catalysts
Article
Catalytic Upgrading of Biomass-Derived Furfuryl
Alcohol to Butyl Levulinate Biofuel over Common
Metal Salts
Lincai Peng *, Ruili Tao and Yu Wu
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China;
[email protected] (R.T.); [email protected] (Y.W.)
* Correspondence: [email protected]; Tel.: +86-871-6746-8910; Fax: +86-871-6592-0171
Academic Editor: Alina M. Balu
Received: 12 August 2016; Accepted: 13 September 2016; Published: 15 September 2016
Abstract: Levulinate ester has been identified as a promising renewable fuel additive and platform
chemical. Here, the use of a wide range of common metal salts as acid catalysts for catalytic
upgrading of biomass-derived furfuryl alcohol to butyl levulinate was explored by conventional
heating. Both alkali and alkaline earth metal chlorides did not lead effectively to the conversion
of furfuryl alcohol, while several transition metal chlorides (CrCl3 , FeCl3 , and CuCl2 ) and AlCl3
exhibited catalytic activity for the synthesis of butyl levulinate. For their sulfates (Cr(III), Fe(III),
Cu(II), and Al(III)), the catalytic activity was low. The reaction performance was correlated with the
Brønsted acidity of the reaction system derived from the hydrolysis/alcoholysis of cations, but was
more dependent on the Lewis acidity from the metal salts. Among these investigated metal salts,
CuCl2 was found to be uniquely effective, leading to the conversion of furfuryl alcohol to butyl
levulinate with an optimized yield of 95%. Moreover, CuCl2 could be recovered efficiently from
the resulting reaction mixture and remained with almost unchanged catalytic activity in multiple
recycling runs.
Keywords: biofuel; furfuryl alcohol; catalytic upgrading; metal salt; levulinate ester
1. Introduction
Given the gradual depletion of fossil fuel resources and rising environmental concerns, increasing
effort has been devoted to the development of alternative fuels and chemicals from abundant and
renewable biomass resources [1–4]. Furfuryl alcohol, one of the most important furan derivatives,
is produced industrially via hydrogenation of furfural derived from the hydrolysis and dehydration
of xylan contained in lignocellulosic biomass [5]. At present, furfuryl alcohol is not fully utilized
and is oversupplied in the chemical market [6]. The development of a feasible and competitive
strategy to reform and upgrade furfuryl alcohol is therefore strongly desired. An attractive option
regarding the one-pot synthesis of levulinate esters via performing the reaction of furfuryl alcohol
with alcohols can be envisioned because the reforming process is atom-economic and convenient,
the alcohols suppress humin by-product formation from furfuryl alcohol polymerization [7], and it
affords versatile platform chemicals. Concretely, levulinate esters have found applications as fuels
or fuel additives [8,9], flavoring agents, solvents, as well as in the area of organic chemistry for the
synthesis of γ-valerolactone [10,11], liquid hydrocarbon transportation fuels [12], and other industrial
chemicals [13].
In recent years, research on the transformation of furfuryl alcohol to levulinate esters has been
reported at an increasing rate [13–15]. Acid catalyst is identified as the key point for the synthesis
of levulinate esters, and is thus the focus of the study. Currently, the utilization of solid acid as
Catalysts 2016, 6, 143; doi:10.3390/catal6090143
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Catalysts 2016, 6, 143
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a catalyst of the reaction has attracted considerable interest due to environmental friendliness and easy
separation from the reaction mixture for reuse. A series of efficient solid acid catalysts including acidic
ion-exchange resin [16], organic-inorganic hybrid acid [17], porous aluminosilicate [18], propylsulfonic
acid-functionalized mesoporous silica [19], mesostructured zirconium-based mixed oxides [20], carbon
and carbon-silica composites containing SO3 H groups [21], graphene oxide [22], organosulfonic
acid-functionalized organosilica hollow nanospheres [23], and hierarchical zeolite [24] have been
successively developed. Beyond that, acidic ionic liquids were found to be efficient and reusable
catalysts for the alcoholysis of furfuryl alcohol to levulinate esters [6,25]. In spite of significant advances
from the persistent studies, the main obstacles for the above-mentioned catalysts include the complex
preparation of the catalyst, expensive catalyst cost, and/or the deactivation of the recovered catalyst in
recycling runs. Consequently, more facile, cheap, efficient and stable catalyst systems are still desirable.
It is well known that many metal salts can endow both strong Lewis and Brønsted acidity in
organic or aqueous solvents. Previously, some interesting and admirable findings on the one-pot
conversion of biomass carbohydrates into fuels and platform chemicals catalyzed by common metal
salts were presented. For example, CrCl2 in ionic liquid solvents was found to be uniquely effective
for glucose conversion, affording a near 70% yield of 5-hydroxymethylfurfural [26]. Analogously,
a high levulinic acid yield of 67% was achieved from cellulose catalyzed by CrCl3 in aqueous
medium [27]. Al2 (SO4 )3 was highly active in the catalytic synthesis of methyl levulinate from
carbohydrates in methanol, giving a 64% yield of methyl levulinate from glucose [28]. These metal
salts are earth-abundant and commercially available, providing inexpensive and environmentally
benign catalyst sources for biomass conversion. Accordingly, common metal salts are expected to be
qualified for the desired purpose in the conversion of furfuryl alcohol to levulinate esters. Recently,
Huang et al. [29] reported Al2 (SO4 )3 with good catalytic activity for the alcoholysis of furfuryl alcohol
under microwave irradiation, affording a 80.6% yield of methyl levulinate. A high butyl levulinate
yield of 92% could be achieved for the alcoholysis of furfuryl alcohol catalyzed by indium (III) triflate;
however, the catalyst is costly [30]. In order to better understand the catalytic mechanism of metal salts
and find more competitive metal salts as alternative catalysts for the conversion of furfuryl alcohol
to levulinate esters, further studies are needed. Herein, the use of a wide range of metal salts as acid
catalysts for the alcoholysis of furfuryl alcohol to butyl levulinate with conventional heating was
evaluated. The results showed that several transition metal chlorides (CrCl3 , FeCl3 , and CuCl2 ) and
AlCl3 were effective for the synthesis of butyl levulinate. In the case of their corresponding sulfates,
the catalytic activity dropped markedly. Among these investigated metal salts, CuCl2 emerged as
a preferred catalyst supplier with unique selectivity for the synthesis of butyl levulinate. Based on
its use, the effect of the process variables was further explored in order to determine the optimal
conditions for the maximized yield of butyl levulinate. Lastly, the reusability of the CuCl2 catalyst was
performed to demonstrate the versatility of the catalytic strategy.
2. Results and Discussion
2.1. Various Metal Salts’ Catalytic Conversion of Furfuryl Alcohol
2.1.1. Catalytic Performance of Various Metal Salts
Firstly, the effect of a wide range of various metal chlorides on the catalytic transformation of
furfuryl alcohol to butyl levulinate in n-butanol was screened at 110 ◦ C after 60 min reaction time
using a 0.3 mol/L furfuryl alcohol solution and a 0.02 mol/L solution of metal chloride as the catalyst.
No catalyst was used as the blank for a reference. The results of the furfuryl alcohol conversion
and butyl levulinate yield for the investigated metal chlorides are depicted in Figure 1. Hardly any
yield of butyl levulinate was observed in the absence of the catalyst. It can also be seen that all
alkali and alkaline earth metal chlorides did not lead effectively to the conversion of furfuryl alcohol,
and there was no obvious difference with the blank (no catalyst). In contrast, several transition
metal chlorides (CrCl3 , FeCl3 , and CuCl2 ) and AlCl3 exhibited catalytic activity in the conversion of
Catalysts 2016, 6, 143 Catalysts 2016, 6, 143 Catalysts
2016, 6, 143
3 of 12 33 of 12 of 12
(CrCl33, FeCl33, and CuCl22) and AlCl33 exhibited catalytic activity in the conversion of furfuryl alcohol to butyl levulinate. Among these four metal chlorides, CrCl
furfuryl
alcohol to butyl levulinate. Among these four metal33 gave a relatively low furfuryl alcohol chlorides, CrCl3 gave a relatively low
conversion (47%) and butyl levulinate yield (28%). Significantly higher conversions (>97%) and yields furfuryl
alcohol conversion (47%) and butyl levulinate yield (28%). Significantly higher conversions
(61%–73%) were found under identical operating conditions for CuCl
22 and AlCl
33. For FeCl
33, a high (>97%)
and yields (61%–73%) were found under identical operating conditions
for
CuCl2 and
AlCl3 .
conversion of 95% could be achieved; however, the butyl levulinate yield was obviously lower (24%) For
FeCl3 , a high conversion of 95% could be achieved; however, the butyl levulinate yield was
than that for CuCl
22 and AlCl
33. The yield of butyl levulinate decreased in the order CuCl
22 > AlCl
33 > obviously
lower (24%)
than that
for CuCl2 and AlCl3 . The yield of butyl levulinate decreased
in the
CrCl33 > FeCl
. These findings suggest that the cation effects in this type of catalyst are quite dramatic order
CuCl2 33>
AlCl3 > CrCl3 > FeCl3 . These findings suggest that the cation effects in this type of
for the synthesis of butyl for
levulinate from of
furfuryl alcohol. However, further investigations using catalyst
are quite dramatic
the synthesis
butyl levulinate
from furfuryl
alcohol.
However, further
metal sulfates instead of metal chlorides as the catalysts indicated that the catalytic performance of investigations using metal sulfates instead of metal chlorides as the catalysts indicated that the catalytic
metal salts is not only correlated with their cation species, but also depends on the anions. Distinct performance
of metal salts is not only correlated with their cation species, but also depends on the
differences in the conversion of conversion
furfuryl alcohol to butyl levulinate observed from Figure anions.
Distinct
differences
in the
of furfuryl
alcohol
to butyl were levulinate
were observed
from2 between the chloride and sulfate salts of four typical metals (Cr(III), Fe(III), Cu(II), and Al(III)). Figure 2 between the chloride and sulfate salts of four typical metals (Cr(III), Fe(III), Cu(II), and Al(III)).
Furfuryl alcohol conversion and butyl levulinate yield for all the above‐investigated metal sulfates Furfuryl
alcohol conversion and butyl levulinate yield for all the above-investigated metal sulfates
were significantly lower than that for their corresponding chlorides, implying that metal chlorides were
significantly lower than that for their corresponding chlorides, implying that metal chlorides
have more potential for the synthesis of levulinate esters from furfuryl alcohol. have
more potential for the synthesis of levulinate esters from furfuryl alcohol.
Conversion and
and yield,
yield, %
%
Conversion
100
100
80
80
Furfuryl
Furfuryl alcohol
alcohol conversion
conversion
Butyl
Butyl levulinate
levulinate yield
yield
60
60
40
40
20
20
BBll
aann
kk
LLii
CCll
KKC
C
NNa ll
aCC
M
M ll
ggCC
l
CCaa l22
CCll
CCrr 22
CC
M
M l3l3
nnCC
l
FFee l22
CCll
CCoo 33
CC
CCuu l2l2
CC
ZZnn l2l2
CCll
AAl 22
lCC
l3l3
00
Figure 1.
1. Catalytic
Catalytic comparison
comparison various metal chlorides on conversion
the conversion of furfuryl alcohol to Figure
ofof various
metal
chlorides
on the
of furfuryl
alcohol
to butyl
levulinate.
ReactionReaction conditions:
0.3 mol/L0.3 furfuryl
in n-butanol;
chloridemetal concentration
butyl levulinate. conditions: mol/L alcohol
furfuryl alcohol in metal
n‐butanol; chloride of
0.02 mol/L; reaction temperature of 110 ◦ C; reaction time of 60 min.
concentration of 0.02 mol/L; reaction temperature of 110 °C; reaction time of 60 min. Conversion and
and yield,
yield, %
%
Conversion
100
100
Furfuryl
Furfuryl alcohol
alcohol conversion
conversion
Butyl
Butyl levulinate
levulinate yield
yield
80
80
60
60
40
40
20
20
44))
33
44
AAl
lCC
l3l3
AAl
l22((
SSOO
CCuu
CCll
22
C
Cuu
SSOO
44))
33
FFee
CCll
FFee
33
22((
S
SOO
CCrr
CCll
CCrr
33
22((
SSOO
44))
33
00
Figure 2.2. Catalytic
Catalytic comparison four typical metal chlorides and sulfates on the conversion of Figure
comparison
of of four
typical
metal
chlorides
and sulfates
on the conversion
of furfuryl
furfuryl alcohol to butyl levulinate. Reaction conditions: 0.3 mol/L furfuryl alcohol in n‐butanol; metal alcohol to butyl levulinate. Reaction conditions: 0.3 mol/L furfuryl alcohol in n-butanol; metal salt
n+
n+); reaction temperature of 110 °C; reaction time of 60 salt concentration of 0.02 mol/L (based on Me
concentration
of 0.02 mol/L (based on Men+ ); reaction
temperature of 110 ◦ C; reaction time of 60 min.
min. 2.1.2. Relationship between Reactivity and Acidity of Reaction System Catalysts 2016, 6, 143
4 of 12
Catalysts 2016, 6, 143 4 of 12 2.1.2. Relationship between Reactivity and Acidity of Reaction System
Although many metal salts belong to Lewis acids, the existence of metal salts as the catalysts in Although
many metal salts belong to Lewis acids, the existence of metal salts as the catalysts
the reaction solution can afford Brønsted acidity derived from the hydrolysis/alcoholysis of cations, 4 of 12 in theCatalysts 2016, 6, 143 reaction solution can afford Brønsted acidity derived from the hydrolysis/alcoholysis
of
which may be related to the reaction performance [28,29,31]. Taking metal chloride as an example, cations, which may be related to the reaction performance [28,29,31]. Taking metal chloride as
Although many metal salts belong to Lewis acids, the existence of metal salts as the catalysts in the example,
possibility hydrolysis/alcoholysis in the reaction to produce species was an
theof possibility
of hydrolysis/alcoholysis
in medium the reaction
medium active to produce
active
the reaction solution can afford Brønsted acidity derived from the hydrolysis/alcoholysis of cations, presumed [32,33], as illustrated in Figure 3. In the reaction medium, Lewis acid metal salts dissociate species
was presumed [32,33], as illustrated in Figure 3. In the reaction medium, Lewis acid metal
which may be related to the reaction performance [28,29,31]. Taking metal chloride as an example, to form complex ions, such as [Me(H
O)
]n+ and O)[Me(ROH)
m]n+. These n+
can ions
be further n+ and [Me(ROH)
salts the dissociate
to form
complex
ions,
such2in as mthe [Me(H
]ions . These
can be
m ]medium 2
possibility of hydrolysis/alcoholysis reaction to produce mactive species was +
hydrolyzed/alcoholyzed, releasing H , leading to a drop in the solution pH. Figure 4 plots the further
hydrolyzed/alcoholyzed, releasing H+ , leading to a drop in the solution pH. Figure 4 plots the
presumed [32,33], as illustrated in Figure 3. In the reaction medium, Lewis acid metal salts dissociate relationship between the butyl levulinate yield (data from Figure 1) and the initial pH value of the n‐
to form between
complex the
ions, such as [Me(Hyield
2O)m]n+
and from
[Me(ROH)
m]n+
. These ions can pH
be value
further relationship
butyl
levulinate
(data
Figure
1)
and the
initial
of the
butanol solution with the employed metal chlorides. It can be found that the pH value of the reaction +, leading to a drop in the solution pH. Figure 4 plots the hydrolyzed/alcoholyzed, releasing H
n-butanol solution with the employed metal chlorides. It can be found that the pH value of the reaction
system of four metal chlorides including CrCl3, FeCl3, CuCl2 and AlCl3 with a high yield of butyl relationship between the butyl levulinate yield (data from Figure 1) and the initial pH value of the n‐
system
of four metal chlorides including CrCl3 , FeCl3 , CuCl2 and AlCl3 with a high yield of butyl
levulinate was relatively low, showing strong Brønsted acidity. In comparison, the other investigated butanol solution with the employed metal chlorides. It can be found that the pH value of the reaction levulinate was relatively low, showing strong Brønsted acidity. In comparison, the other investigated
system of four metal chlorides including CrCl3, FeCl3, CuCl2 and AlCl3 with a high yield of butyl metal chlorides presented weak acidity, and did not effect the conversion of furfuryl alcohol to butyl metal chlorides presented weak acidity, and did not effect the conversion of furfuryl alcohol to butyl
levulinate was relatively low, showing strong Brønsted acidity. In comparison, the other investigated levulinate. It suggests that a certain amount of Brønsted acid sites are required for the synthesis of levulinate.
It suggests that a certain amount of Brønsted acid sites are required for the synthesis
metal chlorides presented weak acidity, and did not effect the conversion of furfuryl alcohol to butyl butyl levulinate. However, this does not mean that the stronger the Brønsted acidity, the better the of butyl
levulinate. However, this does not mean that the stronger the Brønsted acidity, the better
levulinate. It suggests that a certain amount of Brønsted acid sites are required for the synthesis of butyl levulinate formation. For example, the pH value of the reaction system with FeCl
3 was slightly the butyl
levulinate formation. For example, the pH value of the reaction system with FeCl3 was
butyl levulinate. However, this does not mean that the stronger the Brønsted acidity, the better the lower (stronger Brønsted acidity) than that with CuCl2, but the yield of the butyl levulinate was slightly
lower (stronger Brønsted acidity) than that with CuCl2 , but the yield of the3 was slightly butyl levulinate
butyl levulinate formation. For example, the pH value of the reaction system with FeCl
markedly lower. In the case of the four effective metal chlorides (CrCl
3, FeCl3, CuCl2 and AlCl3), lower (stronger acidity) than that with CuCl
2, but the yield of the butyl was was markedly
lower.Brønsted In the case
of the
four
effective
metal
chlorides
(CrCl
CuCl2 and
AlCl3 ),
3 , FeCl
3 ,levulinate further tests were carried out at two effective same pH values (1 and 2) 3adjusted by 2their dosages. It markedly lower. In the of the the metal (CrCl
, by
FeCl
3, CuCl
and AlCl
3), further
tests were
carried
outcase at the
twofour same pH values
(1chlorides and 2) adjusted
their
dosages.
It revealed
revealed from Figure 5 that different metal chlorides as the catalysts produced clearly different yields carried out at the two same values produced
(1 and 2) adjusted by their dosages. from further Figure tests 5 thatwere different
metal
chlorides
as thepH catalysts
clearly different
yields ofIt butyl
of butyl levulinate, and the yield for CuCl
2 was the highest under the condition of two pH values. revealed from Figure 5 that different metal chlorides as the catalysts produced clearly different yields levulinate, and the yield for CuCl2 was the highest under the condition of two pH values. Based on
of butyl levulinate, and the yield for CuCl2 was the highest under the condition of two pH values. Based on these observations, we concluded that the Lewis acidity of metal salts probably plays some these observations, we concluded that the Lewis acidity of metal salts probably plays some role in the
2+ Based on these observations, we concluded that the Lewis acidity of metal salts probably plays some role in the synthesis of butyl levulinate in addition to the Brønsted acidity. The combination of Cu
synthesis
of butyl levulinate in addition to the Brønsted acidity. The combination of Cu2+ and Cl2+− can
role in the synthesis of butyl levulinate in addition to the Brønsted acidity. The combination of Cu
−
and Cl can afford optimal Lewis and Brønsted acid sites for the synthesis of butyl levulinate from affordand Cl
optimal
Lewis and Brønsted acid sites for the synthesis of butyl levulinate from furfuryl alcohol
− can afford optimal Lewis and Brønsted acid sites for the synthesis of butyl levulinate from furfuryl alcohol conversion with unique selectivity. conversion
with unique selectivity.
furfuryl alcohol conversion with unique selectivity. [Me(H O) ]n+ + n Cl-
Hydrolysis：MeCln + m H2O
2 ]mn+ + n Cl[Me(H2O)
m
Hydrolysis：MeCln + m H2O
n+n+
[Me(H
[Me(H
H22O
O
2O)
m ]m ] ++ H
2O)
(n-1)+
(n-1)+
+ O
[Me(H
OH]
[Me(H
OH]
+ H+3OH
2 O)
3
2 O)
m-1m-1
+
n+n+
[Me(ROH)
n Cl
[Me(ROH)
n -Clm]m] + +
Alcoholysis：MeCl
ROH
Alcoholysis：MeCl
n n+ +mmROH
+
(n-1)+
+
[Me(ROH)
+ ROH
m-1 (OR)]
2 [Me(ROH)
(OR)](n-1)+
+ ROH
[Me(ROH)]mn+
]n+ ++ ROH
ROH
[Me(ROH)
m
m-1
2
Figure 3. Presumed hydrolysis/alcoholysis reaction of metal chloride in the reaction medium. Figure 3. Presumed hydrolysis/alcoholysis reaction of metal chloride in the reaction medium. Figure
3. Presumed hydrolysis/alcoholysis reaction of metal chloride in the reaction medium.
Butyl levulinate yield, %
Butyl levulinate yield, %
80
80
60
60
40
40
20
Blank
Blank
LiCl
NaCl
LiCl
KCl
MgCl
2
KCl
NaCl
CaCl
2
MgCl
2
CrCl
3
MnCl
CaCl2 2
FeCl
3 3
CrCl
CoCl
2 2
MnCl
CuCl
2 3
FeCl
ZnCl
2
CoCl
2
AlCl
3
CuCl
2
ZnCl2
AlCl3
20
0
0
0
1
2
3
4
5
pH value of reaction solution
6
7
0
1
2
3
4
5
6
7
Figure 4. Relationship between butyl levulinate yield (data from Figure 1) and initial pH value of n‐
pH value of reaction solution
butanol solution with 0.02 mol/L metal chlorides measured at room temperature. Figure 4. Relationship between butyl levulinate yield (data from Figure 1) and initial pH value of n‐
Figure
4. Relationship between butyl levulinate yield (data from Figure 1) and initial pH value of
butanol solution with 0.02 mol/L metal chlorides measured at room temperature. n-butanol solution with 0.02 mol/L metal chlorides measured at room temperature.
Catalysts 2016, 6, 143
Catalysts 2016, 6, 143 5 of 12
5 of 12 Butyl levulinate yield, %
80
pH=2
pH=1
60
40
20
0
FeCl3
CrCl3
CuCl2
AlCl3
Figure 5. Butyl levulinate yield for four typical metal chlorides at the two same initial pH values of n‐
Figure
5. Butyl levulinate yield for four typical metal chlorides at the two same initial pH values of
butanol solution adjusted by metal chlorides dosage. Reaction conditions: 0.3 mol/L furfuryl alcohol n-butanol solution adjusted by metal chlorides dosage. Reaction conditions: 0.3 mol/L furfuryl alcohol
in n‐butanol; reaction temperature of 110 °C; reaction time of 60 min. in
n-butanol; reaction temperature of 110 ◦ C; reaction time of 60 min.
Next, Figure 6 presents the relationship between the butyl levulinate yield (data from Figure 2) Next,
Figure 6 presents the relationship between the butyl levulinate yield (data from Figure 2)
and the initial pH value
value of
of the
the reaction
reaction system
system with
with various
various metal
metal chlorides
chlorides and
and sulfates.
sulfates. For
For all
all and the initial pH
investigated metal sulfates, the butyl levulinate yield was lower than that for their corresponding investigated
metal sulfates, the butyl levulinate yield was lower than that for their corresponding
chlorides. Accordingly, the pH value of the reaction system with metal sulfates was high, and fewer chlorides. Accordingly, the pH value of the reaction system with metal sulfates was high, and fewer
Brønsted acid sites were available for metal sulfates. This finding suggests that there is a good Brønsted acid sites were available for metal sulfates. This finding suggests that there is a good
agreement between the reaction performance and Brønsted acidity. However, when we conducted agreement
between the reaction performance and Brønsted acidity. However, when we conducted
experiments at the same initial pH value of the reaction system for metal chlorides and sulfates, the experiments at the same initial pH value of the reaction system for metal chlorides and sulfates,
yield of the butyl levulinate for the sulfates of Cr(III), Cu(II) and Al(III) was still significantly lower the yield of the butyl levulinate for the sulfates of Cr(III), Cu(II) and Al(III) was still significantly lower
than that
that their corresponding chlorides (see 7).
Figure 7). A explanation
possible explanation for these than
forfor their
corresponding
chlorides
(see Figure
A possible
for these appearances
appearances is that the reaction performance depends not only on the Brønsted acidity of the reaction is that the reaction performance depends not only on the Brønsted acidity of the reaction system,
system, but also on the Lewis acidity of the metal salts, and the Lewis and Brønsted acidity for metal but
also on the Lewis acidity of the metal salts, and the Lewis and Brønsted acidity for metal sulfates
sulfates are weaker than those for their corresponding chlorides [34,35]. and large, the are weaker
than
those for
their
corresponding
chlorides [34,35].
By and large,
the By presentation
of the
presentation of the optimal Lewis and Brønsted acid sites for the efficient conversion of furfuryl optimal Lewis and Brønsted acid sites for the efficient conversion of furfuryl alcohol to butyl levulinate
alcohol to butyl levulinate is derived from a valid combination of cations and anions. is derived from a valid combination of cations and anions.
Butyl levulinate yield, %
80
60
CrCl3
Cr2(SO4)3
FeCl3
CuCl2
Fe2(SO4)3
CuSO4
AlCl3
Al2(SO4)3
40
20
0
0.5
1.0
1.5
2.0
2.5
pH value of reaction solution
3.0
Figure 6. Relationship between butyl levulinate yield (data from Figure 2) and initial pH value of n‐
Figure
6. Relationship between butyl levulinate yield (data from Figure 2) and initial pH value of
butanol solution with various metal chlorides and sulfates. n-butanol solution with various metal chlorides and sulfates.
Catalysts 2016, 6, 143
Catalysts 2016, 6, 143 6 of 12
6 of 12 pH=0.94
80
6 of 12 pH=0.67
60
100
40
80
pH=1.67
pH=0.67
4)
3
3
A
l2 (
SO
2
lC
l
A
(S
Cu
SO
4)
3
pH=0.78
Fe
2
20
Cu
Cl
3
pH=1.67
O
0 40
4
60
4)
3
20
pH=0.94
pH=0.78
Fe
Cl
Catalysts 2016, 6, 143 Butyl levulinate yield, %
Cr
Cl
Cr
3
2(
SO
Butyl levulinate yield, %
100
0
4)
3
SO
A
l2 (
A
lC
l
3
4
2
Cu
SO
4)
3
Fe
2
Cu
Cl
3
O
Fe
Cl
(S
SO
Cr
2(
Cr
Cl
3
4)
3
Figure 7.7. Butyl
Butyl levulinate
levulinate yield metal chlorides sulfates the initial
same initial pH ofvalue of n‐
Figure
yield
forfor metal
chlorides
and and sulfates
at theat same
pH value
n-butanol
butanol solution adjusted by metal salts dosage. Reaction conditions: 0.3 mol/L furfuryl alcohol in n‐
solution
adjusted by metal salts dosage. Reaction conditions: 0.3 mol/L furfuryl alcohol in n-butanol;
butanol; reaction temperature of 110 °C; reaction time of 60 min. reaction
temperature of 110 ◦ C; reaction time of 60 min.
Figure 7. Butyl levulinate yield for metal chlorides and sulfates at the same initial pH value of n‐
butanol solution adjusted by metal salts dosage. Reaction conditions: 0.3 mol/L furfuryl alcohol in n‐
2.2. Optimization of Butyl Levulinate Production Catalyzed by CuCl
2.2.
Optimization
of Butyl Levulinate Production Catalyzed by CuCl 2 butanol; reaction temperature of 110 °C; reaction time of 60 min. 2
Among the tested metal chlorides as the catalysts, CuCl
2 was found to be uniquely effective in Among
the tested metal chlorides as the catalysts, CuCl2 was
found to be uniquely effective in the
2.2. Optimization of Butyl Levulinate Production Catalyzed by CuCl2 the conversion of furfuryl alcohol to butyl levulinate. Therefore, it was chosen as the most suitable conversion of furfuryl alcohol to butyl levulinate. Therefore, it was chosen as the most suitable catalyst
catalyst for this purpose and was used in the further exploration. To understand the transformation Among the tested metal chlorides as the catalysts, CuCl
was found to be uniquely effective in for
this purpose
and was used in the further exploration. To 2understand
the transformation process
the conversion of furfuryl alcohol to butyl levulinate. Therefore, it was chosen as the most suitable process of alcohol
furfuryl catalyzed
alcohol catalyzed 2 and to achieve highest possible yield of butyl of
furfuryl
by CuCl2 by andCuCl
to achieve
the
highestthe possible
yield
of butyl
levulinate,
catalyst for this purpose and was used in the further exploration. To understand the transformation levulinate, the effects of the process parameters including the catalyst dosage, reaction temperature the effects of the process parameters including the catalyst dosage, reaction temperature and initial
process of furfuryl alcohol catalyzed by CuCl2 and to achieve the highest possible yield of butyl and initial furfuryl alcohol concentration on furfuryl alcohol conversion and butyl levulinate yield furfuryl
alcohol concentration on furfuryl alcohol conversion and butyl levulinate yield were studied
levulinate, the effects of the process parameters including the catalyst dosage, reaction temperature were studied as a function of time. The results are discussed successively in the following sections. as
a function of time. The results are discussed successively in the following sections.
and initial furfuryl alcohol concentration on furfuryl alcohol conversion and butyl levulinate yield were studied as a function of time. The results are discussed successively in the following sections. 2.2.1.
Catalyst Dosage
2.2.1. Catalyst Dosage 80
60
40
20
100
100
(a)
(a)
100
80
CuCl2 concentration, mol/L
60
0.002
CuCl2 concentration,
mol/L
0.005
0.002
0.01
0.005
0.02
0.010.03
0.02
0.03
40
20
0
0
20
40
0
0
20
40
60
80
Time, min
60
80
100
120
levulinate
yield,
ButylButyl
levulinate
yield,
%%
100
Furfuryl alcohol conversion, %
Furfuryl alcohol conversion, %
2.2.1. Catalyst Dosage Figure
8 illustrates the influence of the CuCl22 catalyst dosage, ranging from 0.002 to 0.03 mol/L, catalyst dosage, ranging from 0.002 to 0.03 mol/L,
Figure 8 illustrates the influence of the CuCl
on
the
conversion
of
furfuryl
alcohol
to
butyl
levulinate,
while the reaction temperature was held
on the conversion of furfuryl alcohol to butyl levulinate, while the reaction temperature was held Figure 8 illustrates the influence of the CuCl2 catalyst dosage, ranging from 0.002 to 0.03 mol/L, ◦
on the conversion of furfuryl alcohol to butyl levulinate, while the reaction temperature was held constant
at 110 C. As can be seen, the conversion rate of furfuryl alcohol and butyl levulinate yield
constant at 110 °C. As can be seen, the conversion rate of furfuryl alcohol and butyl levulinate yield constant at 110 °C. As can be seen, the conversion rate of furfuryl alcohol and butyl levulinate yield increased
rapidly with increasing the CuCl22 dosage from 0.002 to 0.02 mol/L. This implies that the dosage from 0.002 to 0.02 mol/L. This implies that the
increased rapidly with increasing the CuCl
increased rapidly with increasing the CuCl
2 dosage from 0.002 to 0.02 mol/L. This implies that the synthesis
of butyl levulinate from furfuryl alcohol involves a more acidic-demanding step. It is worth
synthesis of butyl levulinate from furfuryl alcohol involves a more acidic‐demanding step. It is worth synthesis of butyl levulinate from furfuryl alcohol involves a more acidic‐demanding step. It is worth noting
that furfuryl alcohol was almost entirely consumed after the reaction time of 80 min for all
noting that furfuryl alcohol was almost entirely consumed after the reaction time of 80 min for all noting that furfuryl alcohol was almost entirely consumed after the reaction time of 80 min for all tested
catalyst dosages (Figure 8a). However, the CuCl22 dosage of below 0.01 mol/L cannot seem to dosage of below 0.01 mol/L cannot seem to
tested catalyst dosages (Figure 8a). However, the CuCl
tested catalyst dosages (Figure 8a). However, the CuCl2 dosage of below 0.01 mol/L cannot seem to offer
sufficient acid sites for the synthesis of butyl levulinate from furfuryl alcohol and/or its derived
offer sufficient acid sites for the synthesis of butyl levulinate from furfuryl alcohol and/or its derived offer sufficient acid sites for the synthesis of butyl levulinate from furfuryl alcohol and/or its derived intermediates at a short time, thus giving rise to a low production rate. With increasing the CuCl
intermediates
at a short time, thus giving rise to a low production rate. With increasing the CuCl
22 intermediates at a short time, thus giving rise to a low production rate. With increasing the CuCl
2 dosage, the increased total number of available acid sites resulted in a faster reaction rate to promote dosage,
the increased total number of available acid sites resulted in a faster reaction rate to promote
dosage, the increased total number of available acid sites resulted in a faster reaction rate to promote the synthesis of butyl levulinate. When the CuCl
the
synthesis
of butyl levulinate. When the CuCl222 dosage was 0.02 mol/L, the equilibrium conversion dosage
was 0.02 mol/L, the equilibrium conversion
the synthesis of butyl levulinate. When the CuCl
dosage was 0.02 mol/L, the equilibrium conversion for the formation of butyl levulinate was almost reached 80 min (Figure The loading for the
the formation butyl levulinate almost reached after 80 min (Figure 8b). The loading of for
formation
ofof butyl
levulinate
waswas almost
reached
afterafter 80 min
(Figure
8b).8b). The
loading
of of excess
excess CuCl
2
(0.03 mol/L) had little effect, indicating that an optimum CuCl
2
dosage exists for the excess CuCl
2 (0.03 mol/L) had little effect, indicating that an optimum CuCl
dosage exists for the CuCl
had little effect, indicating that an optimum CuCl2 dosage2exists
for the reaction.
2 (0.03 mol/L)
reaction. The most beneficial CuCl
2 dosage was chosen to be 0.02 mol/L. reaction. The most beneficial CuCl
2 dosage was chosen to be 0.02 mol/L. The
most
beneficial CuCl2 dosage was
chosen to be 0.02 mol/L.
(b)
(b)
80
80
60
60
40
40
20
20
0
0
20
0
100
120
0
20
40
60
80
Time, min
40
60
100
80
120
100
120
Time, min
min
(b) as
FigureFigure 8. Effect of catalyst dosage on furfuryl alcohol conversion (a) and butyl levulinate yield (b) as 8. Effect of catalyst Time,
dosage
on furfuryl alcohol conversion (a) and
butyl levulinate yield
a function of time. Reaction conditions: 0.3 mol/L furfuryl alcohol in n‐butanol; CuCl2 as the catalyst; aFigure 8. Effect of catalyst dosage on furfuryl alcohol conversion (a) and butyl levulinate yield (b) as function of time. Reaction conditions: 0.3 mol/L furfuryl alcohol in n-butanol; CuCl2 as the catalyst;
reaction temperature of 110 °C. reaction
temperature of 110 ◦ C.
a function of time. Reaction conditions: 0.3 mol/L furfuryl alcohol in n‐butanol; CuCl
2 as the catalyst; reaction temperature of 110 °C. Catalysts
2016, 6, 143
Catalysts 2016, 6, 143 77 of 12 of 12
100
100
(a)
Butyl levulinate yield, %
Furfuryl alcohol conversion, %
2.2.2. Reaction Temperature 2.2.2. Reaction Temperature
The effect of the reaction temperature (100–120 °C) on the conversion of furfuryl alcohol to butyl ◦ C) on the conversion of furfuryl alcohol to butyl
The
effect
of the reaction
temperature
(100–120
levulinate is presented in Figure 9. It can be observed that the reaction temperature played an levulinate
is presented in Figure 9. It can be observed that the reaction temperature played
an important
important role in the reaction process, which had similar rules as that of the CuCl
2 dosage for furfuryl role
in theconversion reaction process,
which
had similar
rulesUnder as thatlower of thetemperature CuCl2 dosageconditions, for furfurylthe alcohol
alcohol and butyl levulinate yield. butyl conversion
and
butyl
levulinate
yield.
Under
lower
temperature
conditions,
the
butyl
levulinate
levulinate yield grew relatively slowly with the prolonging of the reaction time. Only 46% and 67% yield
grew
relatively
slowlywere withreached the prolonging
ofreaction the reaction
Only
and°C 67%
yields
of
yields of butyl levulinate after the time time.
of 120 min 46%
at 100 and 105 °C, ◦
◦
butyl
levulinate
reached
afteralcohol the reaction
time oftotally 120 min
at 100 Cwithin and 105
C, respectively.
respectively. In were
contrast, furfuryl was almost converted 60 min. A possible Inexplanation for this appearance is that a large number of intermediates can be accumulated in the contrast, furfuryl alcohol was almost totally converted within 60 min. A possible explanation for this
appearance
is that a large number of intermediates can be accumulated in the reaction process [6,36],
reaction process [6,36], which may need a longer time to achieve a complete reaction. Increasing the which
may need a longer time to achieve a complete reaction. Increasing the temperature to 115 ◦ C,
temperature to 115 °C, the conversion takes place at a faster rate and the yield of butyl levulinate the
conversion takes place at a faster rate and the yield of butyl levulinate increased rapidly with the
increased rapidly with the extension of the reaction time, and the equilibrium point was reached by extension
of the reaction time, and the equilibrium point was reached by 60 min. With an elevated
60 min. With an elevated temperature of 120 °C, the reaction rate and final yield of butyl levulinate temperature
of 120 ◦ C, the reaction rate and final yield of butyl levulinate did not change substantially
did not change substantially compared with that at 115 °C. Thus, it can be seen that the elevation of compared
with that at 115 ◦ C. Thus, it can be seen that the elevation of temperature is also unavailing
temperature is also unavailing for the extent of the reaction. for the extent of the reaction.
80
Temperature, oC
100
105
110
115
120
60
40
20
(b)
80
60
40
20
0
0
0
20
40
60
80
100
Time, min
120
0
20
40
60
80
100
120
Time, min
Figure 9. Effect of reaction temperature on furfuryl alcohol conversion (a) and butyl levulinate (b) Figure
9. Effect of reaction temperature on furfuryl alcohol conversion (a) and butyl levulinate (b) yield
yield as a function of time. conditions:
Reaction conditions: 0.3 mol/L furfuryl alcohol CuCl
in n‐butanol; CuCl2 as a function
of time. Reaction
0.3 mol/L furfuryl
alcohol
in n-butanol;
2 concentration
concentration of 0.02 mol/L. of 0.02 mol/L.
2.2.3. Initial Furfuryl Alcohol Concentration 2.2.3.
Initial Furfuryl Alcohol Concentration
Theoretically, increasing the initial furfuryl alcohol concentration results in more substrate being Theoretically,
increasing the initial furfuryl alcohol concentration results in more substrate being
available for the efficient conversion to produce butyl levulinate. However, yield of butyl available for the efficient
conversion
to produce
butyl levulinate.
However,
the yield the of butyl
levulinate
levulinate dropped clearly with increases in the initial furfuryl alcohol concentration from 0.1 to 2 dropped clearly with increases in the initial furfuryl alcohol concentration from 0.1 to 2 mol/L
mol/L (see Figure 10b), implying that the inhibits
substrate the kinetics in A
this process. A similar (see
Figure
10b),
implying
that the substrate
the inhibits kinetics in
this
process.
similar
phenomenon
phenomenon has also been observed in the previous related reports [7,17,25,27]. When the initial has also been observed in the previous related reports [7,17,25,27]. When the initial furfuryl alcohol
furfuryl alcohol concentration was 0.1 mol/L, the yield of butyl levulinate reached a maximum of 95% concentration was 0.1 mol/L, the yield of butyl levulinate reached a maximum of 95% within 80 min.
within 80 min. Unfortunately, the concentration of butyl levulinate in the reaction mixture was fairly Unfortunately,
the concentration of butyl levulinate in the reaction mixture was fairly low, at around
low, at around 0.09 mol/L (Figure 10c). In reality, a higher concentration of product is desired because 0.09 mol/L (Figure 10c). In reality, a higher concentration of product is desired because it can not only
it can not only improve the production efficiency of butyl levulinate, but can also cut down energy improve
the production efficiency of butyl levulinate, but can also cut down energy consumption
consumption in the purification the product. the alcohol
initial furfuryl alcohol concentration in the purification
of the
product. of When
the initialWhen furfuryl
concentration
increased
from
increased from 0.1 to 1 mol/L, the concentration of butyl levulinate had risen obviously, from 0.09 to 0.1 to 1 mol/L, the concentration of butyl levulinate had risen obviously, from 0.09 to 0.52 mol/L,
0.52 mol/L, but its yield was found to decrease. On the other hand, it can be noticed that the furfuryl but
its yield was found to decrease. On the other hand, it can be noticed that the furfuryl alcohol
alcohol conversion was less influenced by its initial concentration, giving close to 100% conversions conversion
was less influenced by its initial concentration, giving close to 100% conversions after
after for (Figure
all tests (Figure 10a). At a higher initial furfuryl alcohol concentration, more 80
min80 formin all tests
10a).
At a higher
initial
furfuryl
alcohol
concentration,
more intermediates
intermediates (e.g., 2‐butoxymethylfuran, 4,5,5‐tributoxypentan‐2‐one) in the reaction mixture were (e.g.,
2-butoxymethylfuran, 4,5,5-tributoxypentan-2-one) in the reaction mixture were detected by gas
detected by gas chromatography‐mass spectrometer (GC‐MS) ofand the formation of dark‐brown chromatography-mass
spectrometer (GC-MS)
and the formation
dark-brown
insoluble
substances
insoluble substances known as humins from furfuryl alcohol polymerization was also favored [7,25]. known
as humins from furfuryl alcohol polymerization was also favored [7,25]. The reason for these
The reason for these observations is probably due to product feedback inhibition and/or reactivity diminution bringing about an incomplete alcoholysis reaction and the enhancement of the side Catalysts 2016, 6, 143
8 of 12
Catalysts 2016, 6, 143 8 of 12 Furfuryl alcohol conversion, %
observations is probably due to product feedback inhibition and/or reactivity diminution bringing
reaction. Further increasing the initial furfuryl alcohol concentration to 2 mol/L, the concentration of about an incomplete alcoholysis reaction and the enhancement of the side reaction. Further increasing
the butyl levulinate was lower than that of 1 mol/L during the initial stage of the reaction (Figure 10c). the initial furfuryl alcohol concentration to 2 mol/L, the concentration of the butyl levulinate was
This may be because of the insufficient acid for the conversion of excess furfuryl alcohol, thus gaining lower than that of 1 mol/L during the initial stage of the reaction (Figure 10c). This may be because of
a low production rate of butyl levulinate. Based on these findings, it is necessary to have a the insufficient acid for the conversion of excess furfuryl alcohol, thus gaining a low production rate of
comprehensive consideration for choosing an adequate initial furfuryl alcohol concentration in the butyl levulinate. Based on these findings, it is necessary to have a comprehensive consideration for
practical operation. choosing an adequate initial furfuryl alcohol concentration in the practical operation.
100
(a)
80
60
Initial furfuryl alcohol concentration, mol/L
0.1
0.3
0.5
1
2
40
20
0
0
20
40
60
80
100
120
20
40
Time, min
0.6
(b)
Butyl levulinate con., mol/L
Butyl levulinate yield, %
100
80
60
40
20
(c)
0.5
0.4
0.3
0.2
0.1
0.0
0
0
20
40
60
Time, min
80
100
120
0
60
80
100
120
Time, min
Figure 10.
10. Effect
Effect of
of initial
initial furfuryl
furfuryl alcohol
alcohol concentration
concentration on
on furfuryl
furfuryl alcohol
alcohol conversion
conversion (a);
(a); butyl
butyl Figure
levulinate yield (b); and concentration (c) as a function of time. Reaction conditions: CuCl
levulinate yield (b); and concentration (c) as a function of time. Reaction conditions: CuCl2 2 concentration of 0.02 mol/L; reaction temperature of 115 °C. concentration
of 0.02 mol/L; reaction temperature of 115 ◦ C.
2.3. Reusability of CuCl
2.3.
Reusability of CuCl22 Catalyst Catalyst
The reusability
reusability and
and long‐term stability of a catalyst are very important characteristics for
for the
the The
long-term stability
of a catalyst
are very
important characteristics
practical implementation of the technology to reduce the production cost. Previously, several metal practical implementation of the technology to reduce the production cost. Previously, several metal
salts such as Al
2(SO4)3 [28,29], AlCl3 [31,37], and In(OTf)3 [38] were found to be recoverable from the salts
such as Al2 (SO
4 )3 [28,29], AlCl3 [31,37], and In(OTf)3 [38] were found to be recoverable from the
reaction mixture and exhibited fine catalytic stability in multiple cycles of reuse. Herein, the recycle reaction mixture and exhibited fine catalytic stability in multiple cycles of reuse. Herein, the recycle
experiments for the selected CuCl
experiments
for the selected CuCl2 catalyst were carried out under the same reaction conditions. After 2 catalyst were carried out under the same reaction conditions.
the reaction was finished, the solvent and products were removed by a distillation technique that After the reaction was finished, the solvent and products were removed by a distillation technique that
combines an atmospheric distillation with a vacuum distillation where n‐tridecane was added to help combines
an atmospheric distillation with a vacuum distillation where n-tridecane was added to help
distill the heavy fraction. The obtained residue was used as catalyst in the next run. The weight of the distill
the heavy fraction. The obtained residue was used as catalyst in the next run. The weight of
residue was around 1.1 times that of the initial added CuCl
2. This is likely due to the interfusion of the
residue was around 1.1 times that of the initial added CuCl
2 . This is likely due to the interfusion
humins formed from the partial polymerization of the furfuryl alcohol [7], resulting in an increase in of humins formed from the partial polymerization of the furfuryl alcohol [7], resulting in an increase
the residue weight. If so, it is reckoned that about 3.5% furfuryl alcohol was polymerized to form the in
the residue weight. If so, it is reckoned that about 3.5% furfuryl alcohol was polymerized to form
humins. The above recycling process was repeated eight times, and the reusability results of the CuCl
the humins. The above recycling process was repeated eight times, and the reusability results of the 2 are given in Figure 11. The catalytic activity of the recovered catalyst after six cycles was still CuCl
2 are given in Figure 11. The catalytic activity of the recovered catalyst after six cycles was still
comparable to that of the fresh CuCl
catalyst. Furfuryl alcohol was almost fully converted for all six comparable to that of the fresh CuCl2 2catalyst.
Furfuryl alcohol was almost fully converted for all six
runs, giving nearly the same yield of butyl levulinate. In the subsequent two runs, there was a decrease in the yield of butyl levulinate, but the changes were not very noticeable. It suggests that the catalyst species basically appear not to decompose in the reaction, and CuCl2 can serve as a Catalysts 2016, 6, 143
9 of 12
runs, giving nearly the same yield of butyl levulinate. In the subsequent two runs, there was a decrease
Catalysts 2016, 6, 143 9 of 12 in the yield of butyl levulinate, but the changes were not very noticeable. It suggests that the catalyst
species
basically appear not to decompose in the reaction, and CuCl2 can serve as a reusable catalyst
reusable catalyst with excellent catalytic stability for the synthesis of butyl levulinate from furfuryl with
excellent
catalytic stability for the synthesis of butyl levulinate from furfuryl alcohol.
alcohol. Furfuryl alcohol conversion
Butyl levulinate yield
Conversion and yield, %
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Number of cycles
Figure 11.
11. Reusability furfuryl alcohol in in
n‐
Figure
Reusability of of the the CuCl
CuCl22 catalyst. catalyst. Reaction Reactionconditions: conditions:0.1 0.1mol/L mol/L
furfuryl
alcohol
◦
butanol; CuCl
2 concentration of 0.02 mol/L; reaction temperature of 115 °C; reaction time of 60 min. n-butanol; CuCl2 concentration of 0.02 mol/L; reaction temperature of 115 C; reaction time of 60 min.
3. Materials and Methods 3. Materials and Methods
3.1. Materials 3.1.
Materials
Furfuryl alcohol (98% purity) and butyl levulinate (98% purity) were purchased from Aladdin Furfuryl
alcohol (98% purity) and butyl levulinate (98% purity) were purchased from Aladdin
Reagent (Shanghai, China). Metal n‐butanol were
were obtained
obtained from
from Sinopharm
Sinopharm Chemical
Chemical Reagent (Shanghai, China).
Metal salts salts and and n-butanol
Reagent Co., Ltd. (Shanghai, China). The above reagents and chemicals were all of analytical grade Reagent Co., Ltd. (Shanghai, China). The above reagents and chemicals were all of analytical grade
and used without further purification or treatment. and
used without further purification or treatment.
3.2. General Procedure for the Synthesis of Butyl Levulinate 3.2.
General Procedure for the Synthesis of Butyl Levulinate
The synthesis of butyl levulinate from furfuryl alcohol were conducted in a cylindrical stainless The
synthesis of butyl levulinate from furfuryl alcohol were conducted in a cylindrical stainless
steel pressurized reactor with 50 mL total volume. In a typical experiment, 0.53 mL furfuryl alcohol steel pressurized reactor with 50 mL total volume. In a typical experiment, 0.53 mL furfuryl alcohol
(0.3 mol/L working concentration) and 19.47 mL n‐butanol were introduced into the reactor along (0.3
mol/L working concentration) and 19.47 mL n-butanol were introduced into the reactor along
n+). After the reactor was sealed, the above mixture was then with 0.4 mmol metal salt (based on Me
with
0.4 mmol metal salt (based on Men+
). After the reactor was sealed, the above mixture was then
brought to desired temperature by oil‐bath heating and was continuously stirred at 800 rpm with a brought
to desired temperature by oil-bath heating and was continuously stirred at 800 rpm with
amagnetic stirrer. When running the reaction for a desired duration, the reactor was cooled down in magnetic stirrer. When running the reaction for a desired duration, the reactor was cooled down
an ice water bath to terminate the reaction. Then, the resulting reaction mixture taken from the reactor in
an ice water bath to terminate the reaction. Then, the resulting reaction mixture taken from the
was diluted with an ethanol solution containing the internal standard of methyl levulinate (0.01 reactor
was diluted
with
an ethanol
solution
containing
the internal
standard
of methyl
levulinate
mol/L) for GC analysis. (0.01
mol/L) for GC analysis.
3.3.
Products Analysis
3.3. Products Analysis Furfuryl
alcohol and butyl levulinate in the reaction mixture after the reaction were analyzed by
Furfuryl alcohol and butyl levulinate in the reaction mixture after the reaction were analyzed by GC
on
an
Agilent
6890 instrument (Agilent Instruments, Fair Oaks, CA, USA) equipped with a DB-5
GC on an Agilent 6890 instrument (Agilent Instruments, Fair Oaks, CA, USA) equipped with a DB‐5 capillary
column (30.0 m × 320 µm × 0.25 µm) and a flame-ionization detector (FID). The following
capillary column (30.0 m × 320 μm × 0.25 μm) and a flame‐ionization detector (FID). The following operating
conditions were used in the analysis: the carrier gas was nitrogen with a flow rate of
operating conditions were used in the analysis: the carrier gas was nitrogen with a flow rate of 1.0 1.0
mL/min,
the injection port temperature was 250 ◦ C, the oven temperature was programmed from
mL/min, the injection port temperature was 250 °C, the oven temperature was programmed from ◦
◦
60
C to 90 C (6 min) at a heating rate of 5 ◦ C/min and then to 190 ◦ C (5 min) at a heating rate
60 °C to 90 °C (6 min) at a heating rate of 5 °C/min and then to 190 °C (5 min) at a heating rate of ◦ C/min, and the detector temperature was 250 ◦ C. The amount of furfuryl alcohol and butyl
of
20 20
°C/min, and the detector temperature was 250 °C. The amount of furfuryl alcohol and butyl levulinate
determined
using
internal
standard
curvescurves constructed
with theirwith authentic
levulinate were
were determined using internal standard constructed their standards.
authentic standards. Furfuryl alcohol conversion was defined as the ratio of the moles of furfuryl alcohol converted to the moles of furfuryl alcohol loaded in the feed. Butyl levulinate yield was defined as the ratio of the moles of butyl levulinate obtained to the moles of furfuryl alcohol loaded in the feed. Catalysts 2016, 6, 143
10 of 12
Furfuryl alcohol conversion was defined as the ratio of the moles of furfuryl alcohol converted to the
moles of furfuryl alcohol loaded in the feed. Butyl levulinate yield was defined as the ratio of the
moles of butyl levulinate obtained to the moles of furfuryl alcohol loaded in the feed.
3.4. pH Measurement of the Reaction Solution
The pH measurements of the reaction solution with various metal salts were performed at ambient
temperature using a model PHS-3C pH meter (Leici Analytical Instrument Factory, Shanghai, China)
calibrated with standard pH buffer solutions.
3.5. Recycling of the Catalyst
To assess the stability of metal salts as catalysts, a selected CuCl2 was recovered after the reaction
and reused for the next run under comparable conditions. The separation of CuCl2 after the reaction
was as follows: the solvent n-butanol and low boiling substances in the reaction mixture were firstly
removed through rotary evaporation at 120 ◦ C until a dense mixture was formed. Subsequently,
high boiling n-tridecane was added to the residual component to help distil the heavy products
(mainly butyl levulinate), which acted as desorption driving agent for heavy products. At this stage,
the heavy products in the residue were separated out by vacuum distillation of 180 ◦ C. This step was
repeated three times. Finally, the obtained solid residue (i.e., recovered CuCl2 ) was directly dissolved
in a certain amount of furfuryl alcohol and n-butanol with a specified proportion. The formed mixture
was then transferred to reactor for the next run.
4. Conclusions
Different types of metal salts had obviously different impacts on the catalytic conversion of
furfuryl alcohol to butyl levulinate. Both alkali and alkaline earth metal chlorides were not effective in
the alcoholysis of furfuryl alcohol, while several transition metal chlorides (CrCl3 , FeCl3 , and CuCl2 )
and AlCl3 showed catalytic activity for the synthesis of butyl levulinate. For their sulfates (Cr(III),
Fe(III), Cu(II), and Al(III)), the catalytic activity was low. The reaction performance was correlated
with the Brønsted acidity of the reaction system derived from the hydrolysis/alcoholysis of cations,
but was more dependent on the Lewis acidity from the metal salts. The formation of butyl levulinate
was favored in the diluted initial furfuryl alcohol concentration at a relatively high catalyst loading
and reaction temperature. An optimized yield of butyl levulinate of 95% was achieved when catalyzed
by the uniquely effective CuCl2 . After the reaction, the CuCl2 could be recovered efficiently and reused
multiple times without substantial loss of its catalytic activity. The developed strategy using CuCl2 as
a facile, efficient, and reusable catalyst exhibits a promising prospect for the large-scale synthesis of
levulinate ester biofuels from biomass-derived furfuryl alcohol.
Acknowledgments: This work was supported by the National Natural Science Foundation of China
(21406099, 21566019).
Author Contributions: L.P. conceived and designed the experiments; R.T. and Y.W. performed the experiments;
L.P., R.T. and Y.W. analyzed the data; L.P. contributed reagents/materials/analysis tools; L.P. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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