Synthesis and characterization of the n-butyl palmitate as an organic phase change material

Article abstract of DOI:10.1007/s10973-018-7846-y

In this research, the n-butyl palmitate was synthesized using the esterification reaction of the PA with n-butanol. The ¹H nuclear magnetic resonance and Fourier transform infrared illustrated that the hydroxyl group and carboxyl group disappeared, and the ester bond appeared after the reaction, explaining that n-butyl palmitate was successfully fabricated. The differential scanning calorimetry indicated that the phase-transition temperature and latent heat are 12.6?°C and 127.1?J?g^?1, which was suited to use in low-temperature fields such as food, pharmaceutical, and biomedical. The thermogravimetric analysis suggested that it had great thermal stability during the phase change process. In addition, the thermal conductivity of the n-butyl palmitate was slightly higher than other fatty acid ester, and the 500 thermal cycles test results indicated that it had excellent thermal reliability. Therefore, the n-butyl palmitate is deduced to share great thermal energy storage ability in terms of latent heat thermal energy system applications.

Full text of DOI:10.1007/s10973-018-7846-y

Journal of Thermal Analysis and Calorimetry
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Synthesis and characterization of the n-butyl palmitate as an organic
phase change material
Liyun Ma1 • Chuigen Guo1 • Rongxian Ou1 • Qingwen Wang1 • Liping Li1
Received: 3 July 2018 / Accepted: 16 October 2018
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2018
Abstract
1
In this research, the n-butyl palmitate was synthesized using the esterification reaction of the PA with n-butanol. The H
nuclear magnetic resonance and Fourier transform infrared illustrated that the hydroxyl group and carboxyl group dis-
appeared, and the ester bond appeared after the reaction, explaining that n-butyl palmitate was successfully fabricated. The
differential scanning calorimetry indicated that the phase-transition temperature and latent heat are 12.6 °C and
-
1
1
27.1 J g , which was suited to use in low-temperature fields such as food, pharmaceutical, and biomedical. The
thermogravimetric analysis suggested that it had great thermal stability during the phase change process. In addition, the
thermal conductivity of the n-butyl palmitate was slightly higher than other fatty acid ester, and the 500 thermal cycles test
results indicated that it had excellent thermal reliability. Therefore, the n-butyl palmitate is deduced to share great thermal
energy storage ability in terms of latent heat thermal energy system applications.
Keywords Palmitic acid ester Á Phase change material Á Esterification Á Thermal energy storage
Introduction
systems [5], building [6], and air conditioning systems [7]
due to high energy storage density and repeatable utiliza-
tion property [8]. Among the previous reports [9–11], fatty
acids PCMs have been widely investigated as a superior
kind of phase change material (PCM).
At present, the secondary energy such as the wasted heat
from industry, solar power, surface heat, and others still
exists many problems, which have low utilization effi-
ciency, poor economic benefit and other major issues.
Many scholars have found that phase change energy stor-
age technology is an effective means to enhance energy
utilization and economic benefit by using phase change
materials (PCMs) [1], which is a kind of material that can
absorb thermal energy from the environment and release
the thermal energy to the environment during the phase
change process, to achieve the purpose of thermal storage
and release. Most of PCMs, such as organic PCMs [2],
inorganic PCMs [3], or their eutectic mixtures [4] have
been extensively studied as latent heat storage materials for
decades. And it has been applied in the solar water-heating
However, fatty acid ester PCM was relatively much less
studied. From the previous study [12–15], the fatty acid
ester can be fabricated by direct esterification reaction of
fatty acids with alcohols. It has a high latent heat and a
suitable phase-transition temperature as
a
PCM.
Feldman et al. [12] considered esters to be promising
storage media candidates for latent heat storage systems as
they show little to no supercooling, high chemical and
thermal stability, and no corrosiveness. Afterwards,
Karaipekli et al. [13] discovered that several fatty acid
esters equipped with great phase change properties, and the
phase-transition temperatures and the latent heats of them
-
1
were in the range of 21.6–32.3 °C and 35.9–43.3 J g
,
respectively. Sarı et al. [14] reported that the stearic acid,
myristic acid, and palmitic acid were combined with
glycerol to synthesize fatty acid esters through the reaction
&
&
Qingwen Wang
qwwang@scau.edu.cn
Liping Li
lilipingguo@126.com
1
of Fischer esterification. They were characterized by the H
1
nuclear magnetic resonance ( H NMR), Fourier transform
1
College of Materials and Energy, South China Agricultural
University, Guangzhou 510642, People’s Republic of China
infrared (FT-IR), and differential scanning calorimetry
123

L. Ma et al.
(
DSC). The test outcome suggested that the latent heats of
-
Chemical Reagent Factory, China. All of the above
chemical materials were analytical grade reagent.
1
synthesized esters were around 149–185 J g , and the
phase-transition temperatures were found in the range of
3
1–63.1 °C, illustrating that they had optimum properties.
Fabrication of n-butyl palmitate
Sari et al. [15] also investigated that the erythritol with the
melting temperature of 118.4 °C served as alcohol, the
palmitic acid (PA) and stearic acid (SA) with the phase-
transition temperatures of 62.74 °C and 68.86 °C acted as
acids, the erythritol palmitate and erythritol stearate were
prepared by the esterification reaction. The results showed
that the phase-transition temperatures of these two esters
were 21.93 °C and 30.35 °C, respectively. It also had good
thermal reliability, chemical stability, and physical
properties.
The n-butyl palmitate was done in accordance to the
method of the esterification reaction, which was conducted
by taking the calculated amount of PA and n-butanol
(molar ratio: 1:1 for PA:n-butanol) in benzene and a flask
with a reflux condenser and numerical show constant
temperature oil-bathing (the Jiang Su Ke Xi Instrument
Co., Ltd., China). Then, the catalyst p-toluenesulfonic acid
was added when the PA was melted thoroughly. Following,
the mixture was unremittingly stirred at the temperature of
120 °C for 1.5 h in the oil-bathing with a mechanical
stirrer. Water in the reaction medium was gradually
removed by benzene using the distillation method. After-
wards, the product was slowly cooled down to crystal
completely. Finally, the as-prepared n-butyl palmitate was
purified in acetone solution with agitation for 30 min and
then distilled to remove acetone. Subsequently, it was dried
using anhydrous sodium sulphate. In the end, the n-butyl
palmitate was fabricated successfully. The reaction
scheme is shown in Scheme. 1.
Among the fatty acid PCMs, the PA has very superior
performance, such as suitable phase-transition temperature,
high latent heat, and excellent chemical stability, etc. [16].
It has a strong application potential as a PCM such as
water-heating systems and others high-temperature filed
[
5]. However, the PA is limited in some practical field such
as solar heating–cooling of buildings and smart textile due
to the high phase-transition temperature [17].
To overcome the above-mentioned issues, the PA ester
was synthesized as a PCM. In this study, a kind of PA ester
called n-butyl palmitate was obtained successfully by using
the esterification reaction between the PA and n-butanol.
The chemical structures of the PA, n-butanol, and the
obtained n-butyl palmitate were analyzed by the FT-IR,
Characterization
To verify the properties of the n-butyl palmitate, the pre-
pared n-butyl palmitate and its components were system-
atically characterized. The physicochemical compatibility
of the components for the n-butyl palmitate with each other
was investigated by the FT-IR (Perkin Elmer Spectrum
100; USA) on a KBr pellet at the wavenumber range from
1
and the esterification reaction was investigated by the H
NMR technique. The thermal property of the n-butyl
palmitate was characterized by using the DSC and ther-
mogravimetric analysis (TG). The Hot-disk thermal con-
stants analyzer was employed to measure the thermal
conductivity of n-butyl palmitate. After 500 thermal cycles,
the DSC and FT-IR analyses were duplicated to find out the
change of phase change property and chemical structure of
the PA ester.
-
1
400 to 4000 cm . The chemical bond and proton number
1
of the n-butyl palmitate were conducted by the H NMR
1
spectrum (Varian 400MR; USA). The H NMR spectrum
of the n-butyl palmitate was obtained at the frequency of
4
00 MHz, and it was dissolved in acetone-d6 (C D O) with
3 6
-
a concentration of 100 mg 0.5 mL [18].
1
Experimental
The temperatures related with melting and crystalliza-
tion and phase change latent heat values of melting and
cooling were determined for the PA and its esters via the
DSC (NETZSCH STA-449C model instrument, Germany)
in the range of 253–373 K. For the measurement, the
aluminum crucible was filled with the samples around
5 mg. All of the DSC measurements were conducted at a
Materials
PA (C H O ) was supplied by Kermel Chemical Reagent
16 32 2
Co., Ltd., Tianjin, China. n-Butanol (C H O) was pur-
4 10
chased from Guangzhou Chemical Reagent Factory, China.
P-toluenesulfonic acid (TsOH, p-CH C H SO H) was
-
1
melting/cooling rate of 3 K min , under the static nitro-
3
6
4
3
-
1
bought from Guangzhou Jinhuada Chemical Reagent Co.,
Ltd., China, which acted as the catalyst. Benzene was
provided from Tianjin Damao Chemical Reagent Co., Ltd.,
China, which served as water-carrying agent to separate the
water that generated in the reaction. Anhydrous sodium
sulphate as a desiccant was supplied by Guangzhou
gen atmosphere of 20 mL min . All the experiments and
each sample were required to be measured twice, while the
average values were put into use.
The thermal stability of n-butyl palmitate was carried
out using a TG (NETZSCH TG 209 model instrument,
Germany) under
a
static nitrogen atmosphere of
1
23

Synthesis and characterization of the n-butyl palmitate as an organic phase change material
TsOH (cat.)
CH (CH ) COOH + CH (CH ) CH OH
CH (CH ) COOCH (CH ) CH + H O
3
2 14
3
2 2
2
3
2 14
2
2 2
3
2
Scheme 1 The esterification reaction of the PA with n-butanol
-
1
-1
0 mL min , at a heating scanning rate of 10 K min .
2
n-butyl palmitate
The mass of the samples was put into alumina crucible and
kept within 8–12 mg for the measurement in the temper-
ature range of 303–873 K [19]. The thermal conductivity
of n-butyl palmitate was detected by using the Hot-disk
thermal constants analyzer (Hot-disk Tp2500S; Sweden) at
ambient temperature.
1
461
1175
2
856
1739
n-butanol
2926
1463
The thermal reliability is one of the essential parameters
of PCMs. To make sure a good thermal reliability of the n-
butyl palmitate, performing an accelerated test in terms of
thermal cycling was necessary. It was as follows: the n-
butyl palmitate was put into a thermostatic chamber. Then,
it was heated above the melting temperature and cooled
under the crystallization temperature. Thus a thermal cycle
was completed including a melting and cooling process
1
071
2
874
3
361
2
957
PA
2
2921
850
7
20
3443
1467
1294
941
1703
4
000 3600 3200 2800 2400 2000 1600 1200
800
400
[
20]. It is required that the procedure is scheduled to be
Wavenumber/cm–1
continuously repeated 500 times. Finally, the variations of
phase change property and chemical structure of thermal
cycling treated sample were examined by the DSC and FT-
IR, respectively [21].
Fig. 1 FTIR spectra of the PA, n-butanol, and n-butyl palmitate
-
1
The characteristic absorption bands at 2957 and 2874 cm
correspond to the –CH and –CH stretching vibration. The
3
2
-
1
characteristic peak exhibited at 1463 cm is CH asym-
2
metrical bending vibration. The absorption peak emerged
-
Results and discussion
1
at 1071 cm is assigned to stretching vibration of the C–O
group.
The chemical property of the synthesized n-butyl
palmitate as a PCM
However, as for the n-butyl palmitate, the carboxylic
-
acid and hydroxy absorption peaks at 3443 and 3361 cm
1
have disappeared, which indicated that the carboxylic acid
groups of PA and hydroxyl of the n-butanol have trans-
formed into ester bond. As a result, the characteristic peaks
Among PCMs, the synthesized n-butyl palmitate was
1
determined by using the H NMR and FT-IR method. The
FT-IR characteristic peaks of the PA, n-butanol, and n-
butyl palmitate were shown in Fig. 1. At the FT-IR spec-
trum of the PA, the characteristic bands appeared at 3443,
-
1
exhibited at 1739 cm signifies stretching vibration of the
C=O groups of the n-butyl palmitate. The peak located at
-
1
-
1
1175 cm is assigned to stretching vibration of the C–O–
C band for the n-butyl palmitate. In conclusion, the n-butyl
palmitate was successfully synthesized.
2
921, 2850, 1703, 1467, 1294, 941, and 720 cm . The
-
peak at 3443 cm exhibited in the spectrum of PA indi-
1
cates the stretching vibrations of the hydroxyl group. The
1
-
1
-1
peaks at 2921 cm and 2850 cm represent the stretch-
The H NMR spectrum of n-butyl palmitate was shown
in Fig. 2. There was no peak of –OH group at about
ing vibration of –CH and –CH2 group. The peak at
3
-
1
2.24 ppm for the n-butanol. Furthermore, the proton of –
1
703 cm
represents the stretching vibration of C=O.
-
What the absorption band at 1467 cm is attributed to –
1
COO–CH – which is at 4.03 ppm for n-butyl palmitate is
2
related to the appearance of the peak, which play a role in
illustrating the esterification reaction, proved to be suc-
cessful. The number of H proton and chemical shift were in
accordance with the n-butyl palmitate, and all of the above
results verify that the n-butyl palmitate was also
synthesized.
CH asymmetrical bending vibration. The characteristic
2
-
1
-1
absorption bands at 941 cm and 720 cm are associated
with the out-of-plane bending vibration and the in-plane
swinging vibration of the hydroxyl group, respectively
[
16].
As for the n-butanol, the characteristic peaks are much
less due to the short molecular chain. The peak at
-
1
3
361 cm
exhibited in the spectrum of the n-butanol
indicates the stretching vibrations of the hydroxyl group.
123

L. Ma et al.
Table 1 Thermal characteristics of the PA and n-butyl palmitate
Melting process Cooling process
n-butyl palmitate
(
b)
-
1
-1
c
/J g
T
m
/K
4H
m
/J g
c
T /K
4H
PA
335.6
285.6
212.8
127.1
332.5
285.7
216.1
134.5
CH –(CH ) –CH –CH –COO–CH –CH –CH –CH
3
2
12
2
2
2
2
2
3
n-Butyl palmitate
(
a)
(b)
(d) (e)
(f)
(c)
(b)
(a)
suitable melting temperature. As seen in Table 1, the
melting temperature of PA that is 335.6 K could not be
discovered in n-butyl palmitate; this confirmed that there
was no residual PA in the prepared n-butyl palmitate. The
melting and crystallization temperatures of the n-butyl
palmitate are 285.6 K and 285.7 K, respectively. The phase
change latent heats of melting and crystallization of the n-
(
c)
(
a)
(
d)
(
e)
(
f)
6
5
4
3
2
1
0
– 1
-
1
-1
δ/ppm
butyl palmitate are 127.1 J g and 134.5 J g , respec-
tively. These properties make them become a promising
PCM to storage and release thermal energy for the LHTES
applications. In a word, these measurement results also
suggested that the highest potential of n-butyl palmitate lies
in low-temperature application such as food, pharmaceu-
tical, and biomedical [1].
1
Fig. 2 H NMR spectrum of the n-butyl palmitate
8
PA
n–butyl palmitate
Endo
4
0
4
Thermal conductivity of the synthesized n-butyl
palmitate as a PCM
To become a PCM which was widely used in the LHTES
applications, the thermal conductivity is an important
parameter, because the thermal conductivity of the PCM
directly affects the rate of thermal energy storage and
release [22]. The thermal energy storage and release could
not be achieved entirely if the thermal conductivity was
very low. The previous studies [16] reported that the
–
260
280
300
320
340
360
380
-
1
-1
Temperature/K
thermal conductivity of the PA was 0.17 W m
which was too low to be directly used in many fields. So
the high thermal conductivity materials
2–400 W m ) were added to PCMs to enhance
K ,
Fig. 3 DSC curves of the PA and n-butyl palmitate
-
1
-1
(
K
Thermal property of the synthesized n-butyl
palmitate as a PCM
thermal conductivity. The previous works [21, 23, 24]
reported that the activated carbon, expanded graphite,
carbon fiber, and graphene were added to the PCM,
exhibiting a superior thermal conductivity.
The DSC curves of the PA and n-butyl palmitate were
illustrated in Fig. 3. The DSC results were listed in
Table 1. The definition of phase change temperature and
the latent heat (DH) were reported in our previous study
The thermal conductivity of the n-butyl palmitate was
-1 -1
measured at the room temperature as 0.25 W m
K
(Fig. 4). Although it was higher than the PA, it was still too
[
11]. From the Fig. 3 and Table 1, the melting temperature
low to be used in practical applications. Some materials
with high thermal conductivity (expand graphite, graphene,
carbon nanotube, etc.) can be added to the n-butyl palmi-
tate to enhance the thermal conductivity and expand uti-
lization of the n-butyl palmitate. As a result, without
affecting the thermal properties and physicochemical
properties of the n-butyl palmitate, the thermal conduc-
tivity of the n-butyl palmitate should be increased as much
of the PA was higher than the n-butyl palmitate’s. The
latent heat value of n-butyl palmitate was lower than the
PA’s. However, these values obtained from DSC analysis
were still high enough to be a PCM for the latent heat
thermal energy system (LHTES) applications. Therefore, it
can be concluded that the n-butyl palmitate is well suited as
a PCM due to the high phase latent heat and a
1
23

Synthesis and characterization of the n-butyl palmitate as an organic phase change material
as possible. Moreover, it has been shown in Table 2 about
the results of thermal conductivity in terms of n-butyl
palmitate with that from some fatty acid ester in the liter-
ature [1, 14, 15, 25]. By comparing these data, it can be
noted that the n-butyl palmitate in this study has the
slightly higher thermal conductivity than those reported
fatty acid ester due to the different test conditions and
methods, crystallinity, and molecular chain orientation. As
a result, the synthesized n-butyl palmitate as an organic
PCM had great ability in terms of thermal energy storage
for the LHTES applications.
100%. However, the n-butyl palmitate still had superior
thermal stability during the phase change process due to its
phase-transition temperature below the 436.6 K.
Thermal reliability of synthesized n-butyl
palmitate as a PCM
A PCM must be thermally and chemically stable over a
large number of melting and freezing cycling. Therefore, it
was absolutely essential for no or less change in its prop-
erties in the long-term using process [16]. Thermal cycling
test was carried out to verify the change in thermal prop-
erties of the n-butyl palmitate with concerning 500 thermal
cycles and the results were shown in Table 3. Figure 6
depicts the DSC curves of n-butyl palmitate before and
after 500 thermal cycles. From the DSC curves, it is a little
changed for the melting temperature and latent heat. The
Thermal stability of the synthesized n-butyl
palmitate as a PCM
The TG as well as derivative TG (DTG) curves which
belongs to the PA and n-butyl palmitate has been shown in
Fig. 5. The analysis method is referred to the previous
report [26, 27]. As can be seen from the Fig. 5, the mass
loss temperatures (at 5% degradation) of the PA and n-
butyl palmitate occurred at 477.4 K and 436.6 K, respec-
tively. The maximum degraded temperatures of the PA and
n-butyl palmitate occurred at 540.6 K and 524.9 K,
respectively. The appeared mass loss was attributed to the
degradation of the PA and n-butyl palmitate. The mass
losses of the PA and n-butyl palmitate are approximately
5
477.4 K
100
95%
0
4
36.6 K
8
6
4
2
0
0
0
0
0
PA
– 5
– 10
n–butyl palmitate
–
–
–
–
–
15
20
25
30
35
524.9 K
540.6 K
600
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
0
.25
300
400
500
700
800
900
Temperature/K
Fig. 5 TG and DTG curves of the PA and n-butyl palmitate
0
.17
Table 3 Thermal characteristics of the n-butyl palmitate before and
after 500 thermal cycles
Melting process
Cooling process
-
1
-1
c
/J g
T
m
/K
4H
m
/J g
T
c
/K
4H
Before 500 thermal
cycles
285.6 127.1
286.5 113.5
285.7 134.5
285.4 128.0
PA
n-butyl palmitate
After 500 thermal
cycles
Fig. 4 Thermal conductivities of the PA and n-butyl palmitate
Table 2 Comparison of thermal
Fatty acid esters
Thermal conductivities/
-1 -1
References
conductivity of n-butyl
palmitate with that of fatty acid
W m
K
ester in the literatures
Glycerol tripalmitate
0.17
0.16
0.19
0.23
0.25
[14]
Erythritol tetrapalmitate
Galactitol hexa palmitate
Butyl stearate
[15]
[1]
[25]
n-Butyl palmitate
This work
123

L. Ma et al.
6
segment rupture and recombination for the phase change
material, there is no new chemical reaction observed. As a
result, the synthesized n-butyl palmitate had relatively
excellent thermal reliability during utility period.
Endo
Before thermal cycles
After thermal cycles
4
2
Conclusions
0
2
4
In this study, a type of PA ester was investigated as an
organic PCM for LHTES. This PA ester called n-butyl
palmitate was synthesized by the esterification reaction of
PA with n-butanol. The prepared n-butyl palmitate and its
components were systematically characterized by the FT-
–
–
2
60
280
300
Temperature/K
320
340
1
IR, H NMR, DSC, Hot-disk, and TG techniques. The
results revealed that the PA and n-butanol have reacted
fully due to the new ester bond appeared. The DSC results
suggested that the n-butyl palmitate had a suitable temper-
atures and high latent heats for melting or cooling. It is
further explained that synthesized n-butyl palmitate can be
utilized as a PCM for LHTES. Furthermore, the FT-IR and
DSC analyses results displayed that there is no any
degradation occurred in the chemical structure of the n-
butyl palmitate after the duplicated 500 thermal cycles and
no any significant change in thermal properties of the n-
butyl palmitate, respectively. Therefore, as long as the
thermal conductivity was enhanced by adding high thermal
conductivity materials, the n-butyl palmitate can be used in
many applications due to their suitable phase change
temperature, satisfactory thermal properties, and good
thermal reliability.
Fig. 6 DSC curves of the n-butyl palmitate before and after 500
thermal cycles
Before thermal cycles
After thermal cycles
Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (31570572, 31670516
and 31600459).
4
000 3600 3200 2800 2400 2000 1600 1200 800
400
Wavenumber/cm–1
Fig. 7 FTIR spectra of n-butyl palmitate before and after 500 thermal
cycles
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