Oxidative decarboxylation of levulinic acid by silver(I)/persulfate

Article abstract of DOI:10.3390/molecules16032714

The oxidative decarboxylation of levulinic acid (LA) by silver(I)/persulfate [Ag(I)/S₂O₈^2-] has been investigated in this paper. The effects of buffer solution, initial pH value, time and temperature and dosages of Ag(I)/S₂O₈ ^2- on the decarboxylation of LA were examined in batch experiments and a reaction scheme was proposed on basis of the reaction process. The experimental results showed that a solution of NaOH-KH₂PO₄ was comparatively suitable for the LA decarboxylation reaction by silver(I)/persulfate. Under optimum conditions (temperature 160 °C, pH 5.0, and time 0.5 h), the rate of LA conversion in NaOH-KH₂PO₄ solutions with an initial concentration of 0.01 mol LA reached 70.2%, 2-butanone (methyl ethyl ketone) was the single product in the gas phase and the resulted molar yield reached 44.2%.

Full text of DOI:10.3390/molecules16032714

Molecules 2011, 16, 2714-2725; doi:10.3390/molecules16032714
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Oxidative Decarboxylation of Levulinic Acid by
Silver(I)/Persulfate
Yan Gong and Lu Lin *
Department of Resources Science and Engineering, State Key Lab of Pulp and Paper Engineering,
South China University of Technology, Guangzhou 510640, China
* Author to whom correspondence should be addressed; E-Mail: lclulin@scut.edu.cn.
Received: 3 December 2010; in revised form: 11 March 2011 / Accepted: 22 March 2011 /
Published: 23 March 2011
Abstract: The oxidative decarboxylation of levulinic acid (LA) by silver(I)/persulfate
2−
[Ag(I)/S2O8 ] has been investigated in this paper. The effects of buffer solution, initial pH
2−
value, time and temperature and dosages of Ag(I)/S2O8 on the decarboxylation of LA
were examined in batch experiments and a reaction scheme was proposed on basis of the
reaction process. The experimental results showed that a solution of NaOH-KH2PO4 was
comparatively suitable for the LA decarboxylation reaction by silver(I)/persulfate. Under
optimum conditions (temperature 160 °C, pH 5.0, and time 0.5 h), the rate of LA
conversion in NaOH-KH2PO4 solutions with an initial concentration of 0.01 mol LA
reached 70.2%, 2-butanone (methyl ethyl ketone) was the single product in the gas phase
and the resulted molar yield reached 44.2%.
Keywords: oxidative decarboxylation; levulinic acid; silver nitrate; potassium persulfate
1. Introduction
Levulinic acid (4-oxo-pentanoic acid), results from a series of biomass hydrolysis reactions [1,2].
With its wide application profile it is an important chemical platform obtainable from renewable
lignocelluloses [3-5] because of its molecular structure that contains a carbonyl group and a carboxyl
group. Hydrogenation of levulinic acid, can produce γ-valerolactone [6], a potentially useful polyester
monomer (as γ-hydroxyvaleric acid); 1,4-pentanediol, also of value in polyester production;

Molecules 2011, 16
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methyltetrahydrofuran, a valuable solvent or a gasoline blending component; and diphenolic acid with
potential for use in polycarbonate production [7] are other significant derivatives.
Decarboxylation of organic molecules happens by the removal of a carboxyl group from the
carboxylic acids, further replaced by a hydrogen atom. Up to date, oxidative decarboxylation with
various agents have been repeatedly reported as the most common pathway for the decarboxylation of
carboxylic acids and their derivatives, and a variety of oxidants involved in these reactions have been
extensively studied, such as porphyrin combined with manganese (Ш) [8], Schiff base complexes
combined with manganese (III) [9], active hydroxyl radicals (•OH) [10], peroxyl radicals [11], and
high-valent metal ions [12-14]. As for decarboxylation of LA, to the best of our knowledge until now
only two reports exist; one is for the photoelectron-chemical decarboxylation of LA [15], leading to
2-butanone, propionic acid, acetic acid, acetone and acetaldehyde as major products, but the yield of
butanone was not satisfactory. Another is for oxidative decarboxylation of LA catalyzed by cupric oxide
(CuO) [16] which came from our laboratory. As for 2-butanone, it is a product of LA decarboxylation,
which can be directly hydrogenated to butanol [17,18] or be converted to ethyl acetate through
Baeyer-Villiger oxidation [19] and further reduced to ethanol. Therefore, decarboxylation of levulinic
acid to produce butanone and its consequent product of ethanol could play an important role to meet
future energy needs and solve environmental problems caused by fossil fuels.
In the previous report of our laboratory on oxidative decarboxylation of LA [16], it was found that
the yield of butanone can reach 67% (mol ratio), and acetic acid and acetone were also produced as
side products. However, the reaction conditions used were extremely drastic (temperature was over
2−
300 °C). In this paper, a novel pathway for decarboxylation of LA by Ag(I)/S2O8 under mild
2−
conditions was researched. Persulfate anion (S2O8 ) is a strong oxidizing agent with a redox potential
(Eo) of 2.01 V and its reduction results in the production of sulfate anions [20]. It is used in many
fields such as oxidation of trichloroethylene (TCE) from groundwater contaminants [21], degradation
of organic pollutants in wastewater [22]; oxidation–dealkylation of 3,4-dihydropyrimidin-2(1H)-ones
in the presence of Co(NO3)2 [23]. Persulfate anion is also applied in the decarboxylation of carboxylic
acids with or without various transition metal catalysts [24-27], et al. Therefore, utilization of
persulfate anion for the decarboxylation of LA under milder conditions could open a new venue for
prospective application of this reaction and its products in the future.
2. Results and Discussion
2.1. Decarboxylation of Levulinic Acid to 2-Butanone in Different Buffer Solutions
2−
The experiments of LA decarboxylation by Ag(I)/ S2O8 were carried out at 100 °C for 30 min in
an oil bath. From Figure 1A, it could be found that in NaOH-KH2PO4 solution, as pH values increased,
both the rate of LA conversion and yield of butanone increased and reached about 30% and 43%,
respectively, at pH 5.0, but then they decreased with further pH value increases. In C6H8O7-Na2HPO4
solution, as shown in Figure 1B, the rate of LA conversion and the yield of butanone went up with the
pH values ranging from 2.1 to 6.0. Besides 2-butanone, acetone was also produced. At pH 2.1 and 3.9,
the yields of acetone were higher than the rate of LA conversions. It was suggested that citric acid in
the solution reacted with Ag(I)/ S2O82− and caused the production of acetone. Moreover, it was shown

Molecules 2011, 16
2716
in Figure 1C that the only product in the gas phase was acetone. These results indicated that
2−
NaOH-KH2PO4 solution was comparatively suitable for LA decarboxylation by Ag(I)/ S2O8 to
produce butanone.
Figure 1. Effect of initial pH on the decarboxylation of levulinic acid in different buffer
solutions (A: NaOH-KH2PO4; B: C6H8O7-Na2HPO4; C: C6H8O7-Na3C6H5O7; temperature:
100 °C; time: 0.5 h).
45
40
35
30
25
20
15
80
70
60
50
40
30
20
NaOH-KH PO
Butanone
LA
2
4
A
3
4
5
6
7
8
9
10 11
Initial pH
50
40
30
20
10
0
C H O -Na HPO
6 8 7
2
4
80
60
40
20
0
Butanone
Acetone
LA
B
2
3
4
5
6
Initial pH
30
25
20
15
30
25
20
15
C H O -Na C H O
6 8 7 3 6 5 7
C
Acetone
LA
3.2
4.0
4.8
Initial pH

Molecules 2011, 16
2717
2.2. Decarboxylation of Levulinic Acid to 2-Butanone under Different Times and Temperatures
Based on the above results, the effect of reaction time at different temperatures on LA
2−
decarboxylation using Ag(I)/S2O8 in NaOH-KH2PO4 solution was further investigated. The
experiments were carried out at lower temperatures (≤100 °C) in an oil bath. The results obtained from
the experiments are shown in Figure 2A. It was found that the yields of butanone all increased rapidly
within 15 min. After that, the yield of butanone at 45 °C still rose slightly, while it decreased at 60 °C,
80 °C and 100 °C. The highest yield of butanone was 35.2% at 100 °C for 15 min. However, the rate of
LA conversions increased during the 3 h but the ascending trends were diminished after 1 h
(Figure 2B).It was claimed that butanone from the LA decarboxylation also involved in the oxidation
with Ag(I)/S2O82− and was disjoined and converted to other materials with smaller molecule. Besides,
the oxidation of butanone to a certain extent inhibited the decarboxylation of LA.
Figure 2. Effect of time on decarboxylation of levulinic acid to 2-butanone {[A] butanone
yield (≤100 °C); [B] LA conversion(≤100 °C); [C] butanone yield (≥100 °C); [D] LA
conversion (≥100 °C)}.
A
B
35
30
25
20
15
10
70
60
50
40
30
45℃
45℃
60℃
80℃
100℃
60℃
80℃
100℃
0
1
2
3
0
1
2
3
Time (h)
Time (h)
80
45
40
35
30
D
C
70
60
50
120℃
140℃
160℃
120℃
140℃
160℃
Time (h)2
0
1
2
3
0
1
3
Time (h)

Molecules 2011, 16
2718
When reactions were carried out in the pressure reactor at the temperature of 120 °C, 140 °C and
160 °C, it is shown in Figures 2C and 2D that the yields of butanone were apparently improved within
0.5 h and reached 36.2%, 42.8% and 44.2%, respectively. With the increase of time, the yields of
butanone all had a mild decline. The rate of LA conversions went up sharply in 0.5 h and then rose
slowly for the residual reaction time, but LA was not converted completed finally after 3 h. Compared
with the effectiveness in oil bath, it was better in autoclave, that is, the yields of butanone resulted
from LA decarboxylation in autoclave were higher, at the same time, optimum time was prolonged to
0.5 h. We proposed that butanone vaporized at higher temperatures and was separated from the react
2−
solution, which reduced the contact between butanone and Ag(I)/ S2O8 .
The effect of temperature from 45 °C to 180 °C in a time of 0.5 h on LA decarboxylation by
2−
Ag(I)/S2O8 is shown in Figure 3. The yields of butanone and the rate of LA conversions were
gradually improved with the enhancement of temperature. The yield of butanone reached about 43% at
140 °C and then remained stable as temperature rose, while the rate of LA conversion reached 67.2%
at 100 °C and had no distinct change when higher temperatures were applied. At the temperature of
100 °C, the experiments were performed both in oil bath and autoclave and the yields of butanone
were 29.9% and 32.6%, respectively. It is presumed that this resulted from the fact that the autoclave is
a close system and therefore butanone loss was reduced and reaction efficiency was comparatively
higher. Moreover, mechanical agitation in autoclave led to better mass and heat transfer for the
reaction, which diminished side effects.
Figure 3. Effect of temperature to butanone yield (NaOH-KH2PO4; pH: 5.0; time: 0.5 h).
50
80
45
70
40
60
35
50
40
30
Butanone (in autoclave )
25
-●- Butanone (in oil bath)
30
-◇- LA
20
20
40
60
80 100 120 140 160 180
Temperature (℃)
2.3. Decarboxylation of Levulinic Acid to 2-Butanone under Different Ratios of Substrate and Oxidant
2−
As seen in Table 1, when LA was oxidatively decarboxylated by S2O8 without Ag(I) under the
same conditions, the rate of LA conversion and the yield of butanone were similar with those from LA
2−
decarboxylation by Ag(I)/S2O8 .
2−
In contrast, reaction of Ag(I) with LA without S2O8 led to production of butanone at a low
efficiency (LA conversion 12.9%; butanone yield 1.2%). The XRD analyses of the final residues are
shown in Figure 4, where Figures 4A and Figure 4B are the XRD spectra of silver sulfate (Ag2SO4)

Molecules 2011, 16
2719
and silver phosphate (Ag3PO4), respectively, which corresponded to the residues from the
decarboxylation of LA by Ag(I)/S2O82− and Ag(I). The ratios of LA with Ag(I)/S2O82− ranging from 0.5
to 1.5 were investigated, indicating that the efficiency of decarboxylation went up as the ratio was
increased and it was relatively lower at the ratio of 0.5, but less change was found at other ratios.
Table 1. Effect of ratio of substrate and oxidant on the yield of butanone and the rate of LA
conversion (temperature: 120 °C; time: 0.5 h; pH: 5.0).
Metal ion catalysts
Yield of butanone (%)
The Rate of LA conversion (%)
Ag(I)
S2O8
0.5:1 Ag(I)/S2O8
1:1 Ag(I)/S2O8
1.2:1 Ag(I)/S2O8
1.5:1 Ag(I)/S2O8
1.2
12.9
69.3
34.8
69
70
72
2-
35.9
28.8
36.2
36.4
36.7
2-
2-
2-
2-
2−
Figure 4. The X-ray diffraction spectra of the reaction residues [A: Ag(I)/S2O8 ; B: Ag(I)].
6000
●
Ag2SO
Ag PO
A
B
●
*
3
4
4
*
8000
6000
4000
2000
0
4500
3000
1500
0
●
●
*
●
●
●
*
*
*
●
●
*
20
30
40
50
20
30
40
50
2θ(°)
2θ(°)
2.4. Decarboxylation of Levulinic Acid to 2-Butanone under Different Metal Ion Catalysts
2−
Decarboxylation of LA by other metal ions combined with S2O8 was studied further. The
experiments were carried out in a DF-101S oil bath at 100 °C for 0.5 h. It is shown in Table 2 that
2−
Ag(I)/S2O8 was the most effective agent for LA decarboxylation to butanone, followed by
2−
2−
2−
Co(II)/S2O8 . The yields of butanone were lower with Fe(II)/S2O8 and Mn(II)/S2O8 . However,
2−
when LA reacted with Cu(I)/S2O8 , it was shown from the results HS-GC-MS analysis that methyl
vinyl ketone was produced as the main gaseous product (about 10% by HS-GC), other products
included 2,3-butanedione and 2-chloro-1-buten-3-one. Almost no product could be detected by HS-GC
2−
in the reaction system of LA and Cu(II)/S2O8 .

Molecules 2011, 16
2720
Table 2. Effect of metal ions on the yield of butanone and the rate of LA conversion in
S2O82− reaction system (temperature: 100 °C; time: 0.5 h; pH: 5.0).
Metal ion catalysts
Yield of butanone /%
Rate of LA conversion/%
2-
Ag(I) a /S2O8
35.2
6.63
—
—
1.1
5.91
67.2
20.73
25.42
20.36
92.6
2-
Co(II) b / S2O8
2-
Cu(I) c / S2O8
2-
Cu(II) d / S2O8
2-
Fe(II) e / S2O8
2-
Mn(II) f / S2O8
77.42
a AgNO3; b Co(NO3)2; c CuCl; d CuCl2; e FeCl2; f MnCl2
2−
2.5. Reaction scheme for LA decarboxylation by Ag(I)/S2O8
2-
A mechanism for the oxidative decarboxylation of aliphatic carboxylic acids by the Ag(I)/S2O8
system has been described by Anderson et al. [28]. According to the HS-GC/GC-MS experimental
analysis shown in Figures 5A and 6 and the ion chromatography data in Figure 5B, LA undergoes
oxidative decarboxylation by Ag(II) ion which can be produced by persulfate oxidation of Ag(I) as
shown in equations (1–5). In this scheme, the oxidation of Ag(I) by persulfate (eq. 1) is followed in
quick succession by a second oxidation by the sulfate ion radical (eq. 2), in accordance with the kinetic
study of the persulfate oxidation demonstrated by Miller [29]. Direct and facile oxidation of LA by
Ag(II) in a fast follow-up step could bring about an anion of CH3COCH2CH2CO2⋅, which could be
subsequently fragmented to an CH3COCH2CH2⋅ and CO2. Butanone was derived by oxidation of the
CH3COCH2CH2• with silver species as well as by hydrogen transfer to solvent (eqs. 3–5). It was said
that the activation process for Ag(I)-catalyzed oxidative decarboxylation by persulfate was largely
associated with the formation of Ag(II) species [28], but it appears from Table 1 that the yield of
butanone from S2O82− oxidation is 35.9%, near to 36.2% which comes from Ag(I)/S2O82− oxidation. It
2−
is reasonable to assume that S2O8 can oxidize LA independently. We propose the mechanism of
−
S2O82− reacting with LA without Ag(I) in eqs. (6–7) that sulfate ion radical (SO4 ·) is from the thermal
2−
decomposition of S2O8 , and it can directly oxidize LA.
2-
-
2-
Ag(II)
+
+
(1)
(2)
(3)
(4)
(5)
Ag(I) +
S2O8
SO4
+
SO4
2-
-
SO4
Ag(II)
+
SO4
Ag(I)
+
+
+
H
Ag(I) CH3COCH2CH2COO
CH3COCH2CH2
+
CH3COCH2CH2COOH
Ag(II)
+
CH3COCH2CH2COO
CO2
CH3COCH2CH2
+
+
CH3COCH2CH3
H-Solv
Solv
Δ
2-
-
S2O8
SO
2
(6)
(7)
4
-
2-
+
+
+
H
CH3COCH2CH2COO
+
SO4
SO4
CH3COCH2CH2COOH
2−
Selective oxidation of arylacetic acids in the presence of Cu(II)/S2O8 oxidation system has been
reported [30] and the corresponding benzylacetates can be generated. It is conjectured that
Cu(II)/S2O82− may oxidize LA to a kind of ester. The identification of methyl vinyl ketone arising from

Molecules 2011, 16
2721
LA decarboxylation by Cu(I)/S2O82− oxidation indicated that eqs. (8–10) were operative. Oxidation of
LA to alkene can also occur since the second-order rate constant of 108–109 M−1 sec−1 for reaction (11)
has been assigned according to the report by Anderson [28].
2-
-
2-
Cu(II)
+
Cu(I) +
S2O8
SO4
+
(8)
(9)
SO4
-
2-
+
CH3COCH2CH2COO
CH3COCH2CH2COO
SO4
CH3COCH2CH2COO
+
SO4
(10)
+
CH3COCH2CH2
CO2
+
H+
(11)
(12)
CH3COCH2CH2
+
Cu(I)
+ Cu(II)
CH3COCHCH2
2CH3COCH2CH2
CH3COCH2CH2CH2CH2COCH3
Figure 5. Headspace - gas chromatogram (A) and Ion Chromatogram (B) of samples.
LA
B
A
Butanone
Acetone
Acetic acid
2
4
6
8
2
3
4
5
Time(min)
R.Time (min)
Figure 6. Mass spectrograms of butanone and acetone.
700000
43
43
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
600000
Butanone
Acetone
500000
400000
300000
72
58
200000
100000
57
39
50
41
37
96 107
129
60
64
79
86
36
115
53
55
45
0
20 30 40 50 60 70 80 90 100 110 120 130
34 36 38 40 42 44 46 48 50 52 54 56 58 60 62
M
M
Benzylic radicals can be produced from the oxidative decarboxylation of arylacetic acids such as
phenylacetic acid and dimerized further to 1,2-diarylethanes [31], thus, it is deduced that alkyl radical
(CH3COCH2CH2) can dimerize to 2,7- dioctanone (eq. 12) in the liquid phase. However, further

Molecules 2011, 16
2722
studies for a delicate exploration of this reaction procedure are required.
3. Experimental
3.1. Procedure for Decarboxylation of Levulinic Acid
Batch experiments were performed in a DF-101S oil bath (Yu Hua instrument), and a PARR-5050
Pressure Reactor (100 mL, Parr Instrument). Reaction solutions were obtained by adding levulinic acid
(0.01 mol) into a buffer solution and then silver nitrate (0.01 mol) and potassium persulfate (0.005 mol)
were added to the prepared reaction mixtures. After reaction, the samples were taken for HS-GC and
ion chromatography detection. The final residues were analyzed by XRD after washing and
desiccation. All experiments were repeated several times and shown to be reproducible.
3.2. Headspace – gas chromatography detection
Headspace - gas chromatography (HS-GC, HSS 86.50, QP2010, Shimadzu Japan) was used
extensively in the separation and identification of mixtures of volatile compounds. The volatile
products of LA decarboxylation were analyzed by HS-GC using a DB-5 column (30 m × 0.25 mm i.d.
× 0.25 μm film) at a column temperature of 120 °C, a flow rate of 4.7 mL/min and with a flame
ionization detector at the temperature of 220 °C. The analysis was performed at 60 °C for 5-min. The
component concentrations were calculated by the area normalization method. The sample vials was
shaken for 20 min at the temperature of 75 °C. The temperatures of sampling probe and tube were
85 °C and 95 °C; respectively. Automatic sampling was employed. The headspace - gas chromatogram
of volatile products in samples (from C6H8O7-Na2HPO4 solution) was shown in Figure 5A that acetone
and butanone were the gas products and the retention times were about 3.26 min and 3.63 min,
respectively.
3.3. Ion chromatography detection
Ion chromatography (ICS-3000, DIONEX USA) was used for the analysis of levulinic acid in the
aqueous phase to figure out the levulinic acid conversion. Liquid reactant after the reactions was
filtered and diluted before testing. The analysis was conducted in an anion-exchange column (AS11-HC)
with 2 mol/L NaOH leacheate in a conductivity cell of 35 °C. The flow rate was 1.0 mL/min and the
sample size was 50 μL. The calibration was performed using aqueous solutions. Ion Chromatogram of
samples was shown in Figure 5B that the retention time of levulinic acid was 4.06 min.
3.4. X-ray diffraction analysis
The precipitate after reactions was characterized by X-ray diffraction (XRD) (D/max-IIIA, Rigaku,
Japan) (40 kV, 40 mA) using the standard reflection mode with Cu/Ka (λ = 0.154056 nm) in the angle
30° < 2θ < 90° and step angle was 0.02°.

Molecules 2011, 16
2723
4. Conclusions
2−
It was shown that LA could be oxidatively decarboxylated by S2O8 with or without Ag(I) and
leading to 2-butanone as the main gas product. When Ag(I) was replaced with Cu(I), the main product
changed to methyl vinyl ketone. The conclusions from the reaction process are as follows: 2-butanone
can be produced from biomass-derived LA, which is a matter of considerable interest for fuel sources
and environmental problems; when the reaction was performed in NaOH-KH2PO4 solution with a pH
value of 5.0 at a temperature of 160 °C and lasted for 0.5 h, the yield of butanone and the rate of LA
conversion could reach 44.2% and 70.2%, respectively. Although the yield is lower than that obtained
with CuO oxidative decarboxylation of LA (yield 67%), the reaction conditions are milder with shorter
react time.
Acknowledgements
The authors are grateful to the financial support from Natural Science Foundation of China
(50776035, U0733001), National Key R&D Program (2007BAD34B01) and National Key Basic
Research Program (2010CB732201) from the Ministry of Science and Technology of China.
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Sample Availability: Samples of the compounds are available from the authors.
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