Alternative carbon based acid catalyst for selective esterification of glycerol to acetylglycerols

Article abstract of DOI:10.1016/j.apcata.2011.07.027

Carbon-based acid catalysts with porous structure were prepared by sulfonation of carbonized sucrose. The catalysts have an amorphous porous structure with a good acid capacity and high thermal stability. The catalytic activity was evaluated by the esterification of glycerol with acetic acid. The sulfonated carbon catalysts showed that glycerol was completely transformed into a mixture of glycerol esters including a high selectivity of about 50% to triacetylglycerol (TAG).

Full text of DOI:10.1016/j.apcata.2011.07.027

Applied Catalysis A: General 405 (2011) 55–60
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Alternative carbon based acid catalyst for selective esterification of glycerol to
acetylglycerols
Julián A. Sánchez, Diana L. Hernández, Jorge A. Moreno, Fanor Mondragón, Jhon J. Fernández∗
Energy Resources and Environmental Chemistry Group, Institute of Chemistry, University of Antioquia, A.A. 1226, Medellín, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon-based acid catalysts with porous structure were prepared by sulfonation of carbonized sucrose.
The catalysts have an amorphous porous structure with a good acid capacity and high thermal stability.
The catalytic activity was evaluated by the esterification of glycerol with acetic acid. The sulfonated carbon
catalysts showed that glycerol was completely transformed into a mixture of glycerol esters including a
high selectivity of about 50% to triacetylglycerol (TAG).
Received 22 February 2011
Received in revised form 14 July 2011
Accepted 21 July 2011
Available online 28 July 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Catalytic esterification
Glycerol
Triacetylglycerol
Sulfonated carbon catalyst
Carbon-based acid catalyst
1. Introduction
Etherification of glycerol with isobutene over similar resins
produced up to five different ethers [6].
Glycerol is the main byproduct of the biodiesel production
industry. The availability of glycerol can exceed current demand
for the production of fine chemicals, cosmetics, pharmaceutical
and food additives. The low cost of glycerol in addition that it is
nontoxic, edible, and biodegradable has lead researchers to look
for different alternatives for the conversion of glycerol into other
valuable chemicals. It is known that a very large number of chemi-
cals can be derived from glycerol by catalytic conversion processes,
for example, oxidation, hydrogenolysis, dehydration, pyroly-
sis, gasification, transesterification, esterification, etherification,
oligomerization, polymerization and carboxylation. Consequently,
there is a great industrial and economical interest to investigate
new ways for green and chemoselective catalytic materials for
these processes [1–4].
The glycerol etherification reaction can be catalyzed by acidic
homogeneous catalysts or preferentially by heterogeneous acid
catalysts such as zeolites, strong acid ion exchange resins or
sulfonic mesostructured silicas [5–8]. Large pore size materials are
very active catalysts, in particular good results at 100% conversion
of glycerol tert-butylation using isobutylene with selectivi-
ties to di- and tri-ethers up to 92% were obtained over strong
acid macroreticular ion-exchange resins (Amberlyst type) [5].
In the case of glycerol esterification or transesterification, it is
necessary to consider that the glycerol is a symmetrical molecule
with the three hydroxyl groups which are not very different in
reactivity. Therefore, the product of the direct esterification or
transesterification of glycerol with acid and/or basic catalysts is
a mixture of mono-, di-, and some triglyceride, plus glycerol that
has not reacted. A theoretical study of glycerol acetylation using
DFT theory determined the most stable structures of the reactants,
intermediates and products between a large amount of conformers.
Also it was found that the glycerol reaction with acetic acid is a ther-
modynamically favorable and that the outside hydroxyl bonds can
be esterified before the center hydroxyl group [9]. Experimentally,
in order to lower the reaction temperature for the direct esterifica-
tion of glycerol with fatty acids usually an acid catalyst is required,
for example, sulfuric acid, phosphoric acid, or organic sulfonic acids.
It is therefore highly desirable to develop more environmentally
friendly catalytic processes that can improve the yield of the desired
products (e.g. monoester yield) using more economic reaction con-
ditions. Solid basic materials such as metal oxides MgO, CeO2, La2O3
and ZnO; Al–Mg hydrotalcites; Cs-exchanged sepiolite; and meso-
porous MCM-41 have been used as potential basic catalysts for
glycerol transesterification with triglycerides [4,10–14]. In these
investigations it was found that when the basicity is significant,
the catalyst becomes more active. Solid acids such as microporous
zeolites or mesoporous silicas have also been tested for the cat-
alytic esterification of glycerol with fatty acids. Mesoporous silicas
are more easily accessible by large molecules like fatty acids and
∗
Corresponding author. Tel.: +57 4 2196613; fax: +57 4 2196565.
E-mail addresses: jfernan@udea.edu.co, jj.fernan1@gmail.com (J.J. Fernández).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.07.027

56
J.A. Sánchez et al. / Applied Catalysis A: General 405 (2011) 55–60
their esters than the zeolite microporosity. However, mesoporous
silicas have lower strength of the acid sites [10–14].
In summary, the sulfonated porous carbon materials accord-
ing to the evidence catalytic capacity could be a viable alternative
to improve the reactions of esterification of glycerol. Despite the
increasing interest on glycerol conversion to valuable chemicals,
few researches have been working on the esterification of glyc-
erol with acetic acid. There is no information available concerning
the performance of carbon acid catalysts prepared under mild con-
ditions from polysaccharides to obtain selectivity to TAG derivate
from glycerol, which is a biodiesel byproduct. In this research,
sulfonated carbon catalysts were synthesized and used for the
esterification of glycerol. Different methodologies were used for
the carbonization and functionalization of the catalyst promoters.
Catalytic esterification of the glycerol with acetic acid was carried
out and the reaction conditions were optimized.
Similar results were found in other researches [15,16]. It was
found that glycerol esterification with acetic acid reaches conver-
sions over 90% with a TAG selectivity of 13% using resins (such
as Amberlyst-15 and Amberlyst-35) and zeolites (such as HZSM-5
and HUSY9) [15], the results showed that the acid exchange resin,
Amberlyst-15, was the most active catalyst, achieving 97% con-
version, followed by K-10 montmorillonite, niobic acid, HZSM-5
and HUSY. It was assumed that the poor performance of the zeo-
lites might be related to diffusion problems of the acetylated esters
inside the cavities. Also the same commercial resins and the HY
and HZSM-5 were tested for the same reaction. Optimization of the
reaction conditions indicated that an acetic acid to glycerol molar
ratio of 9:1, at 378 K reaction temperature over Amberlyst-35 dur-
ing 4 h yielded almost 100% of glycerol conversion and a selectivity
to TAG near to 35% [16].
2. Experimental
Silica-based mesoporous materials containing R–SO3H groups
have been evaluated for esterification of glycerol with fatty acids
[13]. It was found that an optimum balance exists between the
nature of the organic group which supports the sulfonic acid (aro-
matic or alkyl, length of the alkyl chain), the distance between the
sulfonic group and the silica surface, and also the average pore size
of the material. While mesoporous silica systems continue to be
very important, over the past few years increasing attention has
been paid to porous carbon materials with designed pore architec-
ture. Some researchers consider nanoporous carbons as the next
generation of mesoporous materials [17–19].
Porous carbon is an attractive catalytic material because it can
be prepared from a wide variety of low-cost precursors, it is very
stable under nonoxidizing conditions, it has low density, high ther-
mal conductivity, good electrical conductivity, mechanical stability
and the specific surface-area can be controlled. Compared to meso-
porous silica, mesoporous carbon is more resistant to structural
changes by hydrolytic effects in aqueous environments. Porous car-
bon materials functionalized with sulfonic acid groups have been
investigated as potential, environment-friendly solid–acid cata-
lysts; because functionalized carbons eliminate the need for liquid
acids in certain catalytic reactions, and may be reused many times
[19–21]. Mesoporous carbon can be produced by carbonization of
starch and polysaccharides, and then sulfonated by suspension in
fuming or liquid sulfuric acid at elevated temperature. The catalysts
thus prepared were active and selective for several esterification
reactions [18–24].
Carbon mesoporous materials can also be produced by pyrol-
ysis of expanded starch and subsequently functionalized with
sulfonated groups (Starbon type catalysts) [23]. These materials
were tested for the esterification reaction of different diacids, under
similar reaction conditions. The rates of esterification of diacids
(succinic, fumaric and itaconic) were found to be between 5 and
10 times higher than any commercial alternative solid acid catalyst
(zeolites, sulfated zirconias, acidic clays, etc.), a diester selectivity
improvement (from 35 to 50% range for the majority of the solid
acids to 90% for Starbon-400 at conversion levels of 90%) was also
obtained. The recovered Starbon could be added to fresh substrate
solutions giving almost identical behavior. Recently, a sulfonated
carbon catalyst with mesoporous structure and high specific sur-
face area was prepared by impregnating the cellulosic precursor
(wood powder) with ZnCl2 prior to activation and sulfonation [20].
The performance of the sulfonated porous carbon material as an
acid catalyst was examined through the esterification of acetic acid
in the presence of ethanol (343 K) and the benzylation of toluene
in the presence of benzylchloride (373 K). It was found that the
activity for the benzylation of toluene was depending on both the
specific surface area and the acid density of the sulfonated porous
carbon catalyst.
2.1. Carbon preparation
Two different methods were used to obtain the porous car-
bons. The first one was a direct synthesis carbonization (here after
called DC), in this case pure sucrose powder was carbonized in a
tube reactor for 15 h under nitrogen atmosphere at 673 K. The sec-
ond one was a template assisted carbonization process [25] (here
after called TAC), in this method silica was used as template. The
silica template was prepared in situ using Na2SiO3 and H2O at
343 K, and then sucrose and HCl were added and then the mix-
ture was left during two days for polymerization. The material was
carbonized for 3 h under nitrogen atmosphere at three different
temperatures: 673, 873 and 1123 K. The carbonized solids were
stirred overnight at 393 K with NaOH 3 M solution and finally, fil-
tered and washed with hot water in order to eliminate the template.
The prepared materials were characterized by thermal analysis
using a TA-Instruments Q-500 and IR spectroscopy using a Nicolet
Magna 560 by a KBr disk method (0.5% of sample).
2.2. Carbon functionalization
In the sulfonation step the carbons samples were functionalized
using two sulfonating agents to incorporate –SO3H groups on the
carbon surface. In one approach the carbon sample was left in con-
tact overnight with fuming sulfuric acid (7% of SO3). In the second
approach the carbon sample was mixed with liquid H2SO4 (>98%),
and heated during 10 h. The treated samples were washed repeat-
edly in boiling distilled water until impurities such as sulfate ions
were no longer detected in the washing water, and then the sample
was dried overnight in an oven at 373 K. The catalysts thus prepared
are identified here after as TAC and DC; in reference to the template
assisted and direct carbonization method respectively, following of
the temperature of carbonization.
2.3. Catalyst characterization
Elemental analysis was used to determine the composition of
the catalysts. The carbonaceous samples before and after sulfona-
tion were characterized by infrared spectroscopy, by means of KBr
disk (0.5% of sample) using a Nicolet Magna 560, to identify the
functional groups bonded to the carbon surface. Surface acid capac-
ity was determined by titration with different basic solutions, using
Boehm’s method [26], the concentration value were normalized
and expressed as percentage. The surface and pore structure prop-
erties of sulfonic acid-carbons have been evaluated by means of
nitrogen adsorption and desorption isotherm at 77 K, also carbon
dioxide adsorption isotherm at 273 K using a Micromeritics ASAP
2010 system. Isotherm data were analyzed to obtain pore size dis-
tribution using the DFT model provided by Micromeritcs, the area

J.A. Sánchez et al. / Applied Catalysis A: General 405 (2011) 55–60
57
Table 1
Preparation conditions and sulfonic acid density of sulfonated based carbons.
Table 2
Characteristics of sulfonated carbon catalysts.
Sample
Sulfonic agent
Carbonization
temperature (K)
–SO3Ha density
Sample
ITSa (K)
Surface and pore properties
N2 adsorptionb CO2 adsorptionc
(mmol g−1
)
DC-673
DC-673
H2SO4 (l)
H2SO4 (f)
H2SO4 (l)
H2SO4 (f)
H2SO4 (l)
H2SO4 (f)
H2SO4 (l)
H2SO4 (f)
0.14
0.36
0.45
1.35
0.43
0.77
0.50
0.76
673
673
873
1123
SA
VP
MSA
VP
TAC-673
TAC-673
TAC-873
TAC-873
TAC-1123
TAC-1123
DC-673
4
2
8
49
4
<0.01
0.51
0.57
1.07
nd
nd
TAC-673
TAC-873
TAC-1123
556
616
804
333
432
383
0.13
0.17
0.15
nd: no determined.
a
Index of thermal stability, ITS = (% fixed carbon)/(% volatile material) from TGA
a
Quantified from elemental sulfur.
experiment.
Nitrogen adsorption at 77 K; SA: total surface area (BET) in m2 g−1; VP: pore
b
volume in cm3 g−1 at P/Po = 0.99.
under the curve for every sample was integrated and normalized to
obtain the percentage of every fraction of porosity, according to the
IUPAC definition. Pore volume was taken at P/Po = 0.99 and 0.028
single point respectively to N2 and CO2 adsorption measure.
Carbon dioxide adsorption at 273 K; MSA: micro surface area (DR) in m2 g−1
;
c
VP: pore volume in cm3 g−1 at P/Po = 0.99.
carboxyl groups which predominant at high temperature of prepa-
ration, to carbons obtained under similar temperature condition,
like TAC-673, methylene groups and phenolic OH groups were con-
firmed. Also, it was found in other research that the density of SO3H
groups remains relatively constant with respect to carbonization
temperature in the range of 523–723 K, but decreased with further
increases in carbonization temperature [20]. The scientific litera-
ture reports also that for sulfonated (SO3H-bearing) carbon the best
acid density values ranging from 1.04 to 1.34 mmol of SO3H g−1
[20,21,24]. In particular a value of 1.34 was obtained to a material
prepared from wood powder previously impregnated with ZnCl2
and pyrolyzed at 873 K and then sulfonated with fuming sulfuric
acid; this material with high densities of micro- and mesopores
showed high catalytic performance for the esterification of acetic
acid, the activity was attributed only on the acid density. For a sul-
fonated mesoporous silica there are reports of acid density value
about 1.15 mmol g−1; this material was also used to acetylation of
glycerol and good selectivity to TAG was reported [7,14,20]. As was
said above the material TAC-673 had a value of density acid (as
SO3H) of 1.35 mmol g−1, which makes it comparable to the value
before mentioned, for other catalysts such as carbons and silica
functionalized with sulfonic groups.
2.4. Catalytic performance of the sulfonated carbons
Esterification of oleic acid with high purity methanol was used
as a reference reaction to probe the reaction system. The cat-
alyst with the highest sulfur content was selected to optimize
the time, catalyst load and temperature for the esterification of
glycerol with acetic acid. The previous selected catalysts and a
commercial ion-exchange resin (Amberlyst-15) were used to test
the catalytic activity varying the temperature reaction from 378
to 473 K. These reactions were carried out in a reflux system
using a high purity glycerol and acetic acid (HPLC grade), in a
molar ratio acetic acid:glycerol of 1:9, the time reaction was 4 h
and the catalyst load was 5% (w/w). After reaction the products
were separated by filtration and analyzed by gas chromatogra-
phy using Agilent GC and GC–MS 7890A instruments by means of
a FID detector and a capillary column of polyetylene glycol (HP-
Innowax 30 m × 0.32 mm × 0.25 ␮m), for the quantification of the
main product, TAG, was used a column HP-5; other products were
identified by GS–MS with a capillary column HP-5.
3. Results and discussion
Table 2 shows the correlation of the thermal characteristics, sur-
face area and pore volume of the carbonized materials according
to the method and the temperature of carbonization. It is pre-
sented in this table the index of thermal stability, ITS, which relates
the carbon stable structures and hydrocarbon unstable structures.
The last compounds volatilize after breakdown from the carbona-
ceous parental material in a TGA experiment at 1023 K in an inert
atmosphere. Therefore the increase of the ITS value during the car-
bonization process can be attributed to the formation of random
oriented amorphous carbon structure composed of aromatic sheets
[17–21].
It was possible to observe from Table 2 that the direct carboniza-
tion of the sucrose at 673 K produces a non-porous material with
a superficial area of 4 m2 g−1, which was in the limit of detec-
tion of this analytical technique. It is shown in Table 1 that ITS
was very low for a carbons obtained using DC method, also the
mass yield using this method was very low. Due to these reasons,
the DC method was not used to prepare other catalytic materi-
als. In contrast the method using silica to mold the carbonaceous
pore structural properties allowed obtaining materials with mixed
micro- and mesoporosity material with a high specific surface area
and high thermal stability, depending on the carbonization tem-
perature. This is probably due to the control of the polysaccharide
reactions of dehydration and condensation by the silica-template,
avoiding in this way the massive decomposition of the parental
material. High values of specific surface area (>500 m2 g−1) and
thermal stability were obtained from the carbonization process
3.1. Structure
All of the carbonized materials were functionalized using liq-
uid or fuming sulfuric acid. The percentage of sulfur bonded to the
surface was calculated from elemental sulfur analysis, the results,
expressed as mass acid density, are shown in Table 1. There was
a marked influence of the method used to introduce sulfur func-
tionalities on the carbon surface, as well as of the carbonization
procedure. The low surface area developed by the DC method
induced poor surface sulfonation efficiency. On the other hand the
TAC method developed larger surface areas and because of this, it
was possible to increase the density of acid groups, of the SO3H type,
on the carbon surface. Therefore, the silica played an important role
as a pore structure director. Comparison of both sulfonating agents
leads to the assumption that the molecules in vapor phase have
better diffusion properties and binding characteristics to the sur-
face than the liquid. The highest amount of sulfur incorporated into
the carbon surface was obtained with the material prepared using
the TAC method with a previous carbonization at 673 K, in this way
1.35 mmol SO3H g−1was obtained. This high sulfur uptake can be
explained by the relatively mild carbonization temperature which
produced an amorphous but more active surface towards sulfona-
tion. It was proposed after structural characterization; using XRD,
13C CP-MAS NMR and XPS, that sulfonated carbons prepared like
TACs exhibit characteristics due to polycyclic aromatic carbon, and

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J.A. Sánchez et al. / Applied Catalysis A: General 405 (2011) 55–60
Fig. 1. Infrared spectra of the carbonized materials. Top: DC carbonized material at
673 K. Bottom: TAC carbonized material at 673 K.
at 850 K and 1123 K. It is happened in this kind of materials (i.e.
active carbons) that the development of pore volume is caused
by the widening of the size of pore in the range of mesoporosity
more than the new porosity generation. In this case the volume of
nitrogen adsorbed was major that the volume adsorbed of carbon
dioxide in all materials produced for the TAC method. Therefore
it is expected that TAC materials can have a pore size which can
be accessed by molecules of the functionalizing agent but also by
reactant molecules.
In this work the IR spectra was used to confirm the carboniza-
tion degree and the presence of the characteristics bands to identify
sulfonic, carboxylic and polycyclic aromatic carbons, as was estab-
lished early in this paper it is assumed that they were present in
the carbon surface. Fig. 1 shows the infrared spectra of the car-
bonized materials at 673 K using the DC and TAC methods; the TAC
carbonized material presents a well-resolved spectra. Both sucrose
carbons showed a broad band centered at 3370 cm−1 was assigned
to the –OH stretching vibration mode of the –COOH group. The
main difference is exhibited in the region from 900 to 1900 cm−1
where the DC material shows an incipient resolution, in contrast
TAC carbon present a strong band at around ∼1590 cm−1 and other
at 1368 cm−1due to C C stretching, also a broad band at 1255 cm−1
can be assigned to the aryl-hydroxyl stretching (Ar–OH).
Fig. 3. FTIR spectra of the TAC-673 materials prepared from sucrose before and after
sulfonation.
Table 3
Elemental analysis of the TAC-673 catalyst before and after sulfonation.
Element
Before sulfonation % (w/w)
After sulfonation % (w/w)
C
H
68.9
4.0
46.9
2.8
S
<0.1
27.0
4.3
46.0
O (by difference)
assigned to the SO2 asymmetric and symmetric stretching vibra-
tion mode, which can be assigned to the –SO3H groups. For the
sulfonated carbon the bands assigned to –COOH and C C remained
unaltered. The overall FTIR analysis suggests that the carbon-based
catalysts consist of polycyclic aromatic carbon sheets containing
–SO3H, –COOH, and –OH functionalities, in agreement with the
previous analysis. Elemental analysis of the TAC-673 sample is
presented in Table 3. An increase of 4.3% sulfur content after func-
tionalization is observed, a C/S ratio of 10.9 and O/S ratio of 10.6,
is indicative of the chemical incorporation of sulfur. However the
atomic ratio of oxygen increased in a proportion larger than the
one of sulfonation. Therefore, it is assumed that was oxidation of
the surface as carboxylic-type groups, because this information was
correlated with FTIR data.
Table 4 shows the surface analysis for TAC-673 catalyst. This
material has a bimodal pore size distribution, essentially between
micro and mesoporosity in very similar proportions. Macroporosity
was negligible. This pore size distribution is a desired composition
for use in heterogeneous reactions due to diffusion requirements.
The Boehm method provides evidence of how different bases can be
linked with the functional groups of the surface catalyst, to deter-
mine this interaction NaOH, NaCO3 and NaHCO3 solutions were
used to titrate the acid groups. It was found that there was a bal-
anced distribution of the three kinds of acidic groups on the surface.
As expected the strong acid groups on the surface had predomi-
Fig. 2 shows the effect of the temperature of carbonization on
the surface functional groups for TACs carbons. The intensity bands
at 1590, 1368 and 1260 cm−1 were decreased as the temperature
becomes higher. The infrared spectra before and after sulfonation
of the TAC-673 carbons are shown in Fig. 3. As shown in this figure,
two bands appeared at 1035 and 1186 cm−1 in the sulfonated TAC-
673 catalyst compared with the unsulfonated one that could be
Table 4
Acid capacity and pore size distribution of the TAC-673 catalyst.
Surface characteristics
Mono-functional strong groups: 39
Bi-functional strong groups: 32
Weak groups: 29
Acid capacity distribution
by Boehm’s method (%)
Microporous 0–2 nm: 45
Mesoporous 2–50 nm: 51
Macroporous >50 nm: 4
Pore size distribution by N2
and CO2 adsorption (%)
Fig. 2. Infrared spectra of the TAC carbonized materials at different temperatures.

J.A. Sánchez et al. / Applied Catalysis A: General 405 (2011) 55–60
59
Scheme 1. Reaction for production of actylglycerols.
nance. These characteristics make this material a potential catalyst
for glycerol esterification reactions.
3.2. Catalytic performance
3.2.1. Oleic acid esterification with methanol
In the first step, the esterification reaction of oleic acid with
methanol was used in order to determine the efficiency of the cat-
alyst against a convencional catalyst. With the catalyst TAC-673 a
conversion of 68.4% to the respective ester was obtained, while the
commercial catalyst, Amberlyst-15, the conversion was 30.6%. That
means, the catalyst obtained in this research yielded more than
twice the selectivity of that obtained with Amberlyst-15 resin. This
kind of resin is recognize for the macroporous structure and its
catalytic activity for this reaction [17,27].
Fig. 4. Selectivity to TAG by esterification of glycerol with acetic acid (9:1 molar
relation, at 433 K and 5% of catalyst load). (Black rhombus) effect of the temperature
of carbonization of the catalysts. (Equis) effect of SO3H density sites. The connecting
lines in figure are just a guide for the eyes.
3.2.2. Esterification of glycerol with acetic acid
In this case the reaction of glycerol with acetic acid can yield
products such as mono-, di- and triacetin. Our purpose was directed
towards triacetin because this chemical is widely used in industry
as a fixative of odors and flavors, biocides, cosmetics, plasticizing
agent and recently as additive for biodiesel. Scheme 1 illustrates
the different esterification products that can be obtained: mono-
acetylglycerol (MAG) or monoacetin, di-acetylglycerol (DAG) or
diacetin, tri-acetylglycerol (TAG) or triacetin accompanied by water
elimination. All the reactions gave conversions of glycerol higher
that 99.6%.
The effect of the carbonization temperature on the catalytic
activity of the sulfonated carbons is shown in Fig. 4. The catalyst
activity is almost invariant for the materials carbonized at 873 and
1123 K (23.0%). However, it increases notably for the sample car-
bonized at 673 K (39.6%). Fig. 4 also shows the correlation between
C–SO3H acid density and the carbonization temperature. The com-
parison shows a similar pattern for the selectivity and acid density
sites. The TAG formation takes place over sulfonated sites which
are directly related to the porosity of the material. The samples
carbonized at lower temperatures contain SO3H groups and also
high densities of hydrophilic functional groups such as OH and
COOH, which were confirmed by FTIR data of TAC-673. It is there-
fore assumed that this sample can also adsorb large amounts of
hydrophilic molecules such as alcohols and glycerol, among others.
For the esterification reactions the influence of the amount
of catalyst and the reaction time were investigated keeping the
acid–glycerol ratio of 9:1. Table 5 displays the TAG selectivity as
function of the reaction time. When the time of reaction was 4 h the
amount of catalyst did not affect the selectivity reached to TAG. The
variation is from 37.6% to 41.0% for 1% to 7% respectively (weight of
catalyst to weight of glycerol). To test the optimum reaction time
the catalyst load was kept at 5%. A maximum, at 4 h, with a 39.6%
of selectivity was found.
The effect of the reaction temperature was evaluated from
378 to 473 K. The results are plotted in Fig. 5. The conversion of
glycerol was more than 99.6% for all of the experiments with a
significant increase in the selectivity to triacetin (TAG) as the tem-
perature reaction increased from 17% (at 378 K) to 50% (at 473 K).
The maximum selectivity to TAG was 50% at 453 K. As seen in
Scheme 1, in this particular type of reaction if the conversion of
Table 5
Variation of the reaction time and load of the catalyst again selectivity for the ester-
ification of glycerol with acetic acid at 378 K using TAC-673 catalyst.
Selectivityto TAG (%)
2
4
6
1
5
7
34.6
39.6
31.4
37.6
39.6
41.0
Time (h)
Catalyst load (%)

60
J.A. Sánchez et al. / Applied Catalysis A: General 405 (2011) 55–60
glycerol acetylation reaction. The catalysts prepared exhibited an
appropriated porous system of the interconnected micro- and
mesoporosity that allowed the surface sulfonation using fuming
sulfuric acid to form sufficient but variety catalytic active centers;
type C–SO3H. The combination of low temperature carbonization
at 673 K using silica as template material and the sulfonation with
fuming sulfuric acid produced the best active catalyst. The use of
this catalyst in the esterification reaction of glycerol with acetic acid
allowed conversions above 99% with selectivity towards triacetin
around 50%, while the esterification at the same conditions using a
conventional catalyst like Amberlyst-15 also gave over 99% conver-
sion but with a selectivity to triacetin of about 10%. Therefore, these
materials can be an alternative to other commercial heterogeneous
acid catalyst such as commercial acid exchange resins.
To settle the potential industrial applicability of these solid acid
catalysts, and as a continuation of the current research, the etheri-
fication of glycerol to fuel additives and use of crude glycerol from
biodiesel production with true plant impurities are already under
investigation.
Fig. 5. Selectivity to TAG by esterification of glycerol with acetic acid (at 9:1 molar
relation, 5% of catalyst load and 4 h of reaction time). Black rhombus for TAC-673.
Equis for commercial catalyst (Amberlyts-15). Black triangle for DC-673 catalyst.
The connecting lines in figure are just a guide for the eyes.
Acknowledgements
glycerol was 100%, the mono (MAG) and diacetin (DAG) compounds
were transformed into triacetin with the increasing of the reaction
temperature. In the same Figure, the selectivity to TAG obtained
under the same conditions, at 378 K, using the catalyst prepared
by the direct carbonization method, DC-673, was the lowest value
(9%). This was a consequence of the poor structural characteris-
tics developed; such as a non-porous material with the lowest
increasing of sulfur incorporated into the surface, which is shown
in Tables 1 and 2.
The authors wish to acknowledge the financial support for the
project 200D3347-498, C-013-2007 received from the Ministry of
Agriculture and Rural Development of Colombia and the University
of Antioquia and to the “Sostenibilidad” Program 2009–2011 of the
University of Antioquia. Diana Hernandez thanks the Colombian
Administrative Department of Science, Technology and Innovation
(COLCIENCIAS) for her Ph.D. scholarship.
Finally, in order to make a comparative analysis of the selectivity
between a carbon-based acid catalyst before analyzed (TAC-
673) and a commercial catalyst (Amberlyst-15), the esterification
reaction of both materials was run under the same conditions,
(acid–glycerol molar ratio = 9:1, 5% of catalyst load and to the same
reaction points of temperature). Fig. 5 shows that the selectivity to
TAG using the commercial resin ranged from 10% to 22%. There was
a decrease of 7% in the selectivity in comparison to the TAC-673 to
378 K, and a significant detriment of 28% in the point of maximum
catalytic activity. From this graph, it is possible to affirm that the
selectivity behavior of the commercial catalyst is less than TAC-
673 at the temperature range studied (from 378 to 473 K), though
there is a slight trend to increase the selectivity for the resin above
of 453 K, there is an advantage for the material TAC-673 since its
maximum selectivity was reached about this value of tempera-
ture, which is important thinking about its application. It has been
reported that the acid capacity of Amberlyst is between 4.5 and
5.0 mmol g−1 [14]. That means more than 3 times the capacity of the
TAC-673 catalyst. Nevertheless, the best catalytic performance evi-
denced by TAC-673 catalyst might be explained by the combination
of several factors before approached in other sections; such as the
diversity of active surface sites however the predominance of acid
strong groups like –SO3H and the easy access of reactants caused by
the pore system developed. Other structural studies should be real-
ized to prove that statements. To the knowledge of the authors, the
best catalytic result for the conversion of glycerol to TAG before this
work, reported a selectivity value of 35% [16], using the Amberlyst-
35 resin. In the present research a selectivity of about 50% to TAG
were reached. This is a significant improvement in the selectivity
yield for this application.
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In this research the sulfonation of carbon-based materials pre-
pared by controlled porous structure by sucrose carbonization
produced a highly active, and stable solid acid catalyst for the