Activity and stability of polyaniline-sulfate-based solid acid catalysts for the transesterification of triglycerides and esterification of fatty acids with methanol

Article abstract of DOI:10.1016/j.apcata.2010.05.042

A polymeric catalyst, polyaniline-sulfate, was studied in the transesterification of triglycerides (triacetin, castor oil) and esterification of fatty acid, ricinoleic acid with methanol at mild conditions (temperature of 50-60 °C). Polymer powder (PANI-S) and three samples of various contents of polymer deposited on carbon support were examined. The samples of catalysts, before and after catalytic tests were characterized by BET, FT-IR, XRD and SEM techniques. The acid capacity was also determined. All the samples were found to be active solid acid catalysts in both tested reactions. Catalytic performance of polyaniline-sulfate-based catalysts for methanolysis of triacetin (glycerol triacetate) the shortest triglyceride molecule differed from that for vegetable oil, castor oil, consisting of long chain triglycerides of ricinoleic acid. In transesterification of triacetin PANI-S powder was more active than carbon-supported catalysts. In methanolysis of vegetable oil the outermost surface of catalysts was mainly involved and much higher activity was exhibited by carbon-supported catalysts with deposited polymer, especially with low content of polymer (13.1 wt.%). The activity of polyaniline-sulfate-based catalysts was almost stable during recycling use in tested reactions. After five successive catalytic runs, their activities were found to be ca. 80-95% relative to the activities of fresh catalysts.

Full text of DOI:10.1016/j.apcata.2010.05.042

Applied Catalysis A: General 383 (2010) 169–181
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Activity and stability of polyaniline-sulfate-based solid acid catalysts for the
transesterification of triglycerides and esterification of fatty acids with methanol
A. Zi e˛ ba , A. Drelinkiewicz , E.N. Konyushenko , J. Stejskalb
a
a,∗
b
a
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A polymeric catalyst, polyaniline-sulfate, was studied in the transesterification of triglycerides (triacetin,
Received 10 March 2010
Received in revised form 26 May 2010
Accepted 26 May 2010
castor oil) and esterification of fatty acid, ricinoleic acid with methanol at mild conditions (temperature of
◦
5
0–60 C). Polymer powder (PANI-S) and three samples of various contents of polymer deposited on car-
bon support were examined. The samples of catalysts, before and after catalytic tests were characterized
by BET, FT-IR, XRD and SEM techniques. The acid capacity was also determined. All the samples were found
to be active solid acid catalysts in both tested reactions. Catalytic performance of polyaniline-sulfate-
based catalysts for methanolysis of triacetin (glycerol triacetate) the shortest triglyceride molecule
differed from that for vegetable oil, castor oil, consisting of long chain triglycerides of ricinoleic acid.
In transesterification of triacetin PANI-S powder was more active than carbon-supported catalysts. In
methanolysis of vegetable oil the outermost surface of catalysts was mainly involved and much higher
activity was exhibited by carbon-supported catalysts with deposited polymer, especially with low con-
tent of polymer (13.1 wt.%). The activity of polyaniline-sulfate-based catalysts was almost stable during
recycling use in tested reactions. After five successive catalytic runs, their activities were found to be ca.
Available online 4 June 2010
Keywords:
Biodiesel
Transesterification
Triacetin
Castor oil
Ricinoleic acid
Polyaniline
8
0–95% relative to the activities of fresh catalysts.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Biodiesel fuel, an alternative to classic diesel fuel, belongs to
free fatty acid and transesterification of triglycerides. Their cat-
alytic efficiency has been reviewed in recently published papers
[1–5]. The catalysts based on sulfonic groups were found to be the
most active in methyl ester synthesis. Two types of solid catalysts
are mainly studied: sulfonic acid-functionalized organic resins and
ecological fuels because it consists of methyl esters of fatty acids,
derived from vegetable oils or animal fats. Methyl esters are
obtained by the transesterification of triglycerides with short-chain
alcohols, such as methanol, as illustrated in Scheme 1. The reaction
is catalysed by strong acids or bases, but owing to environmental,
technological, and economic reasons, the substitution of industrial
homogeneous catalysts by heterogeneous catalysts is a desirable
goal.
2−
2−
/ZrO , SO4 /TiO2
2
sulfate treated inorganic oxides, such as SO
4
2−
SO4 /SnO2 [6–8]. They offered a high activity but strongly deac-
tivated because sulfate groups easily leach from these catalysts in
contact with reaction mixture which, in turn, gives rise to a homo-
geneous acid catalysis. On the contrary, non-sulfated superacid
material, WO /ZrO exhibited good acid properties and high stabil-
3
2
The advantage of solid acid catalysts consists in the ease of
product separation, recycling of the catalyst and reduced equip-
ment corrosion. Solid acid catalysts can be used in a packed bed
continuous flow reactor, simplifying the product separation and
purification. Moreover, solid acid catalysts are able to catalyse both
transesterification of triglycerides and esterification of free fatty
acids present in oil feedstock. A variety of solid acids have already
been tested for methyl ester synthesis via both, esterification of
ity, but its activity was lower compared to sulfate-containing oxides
[9,10]. The activities of sulfated mixed titania–zirconia oxides when
used in fixed bed reactor were as high as that of WO /ZrO , and
3
2
they exhibited a very good stability, being only slightly deactivated
during long reaction time [11,12]. Heteropolyacids (H PW12O40
)
3
supported by zirconia, silica, alumina, activated carbon [13] and
Ta O5 [14] were also tested, however, they are easily deactivated
2
especially in the presence of small quantities of water. By enhancing
hydrophobicity of Ta O5/SiO2 support via functionalization with
2
alkyl groups the activity and stability of catalysts increased [14].
Sulfonic acids-based ion-exchange resins are another class of solid
acid catalysts known to be very effective in numerous industrial
reactions, including olefin isomerizations, alkylations, acylations,
∗ Corresponding author.
E-mail addresses: ncdrelin@cyf-kr.edu.pl, drelinki@chemia.uj.edu.pl (A.
Drelinkiewicz).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.05.042

1
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A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
be much higher compared to pure Nafion NR50 polymer because
the majority of the acid sites were accessible to reagents [22]. The
activity of Nafion SAC-13 composite was not reduced in successive
cycle of triacetin methanolysis and esterification of palmitic acid
[
22,23].
By incorporation of organosulfonic groups into mesostructured
silica SBA-15, a new type of solid acid catalysts of well defined
porosity, high surface area and easily accessible active centres were
obtained [24,25]. The activity of propylsulfonic acid-functionalized
SBA-15 catalyst in esterification of free fatty acids present in beef
tallow was comparable to that of Nafion NR50, however the cat-
alysts deactivated due to the condensation of reagents in the
mesopores of catalysts. This was explained by too high polarity
of the catalyst. Bifunctionalization of the SBA-silica support via
simultaneous incorporation of propylsulfonic groups and propyl
groups resulted in enhanced hydrophobicity of the catalyst thereby
diminishing the interaction of the polar molecules with the sul-
fonic acid groups, a reason of catalyst deactivation. Melero et al.
[
25] demonstrated very promising performance of propylsulfonic
Scheme 1. Transesterification of triglyceride with methanol.
acid-modified SBA-15 catalyst towards simultaneous esterification
of fatty acids and transesterification of triglycerides using refined
and crude vegetable oils. Significant activity of these catalysts was
attributed to the large surface area and pore diameter of the meso-
porous support as well as the moderate acid strength. Caetano et
al. [26] studied sulfonic acid-containing resin-based on poly(vinyl
alcohol) (PVA), prepared by crosslinking the PVA with sulfosuccinic
acid. Such resins displayed much better catalytic performance in
term of activity and stability than commercial Nafion NR50 and
Dowex resins in both esterification of palmitic acid and trans-
esterification of soybean oil carried out in a membrane reactor
[27].
esterifications and others [15]. Two main classes of ion-exchange
resins, namely styrene-based sulfonic acids (Amberlyst and Dow
type resins) and perfluorosulfonic acid-based catalysts including
Nafion resin and Nafion resin–silica nanocomposites such as Nafion
SAC-13 [5,15] have been reported to display excellent catalytic
activity in esterification of small organic acids and fatty acids to
produce methyl esters [16]. Recently, systematic study dealing with
catalytic behaviour of different (polystyrene-divinylbenzene) PS-
DVB-based sulfonic acid-containing resins in the esterification of
fatty acids performed as the pre-step prior to the transesterifica-
tion of triglycerides was reported [17]. Low thermal stability of
Amberlyst-series resins, however, limited the utility of these cata-
In the present work, other polymeric catalyst, polyaniline-
sulfate (PANI-S) is evaluated in transesterification of triglycerides
and in esterification of fatty acid. The commonly used term
“polyaniline” refers to a polymer (Scheme 2) prepared by the
oxidative polymerization of aniline [28]. Due to basic character
of nitrogen-containing groups (imine groups –N , pKa = 2.5, amine
ones –NH–, pKa = 5.5 [29]), polyaniline can be easily protonated at
imine sites with protonic acids to give the “polyaniline-salt”, such
as hydrochloride, nitrate, phosphate, sulfate etc. Polyaniline-salts
are organic semiconductors. They are easy to prepare, handle, and
they exhibit good environmental and thermal stability (up temp
◦
lysts for maximum operating limit ca. 120 C. Catalytic performance
of this kind of resin (like Amberlyst, Nafion NR50) was affected
by their swelling in reaction mixture which determined the num-
ber of sites available for the reactants [7,18,19]. In addition, the
activity of Amberlyst-15 significantly decreased after recycling in
◦
triacetin methanolysis (at 60 ) which was ascribed to site blockage
by adsorbed reagents, like more polar intermediates or products
[
7].
de Rezende et al. [20] demonstrated that activity of sul-
◦
fonated poly(styrene-co-divinylbenzene) (poly(S-DVB)-SO H) and
ca. 300 C) and are insoluble in most of organic solvents. These
3
poly(divinylbenzene) (poly(DVB)-SO H) catalysts in transesteri-
fication of Brazilian vegetable oils (at 65 C) was higher than
properties make polyaniline-salts a potential candidate to be used
3
◦
that of commercial Amberlyst-15 and Amberlyst-35. The best
performance was exhibited by (poly(DVB)-SO H) that presented
3
macroporous structure and high surface area. Its activity was com-
parable to that of p-toluenesulfonic acid and even higher than that
of H SO4 under homogeneous conditions. No data of reusing the
2
catalysts were reported. Sulfonated polystyrene-based catalysts
prepared from linear polystyrene were reported by Soldi et al. [21]
to be more efficient than Amberlyst-15 in transesterification of
◦
refined soybean oil and beef tallow (at 64 C) and high excess of
methanol (100:1). A significant reduction in their activity observed
under recycling use was attributed to the impregnation of polar
compounds such as glycerol, partial glycerides.
Pure Nafion NR50 polymer although more thermally stable
2
than Amberlyst, has extremely low surface area (below 1 m /g)
what limits its application. In methanolysis of triacetin no leach-
ing of S-species was observed however its activity changed to
swelling effect [7]. Nafion resin-based silica nanocomposites in
which nanometer sized Nafion resin particles were entrapped
within a highly porous silica network have much larger surface
2
area, ca. 200 m /g [15,22]. As the result the catalytic activity of this
Scheme 2. Polyaniline-base and its two-step protonation with sulfuric acid to
nanocomposite material per unit mass of Nafion resin was found to
polyaniline-sulfate and subsequently to polyaniline-hydrogen sulfate.

A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
171
in optical and microelectronic devices, solar cells, sensors, energy
2. Experimental
storage and energy conversion systems [28]. Doping of PANI with
various mineral and carboxylic acids, among them H SO , HCl,
2.1. Catalysts preparation
2
4
HNO , HClO can therefore be easily realized by protonation (inser-
3
4
tion of counter-anions) to the pre-formed polyaniline-base [29].
The insertion of acids can also be realized during the polymer-
ization of aniline, performed in their presence [30]. In addition,
polyaniline is well known to exhibit very strong adhering ability
towards various inorganic materials so strong that even PANI film
is formed onto glass vessels used during polymerization reaction.
This ability allows easy preparation of composites with polyani-
line film deposited onto various materials including glass, ceramics,
inorganic oxides, metals, plastics, fibers and other textile mate-
rials. Strongly adhering films of PANI are commonly prepared by
the immersion of the supporting material into an aqueous acidic
solution of aniline containing an oxidant, i.e. by “in situ” adsorp-
tion polymerization of aniline [31]. The polymer chains grow as
brushes from the support surface (for example silica, glass) oriented
preferentially perpendicularly to the surface [32–34]. The obtained
polyaniline films are relatively thick and their surface has a granular
morphology.
Polyaniline-sulfate (PANI-S) powder was prepared by the oxida-
tion of 0.1 M aniline sulfate with 0.25 ammonium peroxydisulfate
in 50 vol.% ethanol–water at room temperature. A 100 cm3 of
reaction mixture generates ∼2 g of PANI-sulfate. The solids were
collected after 24 h on filter, rinsed with 0.1 M sulfuric acid, acetone,
and dried. Polyaniline-sulfate was deposited on carbon support
in situ during the preparation of PANI by the modification of
method reported earlier [41]. Support was dispersed in ethanol,
aniline sulfate was added and dissolved, followed by the equal
volume aqueous solution of ammonium peroxydisulfate. The con-
centrations of reactants were the same as in the preparation of
PANI-sulfate in the absence of carbon support. For ∼10 wt.% PANI
loading, 100 cm3 of reaction mixture was used per 20 g of carbon
support. The double volume of reaction mixture led to ∼20 wt.%
loading, etc. Next day the coated carbon was separated, rinsed
with 0.1 M sulfuric acid, acetone, and dried. The content of PANI-
sulfate in the prepared three composites: C/PANI-S-a, C/PANI-S-b
and C/PANI-S-c was calculated from the increase in the mass of the
samples after the coating procedure.
PANI-salts, a hydrochloride, sulfate, nitrate, p-toluenesulfonate
and phosphate, have been studied by Palaniappan et al. [35–39]
as the solid acid catalysts for various organic transformations,
among them for the esterification of carboxylic acids, such as
lauric, caprylic, caproic, myristic, acetic, with alcohols. All the
PANI-salts (except for PANI-phosphate) produced esters at almost
2.2. Characterization of catalysts
The specific surface areas of samples were calculated from the
nitrogen adsorption–desorption isotherms at 77 K in an Autosorb-
1, Quantachrome equipment. Prior to the measurements, the
◦
stoichiometric yields (99%) in reaction performed at 70 C [35].
In transesterification of ketoester (acetoacetate) with hexanol (at
◦
◦
1
10 C) PANI-sulfate gave better conversion than other salts, nitrate
samples were preheated and degassed, under vacuum at 60 C for
and hydrochloride [38]. The reusability of PANI-sulfate checked
with nine consecutive reactions cycles of esterification showed a
fully stable activity and no essential changes in catalyst compo-
sition [36,37]. In case of PANI-sulfate no leaching of sulfate was
observed in methanol medium during catalytic test [39].
18 h. Pore size distribution in sample of PANI-S powder was calcu-
lated using the BJH model based on nitrogen desorption isotherm,
for carbon-supported composites, the DFT/Monte-Carlo differen-
tial model was used.
FT-IR spectra were recorded using Bruker-Equinox 55 spectrom-
eter and standard KBr pellets technique. Morphology of samples
was studied by means of Field Emission Scanning Electron Micro-
scope JEOL JSM–7500 F. X-ray diffraction patterns were obtained
with a Siemens D5005 diffractometer using Cu K␣ radiation (55 kV,
30 mA).
In the present work, the activity of samples based on PANI-
sulfate is examined for the methanolysis of triglycerides and
esterification of free fatty acid with methanol. This is the first
example of the use of PANI-sulfate as the solid acid catalyst for
methanolysis of vegetable oil, castor oil. Polyaniline-sulfate does
not remarkably swell in reagents such as methanol, methyl esters
and triglycerides and similarly to other polymers has relatively low
surface area [40]. Therefore, in order to improve the catalytic per-
formance of PANI-sulfates, the studies have been extended to the
catalysts with polymer deposited on carbon support. The catalysts
were prepared by the in situ polymerization of aniline performed
in the presence of carbon support. By changing the polymeriza-
tion conditions three catalysts differing in the content of deposited
polymer were synthesized.
Elemental analysis was performed by means of CHNS-VarioEL
III apparatus (Elementar Analysensysteme Hanau-Germany).
The total amount of Brönsted acid sites was determined by
acid–base titration. An amount of ca. 0.1 g of each sample was sus-
3
pended in 20 cm of NaOH (0.1 M) for 24 h at room temperature. The
liquid sample obtained after polymer filtration was subsequently
titrated with 0.1 M HCl. The completeness of PANI-sulfate deproto-
nation after treatment by NaOH was checked by the FT-IR method.
Methanolysis was studied for triacetin (glycerol triacetate), a
model triglyceride molecule and for natural oil, castor oil. The
main constituent of castor oil is triglyceride of 12-hydroxy-9-
octadecenoic acid (ricinoleic acid). Due to the presence of OH group
at C-12 carbon, the castor oil exhibits unique chemical and physical
properties, such as good solubility in methanol and it applies also
to methyl esters formed after its transesterification. Similarly, tri-
acetin is also readily soluble in methanol and methyl esters formed.
Thus, during methanolysis of both our triglycerides, there was a
single liquid-phase system. No separate phases of methanol and
triglycerides occurred and mass-transfer effects resulting from the
presence of two phases (oil–methanol) encountered in the transes-
terification reaction of natural oils are in fact negligible. Moreover,
by using a triacetin as the model compound we are able to gain
some insight into reactivity of PANI-sulfate-based catalyst for the
formation of partial glycerides. Ricinoleic acid was selected as the
substrate for esterification reaction.
2.3. Catalytic tests
The transesterification of triglycerides, triacetin (Fluka) and cas-
tor oil (Microfarm, Poland) with methanol was carried out in a
100 cm3 glass reactor at atmospheric pressure following the pro-
cedure reported in our previous papers [42,43].
Reactor was equipped with a reflux condenser, magnetic stir-
rer, and a tube for sampling the solution. In catalytic experiment,
triacetin or castor oil, methanol and internal standard (toluene or
eicosane) were introduced to the reactor, heated up to a given tem-
perature and then the catalyst was added.
The transesterification of triacetin (TACT) with methanol was typ-
◦
3
ically performed at temperature of 50 C using 2.6 cm of triacetin
and 16.2 cm3 of methanol, i.e. at methanol to triacetin molar ratio
3
(MR) of 29. The concentration of catalysts varied from 1.58 g/dm
3
to 24 g/dm , which corresponds to 1–15 wt.% relative to triacetin
mass. In the course of catalytic tests the samples of reaction mix-

1
72
A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
ture were periodically taken and analysed by GC method following
the method described earlier [42]. From the data of GC analysis the
triacetin conversion (CTG), and the yields of partial glycerides (Yi)
diacetin and monoacetin were calculated as follows:
Table 1. The specific surface area and porosity of PANI-S pow-
2
3
der are low, 25.9 m /g and 0.054 cm /g. A bimodal distribution
of the pores with average pore diameter of 4 and 12 nm was
determined in powder polymer sample (Fig. 1). Scanning electron
microscopy shows that the sample of PANI-S powder exhibited
aggregated granular morphology, typical of polyanilines (Fig. 2). All
three carbon-supported composites have higher surface area and
porosity compared to PANI-S powder. Deposition of polymer sig-
nificantly reduced surface area as well as porosity of carbon support
NTG,0 − NTG
CTG
=
× 100%
(1)
(2)
N
TG,0
Ni
Yi =
× 100%
NTG,0
2
3
2
3
(
from 734 m /g and 0.389 cm /g up to 298 m /g and 0.125 cm /g at
where NTG,0 and NTG are the moles of triacetin initially present in the
reactor and the moles of triacetin remaining at time t, respectively.
the highest polymer content 26.7 wt.% in C/PANI-S-c). In all sam-
ples, carbon support as well as the composites with polymer, the
micropores of diameter in range 1.2–1.7 nm are observed (Fig. 1).
Elemental analysis of PANI-S gave the contents of C, N, H and S
equal to 50.15, 9.72, 4.78 and 7.89 wt.%, respectively. The atomic
ratio of C/N = 6.0 calculated from elemental analysis is equal to
the theoretical value (C/N = 6). The data from elemental analysis
for carbon-supported series catalysts are not informative, owing to
high contribution of carbon support. The content of sulfur deter-
mined by elemental analysis was low (0.15–0.32 wt.%), being close
to the detection limit of the used technique. Therefore, the content
of polymer determined as the increase of mass upon preparation of
C-series catalysts has been taken into consideration. This procedure
was commonly used in case of samples with polyaniline-deposited
onto various carrier materials [44].
N presents the number of moles of diacetin (NDI) or monoacetin
i
(
NMONO) in the reaction mixture at time t. The fraction of triacetin
reacted to glycerol (YGL) was calculated from the mass balance:
NTG,0 − (NTG + NDI + NMONO)
(3)
YGL
=
× 100%
N
TG,0
The transesterification of castor oil (CAS) with methanol was per-
3
formed using 6 g of castor oil, 7.6 cm of methanol (MR = 29) and
0
.6 g of catalyst (42.9 g/dm3 equivalent to 10 wt.% relative to oil
mass). The progress of reaction was monitored by means of HPLC
and GC techniques following the procedure reported in our previ-
ous paper [42,43].
The HPLC analysis showed that our castor oil was entirely com-
posed of a triglyceride of ricinoleic acid (87.44 wt.%), traces of
glycerol (below 0.1 wt.%), di- and mono-glycerides of ricinoleic
acid, and free ricinoleic acid (below 0.1 wt.%). From the GC anal-
ysis of methyl esters the average molecular weight of castor oil
was calculated to be 928 g/mol [43]. Apart from the dominating
triglycerides of ricinoleic acid, low amounts of triglycerides of
other fatty acids (linoleic 5.05 wt.%, oleic 3.88 wt.%, stearic 1.4 wt.%,
palmitic 1.28 wt.%, linolenic 0.56 wt.%, and other acids 0.39 wt.%)
were present in castor oil [42]. This makes determination of con-
version data more complex. Therefore, in the discussion of catalytic
results, the yield of methyl esters formed in methanolysis of cas-
tor oil has been taken into account, identically to the procedure
widely used in the case of vegetable oils. The yield of methyl esters
formed in methanolysis of castor oil was expressed in terms of the
percentage of methyl esters produced [43].
+
The acid capacity (mmol H /g of catalyst) measured by
acid–base titration is given in Table 1. Before discussing the acidity
data, some comments pertinent to the formation of polyaniline-
sulfate are needed. Polyaniline-based catalysts were prepared by
the polymerization of aniline in the presence of sulfuric acid and the
obtained product was polyaniline-sulfate. Recent results of Ayad et
al. [44] show that, in fact, two kinds of polyaniline-salts may be
formed (Scheme 2). At lower acidity of the medium, PANI-sulfate is
formed by the association of one molecule of sulfuric acid per two
imine nitrogens in PANI (structure 1, Scheme 2), while at higher
acidity two molecules of sulfuric acid may be introduced yielding
PANI-hydrogen sulfate (structure 2, Scheme 2). The values of H+
capacity calculated assuming the formation of PANI-sulfate (struc-
ture 1) or PANI-hydrogen sulfate (structure 2) are summarized in
Table 1. It should be stressed that to check our titration method, acid
capacity was determined for Amberlyst-15 (a commercial, Aldrich
Esterification of ricinoleic acid with methanol was carried in the
same reactor using 3 g of ricinoleic acid (Fluka) and 11.8 cm3 of
◦
product). Before the measurement, the polymer was dried at 105 C
◦
methanol (MR = 29) and 0.66 g of catalyst at temperature of 60 C.
for 5 h, i.e. following the procedure used by other authors [7,18,19].
Acid capacity of 4.72 mmol/g was determined by us for Amberlyst-
The progress of reaction was monitored by GC at the same analytical
procedure as in castor oil methanolysis.
1
5, in good agreement with the literature data, 4.70 mmol/g [7,19].
The content of H+ determined by titration for PANI-S powder
3
. Results and discussion
sample is equal to 4.75 mmol/g (Table 1) being slightly higher than
theoretical value of structure 1. In addition, sulfur content deter-
mined by elemental analysis, 7.89 wt.% is also between the sulfur
contents of 6.96 wt.% and 11.48 wt.% calculated for both structures 1
and 2, respectively. Thus, in our PANI-S powder sample both struc-
tures 1 and 2 may appear although structure 1 seems to dominate.
The capacity of H+ in PANI-S sample is close to that of Amberlyst-
3.1. Characterization of catalysts based on PANI-sulfate
In the present work, PANI-sulfate powder (PANI-S) and three
catalysts with PANI-sulfate deposited onto carbon support were
tested. Physicochemical properties of catalysts are collected in
Table 1
Physicochemical characterization of Polyaniline-sulfate catalysts.
Catalyst
Surface
area [m /g]
Porosity
[cm /g]
Content of
PANI-S [wt.%]
Measured acid capacity
Calculated acid capacity
mmol H /g catalyst
2
3
+
+
+
mmol H /g
catalyst
mmol H /g PANI-S
in the catalyst
Structure 1
Structure 2
PANI-S
25.9
618
347
298
734
0.054
0.169
0.131
0.125
0.389
100
13.1
20.2
26.7
–
4.75
1.16
1.25
1.40
0.37
4.75
8.85
6.19
5.24
4.33
0.57
0.88
1.16
7.16
0.94
1.45
1.91
C/PANI-S-a
C/PANI-S-b
C/PANI-S-c
Carbon-support

A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
173
Fig. 1. Pore size distribution of catalysts.
1
5, which is commonly considered to be a catalyst of relatively high
In case of C/PANI-S-a catalyst with the lowest content of poly-
mer, protons capacity determined by titration (1.16 mmol H /g) is
+
+
acidity. The comparison of H capacity in case of carbon-supported
samples is more complex, because some of acidity was found in
the carbon carrier (0.37 mmol/g). However, it seems very probable
that carbon acidity was neutralized to high extent (or totally) dur-
ing deposition of polyaniline and the observed acid capacity can be
related to the polyaniline.
ca. two times higher than that of structure 1 but is comparable to
that of structure 2. Similar tendency can be observed for carbon-
supported b and c catalysts. However, as the content of polymer
on carbon support grows the contribution of structure 2 seems to
decrease because experimentally determined content of protons
Fig. 2. Scanning electron micrographs of PANI-S powder (a) and recovered catalyst after methanolysis of triacetin (b).

1
74
A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
Fig. 3. XRD diffraction pattern of as-received PANI-S and PANI base, and after recov-
ery from the methanolysis of triacetin.
locates between that for structures 1 and 2. Thus, in all polyani-
line catalysts both structures 1 and 2 in various proportions appear
and the contribution of structure 2 is especially high in carbon-
supported C/PANI-S-a sample with the lowest amount of polymer.
This seems to be very probable because the access of sulfuric acid
to the polyaniline dispersed on carbon support is easier compared
to the polyaniline powder. However, for simplicity, all the catalysts
are termed as “polyaniline-sulfate”-based samples.
The XRD diffraction pattern of powder PANI-S and the sample of
PANI-base (obtained by treatment of PANI-S with excess of 0.1 M
ammonia solution) are displayed in Fig. 3. In the diffraction pat-
tern of PANI-base the reflections characteristic of crystalline and
amorphous phases of the polymer are observed similarly to the
results reported by other authors [45,46]. The presence of crys-
talline phase is evidenced by small and broad peak centred at
◦
2
ꢀ = 16 . This peak is superimposed on a strong and broad hallo
◦
at 2ꢀ around 20 originating from an amorphous phase (Fig. 3).
Fig. 4. FT-IR spectra of PANI-S powder, PANI base, and catalysts recovered after
the methanolysis of triacetin (a) C/PANI-S-c and the sample after methanolysis of
triacetin compared to the carbon support (b).
The X-ray diffraction pattern of PANI-S essentially differed from
◦
◦
that of PANI-base because of distinct reflexes at 2ꢀ ∼ 15 , 20 and
◦
2
5
appear. This diffraction pattern is characteristic of PANI-salts
in which polyaniline is in protonated form. Literature data showed
that polyaniline-base become more ordered upon doping by acids,
i.e. when polyaniline salts are formed [45]. In fact, an improved
crystallinity was observed after doping of polyaniline-base with
acids, such as sulfuric and p-toluenesulfonic acids [47].
of the protonated form of polyaniline are a metallic polaron energy
band observed as a broad absorption band at wavenumbers higher
−
1
−1
than 2000 cm (Fig. 4) and the distinct peak at 1310 cm , possibly
related to the C–N deformation mode. Thus, the FT-IR spectrum of
PANI-S sample exhibits the spectral features characteristic of PANI-
salt. The band originating from the vibration of sulfonic group is
−
1
In the FT-IR spectrum of PANI-base, the band at 1498 cm
assigned to the stretching vibration of the C–C in benzenoid unit,
−1
−1
the band at 1589 cm
originating from stretching vibrations of
located at 1030–1050 cm , i.e. at the position which is superim-
−
1
C
N and C C bonds in quinonoid units as well as the band at
posed with the strong and broad band at 1125 cm arising from
protonated segments of polyaniline [48]. This makes its observation
difficult.
−
1
ca. 1166 cm
ascribed to C–C in-plane deformation are visible
(
Fig. 4). Upon protonation of nitrogen groups and formation of
salts, the quinonoid and benzenoid bands, are both shifted to lower
Spectral features characteristic of PANI-salt can also be observed
in the FT-IR spectrum of C/PANI-S-c sample (Fig. 4). Owing to a low
content of polymer in C/PANI-S-a and C/PANI-S-b samples (13.1
and 20.2 wt.%, respectively), the intensity of bands originating from
−1
−1
wavenumbers 1479 cm and 1559 cm and a new broad band
−1
located at 1125 cm
attributed to the protonated segments of
polyaniline appeared [28,47]. Other spectral features characteristic

A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
175
3
Fig. 5. The methanolysis of triacetin carried out at various methanol to triacetin molar ratio, MR = 12–35, PANI-S powder: 7.9 g/dm (equivalent to 5 wt.%), C/PANI-S-a:
1
5.8 g/dm3 (equivalent to 10 wt.%); 50 C.
◦
polyaniline was very weak and the FT-IR spectra were not enough
distinct to be considered.
is strongly inhibited. For instance at MR = 12 the yield to diacetin
and monoacetin are similar. Higher excess of methanol, above
MR = 12, is needed to enhance the transformation of monoacetin
to glycerol. At MR = 29 the yield to monoacetin starts to remark-
ably grow when conversion of triacetin is higher than 60% (Fig. 7).
An increase in methanol excess up to MR = 35 further improves the
yield of methyl esters by facilitated transformation of diacetin to
monoacetin. However, taking into account that the high molar ratio
of alcohol to oil interferes with the separation of glycerol because
there is an increase in solubility, further catalytic experiments were
carried out at methanol to triglyceride molar ratio MR = 29.
3.2. Catalytic experiments
It has been shown in our previous paper that in the absence
of catalyst, no reaction proceeded in the case of castor oil and
only a trace activity was observed for triacetin (conversion 0.9%
after 30 min of reaction) [43]. Because the transesterification is
catalysed by alkaline catalysts, polyaniline-base was also tested. At
3
typical reaction conditions (MR = 29, catalyst loading 15.8 g/dm ),
The representative reagents distribution profile i.e. the content
of diacetin, monoacetin, methyl ester and glycerol vs reaction time
carried on PANI-S powder catalyst (MR = 29) is plotted in Fig. 8.
From the very beginning of reaction the diacetin is formed and its
content passes through the maximum at ca. 60% triacetin conver-
sion. Monoacetin is observed only when the conversion of triacetin
is relatively high (50–60%) and its content also passes through the
PANI-base did not exhibit activity for the methanolysis of both
triglycerides triacetin and castor oil.
Heterogeneously catalysed methanolysis needs large excess of
methanol relative to the stoichiometric amount to reach a high
yield of methyl esters. For solid acid catalysts, even as high excess
of methanol as 300: 1 was used in case of Amberlyst catalysed reac-
tion [19]. Here, the methanol to triglyceride molar ratios starting
from 12:1 up to 35:1 were examined in the presence of PANI-S and
C/PANI-S-a catalysts (Fig. 5). By increasing the amount of methanol
the transesterification is faster process and in the presence of both
PANI-S and C/PANI-S-a catalysts the triacetin conversion (after
6
0 min of reaction time) increases linearly with the MR value
(
Fig. 6). One can expect that in case of reaction for vegetable oils,
large excess of methanol may result in change of reaction mixture
viscosity and as a consequence some deviation from the linear-
ity can occur. This effect is less probable in the triacetin–methanol
mixture because the viscosity of triacetin is definitively lower
compared to vegetable oils. In experiments, the concentration of
catalysts is constant (7.9 g/dm3 and 15.8 g/dm ) while the concen-
tration of methanol grows. This means that the ratio of catalyst
to triacetin was changed and higher reaction rates were obtained
when the ratio of catalyst to triacetin was higher, i.e. at conditions
of high MR. This effect is consistent with the observations reported
in [22] regarding the reaction mechanism on solid acid catalysts.
All three reaction steps transforming triglyceride via di- and
mono-glyceride to methyl ester and glycerol are the equilib-
rium processes (Scheme 1). Therefore, the methanol to triacetin
molar ratio (MR values) influenced the course of particular reac-
tion steps. This is evidenced by the change in yields of partial
glycerides, monoacetin and diacetin plotted against conversion of
triacetin (Fig. 7). At lower excess of methanol corresponding to MR
below 29, the transformation of mono-glycerides to methyl ester
3
Fig. 6. The effect of MR on the conversion of triacetin determined after 60 min of
reaction time in the presence of PANI-sulfate and C/PANI-S-a catalysts. Reaction
conditions: see Fig. 5.

1
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A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
Fig. 9. The yield of methyl ester obtained after 60 min of reaction vs catalysts con-
Fig. 7. The role of methanol excess (MR value) on the yield of diacetin and
centration in methanolysis of triacetin and castor oil (MR = 29).
monoacetin in methanolysis of triacetin catalysed by PANI-S (reaction conditions:
◦
3
5
0 C, catalyst loading 7.9 g/dm ).
are displayed in Fig. 9. As expected, with an increase in the con-
centration of catalysts the transesterification of both triglycerides
occurs at a faster rate. A comparison of results for methanolysis of
triacetin and castor oil shows that in agreement with the data of
other authors, at a given loading of catalyst, methanolysis of tri-
acetin, short triglyceride molecule is much faster than the reaction
for vegetable oil consisting of long chain-triglyceride molecules.
maximum at high triacetin conversion. As a result, the triacetin
conversion higher than 95% was attained after 18 h of reaction
time. However, from the linear relationship obtained between tri-
acetin conversion and MR (Fig. 6), one can speculate that at higher
methanol excess, e.g. MR = 100, practically total conversion of tri-
acetin would be attained after shorter reaction time, ca. 60 min.
The effect of catalyst loading was studied in reaction of both
triglycerides triacetin and castor oil. In experiments, the amount
of catalyst was varied keeping other parameters constant. In the
discussion of catalytic results the yield to methyl esters is taken
into account, identically to the procedure widely used in the case of
vegetable oils. Moreover, as described before [42,43] apart from the
dominating triglycerides of ricinoleic acid, low amount of triglyc-
erides of other fatty acids was present in castor oil. This makes
difficult a determination of conversion data.
3.2.1. Transesterification of triacetin with methanol
As shown in Fig. 9 the yield of methyl ester increases proportion-
ally with the increase of the carbon-supported catalysts loading in
the range employed in the studies. The yield of methyl ester also
grows with the increase in concentration of PANI-S powder cata-
lyst. However, at high loading of PANI-S powder catalyst a deviation
from the linearity can be observed showing less effective utilization
of active centres in polymer powder. This may be result of low sur-
2
3
face area (25.9 m /g) and low porosity (0.054 cm /g) of polymer
powder.
The yields of methyl ester (after reaction time of 60 min) vs the
concentration of PANI-S powder and carbon-supported catalysts
It seems interesting to follow the change of partial glycerides
concentrations during the reaction. To better recognize the role of
polyaniline-sulfate solid catalysts, the study was extended towards
homogeneous catalysis using sulfuric acid. Reagents distribution
profiles obtained at the same concentrations of PANI-S powder
3
and C/PANI-S-a catalysts in the reaction mixture (15.8 g/dm ) are
displayed in Fig. 10a. In the presence of polyaniline-sulfate cat-
alysts the course of consecutive reactions transforming triacetin
via partial glycerides to methyl ester and glycerol differs from that
obtained for sulfuric acid catalysed reaction (Fig. 10a). This differ-
ence is manifested by the maximum yields of partial glycerides,
diacetin and monoacetin and the triacetin conversion at which they
were attained. In homogeneously catalysed reaction the maximum
yield of diacetin is low, 12%, and is attained at triacetin conversion
of ca. 40%. The yield of monoacetin also passed over the maximum,
corresponding to ca. 18%. In the presence of PANI-sulfate catalysts
the maximum yields of both partial glycerides attained higher val-
ues and in particular a difference can be observed with respect to
diacetin.
The maximum yields of diacetin on PANI-S powder catalyst
is ca. 20.8% and is reached at triacetin conversion of ca. 60%, i.e.
higher compared to homogeneous conditions. An opposite effect
is observed for monoacetin. At a given triacetin conversion the
yield of monoacetin is lower on PANI-sulfate catalysts, compared
Fig. 8. Reagents distribution profile in methanolysis of triacetin catalysed by PANI-S
powder. Reaction conditions: MR = 29, 50 C, catalysts loading 15.8 g/dm .
◦
3

A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
177
Fig. 10. (a) The yield of partial glycerides (diacetin, monoacetin) and (b) the yield of glycerol vs triacetin conversion in reaction performed on PANI-S powder and C/PANI-S-a
3
catalysts (catalysts concentrations 15.8 g/dm ) and in homogeneous catalysis by sulfuric acid (1 wt.%).
to that in the reaction catalysed by sulfuric acid alone. In the
presence of C/PANI-S-a catalyst the maximum yield of diacetin
is located between that of PANI-S-powder and dissolved H SO .
For the conversion of triacetin in the presence of Nafion SAC-
13 composite, the activation energy of 48.5 kJ/mol was obtained,
which was comparable to that for the reaction catalysed by H SO4
2
4
2
Similar effect is observed for monoacetin. However, the fraction
of triacetin reacted to glycerol is practically the same on all three
catalysts (Fig. 10b). This shows that in the presence of PANI-sulfate-
based catalysts, the course of consecutive reactions changes to
some extent relative to the homogeneous conditions. In particu-
lar the transformation of diacetin to monoacetin is to some extent
inhibited and this unprofitable effect is more pronounced in the
presence of PANI-S powder catalyst. The transformation of diacetin
to monoacetin is less inhibited on C/PANI-S-a catalyst with dis-
persed polymer. The observed effects show that migrations of
glycerides molecules to and from the active sites in the interior
of polymer matrix are hindered. Polyaniline-sulfate powder did
not swell when contacted with reaction mixture and the com-
pact structure of polymer as well as relatively low surface area
of polyaniline-sulfate may be considered to be responsible for the
observed effects.
(46.1 kJ/mol) [22]. It should be mentioned that most of activation
energy data in the literature are reported for triglyceride methanol-
ysis catalysed by alkaline catalysts. For methanolysis of soybean
oil catalysed by homogeneous NaOH, the activation energy in the
range of 33.5–77.5 kJ/mol was calculated by Noureddini and Zhu
[50] by computational simulation. Liu et al. [51] reported that the
solid base catalysts, such as CaO, have almost the same range of acti-
vation energy (38–84 kJ/mol) as the homogeneous base catalysts in
methanolysis of soybean oil. Theoretical model considered by the
authors indicated that the transesterification reaction on solid base
catalysts was controlled by both the mass transfer and reaction [51].
Thus, apparent activation energy obtained on PANI-sulfate-based
catalysts is within the range predicted by other authors.
From the correlations displayed in Fig. 9 it is clear that at
the same loading of catalysts the rate is the highest on powder
Methanolysis of triacetin was studied within the temperature
◦
◦
range from 30 C up to 60 C in the presence of powder PANI-S
and C/PANI-S-a catalysts. In order to evaluate apparent activation
energy the method of calculation reported by Liu et al. [49] is
adopted. This method has been reported for the transesterification
of soybean oil catalysed by heterogeneous calcium ethoxide. The
authors assumed that at excess of methanol, the transesterifica-
tion was a pseudo-first order reaction and the overall rate constant
(
k) was calculated from the equation
ln(1 − ꢁ)
k = −
(4)
t
where ꢁ is the methyl esters yield and t is the time of reaction (min).
From this equation the activation energy of 54.2 kJ/mol was cal-
culated by Liu et al. [49] for the transesterification of soybean oil.
Here, using this equation the overall reaction rate constants were
calculated from the initial data of methyl ester yields (the yield
no higher than 20%) and the obtained Arrhenius plots are reported
in Fig. 11. The linear relationships were obtained throughout the
◦
whole temperature range 30–60 C. By linear regression analysis
the apparent activation energy was estimated to be ca. 50 kJ/mol at
3
3
both low (1.6 g/dm ) and high (15.8 g/dm ) concentrations of PANI-
S powder catalysts. In the presence of C/PANI-S-a catalyst with
Fig. 11. Arrhenius plots for the methanolysis of triacetin in the presence of PANI-S
3
concentration of 15.8 g/dm similar apparent activation energy was
3
3
3
powder (1.6 g/dm and 7.9 g/dm ) and C/PANI-S-a catalysts (15.8 g/dm ). Reaction
◦
calculated, 49.2 kJ/mol.
conditions: MR = 29, temperature: 30, 40, 50, and 60 C.

1
78
A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
differs from that in reaction of triacetin. In the transesterification
of castor oil the most active is carbon-supported C/PANI-S-a
catalyst with the lowest content of polymer. The activity of other
two carbon-supported b and c catalysts is lower, and does not
remarkably differ from that of PANI-S powder catalyst (Fig. 9).
Furthermore, it can be observed from Fig. 12 that at a given content
of protons in the catalysts, the rate of castor oil methanolysis is the
highest in the presence of C/PANI-S-a catalyst and distinctly lower
over PANI-S powder catalyst. This shows an opposite relation to
the one observed in methanolysis of triacetin.
These results may suggest that methanolysis of castor oil com-
posed of long chain triglycerides occurred on the outermost surface
of all PANI-sulfate catalysts, either powder or supported by car-
bon, with restricted penetration of long chain glycerides molecules
inside the polymer matrix. Higher yield of methyl esters on C/PANI-
S-a catalyst with polymer dispersed on carbon support compared
to that obtained on PANI-S powder reveals better utilization of
acid centres when the polymer is deposited onto carrier. This is
the case when the content of dispersed polymer is low (13.1 wt.%)
i.e. in the presence of C/PANI-S-a catalyst. At higher contents of
polymer (20.2 and 26.7 wt.%) i.e. in carbon-supported b and c cat-
alysts the effective utilization of active centres is lower, but still
higher than in case of PANI-S powder catalyst. This also shows that
in case of long chain triglycerides when the penetration of glyc-
erides inside polymer matrix is strongly suppressed, a role of the
type of active centres, structure 1 or structure 2 is less important
than in case of triacetin, short-chain triglyceride. Easier penetra-
tion of small triacetin molecules may explain this effect. Another
property which can be considered is the hydrophobicity of the cata-
lysts. Although polyaniline itself is a hydrophobic material, because
is not wetted by water and other polar solvents like methanol.
However, a hydrophobicity of carbon-supported polyaniline cat-
alysts seems to be higher. The role of increased hydrophobicity of
carbon-supported polyaniline-sulfate catalysts seems to be more
important during methanolysis of castor oil consisting of long chain
triglycerides than in case of triacetin, a short-chain triglyceride. The
role of catalyst hydrophobicity is indeed predicted by number of
authors. However, to the best of our knowledge this role has been
not systematically studied.
Fig. 12. The yield of methyl ester obtained after 60 min of reaction vs the content
of protons in the catalysts Methanolysis of triacetin and castor oil carried out at
MR = 29.
PANI-S catalyst and gradually decreases as the content of polymer
deposited on carbon support decreases. In order to examine the role
of polymer content, the relationship between the yield of methyl
ester and the content of protons in the catalysts is taken into con-
sideration (Fig. 12). It can be clearly seen that at a given number of
+
H in the reaction, the yield of methyl ester depends on the type of
catalyst. The activity relative to proton is the lowest in the case of
C/PANI-a catalyst with the lowest content of polymer (13.1 wt.%).
This activity gradually increases as the content of polymer on car-
bon support grows. Finally, the activity related to proton in case of
carbon-supported C/PANI-S-c sample with 26.7 wt.% of polymer is
only slightly lower than that of PANI-S powder catalyst.
This effect may be explained taking into account two possible
structures (1 and 2) of polyaniline-sulfate, which may be formed
upon catalysts preparation (Scheme 2). These structures differ with
respect to the total number of protons introduced into polyaniline-
base. Moreover, in structure 1, i.e. polyaniline-sulfate, the only
3.2.3. Recycling use of polyaniline-sulfate catalysts
The possibility of recycling and reusing heterogeneous catalysts
in transesterification reaction is one of the key factors for their
practical application.
+
protons are the ones in protonated nitrogen groups –NH – of
polymer. In structure 2, polyaniline-hydrosulfate, apart from the
+
protons in –NH – groups, other protons are the ones in the form
In order to evaluate any possible contribution of some PANI-S
soluble in methanol to the catalytic performance, the PANI-S pow-
−
of HSO4 ions. As described before, structure 2 seems to dominate
in carbon-supported C/PANI-S-a catalyst with the lowest content
of polymer, whereas structure 1 predominates in case of PANI-S
powder catalyst. Thus, structure 1 i.e. polyaniline-sulfate exhibited
higher activity than structure 2, corresponding to the polyaniline-
hydrosulfate.
Another explanation may be given taking into account the effect
of carbon support, similarly to what was observed in the Nafion
resin–silica composites [52]. The content of polymer in these Nafion
resin–silica composites was found to affect the global acidity of the
active phase and the rate of isobutane/2-butene alkylation. This was
ascribed to the interaction between sulfonic groups of polymer and
silanol groups. At the lowest content of polymer but the greatest
dispersion of Nafion, the sulfonic groups of the polymer interact to
greater extent with the silanol groups of the silica, decreasing the
acid strength of the resin with consequent decreases in activity of
the composites.
◦
der was vigorously stirred under methanol reflux (for 3 h at 50 ).
Then, methanol obtained after filtration was used in methanolysis
of triacetin. The conversion of triacetin ca. 2% was observed after
3
0 min of reaction, only slightly higher than that determined in
blank catalytic experiment carried out in the absence of catalysts
0.9%). The methanol-filtrate was also studied for the presence of
(
2−
2+
SO4 ions using very sensitive analytical reaction with Ba ions.
However, after addition of BaCl solution no precipitate was formed
2
2−
confirming no leaching of SO4 ions. It seems therefore, that after
filtration of PANI-S powder, some of shorter polymer chains may
stay in methanol giving rise to the observed low conversion of
triacetin.
Here, recycling experiments were performed for PANI-S and
C/PANI-S-a sample. Catalytic recycling studies were carried out
by recovering the used catalysts samples after 3 h of reaction and
reusing them with fresh reagents in subsequent reaction cycles,
repeated five times. The recovered catalysts were washed with
acetone/THF and dried in air before reusing in next catalytic test.
Additionally, the recovered catalysts (washed with acetone/THF
and dried in air) were characterized by FT-IR, SEM and surface area
3.2.2. Transesterification of castor oil with methanol
The performance of polyaniline-sulfate catalysts during
methanolysis of castor oil composed of large triglyceride molecules

A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
179
Table 2
Recycling experiments of PANI-S and C/PANI-S-a catalysts. Conversion of triacetin and ricinoleic acid after 3 h of reaction time.
Catalyst
Methanolysis of triacetina
PANI-S
Esterification of ricinoleic acidb
C/PANI-S-a
C/PANI-S-a
Activity [min 1]c
−
Conversion [%]
Activity [min ]c
−1
Conversion [%]
Conversion [%]
Run 1
Run 2
Run 3
Run 4
Run 5
32.7
32.2
30.7
29.7
29.2
0.0461
0.0446
14.6
14.3
14.0
13.3
12.8
0.0347
0.0349
95.4
93.4
89.8
84.1
78.7
0.0434
0.0342
a
◦
◦
3
3
Reaction conditions: temperature 50 C, PANI-S: 7.9 g/dm , C/PANI-S-a: 15.8 g/dm .
b
c
3
Reaction conditions: temperature 60 C, catalyst loading 42.9 g/dm .
−1
−1
+
+
Activity is expressed as: mol triacetin × min × mol H , where mol H is the number of active sites determined in reused catalyst.
measurements. The sulfur content and the H+ capacity were also
determined.
Although, the content of H+ slowly decreased, FT-IR spectra
(Fig. 4) and XRD diffraction patterns (Fig. 3) show that polymer in
recovered PANI-S and C/PANI-S-c catalysts was still in the form of
polyaniline-salt, i.e. in the protonated form. In Fig. 2 the SEM images
of fresh and recovered PANI-S-powder catalysts are compared.
Granular morphology of polymer is preserved in recovered catalyst.
However, some “additional material” precipitated onto granular
particles of polymer can be observed in the SEM image of recovered
PANI-S catalyst. This may suggest that some of reaction products
such as glycerol/partial glycerides were difficult to remove upon
acetone/THF washing of the spent catalysts. Their accumulation
may be a reason of the reduced surface area of recovered PANI-
S catalyst. Although methanolysis of triacetin resulted in slight
reduction of acid capacity of the PANI-sulfate, the catalysts main-
tained a high level of activity in successive reusing experiments.
This is graphically illustrated in Fig. 14 showing the percentage
contribution of activity (% act) which was maintained after suc-
cessive use in five catalytic run. The results for PANI-S-powder and
C/PANI-S-a catalysts are close thus showing comparable stability of
both catalysts, regardless of the polymer form, powder or deposited
on carbon support.
The conversion of triacetin (after 3 h of reaction time) obtained
in five successive reaction cycles on PANI-S and C/PANI-S-a cat-
alysts are summarized in Table 2. The activity of both catalysts
slightly decreased upon 5-recycling tests. The similar activity drop
of ca. 11–12% is observed for both PANI-based catalysts after 5-
reusing tests. The recycling use of PANI-S-powder catalyst resulted
2
2
in reductions of specific surface area from 25.9 m /g to 15.4 m /g
after five reaction cycles. The sulfur content in the catalyst recov-
ered after the first catalytic run was 6.40 wt.%, being slightly lower
+
than 7.89 wt.% determined in fresh catalysts. The content of H
determined by acid–base titration for selected samples of recov-
ered catalysts (runs 1–5) is shown in Fig. 13. It can be observed
that H+ capacity slowly decreases during recycling tests. After five
reaction cycles using PANI-S and C/PANI-a catalysts, an acid capac-
ity is determined to be 88.6% and 82.2% from the initial capacities,
+
respectively (Fig. 13). The reduction of H content during methanol-
ysis as well as esterification with methanol was also observed for
other sulfonic acid-based catalysts. This was explained by the pos-
sible esterification of the sulfonic acid sites to methyl sulfonates
[
5,25]. In Table 2 the activity of fresh and reused catalysts expressed
+
as the rate (mol triacetin/min) per active sites (mol H ) is reported.
This activity is practically stable for C/PANI-S-a catalyst and only
slightly decreases for PANI-S catalyst. This reveals that the observed
decrease of triacetin conversion upon recycling experiments was
result of slow and gradual reduction in the content of acid centres,
similarly to the observation of other authors.
3.2.4. Esterification of ricinoleic acid with methanol
Catalytic experiments were further performed using carbon-
supported C/PANI-S-a, b and c catalysts. The conversion of ricinoleic
acid plotted against the esterification time is displayed in Fig. 15.
It can be observed that in the presence of all three catalysts,
Fig. 14. The percentage contribution of catalysts activity after recycling tests of
PANI-sulfate catalysts for methanolysis of triacetin and esterification of ricinoleic
acid (reaction conditions, see Table 2).
Fig. 13. Proton capacity determined by acid–base titration in fresh and recovered
catalysts during recycling activity tests in the methanolysis of triacetin.

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A. Zi e˛ ba et al. / Applied Catalysis A: General 383 (2010) 169–181
4. Conclusions
Polyaniline-sulfate in the form of powder (PANI-S) and
deposited on carbon support was found to be effective and rel-
atively stable catalyst for the transesterification of triglycerides
(triacetin and castor oil) and esterification of ricinoleic acid with
methanol. The catalytic performance of polyaniline-sulfate sam-
ples for the methanolysis of triacetin, the shortest triglyceride
differs from that for vegetable oil, castor oil consisting of long chain
triglycerides. The migration of glycerides to and from inside the
polymer matrix played a role in methanolysis of triacetin, espe-
cially over PANI-S powder catalyst in which protons in the form
of polyaniline-sulfate dominated. This was ascribed to a compact
structure and low surface area of PANI-S powder. Deposition of
polyaniline-sulfate on carbon support gives catalysts in which both
polyaniline-sulfate and polyaniline-hydrosulfate appeared and the
contribution of the latter grows as the content of deposited poly-
mer decreases. This resulted in lower activity of carbon-supported
polyaniline catalysts compared to that of PANI-S powder. A role
of the type of polyaniline-sulfates did not essentially affect the
reactivity of polyaniline-sulfate samples for transesterification of
long chain triglycerides (castor oil) with methanol owing to their
restricted penetration inside the polymer matrix. In methanolysis
of castor oil the outermost surface of all polyaniline-sulfate-based
catalysts played a role and C/PANI-S-a catalyst with the lowest con-
tent of polymer and the highest specific surface area exhibited the
highest activity. The activity of polyaniline-sulfate-based catalysts
was almost stable during recycling use in tested catalytic reactions.
After five successive catalytic runs, their activities were found to be
ca. 80–95% relative to the activities of fresh catalysts.
Fig. 15. The esterification of ricinoleic acid with methanol in the presence of PANI-
S powder and C/PANI-S-a catalysts. Reaction conditions: MR = 29, catalysts loading
3
◦
4
2.9 g/dm ; temperature 60 C.
the conversion achieved ca. 95–98%. With the same loading of
3
catalysts (42.9 g/dm ), the activity of PANI-S-a catalyst with the
lowest content of polymer is the highest and the C/PANI-S-b cata-
lyst with the intermediate loading of polymer shows the lowest
reaction rate. This trend may be explained by the type of acid
sites observed in the structures 1 and 2 (Scheme 2), the strength
of active sites as well as the formation of water inhibitor shell,
etc.
Acknowledgements
To examine the stability of most active C/PANI-S-a catalysts, the
recycling experiments of esterification were repeated five times.
The recovered catalysts were at first rinsed with acetone and then
air-dried before being reused in the subsequent esterification tests.
The activity of C/PANI-a catalyst slightly decreases upon recycling
tests (Table 2) and after five successive reaction runs, the activity
is found to be 82.5% relative to the initial activity of fresh catalyst.
These results are consistent with the data reported by Palaniappan
et al. [35–39] who observed the stable activity of various PANI-
salts in the esterification of small carboxylic acids with alcohols.
The present data prove that PANI-sulfate is also effective and sta-
ble catalyst in the esterification of fatty acid, such as ricinoleic
acid.
A. Zi e˛ ba acknowledges a Ph.D. grant of the Polish Academy of
Sciences. Thanks are also to the Grant Agency of the Academy of
Sciences of the Czech Republic (IAA 400500905).
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