Spectroscopic study of surface properties of various NH4-exchanged X zeolites

Abstract

The nature of hydroxyl groups formed during deammoniation of three NH4-X zeolites has been investigated in terms of their stability during dehydroxylation and their acidity towards bases like pyridine. It appears that besides the two principal bands at 3660 and 3560 cm-' always observed in spectra of decationated X and Y zeolites, a third band is present at 3625 cm-l, the intensity of which depends on the percentage of Na+ ions exchanged and, for a given sample, on the outgassing temperature. This band is attributed to hydroxy1 groups which show acidic properties, are very accessible to large molecules, and are therefore probably located inside the large cavities of the network, but which are very sensitive to dehyroxylation at relatively low outgassing temperatures.

A Variable-Temperature Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study of the Binding of Water and Pyridine to the Surface of Acid-Activated Metakaolin

“…Consequently, there were no appreciable quantities of either isolated, terminal AlOH or SiOH, which exhibits at band at 3745 cm -1 , groups in the AAMK structure. In contrast, the center of the fitted band at 3730 cm -1 correlates well with the presence of hydroxyl groups bridging between two octahedrally coordinated Al atoms, which are routinely observed in Al 2 O 3 , SiO 2 /Al 2 O 3 , and γ-Al 2 O 3 . , The band at 3699 cm -1 was in excellent agreement with that observed at 3700 cm -1 in the spectrum of zeolite X heated at 400 °C, while the fitted band at 3655 cm -1 agreed well with that observed for AlOH groups in zeolite X 36 and H−ZSM-5 at 3660 and 3662 cm -1 , respectively. The band near 3650 cm -1 in zeolites X and Y disappears upon exposure of the zeolite to pyridine vapor and is accompanied by the appearance of the diagnostic band for the pyridinium ion at 1540 cm -1 .…”

Unusual Acid Sites in LSX Zeolite: Formation Features and Physico-Chemical Properties

“…When the sample is heated up to 200 °C, the absorbance band at 3650 cm −1 appears in the spectrum. This band corresponds to bridged OH groups localized in supercages [ 70 , 71 , 72 , 73 , 74 , 75 , 76 ]. This is consistent with N 2 adsorption; the am -200 sample is characterized by the largest adsorption capacity due to the replacement of relatively large and heavy alkaline cations with small and light protons in the SIII positions.…”

“…This is consistent with N 2 adsorption; the am -200 sample is characterized by the largest adsorption capacity due to the replacement of relatively large and heavy alkaline cations with small and light protons in the SIII positions. An increase in the temperature above 200 °C results in the manifestation of the second band at 3550 cm −1 , which corresponds to –Si–O(H)–Al– groups in sodalite cages [ 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 ].…”

Dehydroxylation and Rehydroxylation Mechanisms in Fired Clay Ceramic: A TG‐MS and DRIFTS Investigation

“…20) and from 3609 cm À1 in H-ZSM-5 (Ref. The interpretation of the OH stretching frequencies of ceramic materials becomes more complicated as the number of metallic elements increases.…”

“…The reported OH stretching frequencies for SiOHAl in zeolites range from 3625 cm À1 in zeolite X (Ref. 20) and from 3609 cm À1 in H-ZSM-5 (Ref. 21).…”