FTIR Study on Molecular Motions of Benzene Adsorbed in ZSM-5 Zeolite: Role of Charge-Balancing Cations and Pore Size

Occlusion of benzene in NaZSM-5 zeolite is investigated using in situ FTIR spectroscopy as a function of substitution of Na+ with group IIA cations. At least four pairs of overlapping vibrational bands were observed in the region of out-of-plane C−H bending vibrations (2000−1800 cm-1) on adsorption of benzene in NaZSM-5 at room temperature. Whereas two pairs of these bands, e.g., one pair at around 2007 and 1986 cm-1 and the other at 1969 and 1956 cm-1, correspond to the 1960 cm-1 band of liquid benzene, the other two pairs, e.g., at 1874 and 1852 cm-1 and 1831 and 1810 cm-1, appear in place of the 1815 cm-1 band of liquid benzene in this region. No measurable difference was observed in the frequencies of these bands for adsorption in cation-exchanged samples, suggesting that any specific interaction between cations and benzene molecules is small compared to the effect of benzene−benzene interaction. These multiple bands are therefore attributed to the existence of at least two distinct clustered states o...