Zirconium-based metal-organic framework as an efficiently heterogeneous photocatalyst for oxidation of benzyl halides to aldehydes

Article abstract of DOI:10.1016/j.mcat.2021.111542

The development of efficient heterogeneous catalysts for fine chemical synthesis is critical for practical applications. Herein, for the first time, the zirconium-based metal-organic framework (UiO-66-NH₂) is applied as an efficiently heterogeneous photocatalyst for conversion of benzyl halides to corresponding aldehydes with high selectivity (about 80 %) and conversion (up to 99 %) in the presence of oxygen and DMF as solvent. Through a series of experiments and analysis, the reaction mechanism is proposed to involve nucleophilic attack of the N-oxide. This study provides a general, environmental and high selective method to prepare benzaldehydes and broadens the application fields of UiO-66-NH₂.

Full text of DOI:10.1016/j.mcat.2021.111542

Molecular Catalysis 506 (2021) 111542
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Zirconium-based metal-organic framework as an efficiently heterogeneous
photocatalyst for oxidation of benzyl halides to aldehydes
Ping Xue, Jiming Huang, Liguang Lin, Rong Li, Mi Tang*, Zhengbang Wang*
Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials
Science and Engineering, Hubei University, Wuhan, 430062, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of efficient heterogeneous catalysts for fine chemical synthesis is critical for practical appli-
Metal organic frameworks
2
cations. Herein, for the first time, the zirconium-based metal-organic framework (UiO-66-NH ) is applied as an
UiO-66-NH
2
efficiently heterogeneous photocatalyst for conversion of benzyl halides to corresponding aldehydes with high
selectivity (about 80 %) and conversion (up to 99 %) in the presence of oxygen and DMF as solvent. Through a
series of experiments and analysis, the reaction mechanism is proposed to involve nucleophilic attack of the N-
oxide. This study provides a general, environmental and high selective method to prepare benzaldehydes and
Photocatalysis
Benzyl halides
2
broadens the application fields of UiO-66-NH .
1
. Introduction
photocatalysis for the preparation of corresponding aldehydes from
benzyl halides.
Aromatic aldehydes and its derivatives are extensively used as
Recently, metal-organic frameworks (MOFs) as a new class of crys-
talline porous materials are attracting considerable attention in the field
of heterogeneous catalysis due to their designable structure, large sur-
face area and porosity, abundant catalytic sites and adjustable pore size
[3,16,17]. A variety of well-designed MOF-based heterogeneous cata-
lysts have been prepared and applied in many types of chemical con-
version processes, not only for energy chemistry [18,19], but also in
classic organic synthesis including name reactions and up-to-date
functional reactions [20,21]. In particular, MOFs offer unique features
in heterogeneous photocatalysis that have no comparison with other
types of inorganic or organic photocatalysts, like the well-defined
crystalline structure with uniform channels and nanopores that is
beneficial for catalyst-substrate interaction, and the tunable light ab-
sorption properties via modular interaction between the inorganic metal
clusters and the organic linkers, which could enhance the photoelec-
trons transfer. In addition, MOFs are also easy to separate from the re-
action mixtures to avoid the costly separation step for reusing. Based on
the above, MOF-based photocatalysts exhibited advantages in many
important precursors in synthetic, pharmaceutical, and materials
chemistry [1,2]. To date, variety of methods have been developed to
access aromatic aldehydes, including the direct oxidation of benzyl
alcohol [3–6], benzyl halides [7,8] or toluene [9,10] into corresponding
aldehydes, transition-metal-catalyzed/mediated formation of aromatic
aldehydes [11] and so on. Among them, the direct oxidation of benzyl
halides to aldehydes has attracted widespread attention because of its
convenience, excellent selectivity and efficiency. However, the classical
oxidation approaches, such as Kornblum oxidation [12,13] and Som-
melet
oxidation
[14],
either
generate
unpleasant
and
difficult-separating sulfide byproducts or require excessive hexamine
with low yield and inevitable side reaction (Delepine reaction), which
severely limit their practical applications. Very recently, it was reported
that benzyl halides could be oxidized to corresponding aldehydes cata-
lyzed by benzo[c]cinnoline [15] with molecular oxygen, which showed
excellent conversion and environmentally friendly. We reasoned that
the recently emerging visible-light photocatalysis could promote this
redox process under sustainable and mild conditions. Although there
have been sporadic reports that photocatalysis could promote oxidation
reactions of halides to aldehydes, the reported methods either suffered
from poor selectivity [8] or required the long reaction time [7]. It is
extremely urgent to develop efficient, high selectivity and recyclability
2
catalytic reactions, such as hydrogen generation [18,22], CO reduction
[23], and other organic conversion [24]. However, the direct oxidation
of benzyl halides to aldehydes catalyzed by MOFs has been rarely
explored.
2
Hence, we apply the zirconium-based MOFs (UiO-66-NH ) as the
*
Corresponding author.
E-mail addresses: mtang@huhu.edu.cn (M. Tang), zhengbang.wang@hubu.edu.cn (Z. Wang).
https://doi.org/10.1016/j.mcat.2021.111542
Received 30 December 2020; Received in revised form 11 March 2021; Accepted 21 March 2021
Available online 7 April 2021
2
468-8231/© 2021 Published by Elsevier B.V.

P. Xue et al.
Molecular Catalysis 506 (2021) 111542
2 2 2
Fig. 1. a. PXRD patterns of fresh and simulation UiO-66-NH ; b. FT-IR spectroscope of UiO-66-NH and 2-aminoterephthalic; c. SEM image of UiO-66-NH ; d. TEM
image of UiO-66-NH
2
; e. N
2
adsorption-desorption isotherm of fresh UiO-66-NH
2
2
; f. Pore size of fresh UiO-66-NH .
heterogeneous photocatalyst for the oxidation of benzyl halides to al-
dehydes, which is operationally simple and works under mild and
2. Experimental
environment-friendly conditions. UiO-66-NH
2
is a well-known func-
2.1. Synthesis
tional MOF with high chemical and thermal stability [25], which
showed excellent photocatalytic activity for selectivity oxidation of
benzyl alcohols into aldehydes [26–28]. Under visible light irradiation,
4
The anhydrous zirconium chloride (ZrCl ), 2-aminoterephthalic
acid, 1,4-benzenedicarboxylic and benzyl bromide were purchased
the UiO-66-NH
2
catalyst was excited to produce photogenerated elec-
from Aladdin. Other reagents, such as N, N-dimethylformamide (DMF),
trons and holes, and then the electrons combined with the oxygen to
3 3
acetonitrile (CH CN), 1,4-dioxane and methanol (CH OH), etc. are
•
ꢀ
form superoxide radical anion ( O
2
). Benzyl alcohol was oxidized to
analytical reagent grade from Sinopharm Chemical Reagents Co., Ltd.
The above reagents were used without further purification.
•
ꢀ
2
and holes with a selectivity
benzaldehyde by the combined action of O
of 100 % [27]. And such catalytic reaction could be improved with
higher conversion and shorten reaction time by adding rGO into the
UiO-66-NH
2
was synthesized by the reported method [29]. Typi-
(0.244 g) and an equimolar amount of 2-amino-
cally, 1.04 mmol ZrCl
4
UiO-66-NH
2
catalyst and the reason was attributed to the effective
terephthalic acid (0.188 g) were dissolved in 60 mL DMF. Thereafter, 1.8
mL glacial acetic acid as a modulator was added to the mixture and then
sonication for 20 min. The resulting homogeneous was obtained and
transferred into 100 mL Teflon-lined stainless steel autoclave. Then, the
separation and transport of photogenerated electrons and holes, conse-
quently improving the photocatalytic activity [26,28]. In this study, we
•
ꢀ
further reveal that the O
2
, which is produced by photoelectrons transfer
◦
from UiO-66-NH
2
to O
2
, could promote the oxidation of benzyl halides
autoclave was placed to oven and heated at 120 C for 24 h. After cooled
to aldehydes under mild and environment friendly conditions. In addi-
tion, the possible photocatalytic reaction mechanism is also proposed.
naturally, the pale-yellow particles were separated by centrifugation
and washed three times with DMF and CH
3
OH. Finally, the particles
◦
dried overnight at 80 C under vacuum. UiO-66 was prepared by
replacing 2-aminoterephthalic acid with 1,4-benzenedicarboxylic under
the same conditions.
A mixture of benzyl bromide (0.2 mmol), UiO-66-NH
2
(20 mg) and
◦
DMF (8 mL) in 25 mL glass reactor was stirred at 90 C for 18 h in the air
2

P. Xue et al.
Molecular Catalysis 506 (2021) 111542
under visible light irradiation (26 W helical bulb) to obtain the corre-
sponding aldehydes. At the end of reaction, the mixture was centrifuged,
the liquid phase was used to test the conversion and selectivity, and the
solid phase was used to the next catalytic. The solid catalyst was washed
Table 1
Optimization of the UiO-66-NH
2
-catalyzed conversion of benzyl bromide to
a
benzaldehyde
.
three times with DMF and CH
3
OH, respectively, and then placed in
◦
vacuum oven to dry at 80 C.
2
.2. Characterization
Entry
Solvent
T
Catalyst (mg)
Time
(h)
Conv.
(%)
Sel.
(%)
o
(
C)
X-ray diffraction (XRD) measurements were performed on the Bruker
D8 with a Cu K
radiation (40 KV, 40 mA, λ =1.5418 Å). The
1
2
3
4
5
6
DMF
DMF
DMF
DMF
DMF
1,4-
25
70
90
100
90
90
MOF, 20
MOF, 20
MOF, 20
MOF, 20
MOF, 20
MOF, 20
18
18
18
18
12
18
15.17
54.84
> 99
trace
α
30.78
79.29
65.36
72.36
66.78
morphology of catalyst was performed over field emission scanning
electron microscope (FESEM, Sigma 500, ZEISS, Germany), field emis-
sion transmission electron microscope (FETEM, Talos F200X, America)
and high resolution transmission electron microscope (HRTEM, JEM-
ARM200 F, Japan). The ultraviolet-visible (UV–vis) absorption spec-
trum was obtained by Ultraviolet-visible spectrometer (UV3600, SHI-
MADZU, Japan), while Fourier-transform infrared (FT-IR) spectra were
recorded on the Thermo Fisher Nicolet iS50 FT-IR spectrometer by KBr
> 99
74.09
10.30
dioxane
7
8
9
3
CH CN
90
90
90
MOF, 20
18
18
18
2.44
17.93
42.53
13.64
DMF
DMF
0
> 99
70.53
2-aminoterephthalic
acid, 20
b
1
0
DMF
DMF
DMF
DMF
DMF
DMF
90
90
90
90
90
90
NaHCO
3
18
18
18
18
18
18
> 99
5.00
pellets. Specific BET surface area and pore volumes were determined
c
d
e
e
◦
11
MOF, 20
MOF, 20
MOF, 20
0
45.72
67.55
59.88
60.68
> 99
trace
29.89
8.96
from nitrogen adsorption-desorption isotherms of nitrogen at ꢀ 196
C
1
1
2
3
using automatic Micromeritics apparatus (HJ-BW200B, JWGB Sci &
◦
Tech Ltd., China), the samples were degassed under vacuum at 150 C
14
15
36.64
59.12
for 12 h prior to measurement. The pore size distribution of samples was
UiO-66, 20
•
ꢀ
calculated by the Brunauer-Emmett-Teller (BET) model. The O
2
was
a
2
Reaction conditions: 0.2 mmol of benzyl bromide, 20 mg UiO-66-NH , DMF
confirmed by electron paramagnetic resonance spectrometer (EPR,
Bruker EMXplus, Germany). The electrochemical characterization was
tested on a Autolab electrochemical workstation (Autolab PGSTAT302
N, Metrohm, Switzerland). A standard three-electrode system was used,
Ag/AgCl as the reference electrode, Pt as the counter electrode and the
(8 mL), under visible light (26 w helical bulb) for 18 h at 90 ◦C.
b
2.0 eq NaHCO3 was added.
c
the reaction was in a sealed tube.
d
Avoid light.
e
1
5 mol% benzoquinone was added. Con. (%) = (C
0
– C
i 0 0 i
) / C (C and C are
the molar concentrations of benzyl bromide before and after the photocatalytic
reaction, respectively.) Sel. (%) = C / (C + C ) (C and C are the molar
concentrations of benzaldehyde and side products, respectively).
2 4
sample as the working electrode in 0.2 M Na SO solution. To prepare
a
a
b
a
b
working electrodes, 10 mg sample was added to 1 mL 0.05 % Nafion
ethanol solution. Then, the obtained suspension was fully dispersed by
ultrasound for 30 min. Finally, the slurry was coated on a piece of FTO
2
◦
adsorption of reactants or guest molecular, hence accelerated the reac-
tion progress. Therefore, the porosity of UiO-66-NH was characterized
by nitrogen adsorption-desorption isotherms. As shown in Fig. 1e and f,
UiO-66-NH demonstrated the characteristic Type I shape, indicating
the microporosity. The BET surface area of UiO-66-NH was calculated
to be 997.714 m g , and the average pore was estimated to be 8.0 Å.
glass (1 × 1 cm ) and dried at 80 C for 12 h. The conversion and
selectivity were determined by gas chromatography (GC-2014C, SHI-
MADZU, Japan, FID detector and Rtx-5 chromatographic column). The
2
◦
2
temperature of gasification and detector was 300 C while the initial
◦
2
column was set at 100 C and kept for 1 min, then the temperature was
2
ꢀ 1
◦
◦
raised to 200 C at a rate of 10 C/min and kept for 2 min. GC–MS
GCMS-QP2010 Ultra, SHIMADZU, Japan) was used for the detection of
side products.
These results showed that the obtained UiO-66-NH
with large surface area.
2
is highly porous
(
Next, we probed the catalytic activity of UiO-66-NH
2
for the direct
conversion of benzyl bromide to benzaldehyde. As shown in Table 1
3
. Results and disscussion
(
entries 1–15), catalytic reactions were conducted under different con-
ditions, including different reaction temperatures and time, possible
solvents, catalyst amounts, base, visible light and oxygen. Under the
visible light and air as oxidant, the conversion reaction of benzyl bro-
2
The structures and morphology of UiO-66-NH were confirmed by
XRD, FT-IR, SEM images, TEM images and HRTEM images. As shown in
Fig. 1a, the as-synthesized material showed the characteristic diffraction
◦
◦
◦
◦
◦
mide to benzaldehyde exhibited the best result in DMF with UiO-66-NH
2
peaks at 2θ values of 7.33 , 8.47 , 12.03 , 17.04 and 22.15 corre-
sponding to the (111), (002), (022), (004) and (115) crystal plane,
respectively, which was consistent with previous reports and simulation
◦
◦
(
20 mg) at 90 C for 18 h (Table 1, entry 3). The lower (25 and 70 C) or
◦
a higher temperature (100 C) resulted in reduced selectivity and con-
version (Table 1, entry1, 2 and 4). Shortening the reaction time was also
found to prohibit conversion and selectivity (Table 1, entry 5). Other
[
30]. The FT-IR spectroscopes also confirmed the successful acquisition
ꢀ 1
ꢀ 1
of UiO-66-NH
2
(Fig. 1b). The absorptions at 3458 cm and 3369 cm
relative low polar solvents, such as 1,4-dioxane and CH
to a very low conversion (less than 11 %, Table 1, entry 6 and 7). In
addition, without UiO-66-NH , the reaction showed a very low selec-
3
CN would lead
were assigned to asymmetrical and symmetrical stretching vibration of
ꢀ
1
amino group, while the band at 1570 cm suggested the coordination
between carboxyl group and Zr4 . The SEM and TEM images (Fig. 1c
and d) displayed that UiO-66-NH has uniform and regular spherical
morphologies with about 0.6 m diameter. The relative large particle
size of UiO-66-NH could be attributed to the addition of regulator
acetic acid) in the process of preparing UiO-66-NH based on the
+
2
tivity although it had over 99 % conversion (Table 1, entry 8, the
possible explanation would be discussed in the mechanism part). Other
alkaline catalysts, such as homogeneous ligand 2-aminoterephthalic
2
μ
2
3
acid and inorganic base NaHCO , dramatically decreased selectivity
(
2
(
Table 1, entry 9 and 10). Lack of air or visible light would lead to poor
competitive reaction between the regulator and ligands in the coordi-
nation process [31,32]. The lattice fringes had an interplanar spacing of
conversion and selectivity (Table 1, entry 11 and 12), indicating that
visible light and oxygen were essential for the conversion of benzyl
bromide to benzaldehyde.
0
.99 nm, corresponding to (002) fringes of UiO-66-NH
2
, as shown in
Fig. S1, which could further confirm the structure of UiO-66-NH
2
.
As a heterogeneous catalyst, as shown in Fig. 2a, UiO-66-NH
2
could
The porous properties of the catalysts have an important effect on the
catalytic performance. High surface area was benefit for effective
be recycled, and the conversion and selectivity also could maintain up to
3

P. Xue et al.
Molecular Catalysis 506 (2021) 111542
2 2
Fig. 2. a. Cycle performance of UiO-66-NH for the selective oxidation of benzyl bromide to benzaldehyde; b. PXRD patterns of fresh and reused UiO-66-NH . c. EPR
2 2
spectra of UiO-66-NH dispersed in methanol under dark or light irradiation; d. Leaching tests for benzyl bromide reaction by UiO-66-NH .
blockage of the pores. The crystallinity or structural integrity of MOF
didn’t change during cycles as revealed by XRD pattern and FT-IR
spectroscopes. After three cycles, both of XRD pattern and FT-IR spec-
Table 2
a
The selective oxidation of others substituted benzyl bromides
.
2
troscopes of UiO-66-NH showed negligible changes (Figs. 2b and S3),
indicating that the crystallinity and structure were well maintained after
multiple reuses.
2
The scope of UiO-66-NH -catalyzed conversion of benzyl bromides
to benzaldehydes was further investigated utilizing various substrates.
The results were summarized in Table 2. In the presence of electron-
donating groups, such as methyl group and methoxy group (Table 2,
entry 1 and 2), the selectivity of corresponding aldehyde was higher,
while electron-withdrawing groups (Table 2, entry 3–6) were converted
into corresponding aldehyde with moderate selectivity. The possible
explanation will be discussed in the reaction mechanism.
entry
1
substates
Conv. [%]
Sel. [%]
75.69
> 99
2
3
> 99
> 99
77.77
59.80
In order to explore the reaction mechanism, a series of control ex-
2
periments were carried out. As we know, UiO-66-NH has good photo-
catalytic activity. Therefore, we initially inspected the performance of
•
ꢀ
UiO-66-NH
2
in producing O
2
. On one hand, UV–vis and electrochemical
4
5
6
> 99
> 99
98.44
59.31
66.14
53.51
experiments were performed to demonstrate that the UiO-66-NH
2
could
ꢀ
2
under the
•
be excited by visible light and had the ability to produce O
investigated conditions. As shown in Fig. S4, the absorption edges and
the band gap of UiO-66-NH located at 443 nm and 2.80 eV, respec-
tively. The flat-band potential of UiO-66-NH was measured to be -1.06
V versus Ag/AgCl (-0.86 V vs. NHE, Fig. S5). The conduction band (CB)
of UiO-66-NH was estimated to be -0.96 V, since the bottom of CB of n-
type semiconductors is more negative 0.1 V than the flat-band potential
2
2
2
[
33,34]. The CB of UiO-66-NH
2
is much more negative than the standard
ꢀ
2
(-0.33 V vs. NHE) [35], meeting the produc-
•
a
redox potential of O
2
/ O
Reaction conditions: 0.2 mmol of substrate, 20 mg UiO-66-NH
2
, DMF (8 mL),
•
ꢀ
◦
tion requirements for O
UiO-66-NH had the ability to produce O
shown in Fig. 2c. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as super-
. On the other hand, EPR test also showed the
under visible light (26 w helical bulb) for 18 h at 90 C.
2
•
ꢀ
2
under light irradiation, as
2
about 100 % and 80 %, respectively. However, comparing with the first
cycle, the selectivity of the second and third cycle slightly decreased,
which could be attributed to some degree of blockage in the micropores
oxide radical anion trap, UiO-66-NH
2
methanol dispersed solution dis-
ꢀ
2
signals under light irradiation, which was in
•
played remarkable O
accord with the reported results [27]. In contrast, if we removed the
light source, the signals immediately disappeared. These results recon-
of UiO-66-NH
2
. As shown in Fig. S2, the BET surface area of UiO-66-NH
2
reduced from 997.714 to 302.138 m2
ꢀ 1
g
and lost some microporous
•
ꢀ
firmed that UiO-66-NH
2
did produce O
2
under light irradiation.
properties after three runs. We speculated that the generation of
byproducts for example bromine ions in the reaction progress coordi-
nated with the free Zr sites, which can’t be removed easily, causing the
To explore the catalytic site, the catalyst leaching test was designed
and the result was shown in Fig. 2d. After reacting for 6 h or 12 h, UiO-
4

P. Xue et al.
Molecular Catalysis 506 (2021) 111542
2
Scheme 1. Proposed mechanism for selective oxidation of benzyl bromide to benzaldehyde by UiO-66-NH in DMF.
6
6-NH
2
was separated from the reaction mixture by centrifugation. After
formation of benzaldehyde without adding catalyst. In fact, the benzyl
radical probably took part in the actual reaction without catalyst.
removing catalyst, the reaction was continued for an additional 12 h or 6
h, respectively. Although the conversion could still be up to 99 %, the
selectivity was only 45.72 % and 64.25 %, which clearly showed that the
actual catalytic site for the selective oxidation of benzyl bromide to
•
ꢀ
Moreover, in the above discussion, we have proved that O
2
was being
produced in the reaction. After adding benzoquinone into reaction
mixture (entry 13, Table 1), the selectivity of benzaldehyde significantly
•
ꢀ
benzaldehyde was located on UiO-66-NH
in UiO-66-NH could be regarded as microphotoreactors for the for-
mation of superoxide radical and reactive intermediates. The amino
group in UiO-66-NH have three key functions in the reaction [4,27]: 1)
2
. The nanometer-sized cavities
reduced to 8.96 % because of the almost total consumption of O
2
.
•
ꢀ
2
Therefore, O
2
produced by UiO-66-NH
2
catalyst was the key to improve
the selectivity.
2
Next, air was essential oxidant. As shown in Table1 entry 11, if there
was no oxygen, benzaldehyde can hardly be detected. EPR spectra
act as an auxochromic and bathochromic group to promote the optical
response; 2) as strong electron-donating functional groups to reduce the
band gap. Upon light irradiation, electrons on the HOMO composed of
•
ꢀ
showed that UiO-66-NH
2
can transfer electronic to O
2
to form O
2
under
•
ꢀ
visible light. The O
2
acted as efficient oxidant to oxidize substrates. The
•
ꢀ
O, C and N 2p orbitals jump to the LUMO, and transfer to O
2
molecules
; 3) stabilizing the reactive in-
ꢀ
2
could be stabilized in the cavities of UiO-66-NH
O
2
also could be confirmed by capturing test by using benzoquinone
3
+
•
ꢀ
adsorbed on the Zr sites to form O
2
(Table 1, entry 13). If benzoquinone was added into reaction mixture,
the selectivity of benzaldehyde significantly reduced to 8.96 % because
•
termediates. The O
2
•
ꢀ
due to its interaction with the amine groups. In addition. under the same
conditions, the catalyst performance of UiO-66 was also explored, as
shown in Table 1 entry 15. The result showed that the conversion of
benzyl bromide was over 99 % while the selectivity of benzaldehyde was
only about 59.12 %. In fact, UiO-66 could not be excited under
visible-light irradiation. Therefore, photogenerated electrons and holes
did not generate under this condition [27]. Slightly improving the
selectivity of benzaldehyde was possibly associated with the porosity of
UiO-66. Accelerating the reaction rate and improving the selectivity by
using the porous catalysts were usually found in some heterogeneous
catalytic processes [36,37].
of the almost total consumption of O
2
. Hence, the oxygen was con-
cerned in the reactions.
Finally, we also found DMF might be involved in the reaction. Firstly,
DMF would react with water in the air to produce formic acid and
dimethylamine in the presence of photogenerated electrons and holes
generated by UiO-66-NH
2
under visible-light irradiation [38]. The
•
ꢀ
decomposition product dimethylamine was oxidized by the
O
2
to
generate N-oxide, which has been proved as an effective oxidant for the
oxidization of halides to aldehydes [39–41]. The in situ generating
N-oxide as actual oxidant here reacted with benzyl bromides. Secondly,
in order to confirm the hydrolysis of DMF, we analyzed the side prod-
ucts. Benzyl formate, which possibly come from the esterification re-
action of formic acid and benzyl alcohol (It comes from the hydrolysis of
benzyl bromide.) was found by GC–MS and gas chromatographic (the
retention time was consistent), as shown in Fig. S6. This result recon-
firmed the decomposition of DMF. Lastly, the replacement of solvents
would greatly reduce the yield of benzaldehyde (entry 6 and 7 in
Table 1). Therefore, we proposed that DMF might be involved in the
reaction.
To further illustrate how UiO-66-NH
conducted more capture assay. It should be noted that the reaction
showed some selectivity without catalyst UiO-66-NH (entry 8, Table 1).
However, this selectivity possibly was not attributed to O
discussions seen below). In the absence of UiO-66-NH , benzoquinone
2
improve the selectivity, we
2
•
ꢀ
2
(detailed
2
was also added to the reaction. The results showed that the selectivity of
entry 14 (Table 1, 36.64 %) slightly decreased comparing with entry 8
•
ꢀ
(
Table 1, 42.53 %), which suggested that the O
2
was not involved in the
5

P. Xue et al.
Molecular Catalysis 506 (2021) 111542
Based on the above experimental results and discussions, we pro-
posed a plausible mechanism, as shown in Scheme 1. Under visible-light
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111542.
and heating, UiO-66-NH
2
generated photoelectrons and holes, photo-
References
•
ꢀ
electrons immediately transferred into O
2
to form O
2
. Photogenerated
[
[
[
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This work was supported by National Science Foundation of China
(
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
6