FT-IR Studies of the Interaction between Zeolitic Hydroxyl Groups and Small Molecules. 3. Adsorption of Oxygen, Argon, Nitrogen, and Xenon on H−ZSM-5 at Low Temperatures

Abstract

Adsorption of O 2 , Ar, N 2 , and Xe on H-ZSM-5 at low temperatures has been studied by transmission Fourier transform infrared spectroscopy. These molecules form 1:1 hydrogen-bonded complexes with the acidic OH groups in H-ZSM-5, which is characterized by the downward shift and the broadening of the ν(OH) band due to the acidic OH groups. The accompanying behavior of the adsorbed species has been examined simultaneously for the case of the O 2 and N 2 adsorption; their absorption bands have been observed at 1550 cm -1 for the dioxygen species adsorbed on Brønsted acid sites (BAS) and at 2334 and 2352 cm -1 for the dinitrogen species on BAS and Lewis acid sites (LAS), respectively. The broadening of the ν(OH) band caused by Xe is much larger than those by the other gases, which is tentatively associated with the large polarizability of Xe. The degrees of the shift of the acidic-OH band caused by the adsorption of these molecules were systematically in the order of H-ZSM-5 > H-MOR > H-Y. Using these data, the correlation between the shift and the acidity of the zeolites has been discussed.

Brønsted Acidity in Metal–Organic Frameworks

“…158,159 Besides, it allows one to see the vibrational frequencies of hydroxyl groups, which are not readily identified by other techniques. Since hydroxyl bands have characteristic frequencies in the IR, one can determine their mode of interaction with other molecules, 160 which renders this a good means of characterization for the presence of Brønsted acidity.…”

Selective conversion of methanol to propylene over highly dealuminated mordenite: Al location and crystal morphology effects

“…After dealumination, the band at 3659 cm −1 diminished, indicating that the extra-framework aluminum species were removed, in agreement with the results from 27 Al MAS NMR spectra. Meanwhile, the 3500 cm −1 band assigned to internal silanol nests appeared [41,42], due to the removal of framework Al atoms. The 3610 cm −1 band due to the bridging Si(OH)Al hydroxyl groups decreased significantly in 6)), implying the presence of framework Al species.…”

Molecular orbital model of the influence of interaction between O2 and aluminosilicate sites on the triplet–singlet energy gap and reactivity

“…Silanol groups on amorphous silica and in H-form zeolites have been reported to form H-bondings [28] that represent terminal hydroxyls at the surfaces or defect sites. Therefore, in order to study the nature of terminal silica hydroxyls, the silanol models were used.…”

“…The association has little effect on the geometry of the model and the O1-O2 distance is not significantly affected. Wakabayashi et al [28,29] report that the interaction of O 2 with the bridging hydroxyl groups would occur through O 2 in preferably linear orientation and it is rather weak. At the same time, the frequency of the n(O-O) is not significantly changed after the association.…”

“…Several reports of fundamental absorption data of O 2 in cation exchanged zeolites [22][23][24][25] and in H-form zeolites [26,27] have been published. Oxygen sorption on the pores of HY, H-ZSM-5 and H-MOR has been studied by FT-IR and used to determine which Brønsted acid sites are accessible to oxygen molecule [26,28,29]. O 2 has been used as a shift reagent to prove accessibility of cations and protons in zeolite by Liu et al [26].…”