6
A.S. Amarasekara, M.A. Hasan / Catalysis Communications 60 (2015) 5–7
Table 1
Product yields and reaction conditions used in the Pd/C catalyzed conversion of levulinic
acid (1) to γ-valerolactone (2) using alcohol as a hydrogen donor under microwave and
thermal conditions.
2.00 mmol of levulinic acid, and 4.00 mL of solvent.
Entry Alcohol/
solvent
Base
Catalyst (mg)
Heating
GVL %
yield
(equivalents) per mmol of LAa method
Fig. 1. Pd/C catalyzed reduction of levulinic acid (1) to γ-valerolactone (2) using alcohol as
a hydrogen donor under microwave conditions.
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
KOH (2.0)
KOH (2.0)
–
10
10
10
10
10
10
10
10
Oven, 160 °C (6 h)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
MW (50 s)
0
86
0
2
are available at the supplier's website. Levulinic acid (99.9%), NaOH and
KOH were also purchased from Aldrich Chemical Co. Oven heating
experiments were carried out in 25 mL stainless steel solvothermal
reaction kettles with Teflon inner sleeves, purchased from Lonsino Med-
ical Products Co. Ltd., Jingsu, China. These reaction kettles were heated
in a preheated Cole-Parmer WU-52402-91 microprocessor controlled
convention oven with 1 °C accuracy. Microwave samples were heated
using 920 W, GE model JE635WW microwave oven at full power in 10 s
pulses giving 20 s cooling time between the pulses. 1H and 13C NMR
spectra were recorded in CDCl3 on a Varian Mercury plus spectrometer
operating at 400 and 100 MHz respectively. The mass spectra were re-
corded using Varian Saturn 2100 T GC–MS.
KOH (1.0)
KOH (1.5)
KOH (1.75)
KOH (3.0)
NaOH (2.0)
8
1
47
72
85
82
0
32
83
65
71
66
1
2
2
2
9
Na2CO3 (2.0) 10
KOH (2.0)
10
11
12
13
14
10
10
10
10
5
1
2
1
2
2
iso-PrOH KOH (2.0)
n-BuOH KOH (2.0)
sec-BuOH KOH (2.0)
EtOH KOH (2.0)
a
5% Pd/C from Sigma-Aldrich.
acid. In the first experiment, LA with two equivalents of KOH in ethanol
was heated for 6 h at 160 °C, in the presence of Pd/C catalyst in a high
pressure reactor with a Teflon inner surface as shown in entry 1 of
Table 1. The NMR and GC–MS analysis of the reaction after a work up
showed no reaction under thermal conditions and unreacted levulinic
acid could be recovered. However, a similar reaction mixture under mi-
crowave heating for 50 s (five 10 s MW exposures with 20s cooling in-
tervals between pulses) produced γ-valerolactone as the only isolable
product in 86 2% yield as shown in entry 2 of Table 1. No other prod-
ucts were detected beside GVL in the 1H NMR spectrum of the crude
product. This product was identified by comparison of 1H, 13C NMR
and mass spectra with published data [22]. In the next step we have
studied the role of KOH by performing five separate reactions with no
KOH, 1.0, 1.5, 1.75 and 3.0 equivalents of KOH (entries 3–7). The
reaction with no KOH failed to produce any products; all levulinic acid
was recovered (entry 3). With one equivalent, only a 8 1% yield of
GVL was obtained (entry 4). The experiments using 1.5 and 1.75 equiv-
alents of KOH produced 47 1 and 72 2% yields of GVL respectively
(entries 5 and 6). Whereas the three equivalent reactions (entry 7)
produced a yield similar to 2.0 equivalent reaction (entry 2), showing
that at least 2.0 equivalents of KOH are needed and excess KOH has no
particular advantage in the reaction.
2.2. General procedure for conversion of levulinic acid to γ-valerolactone
A mixture of levulinic acid (232 mg, 2.00 mmol), KOH/NaOH (4.00
mmol), and 20 mg of 5% Pd/C in 4.00 mL of ethanol or iso-propanol in
a closed 20 mL glass vial was subjected to microwaves for 50 s (five
10 s pulses with 20 s cooling time between pulses). Then the contents
were allowed to cool, diluted with 10 mL of water, neutralized with
1 M aq. HCl, centrifuged at 1700 ×g for 5 min to remove the catalyst,
supernatant was extracted to methylene chloride (4 × 10 mL) and
concentrated to give a colorless oily product, which was identified as
γ-valerolactone with at least 99% purity by comparison of 1H, 13C
NMR and mass spectra with published data [22].
2.3. Recovery and reuse of 5% Pd/C catalyst
In the recycling experiments 5% Pd/C catalyst from entry 2 (Table 1)
experiment was separated by centrifuging at 1700 ×g for 5 min, washed
with water (2 × 10 mL), dried in an oven at 90 °C for 3 h, and reused in
the first reuse experiment. The same recycling procedure was repeated
in the subsequent experiments and the GVL yields form reuse experi-
ments are shown in Table 2.
The substitution of KOH with NaOH and Na2CO3 is shown in entries 8
and 9. These two experiments showed that NaOH gives a yield compa-
rable to KOH, whereas Na2CO3 failed to produce any GVL (entries 8 and
9). In order to prove the general applicability of alcohols as hydrogen
donors in the Pd/C catalyzed transformation, we have tested methanol,
iso-propanol, n-butanol and sec-butyl alcohols as shown in entries
10–13. The reduction potentials of reducing alcohols are in the order:
iso-propanol ≈ sec-butanol b n-butanol b ethanol b methanol [16].
Methanol has the highest reduction potential, which resulted in as a
3. Results and discussion
As pointed out earlier, our initial aim was to use Pd/C as a catalyst for
an α-alkylation of levulinic acid to raise the carbon number of levulinic
Table 2
Product yields in reusing Pd/C catalyst. 2.0 mmol LA,
4.0 mmol KOH, 4.0 mL EtOH, 20 mg 5% Pd/C used. Pd/C
catalyst from previous experiment was separated by
centrifuging at 1700 ×g for 5 min., washed, dried and reused
in the next experiment.
Catalytic cycle
GVL % yield
1
2
3
4
5
86
83
82
84
82
2
2
2
2
2
Fig. 2. Reaction pathway for the conversion of levulinic acid (1) to γ-valerolactone (2)
using ethanol as the hydrogen source and Pd/C as the catalyst under microwave heating.