Introduction
Liquid-phase oxidation of hydrocarbons with molecular oxygen to high value-added products such as ketone, carboxylic acids, and alcohols is a very important industrial process.[1-3] The liquid aerobic oxidation of cumene is a most crucial reaction, which can produce cumene hydroperoxide (CHP), acetophenone (AP) and 2-phenyl-2-propanol (PP).[4, 5] Those oxidation products are intermediate platform molecules to produce fine synthetic chemicals such as phenol, fragrances, and polyesters. For instance, over 95% of world production of phenol is produced by the conversion of CHP.[6, 7] To pursue high performance catalysts, a great deal of research about catalytic reaction have been done for decades.[8, 9]
The nonmetallic, excellent catalytic performance and high stability of CNTs made them an excellent catalyst for selective oxidation.[10-12] Based on our previous research, we found that CNTs possess excellent catalytic performance for the oxidation of cumene.[11, 13-15] The advantage of CNTs is that oxygen can be activated on the surface of CNTs to generate ROS, which has been reported.[16, 17] The catalytic performance of CNTs was further improved by doping nitrogen.[18]Because doping nitrogen would cause the strong interaction between CHP and CNTs.[15] The main oxidation product of cumene catalyzed by CNTs is CHP.[11, 13, 19] In that way, cumene and CHP are able to react continuously on the surface of CNTs. Moreover, we recently found that the selectivity of the oxidation products would be changed by mixing different metal cocatalysts (CuCl2, FeCl3, MnCl2 and so on) into the system catalyzed by CNTs, especially, the trace amount of CuCl2 could significantly improve the reactivity and regulate product selectivity for cumene oxidation.[14]
Most recently, we added a small amount of ZnCl2 as a co-catalyst and observed a curious result that the oxidation product could hardly be detected at the end of the reaction. To our best knowledge, this is the first time that this inhibition phenomenon has been found. A reasonable guess is that the key steps about the production of CHP were blocked by ZnCl2, while the generation of AP and BP is related to the participation of CHP. Thus, inhibiting the production of CHP leads to no oxidation products at the end of the reaction. To find out how ZnCl2 works in the oxidation reaction of cumene, the interactions between ZnCl2 and key reactants should be considered by DFT. This work is of great significance to reveal the influence of impurity of metal ion on the oxidation of cumene, and to have a deeper understanding of carbon catalyzed radical oxidation reaction.
Experimental