Conversion of levulinate ester and formic acid into γ-valerolactone using a homogeneous iron catalyst

Article abstract of DOI:10.1055/s-0034-1379462

Abstract An iron-catalyzed hydrogenation of biomass-derived ethyl levulinate (EL) to γ-valerolactone (GVL) has been developed using formic acid as hydrogen source. Ethyl levulinate was converted into γ-valerolactone quantitatively under optimized reaction conditions. This catalytic process does not require the use of any base or additives.

Full text of DOI:10.1055/s-0034-1379462

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Conversion of Levulinate Ester and Formic Acid into γ-Valerolactone Using a
Homogeneous Iron Catalyst
Ming-Chen Fu, Rui Shang, Zheng Huang, Yao Fu*
Anhui Province Key Laboratory of Biomass Clean Energy, Collaborative Innovative Center of Chemistry for Energy Materials, and Department
of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. of China
Fax +86(551)63606689; E-mail: fuyao@ustc.edu.cn
Received: 22.09.2014; Accepted after revision: 27.10.2014
transportation fuels.11a Lange group has confirmed that γ-
Abstract: An iron-catalyzed hydrogenation of biomass-derived
valerolactone can be used to produce ‘valeric biofuels’
ethyl levulinate (EL) to γ-valerolactone (GVL) has been developed
through proper derivatization and esterification.11c All
these researches show the importance of γ-valerolactone
using formic acid as hydrogen source. Ethyl levulinate was convert-
ed into γ-valerolactone quantitatively under optimized reaction con-
ditions. This catalytic process does not require the use of any base production in biomass conversion.
or additives.
γ-Valerolactone can be obtained through hydrogenation
Key words: biomass conversion, iron catalysis, formic acid, γ-
of levulinic acid or levulinate esters. Compared with di-
rect hydrogenation of levulinic acid, hydrogenation of
levulinate ester is more attractive since production of
levulinate ester from lignocellulose has higher yield and
easier separation procedure.13 For hydrogenation process,
the hydrogen source can be formic acid,14a hydrogen
gas,14b or isopropanol.13 Among these hydrogen sources,
formic acid is an ideal one since it is also the major by-
product of levulinic acid production through hydrolysis of
lignocellulose. It is a nontoxic liquid with hydrogen con-
tent of 4.4 wt% (5.22 MJ.Kg–1).15 Formic acid can undergo
highly efficient dehydrogenation to generate hydrogen
and nontoxic carbon dioxide in the presence of a suitable
catalyst. Thus it is considered as a potentially attractive
hydrogen storage compound.16 Hydrogenation of levulin-
ic acid to γ-valerolactone using formic acid as hydrogen
source has drawn considerable attention in recent years.17
valerolactone, levulinate ester
Due to the gradual depletion of fossil fuels in recent years,
society needs to find new sustainable resources to replace
petroleum-derived products.1 As a promising substitute,
lingocellulosic biomass is a renewable carbon resource
and is abundant in nature. Developing catalytic methods
to convert lingocellulosic biomass into biofuels and valu-
able chemicals is an ideal way to relieve such problems.2
After extensive efforts in recent years, various chemicals
such as alcohols,3 5-hydroxymethylfurfural (HMF),4 2,5-
dimethylfuran (DMF),5 levulinic acid (LA),6 lactic acid,7
pentanoic acid esters,8 γ-valerolactone (GVL),9 and short-
chain hydrocarbons10 can all now be produced through a
lingocellulosic biomass based route of transformation.
Among these platform molecules, γ-valerolactone is con-
sidered as the most appealing one since it can be used as
fuel additive, liquid fuel, food additive, solvent and fine
chemical intermediates.11 In various biomass conversion
processes, production of γ-valerolactone has drawn most
attention by chemists.12 Dumesic et al. succeeded in de-
veloping an integrated, large-scale biorefinery system,
which can transfer γ-valerolactone to liquid alkenes for
However, most of the reports on conversion of levulinate
ester and formic acid into γ-valerolactone using homoge-
neous catalyst are based on precious metals, such as Ru
and Ir (Scheme1).14a,18 Although these precious-metal cat-
alysts show high catalytic performances, precious-metal
resources are like fossil oil, their reserves are limited.19
Precious metal resources also face the problem of deple-
tion, standing on the time scale of centuries. Sustainable
O
O
OR
R = Et, Me, H
O
O
O
Fe catalyst
Ru or Ir catalyst
O
O
O
OR
– CO2
– CO2
+
HO
previous work
this work
yield 99%
selectivity 99%
HO
HCOOH
not detected
Scheme 1 Homogeneous hydrogenation of levulinic esters using formic acid in the presence of a transition-metal catalyst
SYNLETT 2014, 25, 2748–2752
Advanced online publication: 10.11.2014
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9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0034-1379462; Art ID: st-2014-w0789-c
© Georg Thieme Verlag Stuttgart · New York

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transformations prefer using sustainable catalysts. Iron is led us to test iron precursors with weakly coordinated an-
the most abundant transition metal on earth and possesses ionic ligand. To our delight, when iron(II) triflate was
a set of advantages such as wide availability at very low used as the iron source, γ-valerolactone was detected in
cost, and no toxicity. Recently, interest in using homoge- 98% yield with complete conversion of ethyl levulinate
neous iron catalysts in biomass conversion has gained sig- (Table1, entries 10 and 11). Changing iron(II) triflate to
nificant attention. Lignin degradation20 has been proved to iron(III) triflate resulted in a sharp drop of the yield to
be viable through homogeneous iron catalysis. Inspired by 28% (see Table SI 3). It is worth noting that when using
the recent developments of iron-catalyzed hydrogen- methyl levulinate under the optimized conditions, γ-
transfer reactions,21 especially iron-catalyzed hydrogena- valerolactone was also generated in 90% yield. But when
tion and hydrogen production using formic acid by Beller levulinic acid was used, no γ-valerolactone was detected
et al., we became interested in realizing the conversion of under the optimized conditions; sodium levulinate also
levulinate ester and formic acid into γ-valerolactone by gave no product. This observation may be attributed to the
using an iron catalyst. Notably, Assary group22 previously coordination of levulinate anion to iron catalyst which de-
studied the feasibility of iron-catalyzed hydrogen transfer creases the reactivity of the catalyst (see Table SI 7).
in the production of γ-valerolactone by DFT calculation.
Table 1 Investigation of Catalytic Conditiona
Although related iron-catalyzed hydrogenation of ni-
troarenes,23a,23b aldehydes23c and alkynes23d had been ad-
HCO2H
catalyst
ligand
dressed by Beller et al. iron-catalyzed hydrogenation of
O
O
O
ketones using formic acid as hydrogen source has not
been achieved yet. Especially, the transformation of
levulinate ester to γ-valerolactone, in which case the ke-
tone functionality needs to be selectively hydrogenated
without further reducing the ester functionality, has never
been achieved by using a homogeneous iron catalyst. In
this paper, we have achieved this transformation for the
first time in high selectivity and high yield by a homoge-
neous iron catalyst.
O
– CO2, – H2O
O
EL
Entry
Catalyst
Conv. (%)b
Yield (%)b
1
2c
3
Fe(BF4)2·6H2O/L1
Fe(CF3SO3)2/L1
Fe(CF3SO3)2/L2
Fe(CF3SO3)2/L3
Fe(CF3SO3)2/L4
Fe(CF3SO3)2/dppb
Fe(CF3SO3)2/dppe
Fe(CF3SO3)2/Ph3P
Fe(CF3SO3)2/BINAP
Fe(CF3SO3)2)/L1
Fe(CF3SO3)2/L1
Fe(CF3SO3)2/L1
Fe(CF3SO3)2/L1
23
65
22
62
<1
6
<1
8
Initially we applied the catalytic system reported by Beller
et al. in the hydrogenation of aldehydes to our reaction,
but the catalytic system composed of Fe(BF4)2·6H2O and
tris[(2-diphenylphosphino)ethyl]phosphine (L1, PP3) at
60 °C in THF solution was entirely ineffective. Ethyl
levulinate (EL) was totally recovered after reaction. This
result suggested that although the catalytic system can be
applied to reduce aldehyde, it is not reactive enough to re-
duce ketone or ester. After we screened different tempera-
tures (see Table SI 1), we found that when the reaction
temperature was increased to 100 °C, γ-valerolactone
could be detected in 3% yield with only 4% conversion of
ethyl levulinate. The low conversion (4%) suggested that
the iron-based catalyst has good selectivity towards the
conversion of ethyl levulinate to γ-valerolactone. Inspired
by this result, we further optimized the temperature. The
yield of γ-valerolactone was increased to 7% at 120 °C.
Due to the limitation of running reactions in THF at high
temperature, we screened different solvents. The solvent
is crucial for this reaction (see Table SI 1). When polar
solvents such as N,N-dimethylacetamide (DMA) or di-
methyl sulfoxide (DMSO) was used as solvent, no γ-
valerolactone was detected. Ether solvents were found to
be effective, with 1,4-dioxane being the optimal solvent
giving γ-valerolactone in 22% yield at 140 °C (Table 1,
entry 1). We then screened different iron sources (see Ta-
ble SI 3). When FeCl2 or FeCl3 was used as pre-catalyst,
no γ-valerolactone was detected, and the GC analysis re-
vealed that ethyl levulinate was recovered. Beller et al.23e
pointed out that the presence of chloride ions can poison
the iron catalyst in dehydrogenation of formic acid, which
4
5
<1
<1
<1
<1
<1
>99
>99
50
<1
<1
<1
<1
<1
98
99
47
<1
6
7
8
9
10
11d
12e
13f
<1
a Reaction conditions: EL (0.5 mmol), catalyst (0.025 mmol, metal/li-
gand ratio = 1:1), HCO2H (2.0 equiv), 1,4-dioxane (0.5 mL), 140 °C,
24 h.
b Determined by GC using N-methylpyrrolidone as an internal stan-
dard.
c Reaction temperature was 120 °C.
d Amount of catalyst used was 0.05 mmol.
e Amount of catalyst used was 0.015 mmol.
f Conditions: 3 MPa H2, no formic acid.
We next screened several ligands commonly used in iron-
catalyzed hydrogenation. The results showed this process
to be highly ligand dependent. Tris[(2-diphenylphosphi-
no)ethyl]phosphine (L1; Figure 1) is uniquely effective
for this process, whereas the other two effective ligands
L2 and L4 (Figure 1) in iron-catalyzed dehydrogenation
© Georg Thieme Verlag Stuttgart · New York
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M.-C. Fu et al.
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of formic acid were found to be ineffective for this trans- triethylamine can sometimes promote the reaction,14a but
fer hydrogenation reaction (Table 1, entries 3 and 5). in the iron-catalyzed reaction, the yield drops sharply
1,1,1-Tris(diphenylphosphinomethyl)ethane (L3; Figure even if catalytic amount of triethylamine is added (Table
1), a triphosphine ligand which is highly effective for Ru- 2, entry 6). Since this iron-catalyzed reaction is quite
catalyzed hydrogenation24 showed very low catalytic ac- clean and does not need any base or additives, we can eas-
tivity (Table 1, entry 4). Other common phosphine ligands ily separate γ-valerolactone from the reaction mixture by
such as dppb (Table 1, entry 6), dppe (Table 1, entry 7), distillation (85–86 °C, 15 Torr).25 1,4-Dioxane can be eas-
triphenylphosphine (Table 1, entry 8) and BINAP (Table ily recovered after distillation. The ligand (L1) can also be
1, entry 9) all failed to give detectable amount of the de- recovered from the residue of distillation by a simple col-
sired product, leading us to use L1 as the preferred ligand. umn chromatography.
Table 2 Effect of Different Reductantsa
PPh2
P
Entry
Reducant
Conv. (%)b Yield (%)b
P
PPh2
Ph2P
1
2
3
4
5
6
none
<1
80
<1
<1
<1
16
<1
77
<1
<1
<1
10
Ph2P
PPh2
PPh2
formic acid
ammonium formate
sodium formate
sodium formate, H2O
formic acid, Et3N (5:1)
L2
L1
H
N
PPh2
PPh2
Ph2P
Pi-Pr2
Pi-Pr2
L4
L3
a Reaction conditions: EL (0.5 mmol), catalyst (0.025 mmol, metal/li-
gand ratio = 1:1), HCO2H (2.0 equiv), 1,4-dioxane (0.5 mL), 140 °C,
16 h.
Figure 1 Ligand structure in Table 1
b Determined by GC using N-methylpyrrolidone as internal standard.
The optimization of the ratio between formic acid and eth-
yl levulinate showed that using 2.0 equivalents of formic
acid gives the best result, as increasing the amount of for-
mic acid to 3.0 equivalents or reducing it into 1.0 equiva-
lent significantly reduced the yield. It should be noted that
larger than 1.0 equivalent amount of formic acid is neces-
sary to push the reaction into complete conversion, since
some amount of formic acid was inevitably decomposed
into hydrogen and carbon dioxide, which were released
from the reaction mixture. It is worthwhile to note that
further increasing the reaction temperature also decreased
the yield of γ-valerolactone (see Table SI 6). This obser-
vation is probably due to the decreasing stability of the
phosphine-coordinated iron complex at higher tempera-
ture.23d Besides, decreasing the catalyst loading resulted in
the decrease of the yield of γ-valerolactone (Table 1, entry
12). For iron catalysts, since it is important to consider
trace amount of metal contamination which can give false
conclusion, we tested the purity of the iron source by AFS
analysis, and also tested the catalytic activity of several
other metals that might be contained in the iron catalyst as
trace impurity. The results showed that the complex of
Co, Pd, Ru or Ir with ligand L1 does not have the catalytic
reactivity under the optimized conditions for iron (see Ta-
ble SI 3).
To understand whether ethyl levulinate is hydrogenated
by iron-catalyzed transfer hydrogenation of formic acid or
hydrogenated by hydrogen gas formed by formic acid de-
hydrogenation,23a we tested this reaction under 3 MPa of
hydrogen gas. The result showed that ethyl levulinate can-
not be hydrogenated by hydrogen gas under this iron cat-
alyst system (Table 1, entry 13). This result suggests that
the Fe–H intermediate generated after dehydrogenation of
formic acid should be the active species to reduce the car-
bonyl group in levulinate ester.
The key to achieve this transformation is the suitable
choice of the iron precursor, iron (II) triflate. To further
understand the reason for the good performance of this
iron pre-catalyst, we conducted control experiments by
adding water and halide anion. The results of these control
experiments showed that adding catalytic amount of water
sharply reduced the yield of γ-valerolactone (Table 3, en-
tries 1 and 2). Adding chloride anion entirely poisoned the
iron catalyst (Table 3, entries 3 and 4). This result is con-
sistent with the lack of catalytic reactivity of iron(II) chlo-
ride (see Table SI 3). We can conclude that a catalytic
system containing anion of weak coordination ability for
iron(II) is the key to maintain the catalytic activity of the
iron catalyst in this reaction.
Based on literature reports23d,23e,26 and our experimental
results, we propose a possible mechanism for this iron-
catalyzed transfer hydrogenation reaction (Scheme 2).
The iron catalyst can be formed by coordination of
iron(II) triflate with PP3 ligand. Triflate anion on iron can
be easily replaced by formate anion due to the weak coor-
In recent studies of Ru-catalyzed conversion of levulinic
acid and formic acid to γ-valerolactone, it has been point-
ed out that adding base to form formate can facilitate the
transformation. In contrast to the Ru-system, formic acid
gives better result compared with formate in this Fe-cata-
lyzed reaction (Table 2, entries 2–5). In Ru-catalyzed
transfer hydrogenation reaction using formic acid, adding
Synlett 2014, 25, 2748–2752
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2751
Table 3 Influence of Water and Additional NaCla
number of the iron catalyst in this reaction is under inves-
tigation in our lab.
Entry
Additive
Conv. (%)b Yield (%)b
Acknowledgment
1
2
3
4
H2O (5 mol%)
H2O (10 mol%)
NaCl (5 mol%)
NaCl (10 mol%)
13
<1
<1
<1
11
<1
<1
<1
This work was supported by the National Basic Research Program
of China (2012CB215305, 2013CB228105), NSFC (21325208,
21172209, 21272050), CAS (KJCX2-EW-J02), and FRFCU
(WK2060190025).
a Reaction conditions: EL (0.5 mmol), catalyst (0.025 mmol, metal/li-
gand = 1:1), HCO2H (2.0 equiv), 1,4-dioxane (0.5 mL), 140 °C, 16 h.
b Determined by GC using N-methylpyrrolidone as internal standard.
Supporting Information for this article is available online
at
http://www.thieme-connect.com/products/ejournals/journal/
10.1055/s-00000083.SunogIipmifrtano
dination ability to form complex 1. The complex 1 can
isomerize to 2 by changing the coordination mode of for-
mate anion. After dehydrogenation of 2, a Fe–H species is
formed. This Fe–H species can further coordinate with
ethyl levulinate, to form complex 3. Please note that since
neutral carbonyl group has weak coordination ability with
iron(II), this step may be easily hampered in the presence
of an anion with stronger coordination ability. After re-
ducing the carbonyl group by Fe–H, alkoxide anion can be
protonated and replaced by formate anion to regenerate
complex 2.
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In conclusion, we have reported an iron-based homoge-
neous catalyst for conversion of levulinate ester and for-
mic acid into γ-valerolactone in high selectivity and high
yield. This iron catalyst system is sustainable and nontox-
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PPh2
Fe
PPh2
O
CO2
OEt
H
P
O
P
Ph2
PPh2
3
PPh2
Fe
O
O
Fe
PPh2
H
P
OEt
H
O
O
P
H
PPh2
Fe
PPh2
P
O
P
O
Ph2
P
Ph2
PPh2
2
1
P
Ph2
4
OH
OEt
OEt
PPh2
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Fe
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P
H
HCOOH
PPh2
P
O
Ph2
O
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EtOH
+
Scheme 2 Proposed mechanism
© Georg Thieme Verlag Stuttgart · New York
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