Efficient oxidation of hindered amines to nitroxide radicals with hydrogen peroxide over layered double hydroxide

Abstract

Alkoxylation of hindered amine light stabilisers is a common strategy to reduce their basicity and expand their application scope. However, the current catalysts used for preparing the nitroxide radical intermediates are limited by their separation and recycling properties. To address this issue, this study proposes a method for the preparation of hindered amine nitroxide radical intermediates using hydrogen peroxide as the oxidant and layered double hydroxides (LDHs) as heterogeneous catalysts. Excitingly, when using only 5.00 wt% LDHs with a Mg/Al ratio of 3 : 1, both the conversion of the hindered amine and the selectivity towards the corresponding nitroxide radical exceeded 99.9%. Moreover, the catalyst was recyclable and could be reused for more than five cycles without significant loss of its activity. Theoretical calculations and radical trapping experiments revealed that LDHs act as solid bases to promote the decomposition of hydrogen peroxide into reactive oxygen species (ROS), thereby synergistically oxidizing hindered amines to nitroxide radical intermediates. The key influencing factors for the catalytic performance were found to be the types and proportions of metal ions in the LDHs. This study has provided a new pathway for the oxidation of hindered amines to the corresponding nitroxide radicals.