(102b) Cyclooctene Cooxidation-Facilitated Co-ZSM-5-Catalyzed Selective Oxidation of Ethylbenzene with Molecular O2

Solventless activation of benzylic C-H bonds by oxidation is of great interest in the synthesis of various industrial chemicals^[1]. However, common liquid-phase methods to oxidize benzylic C-H bonds either use stoichiometric amounts of organic oxidants or catalysts together with radical forming agents such as NDPI. For greener processes, it is much more desirable if neither of these needs to be used. We report here a new method for selective oxidation of ethylbenzene (EB) that is achieved by cooxidation of cyclooctene (COE) selectively to its epoxide, using O₂. In this process, no products from wasteful reagents are produced.

The solvent-free aerobic oxidation of EB was carried out in a batch reactor under atmospheric pressure at 100ËšC. The highest catalytic activity was achieved when Co-ZSM-5 and a Au-containing initiator solution generated from cyclooctene (COE) epoxidation reaction^[2] were simultaneously present in the system. It was confirmed with a radical quenching experiment that radicals were essential in the reaction. However, EB oxidation did not involve a radical chain mechanism. Quantitative EPR experiment and spin-trap experiment were used to analysis the radical intermediates and to gain mechanistic insight of the reaction. The overall conversion of EB depended on the total amount of COE present in the system, which indicated that the two reactions were coupled together. The data suggested that the primary role of Co-ZSM-5 was to activate cyclooctene-3-hydroperoxide, an initiator in-situ generated by Au-cluster catalyzed-COE epoxidation, to form other radicals that accelerate the EB oxidation. This aspect is quite different from traditional initiators like tert-butyl hydroperoxide. Possible mechanistic explanations are presented.

1. A. M. Hossain et al., Advanced Materials Research. (2015) 1109, p.248-252.
2. Qian, Z. Wang, E. V. Beletskiy, J. Liu, H. J. dos Santos, T. Li, M. do C. Rangel, M. C. Kung, and H. H. Kung. Nat. Commun. (2017) 8, 14881.