Metal coordination center mediated microenvironment regulation of cobalt sites on poly(ionic liquid)s enhance selective oxidation of cyclohexane conversion

The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oils) without additional initiator remains a challenge because of its inherent low activity and limited selectivity. To promote the selectivity of KA oils at high reaction conversion, a series of novel cobalt-based poly(ionic liquid)s (Co-PILs) catalysts were designed. The electronic structure and the microenvironment of Co sites in Co-PILs were regulated by tuning the contents of Co. By the regulation of the microenvironment, the cyclohexane conversion and KA oils selectivity are improved by 19% and 47% (Co-PIL-1–2.5) compared with traditional metal salt catalysts, respectively. Mechanistic studies demonstrated that the electronic structure and coordination environment of transition metals can be effectively tailored. Notably, Co-PILs with low metal loading exhibit a remarkable ability to improve the reaction by transitioning from a low-spin to a high-spin state. Moreover, by adjusting the ratio of cobalt ions to ionic liquid monomers derived from PILs, the acidity, oxygen activation, and desorption of reactive oxygen species were also regulated accordingly. These properties play a pivotal role in the oxidation process, ultimately allowing for precise control over the activity and selectivity.