Catalytic Biodiesel Production Mediated by Amino Acid-Based Protic Salts

Article abstract of DOI:10.1002/cssc.201700026

Hetero- and homogeneous acid catalysts are effective catalysts for the production of biodiesel from oils containing high free fatty acids. The protic salts synthesized from natural amino acids were examined for catalytic activity and efficiency for the esterification of oleic acid after structural identification and characterization. In the esterification reaction of oleic acid with methanol, [Asp][NO₃] was the best catalyst, and its high activity correlated to its high Hammett acidity. The optimal reaction conditions for the esterification of oleic acid to achieve 97 % biodiesel yield were: 70 °C, 10 % catalyst loading (w/w, on oleic acid basis), methanol/oleic acid ratio 7.5:1, and 5 h. Generally, [Asp][NO₃] could be a good catalyst for the esterification of oleic acid with alcohols with chain lengths of up to six. The biodiesel yield of 93.86 % obtained from palm fatty acid distillate implies that the catalyst has potential for industrial application. A study of the kinetics indicated that the reaction followed pseudo-first-order kinetics with an activation energy and pre-exponential of 57.36 kJ mol^?1 and 44.24×10⁵ min^?1, respectively. The aspartic acid-derived protic salt is a promising, operationally simply, sustainable, renewable, and possibly biodegradable catalyst for the conversion of free fatty acids into biodiesel.

Full text of DOI:10.1002/cssc.201700026

Accepted Article
Title: Catalytic biodiesel production mediated by amino acid-based
protic salts
Authors: Jingbo Li and Zheng Guo
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ChemSusChem
10.1002/cssc.201700026
FULL PAPER
Catalytic biodiesel production mediated by amino acid-based
protic salts
[
a]
[a]
Jingbo Li , Zheng Guo*
Hetero-/homo-geneous acid catalysts are effective catalysts for biodiesel produced from oils containing high free fatty acids. The protic salts
synthesized from natural amino acids were examined for their catalytic activity and efficiency for esterification of oleic acid after structural
identification and characterization. The melting points of the protic salts were measured. In the esterification reaction of oleic acid with methanol,
[
Asp][NO
3
] performed the best, correlating to its higher Hammett acidity. The optimal reaction conditions for esterification of oleic acid to achieve
9
7% biodiesel yield were: temperature 70 °C, catalyst 10% (w/w, on oleic acid basis), methanol to oleic acid ratio 7.5:1, and 5 h. [Asp][NO ]
3
could be a generally good catalyst for esterification of oleic acid with alcohols with chain length up to 6. The biodiesel yield of 93.86% was
obtained from palm fatty acid distillate, implying potential industrial application of the catalyst. Kinetic study indicated that the reaction followed
5
-1
a pseudo-first order reaction, with activation energy and pre-exponential of 57.36 kJ/mol and 44.24×10 min . In conclusion, the aspartic acid
derived protic salt is a promising, operational simply, sustainable, renewable, and possible biodegradable catalyst for converting high content
of free fatty acids into biodiesel.
Introduction
procedures in the refining of palm oil [11]. However, their high free
fatty acids content impedes the utilization of the most effective
catalysts, basic catalysts, because of the formation of soaps[
.
12]
Fossil fuels supply most of the world’s energy demands currently
and it is likely to remain adequate for the next few generations.
However, the unacceptable long-term consequence forces
Fortunately, acidic catalysts are effective and compatible for
converting free fatty acids into ester forms.
[1]
people to find a solution within this period . Biofuels, regarding
as renewable and sustainable fuel sources, including bioethanol
Sulfuric acid is usually used as the conventional catalyst in
esterification of free fatty acid. However, there are some obvious
drawbacks such as corrosion to the facilitates and its hazardous
effluent to the environment [4,13]. In order to overcome these
problems, heterogeneous catalysts are explored based on their
[2,3],
biobutanol, and biodiesel [2] were thought to be promising
alternative fuels. All the mentioned biofuels could be produced
from biomass, which is an abundant and carbon-neutral resource.
Another advantage of utilization of biofuels is the smaller amount
[14–16]
easy separation and further reuse properties
. These
of greenhouse gases formation [2]
.
catalysts, however, have shortcomings such as high molecular
weight/active-site ratios and rapid deactivation from coking [13]. It
should be noted that the synthesis processes of these
heterogeneous catalysts create wastes and potential pollution as
well. Molten salts, also referred to as ionic liquids (ILs), are
Some biodiesel properties are even superior to petroleum diesel.
Its higher flashpoint ensures safety of biodiesel during
transportation or distribution, while higher cetane number endows
biodiesel a higher combustion efficiency [4]. Biodiesel can be used
directly without modifying existing engines, and produce less
harmful gas emission such as sulfur oxide [5]. Additionally,
combustion of neat biodiesel decreases carbon monoxide
emissions by 46.7%, particulate matter emissions by 66.7% and
attractive in many fields including organic chemistry,
[17]
electrochemistry, catalysis, etc
. More specifically, ILs have
been used in lipid processing [ and analysis . ILs either as
(co)solvents or catalysts are used for biodiesel production [20]. For
example, biodiesel production via transesterification catalyzed by
18]
[19]
unburned hydrocarbons by 45.2% [5,6]
.
[choline][OH] and [TBA][Arg] was found to be very effective [21,22]
Esterification of free fatty acids catalyzed by different
functionalized acidic ILs such as [TMEDAPS][HSO
[13],
, and
] [26] was studied extensively. All of these
acidic ILs contain sulfur, leading to the potential leach to biodiesel.
Additionally, the starting materials of those ILs are synthetic
chemicals or hetero aromatic compounds and are therefore not
as green as desired [27]. Amino acid based ILs were found to be
.
Biodiesel is derived from either transesterification of acylglycerols
including tri-, di-, and mono-glycerides or esterification of free fatty
acids with low molecular weight alcohols [7]. Recently, it is thought
that biofuels are of nonsense based on several standpoints
including the extreme inefficient land use and competition with
4
[25]
]
[DDPA][Tos] [23], [BMIM][HSO
[mimC SO H][SO CF
3 3
] SO ]
[24], [NMP][CH
4
4
3
3
3
[
8]
food supply in biofuels production
. Therefore, biodiesel
production from non-food or byproducts from food industry
[9]
becomes more attractive. Based on this, waste cooking oils or
[
10]
inedible oils
production. Among the inedible oils, palm fatty acid distillate
PFAD) has the most potential because it is a low-value byproduct
generated during the fatty acid stripping and deodorization
are used as main feedstocks in biodiesel
[
28]
[27]
ready for biodegradation
acidity similar to H PO
3 4
, easy to prepare
, and with the
[
29]
(
and was thought to be a green and
sustainable [30]. Although the amino acid derived salts were
classified as ILs [27,29], some of the melting point is higher than
100 °C. For example, the melting point of [Gly][Cl], [Gly][NO
3
],
[
Val]NO ], and [Ile][NO ] is 186, 111, 134, and 105 °C,
3
3
[
a]
Department of Engineering, Faculty of Science and Technology,
Aarhus University, Gustav Wieds Vej 10, 8000, Aarhus C, Denmark
respectively. Therefore, these salts are classified as amino acid-
based protic salts (AAPS) in the present work. The high melting
point could endow advantages such as easer final separation of
synthetic biodiesel from catalyst since the protic salts may be
crystallized out at the end of the reaction when the reaction
Z. Guo. E-mail: guo@eng.au.dk; guo@mb.au.dk; Fax: (+45)
76123178; Tel: (+45) 89425285
J. Li: jingboli@jnu.edu.cn, jingboli@eng.au.dk.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201700026
FULL PAPER
system cools down. With these advantages, the esterification
activity of the new class of AAPS is worth being explored
extensively and intensively.
In the present work, seven AAPS with different amino acids were
synthesized and their structures and compositions were
confirmed by 1H NMR analysis and elemental analysis,
respectively. The acidity and melting point were determined as
well. Catalytic performance for biodiesel production was
examined and the final product was analyzed by H and C NMR
in order to make sure the occurrence of esterification and non-
reactivity of the catalyst or contamination to the product.
Thereafter, the reaction conditions were optimized by using the
selected promising catalyst. Different alcohols with various chain
lengths were tested in the esterification reaction to verify the
general application of the AAPS catalyst. Finally, kinetic study of
the esterification using the selected AAPS was performed to
determine the key parameters including the activation energy and
the pre-exponential factor.
Table 1. Structure, melting point, and acidity of the prepared AAPS .
Cation structure
Melting
point °
C
pH
H
0
in
0
H in
85%
MeOH
aqueou
s
(exp)
aqueou
s
1
[29]
solution
1
13
[29]
solution
O
[
3
Asp][NO
]
81
2.08
(0.02
)
2.17
(0.03)
2.27
2.35
2
OH
1
HO
NH3
O
NH3
[
]
Glu][NO
3
120
NG
NG
3
HO
OH
1
2
O
O
Results and Discussion
O
[
]
His][NO
2
3
164
2.15
(0.02
)
2.10
(0.01)
2.48
H
N
3
4
Synthesis of the AAPS
OH
1
Amino acids are able to serve as either cations or anions due to
2
NH3
HN
their special properties [30]. Herein, they were converted into salts
-
as cations where the anion was NO
3
. The synthesis of such
groups by
without any involvement of organic solvents
implying potential greener and more sustainable process.
AAPS is a protonation reaction of the –NH or –NH
2
H2
[
]
Pro][NO
3
Liquid
at RT
2.26
(0.05
)
2.26
(0.09)
2.49
2.77
N
O
equal mole of HNO
[27,29],
3
4
3
1
2
However, water removal is an energy intensive process.
Nevertheless, if the other types of renewable energy like wind
energy, solar energy, hydropower, tidal power, and geothermal
energy could provide the energy consumption for water removal
in the current case, an overall green and sustainable technology
could be realized because of no involvements of fossil-based
solvents and fuels.
OH
[
3
Phe][NO
]
3
3
151.5
2.35
(0.04
)
NG
2
3
O
1
3
3
H3N
OH
Characterization of the prepared AAPS
H NMR spectra (Fig. A1, Supporting information) of all the AAPS
[
]
Ala][NO
3
2
O
168
68
2.36
(0.04
)
2.41
(0.07)
2.63
2.62
1
1
and their parental amino acids were recorded to certify the
H3N
OH
1
formation of AAPS. Peak assignments of H NMR (400 MHz, D
2
O)
signals of the prepared AAPS are shown below (please see Table
for structure and positions):
Asp][NO ]: δ ppm 4.26 (s, 1H, position 1), 3.00 (s, 2H, position 2).
Glu][NO ]: δ ppm 4.02 (t, J = 6.7 Hz, 1H, position 1), 2.55 (td, J =
.4, 2.2 Hz, 2H, position 3), 2.14 (dp, J = 21.6, 7.3 Hz, 2H, position
). [His][NO : δ ppm 8.69 (s, 1H, position 1), 7.44 (s, 1H, position
), 3.45 (s, 2H, position 3), 4.36 (s, 1H, position 4). [Pro][NO ]: δ
NH3
1
[Ser][NO
]
3
2.38
(0.03
)
2.31
(0.06)
O
[
[
3
1
2
3
OH
OH
7
2
2
3 2
]
1
pH (exp) conditions: 0.010 mol/L aqueous solution, 25 °C, and the data in
parenthesis are the uncertainties (U). NG: not given. RT: room temperature.
3
ppm 4.41 (s, 1H, position 1), 2.27 (d, J = 98.4 Hz, 2H, position 2),
2
4
2
4
.02 (s, 2H, position 3), 3.39 (s, 2H, position 4). [Phe][NO
.39 – 4.25 (m, 1H, position 1), 3.26 (ddd, J = 58.0, 14.5, 6.6 Hz,
H, position 2), 7.48 – 7.24 (m, 5H, position 3). [Ala][NO ]: δ ppm
.11 (d, J = 7.1 Hz, 1H, position 1), 1.55 (d, J = 7.1 Hz, 3H,
3
]: δ ppm
Elemental analysis of all synthesized AAPS was performed.
[
Asp][NO
3
]: Anal. Found: C, 24.44; H, 4.23; N, 13.95; O, 57.38%.
(MW 196.12): C, 24.50; H, 4.11; N, 14.28; O,
]: Anal. Found: C, 28.82; H, 4.79; N, 13.21; O,
3
Calc. for C
4
8 2 7
H N O
5
5
1
2
3
5
7.11%. [Glu][NO
3.18%. Calc. for C
3.31; O, 53.29%. [His][NO
5.07; O, 44.83%. Calc. for C
.94; N, 24.91; O, 45.52%. [Pro][NO
.71; N, 15.76; O, 44.46%. Calc. for C
3
position 2). [Ser][NO
3
]: δ ppm 4.18 (s, 1H, position 1), 4.05 (d, J =
5
H
10
N
2
O
7
(MW 210.14): C, 28.58; H, 4.80; N,
1
23.1 Hz, 2H, position 2). The results from H NMR clearly revealed
3 2
] : Anal. Found: C, 26.19; H, 3.91; N,
the AAPS were formed.
6
H
11
N
5
O
8
(MW 281.18): C, 25.63; H,
]: Anal. Found: C, 34.07; H,
(MW 178.14): C,
3
5
10 2 5
H N O
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ChemSusChem
10.1002/cssc.201700026
FULL PAPER
3
4
2
3.71; H, 5.66; N, 15.73; O, 44.90%. [Phe][NO
7.76; H, 5.31; N, 12.34; O, 34.59%. Calc. for C
28.20): C, 47.37; H, 5.30; N, 12.28; O, 35.05%. [Ala][NO
3
]: Anal. Found: C,
(MW
]: Anal.
apparently higher acidic property gives better catalytic activity for
the esterification reaction. The correlation between the Hammett
acidity measured in 85% methanol and biodiesel yield in the first
60 min was plotted and is shown in Fig. 1b. The close positive
correlation could be observed. In line with the present results, the
esterification efficiencies were found to be positively relevant to
the Hammett acidity of ionic liquids [31]. The longer reaction time,
as expected, gave more biodiesel yield for all the investigated
catalysts (Fig. 1a).
9
12 2 5
H N O
3
Found: C, 24.25; H, 5.27; N, 18.53; O, 51.95%. Calc. for
(MW 152.11): C, 23.69; H, 5.30; N, 18.42; O, 52.59%.
]: Anal. Found: C, 21.67; H, 4.79; N, 16.56; O, 56.99%.
Calc. for C (MW 168.11): C, 21.43; H, 4.80; N, 16.67; O,
7.10%.
Melting point of AAPS was measured and is shown in Table 1 and
Fig. A2. [Asp][NO ] and [Ser][NO ] were liquid after drying at 95 °C
3 8 2 5
C H N O
[
Ser][NO
3
3
8 2 6
H N O
5
3
3
Solid catalysts allows easier separation from products after
reaction and can be further recycled while liquid catalysts give
larger contacting surface between catalysts and reactants. The
because of their lower melting point. They became solid state
when the temperature cooled down to room temperature.
[
Pro][NO
3
] was the only one that was liquid at room temperature.
melting point of [Asp][NO
3
] was found at around 80 °C. It should
The melting point of the others was higher than 120 °C.
be noted that [Asp][NO ] started to melt from 55 °C (Fig. A2),
3
The Brønsted acidity of the prepared AAPS dissolved in 85%
aqueous methanol solution was measured by the Hammett
method [23] and the results are shown in Table 1. Additionally, the
acidity of the AAPS in aqueous solution was adopted from a
previous report [29]. The Hammett acidity measured in 85%
methanol was generally lower than that measured in water. When
indicating that it may be liquid during reaction (70 and 80 °C) while
it was precipitated as solid at room temperature after reaction.
Excessive methanol, formed water, and catalysts were at the
bottom of the reactor whereas pure biodiesel was the upper phase.
Only simple filtration, precipitation, or centrifugation is required for
synthesized biodiesel separation (Fig. A3). This could be another
they were dissolved in water, [Asp][NO
3
29]
] displayed the lowest pH
unique advantage of [Asp][NO
production.
3
] as a catalyst for biodiesel
[
while [His][NO
3
]
2
gave the lowest H
0
. In the system of 85%
] > [Glu][NO ] >
] > [Phe][NO ].
methanol, the order of the acidity was: [Asp][NO
3
3
[
[
His][NO
3
]
2
> [Pro][NO
3
] > [Ser][NO
3
] > [Ala][NO
3
3
+
+
2+
Asp] , [Glu] , and [His] contain acidic groups in side chains
+
+
+
+
whereas [Pro] , [Ser] , [Ala] , and [Phe] are with neutral side
chains. The analysis suggests that the acidity of the AAPS are
partially determined by the structure and property of the side
chains of the parental amino acids. It was reported that the pKa1
values of the AAPS are from 2.14 to 2.42, which are much lower
than those of the corresponding neutral amino acids (>9.10) [29]
.
The low pKa1 values likely originate from the –COOH group of
amino acid cations as suggested [29]. Accordingly, the number and
ionization status of carboxyl group in side chain of amino acids
may profoundly affect the Brønsted acidity of the AAPS.
Selection of AAPS for catalyzing biodiesel production
Seven types of amino acids covering basic, neutral and acidic
amino acid were selected for syntheses of AAPS and the catalytic
performance of the resulting AAPS for biodiesel production from
oleic acid and methanol was examined and the results are shown
in Fig. 1a. The catalytic activity of [Asp][NO
among all tested AAPS, followed by that of [His][NO
Glu][NO ]. The proline based AAPS performed weaker than the
3
] was the highest
3 2
]
, and then
[
3
above mentioned AAPS; however, it was stronger than serine,
alanine, and phenylalanine based AAPS. The pH value of
[
Asp][NO
the lowest compared with other investigated AAPS as shown in
Table 1, which correlates to the catalytic activity. The H of
in water was the lowest according to a previous report
giving it relatively good catalytic activity for biodiesel
value of
in 85% methanol was higher than that of [Glu][NO ],
which was inconsistent with the order of their catalytic activity. The
relatively higher catalytic activity of [His][NO might come from
the two anions while that of [Glu][NO ] and [Asp][NO ] might result
from the two carboxylic groups. If all the AAPS are considered,
3 0 3
] in water and H of [Asp][NO ] in 85% methanol were
0
[
3 2
His][NO ]
[
29],
Figure. 1. a) Catalytic activity of different AAPS in the esterification of oleic acid.
Conditions: n(methanol):n(oleic acid)= 10:1; catalyst loading 4.4% (w/w, based
on oleic acid) and 80 °C. b) Correlation between the Hammett acidity and
production in this study. However, the measured H
His][NO
0
[
3
]
2
3
biodiesel yield within the first 60 min.
1H NMR and 13C NMR of synthesized biodiesel
3
]
2
3
3
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ChemSusChem
10.1002/cssc.201700026
FULL PAPER
Figure. 2. 1H and 13C NMR spectra of oleic acid and final product. a: 13C NMR of oleic acid. b: 13C NMR of product. c: 1H NMR of oleic acid. d: 1H NMR of product.
The upper phase from reaction mixture catalyzed by [Asp][NO
was evaporated under vacuum at 60 °C followed by dissolving an
appropriate amount in CDCl and finally subjected to NMR
3
]
methyl ester. The content of the synthesized biodiesel can be
calculated according to the 1H NMR spectrum as well by the
equation (100×2×peak area of methoxy protons/(3×peak area of
methylene protons)) [34]. The purity of the resultant biodiesel was
95.96%.
It is obvious that biodiesel was formed from oleic acid and
3
methanol using [Asp][NO ] as the catalyst, confirming the
effectiveness of the amino acid based catalyst to synthesize
biodiesel.
3
analyses. Compared the 13C NMR spectrum of oleic acid (Fig. 2a)
with that of final product (Fig. 2b), it clearly indicated the
synthesized compound was oleic acid methyl ester because the
characteristic peak at 51.58 ppm is the diagnose peak for methyl
group (Fig. 2b). The carbonyl carbons of biodiesel resonate
between 172 and 179 ppm (Fig. 2b) while the carboxylic carbon
of free fatty acid resonate at 180.52 ppm (Fig. 2a), which agrees
well the published data [32]. No peak corresponding to free fatty
acids could be observed (Fig. 2b), indicating almost all FFA was
converted into methyl ester form.
Effect of the reaction temperature on biodiesel yield
Generally, an increase in temperature results in an increase in the
initial reaction rate for an irreversible reaction and the magnitude
of the effect of temperature on the reaction rate is governed by
the activation energy according to the Arrhenius equation [ . As
a reversible reaction of esterification, the temperature affects both
forward and backward reactions. However, at the beginning of the
reaction the effect of temperature on the forward reaction is more
significant than that of the backward reaction because of the lower
concentration of products. The evolution of biodiesel formation
against time is shown in Fig. 3a. Within the initial 60 min, biodiesel
yield increased with the increased temperature up to 90 °C,
revealing the initial reaction rate is dependent on the temperature.
With the prolonged reaction time, the temperature higher than
70 °C did not enhance biodiesel yield anymore. The effect of
temperature on reaction rate is more significant at temperatures
1
Figure 2c shows the H NMR spectrum of oleic acid and Fig. 2d
12]
shows that of the product. Protons associated with double bonds
(
H22 and H23) give the chemical shift at 5.34 ppm. The methoxy
group (-O-CH ) in biodiesel gives the chemical shift of the protons
at 3.66 ppm, which was absent in the spectrum of oleic acid (Fig.
c). The proton resonance of all α-CH (methylene group)
3
2
2
including free fatty acid, triglyceride, diglyceride, monoglyceride,
and biodiesel appeared at 2.28-2.32 ppm. Protons on position 8
and 11 resonate at 2.00 ppm and those on position 2 resonate at
1.62 ppm (Fig. 2d). The protons of the terminal methyl group (-
3
CH , position 18) resonate at 0.88 ppm and all the rest protons
resonate between 1.26 and 1.30 ppm [33]. The integral of the
peaks indicated that the synthesized compound was oleic acid
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ChemSusChem
10.1002/cssc.201700026
FULL PAPER
lower than 70 °C, indicating there is an energy barrier at <70 °C
to be overcome for an effective reaction. The biodiesel yield at 5
h was plotted versus temperature and is shown in Fig. 3b. When
the temperature was 40 °C, only 35% biodiesel was formed. The
biodiesel yield increased linearly with increasing temperature,
reaching 80% at 60 °C. Further increase of temperature to 70 °C
gave rise to 10% improvement of biodiesel yield but even higher
temperature did not prompt more (Fig. 3b). Similar pattern was
also found in other works [13,23,25,26]. It was reported that 70 °C
washe optimal reaction temperature for esterification to produce
The effect of catalyst loading on biodiesel yield was investigated
and the results are shown in Fig. 4. At all investigated time
durations, more catalyst resulted in higher biodiesel yield.
However, even when the catalyst loading intervals of increase
were the same, the corresponding enhancement of biodiesel yield
was reduced gradually. The final biodiesel yield for catalyst
loadings of 4.4%, 6%, and 10% was relatively close (94.70%,
96.90%, and 98.32%). However, the biodiesel formation rate
before 3 h for higher catalyst loading was significantly higher than
those lower catalyst loadings. In other words, higher catalyst
loading resulted in less time consumed (5 h for 10%, 8 h for 6%).
When the catalyst loading was higher than 4.4%, it affected the
biodiesel formation rate more significantly than the yield.
biodiesel by using ionic liquids as catalysts such as
[25]
[
TMEDAPS][HSO
optimal reaction temperature for [mimC
DDPA][Tos] [23], 3,30-(octane-1,8-diyl)- bis(4-sulfobenzyl-1H-
imidazol-3-ium) hydrogensulfate [17], and [BMIM][HSO ] [24] was
00 °C, 60 °C, 50 °C, and 87 °C, respectively (Table 2).
4
] [13] and [NMP][CH
3
SO
3
]
(Table 2). The
[26],
4
SO
3
H][SO CF
3
3
]
[
The mechanisms of esterification catalyzed by a typical
4
protonated acid may consist of several steps (Scheme 1). The first
and key step is the protonation of the carboxylic oxygen, resulting
in increased electrophilicity of the adjoining carbon atom, further
making it more susceptible for nucleophilic attack. Then a
nucleophilic attack of the alcohol takes place on that carboxylic
1
Apparently, the optimal temperature of IL-mediated reaction
shows a dependency on the property and structure of the catalytic
ILs used. Therefore, the kinetic study is necessary for a specific
catalyst as conducted in the following section.
carbon, forming
a tetrahedral intermediate. Finally, proton
migration and a breakdown of the intermediate occur, forming
biodiesel and water (Scheme 1) [35]. It is apparent that the
protonation ability of the catalyst
Effect of the catalyst loading on biodiesel yield
Figure. 3. Effect of the reaction temperature on biodiesel yield. a: biodiesel yield versus time. b: biodiesel yield (5 h) versus temperature. Conditions:
n(methanol):n(oleic acid)= 10:1; catalyst loading 4.4% (w/w, based on oleic acid).
to the carboxylic oxygen on free fatty acids determines the
effectiveness of the esterification reactions catalyzed by acid and
the amount of catalyst determines the protonation rate. Combined
with the results in Fig. 4, higher amount of catalyst donates more
protons, resulting in faster and more formation of intermediates
and thus accelerates the reaction. Longer reaction time could
compensate the overall conversion with respect to lower catalyst
loadings. Considering the time-space efficiency of biodiesel
production catalyzed by [Asp][NO
might be a recommended value.
3
], the loading of 10% catalyst
Effect of the methanol/oleic acid mole ratio on biodiesel yield
Scheme 1. Mechanisms of biodiesel production via esterification catalyzed by
acidic catalyst. R=alkyl group of fatty acids.
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As a bi-substrate reaction, high methanol/oleic acid mole ratio
affects the reaction rate and final biodiesel yield by shifting
reaction towards the formation of biodiesel. Methanol loading was
therefore investigated from 1.5:1 to 10:1 (mol/mol, based on oleic
acid) and the results are shown in Fig. 5. Biodiesel yield increased
with the increasing methanol ratio up to 7.5:1. Further increase of
the ratio to 10:1 did not enhance the final biodiesel yield but
decrease the biodiesel formation rate in the first 2 h. The
decreased biodiesel formation rate for the ratio of 10:1 probably
because oleic acid and catalyst were too diluted by excessive
methanol. Many works reported the same observation [17,25,36]
.
Figure. 4. Effect of the catalyst loading (w/w, based on oleic acid) on biodiesel
yield. Conditions: n(methanol):n(oleic acid)= 10:1; temperature 70 °C.
Table 2. Comparison of the results obtained in the esterification of free fatty acids using different functionalized ionic liquids.
Structure
T
Catalyst
loading
20a
Methanol
loading
1.8
Reaction
time (h)
6
Biodiesel
yield (%)
93-96
Reference
(°C)
[13]
[
[
TMEDAPS][HSO
DDPA][Tos]
4
]
70
HO3S
N
N
n
SO3H
HSO4-
HSO4-
n=2
P-CH3(C6H4)SO3-
10a
[23]
[17]
60
50
1.5
2
3
6
92.5-96.5
95
N
SO3H
[
ODBSIM][ HSO
4
]
N
7.5a
N
N
N
HSO4-
n
HSO4-
O
S
O
O
S
O
OH
HO
n=6
[24]
[25]
[
[
BMIM][HSO
4
]
87
70
6a
9
2
5.2
8
81.2
N+
HSO4-
N
O
10b
NMP][CH
3
SO
3
]
93.6-95.3
O
S
O
NH
O-
O
7b
[26]
[
[
mimC
4
SO
3 3 3
H][SO CF ]
100
70
10
5
5
90
97
N+
N
O-
S
OH
O
S
O
O
CF3
O
15.7 , 10b
a
Asp][NO
3
]
7.5
This work
OH
HO
NO3-
NH3
O
a: mol%. : wt%. [ODBSIM][ HSO
b
4
]: 3,30-(octane-1,8-diyl)- bis(4-sulfobenzyl-1H-imidazol-3-ium) hydrogensulfate
Theoretically, methanol to oleic acid ratio of 1:1 is possible to
allow a full conversion of oleic acid into methyl ester form.
However, the biodiesel yield was only 33.46% and 61.86% with
methanol/oleic acid ratio at 1.5:1 and 2.5:1, respectively (Fig. 5).
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This is partially because that low methanol concentration is
unable to shift the reaction equilibrium towards the formation of
biodiesel sufficiently. Meanwhile, evaporation of methanol from
the reaction system at the reaction temperature also decreased
the real methanol/oleic acid ratio to some degree although
condenser was equipped. According to the results in Fig. 5, 7.5:1
was found to be the optimal methanol to oleic acid ratio in the
present case.
3
To verify the catalytic versatility of [Asp][NO ] for production of
alkyl fatty acid esters, the catalytic performance was investigated
by using various alcohols as alternative substrates of methanol
under the obtained optimal conditions and the results are shown
in Fig. 6. No significant difference in yielding alkyl fatty acid esters
from methanol, ethanol, and 1-propanol was observed when the
reaction progressed for 5 h, whereas the reaction activity
decreased remarkably when 1-pentanol and 1-hexanol were used
as alcohol donors. As discussed before, there are 3 steps in
esterification reaction and the 2nd step involves alcohols, which
present nucleophilic attack and then form tetrahedral
intermediates (Scheme 1). The reactivity of the hydroxyl group in
alcohols decreases with the increased chain length, due to the
The optimal conditions for biodiesel production through
esterification process catalyzed by [Asp][NO
7
7
3
] were: temperature
0 °C, catalyst loading 10% (w/w), methanol to oleic acid ratio
.5:1, and 5 h, resulting in the biodiesel yield of 97%. The yield is
equal or higher than those relevant works in which ILs (molten
salts) were used as catalyst as summarized in Table 2. The
reaction temperature, catalyst loading, methanol input, and
reaction time are comparable to other reported values with ILs as
catalysts (Table 2). From industrial application point of view, the
current process (97% biodiesel in 5 h) is effective and promising.
Commonly used ILs are synthetic chemicals and are therefore not
as green as desired [27]. Biorenewable substances like amino
acids are then more sustainable, abundant, and greener
[
30]
resources . Catalyst preparation in the present work is a very
simple protonation reaction carried out by mixing the appropriate
mole ratio of amino acid and HNO
3
in water, followed by
evaporation of the water. In the whole preparation processes, no
organic solvents were involved and no further purification was
required. This makes the whole process more operable, easier
scale-up, and more sustainable due to no utilization of organic
solvents.
Figure. 6. Effect of different alcohols on biodiesel yield. Conditions:
n(methanol):n(oleic acid)= 7.5:1; catalyst loading 10% (w/w, based on oleic
acid); temperature 70 °C.
Amino acids are natural products and therefore are biodegradable.
It has been reported that both plant roots and soil microorganisms
could metabolize amino acid very rapidly [37], which leads to
further treatments of the catalyst unnecessary. The low toxicity
and high biodegradability of cholinium amino acid ionic liquids
were proved [28], implying [Asp][NO
] should be of low toxicity and
3
high biodegradability as well. Green, sustainable, and renewable
production of biodiesel could thus be achieved with AAPS.
Figure. 7. Biodiesel synthesis from palm fatty acid distillate catalyzed by
[
3
Asp][NO ]. Conditions: n(ethanol):n(oleic acid)= 7.5:1; temperature 70 °C;
catalyst loading 10% (w/w, based on oleic acid).
fact that longer alkyl group weakens nucleophilicity of hydroxyl by
generating electronic donation effect and presents larger steric
hindrance; accordingly it makes the formation of tetrahedral
intermediates more difficult. Nevertheless, the results in Fig. 6
Figure. 5. Effect of the methanol molar ratio on biodiesel yield. Conditions:
catalyst loading 10% (w/w, based on oleic acid); temperature 70 °C.
3
demonstrated that [Asp][NO ] could be used as catalyst for
esterification reactions with longer chain alcohols despite of the
decreasing reaction rate.
Catalytic performance of [Asp][NO
oleic acid with different short-chain alcohols
3
] for the esterification of
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FULL PAPER
푑ꢄ
푑푡
=
푘1(ꢃ − 푋) (5)
Biodiesel production from palm fatty acid distillate
Integration of (5), the following equation is obtained:
푙푛|ꢃ − 푋| = 푘 ꢅ + 푎 (6)
where a is a constant value and t is the time.
Production cost of the biodiesel is not economically competitive
with petroleum based fuels due to the relatively high cost of the
lipid feedstocks, which are usually edible oils for foods [38].
Additionally, biofuels production may be of nonsense if we use
land exclusively for planting biofuels feedstocks [8]. In order to
compete with fossil fuels and decrease land use for biofuel
feedstocks, low cost feedstocks such as waste cooking oil, algal
oil, or fatty acid distillate are attractive substrates. Palm fatty acid
distillate consists of 90-95% free fatty acid and little acyglycerides
and impurities [38], which could be used to produce biodiesel by
acidic catalysts. Therefore, biodiesel production from palm fatty
acid distillate and bioethanol was investigated in order to extend
the reaction system to a more practical application and the results
are shown in Fig. 7. In the first 2 h, the formation of biodiesel
increased linearly with increasing time. Thereafter, the biodiesel
formation slowed down gradually until the end of the reaction. The
final biodiesel yield was 93.86% after 6 h reaction under the
optimal reaction conditions. Comparably, biodiesel yield of 93.5%
was obtained at a methanol to palm fatty acid distillate ratio of
−
1
2
0:1, catalyst (sulphonated multi-walled carbon nanotubes)
Figure. 8. Plot of −푙푛|ꢃ − 푋| versus time for determination of reaction rate
constants.
loading of 3 wt%, reaction temperature of 170 °C, and reaction
time of 2 h [ . It is obvious that the current process enables the
reaction at significantly lower temperature and alcohol loading to
achieve comparable result.
11]
The reactions at different temperatures with the other optimal
parameters were carried out for determining k . Based on
1
equation (6), reaction rate constant for each temperature was
obtained by plotting the graph of −푙푛|ꢃ − 푋| versus time, which is
shown in Fig. 8. The reaction rate constants increased with
increased temperature until 70 °C. All reaction rate constants
gave high R-square, verifying the assumption of the process
3
Kinetics study of esterification using [Asp][NO ]
Esterification of free fatty acid with alcohols is a reversible
reaction, and the reaction forms ester and generates water at the
same time.
having pseudo-first order reaction kinetic. Mohammad et al. [
4]
k1 cat
and Berrios et al. [39] also found the same order of the forward
A (FFA)+B(MeOH)
C (ester)+D (water)
(1)
k-1 cat
reaction in esterification of free fatty acid catalyzed by magnetic
ionic liquid and sulfuric acid, respectively.
With the values of the rate constants of the reaction at four
different temperatures, the activation energy could be calculated
according to the Arrhenius equation (7):
where k
backward reaction rate constant.
The reaction rate for biodiesel formation can be expressed as the
following equation:
1
is the forward reaction rate constant and k-1 is the
퐸ꢇ
푑퐶ꢀ
푙푛푘1 = 푙푛ꢆ +
(7)
=
푘 ꢁ ꢁ − 푘 ꢁ ꢁ
(2)
ꢂ푅푇
1
퐴
퐵
ꢂ1 퐶 퐷
푑푡
a
where E is the activation energy, R is the gas constant (J/mol K),
T is the absolute temperature (K), and A is the pre-exponential
factor for the esterification reaction.
Arrhenius plot for oleic acid conversion to biodiesel is shown in
Fig. 9 and accordingly Ea and A could be calculated by the
where C
i
indicates the concentration of the compound i.
Theoretically, 1 mol methanol is required to esterify 1 mol of oleic
acid. Since this is a reversible reaction, higher loading of reactants
could accelerate the forward reaction to form more biodiesel. In
the present study, methanol to oleic acid ratio was 10, which gave
much excessive methanol concentration compared to the other
components (C ≫C , C , and C ). Thus C can be regarded as a
B A C D B
constant value and Cc and Cd could be negligible. To this end,
the reaction is predicted to be a pseudo-first order kinetic reaction
[4].
regressed formula in Fig. 9. The E
4
a
and A were 57.36 kJ/mol and
4.24×10 min , respectively. These values were close to the
results obtained using 5% and 10% H SO
as catalyst [39],
suggesting the activation energy demand of using [Asp][NO ] and
SO is close. When a tungstated zirconia catalyst was used,
the E
A of the forward reaction of esterification catalyzed by 4-
5
-1
2
4
3
H
2
4
and A were 51.90 kJ/mol and 15×10 min-1 [40]. The E
9
and
a
a
The equation (2) could then be simplified as following:
푑퐶ꢀ
8
-2
=
푘 ꢁ
1
(3)
dodecylbenzenesulfonic acid were 58.5 kJ/mol and 6.56×10 M
min-1 [12]. The magnetic ionic liquid, [BMIM][FeCl
], displayed the
lowest E
and A values, which were 17.97 kJ/mol and 181.62 min-
1, respectively [4]. The larger the activation energy, the more
퐴
푑푡
4
ꢁ퐴 = ꢁ퐴0(ꢃ − 푋); ꢁ = ꢁ퐴0푋 (4)
where X is oleic acid conversion.
Combine the equations (3) and (4), the following equation could
be obtained:
퐶
a
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ChemSusChem
10.1002/cssc.201700026
FULL PAPER
(Soeborg, Denmark). Deionized Mili-Q water with 18.2 mΩ resistivity was
used for synthesis of the AAPS. PFAD was obtained from Sime Darby
Jomalina Sdn. Bhd. (Selangor Darul Ehsan, Malaysia). The lipid class (by
TLC-FID) analysis of the PFAD sample gives 95.5% free fatty acids and
total neutral lipids content of 2.32% and the unsaponifiable matter (%) is
1.89%. The fatty acid composition analysis (by GC-FID) suggests that the
dominating fatty acids are palmitic acid (54.22%) and oleic acid (29.45%),
followed by linoleic acid (9.21%) and stearic acid (8.56%).
Preparation of AAPS
3
Amino acid and HNO (69%) with molar ratio of 1:1 or 1:2 (only for
histidine) were mixed in water and stirred at 500 rpm for 24 h at room
temperature in stoppered round-bottom flask, followed by evaporation of
water under vacuum at 95 °C. Subsequently, the resultant compounds
O
O
OH
Figure. 9. Arrhenius plot for oleic acid conversion to biodiesel.
OH
NO3-
+
HNO3
a
temperature sensitive the reaction. In other words, for large E , an
NH2
increase in just a few degrees in temperature can greatly increase
and thus increase the rate of reaction. However, it should be
noted that this relation between temperature and k is only valid
NH3
k
1
Scheme 2. An example of synthesis of the AAPS.
1
in the temperature range of 40-70 °C in the present work.
were further dried at around 10 mbar overnight to remove trace water. The
procedure is a typical atom-economic reaction without any poisonous
byproducts (Scheme 2) [27]. The AAPS were stored in a desiccator to
prevent them from moistening.
Conclusions
Characterization of the AAPS
Amino acid-based protic salts (AAPS) were synthesized through
simple protonation in water. The acidity of the AAPS derived from
acidic amino acids was generally stronger than those from neutral
ones. All investigated AAPS could catalyze the esterification
reaction to produce biodiesel from oleic acid and among which
1H NMR spectra of the AAPS and their parental amino acids in D
recorded by using a Bruker Ascend 400 spectrometer. MestReNova 10
was used to process the spectra and the chemical shift of residual H
2
O were
2
O
was used as the reference. The spectra are shown in supporting materials.
[
3
Asp][NO ] was found to perform the best catalysis. Optimization
The melting point of the AAPS was measured using differential scanning
calorimetry on a Pyris6 DSC system (Perkin-Elmer Cetus, Norwalk, USA).
Approximately 10 mg of each sample was put into an aluminum pan and
placed in the equipment under a purging atmosphere of nitrogen of 20
mL/min, with an empty pan as an inert reference. The heating rate was
of the conditions including temperature, catalyst and methanol
loadings were carried out and the optimal conditions were found
to be: temperature 70 °C, catalyst 10% (w/w, on oleic acid basis),
methanol to oleic acid ratio 7.5:1, and 5 h with biodiesel yield of
9
7%. The catalyst [Asp][NO
reaction of long chain length alcohols as well. Biodiesel yield of
3.84% was obtained from esterifying palm fatty acid distillate
3
] could catalyze esterification
10 °C/min.
9
The C, H, N, and O contents in the synthesized AAPS were analyzed using
an Elementar Vario Macrocube CHNS/O Elemental Analyzer.
with ethanol for testing potential industrial applications.
Separation and recovery of the catalyst from the produced
biodiesel could be realized by simple precipitation and filtration.
Kinetic analysis was performed and the activation energy was
found to be 57.36 kJ/mol, which is very close to that of H
The [Asp][NO can be promising, green, sustainable,
The Hammett acidity of AAPS was measured to compare the acidities of
AAPS using spectroscopic measurements [13]. AAPS and the indicator 4-
nitroaniline were dissolved in 85% methanol aqueous solution at
concentrations of 0.1% and 54 µmol/L, respectively. The solutions were
shaken vigorously and then stand statically for 20 h before measurements.
The UV/Vis spectra were recorded on a Varian Cary® 50 UV-Vis
spectrophotometer at room temperature [23]. The final results were
calculated according to the following equation [13].
2 4
SO .
3
]
a
renewable, and biodegradable catalyst for biodiesel synthesis
from lower quality oils containing high amount of FFA.
/[IH+]
) (8)
s s
Experimental Section
0
H =pK(Iaq)+log([I]
and [IH+]
Materials
where I represents the indicator base (4-nitroaniline), [I] are the
s s
respective molar concentrations of the un-protonated and protonated
forms of the indicator.
HNO
acid, aspartic acid, and glutamic acid were purchased from Sigma-Aldrich
St. Louis, MO, USA) without further purification. Proline, histidine, alanine,
phenylalanine and serine were purchased from VWR Bie & Berntsen
3
(69%), methanol, ethanol, 1-pentanol, 1-propanol, 1-hexanol, oleic
(
Esterification of free fatty acid with alcohols
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Oleic acid, at the first place, was used as model substrate for evaluating
the catalytic effectiveness of AAPS and optimizing the conditions. A certain
amount of oleic acid, alcohol, and catalyst (AAPS) according to
experimental design were added in a double-neck flask charged with a
reflux condenser. The temperature of the condenser was controlled at
around 22 °C in order to prevent moisture in the air from being condensed
and alcohol in the reaction system from being evaporated. The well-
equipped flask was placed in an oil bath with controlled temperature and
magnetic stirring. The stirring speed was 1000 rpm in order to eliminate
potential mass and heat transfer problems. Samples were taken regularly
from one neck of the flask equipped with a syringe. An appropriate amount
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× ꢃꢗꢗ% (9)
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Table of Contents
FULL PAPER
Jingbo Li, Zheng Guo*
Page No. – Page No.
Title: Catalytic biodiesel production
mediated by amino acid-based protic
salts
Biorenewable protic salts for biorenewable fuels: The amino acids derived protic
salts were found to be able to catalyze biodiesel production from free fatty acids.
The preparation of the protic salts is a simple protonation process and only water is
required, leading to green synthesis. The industrial waste oil, palm fatty acid
distillate, was converted into biodiesel, resulting in sustainable process. The
properties of amino acids like renewability, biodegradability, and non-toxicity may
endow this process greener and more sustainable.
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