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Green Chemistry
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ARTICLE
Journal Name
Figure 6. Effect of the reaction time on the oxidation of furfural in OA-ChCl DES without
excess adding water. Reaction condition: furfural (1 mmol), H2O2 (10 mmol),
furfural/OA is 1:5.6, OA/ChCl=1:3, 50 oC.
Experimental
Materials and Characterizations
DOI: 10.1039/C8GC04022B
All reagents and solvents mentioned were purchased from
Furfural (98%), maleic acid (MA, 99.5%), 2-furoic acid (98%),
phosphate (75%), L-proline (99%), betaine (98%), fructose
(99%) were purchased from Aladdin. Hydrogen peroxide
(30%), oxalic acid (OA, 99.5%), fumaric acid (FA, 99%), succinic
acid (99.5%), glucose (AR), xylose (BR) were supplied by
Sinopharm. Choline chloride (ChCl, 98%) was supplied by
Macklin. 2-(5H)-Furanone (98%) was supplied by Sigma-
Aldrich. Methanol (analytical purity) was supplied by
Oceanpak, ultrapure water with resistivity of 18 Mcm was
produced by a Milli-Q (Millipore, USA).
High performance liquid chromatograph (Shimadzu), C-18
Chromatographic column (5µm, 4.6×250mm, purchased from
Agilent Technologies Inc.) with an ultraviolet detector. Mobile
phase: 20% methanol + 80% Phosphoric acid solution(1‰),
column temperature:40 oC, and flow rate: 0.6 mL/min.
The possible reaction mechanism was shown in Scheme 2.
At first, oxalic acid reaction with hydrogen peroxide to
generate peroxalic acid (PA), which works as the oxidant. The
oxidation ability of -O-O- group in PA is stronger than H2O2 due
to the electron-withdrawing effect of the carbonyl group. In
the EDS, the pKa of oxalic acid is 1.23, which makes the solvent
more acidity than other C1-C4 organic acid. According to the
Nernst equation (E = E0 - 0.059pH),8 it should be a strong
oxidant for furfural. The in-situ formed peroxalic acid inserts
an oxygen atom between the furan ring and the aldehyde
group, and then the ester group is hydrolyzed to formic acid
and the unstable intermediate 2-hydroxyfuran which arrange
to get 2(5H)-Furanone rapidly.5 As the allyl hydrogen is easily
oxidized, 2(5H) -furanone may be oxidized to maleic anhydride
in the presence of hydrogen peroxide. Then maleic anhydride
can be hydrolyze to maleic acid under acidic conditions, and it
is easily converted to a more stable fumaric acid. This means
that the yield of fumaric acid is higher than that of maleic acid
in such condition. In HPLC studies, the peaks of formic acid and
2(5H)-Furanone can be directly detected (Fig.S3), which
supports the rationality of the suggested mechanism.
Oxidation of furfural
Firstly, Furfural was purified by vacuum distillation. For
water/ethanol solvent, adding 0.5g oxalate and 4ml
water/ethanol into a 25 ml flask, and heating the container at
50 oC under water bath for several minutes to get a clear
solution. Then, adding 1 mmol furfural (83μL) and 1ml H2O2
o
(30%). The reaction was carried out for 24 h at 50 C. Next,
diluting the reaction solution 100 times to quench the
reaction, and then it was through a 0.22 μm syringe filter for
HPLC analysis.
For DES, adding 0.4-1.0g oxalate and 0.4-3.0g ChCl (or
betaine/proline) into a 25 ml flask, heating the container at 50
oC under water bath for several minutes to form the liquid, and
0-5 mL water was added next. Then, adding 1 mmol furfural
(83μL) and 0.5-4ml H2O2 (30%) to the container. The reaction
o
was carried out for 24 h at 50 C. At the end of the reaction,
diluting the reaction solution 100 times by water to quench
the reaction, and then it was through a 0.22 μm syringe filter
for HPLC analysis.
Scheme 2. Suggested mechanism of the reaction
Conclusions
Here, an efficient route was found to convert furfural to maleic
acid (MA) and fumaric acid (FA) using an oxalic acid (OA) based
deep eutectic solvents (DES) as both solvent and acidic
catalyst. The reaction can be carried out under mild condition
at low temperature (50-60 oC) with hydrogen peroxide as
oxidizer, and the total yield of MA and FA reached 95.7%. It
provides a new and green method to convert biomass to
valuable chemicals.
Conflicts of interest
There are no conflicts to declare.
Figure 7. Effect of the reaction temperature on the oxidation of furfural in OA-ChCl DES
without excess adding water. furfural (1 mmol), furfural/OA is 1:5.6, OA/ChCl=1:3, 50
oC, H2O2 (10 mmol), 6 h.
Acknowledgements
4 | J. Name., 2012, 00, 1-3
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