Communications
FeIII-substituted aluminium phosphate with properly tailored
few works. The kinetics for the radical autoxidation was
shown to relate to the chain length of radical reaction.[10a]
Also, CNTs were found to accelerate the electron-transfer
induced decomposition of radical precursors, including
organic peroxides.[20] The CNTs are expected to accept and
stabilize the radicals by forming new intermediates, which are
able to abstract hydrogen atoms from cyclohexane. An
improved lifetime with respect to that of peroxyl radicals is
probably assured by the large p system of graphitic units,
especially in the presence of N-containing complexes.[21] This
phenomenon can be evidenced by the unchanged ID/IG ratio
in the Raman spectra during recycling of CNT catalysts over
three cycles as an important indicator of the extent of
ordering of carbon materials (Figure S4 in the Supporting
Information). Results from transmission infrared spectrosco-
py and surface functionality analysis by Boehm titration also
show almost the same surface properties before and after
reaction (Figures S5 and S6 in the Supporting Information).
The stabilization of peroxyl radicals by p–p conjugation
between radical and CNTs is also supported by a theoretical
calculation performed at the B3LYP level of DFT theory. Our
calculations have revealed that the C6H11OO· radical, the
most important intermediate in the oxidation of cyclohexane,
can form a p–p stacking complex with a CNT or N-doped
CNT (represented by a nitrogen atom embedded in a
graphene sheet), as the optimized structures shown in
Figure 3. The radical is approximately 3.9 ꢀ away from the
CNT and approximately 3.5 ꢀ away from the N-doped CNT.
The energy of the interaction between the two species was
calculated to be 6.1 kJmolꢀ1 for the radical CNT complex and
60.3 kJmolꢀ1 for the radical N-doped CNT complex. These
results imply that the radical can be stabilized by the CNTand
N-doped CNT. Moreover, the radical N-doped CNT complex
is much more stable than the radical CNT complex, which is
consistent with the higher activity of N-doped CNT.
pore size to facilitate the formation of AA in pores.
Compared with FeAlPO, CNTs and N-doped CNTs have
similar AA selectivity, but lower selectivity of decarboxylated
by-products. Because no metal is involved, carbon-catalyzed
C6H12 oxidation is low-cost, the catalyst is easy to recover, and
the reaction is resistant against pipeline fouling. Owing to
their high activity, controllable selectivity, and outstanding
recyclability, CNTs are promising catalysts for cyclohexane
conversion as well as single-step production of adipic acid.
It is widely accepted that the liquid-phase oxidation of
C6H12 proceeds through a radical-involved autoxidation
process.[10b,c] The product cyclohexanone (C6H10( O)) can
=
also catalyze the initiation of a chain reaction [Eq. (1)].[10a]
C
C
C6H11OOH þ C6H10ð¼OÞ ! C6H11O þ C6H9ð¼OÞ ðꢀaHÞ þ H2O
ð1Þ
To prove the dominant role of radical species in our
system, p-benzoquinone as a typical radical scavenger was
added to the reactor. As shown in Table 2, the reaction was
almost totally suppressed to give a conversion as low as 0.8%
with the addition of p-benzoquinone. Similar to the reported
catalysts,[10a] a positive effect of cyclohexanone product was
also observed. With its presence, the conversion was elevated
from 14.0 to 21.8%. The apparent activation energy (Ea) was
calculated from the reaction data (Figure S3 in the Supporting
Information). The overall Ea value for cyclohexane conver-
sion is (111.5 ꢁ 15.5) kJmolꢀ1, which is extremely close to the
value for Equation (1), that is, in the autoxidation of
cyclohexane,
calculated
by
transition-state
theory
(116.3 kJmolꢀ1).[10a,15] A similar reaction mechanism was
then proved for our CNT-based process. The Ea values of
the oxidation with cyclohexanol and cyclohexanone as start-
ing materials were (37.5 ꢁ 6.2) and (41.7 ꢁ 5.1) kJmolꢀ1,
respectively, showing that the chain initiation is probably
the rate-determining step. It should be emphasized that the
catalytic role of CNTs in the C6H12 oxidation is crucial. Even
Studies on the gas-phase reaction show the crucial role of
surface functionalities in the catalytic performance of
CNTs.[16a,22] For example, in the oxidative dehydrogenation
=
=
without the C6H10( O) initiator, CNTs were an effective
reactions, the Lewis basic C O sites are responsible for
catalyst for the aerobic oxidation of C6H12.
activating hydrocarbons, while the graphene plane may
dissociate oxygen molecules.[16a,c,17] As shown in Table 3, in
contrast, there is a negative effect for the oxygen function-
alities on the activity of C6H12 oxidation. It was also shown
that this negative effect of groups on the activity did not
originate from the weaker adsorption of C6H12 on the
hydrophilic surface of oxidized CNTs (Figure S7 in the
Supporting Information). Such an effect is probably caused
by the localization of electrons as a result of the introduction
of groups and defects, which may be adverse to the p–p
interaction between the radical and graphene planes and is
supported by the catalytic performance of CNTs annealed at
high temperatures (Table 3). The annealing of CNTs can
decompose the oxygenous groups and benefit the repair of
the defects, indicated by the decrease of ID/IG ratios with
annealing temperature. The conversion of C6H12 increased
with annealing temperature, indicating that CNTs with higher
long-range order and electron delocalization are preferred.
Thus, the introduction of electron-donating nitrogen species
Diverse catalytic properties of CNTs have been reported,
including the activation of hydrocarbon,[16] oxygen,[17] and
hydrogen[18] molecules. Electron-donating nitrogen species at
the exposed graphitic defects were reported to facilitate the
adsorption of reactive intermediates and thus improve the
electrocatalysis of oxygen reduction.[19] Discussions on the
mechanism of liquid-phase processes can be found in only a
=
Table 2: Effect of C6H10( O), CNTs, and p-benzoquinone radical scav-
enger on the C6H12 conversion.[a]
[b]
=
Entry
C6H10( O)
CNTs[b]
p-benzoquinone
X[c] [%]
1
2
3
4
*
ꢀ
*
*
ꢀ
ꢀ
ꢀ
ꢀ
2.6
14.0
21.8
0.8
*
*
*
*
=
[a] Conditions: 398 K, 1.5 MPa O2, 93.6 g C6H12, 2.6 g C6H10( O), 64 g
acetone, 16.2 g butanone, 200 mg CNTs, 3 g p-benzoquinone. [b] *:
added; ꢀ: not added. [c] C6H12 conversion at 5 h.
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3978 –3982