Tuned Acidity for Catalytic Reactions: Synthesis and Characterization of Fe- and Al-MFI Zeotypes

Abstract

The addition of elements other than Al in the framework structures of zeotype materials gives the opportunity to tune the acidity of the zeotype and thus might expand the possibilities in the field of acid-catalyzed reactions. Here, we have synthesized silicalite (MFI) with and without Fe and/or Al in the framework structure. To understand the mechanism of the removal of the structure directing agent (SDA) from the as synthesized samples we combine X-ray diffraction, nitrogen sorption and in situ IR spectroscopy experiments. IR spectroscopy is also used to examine the acid species with and without NH3 as probe molecule. Furthermore, NO adsorption is performed to characterize the different metal species. The results of the IR experiments during calcination of the SDA suggest that the SDA first loses contact with the internal surface of the zeotype and then at higher temperature is oxidized to CO2. The IR spectra of the zeotype samples in the H+-form show that the sample containing both Fe and Al shows a broader and less intense peak at 3620 cm−1 indicating the presence of two distinct Brønsted acid sites, one associated to Fe and one to Al. The IR experiments after NH3 adsorption confirm these results and thus the following series of increasing acidity could be outlined: 0 = pure silicalite < Fe-silicalite < Al-silicalite. Finally, the IR experiments after NO adsorption shows the presence of nitrates in the Fe-containing samples suggesting the formation of extra framework Fe species.