A study of the acid sites in Mn+-ZSM-5 zeolites (Mn+=H+, Al3+, Ca2+, and Ba2+), synthesized by ion-exchange from the NH4+ form, has been carried out using X-ray diffraction, Al-27 MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. Al-27 MAS NMR spectroscopy confirms that Lewis acid sites in all of the Mn+-ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H+-ZSM-5 and Al3+-ZSM-5. Bronsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in Mn+-ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Bronsted acid sites in Mn+-ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in Mn+-ZSM-5 increases in the same order as the acidity of cations with Ba2+

Keywords:Zeolite;Acidity;FTIR