Bioresource Technology 116 (2012) 53–57
Contents lists available at SciVerse ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
Preparation of biodiesel from rice bran fatty acids catalyzed by heterogeneous
cesium-exchanged 12-tungstophosphoric acids
K. Srilatha a, Rekha Sree a, B.L.A. Prabhavathi Devi b, P.S. Sai Prasad a, R.B.N. Prasad b, N. Lingaiah a,⇑
a
b
Catalysis Laboratory, Inorganic & Physical Chemistry Division, Hyderabad 500607, India
Centre for Lipid Research, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e
i n f o
Article history:
Received 19 October 2011
Received in revised form 11 April 2012
Accepted 12 April 2012
Available online 21 April 2012
Keywords:
Esterification
Rice bran fatty acids
Biodiesel
12-Tungstophosphoric acid
Cesium salts
a b s t r a c t
Biodiesel synthesis from rice bran fatty acids (RBFA) was carried out using cesium exchanged 12-tungstophosphoric acid (TPA) catalysts. The physico–chemical properties of the catalysts were derived from Xray diffraction (XRD), Fourier transform infrared (FTIR), temperature programmed desorption (TPD) of
NH3 and scanning electron microscopy (SEM). The characterization techniques revealed that the Keggin
structure of TPA remained intact as Cs replaced protons. The partial exchange of Cs for protons resulted in
an increase in acidity and the catalysts with one Cs+ (Cs1H2PW12O40) showed highest acidity. Under optimized conditions about 92% conversion of RBFA was obtained. The catalyst was reused for five times and
retained of its original activity. Pseudo-first order model was applied to correlate the experimental
kinetic data. Modified tungstophosphoric acids are efficient solid acid catalysts for the synthesis of biodiesel from the oils containing high FFA.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The use of cheap and non-edible vegetable oils, animal fats and
waste oils as raw feedstocks for biodiesel production is an effective
way to reduce the cost of biodiesel. Rice bran offers potential as an
alternative low-cost feedstock for biodiesel production as it is a
low-value co-product of rice milling; which contains approximately 15–23% oil. Only a small portion (<10%) of rice bran is currently processed into edible rice bran oil (RBO) due to problems
with instability caused by the presence of lipases and infrastructure issues (Rogers et al., 1993; Shih et al., 1999; Takano, 1993).
Thus it might be justifiable to extract the oil from rice bran and
convert the RBO into biodiesel.
The most common method for the production of biodiesel is
homogeneous alkali-catalyzed transesterification of vegetable oil
and methanol. The main criteria for base-catalyzed transesterification are that both water and FFA contents in the oil must be below
0.5% (Wang et al., 2006; Zhang et al., 2003). Thus, highly refined
vegetable oils are required as feedstock in the alkali-catalyzed production of biodiesel. RBO may contain up to 80% FFA, depending on
storage conditions and history of the bran (Chao-Chin Lai et al.,
2005), making it unsuitable as a feedstock for alkali-catalyzed production of biodiesel. An alternate method is to use homogeneous
acid catalysts for transesterification since they do not show
⇑ Corresponding author. Tel.: +91 40 27191722; fax: +91 40 27160921.
E-mail address: [email protected] (N. Lingaiah).
0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biortech.2012.04.047
measurable susceptibility to FFAs and can catalyze esterification
and transesterification simultaneously.
A number of solid acid catalysts such as WO3/ZrO2 (Suwannakarn
et al., 2009), SO42/TiO2–SiO2 (Peng et al., 2008), TPA/ZrO2 (Kulkarni
et al., 2006), H3PW12O40/Ta2O5 (Xu et al., 2008), H3PW12O40/Nb2O5
(Srilatha et al., 2010), H3PW12O40/SnO2 (Srilatha et al., 2011),
Cs-doped heteropolyacid catalysts (Narasimharao et al., 2007; Pesaresi et al., 2009), carbohydrate-derived solid acid (Lou et al., 2008),
cerium trisdodecyl sulfate (Ghesti et al., 2009) and propylsulfonic
acid-functionalized mesoporous silica (Mbaraka et al., 2006) have
been reported for esterification and transesterification reactions.
In the present study, methanolysis of rice bran fatty acids
(RBFAs) is investigated by employing cesium exchanged TPA as solid acid catalysts. The aim of this study was the determination of
optimal conditions of RBFAs esterification by methanol and the calculation of the kinetic parameters of this process in order to establish the dependency of reaction rate on the amount of the catalyst
and its acidity.
2. Methods
2.1. Catalyst preparation
Cesium salts of 12-tungstophosphoric acid were prepared by
adding 5 M aqueous solution of CsNO3 (S.D. Fine, India) to 1 M
aqueous solution of TPA under vigorous stirring in the desired stoichiometric proportion. The resultant milky suspensions were aged
at room temperature for overnight. White powder was isolated by
54
K. Srilatha et al. / Bioresource Technology 116 (2012) 53–57
Nomenclature
A
[A0]
B
[B0]
C
D
k1
RBFAs
initial concentration of RBFAs, mol/cm3
methanol
initial concentration of methanol, mol/cm3
methyl esters of RBFAs
water
rate constant, (cm3/mol) (cm3/g-cat) (1/s)
slow evaporation of water in a water bath at 100 °C. The samples
were further dried in an oven at 120 °C for 12 h and calcined at
300 °C for 2 h.
2.2. Characterization of catalysts
Brunauer–Emmett–Teller (BET) surface areas of the catalyst
samples were calculated from N2 adsorption data acquired on an
Autosorb-1 instrument (Quantachrome, Boynton Beach, FL) at
liquid N2 temperature. X-ray diffraction (XRD) patterns of the
catalysts were recorded on a Siemens D-5000 diffractometer using
CuKa radiation. The intensity data were collected over a 2h range of
2–80°. The FTIR spectra were recorded on a Nicolet 740 spectrometer using the KBr disc method. Temperature-programmed desorption (TPD) of ammonia was carried out in a laboratory-built
apparatus equipped with a gas chromatograph using a thermal
conductivity detector (TCD) detector as described by Srilatha et
al. (2009). Scanning electron microscopy (SEM) of the catalysts
was carried on a Hitachi S-520 electron microscope at an accelerated voltage of 10 kV. Samples were mounted on aluminum stubs
using double-adhesive tape and gold coated in a Hitachi HUS-5GB
vacuum evaporator.
2.3. Reaction procedure and analysis of products
The experiments were conducted in batch mode using a 100-mL
capacity autoclave (Parr Instrument Co.). Standard experiments
were carried out by combining 0.0181 mol of rice bran fatty acid
(Ramcharan Industries, Hyderabad, India), 0.254 mol of methanol
and 0.041 g/cm3 of catalyst at a temperature of 65 °C. The catalysts
were dried in oven at 120 °C for 2 h before use. The reaction mixture was allowed to reach the desired temperature and agitation
was commenced at a known speed. After the completion of the
reaction, catalyst was separated by simple filtration. The resultant
reaction mixture was passed through anhydrous sodium sulfate,
which was previously dried in an oven at 120 °C for 1 h. The methanol present in the filtrate was removed on a rotary evaporator.
The conversion of fatty acid was estimated by measuring the acid
value of the product. The acid values are measured twice and average value was taken to calculate the conversion. Conversions values were calculated according to Ozbay et al. (2008).
3. Results and discussion
3.1. Characterization of the catalysts
The BET surface area values of cesium exchanged tungstophosphoric acid catalysts are summarized in Table 1. The surface areas
of catalysts increased with their cesium content. The increase in
surface area is related to the change in morphology corresponding
to the formation of small spherical shaped particles obtained by
precipitation of Cs salts of TPA (Langpape et al., 1999; Narasimharao
et al., 2007).
observed rate constant, (1/s)
k2
KA, KB, KC adsorption equilibrium concentrations for A, B, C and
D, respectively
KD
(cm3/mol)
rA
rate of reaction, mol/cm3 s
w
catalyst loading in liquid phase, g/cm3
XA
fractional conversion of fatty acid
XRD patterns of the cesium exchanged tungstophosphoric acid
catalysts are presented in the Supplementary information
(Fig. S1). All the samples were crystalline in nature. For all of the
samples, the main XRD lines were 2h values at 10.5°, 18.3°, 23.7°,
26.1°, 30.2°, 35.6° and 38.8° which are assigned to cubic alkaline
salts of TPA (Langpape et al., 1999). Catalysts with less than two cesium atoms per Keggin Unit exhibited both cubic phase of pure
alkaline HPAs and triclinic phase of TPA, whereas only the cubic
phase was observed for salts with more than two cesium atoms
(Dias et al., 2004; Okuhara et al., 2000). Some peaks of TPA were
interfered with peaks of CsTPA salts. On the whole, XRD results
indicate that the triclinic phase was decreased with an increase
in cesium content.
FT-IR spectra of the catalysts (Supplementary information
Fig. S2) showed bands at 1080, 986, 890 and 820 cm1 that are
related to asymmetric vibrations of P–Oa (Oa – oxygen atoms bound
to three W atoms and to P),W–Ot (Ot – terminal oxygen atom),W–
Ob–W(Ob – corner sharing bridging oxygen atom) and W–Oc–W
(Oc – edge sharing bridging oxygen atom), respectively (Lingaiah
et al., 2009). The split in the band at 986 cm1 has been assigned
to W@O associated with H+(H2O)n species as reported elsewhere
(Dias et al., 2004). The FTIR data imply the retention of a Keggin
structure during exchange of protons of TPA with cesium.
The acidic properties of the catalysts can be evaluated by temperature programmed desorption of NH3. TPD profiles of cesium
exchanged tungstophosphoric acid catalysts are provided in the
Supplementary information (Fig. S3). All catalysts exhibited distinct desorption peaks. The samples with low cesium content
showed a broad low-temperature desorption peak around 150–
280 °C and a high temperature peak at 650 °C. The intensity of
the high-temperature desorption peak was decreased as the cesium content increased and disappeared for catalyst with total exchange of TPA protons by Cs. The high temperature peak could be
related to the strong acidic sites generated from Keggin ions of TPA
with partial substitution of its protons with Cs. Partially substituted catalysts showed high acidity due to the presence of high
mobile residual protons (Narasimharao et al., 2007). The samples
with high cesium content showed a broad low temperature
desorption peaks at 185 °C and high temperature peaks at 410
and 620 °C. The NH3-TPD analysis revealed a decrease in acidity
of the catalyst with an increase in cesium content. This result
was expected, as the numbers of available protons responsible
for acidity are decreasing. Cs3PW12O40 whose protons are fully exchanged with Cs did not contain any considerable amount of
acidity.
Table 1
BET Surface area and acid strength distribution of CsxH3xPW12O40 catalysts.
Catalyst
Surface area (m2/g)
Acidity 104 (mol of NH3/g-cat)
Cs1H2PW12O40
Cs2H1PW12O40
Cs2.5H0.5PW12O40
Cs3PW12O40
4.1
70.3
122.5
155.2
0.840
0.324
0.194
0.098
55
Catalyst
RBFA conversion (%)
Cs1H2PW12O40
Cs2H1PW12O40
Cs2.5H0.5PW12O40
Cs3PW12O40
92.4
36.2
7.5
2.2
SEM pictures of cesium exchanged tungstophosphoric acid catalysts are also given in the Supplementary information (Fig. S4).
These images indicate a decrease in size of the catalysts with increase in cesium content. Thus SEM observations further supported
the measured increased surface area of catalysts with increased cesium contents. Similar variation in size was observed for cesiumexchanged tungstosilic acid catalysts (Pesaresi et al., 2009).
3.2. Catalytic activity
3.2.1. Esterification activity on cesium-exchanged tungstophosphoric
acid catalysts
The cesium exchanged tungstophosphoric acid catalysts were
studied for the esterification of RBFA with methanol and the results
are shown in Table 2. The Cs1TPA catalyst showed maximum esterification activity compared to all other catalysts. The observed catalytic activity is directly related to acidity of the catalysts. The low
activity of the high Cs containing catalysts might be due to formation of cubic phase of CsTPA in which a numbers of available protons are absent. The results revealed that the acidity of the
catalysts is important and pure cesium salt which showed low
acidity is practically inactive for esterification of RBFA. Similar
observations were made for acid-catalyzed iso-propyl alcohol
decomposition over Cs-containing TPA catalysts (Langpape et al.,
1999). The authors reported that the salts of heteropolyacids were
composed of two phases corresponding to the hydrated acid and
the pure cesium salt with the acid phase coating the salt particles
and that acid phase was catalytically active regardless of whether
the reaction was limited to the surface only or took place in the
bulk as well. They also suggested that in the case of H2Cs1PMo12O40
catalyst both the surface and the bulk of the acid participate in the
reaction. As Cs1H2PW12O40 showed high activity, this catalyst was
used for further studies to estimate the reaction parameters.
3.2.2. Effect of agitation speed
In order to quantitate the influence of external resistances to
mass transfer, the effect of stirring speed on the esterification of
rice bran fatty acids was studied and the results are presented in
the Supplementary information. (Fig. S5). The stirrer speed, beyond
which there was no effect on the reaction rate, was considered to
be the minimum speed of agitation required to eliminate external
diffusion effects. The results suggest that the external diffusion
control was negligible for stirrer speed greater than 500 rpm. A
stirring speed of 600 rpm was maintained for all of the reaction kinetic studies reported here. As external mass transfer resistances
were eliminated by providing adequate stirring speed, the rate
may be either surface reaction controlled or intraparticle diffusion
controlled. Therefore, the effect of catalyst loading was studied to
ascertain the influence of intraparticle resistance.
3.2.3. Effect of catalyst loading
In the absence of external mass transfer resistance, the rate of
reaction is directly proportional to catalyst loading based on the
entire liquid phase volume. The catalyst loading was varied over
a range of 0.01–0.08 g/cm3 on the basis of the total volume of
the reaction mixture and the results are presented in Fig. 1a. The
(a)
100
80
60
40
0.014 g/cm3
0.028 g/cm3
0.041 g/cm3
20
0.054 g/cm3
0.067 g/cm3
0
0
50
100
150
200
Time (min)
(b)
RBFAs conversion (%)
Table 2
Efficacies of CsxH3xPW12O40 (x = 1, 2, 2.5 and 3) catalysts in esterification of rice bran
fatty acids with methanol.
RBFAs conversion (%)
K. Srilatha et al. / Bioresource Technology 116 (2012) 53–57
100
80
60
40
5:1
10:1
14:1
20:1
25:1
20
0
0
50
100
150
200
Time (min)
Fig. 1. (a) Effect of catalyst concentration and (b) molar ratio on conversion of rice
bran fatty acids.
conversion of fatty acid increased with an increase in catalyst
amount due to the proportional increase in the number of active
sites. Further reactions were carried out with 0.041 g/cm3 catalyst
loading in all other experiments.
3.2.4. Effect of mole ratio
Stoichiometrically, the methanolysis of fatty acid requires 1 mol
of methanol for 1 mol of acid. As esterification is a reversible reaction, use of excess methanol shifts the equilibrium to products. In
order to improve the rate of esterification, high molar ratios of
alcohol to fatty acid were employed such as 20:1 (Mbaraka et al.,
2006), 30:1 (Chung et al., 2008) and 60:1 (Caetano et al., 2009).
Fig. 1b reflects the effect of the methanol to acid molar ratio on
the conversion of fatty acid, which clearly indicates that, with an
increase in the methanol ratio the ester yield, was increased. The
excess methanol used in the reaction can be recovered for reuse.
Therefore, methanol to RBFAs molar ratio of 14:1 was considered
as optimum molar ratio.
3.2.5. Effect of reaction temperature
The effect of the reaction temperature on the esterification of
RBFA was studied in the range of 40–65 °C and the results are presented in Fig. 2. The results show that an increase in the temperature accelerates the reaction, favouring ester formation. Thus, a
reaction temperature of 65 °C is considered as the optimum temperature for the esterification of rice bran fatty acids. The catalyst
also exhibited considerable conversion even at low reaction temperature suggesting the high activity of the catalysts.
56
K. Srilatha et al. / Bioresource Technology 116 (2012) 53–57
100
r A ¼
60
r A ¼ k2 w½A½B
ð2Þ
where
40
o
40 C
k2 ¼ k1 K A K B
ð3Þ
o
45 C
20
o
55 C
o
65 C
0
0
50
100
150
200
250
Time (min)
Fig. 2. Effect of reaction temperature on the conversion of rice bran fatty acids.
100
Since methanol was taken in molar excess over fatty acid
([B0] [A0]), it becomes a pseudo-first order equation which can
be integrated. When the esterification reaction is considered to follow first-order kinetics, a plot of ln(1 XA) as a function of time
will be linear, where XA is the fractional conversion of fatty acid.
Plots of ln(1 XA) vs. time were made for different temperatures
and are shown in Fig. 4a. The slopes of these lines are equal to
k2w[B0] from which Arrhenius plots were made to determine the
apparent energy of activation [Fig. 4b]. It was 37.09 kJ/mol, which
is comparable to those reported for other solid acid catalysts
(Srilatha et al., 2009; Yadav and Bhagat, 2005). Thus, the present
catalyst appears to be a promising candidate for esterification of
RBFA.
80
5
60
o
40 C
40
o
4
45 C
o
55 C
o
20
65 C
3
0
0
1
2
3
4
5
6
-ln (1-xA)
RBFAs conversion (%)
ð1Þ
Further, it is assumed that there is a weak adsorption of both
reactants and products (1 + KA[A]+KB[B]+KC[C]+KD[D] 1) and thus
a simple second order kinetic equation could fit the data.
80
RBFAs conversion (%)
k1 wK A K B ½A½B
ð1 þ K A ½A þ K B ½B þ K C ½C þ K D ½DÞ2
2
Recycle number
Fig. 3. Reusability studies of Cs1H2PW12O40 catalyst.
1
3.2.6. Reusability of the catalyst
In order to prove the recyclability, the catalyst was recycled for
five times and the results are shown in Fig. 3. After the first use and
before every reuse, the catalyst was filtered from the reaction mixture and then stirred in a mixture of methanol and hexane for 2 h
to remove any polar and non-polar compounds present on the surface of the catalyst. Further, the catalyst was dried for 2 h at 120 °C
before using it for esterification. The recyclability results suggest
that there was a decrease in acid conversion by 15–17% upon recycling up to five cycles. It is important to note that the catalyst was
not reactivated during recycling. These results support that the
present catalyst can be reused with relatively stable activity. Further studies are continuing on the reactivation of the catalysts by
different methods.
0
3.2.7. Kinetics of the esterification reaction
The kinetic data of the esterification reaction was correlated
with the pseudo-first order model as previous studies indicated
the utility of this approach (Kirumakki et al., 2003; Parida and
Mallick, 2007; Srilatha et al., 2009). The reaction and apparent activation energies were also calculated.
In the absence of internal diffusion and external mass transfer
resistances, an intrinsic kinetic equation could be written as follows (Yadav and Bhagat, 2005).
0
50
100
150
200
Time (min)
250
300
-4.0
ln k
-4.5
-5.0
-5.5
0.0029
0.0030
0.0031
0.0032
0.0033
1/T (K-1)
Fig. 4. (a) Plots of ln(1 XA) vs. time at different temperatures (b) Arrhenius plot
of lnk vs. 1/T for reaction of rice bran fatty acids with methanol.
K. Srilatha et al. / Bioresource Technology 116 (2012) 53–57
4. Conclusions
The activity of cesium-exchanged tungstophosphoric acid catalysts was evaluated for esterification of rice bran fatty acids with
methanol. Among all, Cs1H2W12O40 catalyst showed high activity
for esterification. The esterification activity depends upon the acidity of the CsTPA catalysts. The acidic strength of CsTPA catalysts
arises due to the partial substitution of Cs+ ions with the protons
of TPA. The esterification activity also depends on reaction parameters and the reaction conditions were optimized. The catalyst was
found to be reusable with marginal variation in the overall activity.
A pseudo-first order kinetic model was used to analyze the experimental data and the apparent activation energy is 37.09 kJ/mol.
Acknowledgements
Authors would like to acknowledge CSIR (Council for Scientific
and Industrial Research), India, for awarding a Research Associate
Fellowship to KS.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.biortech.2012.
04.047.
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