Photorefining of biomass and its derivatives to value-added chemicals is an alternative solution to address the global energy shortage and environmental issues. Herein, efficient and selective oxidation of 5-hydroxymethylfurfural (HMF, 91.1% conversion) to 2,5-diformylfuran (DFF, 99.4% selectivity) is demonstrated by visible light-driven photocatalysis over cocatalyst-free ZnIn2S4 nanosheets with crystal facet engineering. The spatial accumulation of photogenerated electrons and holes on the (110) and (102) crystal facets triggers a two-electron oxygen reduction reaction (2e-ORR) for H2O2 generation and HMF oxidation into DFF, respectively. The severe attenuation of photostability is caused by the irreversible photocorrosion of Zn–S with the formation of Zn–O chemical bonds by the formation of ˙OH from the in situ decomposition of H2O2. Spontaneous substitution of oxygen with sulfur has been proven to efficiently improve the photostability of ZnIn2S4. This present work provides insights into improving the durability of ZnIn2S4 and sheds new light on biomass valorization via photorefinery.

1. Green Chemistry
PAPER
Cite this: DOI: 10.1039/d2gc04362a
Received 18th November 2022,
Accepted 20th December 2022
DOI: 10.1039/d2gc04362a
rsc.li/greenchem
Spatial charge separation on the (110)/(102) facets
of cocatalyst-free ZnIn2S4 for the selective
conversion of 5-hydroxymethylfurfural to
2,5-diformylfuran†
Heng Zhao,*a
Dhwanil Trivedi,a
Morteza Roostaeinia,a
Xue Yong,b
Jun Chen,c
Pawan Kumar,a
Jing Liu,c
Bao-Lian Su,c,d
Steve Larter, e
Md Golam Kibria *a
and
Jinguang Hu *a
Photorefining of biomass and its derivatives to value-added chemicals is an alternative solution to address
the global energy shortage and environmental issues. Herein, efficient and selective oxidation of
5-hydroxymethylfurfural (HMF, 91.1% conversion) to 2,5-diformylfuran (DFF, 99.4% selectivity) is demon-
strated by visible light-driven photocatalysis over cocatalyst-free ZnIn2S4 nanosheets with crystal facet
engineering. The spatial accumulation of photogenerated electrons and holes on the (110) and (102)
crystal facets triggers a two-electron oxygen reduction reaction (2e-ORR) for H2O2 generation and HMF
oxidation into DFF, respectively. The severe attenuation of photostability is caused by the irreversible
photocorrosion of Zn–S with the formation of Zn–O chemical bonds by the formation of •
OH from the
in situ decomposition of H2O2. Spontaneous substitution of oxygen with sulfur has been proven to
efficiently improve the photostability of ZnIn2S4. This present work provides insights into improving the
durability of ZnIn2S4 and sheds new light on biomass valorization via photorefinery.
1. Introduction
Biomass, as a natural and renewable carbon feedstock, has
been regarded as one of the potential alternatives to traditional
petroleum resources to generate value-added chemicals and
sustainable fuels.1
Effective biomass or biomass-derived inter-
mediate utilization provides a promising perspective to reduce
the current pressure from fossil fuel shortages, global
warming and environmental issues.2
Current biomass valoriza-
tion technologies, such as thermochemical and biological pro-
cesses, are still carbon-intensive and not economically
feasible.3,4
Solar-driven biomass conversion, the so-called
biomass photorefinery, has emerged as a sustainable and feas-
ible approach for biomass utilization due to the mild operat-
ing conditions, inexhaustible solar energy and controllable
product selectivity.5–10
5-Hydroxymethylfurfural (HMF) is one of the top 12 chemi-
cals derived from biomass.11
Due to the dual functional
groups (aldehydes and hydroxyls), numerous furan derivatives
with high values can be produced through the oxidation or
reduction of HMF, such as 2,5-diformylfuran (DFF), 2,5-furan-
dicarboxylic acid (FDCA), 2,5-dihydroxymethyfuran (DHMF),
etc.12–14
DFF is one of the most valuable precursors of furan-
based polyesters, pharmaceutical products, antifungal agents,
etc.15
Herein, the selective oxidation of HMF to DFF by mild
photocatalysis has attracted considerable attention. There are
several reports on selective DFF and/or FDCA production from
the photocatalytic oxidation of HMF over different semi-
conductor photocatalysts.16–22
However, the achieved conver-
sion or selectivity is still unsatisfactory for practical appli-
cations. Two key factors controlling the biomass-derivatives
conversion and product selectivity are the photocatalyst design
and reaction conditions.7
In this regard, developing an
†Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d2gc04362a
a
Department of Chemical and Petroleum Engineering, University of Calgary, 2500
University Drive, NW, Calgary, Alberta T2N 1N4, Canada.
E-mail: heng.zhao1@whut.edu.cn, md.kibria@ucalgary.ca,
jinguang.hu@ucalgary.ca
b
Department of Chemistry, The University of Sheffield, Western Bank, Sheffield S10
2TN, UK
c
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan,
Hubei, China
d
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de
Bruxelles, B-5000 Namur, Belgium
e
Department of Geosciences, University of Calgary, 2500 University Drive, NW,
Calgary, Alberta T2N 1N4, Canada
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2. efficient photocatalyst and using suitable reaction conditions
for the selective oxidation of HMF to DFF still remain a chal-
lenge and have attracted significant attention.
ZnIn2S4, a typical ternary metal sulfide with a suitable band
gap (∼2.4 eV), has been widely used as a visible-light respon-
sive photocatalyst.23–25
The intrinsic two-dimensional
nanosheet structure endows ZnIn2S4 with separation and
transportation properties for photogenerated charge carriers.
Numerous efforts have been devoted to designing a ZnIn2S4-
based heterojunction by introducing noble metals or other
semiconductors to further improve the spatial separation of
electrons and holes.26–28
However, the formation of grain
boundaries from the mismatching of crystal lattices always
increases the electron transfer impedance at the interface.
From this aspect, utilizing the intrinsic formation of homo-
junctions by different crystal facets stands out for excluding
lattice mismatching.29,30
Another common issue for metal sul-
fides is photocorrosion due to the weak chemical bonds
between metals and sulfur.31
Particularly when the photo-
catalytic reaction proceeds in an oxygen atmosphere, the
breakage of metal–sulfur bonds and the displacement of
sulfur with oxygen leads to a dramatic decrease in the photo-
catalytic activity.32
Although ZnIn2S4-based and other compo-
sites have demonstrated the ability to convert HMF into DFF,
exactly determining the active sites and revealing the detailed
mechanism are still challenging. Therefore, realizing efficient
HMF oxidation to selectively produce DFF over pristine
ZnIn2S4 with the assistance of crystal facet engineering and
photocorrosion inhibition is highly desirable.
Herein, we demonstrate the photocatalytic oxidation of
HMF into DFF over ZnIn2S4 without any cocatalyst. Catalyst
screening tests and reaction optimizations reveal that the
highest activity is achieved by pristine ZnIn2S4 using aceto-
nitrile as the solvent under an oxygen atmosphere. However,
the unprecedented activity (91.1% HMF conversion and 99.4%
DFF selectivity) significantly decreases during the cycling test
and is caused by the inevitable photocorrosion of Zn–S with
the formation of Zn–O chemical bonds. The in situ catalyst
regeneration by replacing oxygen atoms with sulfurs is revealed
to improve the photocatalytic stability. Density functional
theory (DFT) calculations demonstrate the spontaneous separ-
ation of electrons and holes between the (110) and (102)
crystal facets, respectively, inducing the ORR into H2O2 and
HMF oxidation, respectively. This present work demonstrates
an example of achieving the sustainable production of value-
added chemicals from a biomass photorefinery with catalyst
regeneration.
2. Experimental
Synthesis of ZnIn2S4
0.5 mmol ZnCl2, 1.0 mmol InCl3·4H2O and 2.0 mmol thioace-
tamide were dissolved in a mixed solvent of 25 mL of de-
ionized water and 25 mL of ethanol. The transparent solution
was transferred into an 80 mL Teflon-lined autoclave and kept
at 180 °C for 2 h. The yellow ZnIn2S4 photocatalyst was finally
obtained after washing with 70% isopropanol three times and
then dried at 40 °C overnight.
Photocatalyst preparation
Different photocatalysts were prepared or purchased as control
samples. Detailed information on the photocatalysts used in
this work is provided in the ESI.†
Synthesis of 3DOM TiO2
3DOM TiO2 was prepared according to our previous report
with slight modifications.33,34
To prepare the precursor, 5 mL
of methanol, 1 mL of hydrochloric acid, and 5 mL of TIPT
were mixed and stirred for 5 min, then 2 mL of DI water was
added to the above solution and the solution mixture was
stirred at room temperature for another 5 min to obtain the
precursor. 5 g of dried polystyrene colloidal template was
placed in a Buchner funnel with a filter paper and the pre-
pared precursor was added to the Buchner funnel by applying
suction. The precursor filtration process was repeated every
8 h three times. After air-drying the mixture of precursor and
template, 3DOM TiO2 could be obtained after calcination at
300 °C for 2 h, 400 °C for 2 h and 550 °C for 2 h at a heating
rate of 2 °C min−1
in air.
Characterization
X-ray diffraction (XRD, Bruker D8 Advanced) analysis was used
to investigate the crystalline structure. Field emission scanning
electron microscopy (FESEM, Hitachi S-4800) was performed
for the morphological investigations. X-ray photoelectron spec-
troscopy (XPS) measurement was performed on the ESCALAB
250Xi instrument (Thermo Scientific) to demonstrate the
surface chemical state of the photocatalyst. The binding
energy for the C (1s) peak at 284.8 eV (relative to adventitious
carbon from the XPS instrument itself) was used to calibrate
all the XPS results. The UV-vis absorbance spectra were
recorded using a UV-vis spectrophotometer (Lambda 365,
PerkinElmer) in the integrating sphere mode with BaSO4 as
the reference. The generated radicals during the photocatalytic
reaction were monitored using electron paramagnetic reso-
nance (EPR) spectroscopy. The specific surface area and pore
distribution were analyzed by nitrogen adsorption–desorption
on a Micrometrics Tri Star II 3020. Soft X-ray absorption spec-
troscopy (sXAS) was performed using a spherical grating
monochromator (SGM) beamline 11ID-1 of the Canadian Light
Source (CLS) synchrotron. Excitation–emission matrix spec-
troscopy (EEMS) measurement to investigate the energy edges
of all the elements present in the materials was performed in
an energy range of 250–2000 eV (energy resolution ∼5 eV).
Photocatalytic measurements
The photocatalytic reactions were performed in closed glass
vials (20 mL) using a 300 W xenon lamp (Excelitas Tech.).
Typically, 20 mg of ZnIn2S4 photocatalyst was dispersed in
10 mL (5 mM) of HMF solution (with acetonitrile as the
solvent) and the suspension was sonicated to fully disperse the
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3. photocatalyst. The reaction system was purged with pure O2
for 0.5 h before light irradiation. The liquid sample was
periodically analyzed with a high-performance liquid chro-
matograph (HPLC) which was equipped with an Aminex
HPX-87H (300 × 7.8 mm, Bio-Rad) column and a refractive
index detector (RID). The photocatalyst loading, HMF concen-
tration, solvent and atmosphere were the variables involved in
this reaction system. Scavenger tests were performed with the
same process as indicated above but with the addition of
5 mM ethylenediaminetetraacetic acid (EDTA), isopropyl
alcohol (IPA), p-benzoquinone (BQ) and sodium azide (NaN3)
to elucidate the roles of photogenerated holes (h+
), hydroxyl
radicals (•
OH), superoxide radical anions (•
O2
−
) and singlet
oxygen (1
O2), respectively. Different wavelengths of monochro-
matic light were provided using 365, 400, 450, 500, 550 and
600 nm filters, respectively.
Cycling tests and photocatalyst regeneration
Cycling tests were performed by collecting the photocatalyst
after one hour from the reaction solution, washing the photo-
catalyst with acetonitrile several times and then re-dispersing
the reaction solution into another fresh reaction solution con-
taining 10 mL (5 mM) of HMF solution. A small amount of a
fresh photocatalyst was added to the resulting solution so that
20 mg of ZnIn2S4 could be used in each cycle. The cycling tests
were conducted four times to check the stability of the photo-
catalyst. To regenerate the photocatalyst after one cycling reac-
tion, the collected photocatalyst was dispersed into a 0.2 M
Na2S aqueous solution and the mixture was stirred at 80 °C for
12 h. The precipitate was washed with DI water several times
and then dried in an oven overnight.
Computational details
All DFT calculations were performed using the Vienna Ab initio
Simulation Package (VASP).35–37
The Perdew–Burke–Ernzerhof
(PBE) functional of the Generalized Gradient Approximation
(GGA) was used to describe the exchange–correlation energy.38
To describe the expansion of the electronic eigenfunctions, the
projector-augmented wave (PAW) method was applied with a
kinetic energy cutoff of 500 eV.39,40
The total energy and force
convergence threshold were set to 10−6
eV and 0.02 eV Å−1
,
respectively. The Brillouin zone was sampled with a 4 × 4 × 1
k-point grid of the Monkhorst–Pack scheme.41
A 15 Å vacuum
was set above the 102 and 101 slabs to prevent the interaction
between the two periodic images. The van der Waals inter-
actions were considered using the empirical correction via the
DFT + D3 scheme.42,43
3. Results and discussion
We first screened different semiconductor photocatalysts on
the performance of HMF conversion to DFF under simulated
light irradiation. These selected photocatalysts have different
band gap structures, light absorption abilities and the capa-
bility to produce active oxygen species (Fig. S1†). All the photo-
catalysts exhibited HMF conversion ability, but few of them
selectively produced DFF (Table 1). Combining the experi-
mental results with the band gap structure, it can be con-
cluded that the photocatalyst with the ability to produce the
hydroxyl radical (•
OH) shows a poor DFF selectivity, which is
likely due to the strong oxidative capability of •
OH to trigger
the over-oxidation reaction. Meanwhile, the ability of the
photocatalyst to produce the superoxide radical anion (•
O2
−
)
leads to a good DFF selectivity. CdS and ZnIn2S4 actually have
similar band gap structures, but the latter is endowed with
both high HMF conversion and DFF selectivity. These results
indicate that the reaction of HMF to DFF driven by photocata-
lysis is not only related to the band gap structure but also
attributed to the atomic composition of the photocatalyst.
Due to its excellent performance, ZnIn2S4 was chosen as the
photocatalyst to investigate the selective oxidation of HMF to
DFF in detail. Upon irradiation, ZnIn2S4 exhibited an excep-
tional ability to convert HMF to DFF with 99.4% selectivity and
91.1% HMF conversion within a 1 h reaction time (Fig. 1),
which demonstrated an outstanding performance compared
with the reported literature (Table S1†). A series of control
experiments was then conducted to confirm the critical effect
of the photocatalytic system (Table 2). It is clear that no HMF
Table 1 Photocatalytic conversion of HMF to DFF over different
photocatalysts
Entry Photocatalyst HMF Con. (%) DFF Sel. (%) DFF yield (%)
1 g-C3N4 31.3 63.5 19.9
2 NiTiO3 12.5 0 0
3 BiVO4 5.3 0 0
4 TiO2 55.1 62.8 34.6
5 ZnS 7.8 0 0
6 In2S3 4.9 0 0
7 CdS 1.6 0 0
8 CdIn2S4 13.6 93.6 12.7
9 NiIn2S4 5.3 92.7 4.9
10 ZnIn2S4 91.1 99.4 90.6
Fig. 1 Photocatalytic HMF conversion and DFF selectivity with reaction
time. Reaction conditions: 5 mM HMF, 10 mL of CH3CN, 20 mg of
ZnIn2S4, O2, and simulated solar light irradiation. Legend: black line indi-
cates HMF conversion and red bars indicate the DFF selectivity.
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4. conversion and DFF production were observed without a
photocatalyst, light or O2, demonstrating the synergistic effects
of these parameters. The solvent was also changed to check
the effect on the selective oxidation of HMF to DFF (entries 4
and 5 in Table 2) and it was found that acetonitrile would be
the best solvent for selective DFF production from HMF photo-
catalytic conversion. The effects of photocatalyst loading and
HMF concentration on the performance were also investigated
(Fig. S2 and S3†).
Reaction conditions: 20 mg of the photocatalyst, 10 mL of
acetonitrile, 5 mM HMF, O2, and 1 h.
The effect of incident light on selective HMF conversion to
DFF was further investigated. The as-prepared ZnIn2S4 exhibi-
ted visible light absorbance with an absorption edge at around
590 nm, corresponding to the band gap energy of 2.1 eV
(Fig. S4†). Monochromatic light with wavelengths of 365, 400,
450, 500 and 550 nm were selected as the light source for the
photocatalytic reaction, respectively. The HMF conversion was
consistent with the light absorption spectrum in the visible
light region (λ > 400 nm) and a high DFF selectivity was
achieved with different monochromatic light irradiation wave-
lengths (∼100% at 400 nm, ∼94% at 450 nm, ∼84% at 500 nm
and ∼83% at 550 nm) (Fig. 2a). However, HMF conversion at
365 nm was an exception with reduced photocatalytic activity
(∼21.4% conversion and ∼94.5% selectivity). This result indi-
cates that the presence of ultraviolet light causes the activity
loss of ZnIn2S4. The cycling tests also demonstrated a similar
result. The HMF conversion dropped to ∼26.8% during the
second cycle and ∼12.8% during the fourth cycle while the
DFF selectivity also finally decreased to ∼48.6% under simu-
lated solar light irradiation (Fig. 2b). To exclude the effect of
ultraviolet light on the loss of photocatalytic activity, control
experiments using visible light (λ > 400 nm) were conducted.
The results demonstrated that ZnIn2S4 also exhibited signifi-
cantly decreased HMF conversion and DFF selectivity during
the cycling test under visible light (Fig. S5†).
It was speculated that the irreversible change of ZnIn2S4
during the reaction under simulated solar light contributed to
the decrease in photocatalytic performance. Herein, the photo-
catalyst was recovered after the reaction and the morphology
and chemical state were analyzed accordingly. There was
almost no difference in the morphology of ZnIn2S4 after the
reaction (Fig. S6†). The high crystallinity with the exposed
(110) and (102) crystal facets remained and each element was
distributed uniformly on the nanosheet-assembled flower
structure (Fig. S7†). However, it is clear from the XPS results
that a typical signal of oxygen was observed in ZnIn2S4 after
the reaction, indicating that ZnIn2S4 suffered from irreversible
photo-corrosion under simulated solar light (Fig. 3a). A typical
Zn–O chemical bond was detected in ZnIn2S4 after the reaction
and the relative area of the Zn–O chemical bond was larger
than that of the pristine Zn–S chemical bond in ZnIn2S4, indi-
cating that ZnIn2S4 suffered from a serious change in chemical
composition under simulated solar light irradiation (Fig. 3b).
The signals of In 3d were almost the same for ZnIn2S4 after the
reaction, indicating the presence of simulated solar light just
broke Zn–S bonds rather than In–S bonds in the ZnIn2S4
crystal (Fig. 3c). The released sulfur from the ZnIn2S4 crystal
was revealed to form sulfate ions (Fig. S8†). However, no metal
ions (Zn2+
or In3+
) were detected using an inductively coupled
plasma emission spectrometer in the solution after the photo-
catalytic reaction, indicating the formation of metal oxides
Fig. 2 (a) Light absorption spectrum of ZnIn2S4 along with HMF con-
version and DFF selectivity under monochromatic light with different
wavelengths and (b) cycling tests of ZnIn2S4 under simulated solar light
irradiation.
Fig. 3 (a) XPS survey spectra and high-resolution XPS spectra of (b) Zn
2p, (c) In 3d, EEMS maps of (d) Zn and (e) In of ZnIn2S4 before the reac-
tion and after the reaction under simulated solar light irradiation.
Table 2 Photocatalytic conversion of HMF to DFF under different conditions
Entry Photo-catalyst Light Solvent Atmo-sphere HMF Con. (%) DFF Sel. (%) DFF yield (%)
1 ZnIn2S4 Xenon MeCN O2 91.1 99.4 90.6
2 No Xenon MeCN O2 0 0 0
3 ZnIn2S4 Xenon MeCN Ar 0 0 0
4 ZnIn2S4 Xenon Water O2 100 47.3 47.3
5 ZnIn2S4 Xenon DMSO O2 24.5 26.8 6.6
6 ZnIn2S4 No MeCN O2 0 0 0
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5. which were attached on the catalyst surface. To further
confirm the formation of Zn–O during the reaction, ZnIn2S4
photocatalysts before and after the reaction were analyzed by
synchrotron-based soft X-ray absorption spectroscopy (sXAS).
The excitation–emission matrix spectroscopy (EEMS) map of
the ZnIn2S4 sample before the photocatalytic reaction dis-
played a bright line in the Zn L-edge region in 1020–1050 eV,
whereas no signals for the O K-edge region were detected, sub-
stantiating the presence of Zn–S in the ZnIn2S4 lattice
(Fig. 3d). Interestingly, after the reaction, in addition to the Zn
L-edge line, the O K-edge line also appeared at 530 eV,
suggesting that a certain fraction of the catalyst was oxidized
to Zn–O species on the surface during the reaction. The SGM
beamline of the Canadian light source operates in the energy
range 250–2000 eV, so no signals can be acquired for the In
L-edge (3730 eV). Hence, we measured the In M-edge, which
displayed a bright line in the energy range of 440–455 eV.
There was no change observed after the reaction, validating
that the lattice structure remained unaltered and only the
surface of Zn–S was oxidized to Zn–O (Fig. 3e).
To figure out the active oxygen species for HMF conversion
to DFF and Zn–O bond formation during the reaction, EPR
was performed with the addition of 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TMP) as
spin traps in the reaction solution. The formation of •
O2
−
from
the oxygen reduction reaction with photogenerated electrons
was confirmed by the characteristic DMPO/•
O2
−
signal
(Fig. 4a). A typical 1 : 1 : 1 triplet signal was observed, and the
corresponding intensity increased with irradiation time, indi-
cating the formation of 1
O2 during the photocatalytic reaction
(Fig. 4b). The formed 1
O2 should come from the oxidation of
•
O2
−
by the photogenerated holes.44,45
In addition, the pres-
ence of •
OH was also confirmed by the typical DMPO/•
OH
signal with a relative intensity of 1 : 2 : 2 : 1 under irradiation
(Fig. 4c). Theoretically, pure ZnIn2S4 does not have enough
valence band potential to directly oxidize absorbed H2O to
produce •
OH (Fig. S1†). The generated •
OH would come from
(1) H2O oxidation by Zn–O species on the photocatalyst surface
and (2) decomposition of the produced H2O2. The produced
H2O and/or H2O2 are from the oxygen reduction reaction by
photogenerated electrons with a combination of protons from
HMF. The effects of the produced active oxygen species on
HMF conversion and DFF selectivity were also investigated by
adding scavengers in the reaction solution (Fig. 4d). The slight
decrease in HMF conversion with the addition of isopropyl
alcohol (IPA) indicates the presence of •
OH is not the main
species for HMF conversion to DFF. The addition of ethylene-
diaminetetraacetic acid (EDTA) leads to a significant decrease
in HMF conversion, indicating the vital effect of photogene-
rated holes (h+
). A similar decrease in HMF conversion could
be achieved with the addition of p-benzoquinone (BQ) and
sodium azide (NaN3) as scavengers of •
O2
−
and 1
O2, respect-
ively. The above results demonstrate the synergistic effect of
h+
, •
O2
−
and 1
O2 in photocatalytic HMF conversion to DFF pro-
duction over ZnIn2S4.
Herein, the photocatalytic mechanism on selective HMF
valorization to DFF over ZnIn2S4 was proposed accordingly
(Fig. 4e). Under the irradiation of simulated solar light, the
photogenerated electrons on the photocatalyst surface activate
molecular O2 into active •
O2
−
, which deprotonates HMF on the
hydroxyl group and forms an alkoxide anion intermediate. The
release proton is combined with •
O2
−
to form the hydroperoxyl
radical (•
OOH). The photogenerated holes at the valence band
(VB) are annihilated by the electrons from the alkoxide anion
intermediate, which then forms the corresponding anionic
radical. The produced •
OOH further deprotonates the anionic
radical to finally produce DFF while H2O2 is the side product,
which follows a similar reaction pathway to that in the
literature.45,46
One the other hand, the 1
O2 produced from the
oxidation of •
O2
−
by photogenerated holes also participates in
the deprotonation of HMF to DFF with the formation of H2O2.
The produced H2O2 was confirmed by adding excess KI,
which reacts with H2O2 to form triiodide (I3
−
) with typical
absorbance at 350 nm (Fig. S9†). The concentration of H2O2
was calculated to be 2.56 mM by the equation H2O2 + 3I−
+
2H+
→ I3
−
+ 2H2O, which was far below the theoretical value
(4.56 mM). This result revealed that the •
OH detected by EPR
originated from the decomposition of H2O2. It is well known
that •
OH has very strong oxidation ability and the formation of
the Zn–O chemical bond after the reaction arises partially
from oxidation by •
OH. To check the •
OH on the crystal struc-
ture of ZnIn2S4 during the reaction, the photocatalyst was
recovered after the reaction and tested by XRD. It is clear that
the photocatalyst still has the typical hexagonal structure
(Fig. 5a). However, the intensity of the (110) facets has
decreased significantly, indicating that the exposed (110) facet
provides more active sites, which is consistent with the
reported investigation.47
To quantify the change in the (110)
facet intensity, we defined and calculated the intensity ratio of
three major XRD diffraction peaks (Table S2†). The intensity
ratio of (110)/(102) shows a consistently linear relation with
photocatalytic HMF conversion (Fig. 5b), indicating the vital
role of (110) and (102) facets in the photocatalytic HMF to DFF
reaction.
Fig. 4 EPR signals of the reaction solution in the dark and simulated
solar light irradiation with different times in the presence of DMPO and
TMP as spin-trapping reagents for (a) •
O2
−
, (b) 1
O2 and (c) •
OH. (d) Effect
of different scavengers on HMF conversion and DFF selectivity. (e)
Proposed photocatalytic mechanism on HMF conversion to selective
DFF production over ZnIn2S4.
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6. To gain insights into the detailed effect of the (102) and
(110) crystal facets on the photocatalytic reaction, density func-
tional theory (DFT) calculations were performed. The (102)
surface is found to have a higher work function (5.71 eV),
which makes it more difficult to lose an electron, whereas the
(110) surface has a lower work function (5.02 eV), allowing
facile O2 reduction on this surface (Fig. 5c). The charge density
plots also demonstrate the less enriched electron densities of
the (102) and (110) surfaces, respectively (Fig. 5d). Therefore,
we believe that the oxidation of HMF mainly occurs on (102)
surface while the reduction of O2 occurs on the (110) surface.
The HMF is found to be absorbed on the Zn sites on the (102)
surface (inset of Fig. 5e). After absorption on the surface, the
HMF is polarized and activated, leading to the easier deproto-
nation of the terminal C–H bond with the formation of a Cα
radical, which is consistent with the proposed reaction
pathway (Fig. 4e). HMF oxidation and O2 reduction proceeded
downhill in the reaction energy profiles, indicating the dual
functionality of the as-prepared ZnIn2S4 photocatalyst (Fig. 5e).
At the same time, the H2O2 formed on (110) is found to be
easily dissociated into OH* radicals with a strong capability to
cleave the Zn–S chemical bonds, leading to decreased activity
(Fig. S10†).
According to above experimental and computational
results, the breakage of Zn–S on (110) during the photo-
catalytic reaction to form Zn–O species is the key factor for the
unsatisfactory stability of ZnIn2S4. Herein, we tried to realize
the in situ decomposition of the produced H2O2 to inhibit the
oxidation of Zn–S chemical bonds. Catalase and CeSO4 were
separately used for H2O2 decomposition. However, due to the
limited solubility of catalase and CeSO4 in acetonitrile, the
activity of ZnIn2S4 still decreased significantly during the
cycling tests (Fig. S11 and S12†). Then, the catalyst regener-
ation was conducted by immersing the used photocatalyst into
a Na2S aqueous solution by the replacement of oxygen species
with sulfur. Due to the significant difference of the solubility
product constant between ZnS (2.93 × 10−25
) and ZnO (6.8 ×
10−17
), the presence of S2−
could theoretically substitute O2−
to
retrieve ZnIn2S4. The cycling tests over the regenerated ZnIn2S4
showed that the improved stability and HMF conversion had
increased from 12.8% to 73.2% even at the fourth cycling reac-
tion (Fig. 6a). The DFF selectivity had also increased from
48.6% to 76.6% at the fourth cycling reaction (Fig. 6b). The
achieved results revealed the positive enhancement of photo-
catalytic HMF conversion into DFF during the long-term
cycling reaction.
4. Conclusions
In summary, ZnIn2S4 was first screened as the best photo-
catalyst for the selective HMF conversion into DFF. The
highest activity (91.1% HMF conversion and 99.4% DFF
selectivity) was achieved in acetonitrile under an oxygen
atmosphere. The photogenerated electrons and holes partici-
pated in the formation of active oxygen species to realize the
oriented reaction. The oxygen molecule and in situ decompo-
sition of H2O2 into •
OH induced the irreversible breakage of
Zn–S with the inactive Zn–O bonds on the (110) and (102)
crystal facets, leading to the significant loss of photostability.
Photocatalytic regeneration by the spontaneous replacement
of O2−
with S2−
was revealed to improve the durability of
ZnIn2S4. This present work demonstrates a good example of
photocatalytic regeneration to retrieve the active site for the
selective biomass or biomass-derivatives valorization into
value-added chemicals.
Conflicts of interest
There are no conflicts to declare.
Fig. 5 (a) XRD patterns of ZnIn2S4 before and after cycling reactions, (b)
relation between the intensity ratio of the (110)/(102) crystal facet and
HMF conversion. (c) Work functions and (d) charge densities of the (102)
and (110) crystal facets. (e) Reaction energy profiles of HMF oxidation on
(102) and O2 reduction on (110).
Fig. 6 Cycling tests of (a) HMF conversion and (b) DFF selectivity over
pristine ZnIn2S4 and regenerated ZnIn2S4 by Na2S.
Paper Green Chemistry
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7. Acknowledgements
This work is financially supported by the Canada First
Research Excellence Fund (CFREF). The authors would like to
thank Dr Tom Regier and Dr James Dynes for their assistance
with soft X-ray absorption spectroscopy measurements.
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