A Kinetic Investigation of Triacetin Methanolysis and Assessment of the Stability of a Sulfated Zirconium Oxide Catalyst

Article abstract of DOI:10.1002/aocs.12085

In this work, the activity and stability of a sulfated zirconium catalyst in transesterification reactions of triacetin, a model molecule, were investigated. This catalyst has Lewis and Br?nsted acid sites and has shown to be highly active in reactions converting triacetin into methyl esters. This catalyst was synthesized using the impregnation method, and systematically characterized using the techniques of x-rays diffraction (XRD), scanning electron microscopy, Fourier transform-infrared spectroscopy (FT-IR), Brunauer, Emmett, and Teller, and thermal gravimetry analysis (TGA). Kinetic studies were carried out to determine the activation energy as well as the reaction order. The effects of the main reaction parameters, such as temperature, the molar ratio, and the catalyst content, were evaluated. The reuse and possible leaching of the catalyst were also investigated. The highest efficiency (ca. 99% of methyl esters) was achieved in the sulfated zirconium oxide-catalyzed transesterification reaction.

Full text of DOI:10.1002/aocs.12085

J Am Oil Chem Soc (2018)
DOI 10.1002/aocs.12085
ORIGINAL ARTICLE
A Kinetic Investigation of Triacetin Methanolysis and
Assessment of the Stability of a Sulfated Zirconium Oxide
Catalyst
1
1
1
1
ꢀ
Rafael L. Temóteo · Marcio J. da Silva · Fabio de Avila Rodrigues · Wagner F. da Silva ·
Deusanilde de Jesus Silva1 · Cesar M. Oliveira1
Received: 18 February 2018 / Revised: 3 May 2018 / Accepted: 4 May 2018
© 2018 AOCS
Abstract In this work, the activity and stability of a sul-
fated zirconium catalyst in transesterification reactions of
triacetin, a model molecule, were investigated. This catalyst
has Lewis and Brønsted acid sites and has shown to be
highly active in reactions converting triacetin into methyl
esters. This catalyst was synthesized using the impregna-
tion method, and systematically characterized using the
techniques of x-rays diffraction (XRD), scanning electron
microscopy, Fourier transform-infrared spectroscopy (FT-
IR), Brunauer, Emmett, and Teller, and thermal gravimetry
analysis (TGA). Kinetic studies were carried out to deter-
mine the activation energy as well as the reaction order.
The effects of the main reaction parameters, such as tem-
perature, the molar ratio, and the catalyst content, were
evaluated. The reuse and possible leaching of the catalyst
were also investigated. The highest efficiency (ca. 99% of
methyl esters) was achieved in the sulfated zirconium
oxide-catalyzed transesterification reaction.
Introduction
The depletion of fossil fuels has motivated the search for
alternative energy sources. Biodiesel obtained from renew-
able raw materials reduces the emission of greenhouse gases,
and is environmentally friendly. Nonetheless, the traditional
processes of biodiesel production are carried out under
homogeneous alkaline catalysis conditions, which generate
large amounts of effluents and neutralization residues (Ilmi,
Hommes, Winkelman, Hidayat, & Heeres, 2016). Conven-
tional alkaline processes require the control of the amount of
free fatty acids (FFA) to avoid the undesirable saponification
reactions (Nakatake, Yazaki, Matsushima, & Ohshima,
2016). Therefore, only raw materials with low content of
fatty acids can be used in the alkaline processes
To solve these problems, an attractive alternative is to
replace the homogeneous catalysts with solid acid catalysts,
which are compatible with lipidic raw materials and are recy-
clable catalysts (Grossi, Jardim, de Araújo, Lago, & da Silva,
2010; Silva, Laier, & da Silva, 2010). These solid catalysts
simplify the process because they make the separation steps
of alkyl esters easier and avoid the need for neutralization
steps (Grossi et al., 2010; Zabeti, Daud, & Aroua, 2009). A
variety of solid acids have already been tested for the synthe-
sis of methyl esters through esterification reactions of FFA
and transesterification of triglycerides (Zabeti et al., 2009;
Zieba, Drelinkiewicz, Chmielarz, Matachowski, & Stejskal,
2010). Amberlyst-15 resins (Pradhan, Chakraborty, & Chak-
raborty, 2016), zeolites (Wang & Chen, 2016), carbon-based
biomass (Mardhiah, Ong, Masjuki, Lim, & Pang, 2017), and
polyaniline sulfate (Intarapong, Luengnaruemitchai, & Jai-
In, 2012) are some examples.
Keywords Sulfated zirconium oxide ꢀ Lewis and
Brønsted acid sites ꢀ Triacetin ꢀ Transesterification
J Am Oil Chem Soc (2018).
Supporting information Additional supporting information may be
found online in the Supporting Information section at the end of the
article.
* Marcio J. da Silva
silvamj2003@ufv.br
1
Chemistry Department, Federal University of Viçosa, University
In this sense, despite low acidity and small porosity and
surface area, zirconium oxide has received a lot of attention
Campus, P.H. Rolfs Avenue, Viçosa, Minas Gerais 36570-000,
Brazil
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due to its high thermal and chemical stability (Lughi &
Sergo, 2010; Miao et al., 2017; Ranjbar, Yousefi,
triacetin with methyl alcohol. We paid special attention to
the main reaction parameters such as temperature, reactant
stoichiometry, and catalyst concentration. A kinetic study
provided the activation energy and the reaction order. The
stability of the catalyst was evaluated as well as its ability
to be recycled.
Lahooti,
& Malekzadeh, 2012; Sigwadi, Mavundla,
Moloto, & Mokrani, 2016). The low acidity and surface
area can be improved by different strategies. The treatment
of ZrO2 with sulfuric acid allows an increase in catalytic
activity of this material, making it a superacid material
(Silva, Moura, Silva, Pallone, & Costa, 2017).
The superacidity of ZrO2 is attributed to the formation of
several Brønsted and Lewis acid sites on its surface. Mean-
while, the Lewis acid sites appear due to the electron with-
drawing effect of the sulfate anions on the zirconium
cations, the Brønsted acid sites are formed by the presence
of protons provided by the treatment with sulfuric acid
(Silva et al., 2017). Conversely, the immobilization of ZrO2
over solids with a high surface area makes them a highly
active catalyst in several acid-catalyzed reactions
(Bandyopadhyay, Tsunoji, & Sano, 2017; Delesma, Cas-
tillo, Sevilla-Camacho, Sebastian, & Muñiz, 2017). Starting
from these two different approaches (i.e., sulfuric acid treat-
ment or supporting on porous solids), some studies have
demonstrated the efficiency of sulfated ZrO2 as an efficient
heterogeneous acid catalyst. Jitputti et al. (2006) investi-
gated the transesterification of renewable feedstock such as
palm kernel and crude coconut oils, and verified that high
methyl ester yield (ca. 90% and 86%, respectively) was
achieved with sulfated ZrO2. Although, when ZrO2 was not
sulfated, a strong decrease occurred in reaction yield
(ca. 64% and 48% was obtained, respectively).
Materials and Methods
Chemicals
All chemicals and solvents were used as received. Zirco-
nium hydroxide was purchased from Sigma–Aldrich (St.
Louis, Missouri, EUA) (97 wt%). Triacetin (98 wt%) was
provided by Buschle and Lepper S.A. (Joinville, Brazil),
ammonium sulfate (99 wt%) was bought from Synth S.A.
(Sao Paulo, Brazil), methyl alcohol (99.8 wt%) was from
Vetec (Sao Paulo, Brazil). Acetone (99.8 wt%) was pur-
chased from Alphatec (Sao Paulo, Brazil).
Synthesis of the Catalyst
The catalyst was synthesized using the impregnation
method. Zirconium hydroxide (ca. 10 g, 31 mmol) was
impregnated with 0.5 M ammonium sulfate solution and
incubated for 18 hours. It was then oven dried for 12 hours
and then calcined at 873 K for 4 hours according to Sun
et al. (2005).
The combination of ZrO2 with other metal oxides
(i.e., WO3, Al2O3) was assessed by Jacobson, Gopinath,
Meher, and Dalai (2008). No gains in terms of mechanical
resistance were obtained, however, the catalytic activity was
enhanced. Furuta, Matsuhashi, and Arata (2006) investigated
the combination of these three oxides (i.e., ZrO2, WO3, and
Al2O3), as well as the effect of sulfuric acid treatment. These
catalysts were used in transesterification reactions of soybean
oil with methyl alcohol at temperatures ranging from 473 to
573 K. Tungsten-zirconium-alumina untreated oxide was
found to have a higher activity in transesterification com-
pared to the sulfated zirconium-alumina oxide catalyst. Park
et al. also verified that the introduction of tungsten improved
the stability of zirconium catalysts (Park, Chung, Eom,
Lee, & Lee, 2010; Park, Dw, Kim, Lee, & Lee, 2008).
Model molecules such as triolein (Wang & Chen, 2016),
triacetin (Park et al., 2010), and tricaprilin (Chantrasa,
Phlernjai, & Goodwin, 2011) are useful to access the activity
of solid acid catalysts. Unlike fatty materials, which contain
glycerides with the carbon chains with different sizes, these
materials present chemical and structural simplicity, facilitat-
ing the identification and quantification of reaction products.
In this work, we have assessed the catalytic activity of
sulfated zirconium oxide in transesterification reactions of
Characterization of Catalysts
Crystalline states of the zirconium catalyst were determined
using a diffractometer model D8-Discover (Billerica, MA,
US) using Cu-Kα (λ = 15,405 Å) operating at 40 kV and
mA. The patterns were recorded from 5ꢁ to 50ꢁ at a scan-
ning rate of 4ꢁ min−1. FT-IR spectra were recorded on a
Varian 660 FT-IR spectrophotometer (P.A. California, US)
in the wavenumber range 4000–400 cm−1
.
Nitrogen adsorption–desorption isotherms were mea-
sured at 77 K using a Micromeritics 2010 instrument (Nor-
cross, G.A., US). The pore-size distribution was calculated
from the desorption isotherm obtained using the Barrett,
Joyner, and Halenda method. Surface area was determined
using desorption–adsorption isotherms of nitrogen
(Brunauer, Emmett, and Teller [BET]) in an ASAP 2010
Micrometrics instrument (Norcross, Georgia, USA).
The thermal decomposition temperatures and weight loss
were measured in a Differential Thermal Gravimetry (DTG)-
60H Perkin Elmer Thermal Gravimetry Analysis (TGA)/Dif-
ferential Scanning Calorimetry (DSC) analyzer (Waltham,
Massachusetts, EUA) at a heating rate of 20 K min−1 under
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a nitrogen atmosphere from room temperature till 1273 K.
The samples were scanned from room temperature to 873 K
at a heating rate of 10 K min−1 under a nitrogen atmosphere.
The catalyst morphology was characterized using scanning
electron microscopy (SEM) equipped with an energy-
dispersive X-ray spectroscopy (EDS) unit (JEOL [Akishima,
Tóquio, Japão], JSM-6010A).
Herein, “A0” is the initial area of the triacetin GC peak,
obtained from the calibration curve; “A” is the area of the
triacetin GC peak present in the chromatogram. The reac-
tion products were identified with coinjection of pure sam-
ples and gas chromatography-mass spectrometry (GC-MS)
analysis (Shimadzu 2010, 70 eV).
Catalyst acidity was measured by potentiometric titra-
tion, in agreement with the procedure reported by Pizzio,
Vásquez, Cáceres, and Blanco (2003). The electrode poten-
tial variation was measured with a potentiometer (i.e., Bel,
model W3B [Sao Paulo, Brazil]). Typically, a solution con-
taining an adequate amount of heteropolyacid (HPA) dis-
solved in CH3CN was titrated with n-butylamine solution
in toluene (ca. 0.025 mol L−1).
Reuse and Recycling of Catalysts; Leaching Tests
To evaluate the reuse and the leaching of the SZr catalyst,
the following procedure was used; after the end of the reac-
tion, the solid catalyst was filtered, washed with acetone,
and dried in an oven at 383 K for 5 hours. Afterward, it
was weighed and reused in another catalytic run. The FT-
IR spectra, the diffractograms, and the micrographs of the
washed samples were recorded prior to the sulfate leaching
test and the elemental analysis.
Catalytic Tests
The sulfated zirconia catalyst, (SZr), was evaluated in
transesterification reactions of triacetin with methyl alco-
hol. The experiments were carried out in a 300 mL Parr
reactor (model 4848, Parr Instrument Company, Molline,
IL, US) equipped with a mechanical stirrer and heated to
393 K over 1 hour under autogenic pressure.
The effects of the main reaction parameters were evaluated
as follows. To assess how the proportion of reactants affects
the reaction, we have performed runs with five different stoi-
chiometries of methyl alcohol to triacetin (ca. 3:1, 6:1, 9:1,
12:1, and 20:1). In these tests, the temperature was kept con-
stant (ca. 393 K) and the catalyst load was ca. 5 wt%. The
temperature effects were assessed performing reactions at
five levels (ca. 343, 363, 383, 393, and 403 K). On these
runs, the molar proportion between methyl alcohol and tria-
cetin was 20:1 and the catalyst load was ca. 5 wt%. Finally,
the effect of the catalyst concentration was studied in the
load range of 1–5 wt%; these reactions were carried out at
1:20 proportion of triacetin to methyl alcohol and at 393 K.
Results and Discussion
Catalyst Characterization
X-Ray Diffraction
Fig. 1 presents the diffractograms of the starting sample
(i.e., Zr(OH)4 (a) and ZrS catalyst (b)).
The literature describes that the main diffraction lines of
pure ZrO2 rise at 2θ values matching the Joint Committee on
Powder Diffraction Standards data mixed of monoclinic and
tetragonal structures. They are present at 2θ = (28.2ꢁ, 31.4ꢁ,
and 34.0ꢁ) and 2θ = (30.2ꢁ, 35.2ꢁ, 50.6ꢁ, and 60.2ꢁ), respec-
tively (Basahel, Ali, Mokhtar, & Narasimharao, 2010).
°
Reaction Monitoring and Product Identification
°
The reaction progress was followed by gas chromatography
analyses using a Shimadzu gas chromatograph equipped with
a flame-ionization detector and a capillary column (Restek
[Bellefonte, PA, US], 30 m × 0.25 mm × 0.25 μm). The car-
rier gas was H2 with a flow of 2.0 mL min−1. The tempera-
ture profile was as follows: an initial temperature of 333 K, a
heating rate of 5 K min−1 until 373 K; 10 K min−1 to 443 K;
5 K min−1 until 483 K, kept constant for 2 min. The injector
and detector temperatures were 333 and 483 K, respectively.
To calculate the reaction yields, Eq. 1 was used.
°
°
a
*
b
0
20
30
40
50
60
70
80
Degree (2θ)
Fig. 1 Powder X-ray diffraction patterns of samples sulfated zirco-
nium oxide (a) and zirconium hydroxide (b); the tetragonal phase (ꢁ)
and the monoclinic phase (*)
A0 −A
A0
Conversionð%Þ =
× 100%
ð1Þ
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As shown in Fig. 1, all these characteristic peaks of the
tetragonal (ꢁ) and monoclinic phases (*) are present in the
diffractogram of the SZr catalyst (Fig. 1). Such peaks are in
accordance with the synthesis and characterization of SZr
previously described (Basahel et al., 2010; Dosuna-Rodrí-
guez, Adriany, & Gaigneaux, 2011; Son, Gwon, & Kim,
2001). According to Brum, Santos, Destro, and Guerreiro
(2011), the tetragonal phase is very desirable in SZr,
because it has high acid strength and, consequently, may
result in great catalytic activity.
electrons (Fig. 2a,b), inducing a charge accumulation, not
showing roughness or porosity on its surface. Conversely,
the samples of the SZ catalyst (Fig. 2c,d) show a morphol-
ogy formed by larger-sized particles and pores on their sur-
face. Structures like these are found in the literature
(Pereira, Júnior, Carvalho, & Silva, 2015; Shi et al., 2016).
FT-IR Analysis
The FT-IR spectrum of SZ (Fig. SP1) shows an absorption
band of low intensity in the region of 3600–3000 cm−1; this
band is assigned to the deformation (δ HOH) of water mol-
ecules associated with the sulfate group (Ivanova, Hariza-
nova, Koutzarova, & Vertruyen, 2010). In addition, we can
see that the sulfated ZrO2 showed bands at 1620 and
3400 cm−1 wavenumbers; the first one is attributed to the
SEM Images
Fig. 2 shows the SEM images of commercial zirconium
hydroxide and sulfated zirconium samples. It is possible to
observe that the sample of Zr(OH)4 has low conduction of
(a)
(b)
(c)
(d)
Fig. 2 SEM images of Zr(OH)4 samples, 2500 times magnified (a), 5000 times magnified (b), and sulfated zirconium, 2500 times magnified (c),
5000 times magnified (d)
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scissor bending mode of water molecules coordinated to
the Zr4+cations. The second band was assigned to the
vibrations of physically adsorbed hydroxyl groups to the
Zr4+ cations (Heshmatpour & Aghakhanpour, 2011; Jaya-
kumar et al., 2011).
500
400
300
200
100
0
Zr(OH)4
ZrO2 / SO42-
ZrO2
A higher intensity for this same absorption band can be
verified in the zirconia hydroxide spectrum (Fig. SP1).
According to Yu et al. (2012), the absorption between 1000
and 1250 cm−1 wavenumbers is assigned to the vibrations
of the S O bonds of the sulfate groups bonded to the zirco-
nium oxide surface (Fig. SP1). This band is absent in the
starting material for this same region. The absorption bands
located around 700–400 cm−1 correspond to the
Zr O O Zr bond deformation modes and Zr O bond
asymmetric stretching, which is very common in crystalline
ZrO2 (Hwang et al., 2003).
-100
-200
0.0
0.5
1.0
1.5
2.0
2.5
Acidic sites (mmol g–1)
Fig. 3 Potentiometric titration with n-butylamine of the precursor and
zirconium catalysts
The absorption band at 1422 cm−1 wavenumber is
referred to the absorption of nonbridging OH groups
(Ibrahim, 2015). The stretching bands at 502 and 570 cm−1
are assigned to tetragonal and monoclinic structures of
ZrO2, respectively (Rouquerol, Rouquerol, & Sing, 1999).
To measure the acid strength of the zirconium catalysts,
the samples were potentiometrically titrated with n-
butylamine solution (Fig. 3). The initial electrode potential
(Ei) allows classifying the acidity strength of acid sites as:
Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong
sites), −100 < Ei < 0 (weak sites), and Ei < −100 mV
(very weak sites) (Pizzio et al., 2003).
The potentiometric titration curves revealed that the
treatment of the zirconium oxide catalysts with sulfuric acid
was highly efficient. After the conversion of Zr(OH)4 to
ZrO2 and their posterior treatment with sulfuric acid, the
acid strength of the zirconium catalysts was notably
increased. The initial electrode potential of 460 mV of the
SZr samples indicates that these catalysts have very strong
acid sites; conversely, their synthetic precursors have weak
acid sites, as demonstrated by the initial electrode potential
close to zero (Pizzio et al., 2003).
Surface Area and Porosity Properties
The pore-size distribution obtained from the desorption branch
of the isotherms is shown in Fig. SP2. The specific surface
area (SBET), total volume, size, and distribution of pore diame-
ter obtained from the N2 adsorption–desorption isotherms
were 36.3 m2 g−1, 0.047 cm3 g−1, and 18.46 Å, respectively.
The isotherms of the sulfated zirconium oxide samples
are stratifying as type II, which is a model of either a non-
porous or a mixed pore system according to the Interna-
tional Union of Pure and Applied Chemistry (IUPAC)
classification (Kamaruzaman & Chin, 2014).
Thermogravimetry Analyses
Fig. SP3 shows the TG curves of the SZr and Zr(OH)4 sam-
ples. Both samples presented considerable losses of mass
between 298 and 573 K, which were assigned to the dehy-
dration and dehydroxylation processes (Saravanan,
Tyagi, & Bajaj, 2014). However, between 873 and 1073 K,
the SZr sample presented another significant loss of weight
due to the decomposition of the sulfate groups (Garcia,
Teixerira, Marciniuk, & Schuchardt, 2008). It is notewor-
thy that the mass loss suffered by the Zr(OH)4 sample is
higher than that observed on TG curve of the SZr, in the
region of 298–573 K. This is evidence that Zr(OH)4 con-
tains more hydroxyl groups than the SZr sample. It was
confirmed by the OH stretching band at wavelength
3100 cm−1 (Fig. SP3). All our results agree with the litera-
ture data (Garcia et al., 2008; Noda, Gonçalves, de
Borba, & Silveira, 2007).
The Reactivity of Triglyceride and the Activity of the SZr
Catalysts
Triacetin was used as a triglyceride model for the kinetic
study of this reaction because it has the same chemical
functionality as any triglyceride molecule, sharing the same
reactivity principles. The transesterification of triacetin with
methyl alcohol occurs in three sequential steps, as depicted
in Scheme 1 (Muñiz, Castillo, Robles, & Sansores, 2016).
Due to the high steric hindrance, the acetyl group on the
secondary carbon atom is more reactive than that on pri-
mary carbon atoms and consequently it was preferentially
transesterified (Corma, 1997).
Initially, we have assessed the catalytic activity of zirco-
nium oxide before and after the sulfuric acid treatment. The
kinetic curves are displayed in Fig. SP4. These results
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O
OH
O
Table 1 Rate constant and the correlation coefficient of curves of the
ln(instantaneous triacetin concentration)/(initial triacetin concentra-
tion) as a function of reaction time
Catalyst
CH3
O
H3C
O
+
+
+
OCH3
H3C
CH3
O
O
+
+
CH3OH
O
CH3
CH3
O
H3C
H3C
O
O
O
O
Diacetylglycerol
Methyl acetate
Triacetin
Methyl alcohol
Catalyst
Temperature (K) Rate constants (s−1
)
R2
1/T
−ln k
O
OH
OH
CH3OH
OH
H3C
O
O
O
OCH3
H3C
343
363
383
403
423
0.003
0.98
0.99
0.99
0.99
2.915
−5.809
O
O
0.008
2.754 −4.828
2.611 −3.681
2.481 −2.776
Diacetin
Methyl acetate
Monoacetylglycerol
Methyl alcohol
Catalyst
0.0252
0.0623
0.1327
O
OH
OH
+ CH3OH
OH
H3C
O
OH
HO
OCH3
H3C
O
0.987 2.364 −2.019
Monoacetin
Methyl alcohol
Glycerol
Methyl acetate
mesoporous silicas (i.e., MCM-48, MCM-41, and
SBA-15).
Scheme 1 Transesterification steps of triacetin with methyl alcohol
The profile of kinetic curves shows that an increasing of
temperature resulted in a higher rate of reaction as well as a
higher final conversion. These data provided the kinetic
parameters to calculate the reaction order (see “Dependence
of the Reaction Rate in Relation to the Triacetin Concentra-
tion”) and the activation energy (see “Effect of Catalyst
Load on the SZr-Catalyzed Transesterification of Triacetin
with Methyl Alcohol”).
agree with the acidic properties of the catalysts; in Fig. 3,
the zirconium oxide presented acidic sites weaker than sul-
fated zirconium oxide and in a lower amount.
Kinetic Studies
Effect of Temperature on SZr-Catalyzed Transesterification
of Triacetin with Methyl Alcohol
Dependence of the Reaction Rate in Relation to the
Triacetin Concentration
To study the effect of the temperature on the reaction rate,
we carried out catalytic runs at temperatures ranging from
343 to 423 K. Fig. 4a shows the kinetic curves obtained.
When carrying out catalytic runs at 343 K, no significant
conversion was achieved, therefore, the curve was omitted
in Fig. 4a. Although the reactions have displayed different
initial rates, the runs performed at 403 and 423 K achieved
almost a complete conversion after 1 hour of the reaction.
These results are superior than those described by Bandyo-
padhyay et al. (2017), which studied the activity of ordered
From the kinetic curves of Fig. 4a, it was possible to deter-
mine the specific reaction rate constants for each tempera-
ture. The dependence of the reaction rate on the
concentration of triacetin was determined from plots of ln
[A/A0] as a function of the reaction time (Fig. 4b).
These experimental data were used to calculate the rate
constant (k) using the first-order rate law. From the angular
coefficients of the curves shown in Fig. 4b, the rate
(a)
(b)
363 K
0
383 k
403 K
423 K
90
-1
-2
-3
75
60
45
30
15
0
-4
343 K
363 K
383 k
403 K
423 K
-5
-6
-7
-8
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
Fig. 4 Effect of temperature on sulfated zirconium oxide-catalyzed transesterification of triacetin with methyl alcohol (a) and curves of
a
ln(instantaneous triacetin concentration)/(initial triacetin concentration) as a function of reaction time (b)a. Reaction conditions: triacetin:methyl
alcohol molar proportion (1:20); catalyst load (5 wt%)
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constants (k) for each process at each temperature were
measured (Table 1). The resulting linear equations pre-
sented higher linear correlation coefficients (R2) for the
first-order equation, thus indicating that the reaction has
first-order dependence in relation to the triacetin concentra-
tion at any temperature.
The angular coefficient (Ea/R) obtained by plotting ln
k × 1/T (K) (Eq. 2; Fig. 5) provided the activation energy
(Ea) of the sulfated zirconium oxide-catalyzed transesterifi-
cation of triacetin with methyl alcohol.
5 wt%
4 wt%
3 wt%
2 wt%
1 wt%
90
75
60
45
30
15
0
ꢀ
ꢁꢀ ꢁ
Ea
R
1
T
lnk = lnA−
ð2Þ
0
10
20
30
40
50
60
The value of the activation energy dictates whether the
reaction rate depends on the mass transfer phenomenon or
chemical dependence. It was reported that the activation
energy for the reactions that are limited by the mass trans-
fer/diffusion processes is of the order of 10–15 kJ mol−1
(Shi et al., 2016). Herein, the activation energy of the sul-
fated zirconium oxide-catalyzed transesterification reaction
Time (min)
Fig. 6 Effect of catalyst load on sulfated zirconium oxide-catalyzed
transesterification of triacetin with methyl alcohola. Reaction condi-
a
tions: triacetin:methyl alcohol molar proportion (1:20); tempera-
ture (393 K)
of the triacetin with methyl alcohol was 58.2 kJ mol−1
,
evidence that the reactions did not reach the equilibrium.
This effect can be attributed to the increase in the number
of active sites due to high catalyst load.
which indicates that the reaction rate was controlled by the
chemical processes (Fig. 5).
Effect of Catalyst Load on the SZr-Catalyzed
Transesterification of Triacetin with Methyl Alcohol
Effects of Reactant Stoichiometry on the SZr-Catalyzed
Transesterification of Triacetin with CH3OH
To assure that only the catalyst concentration was affecting
the reaction rate, all the measurements were done very far
from the equilibrium (i.e., within first hour of the reaction).
The kinetic curves are shown in Fig. 6.
The kinetic curves displayed in Fig. 6 show that the reac-
tion conversion carried out with catalyst load of 4 or 5 wt%
was almost complete. The reaction conversions were
enhanced with the increase of catalyst load; it is clear
The influence of the methanol:triacetin molar ratio in the
reaction conversion is shown in Fig. SP5. The conversion
of the triacetin improves with an increasing of the molar
ratio and reaches a maximum (ca. 99%) when the propor-
tion is 20:1 between alcohol and triacetin.
Despite the positive effect provoked by an increase of
methyl alcohol excess of 9:1 to 12:1, the differences in con-
versions are minimal when comparing the proportions of
3:1 to 9:1. The results also show that the 12:1 stoichiometry
becomes economically more interesting in terms of applica-
tion potential (i.e., initial investment and operating cost),
considering that the results are close to the highest molar
ratio studied in this work.
-2
-3
-4
Studies of Recovery and Reuse of the Sulfated Zirconium
Oxide Catalysts
y = -7.00x + 14.56
-5
R2 = 0.998
The main advantage of the heterogeneous catalysts over
homogeneous ones is their ease of recovery and the possibil-
ity of reuse. In this work, deactivation studies were carried
out aiming to evaluate the reusability of sulfated zirconium
oxide. Fig. 7 shows the results of four successive recycles.
The catalyst activity showed a significant decrease after
it was recovered and reused twice. The reactions with the
first and second recycle achieved conversions of 99%. In
-6
2.3
2.4
2.5
2.6
2.7
2.8
2.9
1/ T (K)
Fig. 5 Arrhenius plot of the sulfated zirconium oxide-catalyzed trans-
esterification of triacetin with methyl alcohol
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J Am Oil Chem Soc
Table 2 Elemental analysis (SEM/EDS) of the sulfated zirconium
oxide catalyst samples
First cycle
Second cycle
Third cycle
Fourth cycle
90
75
60
45
30
15
0
Atom Fresh After first
After second
cycle
After third
cycle
cycle
O
S
72.39
11.18
16.43
70.22
9.24
70.03
8.80
68.30
6.74
Zr
20.14
21.17
24.95
It means that there is no loss or noticeable alteration of the
sulfate ions and that no organic species were adsorbed on
the surface of the reused catalysts.
Our results agree with those described by Garcia
et al. (2008), which demonstrated that the limitation of SZr
is its quick deactivation. Those authors assessed the trans-
esterification of vegetal oils over sulfated zirconium oxide
and verified that the fatty-acid methyl esters yields dropped
off strongly when they recycled the catalyst. The yields
were decreasing as follows: 99%, 59%, 30%, and 14%.
Although our catalyst has also been deactivated, it was
more stable (ca. 99%, 97%, 69%, and 33% in the fourth
cycle).
According to Corma, this deactivation is a drawback
inherent to sulfated metal oxides and is a consequence of
hydrolysis of sulfate groups, which are lost due to the water
generated in the reactions (Corma, 1997). However, Kiss,
Dimian, and Rothenberg (2006) reported that only a small
loss of sulfate groups was detected when the water was at
low concentration.
Therefore, to verify if the structural properties of SZ cat-
alysts were affected during the recycle steps, we performed
elemental analyses using EDS and their recorded SEM
images before and after the reuse (Table 2 and Fig. SP7).
The comparison of SEM images of the fresh catalyst
after the first use shows that no significant change occurred
(Fig. SP7a,b). It is corroborated by elemental analysis of
these catalysts, which were similar (Table 2). Nonetheless,
the SEM images of the catalyst after second reuse
(Fig. SP7c) show that both the crystallite size and rugosity
were very distinct from the fresh catalyst.
0
10
20
30
40
50
60
Time (min)
Fig. 7 Conversion of triacetin after recovery and reuse of sulfated zir-
conium oxide. aReaction conditions: triacetin:methyl alcohol molar
proportion (1:20); temperature (393 K), catalyst (5 wt%)
the third and fourth recycles there was a significant
decrease (ca. 67% and 39%, respectively).
The SZr catalysts were characterized after the reuse in
each reaction cycle to study the possible changes in their
properties. However, the diffractograms of the original
sample and the reused catalysts were similar, indicating that
the crystalline phase remained without significant modifica-
tions (Fig. 8).
The FT-IR spectra of the reused catalysts were like the
original one. Only a little change was observed; the peak at
1080 cm−1 wavenumber underwent a split in two other
peaks (Fig. SP6). Interestingly, this modification was also
observed when the fresh catalysts was subjected to the pro-
cedure of washing and drying, even though it was not used.
°
°
°
First cycle
°
*
Interestingly, the sulfur percentage of the samples after
first and second reuse was close (ca. 9.24% and 8.80%;
Table 2). However, the conversions achieved in the reac-
tions with these catalysts were 97% and 69%, respectively
(Fig. 7). It means that both structural properties and sulfur
content play an important role in the catalytic activity.
Based on these results, we concluded that an acid-washing
step is required to reactivate the acid strength of the catalyst.
Thus, we performed an additional procedure after the
removal of the catalyst from the reaction solution; the cata-
lyst was washed three times with a mixture of methanolic
sulfuric acid solution and then dried in an oven as in the ini-
tial procedure. The reaction performed reusing the acid-
Second cycle
Third cycle
Fourth cycle
0
20
30
40
50
60
70
80
Degree (2θ)
Fig. 8 Powder X-ray diffraction patterns of sulfated zirconium oxide
fresh and after three reuse (ꢁ) the tetragonal phase and (*) the mono-
clinic phase
J Am Oil Chem Soc (2018)

J Am Oil Chem Soc
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Conclusion
The method used in the synthesis of the catalyst was effi-
cient considering the results of FT-IR, XRD, and
SEM/EDS. The experiments to evaluate the activity of the
sulfated zirconium oxide catalyst in the transesterification
reaction of triacetin with methyl alcohol reached high con-
versions (ca. 99%) under optimized conditions; 393 K, cat-
alyst load ca. 5 wt%, and the molar ratio of triacetin to
alcohol equal to 1:20. A first-order dependence in relation
to the triglyceride concentration was obtained in the kinetic
studies performed aiming to determine the rate constant by
adjusting the experimental data. It was observed that the
rate constant increased proportionally with an increase in
temperature and the activation energy was 58.2 kJ mol−1
.
The characterization of the reused catalyst did not reveal a
significant change in its properties, however, after it was
recycled twice there was a significant reduction in the cata-
lytic activity. This drawback was overcome by performing
an acid washing of the catalyst, which reactivated the acid
sites of the catalyst.
Acknowledgments Authors thank the CAPES, FAPEMIG e CNPq.
Ivanova, T., Harizanova, A., Koutzarova, T., & Vertruyen, B. (2010)
Effect of annealing temperatures on properties of sol–gel grown
ZnO–ZrO2 films. Crystal Research and Technology,
45:1154–1160.
Conflict of Interest The authors declare that they have no conflicts
of interest.
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