A Schiff-base-type vanadyl complex grafted on mesoporous carbon nitride: A new efficient catalyst for hydroxylation of benzene to phenol

Article abstract of DOI:10.1039/c5ra21438f

Mesoporous graphitic carbon nitride (g-CN) was utilized as a new support to immobilize vanadyl(iv) acetylacetonate ([VO(acac)₂]). The immobilized vanadyl complex materials (VOac-CND) have been thoroughly characterized by various techniques incl

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A Schiff-base-type Vanadyl Complex Grafted on Mesoporous Carbon
Nitride: A New Efficient Catalyst for Hydroxylation of Benzene to
Phenol
Jie Xu *, Quan Jiang, Jie-Kun Shang, Yue Wang, Yong-Xin Li *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Mesoporous graphitic carbon nitride (gꢀCN) was utilized as a new support to immobilize vanadyl(IV)
acetylacetonate ([VO(acac)2]). The immobilized vanadyl complex materials (VOacꢀCND) have been
thoroughly characterized by various techniques including N2 adsorption–desorption, XRD, SAXS, TEM,
10 Raman, FTꢀIR, XPS and benzeneꢀTPD. The characterization results showed that [VO(acac)2] had been
successfully grafted on the surface of gꢀCN via the reaction between the carbonyl group of the
acetylacetonate ligand and the amino groups of gꢀCN and thus transformed into a Schiffꢀbaseꢀtype
complex. Moreover, after the immobilization of [VO(acac)2], the orderness of mesoporous structures and
graphitic structures of the gꢀCN support have been well retained. The immobilization temperature has
15 been found to be sensitive to the immobilization effect. As heterogeneous catalysts, the immobilized
catalysts exhibited high performances in the direct hydroxylation reactions of benzene to phenol,
affording a maximum phenol yield of ca. 20.0%. The catalytically active sites for the hydroxylation were
proposed as the grafted vanadyl complex while the mesoporous gꢀCN played a crucial role in activating
benzene.
new and efficient catalyst, which can afford high activity and
selectivity to phenol, is a hot topic for the catalytic hydroxylation
of benzene.
In the past decade, a wide range of catalyst systems, including
50 titanium silicalite sieves (TSꢀ1), polyoxometalate (POM),
graphene, carbon nanotubes etc, have been successfully exploited
for the direct hydroxylation of benzene to phenol 7,8. Vanadyl(IV)
20 1. Introduction
Phenol is one of the most important commodity chemicals, which
is widely used as a basic intermediate in the production of resins,
fibers, dyes, and medicines. Commercially, phenol is produced
mainly based on the wellꢀknown threeꢀstep cumene process
25 introduced by Hock and Lang in 1944 1,2, which however suffers
some disadvantages including high pollution, low oneꢀpass yield
of phenol (<90%), low atomic efficiency, and a large amount of
acetylacetonate ([VO(acac)2]) is a typical efficient catalyst
2,9
extensively employed for various oxidative transformations
.
55 However, the thorny issue associated with [VO(acac)2] lies in its
intrinsically homogeneous property, especially the difficulty in
catalyst separation and product purification 9,10. Alternatively,
3
acetone as coꢀproduct . To address this issue, various alternative
strategies have been proposed and up to now, oneꢀstep
30 hydroxylation of benzene to phenol, featuring high atomic
economy and high selectivity towards phenol, has been regarded
as the ideal approach to replace the traditional cumene process 4.
In the cases of oxidants applied for the direct hydroxylation of
benzene, air and oxygen are widely available. However, under
35 such catalytic systems, a relatively high reaction temperature is
necessarily demanded, which may lead to the deep oxidation of
benzene. As for N2O, despite of its high selectivity (nearly
100%), the use of N2O is much more complex in industrial
immobilization of [VO(acac)2] on other solids (including
10,12
mesoporous silicas 2,11, amineꢀfunctionalized clay
or carbon
9
60 materials ), via the reactions between the carbonyl group of the
acetylacetonate ligand and the amino groups of the solids
surfaces, provides an effective strategy to utilize [VO(acac)2]. In
spite of such success in the catalyst preparation, the final
activities received in the corresponding oxidative reactions were
65 not satisfactory. For instance, to achieve a conversion up to 25%,
2
8 h of reactions time was at least required . Furthermore, the
operation 5. Instead, using H2O2 as an oxidant is green and simple
40 and more importantly, water is the only byꢀproduct
Notwithstanding, there remains a challenge to reach a high
conversion (>10%) of benzene in the presence of H2O2. That is,
phenol is liable to transforms into undesired byꢀproducts (e.g.
hydroquinone, benzoquinone) due to the uncatalyzed deep
leaching of the active component, i.e. VO(acac)2, still restrains
the wide application of such grafted complex catalysts. In this
context, it is of interest to explore a new material as an effective
70 solid to immobilize [VO(acac)2].
As a fascinating material to complement traditional carbon
materials, graphitic carbon nitride (gꢀCN) and its based materials
have recently attracted tremendous attention in wide research
6,7
.
5
45 oxidation of phenol . In this sense, how to design and develop a
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communities 13,14. Owing to its combination of several unique
physicochemical natures, gꢀCN material has been applied in
numerous fields including photocatalysis 15, fuel cells 16, gas
storage 17,18, and heterogeneous catalysis 19ꢀ21. As an analogue of
graphite, gꢀCN possesses twoꢀdimensional structures with πꢀ
conjugated planar layers that are constituted of triꢀsꢀtriazine
T, where T represented the reaction temperature between CND
and VO(acac)2.
2.2. Sample characterization
60 Nitrogen adsorption–desorption isotherms were measured at −196
°C using a Micromeritics ASAP 2020 analyzer. The specific
surface area was calculated according to the Brunauer–Emmet–
Teller (BET) method, and pore size distribution was determined
by the Barret–Joyner–Halenda method.
5
moieties bridged by nitrogen atoms 22, and the feature enables gꢀ
23,24
CN with an inherent ability to activate aromatic molecules
.
As reported by Antonietti and Park, gꢀCN materials showed
10 outstanding catalysis in various benzeneꢀinvolved reactions, such
65
Xꢀray diffraction patterns were recorded with a Rigaku D/max
2500 PC Xꢀray diffractometer equipped with graphite
25
as Friedel–Crafts acylation 24, alkylation
and oxidation of
a
27
4
benzene by CO2 26. Very recently, Wang and Han reported
that gꢀCNꢀbased materials (VꢀgꢀC3N4, gꢀC3N4–H5PMo10V2O40)
could also serve promising catalysts for oxidation of benzene to
15 phenol. However, the catalytic activity obtained on the bare gꢀCN
was limited.
monochromator (40 kV, 40 mA) using Niꢀfiltered CuꢀKα
radiation (λ = 1.5418 Å).
Small angle Xꢀray scattering (SAXS) measurements were
70 conducted on a Bruker NanoSTAR U SAXS system equipped
with a highꢀresolution pinhole chamber using CuꢀKα radiation (40
kV, 35 mA).
Fourier transform infrared (FTꢀIR) spectra of the samples were
collected in transmission mode at room temperature on a Bruker
75 Tensor 27 spectrometer with a resolution of 4 cm−1, using 32
scans per spectrum in the region of 400–4000 cm−1. The mass
ratio of every sample to KBr was constant at 1:200.
Xꢀray photoelectron spectroscopy (XPS) measurements were
performed using a Thermo ESCALAB 250XI spectrometer
80 working in the constant analyzer energy mode with Mg Kα
radiation as the excitation source. The atomic content of
vanadium for each Vꢀcontaining sample was calculated based on
the peak areas (survey spectrum) of C, N, O, and V elements, and
their corresponding atomic sensitivity factors.
On the other hand, in terms of the tectonic units of gꢀCN, there
exist abundant uncondensed amines, in the forms of –NH2 and –
NH– groups, at its graphitic edges 28. Indeed, the characteristic
20 compositions of amine species were usually utilized to enable gꢀ
CN as a solid base catalyst 19,28ꢀ30. Since the immobilization of
[VO(acac)2] relies on the aliphatic amines located on the support
while gꢀC3N4 has both innate amines and ability to activate
benzene, we envision that gꢀCN could also be used as a solid to
25 graft [VO(acac)2]. Inspired by this, in the present work, we have
reported for the first time new Schiffꢀbaseꢀtype materials by
immobilizing [VO(acac)2] on mesoporous gꢀCN materials. In the
direct hydroxylation of benzene to phenol, the grafted vanadyl
complex catalysts demonstrated high and stable activity.
85
Raman spectra were recorded on a Raman spectrometer (Jobin
Yvon Lab Ram HR evolution) using 532 nm line as an excitation
source.
30 2. Experimental
2.1. Catalyst preparation
Transmission electron microscopy (TEM) experiments were
conducted on a JEOL 2010 electron microscope operating at 200
Mesoporous gꢀCN was prepared via a previously reported
90 kV. Before being transferred into the TEM chamber, the samples
dispersed in ethanol were deposited onto holey carbon films
supported on Cu grids.
nanocasting approach using SBAꢀ15 as
a template and
dicyandiamide (DCDA) as a precursor 30. In brief, 2.5 g of
35 DCDA was dissolved in 10 mL of anhydrous ethylenediamine
(EDA), and the solution was added dropwisely onto 1 g of dried
SBAꢀ15 (see the ESI for detailed preparation routes). After the
digestion for 6 h, the mixture was heated at 90 °C overnight.
Next, the resultant solid was ground in a mortar, and heated at 2.5
40 °C min−1 up to 550 °C and then treated for further 4 h under N2
atmosphere. Afterwards, the asꢀsynthesized dark powder was
immersed into 200 mL of NH4HF2 solution (4 mol L−1) to etch
SBAꢀ15. Then, the dispersion was centrifuged and the precipitate
was washed with water and ethanol for several times, and dried at
45 50 °C under vacuum. The resulting powder was designated as
CND.
Benzene temperatureꢀprogrammed desorption (benzeneꢀTPD)
experiments were performed on a Quantachrome ChemBETꢀ3000
95 analyzer. 200 mg of the sample was pretreated at 350 °C for 1 h
in dry He flow (30 mL min−1), cooled to 50 °C, and then exposed
to Heꢀcontaining benzene for 0.5 h. After purging the sample
with He for 0.5 h, the TPD data based on the thermal conductivity
detector (TCD) signals was recorded from 50 to 350 °C with a
100 ramping rate of 10 °C min−1.
2.3. Catalytic test
The hydroxylation reaction of benzene to phenol was performed
in a twoꢀneck round bottomed flask (25 mL). 50 mg of catalyst
was added into 1 mL of benzene (11.2 mmol) and then mixed
105 well. After the reactant was heated to 60 °C, 3 mL of aqueous
H2O2 solution (30wt%) was added into the mixture in 30 min via
a peristaltic pump. The reaction was conducted for 4 h. After that,
the mixture was centrifuged and analyzed by GC equipped with a
SEꢀ54 capillary column. The filtered catalyst was washed by
110 acetonitrile (30 mL) for two times, dried at 50 °C for 2 h, and
then investigated for its next running.
175 mg of VO(acac)2 was dissolved into 40 mL of toluene,
followed by addition of 0.6 g of dried CND. The mixture was
transferred into a stainless autoclave and pressured with Ar up to
50 0.7 MPa, and then stirred under a desired temperature (80–120
°C) for 10 h. After that, the dispersion was filtrated and the
precipitate was washed with toluene for several times. To
eliminate the possibly residual VO(acac)2, the solid was further
refluxed with chloroform for 3 h and washed for two times. After
55 the subsequent drying under vacuum at 60 °C for 2 h, CND
grafted with VO(acac)2 was obtained and labeled as VOacꢀCNDꢀ
The molar amount (n) for each component in the catalytic
reaction was calculated by n = A × f, where A and f were the GC
2
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peak area and response factor, respectively. The conversion of 40 the (100) plane was ca. 10.2 nm. In the case of the negative
benzene (Conv.) and selectivity to phenol (Sel.) were calculated
as follows:
replica, CND, the pattern demonstrated a peak and shoulder at q
= 0.683 and 1.15 nm−1, respectively, along with lower intensities
than those of SBAꢀ15 (Fig. 1B). Meanwhile, the a0 of CND
decreased to 9.2 nm, slighter smaller than the value gained in
45 SBAꢀ15. This means that after the hardꢀtemplating process, the
orderness of SBAꢀ15 template declined, probably due to the
partial collapsed or shrinking of mesoporous walls, which was
also found in other gꢀCN materials templated by mesoporous
silicas 33,34. Nevertheless, the whole periodicity the mesoporous
50 structures were successfully replicated. Interestingly, the
introduction of vanadyl complex has not affected the original
ordered mesostructures of CND, since both the scattering vectors
of (100) planes and their intensities of VOacꢀCND samples
showed no obvious change.
nphenol + nbenzoquinone
nphenol
Conv. =
Sel. =
nbenzene + nphenol + nbenzoquinone
nphenol + nbenzoquinone
.
5
3. Results and discussions
3.1. Structure characterization
B
A
e
d
c
55
N2 adsorption–desorption isotherms of CND and VOacꢀCND
materials are depicted in Fig. 2A. The pure CND exhibited
typical IV curves with a steep H1 hysteresis loop at ca. 0.5–0.75
p/p0, inferring that the support had classical mesopores together
with a narrow pore size distribution (Fig. 2B). After the
a
e
d
60 incorporation of vanadyl complex, the quantity of adsorption of
CND decreased obviously. Specifically, as the grafting
temperatures were elevated from 60 to 120 °C, the pore volumes
as well as surface areas of the obtained VOacꢀCND materials
decreased monotonously from 380 to 328 m2 g−1 (Table 1). This
65 implies that higher grafting temperatures would induce much
more vanadyl species to be immobilized into the mesoporous
pores of CND, thereby leading to the progressive decease of the
pore sizes. However, no significant blockage of the mesopores
after the grafting was found, as the corresponding pore size
70 distributions still revealed concentrated peaks in the range of 4.5–
5.0 nm (Fig. 2B).
c
b
a
2500
20
0.5
10
30
40
50
60
70
0.4
0.8
1.2
1.6
2.0
1
2θ (°)
q
(nm−
)
Fig. 1 XRD (A), and SAXS patterns (B) of CND (a) and VOacꢀCNDꢀT
(b–e: 60, 80, 100, and 120) materials.
10 Fig. 1A shows the XRD patterns of CND and VOacꢀCND
materials. Each sample displayed an apparent diffraction peak
centered at 2θ = 25.9 ° (d = 0.343 nm), which was indexed as the
interplanar stacking of graphitic structures, namely (002) plane 24
.
In addition, a shoulder peak with low intensity was also detected
15 at ca. 2θ = 42 °, further suggesting that CND and its grafted
400
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
B
A
CND
vanadyl materials possessed
a high degree of intralayer
VOac-CND-60
VOac-CND-80
VOac-CND-100
VOac-CND-120
350
300
250
200
150
100
50
condensation of graphitic phrase 31. In terms of the diffraction
intensities of all the samples, the graphitic structures of the
grafted VOacꢀCND materials have well remained, and the
20 grafting of VO(acac)2 has not damaged the overall crystallinity of
gꢀCN. Furthermore, referring to the XRD pattern of the pure
VO(acac)2 sample (Fig. S1), no clear diffraction line pertaining to
VO(acac)2 has been observed in the patterns of all the VOacꢀ
CND materials. Besides XRD, Raman spectroscopy was also
25 used to analyze the graphitic structures (Fig. S2). The spectrum
of CND revealed two bands centered at 1350 and 1551 cm−1,
associated with disordered (D) and graphitic (G) modes of
carbon, respectively. The ratio (ID/IG) of relative intensities for
the two peaks was ca. 1.12, further verifying that the mesoporous
30 gꢀCN contained comparatively high graphitic structures. After the
introduction of VO(acac)2, the overall intensities of spectra
declined whereas the ratios of ID/IG were still close to the above
value.
0.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
60
p
/
p0
Pore size (nm)
Fig. 2 N2 adsorption–desorption isotherms (A) and pore size distributions
(B) of CND and VOacꢀCND materials.
75 Table 1 Textural parameters of CND and VOacꢀCND samples.
Sample
SBET (m2 g−1) Pore size a (nm) Pore volume (cm3 g−1)
CND
488
380
363
351
328
5.4
5.0
4.9
4.8
4.6
0.57
0.45
0.43
0.42
0.38
SAXS technique was then employed to analyze the orderness
35 of porosity of the materials. The SAXS pattern of SBAꢀ15
template (Fig. S3) showed four wellꢀresolved scattering peaks in
the range of q = 0.6–1.7 nm−1, corresponding to the (100), (110),
(200), (210) planes of twoꢀdimensional hexagonal structure
(p6mm).32 Wherein, the lattice parameter (a0) calculated based on
VOacꢀCNDꢀ60
VOacꢀCNDꢀ80
VOacꢀCNDꢀ100
VOacꢀCNDꢀ120
a Determined by the adsorption branches.
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The mesoporous morphology of SBAꢀ15 and VOacꢀCND
as VOacꢀCND samples, their FTꢀIR spectra exhibited similar
samples were characterized by TEM. The images of the parent 40 characteristic bands to that of CND. However, the differences in
SBAꢀ15 material revealed highly ordered tubular arrays (Fig. 3A)
and honeycombꢀlike hexagonal structures (Fig. 3B),
corresponding to the (110) and (100) planes. On the other hand,
the images of CND material presented stripꢀlike (Fig. S4A) and
the spectra of the pristine and grafted materials could be also
identified. In detail, a weak yet unique band at 970 cm−1 emerged
for each VOacꢀCND sample; moreover, the intensity of the band
seems to be gradually enhanced with the increase of the grafting
5
ordered sliceꢀlike (Fig. S4B) arrangements. Wherein, the black 45 temperatures. According to the FTꢀIR spectra of vanadiaꢀ
region was gꢀCN walls templated by the mesopores of SBAꢀ15,
while the white one was derived from the original silica walls of
supported catalysts reported by Fehrmann et al 38, the band at 970
cm−1 was actually derived from the stretching modes of V=O
bonds. Considering this fact, it can be inferred that VO(acac)2
compound has been successfully loaded on CND.
10 SBAꢀ15. Compared with SBAꢀ15, a small amount of mesopores
of CND underwent partial blockage. One reason accounting for
this was the shrinkage of mesopores of CND during its rinsing or
drying procedures. Additionally, under the exposure of highꢀ
speed electronic beam, the mesostructure walls of gꢀCN material
15 was prone to be damaged, which was also found in our previous
work 30,34. As for the grafted VOacꢀCND material, the TEM
images (Fig. 3C & Fig. 3D) also exhibited similar patterns like
those of CND. Overall, the TEM characterization manifested that
the CND has replicated the ordered mesostructures of siliceous
20 SBAꢀ15 and moreover, after grafting vanadyl compound, the
ordered structures have been reserved, in a good agreement with
the above N2 adsorption–desorption and SAXS characterization.
e
d
c
b
2212
a
5%
1284
1612
4000
3500
3000
2500
2000
1500
1000
5001000
900
1
Wave number (cm−
)
50
Fig. 4 FTꢀIR spectra of CND (a) and VOacꢀCNDꢀT (b–e: 60, 80, 100, and
120) materials. The right is the magnified region of 1000–900 cm−1.
A
B
b
b
c
a
2000
20000
800
600
400
200
0 526 524 522 520 518 516 514
Binding energy (eV)
Binding energy (eV)
Fig. 3 TEM images of SBAꢀ15 (A&B) and VOacꢀCNDꢀ120 (C&D)
25 materials. The left and right images were recorded along with the [110]
and [100] directions, respectively.
Fig. 5 XPS survey (A) and V 2p (B) spectra of CND (a), VOacꢀCNDꢀ80
55
(b), and VO(acac)2 (c) samples.
The chemical functions of CND and its grafted samples were
analyzed using FTꢀIR spectroscopy. As described in Fig. 4, the
FTꢀIR spectrum of the bare CND represented three distinct zones.
30 The major and strong bands located in 1612 and 1284 cm−1 were
Besides FTꢀIR, we have employed XPS measurement to
further analyze the surface chemical information of the materials.
The XPS survey (Fig. 5A) of CND showed the C, N, and O
attributed to aromatic ring modes and C=N stretching bonds 33,35
,
60 elements, which probably originated from the adsorbed water. In
addition to the three peaks, the survey spectrum of VOacꢀCND
material, for instance VOacꢀCNDꢀ120, also offered a lowꢀ
intensity yet sharp signal near O 1s, which was attributed to the V
2p peak (ca. 516 eV). According to the peak areas under these
65 elemental signals, the surface contents of vanadium per weight of
VOacꢀCNDꢀ120 was 495 ꢁmol gcatal.–1, close to the reported value
of VO(acac)2 grafted on amineꢀfunctionalized active carbon 9. On
respectively. Whilst the lowꢀintensity and broad bands in the
range of 3382 to 3193 cm−1 were assigned to the physically
adsorbed water molecules (νO–H) and N–H stretching of NH2
35 groups attached to the sp2 C atoms 35. Furthermore, the tiny yet
apparent band at 2212 cm−1 was indexed as conjugated N=C−N
units in gꢀCN planar network 36. These FTꢀIR signals agreed well
with mesoporous gꢀCN materials reported elsewhere 30,37. As far
4
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the other hand, the nominal loading amount of VOacꢀCNDꢀ120
summarized in Table 2, the catalytic conversions of benzene
showed a direct dependence on the grafting temperatures. That is,
was ca. 1100 ꢁmol gcatal.–1 (175 mg of VO(acac)2 mixed with 600
mg of CND), which meant that the amount of VO(acac)2 was 50 higher temperatures applied in grafting VO(acac)2 on CND
enough to utilize the amino groups of CND. Fig. 5B gives the
highꢀresolution spectra of V 2p. The similarity between the
profiles of the free and grafted vanadyl complexes indicated that
the complex had kept its integrity after immobilization on
resulted in higher catalytic activities (entries 4–8). The maximum
activity was achieved over VOacꢀCNDꢀ120, affording ca. 20% of
phenol yield. As disclosed above, the grafting of VO(acac)2 on
CND was essentially based on the reaction between terminal
5
mesoporous gꢀCN. Nevertheless, as compared with the V 2p band 55 amine groups and C=O groups of VO(acac)2. Therein, using
of the pure VO(acac)2 (516.3 eV for 2p 3/2), a slight band shift to
higher grafting temperature would inevitably improve the loading
amount of vanadia species, thus increasing the whole amounts of
catalytically active species.
10 lower binding energy in VOacꢀCNDꢀ120 (516.0 eV for 2p 3/2)
has been observed. It is apparent that the chemical environment
of V atoms of vanadyl complex has undergone a change after
Besides grafting temperature, the catalytic activity was found
immobilization, suggesting that the complex has reacted with gꢀ 60 to be related to the grafting amount of VO(acac)2. With the
CN. Nonetheless, one might think that vanadium in such VOacꢀ
15 CND materials existed as vanadia (V2O5) rather than vanadyl
complex. Firstly, in the case of VO(acac)2, its decomposition
increase of the amount from 50 to 150 mg (entries 9–11), the
final benzene conversion increased remarkably but leveled off
when the grafting amount was up to 175 mg. This can infer that,
at that stage, the terminal amino groups of gꢀCN have been
temperature to transform into vanadia (V2O5) is at least 230 °C 39
.
In our experiment to prepare vanadyl complex, the grafting 65 almost fully utilized by reaction with VO(acac)2. Accordingly,
temperature we applied was less than 120 °C, far below the above
20 transformation threshold. Meanwhile, after immobilization of
VO(acac)2, no further calcination was demanded. Secondly, as
mentioned above, the binding energy of V 2p 3/2 was 516.3 eV.
By comparison, the corresponding location of V 2p 3/2 of
vanadia (V2O5) was at least 516.7 eV, as reported in the work
25 involving the supported vanadia materials 40,41. Given in these
two points, it can be excluded that vanadium existed in the form
of vanadia. Namely, like the vanadyl complexes anchored on
amineꢀmodified silicas 11,42, the C=O group of acetylacetonate
ligand of VO(acac)2 reacted with the terminal amines of gꢀCN,
30 thus yielding a Schiffꢀbaseꢀtype complex (Scheme 1). Wherein,
as the electronegativity of N is slightly lower than that of O, the
substitution of O by N in its ligand of vanadium led to the shift of
V 2p band towards lower energy. Such variation in XPS profiles
has also been found in other grafted vanadyl complexes 2.
the FTꢀIR spectra (Fig. S5) of the samples grafted with various
amounts of VO(acac)2 clearly presented a progressive increase of
the intensity of the band at 970 cm−1 as the feeding amount of
VO(acac)2 was elevated.
70 Table 2 Catalytic activities of VOacꢀCND materials in the hydroxylation
of benzene to phenol.
Entry
1 a
2 b
3 c
4
Catalyst
–
CND
Conv. (%)
< 1
1.4
12.0
6.5
Sel. (%)
100
Yield (%)
< 1
1.3
11.6
6.3
90.2
96.8
97.4
94.3
94.4
95.4
96.8
97.0
98.6
99.3
VO(acac)2
VOacꢀCNDꢀ60
VOacꢀCNDꢀ80
VOacꢀCNDꢀ90
VOacꢀCNDꢀ100
VOacꢀCNDꢀ120
VOacꢀCNDꢀ120
VOacꢀCNDꢀ120
VOacꢀCNDꢀ120
5
6
7
8
8.7
9.8
8.2
9.3
10.5
20.7
18.6
10.9
5.2
10.0
20.0
18.0
10.7
5.1
9 d
10 e
11 f
a Without any catalyst. b Reaction conditions: 1 mL of benzene (11.2
mmol), 3 mL of aqueous H2O2 solution (30wt%), 6 mL of acetonitrile as
solvent, Wcatal. = 50 mg, T = 60 °C, and t = 4 h. c 6.6 mg of the catalyst
75 (equivalent to 25 ꢁmol of vanadium). d Grafted with 150 mg of VO(acac)2.
e Grafted with 100 mg of VO(acac)2. f Grafted with 50 mg of VO(acac)2.
On the hand, the pure VO(acac)2 compound exhibited a
moderate phenol yield (11.6%, entry 3). Note, the weight of
VO(acac)2 was indeed based on the actual loading amount of
80 vanadyl species on VOacꢀCNDꢀ120, as analyzed by XPS. In view
of their catalyst weights, the catalytic activities obtained over
VO(acac)2 was exactly superior than VOacꢀCNDꢀ120.
Nonetheless, it is worth noting that the reaction system
employing VO(acac)2 was solely homogeneous, suffering
85 intrinsic difficulty in catalyst–product separation.
35
Scheme 1 A possible Schiffꢀbaseꢀtype vanadyl complex grafted on gꢀCN.
3.2. Catalyst activity
Initially, a blank experiment was performed in the absence of
catalyst (entry 1, Table 2); the result indicated that benzene could
40 be rarely converted into the desired product, phenol.
Alternatively, using the pure CND material (entry 2), the phenol
productivity received was quite poor. In sharp contrast, VOacꢀ
CND complexes offered remarkable catalytic activity for the
hydroxylation of benzene under the same reaction conditions.
45 The target phenol contributed about 95% on the catalytic
selectivities, along with a small amount of benzoquinone that
probably originated from the deep oxidation of phenol. As
The influence of reaction conditions on the catalytic
performances of VOacꢀCND in the hydroxylation reactions was
further investigated. As described in Fig. 6A, the reaction
temperatures were found to be sensitive to the catalytic activity.
90 Upon increasing the temperature, the conversions of benzene
were enhanced progressively, whereas the selectivity to phenol
still remained above 95%. However, much higher temperatures
above 60 °C led to a negative impact on the catalytic activity,
mainly due to the rapid decomposition of H2O2. The catalytic
95 activity was also dependant on reaction time (Fig. 6B). Under the
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conditions adopted in the present study, the highest activity was
received at 4 h, and further prolonging the hydroxylation reaction
showed no apparent improvement for its productivity.
this viewpoint, the VOacꢀCNDꢀ120 catalyst has been submitted
with several consecutive runs, and the corresponding results are
listed in Table 3. During the four runs, the selectivities to phenol
Furthermore, the effect of catalyst amount on the catalytic 15 were above 95%, and the benzene conversions were higher than
5
performances has also been carried out (Fig. 6C), and the
corresponding evaluation exhibited that adding more catalysts
(10–50 mg) could facilitate the activity, while the benzene
conversions were improved smoothly under excessive use of the
19%, showing almost no apparent loss of catalytic activity in
such heterogeneous catalytic examination. According to N2
adsorption–desorption (Fig. S6, and Table S1) and FTꢀIR
characterization (Fig. S7) for the spent VOacꢀCNDꢀ120 catalyst
VOacꢀCND catalyst. In addition to the optimization of reaction 20 after four runs, its surface area, pore size, and main chemical
10 conditions, the recyclability and reproductivity were also
important parameters to evaluate a heterogeneous catalyst. Given
bonding have been well reserved, confirming that the grafted
vanadyl complex was stable in the catalytic reactions.
30
25
20
15
10
5
10030
10030
100
80
60
40
20
0
Conv.
Sel.
25
80
25
80
20
60
20
60
15
15
40
10
40
10
20
5
20
5
A
B
C
0
0
0
0
0
40 45 50 55 60 65 70
1
2
3
4
5
10
30
50
70
90
Temperature (
°
C)
Time (h)
Wcatal. (mg)
Fig. 6 Influence of reaction temperatures (A), time (B), and catalyst amounts on the catalytic activities of VOacꢀCNDꢀ120. Reaction condition: 1 mL of
25
benzene (11.2 mmol), 3 mL of aqueous H2O2 solution (30wt%), 6 mL of acetonitrile as solvent.
with wide transitionꢀmetal complexes. Envisioned by this, we
thereafter grafted another acetylacetonateꢀcontaining transitionꢀ
Table 3 Catalytic performances of VOacꢀCNDꢀ120 during four
consecutive runs
metal compounds, for instance copper (II) acetylacetonate
Run
1
Conv. (%)
20.7
Sel. (%)
96.8
Yield (%)
20.0
45 ([Cu(acac)2]) on CND. The synthesized CuacꢀCND sample
showed similar textual properties and chemical functionalities
with VOacꢀCNDꢀ120 (Fig. S6, Table S1, and Fig. S8). As
expected, CuacꢀCNDꢀ120 demonstrated a moderate benzene
conversion (12.1%, Table 4), together with a high selectivity
50 (97.4%) to phenol. In view of this tentative test, it is anticipated
that mesoporous gꢀCN could serve as a new support for wide
transitionꢀmetal complexes. In this work, in addition to
mesoporous gꢀCN, SBAꢀ15 and carbon nanotubes have been also
applied as supports to immobilize [VO(acac)2] (See the ESI for
55 the detailed preparation routes. FTꢀIR spectra of the bare SBAꢀ15,
SBAꢀ15ꢀNH2, and VOacꢀSBAꢀ15ꢀNH2 materials are given in Fig.
S9.). As summarized in Table 4, under the present reaction
conditions, VOacꢀSBA15ꢀNH2 and VOacꢀCNTsꢀNH2 showed
poor catalytic activities. Upon adding the catalyst amounts, a
60 moderate benzene conversion (11.2%) was retrieved in the case
of VOacꢀSBA15ꢀNH2; however, the activity received over VOacꢀ
CNTsꢀNH2 was still low. The comparison strongly indicated that,
in the hydroxylation of benzene catalyzed by immobilized
vanadyl complexes, besides the reaction conditions, the support
65 itself also played an important role for the catalytic activity.
In addition, we have also calculated the turnover frequency
(TOF, grams of synthesized phenol per gram of the total catalyst
per hour) value in the case of entry 1 of Table 4, and the resultant
value was ca. 1.1 h–1. It is worth noting that in the cases of
2
3
4
19.8
19.6
19.0
95.4
95.0
94.7
18.8
18.6
18.0
a Reaction conditions: 1 mL of benzene (11.2 mmol), 3 mL of aqueous
H2O2 solution (30wt%), 6 mL of acetonitrile as solvent, Wcatal. = 50 mg, T
30 = 60 °C, and t = 4 h.
Table 4 Catalytic activities of the grafted vanadyl complexes in the
hydroxylation of benzene to phenol.
Entry
1 a
Catalyst
Conv. (%)
20.7
12.1
<1
11.2
<1
Sel. (%)
96.8
97.4
100
Yield (%)
20.0
11.7
<1
VOacꢀCNDꢀ120
CuacꢀCNDꢀ120
2 a
3 a VOacꢀSBA15ꢀNH2
4 b VOacꢀSBA15ꢀNH2
81.4
100
100
9.0
<1
4.7
5 a
6 b
VOacꢀCNTsꢀNH2
VOacꢀCNTsꢀNH2
4.7
a Reaction conditions: 1 mL of benzene (11.2 mmol), 3 mL of aqueous
35 H2O2 solution (30wt%), 6 mL of acetonitrile as solvent, Wcatal. = 50 mg, T
= 60 °C, and t = 4 h. b Reaction conditions: 1 mL of benzene (11.2 mmol),
3 mL of aqueous H2O2 solution (30wt%), 6 mL of acetonitrile as solvent,
Wcatal. = 200 mg, T = 60 °C, and t = 4 h.
Since the CND material possesses abundant aliphatic amines at
40 the edges of its graphitic sheets, which could be utilized to graft
VO(acac)2, it is of interest to synthesize gꢀCN materials grafted
6
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DOI: 10.1039/C5RA21438F
43
previously reported supported vanadia catalysts (e.g. V/clay
,
reaction of acetylacetonate ligand and the amino groups of gꢀCN,
VO/TiO2 44, VꢀgꢀC3N4 4, and VO/MCMꢀ41ꢀNH2 45), their TOF 45 and high temperatures could facilitate the immobilization
values were in the range of 0.14–0.73 h–1, whereas the reaction
conditions adopted such as temperatures were similar. Given this
point, the catalytic activity received over the present grafted
vanadyl complex catalyst was relatively high.
efficiency. In the catalytic hydroxylation of benzene, the VOacꢀ
CND materials provided high and stable catalytic conversion of
benzene (20.7%) along with high selectivity (96.8%) to phenol.
Moreover, the catalytic performance was comparably high than
5
To probe a potential effect of the support on the hydroxylation 50 that obtained over vanadyl complex grafted on amineꢀ
reaction, benzeneꢀTPD was conducted to investigate the
adsorption capability of benzene on SBAꢀ15 and CND samples.
functionalized SBAꢀ15. The mesoporous gꢀCN material acted not
only as a support for immobilizing [VO(acac)2] but also a crucial
component in activating benzene.
10 As depicted in Fig. 7, both SBAꢀ15 and its grafted vanadyl
complex represented minor desorption peaks at ca. 120 °C, which
should be due to physically adsorbed benzene molecules. In sharp
contrast, CND offered a pronounced broad yet highꢀintensity
peak centered at 150 °C. Referring to the textual parameters of
15 SBAꢀ15 and CND (Table S1 and Table 1), the surface area of
CND (488 m2 g−1) was actually lower than that of its counterpart
(564 m2 g−1). This means that the desorption peak acquired in
CND was probably contributed from the chemical rather than
physical adsorption of benzene. Based on the benzeneꢀtitration
20 experiment, the adsorbed amount of benzene on CND was ca. 28
ꢁmol gcatal.–1, far beyond the value gained on SBAꢀ15 (ca. 1.9
ꢁmol gcatal.–1). Obviously, the mesoporous gꢀCN possessed a
strong interaction with benzene, definitely owing to the
aforementioned conjugated triꢀsꢀtriazine units of gꢀCN, as also
25 reported by Wang et al 46. Interestingly, the capability to adsorb
benzene molecules remained even after the immobilization of
[VO(acac)2]. Based on the reported work involving hydroxylation
of benzene catalyzed by vanadyl complexes 2,45, a simple
catalytic mechanism can be speculated. Initially, H2O2 oxidized
30 V4+ into a peroxo vanadyl radical, i.e. V5+–O–Oꢂ. On the other
hand, benzene was absorbed by mesoporous gꢀCN support.
Afterwards, the activated benzene underwent an insertion of
oxygen atom in one of its C–H bonds by V5+–O–Oꢂ, therein
yielding phenol and V4+. Finally, the V4+ species was regenerated
35 by another oxidation by H2O2.
Acknowledgments
55 This work was supported by National Natural Science Foundation
of China (21203014), Postgraduate Innovation Project of Jiangsu
Province (KYLX_1097), Jiangsu Key Laboratory of Advanced
Catalytic Materials and Technology (BM2012110), and the
Project Funded by the Priority Academic Program Development
60 of Jiangsu Higher Education Institutions.
Notes and references
Jiangsu Key Laboratory of Advanced Catalytic Materials and
Technology, School of Petrochemical Engineering, Changzhou
University, Gehu Road 1, Changzhou, Jiangsu 213164, PR China. Fax:
65 +86ꢀ519ꢀ86330135; Tel: +86ꢀ519ꢀ86330135; Eꢀmail:
shine6832@163.com (Jie Xu); liyxluck@163.com (YongꢀXin Li)
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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