Comparison of the acidic properties of ZSM-5 zeolites isomorphously substituted by Ga,In, B and Fe

The acidic properties of isomorphously substituted MFI-type zeolites prepared and pretreated in the same way were studied to obtain an exact gradation of the intrinsic acidity caused by the incorporation of Ga, Fe, In and B instead of Al into the framework. The nature, strength and concentration of the acidic sites were studied by temperature-programmed desorption of ammonia (TPDA) and Fourier transform infrared spectroscopy. The assignment of the two peaks observed in the TPDA plots of all zeolite samples required the additional registration of the decomposition plot of the ammonium form in the cases of In-ZSM-5 and B-ZSM-5. A temperature program with two ramps and an intermediate isothermal hold was used to separate the low-temperature peak from the high-temperature peak. In this way, the Brönsted sites could be determined more clearly on samples containing a high portion of non-framework species that inhibit the egress of ammonia from the pores. The nature and the strength of the acidic sites were spectroscopically characterized by recording the stretching vibration of the bridged OH groups of Brönsted sites and the spectra of pyridine adsorbed on Brönsted and Lewis sites. The intrinsic acidity of the Brönsted sites on In-ZSM-5 could be clearly classified to be intermediate between the acid strength of B- and Fe-ZSM-5. A relation could be revealed between the frequencies of the OH-stretching vibration and the temperature of the peak maxima of ammonia desorption from the Brönsted sites of the isomorphously substituted ZSM-5 samples.