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 CÈC and CÈH stretching bands of adsorbed benzene remained una †ected, a splitting was observed in the out-of-plane CÈH bending vibrational bands, a feature reported for the transformation of benzene from liquid to solid phase. Also, the intensity ratio of the in-plane CÈC stretching band of adsorbed benzene at 1479 cm~1 and the bands in the region (l 19 ) 3100È3035 cm~1 due to fundamentals and combination CÈC and CÈH 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 (D3 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 D2.5 ps reported for the liquid and D19 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 conÐned 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 p-electron clouds. No electronic bonding is envisaged between these clusters and the framework or the extra-framework zeolitic sites.

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 of benzene, localized at intersections and in the straight channels of NaZSM-5, respectively. While the frequency of these bands remained unchanged, the intensity of the lower frequency side pair (i.e., 1969, 1956 cm-1 and 1831 and 1810 cm-1) was found to be very sensitive to the nature of the charge-balancing cation and followed a trend NaZSM5 < CaZSM5 > SrZSM5 > BaZSM5, similar to that followed by the pore volume of exchanged samples. These two pairs of bands are therefore identified with the benzene clusters encapsulated in straight zeolitic channels where most of the balancing cations are located. Dose-dependent measurements have shown that such benzene clusters may form at loading as low as ∼1.6 molecules/uc; when a larger fraction is located at intersection sites and at the same time a small fraction also exists in the straight or sinusoidal channels. The concentration in the later locations grows with the increasing benzene loading. Considering these results and in view of the fact that no frequency shift or band splitting was observed in the in-plane C−H/C−C and fundamental ν19 stretching vibrations of adsorbed benzene, we infer that the benzene molecules are packed side by side with their planes parallel to the zeolite channel, the intermolecular interaction occurring through π-electron cloud.

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The in-plane C−H/C−C, out-of-plane C−H, and the fundamental ν19 C−C stretch vibrations of benzene molecules were monitored by FTIR spectroscopy, for adsorption in MCM-41 at room temperature and at different loadings. The results were compared with the corresponding data on ZSM-5 zeolites. The frequency and the relative intensity of IR bands point to the development of a compressed state of benzene in the pores of MCM-41, the density of which depended upon loading and lay in general between that of bulk liquid and a solidlike phase formed in the pores of ZSM-5 zeolite on benzene adsorption. Quasi-elastic neutron scattering results reveal that the pore characteristics play a crucial role in deciding the dynamics of benzene molecules in a confined medium. Thus, in contrast to the molecular motions of benzene in ZSM-5 zeolite where only rotational motion was observed in the instrumental time window of 10-10−10-12 s, the benzene molecules adsorbed in MCM-41 exhibit only translational motion. Further, the value observed for the translational diffusion constant (D = 2.18 × 10-5 cm2/s) of benzene occluded at saturation in MCM-41 confirmed the existence of its condensed state. Because of the unhindered mobility in mesopores, a smaller fraction of benzene gets occluded in MCM-41 compared to ZSM-5 zeolite, under the identical conditions of loading. Also, the parallel studies using MCM-41 with different Si/Al ratios and with different charge balancing cations rule out the possibility of any specific coordination of the occluded benzene with the framework sites of molecular sieves. The results are explained in terms of the current theories of capillary condensation of fluids on confinement in narrow pores.

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