One-step green conversion of benzyl bromide to aldehydes on NaOH-modified g-C3N4 with dioxygen under LED visible light

Article abstract of DOI:10.1039/c9cy00481e

Kornblum oxidation (benzyl bromide to benzaldehyde) can be realized efficiently by NaOH-modified graphitic carbon nitride (g-C₃N₄). Different from the oxidation reaction carried out at high temperature using DMSO as an oxidant, our r

Full text of DOI:10.1039/c9cy00481e

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1
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Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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One-step Green Conversion of Benzyl Bromide to Aldehydes on
NaOH-modified g-C N with Dioxygen under LED Visible Light
3
4
Dan Liu, Chenwei Qiu, Minbo Li, Yanyu Xie, Lu Chen, Huaxiang Lin, Jinlin Long,
*
Zizhong Zhang , Xuxu Wang
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy
and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China

Author to whom all correspondences should be addressed.
E-mail: z.zhang@fzu.edu.cn (Z. Zhang); xwang@fzu.edu.cn
1

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Abstract
Kornblum oxidation (benzyl bromide to benzaldehyde) can be realized efficiently by
NaOH-modified graphitic carbon nitride (g-C N ). Different from the oxidation reaction
3
4
carried out at high temperature using DMSO as oxidant, our reaction is eco-friendly
because it carried out at room temperature under LED light irradiation using molecular
oxygen as an oxidant, which avoids the problems of toxicity, pollution and purification.
The alkali treatment leads to the surface deprotonation of the g-C N to form the
3
4
NaOH-modified g-C N (Na/g-C N ). The modifications of electronic structure and
3
4
3
4
-
surface chemical environment of g-C N facilitate light absorption and formation of •O
2
3
4
and therefore make the conversion of benzyl bromide up to 99%. This work demonstrates
a green and sustainable route for benzaldehyde production and the superiority of g-C N
3
4
for organic syntheses.
Keywords: Photocatalytic oxidation, g-C N , Benzyl bromide, Benzaldehyde
3
4
2

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1
. Introduction
Benzaldehyde, one of the very important chemicals, has been widely used in
medicine, pesticides, dyes, and spices industries. Traditional synthesis methods of
benzaldehyde include benzyl dichloride hydrolysis, toluene/benzyl alcohol/benzyl halide
oxidation and benzoic acid reduction. Among them, the oxidation of benzyl halide to
benzaldehyde is an appealing method because benzyl halide can be easily obtained by the
photohalogenation of aromatic methyl groups.1, 2 However, the oxidation of benzyl halide
to benzaldehyde often needs mercury-based and chromium-based compounds as
stoichiometric oxidants, which are toxic and release considerable amounts of
by-products.3, 4 In addition, the harsh reaction conditions such as the inevitable use of
strong alkali at high temperature in the hydrolysis process limit its further application.
Therefore, it is highly imperative to develop a green and efficient manufacturing process
of benzaldehyde. It is known that the Kornblum oxidation is one of the oxidation of
organic halides to carbonyl compounds, which is a good route to prepare the aldehydes
by benzyl halides with DMSO as an oxidizing agent and base as a deprotonated catalyst
5
at elevated temperature. The reaction creates an alkoxysulfonium ion, which will be
eliminated to form the desired aldehyde in the presence of a base such as triethylamine. It
has been reported that in the Kornblum oxidation at the temperature of 140 °C, Al-Mg
hydrotalcites and DMSO provide an effective combination for the oxidation of benzylic
halides to benzaldehydes. The production of benzaldehyde was further promoted,
although it released a certain amount of by-product.6-9 Since DMSO as oxidizing agent
was reduced into a toxic dimethyl sulfide in this reaction and the reaction requires high
temperature conditions, the Kornblum oxidation is neither environmentally friendly nor
easy for product’s separation and purification. Therefore, to develop a green synthetic
pathway of benzaldehyde with mild reaction conditions, non-polluting oxidants, such as
dioxygen and hydrogen peroxide, has a promising application prospect.
Photochemical transformation has become an important tool in organic synthesis
because of its sustainability, green and milder reaction conditions, compared with
traditional organic synthesis methods.10, 11 The semiconductor photocatalysis has been
shown to selectively activate specific chemical bonds of organic compounds to bring
about oxidation, nitration, coupling, and so on.12, 13 Unfortunately, photocatalytic
3

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oxidation of benzyl halide to benzaldehyde has still typically suffered from low
1
4
efficiency. The weak interaction of O with the surfaces of photocatalysts is a critical
2
issue for the inefficiency. Another critical issue is the strong oxidizing power of
photogenerated holes in photocatalysts, such as TiO (E = +2.7 V, vs. NHE) and BiOCl
2
VB
(EVB=+3.3 V, vs. NHE), which can cause an unavoidable nonselective process to
over-oxidation.15, 16 Therefore, the challenge for achieving selective benzyl halide
oxidation to benzaldehyde is to develop photocatalysts that can interact with O but do
2
not generate strongly oxidative holes. As a non-toxic and metal-free organic polymer
semiconductor photocatalyst, g-C N has been shown to have peculiar potential in the
3
4
green synthesis. For example, the formyl group can be generated from an appropriate
precursor group, such as hydroxymethyl, acid chloride, and halomethyl or methyl group,
1
7-25
on g-C N under visible irradiation.
It has been known that the conduction band (CB)
3
4
and valence band (VB) of g-C N are located at -1.3 V (vs. NHE) and +1.4 V (vs. NHE),
3
4
respectively. Negative reduction potential of CB allows the photogenerated electrons to
-
possess a large thermodynamic driving force for reducing O (E° (O /•O ) = -0.16 V).
2
2
2
Nevertheless, the photogenerated hole in the VB of g-C N is inadequate to oxidize -OH
3
4
to •OH (E° (-OH/•OH) = 2.4 V). Since •OH is highly active but nonselective for
oxidation of substrates, g-C N might use as a photocatalyst for selective photo-oxidation
3
4
and related transformations in aqueous media by inhibiting the undesirable
over-oxidation products. And because of the ability to reductively adsorb O in
2
N-containing materials, it can be applied in moderate photocatalytic conversion. In fact, it
has been reported that photo-oxidation of benzyl alcohol to benzaldehyde showed a
2
6
remarkable selectivity (> 99%) over mesoporous carbon nitride with visible light. The
activity and selectivity of amine, alcohols, and alkane oxidation can be promoted by
2
7-30
varying the doping atoms or nanofillers over g-C N based catalysts.
Recently, it has
3
4
been reported that efficient benzyl alcohol conversion into benzoin could be achieved
over alkalinized g-C N photocatalyst with remarkable nucleophilicity for prompting C-C
3
4
coupling reactions.31
Herein, an alkali-modified g-C N was applied successfully for one-step synthesis of
3
4
benzaldehyde from benzyl bromide with dioxygen as an oxidant at room temperature
under LED visible light. The alkali treatment leads the surface of g-C N to
3
4
4

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deprotonation, which not only facilitate the light harvesting but also the transport of
charge carriers. The dioxygen serves as an active mediator for photoinduced conduction
band electrons, thus it promotes the hydrolysis of benzyl bromide and makes synthesis of
benzaldehyde proceed in one-step. The photocatalyst g-C N is cheap and easy to obtain,
3
4
dioxygen is green and environmentally friendly, and the used visible light can come from
solar energy as an inexhaustible green energy. Therefore, this work provides a green
synthetic pathway for the preparation of benzaldehyde.
2
. Experimental
2
.1 Materials
Benzyl bromide was acquired from Aladdin. Other reagents used in the present
study, including acetonitrile, benzyl alcohol, benzaldehyde, benzoic acid, sodium
hydroxide and cyanimide, were of analytical reagent grade and obtained from Sinopharm
Chemical Reagent Co., Ltd. All of the above chemicals were used without further
purification.
2
.2 Synthesis of g-C N and Na/g-C N
3 4 3 4
In a typical synthesis procedure, cyanimide (5 g) was loaded in a porcelain boat and
-1
then treated in a tubular furnace at 550 °C for 4 h under flowing nitrogen (60 mL∙min )
-1
with a ramp rate of 1 °C∙min . After thermal polymerization, the cyanimide (5 g)
changed into g-C N (2.03 g) and the yield is about 40%. The NaOH treated g-C N
4
3
4
3
sample (Na/g-C N ) was synthesized by impregnation method. 1 g g-C N was added in
3
4
3
4
NaOH 50mL solution (5 wt.%) with continuously agitation about 30 min then the
solution was kept agitating at 200 °C (oil bath) until the complete evaporation of water.
The resulting powder was washed several times with deionized water and then dried in a
vacuum oven at 60 °C. After dried in vacuum oven, 1.013 g Na/g-C N was obtained.
3
4
2
.3 Characterizations
The X-ray diffraction patterns were recorded on a Bruker D8 Advance X-ray
diffractometer (Cu-K irradiation, λ=1.5406 Å). The morphology and microstructure of
α
the samples were investigated by transmission electron microscopy (SEM, JSM-6700F)
and transmission electron microscopy (TEM, JEM-2010). Fourier transform infrared
−
1
(
FT-IR) spectra were recorded in a transmittance mode with a resolution of 4 cm using
a Nicolet Nexus 670 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS)
5

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analysis was performed on an ESCALAB 250 XPS System with a monochromatized
Al-K source. The Zeta potential was measured via Zeta potential analyzer (Malvern
α
Instruments Zetasizer Nano ZS90). UV-vis diffuse-reflectance spectrometry (DRS) of the
powder was obtained on a Cary 500 Scan spectrophotometer using BaSO as the
4
reflectance.
2
.4 Photo-catalytic reaction
The photocatalytic oxidation of benzyl bromide was performed in a sealed tube
reactor. The photocatalyst (30 mg), 4 ml solvent (MeCN/H O = 1:1) and 0.05 mmol
2
benzyl bromide were added in the reaction tube, then the reaction tube was filled with O2
at a flow rate of 5 ml/min through a double row tube for 30 min before illumination. The
reaction tube was irradiated by a white LED light (Beijing Perfect Light, PCX50B, 65
2
mW/cm , the spectrum is shown in Figure S1), its thermal effect is weak for the absence
of infrared radiation of LED light, and our reaction tube is continuously ventilated with a
fan from the bottom and the reactor temperature could be controlled in room temperature
to avoid solvent evaporation. The reaction mixture was filtered through a porous
membrane (200 nm in diameter) and then injected into the GC equipped with FID
detector (Agilent Technologies, GC6890) and a chromatographic column (HP-5) for
analysis. The temperatures of the gasification chamber and the detector are 300 °C and
3
20 °C, respectively. After the sample was injected into the gasification chamber, the
initial column temperature is maintained at 50 °C for 2 min, then raised to 125 °C by the
rate of 30 °C/min and maintained for 3 min, and then raised to 200 °C by the rate of
3
0 °C/min and maintained for 10 min followed by decreasing temperature.
3
. Results and discussion
3
.1. Photocatalytic benzaldehyde synthesis and its key influencing factors
First, the activity of bare g-C N was determined in different solvent at 298 K under
3
4
LED light irradiation. As shown in Figure 1A, the solvent did show influence on the
reaction. The mixed solvent (MeCN/H O = 1:1) is more effective in view of both benzyl
2
bromide conversion and benzaldehyde production as compared with that in MeCN only
after 10 h irradiation. An explanation to its improved performance is that the presence of
suitable amount of H O provides the proton source.32 Moreover, the produced
2
3
3
benzaldehyde can transfer to aqueous phase to avoid the over-oxidation. It is notable
6

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that no benzaldehyde formed when ethanol was used as the solvent, although the
conversion of benzyl bromide reached 42%. Instead, nearly 100% yield of benzyl alcohol
was obtained. This is probably because the alcoholysis reaction of benzyl bromide was in
a dominant position with ethanol as a solvent. Therefore, the mix solution of MeCN and
H O was chosen as the reaction medium in the following studies. As prolonging the
2
irradiation time to 40 h, the conversion of benzyl bromide reached 36%, while the
selectivity of benzaldehyde kept at >99% (Table 1, entry 1). However, a moderate
conversion (36%) of benzyl bromide was still not good enough for industrial applications.
To make the g-C N semiconductor more valuable for heterogeneous photocatalysis, a
3
4
targeted modification is important.
Since the base often abstracts benzylic proton to form the aldehyde, which is
beneficial for the oxidation, the Na/g-C N (g-C N treated by 5 wt.% NaOH solution)
3
4
3
4
was prepared. Indeed, modification with Na ions greatly enhanced the photocatalytic
activity of the sample for oxidation of benzyl bromide to benzaldehyde. Figure 1B shows
the time-dependent conversion of benzyl bromide. The conversion of benzyl bromide was
increased with increasing reaction time and reached a maximum benzyl bromide
conversion of 99% after 40 h illumination (Table 1, entry 2). The reaction stopped after
turning off the LED-light source, and continued after regaining normal irradiation (Figure
1
C). Since the light source has impact on the system, a 300 W Xeon lamp with 65
2
mW/cm was also used for the reaction. After 40 h reaction, both the conversion of
benzyl bromide and the selectivity of benzaldehyde reached up 99% (Table 1, entry 3).
Considering that LED is a cold light source and energy efficient, our subsequent
irradiation reactions were carried out under LED light. No products was detected in the
absence of light or g-C N . These indicate that the transformation of benzyl bromide to
3
4
benzaldehyde is a g-C N -mediated photocatalytic process (Table 1, entries 4 and 5).
3
4
Using nitrogen instead of oxygen atmosphere led to decrease in the yield to only 1%,
indicating that aerobic condition was essential to this reaction (Table 1, entry 6). In order
to clarify the role of alkali in this reaction, effect of different kinds of adding alkalis on
the reaction over g-C N photocatalyst was observed. Although both additional NaOH
3
4
and Et N to the reaction solution could also improve the conversion of benzyl bromide,
3
the selectivity of benzaldehyde was only 61% and 83%, respectively (Table 1, entries 7
7

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and 8). This indicates that the additional bases in the reaction system tend to promote the
hydrolysis of benzyl bromide. The reaction carried out over Na/g-C N was more
3
4
effective in view of both conversion and benzaldehyde selectivity as compared with the
traditional Kornblum reaction that using Et N as base in the solvent of DMSO (Table 1,
3
entries 9 and 10). To test the reusability of the catalyst system, the used Na/g-C N was
3
4
washed with ethanol, dried at 80 °C, and then re-employed with a new reactant mixture.
A decreased catalytic performance was observed during three consecutive experiments
(
Figure 1D). However, the Na/g-C N could be completely regenerated simply by
3 4
immersing into NaOH solution (5%) with continuously agitation at 200 °C to evaporate
water. This result indicates not only surface protonation as the deactivation mechanism,
+
but also proves the promoting role of the introduced Na in photocatalysis.
3
.2. Characterization
The phase structure of the samples was first identified by XRD, as shown in Figure
A. Two characteristic diffraction peaks at 27.8° (strong) and 13.5° (weak and wide)
2
were observed for the g-C N sample. The former is attributed to the (002) inter-planar
3
4
stacking with an interlayer distance of aromatic units of 0.326 nm, while the latter is
attributed to the in-plane ordered arrangement of structure units with the interval distance
of d = 0.675 nm.34 Compared to that of g-C N , the (002) characteristic peak of the
3
4
Na/g-C N shifted from 27.8° to 27.6°, showing increase in the interlayer spacing of the
3
4
+
carbon nitrogen due to the successful introduction of Na . Figure 2B and 2C show the
UV-vis diffuse-reflectance spectra of samples. The absorption spectrum of the g-C N
3
4
consists of a strong intrinsic absorption band showing a steep edge with the extrapolated
onset of 482 nm (band gap of 2.57 eV) and a weak absorption band starting from the
onset to the end at around 620 nm. The former is caused by the excitation of electrons
from valence band to conduction band of g-C N and the latter is related to the excitation
3
4
from band tails/localized states, originating from the incomplete long-range ordering of
structure.35 The NaOH treatment caused the red-shifted of the onset of the intrinsic
absorption band to 510 nm (band gap of 2.43 eV). The addition of Na atoms not only
replaced the H of -NH groups, but also increased the spacing between the layers of
g-C N , leading to the change in the intralayer long-range atomic order of g-C N . This
3
4
3
4
8

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change increased both the density and distribution of band tails/localized states, which is
responsible for the increased visible light absorption.
The SEM images (Figures 3A, 3B) and the TEM images (Figures 3C, 3D) show that
NaOH treatment did not lead to change in the microscopic morphology. The HRTEM
imageand elemental mapping results (Figures 3E, 3F) clearly reveal the homogeneous
+
distribution of Na. In order to further verify the interactions between introduced Na and
g-C N , both samples were analyzed by X-ray photoelectron spectroscopy (XPS). As
3
4
shown Figure 4A, compared to the silent signal of g-C N , Na 1s response at 1072.5 eV
3
4
was obviously detected in Na/g-C N , suggesting successful introduction of Na. The
3
4
content of Na in Na/g-C N was determined to be 3.39 wt.%. As shown in Figure 4B, the
3
4
high-resolution N 1s spectrum of g-C N could be fitted into three sub peaks centred at
3
4
2
3
98.8 eV, 400.0 eV and 401.2 eV, which are ascribed to the sp -hybridized N atoms in
the C=N-C bonds of C N rings, the N atoms located at the bridge sites and the N atoms
3
3
in the center of the heptazine ring, respectively.36, 37 Notably after Na modification, the
+
characteristic peak of N-H bonding shifted to 399.8 eV, suggesting the deprotonation
+
38
effect of Na . The substitution of H in amino groups by Na can be further verified by
+
FTIR spectroscopy (Figure 4C). After Na ions introduction, intensity of the peak at ca.
-1
3
230 cm ascribed to H O and amino groups in g-C N significantly decreased, implying
2 3 4
the substitution of H sites in amino groups by Na atoms. No much change was observed
-1
in the feature bands between 1650 and 1200 cm , which are associated with the trigonal
3
9
N(C) /bridging HN(C) units and heptazine ring of the melon unit in the strands,
3
2
implying the basic atomic structures of the strands was maintained after the
deprotonation. The increased visible light absorption can be explained as a consequence
of the changed microstructure of the Na/g-C N with broken hydrogen bonds. To further
3
4
investigate physicochemical properties of g-C N , the Raman spectroscopy were
3
4
measured. To overcome the fluorescence interference, we employed a laser of 785ꢀnm for
excitation. As shown in Figure 4D, two intensive peaks at 704 and 1231ꢀcm-1
corresponding to heptazine ring breathing mode and stretching vibration modes of C=N
4
0
and C-N heterocycles, respectively can be clearly seen in g-C N . Besides, the peaks
3
4
-1
with lower intensity at 757, 978, 1147 and 1308ꢀcm can be assigned to vibration modes
of melem, an intermediate product of the process to form g-C N from cyanimide
3
4
9

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precursor.41 There is no evident difference in the Raman shift in Na/g-C N compared
3
4
with pure g-C N , indicating that the modification by NaOH do not alter the basic
3
4
structure of the substrate material. According to the above analysis results from XRD,
XPS, FTIR and Raman, successful substitution of amino H by Na can be concluded. As
shown in Figure 5, the zeta potential of the pristine g-C N and the Na/g-C N dispersed
3
4
3
4
in water (pH = 7) were determined to be -12.9 mV and -39.6 mV, respectively. This
+
substantial potential change implies that Na introduction makes the g-C N surface to be
3
4
more negatively charged, which should be favorable for the deprotonation of the
substrate.
3
.3. Possible mechanism
It has been generally believed that the in-situ formation of superoxideradical anion
-
(
•O ) plays an important role in the photo-oxidation reaction over g-C N using
2
3
4
3
1
-
molecular oxygen as oxidant. The capability to form •O over irradiated g-C N was
2
3
4
confirmed by the solvent quenching experiment. The addition of benzoquinone, an
efficient superoxide radical scavenger, led to the trace amount of conversion, as showed
in Table 1 (entry 11). Based on the results, a mechanistic proposal for the photo-oxidation
reaction of benzyl bromide to benzaldehyde is given in Scheme 1. The corresponding
h
(
1) photocatalyst
e
h
(2) O2
e
O2
(
3) ArCH Br
h
ArCHBr (A)
H
2
(4) O2
H
O H (B)
2
(
5) ArCHBr (A)
6) ArC(OOH)HBr (C)
7) HOBr
HBr
8)
O H (B)
ArC(OOH)HBr (C)
2
(
(
ArCHO HOBr
1
2
O2
(
C N NH2 NaBr
C N NHNa HBr
3
4
3
4
Overall reaction:
1
2
h
C N NHNa
C N NH2 NaBr ArCHO
3 4
O2 ArCH Br
3
4
2
chemical equations to the Scheme 1 are described as follows:
1
0

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When irradiated by LED light, the photocatalyst was stimulated to generate electrons
and holes (Eq. (1)). The molecular oxygen was reduced by the excited electrons from the
-
conduction band of the g-C N to form superoxide radical anion (•O ) (Eq. (2)). The
3
4
2
benzyl bromide adsorbed on the surface of the catalyst was oxidized by the
photogenerated holes to form intermediate A and released a hydrogen ion in the meantime
-
(
Eq. (3)). Afterwards, the resulting free hydrogen ion reacted with •O to form •O H (B)
2
2
(Eq. (4)), which then attacked intermediate A to form intermediate C (Eq. (5)). The
intermediate C was very unstable, it quickly released hypobromous acid and form the
final product benzaldehyde (Eq. (6)). The hypobromous acid was further decomposed
into hydrogen bromide and oxygen (Eq. (7)). As the reaction progresses, the sodium ions
in the catalyst Na/g-C N were constantly released to react with hydrogen bromide,
3
4
eventually producing sodium bromide and the original g-C N (Eq. (8)). We further added
3
4
NaOH to the reaction solution to neutralize the HBr in the reaction. However, both the
conversion of benzyl bromide and the selectivity of benzaldehyde were notably
decreased, result of which was added in Table 1 (entry 12 and 13). This is probably
because the addition of alkali improve the alcoholysis reaction of benzyl bromide.
Nevertheless, the catalyst Na/g-C N could be completely recovered by simply washing
3
4
the used photocatalyst with 5 wt.% NaOH, which were consistent with previous literature
reports.13
In order to verify the reaction mechanism, vacuum-heat treated g-C N (g-C N -HV)
3
4
3
4
with different content of surface-termination amino groups was prepared, since the
termination amino groups was proposed to facilitate the reaction of g-C N with
3
4
bromobenzene to product of intermediate A.42 As shown in the Figure S2, the XRD
patterns show that after the heat-vacuum treatment, the g-C N sample was transformed
3
4
into g-C N -HV sample with more ordered structure. What’s more, as shown in Table S1,
3
4
the polycondensation and the structural rearrangement occurred after the heat-vacuum
treatment, which led to decrease in the area of characteristic peak of bridge sites (N2) of
g-C N -HV along with the increase of N atoms in the center of the heptazine ring content
3
4
(
N3). As the method presented above, the sample g-C N -HV treated with NaOH to get
3 4
sample Na/g-C N -HV. Compared to the XRD pattern of g-C N -HV in Figure S2, the
3
4
3
4
+
(
100) diffraction peak at 13.7° of Na/g-C N -HV suggests that Na intercalation barely
3 4
1
1

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damaged the periodic structure of g-C N -HV. However, the (002) characteristic peak in
3
4
+
Na/g-C N shifts from 27.7° to 27.5°, showing the successful introduction of Na .
3
4
+
Notably after Na modification, the characteristic peak of N-H bonding shifted from
99.7 to 399.5 eV, suggesting the substitution of H atom in amino groups by Na atom
Figure S3). Furthermore, Figure S4 shows the zeta potential of the g-C N -HV and the
3
(
3
4
Na/g-C N -HV dispersed in water (pH = 7) were determined to be -10.9 mV and -23.1
3
4
mV, respectively. The zeta potential decreases in order of Na/g-C N > Na/g-C N -HV >
3
4
3
4
g-C N > g-C N -HV, this substantial potential change further validates the position of
3
4
3
4
Na in the g-C N structure. Figure S5 shows the activity of g-C N -HV and
3
4
3
4
Na/g-C N -HV are lower than that of g-C N and Na/g-C N , respectively, which
3
4
3
4
3
4
indicates that the surface-termination amino group plays an important role in this
selective oxidation reaction as its H is replaced by Na, further validating the feasibility of
this reaction mechanism.
4
. Conclusions
In summary, this study for the first time demonstrated that efficient and direct
conversion of benzyl bromide to benzaldehyde can be achieved over Na/g-C N
3
4
photocatalyst with high selectivity using molecular oxygen as oxidant. The Na+
modification induced effective deprotonation of the surface amino groups in g-C N ,
3
4
resulting in the significant improvement of the photocatalytic performance. This study
not only provides a green and sustainable method for benzaldehyde production, but also
highlights the important role of carbon nitrogen as a photocatalyst for organic syntheses.
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Acknowledgements
The work is financially supported by National Natural Science Foundation of China
Grants No. 21673043, 21673042).
(
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Captions for Figures
Figure 1 A. Photocatalytic performance of g-C N in different solvents for the oxidation
3
4
of benzyl bromide with O ; B. Time-dependent changes in the amounts of benzyl
2
bromide and the product benzaldehyde during oxidation reaction under visible-light
irradiation over Na/g-C N ; C. The yield of benzaldehyde in the reaction with alternating
3
4
light irradiation and dark intervals; D. Recycling of the Na/g-C N for photocatalytic
3
4
oxidation of benzyl bromide to benzaldehyde.
Figure 2 A. XRD pattern of g-C N and Na/g-C N samples; B. UV-vis diffuse
3
4
3
4
reflectance spectrum of g-C N and Na/g-C N sample; C. Estimated energy gap of
3
4
3
4
g-C N and Na/g-C N samples.
3
4
3
4
Figure 3 SEM image of A. g-C N and B. Na/g-C N ; TEM image of C. g-C N and D.
3
4
3
4
3
4
Na/g-C N ; E. HRTEM image of Na/g-C N sample; F. TEM image of Na/g-C N and
3
4
3
4
3
4
the corresponding elemental mapping image.
Figure 4 High-resolution XPS spectra of A. Na 1s and B. N 1s of g-C N and Na/g-C N
4
3
4
3
samples; C. FTIR spectra of g-C N and Na/g-C N samples; D. The Raman spectroscopy
3
4
3
4
of g-C N and Na/g-C N sample.
3
4
3
4
Figure 5 Zeta Potential of g-C N and Na/g-C N sample measured in H O (pH=7).
3
4
3
4
2
Table 1 Photocatalytic oxidation of benzyl bromide to benzaldehyde.a
Scheme 1 Proposed reaction mechanism of photocatalytic benzaldehyde synthesis over
Na/g-C N .
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4
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Table 1
O
Br
1 bar O2
H
LED Lamp Irridiation
Room Temperature
b
Entry
Sample
g-C
Base
hν
Oxidant
Conv. (%)
Sel. (%)
1
2
N
3 4
-
+
+
+
+
-
O
O
O
O
O
-
2
36
99
99
2
>99
>99
>99
-
Na/g-C
Na/g-C
-
3
N
4
4
-
2
3c
4
-
3
N
2
2
2
-
5
g-C
g-C
g-C
g-C
3
3
3
3
N
4
-
-
-
-
6
N
4
N
4
N
4
+
+
+
+
-
3
1
7d
8e
9f
NaOH
O
O
58
41
5
61
83
85
92
2
2
Et
3
Et
3
Et
3
N
N
N
2
-
DMSO
DMSO
1
0g:373
1h
-
63
6
1
Na/g-C
Na/g-C
Na/g-C
3
N
N
N
4
4
4
-
+
+
+
O
2
O
2
O
2
1
1
2i
3j
3
3
NaOH
NaOH
42
77
67
63
Reaction conditions: 0.05 mmol of substrates, 30 mg photocatalyst, CH
3
CN (2 mL), H
2
O (2 mL), visible light irradiation for 40 h at
2
98 K. b Selectivity was based on the yield of benzaldehyde with benzyl alcohol as the only by-product. c Xe lamp. d 1.5 mg of NaOH.
e
.2 μL of Et
3 3 3
N. f 30 μL of Et N, DMSO (4 mL), 298 K. g 30 μL of Et N, DMSO (4 mL), 373 K. h 14 mol% benzoquinone was added.
5
i
j
1
0 h. 20 h.
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Scheme 1
B
A
C
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Synopsis
3 4
The NaOH-modified g-C N facilitates a green and sustainable route for the oxidation of benzyl bromide to benzaldehyde with
9
9% conversion.
2
5