2D/2D MXene/g-C3N4 for photocatalytic selective oxidation of 5-hydroxymethylfurfural into 2,5-formylfuran

Article abstract of DOI:10.1016/j.catcom.2020.106152

The selective oxidation of 5-hydroxyfurfural (HMF) into the corresponding aldehyde is one of the key reactions in the production of chemical products. Here, we prepared the MXene/g-C₃N₄ composite (MX/CN) for photocatalytic selective oxidation of HMF into 2,5-diformylfuran (DFF). At the mild reaction conditions, both DFF selectivity and yield can reach up to ≥ 90% over 6% MX/CN. In addition, the MX/CN also shows high activity for selective oxidation of other non-aromatic and aromatic alcohols. Moreover, the reaction conditions are surveyed and the mechanism has been studied. This study provides a way for HMF conversion under mild conditions.

Full text of DOI:10.1016/j.catcom.2020.106152

Catalysis Communications 147 (2020) 106152
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
2
D/2D MXene/g-C N for photocatalytic selective oxidation of 5-
3
4
hydroxymethylfurfural into 2,5-formylfuran
⁎
⁎
Xiang-Xiang Wang, Sugang Meng , Sujuan Zhang, Xiuzhen Zheng, Shifu Chen
Key Lab of Clean Energy and Green Circulation, Huaibei Normal University, Anhui Huaibei 235000, People's Republic of China.
A R T I C L E I N F O
A B S T R A C T
Keywords:
The selective oxidation of 5-hydroxyfurfural (HMF) into the corresponding aldehyde is one of the key reactions
in the production of chemical products. Here, we prepared the MXene/g-C N composite (MX/CN) for photo-
Photocatalysis
Nanocomposite
3
4
catalytic selective oxidation of HMF into 2,5-diformylfuran (DFF). At the mild reaction conditions, both DFF
selectivity and yield can reach up to ≥ 90% over 6% MX/CN. In addition, the MX/CN also shows high activity
for selective oxidation of other non-aromatic and aromatic alcohols. Moreover, the reaction conditions are
surveyed and the mechanism has been studied. This study provides a way for HMF conversion under mild
conditions.
3 4
G-C N
MXene
Selective oxidation
1
. Introduction
composite photocatalysts [10–21].
Herein, we report a noble-metal-free photocatalyst MXene/g-C
3 4
N
The selective oxidation of 5-hydroxyfurfural (HMF) into 2, 5-di-
(MX/CN) for selective conversion of HMF into DFF at mild reaction
formylfuran (DFF) is one of the key reactions in the production of
chemical products [1–7]. However, its industrial synthesis requires
toxic chemical oxidants, expensive precious metals and high tempera-
tures [1–3]. Therefore, it is important to develop the mild reaction to
synthesize DFF.
conditions for the first time, in which 2D MXene (MX) as a cocatalyst is
supported on 2D g-C
3
N (CN). The visible-light-driven CN possesses
4
suitable energy band structure, which can provide feasible redox po-
tential for selective conversion HMF into DFF. The MX can significantly
improve the separation and transfer efficiencies of the photogenerated
charge carriers of the CN. Thus, the MX/CN exhibits high photocatalytic
performance for selective conversion of HMF into DFF under visible
light irradiation.
Recently, visible-light-driven photocatalytic selective oxidation of
alcohols into corresponding aldehydes has attracted great attention due
to its mild reaction conditions, high selectivity and merit to be driven
by solar energy [3–9]. For example, Krivtsov et al. found that metal-free
g-C
3
N
4
can selective oxidize HMF into DFF under visible light irradia-
2. Experimental section
tion [7]. However, the DFF selectivity is low (50%). Later, our group
reported that HMF can be transformed into DFF over the relative p-n
All the chemicals used in the experiment were commercially and
junction CdS/g-C
3
N
4
under visible light irradiation [5], and photo-
without further purification. Firstly, the MXene (Ti
3
C , MX) was ob-
2
catalytic selective oxidation of HMF to simultaneously produce DFF and
tained by etching Ti
3
AlC (Fig. 1). Then, the dispersed MX nanosheets
2
H
2
using 2D/2D-3D NiS/Zn
In
3 2
6
S hierarchical structure under visible
were prepared via ultrasound treatment of MX suspension, and the g-
light irradiation [6]. Compared with traditional synthesis process,
photocatalytic method exhibits promising future with features of sus-
tainability and green. It has been demonstrated that 2D nanomaterials
or structures are the best for photocatalysts [5–7]. More recently,
MXene, a new class of 2D material, has received extensive attention for
C
3
4
N nanosheets were synthesized by the previous report [22]. The
MX/CN composites with different amount of MX were fabricated by a
self-assembly method. The synthesis procedure, characterization of
photocatalysts and photocatalytic activity tests are shown in the sup-
porting information.
photocatalysis such as nitrogen fixation, hydrogen evolution and CO
2
conversion [10–16]. Its wonderful geometry and the extraordinary
electronic structure as well as the controllable surface chemistry in-
dicate that it is a promising and versatile cocatalyst for preparing
3. Results and discussion
XRD patterns of the samples are presented in Fig. S1. After HF
⁎
Corresponding authors.
E-mail addresses: sgmeng@chnu.edu.cn (S. Meng), chshifu@chnu.edu.cn (S. Chen).
https://doi.org/10.1016/j.catcom.2020.106152
Received 24 April 2020; Received in revised form 22 August 2020; Accepted 27 August 2020
Available online 01 September 2020
1566-7367/ © 2020 Elsevier B.V. All rights reserved.

X.-X. Wang, et al.
Catalysis Communications 147 (2020) 106152
Fig. 1. Pictorial representation of the procedure for preparing MX/CN.
etching Ti
3
AlC
2
to remove the Al, the XRD diffraction peak of Ti
AlC has been successfully
(MXene, MX). The as-prepared CN shows two
3
AlC
2
at
o
2
θ = 39 is disappeared, indicating that Ti
3
2
converted into Ti
characteristic diffraction peaks (27.5 and 13.1 ) of the standard g-
, it is consistent with the literature [22]. For the MX/CN compo-
3
C
2
o
o
C
N
3 4
sites, the characteristic diffraction peaks of MX and CN can be clear
observed. It is demonstrated that MX and CN keep pure phases in the
MX/CN composites. The result can be further confirmed by the FT-IR
spectra (Fig. S2). The XPS analysis was used to detect the chemical
composition of MX, CN and MX/CN (Fig. S3). The XPS patterns of MX
including C 1s, N 1s, Ti 2p, O 1s and F 1s are similar with that of MX
and CN. It can be clear observed that the MX/CN composite is mainly
composed of Ti, C, N, O and F. The O and F elements are derived from
the surface group of the MX. This is a comment phenomenon for MX
prepared by HF etching method [23].
As shown in Fig. S4, The MX after HF etching exhibits 2D stack
shape like a book, which is made up of 2D nanosheets and spacings
between 2D nanosheets (Fig. S4a and c). However, after 10 h of soni-
cation, the 2D nanosheets-stack structure is broken and 2D dispersed
nanosheets are obtained (Fig. S4b and d). The dispersed 2D MX na-
nosheets would benefit the deposition of 2D CN nanosheets (Fig. S4e).
Additionally, the crystal lattice distortion between MX and CN is about
3
%, indicating that CN flat can be well combined with 2D MX (Scheme
S1). It can be seen that 2D MX is contacted well with 2D CN (Fig. S4f).
The further HRTEM image analysis confirms it. Two different types of
areas can be observed in Fig. 2. Specifically, one is the crystalline re-
gions with distinctive lattice fringes; the other is amorphous areas with
no lattice fringes. Based on the previous studies [10,17,19,20], the
lattice spacing of 0.265 nm is contributed to the MX, while the area
with no crystal lattice can be assigned to the CN. Moreover, the EDX-
mapping images further confirm the composed elements of the MX/CN
composite (Fig. S5), which is in line with XPS result. In addition, the
distribution of these elements (C, N, Ti, O and F) is consistent with the
shape of the MX/CN composite.
The photocatalytic activities of these catalysts were evaluated by
selective oxidation of HMF into DFF under visible light irradiation. As
shown in Fig. 3a, the yields of DFF over CN and MX are 8.8% and
Fig. 3. (a) Photocatalytic selective oxidation HMF to DFF with MX, CN and MX/
CN under visible light irradiation for 2 h. (b) Cyclic photocatalytic oxidation of
HMF into DFF over the 6% MX/CN composite under visible light irradiation
(
reaction time 6 h per circle).
1
1.8%, respectively. However, the MX/CN composites show higher
activities than CN and MX. Among the MX/CN composites, the 6% MX/
CN shows the highest activity. The DFF yield is about 32.6%, which are
3
.7 times that of CN and 2.8 times that of MX. Importantly, the DFF
yield of the 6% MX/CN can be improved by increasing reaction time
Fig. S6a). The yield of DFF can reach up to 90% with high selectivity of
(
9
7% under visible light irradiation for 10 h. Interestingly, there is still
about half of the yield when the reaction system was performed under
nitrogen conditions (Fig. S6b). It may be caused by the fact that the
surface of MX is rich in free radicals, which would in favor of HMF
conversion. Moreover, the yield and selectivity obtained under pure
Fig. 2. HRTEM image of 6% MX/CN.
2

X.-X. Wang, et al.
Catalysis Communications 147 (2020) 106152
oxygen conditions are basically the same with that under air atmo-
sphere (Fig. S6b). It indicates that this reaction can be happened under
air atmosphere. The stability of the catalyst is an important property for
its application. In order to test the reusability of the MX/CN photo-
catalyst, all reaction conditions are the same as before. Specifically,
after each reaction, the catalyst is separated by centrifugation, washed
to remove the residual original reaction solution and finally dried at
6
0 °C. It can be seen from Fig. 3b that the catalytic activity of the MX/
CN decreases slightly with recycling runs, which may due to the fact
that a small amount of catalyst loss per use.
To investigate the effect of solvents, acetonitrile (CAN), di-
methylsulfoxide (DMSO), ethyl alcohol (ETOH), trifluorotoluene (BTF),
and isopropanol were used as solvents (Table S1). It can be seen that
BTF is the best solvent to convert HMF into DFF. In addition, the dif-
ferent reactants were used as the substrate (Table S2). It can be found
that the MX/CN not only has good activity for non-aromatic alcohols,
but also has good reactivity for aromatic alcohols. For example, 84% of
p-methoxybenzyl alcohol can be selective oxidized into p-methox-
ybenzaldehyde with the selectivity of 96%.
Fig. 4. The illustration of the mechanism for selective oxidation of HMF into
DFF over the MX/CN under visible light irradiation.
The promising activities of MX/CN were then investigated by a
series of control experiments. It is known that the photocatalytic per-
formance is mainly influenced by three crucial factors: light absorption,
surface area or reactive site, and photogenerated charge separation and
transportation [6]. Notably, the last one is demonstrated to be the most
crucial factor [6]. As shown in Fig. S7, the light absorption of MX/CN is
similar to that of CN. Specifically, the band gaps of CN and the 6% MX/
CN are approximately equal to 2.75 and 2.70 eV, respectively. More-
over, the calculated conduction band (CB) and valence band (VB) of CN
are −0.94 and 1.81 V, respectively. After MX loaded on the CN, the CB
−
extension of the reaction time, the intensity of the DMPO-•O
2
signal
over MX/CN is increasing. It is in line with the above results that the
−
•
O
2
is the main active species. However, the ESR signals of DMPO-•OH
cannot be observed over both CN and MX/CN (Fig. S13), implying that
•
OH cannot be created. It is reasonable to get this result, because the
valence band potential of CN is lower than the redox potential of •OH
production [24].
Based on the above experimental data and analysis, we propose a
possible photocatalytic mechanism (Fig. 4). First, the main photo-
catalyst CN provide photogenerated electrons and holes with suitable
redox potentials (that is applicable CB and VB positions). The electrons
photoexcited on the CB transfer to the cocatalyst MX and are captured
(
0.88 V) and VB (1.82 V) of CN are not changed obviously. Compared to
the redox potentials of HMF/DFF (1.61 V) and DFF/oxidized DFF
(
2.03 V), the CN can provide powder and suitable oxidizing ability for
−
by O
2
to form •O
2
on the surface. The holes photogenerated on the VB
selective oxidation of HMF into DFF. Fig. S8 shows the nitrogen ad-
sorption-desorption isotherms and the corresponding pore-size dis-
tribution curves of MX, CN and 6% MX/CN. The surface areas of MX,
meet the selective oxidation of HMF to DFF, and DFF cannot be con-
verted into the corresponding acid. Therefore, under visible light irra-
diation, HMF can be oxidized into DFF over MX/CN composite photo-
catalyst with high selectivity and conversion.
2
−1
CN and 6% MX/CN are 23.9, 7.2 and 19.9 m g , respectively. The
pore volumes of MX, CN and 6% MX/CN are 0.139, 0.060 and
3
−1
0
.132 cm
g
, respectively. Clearly, the increased surface area and
4
. Conclusion
pore volume can promote mass transfer. Fig. S9a shows that the 6%
MX/CN catalyst exhibits the highest photocurrent value than other
samples, which means that the 6% MX/CN has high electron-hole se-
paration efficiency. In addition, the electrochemical impedance spectra
indicate that the transfer efficiency of the photogenerated carriers over
the 6% MX/CN is higher than that over MX and CN (Fig. S9b). The
recombination of the photogenerated electrons and holes is further
analyzed by PL. Among the MX/CN composites, the 6%MX/CN with the
best activity has the lowest emission peak, indicating that the photo-
generated carrier recombination rate of the 6%MX/CN composite is the
lowest. This indicates that the addition of MX in the MX/CN can ef-
fectively suppress the recombination of photogenerated electron-hole
pairs under visible light irradiation, thereby greatly improving the
photocatalytic efficiency. This conclusion is consistent with the pho-
tocatalytic activity and the results of photocurrent and impedance.
In order to study the reactive species during photocatalytic oxida-
tion of HMF, different scavengers were added into the reaction system
to eliminate the corresponding reactive species, and the effect of the
reactive species can be judged by the activity change of the reaction. As
shown in Fig. S11, when the introduction of p-benzoquinone (BQ) or
triethanolamine (TEOA) into the reaction system, the conversion of
HMF and the yield of DFF are significantly decreased. However, the
activity of MX/CN is not reduced as isopropanol (IPA) or carbon tet-
The MX/CN composite with applicable CB and VB positions has
been successful fabricated. The 6% MX/CN composite exhibits the
highest efficiency of separation and transfer of the photogenerated
electron-hole pairs under visible light irradiation. Under visible light
irradiation for 10 h, the yield and selectivity of DFF can reach up to
9
0% and 97%, respectively. Besides HMF, aromatic alcohols and other
non-aromatic alcohols can also be effectively converted into the cor-
responding aldehydes over the MX/CN composite under visible light
irradiation. This study demonstrates that the MXene is an efficient co-
catalyst to improve the reactivity of g-C
3
N for selective oxidation of
4
alcohols into fine chemicals under mild reaction conditions.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgement
This work was supported by the Natural Science Foundation of
China (NSFC, grant Nos. 51772118, 51972134 and 21607027), the
Natural Science Foundation of Educational Committee of Anhui
Province (grant Nos. KJ2018A0387, KJ2019A0602 and KJ2019A0601)
and the Project of Anhui Province for Excellent Young Talents in
Universities (grant No. gxyq2019029), China.
rachloride (CCl
4
) was added into the reaction system. These evidence
−
+
that •O
2
and h are active species for photocatalytic selective oxi-
−
dation of HMF. Moreover, the •OH and •O
2
were measured by ESR
technique. As shown in Fig. S12, the MX/CN and CN show strong
−
DMPO-•O
2
signals under light irradiation. Importantly, with the
3

X.-X. Wang, et al.
Catalysis Communications 147 (2020) 106152
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