Benzene in Zeolite ZSM-5 Studied by Diffuse Reflectance Infrared Spectroscopy

The behavior of benzene molecules in ZSM-5 has been studied in detail with the use of diffuse reflectance FT-IR spectroscopy. The infrared difference spectra indicate that benzene molecules have a high selectivity for adsorption first on the most energetic (Brönsted acid) ZSM-5 sites. The existence of at least two adsorbed benzene species has also been confirmed by infrared spectroscopy. Deuterated benzene (C 6 D 6 ) species located at the ZSM-5 channels and channel intersections are characterized by the C-D stretching vibrations at 2284 and 2276 cm −1 , respectively. Since the high symmetry ( D 6h ) of the adsorbed benzene molecules is preserved, the framework of ZSM-5 may have been distorted. The distortion of the framework of ZSM-5 is also indicated by the red shift of the Si-O (or Al-O) asymmetric stretching vibrations (at ∼1215 cm −1 ) when adsorbed benzene molecules are present.