Studies of hydrogen held by solids. Part 11.—Interaction of simple olefins and pyridine with decationated zeolites

Abstract

Three OH stretching bands, with maxima at about 3745,3650 and 3545 cm-1, are present in infrared spectra of decationated X-and Y-type zeolites. Pyridine reacted with the 3650 cm-1 species to form pyridinium ion, This band underwent a hydrogen-bonding shift when ethylene was adsorbed at room temperature, indicating a strong physical adsorption which was quantitatively and rapidly reversible on raising or lowering the pressure. NO exchange occurred between C2H4 and the OD groups of an exhaustively deuterated zeolite. The hydrogen-bonding shift from 3650 to 3300 cm-1 was identical when C2D4 was substituted for CzQ. The 3745 and 3545 cm-1 bands were not noticeably altered by the adsorption, at least in the pressure region where the 3650 cm-1 band was affected. Adsorption isotherms, calculated from the spectra, could be superimposed on those determined by volumetric techniques. Both were close to Langmuir-type but showed evidence of a superimposed non-selective adsorption at higher pressures. Ethylene may also adsorb, via a charge-transfer interaction, on the residual Na+ ions of the zeolite. At about 500 torr, the total adsorption of C2H4 was about 30 % greater than the available zeolite hydroxyl groups. It was inferred that the strongest interaction was with the acidic hydroxyl groups and that, at low pressures, most of the adsorbed C2H4 was hydrogen-bonded to these. In the CH bending and stretching regions, only the Q-branch of the spectra of the gaseous molecules appeared, indicating that molecular rotation was hindered in the adsorbed state. Qualitatively similar results were obtained with C3H6. The hydrogen-bonding shift was greater by about 100 cm-1, however, and the adsorption was not entirely reversible. Moreover, the irreversible portion increased with time, and products stemming from polymerized olefin could be recovered ; also, c3D6 exchanged with the OH groups of the 3650 and 3545 cm-1 bands, but not with those of the 3745cm-1 band. Preliminary results for these substrates adsorbed on dehydroxylated, decationated zeolites are given.

Characterization of Zeolites — Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction

“…Liengme and Hall [101] adopted this method to investigate the Br0nsted acidity and its conversion to Lewis acidity in faujasite type zeolites. Figure 4.19 shows, as an example, the IR bands of pyridine adsorbed on hydrogen Y-zeolite (faujasite structure), hydrogen mordenite and sodium mordenite [102].…”

“…Other bands, sometimes preferred for investigation of sites indicated by pyridine, appear at 1620 and 1632 cm-1 (mode Sa, sb of the pyridine ring vibration, see [56]), the first resulting from both B-and L-sites, the latter one being specific for B-sites. The complete spectra for pyridine adsorbed on hydrogen-faujasite (H-Y) and hydrogen-mordenite (H-MOR) are, for instance, provided in [5,101] and [56], respectively, where the assignments are also discussed in more detail. The position of the bands typical of pyridine attached to cations depends on the Coulomb potential, i. e., the frequency increases with increasing charge and decreasing radius of the respective cation (vide infra; compare Fig.…”

Intensities of combination IR bands as an indication of the concerted mechanism of proton transfer from acidic hydroxyl groups in zeolites to the ethylene hydrogen-bonded by protons

“…In addition, although ethylene adsorption on both sodium and hydrogen forms of the zeolites starts with formation of similar π-complexes, for the sodium forms it does not result in subsequent chemical activation and oligomerisation. In contrast, ethylene π-bonded to acidic protons is iniitially transformed into surface ethoxy groups and then involved in subsequent oligomerization with formation of the growing polyethylene chains anchored to the surface of the zeolites [4,5]. Therefore, such differing ethylene activation by the hydrogen and sodium forms could be also expected to affect the IR spectra of the π-bonded species.…”

The effect of dehydroxylation of HNaY zeolites on the interaction with ethylene and propylene

“…Thus the transition state of the isotopic exchange differs from that one which stimulates the oligomerization of ethylene; this fact was noted by the authors of ref. 1 on the basis of other experimental data.…”