Synthesis and characterization of calcium methoxide as heterogeneous catalyst for trimethylolpropane esters conversion reaction

Article abstract of DOI:10.1016/j.apcata.2012.03.019

Trimethylolpropane (TMP) esters are potential biodegradable basestock for biolubricant. In order to attain environmental benignity, attention has been focused on utilizing heterogeneous catalysts for production of TMP esters. Alkaline homogeneous catalysts tend to react with free fatty acids to produce unwanted soap, thus reducing the overall product yield. This study had focused on the synthesis of calcium methoxide and investigating its potential as heterogeneous catalyst for the transesterification of TMP and palm oil methyl esters (POME) to TMP esters. The performance of synthesized calcium methoxide as a catalyst was examined by characterizing it through some instrumental techniques. X-ray diffraction (XRD) showed calcium methoxide has been successfully synthesized. Scanning electron microscopy (SEM) displayed thermally resistant surface structure with good porosity; BET showed high surface area; particle size analysis evidenced reasonable size of catalyst particles; and thermogravimetry (TGA) revealed good thermal stability of synthesized calcium methoxide. Moreover, the catalyst was found to possess mesoporous surface by pore size analysis through Barrett-Joyner-Halenda (BJH) method. The results of transesterification reaction indicated satisfactory catalytic activity of synthesized calcium methoxide and the TMP triesters yield obtained was 80.35% after 2 h, 87.48% after 4 h, 91.30% after 6 h and 92.38% after 8 h reaction time.

Full text of DOI:10.1016/j.apcata.2012.03.019

Applied Catalysis A: General 425–426 (2012) 184–190
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis and characterization of calcium methoxide as heterogeneous catalyst
for trimethylolpropane esters conversion reaction
Hassan Masooda, Robiah Yunusa,b,∗, Thomas S.Y. Choonga, Umer Rashidb, Yun H. Taufiq Yapc
a Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
b Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
c Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Trimethylolpropane (TMP) esters are potential biodegradable basestock for biolubricant. In order to attain
environmental benignity, attention has been focused on utilizing heterogeneous catalysts for production
of TMP esters. Alkaline homogeneous catalysts tend to react with free fatty acids to produce unwanted
soap, thus reducing the overall product yield. This study had focused on the synthesis of calcium methox-
ide and investigating its potential as heterogeneous catalyst for the transesterification of TMP and palm oil
methyl esters (POME) to TMP esters. The performance of synthesized calcium methoxide as a catalyst was
examined by characterizing it through some instrumental techniques. X-ray diffraction (XRD) showed
calcium methoxide has been successfully synthesized. Scanning electron microscopy (SEM) displayed
thermally resistant surface structure with good porosity; BET showed high surface area; particle size
analysis evidenced reasonable size of catalyst particles; and thermogravimetry (TGA) revealed good ther-
mal stability of synthesized calcium methoxide. Moreover, the catalyst was found to possess mesoporous
surface by pore size analysis through Barrett–Joyner–Halenda (BJH) method. The results of transesteri-
fication reaction indicated satisfactory catalytic activity of synthesized calcium methoxide and the TMP
triesters yield obtained was 80.35% after 2 h, 87.48% after 4 h, 91.30% after 6 h and 92.38% after 8 h reaction
time.
Received 25 January 2012
Received in revised form 8 March 2012
Accepted 12 March 2012
Available online 21 March 2012
Keywords:
Lubricant
Heterogeneous catalyst
Calcium methoxide
Transesterification
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
performed transesterification of vegetable oil methyl esters with
trimethylolpropane (TMP) to generate esters of TMP, which are
The concern and awareness of the harmful impact of mineral
based lubricants on the environment have pushed the research
towards the production of environmental friendly lubricants from
vegetable oils. Due to some performance limitations, vegetable
oils are not considered suitable to be used directly as lubricants
[1]; nonetheless, the limitations can be minimized by means of
chemical modification of vegetable oils through transesterification
with polyhydric alcohols [2]. In similar fashion, researchers have
generally utilized as base oil for various types of lubricating oils
[3–9].
The transesterification reaction involves three consecutive reac-
tions in the presence of a catalyst. Monoesters (ME) and diesters
(DE) of TMP are formed as intermediate products towards the com-
pletion of the reaction producing TMP triesters. The overall reaction
stoichiometry requires one mole of TMP and three moles of methyl
esters. The overall reaction scheme is shown by Eq. (1).
CH2 – OH
CH2OCOR
CH3CH2 – C -CH2OH + 3 RCOOCH3
CH2 – OH
CH3CH2 – C – CH2OCOR
CH2OCOR
+ 3CH3OH
(1)
TMP
Methyl Esters
TMP triesters Methanol
(1)
Catalyst holds a prominent role in the transesterification reac-
tion of TMP and vegetable oil methyl esters, in order to obtain a
better yield of TMP triesters in less time. Reported catalysts for
the production of TMP esters include homogeneous bases such as
sodium methoxide [10], homogeneous acids such as sulfonic acid
[11], enzymes such as lipases [4,5] and heterogeneous base cal-
cium methoxide [6]. Alkaline homogeneous catalyst was highly
∗
Corresponding author at: Institute of Advanced Technology, Universiti Putra
Malaysia, 43400 UPM Serdang, Selangor, Malaysia. Tel.: +60 3 89467531;
fax: +60 3 89467006.
E-mail address: robiah@eng.upm.edu.my (R. Yunus).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.03.019

H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
185
corrosive in transesterification between TMP and palm oil methyl
2.3. Characterization of catalyst
esters (POME) under vacuum, and also tend to react with free fatty
acids to produce unwanted soap [10,12]. Homogeneous acid cat-
alysts are difficult to recycle and operate at high temperatures,
and also give rise to serious environmental and corrosion problems
[13]. Lipases, used as enzymatic catalysts in the transesterification
between rapeseed oil methyl esters and TMP, were reported to
experience problems with free fatty acids and methanol which had
decreased the operational ability of lipases in the reaction [5,14].
The need of developing heterogeneous catalysts is gaining atten-
tion due to non-toxic nature, ease of separation and reusability [15].
Unfortunately, heterogeneous calcium methoxide developed in an
earlier study by direct reaction between calcium and methanol
proved inadequate for the transesterification reaction due to pro-
longed reaction time [6].
In previous literature, calcium methoxide has also been served
as a catalyst for transesterification reaction between methyl esters
of olive oil and rapeseed oil with neopentyl glycol [6]. The catalyst
was also utilized for biodiesel production by methanolysis of soy-
bean oil [16,17] and tributyrine [18], and for synthesis of aliphatic
polyesters by polymerization of ␧-caprolactone and l-lactide [19].
The current study aims to report the detailed synthesis of calcium
methoxide and its effectiveness as a heterogeneous catalyst in the
transesterification of TMP with POME. The physical and chemical
properties of synthesized calcium methoxide are also character-
ized.
X-ray diffraction (XRD) was performed to examine the con-
stituents of the synthesized catalyst on Shidmazu Model XRD-6000
diffractometer having Cu tube and scintillation counter detector.
The analysis was made over a 2ꢀ range from 5◦ to 40◦ along with
a step size of 0.04◦ and scanning speed was set at 2 min−1. Mea-
surement of IR spectrum was performed using attenuated total
reflection-Fourier transform-infrared (ATR-FTIR) on PerkinElmer
Spectrum 100 FTIR spectrometer to know the surface groups exist-
ing on catalyst surface. The catalyst sample in powder form was
analyzed over the scanning range of 650–4000 cm−1 at a resolution
of 4 cm−1
.
The BET surface area, total pore volume, pore diameter and pore
size distribution were measured with BELsorp-mini II (BEL Japan,
Inc.) automatic specific surface area/pore size distribution analyzer.
A weighed sample of calcium methoxide was prepared for analysis
by outgassing it at 150 ◦C for 6 h under vacuum through BELprep-
vac II. The isotherms were generated by dosing nitrogen onto the
catalyst in a bath containing liquid nitrogen at 77 K. Equipment was
purged with helium gas during operation. The relative pressure
range for adsorption was set at 0.020–0.990 bar and for desorp-
tion the range was set at 0.970–0.306 bar. BET specific area, total
pore volume and average pore diameter were evaluated from the
BET plot, while the pore size distribution was evaluated by using
Barrett–Joyner–Halenda (BJH) plot. The particle size distribution
was measured using Malvern Mastersizer Hydro 2000MU laser par-
ticle size analyzer. These analyses were useful in understanding the
reactivity of catalyst in the reaction.
2. Experimental
Scanning electron microscopy (SEM) was performed on Ther-
moscientificInfravision Quasi-S (Scanning Electron Microscope,
Model No.: S3400N, Hitachi Co., Japan) to observe the shape of
catalyst particles. Energy-dispersive X-ray spectroscopy (EDX) was
conducted on ThermoscientificNoran System 7 with Nanotrace
detector to know chemical composition of elements inside the cat-
alyst. Finally, thermogravimetric and differential thermal analysis
(TG–DTA) was carried out with Mettler Toledo TGA/SDTA851e in
2.1. Chemicals
Precipitated calcium carbonate (analytical grade) was pur-
chased from Systerm Chemicals Malaysia. Trimethylolpropane,
2-ethyl-2-hydroxymethyl-1,3-propanediol (purum grade) and
methanol (GC grade) were obtained from Sigma Aldrich Sdn.
Bhd. (Malaysia). Palm oil methyl esters (POME) were obtained
from Carotech Sdn. Bhd. Malaysia. The average fatty acid com-
positions of POME were determined according to the standard
methods of oil and fats analysis [20]; the results are presented in
Table 1.
an air flow, with heating up to 800 ◦C at the rate of 10◦ min−1
.
2.4. Transesterification reaction procedure
The transesterification reaction was carried out in a 500 mL
flat bottom three-necked flask made of borosilicate glass (as the
reactor) having a PTFE coated magnetic stirrer bar, a reflux con-
denser, a mercury thermometer and a sampling port. A rotary
vane vacuum pump was coupled with the reflux condenser to pro-
vide the necessary vacuum conditions, and the vacuum level was
controlled by an air leakage valve located in between the reflux
condenser and the vacuum pump. An amount equal to 6.5 g TMP
was introduced into the reactor and was heated up to 100 ◦C. The
temperature was maintained at 100 ◦C for 15 min with constant
stirring at 1000 rpm under 20 mbar vacuum in order to remove
moisture trapped by the hygroscopic TMP. At this point, 86.2 g of
moisture free POME was added to the reactor and mixed with TMP.
The molar ratio TMP:POME was 1:6 and the amount of TMP and
POME to be introduced in the reaction was calculated from their
respective molecular weights accordingly. The purpose of using
excess POME was to suppress the reverse reaction [21]. The reac-
tion mixture was heated to 180 ◦C and 0.28 g of calcium methoxide
(0.3%, w/w of reaction mixture) was added. Then, vacuum pressure
was changed to 50 mbar and temperature was maintained at 180 ◦C
for 8 h. Samples were collected through sampling port after every
1 h and were analyzed by gas chromatography (GC) to know the
composition of mono-, di-, and tri-esters of TMP in the reaction
product.
2.2. Synthesis of catalyst
An amount of 5 g calcium carbonate was calcined at 900 ◦C for
1.5 h under helium flow of 50 mL min−1. The calcined product was
reacted with 100 mL methanol under reflux at 65 ◦C for 2 h with
50 mL min−1 nitrogen flow. A continuous stirring of 700 rpm was
provided to facilitate sufficient contact between the reagents. After
the reaction, the resultant precursor was filtered for the removal of
most of the methanol and then dried in the vacuum oven at 105 ◦C
for 1 h.
Table 1
Average fatty acid compositions of palm oil-based methyl esters.
Fatty acid
Composition (wt%)
Capric (C10:0
)
–
0.9
1.5
41.5
2.7
40.6
11.9
0.3
Lauric (C12:0
)
Myristic (C14:0
Palmitic (C16:0
)
)
Stearic (C18:0
Oleic (C18:1
Linoleic (C18:2
)
)
)
Linolenic (C18:3
Others
)
0.2

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H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
Fig. 2. FTIR spectrum of calcium methoxide. (a) Peak assigned to CO stretching vibra-
tion of methanol; (b) Peak resulting from CH bending; (c) Peaks derived from CH3
stretching vibrations; (d) Peak assigned to OH stretching vibration of primary
alcohol.
Fig. 2 shows the FTIR spectrum of synthesized catalyst at
room temperature. A distinct peak observed at around 1070 cm−1
was assigned to
C O stretching vibration of primary alcohol,
while another peak at around 3650 cm−1 was attributed to OH
stretching vibration of primary alcohol [16]. Other peaks indicat-
ing the important features were 2800–3000 cm−1 derived from CH3
Fig. 1. XRD patterns of: (a) calcination product (b) reaction product.
stretching vibrations and 1465 cm−1 concerning
C H (alkane)
bending [17]. Earlier researches suggest the peak appeared at
3650 cm−1 due to adsorption deriving from hydration of the sur-
face [24]; it was peculiar to calcium methoxide. This peak indicates
the existence of OH groups isolated on calcium cation [25]. Since
water was produced as a by-product in the catalyst synthesis reac-
tion, these isolated OH groups might have produced from water
facilitated by the strong basic property of calcium methoxide.
The reactivity of a catalyst is directly related with the surface
area of the catalyst, therefore BET surface area is considered to be
an important characterization of solid catalyst. The analyzed results
for BET surface area, total pore volume and average pore diameter
of synthesized catalyst were found to be 38.6 m2 g−1, 0.24 cm3 g−1
and 25.7 nm respectively. Due to high BET surface area, the catalyst
appears favorable for use in the liquid phase reactions, and can
provide an enough reaction area in stirred reactor [23]. The pore
size distribution of synthesized catalyst in Fig. 3 reveals that a large
part of surface is occupied by mesopores, having diameters within
a range of 20–30 nm, which confirms mesoporous nature of the
catalyst. Mesoporous materials can serve as effective catalysts in
2.5. Analysis of product
The collected samples were analyzed by gas chromatogra-
phy (GC) through a method described in an earlier research
[22]. Approximately 1 ␮L of sample was taken in a 2 mL auto-
sampler vial and diluted with 1 mL ethyl acetate and 0.5 mL
N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA). The vial was
then heated in a water bath for 10 min at 40 ◦C. The sample was
allowed to cool down at room temperature and it was then injected
into the GC system. The capillary column (SGE HT5) was used in GC
system with hydrogen as carrier gas at a flow rate of 26.7 mL min−1
and a split ratio of 1:1. The oven temperature was set initially
at 80 ◦C and was maintained for 3 min, then was increased at
6 ◦C min−1 up to 340 ◦C and was maintained for another 6 min.
The injector and the detector temperatures were set at 300 ◦C and
360 ◦C respectively.
3. Results and discussion
3.1. Characterizations of the synthesized catalyst
The XRD patterns of the substrates obtained after calcination
and synthesis reaction are demonstrated in Fig. 1. The obvious
broad peaks at 2ꢀ values of 32◦ and 37◦ in Fig. 1(a) are attributed to
calcium oxide (JCPDS 37-1497) which was obtained after the cal-
cination of calcium carbonate. After reacting with methanol under
reflux, this calcium oxide was transformed into calcium methoxide
as evident from the characteristic peak of calcium methoxide at 2ꢀ
value of 11◦ in Fig. 1(b). Similar results were reported in earlier stud-
ies confirming the synthesized catalyst to be calcium methoxide
[23]. There was no indication regarding the presence of unreacted
calcium oxide or calcium carbonate in the final product. However,
an insignificant peak at 2ꢀ value of 18◦ hints the existence of cal-
cium hydroxide (JCPDS 1-84-1264) which has possibly appeared
due to the interaction of catalyst with water formed as a by-product
of the synthesis reaction. The reaction which has feasibly occurred
between calcium oxide and methanol is represented in Eq. (2).
◦
C
CaO + 2CH3OH6−5→ Ca(OCH3) + H2O
(2)
2
Fig. 3. Pore size distribution of calcium methoxide.

H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
187
Fig. 4. Particle size distribution of calcium methoxide.
transesterification reactions for adsorbing large organic molecules
due to uniform pore structure and extensively high surface area
[26].
out of the clusters of thin plates which were similar in shape to
those reported in an earlier research [23]. A large number of pores
are visible on the surface. The shape of the particles gives no indi-
cation of the chemical composition for constituents of catalyst, so
EDX analysis was performed and the spectrum can be seen in Fig. 6.
The mass% of calcium, oxygen and carbon on a particular area at
the surface of synthesized catalyst were determined to be 41.38%,
52.79% and 5.82% respectively. The atomic ratio between calcium
and oxygen (Ca:O) was 2.83, while the atomic ratio between cal-
cium and carbon (Ca:C) was 3.04. Peaks at 2.1 keV and 2.9 keV
represent the sputter coating of gold/palladium alloy, and the val-
ues were excluded from the final results. The actual formula for
calcium methoxide is Ca(OCH3)2. The surface of catalyst has strong
basic character and therefore holds tendency to absorb moisture.
The peaks attributed to OH on XRD and FTIR results explain the
presence of more oxygen atoms on surface. Furthermore, carbon
dioxide adsorbed within the pores on the basic catalyst surface has
influence on both Ca:O and Ca:C. Hydrogen was not quantified by
EDX because of small orbital diameter (atomic number = 1), which
reduces catch probability for spectroscopy to almost zero.
Particle size distribution can markedly affect the settling and
filtering characteristics of solid catalyst in a stirred slurry reactor;
particles within a size range of 5–200 ␮m are generally favorable in
this regard [27]. The particle size distribution of catalyst is shown in
Fig. 4, indicating that a large amount of catalyst particles are within
the size range of 2–100 ␮m. These particles can be separated from
the reaction products through filtration or centrifugation, after the
reaction.
The shape and topology of catalyst particles was observed
through SEM, shown in Fig. 5. The surface appears to be formed
In thermogravimetric analysis (TGA), the weight of synthesized
catalyst was measured as the function of temperature while it was
subjected to a controlled heating program. The amount of weight
loss provided a quantitative indication about the composition of
sample. In addition, material behavior was also investigated in
terms of thermal degradation and reaction with oxygen. Temper-
ature of thermal and oxidative degradation was also measured.
Differential thermal analysis (DTA) curve showed the physical and
chemical transitions of the sample by measuring exothermic and
endothermic effects.
Fig. 7 corresponds to the thermal decomposition of synthe-
sized calcium methoxide by heating under air flow. The TGA curve
remained almost constant from 25 ◦C to 396 ◦C, with the slight
weight loss due to loss of moisture or volatile matters. After that,
there was a sudden weight loss of around 4%, possibly due to a
chemical reaction, and an exothermic DTA peak at 450 ◦C further
confirms it. A small DTA peak at 405 ◦C can be attributed to the
loss of water and removal of some adsorbed carbon dioxide. The
reaction at 450 ◦C might resulted in the decomposition of calcium
methoxide with oxygen to calcium carbonate (Eq. (3)), and minor
weight loss was observed due to very little difference in molecu-
lar weights between calcium methoxide (102 g/mol) and calcium
carbonate (100 g/mol) [16,17].
Ca(OCH3)2 + 3O2 → CaCO3 + CO2 + H2O
(3)
After 600 ◦C, the TGA curve had evidenced thermal decomposi-
tion of calcium carbonate with the formation of gaseous products.
At 760 ◦C, the DTA peak showed endothermic transformation of
calcium carbonate to stable calcium oxide, and 40% weight loss
matches with the stoichiometric values of the calcination reaction
Fig. 5. SEM images of surface of calcium methoxide.

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H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
Fig. 6. EDX spectrum on surface of calcium methoxide.
for calcium carbonate. The analysis suggested that the synthesized
calcium methoxide was stable below 400 ◦C.
3.2. Reaction results
To study the effectiveness of calcium methoxide as a catalyst for
production of trimethylolpropane triesters, the transesterification
reaction was conducted between TMP and POME by the method
described earlier. The conditions selected for the reaction were
180 ◦C temperature, 50 mbar vacuum, molar ratio TMP:POME 1:6,
0.3% calcium methoxide by weight of reaction mixture and 8 h reac-
tion time. The transesterification reaction involved the cleavage of
an ester group RCOO , from palm-based methyl esters, by an alco-
hol group of TMP to produce new palm-based esters. Since there are
three alcohol groups in TMP, the process yielded intermediate for-
mation of monoesters (ME), diesters (DE) and triesters (TE) of TMP.
Fig. 7. TG–DTA thermogram of calcium methoxide.
Fig. 8. Formation of ME, DE and TE.

H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
189
Fig. 9 shows the composition of ME, DE and TE when calcium
methoxide synthesized in this study was used in the transesterifi-
cation reaction between TMP and POME. Thirty minutes after the
start of the reaction, the product contained 3.73% ME, 61.86% DE
and 34.40% TE. Thereafter, the composition of TE started to increase
while composition of DE and ME kept on decreasing. The compo-
sition of TE reached 80.35% after 2 h and the increase in yield of TE
was less prominent afterwards. After 8 h reaction time, the product
contained 92.38% TE and the reaction was still in progress due to the
presence of unreacted ME i.e., 0.40%. In comparison with the earlier
studies, the reaction had generated a better yield of TE in less time
[4–6]. However, the yield was lower as compared to Yunus et al.
[10] who managed to get 98% triesters in 1 h by having better mass
transfer in the reaction provided by homogeneous sodium methox-
ide catalyst. The only setback was the formation of unwanted soap
with the use of homogenous catalyst.
The proposed mechanism of the catalysis process is shown in
Fig. 10. The mechanism is based on an earlier study where calcium
ethoxide showed a similar behavior in another transesterification
reaction as calcium methoxide did for the current reaction [27]. It
was assumed that the reaction took place on the surface of catalyst
with Ca and O being the two active catalytic sites participating in the
reaction. Ester and alcohol were adsorbed on these two neighboring
free catalytic sites as indicated by step (a) and step (b) in Fig. 10.
The adsorbed ester had formed an intermediate on the surface of
Fig. 9. Composition of ME, DE and TE obtained from transesterification between
TMP and POME catalyzed with calcium methoxide.
Methanol was produced as a by-product and it was continuously
removed to ensure the completion of process aiding by forward
reaction. After eliminating the unreacted methyl esters, the final
product consisted mainly of TE, minor quantity of DE, and negli-
gible amount of ME. These three consecutive–competitive steps of
transesterification reaction between TMP and POME are shown in
Fig. 8.
catalyst, while Oı− had extracted Hı+ and Caı+ adsorbed
O CH3
from alcohol. These two neighboring adsorbed species had then
reacted with each other, in step (c), which led to the formation
of ME of TMP and a molecule of methanol. The ME followed the
Fig. 10. Possible mechanism for transesterification of POME with TMP catalyzed by calcium methoxide: (a) adsorption of ester on catalyst; (b) adsorption of alcohol on
catalyst; (c) reaction between adsorbed neighboring species.

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H. Masood et al. / Applied Catalysis A: General 425–426 (2012) 184–190
same process on the surface of catalyst and reacted with another
molecule of POME to form DE, which was further converted into
triester of TMP in a similar fashion.
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Developing an environmental benign process for production of
biolubricant using a solid base catalyst has become a global need.
Calcium methoxide was synthesized to serve as heterogeneous cat-
alyst for production of TMP esters as biolubricant basestock. The
XRD, FTIR and EDX results showed that the catalyst was success-
fully synthesized with sufficient purity; however, the surface had
hydroxide functional group indicating the influence of water. The
synthesized catalyst was also found to acquire mesoporous sur-
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The authors are grateful to the Ministry of Science and Tech-
nology, Malaysia, for the financial support under the Techno fund
grant and Science Fund (vot: 5450511). The valuable contribution
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