Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.

1. Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-
Doped Carbon Nitride
Jiu Wang,⊥
Qi Zhao,⊥
Pawan Kumar, Heng Zhao,* Liquan Jing, Devis Di Tommaso,
Rachel Crespo-Otero, Md Golam Kibria,* and Jinguang Hu*
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sı Supporting Information
ABSTRACT: Biomass photorefining is a promising strategy to
address the energy crisis and transition toward carbon carbon-
neutral society. Here, we demonstrate the feasibility of direct
cellulose photorefining into arabinose by a rationally designed
oxygen-doped polymeric carbon nitride, which generates favorable
oxidative species (e.g., O2
−
, •
OH) for selective oxidative reactions
at neutral conditions. In addition, we also illustrate the mechanism
of the photocatalytic cellulose to arabinose conversion by density
functional theory calculations. The oxygen insertion derived from
oxidative radicals at the C1 position of glucose within cellulose
leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in
the subsequent gluconic acid formation. The following decarbox-
ylation process of gluconic acid via C1−C2 α-scissions, triggered
by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic
understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose
photorefining into value-added bioproducts under mild conditions.
KEYWORDS: cellulose photorefining, carbon nitride, oxidative cleavage, arabinose, cellulose photorefining mechanism
Biomass valorization is a viable method to mitigate energy
scarcity as well as environmental problems.1,2
Biomass
photorefinery is an emerging biomass valorization strategy for
transforming biomass into value-added bioproducts together
with sustainable H2 coproduction, by using sunlight as energy
source and water as supplementary substrates.3,4
In the past
few years, we and others have already demonstrated the
feasibility of selective converting biomass-derived soluble
components into a series of value-added biochemicals by
rational photocatalyst design and reaction condition regu-
lation.5−8
For example, we can convert glucose, the basic
chemical building block of cellulose, into arabinose,8
glycerol,9
lactic acid,10
fructose,11
and gluconic acid,12
respectively, by
adjusting photocatalyst band gap energy, morphology, and
surface chemistry, as well as regulating reaction pH,
atmosphere, and solvent media. However, directly and
selectively converting cellulose into high-value bioproducts
via photocatalysis has been challenging. So far, most of studies
only utilized cellulose as a sacrificial agent to facilitate
photocatalytic hydrogen production, where cellulose was
converted to various types of organic acids or overoxidized
to CO2 without selectivity.13−16
While few studies demon-
strated the potential of converting cellulose into lactic acid via
photocatalysis, these could only be achieved at high alkalinity
that caused serious environmental issue and difficulties for
product separation.17,18
Therefore, it is highly desired to design
a photocatalysis process that can directly convert cellulose
macromolecules into specific value-added chemicals under
neutral conditions. In addition, the lack of mechanistic
understanding of cellulose photocatalytic conversion impedes
the practical application of the biomass photorefinery.
Numerous photocatalysts, such as titanium dioxide (TiO2),
zinc oxide (ZnO), zinc indium sulfide (ZnIn2S4), cadmium
sulfide (CdS), and carbon nitride (CN) have been utilized in
biomass photoreforming.19−23
Among these, CN demonstrates
great potential due to its nontoxicity, stable physicochemical
properties, promising band structure, and favorable response to
visible light.24
In addition, CN can be easily modified through
elemental doping to generate a large amount of favorable
oxidative radicals during photocatalysis.12,25
It has been proven
that selective photocatalytic glucose/cellobiose conversion at
neutral pH could be achieved by certain reactive oxidative
species (e.g., O2
−
, •
OH) generated under light irradia-
Received: December 12, 2023
Revised: January 22, 2024
Accepted: February 5, 2024
Research Article
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© XXXX American Chemical Society
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2. tion.12,26,27
Thus, we hypothesize that rational modification of
CN could realize selective cellulose photocatalytic conversion
at neutral pH.
Herein, for the first time, we demonstrated the feasibility of
direct cellulose photocatalytic conversion into value-added
bioproducts (arabinose) at neutral pH, and we explored the
reaction mechanism by density functional theory (DFT)
calculations. This process is realized by an oxygen-doped CN
photocatalyst with favorable oxidative species (O2
−
, •
OH)
generation. This leads to the oxygen insertion at the C1
position of glucose within cellulose and results in the oxidative
cleavage of cellulose β-1,4 glycosidic linkages to eventually
produce gluconolactone. The gluconolactone further hydro-
lyzes into gluconic acid and then decarboxylates via C1−C2 α-
scissions to form arabinose and formic acid, respectively. This
work provides an example to design a reaction pathway for
direct cellulose photocatalytic conversion at a neutral pH.
■ RESULTS AND DISCUSSION
The oxygen-doped CN is synthesized from dicyandiamide and
cyanuric chloride via a facile solvothermal method, which is
marked as the OCN−OH (Figure 1a). The control photo-
catalyst (DCN) is produced through a single-step thermal
polymerization using dicyandiamide as a precursor. As shown
in Figure S1, the DCN shows an irregular bulk structure, while
the morphology of the OCN−OH exhibits well-defined
microspheres with uniform distribution by field emission
scanning electron microscopy (FESEM) (Figure 1b), which is
observed by the transmission electron microscopy (TEM)
image (Figure 1c). The analysis conducted with high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) demonstrates an even distribution of the
elements C, N, and O within the OCN−OH matrix, as
observed in the elemental mappings (Figure 1d).
X-ray diffraction (XRD) patterns and Fourier-transform
infrared (FTIR) spectra confirm the formation of CN. The
XRD peaks observed at 13.1° and 27.5° for DCN correspond
to the (100) and (002) peaks, indicative of the representative
features of CN materials (Figure S2).28
It is worth noting that
the (100) peak of OCN−OH nearly vanishes, presumably
ascribed to disorder packing of graphitic structure since the
polymerization degree of CN is relatively lower than that of the
high-temperature calcination synthesis.29
As shown in Figure
2a, the presence of s-triazine rings in the structure of CN is
supported by a characteristic peak at 810 cm−1
, while signals
within the range of 1150 to 1750 cm−1
are attributed to the
C−N bonds within heterocycles. In addition, it is worth
mentioning that OCN−OH demonstrates an obvious redshift
as well as remarkably increased visible-light adsorption (Figure
2b), suggesting the rearrangement by elemental doping within
the CN structure results in alternation to the electronic and
optical properties of the material.30
By means of the Kubelka−
Munk function, the band gaps of DCN and OCN−OH are
determined to be 2.76 and 2.32 eV, respectively (inset of
Figure 2b). The lower steady-state photoluminescence
observed in OCN−OH, as compared to DCN, indicates an
improvement in the charge carriers separation of the former
(Figure S3). Time-resolved photoluminescence (TRPL)
spectra, photocurrent measurements, and electrochemical
impedance spectroscopy (EIS) have been conducted for
studying the charge dynamics of OCN−OH. In comparison
to DCN, the average lifetime of the OH decreases from 9.83 to
5.42 ns, as shown in Figure 2c and Table S1. This decrease
could be attributed to the fact that photogenerated electrons
rapidly transfer through nonradiative pathways from the bulk
to the interface rather than undergoing recombination, leading
to an improvement in the separation efficiency of charge
carriers.31,32
It can be seen that OCN−OH exhibits a higher
photocurrent and a shorter radius of Nyquist plot in
Figure 1. (a) Synthetic process for oxygen-doped CN through one-step solvothermal synthesis. (b) FESEM image, (c) TEM image, and (d)
HAADF-STEM image of OCN−OH with relevant elemental mappings. The colors of red, green, and blue represent the elemental components of
C, N, and O, respectively.
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3. comparison to DCN (Figure S4), indicating superior charge
separation efficiency. Based on the Mott−Schottky plots, it can
be inferred that both DCN and OCN−OH are categorized as
n-type semiconductors due to their positive slopes (Figure
S5a,b). The flat band positions of DCN and OCN−OH were
determined to be −0.59 and −0.71 eV, respectively, while the
estimated conduction band positions were −0.79 and −0.91
eV, respectively.33
The band structure of the samples is
provided in Figure S5c. The electron paramagnetic resonance
(EPR) spectra can be utilized to further investigate the
paramagnetic properties of CN doping with heteroatoms.34
The oxygen-doped CN shows the enhanced EPR signal than
that of DCN (Figure 2d), presumably due to the paramagnetic
defects in OCN−OH, thus leading to heavier delocalization by
the substitution of heteroatoms.35
This might lead to enhanced
mobility of spins and higher charge carriers density, thus
advantageous during the photocatalytic reaction.36,37
An in-
depth investigation of the electronic characteristics was carried
out to analyze the charge distributions in DCN and OCN−
OH via employing DFT calculations. As shown in Figure 2e
and f, it is found that the bandgap of OCN−OH is narrower
than that of DCN from density of states (DOSs), which aligns
well with experimental measurements involving the extended
optical adsorption (Figure 2b). Upon oxygen doping, the
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of the samples
demonstrate significant alterations in charge distribution.
Evidently, the charges in the LUMO and HOMO orbitals of
DCN show nearly uniform distribution along DCN chains,
indicating the presence of overlapping electrons and holes
(Figure 2g). In contrast, the delocalization of OCN−OH is
more pronounced (Figure 2h), implying improved spatial
separation of charge carriers. The whole HOMO and LUMO
distribution of the catalysts are provided in Figure S6. The
substantial spatial charge separation in HOMO and LUMO
orbits within OCN−OH could effectively distance the redox
sites, thereby reducing the likelihood of charge recombina-
tion.38,39
XPS can be employed to delve deeper into the chemical
environment of elements for the photocatalysts. As depicted in
Figure 3a, impurities of C, C attached to −NH2 groups, and
N−C�N inside rings are represented by the three peaks of C
1s at 284.8, 286.1 (286.4), and 288.3 eV, respectively. In
addition to the common responses, the OCN−OH exhibits a
newly generated peak at 289.2 eV, which could be attributed to
the C−O bond.40
Moreover, the N 1s curve of the samples
Figure 2. (a) FTIR spectra, (b) UV−Vis DRS, (c) TRPL spectra, and (d) EPR spectra of DCN and OCN−OH. Calculated DOS of (e) DCN and
(f) OCN−OH. HOMO and LUMO distributions of (g) DCN and (h) OCN−OH, respectively.
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4. displays four distinct peaks at 398.7 (398.8), 399.5 (399.8),
400.8, and 404.3 eV, which correspond to C−N�C, N−(C)3,
N within −NH2 groups, and charging effects, respectively
(Figure 3b).41−43
Additionally, the O 1s signals can be
assigned to corresponding signals at 531.5, 532.4, and 533.6
eV, associated with C−O, O−H, and adsorbed oxygen (Figure
3c).12,44
Note that the peak at 531.5 eV of OCN−OH is
stronger and sharper than that of DCN, revealing the oxygen
doping during the solvothermal process, thus giving rise to the
substitution of bicoordinated N sites by O atoms.45
Elemental
analysis provides further evidence of the O doping (Table S2).
It is found that the ratio of O grows from 1.6% of DCN to
16.9% of OCN−OH. Solid-state 13
C NMR spectroscopy
further proves the CN heptazine structure (Figure 3d).
Referring to DCN, the characteristic peaks of OCN−OH of
156.4 and 164.1 ppm can correspond to CN3 moieties and
CN2−(NHx), indicating the distinct poly(tri-s-triazine)
structure within OCN−OH.46
Based on the aforementioned
results, the oxygen-functionalized microsphere CN via a one-
step solvothermal process well retains the typical heptazine
structure as a high-temperature calcination method. The
synchrotron-based X-ray absorption near-edge structure
(XANES) spectra are further carried out to investigate the
structural characteristics of DCN and OCN−OH. As shown in
Figure 3e, the C K-edge XANES spectra of DCN and OCN−
OH exhibit three characteristic peaks including N-defects in
the heptazine rings, π* (C�C) and π* (C−N−C),
respectively.47
As compared to DCN, the resonance peak of
π* (C−N−C) shows a slightly positive shift in photon energy,
presumably owing to the loss of some electrons from C atoms
in OCN−OH.48
In N K-edge XANES spectra (Figure 3f), the
observed absorption peaks at 399.4 and 402.5 eV can originate
from N in aromatic C−N−C coordination and N-3C moieties,
respectively. The broad peak at 407.2 eV can be assigned to
electron transition from N 1s to C−N σ* orbital.49
Note that
the intensity of the π* (C−N−C) feature decreases after
structural modulation, which could be attributed to the
substitution of bicoordinated N sites by oxygen doping.50
All
in all, the rationally designed OCN−OH with remarkably
enhanced visible light absorption and promoted charge
separation efficiency might produce favorable oxidative species
for selective oxidative reactions during cellulose photorefining.
When the well-designed OCN−OH is utilized for cellulose
photorefining, it indeed demonstrates the feasibility to obtain
value-added products from direct cellulose valorization. There
are mainly three products after the 24-h photocatalytic reaction
Figure 3. High-resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) O 1s of DCN and OCN−OH. (d) Solid-state 13
C NMR spectra of DCN and
OCN−OH. XANES spectra of the (e) C K-edge and (f) N K-edge for DCN and OCN−OH.
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5. adopting cellulose-II as the initial substrate (1 g/L) with a
conversion of ∼18%, namely, arabinose, formic acid, and acetic
acid (Figure S7). Arabinose is the dominant product, whereas
formic acid and acetic acid as byproducts after the reaction and
the resultant concentration of arabinose, formic acid, and
acetic acid are determined to be 125, 20, and 40 mg/L,
respectively. The tough cellulose depolymerization could serve
as the rate-determining step during the photorefining process.
It should be noticed that a small amount of gluconic acid can
be observed after the cellulose photorefining process, which
could be attributed to the oxygen insertion at the C1 position
of glucose within cellulose, thus leading to the oxidative
cleavage of β-1,4-glycosidic linkage.26
Moreover, this process
can produce glucose as an intermediate during the photo-
reforming, which is most likely happened in the reducing end
position of the cellulose chain.51
These results demonstrate the
feasibility of direct cellulose photorefining into arabinose as a
major product and the nature of OCN−OH with hydroxyl
modification could improve the hydrophilic accessibility
between catalyst and cellulose, thus leading to easier
depolymerization of cellulose fragments.52
In order to figure
out the intrinsic mechanism behind cellulose photorefining
into arabinose, glucose as the basic unit of cellulose can be
rationally adopted. We carried out the experiments for glucose
photoreforming over OCN−OH and it shows ∼10% glucose
conversion under air for 6 h irradiation (Figure 4a), whereas
DCN shows negligible glucose conversion. It is found that
arabinose is the dominant product, and the selectivity of
arabinose remains >90% at the end of 6 h illumination.
Additionally, OCN−OH exhibits exceptional recyclability in
converting glucose and selectively producing arabinose, as
demonstrated by the 18 h cycling test (Figure 4b). Then, the
experiments under the O2 atmosphere are conducted and it is
found that glucose conversion over OCN−OH increases to
∼28%, while DCN still exhibits negligible glucose conversion
(Figure 4c). It is interesting that gluconic acid, arabinose, and
formic acid become the primary components with reaction
time. The selectivity of gluconic acid and arabinose exhibits a
decreasing trend while formic acid increases with time, but
there is >80% total selectivity after a 6 h photocatalytic
reaction (Figure 4d). We extended the glucose photoreforming
reaction to 8 h under an O2 atmosphere, and it was found that
glucose conversion grows over time and achieves a plateau of
∼31% at the end of the reaction, while the arabinose selectivity
Figure 4. (a) Glucose conversion and corresponding arabinose selectivity of OCN−OH upon 6-h irradiation under air atmosphere. (b) The
recyclability of OCN−OH for glucose photoreforming into arabinose during three cycles for 18 h. (c) Glucose conversion under O2 atmosphere,
and corresponding (d) products selectivity, and (e) carbon balance for glucose photoreforming over OCN−OH. (f) Scavenger experiments of
OCN−OH with the addition of BQ, TBA, and EDTA-2Na. (g) ESR spectra of CPH-h+
under N2, (h) DMPO-•
O2
−
and (i) DMPO-•
OH for
OCN−OH. Reaction conditions: 10 mg of photocatalyst, 10 mL of 2g/L glucose solution (pH ∼ 7), air or O2, and 300 W xenon lamp.
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6. continues to decrease to ∼22% (Figure S8). To promote the
high selectivity of arabinose, ion-exchange resin could be
considered for the separation of distinct monosaccharides and
organic acids within the system.53−55
There is negligible CO2
or CH4 formation during the process. Control experiments
without light or photocatalysts exhibit no glucose conversion
and arabinose production, suggesting that the glucose
photoreforming process is indeed triggered by incident
photons and photocatalysts. In addition, the gas product CO
can be detected during the reaction, and the amount is
increasing (Figure S9), reaching ∼80.2 μmol/g at 6 h, which is
most likely originated from formic acid dehydration.56,57
The
presence of hydrogen peroxide (H2O2) is determined during
the photocatalytic reaction (Figure S10), which is a hint that
gluconolactone could be an intermediate from glucose
deprotonation, accompanied by the release of H2O2 during
glucose photoreforming into gluconic acid by modified CN
photocatalysts.27,58
As shown in Figure 4e, the photocatalytic
reaction maintains over 90% of the carbon balance after the
reaction, indicating little carbon loss throughout the process.
Additionally, we utilize gluconic acid as the initial substrate (2
g/L) for conducting the photoreforming process, and it is
found that OCN−OH achieves ∼14% gluconic acid
conversion and ∼0.1 g/L arabinose production at the end of
6 h reaction (Figure S11), suggesting that arabinose can be
derived from gluconic acid during the photoreforming process.
The above results reveal the potential intermediates and gas
products involved in the cellulose photorefining process, which
is beneficial for the intrinsic mechanism explanation. In order
to determine the primary reactive species involved in the
reaction, scavenger experiments are then conducted using
ethylenediaminetetraacetic acid disodium salt (EDTA-2Na),
1,4-benzoquinone (BQ), and tert-butanol (TBA) as hole (h+
),
superoxide radical (•
O2
−
), and hydroxyl radical (•
OH)
scavengers, respectively (Figure 4f). It was found that the
introduction of BQ, TBA, and EDTA-2Na led to the reduction
of glucose conversion to 10.58, 14.02, and 15.59%,
respectively, after the 6-h reaction. These findings suggest
that superoxide radical (•
O2
−
), hydroxyl radical (•
OH), and
hole (h+
) are conducive to arabinose production via photo-
catalytic glucose conversion, and •
O2
−
and •
OH play the major
roles. For additional verification of the reactive species engaged
during photoreforming of OCN−OH, electron spin-resonance
(ESR) spectra were conducted using 1-hydroxy-3-carboxy-
2,2,5,5-tetramethylpyrrolidine (CPH) and DMPO as capture
agents.59−61
As shown in Figure 4g, the typical signals of CPH-
h+
increase over time under a nitrogen atmosphere, indicating
the generation of more h+
during the process. With an increase
in the illumination time, the intensity of characteristic peaks in
the •
O2
−
signal also increases (Figure 4h), indicating the
Figure 5. Charge difference profiles of front views of glucose and O2 adsorption on (a) DCN and (b) OCN−OH, respectively. Enlarged top views
of O2 adsorption on (c) DCN and (d) OCN−OH. The yellow and blue regions represent areas with an increased and decreased charge densities,
respectively. Isosurfaces = 0.001 e Å−3
. (e) Calculated reaction energy diagram for the reaction paths followed by the glucose conversion into
arabinose on DCN (black line) and OCN−OH (red line). The gray, blue, and white balls represent C, N, and H atoms, respectively. The red and
pink balls represent O in reacting intermediates and oxygen doping in CN, respectively. (f) Proposed mechanism of cellulose photorefining into
arabinose over OCN−OH.
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7. generation of more •
O2
−
. The •
O2
−
is easy to be obtained from
the oxygen reduction reaction via one single electron process
during the photoreforming process.62
Similarly, while the
DMPO-•
OH signal is absent in the dark, an escalating signal
can be observed under light illumination (Figure 4i), indicating
that more •
OH is produced during the photocatalytic process.
However, from a thermodynamic perspective, the oxidative
capacity of OCN−OH is not adequate for the direct oxidation
of H2O into •
OH (Figure S5c), implying the existing •
OH
probably derives from •
O2
−
.62,63
The ESR findings demon-
strate the presence of h+
, •
O2
−
, and •
OH throughout the
photocatalytic reaction.
Subsequent to that, DFT calculations are executed to delve
deeper into the generation of arabinose by oxygen-doped CN.
The DFT structural model of the oxygen-doped CN is built
based on experimental characterizations, which is consistent
with the previous study.64
The concentration of oxygen atoms
within the heptazine unit is optimized to be 15.39%, which is
approximately close to 16.9% determined by elemental analysis
(Figure S12). Based on the previous experimental results, it is
found that O2-derived reactive oxidative species play significant
roles in glucose photoreforming into arabinose. Therefore, we
initially investigated the effect of a modified photocatalyst on
O2 adsorption during the glucose photoreforming process.
Compared with the pure CN (Figure 5a), electron
redistribution of O2 adsorption on OCN−OH is larger and
the electrons accumulate toward the surface (Figure 5b),
suggesting a favorable affinity between the catalyst and O2
molecule, which is beneficial for the following activation
process. This can be seen more intuitively in Figure 5c,d, and it
shows an obvious electron accumulation on O2 adsorption
toward the surface and the adsorption energy decreases from
0.87 to 0.58 eV, revealing stronger interaction with O2 for the
subsequent formation of reactive oxygen species over OCN−
OH during glucose photoreforming. Then, the calculated
reaction energy diagram of the reaction paths from glucose
into arabinose is presented. It can be seen from Figure 5e and
Table S3 that the adsorption energies of glucose (Int. 1) on
DCN and OCN−OH are −0.62 and −0.61 eV. The next step
for O2 adsorption (Int. 2) is the rate-limiting step during the
whole reaction path for its highest energy among each step,
which requires 0.87 and 0.58 eV for DCN and OCN−OH,
respectively. Subsequently, the presence of the adsorption of
the OH by the OCN−OH prominently facilitates O2 activating
glucose (Int. 3) with a lower reaction energy of −2.48 eV than
that of DCN (−2.35 eV), which could be attributed to the
electron accumulation on O2 adsorption on OCN−OH as
demonstrated by the calculated charge density difference
results. Afterward, following the conversion of adsorbed
glucose into gluconolactone by deprotonation (Int. 4), the
side product H2O2 desorption requires 0.66 eV on DCN and
0.30 eV on OCN−OH, respectively, indicating a favorable
reaction on OCN−OH. Further, the reaction from glucono-
lactone to gluconic acid (Int. 6) requires 0.24 eV energy on
DCN and −0.38 eV energy on OCN−OH, suggesting a more
advanced performance of modified CN. Up to the last step,
from gluconic acid (Int. 6) to arabinose (Int. 7) triggered by
surface oxygen-doped active sites, OCN−OH (−0.35 eV)
remains advantageous in performance compared to DCN
(−0.18 eV). Thus, Figure 5e illustrates that the modified CN,
namely, the OCN−OH, possesses lower conversion energies
for each step during the reaction from Int. 1 (glucose) to Int. 7
(arabinose) (red line), as compared to DCN, which exhibits
more uphill energies for each step (black line). The above
discussion indicates that the more easily intermediates are
converted on OCN−OH, leading to better performance for
arabinose production. The values of each reaction energy on
DCN and OCN−OH are also provided in the Supporting
Information (Table S3).
Considering both experimental and theoretical outcomes,
we subsequently propose a comprehensive mechanism for
cellulose photorefining. As shown in Figure 5f, oxygen can be
inserted into the C1 position of glucose within cellulose to
form intermediate I under the effect of photogenerated
oxidative radicals derived from O2 followed by the β-1,4
cleavage via an elimination reaction to produce gluconolac-
tone, intermediate II, and glucose, respectively.26,65
This
process is likely to occur at the nonreducing end group to
form gluconolactone or reducing end group position to form
glucose within the cellulose noncrystalline region.51
Then,
glucose could be oxidized into gluconolactone via major
oxidative species accompanied by simultaneous H2O2 gen-
eration followed by spontaneous hydrolyzation into gluconic
acid.27,58
Subsequently, gluconic acid undergoes a decarbox-
ylation through C1−C2 α-scissions, resulting in the production
of arabinose and formic acid.8,63
The possible reaction pathway
for acetic acid production is provided in Figure S13. Glucose
might undergo the step-by-step decarboxylation process to
form glyceraldehyde, giving rise to the pyruvaldehyde
formation via dehydration reaction followed by oxidative
decarboxylation of lactic acid to produce acetic acid.66−68
The
gas product CO is likely to be subsequently produced through
formic acid dehydration.69
Hence, the current study shows the
potentiality of well-designed photocatalysts toward direct
cellulose photorefining into arabinose under mild conditions.
■ CONCLUSIONS
In summary, we have demonstrated the feasibility of direct
cellulose photorefining into arabinose and delved into the
mechanism of photocatalytic cellulose conversion. This
procedure is achieved by utilizing an oxygen-doped CN
photocatalyst, which efficiently generates oxidative species
conducive to cellulose oxidative reaction. The DFT calcu-
lations reveal the superior performance of oxygen-doped CN in
generating arabinose compared with pure CN. The process
involves oxygen insertion at the C1 position of glucose within
cellulose, leading to oxidative cleavage of β-1,4 glycosidic
linkages to generate gluconolactone followed by hydrolyzation
to form gluconic acid and then decarboxylation via C1−C2 α-
scissions to produce arabinose and formic acid. This study not
only provides a mechanistic understanding of cellulose
photorefining but also serves as a protocol to inspire the
potential of direct photocatalytic cellulose conversion into
value-added bioproducts under mild conditions.
■ METHODS
Synthesis of the Photocatalysts. OCN−OH was
prepared in the following manner: typically, 11 mmol of
DCN and 15 mmol of cyanuric chloride powders were
dispersed uniformly in 60 mL of acetonitrile and kept stirring
for 12 h in a 100-mL Teflon-lined autoclave. Afterward, the
autoclave was sealed and sent for solvothermal synthesis at 180
°C for 48 h. The collected powders were thoroughly rinsed
with deionized water and ethanol, subjected to multiple
centrifugation cycles, and subsequently dried overnight at 60
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ACS Catal. 2024, 14, 3376−3386
3382

8. °C. Ultimately, the obtained resulting powders were
designated as OCN−OH.
In contrast, DCN was synthesized through the thermal
polymerization process using DCN as precursor at 550 °C for
2 h in air atmosphere.
Synthesis of Cellulose II. The commercial Avicel was
adopted to obtain type II cellulose under alkali pretreatment.
In detail, 1 g of Avicel powders was mixed with 5 M sodium
hydroxide solution under continuous stirring for 2 h. Then, the
powders were washed with deionized water until the
supernatant is neutral followed by removing the solution and
drying at 70 °C overnight for further tests. The resultant white
powders were named as cellulose-II.
Characterization. A Bruker D8 Advance had been utilized
to record the XRD analysis using Cu−Kα-radiation. The FTIR
spectra are obtained using a Nicolet iS 50 FTIR spectrometer
in the range of 500 ∼ 4000 cm−1
. The UV−visible DRS is
measured with a Lambda spectrometer (PerkinElmer) in the
range of 300−800 nm. The spectrophotometers (F-4700,
FLS920) are utilized to obtain the PL and TRPL spectra, and
the excitation wavelength is 350 nm. XPS had been performed
with equipment from Thermo Scientific (Escalab, 250Xi) using
a monochromatic Al Kα source, with a calibrated binding
energy of 284.8 eV for adventitious carbon. For XANES
measurements, the powder sample was mounted on the
double-sided, conducting carbon tape. For the normalization
procedure, the pre-edge was set to 0, and the edge jump was
normalized to 1. 13
C CP/MAS NMR was executed by a Bruker
AVANCE III 600 spectrometer, operating at a resonance
frequency of 150.9 MHz. The corresponding spectra had been
captured adopting a 4 mm MAS probe with a spinning rate set
at 12 kHz. A contact time of 4 ms and a recycle delay of 2s
were applied for the measurement. The chemical shifts of 13
C
were externally referenced to TMS. The EPR test for solid
samples was conducted on a Bruker A300 instrument in the
absence of light. Instrument settings were modulation
frequency: 100.00 kHz; modulation amplitude: 2.00 G;
sweep width: 100.00 G; time constant: 40.960 ms; conversion:
40.000 ms; sweep time: 60.7s; The microwave power was
20.00 mW, and the frequency was 9.84 GHz. HITACHI
(SU8100) and Talos 200 were adopted for the acquisition of
FESEM and TEM images of the samples. Radical ESR spectra
were generated using the ESR spectrometer (JES-X320,
JEOL), with 5,5-dimethyl-1-pyrroline N-oxide (DMPO)
employed as the spin trap. ESR detection had been carried
out in a methanol solution for DMPO-•
O2
−
and in an aqueous
solution for DMPO-•
OH, respectively. Electrochemical experi-
ments had been performed using a CHI660D workstation
using a 0.1 M Na2SO4 aqueous solution as an electrolyte.
Additionally, the working electrode was connected to the
sample-loaded FTO glass, the counter electrode to platinum,
and the reference electrode to Ag/AgCl. Thermo Scientific
FLASH 2000 was utilized to carry out the elemental analysis.
Photocatalytic Measurements. Photocatalytic reactions
were performed by a glass vial with a volume of 20 mL. In
detail, the glucose solution with 2 g/L concentration (10 mL)
had been prepared, and a uniform dispersion of 10 mg of
catalyst was achieved within the solution. The sealed reactor
was kept in darkness while stirring for 1 h. A 300 W xenon
lamp with a calibration of AM1.5G was conducted to carry out
the photocatalytic reactions. For cellulose photorefining, a
uniform dispersion of 50 mg of catalyst was achieved within
the cellulose-II solution, which had a concentration of 1 g/L
(10 mL). The suspension in the sealed reactor was subjected
to O2 purging for 30 min and sent under the dark for 1 h
stirring followed by light irradiation for photocatalytic
reactions. A refractive index detector (RID) was employed to
analyze glucose, arabinose, and other products. The RID was
coupled by high-performance liquid chromatography (HPLC,
1200 Agilent) together with an Aminex HPX-87H column
(300 × 7.8 mm, Bio-Rad). A flow rate of 0.6 mL/min and 5
mM sulfuric acid were utilized as the mobile phase. The
experiments were replicated three times to acquire error bars.
Furthermore, the cellulose conversion, glucose conversion,
arabinose selectivity, gluconic acid selectivity, formic acid
selectivity, and carbon balance were calculated using the
following equations:
=
[ ] [ ]
[ ]
×
Cellulose conversion
cellulose celluose
cellulose
100%
initial final
initial
=
[ ] [ ]
[ ]
×
Glucose conversion
glucose glucose
glucose
100%
T
O
O
=
[ ]
[ ] [ ]
×
Gluconic acid selectivity
gluconic acid
glucose glucose
100%
T
T
O
=
[ ]
[ ] [ ]
× ×
Arabinose selectivity
arabinose
glucose glucose
5
6
100%
T
T
O
=
[ ]
[ ] [ ]
× ×
Formic acid selectivity
formic acid
glucose glucose
1
6
100%
T
T
O
=
[ ] + [ ]
[ ]
×
Carbon balance
carbon in products carbon in residue substrate
carbon in initial substrate
100%
[glucose]O and [glucose]T denote the molar concentration
of the initial glucose solution at time T. [gluconic acid]T,
[arabinose]T, and [formic acid]T denote the molar concen-
trations of gluconic acid, arabinose, and formic acid,
respectively, at time T. Carbon in both liquid-phase and gas-
phase products were considered.
Computational Details. DFT calculations were per-
formed by the “Vienna ab initio simulation package”
(VASP6.1.1).70
We used the generalized gradient approxima-
tion (GGA) exchange−correlation functional proposed by
Perdew−Burke−Ernzerhof (PBE),71
together with the
Grimme’s-D3 dispersion correction Valence electrons were
described by a plane wave basis set with an energy cutoff (Ecut)
of 450 eV to optimize the intermediates and locate the
intermediates and transition state structures on the reaction
energy profile.72
Optimized intermediates were obtained by
minimizing the forces on each ion until they were less than
0.05 eV/Å. Monkhorst−Pack k-point samplings of (3 × 3 × 1)
were employed for optimized geometries and for the
calculation of intermediates. Monkhorst−Pack k-point sam-
plings of (15 × 15 × 1) were employed for DOSs calculation
ACS Catalysis pubs.acs.org/acscatalysis Research Article
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ACS Catal. 2024, 14, 3376−3386
3383

9. and the projected density of states was dealt with by Vaspkit.73
The HOMO−LUMO orbital of DCN and OCN−OH were
calculated by the Gaussian 16 program together with the
Becke-type three-parameter Lee−Yang−Parr B3LYP exchange-
correlation functional and cc-pVTZ basis set.74
Adsorption
energies for reactions were computed using the following
expression:
= +
E E E E
(adsorbate slab) (adsorbate) (slab)
ads
(1)
where the first term is the energy of the optimized surface slab
with the adsorbate, the second term is the energy of the
isolated optimized adsorbate molecule, and the third term is
the energy of the optimized bare surface slab. Negative values
of Eads correspond to the exothermic adsorption process.
■ ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.3c06046.
FESEM image; TEM image; XRD patterns; PL spectra;
transient photocurrent; EIS spectra; Mott−Schottky
curves; HOMO−LUMO distribution; HPLC spectra;
glucose conversion and arabinose selectivity upon longer
reaction; gas production; photograph of the H2O2 test
paper; glucose conversion and arabinose production
using gluconic acid as substrate; DFT model; possible
reaction pathway; fluorescence lifetimes; elemental
analysis; and elementary reactions and intermediates
(PDF)
■ AUTHOR INFORMATION
Corresponding Authors
Heng Zhao − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; Eastern Institute for Advanced Study, Eastern
Institute of Technology, Ningbo, Zhejiang 315200, China;
orcid.org/0009-0006-6280-3097; Email: heng.zhao1@
ucalgary.ca
Md Golam Kibria − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; orcid.org/0000-0003-3105-5576;
Email: md.kibria@ucalgary.ca
Jinguang Hu − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; orcid.org/0000-0001-8033-7102;
Email: jinguang.hu@ucalgary.ca
Authors
Jiu Wang − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada
Qi Zhao − Department of Chemistry, Queen Mary University
of London, E1 4NS London, U.K.
Pawan Kumar − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; orcid.org/0000-0003-2804-9298
Liquan Jing − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; orcid.org/0009-0008-5897-0823
Devis Di Tommaso − Department of Chemistry, Queen Mary
University of London, E1 4NS London, U.K.; orcid.org/
0000-0002-4485-4468
Rachel Crespo-Otero − Department of Chemistry, University
College London, WC1H 0AJ London, U.K.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.3c06046
Author Contributions
⊥
J.W. and Q.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Canada First Research
Excellence Fund (CFREF). We are grateful to the China
Scholarship Council and the UK Materials and Molecular
Modelling Hub for computational resources, which is partially
funded by EPSRC (EP/P020194/1). Via our membership of
the UK’s HEC Materials Chemistry Consortium, which is
funded by EPSRC (EP/L000202), this work used the
ARCHER UK National Supercomputing Service (https://
www.archer.ac.uk). This research utilized Queen Mary’s
Apocrita HPC facility, supported by QMUL Research-IT. 10.
5281/zenodo.438045.
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