Sp2- and sp3-hybridized carbon materials as catalysts for aerobic oxidation of cyclohexane

Article abstract of DOI:10.1039/c3cy00256j

The catalytic properties of sp²- and sp³-hybridized carbons, represented by graphene and diamond, in the selective oxidation of cyclohexane were investigated to understand the structure dependence of carbon materials in the reaction. sp² carbons showed the higher activity than sp³ carbons. The highest activity was obtained over mesoporous graphene, a representative sp²-hybridized carbon, yielding a weight-normalized activity of 162.6 mmol g^-1 h^-1. The excellent performance of sp² carbons was rationalized by their ability to catalyze the decomposition of peroxide intermediates, such as cyclohexyl hydroperoxide, which promoted the oxidative reaction to produce cyclohexanol and cyclohexanone.

Full text of DOI:10.1039/c3cy00256j

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2
3
sp - and sp -hybridized carbon materials as catalysts
for aerobic oxidation of cyclohexane†
Cite this: DOI: 10.1039/c3cy00256j
a
a
a
a
a
Yonghai Cao, Xianyu Luo, Hao Yu,* Feng Peng,* Hongjuan Wang and
b
Guoqing Ning
2
3
The catalytic properties of sp - and sp -hybridized carbons, represented by graphene and diamond, in the
selective oxidation of cyclohexane were investigated to understand the structure dependence of carbon
2
3
Received 15th April 2013,
Accepted 28th June 2013
materials in the reaction. sp carbons showed the higher activity than sp carbons. The highest activity was
2
obtained over mesoporous graphene, a representative sp -hybridized carbon, yielding a weight-normalized
À1 À1
2
activity of 162.6 mmol g
h
. The excellent performance of sp carbons was rationalized by their
DOI: 10.1039/c3cy00256j
ability to catalyze the decomposition of peroxide intermediates, such as cyclohexyl hydroperoxide,
which promoted the oxidative reaction to produce cyclohexanol and cyclohexanone.
www.rsc.org/catalysis
2
3
catalytic behaviors over sp and sp carbons. Su and co-workers
reported that an sp -hybridized nanodiamond was more active
1. Introduction
3
In the last two decades, carbon materials and their composites, and selective in the gas-phase ODH reaction of ethylbenzene
2
owing to their unique physicochemical properties such as high than sp -hybridized carbon materials, including carbon nano-
specific surface areas, environmental acceptability, corrosion resis- tubes, nanographite, activated carbon, and mesoporous carbon.
3
2
tance, thermal stability under non-oxidative conditions and easy It was due to forming a core–shell structure through an sp -to-sp
1
,2
3,22,23
surface modification, have been proved attractive alternatives to lattice rearrangement.
In the selective oxidation of acrolein
2
conventional metal-based catalysts for oxidative dehydrogenation to acrylic acid in gas phase, sp carbons were demonstrated to
3
–5
6
7
3
(
ODH), ammonia decomposition, Friedel–Crafts reaction, and be efficient for the oxygen insertion reaction, while the sp
8
–10
selective oxidation.
The defects, structure and surface chemistry carbons were detrimental to the selectivity of the formation of
2
4
of carbon materials play significant roles in these reactions. For acrylic acid. However, to the best of our knowledge, the effect
instance, the surface defects and carbonyl/quinone groups have of hybridization of carbons on their catalysis has not been
been proposed as the active sites for the ODH of hydrocarbons,
hydrogenation of nitrobenzene, and the oxidation of benzene
to phenol. Carboxylic acid groups on the surfaces of carbon hybridizations was evaluated by using the oxidation of cyclo-
catalysts were identified as the active sites for the dehydration hexane (CyH) as a model reaction. It is a typical oxidation of
4,5,11
rationalized in liquid-phase oxidative catalysis.
1
2
In this work, the catalytic behavior of carbons with different
1
3
1
4,15
8
of alcohols
and the wet air oxidation of phenols. We have hydrocarbon undergoing a radical mechanism in the liquid
previously found that the surface groups and electron transfer phase, which is applied for the production of cyclohexanol
2
5
6
in graphene sheets have an important effect on the activity in (CyOH), and cyclohexanone (QQO) in 6 Â 10 ton per year in
1
6–21
2
the liquid-phase oxidation of hydrocarbons.
the chemical industry. Various carbon allotropies with sp - and
3
Despite their diverse morphology and structures, the carbon sp -electronic configurations were investigated, including micro-/
materials fall into two main categories, i.e. sp - and sp -hybridized nano-sized diamonds, graphites and graphenes with different size
2
3
carbons. So far, most of the catalytic applications of carbons have and specific surface areas (SSA). By comparing the product
2
been investigated on sp carbons, i.e. graphitic carbons. However, profiles, the reaction pathways on these carbons were determined,
2
3
some recent publications show that there might be different displaying the different functions of sp and sp carbons in the
reaction.
a
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: yuhao@scut.edu.cn,
2
. Experimental
cefpeng@scut.edu.cn; Fax: +86 20 8711 4916; Tel: +86 20 8711 4916
State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Beijing, Changping 102249, China
b
2.1 Materials
Micrometer sized (m-D) and nanosized diamonds (nano-D)
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00256j
were purchased from Alfa Aesar Co. Ltd and Beijing Grish
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Hitech Co., respectively. The diamond samples were treated with CyH, CyOH, and QQO. To determine cyclohexyl hydroperoxide
concentrated nitric acid under reflux for 6 h, repeated 3 times to (CyOOH), the reaction samples were double analyzed, before
2
26
remove sp impurities. Graphite powders in 325 (G325) and and after reducing CyOOH to CyOH with triphenyl phosphine
3
1
4
000 mesh (G4000) were purchased from Alfa Aesar Co. Ltd and (Acros, 99%). Conditions of GC: HP-5 ms capillary column
Qingdao Taineng Graphite Co. Ltd, respectively, and pre-treated (30 m, DF = 0.25 mm, 0.25 mm i.d.), temperature program was
by HCl washing to remove trace metals. Reduced graphite oxide carried out (initial temperature = 90 1C, 7 min; final tempera-
À1
(
r-GO) was prepared from G325 powders by the modified Hummers’ ture = 250 1C, heating rate = 20 1C min , temperature of
27
28
2 2
method and then reduced with hydrazine hydrate (N H ). The injector = 280 1C, temperature of detector = 280 1C). An HPLC
mesoporous graphene (meso-G) was synthesized by the chemical instrument, equipped with an Agilent Hypersil ODS column
vapor deposition (CVD) of methane using MgO powder as a (4.6 Â 250 mm) and UV detector (at 210 nm), was used to
2
9,30
template, as reported elsewhere.
All the samples were analyze the concentrations of adipic acid (AA), glutaric acid
À1
annealed at 600 1C for 6 h in Ar atmosphere before use to (GA) and other acids. The eluent was 0.01 mol L KH PO in a
2
4
reduce the influence of surface functional groups.
1 : 9 methanol–water mixture.
To study the effect of metal impurities, the meso-G was
firstly purified in concentrated HCl for 4 h, then impregnated
2.4 Catalytic decomposition of benzoyl peroxide
with the corresponding sulfates (MgSO
The obtained solids were washed and lyophilized, and then were carried out as follows.
4
Á7H
2
O or MnSO
4
ÁH
2
O). The air-free benzoyl peroxide (BP) decomposition experiments
2
0,32
Typically, 25 mg of catalysts,
heated in Ar at 450 1C for 2 h.
.2 Characterizations
and 50 mL of ortho-dichlorobenzene (o-DCB) as solvent, were
added to a 250 mL three-necked flask. The mixture was
sonicated for 5 min, then placed in an oil bath at 90 1C. After
2
Brunauer–Emmett–Teller (BET) SSAs of the carbons were measured flushing with N for 5 min, 1.5 mmol BP was added to allow the
2
2 2
by N adsorption at liquid N temperature using an ASAP 2010 thermolysis in a closed system. An HPLC instrument, equipped
analyzer. Raman spectra were obtained using a LabRAM aramis with an Agillent Hypersil ODS column (4.6 Â 250 mm) and a UV
micro Raman spectrometer excited at 633 nm with a 2 mm spot size. detector (at 236 nm), was employed to determine the percentage
À1
XPS analysis was performed with a Kratos Axis ultra (DLD) spectro- peroxide remaining after thermolysis. The eluent was 0.05 mol L
meter equipped with an AlKa X-ray source. The binding energies H PO in a 4 : 1 acetonitrile–water mixture.
3
4
2
(Æ0.2 eV) were referenced to the C 1s peak at 284.6 eV for sp
3
carbons (G325, G4000, r-GO and meso-G) and 285.2 eV for sp
carbons (m-D and nano-D). Fourier transform infrared (FTIR)
spectra were obtained on a Nicolet 6700 spectrometer by pelletizing
the samples with KBr. TEM and HRTEM images were obtained
with an FEI Tecnai G2 12 microscope operated at 100 kV and a
JEOL JEM2010 microscope operated at 200 kV, respectively.
Specimens for TEM and HRTEM were prepared by ultrasonically
suspending the sample in acetone and depositing a drop of the
suspension onto a grid and a copper grid covered with amorphous
carbon, respectively. Scanning electron microscope (SEM) images
were obtained with a LEO 1530VP electron microscope. The
concentrations of metal impurities in the catalysts were deter-
mined by ICP-AES (Prodigy SPEC Leeman).
3
. Results and discussion
Fig. 1 displays SEM and TEM images of the carbon materials with
different hybridizations used in this work. The r-GO exhibited a
wrinkled morphology, resulting in agglomerates of thin graphite
nanosheets in 3–10 graphene layers. The meso-G was typically in
one or two graphene layers. Well-developed mesopores can be
observed in the meso-G sample, which were formed by mimicking
the shape of nano-MgO crystallites during the CVD process, giving
2.3 Oxidation of cyclohexane
The CyH oxidation reaction was carried out in a magnetically
stirred, 50 mL, Teflon-lined autoclave in batch mode. Before
reaction, reactants with solvent (acetone or toluene), initiator
QQO and catalysts were loaded in the autoclave, and the reactor
was flushed with N . Then, the reactor was heated to a stable
2
operational temperature. The stirring speed was 1100 rpm to
minimize the effect of mass transfer (Fig. S1, ESI†). Subsequently
2
pure O was fed into the reactor (defining t = 0) to maintain a
reaction pressure of 1.5 MPa. After reaction, the reactor was
cooled to room temperature and the liquid phase was diluted by
acetone and then toluene (butanone if toluene was solvent) was
added as internal standard. The products were analyzed by gas
chromatography (GC) and high-performance liquid chromato-
Fig. 1 TEM and SEM images of carbon materials used in this study: (a) r-GO,
graphy (HPLC). GC was used to analyze the concentrations of (b) meso-G, (c) m-D, (d) nano-D.
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Table 1 BET SSAs and pore volumes (PV) of various carbon allotropes
m-D Nano-D G325 G4000 r-GO
Meso-G
2
À1
SSA/m g
2.1
0.7
374.5
83.6
3.6
1.5
8.3
3.06
293.8 1641.0
À2
3
À1
PV(10 )/m g
77.2
236.8
2
À1
a high SSA of 1641 m g (see Table 1). The primary particles of
m-D were about 1.5 mm in diameter with irregular shapes, as
shown in Fig. 1c. The HRTEM image of purified nano-D in
Fig. 1d presents nanosized particles with an average diameter
of about 7 nm. Clear lattice fringes with intervals of 0.206 nm,
3
3
corresponding to the (111) plane of diamond, evidenced the
3
presence of sp -hybridized diamond phase.
2
N adsorption–desorption isotherms and pore size distribu-
tions of the carbon materials used are presented in Fig. 2. Their
BET SSAs and pore volumes are summarized in Table 1. The
Fig. 3 Raman spectra of m-D, nano-D, G325, G4000, r-GO and meso-G.
3
5
adsorption–desorption isotherms of nano-D, r-GO and meso-G particle size results in more edges, the I
D
/I
G
value of G4000
displayed typical characteristics of mesoporous materials, (0.083) was higher than that of G325 (0.046). The Raman
with pore size distributions centered at about 10, 5 and 4 nm, spectra of the graphenes, r-GO and meso-G, displayed typical
respectively. High SSAs were obtained over them, at 375, G and D bands with I /I values of about 1.88 and 1.86, due to
D
G
2
À1
2
28
2
3
94 and 1641 m g , respectively. m-D and graphite powders in their tiny sizes of sp domains and numerous surface defects.
2
À1
25 and 4000 mesh showed very low SSAs of less than 10 m g
Compared with r-GO by the modified Hummers’ method, the
and a wide pore size distribution from 2 to 100 nm due to random stronger intensity and a downshift of the D band were observed
packing of the particles.
Fig. 3 shows the Raman spectra of m-D, nano-D, G325, sheets,
G4000, r-GO and meso-G. The m-D and nano-D exhibited a
in meso-G, indicating the smaller and thinner graphene
2
9,30
as revealed by the TEM images in Fig. 1.
The above Raman results show that typical sp and sp
2
3
À1
3
single peak at about 1320 cm , a characteristic of sp -carbon in carbons were obtained, allowing for evaluating the effect of
2
6
the diamond structure. The absence of the G band indicated hybridization on the catalytic performances. XPS was further
2
3
that there were no sp impurities in these samples. The G325 and used to detail the surface species, such as sp -type defects in
À1
2
G4000 showed strong G bands at 1594 cm and weak D bands sp carbons and oxygen-containing functionalities. Fig. 4 shows
À1
at 1327 cm , indicating the existence of defects at the edges the C 1s XPS spectra of the carbon materials. The spectra were
3
4
2
3
and in the planes of the graphite sheets. Because the smaller deconvoluted into sp and sp species centered at 284.6 eV and
2
Fig. 2 (a) N adsorption–desorption isotherms and (b) Barret–Joyner–Halenda (BJH) desorption pore size distributions of the carbon materials used in this work.
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Fig. 4 C 1s XPS spectra of m-D, nano-D, G325, G4000, r-GO and meso-G.
3
6
2
85.2 eV, respectively, and oxygen-containing groups, such as can be assigned as the carbons bound to nitrogen (C–N),
37
28,45
C–O, CQO and –COOH. The diamond samples displayed a introduced during the deoxygenation by N H .
2
2
symmetric peak at 285.2 eV, which was fitted by a Gaussian function
The performances of the carbon allotropes as catalysts for
and by approximating the contribution of the background by the the CyH oxidation are summarized in Table 3. With the carbons
Shirley method. Graphites and graphenes showed an asymmetric as catalysts, KA oil, CyOOH and AA were produced. Side-
peak centered at 284.6 eV with a long tail extended to the higher products, such as 6-hydroxyhexanoic acid, e-caprolactone, were
38
energy region. In this work, a Doniac–Sunjic function was used to also detected but were negligible. A series of control experi-
2
39
fit the sp carbon in the graphite basal planes, which is widely ments were conducted to rule out the involvement of metal
38,40,41
42,43
used for graphite,
aromatic molecules and coals.
Three impurities. XPS survey spectra of the carbon materials (see
additional Gaussian peaks centered at 286.2, 287.5, and 288.9 eV Fig. S2, ESI†) demonstrated that there were no detectable metals
were considered to be originated from carbon atoms bound to (lower than 0.1 at%, the detection limit of XPS) on the surfaces of
37
one, two, and three oxygen atoms, respectively. The quantitative the carbons, such as Fe, and Mn, which was used for preparing
results from the XPS deconvolution are shown in Table 2.
GO. ICP-AES results (see Table S1, ESI†) indicate that a small
3
In the diamond samples, only sp carbon was detected, with
about 10% C–O groups caused by exposure to air or HNO
treatment, indicating that the sp impurities on the surfaces
3
pre-
a
Table 3 Catalytic performances of carbon materials in the aerobic CyH oxidation
3
6
2
c
d
e
r
W
r
S
Selectivity [%]
of nano-D have been completely removed by the HNO oxida-
3
b
f
X
[mmol
[mmol
TOF
3
2
tion. sp carbons were observed in all the sp carbons, with
contents from 4% to 9%. G4000 has a higher sp content than
À1 À1
À2 À1
À1
Catalyst [%]
g
h
]
m
h
]
KA
CyOOH AA
8.1
GA [h
]
3
g
Blank
m-D
2.2
—
—
64.8 10.3
31.4 32.0
40.3 27.7
60.1 8.5
67.7 3.3
39.3 2.4
36.7 1.8
34.4 1.6
35.6 3.1
32.4 1.6
—
—
G325, due to the greater number of defects in its graphite
4.4 30.7
8.8 61.3
6.2 43.4
7.4 51.6
17.3 151.6
14.4
0.2
12.2
6.2
0.5
—
0.1
—
—
6.3 8.8 304.7
9.1 6.9 4.3
24.5 5.0 202.8
20.8 4.7 111.0
4
4
3
planes. Meso-G contains about 4.0% sp carbon, caused by pores Nano-D
3
G325
and edges. 7.5% sp carbons were detected in r-GO, indicating the
serious defectiveness due to the strong oxidation. In addition,
G4000
r-GOh
27
48.1 3.4
50.1 6.2
53.9 2.3
50.6 4.5
49.6 4.3
9.4
there was an additional component at 285.9 eV in r-GO, which r-GO
15.8
—
—
Meso-Gi 22.1 162.6
1.7
Meso-G 21.1
—
—
—
—
j
Meso-G 21.2
Table 2 Quantitative results of XPS spectra
a
2
Conditions: 398 K, 1.5 MPa O , 1100 rpm, 14.0 g CyH, 0.39 g QQO,
c
Deconvoluted C 1s [%]
C–
b
9
.6 g acetone, 30 mg catalyst. CyH conversion at 8 h. Initial reaction
d
rate of CyH consumption normalized by catalyst mass. Initial reaction
rate of CyH consumption normalized by catalyst surface (conversion
Carbon
materials
C
O
N
C–
2
3
[at%] [at%] [at%] sp sp
O
CQO COO N
e
o10%). Selectivity of major products at 8 h. KA: cyclohexanol and
m-D
Nano-D
G325
G4000
r-GO
Meso-G
90.2
89.1
98.3
97.0
93.0
96.5
9.8
10.9
1.7
3.0
5.1
—
—
—
—
1.8
—
—
—
90.0 10.0 —
88.0 12.0 —
—
—
—
—
—
—
—
—
cyclohexanone, CyOOH: cyclohexyl hydroperoxide, AA: adipic acid, GA:
glutaric acid. The by-products included succinic acid, hydroxyl hexylic
f
94.9 4.2 0.9 —
88.1 9.0 2.9 —
84.3 7.5 1.9 0.6 1.5 4.2
92.0 4.0 2.7 0.7 0.6
acid and e-caprolactone and other undetected acids. Turnover frequency
per sp or sp carbon atom on surfaces. Without catalyst. Additional
0.598 wt% Mn was impregnated. Additional 1.348 wt% Mg was impreg-
nated. The fourth run.
2
3
g
h
i
j
3.5
—
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quantity of Fe remained in most of the catalysts. Meanwhile, Table 4 Apparent kinetic parameters of the CyH oxidation (see Fig. S4–S7, ESI
for experimental details)
0.225 wt% manganese in r-GO and 0.708 wt% magnesium in
meso-G were detected. The involvement of Fe has been excluded
Reaction order at 398 K
16,18
from the catalysis by carbons in our previous works.
To exclude
À1
Samples
E
a
/kJ mol
CyH
Catalyst
Initiator
the effects of Mn and Mg, an additional 0.598% Mn and 1.348%
Mg were loaded on r-GO and meso-G, respectively, by impregna-
tion. There were no noticeable changes in either the conversion or
selectivity after loading the metals (see Table 3). Thus, the potential
role of metal was unlikely to be essential in this reaction.
Meso-G
Nano-D
79.7 Æ 0.12
91.8 Æ 0.14
À0.3
À0.6
0.7
0.4
0.4
0.3
1
8
observation. The oxidation showed a negative dependence on
Since the solvent acetone may contribute oxygen, we did a the concentration of substrate CyH over both of the carbons.
series of control experiments to identify the source of oxygen in the However, a more sensitive dependence was observed on nano-
products, as summarized in Table S3 (ESI†). No conversion was D, indicating that the reaction may be suppressed significantly
observed in the experiment without the O2 supply in acetone. at high concentrations, e.g. the solvent-free case, upon sp3
15.2% CyH conversion was measured when oxygen-free toluene carbons. The negative order of the substrate has been observed
was used as solvent. Although at the current stage we cannot obtain in metal-catalyzed processes, which may be caused by the
4
7
direct evidence to detail the process of oxygen atom transfer, the formation of inactive dimers on the catalyst surface. In this
above results allow us to conclude that the oxygen from the gas work, it was also probably linked with the competition between
phase plays an essential role in the oxidation reaction.
different pathways of chain propagation. Hermans and co-workers
TOF values were estimated from the SSAs in Table 1 to have determined that the chain propagation reaction of the auto-
evaluate the intrinsic activity of the carbons, assuming that the xidation of CyH goes through the H-abstraction from CyOOH or
2
3
48
ꢀ
average occupied area is 0.0316 nm for an sp carbon atom (in CyH by CyOO radicals, while the former is much faster. Although
2
2
(
100) planes) and 0.0262 nm for an sp one (in honeycomb the higher CyH concentration increases the H-abstraction from
network with C–C bond length of 0.142 nm). Only sp or sp
species were considered for the TOF of sp and sp carbons, formation of peroxide radicals. Regarding the apparent order of
respectively, by applying the quantitative XPS results in Table 2. catalyst loading, the oxidation over meso-G showed a higher order
Compared with the blank test without any catalysts, all the than that over nano-D (0.7 vs. 0.4), suggesting the more crucial role
2
3
CyH, the overall reaction may be suppressed due to the slower
2
3
2
carbons improved the conversion. The m-D and graphite samples of sp carbon in the reaction.
displayed weak to moderate activities. The high SSA of nano-D
To understand the role of hybridization on the catalysis, the
improved the weight-normalized reactivity, while resulting in a main product distribution was measured to unravel the reac-
significant reduction of TOF, implying the decreased accessibility tion pathways with different carbons as catalysts. It was noticed
3
of the catalyst surfaces caused by the tortuosity and steric that, when sp -hybridized diamonds were used, about a 10-fold
effect. Similar surface-area-dependence of conversion was observed higher CyOOH selectivity was observed, suggesting their poor
over graphite and graphene samples. Due to the high SSAs, the ability to induce the decomposition of peroxide. In the radical
graphene samples were the most active among the carbons used in autoxidation of CyH, CyOOH is produced in fast chain propa-
2
5,49
this work. In particular, the meso-G showed the highest weight- gation reactions through eqn (1) and (2).
Its decomposition
À1 À1
48
normalized activity up to 162.6 mmol g
h , which is comparable is responsible for the formation of CyOH, and QQO [eqn (3)].
46
with the reported gold catalysts supported on ZSM-5, displaying If the decomposition is suppressed in the subsequent reaction,
its promise as a high-efficiency, metal-free catalyst. There was the yields of KA oil will decrease, and also the overall reactivity.
no noticeable decline of conversion and selectivity after recycling According to our previous works, the interaction between the
ꢀ
four-times, indicating that the meso-G can be used as a stable and radical species (CyOO ) and carbon nanotubes plays a central
reusable solid catalyst in the oxidation of CyH (see Fig. S3, ESI† for role in the CyH oxidation, which efficiently enhances the
1
8
the Raman spectra of reused meso-G). It should be noted that, catalytic activity. We thus speculated that the different activity
2
under a similar SSA level, the r-GO, as a representative of sp
in catalyzing CyOOH decomposition is probably responsible for
carbons, was of higher conversion than the nano-D, as a represen- their different performances.
3
À1
À1
tative of sp carbons, evidenced by the TOF of 9.4 h vs. 4.3 h
.
ꢀ
ꢀ
2
CyOO + CyH - CyOOH + Cy
(1)
(2)
(3)
This result indicated that sp carbons are intrinsically more active
than sp carbons in the oxidation of CyH.
The kinetic features of the oxidation of CyH catalyzed by sp
and sp carbons were compared by taking meso-G and nano-D
3
ꢀ
ꢀ
Cy + O
2
- CyOO
2
3
ꢀ
ꢀ
CyOO + CyOO - CyOH + QQO + O
2
2
3
as representative sp and sp carbons, respectively. As listed in
Table 4, the meso-G displayed a lower activation energy for the
To verify the role of carbon–peroxide radical interaction, the
2
oxidation of CyH, indicating the positive effect of sp carbon on carbons used in this work were subjected to a catalytic test of
2
3
reducing the energy barrier. Both of the sp and sp carbons peroxide decomposition using benzoyl peroxide (BP) as a model
showed similar dependence on the concentration of initiator, compound. Fig. 5 shows the percentage BP consumed within
implying that the initiation process does not play an important 1 h at 90 1C in the presence of carbons. 15% BP was decomposed
role in the carbon catalysis, being consistent with our previous in 1 h without any catalyst, which is consistent with the result
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the oxidation of CyH. Higher conversions were observed when
2
2
sp carbons were used as catalyst, indicating that sp carbons
3
are more active catalysts than those of sp hybridization. It can
be attributed to their higher activity in catalyzing the decom-
position of CyOOH intermediates, thus enhancing the overall
activity and the formation of CyOH and QQO. By taking BP as a
2
model compound of peroxide, the sp -carbon-catalyzed decomposi-
tion was demonstrated. The results suggested that graphenes may
be a promising catalyst for the oxidation of CyH in the liquid phase,
2
because of their dominant sp carbon atoms in basal planes and
extremely high theoretical SSA values.
Acknowledgements
This work was supported by the National Science Foundation of
China (No. 21133010, 21273079), the Guangdong Provincial
National Science Foundation of China (No. 9251064101000020,
Fig. 5 Effect of different carbons on the thermolysis of benzoyl peroxide.
Table 5 Apparent kinetic parameters of BP decomposition (see Fig. S8–S10, ESI S20120011275) and Program for New Century Excellent Talents
for experimental details)
in University (NCET-12-0190).
Reaction order at 363 K
À1
Samples
a
E /kJ mol
Substrate
Catalyst
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