Esterification of Lactic Acid by Catalytic Extractive Reaction: An Efficient Way to Produce a Biosolvent Composition

Article abstract of DOI:10.1007/s10562-013-1077-4

A biosolvent composition containing ethyl lactate and biodiesel was directly obtained by catalytic extractive esterification of lactic acid. Since the esterification of organic acids with alcohols is a thermodynamically limited reaction, the methodology consists in conducting the esterification and the ester extraction simultaneously thereby shifting the equilibrium towards more esters. The acid catalyzed esterification was performed in a biphasic solvent system composed of (i) a reactive polar phase which contains the esterification constituents, lactic acid, ethanol and an acid catalyst (ii) an extractive solvent selective of the ester, fatty acid methyl ester (biodiesel). This solvents system increases the ethyl lactate yield of more than 30 %. During the reaction progress, the selective solvent, fatty acid methyl ester, is progressively blended with ethyl lactate providing a mixture of esters, which can be directly used as an efficient composition of biosolvents without further purification. The solid acidic potassium salt of 12-phosphotungstic acid, K _2.5H_0.5PW₁₂O₄₀, was shown to be more efficient than the conventional esterification catalysts, H₂SO ₄ or Amberlyst 15. K_2.5H_0.5PW ₁₂O₄₀ gives a yield in ethyl lactate higher than 80 mol% after 2 h of reaction in the biphasic solvent system, using a molar ratio ethanol/lactic acid of 3.3 only. Moreover, K_2.5H_0.5PW ₁₂O₄₀ shows a remarkable higher activity per protonic sites (factor 30) and is recyclable at least two times without apparent loss of activity. These excellent properties were ascribed to its low density of strong Bronsted acid sites and its adequate hydrophobicity which would make this acid more water tolerant than the conventional esterification catalysts, H ₂SO₄ or Amberlyst 15. Graphical Abstract: Acid catalyzed esterification of lactic acid with ethanol was performed in a biphasic system where biodiesel acts as an extractive solvent of ethyl lactate. This extractive reaction produces directly a composition of biosolvent: biodiesel blended with ethyl lactate.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-013-1077-4

Catal Lett (2013) 143:950–956
DOI 10.1007/s10562-013-1077-4
Esterification of Lactic Acid by Catalytic Extractive Reaction:
An Efficient Way to Produce a Biosolvent Composition
•
Thu Ha Thi Vu Hang Thi Au Thuy Ha Thi Nguyen
•
•
•
Thu Trang Thi Nguyen Manh Hung Do
•
•
Ngoc Quynh Bui Nadine Essayem
Received: 24 February 2013 / Accepted: 23 July 2013 / Published online: 7 August 2013
Ó Springer Science+Business Media New York 2013
Abstract A biosolvent composition containing ethyl
lactate and biodiesel was directly obtained by catalytic
extractive esterification of lactic acid. Since the esterifica-
tion of organic acids with alcohols is a thermodynamically
limited reaction, the methodology consists in conducting
the esterification and the ester extraction simultaneously
thereby shifting the equilibrium towards more esters. The
acid catalyzed esterification was performed in a biphasic
solvent system composed of (i) a reactive polar phase
which contains the esterification constituents, lactic acid,
ethanol and an acid catalyst (ii) an extractive solvent
selective of the ester, fatty acid methyl ester (biodiesel).
This solvents system increases the ethyl lactate yield of
more than 30 %. During the reaction progress, the selective
solvent, fatty acid methyl ester, is progressively blended
with ethyl lactate providing a mixture of esters, which can
be directly used as an efficient composition of biosolvents
without further purification. The solid acidic potassium salt
of 12-phosphotungstic acid, K2.5H0.5PW12O40, was shown
to be more efficient than the conventional esterification
catalysts, H2SO4 or Amberlyst 15. K2.5H0.5PW12O40 gives
a yield in ethyl lactate higher than 80 mol% after 2 h of
reaction in the biphasic solvent system, using a molar ratio
ethanol/lactic acid of 3.3 only. Moreover, K2.5H0.5PW12O40
shows a remarkable higher activity per protonic sites
(factor 30) and is recyclable at least two times without
apparent loss of activity. These excellent properties were
ascribed to its low density of strong Brønsted acid sites and
its adequate hydrophobicity which would make this acid
more water tolerant than the conventional esterification
catalysts, H2SO4 or Amberlyst 15.
Keywords Ethyl lactate Á Biosolvent Á Extractive
reaction Á Esterification Á Solid acid catalyst
1 Introduction
Solvents represent an important class of chemicals. They
are used in many industries for their abilities to dissolve,
dilute or remove other compounds without causing chem-
ical changes. Conventional solvents, obtained from fossil
carbons are usually volatile organic compounds. Many of
them are hazardous to human health and the environment
in general. Nowadays, new environmental regulations have
forced the industry to find solutions to reduce their emis-
sion and waste released into the environment. The ideal
solution consists in the replacement of conventional sol-
vents by new alternative environmentally friendly solvents,
which would be non volatile, non toxic to human and the
environment. Importantly, it would provide effective per-
formances and be economically viable and moreover
obtained from renewable feedstock. Most of the biosol-
vents, issued from natural resources, addressed the previ-
ous criteria. The major classes of biosolvents are esters of
natural organic acids produced by fermentation of carbo-
hydrates, fatty acid esters, bioethanol, terpenic compounds,
isosorbide or glycerol derivatives. These biosolvents are
incorporated into different compositions for cleaning
T. H. T. Vu (&) Á H. T. Au Á T. H. T. Nguyen Á
T. T. T. Nguyen Á M. H. Do Á N. Q. Bui
National Key Laboratory for Petrochemical and Refinery
Technologies, 2 rue Pham Ngu Lao, Hanoi, Vietnam
e-mail: ptntd2004@yahoo.fr
N. Q. Bui Á N. Essayem (&)
Institut de Recherche sur la Catalyse et l’environnement de
Lyon, CNRS, Lyon1, 2 Avenue Albert Einstein,
69626 Villeurbanne, France
e-mail: nadine.essayem@ircelyon.univ-lyon1.fr
123

Esterification of Lactic Acid
951
applications, pesticides, printing inks, paints, varnish or
bituminous binders.
reactions due to the over-reactivity of peculiar reactants
and products. This approach is efficient for the synthesis of
HMF, a versatile product which can be easily involved in
successive reactions leading to polymers [11]. By a con-
tinuous extraction of the target product, its yield can be
significantly improved.
Ethyl lactate and esters of fatty acids are biosolvents
which can be used alone or in mixture [1, 2]. Their mixture
provides effective solvency for a broad range of applica-
tions. Their high boiling point, their biodegradability and
non-toxicity make this class of biosolvents of high interest.
Currently, lactic acid manufacture is based on carbohydrate
fermentation. The major problem of this technology is the
salts formation because the fermentation is efficient at
neutral pH, which requires the neutralization of the media.
Thereby, salts of lactic acid are produced instead of lactic
acid. Hence, the main drawback of this technology is the
lactic acid separation and purification, which is hardly
crystallized or distilled [3]. Note that recent developments
report its formation via a thermochemical way from lig-
nocellulosic resources [4]. In other respects, the production
of ethyl lactate of high purity, in high yield, is not an easy
task for two main reasons: first, due to the versatility of
lactic acid, second because of the thermodynamic equi-
librium of the esterification reaction. Lactic acid is a
hydroxy-carboxylic acid with a hydroxyl group born on the
C2 carbon. Under the esterification conditions, this func-
tional group is at the origin of the secondary reactions such
as the self esterification reaction of lactic acid or transe-
sterification reactions between ethyl lactate and any
hydroxy-compounds present in the media producing heavy
oligomers (Schemes 2, 3). To overcome the limitations due
to the thermodynamic equilibrium, more or less complex
technologies were reported. One can mention the use of an
excess amount of alcohol and the continuous removal of
the ethanol–water azeotrope, i.e. by reactive distillation [5,
6]. Note that the ethanol recycling is problematic too and
requires a further dehydration step. The use of catalytic
membranes was also reported to shift the reaction equi-
librium [7, 8]. Earlier patents already reported the appli-
cation of reactive extraction to shift the esterification
equilibrium and promote the yield in the ester product. One
can distinguish two approaches which were applied to
upgrade mixtures of organic acids of low volatility into
mixtures of esters of higher volatility which can be sepa-
rated by distillation. The older one reports the use of water
miscible alcohol and of a water immiscible extractive
solvent [9]. The acid and the alcohol remain in the aqueous
phase while the esters are extracted in the non-aqueous
phase. The preferred solvents were conventional volatile
ones, such as alkanes or aromatics which can be separated
by distillation and reused. A more recent approach reported
the preparation of esters from acids miscible in water and
water immiscible alcohols in the presence of an aprotic
solvent [10].
The concept of extractive esterification applied for ethyl
lactate preparation would present two advantages: the shift
of the esterification reaction equilibrium and the limitation
of secondary reactions due to the functionalities of lactic
acid.
As regard to the catalytic aspect of the esterification,
homogeneous mineral acids, such as sulfuric acid, are
known to promote efficiently the esterification of various
carboxylic acids [9, 10]. Nevertheless, the use of homo-
geneous acid catalysts presents drawbacks such as the
hard catalyst recovery, the production of acidic waste
liquids, and the inescapable corrosion of the equipment.
The use of solid acids is an alternative. Sulfonated acidic
resins are reported to be one of the most efficient solid
acid catalysts for esterification reactions [12]. Although
efficient, the resins remain expensive catalysts which can
be hardly regenerated due to their limited thermal sta-
bility. The heterogeneous acidic potassium salt of
12-phosphotungstic acid (K2.5H0.5PW12O40) was previ-
ously reported as strong Brønsted solid acid of high
potential for petrochemical applications owing to its
strong acid strength, close to that of the parent acid
H3PW12O40 [13]. Moreover, the acidic potassium salt has
a BET surface area higher than 100 m2 g-1, more than
ten time that of H3PW12O40 and is insoluble in polar
media by contrast to the bulk acid [14]. The heteroge-
neous catalysts are non-corrosive, easy to separate from
the reaction mixture, and can be used repeatedly over a
prolonged period without any difficulty in handling and
storing.
In the present work, to overcome these shortcomings of
the esterification of lactic acid, a heterogeneous hetero-
polyacid and other reference catalysts were evaluated in a
biphasic solvent system consisting of a polar reactive
phase, containing the esterification constituents lactic acid/
ethanol/acid catalysts and an extractive phase, fatty acid
methyl ester, in which the ethyl lactate has a greater sol-
ubility than in the reactive phase. This situation would
permit to shift and to reach more rapidly the esterification
equilibrium. In other respect, fatty acid methyl ester was
also selected as regards to the solvent properties of mix-
tures of fatty acid methyl ester and ethyl lactate. Hence, the
addition of fatty acid methyl ester in the esterification
reaction would not only increase the yield of the esterifi-
cation reaction due to the continuous extraction of ethyl
lactate in the biodiesel, but would produce a biosolvent
composition in one step.
Recent articles report the use of biphasic solvents sys-
tems, not to shift an equilibrium but to prevent secondary
123

952
T. H. T. Vu et al.
2 Experimental
2.1 Materials and Catalysts
The reaction solution was obtained by mixing 66.7 g of
ethanol with 39.1 g of lactic acid (molar ratio of EtOH/acid
lactic = 3.3). 0.33 g of 96 % H2SO4 (6.46 mmol of H?)
catalysts was added to the mixture solution or 1.4 g of solid
acid catalysts, K2.5H0.5PW12O40 (0.235 mmol of H?) or
Amberlyst 15 (6.6 mmol of H?). The solid catalysts were
dehydrated before use; for one night in an oven at 110 °C.
The reaction mixture is stirred by means of a magnetic
stirrer and heated at 80 °C. The yield of ethyl lactate was
determined all along the reaction progress by regular GC
analysis.
Lactic acid, ethanol (99.7 wt%) and 12-phosphotungstic
acid (H3PW12O40Á21H2O) were purchased from Sigma
Aldrich. Amberlyst 15 resin was obtained from Rohm &
Hass Co.
The acidic potassium salt of 12-phosphotungstic acid,
K
2.5H0.5PW12O40 (KPW) was prepared according to the
literature procedure [16, 17] by adding dropwise the
required amount of a saturated aqueous solution of KCl to
an aqueous solution of H3PW12O40 (0.1 M) under stirring.
The precipitate obtained was filtrated, washed with distilled
water and then dried at 70 °C for 10 h and then at 120 °C
for 5 h.
To perform the reaction in a biphasic liquid media, 24 g
of methyl ester of rapeseed oil were added.
3 Results and Discussion
3.1 Catalyst Characterizations
2.2 Catalysts Characterizations
The XRD pattern of KPW catalyst confirms that the
potassium salt has the expected cubic phase, in agreement
with previous studies.
The Infrared spectrum of the heteropolyacid sample was
recorded on a Brucker FTIR spectrometer with catalyst
sample diluted in KBr disk, within the 400–4,000 cm-1
wavenumbers range.
The FTIR spectrum indicate also that the Keggin
structure is retained in the KPW catalyst. The FTIR spec-
trum of KPW catalyst (shown in Fig. 1) exhibits the
characteristic vibration bands of the Keggin anion
(Scheme 4). In the range 1,100–600 cm-1, KPW catalyst
presents strong bands at 1,080 and 986 cm-1, which are
characteristic of P–Oa and W–Od vibrations, respectively.
The peaks at 891 and 808 cm-1 are ascribed to W-Ob-W
and W-Oc-W bridges vibrations, respectively (Fig. 1).
The calorimetric curves recorded on KPW present a
short plateau compared to that measured on the Amberlyst
A15 (Fig. 2). Note that, for the latter case, the experiment
The X-ray diffraction pattern of the heteropolyacid
sample was recorded with a Bruker D5005 diffractometer
˚
with the Cu Ka1?a2 (k = 1.54184 A).
Nitrogen adsorption/desorption isotherms were recor-
ded at liquid nitrogen temperature with an equipment
supplied by Micromeritics (ASAP 2020). Before analysis,
K2.5H0.5PW12O40 was vacuum treated at 200 °C for 2 h.
The specific surface area was calculated using the BET
method in the range of relative pressure from 0.03 to 0.2.
The BET surface area of
K2.5H0.5PW12O40 equals
150 m2 g-1, an expected value for acidic alkaline salts of
H3PW12O40 prepared by precipitation using saturated
solutions of precursors [16, 17].
Calorimetry of NH3 adsorption The acid properties were
measured by NH3 adsorption at 80 °C, using a TianCalvet
calorimeter coupled with a volumetric equipment. The
samples of K2.5H0.5PW12O40 or Amberlyst 15 (0.1 g) were
evacuated at 100 °C for 1 h under secondary vacuum.
Then, the pre-treated samples were placed into the calo-
rimeter up to the stabilization of the temperature (one
night), then contacted with small doses of gas up to equi-
librium and the differential enthalpy of adsorption was
recorded together with the amount of adsorbed NH3.
Before reaction
After reaction
0.5
2.3 Esterification of Lactic Acid
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
The synthesis of ethyl lactate was carried out in Pyrex flask
equipped with a condenser and under argon atmosphere.
Fig. 1 FTIR spectrum of fresh and used KPW catalyst
123

Esterification of Lactic Acid
953
3.2 Esterification Reaction
was stopped before the saturation of all the acid sites; when
the experiment was stopped, only 25 % of the expected
total amount of H? (4.7 mmol g-1 according to Rohm &
Hass Co.) was probed. For the KPW sample, the amount of
sites probed with ammonia equals 0.168 mmol g-1, a
value in close agreement with the expected one considering
the stoichiometry of the catalyst: K2.5H0.5PW12O40. Indeed,
for the parent acid H3PW12O40, the amount of titrated acid
sites, probed by calorimetry of NH3 adsorption, achieved
0.850 mmol g-1. This value is five times higher to that
obtained on K2.5H0.5PW12O40 [18] and corresponds to 0.6
protons per P atom instead of 0.5. Both samples present
initially a few amount of sites of very high strength, then a
plateau is observed for both samples. The differential heat
of NH3 adsorption at half NH3 coverage was respectively
of 190 and 135 kJ mol-1 for KPW and Amberlyst A15.
These results clearly demonstrate the higher acid strength
of the KPW sample compared to the Amberlyst A15.
3.2.1 Homogeneous Versus Heterogeneous Acid Catalysts
in Monophasic Solvent Media
The result of esterification of lactic acid using different cata-
lysts is presented in Fig. 3 together with that obtained in
absence of catalyst. Without catalyst addition, the yield in
lactate increased linearly during the first 2 h, then the rate of
esterification reaction slowed down to give an yield in ethyl
lactate of 30 % after 6 h of reaction. The addition of an acid
catalyst, homogeneous or heterogeneous, i.e. H2SO4, Amber-
lyst 15 or KPW, increased significantly the initial rate of
esterification (Fig. 3; Table 1), and the ethyl lactate yield after
6 h of reaction is farabove the value achieved in the absence of
catalyst. The highest initial rate in ethyl lactate formation is
obtained with the homogeneous mineral acid H2SO4,
0.45 mol h-1, more than two times the values obtained with
the heterogeneous acid catalysts. After 6 h of reaction, in the
presence of H2SO4 or Amberlyst 15, the curves representing
the yield in ethyl lactate with time, show a quasi plateau with a
value in ethyl lactate yield of 50 %. With KPW, after 6 h a
reaction, the yield in ethyl lactate still increases slowly, indi-
cating that the thermodynamic equilibrium is not reached, the
ethyl lactate achieved 60 % after 6 h. This also indicates that
part of its superficial acid sites is still active for ethyl lactate
formation. Note that, for this set of experiments, the weight
amount of Amberlyst 15, was chosen in order to adjust the
protonsamounttothatofH2SO4. Thehigherinitialrateofethyl
lactate formation observed with H2SO4 could be explained by
the more accessible protonic sites compared to the heteroge-
neous catalyst and/or to the highest acidity of H2SO4, but any
ranking in terms of acid strength between homogeneous and
heterogeneous catalysts is always questionable. In other
respects, the amount of KPW was equivalent to that of the
resins, but one can remark that the protonic densities of the two
heterogeneous catalysts are different: 0.168 mmol H? g-1 for
KPW and 4.7 mmol H? g-1 for Amberlyst 15. The initial
rates of ethyl lactate formation per acid sites (see Table 1),
evidenced that the value measured on KPW is more than ten
time that of H2SO4 or Amberlyts 15. The higher activity per
acid sites of KPW compared to Amberlyst 15 can logically be
explained by its highest acid strength as shown by calorimetry
of NH3 adsorption (see Section 3.1). More hardly explained, is
the highest yield in ethyl lactate achieved over KPW after 6 h
of reaction. A priori, the value of a (quasi)plateau depends on
the thermodynamic equilibrium and/or on the poisoning of the
acid sites. An alternative explanation could be the occurrence
the side reactions such as the ethyl lactate self-transesterifica-
tion producing heavy products (Scheme 3). It is reasonable to
think that a few amount of this by-product could remain
strongly adsorbed on the superficial acid sites and inhibit the
solid acid activity. In addition, this consecutive secondary
Fig. 2 Calorimetric curves of NH3 adsorption on KPW and Amber-
lyst 15
70
60
50
40
30
without cata
20
10
0
KPW
Amberlyst 15
H2SO4
0
100
200
300
400
Time (min)
Fig. 3 Esterification of lactic acid with ethanol in mono-phasic
solvent medium. Influence of acid catalyst addition. Conditions: m
EtOH = 66.7 g, m lactic acid = 39.1 g, mH2SO4 (96 %) = 0.33 g
(6.46 mmol H?), m A15 = 1.4 g (6.6 mmol H?), m KPW = 1.4 g
(0.235 mmol H?), T = 80 °C (reflux)
123

954
T. H. T. Vu et al.
Table 1 Initial rate of ethyl
lactate formation. Influence of
the nature of the catalysts and of
the biphasic medium composed
of the reactive phase (lactic
acid/ethanol/catalyst) and the
extractive solvent selective of
ethyl lactate (Biodiesel)
Reaction liquid media
Catalysts
Initial rate of ethyl
lactate formation
Initial rate of ethyl lactate
formation per acid sites
(mol h-1
)
[mol (mol H? h)-1
]
Without
H2SO4
0.06
0.45
0.16
0.18
0.59
0.42
0.63
–
70
Mono-phasic
Bi-phasic
Amberlyst 15
KPW
24
766
91
H2SO4
Amberlyst 15
KPW
64
2,680
100
90
80
70
60
50
40
30
20
10
0
bi-phasic
mono-phasic
system
KPW
Amberlyst 15
H2SO4
KPW bi-phasic
Amberlyst 15 bi-phasic
H2SO4 bi-phasic
0
100
200
300
400
Time (min)
Fig. 4 Esterification of lactic acid with ethanol in the presence of
different acid catalysts. Comparison between the bi-phasic and the
mono-phasic solvent system. Conditions: m EtOH = 66.7 g, m lactic
acid = 39.1 g, mH2SO4 (96 %) = 0.33 g (6.46 mmol H?), m A15 =
1.4 g (6.6 mmol H?), m KPW = 1.4 g (0.235 mmol H?), T = 80 °C
(reflux)
Scheme 1 Extractive catalytic reaction concept: application to ethyl
lactate formation
O
reaction might be favored by the high protonic density com-
bined to the absence of steric hindrance over Amberlyst 15.
However, this explanation does not hold true for the homo-
geneous sulfuric acid. Therefore, to rationalize the higher ethyl
lactate yield over KPW as compared to A15 and H2SO4, dis-
tinct water tolerance appears as a more likely explanation.
Since the main difference between the two solid acid catalysts
is their protonic density, one can propose that KPW has cer-
tainly the more hydrophobic surface which might limit its
deactivation by water adsorption during the course of reaction.
KPW would be more tolerant to the competitive adsorption of
water or solvation of the proton by the water produced by the
esterification reaction. KPW should permit to reach more
rapidly the thermodynamic equilibrium compared to the two
other catalysts. This can rationalize the higher lactate yield
obtained on KPW after 6 h of the reaction.
O
O
H
2
2 H2O
O
O
OH
O
lactic acid
dilactide
Scheme 2 Self-esterification of lactic acid
the ethyl lactate. Performing the reaction in a bi-phasic
liquid system leads to remarkable results: the yield in ethyl
lactate was significantly increased. After 6 h of reaction,
for H2SO4 and Amberlyst 15, the yield in ethyl lactate
achieve 80 % while this value is even overreached for
KPW after 2 h of reaction. In the biphasic system, the
initial rates of reaction are strongly increased, by three fold
in the case of the heterogeneous acid catalysts. The initial
rate measured in the presence of H2SO4 is less modified in
the presence of the extractive phase.
3.2.2 Esterification of Lactic Acid Carried Out
in Bi-phasic Solvent System
Figure 4 shows the evolution of the yield of esterification
with the course of reaction, in the absence or presence of
methyl ester of fatty acid as selective extractive solvent of
Hence, these results demonstrate that the esterification
of lactic acid in a bi-phasic solvent system, containing an
123

Esterification of Lactic Acid
955
Scheme 3 Self-
transesterification of ethyl
lactate
O
O
2
Et
O
+
EtOH
O
O
Et
OH
OH
O
Ethyl lactate
90
80
70
60
50
40
30
20
10
0
first run
second run
third run
0
100
200
300
400
Time (min)
Fig. 5 Esterification of lactic acid with ethanol catalyzed by KPW.
Recycling study. Conditions: m EtOH = 66.7 g, m lactic acid =
39.1 g, m KPW = 1.4 g (0.235 mmol H?) T = 80 °C (reflux)
Scheme 4 Keggin structure
extractive solvent selective of the ethyl lactate is a simple
way to shift the equilibrium of esterification leading to high
yield in ethyl lactate. This methodology is certainly operative
for most of the carboxylic acids derived from natural sources
such as succinic, itaconic, aconitic acids and others.
of the vibrations bands of Keggin anions in the range
500–1,200 cm-1
.
In the peculiar case of lactic acid, the presence of bio-
diesel not only increased the yield of the esterification
reaction, but also provided in one reaction step, an efficient
biosolvent composition, made of ethyl lactate and methyl
ethyl esters of fatty acid (biodiesel) (Scheme 1).
4 Conclusion
The results presented in this paper demonstrate that
biosolvent compositions can be prepared in one step by
esterification of lactic acid with ethanol with excellent
yield in the presence of non volatile solvent based on
fatty acid esters, a solvent selective of ethyl lactate. This
is a successful application of the concept of catalytic
extractive reaction. In addition, K2.5H0.5PW12O40 is
shown to be a more efficient catalyst than the conven-
tional esterification catalysts, H2SO4 and Amberlyst 15.
Its efficiency is ascribed to its strong acid strength
associated to a low protonic density which is proposed to
insure an adequate superficial hydrophobicity which
makes this catalyst resistance towards water poisoning.
This strong Brønsted acid catalyst is environmentally
friendly, non-toxic, reusable and in addition, much more
cheaper than its more famous homologue the acidic
cesium salts Cs2.5H0.5PW12O40. The present procedure
represents a clean, practical, and simple method to pre-
pare biosolvents composition in one step [15].
3.2.3 Catalyst Recycling
The recycling investigations were done with the most
efficient catalyst, KPW. After a first esterification reaction
of 420 min, the solid catalyst was separated by filtration,
washed with ethanol, dried at 100 °C for 3 h and then re-
used with fresh reaction mixture under identical conditions
as the first cycle. The recycle procedure was repeated two
times. The results of esterification recycles are shown in
Fig. 5. There were no significant differences in the yield of
the esterification reaction between each cycle performed
over the KPW catalysts, showing that KPW catalyst can be
reused in the esterification reaction of lactic acid in a bi-
phasic solvent system. The excellent recyclability of KPW
can be explained by the catalyst stability in the reaction
medium. Indeed, the FTIR spectrum of KPW recovered
after the first use didn’t show any detectable modification
123

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T. H. T. Vu et al.
Acknowledgments The authors thank the Ministry of Science and
Technology, Programme HoaSen Lotus for support of this work.
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13. Essayem N, Ben Taarit Y, Sapaly G, Naccache C (2001) J Catal
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