QENS and FTIR studies on binding states of benzene molecules adsorbed in zeolite HZSM-5 at room temperature

Abstract

Fourier-transform infrared (FTIR) spectroscopy and quasi-elastic neutron scattering (QENS) were employed for monitoring of the binding states of benzene molecules, adsorbed in HZSM-5 zeolite at 300 K and for loadings of 0.6 to 7 molecules per unit cell. While the in-plane combination C–C and C–H stretching bands of adsorbed benzene remained unaffected, a splitting was observed in the out-of-plane C–H bending vibrational bands, a feature reported for the transformation of benzene from liquid to solid phase. Also, the intensity ratio of the in-plane C–C stretching band (ν₁₉) of adsorbed benzene at 1479 cm⁻¹ and the bands in the region 3100–3035 cm⁻¹ due to fundamentals and combination C–C and C–H stretching vibrations indicated a trend observed typically for a condensed phase of benzene. No shift was observed in the frequency of the above-mentioned IR bands when zeolite samples exchanged with Na⁺ or Ca²⁺ were employed. QENS results suggest that the benzene molecules occluded in zeolitic pores (∽3 molecules per unit cell) undergo a 6-fold rotation but their translation motion is too slow. Also, a high residence time of 16.5 ps was observed for the benzene entrapped in HZSM-5, compared to a time of ∽2.5 ps reported for the liquid and ∽19 ps for the solid state of benzene. These results reveal again the compression of the benzene molecules on adsorption in zeolitic pores. It is suggested that the benzene molecules confined in cavities experience a strong intermolecular interaction, giving rise eventually to their clustered state depending on the loading. In the clustered state, benzene molecules are packed with their plane parallel to zeolitic walls and interact with each other through π-electron clouds. No electronic bonding is envisaged between these clusters and the framework or the extra-framework zeolitic sites.