The ESR technique was employed to investigate the reduction ability of ZSM-5 zeolite. The properties of radicals formed by adsorbing $SO_2$ gas on $NH_4$ ZSM-5 zeolite were studied.
Formation of $SO^-_2$ radicals which have three different thermal stability was observed. The spin concentration of $SO^-_2$ radicals increased linearly with the aluminum content of ZSM-5 zeolite indicating that the aluminum is connected with the $SO^-_2$ formation. This suggests that the active site for reduction is the basic site. $SO^-_2$ radicals formed by adsorbing $SO^-_2$ gas on silicalite showed the best thermal stability. The iron existing as impurity may be responsible for producing those $SO^-_2$ radicals. The fact that the intensity of the ESR spectrum of $SO^-_2$ radicals decrease by dealumination indicate that dealumination makes the active sites inactive possibly by extracting the aluminum from framework.
