A new heterogeneous acid catalyst system for esterification of free fatty acids into methyl esters

Article abstract of DOI:10.1016/j.apcata.2012.04.036

A new heterogeneous acid catalyst system for production of environmentally friendly fuel, biodiesel was created from ferric sulfate and non-toxic, inexpensive source of biopolymer, sodium alginate. The catalyst, in the form of ferric-alginate beads produced from the reaction of 2 wt.% sodium alginate gel with 0.1 M ferric sulfate solution gave excellent methyl ester conversion of 98% with mild reaction conditions. The esterification of 0.5 g lauric acid was carried out at optimum conditions; 16 wt.% of ferric-alginate beads (2.8 wt.% Fe) methanol refluxing temperature, 15:1 methanol to lauric acid molar ratio for 3 h. The ferric-alginate beads were reusable up to 7 times without any pre-treatment. Characterization of the ferric-alginate beads showed the formation of FeOOH that held the alginate chain in place. Thermal analysis showed that the beads are able to withstand the refluxing temperature without degradation. Iron content was found to be 0.175 g Fe/g beads as determined by AAS and 0.189 g Fe/g beads as determined by TGA. Easy catalyst separation, reusability and ability of the ferric-alginate beads to esterify lauric acid to give high conversion of methyl laurate makes this catalyst desirable for biodiesel production from high free fatty acid oils.

Full text of DOI:10.1016/j.apcata.2012.04.036

Applied Catalysis A: General 433–434 (2012) 12–17
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A new heterogeneous acid catalyst system for esterification of free fatty acids
into methyl esters
Peng-Lim Boeya, Shangeetha Ganesana,∗, Gaanty Pragas Maniamb, Melati Khairuddeana, Siew-Ee Leea
a School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
b Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
A new heterogeneous acid catalyst system for production of environmentally friendly fuel, biodiesel
was created from ferric sulfate and non-toxic, inexpensive source of biopolymer, sodium alginate. The
catalyst, in the form of ferric-alginate beads produced from the reaction of 2 wt.% sodium alginate gel with
0.1 M ferric sulfate solution gave excellent methyl ester conversion of 98% with mild reaction conditions.
The esterification of 0.5 g lauric acid was carried out at optimum conditions; 16 wt.% of ferric-alginate
beads (2.8 wt.% Fe) methanol refluxing temperature, 15:1 methanol to lauric acid molar ratio for 3 h.
The ferric-alginate beads were reusable up to 7 times without any pre-treatment. Characterization of
the ferric-alginate beads showed the formation of FeOOH that held the alginate chain in place. Thermal
analysis showed that the beads are able to withstand the refluxing temperature without degradation. Iron
content was found to be 0.175 g Fe/g beads as determined by AAS and 0.189 g Fe/g beads as determined
by TGA. Easy catalyst separation, reusability and ability of the ferric-alginate beads to esterify lauric acid
to give high conversion of methyl laurate makes this catalyst desirable for biodiesel production from high
free fatty acid oils.
Received 26 February 2012
Received in revised form 12 April 2012
Accepted 27 April 2012
Available online 5 May 2012
Keywords:
Sodium alginate
Ferric-alginate
Heterogeneous acid catalyst
Biodiesel
Methyl ester
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
form soap that eventually leads to difficulty in product separation
and decrease in biodiesel yield. The main advantages of heteroge-
Biodiesel, the environmentally friendly, carbon neutral fuel with
low exhaust emissions [1] is the nature’s answer to mankind
for the depletion of the petroleum (fossil) fuel reserve. Biodiesel
(monoalkyl esters) is commonly produced through the transes-
terification of triglycerides with low molecular alcohol (ethanol
or methanol) in the presence of homogeneous basic catalysts
such as sodium or potassium hydroxides, carbonates, sodium and
potassium alkoxides such as sodium methoxides, sodium ethox-
ide, sodium propoxide and sodium butoxide [2,3]. These catalysts
are basic in nature, therefore can only be used for transesterify-
ing low free fatty acid (FFA) oil [2]. However, the production cost
to synthesize biodiesel through this way is higher compared to
petroleum-based diesel fuel due to the use of high quality refined
oils as the feedstock [4].
These problems prompted further research toward biodiesel
production using waste oils and fats of higher FFA and moisture
content [5–7]. The use of such low quality feedstock for biodiesel
production comes with additional concerns on the choice of cat-
alyst. Conventional homogeneous base catalysts are no longer
feasible for transesterification as the base catalyst reacts with FFA to
neous basic catalysts in transesterification are lower production
cost due to catalyst reusability [8–10] and the higher tolerance
toward moisture and FFA [6,9].
Another option to overcome the problem of catalyst poisoning
by FFA and moisture is the usage of homogeneous or heteroge-
neous acid catalysts. Homogeneous acid catalysts are not preferred
because of the long reaction time and difficult catalyst separation
due to their corrosive nature [11]. Therefore, extensive research had
been done over the years to find the most suitable heterogeneous
acid catalyst to convert low quality waste oils and fats into biodiesel
[5,12,13]. Russbueldt and Hoelderich [14] in their work proposed
the use of sulfonic acid ion-exchange resins to pre-esterify free
fatty acids into methyl esters. They achieved good methyl esters
conversion with 1 wt.% catalyst and FFA to methanol ratio of 1:12.
In this work, the authors investigated esterification of lauric acid
to methyl laurate using a new heterogeneous acid catalyst prepared
from sodium alginate and ferric sulfate. Although the discovery of
sodium alginate can be traced to more than 100 years ago [15],
the usage in biodiesel production is practically unheard of except
for the use of a similar biopolymer, ␬-carrageenan in immobilizing
lipases for enzymatic catalysis [16]. However, sodium alginate has
a myriad of other uses in agricultural, biomedical [17] and waste
water treatment [18]. Alginate was vehemently investigated due to
its inexpensive and non-toxic nature. Ferric sulfate on the contrary,
∗
Corresponding author. Tel.: +60 16 9313075; fax: +60 4 6574854.
E-mail address: shangeetha.ganesan@gmail.com (S. Ganesan).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.04.036

P.-L. Boey et al. / Applied Catalysis A: General 433–434 (2012) 12–17
13
has been proved to work excellently as a Lewis acid catalyst for
esterification [19,20]. This paper looks at the possibility of making
use of the gelling property of alginate to trap ferric ions that will
eventually esterify lauric acid into methyl laurate.
Then, small aliquots of reaction mixture were taken out hourly for
methyl esters yield determination using GC analysis. Optimization
of catalyst amount and methanol to oil molar ratio were carried out
at the best reaction time determined at maximum reaction condi-
tions. The amount of beads used was varied from 4 wt.% to 24 wt.%.
Methanol to lauric acid molar ratio was varied from 6:1 to 20:1. The
methyl laurate produced was analyzed using gas chromatography
apparatus (Agilent Technologies, 7890A GC System) fitted with a
flame ionization detector (FID) and the conversion was calculated
according to EN 14103 procedure with methyl heptadecanoate as
the internal standard. The esterification of lauric acid catalyzed by
sodium alginate was carried out as a control experiment using the
optimized reaction conditions.
1.1. Materials
Lauric acid (99%) was obtained from Fluka, iron(III) sulfate
hydrate was supplied by Riedel-de Haen (Switzerland) whereas
sodium alginate, methanol, n-hexane were of QREC brand sup-
plied by Brightchem Sdn. Bhd. (Penang). Anhydrous sodium sulfate
was from BDH Chemical Ltd. (Poole, England). All the chemicals
were of analytical grade. The chromatographic methyl ester stan-
dard, methyl heptadecanoate was supplied by Sigma (Deisenhofen,
Germany).
1.5. Reusability of ferric-alginate beads
1.2. Preparation of ferric-alginate beads
Catalyst reusability was determined by collecting the beads after
each reaction and repeating the esterification reaction with the
used beads (without any further washing or treatment to the used
beads) until very little or no conversion was observed (end product
forms solid after drying).
Approximately, 2 g of sodium alginate was added to 100 mL of
distilled water (heated to 60 ◦C), stirred until a clear viscous solu-
tion was obtained and allowed to cool to 45 ◦C. Then, the viscous
solution was added drop-wise using a Pasteur pipette into 100 mL
of 0.1 M Fe2(SO4)3 solution at room temperature (28–30 ◦C). The
ferric-alginate (FA) beads formed were left to equilibrate in the fer-
ric sulfate solution for 2 h, washed and dried in oven at 60 ◦C for
two days.
2. Results and discussion
2.1. Characterization of the catalyst
1.3. Characterization of the catalyst
When sodium alginate gel is dropped into the ferric sulfate
solution, FA bead is formed immediately. The gelling property of
alginate in the presence of multivalent cation is due to the ionic
crosslinking that produces alginate gels with well-defined forms
[21]. The amount of Fe in the beads produced was determined to
be 0.175 0.009 g Fe per g of beads as determined by AAS. Sreeram
et al. [22] in their study on the nature of interaction of iron(III)
with alginates, concluded that the interaction is better explained
by assuming a site binding model. In this model, they explained that
iron(III) ions are bound to the binding sites on the alginate form-
ing spatially separated iron(III) centers on the alginate backbone.
The number of binding sites per molecule of alginate of this type
(brown algae) has been estimated to be 66 [22]. They also reported
that iron(III) are stabilized against hydrolysis in the presence of
alginate, therefore opening a myriad of industrial applications. This
stabilization property of iron(III)-alginate complexes (as depicted
in Fig. 1) has been utilized in the formation of the FA beads for
esterification.
Based on Fig. 2, it can be seen that sodium alginate does not
have any distinguished peaks for the polymers as expected of
polysaccharides [23]. The XRD of the FA beads does not show any
sharp defined peaks; only broad, weak XRD signals were obtained
which are similar to iron oxyhydroxide–polysaccharide complexes
reported [23–25]. The line broadening could be attributed to inter-
nal strain as observed by Finotelli et al. [26]. Jones et al. [27] and
Coe et al. [25] also reported the possibility of the iron oxyhydroxide
phase to be in the form of ‘cell contracted akaganeite’ or two-line
ferrihydrite. The peaks at 2ꢀ angles of approximately 12◦, 18◦, 27◦,
35◦, 38◦, 47◦, 52◦ and 58◦ are similar to the pure akaganeite (␤-
FeOOH) peaks as reported by Glotch and Kraft [28]. The XRD pattern
of FA beads observed in Fig. 2 can be matched with typical iron
oxyhydroxides but the bonding with alginate makes the peaks less
defined as they are linked via the carboxylate groups of alginate
(Fig. 1). Finotelli et al. [26] reported that the iron is attached to algi-
nate as Fe(III) based on the MS and EPR spectroscopy. As expected,
the EPR suggested that isolated Fe(III) ion substituted the sodium
in the alginate polymeric structure, inducing the linkage of algi-
nate units forming alginate microspheres. They also observed that
The Fe content of the FA beads was determined by whole strip-
ping 5 beads (×5 replicates) with 36% H2SO4. Sodium alginate,
iron(III) sulfate and FA beads were subjected to X-ray diffraction
(XRD) analysis on Siemens Diffractometer D5000 using Cu K␣, 2ꢀ
range from 25◦ to 125◦ with step sizes of 0.1◦, at a scanning speed
of 1◦ min−1. Transmission electron microscopy (TEM) measure-
ments were performed using Philips CM12 equipped with analySIS
Docu ver 3.2 image analysis system; where the 0.1 M ferric sulfate
solution was dropped on to the copper-grid followed by dropping
sodium alginate gel onto the grid and washing with distilled water
after 2 h. The grid is then heated in oven at 60 ◦C for two days
prior to analysis. Sample for TEM analysis was carefully prepared
to emulate the preparation of the beads. Functional group analysis
of sodium alginate and FA beads were done on PerkinElmer System
2000 using potassium bromide (KBr) as standard pellet technique
where the spectra were recorded with a resolution of 4 cm−1 and
in the scan range of 400–4000 cm−1. Thermogravimetric analysis
(TGA) was done on the FA beads to understand the thermostability
of the catalyst using Mettler Toledo TGA/SDTA 851e instrument,
from 30 ◦C to 900 ◦C with 10 ◦C min−1 heating rate, under air.
1.4. Esterification of lauric acid with ferric-alginate (FA) beads as
a catalyst
Lauric acid (0.5 g) was esterified with the FA beads at methanol
refluxing temperature using a stirred reflux system consisting of
a 50 ml round bottom flask fixed to a condenser. The esterified
product was pipetted out from the reaction vessel and allowed to
air dry. Once the methanol was evaporated off, the methyl laurate
was clearly separated from the water produced during the ester-
ification reaction. The methyl laurate was carefully pipetted out
and added with a small amount of sodium bicarbonate (to absorb
water produced during esterification) and left overnight. Then, the
methyl laurate was filtered to remove the solids and kept in a vial
in the refrigerator for further analysis. Firstly, the reaction time
was optimized from 0 h to 4 h for maximum reaction conditions
of 24 wt.% FA beads and 20:1 methanol to lauric acid molar ratio.

14
P.-L. Boey et al. / Applied Catalysis A: General 433–434 (2012) 12–17
Fig. 1. Plausible structure of the beads based on the bonding scheme as suggested by Zaafarany [34].
during the formation of the microsphere, iron hydroxide particles
are precipitated inside the polymer network.
with alginate molecules during the ion exchange interaction. Fe(III)
ion in the ferric sulfate solution is exchanged with Na ion from the
sodium alginate solution. The Fe(III) ion forms bonds with the car-
boxylate groups of alginate molecules. The sulfate ion will form
bonds with sodium ions and are then washed off. Table 1 lists the
functional groups identified in FTIR analysis of sodium alginate and
FA beads found in this study and compared to functional groups of
sodium alginate molecule reported in previous studies [31]. Fil-
ipiuk et al. [31] in their study found that the broad absorption
peak at 3429 cm−1 is due to –OH; similar to the 3450 cm−1 and
3393 cm−1 vibrations found in this study (Fig. 4) for FA beads and
sodium alginate, respectively. Two bands are exhibited by com-
plexed carboxylate group; strong asymmetrical stretching band
(1618 cm−1 for sodium alginate and 1636 cm−1 for FA beads) and
a weaker symmetrical stretching band (1419 cm−1 for sodium
alginate and 1403 cm−1 for FA beads). An obvious peak around
A TEM picture of FA in Fig. 3 clearly shows the formation of iron
oxyhydroxide particles on the grid. The poly-dispersed, dark spher-
ical electron dense particles are the iron core (5–15 nm) that gels
the alginate chains together [23,29]. As the FA sample was pre-
pared directly on the grid using the exact same methods for FA
beads preparation, the tendency for aggregation shown by the com-
plexes on the grid is similar to the structure of the beads. The TEM
shows completely separated nanoparticles as reported previously
by Jones et al. [23] and Sipos et al. [29]. During the preparation of
the FA beads, the alginate gel droplet comes into contact with ferric
sulfate solution and forms beads immediately, similar to the aggre-
gation of the complex on the grid. This is one of the well-known
properties of alginate gels in the presence of divalent or polyvalent
cations. The resultant is an ionotropic metal alginate gel where the
polyvalent metal ion is chelated by the macromolecular chain [30].
The binding model of Fe(III) to alginate molecules were con-
firmed by FTIR analysis. Iron(III) forms metal carboxylate bonds
Fig. 2. XRD patterns of ferric-alginate (FA) beads, ferric sulfate and sodium alginate.
Fig. 3. TEM image of poly-dispersed iron particles in ferric-alginate.

P.-L. Boey et al. / Applied Catalysis A: General 433–434 (2012) 12–17
15
Fig. 4. FTIR spectra of (a) sodium alginate and (b) ferric-alginate (FA) beads.
1743 cm−1, found only in the FA beads corresponds to C O stretch-
ing vibration of free carboxylate anions. However, this band was
not observed in the spectrum of sodium alginate. The C O stretch-
ing bonds of alcoholic and ether groups produces broad absorption
bands around 1095 cm−1 for sodium alginate and 1135 cm−1 for
FA beads. The large absorption bands around 500–700 cm−1 in FA
beads are attributed to the stretching vibrations of Fe O bond in
iron oxide as reported by Wu et al. [32]. Absorption bands around
667 cm−1 and 490 cm−1 are characteristics peaks of Fe O stretch
of akaganeite [33]. These strong broad bands are clearly absent
in sodium alginate. This corresponds well with compound iden-
tification of XRD where the peaks are similar to cell contracted
akaganiete. Zaafarany [34] explained in his work that a stable
coordination geometry is achieved by the polyvalent metal ion by
inter- or intramolecular bonding with the carboxylate and hydroxyl
functional groups of alginate, respectively. Iron(III) is restricted
to cross-linking via intermolecular association in their alginate
complexes. This tendency toward intermolecular bonding explains
the more elongated metal–oxygen bonds that decreased the bond
stretching as observed in Fig. 4.
the beads remains intact without any degradation. Approximately
4.50 mg of beads was used for TGA analysis and the total residue at
the end of the forth step is 1.22 mg. Assuming the thermal decom-
position is complete and the residue is only made up Fe2O3; 0.189 g
Fe per g of beads was obtained by stoichiometric calculations. This
value is similar to the Fe content determined using AAS analysis
method mentioned above.
2.2. Optimization of ferric-alginate beads catalyzed esterification
of lauric acid
Optimization of reaction time for esterification of lauric acid was
carried out at maximum conditions of 24 wt.% (4.2 wt.% of Fe) FA
beads and 20:1 methanol to lauric acid molar ratio. Fig. 6a shows
that as the reaction progresses above 3 h, the reaction reached a
plateau as there are no significant differences in the yield of methyl
esters. Using this as an indicator for the completion of the esteri-
fication reaction, all other optimization reactions were carried out
for 3 h. As depicted in Fig. 6b, low methanol to lauric acid molar
ratio does not give satisfactory methyl ester conversion. Although
theoretically, for esterification of 1 mol of lauric acid requires 1 mol
of methanol to produce 1 mol of methyl laurate and 1 mol of water,
the reaction will not reach completion as it is reversible. There-
fore, excess methanol is required to drive the reaction forward and
toward completion.
Catalyst amount more than 16 wt.% (2.8 wt.% Fe) did not make
much difference to the methyl laurate conversion as almost 100%
yield had been obtained. The ferric ion in the beads that holds the
alginate structure together is postulated to act as a Lewis acid site
for attachment of fatty acids for formation of methyl esters [36].
Carbonyl oxygen of lauric acid molecule attaches to the ferric ion
making the carbonyl carbon susceptible for nucleophilic attack by
methanol. Comparing the results of this experiment to esterifica-
tion of waste cooking oil using ferric sulfate by Gan et al. [20], the
authors reported 59% free fatty acids conversion in a 1 h reaction
with methanol to oil molar ratio of 15:1 and catalyst concentration
of 2 wt.% at 60 ◦C. Wang et al. [19] in their two-step catalyzed pro-
cess for biodiesel production from waste cooking oil obtained 97%
free fatty acid conversion in 4 h at methanol to oil molar ratio of
10:1 and 2 wt.% ferric sulfate concentration at 95 ◦C. Based on the
optimization results of this study, the best conditions for esterifica-
tion of lauric acid (0.5 g) using ferric-alginate beads was found to be
16 wt.% beads (2.8 wt.% Fe, catalyst amount to lauric acid mass ratio
The TG–DTG curves given in Fig. 5 is similar to the typical
TG–DTG curves of Fe(III)-alginate [34] and calcium alginate curves
[32]. The first weight loss in the range of 30–149 ◦C can be attributed
to evaporation of water in the sample. Weight loss ranging from
149 to 380 ◦C is due to the preliminary degradation of alginate and
the subsequent two weight loss around 380–560 ◦C and 560–895 ◦C
are due to further degradation of alginate [32,35]. This shows that
at methanol refluxing temperature during esterification reaction,
Table 1
The main vibrational modes for sodium alginate and ferric-alginate.
Vibration
Sodium
Sodium
alginate
Ferric-
alginate
alginatea
ꢁ(OH)
3429
2921
1743
1629.3
1419
1320.7
1102
1026.9
889.7
821.5
3450
2933
–
1618
1419
1319
1095
1026
903
3393
ꢁ(CH)-anomer
2933
1743
1636
1403
1230
1135
1000
825
ꢁ(COO−
)
sym
ꢁ(COO)sym
ꢁ(COO)asym
ı(CCH) + ı(COH)
ꢁ(CO) + ꢁ(CCC)
ꢁ(CO) + ı(CCO) + ı(CC)
Mannuronic acid residues
Guluronic acid residues
813
737
a
Literature data [31].

16
P.-L. Boey et al. / Applied Catalysis A: General 433–434 (2012) 12–17
Fig. 5. Typical TGA curve of ferric-alginate (FA) beads.
of 0.16:1) and 15:1 methanol to lauric acid molar ratio refluxed at
methanol refluxing temperature for 3 h with methyl ester yield of
98 0.7%. Esterification of lauric acid catalyzed by sodium alginate
did not show any catalytic activity proving that the esterification
reaction was catalyzed by FA beads. The presence of an obvious
peak at 1748 cm−1 for free carboxylate anions in FTIR spectrum
(attached as supplementary data) of reused FA beads clearly shows
that the anion did not take part in the esterification reaction.
2.3. Catalyst reusability
The reusability test showed that the FA beads can be used
up to 8 times with more than 90% conversion of methyl esters
(Fig. 6c). After 7 times reuse, the beads were found to start disinte-
grating. This could be due to the repeated heating and agitation
of the beads during the esterification reaction. Gan et al. [20]
reported that at 2 wt.% ferric sulfate concentration, the catalyst is
Fig. 6. Optimization graphs of (a) esterification reaction time (0.5 g lauric acid, 0.12 g beads, 20:1 methanol to lauric acid molar ratio, 60 ◦C), (b) methanol to lauric acid molar
ratio and catalyst amount (0.5 g lauric acid, 3 h, 60 ◦C) and (c) catalyst reusability (0.5 g lauric acid, 0.08 g beads, 15:1 methanol to lauric acid molar ratio, 60 ◦C, 3 h).

P.-L. Boey et al. / Applied Catalysis A: General 433–434 (2012) 12–17
17
fully dissolved in the reaction mixture. The reaction was reported
References
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treatment adds appeal to FA beads to be used as a catalyst for ester-
ification. Crosslinking agents (multivalent cations), as in this case
Fe (III), increases the water resistance of alginate [37]. Therefore,
catalyst removal and reuse is done with ease. AAS analysis of the
reused beads shows that the beads contain 0.168 0.0025 g Fe/g
beads.
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3. Conclusion
FA beads prepared from reaction of sodium alginate and fer-
ric sulfate showed great potential as a heterogeneous acid catalyst
for esterification of free fatty acids. Thermal analysis showed that
the prepared catalyst is able to withstand refluxing temperature
enabling esterification reaction without catalyst destruction. The
prepared ferric-alginate beads as catalyst for esterification of lau-
ric acid gave 98% methyl esters yield in a 3 h reaction, with catalyst
amount to lauric acid mass ratio of 0.16 (2.8 wt.% Fe) and methanol
to lauric acid molar ratio of 15:1. Catalyst separation was done eas-
ily and reused without any treatment up to 7 times with more than
90% methyl ester yield. Iron content determined by AAS and TGA
was 0.175 g Fe per g of beads and 0.189 g Fe per g of beads, respec-
tively. This new catalyst system paves way for biodiesel production
from high free fatty acids oils.
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Universiti
Sains
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Appendix A. Supplementary data
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