Highly efficient metal salt catalyst for the esterification of biomass derived levulinic acid under microwave irradiation

Article abstract of DOI:10.1039/c5ra24305j

The esterification of levulinic acid (LA) to alkyl levulinates has been investigated in the presence of various metal salt catalysts under microwave irradiation. The reaction obtained 99.4% yield of methyl levulinate (ML) in the presence of Al₂(SO₄)₃ catalyst in methanol solution under microwave conditions. The optimized reaction conditions were 110 °C and 10 minutes with a 20 mol% catalyst loading. Alcohols with longer carbon chains showed lower reactivities in the microwave electromagnetic field due to their poorer abilities to absorb and transmit microwave energy. Moreover, microwave irradiation provided a significantly higher reaction rate compared to conventional oil bath heating. LA aqueous solution was also converted to ML with high yields. The Al₂(SO₄)₃ catalyst was successfully applied to the esterification of other biomass derived organic acids to their corresponding esters in high yields. Finally, the catalyst was recycled 5 times without much decrease in activity.

Full text of DOI:10.1039/c5ra24305j

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: Y. Huang, T. Yang,
B. Cai, X. Chang and H. Pan, RSC Adv., 2015, DOI: 10.1039/C5RA24305J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Please do not adjust margins
Page 1 of 7
RSC Advances
View Article Online
DOI: 10.1039/C5RA24305J
Journal Name
ARTICLE
Highly efficient metal salt catalyst for the esterification of biomass
derived levulinic acid under microwave irradiation
Yao-Bing Huang, Tao Yang, Bo Cai, Xin Chang and Hui Pan*a
Received 00th January 20xx,
Accepted 00th January 20xx
The esterification of levulinic acid (LA) to alkyl levulinates has been investigated in the presence of various metal salts
catalysts under microwave irradation. The reaction reached a 99.4% yield of methyl levulinate (ML) in the presence of
DOI: 10.1039/x0xx00000x
www.rsc.org/
o
Al2(SO4)3 catalyst in methanol solution under microwave condition. The optimized reaction condition was 110 C, 10
minutes with a 20 mmol% catalyst loading. Alcohols with longer carbon chains showed lower reactivities in the microwave
electromagnetic field due to their poorer abilities in absorbing and transimitting the microwave energies. Besides,
microwave irradiation provided significantly higher reaction rate compared to conventional oil bath heating. The LA
aqueous soluton were also converted to ML with high yields. The Al2(SO4)3 catalyst was sucessfully applied to the
esterification of other biomass derived organic acids to their corresponding esters in high yields. Finally, the catalyst was
recycled for 5 times withour much drop in the activity.
Kuwahara et al.7a used sulfated Zr–SBA-15 to catalyze the
esterification of levulinic acid in ethanol; 80% yield of ethyl
levulinate (EL) was obtained at 70oC for 24h. Baronetti et al.8c
Introduction
The production of fuels and chemicals from renewable
lignocellulosic feedstocks has been considered as sustainable
ways to replace the traditional fossil resource based industrial
systems. Among various chemicals produced from biomass,
levulinic acid (LA) and its esters are gaining widely attentions
as versatile platform chemicals for their potential applications
for biofuel, polymer monomers and pharmaceutical
intermediates.1 Levulinic acid can be produced by acid
hydrolysis of (ligno)cellulose. And it can be further converted
into a range of important derivatives like levulinate esters,2 γ-
valerolactone3 and diphenolic acid.4 In particular, levulinate
esters, especially ethyl levulinate (EL), can be as diesel miscible
fuel up to 5 wt% in regular car engines.5 Therefore, the
esterification of levulinic acid with different alcohols to
produce alkyl levulinates is highly desirable.
In general, esterification of LA with alcohols can be
accomplished in the presence of Brønsted acid catalysts such
as H2SO4.6 However, the use of liquid mineral acids always
suffered from corrosion and disposal problems. On the
contrary, heterogeneous catalysts are more favourable due to
their easier separability and recyclability. Sulfated oxides,7
heteropolyacids,8 zeolites9 and sulfonic resins5 were widely
used as acid catalysts for esterification reaction. For example,
employed silica-supported Wells−Dawson heteropolyacid
(H3PW12O40/ZSM-5) as the catalyst to synthesize ethyl
levulinate at 78oC. They obtained 76% conversion of LA within
5h. These works indicated that the acid strength and the
dispersibility of the acid sites have a big impact on the overall
activity of the catalysts. Recently, Fernandes et al.5 used
several different types of zeolites, sulfated oxides and
commercially available resins in the esterification reaction.
They found Amberlyst-15 showed the highest activity.
Sulfonated carbon nanotube has also been employed in the
esterification of LA and achieved moderate to good product
yields.10
On the other hand, multivalent metal salts such as CrCl3,
SnCl4, ZnCl2, and AlCl3 have been widely used in the conversion
of carbohydrates (e.g. cellulose, glucose and fructose) into
platform molecules (e.g. HMF) through their Lewis acidity and
Brønsted acidity.11 The catalytic effect of these metal salts can
be compared to those homogeneous or heterogeneous acid
catalysts under specific reaction conditions. Especially, the
Brønsted acidic site can be generated through the
hydrolysis/alcoholysis of the metal ions in solvents at higher
reaction temperature.12 Apart from that, the metal ions of
these salts were also preferable catalytic sites for esterification
reactions. Combing these two advantages, it’s appealing to
establish a new catalytic system for the esterification of LA
over metal salts catalysts, especially the earth-abundant metal
salts.
a.College of Chemical Engineering, Nanjing Forestry University, 159 Longpan Road,
210037, Nanjing, China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 2 of 7
ARTICLE
Journal Name
Herein, we demonstrated that some inexpensive metal dichloromethane (10mL×3). Residue dichloromethane was
View Article Online
DOI: 10.1039/C5RA24305J
salts have shown outstanding activity, efficiency and stability then evaporated. The dried catalyst was directly dissolved in
in the esterification of LA with different primary alcohols. MeOH and transferred to the reactor.
Microwave irradiation, an efficient and green technology for
Measurement of dielectric properties
biomass conversion, has also been employed in the model
Measurement of the dielectric properties of different
reaction to accelerate the reaction rate. Among all these metal
alcohols were carried out on an Agilent E5071C Vector
salts we have tested, Al2(SO4)3 catalyst gave the highest yield
Network Analyzer with Agilent dielectric probe 85070E. The
of methyl levulinate (99.4%). In addition, heating from
sample was measured at room temperature and the frequency
microwave irradiation had dramatically increased the reaction
was 2.45 GHz.
rate compared to the conventional oil bath heating. Finally,
the effects of catalyst loading, reaction time, temperature,
reusability of catalyst on the esterification of LA were also
Results and discussion
investigated.
Esterification of LA with different catalysts
Initially, esterification of LA was carried out with different
metal salt catalysts in methanol under microwave irradiation.
The results were presented in Table 1. A blank experiment was
firstly carried out and only trace amount of methyl levulinate
(ML) was detected, suggesting that the esterification of LA
reaction cannot proceed without any catalyst (Table 1, entry 1).
When CrCl3 was used as catalyst, the esterification reaction
afforded 91.5% yield of ML with 96.9% LA conversion at 110oC
for 10 minutes. This result suggested that CrCl3 was a
preferable metal salts with both good Lewis and Brønsted
acidity in MeOH. A much lower ML yield was observed when
Cr2(SO4)3was used, which was due to its poor solubility in
Experimental
Materials
Levulinic acid (97%), methyl levulinate (99%), ethyl
levulinate (98%) and butyl levulinate (98%) and were obtained
from TCI Chemicals Co. Ltd (Shanghai China). Amberlyst-15 was
purchased from Alfa Aester (H+, ~4.8 mmol/g). Alcohols and
Al2(SO4)3·18H2O and other metal salts were all of analytical
grade from Sinopharm Chemical Reagent Co., Ltd. (Shanghai
China), and used without further purification.
Esterification of levulinic acid
Esterification of levulinic acid were carried out in a methanol. Similar result was found for SnSO4. On the other
MILESTONE Ethos A microwave reactor (MA039). In brief, hand, soluble SnCl4 gave a much higher ML yield (96.8%). This
levulinic acid (116 mg, 1 mmol), MeOH (14 mL), and a given result was consistent with many previous literatures that Sn4+
amount of catalyst were loaded into a 100 mL sealed Teflon was a good Lewis acidic metal ions.11d In the case of Fe3+ and
tube followed by heating the tube in the microwave reactor, to Cu2+ salts, moderate ML yields were obtained. The different
the desired temperature. The zero time was defined as the Lewis acidity strength of these metal salts may be the main
reactor was heated to set temperature. Then, the reaction was reason for their activities in the esterification reactions.
performed for a given reaction time. After the reaction was Noteworthy, a near 100% ML yield was obtained when
finished, the reactor was cooled down to room temperature. Al2(SO4)3 was submitted to the esterification reaction, which
All experiments were performed in duplicates and the average could be attribute to the excellent Lewis acidity of Al3+ and the
values were reported.
in situ generated Brønsted acidity, as reported in the previous
For the conventional oil heating reaction, a Parr reactor work.12a When the anion changed to Cl- or NO3 , Al3+ salts
with a temperature probe was used and the reactor was provided lower ML yields compared to that of SO42-. This result
placed in the oil bath, the zero time was defined as the might be resulted from the different acid strength of these
reaction mixture was heated to the desired reaction metal salts. Finally, Zn2+ salts were tested in the reaction and
temperature. After the reaction was finished, the rector was only achieved a much lower ML yield which was due to their
-
quickly cooled down in an ice bath.
weak Lewis and Brønsted acid strengthen in the reaction
media. These results further demonstrated that the Lewis and
Brønsted acidity of the reaction system were the key factors to
the esterification of LA to alkyl levulinates.
Analytic methods
Analyses of reaction products were conducted by using a
GC system (Agilent, 7890A) equipped with a flame ionization
detector (FID) and a DB-5 capillary column (30 m × 0.25 mm ×
0.25 μm, Agilent).The injection port and the detector were
operating at 250 oC. The temperature of the column was
programmed to increase from 80oC to 200oC at a rate of
10oC/min. Naphthalene was used as the internal standard to
calculate the yield of reaction products.
For comparison, commercial available solid catalyst
Amberlyst-15 was also tested in the esterification reaction, the
reaction achieved a 99% ML yield with an extended reaction
time which might be caused by the mass transfer limitation of
reactants with the acid sites on the surface of the solid catalyst.
However, the structure of Amberlyst-15 was destroyed to
powders, which was probably caused by the “hot spots” that
generated in the MW condition. For the solid catalyst HNS-
PrSO3H,13 the reaction could also achieve a 98% ML yield in 10
min. However, the high cost may prevent it from wider
Recycle and reuse of catalyst
Recycle of the catalyst was as follows: methanol and
methyl levulinate were removed from the reaction mixture by
rotary evaporation. The catalyst was then washed with
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Page 3 of 7
RSC Advances
Journal Name
ARTICLE
a
Table 2.Effect of catalyst loading.
Table 1. Esterification of LA over different metal salts.
View Article Online
O
DOI: 10.1039/C5RA24305J
Entry
Catalyst loading
(mmol)
Conv.(%)
ML(%)
O
metal salts
OH
O
CH3OH
MW
O
10%
15%
20%
25%
30%
89.4
95.2
100
100
100
87.3
94.3
99.4
99.6
98.5
O
1
2
3
4
5
Entry
catalyst
Conv.(%)
ML yield (%)
1
—
5.0
1.8
2
CrCl3
96.9
45.2
100
91.5
4.1
3
Cr2(SO4)3
SnCl4
Conditions：LA 1.0 mmol, MeOH 14 mL (MeOH/metal ions=864 (mole ratio)), 400W,
4
96.8
15.7
89.6
81.7
49.5
66.1
83.9
99.4
70.9
8.1
110oC, 10 min.
5
SnSO4
58.6
87.6
92.7
70.9
71.8
83.7
100
6
FeCl3
temperature, with an 84.8% ML yield at 90 oC. Further increase
the reaction temperature to 110 C, a maximum ML yield of
99.4% was obtained, suggesting that higher reaction
temperature can promote the reaction rate. The ML yield
remained almost unchanged as the reaction temperature was
further increased to 130 oC. Thus, the optimum reaction
temperature for the esterification of LA is 110 oC.
7
Fe2(SO4)3
CuSO4
o
8
9
CuCl2
10
11
12
13
14
15b
16c
AlCl3
Al2(SO4)3
Al(NO3)3
ZnCl2
79.1
41.4
44.0
100
Effect of catalyst loading
ZnSO4
10.8
98.0
99.6
HNS-PrSO3H
Amberlyst-15
The catalyst loading clearly determines the amount of
Lewis and Brønsted acid sites of the methanol solution, and
subsequently would affect the yield of ML. In order to find out
the optimized catalyst loading, experiments were carried out
by changing the Al2(SO4)3 loading from 10 mmol% to 30
mmol% while other conditions remained unchanged. As shown
in Table 2, the yield of ML firstly increased from 87.3% to
99.4% when the catalyst loading increased from 10 mmol% to
20mmol%. Further increase of the catalyst loading didn’t make
a significant improvement in the ML yield. In addition, an
excess amount of catalyst loading decreased the product yield
slightly, which could be due to the formation of side products.
To sum up, the catalyst loading of 20 mmol% was sufficient to
achieve a high ML yield.
100
Conditions: LA 1.0 mmol, catalyst (metal ions, 0.4 mmol), MeOH 14mL (MeOH/metal
ions=864 (mole ratio)), 400W, 110 oC, 10 min. a: analysed by GC. b: catalyst 80 mg. c:
catalyst 80 mg, 30 min.
application in the industrial production. It is noteworthy that
the auto-esterification of methanol in these reactions can be
ignored since the reaction temperature was relatively lower. In
our previous work, we had revealed that higher reaction
o
temperature (150 C) would lead to the auto-etherification of
alcohols in the reaction. In spite of that, metal salt catalyst
provided the minimum etherification side reactions compared
to traditional liquid or solid acid catalysts.
Effect of reaction temperature
Comparison of two different heating methods
The effect of reaction temperature on esterification of LA
was investigated by using Al2(SO4)3 as the catalyst under
The esterification reactions with microwave heating and
conventional oil bath heating were also investigated to
compare their heating efficiency. The results were presented
in Figure 2. When the esterification reaction was carried out
under microwave irradiation condition, a 99.4% yield of ML
was obtained in just 10 minutes. For conventional oil bath
heating, we used Parr reactor to carry out the reaction with a
temperature probe to monitor the reaction temperature of
the reaction mixture. The ML yield was lower (74.1% for 10min,
91.4% for 60min and 96.6% for 90min) than that with
microwave heating. Even further prolonging the reaction time,
the yield of ML remained almost unchanged (97.1% for 120min,
96.6% for 150min). And some of the side products were also
detected from the reaction mixture as the reaction time
extended and led to a decrease in the reaction selectivity. Thus,
a proper reaction time was crucial to maintain a high
selectivity of the esterification of LA. These results
demonstrated that microwave radiation not only promoted
the esterification reaction rate but also improve the selectivity
of the target product. Microwave irradiation heating method
provides internal heating which can directly couple microwave
energy with the molecules (solvents, reagents and catalysts)
and promote the reaction rate.
o
microwave irradiation from 90 oC to 130 C (Fig.1). It can be
found that the esterification of LA can proceed at lower
Figure 1. Influence of the reaction temperature. Conditions: LA 1.0 mmol,
Al2(SO4)3 0.2 mmol, MeOH 14 mL (MeOH/metal ions=864 (mole ratio)), 400W, 10
min.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 7
ARTICLE
Journal Name
Table 3. Esterification of LA with different alcohols of different dielectric properties.a
View Article Online
DOI: 10.1039/C5RA24305J
ε`
ε``
ester yield/%
Entry
Alcohols
Polarity
1b
2c
3d
4e
MeOH
EtOH
6.6
4.3
4.0
3.7
26.95
9.21
5.29
4.77
14.55
8.26
3.83
2.64
99.4
97.7
96.5
96.2
n-PrOH
n-BuOH
Conditions: LA 1.0 mmol, Al2(SO4)3 0.2 mmol, Solvent 14 mL. a: dielectric properties
o
o
were measured at room temperature. b: 110 oC, 5 min, c: 130 C, 15 min, d: 150 C, 20
min, e: 150 oC, 35 min.
Esterification of Levulinic acid in aqueous solution
Generally, esterification of pure LA was not practical for
the industrial production of alkyl levulinates because of the
high cost to separate LA from the aqueous solution, which was
produced by the hydrolysis of sugars. To investigate whether
the Al2(SO4)3 system could be applied to the direct
esterification of carbohydrate hydrolyzate, the esterification of
LA aqueous solution with methanol was also carried out. Our
previous work on the hydrolysis of glucose in water showed
that about 42 wt% of LA aqueous solution could be obtained
after simple treatment.16 Therefore, a LA aqueous solution of
the same concentration was prepared to simulate the
carbohydrate hydrolyzate resulting from the hydrolysis of
glucose. And this mixture was submitted it to esterification
reaction with the Al2(SO4)3 system. The results were presented
in Figure 3. The amount of methanol and the mole ratio of the
catalyst Al2(SO4)3 to the acid LA were constant. With the
increase loading of the LA a.q. in MeOH solution, the ML’s
yield first decreased slightly and still kept at higher levels
(>80%) till the weight ratio reached 17.5:100, which indicated
that a proper percentage of water in the reaction system did
not have much effect on the esterification reaction. However,
when the ratio further increased to 22.5:100, the ML yield
decreased to 65.3%. Nevertheless, extension of reaction from
10 min to 30 min could provid a much higher ML yield of 96.7%,
implying that the addition of water to esterification system
might can decrease the reaction rate and the reaction could
reach high product yield at prolonged reaction time. The above
results showed that the Al2(SO4)3 system is of high potential of
being used in the esterification of the acids in the
carbohydrate hydrolysate.
Figure 2.Time course of the esterification of LA by using microwave and
conventional heating. Conditions: LA 1.0 mmol, Al2(SO4)3 0.2 mmol, MeOH 14 mL
(MeOH/metal ions=864 (mole ratio)), 110oC, 10 min.
Preparation of different alkyl levulinates
Different alkyl levulinates were synthesized with the same
catalyst and various alcohols. Alcohols with different carbons
(e.g. ethanol, n-propanol and n-butanol) were tested in this
study. The results are shown in Table 3. A complete conversion
of LA and a high selectivity of alkyl levulinates were achieved
for all tested alcohols. However, the reaction conditions need
to be varied with different alcohols to achieve higher products’
yields. This may be caused by two reasons: the different
reactivities of alcohols and their dielectric properties. As
known, dielectric property of a material determines its heating
efficiency when it is subjected to reaction under microwave
irradiation. Dielectric constant (ε`) and dielectric loss (ε``) are
two key parameters to reflect a medium’s abilities of
absorbing microwave energy and converting the energy to
heat, respectively.14 Therefore, to figure out the relationship
between dielectric properties and reaction efficiency, we
carried out the measurement of dielectric properties of
different alcohols used in our reactions (Table 3). Dielectric
constants of alcohols followed the order of methanol
﹥
ethanol n-propanol n-butanol, as well as the dielectric loss
﹥
﹥
at a commonly used microwave frequency 2.45 GHz. This
result indicated that alcohols with larger polarity have larger
dielectric constants/loss, which then can easily absorb the
microwave energy and generate internal heat, perhaps with
more hot pots (local high temperature). Besides, the collision
frequency of the molecular (nonthermal effects) may also be
greatly increased in the alcohols with higher dielectric
constants.15 Thus, for alcohols with lower dielectric
constants/loss, the “promotion effect” of microwave
irradiation were relatively weaker compared with those with
higher dielectric constants/loss. Therefore, to achieve higher
products’ yields, the reaction conditions need to be varied. By
increasing the reaction temperature or prolonging the reaction
time, high alkyl levulintes’ yields could be obtained when
ethanol, n-propanol and n-butanol were employed as the
solvents.
Yield/%
0
20
40
60
80
100
2.5:100
7.5:100
98.9%
96.3%
90.5%
12.5:100
17.5:100
22.5:100
80.2%
65.3%
22.5:100a
Extension of reaction time
96.7%
Figure 3. Esterification of LA aqueous soltuion. Conditions: LA:H2O=42:58 (weight ratio),
Al2(SO4)3:LA= 0.2 (mole ratio), MeOH 11.06g (14 mL), 400W, 110oC, 10 min. a: reaction
time 40 min.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Page 5 of 7
RSC Advances
Journal Name
ARTICLE
Esterification of different carboxylic acids
time, and then it remained almost unchanged as time prolonged.
View Article Online
Similar trend was also found in dielectric lossDoOfI: 10.1039/C5RA24305J
Given the high conversion and yield of the esterification of
LA, the method was applied to the esterification of other
biomass derived acids. Straight-chain aliphatic acids could be
converted to corresponding esters with high yield under
optimized conditions (Table 4, entries 1, 2, 4). For isovaleric
acid, increased temperature of 130 oC was necessary to
achieve a higher product yield. The esterification of aromatic
acid benzoic acid was much more difficult due to its steric
hindrance effect of phenyl ring compared to aliphatic acid.
Therefore, higher reaction temperature and longer reaction
time were used to obtain better result. A good product yield
could also be achieved at 150 oC for 25 minutes for 2, 5-
furandicarboxylic acid. The above results further suggested
that Al2(SO4)3 was an effective catalyst with good compatibility
for the esterification of different acids.
9.2
MeOH
MeOH+Al2(SO4)3
MeOH
MeOH+Al2(SO4)3
9.1
9.0
8.9
8.8
8.7
8.6
8.5
8.4
8.3
8.2
24.82
24.80
24.78
24.76
24.74
24.72
24.70
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
12
14
16
18
20
Time/min
Time/min
Figure 4. Dielectric properties of MeOH and MeOH+20 mmol% Al2(SO4)3 at 60 oC.
Dielectric constants (left) and Dielectric loss (right).
these two systems. The changes of the dielectric properties of
the mixture suggested that metal salt underwent an
alcoholysis/hydrolysis process in MeOH. The alcoholysis of Al3+
in the mixture led to a gradually change of the mixture’s
composition (more H+ and Al species formed), therefore
causing the change of the mixture’s dielectric property. When
this process reached an equilibrium, the composition of the
mixture would remain unchanged and therefore the mixture’s
dielectric properties didn’t change, either. These results
provided more information about the dielectric properties the
alcohol systems with metal salt catalysts, which might be
useful when designing microwave assisted reactions with
metal salt catalysts.
Table 4. Esterification of different carboxylic acids.
Entry
Product
Conv.(%)
100
Yield(%)
99.7
1a
2a
3b
4a
5c
6c
100
100
100
100
100
99.5
98.1
97.7
98.2
98.5
O
O
O
O
O
Reusability of catalyst
The reusability of catalysts is crucial for building a green,
economical, and sustainable catalytic system. The reusability
of Al2(SO4)3 was evaluated by using the esterification of LA in
methanol as a model reaction. The catalyst was recycled and
reused for 5 times and the results were presented in Fig.5. It
was found that the yield of ML can maintain at high level
(99.4% yield for fresh, 97.8% after five runs). The recycling
experiments further demonstrated that metal salts were
economic and efficient catalysts for the esterification of LA for
the production of alkyl levulinates.
O
O
O
O
Conditions: acid 1 mmol, Al2(SO4)3 0.2 mmol, MeOH 14 mL (MeOH/metal ions=864
(mole ratio)), a: 400W, 110 oC, 10min, b: 400W, 130 oC, 15 min, c: 600 W, 150 oC, 25 min.
Alcoholysis/hydrolysis of metal salts in alcohols
The high catalytic activity of Al2(SO4)3 in the esterification
reaction may be resulted from the alcoholysis/hydrolysis of metal
salts in alcohols, which can generate H+ and then participate in the
esterification reaction. Our previous work on the alcoholysis of
furfuryl alcohol with metal salt catalyst showed that the solvent
became acidic when the reaction mixture was heated to higher
temperature, which indicate the generation of H+.17 To get further
insight into this process, the changes of the dielectric property of
the reaction solvent MeOH and MeOH with the catalyst were
monitored by a Network Analyzer. The dielectric constants/loss of
pure MeOH and MeOH+Al2(SO4)3 were measured at a fixed
temperature of 60 oC (Figure 4). For dielectric constant, pure MeOH
remained almost unchanged during the measuring time. When
Al2(SO4)3 was added to MeOH at 60 oC, the dielectric constant of the
mixture gradually decreased during the first 7 min of the mixing
100
80
60
40
20
0
1
2
3
4
5
Runs
Figure 5. Recycling of the catalyst. Conditions: LA 1.0 mmol, Al2(SO4)3 0.2 mmol, MeOH
14 mL (MeOH/metal ions=864 (mole ratio)), 400W, 110oC, 10 min.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 6 of 7
ARTICLE
Journal Name
C. R.Patil, P. S.Niphadkar,V. V. Bokade and P. N.Joshi, Catal.
Commun., 2014, 43, 188; (c)K. Y. NandDiwOaI:le10, .P10.3VS9ie./wNCA5iprRtihAcl2ae4dO3kn0ali5nrJe,
S. S. Deshpande and V. V. Bokade, J. Chem. Technol.
Biotechnol, 2014, 89, 1507.
Conclusions
In summary, the esterification of LA by using commercially
available metal salts as catalysts was studied. A highest ML
yield of 99.4% could be achieved over Al2(SO4)3 catalyst at
110oC for 10min. Comparison of two heating methods, namely
microwave and conventional heating, in the esterification
reaction indicated that microwave radiation could significantly
enhance the efficiency of the esterification of LA. Other
organic carboxylic acids were also successfully converted to
their esters with high yields. The esterification of LA aqueous
solution in methanol was achieved which provided a potential
solution to the esterification of carbohydrate hydrolyzate to
generate alkyl levulinates. At last, the catalyst can be easily
recycled and reused without much drop in the activity. The
current findings provide a new economically viable and
environmentally friendly system for the esterification of LA or
its aqueous solution to alkyl levulinates.
10 B. L. Oliveira and V.Teixeira da Silva, Catal. Today, 2014, 234
,
257.
11 a) Y. Yang, C. Hu and M. M. Abu-Omar, Green Chem., 2012,
14, 509. b) S. De, S. Dutta and B. Saha, Green Chem., 2011,
13, 2859.c) N. Shi, Q. Liu, Q. Zhang, T. Wang and L. Ma, Green
Chem., 2013, 15, 1967. d) S. Hu, Z. Zhang, J. Song, Y. Zhou
and B. Han, Green Chem., 2009, 11, 1746.
12 a)L. Zhou, H. Zou, J. Nan, L. Wu, X. Yang, Y. Su, T. Lu, J. Xu,
Catal. Commun. 2014, 50, 13.b) M. Schwiderskia, A. Kruse, J
Mol. Catal. A-Chem., 2015, 402, 64.
13 B. Lu, S. An, D. Song, F. Su, X. Yang and Y. Guo, Green Chem.,
2015, 17, 1767.
14 a) J. D. Moseley and C. Oliver Kappe, Green Chem., 2011, 13
794. b) M. Nüchter, B. Ondruschka, W. Bonrath and A. Gum,
Green Chem., 2004, , 128.
,
6
15 S. Tabasso, D. Carnaroglio, E. Calcio Gaudinob and G.
Cravotto, Green Chem., 2015, 17, 684.
16 Y. Huang, J. Dai, X. Deng, Y. Qu, Q. Guo and Y. Fu,
ChemSusChem 2011, 4, 1578.
17 Y. Huang, T. Yang, M. Zhou, H. Pan, Y. Fu, Green Chem. 2015,
Acknowledgements
DOI: 10.1039/C5GC01581B.
The authors are grateful for the financial support by National
Natural Science Foundation of China (NSFC 21502095), Natural
Science Foundation of Jiangsu Province (BK20150872) and the
Jiangsu Specially-Appointed Professor program of the State
Minister of Education of Jiangsu Province.
Notes and references
1
G. M. G. Maldonado, R. S. Assary, J. Dumesic and L. A. Curtiss,
Energy Environ. Sci., 2012, 5, 6981.
2
a) D. R. Fernandes,A.S. Rocha, E.F. Mai, J.A.M. Claudio and V.
T. da Silva, Appl.Catal., A, 2012, 425,199. b) J. Zhang, S. Wu,
B. Li and H. Zhang, ChemCatChem, 2012, 4, 1230.
3
a) D. J. Braden, C. A. Henao, J. Heltzel, C. C. Maravelias and J.
A. Dumesic, Green Chem., 2011, 13, 1755. b) Z. Yang, Y.-B.
Huang, Q.-X. Guo, Yao Fu, Chem. Commun., 2013, 49, 5328.
K. Li, J. Hu, W. Li, F. Ma, L. Xu and Y. Guo, J. Mater. Chem.,
2009, 19, 8628.
4
5
6
D. R. Fernandes, A.S. Rocha, E.F. Mai, C. J. A. Mota and V.
Teixeira da Silva, Appl. Catal., A, 2012, 425-426, 199.
(a)H. J. Bart, J. Reidetschlager, K.Schatka, A. Lehmann, Ind.
Eng. Chem. Res., 1994, 33, 21; (b) Y. Liu, E. Lotero, J.G.
Goodwin Jr., J. Catal., 2006, 242, 278; (c) E. I. Gürbüz, D. M.
Alonso, J. Q. Bond and J. A. Dumesic, ChemSusChem, 2011, 4,
357.
7
8
(a)Y. Kuwahara, T. Fujitani and H. Yamashita, Catal. Today,
2013, 476, 186; (b)Z. L. Li, R. Wnetrzak, W. Kwapinski and J. J.
Leahy, ACS Appl. Mater. Interfaces, 2012,
4, 4499.
(a) S. Dharne and V. V.Bokade, J. Nat. Gas. Chem., 2011, 20
,
18; (b) K. Y. Nandiwale, P. S.Niphadkar, S. S. Deshpande and
V. V. Bokade, Appl. Catal., A, 2013, 460-461, 90; (c) G.
Pasquale, P. Vꢀzquez, G. Romanelli and G. Baronetti, Catal.
Commun., 2012, 18, 115; (d) K. Yan, G. Wu, J. Wen and A.
Chen, Catal. Commun., 2013, 34, 58; (e) F. Su, L. Ma, D. Song,
X. Zhang and Y. Guo, Green Chem., 2013,15, 885; (f) F. Su, Q.
Wu, D. Song, X. Zhang, M. Wang and Y. Guo, J. Mater. Chem.
A, 2013, 1, 13209; (g) J. Zhao, H. Guan, W. Shi, M. Cheng, X.
Wang and S. Li, Catal. Commun., 2012, 20, 103; (h) Y.Leng, P.
Jiang, J. Wang, Catal. Commun., 2012, 25, 41.
(a) J. A.Melero, G.Morales, J.Iglesias, M.Paniagua, B.
Hernandez and S. Penedo, Appl. Catal., A, 2013, 466, 116; (b)
9
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 7
RSC Advances
View Article Online
DOI: 10.1039/C5RA24305J
The esterification of biomass derived platform molecule levulinic aicd to its alkyl levulinates were
carried out over metal salt catalysts under microwave condition, achieving near-quantitative
product yields in few minutes.