Highly efficient and tunable visible-light-catalytic synthesis of 2,5-diformylfuran using HBr and molecular oxygen

Article abstract of DOI:10.1039/d1ra00865j

This paper discloses that inexpensive hydrobromic acid (HBr) is active and highly selective to the photo-oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with dioxygen (O₂) or even with water under visible light illumination, which can achieve the highest 89.1% DFF yield in DMSO at 80 °C under pure O₂atmosphere. More importantly, under bifunctional acid-photooxidation catalysis of HBr, fructose can be directly converted to DFF and its two-step cascade conversion in DMSO provides a far higher DFF yield (80.2%) than the one-step cascade conversion in MeCN (42.1%). The results of HMF photooxidation catalyzed by hydrohalic acids, free radical quenching tests and EPR spectrum support that the Br atom and superoxide (O₂^?˙) anion radicals generated by HBr photolysis in O₂are active species for the oxidation of HMF to DFF and their activities are adjusted by the reaction medium. This photo-synthetic protocol is very simple and practical, especially with low operating costs, showing a good industrial application prospect.

Full text of DOI:10.1039/d1ra00865j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Highly efficient and tunable visible-light-catalytic
synthesis of 2,5-diformylfuran using HBr and
molecular oxygen†
Cite this: RSC Adv., 2021, 11, 23365
b
a
a
Bo Yang,a Huanhuan Zhanga
*
*
Wenwei Hu, Jialuo She, Zaihui Fu,
and Dabo Jianga
This paper discloses that inexpensive hydrobromic acid (HBr) is active and highly selective to the photo-
oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with dioxygen (O2) or even with
water under visible light illumination, which can achieve the highest 89.1% DFF yield in DMSO at 80 ꢀC
under pure O2 atmosphere. More importantly, under bifunctional acid-photooxidation catalysis of HBr,
fructose can be directly converted to DFF and its two-step cascade conversion in DMSO provides a far
higher DFF yield (80.2%) than the one-step cascade conversion in MeCN (42.1%). The results of HMF
photooxidation catalyzed by hydrohalic acids, free radical quenching tests and EPR spectrum support
that the Br atom and superoxide (O2ꢁc) anion radicals generated by HBr photolysis in O2 are active
species for the oxidation of HMF to DFF and their activities are adjusted by the reaction medium. This
photo-synthetic protocol is very simple and practical, especially with low operating costs, showing
a good industrial application prospect.
Received 1st February 2021
Accepted 12th May 2021
DOI: 10.1039/d1ra00865j
rsc.li/rsc-advances
is easily synthesized in high yield by direct selective oxidation
of HMF under thermo-catalytic conditions, and the reported
some homogeneous11,12 or heterogeneous13 catalysts are very
Introduction
With growing concerns about diminishing fossil, global
warming and environmental pollution, the development of
new and clean technical routes to produce chemicals from
renewable and abundant biomass has attracted worldwide
efficient for this selective oxidation. In addition to the tradi-
tional thermo-catalytic process, some new technologies, such
as bio-catalysis14 and electro-catalysis,15,16 have been applied
for this selective oxidation. However, it is hard to recover the
used homogeneous catalyst from the reaction system. In
addition, this synthesis protocol is still limited to the fact that
HMF is not readily available and it is easy to perform poly-
condensation at high concentration.17 Therefore, the direct
synthesis of DFF from easily available carbohydrates such as
fructose and glucose is highly appreciated and has received
widespread attention.18–21 The direct synthesis of DFF from
fructose, as shown in Scheme 1, is a two-step tandem reaction
that typically involves fructose dehydration to HMF catalyzed
by an acid22–24 and subsequent HMF oxidation to DFF catalyzed
by a redox catalyst.25–27 It is expected that this direct synthesis
protocol for the fructose-derived DFF, the above HMF
attention.1–5 5-Hydroxymethylfurfural (HMF),
a “bridge”
molecule, has been widely recognized as one of the top bio-
based platform chemicals in the future chemical industry,
not only its derivatives can serve as fuels, but can also replace
oil-based building blocks in the production of polymers.6,7 2,5-
diformylfuran (DFF), one of the HMF-derived oxygenated
products, also is an important platform chemical and has been
used to synthesize many useful compounds and new polymer
materials through hydrogenation, oxidation, polymerization,
hydrolysis and other chemical reactions. These DFF-derived
chemicals have extensive potential applications in medicine,
furan-based biopolymers, organic conductors, uorescent
whitening agents and macrocyclic ligands.8–10 In general, DFF
aNational & Local Joint Engineering Laboratory for New Petro-chemical Materials and
Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, P. R. China. E-mail: dingbang53191@163.com;
fzhhnnu@126.com
bCollege of Chemical Engineering, Hunan Chemical Vocational Technology College,
Zhuzhou 412000, P. R. China
† Electronic supplementary information (ESI) available: The HBr-photocatalyzed
fructose to DFF in MeCN, UV-vis spectrum of HBr in different solvents,
synthesis of HMF from fructose using HBr and isotope tracing test of heavy
oxygen water (H218O). See DOI: 10.1039/d1ra00865j
Scheme 1 Synthesis of DFF from fructose.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23365–23373 | 23365

View Article Online
RSC Advances
Paper
polycondensation is signicantly reduced due to its very low a direct synthesis protocol of DFF from readily available fruc-
concentration in situ.
tose, O2 and metal-free catalysts under mild conditions.
Photocatalysis techniques have received enormous attention
Some efficient catalytic protocols using molecular oxygen as
an oxidant, which are summarized in Table 1, have been during recent decades because of their potential applications in
developed to directly synthesize DFF from fructose by one-pot, environmental treatment44,45 and the synthesis of fuel and ne
two-steps28–36 or one-step,37–43 affording medium to high DFF chemicals.46–48 Semiconductor materials have outstanding
yields of 49–87%. However, these synthesis protocols usually advantages in cost, stability and ease of ne-tuning optical
employ relatively complex catalysts and high reaction temper- properties,49,50 which have been widely used in heterogeneous
atures above 110 ꢀC. The two-step synthesis protocol usually photo-catalysis processes. Among them, g-C3N4, Nb2O5, and
requires the stepwise addition of acidic and oxidative catalysts TiO2 have been reported to efficiently photo-catalyze HMF
in order to improve the DFF yield. The use of the toxic V- oxidation with O2, achieving ca. 52–91% conversion of HMF and
containing Cs3HPMo11V1O40, a-CuV2O6, PMo11V1, and 16–40% yield to DFF51–53 but their heterogeneous photocatalytic
PMoV2@CP-3.5-400 catalysts does not conform to the current properties usually give relatively low quantum efficiency and are
subject of environmental protection. The metal-containing accompanied by serious by-reactions, such as deep oxidation,53
catalysts oen give a relatively high DFF yield, such as f-Ce9- which results in their photocatalytic efficiency is much lower
Mo1Od, Au0.5Ru2.5/rGO, MoO3–ZrO2 and Co–Al hydrotalcites. than most of the above thermal catalytic systems. Hydrobromic
However, the transition metal and especially precious-metal- acid (HBr) is not only an inexpensive and easy to handle strong
based catalysts applied in tandem transformations are usually acid catalyst but is also a good homogeneous photo-catalyst,
expensive and contaminative. Some metal-free catalysts such as which can efficiently achieve the aerobic oxidation of alcohols
GO, NaBr, GN-NS and CC-SO3H–NH2 are appreciated, but a high to the corresponding aldehydes under visible light.54–56 We
ꢀ
reaction temperature (140–150 C) and long reaction time (19– suggest that HBr combined O2 and visible light may construct
24 h) are required to achieve medium DFF yields, thus leading a very simple and practical synthesis protocol for achieving the
to large energy consumption in industrial production. These direct transformation of fructose to DFF under relatively mild
defects described above are hampering their future industrial conditions. Here, we report the initial results obtained using
applications. Therefore, it is still highly desirable to develop this photoreaction protocol.
Table 1 One-pot synthesis of 2,5-diformylfuran from fructose using various methods
Catalyst
Procedure
Two-step
Two-step
Two-step
Two-step
Two-step
Two-step
Two-step
Two-step
Two-step
Reaction conditions
DFF yield (%)
49.0
Ref.
28
29
30
31
32
33
34
35
36
Amberlyst-15 and Ru/HT
Cs3HPMo11V1O40
H2SO4/V-CP
Step 1: 100 ꢀC, 3 h, O2 (20 mL minꢁ1) with Amberlyst-15
Step 2: 100 ꢀC, 6 h, O2 (20 mL minꢁ1) with Ru/HT
Step 1: 110 ꢀC, 2 h, N2 (0.1 MPa)
60.0
Step 2: 120 ꢀC, 6 h, O2 (0.1 MPa)
Step 1: 140 ꢀC, 20 min, air with H2SO4
Step 2: 140 ꢀC; 6 h, O2 (40 mL minꢁ1) with V-CP
Step 1: 140 ꢀC, 2 h, N2 atmosphere
68.4
CC-SO3H–NH2
GO
69.0
Step 2: 140 ꢀC, 17 h, O2 (20 mL minꢁ1
)
)
)
Step 1: 140 ꢀC, 2 h, N2 (20 mL minꢁ1
Step 2: 140 ꢀC, 22 h, O2 (20 mL minꢁ1
Step 1: 120 ꢀC, 2 h, N2 (10 mL minꢁ1
Step 2: 120 ꢀC, 10 h, O2 (10 mL minꢁ1
)
72.5
f-Ce9Mo1Od
)
74.0
Fe3O4-SBA-SO3H and K-OMS-2
Au0.5Ru2.5/rGO
Co–Al hydrotalcites
Step 1: 110 ꢀC, 2 h, air with Fe3O4-SBA-SO3H
Step 2: 110 ꢀC, 6 h, O2 (10 mL minꢁ1) with K-OMS-2
Step 1: 110 ꢀC, 2 h, air with visible light
Step 2: 110 ꢀC; 6 h, O2 (5 bar) with visible light
Step 1: 120 ꢀC, 2 h, N2 (1 bar)
80.0
86.0
87.0
Step 2: 120 ꢀC; 8 h, O2 (3 bar)
P(EVPI-Br)/a-CuV2O6
NaBr
GN-NS
MoO3–ZrO2
Cs0.5H2.5PMo12
PMA-MIL-101
PMoV2@CP-3.5-400
HBr
One-step
One-step
One-step
One-step
One-step
One-step
One-step
One-step
Two-step
135 ꢀC, 3.5 h, O2 (1 bar)
63.1
67.0
70.3
74.0
69.3
75.1
87.3
42.1
79.7
37
38
39
40
41
42
43
150 ꢀC, 23 h, O2 atmosphere
150 ꢀC, 25 h, O2 (20 mL minꢁ1
150 ꢀC, 10 h, O2 (20 mL minꢁ1
160 ꢀC, 2 h, air
)
)
150 ꢀC, 7 h, O2 (20 mL minꢁ1
120 ꢀC, 4 h, O2 atmosphere
)
50 ꢀC, 12 h, O2 (1 atm), light, MeCN
This work
HBr-Amberlyst-15
Step 1: 120 ꢀC, 2 h, air, Amberlyst-15, DMSO
Step 2: 80 ꢀC, O2 (1 atm), 8 h, visible light with HBr
Step 1: 120 ꢀC, 2 h, N2 atmosphere, DMSO
Step 2: 80 ꢀC, 8 h, O2 (1 atm), visible light
HBr
Two-step
80.2
23366 | RSC Adv., 2021, 11, 23365–23373
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
ꢀ
activated carbon (AC, 4.2 g) at 80 C for 4 h, then ltered and
diluted with DMSO to obtain 0.02 M HMF solution. The
following photo-catalytic oxidation conditions for the synthesis
of DFF from fructose-derived HMF were the same as described
in Aerobic oxidation of HMF to DFF.
Experimental
Reagents
The main reagents used in this work were of analytical grade,
which included hydrobromic acid (40%, HBr), hydrochloric
acid (40%, HCl), hydroiodic acid (47.0%, HI), commercial
Amberlyst-15 (SO3H density, 5 mmol gꢁ1, diameter in a range of
0.15–0.053 mm), fructose (C6H12O6), 5-hydroxymethyl furfural
(HMF), acetonitrile (MeCN), dimethyl sulfoxide (DMSO, dehy-
drated with molecular sieve 3A), N,N-dimethylformamide
(DMF), N,N-dimethylacetamide (DMA). Distilled water was used
throughout the experiment.
Analysis of products
The quantitative analysis of fructose-derived products was per-
formed on a high-performance liquid chromatography (All-
tech1500 HPLC) instrument equipped with a C18 AQ column
and an ultraviolet detector at 280 nm. The mixture of acetoni-
trile and water (containing 0.1 wt% acetic acid) whose volume
ratio was 60 : 40, was used as the mobile phase at a rate of 0.5
mL minꢁ1, and column oven temperature was maintained at
30 ꢀC. The analyses were done in triplicate to get more accurate
results. The obtained results were shown as average values ꢂ
standard deviations. The yield of products are dened as
follows:
Aerobic oxidation of HMF to DFF
The photoreaction for the synthesis of DFF from HMF was
carried out in a self-assembly photo-reactor equipped with
a constant temperature water circulation device and an oxygen
storage vessel (1 atm, see Fig. 1). A 35 W tungsten-bromine lamp
(with the UV light lter, light intensity, 535 mW cmꢁ2) was used
as a built-in light source. The whole photocatalytic reactions
were operated in the closed reactor under constant temperature
and normal pressure. Briey, HMF (18.0 mg, 0.1 mmol), HBr
(40 wt%, 0.3 mmol) and DMSO or MeCN (5 mL) were mixed in
theꢀself-assembly photo-reactor. The mixture was incubated at
Yield of HMF (%) ¼ (mole of HMF produced)/(mole of starting
fructose) ꢃ 100%
Yield of DFF (%) ¼ (mole of DFF produced)/(mole of starting
fructose or HMF) ꢃ 100%
ꢀ
80 C (DMSO solvent, 8 h) or 50 C (MeCN solvent, 12 h) with
continuous stirring under visible light illumination and O2
atmosphere. Aer the reaction, the mixture was gradually
cooled to room temperature.
Results and discussion
Aerobic oxidation of HMF to DFF under different conditions
Direct synthesis of DFF from fructose by one step or two steps
Table 2 lists the data for the synthesis of DFF from HMF under
various reaction conditions. Entry 1 shows that the HBr-based
photo-synthetic protocol was very efficient at the photo-
oxidation of HMF to DFF and could achieve 90.8% HMF
conversion and up to 89.1% DFF yield under the optimal stan-
dard reaction conditions. The data listed in parentheses of entry
1 illuminate that dry HBr gas dissolved in anhydrous DMSO also
gave almost the same photo-catalytic efficiency as that of 40%
HBr solution. Two control experiments illuminate that HMF
photo-oxidation could not proceed in the absence of HBr (entry
2) or without visible light irradiation (entry 3), but the
consumption (or conversion) of HMF caused by its polymeri-
zation was up to 37.4 and 7.8%, respectively. Entries 4 and 5
further show that HF and HCl were not active for the photo-
oxidation of HMF, except that compared to entry 2, HMF
consumption was slightly reduced by 5–8%. HI only achieved
a very low 4.4% DFF yield and 4.6% HMF consumption (entry 6).
These ndings support that HBr and visible light irradiation are
the key inuencing elements for the photo-oxidation of HMF.
On the other hand, when the photo-oxidation of HMF hardly
proceeds, its polymerization side reaction occurs easily and
becomes more serious under light irradiation than under the
darkness, but this side reaction can be gradually and efficiently
restrained with a change of adding acid from HF, HCl to HBr or
HI. Notably, the selective oxidation of HMF photocatalyzed by
HBr also proceeded well under air, even N2 atmosphere,
providing 73.3 and 71.8% DFF yields, respectively (entries 7 and
One step. Direct synthesis of DFF from fructose by one step
was carried out using the above photo-reaction device. In
a typical reaction, fructose (18 mg, 0.1 mmol), HBr (40 wt%, 2.5
mmol), water (0.3 mL) and MeCN (5 mL) were mixed in the
ꢀ
photo-reactor and the mixture was incubated at 50 C for 12 h
under continuous stirring under visible light illumination and
O2 atmosphere. Aer the reaction, the mixture was gradually
cooled to room temperature.
Two steps. Dehydration of fructose to HMF was carried out
in a two-necked ask under heating and stirring conditions. In
a typical reaction, fructose (306 mg, 1.7 mmol), HBr (40 wt%
0.085 mmol) or Amberlyst-15 (0.02 g), and DMSO (10 g) were
added to a two-necked ask (25 mL) equipped with a magnetic
stirrer bar. The mixture was heated at 120 ꢀC with vigorous
stirring for 2 h. Aer the reaction, the mixture was treated with
Fig. 1 Schematic diagram of the photoirradiation reaction system.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23365–23373 | 23367

View Article Online
RSC Advances
Paper
Table 2 Synthesis of DFF from HMF photo-oxidation under different conditions
Entry
Change from standard conditions
Conversion of HMF (%)
Yield of DFF (%)
1a
2
3
4
5
6
7
8
9
10
11
12
13c
14c
15d
16e
Standard
Without HBr
90.8(89.1)b
37.4
7.8
32.4
29.3
4.6
75.2
74.0
41.0
39.6
83.9
78.9
83.4
56.4
10.3f
0f
89.1(88.6)b
0
0
0
Without light illumination
HF instead of HBr
HCl instead of HBr
HI instead of HBr
Air instead of O2
0
4.4
73.3
71.8
5.6
0.4
83.8
78.8
56.7
40.3
42.1
0
N2 instead of O2
DMA instead of DMSO
DMF instead of DMSO
Adding 0.2 mmol DMA
Adding 0.2 mmol DMF
MeCN instead of DMSO at 50 ꢀ
MeCN instead of DMSO at 25 ꢀ
C
C
Fructose in MeCN at 50 ꢀ
Fructose in DMSO at 80 ꢀ
C
C
a
b
Standard reaction condition: HMF, 0.1 mmol; 40% HBr, 0.3 mmol; DMSO, 5 mL; O2, 1 atm; temperature, 80 ꢀC; time, 8 h. Using dry HBr gas
dissolved in anhydrous DMSO. c HMF, 0.1 mmol; 40% HBr, 0.2 mmol; MeCN, 5 mL; O2, 1 atm; time, 12 h. d Fructose, 0.1 mmol; 40% HBr, 2.5 mmol;
H2O, 0.3 mL; MeCN, 5 mL; O2, 1 atm; temperature, 50 ꢀC; time, 12 h. e Fructose, 0.1 mmol; 40% HBr, 0.3 mmol; DMSO, 5 mL; temperature, 80 ꢀC;
time, 8 h; O2, 1 atm. f Yield of HMF.
8). The above experimental results strongly support that the and DMF. Additionally, the HBr-catalyzed fructose dehydration
active Br atom generated by HBr photolysis likely plays a vital to HMF is drastically affected by reaction media, HBr in MeCN
role in achieving HMF oxidation with O2. The introduced water shows a very high acid-catalytic activity for this dehydration at
may provide an efficient oxygen source for the HBr-catalyzed even room temperature (Table S1†), accompanied by the poly-
HMF photooxidation under the N2 atmosphere. Entries 9 and condensation of product HMF, but this dehydration catalyzed
ꢀ
10 show that using DMA or especially DMF instead of DMSO as by HBr cannot occur in DMSO at even 80 C (see the following
a solvent, the yield of DFF was drastically reduced to 5.6 and Table 3).
0.4%, respectively, along with a severe polymerization side
reaction. Even if only adding 0.02 mmol DMF or DMA in entry 1
also resulted in DFF yield decreasing by 6 or 10% (entries 11 and
Table 3 Dehydration of fructose to HMF under different conditionsa
12). Notably, HBr was very active in the MeCN medium and
could efficiently function at a mild temperature of 50 ꢀC and
even 25 ꢀC although DFF yield decreased to 56.7% (entry 13) and
40.3% (entry 14), respectively, owing to severe polymerization of
HMF. The HBr-based photo-synthetic protocol in MeCN could
realize a one-step cascade conversion of fructose, affording
42.1% DFF yield under optimal conditions (entry 15 and
Fig. S1†). But it could not proceed in DMSO even at 80 ꢀC. These
ndings support that the photocatalytic performances of HBr in
the synthesis of DFF from HMF and fructose are drastically
affected by solvents. Fig. S2† illuminates that the UV-vis spectral
curve of HBr in MeCN and especially DMSO solvents clearly 5b
displays a tail extending out into the visible region of 400–
550 nm, but its absorption in this visible region was negligible
in DMF and DMA solvents, supporting that the photolysis of
9
HBr can proceed more effectively in MeCN and DMSO than in
DMA and DMF. On the other hand, the Br atom radicals
generated by HBr photolysis may be very active in MeCN, but
they become relatively stable in DMSO and even inert in DMA
Entry HBr (mmol) Temperature (ꢀC) Time (h) Yield of HMF (%)
1
2
3
4
0.340
0.170
0.085
0.085
0.085
0.340
0.170
0.085
0.085
100
100
100
100
100
120
120
120
80
2
2
2
4
6
2
2
2
4
2
66.9
80.4
86.9
90.4
87.8
50.9
76.2
95.4
0
6
7
8
10c
Amberlyst-15 120
99.6
a
b
Reaction condition: fructose, 1.7 mmol; DMSO, 10 g. 0.7% yield of
c
DFF was detected out. Using 20 mg Amberlyst-15 as a catalyst.
23368 | RSC Adv., 2021, 11, 23365–23373
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
its occurrence gradually increases upon elevating the tempera-
ture. Fig. 3b displays the inuence of illumination time on HBr-
photocatalyzed HMF to DFF. In that, HMF conversion and DFF
yield almost presented a rapid and synchronous increase upon
prolonging the time, respectively, achieving 90.8 and 89.1% at
8 h. Thereaer, as the photo-reaction progressed, DFF yield
almost no longer increased but HMF conversion was slightly
enhanced to 94.5% at 10–12 h, indicating that only HMF poly-
merization could proceed at this reaction stage, but its photo-
oxidation is perhaps completely blocked by cumulative
polymers.
Effect of various variables
The effects of various variables such as HBr and HMF dosages,
temperature and time on the photo-oxidation of HMF by O2
were further investigated using the best DMSO as a solvent and
the obtained results are shown in Fig. 2 and 3. Fig. 2a shows that
when the amount of HBr was enhanced from 0 to 0.3 mmol,
HMF conversion and DFF yield gradually and drastically
increased from 0 to 90.8% and from 0 to 89.1%, respectively,
which was indicative of an excellent photo-catalytic function of
HBr. Attempting to further increase the amount reversely
resulted in a signicant decrease in photo-oxidation efficiency
(73.5% HMF conversion and 59.4% DFF yield), which may be
due to the following reasons: based on a fact that, HMF
conversion at a high HBr amount was 14% higher than DFF
yield, we propose that the polymerization side reaction of HMF
occurs more easily under the excessive use of HBr57,58 and the
by-products seem to slow down the photo-oxidation based on
the subsequent experiments. On the other hand, the control test
of adding NaBr to the standard photoreaction system, as shown
in Table S2,† supports that the introduction of excessive Brꢁ
ions play an inhibiting role in the photo-oxidation of HMF to
DFF. Fig. 2b shows that DFF yield was almost unchanged (about
89.0%) when HMF dosage was enhanced from 0.05 to
0.10 mmol. Thereaer, it gradually and slightly reduced with
increasing HMF amount to 0.4 mmol and then stayed almost
86% when HMF amount was between 0.4 and 0.6 mmol.
However, HMF conversion always increased continuously from
90.8 to 99.6% with enhancing HMF amounted from 0.05 to
0.6 mmol. This is likely due to the fact that HMF polymerization
would occur at a higher frequency as its dosage increases, thus
leading to a slight decrease in its photo-oxidation efficiency. It is
worth noting that the turnover number (TON, the molar ratio of
DFF to HBr) in Fig. 2b was almost linearly enhanced with the
increase of HMF dosage, which can be indicative of a contin-
uous improvement in the photocatalytic efficiency of HBr.
Fig. 3a displays that as the photoreaction temperature rose from
Synthesis of DFF from fructose by two steps
As described above, the synthesis of DFF from fructose in the
HBr–MeCN photoreaction system could be achieved by one step
under a relatively mild temperature, but providing a low 42.1%
DFF yield owing to severe polymerization for the generated
HMF. Also, this one-step approach using DMSO as a medium
has failed even if the temperature rises to a maximum of 80 ꢀC
that the current photo-reactors can withstand, this is because
the dehydration of fructose to HMF catalyzed by HBr in DMSO
cannot occur efficiently at 80 ꢀC, which can be conrmed from
the following experiments (Table 3, entry 9). In order to over-
come this obstacle, the synthesis of DFF from fructose was
carried out by the following two steps.
In the rst step, the dehydration of fructose to HMF was
carried out using HBr or Amberlyst-15 as a catalyst. As shown in
Table 3, HBr showed good activity for this dehydration at 100–
120 ꢀC, achieving about 50–95% HMF yield (entries 1–8), but it
became invalid at 80 ꢀC (entry 9). The HMF yield at 100 and
especially 120 ꢀC was gradually and signicantly reduced upon
increasing HBr dosage from 0.085 to 0.34 mmol, this abnormal
inverse relationship between HMF yield and catalyst dosage
may be because the use of excessive HBr easily causes serious
polymerization of fructose with dehydrated product HMF and
especially HMF itself,59 and these polymerizations becomes
ꢀ
ꢀ
50 to 80 C, HMF conversion and DFF yield were continuously
more serious at a higher temperature of 120 C. The effect of
reaction time on the dehydration of fructose at 100 ꢀC was also
checked using 0.085 mmol HBr. As shown in entries 3–5, HMF
yield achieved a maximum of 90.4% at 4 h and a further pro-
longing the time led to the slight reduction of HMF yield owing
improved from 47.4 to 90.3% and from 12.8 to 89.1%, respec-
tively, presenting a gradual approaching trend for both. This
likely implies that the activation energy of HMF photooxidation
is higher than that of its polymerization and the probability of
Fig. 2 Influence of HBr (a) or HMF dosage (b) on the HBr-photocatalyzed HMF oxidation to DFF (by the use of the standard reaction conditions of
Table 2 except for HBr or HMF dosage).
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23365–23373 | 23369

View Article Online
RSC Advances
Paper
Fig. 3 Influence of reaction temperature (a) and reaction time (b) on the HBr-photocatalyzed HMF oxidation to DFF (by use of the standard
reaction conditions of Table 2 except for temperature or time).
to its subsequent oxidation to DFF (about 0.73%) and entries 1 and 2, which could afford DFF yields of 84.2 and
polymerization.
80.2%, respectively (entries 3 and 4), thus providing about 80%
The highest HMF yield of 95.4% could be obtained using yield of fructose-derived DFF from two-step conversions.
0.085 mmol HBr at 120 ꢀC for 2 h. Besides, Amberlyst-15, as
a solid acid, even achieved 99.6% HMF yield under the same
reaction conditions (entry 10), in consistence with the result
reported by Shimizu et al.60
The proposed photo-catalytic mechanism for the conversion
of HMF to DFF
In the second step, the above two dehydration solutions
obtained in entries 8 and 10 of Table 3 were diluted to 0.02 M
HMF solutions with DMSO and then these two diluted solutions
were oxidized to DFF under the standard photoreaction condi-
tions of Table 2 and the results are listed in Table 4. Entries 1
and 2 show that the photo-oxidation of two fructose-derived
HMF solutions were also efficiently catalyzed by HBr, but
achieving a relatively low DFF yield (72–74%) compared to the
use of analytical pure HMF (89% DFF yield), which may be due
to the inhibition of trace byproducts produced during fructose
dehydration,61 as supported in the following control tests. Aer
the two dehydrated stock solutions were treated with a large
amount of active carbon (AC) to remove the possible soluble
polymers as much as possible, their diluted solutions (0.02 M
HMF) exhibited more reactivity than the corresponding ones in
Three radical scavengers were used to conrm the active species
for the HBr-photocatalytic oxidation of HMF by O2 and the
results are shown in Fig. 4. In the presence of t-BuOH as
a
hydroxyl radical (cOH) scavenger,62 this photo-catalytic
oxidation could still efficiently proceed although the DFF yield
was slightly reduced. But it hardly happened in the presence of
TEMPO (a quencher of superoxide anion radical (O2ꢁc)ꢁand
cOH)63, especially benzoquinone (BQ, a quencher of O2 c)64,
suggesting that the O2ꢁc radicals, perhaps also including Br
atom radicals, are the dominant species in the HBr-based
photo-catalytic oxidation.
In addition, Fig. 5 shows that aer the HBr-based photo-
catalytic solution with DMPO (50 mM) was stirred for 5 min
with illumination, its EPR spectrum clearly displayed the
DMPO-O2cꢁ EPR signal. This provides direct proof for the
existence of O2ꢁc species in the photooxidation protocol.
Table 4 Direct conversion of fructose to DFF by two steps
Step 1: yield
of HMF (%)
Yield of DFF from
fructose (%)
Entry
Step 2a: yield of DFF (%)
1b
95.4
99.6
95.4
99.4
72.1
74.6
84.2
80.2
68.7
74.2
80.2
79.7
2c
3b,d
4c,d
a
b
Using standard reaction conditions of Table 2. Using fructose-
c
derived HMF in entry 8 of Table 3. Using fructose-derived HMF in
d
Fig.
4 Effect of various scavengers (0.2 mmol) on HBr-photo-
entry 10 of Table 3. Using fructose-derived HMF treated with AC at
80 ꢀC.
catalyzed HMF oxidation to DFF under standard reaction conditions of
Table 2.
23370 | RSC Adv., 2021, 11, 23365–23373
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Based on a fact that the mass spectrum of DFF obtained from
the isotope-tracing test of H218O under N2 lacks the 18O-labeled
fragment peaks (see Fig. S3†), we suggest that the generated O2
by decomposition of HBrO under visible light is likely to be
ruled out as another way for water to participate in HMF
oxidation. It is reasonable that the O2ꢁc anion radical combines
with benzyl radical (1) much faster than O2, thus accelerating to
generate benzyl peroxy anion as the main pathway,35 as sup-
ported by the radical quenching experiment conducted by BQ.
The photo-generated Br atom and O2ꢁc anion radicals are likely
to play a synergistic role in the selective oxidation of HMF to
DFF. Namely, the Br atom dominates the activation of HMF to
benzyl radicals and O2ꢁc anion accelerates the conversion of
benzyl radical to benzyl with HBr/HMF to yield species 4. These
two redox processes can peroxy anion, thus signicantly
improving the photo-oxidation efficiency. This synergistic
mechanism can reasonably explain the photo-catalytic behav-
iors of the other two hydrohalic acids HCl and HI in HMF
oxidation. The reason why HCl lacks photo-catalytic activity
should be due to its poor reducibility so that it is difficult to
produce the active Cl atom and the derived O2ꢁc anion radicals
through the photolysis pathway of HCl in eqn (1). In sharp
contrast, due to the strong reducibility, HI should be easy to
generate I atom and O2ꢁc anion radicals via its photolysis
pathway in O2. But the relatively inert I atom radical should
hardly initiate the selective oxidation of HMF and even if the
derived O2ꢁc anion radical in the photolysis is also difficult to
initiate this reaction, otherwise, the good DFF yield should be
obtained in a HI-based photocatalytic system.
Fig. 5 EPR spectra of DMPO-captured O2ꢁc in HBr-photocatalyzed
HMF oxidation.
From the present experimental results and previous litera-
ture,54,55,65–68 a plausible reaction mechanism has been proposed
for the selective synthesis of DFF from HMF. As shown in
Scheme 2. The photolysis of HBr in O2 under visible light can
genꢁerate the active Br atom and the derived superoxide anion
(O2 c) radicals54,55 (eqn (1)). Then, the Br atom radical may
abstract one hydrogen atom of HMF to a benzyl radical species
(1), along with the regeneration of HBr. Followed by that, the
ꢁ
species 1 trap O2 c (the main pathway35) or O2 (minor
pathway65) to afford its peroxy anion or peroxy radical (2), and
the latter may combine with a proton of HBr or react with HBr/
HMF to give peroxygen species (3), subsequently, the species 3
further reacts to achieve the regeneration of the Br atom or
benzyl radical species. Species 4 are eventually converted into
the goal product DFF through intramolecular dehydration66
(eqn (2)). Notably, the introduced water, as supported in the
HBr-photocatalyzed HMF oxidation under N2 (entry 8 of Table
2), may provide an efficient oxygen source in the form of
hypobromous acid (HBrO) through the following reactions: the
combination of two Br atoms generates a Br2 molecule and the
latter easily reacts with water to produce HBrO and HBr, then,
HBrO, as a strong oxidant, may directly deprive the hydroxyl
hydrogen atom of the species 1 to yield HBr and DFF (eqn (3)).
Conclusion
In conclusion, for the rst time, we have developed a simple and
efficient photo-reaction protocol for the synthesis of DFF from
HMF and especially fructose under relatively mild conditions,
which has the following advantages: (i) this protocol uses
readily available HBr as a catalyst and molecular oxygen/or even
water as a green oxygen donor. (ii) This protocol can function
well under visible light, which achieves a very good efficiency for
HMF photo-oxidation and 100% DFF selectivity in DMSO at
80 ꢀC. (iii) This protocol can efficiently synthesize DFF (80%
yield) from the two-step cascade conversion of fructose in
DMSO. With such kind of robust photo-reaction protocol in
hand, we are interested in further exploring its potential
applications for highly efficient synthesis of DFF from the one-
step conversion of fructose or two-step conversion of glucose,
sucrose, cellobiose and cellulose.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Scheme 2 Possible mechanism for HBr-photocatalyzed HMF oxida-
tion to DFF under visible light irradiation. (eqn (1): The generation of
Free radical; eqn (2): Selective oxidation of HMF to DFF; eqn (3):
Possible reaction pathways under N2 atmosphere).
We acknowledge the nancial support for this work by the
National Natural Science Fund of China (21546010, 21676079,
22008062), and the Natural Science Fund of Hunan Province
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23365–23373 | 23371

View Article Online
RSC Advances
Paper
(2018JJ3335, 2020JJ4425). The Innovation Platform Open Fund 25 J. Ma, Z. Du, J. Xu, Q. Chu and Y. Pang, ChemSusChem, 2011,
of Hunan College (18K016), Hunan 2011 Collaborative Innova- 4, 51–54.
tion Center of Chemical Engineering & Technology with Envi- 26 A. S. Amarasekara, D. Green and E. McMillan, Catal.
ronmental Benignity and Effective Resource Utilization, Commun., 2008, 9, 286–288.
Program for Science and Technology Innovative Research Team 27 J. Artz, S. Mallmann and R. Palkovits, ChemSusChem, 2015, 8,
in Higher Educational Institutions of Hunan Province.
672–679.
28 A. Takagaki, M. Takahashi, S. Nishimura and K. Ebitani, ACS
Catal., 2011, 1, 1562–1565.
29 R. Liu, J. Chen, L. Chen, Y. Guo and J. Zhong,
ChemPlusChem, 2014, 79, 1448–1454.
30 M. Cui, R. Huang, W. Qi, R. Su and Z. He, RSC Adv., 2017, 7,
7560–7566.
31 P. V. Rathod, S. D. Nale and V. H. Jadhav, ACS Sustainable
Chem. Eng., 2016, 5, 701–707.
32 G. Lv, H. Wang, Y. Yang, T. Deng, C. Chen, Y. Zhu and
X. Hou, Green Chem., 2016, 18, 2302–2307.
33 Z. Yang, W. Qi, R. Su and Z. He, ACS Sustainable Chem. Eng.,
2017, 5, 4179–4187.
34 Z.-Z. Yang, J. Deng, T. Pan, Q.-X. Guo and Y. Fu, Green Chem.,
2012, 14, 2986–2989.
References
1 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106,
4044–4098.
2 C. Liu, C. Zhang, K. Liu, Y. Wang, G. Fan, S. Sun, J. Xu, Y. Zhu
and Y. Li, Biomass Bioenergy, 2015, 72, 189–199.
3 J. Song, H. Fan, J. Ma and B. Han, Green Chem., 2013, 15,
2619–2635.
4 X. Tong, Y. Ma and Y. Li, Carbohydr. Res., 2010, 345, 1698–
1701.
5 S. Hu, Z. Zhang, Y. Zhou, B. Han, H. Fan, W. Li, J. Song and
Y. Xie, Green Chem., 2008, 10, 1280–1283.
6 A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and
C. A. M. Afonso, Green Chem., 2011, 13, 754–793.
35 B. Ma, Y. Wang, X. Guo, X. Tong, C. Liu, Y. Wang and X. Guo,
Appl. Catal., A, 2018, 552, 70–76.
´
´
´
7 T. S. Hansen, I. Sadaba, E. J. Garcıa-Suarez and A. Riisager,
Appl. Catal., A, 2013, 456, 44–50.
8 M. Del Poeta, W. A. Schell, C. C. Dykstra, S. Jones,
36 A. B. Raut and B. M. Bhanage, ChemistrySelect, 2018, 3,
11388–11397.
R. R. Tidwell, A. Czarny, M. Bajic, M. Bajic, A. Kumar, 37 W. Hou, Q. Wang, Z. Guo, J. Li, Y. Zhou and J. Wang, Catal.
D. Boykin and J. R. Perfect, Antimicrob. Agents Chemother.,
1998, 42, 2495.
9 K. T. Hopkins, W. D. Wilson, B. C. Bender, D. R. McCurdy,
Sci. Technol., 2017, 7, 1006–1016.
38 C. Laugel, B. Estrine, J. Le Bras, N. Hoffmann, S. Marinkovic
and J. Muzart, ChemCatChem, 2014, 1195–1198.
J. E. Hall, R. R. Tidwell, A. Kumar, M. Bajic and 39 J. Zhao, Y. Yan, Z.-T. Hu, V. Jose, X. Chen and J.-M. Lee,
D. W. Boykin, J. Med. Chem., 1998, 41, 3872–3878.
10 F. W. Lichtenthaler, Acc. Chem. Res., 2002, 35, 728–737.
11 Y. Yan, K. Li, J. Zhao, W. Cai, Y. Yang and J.-M. Lee, Appl.
Catal., B, 2017, 207, 358–365.
12 J. Zhao, X. Chen, Y. Du, Y. Yang and J.-M. Lee, Appl. Catal., A,
2018, 568, 16–22.
13 Z. Du, J. Ma, F. Wang, J. Liu and J. Xu, Green Chem., 2011, 13,
554–557.
14 R. Qu, W. Zhang, N. Liu, Q. Zhang, Y. Liu, X. Li, Y. Wei and
L. Feng, ACS Sustainable Chem. Eng., 2018, 6, 8019–8028.
Catal. Sci. Technol., 2020, 10, 4179–4183.
40 J. Zhao, A. Jayakumar and J.-M. Lee, ACS Sustainable Chem.
Eng., 2018, 6, 2976–2982.
41 Y. Liu, L. Zhu, J. Tang, M. Liu, R. Cheng and C. Hu,
ChemSusChem, 2014, 7, 3541–3547.
42 J. Zhao, J. Anjali, Y. Yan and J.-M. Lee, ChemCatChem, 2017,
9, 1187–1191.
43 Q. Wang, W. Hou, T. Meng, Q. Hou, Y. Zhou and J. Wang,
Catal. Today, 2019, 319, 57–65.
44 X. Li, G. Chen, Y. Po-Lock and C. Kutal, J. Chem. Technol.
Biotechnol., 2003, 78, 1246–1251.
´
15 T. Cao, M. Wu, V. V. Ordomsky, X. Xin, H. Wang, P. Metivier
and M. Pera-Titus, ChemSusChem, 2017, 10, 4851–4854.
45 A. Zertal, J. Environ. Sci. Eng., 2012, 844–8522.
16 P. Prabhu, Y. Wan and J.-M. Lee, Matter, 2020, 3, 1162–1177. 46 A. Maldotti, A. Molinari and R. Amadelli, Chem. Rev., 2002,
17 L. Hu, A. He, X. Liu, J. Xia, J. Xu, S. Zhou and J. Xu, ACS
Sustainable Chem. Eng., 2018, 6, 15915–15935.
18 V. P. Kashparova, Russ. Chem. Bull., 2015, 64, 1069–1073.
102, 3811–3836.
47 S. Tang, J. She, Z. Fu, S. Zhang, Z. Tang, C. Zhang, Y. Liu,
D. Yin and J. Li, Appl. Catal., B, 2017, 214, 89–99.
19 W. Zhang, T. Meng, J. Tang, W. Zhuang, Y. Zhou and 48 B. Yang, Z. Fu, A. Su, J. She, M. Chen, S. Tang, W. Hu,
J. Wang, ACS Sustainable Chem. Eng., 2017, 5, 10029–10037. C. Zhang and Y. Liu, Appl. Catal., B, 2019, 242, 249–257.
20 J. Zhao, A. Jayakumar, Z.-T. Hu, Y. Yan, Y. Yang and J.-M. Lee, 49 H. Wu, H. L. Tan, C. Y. Toe, J. Scott, L. Wang, R. Amal and
ACS Sustainable Chem. Eng., 2017, 6, 284–291. Y. H. Ng, Adv. Mater., 2020, 32, 1–21.
21 Q. Girka, B. Estrine, N. Hoffmann, J. Le Bras, S. Marinkovic 50 H. L. Tan, F. F. Abdi and Y. H. Ng, Chem. Soc. Rev., 2019, 48,
and J. Muzart, React. Chem. Eng., 2016, 1, 176–182.
1255–1271.
22 V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko, 51 I. Krivtsov, E. I. Garcıa-Lopez, G. Marcı, L. Palmisano,
´
´
´
`
´
´˜
´
V. Nikolakis, N. S. Marinkovic, A. I. Frenkel, S. I. Sandler
Z. Amghouz, J. R. Garcıa, S. Ordonez and E. Dıaz, Appl.
and D. G. Vlachos, J. Am. Chem. Soc., 2013, 135, 3997–4006.
Catal., B, 2017, 204, 430–439.
23 K. Tsutsumi, N. Kurata, E. Takata, K. Furuichi, M. Nagano 52 H. Zhang, Q. Wu, C. Guo, Y. Wu and T. Wu, ACS Sustainable
and K. Tabata, Appl. Catal., B, 2014, 147, 1009–1014.
Chem. Eng., 2017, 5, 3517–3523.
24 Y. Xiao and Y.-F. Song, Appl. Catal., A, 2014, 484, 74–78.
23372 | RSC Adv., 2021, 11, 23365–23373
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
¨
53 S. Yurdakal, B. S. Tek, O. Alagoz, V. Augugliaro, V. Loddo, 60 K.-i. Shimizu, R. Uozumi and A. Satsuma, Catal. Commun.,
G. Palmisano and L. Palmisano, ACS Sustainable Chem.
Eng., 2013, 1, 456–461.
2009, 10, 1849–1853.
61 G. Sampath and S. Kannan, Catal. Commun., 2013, 37, 41–44.
54 S. Gazi and R. Ananthakrishnan, RSC Adv., 2012, 2, 7781– 62 Q. Wu, Y. He, H. Zhang, Z. Feng, Y. Wu and T. Wu, Mol.
7891. Catal., 2017, 436, 10–18.
55 S.-i. Hirashima, S. Hashimoto, Y. Masaki and A. Itoh, 63 Q. Chen, Y. Huang, X. Wang, J. Wu and G. Yu, Org. Biomol.
Tetrahedron, 2006, 62, 7887–7891. Chem., 2018, 16, 1713–1719.
56 N. Tada, K. Ban, T. Ishigami, T. Nobuta, T. Miura and A. Itoh, 64 H. Zhang, Z. Feng, Y. Zhu, Y. Wu and T. Wu, J. Photochem.
Tetrahedron Lett., 2011, 52, 3821–3824. Photobiol., A, 2019, 371, 1–9.
57 I. K. M. Yu, D. C. W. Tsang, A. C. K. Yip, S. S. Chen, L. Wang, 65 A. Itoh and S.-i. Hirashima, Synthesis, 2006, 2006, 1757–1759.
Y. S. Ok and C. S. Poon, Bioresour. Technol., 2017, 237, 222– 66 N. Tada, K. Ban, M. Yoshida, S.-i. Hirashima, T. Miura and
230.
A. Itoh, Tetrahedron Lett., 2010, 51, 6098–6100.
58 X. Zhang, B. B. Hewetson and N. S. Mosier, Energy Fuels, 67 J. Sun, Y. Wang, L. Han, D. Xu, Y. Chen, X. Peng and H. Guo,
2015, 29, 2387–2393.
Org. Chem. Front., 2014, 1, 1201–1204.
59 S. J. Dee and A. T. Bell, ChemSusChem, 2011, 4, 1166–1173.
68 A. Itoh, N. Tada, K. Ban, T. Nobuta, S.-i. Hirashima and
T. Miura, Synlett, 2011, 2011, 1381–1384.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23365–23373 | 23373