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

Abstract

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.

Adsorption and diffusion of benzene in the nanoporous catalysts FAU, ZSM-5 and MCM-22: A molecular dynamics study

“…Therefore, it is not unreasonable that this loading is indeed close to the saturation limit. Similar values were found or discussed in previous works [34][35][36][37].…”

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

“…22 This is in agreement with our contention that in their occluded state, the hexagonal plane of benzene molecules is parallel to the walls of MCM pores, causing no direct binding to the internal hydroxy groups. 18 The results of our QENS and IR studies are in line with the process of capillary condensation of fluids in narrow pores. It is well reported that the average density of the capillary gas is usually larger than that of the bulk gas.…”

“…Further, the width of this band is found to be ∼12.7 cm -1 in the case of NaZSM-5, which is in conformity with the reported behavior of benzene molecules in the pores of ZSM-5 zeolites. 18,19 5. As discussed in our earlier publications, the increase in intensity ratio of the fundamental vibration (ν 19 ) at 1479 cm -1 and the other in-plane vibrations at 3092, 3071, and 3039 cm -1 is a feature associated with the transformation of benzene to a more condensed state.…”

Optimizations of polyester degradation process of waste copper wire for the application of Cu recycling

“…Figure b shows the FTIR spectra of heat‐treated polymer at 969 V. This spectrum shows characteristic peak at 3650–3730 cm −1 because of absorbed water , and the peak around 2150 and 2000 cm −1 for the strong ketene carbonyl stretch absorption band of Dioxine or C–H out of plane bending vibrations band for benzene or ethylene . Another peaks around 663 cm −1 may be because of C=C aromatic torsion and deformation or trans unsaturation bending of CH=CH . And rest of the peaks for polyester were removed to indicate that polyester has been removed successfully after heat treatment.…”

“…Based on this comparison, the polymer coated on WCW was found to be polyester. Figure 2b shows the FTIR spectra of heat-treated polymer at 969 V. This spectrum shows characteristic peak at 3650-3730 cm 21 because of absorbed water [20], and the peak around 2150 and 2000 cm 21 for the strong ketene carbonyl stretch absorption band of Dioxine or C-H out of plane bending vibrations band for benzene or ethylene [21,22]. Another peaks around 663 cm 21 may be because of C¼C aromatic torsion and deformation [16,19] or trans unsaturation bending of CH¼CH [24].…”