Effects of tert-Butyl Halide Molecular Siting in Crystalline NaX Faujasite on The Infrared Vibrational Spectra

Abstract

Experimental, analytical, and modeling techniques employed in this study elucidate interactions between adsorbate molecules and the interior surfaces of the porous host faujasite. The vibrational spectroscopies of guest and host offer opportunities to locate the guest site in the host. We present Fourier transform (FT) infrared (IR) studies of sodium-X (NaX) faujasite supercage-included tert-butyl halides, (CH(3))(3)C-X (X=Cl, Br, I) in comparison with the adsorbate molecular gas-phase and host solid-state spectra at 295 K. Four observations of guest (nu(5,) nu(6), nu(7), and {nu(3), nu(16), nu(17)}) vibrational mode changes, three of them concomitant with host mode changes, together with modeling studies, point to a particular preferred siting of the guest molecules at host hexagonal prisms (D6R). The siting involved simultaneous interactions of the host with methyl group axial protons and the halide atom. All three methyl group axial protons interact preferentially with a single D6R O1 oxygen atom via C-H...O bonding. The halide atom also interacts with a site III' Na cation. The cation, in turn, is coordinated by three O atoms (two O1 and an O4). Two of these O atoms (O1) bridge the double six-rings that form the hexagonal prism part of the NaX substructure. O4 connects the two D6R units.

Infrared vibrational spectra of tert-butyl halides in low-aluminum H-Y Faujasite: Vibrational excitation exchange and other effects of guest–host interactions

“…It will be seen that the weaker framework oxygen-guest interactions, in the absence of supercage cations, give rise to a distinctly different siting in the supercage and a distinctly different pattern of guest-host vibrational mode interactions than were observed for the TBH in Na-X [14]. The weaker interactions in the LAHY host appear to lead to a vibrational mode of the guest molecule interacting with several more-or-less degenerate host modes of comparable frequencies, or with another guest mode, leading to an exchange of quanta of vibrational excitation between guest modes, or between guest and host modes.…”

“…The relationships between the guest functional group interactions with Na-X and the guest-host vibrational mode effects were often very characteristic of the siting of the guest in Na-X, as determined in companion with molecular modeling studies. Thus, the present FTIR and molecular modeling studies of the adsorbed TBH series in LAHY serve to isolate the effect of guest-framework oxygen interactions from the guest-cation interactions that were present, together with the guest-framework oxygen interactions, in this previous study of the TBH series in Na-X [14].…”

“…Two of the studies examine chlorocyclohexanes [11,12] and n-hexane [13] by FT-Raman spectroscopy. These studies do not include a C CH 3 CH 3 X CH 3 TBH systematic study of halogen variation within a single type of organohalogen molecule, as was done previously in Na-X with the tert-butyl halide (TBH) series [14], nor do they constitute a systematic comparison of the organohalide series in LAHY with the series in Na-X, in whose supercages chargecompensating cations are present. It can be said, however, that a unifying theme in these previous LAHY studies is that the site-dependent and orientation-dependent guesthost interactions are generally weaker than is observed in lower Si/Al ratio faujasite zeolites.…”

“…It can be said, however, that a unifying theme in these previous LAHY studies is that the site-dependent and orientation-dependent guesthost interactions are generally weaker than is observed in lower Si/Al ratio faujasite zeolites. It is well known that the three basic effects governing the adsorption of guests in faujasites are guest interactions with cations, guest interactions with framework oxygen atoms facing into the supercages, and competitive interactions of these kinds with adsorbed water [14]. A previous FTIR study of the TBH series in dry Na-X showed that the cation-molecule and framework-molecule interactions lead to specific sitings of the guests in the supercage [14].…”

“…It is well known that the three basic effects governing the adsorption of guests in faujasites are guest interactions with cations, guest interactions with framework oxygen atoms facing into the supercages, and competitive interactions of these kinds with adsorbed water [14]. A previous FTIR study of the TBH series in dry Na-X showed that the cation-molecule and framework-molecule interactions lead to specific sitings of the guests in the supercage [14]. These sitings were shown often to exhibit characteristic mutual perturbations between guest and host vibrational bands that have proximate frequencies.…”