Figure 4 Optimization of different zinc compounds and ROS diffused from the surface of the CNTs. (OH simply represents ROS).
To prove this view, an experiment has been designed to add zinc acetate (Zn(CH3COO)2) into the cumene oxidation reaction catalyzed by CNTs. The result shows that the conversion of cumene with CNTs and Zn(CH3COO)2 was 37.6%, which was 4% lower than that of only with CNTs. But, compared with zinc halide, the inhibition effect of zinc acetate is very weak because Zn2+ is already saturated with oxygen-containing CH3COO- groups. This further elucidates the relationship between the coordination of Zn2+ with oxygen-containing groups and the inhibition of Zn2+ on cumene oxidation. Zn2+combines with ROS in the system, making it hard to extract H atom from cumene.
The reaction (1) is the dissociation of hydrogen radical (H· ) and the production of R·. The production of R· is directly related to the generation of RO· and ROO·. However, the bonding energy of H· with tertiary carbon is about 3.8 eV (Table S3). Besides that, even if the initial reaction of cumene successfully occurs. The radicals generated from the initial reaction will quickly combine to cumene because of its instability. This is the reason why it is difficult for cumene to oxidize without catalyst. The energy barrier of the initial reaction on different catalysts is shown in Figure 5 and Table S4. ROS diffused from the surface of the CNTs effectively reduce the initial reaction energy barrier to 0.55 eV, which can account for the conversion of cumene by CNTs. On this basis, after adding Cu2+, the energy barrier is lower as 0.18 eV which can explain the promotion of Cu2+.[14] The situation with adding Zn2+ is completely the opposite, the energy barrier goes up to 1.90 eV with adding Zn2+. It is reasonable to deduce that the inhibition effect of Zn2+ ions on cumene oxidation is due to the strong interaction of Zn2+ between ROS in the system, which leads to the increase of the energy barrier of the initial reaction.
Figure 5 Potential energy surface of the initial reaction of cumene with different metal ion.
According to the above theoretical calculations, Zn2+can strongly coordinate ROS due to its positive charge and sufficient space. According to the bonding energy between Zn2+and ROS, it is difficult to break out ROS and Zn2+. ROS is the key substance for the oxidation of cumene catalyzed by carbon nanotubes, so the initial reaction of cumene cannot be carried out. Moreover, it is more difficult to extract H atom from cumene to generate oxygen-containing free radicals (RO· and ROO·). This indicates that the interaction characteristic between the catalyst and reactive oxygen should be considered for the rational design of catalysts. In addition to Zn2+, the role of other metal ions in the cumene oxidation may also be worth considering, including the coordination ability to ROS and the inducted chain growth process.
Conclusions
In this work, the inhibition effect of Zn2+ on the catalytic oxidation of cumene was found by experiments, and the reason was studied by DFT. There are two inhibitory effects of Zn2+ on cumene oxidation. Firstly, Zn2+ is able to strongly coordinate ROS activated by CNTs, inhibiting this critical chain-initiation process of cumene oxidation. After adding Zn2+, the energy barrier of initial reaction increases to 1.90 eV, which is nearly 4 times higher than that of the ROS promoted-process. Secondly, the interaction of radicals (RO·, ROO·) and Zn2+ leads to those radicals out of chain propagation reaction and annihilate. This work provides a better understanding of carbon catalyzed cumene radical reactions. At the same time, it is noteworthy that the influence of metal ion impurity on the oxidation activity of cumene in industrial application.
Conflicts of interest
There are no conflicts to declare.