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

Fourier-transform infrared (FTIR) spectroscopy and quasi-elastic neutron sc attering (QENS) were employed for monitoring of the binding states of benze ne molecules, adsorbed in HZSM-5 zeolite at 300 K and for loadings of 0.6 t o 7 molecules per unit cell. While the in-plane combination C-C and C-H str etching bands of adsorbed benzene remained unaffected, a splitting was obse rved 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 int ensity ratio of the in-plane C-C stretching band (nu (19 nu)) of adsorbed b enzene at 1479 cm(-1) and the bands in the region 3100-3035 cm(-1) due to f undamentals and combination C-C and C-H stretching vibrations indicated a t rend observed typically for a condensed phase of benzene. No shift was obse rved in the frequency of the above-mentioned IR bands when zeolite samples exchanged with Na+ or Ca2+ were employed. QENS results suggest that the ben zene molecules occluded in zeolitic pores (D similar to3 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 entrapp ed in HZSM-5, compared to a time of similar to2.5 ps reported for the liqui d and similar to 19 ps for the solid state of benzene. These results reveal again the compression of the benzene molecules on adsorption in zeolitic p ores. It is suggested that the benzene molecules confined in cavities exper ience a strong intermolecular interaction, giving rise eventually to their clustered state depending on the loading. In the clustered state, benzene m olecules are packed with their plane parallel to zeolitic walls and interac t with each other through pi -electron clouds. No electronic bonding is env isaged between these clusters and the framework or the extra-framework zeol itic sites.