High yield of ethyl valerate from the esterification of renewable valeric acid catalyzed by amino acid ionic liquids

Article abstract of DOI:10.1039/c3ra23034a

Ethyl valerate (EV) as a promising fuel additive was produced by esterification of valeric acid with ethanol over Bronsted acidic amino acid ionic liquids. Hammett method and density functional theory (DFT) calculations were preformed to evaluate the acidities of the catalysts. The composition of catalyst, reaction temperature, reaction time, molar ratio of reactants, amount of catalyst, and recycling ability of the catalyst were investigated. Proline bisulfate (ProHSO₄) ionic liquid has the highest catalytic activity and the best recyclability under the optimized esterification conditions. A high conversion of valeric acid (>99.9%) was obtained for 7 h at 80 °C, with 100% selectivity of EV. The density, viscosity, melting point, boiling point, elemental analysis and heat of combustion of the EV product were measured. The density of EV is 0.896 g cm ^-3. The viscosity of EV was 1.7 cP at room temperature. The heating values of EV are 4158.1 kJ mol^-1 and 31.9 kJ g^-1. EV obtained from esterification has higher energy density than methanol, ethanol, γ-valerolactone, and valeric acid, which illustrates that EV is a promising biofuel candidate.

Full text of DOI:10.1039/c3ra23034a

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High yield of ethyl valerate from the esterification of
renewable valeric acid catalyzed by amino acid ionic
liquids3
Cite this: RSC Advances, 2013, 3,
4806
Lin-Lin Dong, Ling He, Guo-Hong Tao* and Changwei Hu*
Ethyl valerate (EV) as a promising fuel additive was produced by esterification of valeric acid with ethanol
over Brønsted acidic amino acid ionic liquids. Hammett method and density functional theory (DFT)
calculations were preformed to evaluate the acidities of the catalysts. The composition of catalyst, reaction
temperature, reaction time, molar ratio of reactants, amount of catalyst, and recycling ability of the
catalyst were investigated. Proline bisulfate (ProHSO4) ionic liquid has the highest catalytic activity and the
best recyclability under the optimized esterification conditions. A high conversion of valeric acid (.99.9%)
was obtained for 7 h at 80 uC, with 100% selectivity of EV. The density, viscosity, melting point, boiling
point, elemental analysis and heat of combustion of the EV product were measured. The density of EV is
0.896 g cm23. The viscosity of EV was 1.7 cP at room temperature. The heating values of EV are 4158.1 kJ
mol21 and 31.9 kJ g21. EV obtained from esterification has higher energy density than methanol, ethanol,
c-valerolactone, and valeric acid, which illustrates that EV is a promising biofuel candidate.
Received 23rd November 2012,
Accepted 29th January 2013
DOI: 10.1039/c3ra23034a
www.rsc.org/advances
Ethyl valerate (EV) is the product of the esterification of VA
and ethanol, and was considered as a possible second
Introduction
generation biofuel (Scheme 1).4 An experimental and kinetic
modeling study for the combustion characteristics of EV was
carried out.4c The acceptable energy density and more
appropriate polarity of EV make it suitable for gasoline
applications. Furthermore, the material safety data sheet
(MSDS) shows that EV is a compound with low hazardousness
and it is mild irritant. The acceptable low polarity of EV makes
it less sensitive to elastomer swell or water pickup than
ethanol. 10–20 vol% of EV splash-blended in regular gasoline
The depletion of fossil fuels, environmental issues, and future
energy requirements have propelled biomass as the most
possible alternative resource to petroleum for fuels and
chemicals. Starch based plants as renewable energy-storage
biomass have been converted into biofuels in recent years.1
However, the large-scale supply of starch based plants as the
feedstock for fuels has resulted in direct competition with
food. Cellulosic materials, a sort of cheap and abundant
renewable feedstock, can serve as a sustainable source of
carbon for the fuel and chemical industries. From lignocellu-
lose, several important platform molecules including 5-hydro-
xymethylfurfural (HMF), levulinic acid (LA), c-valerolactone
(GVL), valeric acid (VA) and some important chemicals like 1,2-
alkanediols have been obtained (Scheme 1).2 GVL was
considered as an important feedstock for its potential
application in the sustainable utilization of energy and has
been widely investigated since 2008.3 VA is the key component
in the hydrogenated product of GVL. This acid itself has 31%
oxygen content, which is still too high to be a good biofuel
candidate. Dumesic et al. described a series of conversion
routes of cellulose and got LA, GVL, VA, a mixture of C9 and
C18 olefins and a mixture of alkanes stepwise.3e
College of Chemistry, Sichuan University, Chengdu 610064, China.
E-mail: taogh@scu.edu.cn; gchem@scu.edu.cn; Fax: (+86)-28-85470368;
Tel: (+86)-28-85470368
Electronic supplementary information (ESI) available: See DOI: 10.1039/
3
Scheme 1 Schematic strategy of ‘‘valeric biofuels’’.
c3ra23034a
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still meets the research and motor octane number (RON and
MON) specification for European gasoline (EN 228). Road
trials using a 15% blend of EV in regular gasoline were carried
out with 10 vehicles and a total of about 250 000 km of driving.
Esterification is an important industrial process for ester
productions in chemical and pharmaceutical industries,
which has also attracted much fundamental academic inter-
est. In esterification, reactive acid derivatives or coupling
agents are often required to shift the equilibrium toward the
desired ester in high yield. Dehydrating agents such as
surfactants, zeolite, scCO2, and fluorous solvents5 have been
investigated to remove water immediately from the reaction
system. However, homogeneous processes using inorganic
acids or organic acids6 as catalysts have the disadvantages of
corrosiveness, high susceptibility to water, difficulty in catalyst
recovery, and deterioration of the side reactions, while
heterogeneous esterification catalyzed by supported metal
oxide, zeolites, heteropolyacids, or resins5c,7 is also proble-
matic with the removal of adsorbed products and rapid
deactivation as well as restricted acidic sites of the catalysts.
Furthermore, lipase enzymes were also used to catalyse
esterification, while their high costs and tough purification
procedures render the enzymes economically unattractive.8
Therefore, an ideal greener approach would be a biphase
esterification employing an environment-friendly catalyst
without any additional dehydrating agent. Such a system
could be auto-phase-separation even in the presence of
carboxylic acid, alcohol and water. Some acidic ionic liquids
have acted as Brønsted acids in biphase esterification,9 with
auto-phase-separation after the esterification reaction. That is,
because the Brønsted acidic ionic liquids could mix well with
carboxylic acids and short-chain alcohols but are immiscible
with esters, the system was a monophasic solution at the
beginning of the esterification reaction, but ended in a
biphasic mixture. They were more selective to the desired
esters accordingly.
In 2005, Kou et al. reported the ‘‘green’’ amino acid ionic
liquids ([AA]X and [AAE]X)10 and their intrinsic Brønsted
acidity. Compared with previous Brønsted acidic ionic
liquids,9 L-amino acids derived from natural materials were
chosen as precursors. Protic amino acid ionic liquids [AA]X
can be easily obtained by one-step protonation reactions
without any byproduct or complicated post-processing. In
addition, they are low cost and can be produced in significant
quantities.10 Herein, three [AA]X amino acid ionic liquids
based on bisulfate anion were prepared. Their acidities were
studied by the Hammett method and theoretical calculation.
These amino acid ionic liquids were used as acidic catalysts for
the esterification of VA with ethanol to obtain EV. The
esterification reaction parameters including composition of
catalyst, reaction time, reaction temperature, molar ratio of
reactants, amount of catalyst as well as recycling ability were
investigated. The fuel performances of EV products were also
evaluated.
Experimental
Chemicals and instruments
All chemicals of analytical grade were obtained commercially
and used as received. Solvents were dried by standard
procedures. Infrared spectra (IR) were recorded by using KBr
plates on a Nicolet NEXUS 670 FT-IR spectrometer. 1H and 13
C
NMR spectra were recorded on a Bruker 400 MHz nuclear
magnetic resonance spectrometer with d6-DMSO as locking
solvent unless otherwise stated. 1H and 13C chemical shifts
were reported in ppm relative to TMS. The products obtained
in each experiment were analyzed using a GC9790II equipped
with an AT SE-54 column (30 m 6 0.25 mm 6 0.25 mm) and
FID detector. The ester yield was quantified by GC using
cyclohexane as internal standard. The density of EV was
measured at 25 uC on a Micromeritics Accupyc 1330 gas
pycnometer. Elemental analysis of EV was performed on a
CARLO ERBA 1106 Elemental Analyzer. The viscosity was
measured with a NDJ-1B-1 viscometer at room temperature.
Synthesis of the catalysts
The amino acid ionic liquids were synthesized similar to the
literature procedure10 by direct protonation reactions of the
corresponding amino acid precursors with dilute sulfuric acid.
The ratio of amino acid and sulfuric acid must be kept to 1 : 1
in mole to obtain bisulfate ionic liquids. The preparation is
one-step protonation reaction in water without any byproduct
or complicated post-processing, which is an atom-economic
reaction (Scheme 2). Thus the preparation of the amino acid
ionic liquids is highly efficient and low cost.
Proline bisulfate (ProHSO4). To a 50 mL round flask, proline
(1.151 g, 10 mmol) was dissolved in 40 mL water with stirring.
An equal molar ratio of sulfuric acid (3.0 mol L21 aqueous
solution) was dropped into the flask and kept stirring for 1 h at
room temperature. The mixture was concentrated and
evaporated under vacuum to obtain a colorless transparent
liquid in high yield (yield 99%).
1H NMR, d (ppm): 4.27 (m, 1H), 3.21 (m, 2H), 2.27 (m, 1H),
1.93 (m, 3H); 13C NMR, d (ppm): 170.33, 58.82, 45.46, 27.78,
23.01. IR (KBr, cm21): n = 3172, 1739, 1584, 1420, 1335, 1140,
1053, 875, 659, 577.
Glycine bisulfate (GlyHSO4). The same procedure was
followed as that described above for ProHSO4. Glycine (751
mg, 10 mmol) and an equal ratio of sulfuric acid (3.0 mol L21
aqueous solution) were reacted for 1 h to obtain a white solid
(yield 99%).
Scheme 2 The preparation of the amino acid ionic liquids ProHSO4, GlyHSO4,
and AlaHSO4.
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1H NMR, d (ppm): 3.66 (s, 2H, CH2); 13C NMR, d (ppm):
with standard deviations calculated as a measure of experi-
mental uncertainty. The bomb was examined for no evidence
of unburned carbon after each run. In all measurements, the
calorimeter was calibrated by the combustion of certified
benzoic acid in an oxygen atmosphere at the same pressure of
3.05 MPa, combined with a correction for burned wire (1400
cal cm21).
169.19, 40.17. IR (KBr, cm21): n = 3473, 3225, 2362, 1750, 1606,
1510, 1433, 1175, 1048, 866, 583, 476.
Alanine bisulfate (AlaHSO4). The same procedure was
followed as that described above for ProHSO4. Alanine (891
g, 10 mmol) and an equal ratio of sulfuric acid (3.0 mol L21
aqueous solution) were reacted for 1 h to obtain a colorless
transparent liquid (yield 99%).
1H NMR, d (ppm): 3.95 (m, 1H, CH), 1.40 (d, 3H, CH3); 13C
NMR, d (ppm): 171.46, 47.87, 15.70. IR (KBr, cm21): n = 3281,
1748, 1609, 1511, 1386, 1112, 871, 611, 516.
Results and discussion
The acidity of amino acid ionic liquids
Hammett acidity determination
The acidity determination by the Hammett method. The
efficiency of esterification is closely associated with the acidity
of the catalysts. For nonaqueous Brønsted acids, the acidity
could be studied using electrochemical windows or Hammett
acidity function.12 Herein, the amino acid ionic liquids with
labile protons exhibit Brønsted super acidity. Their acidity
could not be determined using a pH meter. The Hammett
acidity function method given by Gilbert et al.12d was used to
evaluate the acidity of the amino acid ionic liquids. Samples in
ethanol and methanol solution were measured by UV/Vis
spectroscopy. The Hammett function (H0) values of ionic
liquids and sulfuric acid are summarized in Table 1. The
Hammett acidity function (H0) can be expressed by the
equation: H0 = pKa(In) + log([In]/[InH+]). pKa(In) is the pKa
value of the p-nitroaniline indicator solution. [In] and [InH+]
are the molar concentrations of the protonated and unproto-
nated forms of p-nitroaniline indicator, respectively. In
ethanol system, a solution of 0.05 mol L21 sulfuric acid gave
a H0 value of 1.20. Amino acid ionic liquid ProHSO4 gave a H0
value of 2.74. AlaHSO4 gave a similar H0 value of 2.78.
However, the solubility of GlyHSO4 in ethanol solution was
very limited, and 0.05 mol L21 solution was not able to be
obtained. Thus, the value of GlyHSO4 was not comparable with
the values of ProHSO4 and AlaHSO4. Methanol as a good
solvent for all of the three amino acid ionic liquids was
selected. In methanol system, a solution of 0.05 mol L21
sulfuric acid gave a H0 value of 1.00. Amino acid ionic liquid
ProHSO4 gave a H0 value of 1.76. GlyHSO4 gave the same H0
value of 1.76 as that of ProHSO4. AlaHSO4 gave a H0 value of
1.82, which was a little higher. The values of the three amino
Hammett acidity function study of the ionic liquids was
performed on a HITACHI Model U-4100 UV/Vis spectrometer.
Samples were measured in sealed 1 mm quartz cuvettes
(Helma). Absorbance values, A, of indicator based on the ionic
liquids in methanol and ethanol were recorded between 350
and 400 nm in steps of 0.1 nm. The dye p-nitroaniline (Fluka,
¢99%) was used as an indicator and molecular probe for the
determination of acidity, H0, with 7.5 6 1025 mol L21. The
concentration of ionic liquid was 5.0 6 1022 mol L21. An
appropriate amount of solution was placed into vials which
were sealed for determination.
General procedure for EV production
In a typical esterification procedure, appropriate amounts of
VA, ethanol and ionic liquids were placed in a 25 mL round
bottom flask. The reaction was refluxed under the desired
temperature in an oil bath equipped with a thermostat. After
reaction, the reaction mixture formed two liquid phases. The
product layer was directly separated and detected by GC to
provide conversion and selectivity data. Conversions were
calculated from the amount of EV/the amount of VA in moles.
The experiments were repeated to control the error less than
¡1%. Each data set was repeated at least three times. The
separated ionic liquid layer was treated at 100 uC under
vacuum to remove water and reused immediately (see Fig. 1).
Bomb calorimetry
The calorimetric measurement was performed using a Parr
6725 bomb calorimeter (static jacket) equipped with a Parr 207
A oxygen bomb for the combustion of EV products. EV
products were carefully dropped on an analytical grade
benzoic acid pellet, which was subsequently burned in a 3.05
MPa atmosphere of pure oxygen. The pellet (y100 mg) was
loaded in a platinum pan and a Parr 45C10 alloy fuse wire was
used for ignition. The experimental heat of combustion was
obtained from the average of three separate measurements
Table 1 Hammett function values of various ionic liquidsa
e
Acid
Absorbanceb [In] (%)c
[InH+] (%)d H0
none
H2SO4
1.20/1.13
0.74/0.63
1.18/1.06
—/1.06
100/100
61.7/50.8 38.3/49.1
98.3/85.5 1.7/14.5
—/85.5
98.4/87.1 1.6/12.9
99.1
87.6
0/0
—/—
1.20/1.00
2.74/1.76
—/1.76
2.78/1.82
3.03
ProHSO4
GlyHSO4
AlaHSO4
—/14.5
1.18/1.08
[MIM][CH3SO3]f 1.12
[NMP][CH3SO3]f 0.99
0.9
12.4
1.84
a
The Hammett function values were obtained in ethanol/methanol
solution. Determination conditions (25 uC): c(In) = 7.5 6 1024 mol
L
molar concentration of the p-nitroaniline indicator. The molar
b
c
21, c(H2SO4) = 0.05 mol L21, c(ionic liquids) = 0.05 mol L21
.
The
d
e
concentration of the protonated p-nitroaniline. H0 = pKa(In) +
log([In]/[InH+]), pKa(In) = 0.99. See ref. 11.
f
Fig. 1 The esterification for EV catalyzed by amino acid ionic liquids.
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acid ionic liquids are all higher than that of sulfuric acid,
which means their acidities are a little weaker than that of
sulfuric acid. The acid strengths are similar to those of
common Brønsted acidic ionic liquids like [NMP][HSO4], and
much stronger than [MIM][CH3SO3], which gave a H0 value of
3.03.
Density functional theory (DFT) calculations of the acidity.
On further investigation of the acidity of the amino acid ionic
liquids, the acid dissociation constants (pKa) of the amino acid
ionic liquids were accurately calculated using the Gaussian(R)
09 program.13 The protonated amino acid ionic liquids and
deprotonated amino acid ionic liquids were optimized in the
gas phase using the popular Becke three-parameter hybrid
functional Lee–Yang–Parr (B3LYP) analyses up to the
Scheme 3 Thermodynamic cycle used in the calculation of the pKa of the amino
acid ionic liquids.
the lowest (22.0) among the three amino acid ionic liquids,
while the pKa value of GlyHSO42H was the highest (21.8).
According to the definition of the pKa value, the H+ in ProHSO4
was the easiest one to dissociate, and the acidity of ProHSO4
was the strongest accordingly.
6-311+G** basis set,14 and G(A2
) and G(AHgas) were
gas
obtained accordingly. Then the PCM solvation model devel-
oped by Tomasi and co-workers15 was used to calculate the
solvation free energies of amino acid ionic liquids and
deprotonated amino acid ionic liquids in ethanol.
The definition of pKa is
Esterification to produce EV
The composition of catalysts. As a comparison, no EV could
be detected from the reaction of VA and ethanol at 25 uC in the
absence of acidic catalyst (Table 3, entry 1). Concentrated
sulfuric acid (98%) was also used in the esterification for
comparison where less than 1% EV was detected at room
temperature (entry 2), and only about 67.0% of EV was found
even at 80 uC (entry 4). As a comparison, the esterification
results of VA and ethanol by lipase enzyme are listed in
Table 3. The lipase process needs up to 9 days to reach 97.8%
esterification. Amino acid ionic liquids show better catalytic
reactivity than H2SO4 at both room temperature and 80 uC
(entries 3, 5–7). Their efficiencies are also much higher than
pKa = 2logKa
and in gas phase standard state
(1)
DGu = 2.303RT logKa
pKa = DGu/2.303RT
(2)
(3)
so,
From the thermodynamic cycle, we get
pKa = DGaq/2.303RT
(4)
(5)
Table 2 Theoretical gas-phase data of amino acid ionic liquids and their pKas in
ethanola
DGaq = Gaq(H+) + Gaq(A2) 2 Gaq(AH)
DG* (kcal mol21
)
pKa
So, in the standard state of
temperature,
1
mol L21 and room
Ggas(a.u)
Gs(a.u)
s
GlyHSO42H 2984.809054 2984.938394 281.162079
AlaHSO42H 21024.112701 21024.26728 297.001046
ProHSO42H 21101.51296 21101.70059 2117.74023
21.8
21.9
22.0
DG* = G*aq(H+) + G*aq(A2) 2 G*aq(AH)
(6)
aq
a
It is necessary to add 1.89 kcal mol21 (RTln(1/RgT)) to
convert the calculated gas phase standard free energy, Gugas
from its standard state of 1 atm gas phase/1 mol L21 solution
The protonated amino acid ionic liquids and deprotonated amino
acid ionic liquids were optimized in the gas phase at B3LYP/6-
311++G(d,p).
,
to G*
with a standard state of 1 mol L21 gas/1 mol L21
gas
solution phase. The values of G(H+gas) and DGs(H+) are derived
from literature as 26.28 kcal mol21 and 2265.9 kcal mol21
respectively.16
Table 3 The esterification to produce EV over different catalystsa
G* = G* + DG* = (Gugas + 1.89) + DG*
sol
(7)
Entry
Catalyst
Blank
T/uC
Conversion (%)b
Selectivity (%)b
aq
gas
sol
1
2
3
4
5
6
7
8
25
25
25
80
80
80
80
37
0
,1
—
—
100
88
100
100
100
—
c
Combining these values and the equations, the pKa values
using the thermodynamic cycle (Scheme 3) were finally
obtained using eqn (8).
H2SO4
ProHSO4
62.4
67.0
.99.9
84.7
87.1
97.8
c
H2SO4
ProHSO4
GlyHSO4
AlaHSO4
Lipased
pKa = [Gu(A2gas) + Gu(AHgas) + DG* (A) 2 DG* (AH) 2 270.29]/
s
s
2.303RT
(8)
a
Reaction conditions: catalyst: 50 mol%, based on VA, VA/ethanol
The results are listed in Table 2. We can see that the
theoretical pKa values of the three amino acid ionic liquids are
all less than 0. The theoretical pKa value of ProHSO42H was
b
(molar ratio) = 1, 7 h. Conversion and selectivity of EV determined
by GC analysis. Concentrated sulfuric acid (98%). pH 7.2, 150
rpm, 9 days, ref. 8.
c
d
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that of lipase.8 Their reaction mechanisms are difference. The
sulfuric acid system is a typical homogenous catalysis reaction.
After reaction, the catalyst, the unreacted reactants, the
byproduct water and the product EV were all in the same
liquid phase. However, the amino acid ionic liquid system is a
biphasic catalysis reaction. For example, in the ProHSO4
system, the reactants (VA and ethanol) and the catalyst were
miscible in a single phase before reaction, which was a
homogenous reaction at the beginning. Because of the poor
solubility of EV in the ionic liquid system, as the reaction
proceeded, a biphasic system formed. Since the reactants VA
and ethanol have good solubility in ProHSO4, even though the
conversion of VA was not 100%, the unreacted reactants may
stay in the ionic liquid layer. Moreover, water is also miscible
with ProHSO4, and as a byproduct stayed in the ionic liquid
layer. In the esterification process, ProHSO4 acts not only as a
catalyst but also as a water absorbent. Thus the equilibrium of
the esterification reaction was shifted automatically to the
product side, without any simultaneous water removal. As a
result, ProHSO4 would facilitate the phase separation of the
product from the reactants and the byproduct during the
esterification, which favours the conversion of VA to achieve
high yield of EV. EV was concentrated into the upper layer by
auto-phase separation, and the reversible reaction of the ester
was restrained.
Compared with the AlaHSO4 and GlyHSO4 systems, the
ProHSO4 system gave the highest conversion of VA, more than
99.9%. The EV yield in the ProHSO4 system is also more than
99.9%. The reason may be the effect of the different strengths
of acidity of the three amino acid ionic liquids. In ethanol,
ProHSO4 has a stronger Brønsted super acidity than the other
two amino acid ionic liquids, based on the experimental and
theoretical evaluation of Brønsted acidity above.
The reaction time. The effect of the reaction time on the
esterification was determined using 50 mol% of ProHSO4 with
1 : 1 (molar ratio) VA/ethanol at room temperature. The results
of the esterification at different reaction times are shown in
Fig. 2. The conversion of VA increased from 15.0% to 62.4%
with the extension of reaction time from 1 h to 7 h. During this
procedure, the selectivity of EV remained at 100%. After 7 h,
the conversion of VA did not clearly increase along with the
longer reaction time. When the time was extended to 24 h, the
conversion remained steady at 63.8%. Therefore, the ideal
reaction time of the esterification for EV production was
selected as 7 h.
The reaction temperature. Higher temperature will increase
the speed to reach the equilibrium of a reaction. The effect of
the reaction temperature on the esterification is shown in
Table 4. EV was obtained from the esterification of VA with
ethanol over ProHSO4 ionic liquid from room temperature to
80 uC after 7 h. The high reaction temperature promotes the
efficiency of the esterification significantly. The yield of EV
increased gradually from 62.4% (entry 9) to more than 99.9%
(entry 14) as the temperature increased from 25 uC to 80 uC. At
80 uC, the conversion of VA was up to .99.9% with 100%
selectivity of EV (entry 14).
The amount of catalyst. The yields of EV from the
esterification over various amounts of ProHSO4 are summar-
ized in Table 5. The yield increased upon the increasing the
amount of catalyst from 2% to 50% (based on the mol of VA).
An increase in the amount of catalyst clearly increases the
number of acid sites available. On the other hand, because the
ionic liquids are also water absorbent, the amount of catalyst
was associated with the water produced. More ProHSO4
catalyst results in easier auto-phase-separation, which effec-
tively moves the reaction equilibrium toward EV. For 1 : 1
mole ratio of VA/ethanol, more than 99.9% EV was obtained
when 50 mol% ProHSO4 was used (entry 19).
The molar ratio of the reactants. The high ethanol/acid
molar ratio is favorable for normal acid catalyzed esterifica-
Table 4 The esterification for EV production at different temperaturesa
Entry
T/uC
Conversion (%)b
Selectivity (%)b
9
10
12
13
14
14
25
40
50
60
70
80
62.4
72.6
82.9
88.9
95.9
.99.9
100
100
100
100
100
100
a
Reaction conditions: catalyst = ProHSO4 (50 mol%, based on VA),
b
VA/ethanol (molar ratio) = 1 : 1, 7 h. Conversion and selectivity of
EV determined by GC analysis.
Table 5 The esterification for EV production over different amounts of catalysta
Entry
mol%
Conversion (%)b
Selectivity (%)b
15
16
17
18
19
2
5
10
20
50
72.1
79.8
81.3
83.2
.99.9
100
100
100
100
100
Fig. 2 The esterification to produce EV after different reaction times at room
temperature (reaction conditions: catalyst = ProHSO4 (50 mol%, based on VA),
VA/ethanol (molar ratio) = 1; conversion and selectivity of EV determined by GC
analysis).
a
Reaction conditions: catalyst = ProHSO4, VA/ethanol (molar ratio) =
b
1, 80 uC, 7 h. Conversion and selectivity of EV determined by GC
analysis.
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recovered catalyst exhibited similar catalytic activity to that of
the original (Fig. 3). A consistent EV yield of higher than 95%
with 100% selectivity was obtained in each cycle. No obvious
activity loss of ProHSO4 was found even after five cycles.
Compared with the traditional organic solvents and inorganic
acid catalysts, the easy recycling performance of the amino
acid ionic liquid catalysts is also attractive from environmental
protection and economic aspects.
Table 6 The esterification for EV production over different molar ratios of
ethanol/VAa
Conversion Selectivity
Entry Molar ratio (%)b
(%)b
Status after reaction
20
21
22
23
24
25
26
27
28
1 : 1
2 : 1
3 : 1
4 : 1
5 : 1
6 : 1
7 : 1
8 : 1
9 : 1
.99.9
79.8
100
100
100
100
100
100
100
100
100
Two phases
Two phases
Single phase
Single phase
Single phase
Single phase
Single phase
Single phase
Single phase
58.1
67.2
76.0
84.7
88.1
91.5
94.6
The fuel performances of EV
In most cases, the ester products of the esterification were
pure EV. Because the amino acid ionic liquid can help to
separate the product from the reactants and the byproduct,
impurities would stay in the ionic liquid layer. Therefore, the
esterification procedure catalyzed by amino acid ionic liquid is
an efficient route to obtain high quality EV products.
a
Reaction conditions: catalyst = ProHSO4, 80 uC, 7 h, molar ratio =
b
ethanol/VA. Conversion and selectivity of EV determined by GC
analysis.
The fuel performances of EV are important parameters for
fuel applications. Our products from the esterification were
evaluated and the measured properties are listed in Table 7.
The colorless transparent liquid samples were taken from the
esterification at 80 uC for 7 h catalyzed by 50 mol% ProHSO4.
The density of EV product was 0.897 g mL21. The viscosity of
EV was 1.7 cP at room temperature, as determined using a
rotational viscometer. Thus, the product has good fluidity. The
low viscosity EV is suitable to be transported and utilized as a
fuel component. The melting point of EV was found at 291 uC,
measured by differential scanning calorimetry (DSC) analysis.
The boiling point of EV is 146 uC as recorded by the
thermometer in the atmospheric distillation set. No impurity
was found in the distillation flask after EV was distilled to the
receiving flask.
tion. The effect of the ethanol/VA molar ratio on the
esterification reaction was studied in the range of 1 : 1–9 : 1
(Table 6). The reaction was carried out at 80 uC for 7 h. Finally,
the EV yield dramatically decreases from .99.9% to 58.1%
upon increasing the ethanol/VA molar ratio from 1 : 1 to 3 : 1
(entries 20–22). When ethanol/VA was 3 : 1, auto-phase-
separation did not occur like biphasic catalysis. The reaction
became a homogeneous process again. Even when ethanol/VA
increased from 3 : 1 to 9 : 1, the EV yield increased gradually
to 94.6% (entry 28), but EV still can not be separated
automatically after the reaction. The result reveals that for
homogeneous esterification, the increase of ethanol would
shift the reaction equilibrium toward the product. However,
for ionic liquid involved biphasic esterification, excess ethanol
may not help the production of EV but block the occurrence of
auto-phase-separation, leading to low ester yield.
The heating value of the EV product was obtained by two
different methods. The heating value was calculated using the
following formula proposed by Friedl et al. in 2005:17
The recycling ability of catalyst. We further explore the
recycling ability of the ProHSO4 catalyst for EV production.
The recycling reaction was performed under the optimized
reaction conditions above. The esterification of VA with
ethanol was carried out on 1 : 1 mole ratio of VA/ethanol
using 50 mol% ProHSO4 at 80 uC for 7 h. ProHSO4 was easily
isolated from the product and then reused by simple removal
of water under vacuum by heating at 100 uC for 2 h. The
HHV (kJ g21) = (3.55C2 2 232C 2 2230H +
51.2C 6 H + 131N + 20 600)/1000
(9)
The calculated elemental analysis for EV (C7H14O2, 130.18)
is C 64.58%, H 10.84%, and O 24.58%. The theoretical
calculated heating value of EV according to eqn (9) was 4177.8
kJ mol21 and 32.2 kJ g21. From elemental analysis, our EV
product without any post-processing was found to consist of C
64.13%, H 11.28%, and O 24.59%. The result confirms that the
EV product obtained from the ProHSO4-catalyzed esterification
under optimal conditions is pure. Using the data obtained
from the element analysis and eqn (9), the calculated heating
value of EV is 4192.4 kJ mol21 and 32.2 kJ g21, which is very
close to the theoretical heating value of EV.
Meanwhile, we obtained the heating value from the
measured constant-volume combustion energy of EV using a
Parr 6725 bomb calorimeter. The mean constant-volume
combustion energy (DcU) of EV could be directly obtained by
three combustion experiments of EV in the bomb calorimeter,
and was found to be 4152.0 kJ mol21. Then the standard molar
enthalpy of combustion (DcHu) of EV could be obtained by
using the expression: DcHu = DcU + DnRT (Dn = gni (products,
g) 2 gni (reactants, g); gni is the total molar amount of gases
Fig. 3 Reaction recycling of the esterification for EV production over ProHSO4
catalyst.
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Table 7 Physical properties of methanol, ethanol, GVL, VA and EV
Methanol (CH4O)4b
Ethanol (C2H6O)4b
GVL (C5H8O2)4b
VA (C5H10O2)4b
EV (C7H14O2)
Molecular weight (g mol21
Density (g cm23
Melting point (uC)
Boiling point (uC)
)
32.04
0.79
297.8
64.8
46.07
0.79
2114.3
78.4
100.12
1.05
231
207–208
96
59.98
8.05
31.96
soluble
2625.7
26.3a
102.13
0.94
234.5
185
95
58.80
9.87
31.33
soluble
2751.0
26.9a
130.18
0.90
291
146.1
38
64.58
10.84
24.58
insoluble
4158.1
31.9
)
Flashing point (uC)
Carbon content (%)
Hydrogen content (%)
Oxygen content (%)
Solubility in water (%)
Heating value (kJ mol21
11
12
37.48
12.59
49.93
soluble
415.7
12.8a
52.14
13.13
34.73
soluble
1102.3
23.9a
)
Heating value (kJ g21
)
a
Heating value was calculated using the following formula: HHV (kJ g21) = (3.55C2 2 232C 2 2230H + 51.2C 6 H + 131N + 20 600)/1000.17
in the products or reactants),18 which gave values of 4158.1 kJ
Acknowledgements
mol21 and 31.9 kJ g21
.
The authors gratefully acknowledge the support of the
National Natural Science Foundation of China (No.
21103116), and the Specialized Research Fund for the
Doctoral Program of Higher Education (No. 20100181120042).
The heating value of EV obtained by elemental analysis data
is in good agreement with that from the experimental heat of
combustion. By using eqn (9), the heating values of methanol,
ethanol, GVL, and VA were calculated to be 12.8 kJ g21, 23.9 kJ
g
21,26.3 kJ g21 and 26.9 kJ g21 respectively. Compared with
the heating values of methanol, ethanol, GVL, and VA, the
heating value of our EV product is the highest one among the
five molecules (Table 7). The esterification products may be
sufficiently pure to be directly used as fuel additive. With the
low melting point, relatively high boiling point, low viscosity
and high heating value, EV is a promising high energy density
fuel additive, better than the commercial ethanol fuel additive.
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High yield of EV as a promising fuel additive was obtained by
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