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