Single pot conversion of furfuryl alcohol to levulinic esters and γ-valerolactone in the presence of sulfonic acid functionalized ILs and metal catalysts

Article abstract of DOI:10.1039/c3gc41098f

Ionic liquids functionalized with acidic anions, HSO₄, ClSO ₃H, PTSA, TFA (MIm), HSO₄ and TFA (NMP) were found to efficiently (99% conversion) catalyze the alcoholysis of furfuryl alcohol (FAL) in the presence of methanol, ethanol, n-butanol and isopropyl alcohol (IPA) to the corresponding levulinic acid esters under mild temperature (90-130 °C) conditions. The extended alkyl chain length of [MIm] using 1,4-butane sultone enhanced the Bronsted acidity of [BMIm-SH][HSO₄] catalyst resulting into the highest selectivity of >95% to Me-LA. An increase in both temperature and catalyst concentration increased the furfuryl alcohol conversion and selectivity to levulinate esters. In contrast, an increase in the substrate concentration from 5 to 15% caused a decrease in Me-LA selectivity due to accumulation of intermediate ethers of furfuryl alcohol. Using a combination of [BMIm-SH][HSO₄] and 5% Ru/C catalyst, direct conversion of FAL to γ-valerolactone (GVL) is shown for the first time. A complete conversion of FAL with the highest selectivity of 68% to GVL could be achieved under optimum conditions while higher Ru loading enhanced the GVL selectivity to 94% in the hydrogenation step of this tandem approach. Our catalyst system could be efficiently recycled five times retaining the original activity and selectivity levels.

Full text of DOI:10.1039/c3gc41098f

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Single pot conversion of furfuryl alcohol to levulinic
esters and γ-valerolactone in the presence of sulfonic
acid functionalized ILs and metal catalysts†
Cite this: DOI: 10.1039/c3gc41098f
Amol M. Hengne, Sumit B. Kamble and Chandrashekhar V. Rode*
Ionic liquids functionalized with acidic anions, HSO4, ClSO3H, PTSA, TFA (MIm), HSO4 and TFA (NMP)
were found to efficiently (99% conversion) catalyze the alcoholysis of furfuryl alcohol (FAL) in the pres-
ence of methanol, ethanol, n-butanol and isopropyl alcohol (IPA) to the corresponding levulinic acid
esters under mild temperature (90–130 °C) conditions. The extended alkyl chain length of [MIm] using
1,4-butane sultone enhanced the Brønsted acidity of [BMIm-SH][HSO4] catalyst resulting into the highest
selectivity of >95% to Me-LA. An increase in both temperature and catalyst concentration increased the
furfuryl alcohol conversion and selectivity to levulinate esters. In contrast, an increase in the substrate
concentration from 5 to 15% caused a decrease in Me-LA selectivity due to accumulation of intermediate
ethers of furfuryl alcohol. Using a combination of [BMIm-SH][HSO4] and 5% Ru/C catalyst, direct conver-
sion of FAL to γ-valerolactone (GVL) is shown for the first time. A complete conversion of FAL with the
highest selectivity of 68% to GVL could be achieved under optimum conditions while higher Ru loading
enhanced the GVL selectivity to 94% in the hydrogenation step of this tandem approach. Our catalyst
system could be efficiently recycled five times retaining the original activity and selectivity levels.
Received 8th June 2013,
Accepted 11th July 2013
DOI: 10.1039/c3gc41098f
www.rsc.org/greenchem
furfural and levulinic acid into the potential fuel additives.5,6
Furfural produced by the hydrolysis and dehydration of xylan
Introduction
The exploitation of bio-based fuels and fuel additives is contained in lignocellulose can be easily hydrogenated to fur-
imperative due to its role in effective remediation of green- furyl alcohol.
house gas levels in the atmosphere and to mitigate the econ-
Hence, direct conversion of furfuryl alcohol to levulinic
omic consequences of the depleting fossil resources.1,2 First- esters followed by its in situ hydrogenation to γ-valerolactone
generation biofuels (i.e. ethanol and biodiesel) are presently (GVL) becomes a new alternative route for the production of
produced from sugars, starch and vegetable oils, creating biofuel.7 Levulinic esters (LA) are mainly obtained by a series
important compatibility and energy density issues. One promi- of reactions involving, first, acid catalyzed esterification of
sing alternative to overcome these limitations of oxygenated levulinic acid produced from hexose sugars. LA is then hydro-
fuels is to use more abundant, cheaper and potentially more genated to give 4-hydroxy LA followed by its ring cyclization to
sustainable non-edible lignocellulosic biomass.3,4 The present GVL.8–10 GVL, along with levulinic esters which also can be
paper reports the most promising route to achieve this trans- used as diesel additives, form sustainable liquid transpor-
formation by developing a selective and versatile catalyst tation fuels replacing ethanol in a gasoline ethanol blend.11,12
system involving acid and metal functions together for alcoho- In addition, another product of single step catalytic hydrogen-
lysis and hydrogenation reactions, respectively, in a single ation of levulinic acid is methyl tetrahydrofuran (MTHF) which
step. These two are the primary unit processes for the conver- has been also approved by the US-DOE as a component of
sion of the monomeric carbohydrate building blocks, i.e., P Series type fuels.13,14 The formation of levulinic acid and its
mainly C5 sugars, such as xylose and arabinose, and C6 sugars esters from cellulose and hemicelluloses is possible by two
in the form of glucose and their respective secondary products, different routes: (i) hexose triple dehydration to 5-hydroxy-
methylfurfural (5-HMF) and its rehydration with two water
molecules to produce levulinic and formic acids, and (ii)
Chemical Engineering and Process Development Division, CSIR-National Chemical
hydrolysis of pentose sugars to produce a mixture of C5 mono-
Laboratory, Pune 411008, India. E-mail: cv.rode@ncl.res.in; Fax: +91 20 2590 2621;
mers, mainly furfural which is then reduced to furfuryl alcohol
Tel: +91 20 2590 2349
followed by its rehydration to produce levulinic acid/ester.15–18
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
A commercial Biofine process for the production of levulinic
c3gc41098f
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acid requires 3.5 tons of 3.5 wt% sulfuric acid, thus creating
serious environmental pollution and involving tedious work
up for the recovery of pure LA.19,20 Attempts are being made to
overcome such drawbacks by developing heterogeneous cata-
lysts. For example, Lange et al. have reported ion-exchange
resins and zeolites as solid acid catalysts for the conversion of
furfuryl alcohol to ethyl levulinate, but with a very low selecti-
vity to ethyl levulinate due to the formation of compounds
with high boiling points as byproducts.21 Organo-inorganic
hybrid solid acids were proposed by Zhang et al. for the pro-
duction of butyl levulinate with 93% yield but without
showing the catalyst reusability.22 Some of the other novel
approaches include the use of biphasic systems with new sol-
vents such as 2-sec-butylphenol (SBP) and 4-propyl guaiacol
(PG) for the recycling of mineral acid in the production of fur-
fural and levulinic acid from the hemicellulose portion of
lignocellulose giving a yield of 60–70%.23 The direct conver-
sion of the cellulose unit of glucose to methyl levulinate was
reported by Tominaga et al. using homogeneous mixed Lewis
and Brønsted acid systems resulting in a poor selectivity of
20–50% to methyl levulinate.24 The imidazolium and pyridi-
num based ILs functionalized with (SO3H, PTSA and ClSO3H)
acidic groups studied by Saravanamurugan et al. in the pres-
ence of naphthalene as an added solvent showed 70% selecti-
vity to methyl levulinate with hydroxy methyl furfural ether as
a byproduct.25 A tandem approach combining acid hydrolysis
and hydrogenation reactions together over heterogeneous (Ru/C)
and acid catalysts (mineral acid) has also been proposed for
the catalytic conversion of cellulose into platform chemicals.26
In the present paper, we report a single pot catalytic conver-
sion of furfuryl alcohol to levulinic acid ester and GVL via the
alcoholysis/hydrogenation sequence using sulfonic acid-func-
tionalized ionic liquids (SO3H-ILs) and carbon supported Ru,
Re, Ir and Ag catalysts. Our novel approach for direct conver-
sion of furfuryl alcohol to GVL has several advantages: (i) the
yield of methyl levulinate is higher than that for hydrolysis of
C6 sugars due to humin elimination in alcohol medium, (ii)
no separation step is required for downstream conversion of
levulinic acid/esters, (iii) complete atom efficiency in the con-
version of FAL to GVL and (iv) elimination of metal leaching
and deactivation of the catalysts used for LA hydrogenation to
GVL. To the best of our knowledge, this approach for direct
conversion of C5 furfuryl alcohol to C5 GVL has not been
reported so far. We found that 5% Ru/C + [BMIm-SH][HSO4]
showed complete alcoholysis of furfuryl alcohol with 95%
selectivity to methyl levulinate followed by its hydrogenation
with the highest selectivity of 94% to GVL in a batch reactor.
Table 1 Catalyst screening for alcoholysis of furfuryl alcohol to methyl
levulinatea
% Selectivity
%
Methyl Methoxy
Entry Catalyst
Conversion LA
FAL
Other
1
2
3
4
5
6
7
8
No catalyst
[H2SO4]
Amberlyst-15
SO4-ZrO2
20% DTP-SiO2
20% DTP-MMT
[MIm][HSO4]
[MIm][TFA]
[NMP][HSO4]
[NMP][TFA]
[BMIm-SH][ClSO3H] 98
[BMIm-SH][PTSA]
[BMIm-SH][HSO4]
2
68
99
99
69
73
96
10
99
11
<0.01
81
16
51
19
61
63
21
95
1
94
4
11
2
19
10
9
74
40
74
25
24
65
1
98
2
90
76
95
7
14
13
14
4
1
4
6
13
3
9
10
11
12
13
95
99
a Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH
(95 mL); temperature, 130 °C; N2 atm., catalyst, 0.3 g; reaction time,
2 h.
several acid functionalized ILs and their performance evalu-
ation results are shown in Table 1. The initial control exper-
iment without any catalyst (entry 1, Table 1) showed that
furfuryl alcohol conversion was <2% confirming that the alco-
holysis of furfuryl alcohol was a catalytic reaction. Among the
various solid acid catalysts, almost complete conversion was
obtained with Amberlyst-15 and SO4-ZrO2 but the selectivity to
MeLA was only 74% with the remaining FAL ethers (entries 3
and 4, Table 1). With the other two solid acid catalysts, the
conversion achieved was less than 75% with a very poor selecti-
vity of 25% to Me-LA. Thus, a simultaneous high activity and
selectivity for furfuryl alcoholysis to Me-LA was not possible
using any of the solid acid catalysts screened in this work
(entries 5 and 6, Table 1). This fact is also evident from the
reported literature.10,22,26 Therefore, we explored the use of ILs
having Brønsted acidity which was systematically varied with
anions such as ClSO3H, SO3H, PTSA, and TFA with the same
cation MIm. It was observed that a furfuryl alcohol conversion
of >95% was achieved with cations such as HSO4 and Cl-SO3H
(entries 7 and 11, Table 1) while only 10% conversion was
obtained with the TFA anion (entry 8, Table 1). However, with
only the HSO4 anion, the selectivity to methyl levulinate was
the highest, 98% (entry 9, Table 1). As can be seen from
Table 1, the choice of the anion was critical, because with TFA
both the cations MIm and NMP exhibited very poor activities
in terms of furfuryl alcohol conversion in the range of only
10–11%. HSO4 with both the cations, MIm and NMP, showed
very high conversion (>95%) while the selectivity to methyl
levulinate was only 65% in the case of [MIm][HSO4]. Hence
this catalyst was further modified by extending its alkyl chain
with 1,4-butane sultone so that the resulting IL (BMIm-SH)
would be more stable and also acquire the enhanced Brønsted
acidity. To our expectation, this was actually observed, as the
[BMIm-SH][HSO4] catalyst gave a dramatic increase in selecti-
vity of methyl levulinate up to 95% [entry 13, Table 1]. The
Results and discussion
Two major objectives of the present work were (i) to design a
novel, environmentally benign acid catalyst for the efficient
alcoholysis of furfuryl alcohol to levulinic esters and (ii) to
develop a tandem approach for synthesizing GVL directly from
furfuryl alcohol. For the first stage of this work, we prepared
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Scheme 1 Alcoholysis of furfuryl alcohol to methyl-levulinate.
appropriate combination of the anion and the cation is
emphasized by the fact that incomplete alcoholysis gave
appreciable formation of intermediate FAL ethers (Scheme 1)
in the case of [MIm][HSO4] and [BMIm-SH][PTSA] catalysts
[entries 7 and 12, Table 1]. Using HSO4 as the anion, NMP
showed almost complete conversion of furfuryl alcohol and
selectivity to methyl levulinate [entry 9, Table 1]. However,
when this catalyst was recovered and was attempted to be
recycled, its activity dropped by 50% [ESI, Fig. 1†] as against
the successful complete recyclability of [BMIm-SH][HSO4] cata-
lyst several times (discussed later). Hence, a further study on
the effect of parameters was carried out using [BMIm-SH]-
[HSO4] catalyst. Along with the catalyst, equally important was
the choice of reaction medium as the selectivity to levulinic
acid dropped to 31% in an aqueous medium over the same
active catalyst [BMIm-SH][HSO4], obviously due to humin for-
mation.27 Humin formation also makes separation of the cata-
lyst and the product very much tedious. Hence, direct
alcoholysis of furfuryl alcohol in the presence of methanol
without the formation of levulinic acid is the most advan-
tageous strategy from a process point of view.8,28
Fig. 2 Effect of temperature on the alcoholysis of furfuryl alcohol. Reaction
conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH, (95 mL); temperature,
90–130 °C; N2 atm., catalyst, 0.3 g (acidic IL); reaction time, 2 h.
However, alkyl levulinate selectivity was found to decrease
from 99 to 85% with an increase in alcohol chain length from
ethanol to n-butanol. The decrease in Me-LA selectivity was
associated with the formation of respective ethers of FAL
depending on the alcohols used (shown as blue and green bars
in Fig. 1). In the case of a branched alcohol such as isopro-
panol, the corresponding ester (IPA-LA) selectivity decreased to
the extent of 68%. This indicates that the steric factor played an
important role in the alcoholysis of furfuryl alcohol.
The influence of temperature on the alcoholysis of furfuryl
alcohol was studied in a range from 90 to 130 °C keeping
other parameters constant and the results are shown in Fig. 2.
Almost complete conversion could be achieved as the tempera-
ture increased from 90 to 110 °C, and remained constant
beyond 110 °C up to 130 °C. Similarly, the selectivity to Me-LA
also increased linearly from 65 to 95% with an increase in
temperature from 90 to 130 °C. At higher temperature, the con-
version of ethers of FAL to Me-LA was facilitated, proving that
the reaction pathway (Scheme 1) involves first the etherifi-
cation of FAL followed by rehydration of the intermediate
ethers to Me-LA.
The efficiency of our [BMIm-SH][HSO4] catalyst was further
established for the synthesis of a variety of alkyl levulinates by
carrying out the alcoholysis of furfuryl alcohol in the presence
of different alcohols. Fig. 1 shows that a consistent activity
(99% conversion) was achieved for all the alcohols studied in
this work.
The study on the effect of catalyst loading on the alcoholysis
of furfuryl alcohol showed that the conversion of furfuryl
alcohol increased from 84 to 99% with an increase in catalyst
loading from 0.1 to 0.3 g (Fig. 3). Moreover, the selectivity to
methyl levulinate increased by 1.5 times (>80%) due to
increased rehydration of the intermediate, FAL ethers. The
higher catalyst concentration resulted in the higher availability
of Brønsted acid sites facilitating a faster consumption of fur-
furyl alcohol and rehydration of the intermediates to methyl
levulinate.
One of the important process parameters was the substrate
concentration, the effect of which on conversion and selectivity
pattern was studied by systematically varying the furfuryl
alcohol concentration from 5 to 15% and the results are
shown in Fig. 4. With an increase in substrate concentration
Fig. 1 Screening of solvents for the alcoholysis of furfuryl alcohol. Reaction
conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH, EtOH, n-PrOH, n-BuOH
and IsoPrOH (95 mL); temperature, 130 °C; N2 atm., catalyst, 0.3 g (acidic IL);
reaction time, 2 h.
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from 5 to 15%, both the conversion of furfuryl alcohol and the
Me-LA selectivity decreased significantly. At higher substrate
concentration, the accumulation of higher concentration of
the first step alcoholysis products viz. FAL ethers on the active
sites retarded further rehydration to Me-LA. The substrate con-
centration of 5% was the optimum to obtain complete conver-
sion and suppression of methoxy FAL intermediates. This
minimum substrate concentration can be compensated for by
higher productivity as our catalyst system was found to show
more than three reuses as discussed later.
Encouraged by the results on modified ILs for efficient
alcoholysis of furfuryl alcohol to Me-levulinate, we thought it
most desirable to obtain GVL directly from furfuryl alcohol in
a single pot synthesis. For this purpose, a combination of
acidic ILs and metal catalyst was used in a tandem synthesis of
GVL. We therefore screened several combinations of various
supported noble metals and the ILs selected from Table 1. All
the combinations studied in this work were indeed very suc-
cessful showing an excellent activity in terms of furfuryl
alcohol conversion >99% (see Table 2). The selectivity to GVL
varied depending on the noble metal used for the hydrogen-
ation of the intermediate Me-LA. It is very interesting to note
that under hydrogenation conditions, selectivity to GVL was
only 58% and to the intermediate ester was also only 12%
while almost all the remaining (26%) byproduct was (entry 2,
Table 2) tetrahydrofurfuryl alcohol (THFAL). THFAL was
formed due to the ring hydrogenation of furfuryl alcohol and
it was found to be a very stable product as observed from the
conversion/selectivity vs. time profile in Fig. 5. A novel strategy
adopted by us involved conducting the initial alcoholysis reac-
tion under an N2 atmosphere for 2 h after which N2 was
replaced by H2. This time period of changeover was arrived at
from the optimum time of alcoholysis as studied previously
(Table 1). This approach worked very well to completely sup-
Fig. 3 Effect of catalyst dosage on the alcoholysis of furfuryl alcohol. Reaction
conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH, (95 mL); temperature,
130 °C; N2 atm., catalyst, 0.15–0.3 g (acidic IL); reaction time, 2 h.
Fig. 4 Effect of substrate loading on the alcoholysis of furfuryl alcohol. Reac- press the ring hydrogenation product, THFAL (entry 2,
tion conditions: furfuryl alcohol, 5–15% (w/w); solvent, MeOH, (95 mL); temp-
Table 2), enhancing the selectivities to the desired products
such as MeLA and GVL. Hence, further studies were carried
erature, 130 °C; N2 atm., catalyst, 0.3 g (acidic IL); reaction time, 2 h.
Table 2 Screening of catalysts for alcoholysisa/hydrogenationb of furfuryl alcohol
Selectivity (%)
Entry
Catalyst
Additive
Conversion (%)
MeLA
GVL
4-HMeLA
FAL ethers
Others
1
2
3
4
5
6
7
8
5% Ru/C
5% Ru/C
5% Ru/C
5% Rh/C
5% Re/C
5% Pd/C
5% Ir/C
10% Ag/C
10% Ag/ZrO2
5% Ru/C+
5% Ru/C+
H2SO4
99
99
99
99
99
99
99
99
99
99
99
47
24
12
92
85
94
65
84
81
76
79
31
68
58
2
9
8
1
2
13
<0.01
3
4
4
6
3
4
7
<0.01
<0.01
26
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]b
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]
[BMIm-SH][HSO4]
Amberlyst-15
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
9
2
<0.01
6
8
9
14
18
<0.01
26
2
3
6
9
10
11
14
2
SO4-ZrO2
1
a Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL); temperature, 130 °C; N2 atm. + H2 pressure, 500 psi; catalyst, 0.3 +
0.5 g; reaction time, 2 h + 3 h. b Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL); temperature, 130 °C; H2 pressure,
500 psi; catalyst, 0.3 + 0.5 g; reaction time, 5 h.
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Fig. 6 Effect of the catalyst ratio on the hydrogenation of furfuryl alcohol to
GVL. Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL);
temperature, 403 K; H2 pressure, 500 psi; catalyst, 0.15–0.3 g (acidic IL) +
0.25–0.75 g (5% metal catalyst); reaction time, 5 h.
Fig. 5 CT profile for direct hydrogenation of furfuryl alcohol. Reaction con-
ditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL); temperature, 403 K;
H2 pressure, 500 psi; catalyst, 0.3 (acidic IL) + 0.75 g (5% metal catalyst); reaction
time, 5 h.
out initially under N2 and then replacing it by H2. Under these
conditions, among several noble metal catalysts, 5% Ru/C
showed the highest selectivity to GVL (68%) in combination
with IL [BMIm-SH][HSO4]. For the same IL, other metals gave
GVL selectivity in the range of 2–9%, which was in accordance
with our earlier results.28
Scheme 2 Direct conversion of furfuryl alcohol to GVL.
The GVL selectivity is important not only from the process
point of view but also it is necessary to dissolve the unwanted
humin associated with cellulose hydrolysis.29
The best performance of our Ru/C + [BMIm-SH][HSO4] cata-
lyst system for the direct conversion of FAL to GVL could be
distinctly visible when compared with the performance of Ru/
C in combination with the best solid acids chosen from
Table 1 (entries 10 and 11, Table 2). Ru/C with both amberlyst
and SO4-ZrO2 gave <20% selectivity to GVL, as the major
product was Me-LA. Our catalyst showed ten times higher
activity (TOF, 160 h−1) than the reported catalysts such as Ru/C
+ H2SO4 for the synthesis of GVL (ESI, Table 2†). This was
because of deactivation of Ru catalyst due to sulfur leaching
from H2SO4, which was evidenced by an intense peak at 169.3
eV of S(2p) (ESI, Fig. 2†).30,31
Since both acid as well as metal sites are important for the
direct conversion of FAL to GVL, the effect of compositions of
5% Ru/C + ILs was also studied. For this purpose, loading of
5% Ru/C was varied from 0.25 g to 0.75 g at a constant
loading IL of 0.3 g. Fig. 6 shows that for all the loadings of
Fig. 7 Effect of hydrogen pressure on the hydrogenation of furfuryl alcohol to
GVL. Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL);
temperature, 130 °C; H2 pressure, 250–750 psi; catalyst, 0.3 g (acidic IL) + 0.5 g
(5% metal catalyst); reaction time, 5 h.
Ru, the conversion of FAL remained constant at >99% while in turn adversely affected the selectivity to GVL. This study
the product selectivity was significantly affected. With an suggests that the direct conversion of furfuryl alcohol to GVL
increase in 5% Ru/C catalyst loading from 0.25 to 0.75 g, the involves acid catalyzed first step alcoholysis of furfuryl alcohol
selectivity to hydrogenation product GVL gradually increased to Me-LA followed by its metal catalyzed hydrogenation to 4-
from 35 to 83% at the cost of Me-LA indicating that an hydroxy methyl levulinate and subsequent cyclization to GVL
optimum Ru catalyst loading was necessary to achieve the (Scheme 2). During the cyclization, R–O is released as the
highest selectivity to GVL. In another control experiment, IL corresponding alcohol, methanol in this case.
loading was decreased to 0.15 g which resulted in significant
In order to maximize the selectivity to GVL, hydrogen
lowering of the furfuryl alcohol conversion to 80%. More pressure was varied from 250 to 750 psi and the results are
importantly, Me-LA selectivity was also minimum (10%) which shown in Fig. 7. A complete conversion of furfuryl alcohol was
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Fig. 8 Effect of metal loading on the hydrogenation of furfuryl alcohol to GVL.
Reaction conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL); temp-
erature, 130 °C; H2 pressure, 500 psi; catalyst, 0.3 g (acidic IL) + 0.5 g (5–10%
metal catalyst); reaction time, 5 h.
Fig. 9 CT profile for furfuryl alcohol alcoholysis and hydrogenation. Reaction
conditions: furfuryl alcohol, 5% (w/w); solvent, MeOH (95 mL); temperature,
130 °C; N2 atm. + H2 pressure, 500 psi; catalyst, 0.3 (acidic IL) + 0.75 g (5%
metal catalyst); reaction time, 2 h + 3 h.
achieved over the entire range of H2 pressures studied in this
work. The H2 pressure effect was very positive to enhance the
selectivity to GVL from 35 to 85% with complete disappearance
of FAL ethers. However, even at the highest H2 pressure of
750 psi, marginal Me-LA and 4-MeLA remained unreacted indi-
cating that rather than higher H2 concentration, a higher
extent of metal sites would be necessary for complete hydro-
genation of the intermediates to GVL. Therefore, the effect of
active metal (Ru) loading on carbon was varied in the range of
5–10% with a constant amount of IL. As can be seen from
Fig. 8, the conversion of the intermediate methyl LA increased
from 76 to 95% with an increase in selectivity to GVL up to
94% for the active metal (Ru) loading of 10% on the carbon
support. Thus, the availability of active metal content plays an
important role in hydrogenation of methyl levulinate to GVL.
The role of acidic and metal sites could be further under-
stood for the direct single pot conversion of furfuryl alcohol to
GVL under a nitrogen and a hydrogen atmosphere. Fig. 9
shows the conversion and selectivity vs. time pattern for a
single pot conversion of furfuryl alcohol to GVL over acidic as
well as metal sites. Initially under an N2 atmosphere, Brønsted
acid sites of IL gave complete formation of methyl levulinate
within the first 2 h of the reaction with complete conversion of
furfuryl alcohol while changeover from N2 to H2 facilitated the
hydrogenation of MeLA to 4-hydroxy MeLA followed by its
cyclization to GVL. However, under only hydrogenation con-
ditions, the ring hydrogenation product THF alcohol was
obtained to the extent of 25–31% which remained stable,
affecting the selectivity to GVL (entry 3, Table 2). The mechan-
istic pathway for alcoholysis of furfuryl alcohol to methyl levu-
linate in alcoholic medium involves first the formation of
methoxy FAL followed by ring hydration in the form of 1,4-
addition to cyclic oxonium (ESI, Scheme 1†). The cyclic
oxonium compound then undergoes deprotonation to form
cyclic diene followed by its isomerisation in acidic medium to
give the final product, methyl levulinate.
Fig. 10 Recycle study of (A) rehydration of furfuryl alcohol to methyl levulinate
and (B) direct hydrogenation of FAL to GVL. Reaction conditions: (a) furfuryl
alcohol, 5% (w/w); solvent, MeOH (95 mL); temperature, 130 °C; N2 atm., cata-
lyst, 0.3 g; reaction time, 2 h. (b) H2 pressure, 500 psi; catalyst, 0.75 g.
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The recycle experiments for [BMIm-SH][HSO4] catalyst were
also carried out at 130 °C. After completion of the reaction,
methanol was evaporated using a rotavapour. To this concen-
Experimental
Materials
trated solution, 80 mL of ethyl acetate was added and the IL Furfuryl alcohol (99%), methyl levulinate metal salts, 1-methyl
was extracted with three portions 40 mL each of distilled imidazole and 1,4-butane sultone were purchased from Sigma-
water, in a separating funnel. After evaporation of the aqueous Aldrich, Bangalore, India, while methanol (>99.9%) was pur-
layer, IL catalyst was recovered for its reuse in the subsequent chased from Rankem. Hydrogen (>99.99% purity) was
alcoholysis experiment. This procedure was repeated up to the obtained from Inox, India.
third recycle of the catalyst which showed the consistent
activity (Fig. 10A). A decrease in selectivity to methyl levulinate
from 99 to 83% associated with the corresponding increase in
intermediate selectivity was observed due to handling loss
from time to time extraction. This was confirmed by adding a
make up amount (∼20%) of the catalyst after the third recycle
due to which the original selectivity to methyl levulinate was
regained in the fourth recycle. The results of a catalyst recycle
study for single pot conversion of furfuryl alcohol to GVL are
also shown in Fig. 10B. Both the activity and selectivity were
consistent for 5% Ru/C in combination with [BMIm-SH][HSO4]
up to three recycles, confirming the stability of our catalyst
system under reaction conditions. GC analysis of the organic
layer showed GVL with 100% purity while the aqueous layer
did not show any GVL (ESI, Fig. 6A and B†). This confirms the
easy protocol of solvent extraction for the recovery of pure
product (GVL).
Catalyst preparation
1-Methylimidazole and 1,4-butane sultone (0.2 mol) were
charged into a 100 mL round bottomed flask. The mixture was
then stirred at 40–80 °C for 1 h. The solid zwitterion was recov-
ered by filtration and washed with diethyl ether to completely
remove the unreacted reactants and then dried under vacuum
overnight. A stoichiometric amount of respective sulfonic acid
was subsequently added dropwise to the respective zwitterion
and the mixture was again stirred at 80 °C for 6 h. The viscous
liquid thus obtained was washed with diethyl ether and dried
under vacuum overnight. All the prepared sulfonic acid func-
1
tionalized ionic liquids were confirmed by H NMR (200 MHz)
in D2O at 25 °C.
Supported Ru, Pd, Re, Pd and Ag catalysts were prepared by
the impregnation method. In a typical procedure, 2 g of the
support, carbon, was suspended in aqueous medium contain-
ing a calculated amount of the respective metal precursor
under stirring for 1 h. It was subsequently reduced using 5 mL
of NaBH4 (1 mol). The catalyst was then filtered and dried at
110 °C for 12 h.
Conclusions
A novel approach for direct conversion of furfuryl alcohol
to GVL via the alcoholysis/hydrogenation sequence using
sulfonic acid-functionalized ionic liquids (SO3H-ILs) and
5% Ru/C catalyst system has been demonstrated. Furfuryl
alcohol conversion as high as 99% could be achieved with
ILs modified with HSO4 and Cl-HSO3 anions in the alcoholysis
reaction. The catalyst could be further tuned by increasing
the alkyl chain of IL 1-methyl imidazole with 1,4-sultone
to give an enhanced selectivity of 95% to methyl levulinate.
Combining furfural alcoholysis with a subsequent hydro-
genation of Me-LA in a single pot synthesis using our
catalyst system comprising [BMIm-SH][HSO4] + 5% Ru/C
successfully resulted in the formation of GVL with 68% selecti-
vity and complete conversion of furfuryl alcohol. Under
the hydrogenation conditions, the ring hydrogenation to
Hydrogenation experiment and analysis
Single pot furfuryl alcohol alcoholysis and hydrogenation were
carried out in a 300 mL capacity autoclave (Parr Instruments Co.,
USA) at a stirring speed of 1000 rpm. The typical reaction con-
ditions were: temperature, 130 °C; furfuryl alcohol concentration,
5 wt%; solvent, 95 mL; total volume, 100 mL; catalyst loading,
0.3 g (acidic IL) + 0.5 g (supported metal catalyst); substrate :
catalyst mole ratio (10 : 1); and hydrogen pressure, 500 psi.
Liquid samples were withdrawn periodically and analyzed by GC
(Thermo-Trace-700) having an HP-5 column with a FID detector.
THFAL is
a competing reaction adversely affecting the
GVL selectivity; hence, the first stage alcoholysis can be
carried out under N2 and then changeover to H2 eliminates
the THFAL formation. Among the various parameters
Abbreviations
studied in this work, an increase in Ru loading from 5 to GVL
10% dramatically enhanced the GVL selectivity from 68 to LA
94%, indicating that the availability of active metal sites is MeLA
more significant in the tandem reactions of alcoholysis and 4-MeLA
hydrogenation of methyl levulinate. 5% Ru/C in combination [MIm]
with IL could be satisfactorily recycled three times with con- [NMP]
γ-Valerolactone
Levulinate or levulinic acid
Methyl levulinate
4-Hydroxy methyl levulinate
1-Methyl imidazolium
N-Methyl 2-pyrollidonium
sistent performance, confirming its stability under reaction [BMIm-SH] 1-Butyl sulfonic acid, 3-methyl imidazolium
conditions.
[MMT]
Montmorillonite
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