Synthesis of Layered Disodium (or Dipotassium) Tetrakis-(octanoate-o)-Zinc(II) and Preliminary Investigation of the Catalytic Activity in the Esterification of Octanoic Acid with Isopropanol

Article abstract of DOI:10.1134/S0023158417060076

This paper reports the synthesis of layered zinc/sodium and zinc/potassium octanoates (A₂[Zn(C₈H₁₅O₂)₄], A = Na⁺ or K⁺), two underexplored carboxylates, through reaction of the octanoate of the desired alkali metal with a zinc salt. After drying and chemical characterization, their catalytic activities were tested in the esterification of octanoic acid with isopropanol. Conversion values were high for the reactions conducted under following conditions: temperature 155°C, isopropanol/octanoic acid molar ratio = 8: 1, catalyst/ octanoic acid ratio 7 wt %, and reaction time 2 h. Isolation of the catalysts after esterification showed that the materials were converted into zinc octanoate in situ.

Full text of DOI:10.1134/S0023158417060076

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 6, pp. 726–733. © Pleiades Publishing, Ltd., 2017.
Synthesis of Layered Disodium (or Dipotassium)
Tetrakis-(octanoate-o)-Zinc(II) and Preliminary Investigation
of the Catalytic Activity in the Esterification
1
of Octanoic Acid with Isopropanol
Swami Arêa Maruyama*, Sonia Faria Zawadzki, and Fernando Wypych
Universidade Federal do Paraná, Departamento de Química, Caixa Postal 19032, Curitiba, República Federativa do Brasil
*
e-mail: swamimaruyama@yahoo.com.br
Received August 21, 2016
Abstract—This paper reports the synthesis of layered zinc/sodium and zinc/potassium octanoates
+
+
(
A [Zn(C H O ) ], A = Na or K ), two underexplored carboxylates, through reaction of the octanoate of
2 8 15 2 4
the desired alkali metal with a zinc salt. After drying and chemical characterization, their catalytic activities
were tested in the esterification of octanoic acid with isopropanol. Conversion values were high for the reactions
conducted under following conditions: temperature 155°C, isopropanol/octanoic acid molar ratio = 8 : 1, cat-
alyst/octanoic acid ratio 7 wt %, and reaction time 2 h. Isolation of the catalysts after esterification showed
that the materials were converted into zinc octanoate in situ.
Keywords: zinc/sodium and zinc/potassium octanoates, esterification, octanoic acid, conversion in esterifi-
cation
DOI: 10.1134/S0023158417060076
INTRODUCTION
esters such as isopropyl octanoate produced by react-
ing octanoic acid with isopropanol [9].
Alkanoates or metal carboxylates (metallic soaps)
combine anions derived from fatty acids with metals in
various oxidation states and have the generic formula
Considering that the structures of carboxylates
2+
involving alkali metals and Zn ions have been
described in the literature [10, 11], and that these com-
pounds were identified in paints used in works of art [7],
this study is aimed at evaluating this class of materials
as alternative, highly active catalysts for the produc-
tion of isopropyl octanoate from octanoic acid and
isopropanol. It is noteworthy that to date neither
Na [Zn(C H O ) ] nor K [Zn(C H O ) ] were used
–
x+
x+
(
C H
COO ) M , where M corresponds to a
n
2n + 1
x
+
metal with oxidation state x , with x ranging from 1 to
3
[1, 2]. These compounds have wide industrial appli-
cations including their use as polymers, components
of paints with hydrophobic properties, thickener of
lubricating greases, lubricants employed in various
industrial branches, and liquid crystals [3, 4].
2
8
15 2 4
2
8
15 2 4
in other applications.
More recently, layered carboxylates containing
2
+
metals in the oxidation state 2+, especially Zn , have
proven to be active catalysts during the esterification of
fatty acids with alcohols of different chain sizes [5–8].
This class of catalysts has many advantages: they can
be added to the reaction medium in the solid state,
they are solubilized/merged during the reaction, and
they are recovered intact after reaction cooling. There-
fore, layered carboxylates offer the best advantages of
both heterogeneous and homogeneous catalysts.
EXPERIMENTAL
Layered disodium (or dipotassium) tetrakis-(octa-
noate-O)-zinc(II) (A [Zn(C H O ) ], designated
2
8
15 2 4
+
A [Zn(B) ] hereafter (where A corresponds to Na or
2
+
4
−
2
K and B refers to the anion octanoate (C H O ))
8
15
and denoted zinc/sodium (or zinc/potassium) octa-
noates, were synthesized according to the procedure
adapted from the literature on the synthesis of
zinc/sodium (or zinc/potassium) hexanoates [10, 11].
Although the main goal of research into esterifica-
tion reactions is to obtain fatty esters for biodiesel pur-
Sodium or potassium octanoates were produced by
poses, esterification can also afford other valuable reacting octanoic acid (120.3 mmol) solubilized in
methanol (40 mL) with methanolic NaOH or KOH
solutions taken in stoichiometric amounts and pre-
1
The article is published in the original.
726

SYNTHESIS OF LAYERED DISODIUM
727
(
a)
size. In the industry, this ester is obtained with low
yields and high market value. In the experiments,
octanoic acid (98%) and isopropanol (98%) were
placed in a BüchiGlasUster reactor miniclave driven
with rotation of 500 rpm and connected to a Julabo
HE-4 oil bath. Based on previous tests, 500 rpm was
chosen as the ideal stirring rate in order to ensure a
potential impact of mass transfer, and because higher
stirring rates (750 rpm, 1000 rpm, etc.) did not lead to
a significant increase in the acid-ester conversion. The
reaction conditions: alcohol/acid molar ratio = 8 : 1,
catalyst/octanoic acid ratio = 7 wt %, temperature
2
1
1
55°C, and reaction time 2 h. Acid conversion into the
ester was quantified by titration of the resulting mix-
ture with NaOH 0.1 mol/L previously standardized
with potassium biphthalate.
5
10 15 20 25 30 35 40 45 50
θ, deg
X-ray powder diffraction (XPRD) patterns of all
2
samples were recorded on a Shimadzu XDR-6000
diffractometer using CuK radiation (λ = 1.5418 Å),
α
(b)
at 40 keV, 30 mA, and a scanning rate of 2°/min. The
samples were placed on glass sample holders and
lightly hand pressed so that the crystals were per-
fectly set in the holder plane.
2
958, 2925
668
2871, 2854
1
624, 1598
2
1397, 1386
Fourier-transform infrared spectroscopic mea-
surements (FTIR) were conducted by using the sam-
ples pressed as KBr discs (spectroscopic grade, Vetec).
The spectra were acquired on a Bio-Rad FTS 3500GX
1
7
23
1107
–1
spectrophotometer between 400 and 4000 cm , with a
6
80 δ(COO–)
–1
resolution of 4 cm and accumulation of 32 scans.
Thermal analysis measurements (simultaneous
thermogravimetry, TGA, and differential scanning cal-
orimetry, DSC) were performed on a Netzsch STA 449
F3 Jupiter analyzer under the following conditions: syn-
thetic air flow 50 mL/min, heating rate 10°C/min, and
temperature ranging from 30 to 1000°C. The samples
were placed in alumina crucibles.
2
872, 2857, 2850
–
2
964, 2957, 2923
1410, 1395 (νCOO )sym.
–
1624, 1607, 1597 (νCOO )asym.
3
200 3000 2800 1800 1500 1200 900 600
Wavenumber, cm–1
Energy dispersive spectroscopy (EDS) was accom-
Fig. 1. X-ray diffraction patterns (a) and FTIR spectra (b)
of zinc/sodium octanoate (1) and zinc/potassium octa-
noate (2).
plished with the aid of a microscope VEGA3 Tescan
LMU (EDS-Oxiford) equipped with the AZ Tech
2
(
Advanced) software, an 80 mm SDD detector, and
voltage of 15 kV. Powder samples were placed in the
sample holder and fixed with commercial glue. The
scans were collected from 0 to 11 keV. After EDS anal-
ysis, the samples were metal sputtered, and micro-
graphs were obtained by scanning electron microscopy
pared in advance at 50°C under vigorous stirring. The
resulting sodium octanoate precipitate was solubilized
in 50 mL of distilled water at room temperature under
magnetic stirring.
(SEM) under the same microscope.
Zinc/sodium (or potassium) octanoates were syn-
thesized by slow adding an anhydrous zinc chloride
High-resolution DSC analyses were carried out on
a Netzsch 200F3 calorimeter with the samples placed
in aluminum crucibles. The conditions were as fol-
(
98%) solution containing 60.15 mmol of the zinc salt in
1
00 mL of distilled water to the solution containing
lows: N flow 20 mL/min, heating rate 10°C/min, and
2
sodium (or potassium) octanoate, under magnetic stir-
ring. After zinc chloride was added, the mixture was
stirred for 30 min. The resulting white solid was washed
with distilled water and centrifuged twice followed by
drying in a vacuum oven at 60°C to a constant weight.
temperature ranging from 20 to 160°C.
RESULTS AND DISCUSSION
The X-ray diffraction patterns of zinc/sodium
The catalytic activity assays were restricted to the octanoate and zinc/potassium octanoate (Fig. 1a) are
synthesis of isopropyl octanoate, an aliphatic ester typical of layered materials: a series of diffraction
that is used as food flavoring and is difficult to synthe- peaks emerge at low angles.
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

7
28
MARUYAMA et al.
To avoid errors associated with low angle diffrac-
(a)
tion peak positions, the interplanar spacing was deter-
mined from the peak positions at higher Bragg angles.
Unit cell dimensions of 23.94 and 20.77 Å characterize
zinc/sodium octanoate and zinc/potassium octa-
noate, respectively, which agree with literature data
1
00
0
8
0
DSC
Exot.
–
–
–
5
(24.03 and 20.85 Å, respectively [11]). A unit cell
60
dimension found for zinc/potassium octanoateis
close to the value reported for zinc octanoate (20.83
or 20.88 Å) [9, 11].
10
15
4
0
0
The FTIR spectrum of zinc/sodium octanoate
2
TGA
–
1
(
Fig. 1b, 1) displays bands at 1395 and 1410 cm , due
to symmetric carboxylate vibration; at 1597, 1607,
0
200
400
600
800
1000
–
1
and 1624 cm , ascribed to asymmetric carboxylate
vibration; and at 2872, 2857, 2850, 2923, 2957, and
Temperature, °C
–
1
2
964 cm , attributed to C–H stretching vibration of
(b)
CH and CH .
2
3
100
80
0
The FTIR spectrum of zinc/potassium octanoate
–1
(
Fig. 1b, 2) presents bands at 1386 and 1397 cm , cor-
–
5
responding to symmetric carboxylate vibration; at
DSC
–1
1
598 and 1624 cm , assigned to asymmetric carboxyl-
6
0
0
–1
Exot.
ate vibration, and at 2854, 2871, 2925, and 2958 cm ,
referring to C–H stretching vibration of CH and CH
–10
2
3
4
[
11, 12]. Splitting of the carboxylate bands in the FTIR
–
15
20
spectra of both octanoates results from two non-
equivalent carboxylate moieties, in which the sodium
or potassium and zinc atoms share the carboxylate
groups [11, 12].
20
TGA
–
1000
0
200
400
600
800
Temperature, °C
Considering that zinc/sodium and zinc/potassium
octanoates are isostructural with zinc/sodium hexa-
noate, the Zn atoms in the layers of the materials have
tetrahedral coordination and are bonded to four dif-
ferent carboxylates via an oxygen atom of each carbox-
ylate ion (monodentate form). The other oxygen
atoms of each carboxylate ion are bonded to sodium or
potassium atoms that occupy octahedral sites, and
therefore bonded to six different carboxylate ions.
Fig. 2. Thermal analysis (TGA/DSC) curves of zinc/sodium
octanoate (a) and zinc/potassium octanoate (b).
at 900°C, a process that is not complete even at
1
000°C. Based on the optimal formulas of the materi-
+
+
als, A [Zn(C H O ) ] (A = Na or K ), and consid-
2
8
15 2 4
ering the presence of ZnO and the respective carbon-
The TGA curve of zinc/sodium octanoate reveals a ates in the final residues, the theoretical values for resi-
mass loss of 3.9% up to 215°C, attributed to the loss of dues should correspond to 27.39 and 30.65% of the
adsorbed water. In this same temperature range, the initial sample weight of zinc/potassium and
low-intensity DSC endothermic peaks emerging at 50 zinc/sodium octanoates, respectively, so the deviations
and 114°C are associated with the enantiotropic phase from the expected values are 2.3 and 1.7%, respectively.
transitions and melting point. An endothermic peak at
High-resolution DSC measurements helped to
study the phase transitions observed in the TGA
curves more systematically (Fig. 3). Multiple phase
transitions occur in both zinc/sodium octanoate
2
02°C is also evident in the DSC curve (Fig. 2a).
Equivalent peaks arise almost in the same positions for
zinc/potassium octanoate and correspond to a mass
loss of 3.7% (Fig. 2b).
(
Fig. 3a) and zinc/potassium octanoate (Fig. 3b);
these transitions resemble those observed for zinc car-
boxylates [13, 14]. The first heating effect indicates
water loss from the sample, which cannot be observed
during the second heating. Other structural phase
transitions resemble transitions experienced by euro-
pium carboxylates [15].
For zinc/sodium octanoate, various exothermic
peaks appear near 400°C, the same picture character-
izes zinc/potassium octanoate (Fig. 2b). Heating
zinc/sodium octanoate to 800°C produced a residue
(
dry basis) that accounts for 28.04% of initial sample
weight. As for zinc/potassium octanoate, the experi-
mental residue produced by heating to 500°C makes
In an enantiotropic polymorphic system, transition
up 30.14% of initial sample weight. Both residual between two phases happens below the melting tem-
materials consist of a mixture of ZnO and alkali metal peratures of the two phases. In the second and third
carbonates, which subsequently begin to decompose heating/cooling cycles, these transitions probably cor-
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SYNTHESIS OF LAYERED DISODIUM
729
(
a)
heating, zinc/sodium octanoate (Fig. 3a) undergoes
four phase transitions at 73, 97, 102 (shoulder), and
0
0
0
0
.40
.35
.30
.25
118°C. Hysteresis leads to peaks at 44 and 79°C in the
Cooling
1
Exo
first and second cooling steps, and transitions at 58,
82, 95, and 115°C emerge in the second and third
heating steps.
2
Endo
In the second heating/cooling cycle of zinc/potas-
sium octanoate, two phenomena with an enthalpy of
3
–
–
–
0.25
0.30
0.35
1
1.58 J/g take place during heating, whereas the
enthalpy of cooling is 15.35 J/g. Therefore, the pro-
cesses can be considered as partially reversible.
2
1
Heating
For zinc/potassium octanoate (Fig. 3b), peaks
appear at 50 and 115°C at the first heating step, while
only one peak emerges at 78°C during cooling. At the
second and third heating steps, peaks at 41 and 113°C
arise, and only one peak appears at 79°C during the
cooling step. Zinc/sodium octanoate and zinc/potas-
sium octanoate melt at 118 and 115°C in the first heat-
ing step, respectively, and at 115 and 113°C in the other
steps, respectively.
20
40
60
80
100 120 140
Temperature, °C
(b)
0.6
0.5
0.4
0.3
0.2
Exo
2
Cooling
1
Endo
Table 1 lists the results of the esterification of octa-
noic acid with isopropanol catalyzed by the prepared
octanoates.
Reactions were carried out at a temperature above
the melting point of both octanoates to ensure that
they acted as catalysts in the liquid phase; i.e., dis-
–
0.2
0.3
0.4
0.5
3
2
–
–
–
2+
persed in solution, leaving the Zn ions available to
participate as catalyst. In literature an extensive infor-
mation can be found analyzing the features of the reac-
tion medium with octanoates [5, 6, 16, 17]. The alco-
hol/acid mixture, before esterification, has a totally
homogenous nature. The alcohol/acid/ester/water
mixture, obtained after esterification, also has a
homogenous character. Therefore, it is deducted that
water formed in the reaction cannot significantly
affect the solubility of octanoic acid in isopropanol.
Both materials afford high conversions: 52.2 and
Heating
1
20
40
60
80
100 120 140
Temperature, °C
Fig. 3. DSC curves of zinc/sodium octanoate (a) and
zinc/potassium octanoate (b). 1, 2, and 3 are the first, sec-
ond, and third heating/cooling cycles, respectively.
respond to transitions associated with layered crystal- 54.2% for zinc/sodium octanoate and zinc/potassium
phases I and II (enantiotropic transitions) and isotro- octanoate, respectively, corresponding to conversion
pic liquid-phase 2 (melting) [13, 14]. During the first gains of 30.1 and 32.1%, respectively.
Table 1. Ester conversions*
Conversion
Conversion gain**
No.
Catalyst
%
1
2
3
4
5
6
Zn/Na octanoate
Zn/Na octanoate (first reuse)
Zn/K octanoate
Zn/K octanoate (first reuse)
Zinc octanoate
Without catalyst
52.2
43.1
54.2
52.6
50.7
22.1
30.1
21.0
32.1
30.5
28.6
–
*
All the experiments were performed under the following conditions: isopropanol/octanoic acid molar ratio = 8 : 1, catalyst/octanoic
acid ratio = 7 wt %, temperature 155°C, and reaction time 2 h.
*
* Conversion gain was obtained by subtracting the catalytic conversion from the thermal conversion (experiment 5 was conducted
without catalyst).
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7
30
MARUYAMA et al.
The esterification reaction is nearly thermoneutral,
(a)
as calculated from the enthalpies of formation of dif-
ferent esters, acids and alcohols. The average value of
ΔH° is −3 ± 2 kJ/mol. A heat flux of only 0.8 kW
would be generated, if these processes were designed
to take place with a mean conversion rate of 1 kmol/h.
From the calculation of the effect of temperature on
the degree of esterification with the Van’t-Hoff’s
equation, 0.4% decrease is expected around ambient
temperatures for the equilibrium constant per one
Kelvin increase [18].
2
1.14 A
3
2
20.89 A
However, some results [19] show that, when the
reaction conditions correspond to the equilibrium
state, the water produced in the reaction is mainly dis-
tributed in an alcohol-rich phase, with a small amount
remaining in an ester-rich phase. The presence of
water in a monophasic system would shift reaction
equilibrium to the reactants because of the hydrolysis
reaction. However, when two liquid phases are consid-
ered, the water formed in the reaction medium is not
available in the ester-rich phase, allowing the reaction
to proceed to products. This fact therefore provides an
explanation for the situation, when the experimental
results exceed the calculated data.
2
3.94 A
1
5
10
15
20
25
30
35
40
2θ, deg
(
b)
Reuse tests (Table 1, first reuse) demonstrate that
after the first reaction the catalytic activity of
zinc/sodium octanoate and zinc/potassium octanoate
decreases from 52.2 to 43.1% and from 54.2 to 52.6%,
respectively. X-ray diffraction helped to investigate the
catalysts recovered after the first reaction (Fig. 4).
21.14 A
1.19; 20.91 A
3
2
2
20.77 A
1
After the first esterification reaction, zinc/sodium
octanoate with a lattice constant of 23.94 Å (Fig. 4a, 1)
is transformed into a material with a unit cell parame-
ter of 20.89 Å close to the interplanar spacing of zinc
octanoate (Fig. 4a, 2), the XRD peaks shift slightly to
higher angles as compared to peak positions of zinc
octanoate characterized by the unit cell constant of
5
10
15
20
2
25
30
35
40
θ, deg
Fig. 4. X-ray diffraction patterns of zinc/sodium octa-
noate (a) and zinc/potassium octanoate (b) before (2)
and after use as catalyst in the esterification of octanoic
acid with isopropanol (1). (3) Zinc octanoate.
2
1.14 Å (Fig. 4a, 3). Zinc/potassium octanoate having
a lattice dimension of 20.77 Å (Fig. 4b, 1) partly pre-
serves its structure with a lattice parameter of 20.91 Å
conversion of 43.1% and conversion gain of 21.0%).
This difference in catalytic conversions is presumably
due to a smaller number of zinc sites in the
(Fig. 4b, 2) with a fraction of this material transformed
into zinc octanoate identified by an interplanar spac-
ing of 21.19 Å (Fig. 4b, 2). Our measurements indi-
cated 21.14 Å as the interplanar spacing of zinc octa-
noate (Fig. 4b, 3) whereas the literature reports lattice
dimensions of 20.88 Å [11] and 20.83Å [9]. It appears
that the lattice dimensions are subject to variations,
the range of which depends on the conditions used to
synthesize the sample and on sample positioning in
the sample holder during data collection.
“
zinc/sodium octanoate” taken from the first reuse.
As for zinc/potassium octanoate in the first reuse,
conversion and conversion gain were 52.6 and 30.5%,
respectively. These results can be explained by partial
conversion of the starting zinc/potassium octanoate to
zinc octanoate. Given that zinc/potassium octanoate
and zinc octanoate are characterized by the different
level of catalytic activity, an intermediate value of
activity can be expected from the mixture.
To test the hypothesis that zinc/sodium octanoate
and zinc/potassium octanoate are transformed into
zinc octanoate during the esterification reactions, zinc
The FTIR spectra of zinc/sodium octanoate
octanoate was used as catalyst under the same experi- (Fig. 5a, 1) and zinc/potassium octanoate (Fig. 5b, 1)
mental conditions (Table 1, entry 5). A catalytic con- are different, but the spectra recorded for the catalysts
version of 50.7% and a conversion gain of 28.6% were recovered after the first esterification reaction are sim-
obtained, which are close to the values obtained for ilar (Figs. 5a, 2 and 5b, 2) and retain the similarity
zinc/sodium octanoate in the first reuse (catalyzed after the first reuse (Figs. 5a, 3 and 5b, 3). Hence,
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SYNTHESIS OF LAYERED DISODIUM
731
(a)
C
O
Zn
Zn
4
3
2
Zn
Zn
4
3
C
O
746
Zn
Na
1
408, 1397
K
C
1
467, 1457
O
1549, 1528
K
Zn
Zn
2
1
1
Zn
O
680
1
410, 1395
1
625, 1607, 1597
0
1
2
3
4
5
6
7
8
9
10
keV
320030002800
1600
1200
800
400
Wavenumber, cm–1
Fig. 6. EDS spectra of zinc/sodium octanoate (1),
zinc/potassium octanoate (2), and recovered after the ester-
ification of octanoic acid with isopropanol the zinc/sodium
octanoate (3) and zinc/potassium octanoate (4).
(b)
4
peaks at 116 and 136°C correspond to phase transitions
3
2
in the solid state. Two large exothermic peaks at 290 and
3
43°C correspond to decomposition of the material and
ZnO formation. Similarly, the TGA/DSC curve of pure
zinc octanoate not shown in this work displays three
small endothermic peaks at 101, 120, and 136°C in addi-
tion to an exothermic peak at 383°C.
1
408, 1398
1
466, 1457
1549, 1529
1
The weight of the final residues formed after octa-
noates were used as catalysts in the esterification reac-
tion was expected to be 20.06% for zinc octanoate and
1
397, 1386
1
625, 1598
20.96% for zinc/potassium octanoate. These values
3
200
2800 1600
1200
800
400
are lower than the experimentally found amount of
23.13%. Adsolubilization of some organic material
between the pillars of intercalated octanoate anions
Wavenumber, cm–1
(
in this case, potassium octanoate) possibly occurs.
Fig. 5. FTIR spectra of zinc/sodium octanoate (a) and
zinc/potassium octanoate (b) before (1) and after use as cat-
alyst the first esterification reaction between octanoic acid
and isopropanol (2) and after the first reuse (3). 4—Zinc
octanoate.
Indeed, adsolubilization of neutral organic molecules
between pillars of surfactants in layered double hydrox-
ides (LDH) has already been described [20, 21]. There-
fore, this technique also suggests that zinc/potassium
octanoate is transformed into zinc octanoate in situ
during the esterification reaction.
during esterification, zinc/sodium octanoate and
zinc/potassium octanoate are transformed into the
To corroborate the facts verified by FTIR and
same compound, the FTIR spectrum of which contains TGA/DSC, EDS analysis of the catalysts before and
the same bands as the FTIR spectrum of zinc octanoate after the esterification reactions was conducted (Fig. 6).
(
4).
According to the EDS spectra the starting catalysts con-
tain sodium or potassium, carbon, oxygen, and zinc,
whereas the catalysts recovered at the end of the esteri-
fication reactions contain only carbon, oxygen, and
zinc. Along with FTIR and TGA/DSC results, the
EDS data confirm that zinc/sodium octanoate and
zinc/potassium octanoate are converted into zinc octa-
noate during the esterification reaction.
To confirm that the material isolated after the ester-
ification reactions is actually zinc octanoate, the mate-
rial derived from zinc/potassium octanoate was submit-
ted to TGA/DSC analysis. The results of the analysis
not shown in this work can nevertheless be compared
with TGA/DSC data available for zinc octanoate.
In contrast to the TGA/DSC analysis of zinc/potas-
sium octanoate before its use as catalyst in the esterifi-
As for the characterization of the starting solids by
cation reaction (Fig. 4b), the material isolated after the SEM, with results not shown in this work, the struc-
reaction is anhydrous and does not experience signifi- ture of these materials expectedly encompasses
cant mass loss up to about 180°C. Two endothermic stacked layers. The crystals of both zinc/sodium octa-
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

7
32
MARUYAMA et al.
noate and zinc/potassium octanoate resemble plates. atoms of this intermediate, producing a water mole-
The particles located over the planes of the layered cule. This molecule is eliminated as a result of forma-
crystals are much larger than the particles accommo- tion of isopropyl octanoate. Finally, as the acid-base
dated along the stacked layers.
interaction between the zinc atom from the catalyst
and the carbonyl group from isopropyl octanoate falls
off this zinc atom is free to begin another catalytic
cycle [5–7, 22].
Finally, in order to prove that zinc/sodium octa-
noate and zinc/potassium octanoate were really trans-
formed into zinc octanoate after they were used as cat-
alysts in esterification reactions, the materials recov-
However, in the case of zinc/sodium octanoate and
ered at the end of esterification were analyzed using zinc/potassium octanoate, the sodium and/or potas-
the DSC technique.
sium atoms are also available for the acid-base interac-
tion between sodium and/or potassium atom from the
catalyst and the carbonyl group from the octanoic
acid. Therefore, unlike pure zinc octanoate,
zinc/sodium octanoate and zinc/potassium octanoate
have two kinds of catalytically active sites, the zinc
atom and the sodium and/or potassium atom. This
accounts for the difference in the catalytic activities of
zinc/sodium octanoate, zinc/potassium octanoate
and zinc octanoate.
Based on the results derived from the DSC measure-
ments and not shown in this work, on the first heating
zinc/sodium octanoate and zinc/potassium octanoate
isolated after use as catalyst display two endothermic
peaks at 121 and 139°C, and two exothermic peaks at 93
and 113°C. In the second and third heating runs, three
endothermic peaks occur at 100, 139, and 199°C, and
two exothermic peaks are exhibited at 93 and 113°C in
the course of the second cooling.
DSC curves obtained for zinc octanoate and not
shown in this work indicate that during the first heat-
ing run two endothermic peaks appear at 95 and
CONCLUSIONS
Both Na [Zn(C H O ) ] and K [Zn(C H O ) ]
2
8
15 2 4
2
8
15 2 4
1
27°C, and two exothermic peaks occur at 65 and
were synthesized, characterized, and showed promis-
ing catalytic activity in the esterification of octanoic
acid with isopropanol under the following conditions:
isopropanol/octanoic acid molar ratio = 8 : 1, cata-
lyst/octanoic acid ratio = 7 wt %, temperature 155°C,
and reaction time 2 h. Both catalysts are converted to
zinc octanoate during esterification. It was concluded
that the materials tested as catalysts possess character-
istics of both heterogeneous and homogeneous cata-
lysts, although they are converted into zinc octanoate
during the reaction.
47°C in the course of the first cooling. Following the
second and third heating runs, three endothermic
peaks at 62, 90, and 122°C occur, and the same peaks
were observed on the curve obtained after the first
cooling appear on the second cooling. The literature
brings more details of the attribution of phase transi-
tions undergone by zinc octanoate [13, 14].
Since the thermal behavior of starting zinc/sodium
octanoate and zinc/potassium octanoate is markedly
different from that shown by pure zinc octanoate
(
Fig. 4a,b), it was concluded that both starting mate-
rials are unstable and are transformed into zinc octa-
noate during the esterification reactions. However,
the reasons of different thermal behavior of the cata-
lysts recovered after transesterification and that of
zinc octanoate remain unclear. It can be speculated
that the recovered catalysts are alternative forms of
zinc octanoate, in which stronger intermolecular
forces between carbon chains are developed. Accord-
ingly, additional energy efforts are needed to
approach phase transitions on heating with solid
structures recovered at higher temperatures.
ACKNOWLEDGMENTS
The present work received financial support from
CNPq, Conselho Nacional de Desenvolvimento
Científico e Tecnológico (Brazil) through postdoc-
toral grant process number 168161/2014-1. The finan-
cial funding by CAPES and FINEP is also acknowl-
edged.
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Finally, the reaction mechanism proposed for the
synthesis of isopropyl octanoate with zinc octanoate is
similar to that described in literature for esterification
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carbon. Isopropanol attacks the polarized carbonyl
group in order to form a tetrahedral intermediate.
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