Transesterification of glyceryl tributyrate with methanol using strontium borate as the solid base catalyst

Article abstract of DOI:10.1007/s11426-013-4977-7

Four kinds of strontium borates were prepared and characterized by XRD, SEM, EDS, TG-DTA and Hammett titration method, and their catalytic activities were examined in the transesterification of glyceryl tributyrate with methanol for the first time. The separate effects of the molar ratio of methanol to oil, the reaction time, and reusability were investigated. In addition, the catalytic activities of Sr(OH)₂ and SrCO₃ were also examined for the comparison. The results showed that the basicity and catalytic activity of these catalysts were decreased as the following order: Sr(OH) ₂ > SrB₂O₄4H₂O >SrB₆O ₁₀5H₂O >SrB₂O₄ >SrB ₆O₁₀ >SrCO₃, and the reusability decreases as the following order: anhydrous strontium borates (SrB₂O ₄, SrB₆O₁₀) >hydrated strontium borates (SrB₂O₄4H₂O, SrB₆O ₁₀5H₂O) >Sr(OH) ₂. The results indicate that the SrB₂O₄4H₂O with regular morphology, which was obtained at low temperature by a simple preparation method, might be as one kind of good potential alkaline earth salts catalyst for transesterification. Moreover, the possible reaction mechanism is proposed and analyzed. Science China Press and Springer-Verlag Berlin Heidelberg 2013.

Full text of DOI:10.1007/s11426-013-4977-7

SCIENCE CHINA
Chemistry
December 2013 Vol.56 No.12: 1727–1734
doi: 10.1007/s11426-013-4977-7
• ARTICLES •
Transesterification of glyceryl tributyrate with methanol using
strontium borate as the solid base catalyst
HUANG HongSheng1,2 & LIU ZhiHong1*
1Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an 710062, China
2Department of Chemisitry and Chemical Engineering, Ankang University, Ankang 725000, China
Received May 16, 2013; accepted August 16, 2013; published online October 14, 2013
Four kinds of strontium borates were prepared and characterized by XRD, SEM, EDS, TG-DTA and Hammett titration method,
and their catalytic activities were examined in the transesterification of glyceryl tributyrate with methanol for the first time.
The separate effects of the molar ratio of methanol to oil, the reaction time, and reusability were investigated. In addition, the
catalytic activities of Sr(OH)2 and SrCO3 were also examined for the comparison. The results showed that the basicity and cat-
alytic activity of these catalysts were decreased as the following order: Sr(OH)2 > SrB2O4·4H2O > SrB6O10·5H2O > SrB2O4 >
SrB6O10 > SrCO3, and the reusability decreases as the following order: anhydrous strontium borates (SrB2O4, SrB6O10) > hy-
drated strontium borates (SrB2O4·4H2O, SrB6O10·5H2O) > Sr(OH)2. The results indicate that the SrB2O4·4H2O with regular
morphology, which was obtained at low temperature by a simple preparation method, might be as one kind of good potential
alkaline earth salts catalyst for transesterification. Moreover, the possible reaction mechanism is proposed and analyzed.
strontium borate, solid base catalyst, transesterification, glyceryl tributyrate
and shows weak catalytic activity [4]. On the other hand,
1 Introduction
basic heterogeneous catalysts exhibit high catalytic activity
in transesterification. A variety of solid base catalysts are
known, including alkali or alkaline earth oxides, supported
alkali metals, basic zeolites, clay minerals and supported
organic bases [5].
As for alkaline earth compounds, the studies on hetero-
geneously catalyzed transesterification were focused on the
catalysis by alkaline earth oxides, which usually exhibites
high catalytic activity. However, they usually require high
temperature for the preparation or modification of the active
phases because their precursors usually have poor or no
catalytic activity and these catalysts are easily contaminated
by the acidic gases in air [6–8]. For example, CaO as a base
catalyst, the poisonous species must be removed at more
than 973 K, and exposure to the atmosphere should be
strictly prevented before its use because its active surface is
chemisorbed very easily by H2O and CO2 in the atmosphere
Nowadays, biodiesel (FAMEs) as a biofuel, has become
very attractive because of its environmental and renewable
benefits [1, 2]. Transesterification is a universal and estab-
lished method for biodiesel production from vegetable and
other fatty oils. Homogeneous acid and base catalysts are
usually used in transesterification reactions. However, these
catalysts have quite a few disadvantages, where the cost of
production increases due to the catalyst consumption and a
series of purification steps.
Heterogeneous catalysis solves many of these problems
associated with homogeneous catalysts [3], therefore, recent
research has focused on heterogeneous catalysts. However,
acid heterogeneous catalysis requires a high reaction tem-
perature, high alcohol to oil ratios and long reaction time,
*Corresponding author (email: lzh58@sina.com)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
[9]. In addition, it is not easy to control their morphology
and particle size which are important factors in the catalytic
activity [10]. So, searching for the catalysts with high
chemical stability, longer durability, mild preparation con-
ditions, regular morphology and small particle size becomes
an important research direction. In fact, many alkaline earth
metal salts can keep stable in the air, and can be synthesized
at mild preparation condition and in a variety of presenta-
tion morphologies [11–13], so the alkaline earth metal salts
might be as one kind of potential catalysts. However, there
were few researches on them, mainly on alkaline earth car-
bonates, and their catalytic activity for transesterification is
not good, like CaCO3 [14]. It has been found that some bo-
ron compounds, such as boric acid, boria and borate exhibit
good catalytic activity for transesterification [15–18]. Very
few boron compounds [19, 20] were reported as the catalyst
for catalyzing transesterification of glyceride, which is the
reaction for biodiesel production. However, the catalytic
property of alkaline earth metal borates for catalyzing
transesterification reactions was not reported.
Sr(OH)2·8H2O, 0.001 mol of Na2B4O7·10H2O, and 0.024
mol of H3BO3 were mixed in 60 mL distilled water with
constant stirring for 24 h at room temperature, and the pH of
mixed solution was 8. Then the resulting white precipitate
was filtered and washed several times with distilled water
and absolute alcohol, respectively. Finally, it was dried at
40 °C for 24 h.
The SrB6O10 was prepared by calcining SrB6O10·5H2O in
air for 3 h at 800 °C.
2.2 Characterization of catalysts
The phase composition and crystalline of synthesized cata-
lysts were characterized by X-ray powder diffraction (XRD,
Rigaku D/MAX-C, Cu Kα radiation), energy dispersive
X-ray spectrometer (EDS, Hitachi, JEOL-6700F) and ther-
mogravimetric and differential thermal analysis (TG-DTA,
TA, SDTQ600 simultaneous thermal analyzer at a heating
rate of 10 °C/min in flowing N2). The morphologies and
sizes of the catalysts were investigated by scanning electron
microscopy (SEM, Philips-FEI, Quanta200). The basic
strength of the catalysts (H_) was determined by Hammett
indicators method as reported by Watkins et al. [21]. Ap-
proximately 50 mg of the catalyst was shaken with the ap-
propriate amount of Hammett indicator diluted with metha-
nol and left to equilibrate for 2 h until no further color
changes were observed. The color of the catalyst was then
recorded. The following Hammett indicators were used:
bromothymol blue (H_ = 7.2), phenolphthalein (H_ = 9.3),
2,4-dinitroaniline (H_ = 15.0) and 4-nitroaniline (H_ =
18.4). The basicity of the catalyst was also accurately de-
termined by using Hammett indicator-benzoic acid (0.02
mol/L anhydrous methanol solution) titration test.
In this paper, we investigated the catalytic activities of
the strontium borates (SrB2O4, SrB6O10), and their precur-
sors (SrB2O4·4H2O, SrB6O10·5H2O) in the transesterifica-
tion of glyceryl tributyrate with methanol for the first time.
Furthermore, for comparison, the catalytic activities of
strontium carbonate and strontium hydroxide were also
studied.
2 Experimental
2.1 Materials and preparation of catalysts
Strontium carbonate (SrCO3, 99%), boric acid (H3BO3,
99%), borax (Na2B4O7·10H2O, 99%), sodium hydroxide
(NaOH, 99%), strontium chloride (SrCl2·2H2O, 98%), and
anhydrous methanol (CH3OH, 99%) were purchased from
Sinopharm Chemical Reagent Co. Ltd. Glyceryl tributyrate
(C15H26O6, 98%), dihexylether (C12H26O, 97%), methyl
butyrate (C5H10O2, 99%) and strontium hydroxide (Sr(OH)2,
99%), were purchased from Sigm-Aldrich Co. Llc.
The preparation process for SrB2O4·4H2O: 0.025 mol of
NaOH and 0.01 mol of Na2B4O7·10H2O were mixed in 100
mL distilled water forming solution A, and 0.01 mol of
SrCl2·2H2O was dissolved in 40 mL of distilled water
forming solution B. Solution B was poured into solution A
slowly with constant stirring to form a homogenous mixture,
which was stirred for 24 h at room temperature, and the pH
of mixed solution was 11. Then the resulting white precipi-
tate was filtered, washed several times with distilled water
and absolute alcohol respectively, and finally, dried at 40 °C
to constant mass.
2.3 Catalytic experiments
Transesterification reactions were performed in stirred
radleys batch reactor at 60 °C. Reaction tubes were charged
with 0.0100 mol (3.00 mL) of glyceryl tributyrate, 0.0025
mol (0.59 mL) of dihexylether as an internal standard and
0.3036 mol (12.50 mL) of methanol. Samples were used as
prepared without further activation. Aliquots of 0.1 mL
were regularly sampled and analyzed by gas chromatog-
raphy using a Varian 450-GC equipped with a VF-1ms 25
m × 0.25 mm capillary column. Product yields were calcu-
lated using reactant and product response factors derived for
glyceryl tributyrate and methyl butyrate. Yield of methyl
butyrate was calculated according to the following formula:
MB
YMB

100%
3GT0
The SrB2O4 was prepared by calcining SrB2O4·4H2O in
air for 3 h at 700 °C.
where GT0 is the initial amount (mmol) of glyceryl tributyr-
ate, MB is the amount (mmol) of methyl butyrate at the time
of sampling.
The preparation process for SrB6O10·5H2O: 0.002 mol of

Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
1729
dles of rods with the thickness of about 250 nm, but there
are many voids on the rods surface of SrB2O4, which might
be resulted from the high temperature dehydration. Figure
2(c) shows that the SrB6O10·5H2O sample exhibits a flow-
er-like nanostructure formed by regular nanosheets with the
thickness of about 100 nm . Figure 2(d) shows that the
SrB6O10 consists of aggregated nanosheets.
The simultaneous TG-DTA curves of fresh catalysts and
the spent catalysts which were reused after 3 circles are
shown in Figure 3. As shown in Figure 3(a), fresh
SrB2O4·4H2O has a one-step decomposition from 100 °C to
800 °C with a total mass loss of 29.15%, which corresponds
to the loss of four water molecules and agrees with the cal-
culated value of 29.38%. In the DTA curve, the first endo-
thermic peak appearing at 163 °C is related to the elimina-
3 Results and discussion
3.1 Materials characterization
The basic strength and basicity of the fresh catalysts and the
spent catalysts which were reused after 3 cycles were de-
termined by the Hammett indicator method, and the results
were summarized in Table 1. It was observed that fresh and
spent SrB2O4·4H2O, SrB2O4, SrB6O10·5H2O, SrB6O10 and
SrCO3 exhibited the basic strengths in the range of 7.2–9.3.
The fresh Sr(OH)2 exhibited the basic strengths in the range
of 7.2–18.4, however, the basic strength of spent Sr(OH)2
was decreased to 7.2–15. Compared with the fresh catalyst,
the basicity of spent catalysts was decreased. It can be also
observed the total basicity of these fresh catalysts and spent
catalysts was decreased as the following order: Sr(OH)2 >
SrB2O4·4H2O > SrB6O10·5H2O >SrB2O4 > SrB6O10
>
SrCO3. The relationship between the basic strength and ba-
sicity of catalysts and the catalytic results of catalysts is
discussed later in this paper.
Figure 1 presents the XRD patterns of the prepared cata-
lysts. All diffraction peaks ((a)–(d)) in Figure 1 correspond
to the SrB2O4·4H2O (JCPDS.No.41-0538), SrB2O4
(JCPDS.No.15-0779), SrB6O10·5H2O (JCPDS.No.16-0495),
and SrB6O10 (JCPDS.No.20-1190), respectively. The strong
and sharp diffraction peaks indicate the good crystallinity of
the samples.
SEM images and EDS patterns of SrB2O4·4H2O, SrB2O4,
SrB6O10·5H2O and SrB6O10 samples are shown in Figure 2.
The EDS patterns of the prepared samples clearly indicate
that all the catalysts were composed of Sr, B, and O, and no
indication for other elements. Figure 2 (a, b) shows that
SrB2O4·4H2O and SrB2O4 samples are composed of bun-
Figure 1 X-ray diffraction pattern of catalysts. (a) SrB2O4·4H2O; (b)
SrB2O4; (c) SrB6O10·5H2O; (d) SrB6O10.
Table 1 The basic strength and basicity of catalysts
Basicity (mmol/g)
Total basicity
Basic strength
Catalyst
(H_)
(mmol/g)
7.2 < H_ < 9.3
9.3 < H_ <15
13.52
15 < H_ < 18.4
Sr(OH)2 (fresh)
Sr(OH)2 (spent)
7.2 < H_< 18.4
7.2 < H_< 15
13.86
4.12
3.01
0
30.39
7.79
3.67
SrB2O4·4H2O
(fresh)
SrB2O4 ·4H2O
(spent)
7.2 < H_<9.3
7.2 < H_ < 9.3
3.16
2.12
0
0
0
0
3.16
2.12
SrB2O4 (fresh)
SrB2O4 (spent)
SrB6O10·5H2O
7.2 < H_ < 9.3
7.2 < H_ < 9.3
7.2 < H_ < 9.3
0.55
0.44
0.79
0
0
0
0
0
0
0.55
0.44
0.79
SrB6O10·5H2O
7.2 < H_ < 9.3
0.45
0
0
0.45
(spent)
SrB6O10 (fresh)
SrB6O10 (spent)
SrCO3 (fresh)
SrCO3 (spent)
7.2 < H_ < 9.3
7.2 < H_ < 9.3
7.2 < H_ < 9.3
7.2 < H_ < 9.3
0.31
0.24
0.14
0.09
0
0
0
0
0
0
0
0
0.31
0.24
0.14
0.09

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Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
Figure 2 SEM images and EDS pattern of catalysts. (a) SrB2O4·4H2O; (b) SrB2O4 ; (c) SrB6O10·5H2O; (d) SrB6O10.
tion of crystal water, forming the amorphous phase SrB2O4,
the exothermic peaks at 631 and 690 °C are related to the
recrystallization of SrB2O4. The TG-DTA curve of spent
SrB2O4·4H2O (Figure 3(c)) is similar to the fresh one, and
its total mass loss from 100 °C to 800 °C is 29.44%, which
means it is stable and no water is lost during reaction and
pretreatment. As shown in Figure 3(b), SrB6O10·5H2O
sample has a two-step decomposition from 30 °C to 800°C
with a total mass loss of 22.04%, which corresponds to the
loss of five water molecules and agrees with the calculated
value of 22.38%. In the DTA curve, the endothermic peaks
appearing at 102, 136, and 434 °C are related to the elimi-
nation of crystal water and hydroxyl, forming the amor-
phous phase SrB6O10, the exothermic peak at 743 °C is re-
lated to the recrystallization of SrB6O10. As shown in Figure
3(d), the total mass loss of spent SrB6O10·5H2O is decreased
to 19.92%, which means a little water was lost during the
reaction and pretreatment. In addition, it can be observed in
Figure 3(e, f) that there are some mass losses in the spent
SrB2O4 and SrB6O10, which might be result from the water
molecules being adsorbed on them. From the above results,
it can be found that the thermal stability is increased as the
following order: SrB6O10·5H2O < SrB2O4·4H2O < SrB6O10,
SrB2O4, and the chemical stability of SrB2O4·4H2O during
the reaction and pretreatment is higher than other catalysts.
3.2 Catalytic activity study
3.2.1 Effect of molar ratio of methanol to oil (glyceryl
tributyrate) on methyl butyrate yield
The stoichiometry of this reaction requires 3 mol methanol
per mol triglyceride to yield 3 mol methyl butyrate and 1
mol glycerol. The methyl butyrate yield could be elevated
by introducing an excess amount of methanol to shift the
equilibrium to the right-hand side. When the catalyst/oil
mass ratio was 1.7% at 60 °C for 24 h, the effect of the mo-
lar ratio of methanol to oil (glyceryl tributyrate) on methyl
butyrate yield is illustrated in Figure 4. This shows that the
methyl butyrate yields increased when the molar ratio of
methanol to oil was increased. For the SrB2O4·4H2O,
SrB6O10·5H2O and Sr(OH)2 as the catalysts, the maximum
yields were obtained at 30:1 molar ratio. However, for the
SrB2O4·4H2O, the methyl butyrate yield are almost the
same when the molar ratio of methanol to oil was over 20:1.
For the SrB2O4, SrB6O10 and SrCO3 as the catalysts, the
maximum yields were obtained at 40:1 molar ratio. Howev-
er, when the molar ratio of methanol to oil was over 30:1,
the yield increased slightly. Based on these results, the low-
er molar ratio of methanol to oil was needed when the
SrB2O4·4H2O was used as the catalyst, and the optimum
molar ratio of methanol to oil for catalytic transesterifica-

Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
1731
Figure 3 Simultaneous TG-DTA curves of prepared catalysts. (a) fresh SrB2O4·4H2O; (b) fresh SrB6O10·5H2O; (c) spent SrB2O4·4H2O; (d) spent
SrB6O10·5H2O; (e) spent SrB2O4; (f) spent SrB6O10.
tion of glyceryl tributyrate using other catalysts is 30:1.
and 13% at 1440 min, respectively. However, SrCO3
seemed to be useless in catalyzing the transesterification
because only 0.3% methyl butyrate was yielded. From these
results, the catalytic activity of these catalysts increases as
the following order, strontium carbonate < strontium borate
< strontium hydroxide. As for strontium borates, the cata-
lytic activity of strontium hexaborate is weaker than that of
strontium metaborate. The catalytic activity of anhydrous
strontium borate is weaker than that of hydrated strontium
borate (the precursor), which means that the better catalyst
3.2.2 Effect of reaction time on methyl butyrate yield
Transesterification experiments for glyceryl tributyrate were
carried out at a constant agitation for various periods of
times between 0 and 1440 min (24 h). Figure 5 depicts the
influence of the reaction time on the yield of methyl butyr-
ate. It can be observed that the maximum yield for glyceryl
tributyrate using Sr(OH)2, SrB2O4·4H2O, SrB6O10·5H2O,
SrB2O4, and SrB6O10 was reached 98%, 44%, 26%, 20%,

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Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
Figure 4 Effect of molar ratio of methanol to oil (glyceryl tributyrate) on
methyl butyrate yield. Catalyst/oil mass ratio: 1.7%; reaction temperature:
60 °C; reaction time: 24 h.
Figure 6 Effect of repeated use of catalyst on the methyl butyrate yield.
Reaction condition: methanol to oil (glyceryl tributyrate) is 30:1; catalyst/oil
mass ratio is 1.7%; reaction temperature is 60 oC; reaction time is 24 h.
thyl butyrate yield were decreased from 44.1% to 25.5%,
20.3% to 15.6%, 26.2% to 14.4% and 12.9% to 9.1% when
SrB2O4·4H2O, SrB2O4, SrB6O10·5H2O and SrB6O10 cata-
lysts were reused in the third run, respectively. In addition,
the significant deactivation was observed over the reused
Sr(OH)2, and the methyl butyrate yield is only 29.2% when
it was reused in the third run, which is only a little higher
than that of SrB2O4·4H2O (25.5%). The above results indi-
cate that the Sr(OH)2 catalyst is consumable in this reaction
and the reusability of anhydrous strontium borates is better
than that of hydrated strontium borates although its catalytic
activity is weaker than hydrated strontium borates.
3.3 Reaction mechanism
Figure 5 Effect of the reaction time on the methyl butyrate yield. Reac-
tion condition: mass ratio of methanol to oil (glyceryl tributyrate) is 30:1;
catalyst/oil mass ratio is 1.7%; reaction temperature is 60 oC.
Understanding the reaction mechanisms can help to design a
suitable catalyst for reaction. The mechanisms of transester-
ification using acid or base heterogeneous catalysts have
been discussed [22, 23], and the mechanisms of the trans-
esterification of triglyceride with alcohol in the presence of
a base or acid catalyst are different [24]. The transesterifica-
tion catalyzed by strontium borates, strontium hydroxide,
and strontium carbonate were considered to proceed via a
similar mechanism as that of solid base catalysts. A possible
mechanism follows four main steps.
could be obtained at low temperature by a simple prepara-
tion method. In conclusion, the maximum yield of methyl
butyrate was decreased as the following order: Sr(OH)2 >
SrB2O4·4H2O > SrB6O10·5H2O > SrB2O4 > SrB6O10
>
SrCO3, and it is in good agreement with the total basicity
order of these catalysts, indicating that the catalytic activity
was mainly determined by the basicity of the catalyst and
showing that the higher basicity could result in the higher
products yield or better catalytic effect.
Step 1:
(1-1)
(1-2)
3.2.3 Effect of repeated experiments on methyl butyrate
yield
The reusability of SrB2O4·4H2O, SrB2O4, SrB6O10·5H2O,
SrB6O10, Sr(OH)2 and SrCO3 catalysts was also examined.
After separated from the solution, the catalysts were washed
and dried before retesting the reaction for 3 cycles under the
same operating conditions. As shown in Figure 6, the me-
(1-3)

Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
1733
surface methoxide anions (Eqilibrium (1-1)) in this step. As
for hydrated strontium borates, including SrB2O4·4H2O
(1-4)
and SrB6O10·5H2O, the surface O2 can extract H+ not only
from the hydroxyl group of methanol to form surface
methoxide anions (Eqilibrium (1-1)), but also from their
crystal water (H2O) to form surface OH (Eqilibrium (1-2))
which was more easily adsorbed on the surface of the cata-
lyst than methoxide [26], then, the OH extracts H+ from
methanol to generate methoxide anion (CH3O) and H2O
(Eqilibrium (1-3)). The stronger catalytic activity of
SrB2O4·4H2O than SrB6O10·5H2O could be due to the
stronger polarization of water in SrB2O4·4H2O as the po-
larization of water in crystal lattice have directly impact on
the formation of Bronsted base. As we know, the catalytic
activity usually enhanced after calcining at high temperature
as the acidic molecules such as H2O were removed, however,
as shown in Figure 5, the catalytic activity of anhydrous
strontium borate is lower than that of precursor (hydrated
strontium borate). This special result could be explained by
the extra reaction mechanism of hydrated strontium borate
((Eqilibrium (1-2, 1-3)) which was produced by the crystal
water. The reason attributed for the decreased catalytic ac-
tivity of spent SrB6O10·5H2O is thought to be the loss of
crystal water during the pretreatment (drying), which is
shown in Figure 3. However, excess water will result in the
catalyst poisoning [27], as shown in Figure 3, the spent
SrB2O4 and SrB6O10 exhibit 2.82% and 5.76% mass loss,
respectively, indicating that a little water molecules are as-
sumed to be adsorbed on them, so the reason for the de-
creased catalytic activity of spent SrB6O10 and SrB2O4 could
be the excess water on catalysts. Compared with the fresh
SrB2O4·4H2O, the spent SrB2O4·4H2O shows no weight
loss but basicity decrease, the reason attributed for the de-
creased catalytic activity of spent SrB2O4·4H2O is most
likely due to the reduction of the number of active sites on
the catalyst surface. As for strontium metaborate and stron-
tium hexaborate, including hydrated and anhydrous stron-
tium borate, the catalytic activity of catalyst was mainly
determined by its basicity as the reaction mechanism of
them in this step is similar, and it was confirmed by the cat-
alytic results that the strontium hexaborate with lower ba-
sicity exhibits weaker catalytic activity than strontium
metaborate. Strontium hydroxide as a strong base which is
similar to barium hydroxide [28], the HO–Sr–OH bonds are
strongly polarized and show ionic character, so that small
amounts of Sr(OH)2 could be converted into Sr(OCH3)2
(Eqilibrium (1-4)) which exhibits higher catalytic activity
than Sr(OH)2 because of its stronger basicity.
Step 2:
(2-1)
(2-2)
(2-3)
Step 3:
(3-1)
(3-2)
Step4:
(4-1)
(4-2)
(4-3)
The first step is the generation of methoxide anion
(CH3O) through proton abstraction from alcohol by cata-
lyst. As the strong nucleophiles, the methoxide ions attack
the carbonyl moiety in glyceride molecules to produce the
alkyl esters [25]. So the first step is a key step in determin-
ing its catalytic activity in transesterification. The basicity
of the catalyst is the determining factor in generating meth-
oxide anion, therefore, in the reaction of this step, the ability
difference of generating methoxide ion is mainly due to the
different basicity of the catalyst. As for anhydrous strontium
borates and strontium carbonate, the surface O2 of catalyst
extracts H+ from the hydroxyl group of methanol to form
In the second step, the methoxide anion attacks to the
carbonyl carbon atom of the triglyceride molecule to form a
tetrahedral intermediate, which is shown in Eqilibrium (2-1)
for anhydrous strontium borate and strontium carbonate,
Eqilibriums (2-1, 2-2) for hydrated strontium borate, and
Eqilibriums (2-1, 2-3) for strontium hydroxide, respectively.

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Huang HS, et al. Sci China Chem December (2013) Vol.56 No.12
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from the surface of the catalyst. In addition, the tetrahedral
intermediate can also react with methanol to generate
methoxide anions. The reactions of this step catalyzed by
anhydrous strontium borate, strontium carbonate, hydrated
strontium borate and strontium hydroxide are shown in Eqi-
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Eqilibrium (3-1), respectively.
In the last step, the rearrangement of the tetrahedral in-
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glycerol. The reactions of this step catalyzed by anhydrous
strontium borate, strontium carbonate, and hydrated stron-
tium borate are similar, as shown in Eqilibrium (4-1).
However, the reaction of this step catalyzed by strontium
hydroxide, which is shown in Eqilibriums (4-1, 4-2, and
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solubility in methanol.
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the first time. The hydrated strontium borates, especially the
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have better reusability than strontium hydroxide. This indi-
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