Tetraalkylammonium Ionic Liquids as Dual Solvents–Catalysts for Direct Synthesis of Sugar Fatty Acid Esters

Article abstract of DOI:10.1007/s11743-016-1798-7

As nonionic surfactants derived from naturally renewable resources such as sugars and fatty acids, sugar fatty acid esters have been widely utilized in food, cosmetic and pharmaceutical industries. Our present study has demonstrated that the inexpensive and halogen-free tetraalkylammonium salts (e.g., [Bu₄N][Ac], [Et₄N][Ac] and [Me₄N][Ac]) can act as dual solvents–catalysts for regioselective acylation to produce glucose laurate. This non-enzymatic synthesis can proceed under mild conditions with high specificity, and the conversions obtained were superior to that when an enzyme catalyst (lipase, EC 3.1.1.3) was added. A higher yield was obtained in the ammonium salt with a longer alkyl chain on the cation, while no product was obtained in [Bu₄N][HSO₄] and [Bu₄P][Ac]. A reaction mechanism has been proposed, which is supported by phase-transfer catalysis and law of matching water affinity.

Full text of DOI:10.1007/s11743-016-1798-7

J Surfact Deterg
DOI 10.1007/s11743-016-1798-7
ORIGINAL ARTICLE
Tetraalkylammonium Ionic Liquids as Dual Solvents–Catalysts
for Direct Synthesis of Sugar Fatty Acid Esters
Xiao-Sheng Lin1 Yong Zou2 Kai-Hua Zhao1 Tao-Xiang Yang1
•
•
•
•
Peter Halling3 Zhen Yang1
•
Received: 9 September 2015 / Accepted: 26 January 2016
Ó AOCS 2016
Abstract As nonionic surfactants derived from naturally
renewable resources such as sugars and fatty acids, sugar
fatty acid esters have been widely utilized in food, cos-
metic and pharmaceutical industries. Our present study has
demonstrated that the inexpensive and halogen-free
tetraalkylammonium salts (e.g., [Bu4N][Ac], [Et4N][Ac]
and [Me4N][Ac]) can act as dual solvents–catalysts for
regioselective acylation to produce glucose laurate. This
non-enzymatic synthesis can proceed under mild condi-
tions with high specificity, and the conversions obtained
were superior to that when an enzyme catalyst (lipase, EC
3.1.1.3) was added. A higher yield was obtained in the
ammonium salt with a longer alkyl chain on the cation,
while no product was obtained in [Bu4N][HSO4] and
[Bu4P][Ac]. A reaction mechanism has been proposed,
which is supported by phase-transfer catalysis and law of
matching water affinity.
Introduction
Sugar fatty acid esters (SFAE) are nonionic surfactants
produced from naturally renewable resources such as car-
bohydrates and fatty acids through O-acylation. They are
biodegradable, biocompatible and nontoxic with broad
applications in food, cosmetic and pharmaceutical indus-
tries [1, 2]. Hydroxy group O-acylation is also extensively
employed in carbohydrate chemistry as a protection strat-
egy and for isolation and identification of sugars. However,
the synthetic processes require the use of organic solvents
and catalysts that usually cause concerns regarding their
environmental health risks and have to proceed under harsh
conditions [3]. In particular, it is hard to find an appropriate
solvent that can solubilize the two substrates of opposite
polarity. Therefore, a search for greener solvents and cat-
alysts for this application has always been an ongoing
issue.
Keywords Ionic liquid (IL) Á Sugar fatty acid ester
(SFAE) Á Glucose laurate Á Tetraalkylammonium salts Á
Dual solvent–catalyst Á Phase-transfer catalysis
Ionic liquids (IL) have gained intense attraction as a
promising new reaction medium for a variety of synthetic
processes due to their low volatility, high chemical and
thermal stability, superior solubilizing power, and tunable
solvent properties [4, 5]. Some IL have also been shown to
act as catalysts further augmenting their already wide-
spread applications [6]. For instance, O-acylation of alco-
hols and carbohydrates can proceed rapidly in high yield
under mild conditions in a dicyanamide-based imidazolium
IL, which is both a solvent and a base catalyst [6], and a
few Lewis/Brønsted acidic IL have shown the ability of
working as dual solvents–catalysts for esterification/trans-
esterification reactions [7–17].
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-016-1798-7) contains supplementary
material, which is available to authorized users.
& Zhen Yang
zyang@szu.edu.cn
1
College of Life Sciences, Shenzhen University,
Shenzhen 518060, Guangdong, China
2
In this work, we report for the first time the use of
tetraalkylammonium IL as dual solvents–catalysts for the
synthesis of SFAE, and the synthesis of glucose laurate in
this unique solvent system with and without addition of an
School of Pharmaceutical Sciences, Sun Yat-Sen University,
Guangzhou 510006, Guangdong, China
3
Department of Pure and Applied Chemistry, University of
Strathclyde, Glasgow G1 1XL, UK
123

J Surfact Deterg
Synthetic Reaction
enzyme catalyst has been compared and discussed. Our
recent study in enzymatic synthesis of glucose laurate has
revealed that when lipase (EC 3.1.1.3) was added as the
catalyst, among the 16 IL screened tetrabutylammonium
acetate ([Bu4N][Ac]) was the one in which the highest
conversion was obtained [18]. A serendipitous experiment
later has revealed that glucose laurate could also be pro-
duced in this particular IL in the absence of the enzyme.
A typical reaction was carried out by adding 0.054 g
glucose (corresponding to 0.3 mol/l of the reaction sys-
tem) to a 5-ml capped test tube containing 0.3 M VL in
1 ml solvent (pure IL or IL/2M2B mixture), which was
placed in an incubator/shaker with agitation of 300 rpm
at 40 °C for the reaction to start. Periodically, a 10-ll
sample was taken and four times diluted with dimethyl
sulfoxide, from which 10 ll was injected for HPLC
analysis, which was performed on a Shimadzu LC-20AT
HPLC system equipped with a refractive index detector
(Shimadzu RID-10A) and a 150 9 4.6 mm, 5 lm inertsil
ODS-SP column (GL Sciences Inc. Japan). A solvent
mixture of methanol/water (85:15 v/v, pH adjusted to 2.3
with acetic acid prior to addition of methanol) was
employed as the mobile phase with a flow rate of 1.0 ml/
min. The conversion was calculated based on the total
amount of glucose added to the reaction system. For
reactions to study the effect of water content, water was
added to the IL prior to addition of the reactants, and the
water content (%, w/w) was determined by using a
Metrohm 831 KF Coulometer. For reactions to study the
impact of enzyme addition, Novozym 435 (100 mg) was
added to the reaction mixture to start the reaction. All
experiments were repeated at least twice with the results
subjected to \10 % error.
Experimental
Materials
Ionic liquids (99 %) were obtained from shanghai Cheng
Jie Chemical Co., Ltd. Vinyl laurate (VL) and the Reich-
ardt’s
dye
(2,6-diphenyl-4-(2,4,6-triphenyl-1-pyri-
dinio)phenolate) were from Sigma-Aldrich China Inc. a-D-
Glucose (Glc), lauric acid (LA), 2-methyl-2-butanol
(2M2B) and all other reagents used were of analytical
grade from local manufacturers in China. Novozym 435
(Candida antarctica lipase B immobilized on acrylic
resins) was purchased from Novozymes (China) Invest-
ment Co., Ltd.
Measurement of Electron Pair Acceptor Strength
Electron pair acceptor strength is commonly used as a
measure of the polarity of a solvent and is determined
spectrophotometrically by means of the Reichardt’s dye
[19]. The dye was dissolved in an IL at 0.5 mM. After
centrifugation, the visible spectrum of the solution was
scanned at 25 °C with a PerkinElmer Lambda 25 UV–Vis
spectrophotometer. The wavelength of the absorption peak
(kmax, in nm) was recorded for determination of the IL’s
acceptor strength (ETN) following the equation:
ENT = [28,591/kmax - 30.7]/32.4, where ENT is set on a
dimensionless normalized scale between 0.0 for tetram-
ethylsilane and 1.0 for H2O.
Product Characterization
The glucose laurate product was isolated as a white
powder and identified as 6-O-laurate-a-D-glucopyra-
nose by structural analysis including IR (Nicolet 6700
FT-IR spectrometer, Thermo Scientific), 13C NMR
(Avance-400 NMR spectrometer, Bruker), and MS (AB
SCIEX TripleTOFÒ 5600 System). IR (KBr) confirms
the presence of an ester bond in the product: mmax
1,731 cm-1 (C=O ester) (Fig. S1); 13C-NMR
(400 MHz, DMSO, TMS) (Fig. S2) suggests that a-
anomer predominates with the esterification occurring
at the C6 position: d 13.869 (1C, C-120); d 22.012 (1C,
C-110); d 24.412 (1C, C-100); d 28.369, 28.640,
28.823, 28.920 (6C, C-40 * 90), d 31.219 (1C, C-30), d
33.411 (1C, C-20), d 63.787 (1C, C-6); d 69.097 (1C,
C-4), d 70.519 (1C, C-5), d 72.155 (1C, C-2), 72.833
(1C, C-3); d 92.234 (1C, C-1a); d 172.850 (1C, C-10).
MS (Fig. S3) gave molecular ion peaks at m/z 385 and
363, consistent with the formulations [NaGL]? and
[HGL]? and the monoesterification of glucose.
Determination of log P Values
log P, the logarithm of the partition coefficient of a solvent
in an octanol/H2O biphasic system, is commonly used as a
measure of the solvent hydrophobicity [20]. In this study, the
KowWin software (http://www.epa.gov/opptintr/exposure/
pubs/episuite.htm) was applied to estimate the log P value
for each IL. This software has been found to give the best
results for log P prediction among the four software pack-
ages (HyperChem, Pallas, KowWin and TOPKAT) [21].
123

J Surfact Deterg
0.15
0.12
0.09
0.06
0.03
0
Results and Discussion
GL, w/o enzyme
LA, w/o enzyme
GL, w/ enzyme
LA, w/ enzyme
[Bu4N][Ac] as Dual Solvent–Catalyst for Synthesis
of Glucose Laurate
When the two reactants (glucose and VL) were placed in a
bisolvent mixture ([Bu4N][Ac]/2M2B, 0.05/0.95, v/v) for
24 h, the HPLC analysis revealed that the glucose laurate
product was produced with a conversion of 11.8 %
(Fig. 1). This serendipitous experiment revealed that the
transesterification reaction had occurred between the two
substrates with no added catalyst. Later, a controlled
experiment (Table 1) showed that in pure [Bu4N][Ac],
addition of the two substrates yielded the production of
glucose laurate regardless of whether the lipase catalyst
was present or not, while in 2M2B no product was
detectable in the absence of the enzyme. This suggests that
the IL [Bu4N][Ac] acts as both a solvent and a catalyst for
the transesterification reaction. The product obtained in the
non-enzymatic reaction possessed the same retention time
in HPLC analysis as the one produced from the enzymatic
reaction (which has been verified as 6-O-lauroyl-D-
Reaction time (h)
Fig. 2 Production of glucose laurate (GL) and lauric acid (LA) in
[Bu4N][Ac] with and without addition of the enzyme. 0.054 g glucose
was added to 1.0 ml [Bu4N][Ac] (water content 16.0 %) containing
0.3 M vinyl laurate, and the reaction was carried out by agitation of
300 rpm at 40 °C, with and without addition of 100 mg enzyme
glucopyranose by structural analyses with NMR, IR and
MS), suggesting that the specificity is retained.
A comparison of the two reactions with and without addi-
tion of the enzyme further confirms the above findings
(Fig. 2). Both reactions resulted in the production of glucose
laurate, but witha slightly higher yield obtained in the absence
of the enzyme (a similar phenomenon was also observed in
Table 1). Interestingly, during the reaction course a by-pro-
duct, LA, was also gradually generated, with a slightly lower
amount obtained in the enzyme-free reaction. LA is formed
mainly because both the co-substrate (VL) and the product
(glucose laurate) are easily subjected to hydrolysis, as long as
there is a trace amount of water present in the reaction system
(see Scheme 1). Therefore, this experiment gives us an indi-
cation that the IL [Bu4N][Ac] was capable of catalyzing the
transesterification reaction as well as a side-reaction of
hydrolysis, the latter being more favored in the presence of the
enzyme. In support of this, Fig. 3 has clearly demonstrated
that addition of the enzyme led to a gradual decrease in the
production of glucose laurate and a significant increase in the
production of LA; in particular when 100 mg of Novozym
435 was added, within 24 h a negligible amount of glucose
laurate was produced while almost all VL was converted to
LA. The fact that a lower yield of the glucose laurate was
obtained in the presence of the enzyme might be related to the
preference of Novozym 435 in catalyzing the hydrolysis side-
reaction.
50000
VL
40000
30000
GL
20000
10000
LA
0
Time (min)
Fig. 1 HPLC chromatogram. 0.054 g glucose was added to 1 ml
solvent ([Bu4N][Ac]/2M2B, 0.05/0.95, v/v) containing 0.3 M vinyl
laurate, and the reaction was carried out in an incubator/shaker with
agitation of 300 rpm at 40 °C for 24 h. GL glucose laurate, VL vinyl
laurate, LA lauric acid
Table 1 A controlled experiment to compare the production of
glucose laurate in both solvents ([Bu4N][Ac] and 2M2B, 1.0 ml each)
with and without addition of the enzyme (Novozym 435)
Test no.
1
2
3
4
Solvent
[Bu4N][Ac] [Bu4N][Ac] 2M2B 2M2B
Substrate 1: Glc (M)
Substrate 2: VL (M)
0.3
0.3
100
0.3
0.3
0
0.3
0.3
100
0.3
0.3
0
Effect of Different Factors on the Non-Enzymatic
Synthetic Reactions in [Bu4N][Ac]
Enzyme: Novozym
435 (mg)
[GL] obtained at 24 h 0.10
(M)
0.11
0.10
0
The non-enzymatic reaction can be affected by factors such
as water content, IL/2M2B volumetric ratio, and reaction
123

J Surfact Deterg
Scheme 1 Transesterification reaction of vinyl laurate and glucose to produce glucose laurate, accompanied by side reactions of hydrolysis to
produce lauric acid
method [22]) and lowering the viscosity of the reaction
0.3
medium [549.3 cP for [Bu4N][Ac] vs. 3.5 cP for 2M2B,
0.25
0.2
0.15
0.1
0.05
0
determined by using an AR1000 rheometer (TA Instru-
ments, USA)]. Adding 2M2B to the IL can also reduce the
cost of the synthetic process. As shown in Fig. 4b, the
optimal conversion (42 %) was obtained in the 7:3 [Bu4-
N][Ac]/2M2B bisolvent system, but for the sake of low-
ering the formation of the hydrolytic by-product (LA) a
volumetric ratio of 3:7 was used for later experiments.
An increase in the reaction temperature seemed to be
unfavorable (Fig. 4c), because while the yield of glucose
laurate was fairly unchanged, the concentration of LA was
dramatically enhanced, indicative of a stronger tendency
for hydrolysis.
0
20
40
60
80
100
120
Fig. 3 Effect of enzyme dosage on the production of glucose laurate
(GL) and lauric acid (LA) in [Bu4N][Ac]/2M2B (3:7, v/v) bisolvent
system. 0.054 g glucose was added to 1.0 ml [Bu4N][Ac] (water
content 16.0 %) containing 0.3 M vinyl laurate, and different
amounts of enzyme (0–100 mg) were added to start the reaction,
which was carried out with agitation of 300 rpm at 40 °C
Use of IL Analogous to [Bu4N][Ac]
The above experiments clearly demonstrated that [Bu4-
N][Ac] possesses a catalytic power, thus prompting us to
try some other analogous IL such as [Me4N][Ac], [Et4-
N][Ac], [Bu4N][HSO4] and [Bu4P][Ac]. A screening test
has revealed that the performance of these IL was depen-
dent on both the cation and the anion. The glucose laurate
product was produced in all the three tetraalkylammonium
acetate salts, whereas neither [Bu4P][Ac] nor [Bu4-
N][HSO4] yielded any product. For the former three acetate
salts, the conversion increased as the length of the alkyl
chain on the cation increased ([Me4N][Ac] \ [Et4-
N][Ac] \ [Bu4N][Ac]) (Fig. 5). This is in contrast to the
findings reported by Fang et al. [11] when they evaluated
temperature (Fig. 4). It appears that the presence of a
threshold quantity of water in the IL contributed to higher
conversions (Fig. 4a). For a transesterification reaction
which is accompanied by hydrolysis as a side reaction, the
initial increase in the ester yield along with a gradual
decrease in the formation of the hydrolytic by-product
(LA) was unexpected. For latter experiments the water
content was fixed at 16 % (w/w).
Mixing the IL with 2M2B is advantageous in enhancing
the glucose solubility (13 mM in 2M2B vs 173 mM in
[Bu4N][Ac], determined by using the dinitrosalicylic acid
123

J Surfact Deterg
a
w/o enzyme
w/ enzyme
0.12
0.1
0.08
0.06
0.04
0.02
0
0
5
10
15
20
25
[Me4N][Ac]
[Et4N][Ac]
[Bu4N][Ac]
H2O (%, w/w)
Fig. 5 Production of glucose laurate (GL) in the three tetraalkylam-
monium IL with and without addition of the enzyme catalyst.
Reactions were carried out in the three IL using the conditions the
same as those in Fig. 2
b
0.15
0.12
0.09
0.06
0.03
0
A Plausible Mechanism
A plausible mechanism is postulated in Scheme 2 to
illustrate the catalytic role of the IL. Dissociated from the
salt in the presence of water, the acetate anion would
facilitate the nucleophilic attack of the hydroxyl group of
glucose to the carbonyl group of the ester. The negatively
charged tetrahedral intermediate would be stabilized by the
tetraalkylammonium cation, thus leading to the production
of glucose laurate and the release of the enolate anion of
VL, which subsequently tautomerizes to acetaldehyde.
This mechanism can directly explain why a certain
amount of water was required for the non-enzymatic
reaction. On the other hand, according to the law of
matching water affinities [23], oppositely charged ions in
aqueous solution spontaneously form inner sphere ion pairs
only when they have equal water affinities. As the acetate
anion is highly affinitive to water while the three
tetraalkylammonium cations vary in their hydrophobicity
in the order of Me4N? \ Et4N? \ Bu4N? (which can also
be judged from their log P values listed in Table 2), it is
reasonable to assume that the interaction between Bu4N?
and Ac- is weaker than the one between Me4N? and Ac-.
The enforced large distance between Bu4N? and Ac-, due
to the larger butyl chains in the cation, is another way of
seeing the weaker interaction. Therefore, in the presence of
water [Bu4N][Ac] has a higher propensity of dissociating
and ionizing, thus promoting the catalysis by making the
glucose hydroxyl group nucleophilic with the aid of the
acetate anion and by stabilizing the negatively charged
intermediate with the tetraalkylammonium cation. No ester
product was produced in the presence of [Bu4N][HSO4]
3
.
/0
0/1
0.7
0.5
0
/
1
/
/
/0.95
5
.
.2/0.8 .3
0.1/0.9
0
0
0
0.7
.05
0
[Bu4N][Ac]/2M2B volumetric ratio (v/v)
c
0.16
GL
LA
0.12
0.08
0.04
0
40
50
60
70
Reaction temperature (oC)
Fig. 4 Study on factors affecting the production of glucose laurate
(GL) and lauric acid (LA): a effect of water content (%, w/w), in pure
[Bu4N][Ac]; b effect of [Bu4N][Ac]/2M2B volumetric ratio (v/v), in
the bisolvent system; and c effect of temperature (°C), in 3:7
[Bu4N][Ac]/2M2B bisolvent system. Reaction conditions were the
same as in the above figures
the use of some HSO3-functionalized Brønsted acidic
ammonium IL as dual solvents–catalysts for Fischer
esterification reactions. Again, the conversions obtained in
the three tetraalkylammonium salts became, respectively,
slightly lower when the enzyme was added (Fig. 5), in
support of our assumption above that Novozym 435 cat-
alyzes the hydrolysis side-reaction preferentially, hence
depressing the production of the glucose laurate product.
-
presumably because HSO4 is much less basic than acet-
ate. The inability of [PBu4][Ac] to catalyze the reaction
123

J Surfact Deterg
Scheme 2 A proposed
mechanism to illustrate the
catalytic role of
H2O
Q+Ac-
Q+ + Ac-
tetraalkylammonium salts in the
transesterification reaction for
sugar ester synthesis
Q+
:O-
O
O
(Q = tetraalkylammonium)
O
R
O
Glc
O
R
O
R
-O
Glc
Glc
OH
Ac-
Table 2 Properties of the three tetraalkylammonium IL
order of [Bu4N][Ac] [ [Et4N][Ac] [ [Me4N][Ac], so did
the conversions obtained in these IL, regardless of the
slightly lower polarity and higher viscosity (Table 2).
ENT
log P
Viscosity (cP)
[Me4N][Ac]
[Et4N][Ac]
[Bu4N][Ac]
0.69
0.51
0.45
-2.09
-0.13
3.8
16.49
44.03
62.69
Conclusions
Both polarity (ETN) and viscosity data were obtained in the IL con-
taining 16 % (w/w) H2O
This study has demonstrated that tetraalkylammonium IL
such as [Bu4N][Ac], [Et4N][Ac] and [Me4N][Ac] possess
the potential of acting as both solvents and catalysts for
regioselective acylation to synthesize SFAE. This catalytic
process offers several advantages: (1) no additional catalyst
is required; (2) the IL are inexpensive and halogen-free; (3)
the synthetic reaction can proceed directly under mild
conditions with high specificity; and (4) water produced
does not need to be removed during the reactions. Our
experimental results and the proposed mechanism have
rationalized the structure–function relationship between the
conversions and the choice of the cation and anion of the
IL, thus providing useful information regarding the design
of these catalytic IL for such applications.
may be related to the weaker electrophilicity of the cation
due to the larger size of P versus N and hence more dis-
persed distribution of its electron cloud. Moreover, this
mechanism can also provide explanation about the
regioselectivity of this chemical synthesis: the preferential
acylation of primary over secondary hydroxyl groups in the
sugar may be due to the easier deprotonation of the former
with the aid of the acetate anion.
In fact, as early as half a century ago quaternary
ammonium and phosphonium salts (Q?X-) have been
discovered to play a catalytic role in reactions between two
reactants located in two immiscible phases such as organic
solvent and aqueous solution, and Starks [24] had intro-
duced a concept ‘‘phase-transfer catalysis’’ to explain this
phenomenon. The accelerating rate of the reaction is
believed to be mainly due to the two characteristic features
of the pairing cation (Q?): the high lipophilicity and the
large ionic radius [25]. This can be applied to account for
our experimental observation: because the glucose mole-
cule tends to be deprotonated with the aid of the acetate
anion, the hydrophobic quaternary ammonium cation may
have the propensity to associate with the glucose, thus
making it more compatible with the hydrophobic VL and in
turn facilitating the reaction between them two; this ben-
eficial effect would be stronger when the alkyl chains
attached on the cation are longer and hence more
hydrophobic. Therefore, it is understandable that as the
lipophilicity and the size of the cation varied following the
Acknowledgments The authors are grateful to the National Natural
Science Foundation of China for its financial support (No. 21276159).
References
1. Kobayashi T (2011) Lipase-catalyzed syntheses of sugar esters in
non-aqueous media. Biotechnol Lett 33:1911–1919
2. Gumel AM, Annuar MSM, Heidelberg T, Chisti Y (2011) Lipase
mediated synthesis of sugar fatty acid esters. Process Biochem
46:2079–2090
3. Plat T, Linhardt RJ (2001) Syntheses and applications of sucrose-
based esters. J Surfactants Deterg 4:415–421
4. Yang Z, Pan W (2005) Ionic liquids: green solvents for non-
aqueous biocatalysis. Enzyme Microb Technol 37:19–28
´
´
5. Domınguez de Marıa P (2012) Ionic liquids in biotransfor-
mations and organocatalysis: solvents and beyond. Wiley,
Hoboken
123

J Surfact Deterg
6. Nowicki J, Muszyn˜ski M (2014) Ionic liquids as catalysts and
reaction media in oleochemical raw materials processing: a
review. Curr Org Chem 18:2797–2807
7. Forsyth SA, MacFarlane DR, Thomson RJ, von Itzstein M (2002)
Rapid, clean, and mild O-acetylation of alcohols and carbohy-
drates in an ionic liquid. Chem Commun 714–715
8. Cole AC, Jensen JL, Ntai I, Tran KLT, Weaver KJ, Forbes DC,
Davis JH (2002) Novel Brønsted acidic ionic liquids and their use
as dual solvent–catalysts. J Am Chem Soc 124:5962–5963
9. Zhu H-P, Yang F, Tang J, He M-Y (2003) Brønsted acidic
ionic liquid 1-methylimidazolium tetrafluoroborate: a green
catalyst and recyclable medium for esterification. Green Chem
5:38–39
10. Gui J, Cong X, Liu D, Zhang X, Hu Z, Sun Z (2004) Novel
Brønsted acidic ionic liquid as efficient and reusable catalyst
system for esterification. Catal Commun 5:473–477
11. Fang D, Zhou X-L, Ye Z-W, Liu Z-L (2006) Brønsted acidic
ionic liquids and their use as dual solvent–catalysts for Fischer
esterifications. Ind Eng Chem Res 45:7982–7984
22. Miller GL (1959) Use of dinitrosalicylic acid reagent for deter-
mination of reducing sugar. Anal Chem 31:426–428
23. Collins KD (2004) Ions from the Hofmeister series and osmo-
lytes: effects on proteins in solution and in the crystallization
process. Methods 34:300–311
24. Starks CM (1971) Phase-transfer catalysis. I. Heterogeneous
reactions involving anion transfer by quaternary ammonium and
phosphonium salts. J Am Chem Soc 93:195–199
25. Ooi T, Maruoka K (2007) Recent advances in asymmetric phase-
transfer catalysis. Angew Chem Int Ed 46:4222–4266
Xiao-Sheng Lin is a postgraduate student in the College of Life
Sciences, Shenzhen University, working on research projects in
chemical and enzymatic synthesis of sugar fatty acid esters.
Yong Zou obtained his B.Sc. degree in organic chemistry from
Sichuan University, China, in 1988 and Ph.D. degree in pharmaceu-
tical chemistry from West China University of Medical Sciences,
China, in 1997. He is currently working as a full professor in the
School of Pharmaceutical Sciences, Sun Yat-sen University, China,
with his research focusing on conversion and utilization of biomass,
rational design and synthesis of drugs, and development of new
synthetic strategies for bioactive molecules and APIs.
12. Li H, Yu S, Liu F, Xie C, Li L (2007) Synthesis of dioctyl
phthalate using acid functionalized ionic liquid as catalyst. Catal
Commun 8:1759–1762
13. Zhang H, Xu F, Zhou X, Zhang G, Wang C (2007) A Brønsted
acidic ionic liquid as an efficient and reusable catalyst system for
esterification. Green Chem 9:1208–1211
14. Xie C, Li H, Li L, Yu S, Liu F (2008) Synthesis of plasticizer
ester using acid-functionalized ionic liquid as catalyst. J Hazard
Mater 151:847–850
Kai-Hua Zhao is a postgraduate student in the College of Life
Sciences, Shenzhen University, working on research projects in
chemical and enzymatic synthesis of sugar fatty acid esters.
15. Zhang L, Xian M, He Y, Li L, Yang J, Yu S, Xu X (2009) A
Brønsted acidic ionic liquid as an efficient and environmentally
benign catalyst for biodiesel synthesis from free fatty acids and
alcohols. Bioresour Technol 100:4368–4373
Tao-Xiang Yang is a postgraduate student in the College of Life
Sciences, Shenzhen University, working on research projects in
chemical and enzymatic synthesis of sugar fatty acid esters.
16. Junming XU, Jiang J, Zuo Z, Li J (2010) Synthesis of tributyl
citrate using acid ionic liquid as catalyst. Process Saf Environ
Prot 88:28–30
17. Yang Y, He W, Jia C, Ma Y, Zhang X, Feng B (2012) Efficient
synthesis of phytosteryl esters using the Lewis acidic ionic liquid.
J Mol Catal A Chem 357:39–43
18. Lin X-S, Cai Y-Z, Halling PJ, Yang Z (2015) Impacts of ionic
liquids on enzymatic synthesis of glucose laurate and optimiza-
tion with superior productivity by response surface methodology.
Process Biochem 50:1852–1858
19. Reichardt C (2005) Polarity of ionic liquids determined empiri-
cally by means of solvatochromic pyridinium N-phenolate
betaine dyes. Green Chem 7:339–351
Peter Halling obtained his bachelor’s degree from Cambridge
University (1972) and a Ph.D. from Bristol University (1975). He is
a professor in the Department of Pure and Applied Chemistry,
University of Strathclyde, UK, and his current research interest is in
biophysical chemistry.
Zhen Yang obtained her B.SC. degree in chemistry from Zhongshan
University, China, in 1985 and Ph.D. degree in biochemistry from the
University of Strathclyde, UK, in 1992. This was followed by
postdoctoral research at the University of Pittsburgh, USA. She is
now a professor at Shenzhen University, China. Her current research
has focused on biocatalysis in nonconventional media (especially in
ionic liquids and deep eutectic solvents): catalytic performance,
biophysical chemistry, and applications.
20. Laane C, Boeren S, Vos K, Veeger C (1987) Rules for opti-
mization of biocatalysis in organic solvents. Biotechnol Bioeng
30:81–87
´
21. Benfenati E, Gini G, Piclin N, Roncaglioni A, Varı MR (2003)
Predicting log P of pesticides using different software. Chemo-
sphere 53:1155–1164
123