Efficient and selective alcoholysis of furfuryl alcohol to alkyl levulinates catalyzed by double SO3H-functionalized ionic liquids

Article abstract of DOI:10.1039/c3gc41693c

The production of alkyl levulinates from furfuryl alcohol (FAL) in alcohol media was investigated at moderate temperature in the presence of Bronsted acidic ionic liquids. The reaction was examined and optimized under batch conditions, where it was found that furfuryl alcohol was rapidly and almost quantitatively converted into intermediate products including 2-alkoxymethylfuran and 4,5,5-trialkoxypentan-2-one, and high alkyl levulinates yield of 95% can be achieved after reaching a steady state in 2 h. An advantage of this catalyst system is that undesired dialkyl ether (DEE) formed by a side reaction of the dehydration of alcohol is negligible. The Hammett method was used to determine the acidities of these ionic liquids, which indicated that the acidity and the molecular structure have strong effects on the catalytic activity of ionic liquids. Based on the experimental results, a possible mechanism for the alcoholysis of FAL is proposed.

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Efficient and selective alcoholysis of furfuryl alcohol to alkyl levulinates
catalyzed by double SO3H-functionalized ionic liquids
Guofeng Wang a, Zhanquan Zhang b, Linhua Song a *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The production of alkyl levulinates from furfuryl alcohol (FAL) in alcohol media was investigated at moderate temperature in the
presence of Brønsted acidic ionic liquids. The reaction was examined and optimized under batch conditions, where it was found that
furfuryl alcohol was rapidly and almost quantitatively converted into intermediate products including 2-alkoxymethylfuran and 4, 5, 5-
10 trialkoxypentan-2-one, and a high alkyl levulinates yield of 95% can be achieved after achieving steady state in 2 h. An advantage of this
catalyst system is that undesired dialkyl ether (DEE) formed by side reaction of the dehydration of alcohol is negligible. The Hammett
method was used to determine the acidities of these ionic liquids, which indicated that the acidity and molecular structure have strong
effects on the catalytic activity of ionic liquids. Based on the experimental results, a possible mechanism for the alcoholysis of FAL is
proposed.
acid plant by converting a minor coproduct, furfural, into its main
product.
15 1. Introduction
50
In industry, alkyl levulinates was mainly obtained by
esterification of levulinic acid with the corresponding alkyl
alcohol in the presence of acid catalysts. However, levulinic acid
is of high cost as raw material for this purpose. So converting
carbohydrates to alkyl levulinates in the presence of alcohol is
With the diminishing fossil fuels and concern from greenhouse
gas caused global warming, the strategy of utilizing renewable
biomass resources via catalytic processes is promising, which not
only provides the chemical product as feedstocks for chemical
20 industry, but also can ease the CO2 burden due to the neutral
characteristic of this strategy 1. Of the numerous approaches,
biomass-derived carbohydrates are attractive as fuel and
chemicals because they are cost-effective, sustainable and
55 addressed as well, because alkyl levulinates have been
successfully used as plasticizing agents, solvents, odorous
6
substances, and fuel additives . In addition, alkyl levulinates are
2
also the feedstock/precursors towards the synthesis of various
abundant . Moreover, these carbohydrates do not compete with
7
chemicals and drugs because of their functional groups
.
25 the food chain, in contrast to the sugar, starch, and vegetable oils
that are used for the first generation of bio-fuels and biochemicals
3. In particular, Levulinic acid derived from these carbohydrates
has been recognized as the promising platform molecule for fuels,
e.g., ethyl levulinate and methyl tetrahydrofuran, as well as
30 chemical intermediates including γ-valerolactone, methylene
60 Numbers methods have been developed for the production of
alkyl levulinates with a wide range of source including renewable
biomass feedstock such as cellulose, saccharides, HMF, and
furfuryl alcohol catalyzed with various acids in alcohols. Up to
date, homogenous or heterogeneous catalysts including sulfonic
65 acid-functionalized ionic liquids, zeolites, mixed-acid,
mesoporous aluminosilicates Al-TUD-1, SO42-/ZrO2 were
reported. Riisager group achieved a direct conversion of mono-
4
valerolactone, pyrrolidones, and pentenoate esters . Efforts have
been made to produce levulinic acid from carbohydrates
including glucose, sucrose, fructose and biomass, such as wood,
starch, grain sorghum and agricultural wastes. Jow et al. studied
35 the production of levulinic acid from fructose over a LZY zeolite
catalyst, and high yield of 56% was obtained after 13 h in a batch
reactor at 140 oC due to the strong Lewis acidity of the
silica/alumina support, as well as the molecular sieving capability
of the Y-type zeolite. Van de Vyver et al. showed that levulinic
40 acid can be produced directly from cellulose with 25% carbon
molar yield at mild temperatures using sulfonated hyperbranched
poly(arylene oxindole)s, this value is very close to the theoretical
value (30%). Recently, the literature discloses the strategies of
converting furfural to levulinic acid via hydrogenation to FAL
and disaccharides into ethyl levulinate (EL) with sulfonic acid-
8
functionalized ionic liquids and zeolite
, Tominaga and
70 coworkers studied mixed-acid systems, consisting of both Lewis
and Brønsted acids, as efficient catalysts for direct synthesis of
9
methyl levulinate from cellulose . Valente and coworkers found
that mesoporous aluminosilicates (Al-TUD-1) are effective for
the production of useful bio-based levulinate esters from 5-
75 hydroxymethyl-2-furfural (HMF) or furfuryl alcohol (FAL) with
aliphatic alcohols 10. Lin et al. reported the ethyl levulinate
production from glucose in ethanol using a series of SO42-/ZrO2
based solid acid catalysts 11. Recently, Chen and coworkers
studied sulfonic acid-functionalized carbon materials (C-SO3H)
80 catalyzed conversion of fructose to alkyl levulinates and the
catalyst can preserve the catalytic efficiency after five cycles 12. It
45 over copper-based catalysts and subsequent acid catalyzed
2, 4a, 5
conversion of FAL to levulinic acid in aqueous solution
.
Such a reaction would allow increasing the yield of a levulinic
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is revealed that the acidic catalysts, especially the strength and
ethoxymethylfuran (EMF) is converted to desired products ethyl
amount of acid, are crucial to the production of alkyl levulinates. 60 levulinate together were proposed based on experimental results.
Therefore, it is inspired reveals that the development of an
efficient acid catalyst and suitable feedstock is the key to this
2. Experimental
5
progress. Based on chemical statistics, FAL is not fully utilized
2.1 Materials
5,
and oversupplied in the chemical market 13. Therefore it is
Furfuryl alcohol (98%), 1, 3-propanesulfonate (99%),
trimethylsilylimidazole (99%), 1, 1, 3, 3-tetramethylguanidine
65 (99%), trifluoroacetic acid (99%) were purchased from Aladdin
Industrial Corporation and 1, 4-butane sultone (99%) from J&K
Chemical Ltd. Sulphuric acid (95-98%, AR), tetrafluoroboric acid
(AR) ethyl ether (99.5%), acetic ether (99.5%) were supplied by
Sinopharm Chemical Reagent Co., Ltd.. All alkyl alcohols were
70 freshly distilled before use.
necessary to find a new approach to convert low value added
FAL to alkyl levulinates. Consequently, several popular catalysts
15
including mineral acid 14, organic-inorganic hybrid solid acid
,
16
10 ion-exchange resins 3a, commercial sulfonic resin
and solid
11b, 17
superacid
were extensively studied in terms of their
catalytic performances. The mineral acids, such as H2SO4, HF,
and H3PO4 are the least inexpensive and homogeneous catalysts
for the synthesis of alkyl levulinates. However, these acids are
15 extremely corrosive and contaminative, and thus the
neutralization is required 18. Organic-inorganic hybrid solid acids
2.2 Synthesis of functionalized ILs
Ionic
liquid
1,3-bis(3-sulfopropyl)-1H-imidazol-3-ium
including
methylimidazolebutylsulfate
phosphotungstate
hydrogensulfate, [(HSO3-p)2im][HSO4] was prepared according
([MIMBS]3PW12O40) demonstrated an efficient catalyst for the
alcoholysis of FAL with a high yield of 93%. However, the
20 catalyst shows a poor recyclability and the catalytic activity could
not be restored because the catalyst can easily adsorb insoluble
substances. Ion-exchange resins, commercial sulfonic resin and
solid superacid have been widely used due to their excellent
20a,
to George methods
22: trimethylsilylimidazole (150 mmol)
75 was added to a solution of 1, 3-propanesultone (150 mmol)
o
dissolved in anhydrous toluene (100 mL) by dropwise at 0 C.
o
The resulting solution was slowly heated to 65 C and stirred for
24 h. Afterwards, the white products were obtained after filtration,
washing with ethyl ether and acetic ether three times, respectively,
80 followed by drying at 65 oC under vacuum (0.1 mmHg) for 24 h.
150 mmol of zwitterionic intermediate (150 mmol) was dissolved
in anhydrous toluene (50 mL), a stoichiometric amount of
sulfuric acid was slowly added and the solution was stirred first at
25 oC for 30 min, then at 70 oC for 24 h. The product was finally
85 obtained via rotary evaporation and drying at 65 oC in vacuum for
24 h.
thermal stability and low cost characteristics for industrialization
11,
25
19. However, this process still suffers from activity loss,
catalyst deactivation and mass transfer issues.
In view of both the advantages and disadvantages of
homogenous and heterogeneous catalysts, and to be aware of the
importance and applications of FAL alcoholysis reaction from
30 industrial as well as academic viewpoints, it is urgent to find an
efficient, inexpensive, stable and environmentally benign catalyst
system for this imperative reaction. Ionic liquids have been
widely recognized as green reaction catalysts or media due to
their negligible volatility and excellent thermal stability. SO3H-
35 functionalized ionic liquids (SFILs) with strong Brønsted acidity
were firstly reported as the competing acid catalyst in comparison
of conventional acidic catalysts, because they are flexible,
recyclable, and can act as solvent and catalysts 20. Therefore, the
use of recoverable SFILs has economical and environmental
40 benefits. Rode and coworkers studied single SO3H-functionalized
Other ionic liquids were prepared under the same protocols as
described above.
2.3 Alcoholysis of furfuryl alcohol
90 Alcoholysis of furfuryl alcohol was operated in a Teflon-lined
autoclave reactor. 20 mmol of furfuryl alcohol (2.0 g, 20 mmol),
0.6mmol of [(HSO3-p)2im][HSO4] (0.246 g, 0.6 mmol), and 0.88
mol of ethanol were added into the reactor, and then the reactor
o
was heated to 110 C in an oil batch and held isothermal for 2 h.
95 After cooling down to room temperature, the IL was extracted
with n-hexanes (10 × 15 mL) from the solution. Products were
identified on a HP 6890/5793 GC-MS with a DB-5MS column
and on a Varian NMR spectrometer (9.4 T, 400 MHz for 1H, 100
MHz for 13C) equipped with a 13C-{1H, 19F} TXO probe head.
100 The detected products were EL, ethoxymethylfuran, 4, 5, 5-
triethoxypentan-2-one (TOP), diethyl ether and a small amount of
furfuran resin.
ionic liquids varied with different anions as efficient catalysts for
21
direct synthesis of methyl levulinate from furfuryl alcohol
.
However, none all these ILs can achieve a simultaneous high
conversion and selectivity from FAL to Me-LA because of
45 divergences in Brønsted acidity. The introduction of multi-alkyl
sulfonic acid groups to cations enhanced the Brønsted acidity,
thus resulting in high activity. Here, four kinds of SFILs based on
trimethylsilylimidazole,
pyridine
and
1,
1,
3,
3-
tetramethylguanidine are applied to catalyze the alcoholysis of
50 FAL with various alcohols. It is proved that this is a simple
method of synthesis of alkyl levulinates with excellent yields
(95%) in the presence of ILs (~3 mol%) for a relatively short
reaction time (2 h). The acidities of SFILs based on various
2.4 Characterization
1-(4-sulfobutyl) pyridinium hydrogensulfate, ([BsPy][HSO4]): 1H
105 NMR (400 MHz, D2O), δ (TMS, ppm): 2.21-2.26 (m, 2H), 2.79 (t,
J = 7.2 Hz, 2H), 4.56 (t, J = 7.2 Hz, 2H), 7.84 (t, 2H), 8.30 (t, 1H),
8.62 (d, J = 7.6 Hz, 2H); 13C NMR (100 MHz, D2O), δ (TMS,
ppm): 27.23, 48.34, 61.55, 129.73, 145.43, 147.56.
-
anions including HSO4 , CF3COO- were determined using the
55 Hammett method with UV-vis spectroscopy and their acidity-
catalytic activity relationships were systematically discussed.
Moreover, multiple reaction pathways where formation of 4, 5, 5-
triethoxypentan-2-one in parallel to the other intermediate
N-(bis(dimethylamino)methylene)-4-sulfobutan-1-aminium
110 trifluoromethanesulfonate, ([BsTmG][CF3COO]): H NMR (400
1
MHz, D2O), δ (TMS, ppm): 1.38-1.50 (m, 4H), 2.63 (t, J = 13.6
Hz, 2H) 2.66 (s, 12H), 2.94 (t, J = 7.2 Hz, 2H); 13C NMR (100
2
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Table 1. Reactive correction factors of the products of FAL conversion
MHz, D2O), δ (TMS, ppm): 21.62, 28.73, 40.51, 45.31, 51.33,
120.58, 163.24, 165.46.
N-(bis(dimethylamino)methylene)-4-sulfobutan-1-aminium
hydrogensulfate, ([BsTmG][HSO4]): H NMR (400 MHz, D2O),
δ (TMS, ppm): 2.77-2.75 (s, 3H), 1.63-1.51 (m, 2H), 3.05 (t, J =
7.2 Hz, 2H), 2.73 (t, J = 7.2 Hz, 2H); 13C NMR (D2O, 400 MHz),
δ (TMS, ppm): 21.72, 28.34, 39.53, 44.36, 50.62, 161.83.
Entry
Name
Reactive correction factor
1
1
2
3
4
5
FAL
1.416
1.303
1.591
1.294
1.312
DEE
5
EL
EMF
1-methyl-3-(4-sulfobutyl)-1H-imidazol-3-ium hydrogensulfate,
4,5,5-trithoxypentan-2-one
1
([BsMIm][HSO4]): H NMR (400 MHz, D2O), δ (TMS, ppm):
10 1.37-1.45 (m, 2H), 1.65-1.73 (m, 2H), 2.61 (t, J = 7.2 Hz, 2H),
3.56 (s, 3H), 3.92 (t, J = 7.3 Hz, 2H), 7.11 (s, 1H), 7.16 (s, 1H),
8.39 (s, 1H); 13C NMR (100 MHz, D2O), δ (TMS, ppm): 21.11,
28.32, 35.39, 49.51, 50.63, 122.23, 123.84, 136.13.
2.6 Hammett acidity of Brønsted acidic ILs
The acidities of Brønsted-acidic ILs were measured at room
temperature on
a
Perkin-Elmer Lambda 650S UV-Vis
60 spectrometer equipped with Labsphere intergrating over the
spectral range 300-650 nm using p-nitroaniline as the indicator
and molecular probe 23. The ILs and p-nitroaniline were dissolved
in methanol to achieve the concentration of 3.0 × 10-2 mol·L-1 and
7.5 × 10-5 mol·L-1, respectively. Then, the solutions were mixed
65 for 2 h with magnetic stirring, and then the UV spectrum of
mixed solution was recorded. The Hammett acidity function (H0)
can be expressed on the basis of the following equation:
1,3-bis(3-sulfopropyl)-1H-imidazol-3-ium
hydrogensulfate,
15 [(HSO3-p)2im][HSO4]: 1H NMR (400 MHz, D2O), δ (TMS, ppm):
2.05-2.08 (t, J = 13.2 Hz, 4H), 2.65-2.69 (t, J = 13.6 Hz, 4H),
4.09-4.13 (t, J = 13.6 Hz, 4H), 7.31 (s, 2H), 8.59 (s, 1H); 13C
NMR (100 MHz, D2O), δ (TMS, ppm): 24.45, 46.48, 47.29,
121.99, 135.01.
20 3-butyl-1-methyl-1H-imidazol-3-ium
hydrogensulfate,
([BMIm][HSO4]): 1H NMR (400 MHz, D2O), δ (TMS, ppm):
0.69 (t, J = 7.4 Hz, 3H), 0.98 (m, 2H), 1.61 (m, 2H), 3.65 (s, 3H),
3.96 (t, J = 7.2 Hz, 2H), 7.19 (s, 1H), 7.24 (s, 1H), 8.47 (s, 1H);
13C NMR (100 MHz, D2O), δ (TMS, ppm): 12.82, 18.93, 31.44,
25 35.82, 49.56, 122.47, 123.78, 136.08.
H0=pKa(In)+log([In]/[InH+])
70
Where pKa(In) is the pKa value of the p-nitroaniline indicator
solution (about 0.99), [In] and [InH+] are the molar concentration
of the protonated and unprotonated form of p-nitroaniline indictor,
respectively.
1
4, 5, 5-triethoxypentan-2-one: H NMR (400 MHz, CDCl3), δ
(TMS, ppm): 1.15 (t, J = 6.8 Hz, 3H), 1.20 (t, J = 9.8 Hz, 3H),
1.23 (t, J = 13.6 Hz, 3H), 2.09 (s, 3H), 2.68 (d, J = 5.4 Hz, 2H),
3.59 (m, 3H), 3.72 (m, 3H), 3.84 (m, 1H), 4.41 (d, J = 7.8 Hz,
30 1H); 13C NMR (100 MHz, CDCl3), δ (TMS, ppm): 15.04, 15.05,
15.33, 31.01, 43.96, 64.08, 64.09, 66.61, 76.16, 103.35, 208.28.
75 2.7 Recycling and reuse of the ionic liquid
After the reaction, the water was firstly removed by molecular
sieve via adsorption dehydration (3 Å). The resulting low boiling
1
2-ethoxymethylfuran: H NMR (400 MHz, CDCl3), δ (TMS,
o
point components including diethyl ether (34.6 C) and ethanol
ppm): 1.23 (t, J = 4.1 Hz, 3H), 3.54 (q, J = 7.2 Hz, 2H), 4.44 (s,
2H), 7.40 (m, 1H); 13C NMR (100 MHz, CDCl3), δ (TMS, ppm):
35 142.62, 110.53, 108.94, 152.21, 70.23, 64.84, 14.55.
(78.4 oC) were separated depending on their different boiling
80 points. Then IL was extracted from the remaining EL,
ethoxymethylfuran (EMF), 4, 5, 5-triethoxypentan-2-one (TOP)
with 5 mL of n-hexane 10 times. Finally, the catalyst was then
2.5 Analytic methods
o
heated at 65 C in vacuum until the water in this system was
The products were analyzed on gas chromatography instrument
(Agilent, 7890A) equipped with a HP-5 column (30 m × 0.320
mm × 0.25 µm) and a flame ionization detector. In terms of the
40 operation conditions, N2 as the carrier gas was 1.0 mL·min-1, the
removed absolutely. The dried catalyst was re-used directly for
85 the next run by adding fresh feedstock under the same reaction
conditions.
o
injection port temperature was 250 C, the oven temperature was
3. Results and discussion
o
190 C, and the detector temperature was 300 oC. Combining the
3.1 The Hammett acidity function-activity relationship
analyses of GC-MS and NMR, FAL, DEE, EL, EMF and 4, 5, 5-
trithoxypentan-2-one were identified. In order to calculate FAL
45 conversion and product yield accurately, ethyl benzoate as
internal standard was applied and the reactive correction factors
of the products were summarized in Table 1. FAL conversion and
products selectivity were calculated using the following equations:
Characterization including Hammett method, and NH3-TPD
90 were employed to analyze the amount and strength of the acidity
and the acidity is very important to FAL alcoholysis 6b. Haan et al.
3a
found that EL was preferentially produced at the acid sites at
the surface of the pores. Valenta 10 tested porous aluminosilicates
as acid catalysts for the reaction of FAL with ethanol, higher EL
95 yield was obtained on the catalysts with the higher total amount
of acid sites (L + B) identified with pyridine-adsorption. Taking
the results (Table 2) and literature together, the functionalized
ionic acids influences the catalytic yield of EL to different extent
because of the acidic properties of functionalized ionic acids
100 stemming from divergences in structures of cations (e.g., [(HSO3-
p)2im], [BsMIm] and [BsTmG]) and varieties of anions([HSO4],
and [CF3COO]). To shed light on the impact of acidity on
50
Only selectivity to EMF, 4, 5, 5-triethoxypentan-2-one and DEE
was discussed in this study in view of their relative large amounts
55 detected in the products.
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catalytic performance (EL yield), Brønsted acidity was measured
with UV visible spectroscopy and summarized in Figure 1 and
Table 2. High intensity of absorbance of p-nitroaniline indicates
the synthesized ionic acid has poor acidity. The ILs bearing none
alkyl sulfonic acid group ([BMIm][HSO4]) on the cation
exhibited lower activity than these corresponding single SO3H-
functionalized ionic acids ([BsMIm][HSO4], [BsTmG][HSO4]
and [BsPy][HSO4]). While the ILs bearing two alkyl sulfonic acid
groups have higher activity than single SO3H-functionalized ionic
10 acids, this indicates the significant effect of the number of alkyl
sulfonic acid groups on the catalytic performance of the ILs. The
catalytic performance of the ILs with the anions of [HSO4] was
stronger than that of the corresponding ILs with [CF3COO]. This
is possibly due to the weak acidity of trifluoroacetic acid
15 compared with H2SO4. Therefore, double SO3H-functionalized
ILs exhibited a stronger catalytic performance than other tested
ILs. As shown in Table 2, the acidity of ionic acids decreases in
1.8
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
5
360
380
400
Wavelength/nm
no sample
[BsTmG][HSO4]
[BsPy][HSO4]
[BMIm][HSO4]
[BsMIm][HSO4]
[(HSO3-p)2im][HSO4]
[BsTmG][CF3COO]
300
350
400
450
500
550
600
650
Wavelength/nm
Figure 1. The UV/Vis spectra according to the Hammett method (the
amplified peaks are presented at the top right corner).
Table 3 Impact of substrate on catalytic yield of alkyl levulinates
the order: [(HSO3-p)2im][HSO4]
[BsTmG][HSO4] [BsPy][HSO4]
>
>
[BsMIm][HSO4]
[BMIm][HSO4]
≈
>
Boiling point [oC]
≈
Time [h]
No.
1
Substrate
ethanol
Yield [%]
20 [BsTmG][CF3COO], and the yield of EL also follows the same
trend. It is concluded that the sequence of the catalytic
performance observed in the alcoholysis of FAL was in good
agreement with the Brønsted acidity order determined by the
Hammett method, and [(HSO3-p)2im][HSO4] was determined to
25 be the most suitable catalyst for the alcoholysis of FAL to EL in
78.4
78.4
0.5
2
60
92
59
95
57
90
55
93
70
92
2
ethanol
3
n-propanol
n-propanol
isopropanol
isopropanol
n-butanol
n-butanol
isobutanol
isobutanol
97.1
0.5
2
4
97.1
5
82.5
0.5
2
ethanol 24
.
6
82.5
3.2 Influence of substrate on catalytic yield of alkyl levulinate
7
117.3
117.3
107.9
107.9
0.5
2
The alcoholysis of furfuryl alcohol with different alkyl
alcohols in order to explore the scope of this method was carried
30 out using catalytic amount of ionic liquids [(HSO3-p)2im][HSO4].
When the reactions were carried out for 2 h, yields were achieved
for the investigated solvents (80%-95%), this difference is
probably attributed to the boiling point and steric effects of the
substrates 17. To attenuate the differences of activities, shorter
35 reaction time (0.5 h) was employed (Table 3, entries 1, 3, 7 and
15). In the case of linear alcohol, with the increasing of carbon
length, the alkyl levulinates yield was decreased gradually (Table
3, entries 2, 4, 8 and 16). This difference is associated with the
difficulty of the conversion from 2-alkoxymethylfuran and 4, 5,
40 5-trialkoxypentan-2-one to alkyl levulinates and it is confirmed
with the fast consumption of FAL in the first several minutes.
8
9
0.5
2
10
2-butyl
alcohol
2-butyl
alcohol
11
12
108
108
0.5
2
67
89
13
14
15
16
tert-butanol
tert-butanol
1-pentanol
1-pentanol
83
83
0.5
2
53
87
56
80
137.3
137.3
0.5
2
45 Reaction conditions: Furfuryl alcohol (20 mmol) and catalyst [(HSO3-
p)2im][HSO4] (0.6 mmol) in different alcohol (40 mL), reaction time = 2
h, temperature = 110 oC.
Table 2 Hammett acidity of synthesized ionic acid catalysts and yield of ethyl levulinate
[InH+] [%]
No.
1
Catalysts
none
Absorbance
1.44
[In] [%]
100
Hammett acidity
Yield [%]
0
-
0
2
[BsTmG][CF3COO]
[BMIm][HSO4]
1.43
1.40
0.79
0.87
0.90
0.92
99.3
97.2
54.9
60.4
62.5
63.9
0.7
2.8
3.1
2.5
1.0
1.2
1.2
1.2
24
34
95
92
93
93
3
4
45.1
39.6
37.5
36.1
[(HSO3-p)2im][HSO4]
[BsMIm][HSO4]
5
6
[BsTmG][HSO4]
[BsPy][HSO4]
7
Reaction conditions: Furfuryl alcohol (20 mmol) and catalyst [(HSO3-p)2im][HSO4] (0.6 mmol) in alcohol (40 mL), reaction time = 2 h, temperature =
50 110 oC.
4
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DOI: 10.1039/C3GC41693C
Page 5 of 9
Green Chemistry
The effect of boiling point becomes controlling factor, thus
solvents with longer carbon length results in lower activity.
Increasing the amount of catalyst further, i.e. 0.6 mmol, the
yields of EMF and 4, 5, 5-triethoxypentan-2-one begin to
decrease. Meanwhile, DEE yield increases. It was worthy of
noting that EMF and 4, 5, 5-triethoxypentan-2-one were first
25 formed, and then decreased with reaction time for all the
experiments in Figure 2, and the yield of EL increased rapidly
from zero to stable concentration. This demonstrates that
alcoholysis of FAL to EL proceeded mainly through EMF and 4,
The steric effect becomes obvious in the case of C4 alcohol (n-
butanol, isobutanol, 2-butyl alcohol, tert-butanol). With the
increasing of molecular volume, the conversion of intermediates
required in the production of corresponding alkyl levulinates
becomes restricted and hindered.
5
5, 5-triethoxypentan-2-one as suggested by Dumesic 16
.
3.3 The effect of catalyst amount on the yield of product
distribution
30 3.4 Influence of reaction temperature on conversion and
product distribution
10
The amount of catalyst definitely impacts the acidic
concentration in reaction solution, correspondingly influences the
product distribution. Additionally, the reaction time is a crucial
factor influencing product species. As shown in Figure 2, the
concentration of [(HSO3-p)2im][HSO4] ranges from 0.1 to 0.6
15 mmol, while the other conditions keep constants. A maximum EL
yield of 65.5% to 94% within 3 h with ILs ranging from 0.1
mmol and 0.6 mmol is achieved respectively (Figure 2d),
attributed to an increase of the number of active sites. The yields
of EMF, 4, 5, 5-triethoxypentan-2-one could respectively reach to
20 maxima of 36.4% and 34.7% with 0.1 mmol catalyst in 7 minutes.
Temperature appears to have a profound effect on both the
reaction rate and the product distribution. Therefore, a systematic
study was conducted to understand the effect of reaction
35 temperature on FAL alcoholysis and abundance of the main
o
o
products at 70 oC, 90 C, 110 oC and 130 C respectively. Figure
3 shows the temporal evaluation of the products. As expected, the
higher temperature there is, the higher the conversion of EL is.
FAL not only form intermediates including EMF and 4, 5, 5-
40 triethoxypentan-2-one but also is converted to EL simultaneously.
Moreover, the consumption rate of FAL at the initial time and EL
100
(a)
40
0.1 mmol
0.2 mmol
0.4 mmol
0.6 mmol
(b)
80
60
32
24
16
8
0.1 mmol
0.2 mmol
0.4 mmol
0.6 mmol
40
20
0
0
0
25
50
75
100
125
150
175
(c)
200
0
40
80
120
Time (min)
160
200
Time (min)
48
40
32
24
16
8
2.0
1.6
1.2
0.8
0.4
0.0
(d)
0.1 mmol
0.2 mmol
0.4 mmol
0.6 mmol
0.1 mmol
0.2 mmol
0.4 mmol
0.6 mmol
0
0
40
80
120
160
200
0
40
80
120
Time (min)
160
200
Time (min)
Figure 2. The influences of catalyst amounts in the reaction on time-dependent product distribution in the presence of [(HSO3-p)2im][HSO4]: (a) yield of
45 EL, (b) yield of EMF, (c) yield of 4, 5, 5-triethoxypentan-2-one, (d) yield of DEE. Reaction conditions: Furfuryl alcohol (20 mmol) and catalyst [(HSO3-
p)2im][HSO4] (set amount) in alcohol (40 mL), reaction temperature = 110 oC
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C3GC41P69a3gCe 6 of 9
yield increase with the reaction temperature increasing. The ratio
95%, and equilibriums reached. Interestingly, despite the increase
of EL formation was rather slow at 70 oC (Figure 3b), Even with 15 of reaction rate was observed as expected, the yield of EL was
a prolonged reaction time, a low yield of 32% was achieved with
6 h. It is speculated that the alcoholysis of furfuryl alcohol with
ethanol may experience the formation of intermediates firstly,
which then was converted into the final product, and the step
from intermediates to EL is the rate-determining, this is
confirmed by the relatively amount of EMF and 4, 5, 5-
triethoxypentan-2-one (38% and 33%, respectively).
constant (95%). To sum up, the distribution of products can be
kinetically adjusted by controlling the temperature.
5
3.5 Recycling and reuse of [(HSO3-p)2im][HSO4]
Recycle experiments were conducted to investigate the
20 catalyst’s stability and lifetime of the ionic liquids, the cyclic
tests were conducted for six times. As show in Figure 4, the
catalytic activity of [(HSO3-p)2im][HSO4] for the alcoholysis of
FAL did not decrease after the six run, demonstrating that the
catalyst was stable in this reaction. The EL yield in the recycling
25 test was even higher than in the 1st run, which might be due to the
o
o
o
10
Increasing the temperature to 90 C, 110 C and 130 C, the
reaction rate of FAL alcoholysis and formation rate of EL were
enhanced significantly. As depicted in Figure 3b, FAL was
almost completely converted into the EL after 3 h with a yield of
retention of EL and unreacted feedstock in the previous run 24
.
100
100
(b)
(a)
70 oC
80
80
90 oC
70 oC
110 oC
130 oC
90 oC
60
60
110 oC
130 oC
40
20
0
40
20
0
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
Time (min)
Time (min)
45
40
35
30
25
20
15
10
5
48
40
32
24
16
8
(d)
(c)
70 oC
90 oC
110 oC
130 oC
70 oC
90 oC
110 oC
130 oC
0
0
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
Time (min)
Time (min)
30 Figure 3. Influences of reaction temperatures on time-dependent product distribution in the presence of [(HSO3-p)2im][HSO4], (a) FAL conversion, (b)
yield of EL, (c) yield of EMF, (d) yield of 4, 5, 5-triethoxypentan-2-one. Reaction conditions: Furfuryl alcohol (20 mmol) and catalyst (0.6 mmol) in
alcohol (40 mL).
6
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C3GC41693C
Page 7 of 9
Green Chemistry
hydroxyl group to form intermediates including EMF and 4, 5, 5-
triethoxypentan-2-one and this takes place in parallel to
production of EL. In our conditions, GC-MS analyses allowed
the identification of various products continuously, including
30 diethyl ether (DEE), ethyl levulinate and the excessive alcohol.
The DEE was the product of the inter-molecular dehydration
reaction with ethanol. EMF and 4, 5, 5-triethoxypentan-2-one
were detected and quantified using NMR. These both products
indicate that the reaction is likely to proceed via the latter route,
35 although our experimental results suggest the possible reaction
mechanism is not in accord with above-mentioned. The results in
Table 4 show that the amount of DEE is not equal to EMF or 4, 5,
5-triethoxypentan-2-one, this indicates that the conversion of
FAL to EL are finished mainly through EMF and 4, 5, 5-
40 triethoxypentan-2-one and the main pathway does not exist for
the formation DEE. A reaction pathway for acid-catalyzed
conversion of FAL into ethyl levulinate in ethanol is proposed, as
illustrated in Scheme 1. In the first step, Protonation FAL with
acidic group yields A1 and B1, which are attacked by EtOH to
45 form EMF and B2, respectively. The next intermediate A2
formed from the partially detached protonated ethoxy group was
then isomerized to form part of the product. Meanwhile,
deprotonation of B2 results in the formation of the other
intermediate 4, 5, 5-triethoxypentan-2-one which leads to species
50 B4 by acid-catalyzed removal of ethanol from B3 or DEE
directly. Subsequently, B4 is converted in a serial mode to ethyl
levulinate due to tautomerization.
100
80
60
40
20
0
5
10
1
2
3
4
5
6
7
8
Cycles number
Figure 4. Cyclic performance of [(HSO3-p)2im][HSO4] in the alcoholysis
of FAL with ethanol. Reaction conditions: Furfuryl alcohol (20 mmol)
15 and catalyst (0.6 mmol) in alcohol (40 mL).
3.6 Proposed mechanism for FAL alcoholysis
According to the literature, two possible reaction mechanisms for
the alcoholysis of FAL to produce EL are proposed: (1) EMF
route and (2) EMF and 4, 5, 5-triethoxypentan-2-one multi-path.
20 The EMF route proposed by Zhao and co-workers 15 involves the
first step of alcoholysis of FAL to form an intermediate of EMF
and the second step of acidification of the intermediate to EL.
The latter route proposed by James A. Dumesic et al.
indicates there are three different pathways for the production of
25 EL. The initial step of the entire pathway is the protonation of the
4b, 14
Table 4 The influences of catalyst amounts in the reaction on the DEE to EMF and
DEE to 4, 5, 5-triethoxypetan-2-one ratio in the presence of [(HSO3-p)2im][HSO4]
4,5,5-triethoxypentan-2-
DEE/4, 5, 5-
triethoxypentan-2-one
Catalyst amount/mmol
DEE yield/%
EMF yield/%
DEE/EMF
one yield/%
34.7
0.1
0.2
0.4
0.6
0.2
0.5
1.4
1.7
37.6
36.7
37.7
45.1
0.009
0.023
0.063
0.064
0.017
0.045
0.155
0.212
32.5
26.5
23.6
55
O
H2
C
O
Et
C
HC
CH3
DEE
tautomerization
OH
EtO
OH
B4
HO
A2
O
O
H2
C
H3C
H+
EtO
EtO
C
EtOH
O
CH3
CH
CH3
H2O
O
OEt
H2O
O
C
EtOH
deprotonation
CH3
H2
C
H2
C
O
EtO
EtO
O
CH3
HC
CH
C
CH
CH
OH
H2
C
EMF
O
B3
OH
C
OH
HC
CH
C
H2
EtO
EtO
C
CH
EtOH
CH
CH3
H2O
OEt
B2
H2
C
O
H+
H+
H2
C
HC
HC
C
OH2
O
OH
CH
HC
HC
CH
CH
A1
EtOH
B1
Scheme 1. proposed reaction pathway for the acid-catalyzed conversion of furfuryl alcohol to ethyl levulinate in alcohol.
This journal is © The Royal Society of Chemistry [year]
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Green Chemistry
DOI: 10.1039/C3GC41P69a3gCe 8 of 9
65 13 a) J. Kijeński, P. Winiarek, T. Paryjczak, A. Lewicki and A.
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Efficient conversion of furfuryl alcohol to alkyl levulinates
was experimentally studied using double SO3H-functionalized
ILs as catalysts under mild conditions, where the highest yield of
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70
75
80
o
5
95% was obtained when it was employed at 110 C after 3 h.
Double SO3H-functionlized ILs are efficient catalysts for FAL
conversion into EL, which facilitates the separation of ethyl
levulinate and reuse of ionic liquids. Higher acidity of ILs
measured by Hammett method promotes the conversion from
10 FAL to EL. This approach offers significant improvements for
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View Article Online
DOI: 10.1039/C3GC41693C
Efficient and selective alcoholysis of furfuryl alcohol to alkyl
levulinates catalyzed by double SO3H-functionalized ionic
liquids
Guofeng Wang, Zhanquan Zhang, Linhua Song *
An efficient synthesis of alkyl levulinates from furfuryl alcohol has been developed using double
SO3H-functionalized ionic liquids.