Highly Efficient Hydrogenation of Levulinic Acid into Γ-Valerolactone using an Iron Pincer Complex

Article abstract of DOI:10.1002/cssc.201800435

The search for nonprecious-metal-based catalysts for the synthesis of γ-valerolactone (GVL) through hydrogenation of levulinic acid and its derivatives in an efficient fashion is of great interest and importance, as GVL is an important a sustainable liquid. We herein report a pincer iron complex that can efficiently catalyze the hydrogenation of levulinic acid and methyl levulinate into GVL, achieving a turnover number of up to 23 000 and a turnover frequency of 1917 h^?1. This iron-based catalyst also enabled the formation of GVL from various biomass-derived carbohydrates in aqueous solution, thus paving a new way toward a renewable chemical industry.

Full text of DOI:10.1002/cssc.201800435

Accepted Article
Title: Highly Efficient Hydrogenation of Levulinic Acid into γ-
Valerolactone with an Iron Pincer Complex
Authors: Yuxuan Yi, Huiying Liu, Ling-Ping Xiao, Bo Wang, and
Guoyong Song
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ChemSusChem
10.1002/cssc.201800435
FULL PAPER
Highly Efficient Hydrogenation of Levulinic Acid into γ-
Valerolactone with an Iron Pincer Complex
[a]
[a]
[a]
[a]
*[a]
Yuxuan Yi, Huiying Liu, Ling-Ping Xiao, Bo Wang and Guoyong Song
Abstract: The search for nonprecious metal catalysts for the
synthesis of γ-valerolactone (GVL) through hydrogenation of
levulinic acid and its derivatives in an efficient fashion is of great
interest and importance, as GVL is an important a sustainable liquid.
We herein report a pincer iron complex can efficiently catalyze the
hydrogenation of levulinic acid and methyl levulinate into GVL,
-
1
achieved up to 23000 turnover numbers and 1917 h turnover
frequencies. This iron catalyst also enabled the formation of GVL
from various biomass-derived carbohydrates in aqueous solution,
thus paving a new way toward a renewable chemical industry.
Introduction
The depletion of fossil fuels with increasing emission of carbon
dioxide has led to the search for alternative sustainable energy
[1]
and chemicals sources such as biomass. As a sustainable
liquid, γ-valerolactone (GVL) has attractive physical and
chemical property and is used as solvent, flavoring agent and a
[2]
precursor for high-value chemicals and fuels. In the presence
of a heterogeneous or homogeneous metal catalyst, levulinic
acid (LA), which is produced by acid-catalyzed hydrolysis of
cellulose to glucose followed by in situ isomerization (to form
fructose), dehydration (to form 5-HMF) and deformylation in a
Scheme 1. Transition metal catalysts for hydrogenation of LA to GVL. (TPP =
(
1,2-(bis-di-tert-butylphospino)ethane), DPPB = 1,4-bis (diphenylphosphino)
butane, DTBPE 1,2-bis(ditertbutylphosphino)ethane, tetraphos tris((2-
diphenylphosphino)ethyl)phosphine)
=
=
[1]
biorefinery process, can be converted into GVL through
[
3]
hydrogenation and dehydrative ring closure.
As
a
consequence, the development of efficient, economical catalytic
processes for preparation of GVL from levulinic acid has
[
10]
23. Noticeably, compared to reported Ru or Ir-based catalysts,
the catalytic activities of nonprecious metal catalysts are too low
to use in a practical manner, whereas the TON do not exceed
100 (Scheme 1). Therefore, the search of non-precious metal
catalysts enables the advancement this transformation in a high
efficient fashion is of great interest and importance.
[
3-7]
received intensive attention.
Compared to heterogeneous catalysts,
[
3,4]
homogeneous
catalysts have high catalytic efficiency towards hydrogenation of
levulinic acid or levulinate esters and turnover numbers (TON) of
[
3,5-7]
approximately 174000 has been achieved (Scheme 1).
The
most efficient catalysts are typically based on precious and
Several group recently described that the iron complexes
supported on a PNP-pincer ligand (such as 1-4 in chart 1 and
related species) can act as efficient catalysts for de/hydroge-
[
5]
[6]
[7]
heavy metals such as ruthenium , iridium and palladium . In
comparison, the same transformation with nonprecious, earth-
abundant metal catalysts is a relatively unexplored area in
[
11-15]
nation transformations.
In 2011, Milstein reported that a
[
8-10]
literature.
and the combination of Fe(OTf)
could catalyze the conversion of levulinate esters into GVL.
Burtoloso and co-workers reported Fe (CO)12-catalyzed
hydrogenation of LA, thus giving GVL in a high yield with TON of
Fu and co-workers showed that Casey’s catalyst
pincer iron complex 1 was capable of 1880 turnover numbers in
[
12a]
2
with a tertraphosphine ligand
hydrogenation of ketones.
In 2013, Beller and co-workers
[
8,9]
used an aliphatic PNP-Fe complex 2 for reforming of methanol
[
13a]
3
to produce
H
2
and CO
2
with TON of nearly 10000.
Fairweather and Guan found that the borohydride analog of 2
[
13b]
could catalyze hydrogenation of esters to form alcohols.
[12b]
[13h]
Milstein
,
Hazari and Bernskoetter
,
Kirchner and
independently demonstrated that iron complexes
[
14b]
Gonsalvi
[
a]
Yuxuan Yi, Huiying Liu, Dr. Ling-Ping Xiao, Dr. Bo Wang, Prof. Dr.
Guoyong Song
embedded in a PNP-pincer ligand efficiently catalyzed the
Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry
University, Beijing, 100083, China
Email: songg@bjfu.edu.cn
hydrogenation of CO
numbers.
to formate, achieved up to 60000 turnover
2
[13h]
Supporting information for this article is given via a link at the end of
the document.
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ChemSusChem
10.1002/cssc.201800435
FULL PAPER
Table
valerolactone
1
PNP-Fe-catalyzed hydrogenation of levulinic acid into γ-
[
a]
Chart 1. Selected examples of PNP-pincer iron complexes
-
[
b]
TOF (h
)
Entry
[Fe]
Solvent
Time (h) Yield (%)
TON
1
We envisioned that the pincer Fe complexes might serve as
unique catalysts for preparation of GVL through hydrogenation
of LA. Herein, we report that hydrogenation of levulinic acid and
methyl levulinate (ML) catalyzed by iron complexes with pincer
ligands, thus yielding GVL in excellent yield (up to 97%) with
turnover numbers up to 23000 and turnover frequencies up to
1
2
3
4
1
2
3
4
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
2
90
27
97(95
0
1800
540
900
270
970
5
[
c]
2
)
1940
12
2
-1
1917 h under relatively mild reaction conditions. This iron
[
d]
e]
catalyst is also compatible with the levulinic acid aqueous
solution generated from carbohydrates hydrolysis, to afford GVL
in high yields.
5
6
7
8
9
1
1
3/AgOTf
95
0
1900
950
[
3/AgOTf
12
5
3
3
3
3
3
92
94
90
90
15
1840
1880
1800
1800
300
368
940
900
900
25
i
PrOH
2
Results and Discussion
THF/H
2
O
2
At first, we examined the hydrogenation reaction of levulinic acid
by using a series of iron pincer complexes (Chart 1) in MeOH at
0
1
H
2
O
2
[16]
2
100 °C, 50 atm H in the presence of 1.0 equiv of KOH.
THF
12
Significant influence of the ligands was observed (Table 1),
suggesting that an appropriate pincer ligand is highly important
for this reaction. The Fe complex 1 (0.05 mol %) bearing a
[
a] Reaction conditions: levulinic acid (580 mg, 5 mmol), [Fe] (0.05 mol %),
KOH (280 mg, 5 mmol), solvent (2 mL), H (50 atm). [b] GC yield using N-
2
[
12]
bis(diisopropyl-phosphinomethyl)pyridine ligand afforded GVL
in 90% yield in 2 h (Table 1, entry 1), while the aliphatic PNP-Fe
methyl pyrrolidone (NMP) as an internal standard. [c] An anhydrous KOH
reagent where the KOH composition was determined as 91% was used. [d]
0.1 mol % AgOTf was added. [e] 0.1 equiv KOH was used.
[13]
[14]
2
gave a low yield of 27% (entry 2). When complex 3, which
ligates a 2,6-diaminopyridine scaffold, was used as a catalyst,
the yield of GVL improved to 97% with a turnover numbers of
-
1
1
940 and a turnover frequencies of 970 h (entry 3). In contrast,
complex embedded in 2,6-bis(phosphinito)pyridine
backbone, does not show any activity (entry 4). This could
be because the acidic CH or NH spacer in 1-3 pincer ligands
can assist the formation of a five-coordinated Fe hydride specie
4
100
80
[
15]
[17]
2
[
12-14]
through an acid-base reaction,
4
while the O linker in complex
The combination of 3 and
AgOTf, which possibly leads to the cationic iron specie,
capable of the formation of GVL in high yield in the presence of
.0 equiv KOH. On the contrary, no GVL was observed when
60
[
18,19]
prevents such a possibility.
[
12b]
is
40
1
20
catalytic KOH (0.1 equiv) was used (entries 5-6), suggesting that
the cationic iron specie may be not involved in current catalytic
cycle. A control experiment without catalyst led to only methyl
0
[19]
t
t
t
Na
CO
K
CO
Cs
CO
NaOH KO Bu NaO Bu LiO Bu Et N
3
2
3
2
3
2
3
levulinate in ca. 40% yield, suggesting that ML is a possible
[
3c]
intermediate in methanol.
On the basis of the catalyst
Figure 1. The effect of base for 3-catalyzed hydrogenation of LA (reaction
conditions: levulinic acid (580 mg, 5 mmol), 3 (0.05 mol %), base (5 mmol),
MeOH (2 mL), H (50 atm), 5 h.
2
screening results, solvents screening was then studied with
complex 3. Analogous to the case for MeOH, the hydrogenation
reaction in EtOH, PrOH, THF/H
i
2
O (1/4) and water proceeded to
give GVL in 90-94% yields (entries 7-10). When neat THF was
used, the yield was decreased to 15% in 12 h, indicating the
need for a protic solvent (entry 11). Of note, 4-hydroxypentanoic
acid could be detected as an possible intermediate in protic
solvent (see SI), which can be converted into GVL through ring
A
screening of the effect of base for 3-catalyzed
hydrogenation of LA was carried out (Figure 1, see also SI).
Performing the reaction by using NaOH, K CO and Cs CO
gave GVL in desirable yields. Changing the base from KOH to
Na CO led to a lower yield of GVL (8%) under analogous
2
3
2
3
[
4e,18]
2
3
closure under catalyst-free condition.
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ChemSusChem
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FULL PAPER
t
t
reaction conditions. In the case of 1.0 equiv of KO Bu, NaO Bu or
alive during 15 h under such conditions and the solubility of H
may dominate the rate of hydrogenation of LA.
2
t
LiO Bu, 0.05 mol% of complex 3 was capable of hydrogenating
LA to GVL with 84-91% yields. GVL was not formed using either
The attempt to reuse the catalyst through either isolation of
TEA (triethylamine) or DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), iron complex or addition of more LA to the reaction mixture is not
probably because the weak base does not enable the formation
successful, probably because the active iron specie is readily
decomposed. We then focused on the evaluation of the potential
of complex 3 in reduction of LA by using low catalyst dosage in
MeOH. In the presence of 0.01 mol% of 3, GVL was formed in
[
14a]
of a five-coordinated Fe−H through an acid-base reaction.
It
is worth noting that when sub-stoichiometric KOH (such as 0.9
[
16]
equiv) was used,
2% yield), with no observation of hydrogenation product,
attributing to the fact that excess LA inhibits the formation of
only methyl levulinate was obtained (ca.
4
2 2
40% yield at 50 atm H ; increasing the H pressure to 100 atm
led to an excellent GVL yield (91%) with a 9100 TON after 12 h
(Table 2, entries 1 and 2). On further reduction of the catalyst
loading to 0.002 mol%, hydrogenation of LA (50 mmol, 5.8 g)
[
14a]
five-coordinated Fe species.
with different amounts of base. In the case of 0.9 equiv KOH
where pH is ca. 6.0), no GVL was generated, being contrasts
with the observation of 1.0 equiv KOH dosage (where pH is ca.
4 and GVL yield is 90%). These results confirmed again that
We tested the reactions in water
-1
(
was still achieved with a high TON of 23000 and TOF of 1917 h ,
albeit in moderate yield (46%) (entry 4). Lowering the catalyst
dosage further (0.001 mol %) under the same conditions led to a
lower yield (10%) and TON of 10000 (entry 7). The catalytic
1
the five-coordinated iron active specie could only be generated
under basic condition.
The hydrogenation reaction of LA was further carried out over
activity of complex 3 with 0.002 mol % dosage was also tested
i
in H
2
O and PrOH, which gave GVL in 45% and 23% yields,
3
in MeOH under 50 atm H
2
at different temperatures, as shown
respectively (entries 6 and 7). The observed activity of 3 is
in Figure 2. To our surprise, the reaction proceeded even at
room temperature with 0.05 mol% Fe catalyst, giving GVL in
comparable to some reported precious metal catalysts, such as
-1 [6a]
PNP-pincer iridium complex (TON: 71000, TOF: 1480 h ),
-1 [5f]
7
1% yield in 5 h and 95% yield in 15 h, respectively. Increasing
the temperature to 150 °C, a drop in activity was observed
61%) probably because of the decomposition of iron complex.
The influence of H pressure of 0.05 mol% complex 3 at 100 °C
was also investigated (Figure 2). No transfer hydrogenation
reaction of LA was observed in the absence of H and only 5%
GVL was observed after 48 h under 1 atm H . In 5 h reaction
2
Ru/DPPB (TON: 12740, TOF: 7077 h ), and [Pd(DTBPE)Cl ]
-1 [7]
(TON: 2100, TOF: 2100 h ).
(
2
[
a]
Table 2. Hydrogenation of LA to GVL with 3 on larger scale.
2
LA
3
H
(atm)
2
Yield
(%)
TOF
(h )
2
Entry
TON
-1
(mmol)
(mol %)
time situation, the reduction of LA seemed to slightly depend on
the hydrogen pressure above 40 atm, and became independent
over 50 atm, which is consistent with previous report for
1
2
3
4
5
10
10
20
50
50
50
50
0.01
50
40
91
70
46
10
45
23
4000
333
0.01
100
100
100
100
100
100
9100
758
reduction of LA by using the combination of Ru(acac)
3
and
[
5d]
sulfonated-phosphine ligand.
In the case of a relative low H
2
0.005
0.002
0.001
0.002
0.002
14000
23000
10000
22500
11500
1166
1917
833
pressure (10 atm), 3-catalyzed hydrogenation of LA as a
o
function of time at 100 C was performed, from which 38%, 50%
and 96% yield of GVL were obtained in 2 h, 5 h and 15 h,
respectively. These results indicated that the catalyst can keep
[
b]
6
7
1875
958
Temeperature / °C
20
40
60
80
100
120
140
160
[c]
100
[
1
a] Reaction conditions: [LA] = 2.5 mol/L in MeOH, 1.0 equiv of KOH,
i
2
00 °C, 12 h. [b] [LA] = 2.5 mol/L in H O. [c] [LA] = 2.5 mol/L in PrOH.
8
6
4
2
0
0
0
0
0
When methyl levulinate (ML) is utilized instead of LA under
the similar reaction conditions, GVL could be formed by using
catalytic amount of KOH. At 50 atm
H
2
and 100 °C,
hydrogenation of ML with 0.05 mol% of 3 and 0.1 equiv of KOH
afforded GVL in 70% and 85% yield after 5 h and 12 h
respectively (Table 3, entries 1 and 2). In the case of low dosage
of 3 (0.002 mol%), a drop in GVL yield (11%) was observed,
while increasing KOH to 1.0 equiv resulted in GVL in 44% yield
0
10 20 30 40 50 60 70 80 90 100
H2 pressure / atm
-1
with TON as 22000 and TOF as 1833 h . Obviously, the
production of GVL from ML with complex 3 under hydrogen is
more efficient than previously reported iron catalysts under
2
Figure 2. The influences of temperature (blue line) and H pressure (red line)
for 3-catalyzed hydrogenation of LA during 5 h. (Reaction conditions: levulinic
acid (580 mg, 5 mmol), 3 (0.05 mol %), KOH (5 mmol), MeOH (2 mL); for
temperature line,
temperature is 100 °C).
[9]
transfer hydrogenation conditions or using formic acid as
2 2
H pressure is 50 atm; for H pressure line, reaction
[
8,10]
hydrogen source.
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ChemSusChem
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FULL PAPER
[
a]
[a]
Table 3. Hydrogenation of methyl levulinate into γ-valerolactone with 3.
Table 4. Dehydration of carbohydrates and subsequent reduction of
[b]
carbohydrate-derived LA with 3.
[
c]
[d]
ML
3
H
2
Time
(h)
Yield
(%)
TOF
(h )
Entry
Carbohydrate
fructose
glucose
LA yield (%)
GVL yield(%)
Entry
TON
-1
(mmol) (mol%) (atm)
1
2
3
4
5
50
47
45
31
24
48 (96)
1
2
3
4
5
0.05
50
12
5
85
70
81
45
67
11
44
1700
1400
8100
9000
13400
5500
22000
141
280
675
750
1116
458
1833
45 (96)
5
0.05
50
sucrose
43 (95)
10
25
25
50
50
0.01
100
100
100
100
100
12
12
12
12
12
starch
29 (94)
0.005
0.005
0.002
0.005
cellulose
23 (96)
[
b]
b]
5
6
7
[
a] Reaction conditions: carbohydrate (1.8 g), H
2
SO
4
aqueous (0.5 M, 10
mL), N
2
, 170 °C, 3 h. [b] Reaction conditions: filtrate from hydrolysis with
pH value as 14, complex 3 (0.05 mol%), 100 °C, 5 h. [c] Yields based on
carbohydrate. [d] Overall yield for the two-stage conversion of
carbohydrate. The yield of LA is given in parenthesis.
[
[
a] Reaction conditions: [ML] = 2.5 mol/L in MeOH, 100 °C. [b] 1.0 equiv of KOH
was used.
Since levulinic acid is available from carbohydrates via
Experimental Section
[
1]
hydrolysis, dehydration and deformylation, we then explored
the direct conversion of upstream substrates into γ-valerolactone
by using iron complex 3 (Table 4). The treatment of fructose with
All the reaction dealing with air or moisture-sensitive compounds were
performed by standard Schlenk techniques or in the nitrogen-filled
glovebox. The iron complexes 1-4 were prepared according to reported
H
2
SO
containing LA (50% yield).
with KOH, the LA could converted into GVL in the presence of
complex 3 (0.05 mol%) at 100 °C and 50 atm H in 5 h
4
in water at 170 °C led to an acidic aqueous solution
[
6b,21]
[
12-16]
After filtration and alkalization
literatures,
and were used as catalyst in the crystal form after
recrystallization. GC-MS analysis (Shimadu GCMS-QP2010SE)
equipped with a HP-5 MS (30 m × 0.25 mm × 0.25 μm) column. GC
analysis were obtained on a Shimadzu Model 2010 plus equipped with a
HP-5 column (30 m × 0.25 mm × 0.25 μm) using a flame ionization
detector (FID). HPLC analysis was conducted on an Agilent 1260
chromatography system equipped an Aminex HPX-87H column.
2
quantitatively (Table 4, entry 1). Glucose and sucrose were
compatible with the combined procedure, to afford GVL with
overall yields of 45% and 43%, respectively (entries 2-3). In the
case of polymeric carbohydrates, a drop in formation of levulinic
acid was observed, as seen in the case of starch (31%) and
cellulose (24%), while the activity of 3-catalyzed hydrogenation
of LA remained high (94% and 96%) (entries 4-5). These results
indicated that complex 3 could be used for the generation of
GVL from carbohydrate biomass resources with the addition of
hydrogen, without the need for separation of LA.
General procedure for hydrogenation of LA to GVL: In a glove-box,
levulinic acid (581 mg, 5 mmol), KOH (330.1 mg, 85 % purity, 5 mmol),
complex 3 (1.3 mg, 2.5 μmol) and MeOH (2 mL) were charged into a
steel autoclave. The autoclave was tightened and flushed with hydrogen
three times and finally charged with hydrogen at 50 atm. The reaction
mixture was stirred (600 rpm) at 100 °C for desired time. After reaction,
the autoclave was cooled to room temperature and the pressure was
released carefully. After acidification, N-methyl-pyrrolidinone (NMP) was
added as a standard. The identification and quantification of the products
was performed on GC-MS and GC.
Conclusions
In summary, we report that iron complexes having a pincer
PNP ligand can serve as an excellent catalyst for hydrogenation
of levulinic acid and methyl levulinate into γ-valerolactone,
where turnover numbers of approximately 23000 and turnover
frequencies of 1917 h were achieved. The catalyst also
showed high activity for formation of GVL from LA aqueous
solution generated from various biomass-derived carbohydrates.
This high performance of these iron-based non-precious
catalysts highlights the enormous potential for possible
applications in biomass conversion.
Dehydration of carbohydrates with
hydrogenation: Carbohydrate (1.8 g) and 0.5 M H SO (10 mL) were
H
2
SO
4
and subsequent
2
4
loaded in a Teflon-lined stainless steel autoclave, which was heated at
170 °C under N atmosphere for 3 h. After removal the insoluble fraction
2
-1
by filtration, KOH was added to the solution until pH value reached to 14.
The aqueous solution was then transferred to the 50 mL autoclave
reactor containing complex 3 (0.05 mol%, based on the yield of LA from
carbohydrate, which was determined by HPLC), which was then charged
with hydrogen at 50 atm and heated to 100 °C for 5 h. The GVL was
analyzed by using GC as described above.
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FULL PAPER
[
[
[
8]
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This work was supported by Fundamental Research Funds for
the Central Universities (No. 2017ZY25), National Natural
Science Foundation of China (No. 21776020) and National
Program for Thousand Young Talents of China.
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Keywords: levulinic acid • γ-valerolactone • iron • pincer •
hydrogenation
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ChemSusChem
10.1002/cssc.201800435
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Yuxuan Yi, Huiying Liu, Ling-Ping Xiao,
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Highly Efficient Hydrogenation of
Levulinic Acid into γ-Valerolactone
with an Iron Pincer Complex
A pincer iron complex can serve as an excellent catalyst for preparation of γ-
valerolactone via hydrogenation of levulinic acid and methyl levulinate, achieving up
to 23000 turnover numbers and 1917 h turnover frequencies.
-1
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