Green oxidation of bio-lactic acid with H2O2 into tartronic acid under UV irradiation

Article abstract of DOI:10.1039/c6ra05028j

Tartronic acid (TA) is a high value-added chemical widely used as a pharmaceutical product and a preservative; however, its synthesis technology is complicated and high cost. In this study, aqueous solutions of lactic acid were photochemically converted into TA via green oxidation by using hydrogen peroxide (H₂O₂).

Full text of DOI:10.1039/c6ra05028j

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Green oxidation of bio-lactic acid with H2O2 into
tartronic acid under UV irradiation†
Cite this: RSC Adv., 2016, 6, 41007
Xuxia Tian,ab Zhijian Wang, a Pengju Yang,ab Ruipeng Hao,ab Suping Jia,a Na Li,a Li Lia
*
a
*
and Zhenping Zhu
Received 25th February 2016
Accepted 18th April 2016
DOI: 10.1039/c6ra05028j
www.rsc.org/advances
Tartronic acid (TA) is a high value-added chemical widely used as
Biomass8–11 is a promising candidate that can serve as
a pharmaceutical product and a preservative; however, its synthesis a sustainable source of organic carbon to produce fuels, chem-
technology is complicated and high cost. In this study, aqueous icals, and carbon-based materials. Lactic acid12,13 (2-hydrox-
solutions of lactic acid were photochemically converted into TA via ypropanoic acid) containing adjacent hydroxyl and carboxylic
green oxidation by using hydrogen peroxide (H2O2).
groups can be considered to be a model of an over-
functionalized biomass-derived molecule. It can be obtained
from biomass at a low cost by bacterial fermentation and has
a growing market because of its multiple applications. Previous
studies have reported the direct conversion of lactic acid to
products such as acrylic acid,14,15 propanoic acid,16 pyruvic acid,17
and 2,3-pentanedione18 by dehydration, reduction, oxidation
and condensation. Traditional catalytic methods oen exhibit
poor atomic economy and are carried out under rather harsh
(e.g., strongly basic) conditions, so it is better to develop new
methods that could be operated under mild conditions with
rapid reaction and little environmental pollution.
In the traditional lactic acid oxidation reaction, the a-C could
be selective catalytic activated17 in the presence of catalysts,
which means lactic acid were dehydrogenated and converted to
pyruvic acid. In this study, we propose a new strategy in which
lactic acid was oxidized by hydroxyl radicals (cOH) and selec-
tively converted to TA. Under UV irradiation, hydrogen peroxide
(H2O2) aqueous solution led to the production of hydroxyl
radicals (cOH), which have the capability of oxidizing a variety of
organic molecule. But, in our research we were able to selec-
tively oxidize the methyl of lactic acid molecules to carboxyl
group by controlling the reaction time and the amount of H2O2.
As a result, selective oxidation of the methyl group of the lactic
acid molecule was the main reaction path.
Tartronic acid (TA) or hydroxymalonic acid is a high value-
added ne chemical1–3 used to treat various disorders, such as
osteoporosis and obesity. Dompe Farmaceutici, an Italian
pharmaceutical company, was the rst to report that TA deriv-
atives exhibit positive effects on bone metabolism; this property
allows the use of TA derivatives as bone-sparing agents to treat
osteopenic disorders.4 TA is an anticorrosive and protective
agent used to prevent oxidative decomposition of food.5,6 The
applications of TA are currently limited to high value-added
eld because of the high cost1,7 of this reagent (US $
1564 gÀ1). The TA market is dominated by Johnson Matthey
Company and Alfa Aesar, which use maleic acid as a starting
material via permanganate oxidation to prepare this reagent.
The selective partial oxidation of glycerol with heterogeneous
catalysts is a more cost-efficient way to prepare and obtain TA.
However, many products can be obtained during glycerol
oxidation; thus, the challenge is to nd suitable catalysts that
are selective to the desired product. Kimura et al. employed
cerium and bismuth as promoters for palladium- or platinum-
based catalysts with active carbon as a support for glycerol
oxidation and achieved 58% yield of sodium tartronate at full
conversion. Cai et al.7 reported that glycerol oxidation over Au-
based catalyst yielded 80% TA at 60 ꢀC under 0.3 MPa O2.
Thus, the development of inexpensive and environment-
friendly methods to prepare TA presents a challenge.
Table 1 summarizes the experimental results of the conver-
sion of a 0.5 M lactic acid aqueous solution under different
conditions. When a small volume of hydrogen peroxide solution
was added into the solution, the lactic acid was oxidized under
UV light irradiation (entry 1 in Table 1). The data indicate that the
main liquid products of lactic acid oxidation were TA, mesoxalic
acid (MA), acetic acid (HAc), and gas products mainly consisting
of CO2, as well as small amounts of CO and CH4. Control
experiments were carried out to investigate the inuence of H2O2
aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, P. R. China. E-mail: wangzhijian@sxicc.ac.
cn; zpzhu@sxicc.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
† Electronic supplementary information (ESI) available: Experimental section and
product analysis. See DOI: 10.1039/c6ra05028j
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Table 1 The conversion and selectivity for lactic acid oxidation in different experimental conditionsa
Sel. [%]
Entry
Reactantd [mol]
Conv. [%]
TA
MA
HAc
CO2
CO
CH4
1
0.04H2O2 + 0.1HL
0.30H2O2 + 0.1HL
0.1HL
37.07
84.16
5.13
0
39.01
15.78
0
16.36
10.45
0
5.67
14.42
Trace
0
35.53
48.75
30.10
0
0.87
4.08
0
Trace
Trace
0
2
3b
4c
0.04H2O2 + 0.1HL
0
0
0
0
a
Reaction condition: 0.5 M lactic acid aqueous solution, 200 mL; H2O2 with different quantity; atmosphere, argon; temperature, 20 ꢀC; light source,
b
c
300 W high-pressure Hg-lamp; irradiation time, 30 min. 0.5 M lactic acid aqueous solution without added H2O2. Dark reaction of the same
reactant. d The addition amount of reactants are listed in the table.
and light, a single reaction of the lactic acid solution mainly improving TA selectivity. The inuence of reaction duration on
produced HAc and CO2 under UV light, and the conversion rate TA selectivity was investigated. As illustrated in Fig. 1b, TA
was very low (entry 3 in Table 1). When appropriate of hydrogen selectivity increased with increasing reaction duration and
peroxide solution was added into the lactic acid solution, no reached 39.9% aer 20 min. However, when the reaction
chemical reaction occurred in the dark (entry 4 in Table 1). duration was prolonged, the TA selectivity gradually decreased
Control experiments indicated hydroxyl radicals produced from while the acetic acid selectivity slowly increased. The TA selec-
the photolysis of H2O2 is required to initiate this reactions.
tivity reached a maximum and then gradually prolongation of
As a safe and clean oxidizing agent, hydrogen peroxide the reaction time could be due to the content of H2O2 decreased
exhibits active chemical properties. This compound presents an because the photolysis speed of H2O2 is very fast during the
environmentally benign prole because it decomposes into experiments. Thus, we examined the changes in H2O2 and cOH
water and oxygen as its only reaction products. The UV/H2O2 concentrations over time. Fig. 2a shows the changes in H2O2
system is an advanced oxidation process in which H2O2 is concentrations over time; the curve shows the rapid occurrence
decomposed under UV light to generate hydroxyl radicals (cOH) of H2O2 photolysis during the rst few minutes and the
as highly reactive oxygen species. Generally, the UV/H2O2 complete decomposition of H2O2 aer 30 min. Fig. 2b shows the
process19–21 can lead to complete mineralization of organic uorescence intensity at 425 nm originating from cOH, which
carbon to CO2 and H2O. Thus, the control of hydroxyl radical increased with reaction time; the intensity reached its
reaction and attainment of a highly value-added chemical maximum at 15 min and then decreased rapidly. H2O2 was
present a challenge. In our study, we found methyl of lactic acid
was selectively oxidized to carboxyl, thus make lactic acid
conversion into TA. UV light catalyzes the dissociation of H2O2
into hydroxyl radicals through chain reactions; these oxidants
then react with lactic acid molecules in aqueous solutions.
However, optimization of the peroxide dose is highly critical
because excess peroxide can further oxidize lactic acid into CO2
(entries 2 in Table 1). To evaluate the effect of H2O2 supply on
TA selectivity from the oxidation of lactic acid, a series of
experiments were performed using 0.5 mol LÀ1 lactic acid.
The effects of H2O2 supply on lactic acid conversion and TA
selectivity are shown in Fig. 1a. TA selectivity reached
a maximum of 0.04 mol H2O2 supply and then gradually
decreased when the H2O2 supply further increased. Meanwhile,
the selectivity of acetic acid slowly increased. The effects of
H2O2 supply on CO2 formation during the experiments were
evaluated using GC techniques. As shown in Fig. S1,† increasing
the H2O2 supply resulted in a steady increase in CO2 formation.
With an increase in H2O2 dosage, plenty of hydroxyl radicals
generated within a short time prompt the over-oxidation of
lactic acid, which can produce CO2 in large quantities. Thus,
Fig. 1 (a) Effect of the H2O2 supply on the selectivity of liquid products
from the oxidation of lactic acid, (b) effect of reaction time on the
effectively control the amount of H2O2 was key point of
production from the oxidation of lactic acid.
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Fig. 2 (a) Effect of illumination time on the photolysis of H2O2, (b) PL
spectral changes observed during irradiation of H2O2 in the solution of
terephthalic acid (excitation at 315 nm).
Fig. 3 The effect of the TA selectivity (a) and concentration (b) by
intermittent supplementation of 2 mL lactic acid and 4 mL H2O2.
depleted in the solution; such depletion might have caused the
decline in TA selectivity over time. Fig. S2† shows that the
production rate of CO2 varied with time; this rate rapidly
increased during the rst 10 min and then decreased slowly
over time. At the start of the rst reaction time, lactic acid was
further oxidized into CO2 because of the excessive cOH in the
reaction system; the rate of CO2 production began to decrease
as H2O2 was gradually consumed.
Therefore, we compared the photolysis of lactic acid and TA in
the UV/H2O2 process. As shown in Fig. 4b, the photolytic rate of
lactic acid was higher than that of TA; however, photolysis was
markedly accelerated with the addition of H2O2. This result may
explain why the TA selectivity was maintained to a certain extent.
The generation of TA under UV light irradiation in the
presence of H2O2 is explained by proposing the following
reactions. UV irradiation of H2O2 in aqueous solution leads to
the production of hydroxyl radicals (cOH) (eqn (1)), thus initi-
ating the cleavage of C–H bonds22 by dehydrogenating the
organic substance; this process can simultaneously accept
The intermittent supplementation of lactic acid and H2O2 is
benecial to keep these compounds at optimal proportions,
thereby effectively increasing the TA selectivity because of the
high photolytic rate of H2O2 during the experiments. Fig. 3a
shows the inuence of the intermittent supplementation of
lactic acid and H2O2 within a 30 min interval during the
oxidation of lactic acid. During these operations, lactic acid
conversion (Fig. S3†) tended to exhibit the same increasing
tendency in each cycle, and the TA selectivity (Fig. 3a) remained
constant while the concentration of TA (Fig. 3b) showed upward
trend throughout the experiment. The MA selectivity also
remained fairly constant; meanwhile, the HAc selectivity grad-
ually increased during the process (Fig. S4†). TA gradually
accumulated through the supplementation of lactic acid and
H2O2 at the 30 min interval; hence, the process has great
signicance in practical applications for mass production.
As shown in the aforementioned experiments, lactic acid
conversion slowly declined, whereas the TA selectivity exhibited
no increasing trend over time with intermittent supplementa-
tion of lactic acid and H2O2 (Fig. 3), possibly because TA was
photolyzed under UV light irradiation. Thus, we examined TA
photolysis at varying concentrations. Fig. 4a shows that the
photolysis rate of TA was lower under UV irradiation. So, the
effect of photolysis on TA selectivity can be ignored, possibly
because TA photolysis occurred faster when H2O2 was added in
the presence of UV. Moreover, UV/H2O2 has been found to
increase the oxidation rate of various organic compounds.
Fig. 4 (a) The photolysis of TA in the different concentrations, (b) the
photolysis of lactic acid and TA in the UV/H2O2 process (n means the
photolysis amount of TA or lactic acid, n0 means the initial amount of
TA or lactic acid, n/n0 means the degradation ratio).
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another cOH to form intergradation. In our experiment, cOH results present important implications for the development of
reacting with CH3–CH(OH)–COOH formed (OH)CH2–CO–COOH a green route for biomass transformation with high efficiency.
intermediates (eqn (2)). In this process, the methyl group of
lactic acid dehydrogenated and combined with hydroxyl radicals
to form (OH)CH2–CO–COOH. These intermediates further
Acknowledgements
We acknowledge the nancial support from National Natural
Science Foundation of China (no. 91545116 and U1510108).
reacted with cOH to form (OH)2CH–CH(OH)–COOH (eqn (3)),
which was highly unstable and rapidly converted into HOC–
CH(OH)–COOH (eqn (4)). Furthermore, HOOC–CH(OH)–COOH
was obtained by further oxidation of HOC–CH(OH)–COOH (eqn
(5)). The potential side reactions involved are expressed in eqn
(7)–(11). In these pathways, CH3–CH(OH)–COOH was further
oxidized into HOOC–C(OH)2–COOH, CH3COOH, and CO2.
Notes and references
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H2O2 / 2cOH
(1)
CH3–CH(OH)–COOH + 2cOH
/ (OH)CH2–CH(OH)–COOH + H2O (2)
(OH)CH2–CH(OH)–COOH + 2cOH
/ (OH)2CH–CH(OH)–COOH + H2O (3)
(OH)2CH–CH(OH)–COOH
/ HCO–CH(OH)–COOH + H2O
(4)
(5)
(6)
(7)
HCO–CH(OH)–COOH + 2cOH
/ HOOC–CH(OH)–COOH + H2O
CH3–CH(OH)–COOH + 3H2O2
/ HOOC–CH(OH)–COOH + 4H2O
HOOC–CH(OH)–COOH + 2cOH
/ HOOC–C(OH)2–COOH + H2O
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(8)
(9)
cCH(OH)CH3 + cOH / CH3CHO + H2O
CH3CHO + 2cOH / CH3COOH + H2O
2CH3COOH + 6cOH / 4CO2 + 2H2O + 5H2
(10)
(11)
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418–426.
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