Synthesis of Ag/g-C3N4Composite as Highly Efficient Visible-Light Photocatalyst for Oxidative Amidation of Aromatic Aldehydes

Article abstract of DOI:10.1002/adsc.201600138

In this contribution, an Ag/g-C₃N₄nanocomposite was synthesized and utilized as highly efficient and green photocatalyst for organic reactions under visible light irradiation. A layered, porous g-C₃N₄was synthes

Full text of DOI:10.1002/adsc.201600138

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DOI: 10.1002/adsc.201600138
Synthesis of Ag/g-C3N4 Composite as Highly Efficient Visible-
Light Photocatalyst for Oxidative Amidation of Aromatic
Aldehydes
Lingling Wang,a Min Yu,b Chaolong Wu,a Nan Deng,a Chao Wang,a
and Xiaoquan Yaoa,*
a
Department of Applied Chemistry, College of Material Science and Technology, Nanjing University of Aeronautics &
Astronautics, Nanjing, Jiangsu 210016, Peopleꢀs Republic of China
E-mail: yaoxq@nuaa.edu.cn
School of Environmental Sciences, Nanjing Xiaozhuang University, Nanjing, Jiangsu 211171, Peopleꢀs Republic of China
b
Received: February 1, 2016; Revised: May 12, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600138.
Abstract: In this contribution, an Ag/g-C3N4 nano- effect of Ag NPs exhibited high photocatalytic activi-
composite was synthesized and utilized as highly effi- ties toward aerobic oxidative amidation of aromatic
cient and green photocatalyst for organic reactions aldehydes under visible light irradiation. Good to ex-
under visible light irradiation. A layered, porous g- cellent yields were achieved for various substrates
C3N4 was synthesized following a modified solvother- under the light of a 25W compact fluorescent light
mal-roasting process by using melamine and cyanuric (CFL) bulb in air. The operationally easy procedure
chloride as precursor. Silver nanoparticles (NPs) provides an economical, green, and mild alternative
were well anchored on g-C3N4 nanosheets, which for the formation of amide bonds.
were prepared by a facile impregnation–roasting
method. The inexpensive, stable g-C3N4 coupled with Keywords: g-C3N4; oxidative amidation; photocataly-
the localized surface plasmon resonance (LSPR) sis; silver nanoparticles; visible light
Introduction
Graphitic CN (g-C3N4) is a novel aggregate type
semiconductor material and widely reported recently
The use of solar radiant energy to drive organic reac- in water splitting, heterogeneous organocatalysis, and
tions provides a sustainable pathway for green synthe- environmental remediation.[4] Because of its smallest
sis and has attracted significant interest in recent band gap among various CN allotropes and its solid-
years.[1] However, in most cases, ultraviolet (UV) light state aromatic system which is inert against most
is usually required to drive photochemical reactions. acids and bases,[5] the material was believed to be the
The UV radiation can activate chemical bonds in basis for a new family of solar energy transducers.[6,7]
many molecules and result in undesirable consequen- However, pure g-C3N4 suffers from a high recombina-
ces in many cases.[1] Thus, to develop the direct uti- tion rate of photogenerated electron-hole pairs, low
lization of visible light in organic synthesis, which ac- specific surface area, and low visible-light utilization
counts for 43% of the incoming solar spectrum, might efficiency, which limited its photocatalytic per-
provide a more efficient and greener approach. How- formance.[8] To resolve these problems, many methods
ever, since most common organic molecules cannot have been exploited, such as ion doping,[9] noble-
absorb light in the visible light region, there are many metal nanoparticle deposition,[10] morphology or pore
difficulties to utilize visible light.[2] Thus, to investigate structure design,[11] acid or alkali treatment,[12] dye
novel visible light catalysts has proved to be the key sensitization,[13] conjugated polymer coupling,[14] and
factor in the approach. Currently, only several kinds heterojunction fabrication.[4,15]
of visible light catalysts have been reported, mainly
In recent years, much attention has been directed
about Ru(II) or Ir(III) complexes, and usually suf- to the use of nanoparticles as catalysts in organic re-
fered many difficulties such as high cost, poor stabili- actions.[16] Because of their easy preparation and rela-
ties, and worse recyclabilities.[3]
tive stabilities in air, the nanoparticles of coinage
metals, such as copper, silver and gold, were widely
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reported, and there are many excellent examples of
their applications as catalysts in organic reactions.[17]
Silver and gold nanoparticles were also found to ex-
hibit strong interactions with resonant incident pho-
tons through excitation of the localized surface plas-
mon resonance (LSPR),[17a,18] and were recognized as
a new form of medium that is particularly efficient in
harvesting light energy for chemical processes due to
their strong light absorption over a wide range of the
visible and UV regions of the solar spectrum.[18,19] The
combination of Ag NPs and g-C3N4, provides a novel
strategy to enhance the photocatalytic performance of
g-C3N4.[20a,21] Recently, there have been several exam-
ples of the use of Ag NPs-loaded graphitic carbon ni-
tride (g-C3N4) as a green and stable photocatalyst for
the degradation of pollutants.[20] However, to the best
of our knowledge, there are few examples on their ap-
plication in photocatalytic organic reaction.
Herein, we have synthesized layered g-C3N4 materi-
al by a modified method with melamine and cyanuric
chloride as precursors, and first demonstrated the syn-
thesis of Ag/g-C3N4 hybrid material by a facile im-
pregnation–roasting method. The materials were char-
acterized by TEM, XRD, XPS, nitrogen sorption,
Scheme 1. The preparation process for porous g-C3N4.
UV-Vis and PL. The inexpensive, stable g-C3N4 cou- Table 1. Physicochemical properties of as-prepared samples
pled with the localized surface plasmon resonance
Catalyst
Molar
ratio C/N
Surface
Band gap
[eV]
(LSPR) effect of Ag NPs, exhibited high catalytic ac-
tivity toward the aerobic oxidative amidation of aro-
matic aldehydes under visible light irradiation. Good
to excellent yields were achieved for various sub-
strates under the light of a 25W compact fluorescent
light (CFL) bulb in air. Furthermore, the Ag/g-C3N4
nanocomposite catalyst could be recycled effectively.
area [m2 gÀ1]
bulk g-C3N4
g-C3N4
Ag/g-C3N4
0.71
0.73
0.73
10
34
32
2.71
1.98
1.91
the impregnation–roasting procedure to introduce Ag
NPs, the C/N molar ratio in Ag/g-C3N4 was not
changed.
To further study the structures of the materials, the
Brunauer–Emmett–Teller (BET) specific surface area
and the Barrett–Joyner–Halenda (BJH) pore size dis-
Results and Discussion
Material Characterization
There are many ways for the preparation of g-C3N4, tribution were measured to provide concrete evidence
the most common is to heat melamine directly under (Figure S1, Supporting Information). The nitrogen
an N2 atmosphere (for comparison, g-C3N4 was also sorption isotherm of as-prepared samples exhibited
synthesized following this procedure, and named as typical V shape isotherms and H3 shape hysteresis
bulk g-C3N4).[22] A soft solution-processing method loop, which indicated that the materials have a slit
with melamine and cyanuric chloride as starting mate- mesoporous structure (Figure S1a, Supporting Infor-
rial was also reported and nanobelt g-C3N4 was ob- mation). The specific surface area of the as-synthe-
tained.[23] In our present work, a layered, porous g- sized g-C3N4 sample (34 m2 gÀ1) is approximately three
C3N4 was synthesized following a modified solvother- times higher than that of bulk g-C3N4, whereas the
mal–roasting processes by using melamine and cyanu- Ag/g-C3N4 composite exhibited a similar BET surface
ric chloride as precursors. The procedure is described area (32 m2 gÀ1). The pore size distributions of the
in Scheme 1. With the porous g-C3N4 as support, Ag/ samples are shown in the Supporting Information,
g-C3N4 hybrid material was then prepared by a facile Figure S1b, which revealed that the pore size of meso-
impregnation–roasting method.
pores was concentrated in 2–5 nm and the material
Carbon and nitrogen stoichiometry of the samples still possessed a large number of pores after introduc-
was determined by elemental analysis (Table 1). Re- ing Ag nanoparticles.
sults show that g-C3N4 possess a C/N molar ratio ap-
Figure 1A shows a TEM image of the porous g-
proaching 0.73 (theoretical value is 0.75). Even after C3N4 employed in our studies. As can be seen, g-C3N4
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Figure 1. TEM images of A) g-C3N4, B) Ag/g-C3N4.
possessed a layered structure, and it presents in the spectra, new Ag peaks were observed, and the deco-
form of a thin sheet with irregular morphology. Ag ration of Ag nanoparticles on the g-C3N4 surface was
nanoparticles (Ag NPs) were well anchored on g- further confirmed, which is in good agreement with
C3N4 nanosheets, which were prepared by a facile im- the TEM and XRD results. The XPS Ag 3d spectrum
pregnation–roasting method; Figure 1B indicates a rel- of Ag/g-C3N4 and Ag NPs is shown in Figure 3B, in
atively uniform size and shape of the Ag NPs, with which the peaks at 367.8 and 373.9 eV are ascribed to
the particle diameter of about 15 nm, generated on Ag 3d3/2 [Ag(0)] and Ag 3d5/2 [Ag(0)] and the result is
the surfaces of g-C3N4. On the other hand, as the ref- in accordance with previous reports.[25] Moreover, the
erence samples, Ag NPs and Ag NPs/AC, were also binding energy values of Ag 3d in Ag/g-C3N4 are
synthesized. The TEM images of them showed that slightly lower than those of Ag NPs. The shift might
the Ag NPs in both of the samples had similar sizes result from the strong interaction between Ag NPs
to those depicted in Figure 1B (Figure S2a and S2b, and g-C3N4.
Supporting Information).
Figure 3C and D show the high resolution C1s and
XRD patterns of the as-prepared Ag/g-C3N4 com- N1s spectra of the Ag/g-C3N4, which can be divided
posite and g-C3N4 are shown in Figure 2, the results
confirmed graphitic-like layer structures in Ag/g-
C3N4. The diffraction peaks at 2q of 138[(100) diffrac-
tion plane] and 288 [(002) diffraction plane] are the
characteristic peaks of g-C3N4 (Figure 2), which can
be attributed to the in-plane structural packing motif
of tri-s-triazine units and the interlayer stacking of
the conjugated aromatic system, respectively.[24] How-
ever, the diffraction peaks of g-C3N4 were very weak
in the XRD pattern of Ag/g-C3N4, this might be at-
tributed to the fact that the XRD signals were cov-
ered by Ag NPs. The characteristic peaks are at 388,
448, 658, and 778 which can be attributed to the (111),
(200), (220), and (311) lattice planes of metallic Ag,
respectively. Such observations indicated that the Ag
NPs were successfully loaded on the g-C3N4 after the
impregnation–roasting procedure.
The surface composition and chemical state of ele-
ments in the as-prepared g-C3N4 and Ag/g-C3N4 nano-
composite were further investigated by XPS spectra,
as shown in Figure 3 and Figure S3 (Supporting Infor-
mation).
Figure 3A shows the XPS survey spectra of the g-
C3N4 and Ag/g-C3N4 nanocomposite. Comparing these Figure 2. XRD patterns of the as-prepared Ag/g-C3N4.
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Figure 3. A) XPS survey spectra of the g-C3N4 and Ag/g-C3N4, high resolution XPS spectra Ag 3d. B) Of as-prepared Ag/g-
C3N4 and Ag NPS, high resolution XPS spectra C1s (C) and N1s (D) of Ag/g-C3N4.
into three peaks, respectively. C1s at 286.3 eV and
N1s at 397.4 eV are assigned to the sp2 C=N bond in
the s-triazine ring. The peaks at 286.9 eV and
283.5 eV in the C1s zone are attributed to electrons
originating from an sp2 C atom attached to an NH2
group and to an aromatic carbon atom.[23,26] The N1s
peaks centered at 399.2 eV and 403.2 eV are ascribed
to N atoms bonded to three C atoms [N-(C)3] and the
N atoms located in the heptazine ring and as bridging
atom, respectively.[23,26] The high resolution C1s and
N1s spectra of the Ag/g-C3N4 are in agreement with
those of g-C3N4 (Figure S3a and b, Supporting Infor-
mation), which indicated that after loading Ag nano-
particles, there were no significant changes to the
graphitic C–N network.
Since the spectral absorption property of a photoca-
talyst is very important, UV-vis diffuse reflectance
spectra (DRS) was used to examine the optical ab-
sorption properties of the photocatalysts. The UV-vis
DRS of Ag NPs, Ag/g-C3N4, g-C3N4 and bulk g-C3N4
samples are demonstrated in Figure 4.
Figure 4. UV-vis diffuse reflectance spectra of g-C3N4, bulk
g-C3N4, Ag NPs, and Ag/g-C3N4 nanocomposites.
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The absorption edge of the porous g-C3N4 nano- of the Ag/g-C3N4 indicated that the separation of pho-
sheets indicated a bathochromic shift from approxi- togenerated electron-hole pairs in Ag/g-C3N4 was
mately 454 to 626 nm as compared with the bulk g- more efficient, because the excited electron could be
C3N4. After loading Ag nanoparticles on the surface transferred directly between the Ag NPs and g-C3N4
of g-C3N4, the absorption intensities of Ag/g-C3N4 through their mutual interface.[29] It might indicate
composite became stronger in the whole spectrum that the Ag/g-C3N4 composite possessed higher photo-
window of interest, especially in the visible-light catalytic activity under visible light irradiation than g-
region, which might be due to the strong LSPR ab- C3N4 or Ag Nps.
sorption of Ag nanocrystals and further confirmed
the formation of Ag NPs.[27] In addition, the absorp-
tion spectra were largely red-shifted, and a broad ab- Oxidative Amidation of Aldehydes
sorption ranging from ~465 to ~660 nm was detected.
As for the as-prepared Ag NPs sample, it clearly ex- Amides are functional groups of great importance in
hibits a wide light absorption in the whole UV-vis polymers, natural products, and pharmaceuticals.[30]
range of 200–800 nm. The results of UV-vis DRS Recently, the oxidative transformation of aldehydes
show that the fabrication of the Ag/g-C3N4 composite to amides has received much attention. In 2006, Liꢀs
can greatly improve the optical absorption property groups reported an excellent example with Cu(I) as
and increase the utilized efficiency of visible light, catalyst and AgIO3 as oxidant.[31] Subsequently, Marks
which is favorable for the enhancement of the photo- and co-workers described the catalytic amidation of
catalytic activity.
aldehydes and amines by a lanthanide catalyst.[32] Ya-
The recombination process of electron-hole pairs of maguchi et al. also developed a CuI/2-pyridonate cat-
semiconductors can release energy, which can be de- alytic system.[33] Other catalyst-free methods using
tected by PL emission. A lower photoluminescence stoichiometric oxidants, such as TBHP,[34] Oxone[35]
intensity is a general indication of a lower recombina- and hydrogen peroxide[36] were also reported. Howev-
tion rate of electron-hole pairs, and usually results in er, most of these methods still have some limitations,
higher photocatalytic activity.[28]
such as the need for expensive noble transition
Figure 5 shows the PL spectra of bulk g-C3N4, g- metals, stoichiometric oxidants, and poor substrate
C3N4 and Ag/g-C3N4 samples excited at 320 nm. The scope as well as low atom efficiency. Thus, the visible
PL spectra of bulk g-C3N4 have a strong emission light-promoted oxidative amidation of aldehydes
peak at around 454 nm, which is assigned to the re- might provide a sustainable approach. Until now,
combination of the photoinduced electron-hole pairs there are only a few examples on the transformation
of bulk g-C3N4. As can be seen from the figure, there reported. Au@SiO2 was reported as a highly efficient
is an obvious decrease in the PL intensity of g-C3N4 photocatalyst for the amidation of benzaldehyde and
and Ag/g-C3N4 compared with that of bulk g-C3N4. morpholine under green laser irradiation with H2O2
Notably, the much lower photoluminescence intensity as oxidant.[37] Recently, Leow also presented an ele-
gant approach to the reaction by using phenazine
ethosulfate as visible light photocatalyst in air via an
in situ generation of H2O2.[38] However, the photocata-
lyst seemed easy to be decomposed and could not be
recovered.
In our present work, the as-prepared Ag/g-C3N4
was utilized as visible light photocatalyst, to explore
the oxidative amidation of aldehydes. Our initial re-
search focused on the model reaction of 4-bromoben-
zaldehyde (1b) with pyrrolidine (2a) at ambient tem-
perature (Table 2).
At beginning, the model reaction was performed
with 5 mol% of Ag/g-C3N4 as catalyst under air at-
mosphere and a 25W CFL was used as the source of
visible light. Several kinds of solvents were scanned at
first. It can be seen from Table 2 that toluene,
CH3CN, DMF, DCM were not suitable for the reac-
tion and a 92% of yield was achieved in THF solution
(entries 1–4 vs. entry 5, Table 2). Subsequently, to fur-
ther investigate the photocatalytic property of Ag/g-
C3N4, a series of control experiments were carried
out: g-C3N4, Ag NPs and even AgNO3 were used as
Figure 5. Photoluminescence (PL) spectra of bulk g-C3N4, g-
C3N4 Ag/g-C3N4 samples (excitation wavelength: l=
320 nm)
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Table 2. Optimization of the reaction conditions.[a]
copper gave much worse photocatalytic activity than
that of silver (entry 16).
Having identified our optimized reaction condi-
tions, the Ag/g-C3N4 catalyzed oxidative amidation of
aldehydes under visible light irradiation was extended
to various substrates successfully and good functional-
group tolerance was observed. All of the results are
summarized in Scheme 2.
Entry Light Photocat. (mol%) Solvent Yield [%][b]
1
2
3
4
+
+
+
+
+
+
+
+
+
+
Ag/g-C3N4 (5)
Ag/g-C3N4 (5)
Ag/g-C3N4 (5)
Ag/g-C3N4 (5)
Ag/g-C3N4 (5)
g-C3N4
Ag NPs (10)
AgNO3 (10)
Ag NPs/AC (10)
Ag NPs-TiO2 (10) THF
Ag/g-C3N4 (5)
none
Ag/g-C3N4 (10)
Ag/g-C3N4 (20)
Ag/g-C3N4 (5)
Cu/g-C3N4 (10)
toluene n.r.
CH3CN 10
As can be seen, benzaldehydes bearing Br, Cl, F,
NO2, CF3, MeO, and Me groups at para-positions re-
acted with pyrrolidine (2a) to afford the products 3b–
3h in 65–95% yields. The results indicated that ben-
zaldehydes with electron-withdrawing groups on the
benzene rings seemed to react more efficiently than
those with electron-donating groups (3b–3f vs. 3g and
DMF
DCM
THF
THF
THF
THF
THF
trace
trace
92
35
28
5
6[c]
7
8
9
29
trace
trace
10
10
11
12
13
14
15[d]
16
–
THF
THF
THF
THF
THF
THF
+
+
+
+
+
n.r.
89
85
98
45
[a]
Reaction conditions, unless otherwise noted: aldehyde 1b
(0.10 mmol), amine 2a (2.0 equiv.), solvent (1.0 mL), pho-
tocatalyst, 25W CFL at ambient temperature in air for
30 h.
Yield of isolated product.
g-C3N4 (5 mg).
O2 (with balloon). Abbreviations: CFL=compact fluo-
rescent lamp, THF=tetrahydrofuran, DMF=N,N-dime-
thylformamide, DCM=dichloromethane, n.r.=no reac-
tion.
[b]
[c]
[d]
photocatalyst in the oxidative amidation, respectively.
However, only poor yields were observed in these re-
actions (entries 6–8). Considering the possible support
effect on Ag Nps, Ag NPs/AC and Ag Nps/TiO2 were
also tested. However, only trace reaction occurred
(entries 9 and 10). Furthermore, even after 48 h reac-
tion, only 10% yield was obtained in the dark
(entry 11). Moreover, a blanket reaction was also set
up under visible light, and no reaction was observed
in the absence of a photocatalyst (entry 12). These re-
sults above indicated that the combination of Ag NPs
and g-C3N4 did enhance the photocatalytic per-
formance of g-C3N4.
On the other hand, the loadings of the catalyst
were also examined. It can be seen that 5 mol% load-
ing is the best choice (entries 13 and 14 vs. entry 5).
The reaction was also carried out under an oxygen at-
mosphere instead of air, a 98% of yield was achieved
after 15 h (entry 15). Considering more convenient
handling, we still selected to carry out the reaction in
air. Specifically, we also prepared Cu/g-C3N4 as cata-
Scheme 2. Ag/g-C3N4-catalyzed aerobic oxidative amidation
lyst in the reaction. Obviously, the introduction of of aromatic aldehydes and amines.
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3h). Treatment of substrates bearing different substi-
tution positions also furnished the desired products in
good to excellent yields (3i–3l). Disubstituted aromat-
ic aldehyde 3m also gave the targeted product in 93%
yield. Both b- and a-naphthaldehydes were also
tested in the reaction, providing 3n and 3o in 77%
and 90% yields, respectively.
Reactions of morpholine (2b) with a series of para-
and meta-substituted benzaldehydes were well-tolerat-
ed, and provided the corresponding products 3q–3u in
60–90% yields. Piperidine (2c) was also utilized in the
photocatalytic reaction. The transformation still pro-
ceeded very well and the desired products 3v–3z were Scheme 4. A) Oxidative amidation of 4-bromobenzaldehyde
in sunlight. B) Expanding the reaction of 4-bromobenzalde-
hyde under standard conditions.
obtained in good to excellent yields.
On the other hand, it is worthy to note that dibutyl-
amine was also examined instead of cyclic amines in
the oxidative amidation, but there was no desired
product observed.
One of the advantages of heterogeneous catalysts is
Some heteroaromatic aldehydes were then investi- their easy separation from the reaction mixture. The
gated in the reaction (Scheme 3). Generally, the Ag/g- Ag/g-C3N4 catalyst could be separated and recovered
C3N4 catalyzed oxidative amidation of heteroaromatic conveniently by centrifugation from the reaction mix-
aldehydes and pyrrolidine afforded the desired ture, and then, fresh substrates and solvent were
amides 4a–e in moderate to good yields under visible added to set up a new reaction. Following this proce-
light irradiation. It should be noted that picolinalde- dure, the catalyst was recycled effectively, and the re-
hyde reacted with 2a to afford the corresponding sults are summarized in Scheme 5. We found that the
product 4c in 42% yield under the present conditions. yields of the second and third runs are lower but very
Under standard conditions but with sunlight irradi- close. It might be due to some silver nanoparticles
ation instead of CFL, aldehyde 1b reacted with pyrro- falling off from the Ag/g-C3N4 composite in the first
lidine 2a smoothly to give 3b in moderate yield in washing process, and then, the content of silver in the
sunlight (about 10 h) (Scheme 4A). Moreover, in catalyst tended to remain stable.[39]
order to prove the scalability and practicality of the
novel photocatalytic system, a 1-mmol scale reaction
was performed under the standard conditions: an ex-
cellent yield was achieved after 40 h (Scheme 4B).
Scheme 5. The recyclability of Ag/g-C3N4 photocatalyst.
To investigate the mechanism of the reaction, sever-
al control reactions were carried out (Scheme 6). The
freshly distilled THF solvent (refluxed and distilled
from sodium) was stirred with the Ag NPs/C3N4 cata-
lyst under an oxygen atmosphere and visible light for
1 day, and ca. 10% peroxide of THF (5) was observed
1
from H NMR (Figure S5 and S6, Supporting Infor-
mation); and then, 1b and 2a were added and the
Scheme 3. Ag/g-C3N4-catalyzed aerobic oxidative amidation
of heteroaromatic aldehydes and pyrrolidine.
Scheme 6. Control reactions for mechanism investigation.
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mixture was stirred and monitored by TLC. The reac- mechanism for the visible light-promoted oxidative
tion was finished in 3 h, and 98% of 3b was obtained. amidation was proposed as the following: at first, Ag
In the classical mechanism for the photo-promoted NPs/C3N4 catalyzed the oxidation of THF with the
auto-oxidation of THF,[40] the formation and the exci- promotion of light; and then, the THF hydroperoxide
tation of the charge-transfer complex by the light, fol- (5) formed in situ worked as a highly efficient oxidant
lowed by the dissociation of the excited-state com- reacting with the enamine intermediate (3’) to give
plex, were believed to be the key steps for the auto- target molecule (Scheme 8).
oxidation of THF to give the hydroperoxide product
5 (Scheme 7A). Thus, referring to the mechanism for
the Ag/g-C3N4-catalyzed oxidative decomposition of
organic compounds,[20a] we propose that the charge-
transfer between oxygen and THF would occur in two
separate steps on the surface of Ag/g-C3N4 under visi-
ble light. The detailed description is shown in
Scheme 7B).
Scheme 8. Plausible mechanism of the Ag/g-C3N4-catalyzed
oxidative amidation of aldehydes and amines in THF under
visible light irradiation.
Conclusions
In conclusion, we have synthesized a layered porous
g-C3N4 via a solution-process and roasting approach,
and then, Ag NPs were successfully anchored on the
surface of g-C3N4 by a facile impregnation–roasting
method. An Ag/g-C3N4-catalyzed aerobic oxidative
amidation of aldehydes under visible light irradiation
has been developed. Mainly because of the synergetic
effects between g-C3N4 and Ag NPs, the photocatalyt-
ic system can realize an excellent catalytic per-
formance to generate benzamide bonds. This study
Scheme 7. A) The key steps for the classical mechanism of
photo-oxidation of THF. B) The electron-transfer approach
on Ag/g-C3N4 surface under visible light irradiation.
Under visible light irradiation, g-C3N4 can be excit- might provide a new perspective for photocatalytic or-
ed to generate conduction band electrons (eÀ) and va- ganic synthesis and environment-friendly transforma-
lence band holes (h+).[20a,8] Furthermore, with the in- tions. Further studies to gain more mechanistic details
troduction of Ag NPs, the recombination of photo- of this reaction and apply this strategy to other organ-
generated electron-hole pairs was inhibited efficiently, ic transformations are ongoing in our laboratory.
because the excited electron could be transferred di-
rectly between the Ag NPs and g-C3N4 through their
mutual interface (refer to the PL spectra in Figure 5). Experimental Section
Thus, the photogenerated electrons excited to the
All chemicals and solvents as reference were procured from
Aladdin (Shanghai, China), Alfa Aesar (Shanghai, China),
TCI (Japan) and used as received.
conduction band were entrapped by Ag NPs, and
then, could reduce the electron acceptor O2 to super-
oxide radical anion. On the other hand, THF released
an electron on the valence band holes to give a THF
cation. Following the approach above, the rate for the
auto-oxidation of THF to 5 was improved efficiently.
On the other hand, in order to learn more details
for the reaction, a standard reaction of 1b and 2a was
Characterization
The morphology of the photocatalysts was observed by
transmission electron microscopy (TEM, JEOL JEM-
2100F). The crystal structures of the samples were deter-
mined using an X-ray diffractometer (XRD, Bruker D8 AD-
VANCE) with a 2 scope of 5–908at 40 kV and 40 mA using
1
also carried out in THF-d8. From the crude H NMR
spectra (Figure S7, Supporting Information), an in-
creased water peak was observed.[41] Thus, a plausible Cu-Ka as the irradiation source (l=1.54 ꢂ). X-ray photo-
Adv. Synth. Catal. 0000, 000, 0 – 0
8
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electron spectroscopy (XPS: PHI Quantera II, Japan) was
used to examine the surface properties of the catalysts. The
Brunauer–Emmett–Teller (BET) method was used to calcu-
late the specific surface areas of the samples. The N2 adsorp-
tion–desorption isotherms at 77 K were measured using an
adsorption instrument (ASAP 2020, Micromeritics Compa-
ny, USA) to evaluate their pore structures. The UV-vis dif-
fuse reflectance spectra (UV-vis DRS) of the samples were
recorded on a UV-vis spectrometer (Shimadzu UV-3600)
with an integrating sphere. BaSO4 was used as a reference
sample. The photoluminescence (PL) spectra were recorded
on a Varian Cary-Eclipse 500. NMR spectra were recorded
Typical Procedure for Ag/g-C3N4-Catalyzed Oxidative
Amidation of Aromatic Aldehydes under Visible-
Light Irradiation
A 40-mL oven-dried sealed tube equipped with a magnetic
stir bar was charged with aldehyde 1 (0.1 mmol), amine 2
(0.20 mmol, 2 equiv.), Ag/g-C3N4 (5 mol%), and freshly dis-
tilled THF (1.0 mL) in air. The tube was capped and ex-
posed to a 25W CFL bulb placed approximately 10 cm from
the tube at ambient temperature, the reaction time was 30 h
unless otherwise specified. After completion of the reaction,
the mixture was concentrated to yield the crude product,
which was further purified by flash chromatography (silica
gel, petroleum ether/ethyl acetate=10:1) to give the desired
product 3; yield: 60–95%.
1
on a Bruker AVANCE III-400 (400 MHz for H, 101 MHz
for 13C) instrument in the indicated solvent. Chemical shifts
are reported in units of parts per million (ppm) relative to
1
the signal for internal tetramethylsilane (0 ppm for H) for
solutions in CDCl3. Multiplicities are reported by using the
following abbreviations: s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; m, multiplet; br, broad singlet; and, J,
coupling constants in Hertz. High resolution mass spectros-
copy data of the product were collected on an Agilent Tech-
nologies 6540 UHD Accurate-Mass Q-TOF LC/MS (ESI).
The detailed characterization data for 3a–3z are provided
in the Supporting Information.
Synthesis of Bulk g-C3N4 and Porous g-C3N4
Bulk g-C3N4 was prepared by heating melamine (5.0 g) to
5508C for 4 h with a ramping rate of 2.58CminÀ1 under N2
atmosphere.[22]
The porous g-C3N4 was obtained by a solution-process
and then roasting processes. Typically, 10 mmol 1,3,5-tri-
chlorotriazine and 5 mmol melamine powders were put into
a 100-mL Teflon-lined autoclave, which was then filled with
acetonitrile up to 60% of the total volume. The mixture was
stirred at 1808C for 48 h, and then the resulting sample was
sequentially washed with acetonitrile, distilled water and ab-
solute ethanol several times and dried under 608C for 12 h
to get the orange power.[23] Then, the power was added to
a porcelain cup and calcined under N2 flow at 5008C for 2 h
with the heating rate being 28CminÀ1, the as-prepared sam-
ples were collected and ground carefully to get the final
products.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21472092 & 21172107 to
X.Y.), Natural Science Fund for Colleges and Universities in
Jiangsu Province (15 KJD150003 to M.Y.). This is a project
funded by the Priority Academic Program Development of
Jiangsu higher education institutions.
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12 Synthesis of Ag/g-C3N4 Composite as Highly Efficient
Visible-Light Photocatalyst for Oxidative Amidation of
Aromatic Aldehydes
Adv. Synth. Catal. 2016, 358, 1 – 12
Lingling Wang, Min Yu, Chaolong Wu, Nan Deng,
Chao Wang, Xiaoquan Yao*
Adv. Synth. Catal. 0000, 000, 0 – 0
12
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