Conversion of glycerin into lactic acid by alkaline hydrothermal reaction

Article abstract of DOI:10.1246/cl.2005.1560

Hydrothermal decomposition experiments of glycerin with an alkali showed that glycerin could be converted into lactic acid with a high yield of about 9.0 mol % based on glycerin used. Discussion on the pathway for the conversion of glycerin to lactic acid suggests that glycerin is first decomposed to pyruvaldehyde with elimination of hydrogen by a H^- shift to the adjacent hydrogen. Pyruvaldehyde formed is then converted into lactic acid ion by the benzilic acid rearrangement. Copyright

Full text of DOI:10.1246/cl.2005.1560

1560
Chemistry Letters Vol.34, No.11 (2005)
Conversion of Glycerin into Lactic Acid by Alkaline Hydrothermal Reaction
Hisanori Kishida,ꢀ Fangming Jin,y Zhouyu Zhou,y Takehiko Moriya,yy and Heiji Enomotoy
Environmental Systems Headquarters, Environmental Research and Development Center Hitachi Zosen Corporation, Kyoto 625-8501
yDeparment of Environmental Science and Technology, Graduate School of Environmental Studies,
Tohoku University, Sendai 980-8579
yyResearch and Development Center, Tohoku Electric Power Co., Inc., Sendai 981-0952
(Received August 30, 2005; CL-051115)
Hydrothermal decomposition experiments of glycerin with
an alkali showed that glycerin could be converted into lactic acid
with a high yield of about 90 mol % based on glycerin used. Dis-
cussion on the pathway for the conversion of glycerin to lactic
acid suggests that glycerin is first decomposed to pyruvaldehyde
with elimination of hydrogen by a Hꢁ shift to the adjacent
hydrogen. Pyruvaldehyde formed is then converted into lactic
acid ion by the benzilic acid rearrangement.
was analyzed by high-performance liquid chromatography
(HPLC). Two Shodex KC811 columns were used in series.
The solvent was 1 mM HClO4 with a flow rate of 1.0 mL/min.
The yield of product is defined as a molar fraction of product
referring to the initial glycerin.
First, reaction products from glycerin were identified by
HPLC analysis. Figure 1 shows a result for liquid samples after
reaction of glycerin at a temperature of 300 ꢂC, for a reaction
time of 60 min with 0.25 M NaOH. The main product was lactic
acid. In addition, small amounts of pyruvaldehyde, acetic acid,
and formic acid were detected. Lactic acid has recently been get-
ting attention as a biodegradable lactic acid polymer material
with limited environmental impact.
So, further experiments were performed to optimize the re-
action conditions by varying the reaction temperature and time,
and the concentration of NaOH. Figure 2 gives the yield of lactic
acid. As seen in Figure 2, increasing the concentration of NaOH
led to a great increase in the yield of lactic acid with a very high
lactic acid yield of 90 mol % at 300 ꢂC with a NaOH concentra-
tion of 1.25 M. It should be noted that almost no lactic acid was
formed without addition of NaOH and the yield of lactic acid at
260 ꢂC was low even with 1.25 M NaOH. These results show that
lactic acid can be produced in a large quantity, and both the tem-
perature and the concentration of NaOH have a great effect on
the conversion of glycerin into lactic acid. A possible reason that
a high concentration of NaOH is required to obtain a high yield
of lactic acid is likely that the formed lactic acid can consume
NaOH in neutralization.
In recent years, bio-diesel fuel (BDF) derived from the
transesterification of oil and fat with alcohols in the presence
of an alkaline catalyst has been gathering attention, because
BDF has a good potential as an alternative diesel fuel. It is re-
ported that 20 million tons of palm oil is produced per year in
Southeast Asia, and it is expected that a portion of this palm
oil can be converted into BDF. As the production of BDF in-
creases, an effective and acceptable conversion of glycerin,
which is a by-product of the BDF manufacturing process, to use-
ful substances is becoming increasingly important. It has been
reported that glycerin can be converted easily into acrolein when
treated in a hydrothermal reaction with an acid catalyst.1,2 How-
ever, few studies have been reported about the hydrothermal
conversion of glycerin with an alkali. Since the glycerin of the
by-product from BDF production process contains a large
amount of alkali, a study on the hydrothermal conversion of
glycerin with an alkali was performed. In this paper, the hydro-
thermal conversion of glycerin with an alkali catalyst is dis-
cussed.
The reactor used in this study was a Swagelok SUS 316
tube, sealed with two caps. The cap is equipped with a thin
nozzle and a pressure valve in order to collect gas samples.
The outside diameter of the reactor was 12.7 mm, wall thickness
1 mm, length 111 mm, and capacity 10 mL. The reaction temper-
ature was controlled using a molten salt bath. First, the desired
amount of glycerin and sodium hydroxide (NaOH) solution were
put into the reactor, and then sealed. Concentrations of glycerin
and NaOH solution were 0.33 M (mol/L) and 0.25–1.25 M, re-
spectively. The solution put into the reactor was 5 mL in volume.
Then, the reactor was put into a salt bath that had been preheated
to the desired temperature, and was kept shaking for a desired
reaction time. All experiments were performed with degassed
water and by purging the reactor with nitrogen. After the reac-
tion, the reactor was quickly cooled down in cold water. After
cooling, liquid and gas samples were collected for analyses.
The liquid sample was filtered (to remove solids) through a
0.5 mm filter, and then the pH of the solution was adjusted to
6–7 with sulfuric acid. The gas sample was analyzed with a
gas chromatography with a POLA column. The liquid sample
Next, the reaction pathway of the conversion of glycerin into
lactic acid is discussed. As shown in Figure 1, a little amount of
pyruvaldehyde was detected. It is generally known in sugar
chemistry that pyruvaldehyde readily undergoes the benzilic
0.16
Lactic acid
0.12
Pyruvaldehyde
Pyruvic aid
Formic acid
0.08
0.04
0
Acetic acid
Acrylic acid
solvent
0
10
20
30
Retention time /min
Figure 1. HPLC chromatogram of solution after reaction at
300 ꢂC temperature, 60 min reaction time, with 0.25 M NaOH
(Detector UV-210 nm). Retention.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.11 (2005)
1561
100%
80%
60%
40%
20%
0%
100%
80%
60%
Lactic acid
Hydrogen
Glycerin
300 °C_1.25 M
260 °C_1.25 M
300 °C_0.25 M
300 °C_0 M
40%
20%
0%
0
20
40
60
80
100
Reaction time /min
0
30
60
90
120
150
Reaction time /min
Figure 4. Yields of hydrogen and lactic acid and glycerin by an
alkaline hydrothermal treatment of glycerin. (Temperature;
300 ꢂC, NaOH; 1.25 M).
Figure 2. Yield of lactic acid by an alkaline hydrothermal treat-
ment of glycerin. (NaOH; 0–1.25 M) based on the glycerin used.
First of all, glycerin may produce glycerin alkoxide because
the acidity constant of glycerin (pKa of glycerin is 14.4)4 is sim-
ilar to that of water. Then, glycerin alkoxide formally lose a Hꢁ
to yield glyceraldehyde, which is then dehydrated in E2 mecha-
nism via ꢀ-hydrogen abstraction by the Hꢁ followed by OHꢁ
elimination. The ꢀ-hydrogen abstraction by the Hꢁ may be rapid
due to the high acidity of ꢀ-hydrogen. As the result, glycerin
eliminates H2 and water to give 2-hydroxypropenal which has
a conjugated double bond and is thermodynamically stable,
which could be the reason why this reaction shifts toward the
formation of 2-hydroxypropenal. Pyruvaldehyde is formed by
keto–enol tautomerization of 2-hydroxypropenal.
Glycerin
H
Glyceraldehyde
H
H
H
H−
O
O
H
OH
OH
OH
H
H
H
O
OH
H
H
OH
OH
H
H
OH
OH
H
H
H
H
+
OH−
H
+
+
+
H2O
H2O
H2O + OH− + H2 ↑
Elimination of H2
According to the mechanism proposed in Figure 3, H2
would be formed in a hydrothermal reaction of glycerin with
an alkali and one mole of glycerin would generate one mole of
hydrogen and one mole of lactic acid as follows:
2-Hydroxypropenal
Pyruvaldehyde
H
H
Lactic acid ion
O
O
H
H
O
O
O
OH
OH
H
H
H
H
C H O þ NaOH ! C H O Na þ H " þ H O
ð1Þ
3ꢃ
3
8
3
3
5
2
2
H
+
H
+
H
+
To test this, gas composition was analyzed for reactions at
300 ꢂC with 1.25 M NaOH, by varying the reaction time from
5 to 90 min. As shown in Figure 4, a large amount of hydrogen
was formed, and the yield was almost the same as that of lactic
acid formed. The slightly lower yields for lactic acid than those
for hydrogen after 40 min may be caused by decomposition of
lactic acid. These results strongly support the formation mecha-
nism of lactic acid from glycerin shown in Figure 3.
H2O + OH− + H2 ↑
H2O + OH− + H2 ↑
H2O + H2 ↑
Keto−enol tautomerization
Benzilic acid rearrangement
Figure 3. Proposed reaction pathway for conversion of glycerin
into lactic acid.
acid rearrangement to give lactic acid in an alkaline solution, and
our previous study found that pyruvaldehyde can also be con-
verted into lactic acid in a hydrothermal reaction at 300 ꢂC with-
out using an alkaline catalyst by a mechanism similar to that of
the benzilic acid rearrangement.3 Therefore, pyruvaldehyde is
likely an intermediate product involved in the formation of lactic
acid from glycerin.
Then, the formation mechanism of pyruvaldehyde from
glycerin is discussed. A reaction scheme that may appear best
to explain the formation mechanism of pyruvaldehyde from
glycerin is shown in Figure 3.
References
1
2
3
4
W. Buhler, E. Dinjius, H. J. Ederer, A. Kruse, and C. Mas,
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M. J. Antal, Jr., W. S. L. Mok, J. C. Roy, and A. T. Raissi,
J. Anal. Appl. Pyrolysis, 8, 291 (1985).
F. Jin, Z. Zhou, H. Enomoto, T. Moriya, and H. Higashijima,
Chem. Lett., 33, 126 (2004).
‘‘Dictionary of Organic Compounds,’’ 6th ed., Chapman,
Hall (1995), Vol. 4.
Published on the web (Advance View) October 15, 2005; DOI 10.1246/cl.2005.1560