2.2 Computational methods
As the CNTs using in reaction were
100 mg. Each radical reaction on the surface of CNTs is approximately on
the two-dimensional plane, so DFT calculation is simulated on the
graphene surface which belong to periodic system. DFT calculation were
performed with the Dmol3 package.[21] The
configurations were optimized with GGA/PBE correlation exchange
functional under DNP basis set.[22] The Grimme’
DFT-D was included as the long-range dispersion correction
method.[23] The k-point was set to 4 × 4 × 1 for
the self-consistent field (SCF) procedure. The conductor-like screening
model (COSMO) with a permittivity of 2.4 was invoked to simulate the
cumene solvent environment.[24] The global orbital
cutoff was 5.0 Å. Furthermore, the optimized structures were used for
energy calculations and TS search with the same computational
parameters. The LST/QST method was exploited to search the transition
state of the reaction and the NEB method was adopted to confirm the
transition state.
The relative system energy
(ΔE)
is calculated by equation (1):
ΔE = EZn2+-RO· -
EZn2+ -
ERO· + EBSSE (1)
where, EZn2+-RO· is the energy of the
Zn2+ ion and RO· radical system, EZn2+is the energy of the Zn2+ ion,
ERO· is the energy of RO· and
EBSSE is the energy of Basis Set Superposition Error.
When the value of ΔE is negative, it means that the interaction between
two substances is favorable.
The energy barrier (Ea) is calculated by equation (2):
Ea = ETS – Ereactant(2)
Where, ETS is the total energy of the transition state,
Ereactant is the energy of the initiation state. The
more positive the value of Ea, the harder this reaction
is to do.
Results and discussion
The inhibition
phenomenon ofZn2+ oncumene oxidation
The catalytic properties of cumene oxidation on different conditions are
summed up in Table 1. Without any catalysts, cumene proceeds through an
aerobic autoxidation. And the conversion was as low as 1.3% (entry
1). The conversion of cumene was
increased to 41.8% after adding CNTs (entry 2). This illustrates that
CNTs play a key role in the cumene oxidation. The studies found that
CNTs had an active effect on O2.[2,
25, 26] However, after adding 10 mg ZnCl2 to the
system (entry 3), the conversion of cumene was almost zero. No oxidative
products were detected by iodometric method and gas chromatography,
which means that the oxidation of cumene was totally stopped by
ZnCl2. At the beginning, Cl- ion was
suspected to be involved in inhibiting the oxidation reaction of cumene.
While, CuCl2, FeCl3 and
CoCl2 were added into cumene oxidation system catalyzed
by CNTs, the conversion of cumene was not reduced, and even the activity
was increased.[11, 13-15] Besides, the
ZnCl2 was replaced with ZnBr2 on
oxidation experimental and still none of any oxidation products could be
detected at the end of reaction (entry 4). It is widely known that the
radius of Br- ion is larger than that of
Cl- ion, namely ZnBr2 would more
likely dissociate Zn2+ into the system. Then
Zn2+ is the most likely source of inhibition of cumene
oxidation. In general, metals ions (Cu2+,
Co2+ etc.) can promote the active sites of aromatics
oxidation.[27-29]This particular inhibition of
metal ions has received little attention. In practical industrial
application, it is very important to analyze the effect of metal ion
impurity on catalytic performance. In order to explore the inhibition
effect of Zn2+, it is necessary to analyze the
oxidation mechanism of cumene.
Table 1 Cumene oxidation catalyzed by CNTs with zinc
compoundsa