Visible-light assisted of nano Ni/g-C3N4 with efficient photocatalytic activity and stability for selective aerobic C?H activation and epoxidation

Article abstract of DOI:10.1016/j.jorganchem.2020.121549

A selective, economical, and ecological protocol has been described for the oxidation of methyl arenes and their analogs to the corresponding carbonyl compounds and epoxidation reactions of alkenes with molecular oxygen (O₂) or air as a green oxygen source, under mild reaction conditions. The nano Ni/g-C₃N₄ exhibited high photocatalytic activity, stability, and selectivity in the C?H activation of methyl arenes, methylene arenes, and epoxidation of various alkenes under visible- light irradiation without the use of an oxidizing agent and under base free conditions.

Full text of DOI:10.1016/j.jorganchem.2020.121549

Journal Pre-proof
Visible-light assisted of nano Ni/g-C N with efficient photocatalytic
3
4
activity and stability for selective aerobic C−H activation and
epoxidation
Mona Hosseini-Sarvari , Zahra Akrami
PII:
S0022-328X(20)30452-6
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2020.121549
JOM 121549
To appear in:
Journal of Organometallic Chemistry
Received date:
Revised date:
Accepted date:
11 July 2020
5 September 2020
27 September 2020
Please cite this article as: Mona Hosseini-Sarvari , Zahra Akrami , Visible-light assisted
of nano Ni/g-C N4 with efficient photocatalytic activity and stability for selective aero-
3
bic C−H activation and epoxidation, Journal of Organometallic Chemistry (2020), doi:
https://doi.org/10.1016/j.jorganchem.2020.121549
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Highlights

A procedure for the performance of a metal-organic catalyst with high photocatalytic activity is
described.


Ligand-free nano Ni/g-C N was synthesized by using the photodeposition procedure.
3
4
Ligand-free nano Ni/g-C N exhibited high photocatalytic performance in the selective aerobic
3
4
C−H activation and epoxidation.

The high yields of products, economical, environmentally friendly, and simple procedures are
the advantages of the method.
1

Visible-light assisted of nano Ni/g-C N with efficient photocatalytic activity
3
4
and stability for selective aerobic C−H activation and epoxidation
Mona Hosseini-Sarvari*, Zahra Akrami
Department of Chemistry, Faculty of Science, Shiraz University, Shiraz, 7194684795 I.R. Iran. E-mail:
hossaini@shirazu.ac.ir
Abstract
A selective, economical, and ecological protocol has been described for the oxidation of methyl arenes
and their analogs to the corresponding carbonyl compounds and epoxidation reactions of alkenes with
molecular oxygen (O ) or air as a green oxygen source, under mild reaction conditions. The nano Ni/g-
2
C N exhibited high photocatalytic activity, stability, and selectivity in the C−H activation of methyl
3
4
arenes, methylene arenes, and epoxidation of various alkenes under visible- light irradiation without the
use of an oxidizing agent and under base free conditions.
Keywords
C-H activation; Epoxidation; Nano Ni/g-C N ; Photocatalyst; Visible light
3
4
1
. Introduction
The one-pot addition of molecular oxygen as a “green” oxidant into hydrocarbons (alkanes and
alkenes) to obtain a family of aldehydes, ketones, and epoxy derivatives is an extremely desirable
reaction. So a lot of attention has been paid to the search for novel raw materials for the chemical
industry including drugs, polymers, agricultural and synthetic materials [1]. Oxygen, particularly
if received from the air, demonstrates the perfect oxidant even more efficiently than the
commonly mild oxidant “hydrogen peroxide” [2]. Besides, O has a wide range of derivatives
2
while retaining high selectivity, reduced environmental impact, and high atom economy [3].
Besides, if molecular oxygen can be actuated at room temperatures under standard pressure
condition it will be the most appropriate oxidizing agent. So photocatalytic processes can be
applied for selective and greener oxidation by O as the oxidant [4]. The selective oxidation of
2
3
methyl arenes and the oxidation of benzylic (C-sp ) hydrocarbons to the analogous carbonyl
groups is a key reaction in chemical synthesis [5,6]. Likewise, styrene epoxidation reactions show
a vital part in industrial processes. Styrene oxide is usually synthesized directly by epoxidation of
alkenes with peroxy acids or by chlorohydrin processes which are highly-cost, low-yield of
epoxide, and leads to serious environmental contamination [7]. Most of these reactions have tough
2

conditions that are briefly mentioned in the Scheme 1. As a result, we have decided to apply an
efficient technique in organic synthesis [8,9] for avoiding the disadvantages of current protocols
for the oxidative C−H activation.
Non-photochemical
stoichiometric amounts of reagents [10],
continuous oxygen pressure and using H O2
2
[
11] or using TBHP [12], using catalytic
,
amount of precious metals (such as Au, Pd
etc.) [13], high reaction temperatures
homogeneous catalysts
Previous works
Photochemical approaches
continuous stream of O2 [14], using
temperatures [14,15], Ruthenium complex
[
16], long reaction time [17] using Variac
Cermax 300 W xenon lamp (λ > 320 nm)
18]
[
This work
Nano
Ni/g-C N4
as
economic
3
heterogeneous photocatalyst
no additives, no reagents, room
temperature, air
Scheme 1. The synthesis of aldehyde, ketone, and epoxide derivatives in previous reports and this work
Natural Photosynthesis is a model for the development of non-natural photoredox systems by
utilizing several semiconductor photocatalysts [19]. Nowadays heterogeneous photocatalytic
based on the use of noble metal-semiconductor hybrid nanoparticles are used for new organic
synthesis and many other uses [20]. Among all reported photocatalysts g-C N (a semiconductor)
3
4
3

has attracted extensive attention [21-24]. The g-C N owns a graphitic piling of C N layers, that
3
4
3
4
are created from fused tris-triazine cycles by tertiary amines, lead to its heptazine units and a high
2
density [25,26]. As a result of the sp hybridization of nitrogen and carbon atoms that form π-
conjugated graphitic systems, g-C N is so remarkable as a visible light absorption range of
3
4
4
50−460 nm with an optical band gap of 2.7 eV [27]. Thermal and chemical stability, numerous
interesting electric properties, non-toxicity, low cost, and easy to get from abundant sources of
organic nitrogen like melamine and urea are unique features of g-C N [28]. These polymers
3
4
possess efficient transportation and separation of electron-hole pairs which causes photoactivity
29]. But despite these benefits doping cocatalyst on the g-C N nanosheets is a highly desired
[
3
4
method as the electron-hole recombination rate is high in pure g-C N and surface area
3
4
photocatalytic activity is quite low [30]. Metal doping has several advantages such as:
accelerating the separation and transfer of charge, enhancing the visible-light absorption, lowering
the bandgap, and prolonging the charge carriers' lifetime for g-C N , that all of these features are
3
4
required for the efficient photocatalytic process [31]. Ni is abundant, cheap, good stability, and is
also high activity metal. In the presence of nickel, the separation of electron-hole pairs can be
done successfully, arising from the alteration of the surface band bending which can be faster
surface reaction rates and can suppress back reactions, which results in good photocatalytic
performance in contrast to the pure g-C N [32]. So, loading Ni on the g-C N is an efficient
3
4
3
4
strategy and we understood that the nano Ni/g-C N photocatalyst is an applicable choice for the
3
4
oxidative C−H activation, and leads to the selective oxidation and preparation of organic
structures containing oxygen (Scheme 1).
2
. Experimental
2.1. Materials
Total chemicals were either provided in our laboratories or were gained from Fluka, Aldrich, and Merck.
All the solvents dried in agreement with standard procedures.
2.2 Analysis methods
The proceeds of the reactions were controlled by thin-layer chromatography (TLC) employing silica gel
SILG/UV 254 plates. Column chromatography was run on minor columns of silica gel. IR spectra of the
samples were registered on a Shimadzu FTIR-8300 spectrophotometer by KBr pellets. The amount of
nickel nano Ni/g-C N was assessed using inductively coupled plasma (ICP). Power X-ray diffraction
3
4
analysis (XRD) was measure with Cu Kα (λ= 1.54178 Å) irradiation. The distribution morphology of the
nano Ni/g-C N was established by applying scanning electron microscopy (SEM). The specific surface
3
4
4

areas (SSABET; (m2 /g)) of the nanopowders were identified with the nitrogen adsorption measurement
utilizing the BET procedure at 77 K (BELsorp-mini II). Shimadzu UV-2450 spectrophotometer was
carried out the UV–vis diffuse reflectance spectrum. Melting points were measured by Buchi Melting
1
13
Point B-545 electrical melting point equipment. H NMR and C NMR spectra were registered in CDCl
3
13
as solvent using Bruker, Avance 400 (operating at 400 MHz for 1H and 100 MHz for C), with TMS as
an internal standard (multiplicity: s = singlet, d= doublet, t = triplet, dd = doublet of doublets, m =
multiplet). The lamps applied for photochemical reactions were 12 W white, blue, red LED and 11 W
green LED. A blue LUX* plus (made from china) LED A 65 with a voltage of 220- 240 V, frequency of
50/60 Hz, lumens 1150 lm, and a power of 12 watts was used.
2
.3. Preparation of g-C N
3
4
The g-C N was prepared according to the procedure reported by Li [33]. A Coors high-alumina crucible
3
4
was charged with 10 g of melamine powder, next placed in a 673in³ muffle furnace (115V / 14 Amps) at a
heating rate of 20
ͦ
C /min to 500
ͦ
C in 2h. In the following for formation better structure, it was put at
520 ͦ C for 2 h.
2
.4. Preparation of nano Ni/g-C N
3
4
The nano Ni/g-C N was prepared by an altered photodeposition procedure [34]. 10 mg g-C N , 3 mL
3
4
3
4
triethanolamine (TEOA), 250 μL NiCl (0.1 M) solution, 2 mL NaH PO (0.1 M) aqueous solution, and
2
2
2
4
.9 mL H O were blended in a round bottom flask at room temperature. The system was stirred for 40
2
min under nitrogen atmosphere. Then, the mixed solution was irradiated under UV-vis light (300 W Xe
lamp) for 30 min. The final point involved washing collected precipitates with distilled water to dispel the
residues of reactants.
2.5. General procedure for C−H activation reactions
A mixture of toluene (1 mmol) in EtOH (2 mL)) placed in 10 mL round bottom flask, nano Ni/g-C N4
3
(
6wt%, 0.01 g) was added and the set of the reaction was illuminated by 12 W blue LED Lamp at a
distance 3 cm. Generally, optimized conditions were maintained (air atmosphere and room temperature).
The completion of the reaction was specified by thin-layer chromatography. After centrifuging and
separating the photocatalyst, 12 mL ethyl acetate diluted the mixture of reactions. The pure product was
obtained through silica gel column chromatography of crude mixture (petroleum ether/ethyl acetate).
2.6. General procedure for epoxidation reactions
5

A mixture of styrene (1 mmol) in CH CN (2 mL)) placed in 10 mL round bottom flask, nano Ni/g-C N
4
3
3
(
6 wt%, 0.012 g) was added and the set of the reaction was illuminated by 12 W blue LED Lamp at a
distance 3 cm. Generally, optimized conditions were maintained (air atmosphere and room temperature).
The completion of the reaction was specified by thin-layer chromatography. After centrifuging and
separating the photocatalyst, 12 mL ethyl acetate diluted the mixture of reactions. The pure product was
obtained through silica gel column chromatography of crude mixture (petroleum ether/ethyl acetate).
3
. Results and discussion
.1. Mechanism
3
3
We have used this photocatalyst to deliver a selective oxidation of benzylic (C- sp ) hydrocarbons and
epoxidation of alkenes in the presence of O or air. With this in mind, we questioned whether selective
2
oxidation might be possible using the O in combination with photoredox and nickel catalysis. In this
2
.
-
photocatalytic reaction system, O2 played a significant role. The photocatalytic mechanism of this
oxidation reaction can involve two main stapes: 1) formation of activated oxygen by nano Ni/g-C N ; 2)
3
4
selective oxidation of C-C and C=C bonds, therefore a plausible conclusive photocatalytic mechanism is
outlined in Figure 1. Upon exposure g-C N , to visible light, the holes begot in its VB by transferred
3
4
electrons from VB of g-C N to CB. Additionally, the doping of metallic Ni on g-C N displays the role
3
4
3
4
of co-catalysts can boost its VB level. In comparison to the potential of CB, g-C N composites (around -
3
4
1.1 vs Ag/AgCl, at pH=7) the potential of metallic Ni (-1.0 V vs Ag/AgCl, at pH=7) more negative. Thus
Nickel-metal takes the photo-generated electrons from g-C N , which leads to the enhancement of charge
3
4
disassociation and improved activity [35] As a result, it causes better oxidation on the g-C N surface.
3
4
.
2
-
Thus, activated oxygen (O ) will be generated via grabbing electrons of Ni by O . Subsequently, a
2
single-electron transfer to the hole obtained in the g-C N nanoparticles oxidizes the substrate to the
3
4
intended radical cation. The activated oxygen species selectively oxidize the cationic radicals (a) and (b),
leading to the formation of benzaldehyde or olefin oxides via rearrangement or splitting of a cyclic
peroxide intermediate. Therefore, we began our investigation into the photooxidative hydrocarbons and
epoxidation alkenes by preparing nano Ni/g-C N . In this route, Ni(II) is reduced to Ni(0) by
3
4
photogenerated electrons and was doped on the electron external surface of g-C N . (with particle size
3
4
∼
5-9 nm).
6

Figure 1. Proposed mechanism for the photocatalytic oxidation and epoxidation reaction.
.2. Catalyst characterization
3
With this proposed mechanism in mind, first, the catalyst was prepared and then the characterization
was performed by energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), SEM,
transmission electron microscopy (TEM), BET, and UV-visible analysis. The optical absorption of non-
modified g-C N and Ni-6 wt% g-C N was characterized by UV-vis diffuse reflectance spectroscopy As
3
4
3
4
remarked in Figure 2, with Ni-loading in g- C N support, the absorption peak of modified g- C N
4
3
4
3
progressively switches to the longer wavelength region. The optical band gaps are 2.76 and 2.38 for g-
C N and nano Ni/g-C N , respectively. Decreasing the bandgap after loading Ni implies a high level of
3
4
3
4
interaction between Ni species and g- C N support. Also the stronger visible light absorption than non-
3
4
modified g-C N , in fine accord with their color shift of the compounds (Figure 2).
3
4
7

Figure 2. UV–vis absorption spectra of g-C N and nano Ni/g-C N .
3
4
3
4
Figure 3 shows the XRD patterns of pure g- C N and nano Ni/g-C N (6 wt%) nanoparticles. The minor
3
4
3
4
signal at 13.08, conforming to the interplanar length that is related to interlayer packing. The sharper
signal at 27.41 is a distinctive interlayer amassing peak of graphitic systems. Pure g-C N and Ni
3
4
supported on g-C N represented distinguished signal at 27.41, it indexed that the structural characteristics
3
4
of g-C N were reserved well in the process doping of nickel. For its part, the pure g-C N signal intensity
3
4
3
4
decreased slightly with the doping of the Ni, which showed that the surface of g-C N was loaded by Ni.
3
4
Also, the weakening and broadening of this peak proposed Ni on the surface of g-C N slightly prevented
3
4
the diffraction of crystallinity of g-C N [34]. It can be regarded that for Ni supported on g-C N , new
3
4
3
4
signals at 2θ= 44.5°, 51.8°, and 76.4° become visible respectively, in proportion to (111), (200), and (220)
crystal planes of Ni, which indicates compliance with the standard sheets of metallic Ni(JCPDS 87-0712)
(
Fig 3). The average particle sizes of the unmodified and nano Ni/g-C N , calculated by the Scherre’r
3 4
formula, are around 4.6 nm.
Figure 3. XRD pattern of g-C N and nano Ni/g-C N at room temperature.
3
4
3
4
8

EDX analysis revealed the presence of nickel. Meanwhile, the P element was not displayed in the
evaluation data of energy disperse X-ray, which indicated that no phosphorus compounds were created on
the surface of pure g-C N (Figure 4). Also, the induced coupled plasma (ICP) analysis confirmed that the
3
4
nano Ni/g-C N contains only 6 wt % Ni.
3
4
Figure 4. EDX spectrum of nano Ni/g-C N .
3
4
The scanning electron microscopy (SEM) image of freshly prepared nano Ni/g-C N appears in Figure 5.
3
4
The nano Ni/g-C N with a diameter of nearly 10–20 nm and also mixed cubic and spherical morphology
3
4
can be noted. Cubic morphology increases the absorption in the visible light range [36].
Figure 5. SEM images of nano Ni/g-C N4
3
Similarly, Figure 6 is followed by a transmission electron microscopy (TEM) image of the synthesized
nano Ni/g-C N . We could determine that Ni is coated conveniently by g-C N with cube and sphere form
3
4
3
4
9

and the range of size obtained from TEM is roughly between 5-9 nm which affirms the data in the XRD
pattern. The ratio of surface /sub-surface defects can be elevated to allow charge-carrier separation to
increase which is achieved with the sub-10 nm size of the particle [37].
30
25
20
15
10
5
0
Nanoparticle Size (nm)
Figure 6. TEM image of nano Ni/g-C N (left) showing the distribution of nanoparticle sizes (right).
3
4
Also, the result of the BET surface area and pore volumes for nano Ni/g-C N measurements is displayed
3
4
-1
1
in Table 1. The BET surface area of nano Ni/g-C N was detected to be 12.884 m².g , and 15.233 m².g-
3
4
represented BJH adsorption surface area of pores. Pore size distribution (7.98 nm) and single point
3
-1
adsorption pore volume of pores (0.1129 cm .g ) were distinguished according to Table 1.
Table 1. Results BET for nano Ni/g-C N .
3
4
2
-1
Surface area BET surface area (m .g )
12.884
BJH adsorption curnulative 15.233
2
-1
surface area of pores (m . g )
Mean pore diameter (nm)
Pore size
35.051
7.98
Pore size distribution (nm)
Pore volume Single point adsorption pore 0.1129
3
-1
volume of pores (cm . g ) 0.1147
BJH adsorption curnulative
2
-1
volume of pores (m . g )
3.3. Catalytic application
1
0

After the successful preparation of the catalyst, we began our investigation by exposing toluene at
visible-light irradiation in the presence of nano Ni/g-C N . To realize the performance of the
3
4
catalyst with the various amount of nickel, a set of nano Ni/g-C N were prepared with the
3
4
percentage diverse values of nickel over g-C N . According to Table 2, it became obvious that
3
4
nano Ni/g-C N with 6 wt% of nickel over the g-C N surface is the most effective catalyst,
3
4
3
4
because it was given impressive conversion into 8 h of stirring under 12 W white LED irradiation
at room temperature (Table 2, entry 2). Also, the amounts of nano Ni/g-C N were optimized, and
3
4
we were chosen 0.01 g of the catalyst for expediting of the experiment (Table 2, entry 6), because
the efficiency of the reaction in the presence of less than 0.01 g of nano Ni/g-C N and more than
3
4
0
.01 g of the catalyst, did not illustrate significant progress on the synthesis of desired products.
(
Table 2, entries 5, 7). in addition, other accessible photoactive supports such as ZnO and TiO2,
were investigated to afford this reaction under visible light, (Table 2, entries 8 and 9), both of
them delivered only <10% of the product, indeed pure ZnO [38, 39] and pure TiO [40, 41]
2
nanoparticles render negligible photocatalytic activity because of their related to high
-
+
recombination of e /h transfer and low charge separation proficiency, so they could not absorb
the light under the visible light region. In the following (Table 2, entry 10 and 11) neither the g-
C N nor NiCl alone, were not achieved this transformation. So, it was confirmed that nano Ni/g-
3
4
2
C N is the best catalyst for this reaction.
3
4
Table 2. Optimization conditions:
Entry
1
2
Catalyst (wt%, g)
Yield%
48
83
68
50
70
95
92
<10
<10
<10
-
Ni/g-C N (3, 0.008 )
3
4
Ni/g-C N (6, 0.008)
3
4
b
3
4
5
6
Ni/g-C N (9, 0.008)
3
4
b
Ni/g-C N (12 ,0.008)
3
4
b
Ni/g-C N (6, 0.006)
3
4
c
Ni/g-C N (6, 0.01)
3
4
d
7
Ni/g-C N (6, 0.012)
3
4
e
8
9
Ni/ZnO (6, 0.01)
e
Ni/TiO (6, 0.01)
2
f
1
1
0
1
g-C N (0.01g)
3
4
f
NiCl (0.01g)
2
1
1

a
Reaction conditions: 1a, (1 mmol), nano Ni/g-C N , CH CN (2 mL), at room temperature, 12 W white LED,
3
4
3
b, d, f
c
e
isolated product.
12 h. 8 h. These catalysts have been prepared according to the literature [42,43], 12h.
According to Tables 3, various solvents and atmospheres were also examined. A series of solvents
containing CH CN, THF, DMF, 1,4 -Dioxane, EtOAc, CHCl , EtOH, H O, and solvent-free conditions in
3
3
2
the presence of nano Ni/g-C N (6 wt%) under white light was checked. As can be shown in Table 3, the
3
4
outstanding results have been attained with ethanol and acetonitrile (entries 1 and 7). We would rather use
EtOH because it is safer and greener than CH CN. In terms of atmospheres, almost the same data were
3
obtained for air and O . (entries 7, 10). Under an argon atmosphere, there was no favorable product,
2
which demonstrated that the required oxidant in this system is oxygen. (Table 3 entry 11). The effects of
light irradiation were also investigated and it was understood that the reaction did not accomplish without
light (Table 3, entry 12). This outcome is proof for exciting of nano Ni/g-C N by light radiation. Also,
3
4
the reactions have not proceeded without the use of any photocatalyst. (Tables 3 entry 13).
a
Table 3. Solvent optimization data (C−H Activation)
Entry
Solvent
Atmosphere Yield 2a%
1
2
3
4
5
6
7
8
9
CH CN
THF
DMF
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
95
80
75
90
85
87
95
80
65
98
0
3
1,4 Dioxane
EtOAc
CHCl3
EtOH
H O
2
Solvent-free
EtOH
1
1
0
1
EtOH
EtOH
EtOH
Argon
O2
O2
b
1
1
2
3
0
0
c
1
2

a
Reaction conditions: 1a (1 mmol), nano Ni/g-C N (6wt%, 0.01g), solvent (2 mL), at room temperature, 12 W
3
4
b
c
white LED, 8 h, isolated product. In dark condition. Without catalyst after 24 h.
3.4. Influence of light
We investigated the photocatalytic reactions under light irradiation with different wavelengths. Studies
have shown the same results were obtained when the reaction mixture was exposed to blue 12 W LED
and 12 W white LED irradiation (nano Ni/g-C N as photocatalyst and in EtOH), also the selectivity
3
4
depends on the wavelength when toluene (1a) is oxidized by a blue LED irradiation at 400-495 nm, we
observed the maximum selectivity, so the blue LED irradiation was selected as a source light. The more
effective stimulation of photocatalyst by short wavelengths (higher energy, such as blue) proves the
dependence of photocatalytic activity on light-irradiation wavelength. (Figure 7).
98% 98%
1
00%
0%
0%
0%
0%
8
68%
50%
6
4
2
0%
0%
wavelength (nm)
Figure 7. Dependence of the catalytic activity of nano Ni/g-C N for C−H activation on the wavelength
3
4
of the light irradiation. Toluene (1 mmol), nano Ni/g-C N (6 wt%, 0.01g), EtOH (2 ml), at room
3
4
a
temperature, 8 h. Isolated yield.
3.5. Reaction under sunlight
On a summer day of July in Shiraz city at 68.5° zenith angles with temperature 26-37°C ranges the
capability of sunlight nano Ni/g-C N was checked in the open air. As can be seen in figure 8, these
3
4
results indicate the high ability of the photocatalyst under sunlight.
1
3

78%
80%
60%
40%
20%
57%
40%
30%
14%
0%
6
8
9
10
11
Time(h)
Figure 8. C−H activation under natural sunlight; reaction conditions: Toluene (1 mmol), nano Ni/g-C N
3
4
a
(
6 wt%, 0.01g), EtOH (2 mL), Isolated yield.
To extend of oxidative C−H activation and study of photocatalytic proficiency, with the optimized
conditions obtained a variety of carbonyl compounds have been synthesized. All the reactions were
completed at 25 °C under the visible light and condition optimized (ethanol (2 mL) and nano Ni/g-C N4
3
(
6 wt%, 0.01g) as photocatalyst). In the following, the selectivity of these structures for oxidation under
heterogeneous conditions in the absence of hydrogen peroxide and the visible-light photocatalytic activity
will be described. Different derivatives of methyl arenes were exposed to oxygen by using nano Ni/g-
C N under visible light (Table 4, entries 2a-2s); all of the corresponding products were synthesized with
3
4
good to excellent yields and selectively. The results show that electron withdraw and electron-donating
substituents display almost the same effect in reactivity templates. All of the corresponding ketones were
prepared via previous conditions in quantitative yields by using substituted methylene arenes.
Conforming to observed data, we found that the most appropriate support for prospering of this type of
reaction is graphene carbon nitride. As a result, the aromatic system containing a methylene moiety in its
1
4

structure (Table 4, entry 4p) is more reactive in a shorter time (5 h) than the corresponding open-chain
substrate (Table 4, entries 4a- 4o) in time (8 h).
3
a, b
Table 4. Scope of the oxidative (C sp -H) activation
2
a, 98%
2b, 95%
2g, 87%
2l, 95%
2c, 92%
2h, 89%%
2m, 89%
2d, 94%
2e, 90%
2j, 85%
2o, 92%
2
f, 90%
2i, 87%
2n, 87%
2
k, 92%
2
4
p, 95%
b, 98%
2q, 89%
4c, 98%
2r, 82%
4d, 93%
2s, 85%
4e, 90%
4a, 98%
4f, 94%
1
5

4
g, 92%
4h, 90%
4i, 90%
4j, 83%
4k, 80%
4
l, 80%
4m, 80%
4n, 87%
4o, 80%
4p, 95%
a
Reaction conditions: 1 (1 mmol), nano Ni/g-C N (6 wt%, 0.01g), EtOH (2 mL) at room temperature, 12 W blue
3
4
b
LED, 8 h. Isolated yield.
3
This interesting result of the selective oxidation of C-sp hydrocarbons causes us to test our system in
photo epoxidation of diverse alkenes by utilizing the oxidant agent (air or molecular oxygen) under
visible light irradiation at room temperature. In the following, into the epoxidation of alkene substituted,
styrene was applied as a model system. Several determinative tests were accomplished to depict different
amounts of nickel catalysts, solvent, and atmosphere at ambient temperature. The experimental
consequences are represented in Table 5. As evident, the reactivity and activity of the catalysts for the
styrene were approximately similar to those obtained from toluene. The only difference was the amount
of catalyst and we were chosen 0.012 g of the catalyst for other investigations (Table 5, entry 3). Besides,
investigation of the solvent effect in different solvents and solvent-free conditions in the presence of nano
Ni/g-C N (6 wt%) disclosed the following resultants in the proceeding of the reaction. Yields of products
3
4
(
(
entries 9-16) were around 50% to 62%. In this manner, acetonitrile was shown the best effect as a solvent
Table 5, entry 3) for epoxidation of styrene. In this reaction, like the C-H activation, oxygen performs an
obvious role as the oxidant agent whereas the argon atmosphere does not show promising results (Table 5
entry 18). Also, according to Table 5 entries 19, 20 light absorption and photocatalyst are the major
driving force for the reaction because any products were not formed in the absence of them. Besides,
toward epoxidation reaction 75% selectivity with 82% conversion was obtained and 7% of styrene was
converted to benzaldehyde. (Table 5, entry 17).
1
6

a
Table 5. Selected optimization data
Atmosphere Yield 6a Yield 7a
Entry Photocatalyst (wt%,g)
Solvent
1
2
3
Ni/g-C N (6, 0.008)
CH CN
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
74
78
80
12
12
8
10
0
0
0
3
4
3
Ni/g-C N (6, 0.01)
CH CN
3
4
3
Ni/g-C N (6, 0.012)
CH CN
3
4
3
b
4
5
6
7
8
Ni/g-C N (6, 0.014)
CH CN
76
3
4
3
b
b
c
g-C N (0.01g)
CH CN
trace
trace
trace
trace
62
50
55
60
60
53
50
3
4
3
NiCl (0.01g)
CH CN
2
3
Ni/ZnO (6, 0.01)
CH CN
3
c
Ni/TiO (6, 0.01)
CH CN
0
8
2
3
9
Ni/g-C N (6, 0.012)
THF
DMF
1,4 Dioxane
EtOAc
3
4
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
Ni/g-C N (6, 0.012)
16
12
10
8
13
20
23
7
3
4
Ni/g-C N (6, 0.012)
3
4
Ni/g-C N (6, 0.012)
3
4
Ni/g-C N (6, 0.012)
CHCl3
EtOH
3
4
Ni/g-C N (6, 0.012)
3
4
Ni/g-C N (6, 0.012)
H O
3
4
2
Ni/g-C N (6, 0.012)
Solvent-free
50
82
0
0
3
4
Ni/g-C N (6, 0.012)
CH CN
3
4
3
Ni/g-C N (6, 0.012)
CH CN
Argon
O2
0
3
4
3
d
1
2
9
0
Ni/g-C N (6, 0.012)
CH CN
Trace
0
3
4
3
e
None
CH CN
O
0
3
2
a
Reaction conditions: 1b (1 mmol), nano Ni/g-C N (6 wt%,0.012 g), solvent (2 ml), at room temperature, 12 W blue
3
4
b,c
c
d
LED, 8 h, isolated product. 12h. These catalysts have been prepared according to the literature [42,43], In dark
e
condition. Without catalyst after 24 h.
Likewise, the influence of light and reaction under sunlight is checked and the results are shown in Figure
9
. As can be seen from Figure 9a, by using blue 12 W LED the best results were obtained due to the
maximum absorption of light by the photocatalyst, however, photons with long wavelengths green (495-
70 nm) and red (620-750 nm) cannot transport enough energy to activate the photocatalyst and the
5
epoxidation reaction. Besides in dark conditions, the reaction did not occur. Figure 9b is shown a summer
day of August in Shiraz city at 68.5° zenith angles with temperature 28-38 °C ranges and the
photocatalytic ability of the nano Ni/g-C N under sunlight in the outdoor (Figure 9b).
3
4
1
7

a)
100%
0%
b)
78% 80%
68%
8
0%
57%
42%
50%
5
60%
34%
0%
28%
4
0%
0%
0%
10%
2
0%
6
8
9
10
11
Time (h)
Wavelength (nm)
Figure 9. (a) Dependence of the catalytic activity of nano Ni/g-C N for epoxidation reaction on the
3
4
wavelength of the light irradiation, styrene (1 mmol), nano Ni/g-C N (6 wt%, 0.012g), CH CN (2 ml), at
3
4
3
room temperature, 8 h. (b) Epoxidation reactions under natural sunlight; reaction conditions: Styrene (1
a
mmol), nano Ni/g-C N (6 wt%, 0.012 g), CH CN (2 mL). Isolated yields.
3
4
3
Subsequently, nano Ni/g-C N (6 wt%, 0.012 g) as photocatalyst under blue LED (12 W) irradiation in
3
4
CH CN, at room temperature and under O or air were obtained as the best reaction condition. Some
3
2
diverse alkenes were assigned to the generalizing of this procedure (Table 6). The results show that for
terminal alkenes and cycloalkenes, the epoxidation process was successfully performed with excellent
efficiency (Table 6 entries 6a-6j). Also, the trans-stilbene is more reactive than cis-stilbene (Table 6,
entries 6k-6l). Potent superficial adsorption of π-conjugated molecules onto the Ni metal for reactant
with terminal conjugated double bonds (Table 6, entry 6m), cause Nano Ni/g-C N to show worthy
3
4
implementation. In the case of substrate 5h, 5i, & 5m, no further oxidized products were observed
even by increasing the time from 8 hours to 24 hours. Epoxidation of electron-deficient alkenes was
shown little activity and lower conversion in comparison with the terminal alkenes (Table 6, entries 6n
and 6o).
a,b
Table 6. Scope of the epoxidation reaction of alkenes
1
8

6
a, 82%
6b, 80%
6g, 88%
6l, 72%
6c, 70%
6h, 85%
6m, 70%
6d, 60%
6i, 64%
6n, 56%
6e, 72%
6j, 84%
6o, 53%
6
f, 89%
6
k, 80%
a
Reaction conditions: 5 (1 mmol), nano Ni/g-C N (6 wt%, 0.012 g), CH CN (2 mL) at room temperature, 12 W
3
4
3
b
blue LED, 8 h, Isolated yield; The yield of products 6f, 6g, 6i, 6j, 6m were determined by HPLC analysis.
3.6. Catalyst reusability
The reusability of the nano Ni/g-C N photocatalyst was also inquired and the obtained data is exhibited
3
4
in Figure 10. To recover the photocatalyst, after the accomplishment of each period, it was separated by
centrifuging, rinsed with water and ethanol, and then dried at 80°C. In this method, nano Ni/g-C N4
3
exhibits excellent reusability and any remarkable loss of the catalytic activity was not beholden even after
six runs
(
Figure 10).
98% 98%
9
6%
9
5%
[VALUE]
[VALUE]
95%
1
00%
94%
100%
50%
0%
[VALUE]
VALUE]
VALUE]
VALUE]
[
[
[
50%
0%
1
1
2
2
3
3
4
4
5
5
6
6
Runs
Runs
1
9

Figure 10. Reusability of the recovered catalyst in C−H activation and epoxidation reactions: Substrate (1
a
mmol), nano Ni/g-C N (6 wt%, 0.012 g), solvent 2 (mL); Isolated yields.
3
4
3.7. Leaching test
Details of the weight of the photocatalyst and the wt% of the nickel leaching after each run are given in
Tables 7. As can be seen from this table the nano Ni/g-C N exhibits excellent reusability and the catalyst
3
4
sufficiency was not considerably lessened even after 6 sequences. Also, ICP analysis designated the
amount of leached nickel. (Tables 7).
A mixture of toluene (1 mmol), nano Ni/g-C N ( 10 mg, 6wt % Ni), EtOH (2 mL), placed in a glass test
3
4
tube with a magnetic stirrer bar under 12 W blue LED irradiation (distance 3.0 cm). Similarly, the air
atmosphere and room temperature were considered as a reaction condition. The reactant mixture was
performed for 8 h. After the accomplishment of time reaction, nano Ni/g-C N was separated by
3
4
centrifuging from the solution and the solvent reaction mixture was used to analyze the Ni content using
Inductively Coupled Plasma (ICP) spectroscopy, which indicates that no Ni was detected in the filtrate
solution. (Table 7a).
A mixture of styrene (1 mmol), nano Ni/g-C N ( 12 mg, 6wt % Ni), CH CN (2 mL), placed in a glass test
3
4
3
tube with a magnetic stirrer bar under 12 W blue LED irradiation (distance 3.0 cm). Similarly, the air
atmosphere and room temperature were considered as a reaction condition. The reactant mixture was
performed for 8 h. After the accomplishment of time reaction, nano Ni/g-C N was separated by
3
4
centrifuging from the solution and the solvent reaction mixture was used to analyze the Ni content using
Inductively Coupled Plasma (ICP) spectroscopy, which indicates that no Ni was detected in the filtrate
solution. (Table 7b).
Table 7. (a) Catalyst recyclability for C−H activation reaction and (b)Catalyst recyclability for
epoxidation reaction.
Runs
The catalyst
weight after
each run (mg)
10
Nickel leaching
(wt%)
Runs (b)
The catalyst
weight after
each run (mg)
12
Nickel leaching
(wt%)
(a)
1
2
3
4
5
6
0.000
0.002
0.004
0.007
0.009
0.011
1
2
3
4
5
6
0.000
0.004
0.007
0.009
0.012
0.014
9.8
9.5
9.1
8.7
8.4
11.7
11.4
11
10.7
10.2
2
0

Affordable and recyclable nano Ni/g-C N , availability of toluene, and no need for any additives cause
3
4
process scale to be carried out. Satisfactory results using 6.36 mL (60 mmol) of toluene which resulted in
8% benzaldehyde yield (5.60 g). This successful phenomenon suggests that this reaction can be used on
a large-scale (scheme 2).
8
Scheme 2. Gram-scale C−H activation of hydrocarbons.
3.8. Control experiments
A batch of control experiments was runned, to figure out the mechanism of the reaction. (Figure 11).
When 2 mmol of a radical scavenger such as 2,2,6,6-Tetramethylpiperidinyl-1-oxy (TEMPO•), which is a
stable nitroxide radical and the electron-trapping standard agent, was added to the reactions system, it
could capture electrons, subsequently, the favorable yield of the product in 8h decreased. Also,
superoxide radical scavenger (benzoquinone) or hole scavenger (Triethanolamine (TEA), methanol, and
ethylenediaminetetraacetic acid) were added. The drastic decrease in the efficiency of the reaction can be
+
a piece of strong evidence that photogenerated hole (h ) and superoxide radical anions are the progressive
factors for the convert of toluene to benzaldehyde through oxidation reaction and styrene to 2-
phenyloxirane (Figure 11).
Quencher
TEMPO (2 mmol)
TEA (2 mL)
Yields
0%
0%
EDTA (2 mmol)
0%
CH OH (2 mL)
15%
Yield, 100%
Without Ni/g-C N , 0%
3
Benzoquinone
5%
3
4
Without air, 0%
Quencher
Yields
TEMPO (2 mmol)
TEA (2 mL)
EDTA (2 mmol)
0%
0%
0%
20%
2
1
CH OH (2 mL)
3
Benzoquinone
5%

Yield, 82%
Without Ni/g-C N , 0%
3
4
Without air, 0%
Figure 11. Control experiments
4. Conclusion
As a result, we have demonstrated the first example of a novel and economic ligand-free nano Ni/g-C N4
3
as photocatalyst under visible light irradiation for C−H activation and epoxidation reaction, with a variety
of substrates by using air or O as a green oxygen source and without the use of any reagent or base.
2
Similarly, the reusability of the photocatalyst and the utilization of visible light as attainable energy, are
distinctive attributes of this procedure from the eco- friendly viewpoint.
Acknowledgments
This work was supported by Shiraz University Research Council.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
2
2

Graphical abstract
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