Fourier Transform Infrared and Quasi-Elastic Neutron Scattering Investigations on the Binding States and the Dynamics of Benzene Molecules in the Pores of MCM-41 Molecular Sieves

Abstract

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.

Dramatically Enhanced Phenol Degradation on Alkali Cation‐Anchored TiO2/SiO2 Hybrids: Effect of Cation‐π Interaction as a Diffusion‐Controlling Tool in Heterogeneous Catalysis

“…Figure (b‐1) shows differential IR spectra obtained after introduction of gaseous benzene over TiO 2 / m ‐SiO 2 , H + ‐TiO 2 / m ‐SiO 2 and Cs + ‐TiO 2 / m ‐SiO 2 samples measured at room temperature. The bands observed at 1480 and 1471 cm −1 are assignable to the stretching vibration of benzene ring physisorbed and chemisorbed on samples, respectively . Over the series of M + ‐TiO 2 / m ‐SiO 2 samples, the intensity of the latter IR band decreased in the order of H + > Li + > Na + > K + > Rb + > Cs + (Figure (b‐2)), which well corresponds to the desorption temperature of chemisorbed benzene molecules determined from TG analysis.…”

“…The bands observed at 1480 and 1471 cm À1 are assignable to the stretching vibration of benzene ring physisorbed and chemisorbed on samples, respectively. [38][39] Over the series of M + -TiO 2 /m-SiO 2 samples, the intensity of the latter IR band decreased in the order of H + > Li + > Na + > K + > Rb + > Cs + (Figure 4 (b-2)), which well corresponds to the desorption temperature of chemisorbed benzene molecules determined from TG analysis. This result again confirms that the interaction between benzene molecule and the solid surface decreases as the size of alkali metal cation increases.…”

Interaction of benzene with hydroxyl groups in zeolites: A FTIR study of C6H6 and C6D6 adsorption on H-ZSM-5 and D-ZSM-5

“…Due to steric reasons, solvation is not probable with benzene molecules. There are several investigations pointing out that, at high coverage, benzene forms ordered structures in H-ZSM-5 [43,44,58]. We thus speculate that the band at 3175 cm À1 is due to interaction of bridging hydroxyls with benzene oligomers where the joint effect of two or more benzene molecules leads to an enhanced shift of the OH modes.…”

Probing the dynamics and structure of confined benzene in MCM-41 based catalysts

“…The dynamics of benzene in MCM-41 type materials has been investigated previously through both QENS and MD simulations; however, these investigations were limited to MCM-41 and not the catalyst Pt/MCM-41. The value of the diffusion coefficient of bulk liquid benzene has been thoroughly investigated through both experimental [5][6][7][8] and theoretical methods 9,10 and found to have a value of around 2.2 Â 10 À9 m 2 s À1 under ambient conditions. Earlier QENS studies 7,8 of benzene in MCM-41 found no difference between the bulk liquid diffusion coefficient and that of confined benzene.…”