Conformation and Protonation of 2,2‘-Bipyridine and 4,4‘-Bipyridine in Acidic Aqueous Media and Acidic ZSM-5 Zeolites: A Raman Scattering Study

Abstract

In situ FT-Raman scattering spectroscopy was used to monitor the sorption kinetics of 2,2'- and 4,4'-bipyridine in acidic ZSM-5 zeolites. The data processing of all the Raman spectra was applied to extract the characteristic Raman spectra of occluded species and respective Raman contribution generated from many spectral data which resolves spectrum of mixture into pure component spectra without any prior information. The assignment of the extracted spectra was performed according to careful comparison with corresponding spectra extracted from a set of Raman spectra recorded during the protonation of 2,2'- or 4,4'-bipyridine (bpy) in hydrochloric acid aqueous solutions. The data processing of the Raman spectra recorded during the slow sorption of 4,4'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of N,N'-diprotonated dication 4,4'-bpyH(2)(2+) as unique species generated in the void space of acidic ZSM-5 zeolites. No evidence of Lewis acid sites was found during the sorption of 4,4'-bpy by Raman scattering spectroscopy. The data processing of the Raman spectra recorded during the slow sorption of 2,2'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of trans-N-monoprotonated cation 2,2'-bpyH+ as major species generated in the void space of acidic ZSM-5 zeolites at loading corresponding to 1 mol per unit cell. The trans/cis interconversion occurs at higher loading even after the complete uptake of the sorbate and indicates some rearrangement in the void space over a long time. The cations were found to be located in straight channels in the vicinity of the intersection with the zigzag channel of the porous materials with the expected conformations deduced from ab initio calculations. However, the motions of occluded species within the channel of ZSM-5 are hindered but remain in the range of the isotropic limit of a liquid at room temperature.

Synthesis, structure and catalytic olefin epoxidation activity of a dinuclear oxo-bridged oxodiperoxomolybdenum(VI) complex containing coordinated 4,4′-bipyridinium

“…The Raman spectrum of 4,4 -bipy exhibits bands at 1000, 1300 and 1604 cm −1 , assigned as ring deformation, ( ring + ı CH ) combination, and coupled ( CC / CN ) modes, respectively [33]. In the Raman spectrum of [4,4 -Hbipy] + , a blueshifted ring stretching band at 1644 cm −1 is characteristic of the protonated ring, while the band at 1603 cm −1 arises from the nonprotonated pyridine ring [34]. Accordingly, only one band is observed for [4,4 -H 2 bipy] 2+ at 1650 cm −1 .…”

High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4′-Bipyridine Co-crystal

“…2. The corresponding peak assignments are listed in Table 1 50–53 . It is obvious that the most prominent Raman bands of SQBP are attributed to BP, with some weak bands originating from SQ.…”

Probing the spontaneous ionization of aromatic amines by adsorption in activated acidic zeolite HZSM‐5

“…The zeolite structural parameters, force field and calculation details used to carry out the minimization procedure of the non-bonding energy were as described previously. 12 The structural parameters and set of atomic charges of the guests were derived from previous structural and theoretical work. 13…”

“…However, the 27 Al MAS-NMR spectra of hydrated H 6.6 ZSM-5 sample provide evidence for a small amount of extra-framework hexacoordinated Al species. 12 The dehydrated acidic ZSM-5 zeolite sample was prepared by dehydration under vacuum at 120°C followed by calcination at 600°C under O 2 for 12 h. The sample was then held under vacuum and cooled to room temperature under dry argon. The crystallinity of the samples was checked by XRD and was not reduced by this treatment.…”