A comparative investigation of palmitic acid esterification over p-sulfonic acid calix[4]arene and sulfuric acid catalysts via 1H NMR spectroscopy

Article abstract of DOI:10.1016/j.catcom.2012.05.007

This work reports the novel results of p-sulfonic acid calix[4]arene- catalyzed esterification reactions of palmitic acid with deuterated methanol. The p-sulfonic acid calix[4]arene organocatalyst is easily obtained and handled, and is significantly less corrosive than mineral acids commonly used in esterification reactions. Although used in homogeneous conditions, p-sulfonic acid calix[4]arene catalyst can be recovered and reused without losing its activity and may therefore be potentially useful in biodiesel production. A comparative study by use of in situ ¹H NMR spectroscopy between the kinetic and thermodynamic parameters obtained from the palmitic acid esterification reaction, showed that at 0.5 mol% concentration the activation energy of the p-sulfonic acid calix[4]arene-catalyzed reaction is lower than that measured for sulfuric acid. However, a higher dependence in relation to sulfuric acid concentration was observed. Conversely, ΔG values obtained suggested that this reaction becomes more spontaneous with an increase in temperature than calix[4]arene catalyzed esterification reactions.

Full text of DOI:10.1016/j.catcom.2012.05.007

Catalysis Communications 26 (2012) 127–131
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A comparative investigation of palmitic acid esterification over p-sulfonic acid
1
calix[4]arene and sulfuric acid catalysts via H NMR spectroscopy
⁎
Sergio Antonio Fernandes , Ricardo Natalino, Márcio José da Silva, Claudio Ferreira Lima
Grupo de Química Supramolecular e Biomimética (GQSB), Departamento de Química, CCE, Universidade Federal de Viçosa, Viçosa, MG 36570‐000 Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 March 2012
Received in revised form 27 April 2012
Accepted 11 May 2012
Available online 17 May 2012
This work reports the novel results of p-sulfonic acid calix[4]arene-catalyzed esterification reactions of
palmitic acid with deuterated methanol. The p-sulfonic acid calix[4]arene organocatalyst is easily obtained
and handled, and is significantly less corrosive than mineral acids commonly used in esterification reactions.
Although used in homogeneous conditions, p-sulfonic acid calix[4]arene catalyst can be recovered and reused
without losing its activity and may therefore be potentially useful in biodiesel production. A comparative
study by use of in situ 1H NMR spectroscopy between the kinetic and thermodynamic parameters obtained
from the palmitic acid esterification reaction, showed that at 0.5 mol% concentration the activation energy
of the p-sulfonic acid calix[4]arene-catalyzed reaction is lower than that measured for sulfuric acid. However,
a higher dependence in relation to sulfuric acid concentration was observed. Conversely, ΔG values obtained
suggested that this reaction becomes more spontaneous with an increase in temperature than calix[4]arene
catalyzed esterification reactions.
Keywords:
p-Sulfonic acid calix[4]arene
NMR spectroscopy
Organocatalysts
Fatty acid esterification
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Calix[n]arene macrocycles are formed from the condensation of para-
substituted phenols with formaldehyde in basic medium and have been
Esterification is one of the most fundamental and important reac-
tions in organic synthesis [1,2]. Organic esters are important fine
chemicals used widely in the manufacturing of flavors, pharmaceuti-
cals, plasticizers, polymerization monomers, emulsifiers in the food
and cosmetic industries, and biodiesel [3–5]. Specifically, biodiesel is
a renewable fuel comprised of monoalkyl esters of fatty acids. Interest
in biodiesel as an alternative fuel has accelerated tremendously due
to depletion of fossil fuels [6,7]. Biodiesel is now recognized as a
“green fuel” that has several advantages over conventional diesel
[8]. Non-edible oils make up a sustainable feedstock for biodiesel pro-
duction; however, these oils contain large amounts of free fatty acids
(FFA), which complicate the use of conventional alkaline catalysts
due to undesirable saponification reactions [9].
widely used as ligands in organometallic catalysis [16]. On the other
hand, examples of calix[n]arenes as organocatalysts have been even
less explored [17–19]. Currently, the use of calix[n]arenes still remains
little explored for esterification reactions [14,15].
Thus, this paper describes the data obtained from kinetic study of
palmitic acid esterification with deuterated methanol, in the presence
of the p-sulfonic acid calix[4]arene and sulfuric acid, using in situ 1H
NMR spectroscopy. It was possible to determine the main thermody-
namic properties such as activation energy (ΔE), entropy variation
(ΔS), enthalpy variation (ΔH) and Gibbs free energy variation (ΔG).
From this data, an accurate discussion was conducted on the catalytic
activity of p-sulfonic acid calix[4]arene and sulfuric acid.
The most commonly used methodology for ester synthesis is di-
rect esterification of carboxylic acids with alcohols in the presence
of acid-catalysts. Although efficient, homogeneous catalysts such as
mineral acids are toxic, non-reusable, corrosive and often hard to re-
move from the products [10–11]. Organocatalysts are an attractive al-
ternative, however, their use is still scarce in ester synthesis as well as
in biofuel production [12–15].
2. Experimental section
2.1. Chemicals
Palmitic acid (95% wt) was purchased from General Purpose Reagent.
This fatty acid (FA) was selected due to it being commonly present in
several biodiesel feedstocks. Deuterated methanol (99.75% w/w) and
deuterated chloroform (99.8% w/w) were purchased from Sigma Aldrich.
Toluene (99% w/w) and sulfuric acid (98.5% w/w) were acquired from
Quimica Moderna and Sigma Aldrich, respectively, and used as received.
All other reagents were of analytical grade and used without prior
purification.
⁎
Corresponding author at: Departamento de Química, Universidade Federal de Viçosa
(UFV), Campus Universitário, Avenida P.H. Rolfs, s/n, Viçosa, MG 36570‐000, Brazil.
Tel.: +55 31 3899 3071; fax: +55 31 3899 3065.
E-mail addresses: santonio@ufv.br, sefernandes@gmail.com (S.A. Fernandes).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.05.007

128
S.A. Fernandes et al. / Catalysis Communications 26 (2012) 127–131
SO3H
SO3H
acquisition time of 3.3 s and relaxation delay of 10 ms and 32 scans
SO3H
HO3S
in a 5 mm probe with direct detection.
In general, the methylenic group neighboring the carbonyl groups of
palmitic acid or methyl palmitate (R-CH2-CO2R′(H) show 1H NMR sig-
nals (triplet) in the chemical shift (δ) range of 2.4–2.2 ppm. Thus, reac-
tion progress was monitored according to the resonance signal intensity
(triplet) of these methylenic groups along the reactions. Intensity of the
two signals to the right of each triplet provides an adequate parameter
to quantify substrate conversion [23]. Acid conversion can be estimated
by comparing the areas of these signals because of R-CH2-CO2R′(H), as
defined by the equation given below: (Eq. (1)):
O
S
HO
OH
O
HO
OH
OH
OH
p-Sulfonic acid calix[4]arene
Sulfuric acid
Fig. 1. Structure of palmitic acid (FA model), p-sulfonic acid calix[4]arene and sulfuric
acid catalysts.
RCH2CO2CD3
RCH2CO2H
Fatty acid conversion ð%Þ ¼
ꢀ 100
ð1Þ
2.2. Catalyst synthesis
The p-sulfonic acid calix[4]arene was synthesized in our laborato-
ry according to literature procedures. First, p-tert-butylcalix[4]arene
was prepared by the Gutsche and Iqbal method [20]. Secondly, the
p-tert-butylcalix[4]arene was dealkylated by treatment with alumi-
num chloride in the presence of toluene and phenol according to
the method described by Ungaro and co-works [21]. Preparation of
the p-sulfonic acid calix[4]arene was performed by treatment of
calix[4]arene with concentrated sulfuric acid (98.5% w/w) [22]. The
reaction mixture was agitated under a nitrogen environment and
the temperature was maintained at 353 K. The reaction was followed
by taking samples, which were then washed in a minimum amount of
water to test their solubility. Complete solubility of the sample indi-
cated complete sulfonation and then the reaction stopped. In total
this took about 4 h.
3. Results and discussion
3.1. General aspects
Acid-catalyzed esterification is an equilibrium limited reaction.
Palmitic acid reacts with deuterated methanol in the presence of
p-sulfonic acid calix[4]arene or sulfuric acid catalysts to produce deu-
terated methyl palmitate and water. The catalyst initiates the esterifica-
tion reaction by donating a proton to the carboxylic acid molecule.
Carboxylic acid is then subjected to nucleophilic attack by the hydroxyl
group of alcohol (CD3OD), and the reaction continues with water elim-
ination. Here kinetic and thermodynamic parameters are obtained re-
lated to the performance of two acid catalysts: p-sulfonic acid calix[4]
arene and sulfuric acid (Fig. 1). Moreover, when directly comparing ex-
perimental data a more accurate discussion regarding catalytic activity
is possible.
2.3. Methods
2.3.1. General procedure of the kinetic measurements
The reactions were performed in a NMR tube, where 0.08 mmol of
palmitic acid (FA) was dissolved in 0.6 mL of the deuterated methanol
and heated to reaction temperature in the presence of the catalyst.
Kinetic data was obtained from 1H NMR spectra acquired during
5 minute time intervals in a NMR spectrometer (Mercury-300 Varian
Spectrometer) which were processed using the Varian “software”.
The operation frequency was 300.069 MHz for 1H (64 k data points),
30° excitation pulse duration of 2.2 μs, spectral width of 6 kHz,
3.2. Kinetic studies
Palmitic acid esterification with deuterated methanol was evaluated
via in situ 1H NMR spectroscopy (Fig. 2) in the presence of p-sulfonic
acid calix[4]arene or sulfuric acid catalysts. NMR spectroscopy showed
to be an efficient monitoring method and allowed for obtaining precise
reaction kinetic data for the two catalysts [23]. Good correlations
(R2≥0.996) were obtained for both catalysts (Fig. 2).
Fig. 2. Representative 1H NMR spectra (300.069, CD3OD, 318 K) obtained after partial esterification of palmitic acid. Legend: RCH2CO2H (palmitic acid) (*); RCH2CO2CD3 (deuterated
methyl palmitate) (•).

S.A. Fernandes et al. / Catalysis Communications 26 (2012) 127–131
129
100
100
90
80
70
60
50
40
30
20
10
0
b)
a)
90
80
70
60
50
333 K
328 K
323 K
318 K
313 K
333 K
328 K
323 K
318 K
313 K
40
30
20
10
0
0
40 80 120 160 200 240 280 320 360 400 440 480
Time (minutes)
0
40 80 120 160 200 240 280 320 360 400 440 480
Time (minutes)
-8.8
-8.9
-9.0
-9.1
-9.2
-9.3
-9.4
c)
d)
-8.6
-8.8
-9.0
-9.2
-9.4
-9.6
-9.8
-10.0
y = - 0.9823 - 2617.39 x
2 = 0.994
y = 11.218 - 6614.77 x
R2 = 0.990
R
30.0
30.5
31.0
31.5
32.0
30.0
30.5
31.0
31.5
32.0
1/T x 10-4 (K)
1/T x 10-4 (K)
Fig. 3. Kinetic studies of palmitic acid esterification with deuterated methanol at temperatures from 313 to 333 K over the catalysts a) p-sulfonic acid calix[4]arene and b) sulfuric
acid, and their respective Arrhenius plots: c) p-sulfonic acid calix[4]arene and d) sulfuric acid.
Table 1
Kinetic parameters for palmitic acid esterification with deuterated methanol over p-sulfonic acid calix[4]arene and sulfuric acid catalystsa.
p-Sulfonic acid calix[4]arene catalyst
Sulfuric acid catalyst
T
k (×10−5
)
TOFa
Ea
T
k (×10−5
)
TOFa
Ea
(K)
(L mol−1 s−1
)
(mol s−1
)
(kJ mol−1
)
(K)
(L mol−1 s−1
)
(mol s−1
)
(kJ mol−1
)
313
318
323
328
333
8.7
10.0
12.1
12.9
19.9
162
162
166
168
172
313
318
323
328
333
6.6
6.6
9.1
12.8
18.1
170
172
174
178
184
22.0
54.6
a
The turnover frequency (TOF, mol s−1) is moles of palmitic acid converted per mole of acid sites (H+) on the catalyst per second (i.e. four to calix[4]arene and two to sulfuric
acid catalyst).
110
100
90
80
70
60
50
40
30
20
10
0
110
100
90
80
70
60
50
40
30
20
10
0
a)
b)
5.0 mol%
2.5 mol%
2.0 mol%
1.0 mol%
0.5 mol%
5.0 mol%
2.5 mol%
2.0 mol%
1.0 mol%
0.5 mol%
0
50 100 150 200 250 300 350 400 450 500
Time (minutes)
0
50 100 150 200 250 300 350 400 450 500
Time (minutes)
Fig. 4. Effects of catalyst concentrations on palmitic acid esterification with deuterated methanol: a) p-sulfonic acid calix[4]arene and b) sulfuric acid.

130
S.A. Fernandes et al. / Catalysis Communications 26 (2012) 127–131
2.1
a)
b)
0.6
0.4
1.8
1.5
1.2
0.9
0.6
0.3
0.2
0.0
-0.2
-0.4
-0.6
-0.8
y = 3.7398 + 0.5741 x
R2 = 0.994
y = -3.4848 - 1.0283 x
2 = 0.998
0.0
R
-0.3
-8.1 -7.8 -7.5 -7.2 -6.9 -6.6 -6.3 -6.0 -5.7 -5.4
Ln[Catalyst]
-6.0 -5.7 -5.4 -5.1 -4.8 -4.5 -4.2 -3.9 -3.6 -3.3 -3.0
Ln[Catalyst]
Fig. 5. Order of reaction with respect to catalyst concentration: a) p-sulfonic acid calix[4]arene and b) sulfuric acid.
The kinetic parameters were determined by carrying out the esteri-
From the data shown in Fig. 4, a linear plot of ln (reaction rate)
calculated in terms of mmol palmitic acid consumed/1800 s of reac-
tion versus ln [catalyst concentration], expressed in mmol of the cat-
alysts, the order of reaction was determined with regard to catalyst
concentration (Fig. 5).
The slope of the curve for the initial reaction rate versus concentra-
tion, which was approximately 0.57 (R2=0.994) for the p-sulfonic acid
calix[4]arene, and 1.0 for sulfuric acid, confirms the higher efficiency of
sulfuric acid (Fig. 5). It should be emphasized that the measured ΔE
values should not necessarily agree with the order of catalyst concen-
tration; this observed dependence suggests that in the range concentra-
tion studied, reactions in the presence of calix[4]arene are less sensible
to high catalyst loadings. Contrarily, when utilizing sulfuric acid as the
catalyst, an increase in its concentration more significantly increases
the initial reaction. This effect can be attributed to the absence of diffu-
sion problems, which are caused by higher steric volume of p-sulfonic
acid calix[4]arene.
fication reactions at similar conversion levels (ca. 85–95%). All catalysts
were used at the same hydrogenionic concentration (molar ratio of
fatty acid:H+ equal to 800:1). All experimental data points collected
over 8 h were fitted to a curve passing through the origin. From this
curve (conversion versus time), initial rates were determined. Only
the rates obtained during the initial period of the reactions were taken
into account (first 30 min).
As seen in Fig. 3a and b, the reaction rate strongly depends on tem-
perature. Note that the reaction conversion increases steadily, reaches
maximum values (ca. 90%) after a reaction time of ca. 450 min and
remains almost invariable afterwards. Furthermore, observe that the
two acidic catalysts, although bearing significantly different structures,
displayed quite similar activities as can be confirmed by the comparable
final conversion at the end of the reaction (Fig. 1).
However, it should be highlighted that only the initial rate will be
used for obtaining kinetic parameters. This data was then used to cal-
culate the rate constant (k) using the pseudo-first order rate law.
Employing the Arrhenius equation and the k values determined at
different temperatures, activation energy values (ΔE) were then de-
termined. According to Table 1, we can see that for all temperatures
the rate constants (k) of p-sulfonic acid calix[4]arene-catalyzed ester-
ification were higher than those observed in sulfuric acid-catalyzed
esterification reaction. Consequently, the observed activation energy
value (22.0 kJ mol−1) for esterification of palmitic acid with deuter-
ated methanol over p-sulfonic acid calix[4]arene is much lower than
the activation energy obtained by using the traditional sulfuric acid
catalyst (54.6 kJ mol−1). This result suggests that temperature more
drastically affects the initial rate of reaction for the sulfuric acid cata-
lyst; however during the firsts 13 min, the initial rate of calix[n]
arene-catalyzed reactions was higher than that of sulfuric acid at all
temperatures.
3.3. Thermodynamic studies
When writing the equilibrium constant for the palmitic acid esterifi-
cation reaction with deuterated methanol (Keq) some considerations
must be made. Firstly, it should be considered that it does not depend
on methanol concentration (due to its excess), and secondly that both
water and deuterated methyl palmitate concentrations are equal.
Thus, Eq. (2) can be defined as:
Keq ¼ ½deuterated methyl palmitateꢁ2=½palmitic acidꢁ
ð2Þ
Fig. 3 displays the kinetic curves obtained from measuring the
conversion of palmitic acid in deuterated methyl palmitate in the
presence of p-sulfonic acid calix[4]arene or sulfuric acid at different
temperatures (313–333 K) via in situ 1H NMR spectroscopy (Eq (1)).
Table 2
Values of Gibbs free energy for the p-sulfonic acid calix[4]arene and sulfuric acid-
catalyzed palmitic acid esterification reactions with deuterated methanol.
Temperature (K)
ΔG (kJ mol−1
)
ΔH (kJ mol−1
)
ΔS (kJ mol−1
)
p-Sulfonic acid calix[4]arene catalyst
3.3. Dependence of reaction rate in relation to catalyst concentration
313
318
323
328
333
−2.0
−3.0
−4.0
−5.0
−6.0
Identical reaction conditions as those described above were employed
with the exception of the amounts of the p-sulfonic acid calix[4]arene or
sulfuric acid catalysts, which varied from 0.5 to 5.0 mol% (Fig. 4). Again,
only the rate obtained during the initial period of the reaction was
taken in account. All the experimental data points collected were fit to a
curve that should be passing through the origin. However, a time interval
of 3 min (NMR signal adjust) was required prior to performing the mea-
surements. From this curve (conversion versus time plot), initial rates
were determined.
+60.6
+0.2
Sulfuric acid catalyst
313
318
323
328
333
−5.9
−6.4
−6.9
−7.4
−7.9
+25.4
+0.1

S.A. Fernandes et al. / Catalysis Communications 26 (2012) 127–131
131
2.1
a)
b)
0.6
0.4
1.8
1.5
1.2
0.9
0.6
0.3
0.2
0.0
-0.2
-0.4
-0.6
-0.8
y = 3.7398 + 0.5741 x
R2 = 0.994
y = -3.4848 - 1.0283 x
R2 = 0.998
0.0
-0.3
-8.1 -7.8 -7.5 -7.2 -6.9 -6.6 -6.3 -6.0 -5.7 -5.4
Ln[Catalyst]
-6.0 -5.7 -5.4 -5.1 -4.8 -4.5 -4.2 -3.9 -3.6 -3.3 -3.0
Ln[Catalyst]
Fig. 6. Linear plots of ln Keq versus 1/T for the palmitic acid esterification reaction with deuterated methanol: a) p-sulfonic acid calix[4]arene and b) sulfuric acid.
The initial concentration of palmitic acid was equal to 0.13 mol L−1
Table 2 shows the equilibrium constants calculated for each reaction at
the different temperatures studied.
Eq (3) is valid if the enthalpy change of the reaction is assumed to
be constant with temperature, and was obtained from the second law
of thermodynamic and definition of Gibbs free energy (ΔG=−RT
lnKeq):
.
in temperature than those performed in the presence of the p-sulfonic
acid calix[4]arene catalyst. Hence, the p-sulfonic acid calix[4]arene can
be recovered and reused without losing catalytic activity and can be po-
tentially employed as an efficient and environmentally benign catalyst
in biodiesel production from FFA esterification reactions.
Acknowledgments
ΔS ΔH
This work was financially supported by CAPES, FUNARBE, FAPEMIG
and CNPq. This work is part of a collaboration research project of mem-
bers of the Rede Mineira de Química (RQ-MG) supported by FAPEMIG
(Project: REDE-113/10).
lnKeq ¼
−
lim :
ð3Þ
x→∞
R
RT
From the data shown in Table 2, it was possible to build the linear
plots of ln Keq versus ln 1/T which is presented in Fig. 6, and by using
Eq. (3) the respective values of ΔS and ΔH were calculated and dis-
played in Table 2.
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Kinetic studies utilizing in situ 1H NMR spectroscopy showed that
the catalysts studied here (i.e. p-sulfonic acid calix[4]arene and sulfuric
acid) presented a different behavior in the palmitic acid esterification re-
action. These catalysts were differently affected when main reaction pa-
rameters such as temperature and catalyst concentration are varied. It
was found that the activation energy of the p-sulfonic acid calix[4]
arene-catalyzed esterification reaction is lower than that observed in re-
actions in the presence of sulfuric acid at catalyst concentrations equal to
0.5 mol%. On the other hand, the dependence of reaction rate on sulfuric
acid concentration is higher than on that of p-sulfonic acid calix[4]arene
(1.0 against 0.57 respectively), in the concentration range of 0.5 to
5.0 mol%. Conversely, ΔG values obtained suggested that this reaction
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