Sustainable process for the production of symmetrical dibasic acid ester-based thermal energy storage materials from lipids

Article abstract of DOI:10.3906/kim-1811-44

Oleochemical-based long-chain diesters are potential organic phase change materials convenient for thermal energy storage. The development of efficient and green methods is an important requirement for the preparation of these environmentally friendly compounds, which also are finding wide industrial application. Thus, in the present work, a series of dibasic acid esters were synthesized through ionic liquid catalyzed scalable chemical esterification or transesterification reactions in a solvent-free medium. The starting compounds, dicarboxylic acids or their methyl esters and fatty alcohols or cholesterol, were reacted in the ratio of 1:2 respectively and the synthesized esters were characterized by FT-IR, ¹ H NMR, and ¹³ C NMR.

Full text of DOI:10.3906/kim-1811-44

Turk J Chem
Turkish Journal of Chemistry
(2019) 43: 802 – 808
© TÜBİTAK
http://journals.tubitak.gov.tr/chem/
doi:10.3906/kim-1811-44
Research Article
Sustainable process for the production of symmetrical dibasic acid ester-based
thermal energy storage materials from lipids
Ayhan YILDIRIM∗,, Kaan KIRAYLAR,
Department of Chemistry, Faculty of Arts and Science, Bursa Uludağ University, Bursa, Turkey
Received: 22.11.2018
•
Accepted/Published Online: 27.02.2019
•
Final Version: 11.06.2019
Abstract: Oleochemical-based long-chain diesters are potential organic phase change materials convenient for thermal
energy storage. The development of efficient and green methods is an important requirement for the preparation of
these environmentally friendly compounds, which also are finding wide industrial application. Thus, in the present
work, a series of dibasic acid esters were synthesized through ionic liquid catalyzed scalable chemical esterification or
transesterification reactions in a solvent-free medium. The starting compounds, dicarboxylic acids or their methyl esters
and fatty alcohols or cholesterol, were reacted in the ratio of 1:2 respectively and the synthesized esters were characterized
1
13
by FT-IR, H NMR, and C NMR.
Key words: Diester, green chemistry, ionic liquid, organic phase change materials, thermal energy storage
1. Introduction
Along with the rapid political, economic, and technological developments observed worldwide, the demand for
1,2
energy is increasing day by day.
Therefore, it is important to meet this demand and generate energy by
more environmentally friendly processes. In many countries today, a large part of energy production is from
fossil fuels. However, this method has several disadvantages, the most important of which is that fossil fuels
are very costly and not renewable resources. On the other hand, the negative effects of nuclear power plants
on the environment and human health are well known. Therefore, one of the ways to overcome these problems
encountered in energy production is to reduce the loss and excess consumption by storing energy.
As it is essential to explore new energy sources, it is also important to develop cheap and effective
energy storage materials. Recently, many materials capable of storing thermal energy have been researched
3
and developed. While some of these materials are suitable for use as energy storage materials, some are not
suitable due to their low chemical stability and high cost. Among these materials, some specific nonparaffin
organic compounds are particularly notable. Fatty acids as well as their derivatives are significant renewable raw
4
materials for the chemical industry. Recently, the latent thermal energy storage potential of these compounds
5−12
has been reported.
As is known, phase change materials (PCMs) are inorganic or organic compounds
13
that absorb and release latent thermal energy during melting and freezing processes. Fatty acids and their
derivatives possess some superior properties among such organic compounds. The preparation methods of these
compounds are inexpensive and generally fatty acids and their derivatives have no toxic properties. In addition,
these compounds have low corrosive effects and they are highly resistant to chemical degradation under storage
14
conditions.
∗Correspondence: yildirim@uludag.edu.tr
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This work is licensed under a Creative Commons Attribution 4.0 International License.

YILDIRIM and KIRAYLAR/Turk J Chem
Several studies have reported that diesters derived from fatty acids and fatty alcohols can be used as
15,16
thermal energy storage materials.
Figure 1 shows the principle of PCMs. In this context, developing suitable
methods for the synthesis of these compounds is an important goal. To prepare long-chain diesters, there are
17−20
only a few methods available in the literature and they have certain disadvantages.
The use of aromatic
toxic solvents such as toluene and azeotropic removal of water formed in the reaction, use of aggressive reagents
such as SOCl2 , and difficult stages in product purification are only some of them. On the other hand, in the
transesterification reactions carried out in basic medium, the formation of by-products decreases the diester
yield. Therefore, it is important to investigate the synthesis of these esters with efficient and environmentally
friendly methods.
Figure 1. Principle of PCMs.
2. Results and discussion
In order to achieve maximum yield in esterification reactions, usually one of the reagents is used in excessive
amounts. Another method is to remove the water formed during the reaction by Dean–Stark apparatus
azeotropically from the medium. In this case, however, it is generally necessary to use toxic aromatic solvents
17,18
such as toluene, xylene, or mesitylene.
In alternative methods, more active derivatives of carboxylic acids
19
are used in the esterification reactions. It is therefore necessary to develop more environmentally friendly and
practical methods of esterification and transesterification reactions.
In the present study, some long-chain dibasic carboxylic acid esters that may be potent PCMs were
prepared from long-chain dicarboxylic acids or dimethyl esters thereof and fatty alcohols or cholesterol by
esterification or transesterification reactions (Schemes 1 and 2).
The IR spectrum of 1a shows the characteristic band for the ester group at 1733 cm−1 . Some of the
1
characteristic H NMR peaks of 1a are a triplet at δ 4.07 ppm for the 2xCH2 O protons, a triplet at δ 2.30
ppm for the 2xCH2 CO protons, and a quintet at δ 1.61 ppm for the 2xCH2 CH2 O and 2xCH2 CH2 CO protons
13
(Figure 2). On the other hand, the C NMR spectrum showed a peak at δ 173.98 ppm for the carbon atom
of the ester functional group.
The IR spectrum of 2a shows the characteristic band for the ester group at 1733 cm−1 . Some of the
1
characteristic H NMR peaks of 2a are a triplet at δ 5.39 ppm for the cholesteryl CH= protons, a multiplet
at δ 4.66–4.60 ppm for the cholesteryl CHO protons, a triplet at δ 2.30 ppm for CH2 CH2 COO protons of the
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YILDIRIM and KIRAYLAR/Turk J Chem
Scheme 1. Synthesis of fatty alcohol-based diesters.
Scheme 2. Synthesis of cholesterol-based diesters.
13
acid chain, and a doublet of doublets at δ 0.89 ppm for the cholesteryl CH(CH3)2 protons. The C NMR
spectrum showed characteristic peaks at δ 173.30 ppm for the carbon atom of the ester functional group, and
at δ 139.73 and 122.61 ppm for the cholesteryl olefinic carbon atoms. Cholesterol found in living cells has many
functions, mainly as hormones. Symmetric liquid crystals based on cholesterol can find different uses due to
21−25
their unique structure
(Figure 3).
As is well known, ionic liquids, especially those functionalized with a –SO3 H group, are widely used as
26
catalysts in organic synthesis. The Brønsted acid ionic liquid catalyst used in the present study was previously
27
synthesized and characterized in our laboratory.
The optimum reaction conditions for the esterification
reactions performed are given in Table 1. The highest diester yield was achieved under the conditions given in
entry 3. While extension of the reaction time did not change the yield of the product, a reduction in the amount
of catalyst by half resulted in a slight increase in yield. On the other hand, transesterification reactions were
also carried out under similar conditions of esterification, but with slightly lower yields of diesters (Table 2). In
both ester synthesis methods, cholesterol esters were obtained at lower yields than fatty alcohol esters, because
the cholesterol molecule is a sterically hindered molecule. The minimum amount of toluene was used as the
solvent in order to reduce the amount of colored impurities formed in the esterification and transesterification
reactions of cholesterol.
In order to investigate the synthesized catalyst’s reusability, sebacic acid and lauryl alcohol were esterified
under solventless conditions in the presence of catalyst at 110 °C for 16 h. The crude diester dilauryl sebacate
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YILDIRIM and KIRAYLAR/Turk J Chem
1
Figure 2. H NMR spectrum of 1a.
Figure 3. Molecular structure of cholesterol-based diester 2a.
obtained as a result of the esterification reaction was purified by crystallization from THF/MeOH to give pure
diester 1a with 83% yield. The resulting filtrate was concentrated under reduced pressure and the residue
was dissolved in a minimum amount of diethyl ether and then the catalyst was precipitated with hexane and
used without further purification for the next esterification procedure under the same reaction conditions. The
experimental results obtained are given in Figure 4. According to the yields of the esterification reaction
obtained after crystallization from MeOH/THF, it is seen that the efficiency of the catalyst after three uses is
reduced.
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YILDIRIM and KIRAYLAR/Turk J Chem
Table 1. Optimization of conditions for the esterification reaction.
a
Entry Catalyst loading Time (h) Temperature (°C) Yield (%)
1
2
3
5 mmol %
5 mmol %
2.5 mmol %
16
24
16
110
110
110
81
81
83
a
Yields obtained after crystallization from THF/MeOH.
Table 2. Diesters obtained by the esterification or transesterification procedures.
a
a
Entry Diester Name
Yield (%) (Esterification) Yield (%) (Transesterification)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
Dilauryl sebacate
Dimyristyl sebacate
Distearyl sebacate
Dilauryl dodecanedioate
Dimyristyl dodecanedioate
Distearyl dodecanedioate
Dicholesteryl sebacate
83
90
70
81
88
99
60
43
68
84
53
64
94
58
51
b
b
2a
2b
Dicholesteryl dodecanedioate 56
a
b
Yields obtained after crystallization from THF/MeOH. Small amounts of toluene were used as a reaction solvent.
Figure 4. Reusability of the ionic liquid catalyst in the esterification reaction.
On the other hand, it was also investigated whether the developed methods are suitable for large-scale
preparation of related diesters. Therefore, 15 g of sebacic acid was reacted with 27.6 g of lauryl alcohol in the
presence of 2.5 mmol % of the catalyst. For the esterification reaction, the above reagents were heated at 110
°C for 16 h, and 32.1 g of dilauryl sebacate diester was obtained (81% yield).
2.1. Conclusions
For the synthesis of a series of long-chain diesters as potential PCMs, long-chain dicarboxylic acids and fatty
alcohols or cholesterol at a ratio of 1:2 and in the presence of an ionic liquid catalyst were chemically esterified
in a solvent-free medium. Relevant products were obtained in fairly good yields even after crystallization. The
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corresponding compounds were also synthesized from dicarboxylic acids or dimethyl esters and fatty alcohols
or cholesterol under similar conditions via transesterification reactions. In addition, catalyst reuse studies and
large-scale synthesis of diesters were carried out and as a consequence the catalyst used in this work is also
suitable for scale-up synthesis.
3. Experimental
3.1. Reagents and chemicals
All reagents and solvents were purchased from either Merck or Sigma-Aldrich (St. Louis, MO, USA) and used
without further purification. Thin-layer chromatography (TLC) was performed using silica gel plates (60 F254,
Merck, Darmstadt, Germany). The catalyst 1-hexadecyl-3-sulfo-1H -imidazol-3-ium chloride [C16 ImSO3 H]Cl
27
was prepared according to the procedure given in the literature.
3.2. Analytical techniques
Melting points were recorded by BÜCHI melting point B-540 apparatus (BÜCHI Labortechnik AG, Flawil,
Switzerland). The IR spectra were measured using a PerkinElmer Spectrum 100 spectrometer, while the NMR
spectra were measured using a Bruker Avance NEO 600 MHz NMR spectrometer (Santa Clara, CA, USA) using
tetramethylsilane (TMS) as internal standard. Chemical shifts (δ) are reported in ppm and J values in Hertz.
The elemental analyses were performed using an LECO CHNS-932 elemental analyzer (St. Joseph, MI, USA).
3.3. General esterification procedure
Into a reaction flask dicarboxylic acid and fatty alcohol or cholesterol were added at the ratio of 1:2, respectively.
After addition of the ionic liquid catalyst (2.5 mmol %) the resulting mixture was heated with stirring at 110 °C
for 16 h. At the end of this, the flask was cooled to room temperature and the resulting solid reaction mixture
was crystallized from THF/MeOH to afford pure crystalline product.
3.4. General transesterification procedure
Into a reaction flask dicarboxylic acid dimethyl ester and fatty alcohol or cholesterol were added at the ratio
of 1:2, respectively. After addition of the ionic liquid catalyst (2.5 mmol %) the resulting mixture was heated
with stirring at 110 °C for 16 h. At the end of this, the flask was cooled to room temperature and the resulting
solid reaction mixture was crystallized from THF/MeOH to afford pure crystalline product.
Acknowledgment
This work was financially supported by the Bursa Uludağ University Scientific Research Projects Unit (OUAP(F)-
2018/3).
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