Selective synthesis of glyceryl monolaurate intensified by boric acid based deep eutectic solvent

Article abstract of DOI:10.1016/j.cattod.2021.05.018

Glyceryl monolaurate (GML), as one of widely used industrial chemicals, can be synthesized via Fischer esterification of lauric acid (LA) and glycerol (GL). In this work, we screened boric acid-based deep eutectic solvent (DES) formed from tetrapropylammonium bromide (TPAB) and boric acid to intensify the selective Fischer esterification through conductor-like screening model for realistic solvents (COSMO-RS) theory. The solid liquid equilibrium (SLE) phase behavior, hydrogen bond formation process, and thermal stability of the DES were characterized by differential scanning calorimetry (DSC), Fourier transform infrared spectrometry (FT-IR), and thermogravimetric analyzer (TGA). The results verified that equimolar TPAB and boric acid formed DES [TPAB:B(OH)₃] with the lowest freezing point and very good thermal stability could efficiently intensify the Fischer esterification of GL and LA to GML. The hydrogen bond accepter TPAB of the DES made the esterification system homogeneous, which promoted the selective esterification process. In addition, the hydrogen bond donor B(OH)₃ of the [TPAB:B(OH)₃] combined with diol unit of GL to cyclic boric esters and led to a special selective space effect, which also improved the selectivity to GML. Thus, the esterification intensified of GL and LA by 20 wt% DES achieved GML selectivity of 85.7 % and yield of 76.4 % under the optimal reaction conditions. The esterification kinetics suggested the reaction followed pseudo-second-order model and the reaction activation energy was 33.98 kJ mol^?1, similar to and/or lower than those reported works. Furthermore, DES and unreacted glycerol were readily recovered and reused through temperature controlled gravity-based separation due to the ability of homogeneity under reaction conditions and splitting phase behavior during the DES recovery. The results can expand the boric acid-based DESs for other potential intensification applications.

Full text of DOI:10.1016/j.cattod.2021.05.018

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Selective synthesis of glyceryl monolaurate intensified by boric acid based
deep eutectic solvent
Zilong Shen, Lifang Chen*, Hongye Cheng, Zhiwen Qi*
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Glyceryl monolaurate (GML), as one of widely used industrial chemicals, can be synthesized via Fischer ester-
ification of lauric acid (LA) and glycerol (GL). In this work, we screened boric acid-based deep eutectic solvent
(DES) formed from tetrapropylammonium bromide (TPAB) and boric acid to intensify the selective Fischer
esterification through conductor-like screening model for realistic solvents (COSMO-RS) theory. The solid liquid
equilibrium (SLE) phase behavior, hydrogen bond formation process, and thermal stability of the DES were
characterized by differential scanning calorimetry (DSC), Fourier transform infrared spectrometry (FT-IR), and
thermogravimetric analyzer (TGA). The results verified that equimolar TPAB and boric acid formed DES [TPAB:B
(OH)3] with the lowest freezing point and very good thermal stability could efficiently intensify the Fischer
esterification of GL and LA to GML. The hydrogen bond accepter TPAB of the DES made the esterification system
homogeneous, which promoted the selective esterification process. In addition, the hydrogen bond donor B(OH)3
of the [TPAB:B(OH)3] combined with diol unit of GL to cyclic boric esters and led to a special selective space
effect, which also improved the selectivity to GML. Thus, the esterification intensified of GL and LA by 20 wt%
DES achieved GML selectivity of 85.7 % and yield of 76.4 % under the optimal reaction conditions. The ester-
ification kinetics suggested the reaction followed pseudo-second-order model and the reaction activation energy
was 33.98 kJ molꢀ 1, similar to and/or lower than those reported works. Furthermore, DES and unreacted
glycerol were readily recovered and reused through temperature controlled gravity-based separation due to the
ability of homogeneity under reaction conditions and splitting phase behavior during the DES recovery. The
results can expand the boric acid-based DESs for other potential intensification applications.
Esterification
Lauric acid
Glycerol
Monolaurate
Deep eutectic solvents
COSMO-RS
1. Introduction
esterification of LA and GL for improving value added product and
facilitating product purification.
The esterification of renewable resources, glycerol (GL) and fatty
acids, to produce long-chain esters is a sustainable process due to the low
cost of biomass-derived feedstock [1]. Among long-chain esters, glyceryl
monolaurate (GML) containing both hydroxyl groups and long-chain
alkyl groups, has wide applications as nonionic surfactant, emulsifier,
antibacterial agent, and thickener [2,3]. Generally, the esterification of
lauric acid (LA) and GL to GML can be catalyzed by homogeneous strong
acid catalysts at high temperature [4]. Due to the thermodynamic
immiscibility of LA and GL, the esterification process is separated into
two phases, the lower GL phase and the upper LA phase [5]. GML readily
reacts with LA to produce di-ester, and di-ester further reacts with LA to
obtain tri-ester in the upper phase. The monoester selectivity is limited
to 40–50 % owing to the formation of di- and tri- esters [6,7]. Therefore,
it would be desirable to synthesize GML through highly selective
According to the viewpoint of Szeląg and Zwierzykowski, the ester-
ification of GL and LA follows the Fischer esterification mechanism [8].
The main product of the reaction is GML, but glyceryl dilaurate (GDL)
and glyceryl trilaurate (GTL) are readily obtained because of the further
esterification of GML, as shown in Fig. 1. Besides conventional strong
acids, various types of acidic catalysts have also been developed and
applied in GML synthesis from the esterification of GL and LA, for
example, ion exchange resins, organo-montmorillonites, zeolites mo-
lecular sieves, heteropolyacids, acidic ionic liquids and sulphonated
hydrothermal carbon [9–14]. However, these solid acid catalysts have
mass transfer limitation ascribing to the presence of solid-liquid-liquid
three phases. Ionic liquids as solvent and catalyst have been widely re-
ported for the esterification of long-chain esters, such as butyl hexanoate
[15] and biodiesels [16,17]. However, high costs in preparation and
* Corresponding authors.
E-mail addresses: lchen@ecust.edu.cn (L. Chen), zwqi@ecust.edu.cn (Z. Qi).
https://doi.org/10.1016/j.cattod.2021.05.018
Received 11 March 2021; Received in revised form 25 April 2021; Accepted 31 May 2021
Available online 10 June 2021
0920-5861/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: Zilong Shen, Catalysis Today, https://doi.org/10.1016/j.cattod.2021.05.018

Z. Shen et al.
Catalysis Today xxx (xxxx) xxx
purification, humidity sensitivity, and low biodegradability limit the
application of ionic liquids in the esterification of LA with GL.
significantly improved (Fig. 2) [41,42]. However, Manhas et al.
attempted to directly apply the boric acid mediated esterification and
the monoester selectivity could not be notably improved [42]. Thus, the
utilization of hydrogen bond donor ability and the restriction on diol
unit, it would be a main challenge to design boric acid-based DESs for
the intensification of the highly selective GL esterification to monoester.
Herein, we designed four boric acid-based DESs combined with
different chain length of quaternary ammonium salts to intensify the
selective esterification of LA and GL to GML. Conductor-like screening
model for realistic solvents (COSMO-RS) calculation and experiment
verification showed the DES formed from tetrapropylammonium bro-
mide (TPAB) and B(OH)3 was the best solvent for the production of
monoester from the selective esterification. The equimolar TPAB and B
(OH)3 formed DES [TPAB:B(OH)3] with the lowest freezing point and
very good thermal stability could efficiently intensify the Fischer
esterification of GL and LA to GML. The optimal reaction conditions
were been considered by performing various monofactor analysis. In
addition, the esterification kinetics were been investigated and gravity-
based separation of temperature controlled DES and unreacted LA were
also been evaluated.
In 2003, a binary mixture of urea and quaternary ammonium salts
formed liquid phase at room temperature and was firstly defined deep
eutectic solvents (DESs) [18]. Since then, DESs have received extensive
attention as a new class of ionic liquids analogues because they have
similar properties to ionic liquids [19–21]. Furthermore, DESs can
overcome the drawbacks of ionic liquids such as complicated synthetic
approach, high costs, poor biocompatibility, and corrosivity. In general,
DESs are prepared by facile mixing and heating hydrogen bond acceptor
(HBA, e.g. quaternary ammonium or phosphorus salt) and hydrogen
bond donor (HBD, e.g. carboxylic acid or amide) with appropriate molar
ratio [22]. Additionally, the property of DESs depends on the category of
the composed HBAs and HBDs [23]. Acidic DESs can be divided into
metal chlorides containing Lewis acidic DESs such as aluminum chloride
and zinc chloride [24,25], and Brønsted acidic DESs represented
composition including p-toluenesulfonic acid (PTSA) and benzene-
sulfonic acid (BSA) [26,27]. As solvent and catalysts, acidic DESs have
been widely applied to intensify acid catalyzed esterification reaction.
For example, Sunitha et al. have prepared choline chloride (ChCl) and
ZnCl2 with molar ration of 1:2 formed Lewis acidic DES [ChCl:2ZnCl2] as
dual solvent and catalyst for the esterification of wax esters with a high
yield of 93 % [24]. Carboxylic acids, panthenol, and methanol con-
taining DESs as excellent reaction media have been reported for the in
situ formation of various esters [28–30]. Our group previously used
imidazole (IM) and PTSA formed bifunctional acidic DES [IM:2PTSA]
for Prins condensation of styrene with formaldehyde, the synthesis of
isobutyl isobutyrate from isobutyric acid and isobutanol, and basic DES
[3IM:PTSA] for efficient Knoevenagel condensation at room tempera-
ture [31–33]. In addition, we also synthesized IM and BSA formed DESs
as acidic catalyst and solvent for the dehydration of fructose to
5-hydroxymethylfurfural [34]. PTSA-based DESs were reported for the
esterification of GL with oleic acid and/or palmitic acid, however the
catalytic activities were high and the selectivity towards monoester were
low [35,36]. Nevertheless, DES with advantages of ionic liquids, facile
preparation methods, and low costs would be attractive to intensify the
esterification process of LA and GL.
2. Material and methods
2.1. Material
ChCl (≥ 99 %), tetraethylammonium bromide (TEAB, ≥ 98 %), TPAB
(≥ 98 %), tetrabutylammonium bromide (TBAB, ≥ 98 %), GML (≥ 98
%), and GDL (≥ 96 %) were all purchased from TCI (Japan). LA (≥ 99
%), GL (≥ 99.5 %), B(OH)3 (≥ 99.5 %), and hydrobromic acid (HBr-
water, 40 wt%) were obtained from Adamas-beta (China).
2.2. DES preparation and characterization
Boric acid-based DESs with equimolar boric acid four ammonium
salts (ChCl, TEAB, TPAB, and TBAB) were weighted and mixed. The
mixtures were placed in screw-mouth bottles and heated in the oil bath
until completely dissolved and transparent, as shown in Supporting In-
formation, Fig. S1. The Temperature was controlled in 100 ◦C by Huber
Ministat 230 with the precision of ±0.1 ◦C.
Boric acid is known as a medium-strength Lewis acid due to the
electron deficient nature of the boron atom and has three hydroxyl
groups with strong hydrogen bond donor ability [37]. As hydrogen bond
donor, B(OH)3 combined with HBA component can form weak acidic
DESs. A class of ternary DESs formed by B(OH)3, ChCl, and polyethylene
glycol offered high activity for extraction and oxidative desulfurization
of diesel fuel [38]. Vuoti et al. used ChCl and B(OH)3 formed DES [ChCl:
B(OH)3] to fractionate lignin and cellulose from wood biomass [39]. In
addition, B(OH)3 can also react with diol unit to generate cyclic boric
esters and lead to a special selective space effect [40]. Thus, the Fischer
esterification of polyhydric alcohols and long-chain fatty acids can be
intensified by B(OH)3 and the selectivity of monoesters may be
The freezing points of DESs were determined by differential scanning
calorimetry (DSC; Q2000, TA, USA). Typically, 5 mg of the sample into a
40 μl aluminum crucible was measured under N2 atmosphere of 50
mL∙minꢀ 1 from 40 to 200 ◦C with a heating rate of 5 ◦C minꢀ 1. The
Fourier transform infrared spectroscopy (FT-IR; Spectrum Two, Perki-
nElmer, USA) using pure KBr disc as the background reference was used
to investigate the structure and interaction of TPAB, boric acid, and DES
at ambient temperature. The range of spectra was between the wave-
numbers of 4000 and 400 cmꢀ 1. The thermal stability of DESs was
carried out on a thermo gravimetric analyzer (TGA; STA 449 F3,
Fig. 1. The esterification process of lauric acid and glycerol.
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Z. Shen et al.
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Fig. 2. Proposed strategy for boric acid promoted selective Fischer esterification.
Netzsch, DE) in flowing N2 atmosphere of 50 mL∙minꢀ 1 with the heating
homogeneous under reaction condition to phase splitting state after the
reaction mixture was cooled to 80 ◦C. The lower phase (DES phase)
included excessive GL and DES, and the upper phase (ester phase)
contained unreacted LA, products GML, GDL, GTL, and a little bit DES
from the lower phase. In order to recover the DES from the upper phase,
50 mL of 80 ◦C water was added into the mixture and used to extract DES
and GL. The water phase containing DES and GL was separated by a 125
mL separatory funnel. The extraction process with 3 times ensured that
the content of nitrogen in the ester phase was less than 0.02 %, which
could be measured by a chemiluminescence nitrogen analyzer (Antek
9000, USA). Then, the mixed water, GL, and DES were vacuum dis-
tillated by rotary evaporation for 8 h. To ensure the water removed
completely, the water content in the recovered DES phase was less than
0.20 % by a moisture analyzer (AQV-300 Karl Fischer, Hiranuma,
Japan). In the recycle experimental, the added amount of LA was 0.05
mol and supplied GL was calculated by consumed GL of the previous
round experimental to make sure the molar ratio of LA and GL was 1:4.
The added HBr was 0.24 wt% (0.2 g 40 % HBr-water) because HBr could
be completely removed through water washing and rotary evaporation.
The next reaction was proceeded similar to the method described above.
rate of 10 ◦C minꢀ 1 from room temperature to 600 ◦C.
2.3. COSMO-RS theory
COSMO-RS was used to calculate interactions energy among com-
ponents. The COSMO thermX (version C30_1401) software was applied
for screening of quaternary ammonium salts. The. cosmo files required
for calculation could be obtained directly from the database with BP86-
TZVP parametrization.
2.4. Procedure for esterification of LA and GL
The esterification of LA and GL was conducted by a 100 m-L three-
necked flask. Typically, 0.1ꢀ 0.25 mol GL and 0.05 mol LA with
different molar ratios (2:1, 3:1, 4:1, and 5:1), HBr dosage (0.12, 0.24,
and 0.36 wt%, wt% is weight percentage of total LA and GL weight), and
DES dosage with 10, 20, and 30 wt% of total LA and GL total weight
were charged with simple reflux condenser and the mixture was pre-
heated and controlled from 130 ◦C to 160 ◦C by oil bath with stirring
speed of 800 rpm.
3. Results and discussion
The liquid sample of 300 μL was carefully taken out with a pipette at
regular intervals and was cooled rapidly using ice water to avoid further
reaction. The samples were diluted by ethanol and analyzed by high
performance liquid chromatography (HPLC) gradient elution to deter-
mine the concentrations of LA, GML, GDL, and GTL. The HPLC mea-
surement was using LC-2010AHT (Shimadzu, Japan) equipped with a
UV–vis detector and separated by C-18 reverse column with the detec-
tion wavelength of 220 nm. The mobile phase was methanol-water at a
flow rate of 1.0 mL minꢀ 1 and the oven temperature was 45 ◦C. Detailed
information on the gradient elution and calibration for LA, GML, and
GDL were described in the Supporting Information with a representative
chromatogram as shown in Fig. S2. The conversion was calculated based
on the amount of LA converted into the formation of GML, GDL, and GTL
using external standard method and the selectivity was corrected by
reaction coefficients on the ratio of esters to all reaction products, as
shown in following Eqs. (1)–(4) [11,43].
3.1. Screening of boric acid-based DES
For the esterification, the upper phase is dominated by LA and the
lower phase is GL, while DES tends to dissolve in the lower GL phase.
The screening of the DES depends on its HBA ability which should have
interaction with LA due to the HBD ability of LA in order to enhance the
mutual solubility of both phases [44]. Boric acid has good HBD ability
and can form DESs with common quaternary ammonium salts, which
could be used to intensify the esterification. Our group have previously
compared a variety of chloride and bromide salts formed DESs for in situ
extraction of citrus essential oils [45]. The calculations showed that the
chloride salts exhibited good HBA ability, however, their strong mois-
ture absorptions couldn’t be ignored [46,47]. In addition, ChCl as one of
the most common HBAs was used to form typical DES. Based on the
esterification process of LA and GL, the screening DESs of HBA ability
are initially identified with ChCl and quaternary ammonium bromide
salts with different alkyl chain lengths, including TEAB, TPAB, and
TBAB. The COSMO-RS model was used to calculate the interaction be-
tween the above ammonium salts and LA, and then verify the screening
boric acid-based DES for the intensified esterification process.
CGML + 2CGDL + 3CGTL
ConLA(%) =
SGML(%) =
SGDL(%) =
SGTL(%) =
× 100
(1)
(2)
(3)
(4)
CLA + CGML + 2CGDL + 3CGTL
CGML
× 100
CGML + 2CGDL + 3CGTL
In the COSMO-RS theory, three types of molecular interaction en-
ergies, misfit, van der Waals (VDW), and hydrogen-bond (HB) in-
teractions can be calculated and obtained [48]. Positive and negative
energies represent repulsion and attraction respectively, and the larger
the absolute negative value means the stronger interaction. Fig. 3 is
calculated results of the interaction energies between quaternary
ammonium salts and LA. The HB energy is related to the ammonium salt
anion according to the intermolecular interaction. The HB energies of
three bromide salts and LA are roughly similar, while those of ChCl and
LA perform slightly better. As the length of the alkyl carbon chain in-
creases, the energy of misfit is ranked as ChCl > TEAB > TPAB > TBAB,
which shows that the longer carbon chain of the ammonium salt implies
2CGDL
× 100
CGML + 2CGDL + 3CGTL
3CGTL
× 100
CGML + 2CGDL + 3CGTL
2.5. Temperature controlled recovery of DES and unreacted GL
For the recovery of DES and unreacted GL, the unreacted glycerol
phase containing DES was separated, recovered, and reused in the next
reaction batch due to the excessive amount of glycerol. The reaction
system could be temperature controlled and changed from
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Z. Shen et al.
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predicted by COSMO-RS.
In order to verify the results of the COSMO-RS model, ChCl, TEAB,
TPAB, and TBAB formed DESs with boric acid were prepared to prefill
with molar rate 3:1 of GL and LA. The mixture was just heated to the
reaction temperature (150 ◦C) and kept at this temperature for 2 h
without any stirring, and the phase state distributions are shown in
Fig. 5. The esterification is difficult to occur due to the thermodynamic
immiscibility of LA and GL (Fig. 5a). With the addition of DES, [ChCl:B
(OH)3] and [TEAB:B(OH)3] can not form homogeneous reaction system
(Fig. 5b-c), while [TPAB:B(OH)3] and [TBAB:B(OH)3] can make the
reaction system homogeneous (Fig. 5d-e), which are consistent with the
calculated result of COSMO-RS. However, TBAB is readily decomposed
at high temperature, and the color of the mixture of [TBAB:B(OH)3], LA,
and GL changes from colorless to dark at the reaction temperature of 150
◦C for 3 h. Obviously, the long carbon chain structure of quaternary
ammonium bromide salt formed DES, [TBAB:B(OH)3] is not suitable for
the reaction temperature required for the esterification. Therefore,
[TPAB:B(OH)3] is preferable solvent to intensify the esterification based
on the theory calculation and present reaction conditions.
Fig. 3. The interaction energies of quaternary ammonium salts with lauric acid.
3.2. Characterization of [TPAB:B(OH)3]
the smaller repelling effect on LA. The total energy follows ChCl < TEAB
< TPAB ≈ TBAB, which indicate that TPAB and TBAB would be suitable
candidates from the total interaction.
DESs have lower freezing points relative to each component formed,
which depend on the molar ratios of HBA and HBD [52]. SLE diagram
can be obtained by measuring melting temperature of the screening DES
[TPAB:B(OH)3] with different molar ratios and the determined phase
behavior of the DES is shown in Fig. 6. Obviously, the lowest eutectic
point is 77 ◦C with the 1:1 mole composition of TPAB and boric acid,
which is much lower than the melting point of TPAB (252 ◦C) and boric
acid (171 ◦C) from the PubChem database. The freezing points of other
compositions with low concentration of B(OH)3 less than 50 % are lower
than that of TPAB, while the freezing points of DESs with high content of
B(OH)3 larger than 50 % are lower that of B(OH)3. Thus, the equimolar
DES [TPAB:B(OH)3] with the lowest freezing point is considered for the
intensification of the esterification reaction.
In addition, a COSMO-RS descriptor (σ-profile) based on molecular
structure information was calculated to analyze different intermolecular
interactions between compounds in the esterification system [49,50].
The
σ
-profile represents the distribution of molecular charge density (
σ
),
which is divided into three regions, nonpolar region (ꢀ 0.0082 e/Å2 <
σ
< 0.0082 e/Å2), HBD region (
σ
< ꢀ 0.0082 e/Å2), and HBA region (
σ
>
0.0082 e/Å2) [51]. As shown in Fig. S3, the strong peaks in the HBA
region indicate that the acceptor capacity of the ammonium salts is
provided by Brꢀ and Clꢀ for the four quaternary ammonium salts. In the
non-polar region, the non-polarity of the molecules increases with
increased carbon chain length of the ammonium salts. Fig. 4 shows
σ-profiles of each component in the esterification reaction, LA, GL, GML,
The DESs formation by hydrogen bond interaction between HBA and
HBD can be confirmed by FT-IR spectra of TPAB, boric acid, and [TPAB:
B(OH)3], as shown in Fig. 7. For typical TPAB, the peaks at 2860 and
2920 cmꢀ 1 are ascribed to the stretching vibration and asymmetric
stretching vibration of C–H, while those at 1480 cmꢀ 1 and 1381 cmꢀ 1
are assigned to N+ stretching in quaternary ammonium compounds
(Fig. 7a). The peaks at 971 cmꢀ 1 and 767 cmꢀ 1 contribute to asymmetric
bending vibrations of ꢀ CH2- and the ꢀ CH3 rocking vibration in the
ammonium salt [53]. As for B(OH)3, the broad peak in the region
3100ꢀ 3600 cmꢀ 1, the peaks at 1423, 1193, and 546 cm-1 correspond to
the ꢀ OH stretching, asymmetric Oꢀ Bꢀ O stretching, planar Bꢀ OH
bending, and Oꢀ Bꢀ O vibration bands of B(OH)3, respectively (Fig. 7b)
[53]. Combined equimolar B(OH)3 and TPAB, the broad ꢀ OH stretching
vibration shifted towards higher wavenumbers of 3390 and 3185 cm-1 in
the [TPAB:B(OH)3] indicates strong hydrogen bond formation between
the screening boric acid-based DES [TPAB:B(OH)3] and [TBAB:B(OH)3].
In the non-polar region, LA is located between the two DESs, while GML
has a better coincidence with DES [TBAB:B(OH)3]. However, [TBAB:B
(OH)3] possessing strong binding ability with product GML is not
conducive to the recovery of DES and the purification of GML. There-
fore, [TPAB:B(OH)3] can be used as a much appropriate DES to intensify
the esterification of LA and GL based on interaction energy and σ-profile
–
TPAB and B(OH)3 (Fig. 7c). Simultaneously, the planar B OH bending
and Oꢀ Bꢀ O vibration bands still maintain and cause a slight offset to
1183 and 521 cmꢀ 1. Therefore, the FT-IR results confirm the hydrogen
bond interaction between TPAB and boric acid, and the formation of the
DES [TPAB:B(OH)3].
Thermal stability of the DES is very important for the esterification at
a given reaction temperature and thus studied by thermogravimetric
and derivative thermogravimetry (TG-DTG). In general, boric acid starts
to dehydrate with heating over 100 ◦C and converts to metaboric acid
(HBO2) and then to boron oxide (B2O3) [54,55]. In comparison with
boric acid, the interaction between HBA and HBD can enhance the
thermal stability of the formed boric acid-based DESs [56]. The TG-DTG
of [TPAB:B(OH)3] shows three stages and the initial stage is a small
amount of free water evaporation with a gentle weight loss of 5% below
194 ◦C (Fig. 8). The result demonstrates that no much water loses in the
[TPAB:B(OH)3] due to the interaction of TPAB and B(OH)3. In the
Fig. 4.
σ-Profiles of LA, GL, GML, [TPAB:B(OH)3], and [TBAB:B(OH)3].
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Fig. 5. Phase states of the mixture of LA and GL without addition of DES (a) blank, with the addition of 20 wt% DESs (b) [ChCl:B(OH)3], (c) [TEAB:B(OH)3], (d)
[TPAB:B(OH)3], and (e) [TBAB:B(OH)3] at 150 ◦C for 2 h.
Fig. 8. TGA and DTG curves of [TPAB:B(OH)3].
Fig. 6. Solid-liquid equilibrium phase diagram of TPAB and B(OH)3. L and S
represent liquid and solid regions, respectively.
second stage, a weight loss of 80 % is observed with the temperature
heated to 260 ◦C, which indicates the decomposition of ammonium salt
and the loss of water from boric acid to metaboric acid. The residue
further loses water until 400 ◦C and only B2O3 remains in the third stage.
These results are in good agreement with Virtanen et al. reported, who
presented a weight loss of 10 % based on ChCl and B(OH)3 formed DESs
under different water content [54]. They found that the temperature of
weight loss of 10 % increases from 120 to 184 ◦C for pure boric acid and
DES [ChCl:B(OH)3] with a 5% of water. The decomposition of the DES
occurred in similar range indicates that [TPAB:B(OH)3] has good ther-
mal stability and is favorable for the reaction condition required for the
esterification of LA and GL.
3.3. Intensified esterification based on boric acid-based DES
Boric acid-based DES as solvent intensify the reaction while HBr as
Brønsted acid catalyst catalyze the esterification of GL with LA. The
intensified esterification of GL with LA based on boric acid-based DESs is
shown in Table 1. In the intensification process, HBr as catalyst provides
Brønsted H+ proton whereas residue Brꢀ anions doesn’t have any in-
fluence on the reaction due to the existence of Brꢀ ions in the DESs. LA
conversion is 43.6 % in reaction time of 8 h without the addition of
catalyst and DES, indicating an autocatalytic reaction (Entry 1). When
HBr is added and the required H+ proton is provided for the reaction, LA
Fig. 7. FT-IR spectra of (a) TPAB, (b) B(OH)3, and (c) [TPAB:B(OH)3].
conversion rises to 91.5 %. However, HBr also accelerates the further
esterification of the monoester and leads to low selectivity to GML with
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selectivity of GDL is much higher than that of GML (Entry 9), which is
similar to the research of Abbott et al. [57]. The reason maybe due to the
fact that the repelling effect between ChCl and LA inhibits the mass
transfer of reactants, reduces the reaction rate, and promotes the for-
mation of diester. For the [TEAB:B(OH)3], as the reaction system is also
biphasic, the intensification effect of [TEAB:B(OH)3] is better than that
of [ChCl:B(OH)3] owing to the increased interaction of TEAB and LA
(Entry 10). However, with increased chain length of quaternary
ammonium salts to TBAB, the TBAB in the DES [TBAB:B(OH)3] is readily
decomposed at the reaction temperature of 150 ◦C for a long reaction
time. Therefore, the results demonstrate the DES [TPAB:B(OH)3] is
desirable solvent to intensify the selective esterification of GL and LA to
GML, which is in good agreement with the evaluation of COSMO-RS
theory and experimental.
Table 1
Catalytic performances of the esterification of LA with glycerol GL.a.
LA
Selectivity (%)
GML
yield
(%)
Reaction
time (h)
Entry
Catalysts
conv.
(%)
GML
GDL
GTL-
1
2
3
4
Blank
HBr
8
8
8
8
43.6
91.5
43.3
91.1
82.9
53.6
60.9
51.9
17.1
46.4
39.1
48.1
–
36.1
49.0
26.7
47.3
trace
B(OH)3
–
–
B(OH)3
HBr
+
5
6
7
8
TPAB +
HBr
8
86.9
51.0
71.5
89.1
78.3
91.4
86.8
85.7
21.7
8.6
–
–
–
–
68.0
46.6
62.1
76.4
[TPAB:B
(OH)3]
[TPAB:B
(OH)3]
[TPAB:B
(OH)3] +
HBr
8
24
8
13.2
14.3
3.4. Influence of reaction parameters on the intensified esterification
9
[ChCl:B
(OH)3] +
HBr
8
8
55.5
82.5
38.4
53.9
61.6
46.1
trace
21.3
45.3
The DES [TPAB:B(OH)3] intensified esterification of GL with LA
catalyzed by HBr was further investigated to produce GML at the opti-
mized reaction time of 8 h. Since the GML yield and selectivity are
affected by not only the DES dosage and reaction temperature but also
the amounts of catalyst and reactant molar composition, the GML yield
as function of reaction time was carried out by just varying one variable
under constant reaction conditions of 0.05 mol LA and 0.2 mol GL, 0.24
wt% HBr, 20 wt% DES dosage, reaction temperature of 150 ◦C, and
reaction time of 8 h.
10
[TEAB:B
(OH)3] +
HBr
–
a
Reaction conditions: 0.05 mol LA, 0.2 mol GL, molar ratio of GL:LA (4:1), 20
wt% DES dosage, 0.24 wt% HBr, and reaction temperature of 150 ◦C.
The effect of H+ proton concentration on the esterification activity
by HBr as the representative catalyst is shown in Figs. 9a and S4. LA
conversion and GML yield are low without the addition of HBr and the
presence of a little amount of H+ can greatly promote esterification.
When H+ proton concentration increases from 0.12 wt% to 0.36 wt% at
the reaction time of 3 h, the LA conversion increases from 80.8 %–89.0
% due to the increased number of acid sites available for the reaction.
The LA conversion gradually achieves the reaction equilibrium with
excessive reaction time of 3 h. Excessive acid sites lead to further
esterification of GML, and thus the GML yield starts to decrease because
of the formation of GDL. When H+ proton concentration is 0.24 wt%, the
reaction time of the reaction equilibrium increases whereas the GDL
yield maintains high values of about 89.0 %. Similar results and trend
have been demonstrated previously for the esterification of GL and LA
using propyl sulfonic acid post-synthesis functionalized SBA-15 and
silica gel-based sulfonic acid functionalized ionic liquid as catalysts [11,
12]. Therefore, the catalyst amount is 0.24 wt% for the esterification of
GL and LA with optimal LA conversion and GML yield.
only 53.6 %. The decreased selectivity to GML is due to the esterification
including main and side reactions presented in the upper phase. The
addition of boric acid in either the blank or the catalysis system hasn’t
any influence on the reaction process, which has similar activity to
Entries 1 and 2 (Entries 3, 4). It is beneficial to improve the selectivity of
GML since the addition of TPAB (HBA) made the catalytic system ho-
mogeneous, owing to that the esterification transforms from an interface
reaction to homogeneous catalysis (Entry 5).
In addition, the HBD B(OH)3 of the DES [TPAB:B(OH)3] can also
combine with diol unit of GL to generate cyclic boric esters and lead to a
special selective space effect, which also improve the selectivity of GML.
Since the theoretical calculation and experimental show that boric acid-
based DESs may intensify the esterification, the addition of [TPAB:B
(OH)3] alone can received GML selectivity as high as 91.1 % after 8 h of
reaction time (Entry 6). The LA conversion is only 51.0 %, slightly
higher than that of the blank experiment. Further extending the reaction
time to 24 h, the conversion increases to 71.5 % and the selectivity of
GML reduces to 86.8 % (Entry 7). It is obvious that the medium-strength
acidity of Lewis boric acid cannot promote the Fischer esterification.
Thus, a small amount of HBr added can catalyze the esterification re-
action with the intensification of the DES [TPAB:B(OH)3] (Entry 8).
Notably, both LA conversion and monoester selectivity achieve prom-
ising results by the intensification of the DES [TPAB:B(OH)3]. The LA
conversion is 89.1 %, the GML selectivity is 85.7 %, and the GML yield is
high up to 76.4 % with reaction time of 8 h.
Reaction temperature is a very important factor for the Fischer
esterification because the reaction temperature can change kinetic
mobility of reactant molecules and thus alter contact frequency with
active sites [58]. The reaction rate can be accelerated by increased re-
action temperature, and the reaction temperature also affects equilib-
rium state of the esterification. The influence of the reaction
temperature carried out at 130, 140, 150, and 160 ◦C under fixed re-
action conditions is shown in Fig. 9b. The reaction temperature signif-
icantly influence the GML yield increases from 70.0 % to 76.3 % under
increased reaction temperature from 130 to 150 ◦C with reaction time of
8 h. However, the GML yield decreases to 68.1 % with further increased
reaction temperature up to 160 ◦C. The high reaction temperature can
accelerate molecular motion, improve contact frequency, and reduce
mass-transfer resistance of reactant molecules according to molecular
dynamic theory [11,12,59,60]. However, it also accelerates the thermal
motion of product molecules and thus increases further esterification of
GML with LA, leading to decreased GML yield. Accordingly, it is indic-
ative that the intensified esterification of GL and LA based on the DES
[TPAB:B(OH)3] over the HBr catalyst would be eminent at the reaction
temperature of 150 ◦C in consideration of GML yield under other con-
stant reaction condition.
For esterification of GL and LA, Brønsted acids are usually used as
active sites to catalyze this reaction, however, GML selectivity and yield
are low due to the formation GDL and GTL. For example, PTSA as
conventional homogeneous catalyst has been used to catalyze the
esterification of GL with LA. When the LA conversion is 10 %, GML
selectivity is 65 %. However, the GML selectivity reduces to 49 % with a
LA conversion of 50 % [4]. In addition, GML yield is only 44 % in the
presence of PTSA under reaction conditions of GL:LA = 1:1, 5 wt%
catalyst relative to GL at 112 ◦C for 4 h [9]. Therefore, the DES [TPAB:B
(OH)3] with dual effect of HBA and HBD can efficiently intensify the
esterification and improve the selectivity towards GML.
For comparison, four boric acid-based DESs combined with different
chain length of quaternary ammonium salts were investigated to
intensify the selective esterification of LA and GL to GML. With the
addition of [ChCl:B(OH)3], the LA conversion is only 55.5 % and the
The esterification is a reversible reaction and molar ratio of alcohol
6

Z. Shen et al.
Catalysis Today xxx (xxxx) xxx
Fig. 9. GML yield as function of reaction time at different (a) H+ proton concentration, (b) reaction temperature, (c) molar ratio of GL to LA, and (d) DES [TPAB:B
(OH)3] dosage. Reaction conditions: varying one variable under constant reaction conditions of 0.05 mol LA, 0.2 mol GL, 0.24 wt% of HBr, 20 wt% DES dosage.
to fatty acid would influence the reaction equilibrium. Generally, the
excessive stoichiometric proportion of GL and LA would be necessary,
which not only improves the reaction rate but also shifts the reaction
equilibrium towards the formation of esters. Herein, the effect of
different molar ratios of GL to LA from 2:1 to 5:1 is compared while
other reaction parameters keep constant, as shown in Figs. 9c and S5.
The effect of GL to LA molar ratio on LA conversion and GML yield is
significant when the molar ratio initially increases from 2:1 to 5:1,
especially at the reaction time from 2 to 8 h. The GML yield reaches the
maximum of 76.4 % with a LA conversion of 89.1 % at the GL/LA molar
ratio of 4:1. When the molar ratio of GL to LA increases up to 5:1, LA
conversion and GML yield are lower than those of the other molar ratios
in the initial reaction time of 2 h. when the reaction reaches the equi-
librium state, LA conversion with molar ratio of 5:1 is similar to 4:1,
however, GML yield is lower than that of 4:1. The decreased LA con-
version and GML yield are due to the fact that the amount of DES can no
longer make the system homogeneous and can’t intensify the esterifi-
cation under GL/LA molar ratio of 5:1. These results demonstrate the
intensification of the DES [TPAB:B(OH)3] for the esterification of LA and
GL, in agreement with aforementioned COSMO-RS theory.
0, 5 and 10 wt% DES dosages are still biphasic, while the systems with
20 and 30 wt% DES dosages are homogeneous with increased reaction
time from 2 to 8 h. Obviously, for the DES intensified esterification, the
LA converted to GML is favorable to form homogeneous phase, indi-
cating the intensification process is relative to the added amounts of the
DES [TPAB:B(OH)3], as shown in Figs. 9d and S6. In the initial stage of
the reaction, the LA conversion increases with increased DES dosage,
and the intensification effect of DES accelerates the reaction rate. When
DES dosage is 20 wt%, GML yield achieves the maximum of 76.4 %,
which is due to the intensification by optimal solvation of TPAB and a
special selective apace effect of B(OH)3 combined with diol unit of GL.
However, for esterification equilibrium, solvent effect may promotes
or inhibits the chemical equilibrium by thermodynamic investigation
[61]. In the absence of DES, LA conversion is 91.5 % at reaction time of 8
h, but it decreases to 84.8 % with addition of 30 wt% DES. Excessive
[TPAB:B(OH)3] has a negative effect on the chemical equilibrium con-
version, and thus the maximum yield appears with the addition of 20 wt
% DES.
In order to obtain optimized reaction time, the desired DES [TPAB:B
(OH)3] intensified esterification of GL and LA as function of reaction
time is show in Fig. 10. The LA conversion firstly increases sharply to
70.8 % at initial reaction time of 2 h, ascends slowly to 89.1 % at the
reaction time of 8 h, and approaches the reaction equilibrium with the
prolonging reaction time. The GML selectivity gradually decreases and
GDL yield slowly increases due to the further esterification of GML with
increased reaction time. GML yield has similar trend with LA conversion
at the reaction time of 8 h and achieves the maximum of 76.4 %. Then
GML yield starts to decline when the reaction time exceeds 8 h. Obvi-
ously, the overlong reaction time is favorable to the side reactions and
the optimized reaction time is 8 h based on the intensified reaction
process.
The intensification of the DES [TPAB:B(OH)3] on the synthesis of
GML can be divided into two functions. On the one hand, the solvent
effect of [TPAB:B(OH)3] creates a homogeneous system with excess
glycerol, which promotes selectivity of GML. On the other hand, boric
acid of the DES [TPAB:B(OH)3] combined with diol unit of GL to
generate cyclic boric esters leads to a special selective space effect,
which also improve the selectivity of GML. Since the intensification of
the DES [TPAB:B(OH)3] depends on the HBA and HBD, the effect of DES
dosage is important and studied varying from 0 to 30 wt% based on the
initial total mass of GL and LA at the reaction temperature of 150 ◦C.
The reaction systems are biphasic with all DES dosages in initial
reaction time of 2 h, indicating the absence of intensification process for
the esterification at initial reaction time. Then, the reaction systems with
7

Z. Shen et al.
Catalysis Today xxx (xxxx) xxx
considered under the reaction time from 2 to 8 h.
Ignoring the effect of glycerol, it is found that there is a linear rela-
tionship between (1/CLA-1/CLA,0) and the reaction time at different re-
action temperatures, as shown in Fig. 11b and Table S1. CLA,0 and CLA
are LA concentration at initial and real reaction time, respectively. There
is a little deviation of data points from the straight line with a relative
linear correlative coefficient (R2 > 0.98) because of the existence of the
mass transfer influence at relative low reaction temperature of 130 ◦C.
The model fits well at the reaction temperature of 140, 150, and 160 ◦C
with R2 > 0.99, as it is a kinetically controlled process. Therefore, these
data considered with deviation agree well with pseudo-second-order
kinetic line corresponding LA for the [TPAB:B(OH)3] intensified
esterification.
Fig. 12 compares the results between the calculated and experi-
mental LA conversion under optimal reaction condition within reaction
temperatures from 130 to 160 ◦C. The root mean square deviation
(RMSD) in Eq. (5) is calculated to be only 0.0105, in which XLA,cal,i and
XLA,exp,i are calculated and experimental LA conversion, respectively,
and N is the number of experimental. The excellent agreement between
experimental and calculated LA conversion demonstrates that pseudo-
second-order kinetic Eq. (6) can accurately describe the obtained
experimental results. The rate constants (k) calculated are 0.426, 0.545,
0.708, and 0.851 L molꢀ 1 hꢀ 1 at the reaction temperature of 130, 140,
150, and 160 ◦C, respectively.
Fig. 10. The [TPAB:B(OH)3] intensified esterification of LA and GL as function
of reaction time. Reaction conditions: 0.05 mol LA, 0.2 mol GL, molar ratio of
GL:LA (4:1), 20 wt% DES dosage, 0.24 wt% HBr, and reaction temperature of
150 ◦C.
3.5. Catalytic kinetics and temperature controlled recovery of DES
According to the reaction temperature on GML yield, the reaction
temperature plays a vital role and may provide a balance between LA
conversion and GML yield. It is reported that the esterification of LA and
GL over propyl sulfonic acid functionalized SBA-15 could be modeled as
irreversible parallel reactions and fitted to a second order kinetic model
[11]. Nevertheless, the reaction followed the pseudo-first-order kinetic
model using silica gel supported sulfonic acid functionalized ionic liquid
as solid catalyst [12]. These reports indicate that the kinetics of the
esterification not only are relative to reaction temperature but also rely
on the employed catalysts.
To establish the kinetics of the esterification, the DES intensified
reaction was performed under optimal reaction conditions by 20 wt%
[TPAB:B(OH)3], 0.24 wt% HBr, GL to LA molar ratio of 4:1 at different
reaction temperatures (130, 140, 150, and 160 ◦C) for various reaction
time. As shown in Fig. 11a, LA conversion increases with increased re-
action temperature and time. However, the phase transformations from
liquid-liquid phases to homogeneous phase requires some time under
present reaction conditions. A certain amount of LA may be consumed to
achieve a homogeneous system and thus it takes nearly 2 h to form a
homogeneous phase. We believe that a homogeneous system should be
stable for intrinsically and kinetically controlled measurement, which
can efficiently eliminate mass transfer from liquid-liquid interface. Thus,
the kinetic reaction of the DES intensified esterification would be
Fig. 12. Parity plot for LA conversion between calculated and experimental LA
conversion data.
Fig. 11. (a) LA conversions at different reaction temperature and time and (b) variations of (1/CLA-1/CLA,0) vs reaction time.
8

Z. Shen et al.
Catalysis Today xxx (xxxx) xxx
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
√
N
√∑
(XLA,cal,i - XLA,exp,i)2/N
(5)
(6)
√
RMSD =
i=1
1
1
ꢀ
= kt
CLA CLA,0
In addition, the activation energy for the esterification of LA with GL
to GML can be obtained using Arrhenius plot of ln(k) vs (1/T) and the
result is shown in Fig. S7. The activation energy (Ea) of the intensified
esterification calculated from the slope is only 33.9 KJ molꢀ 1 with a high
correlation coefficient of 0.997. The DES [TPAB:B(OH)3] intensified
esterification process has a similar activation energy to the reported
silica gel-based sulfonic acid functionalized ionic liquid catalysts (31.5
KJ molꢀ 1) [12]. However, the activation energy is far smaller than those
of zinc carboxylate (51 KJ molꢀ 1) [62], Ag2H2SiW12O40 (38.1 KJ molꢀ 1
)
[63] and propyl sulfonic acid functionalized SBA-15 catalysts (42 KJ
molꢀ 1) [11], indicating the DES [TPAB:B(OH)3] can efficiently intensify
the esterification at relative low temperatures.
The reusability of [TPAB:B(OH)3] for the selective esterification of
LA and GL was investigated in Fig. 13 and the results are shown in
Fig. S8. Under the initial reaction conditions, the reactants LA and GL
are miscible with the addition of the DES. With the reaction progressed,
reaction system changes from biphasic to homogeneous. After desired
reaction time, reaction mixture is cooled to 80 ◦C and separated two
phases again, where the lower phase (DES phase) includes excessive GL
and DES, and the upper phase (ester phase) contains unreacted LA,
products GML, GDL, GTL, and a little bit DES from the DES phase. The
DES tends to segregate from the system to form distinct oil-aqueous
layers after the reaction, which is highly efficient for monoester pro-
duction under continuous operation conditions [64]. Therefore, the DES
phase including unreacted GL can be gravity-based separated through
temperature controlled intensification and water-washed lower phase
and evaporated by rotary to remove water for 8 h. The recycle reaction is
proceeded after adding calculated amount of reactants and 0.24 wt%
HBr, similar to the first intensified esterification (Fig. 13). As the re-
covery experiment proceeds, LA conversion increases slightly from 90 %
to 92 % and GML yield declines from 76 % to 72 %. It can be considered
that the minor change in the composition of DES recovery and separa-
tion delayed the time of system homogeneity point, which affects the
monoester selectivity.
Fig. 13. Recovery scheme of DES and unreacted GL for intensified esterifica-
tion of LA and GL.
original draft. Lifang Chen: Conceptualization, Writing - review &
editing, Funding acquisition. Hongye Cheng: Resources, Data curation.
Zhiwen Qi: Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors gratefully acknowledge the support from the National
Natural Science Foundation of China (No. 21776084, 21861132019),
Natural Science Foundation of Shanghai (19ZR1412500) and 111
Project (B08021).
4. Conclusion
Appendix A. Supplementary data
Boric acid-based DESs were designed to intensify the selective
esterification to monoester. The COSMO-RS model was used to screen
boric acid and four ammonium salts with different chain length formed
DESs, and it demonstrated that the DES formed from TPAB and B(OH)3
was preferable for the intensified esterification of GL with LA. In addi-
tion, SLE phase diagram clarified the equimolar DES [TPAB:B(OH)3]
with the lowest freezing point can be considered for the intensification
of the esterification reaction. For the intensified esterification, various
reaction parameters were used to explore the influence of H+ concen-
tration, reaction temperature, GL/LA molar ratio, and DES dosage for
the conversion of LA and the yield of GML. The results showed the LA
conversion was 89.1 % and the GML yield was 76.4 % under optimal
conditions of 0.24 wt% HBr, LA and GL molar ratio of 4:1, and 20 wt%
DES dosage at reaction temperature of 150 ◦C for 8 h. The DES and
unreacted GL could be readily gravity-based separated through tem-
perature controlled intensification process with good recyclability,
which provide a potential for the intensification applications of boric
acid-based DESs.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2021.05.018.
References
[1] B. Larock, Comprehensive Organic Transformations: A Guide to Functional Group
Preparations, 2nd edition, John Wiley & Sons, New York, 1999.
[2] R. Nakamura, K. Komura, Y. Sugi, Catal. Commun. 9 (2008) 511–515.
[3] F. Klevenhusen, L. Meile, M. Kreuzer, C.R. Soliva, Anaerobe 17 (2011) 232–238.
[4] E. Heykants, W.H. Verrelst, R.F. Parton, P.A. Jacobs, Stud. Surf. Sci. Catal. (1997)
1277–1284.
[5] S. Baum, E. Ritter, I. Smirnova, M. Schilling, A. Liese, RSC Adv. 6 (2016)
32422–32429.
[6] R. Radhakrishnan, J. Wu, S. Jaenicke, G.K. Chuah, ChemCatChem 3 (2011)
761–770.
[7] H.E. Hoydonckx, D.E. De Vos, S.A. Chavan, P.A. Jacobs, Top. Catal. 27 (2004)
83–96.
[8] H. Szeląg, W. Zwierzykowski, Lipid-Fett 100 (1998) 302–307.
[9] W.D. Bossaert, D.E. De Vos, W.M. Van Rhijn, J. Bullen, P.J. Grobet, P.A. Jacobs,
J. Catal. 182 (1999) 156–164.
[10] S. Abro, Y. Pouilloux, J. Barrault, Stud. Surf. Sci. Catal. (1997) 539–546.
[11] L. Hermida, A.Z. Abdullah, A.R. Mohamed, Chem. Eng. J. 174 (2011) 668–676.
[12] X. Han, G. Zhu, Y. Ding, Y. Miao, K. Wang, H. Zhang, Y. Wang, S.-B. Liu, Chem.
Eng. J. 359 (2019) 733–745.
CRediT authorship contribution statement
[13] N. Zou, X. Lin, M. Li, L. Li, C. Ye, J. Chen, T. Qiu, ACS Sustain. Chem. Eng. 8 (2020)
18054–18061.
´
´
[14] C. de la Calle, J.M. Fraile, E. García-Bordeje, E. Pires, L. Roldan, Catal. Sci. Technol.
Zilong Shen: Validation, Formal analysis, Investigation, Writing -
5 (2015) 2897–2903.
9

Z. Shen et al.
Catalysis Today xxx (xxxx) xxx
[15] Q. Zeng, Z. Song, H. Qin, H. Cheng, L. Chen, M. Pan, Y. Heng, Z. Qi, Catal. Today
339 (2020) 113–119.
[40] J. Wang, Y. Zhang, ACS Catal. 6 (2016) 4871–4876.
[41] S. Manhas, M.S. Taylor, J. Org. Chem. 82 (2017) 11406–11417.
[42] S. Manhas, Y.C. Lin, G. Wang, L.T. Kyne, M.S. Taylor, Green Chem. 21 (2019)
5363–5367.
[16] D. Fang, J. Yang, C. Jiao, ACS Catal. 1 (2011) 42–47.
[17] M. Li, J. Chen, L. Li, C. Ye, X. Lin, T. Qiu, Green Energy Environ. 6 (2021) 271–282,
https://doi.org/10.1016/j.gee.2020.05.004.
[43] Y. Sun, J. Hu, S. An, Q. Zhang, Y. Guo, D. Song, Q. Shang, Fuel 207 (2017)
136–145.
[18] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem.
Commun. (2003) 70–71.
[44] P.V.A. Pontes, E.A. Crespo, M.A.R. Martins, L.P. Silva, C.M.S.S. Neves, G.
J. Maximo, M.D. Hubinger, E.A.C. Batista, S.P. Pinho, J.A.P. Coutinho,
G. Sadowski, C. Held, Fluid Phase Equilib. 448 (2017) 69–80.
[45] J. Li, J. Wang, M. Wu, H. Cheng, L. Chen, Z. Qi, Ind. Eng. Chem. Res. 59 (2020)
9223–9232.
[19] E.L. Smith, A.P. Abbott, K.S. Ryder, Chem. Rev. 114 (2014) 11060–11082.
[20] S. Mukhopadhyay, S. Mukherjee, N.F. Adnan, A. Hayyan, M. Hayyan, M.
A. Hashim, B. Sen Gupta, Chem. Eng. J. 294 (2016) 316–322.
[21] X. Hu, J. Wang, M. Mei, Z. Song, H. Cheng, L. Chen, Z. Qi, Chem. Eng. J. (2020),
127469.
[46] S.-H. Ren, Y. Xiao, Y.-M. Wang, J. Kong, Y.-C. Hou, W.-Z. Wu, Fuel Process.
Technol. 137 (2015) 104–108.
[22] M.A.R. Martins, E.A. Crespo, P.V.A. Pontes, L.P. Silva, M. Bülow, G.J. Maximo, E.A.
C. Batista, C. Held, S.P. Pinho, J.A.P. Coutinho, ACS Sustain. Chem. Eng. 6 (2018)
8836–8846.
´
[47] W. Dołębska, T. Jaron, J. Mol. Struct. 1206 (2020), 127748.
[48] F. Eckert, A. Klamt, AIChE J. 48 (2002) 369–385.
[49] L. Chen, T. Zhou, L. Chen, Y. Ye, Z. Qi, H. Freund, K. Sundmacher, Chem. Commun.
47 (2011) 9354–9356.
[23] D.O. Abranches, M.A.R. Martins, L.P. Silva, N. Schaeffer, S.P. Pinho, J.A.
P. Coutinho, Chem. Commun. 55 (2019) 10253–10256.
[24] S. Sunitha, S. Kanjilal, P.S. Reddy, R.B.N. Prasad, Tetrahedron Lett. 48 (2007)
6962–6965.
[50] M. Diedenhofen, A. Klamt, Fluid Phase Equilib. 294 (2010) 31–38.
[51] X. Liu, T. Zhou, X. Zhang, S. Zhang, X. Liang, R. Gani, G.M. Kontogeorgis, Chem.
Eng. Sci. 192 (2018) 816–828.
[25] M. Shaibuna, L.V. Theresa, K. Sreekumar, Catal. Lett. 148 (2018) 2359–2372.
[26] H. Qin, X. Hu, J. Wang, H. Cheng, L. Chen, Z. Qi, Green Energy Environ. 5 (2020)
8–21.
[52] M.A.R. Martins, S.P. Pinho, J.A.P. Coutinho, J. Solution Chem. 48 (2018) 962–982.
[53] S. Chauhan, M. Kaur, K. Singh, M.S. Chauhan, P. Kohli, Colloids Surf. A 535 (2017)
232–241.
[27] V. De Santi, F. Cardellini, L. Brinchi, R. Germani, Tetrahedron Lett. 53 (2012)
5151–5155.
¨
¨
[54] R. Hakkinen, P. Willberg-Keyrilainen, J. Ropponen, T. Virtanen, J. Mol. Liq. 280
[28] P. Lozano, E. Alvarez, S. Nieto, R. Villa, J.M. Bernal, Green Chem. 21 (2019)
3353–3361.
(2019) 104–110.
[55] R.P. Pohanish, in: William Andrew (Ed.), Sittig’s Handbook of Toxic and
Hazardous Chemicals and Carcinogens, 7th ed, Elsevier, Amsterdam, 2017,
pp. 451–452.
[29] Y. Pan, M.A. Alam, Z. Wang, J. Wu, Y. Zhang, Z. Yuan, Bioresour. Technol. 220
(2016) 543–548.
ˆ
[30] R. Craveiro, L. Meneses, L. Durazzo, A. Rocha, J.M. Silva, R.L. Reis, S. Barreiros, A.
[56] H. Ghaedi, M. Ayoub, S. Sufian, B. Lal, Y. Uemura, J. Mol. Liq. 242 (2017)
395–403.
R.C. Duarte, A. Paiva, ACS Sustain. Chem. Eng. 7 (2019) 19943–19950.
[31] L. Chen, F. Mo, H. Cheng, Z. Qi, Chem. Lett. 50 (2020) 1194–1197.
[32] H. Qin, Z. Song, Q. Zeng, H. Cheng, L. Chen, Z. Qi, AIChE J. 65 (2018) 675–683.
[33] H. Qin, Y. Zhou, Q. Zeng, H. Cheng, L. Chen, B. Zhang, Z. Qi, Green Energy
Environ. 5 (2020) 124–129.
[57] A.P. Abbott, R.C. Harris, K.S. Ryder, C. D’Agostino, L.F. Gladden, M.D. Mantle,
Green Chem. 13 (2011) 82–90.
[58] Y. Ma, T. Zhang, L. Chen, H. Cheng, Z. Qi, Ind. Eng. Chem. Res. 58 (2019)
13122–13132.
[34] C. Ruan, F. Mo, H. Qin, H. Cheng, L. Chen, Z. Qi, Catal. Lett. 151 (2021) 445–453.
[35] T. Zhang, K. Shahbaz, M.M. Farid, Renew. Energ. 160 (2020) 363–373.
[36] K. Mamtani, K. Shahbaz, M.M. Farid, Renew. Sust. Energ. Rev. 137 (2021),
110501.
[59] I. Sadaba, Y. Gorbanev, S. Kegnæs, S. Putluru, R. Berg, A. Riisager, ChemCatChem
5 (2013) 284–293.
[60] L. Ding, W. Yang, L. Chen, H. Cheng, Z. Qi, Catal. Today 347 (2020) 39–47.
[61] O. Riechert, M. Husham, G. Sadowski, T. Zeiner, AIChE J. 61 (2015) 3000–3011.
[37] S.D. Bull, M.G. Davidson, J.M.H. van den Elsen, J.S. Fossey, A.T.A. Jenkins, Y.-
B. Jiang, Y. Kubo, F. Marken, K. Sakurai, J. Zhao, T.D. James, Acc. Chem. Res. 46
(2013) 312–326.
[62] A. Macierzanka, H. Szela̧g, Ind. Eng. Chem. Res. 43 (2004) 7744–7753.
[63] C. Xu, J. Gan, X. Mei, Y. Zhou, J. Duanmu, G. Zhu, H. Zhang, X. Han, Y. Wang, S.-
B. Liu, Catal. Lett. 150 (2020) 3584–3597.
[38] W. Jiang, H. Jia, H. Li, L. Zhu, R. Tao, W. Zhu, H. Li, S. Dai, Green Chem. 21 (2019)
3074–3080.
[64] M. Huang, X. Han, C. Hung, J.C. Lin, P. Wu, J. Wu, S.-B. Liu, J. Catal. 320 (2014)
42–51.
¨
[39] R. Wahlstrom, J. Hiltunen, M. Pitaluga de Souza Nascente Sirkka, S. Vuoti,
K. Kruus, RSC Adv. 6 (2016) 68100–68110.
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