22) Yaakob, Z.; Mohammad, M.; Alherbawi, M.; Alam, Z.; Sopian,
K. Overview of the production of biodiesel from Waste cooking oil.
Renewable Sustainable Energy Rev. 2013, 18, 184−193.
(23) Borges, M. E.; Díaz, L. Recent developments on heterogeneous
catalysts for biodiesel production by oil esterification and trans-
esterification reactions: A review. Renewable Sustainable Energy Rev.
2012, 16, 2839−2849.
(24) Talebian-Kiakalaieh, A.; Aishah, N.; Amin, S.; Mazaheri, H. A
review on novel processes of biodiesel production from waste cooking
oil. Appl. Energy 2013, 104, 683−710.
(25) Li, M.; Zheng, Y.; Chen, Y.; Zhu, X. Biodiesel production from
waste cooking oil using a heterogeneous catalyst from pyrolyzed rice
husk. Bioresour. Technol. 2014, 154, 345−348.
(26) Atadashi, I. M.; Aroua, M. K.; Aziz, A. R. A.; Sulaiman, N. M. N.
Production of biodiesel using high free fatty acid feedstocks. Renewable
Sustainable Energy Rev. 2012, 16, 3275−3285.
(27) Srilatha, K.; Prabhavathi Devi, B. L. A.; Lingaiah, N.; Prasad, R.
B. N.; Prasad, P. S. S. Biodiesel production from used cooking oil by
two-step heterogeneous catalyzed process. Bioresour. Technol. 2012,
119, 306−311.
(28) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi,
M.; Santacesaria, E. Synthesis of biodiesel via homogeneous Lewis acid
catalyst. J. Mol. Catal. A: Chem. 2005, 239, 111−115.
(29) Atadashi, I. M.; Aroua, M. K.; Aziz, A. R. A.; Sulaiman, N. M. N.
The effects of catalysts in biodiesel production: A review. J. Ind. Eng.
Chem. 2013, 19, 14−26.
(30) Di Serio, M.; Cozzolino, M.; Giordano, M.; Tesser, R.; Patrono,
P.; Santacesaria, E. From Homogeneous to Heterogeneous Catalysts
in Biodiesel Production. Ind. Eng. Chem. Res. 2007, 46, 6379−6384.
(31) Veljković, V. B.; Stamenković, O. S.; Tasić, M. B. The
wastewater treatment in the biodiesel production with alkali-catalyzed
transesterification. Renewable Sustainable Energy Rev. 2014, 32, 40−60.
(32) Olutoye, M. A.; Wong, C. P.; Chin, L. H.; Hameed, B. H.
Synthesis of FAME from the methanolysis of palm fatty acid distillate
using highly active solid oxide acid catalyst. Fuel Process. Technol. 2014,
124, 54−60.
(33) Melero, J. A.; Bautista, L. F.; Iglesias, J.; Morales, G.; Sánchez-
Vazquez, R. Production of biodiesel from waste cooking oil in a
continuous packed bed reactor with an agglomerated Zr-SBA-15/
bentonite catalyst. Appl. Catal., B 2014, 145, 197−204.
(34) Iglesias, J.; Melero, J. A.; Bautista, L. F.; Morales, G.; Sánchez-
Vázquez, R. Continuous production of biodiesel from low grade
feedstock in presence of Zr-SBA-15: Catalyst performance and
resistance against deactivation. Catal. Today 2014, 234, 174−181.
(35) Malins, K.; Kampars, V.; Brinks, J.; Neibolte, I.; Murnieks, R.
Synthesis of activated carbon based heterogenous acid catalyst for
biodiesel preparation. Appl. Catal., B 2015, 176−177, 553−558.
(36) Zabeti, M.; Mohd, W.; Daud, A. W.; Aroua, M. K. Activity of
solid catalysts for biodiesel production: A review. Fuel Process. Technol.
2009, 90, 770−777.
(37) Shibasaki-Kitakawa, N.; Hiromori, K.; Ihara, T.; Nakashima, K.;
Yonemoto, T. Production of high quality biodiesel from waste acid oil
obtained during edible oil refining using ion-exchange resin catalysts.
Fuel 2015, 139, 11−17.
(38) Ren, Y.; He, B.; Yan, F.; Wang, H.; Cheng, Y.; Lin, L.; Feng, Y.;
Li, J. Continuous biodiesel production in a fixed bed reactor packed
with anion-exchange resin as heterogeneous catalyst. Bioresour.
Technol. 2012, 113, 19−22.
(39) Singh, A. K.; Fernando, S. D. Reaction Kinetics of Soybean Oil
Transesterification Using Heterogeneous Metal Oxide Catalysts.
Chem. Eng. Technol. 2007, 30 (12), 1716−1720.
(40) Patil, P. D.; Deng, S. Transesterification of Camelina Sativa Oil
Using Heterogeneous Metal Oxide Catalysts. Energy Fuels 2009, 23,
4619−4624.
(41) Lee, H. V.; Ching Juan, J.; Fitriyah, N.; Abdullah, B.; Mf, R. N.;
Taufiq-Yap, Y. H. Heterogeneous base catalysts for edible palm and
non-edible Jatropha-based biodiesel production. Chem. Cent. J. 2014, 8,
30.
(42) Chang, F.; Zhou, Q.; Pan, H.; Liu, X.-F.; Zhang, H.; Xue, W.;
Yang, S. Solid Mixed-Metal-Oxide Catalysts for Biodiesel Production:
A Review. Energy Technol. 2014, 2, 865−873.
(43) Zhang, Y.; Jin, L.; Sterling, K.; Luo, Z.; Jiang, T.; Miao, R.;
Guild, C.; Suib, S. L. Potassium modified layered Ln 2 O 2 CO 3 (Ln:
La, Nd, Sm, Eu) materials: efficient and stable heterogeneous catalysts
for biofuel production. Green Chem. 2015, 17 (6), 3600−3608.
(44) Sun, K.; Lu, J.; Ma, L.; Han, Y.; Fu, Z.; Ding, J. A comparative
study on the catalytic performance of different types of zeolites for
biodiesel production. Fuel 2015, 158, 848−854.
(45) Mierczynski, P.; Chalupka, K. A.; Maniukiewicz, W.; Kubicki, J.;
Szynkowska, M. I.; Maniecki, T. P. SrAl 2 O 4 spinel phase as active
phase of transesterification of rapeseed oil. Appl. Catal., B 2015, 164,
176−183.
(46) Sankaranarayanan, T. M.; Shanthi, R. V.; Thirunavukkarasu, K.;
Pandurangan, A.; Sivasanker, S. Catalytic properties of spinel-type
mixed oxides in transesterification of vegetable oils. J. Mol. Catal. A:
Chem. 2013, 379, 234−242.
(47) Lertpanyapornchai, B.; Ngamcharussrivichai, C. Mesostructured
Sr and Ti mixed oxides as heterogeneous base catalysts for
transesterification of palm kernel oil with methanol. Chem. Eng. J.
2015, 264, 789−796.
(48) Yan, S.; DiMaggio, C.; Siddharth, M.; Kim, M.; Salley, S. O.; Ng,
K. Y. S. Advancements in Heterogeneous Catalysis for Biodiesel
Synthesis. Top. Catal. 2010, 53, 721−736.
(49) Refaat, A. A. Biodiesel production using solid metal oxide
catalysts. Int. J. Int. J. Environ. Sci. Technol. 2011, 8 (1), 203−221.
(50) Buyevskaya, O.; Baerns, M. Catalytic selective oxidation of
propane. Catal. Today 1998, 42 (3), 315−323.
(51) D’Cruz, A.; Kulkarni, A.; Meher, M. G.; Charan, L.; Dalai, A. K.
Synthesis of Biodiesel from Canola Oil Using Heterogeneous Base
Catalyst. JAOCS J. J. Am. Oil Chem. Soc. 2007, 84 (10), 937−943.
(52) Faungnawakij, K.; Yoosuk, B.; Namuangruk, S.; Krasae, P.;
Viriya-Empikul, N.; Puttasawat, B. Sr−Mg Mixed Oxides as Biodiesel
Production Catalysts. ChemCatChem 2012, 4, 209−216.
(53) Nasreen, S.; Liu, H.; Khan, R.; Zhu, X.-C.; Skala, D.
Transesterification of soybean oil catalyzed by Sr-doped cinder. Energy
Convers. Manage. 2015, 95, 272−280.
(54) Koberg, M.; Abu-Much, R.; Gedanken, A. Optimization of bio-
diesel production from soybean and wastes of cooked oil: Combining
dielectric microwave irradiation and a SrO catalyst. Bioresour. Technol.
2011, 102, 1073−1078.
(55) Patil, P. D.; Gude, V. G.; Camacho, L. M.; Deng, S. Microwave-
Assisted Catalytic Transesterification of Camelina Sativa Oil. Energy
Fuels 2010, 24, 1298−1304.
(56) Patil, P.; Gude, V. G.; Pinappu, S.; Deng, S. Transesterification
kinetics of Camelina Sativa oil on metal oxide catalysts under
conventional and microwave hating conditions. Chem. Eng. J. 2011,
168, 1296−1300.
(57) Muley, P. D.; Boldor, D. Investigation of microwave dielectric
properties of biodiesel components. Bioresour. Technol. 2013, 127,
165−174.
(58) Gude, V. G.; Patil, P.; Martinez-Guerra, E.; Deng, S.;
Nirmalakhandan, N. Microwave energy potential for biodiesel
production. Sustain. Sustainable Chem. Processes 2013, 1, 5.
(59) Islam, A.; Hin Taufiq-Yap, Y.; Ravindra, P.; Hwa Teo, S.;
Sivasangar, S.; Chan, E.-S. Biodiesel synthesis over millimetric γ-
Al2O3/KI catalyst. Energy 2015, 89, 965−973.
(60) Islam, A.; Hui Taufiq-Yap, Y.; Chu, C.-M.; Ravindra, P.; Chan,
E.-S. Transesterification of palm oil using KF and NaNO3 catalysts
supported on spherical millimetric γ-Al2O3. Renewable Energy 2013, 59,
23−29.
(61) Aricetti, J. A.; Tubino, M. A Visual Titration Method for the
Determination of the Acid Number of Oils and Fats: a Green
Alternative. JAOCS. J. Am. Oil Chem. Soc. 2012, 89, 2113−2115.
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b00256
Energy Fuels XXXX, XXX, XXX−XXX
I
10. (62) Patil, P. D.; Gude, V. G.; Deng, S. Biodiesel Production from
Jatropha Curcas, Waste Cooking, and Camelina Sativa Oils. Ind. Eng.
Chem. Res. 2009, 48, 10850−10856.
(63) Berrios, M.; Siles, J.; Martın, M. A.; Martın, A. A kinetic study of
the esterification of free fatty acids (FFA) in sunflower oil. Fuel 2007,
86, 2383−2388.
(64) Ma, M.-G.; Zhu, Y.-J. Microwave synthesis of SrCO3 one-
dimensional nanostructures assembled from nanocrystals using
ethylenediamine additive. Mater. Lett. 2008, 62, 2512−2515.
(65) Koberg, M.; Cohen, M.; Ben-Amotz, A.; Gedanken, A. Bio-
diesel production directly from the microalgae biomass of Nanno-
chloropsis by microwave and ultrasound radiation. Bioresour. Technol.
2011, 102, 4265−4269.
(66) Koberg, M.; Gedanken, A. Direct transesterification of Castor
and Jatropha seeds for FAME production by microwave and
ultrasound radiation using a SrO catalyst. BioEnergy Res. 2012, 5,
958−968.
(67) Koberg, M.; Gedanken, A. Optimization of bio-diesel
production from oils, cooking oils, microalgae and castor and jatropa
seeds: probing various heating sources and catalysts. Energy Environ.
Sci. 2012, 5, 7460−7469.
(68) Ptáček, P.; Bartoníčková, E.; Švec, J.; Opravil, T.; Šoukal, F.;
Frajkorová, F. The kinetics and mechanism of thermal decomposition
of SrCO3 polymorphs. Ceram. Int. 2015, 41, 115−126.
(69) Zhang, J.; Xu, J.; Zhang, H.; Yin, X.; Yang, D.; Qian, J.; Liu, L.;
Liu, X. Chemical synthesis of SrCO3 microcrystals via a homogeneous
precipitation method. Micro Nano Lett. 2011, 6 (4), 205−208.
(70) Raja, P. K.; Chokkalingam, A.; Priya, S. V.; Wahab, M. A.;
Dhawale, D. S.; Lawrence, G.; Ariga, K.; Jayavel, R.; Vinu, A.
Mesoporous Carbon Encapsulated with SrO Nanoparticles for the
Transesterification of Ethyl Acetoacetate. J. Nanosci. Nanotechnol.
2012, 12, 8467−8474.
(71) Shen, B.; Zhai, W.; Tao, M.; Ling, J.; Zheng, W. Lightweight,
multifunctional polyetherimide/graphene@Fe3O4 composite foams for
shielding of electromagnetic pollution. ACS Appl. Mater. Interfaces
2013, 5, 11383−11391.
(72) Viola, E.; Blasi, A.; Valerio, V.; Guidi, I.; Zimbardi, F.; Braccio,
G.; Giordano, G. Biodiesel from fried vegetable oils via trans-
esterification by heterogeneous catalysis. Catal. Today 2012, 179,
185−190.
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b00256
Energy Fuels XXXX, XXX, XXX−XXX
J
