Role of internal coke for deactivation of ZSM-5 catalysts after low temperature removal of coke with NO2

Abstract

By treating a deactivated ZSM-5 catalyst for the conversion of methanol to hydrocarbons with NO2, coke deposits can be removed at around 350 °C, which potentially enables catalyst regeneration at 350–400 °C, which is about 200 °C lower compared to a conventional regeneration in oxygen. To evaluate the regeneration with NO2 at 350 °C, the activity of a used ZSM-5 catalyst was measured after treatment with 1% NO2/He and 0.7% NO2/7% O2/He at 350 °C, and 2% O2/He at 550 °C. After the treatments with NO2 at 350 °C, some activity was restored, but the catalysts showed a fast deactivation. Temperature programmed desorption of ammonia and 27Al MAS NMR measurements indicate that the amount of framework aluminium in the regenerated catalysts is about 60% of that in the fresh catalysts, and some redistribution of the aluminium takes place. Gravimetric temperature programmed oxidation showed that the catalysts still contain 0.3–0.6 wt% coke. GC-MS analysis of the retained species and very high-speed 1H MAS NMR revealed that the remaining coke species are methyl benzenes, which are located inside the micropores of the ZSM-5 zeolite. It is concluded that the deactivation not only depends on the amount of coke, but also on the location of the coke in the catalyst.