a Reaction conditions: 10 mL cumene, 25 mL min-1 O2, 353 K, 8 h.
b Oxidation products cannot be detected.
The analysis of oxidation mechanism of cumene oxidation
For decades, autoxidation chemistry has explained the oxidation of hydrocarbons in the absence of a catalyst.[30-33] Now, it has been proposed to explain the oxidation mechanism of cumene with CNTs as catalysts, as shown in Figure 1. The reactions (1-5) that in network are the major reaction of cumene.[32, 34] The initial reaction (1) can be a unimolecular reaction or catalytic reaction. If it is the former, not only is this unimolecular dissociation very slow, but also the radical-pair (cumenyl radical (R·), H·) quickly recombines because of the effect of solvent cage.[35] It has been reported that CNTs can activate O2, resulting in ROS catalyzing the reaction (1). Then the R· radical combines with O2 to produce the cumyl peroxy radicals (ROO·) (reaction 2). ROO· is a reactive radical can abstract a H atom from cumene to produce CHP and R· (reaction 3). Once CHP is produced, the CHP adsorbed on CNTs tends to decompose into cumenyl oxygen radical (RO·) (reaction 4), which can abstract a H atom from cumene to produce R·. The formed R· will go on enter the reaction (2). Combined with the above, there are three ways to the generation of crucial R· in reactions (1, 3, 5) involving cumene. In these three reactions, the reactive species (ROO·, RO·, ROS) induce abstraction of H atom. It is possible that Zn2+inhibits the reactivity of those species, thus completely inhibits the oxidation of cumene. To prove the point, the interaction of Zn2+ and those reactive substrates (ROO·, RO·, ROS) are calculated by DFT.