2
S. Liao et al. / Applied Catalysis A: General 478 (2014) 1–8
surface of CNTs played an important role in CWAO of phenol [15].
Meanwhile, basis surface was also thought to be responsible for
improved catalytic performance in CWAO [16,17]. More recently,
carbon materials were found to be active for the direct liquid-
phase aerobic oxidation of aromatic hydrocarbon. In our previous
work, we proved that pristine carbon nanotubes could directly
catalyze the aerobic oxidation of cyclohexane to adipic acid and
ethylbenzene to acetophenone, and the surface defects and oxygen
functionalities had a negative effect on the activity [19,20]. Jing-He
Yang etc. also proved that metal-free graphene-based catalyst was
effective for one-step oxidation of benzene to phenol with hydro-
gen peroxide as the oxidant [21]. However, up to now, it is rarely
reported that nanostructured carbon as catalyst for selective oxi-
dation reaction of cumene, and the basic knowledge about carbon
catalysis remains limited.
Herein, by using cost-cheap commercial carbon nanotubes as
catalyst and oxygen as oxidant, we explored their catalytic activity
in the aerobic oxidation of cumene without solvent at atmospheric
pressure. The influence of catalyst surface oxygen-containing func-
tional groups on catalytic performance was also discussed. The
high reaction activity of cumene oxidation and the recyclability of
carbon catalyst make this reaction system attractive for potential
industrial application.
into the flask, sonicated for 5 min, and then heated to preconcerted
temperature followed by the bubbling of oxygen at a constant flow
rate. The main product of cumene oxidation is CHP, the by-products
are acetophenone (AP) and 2-benzyl-2-propanol (BP), as shown
in Scheme 1. The CHP concentration was determined according
to the iodometric method [24]. After the reduction of generated
CHP to BP via triphenylphosphine reaction [25], the other prod-
ucts in the liquid phase were detected by gas chromatography (an
Agilent GC-6820) equipped with a 30 m × 0.25 mm × 0.25 m HP-5
capillary column and a flame ionization detector (detector tem-
perature 553 K, injector temperature 553 K, and oven temperature
413 K) using toluene as external standard. Cumene conversion (X)
and products selectivity (S) were calculated using the following
equations (I)–(IV).
ni,cumene − nf,cumene
X =
× 100%
(I)
(II)
n
i,cumene
nf,CHP − ni,CHP
SCHP
=
× 100%
nf,CHP − ni,CHP + nf,BP + nf,AP
nf,BP
SBP = n
SAP = n
× 100%
(III)
(IV)
f,CHP
− ni,BP + nf,BP + nf,AP
nf,AP
× 100%
f,CHP
− ni,CHP + nf,BP + nf,AP
2
. Experimental
where ni,m and nf,m, respectively, mean the moles of m in initial
reactant and final product.
2.1. Preparation of catalysts
The thermolysis tests of CHP were conducted according to Ref.
[
26]. 8 mL acetonitrile and 0.1 g catalyst were added in a flask,
The commercial carbon nanotubes (denoted as CNTs,
2
−1
sonicated for 5 min and then placed in an oil bath at 353 K. After
flushing with N2 for 5 min, CHP (2 g) was added and the reaction
was conducted in a closed system. The unreacted CHP was tested
by aforementioned iodometric method.
SBET = 85.3 m g
, d = 30–50 nm) were purchased from Shen-
zhen Nanotech Port Co. Ltd (NTP). They were stirred to purify in
hydrochloric acid or heated to reflux in 9 mol/L HNO3 for 0.5, 2
and 4 h to introduce oxygen functional groups, then washed with
deionized water until pH reached 6–7, and dried in air at 383 K
overnight. The obtained materials were denoted as CNTs-HNO3-
3. Results and discussion
0
.5, CNTs-HNO -2, and CNTs-HNO -4, respectively. In addition,
3
3
CNTs-HNO -2 samples were subjected to heat treatment in a
3.1. The optimization of reaction conditions
3
horizontal tubular quartz furnace with 4 cm inner diameter under
argon atmosphere at 873 K or 1173 K for 2 h and then cooled under
argon, denoted as CNTs-HNO -2-873 and CNTs-HNO -2-1173,
Cumene liquid-phase oxidation reaction was operated at atmo-
spheric pressure with oxygen as oxidant with carbon nanotubes as
catalyst and no solvent was added. In order to optimize the reaction
parameters and provide a basis for further mechanism research, the
effects of reaction conditions, including catalyst amount, reaction
temperature, oxygen flow rate and reaction time on the catalytic
performance were investigated in detail.
3
3
respectively [22,23].
.2. Characterizations
Brunauer-Emmett-Teller (BET) specific surface areas were mea-
2
sured by N2 adsorption at liquid N2 temperature in an ASAP 2010
analyzer. Raman spectra were obtained in a LabRAM Aramis micro-
Raman spectrometer with an excitation wave-length at 532 nm
with 2 m spot size. TEM (transmission electron microscopy)
and HRTEM (high resolution transmission electron microscopy)
images were obtained with a FEI Tecnai G2 12 microscope oper-
ated at 100 kV and a JEOL JEM2010 microscope operated at 200 kV.
Specimens for TEM and HRTEM were prepared by ultrasonically
suspending the sample in acetone and depositing a drop of the
suspension onto a grid. X-ray photoelectron spectroscopy (XPS)
analysis was performed in a Kratos Axis ultra (DLD) spectrometer
equipped with an Al K␣ X-ray source in ultrahigh vacuum (UHV)
The effect of catalyst amount on the catalytic activity of cumene
under 353 K was shown in Fig. 1A. The flow rate of O2 was con-
trolled at 10 cm /min. In the absence of catalyst, the autoxidation
3
rate of cumene is very slow (X = 2.7%, 8 h). After the addition of
50 mg CNTs, the conversion of cumene increased to 17.8% with an
outstanding selectivity of 90.8% to CHP. With the increase of CNTs
content, the conversion of cumene also increased. On the contrary,
too much catalyst leads to decline in selectivity to CHP. With 200 mg
CNTs, the 35.0% conversion of cumene was obtained, whereas the
selectivity to CHP obviously decreased to 76.2%. This indicates that
CNTs play an important role in cumene oxidation. To get an accept-
able selectivity to CHP, 100 mg CNTs under reaction condition was
chosen.
The effect of reaction temperature on the catalytic performance
of cumene was shown in Fig. 1B. It revealed that cumene oxida-
tion could be well carried out under a rather low temperature
as 343 K. The conversion of cumene increased significantly with
the reaction temperature increasing. Under 353 K, the selectivity
to CHP remained stable value of 88.4%. When the temperature
reached 373 K, the selectivity to CHP dropped dramatically possi-
bly due to the accelerated decomposition of CHP. Thus, the reaction
−
10
(
<10
Torr). The binding energies (± 0.2 eV) were referenced to
the C1s peak at 284.6 eV. The surfaces of samples were cleaned by
heat treatment at 373 K in UHV prior to the measurements.
2.3. Catalytic tests
The liquid oxidation reactions were carried out in a three-
necked flask (20 mL), supplied with a magnetic stirrer, reflux
condenser and the oil bath. Cumene (10 mL) and catalyst were put