We study the oxidation of NO2- to NO3- by dioxygen in the cages of sodalite, by using the combined blue moon ensemble (BME) and Car Parrinello approaches. Our results indicate the active participation of the zeolite framework in the reaction via peroxy-like defects. Moreover, a molecular level explanation of the experimentally found first-order kinetics is given. A spin-unpolarized density functional approach has been adopted. However, our results suggest that interactions in the zeolite cage may reduce the O-2 triplet-singlet energy gap, therefore, justifying the adopted approximation.