Figure 1 Reaction network of cumene catalyzed by CNTs.
RH → H· + R· (1)
R· + O2 → ROO· (2)
ROO· + RH → CHP + R· (3)
CHP → RO· +·OH (4)
RO· + RH → ROH + R· (5)
Theoretical calculation of inhibition effect of Zn2+ on cumene oxidation
Firstly, a hypothesis is proposed that the life of free radicals is long enough to interact with Zn2+. This is conducive to calculating the interaction between Zn2+ and active radicals. Finally, combined with the actual situation, the revised results are discussed. So, whilst a Zn2+ is in cumene solvent, a Zn2+ is able to interacts with ROO· and RO·. Considering the multiple coordination ability of Zn2+, each Zn2+ can adsorb three oxygen-containing intermediates one by one, as shown in Figure 2. Total energies for the interaction of RO· and ROO· on Zn2+are shown in Table S1. The corresponding bonding energies for each coordination process of RO· and ROO· are shown in Figure 3. It is worth noting that a Zn2+ interacts with the first RO· and the energy of the system is -7.15 eV negative than that of before. When the second RO· is coordinated, the bonding energy is -5.41 eV. The coordination energy of Zn2+ to the third RO· is -1.73 eV. It is conventionally accepted that the energetic of chemisorption is more negative than -0.5 eV.[36] For -1.73 eV is much more negative than -0.5 eV, it is reasonable that Zn2+ might coordinates the fourth RO·. This strongly suggests that the more RO· is interacted with Zn2+, the more positive the system energy is. On the other hand, a Zn2+ coordinates with three ROO· one by one, with the energy of -6.39 eV, -3.12 eV and -1.53 eV respectively. Obviously, Zn2+ possess a great interaction effect on RO· and ROO·. Considering these radicals (RO· and ROO·) with high activity and short lifetime, a Zn2+ nearly cannot coordinate with 2 or 3 radical intermediates. Reasonably, the first coordination of Zn2+ with RO· or ROO· is closer to the actual experiment. The first coordination energy is much more negative, which will affect them to abstract H atom from cumene. Therefore, the coordination effect of Zn2+ affects the reaction (3) and (5). The experimental result shows that there is no oxidation product, so it could be inferred that Zn2+ directly inhibited the steps involved in cumene. Moreover, RO· and ROO· are both produced by R·, the effect of Zn2+ on extraction of H atom from cumene should be also very vital.