Structure-deactivation relationship for ZSM-5 catalysts governed by framework defects

Zeolites are well known catalysts for the conversion of methanol to hydrocarbons. By adjusting the synthesis procedure, the activity and deactivation of the ZSM-5 zeolite in the conversion of methanol to gasoline can be varied significantly. We found that internal framework defects are crucial for the deactivation behavior of the ZSM-5 catalysts. Adsorption of pyridine and 2,4,6-collidine shows that the infrared band at 3745 cm-1 can be assigned to isolated silanol groups on the external surface and the bands at 3726 and 3700 cm-1 to silanol groups inside the zeolite crystals. While the activity is related to the acid site density, the deactivation rate correlates with the measured intensity ratio of the IR bands for internal silanols at 3726 cm-1 and for external silanol groups at 3745 cm-1 (I3726/I3745). The activity and deactivation, therefore, depend on a different physical property, which means that they can be varied independently.