Zdenka Tvarůžková

Monitoring of skeletal T-O-T vibrations of metal ion exchanged zeolites : An attempt at quantitative evaluation

Abstract The ratio of the intensity of the T–O–T band perturbed by metal ion and the total antisymmetric T–O–T band measured by FTIR spectroscopy is evaluated for cobalt exchanged ferrierites. The limits of the method for detection of exchanged metal cations in zeolites with low content of Al (Si/Al>6), as related to published data, are given.

Chromium ions in zeolites. Part 4.—X-ray photoelectron spectroscopic study of chromium valence states in the surface layers of CrY zeolites

X-ray photoelectron spectroscopy has been used to elucidate the state of chromium in the surface layers of CrHY and stabilized CrSY zeolites. Values of both the binding energy (EB) and the spin–orbit splitting (ΔE) of the Cr 2p level for chromium in the zeolite differ from those of bulk chromium oxide compounds or those of chromium supported on silica. The counter-CrIII ions in the surface layers of Y zeolites are reduced by zeolite heat treatment both in carbon monoxide and in vacuo to CrII, unlike the CrIII ions in the zeolite bulk, which are not self-reduced. The stable isolated CrV and CrVI ions present in the oxidized zeolite bulk were not detected by X.p.s. in the surface layers because of their reduction in vacuo and/or by X-ray irradiation in the ESCA system.

Cu coordination in zeolitic matrix. Relationship to nitric oxide binding and decomposition

Cu + luminescence spectra reflecting Cu siting and IR study of adsorbed NO reveal that single Cu ions in zeolite matrices of ZSM-5, Y and erionite form Cu 2+ and Cu + mononitrosyl charge transfer complexes, while (-Cu-O-)n species formed in ZSM-5 yield in adittion Cu + dinitrosyl complexes. From a comparison of the NO decomposition activity of the Cu zeolites of various structures with these findings it follows that NO can be decomposed also on the single Cu ions, however, the presence of (-Cu-O-)n species bearing an extralattice oxygen and possessing close Cu ions contribute to the unique and stable activity of the Cu-ZSM-5 zeolites in nitric oxide decomposition.

Contribution of framework and extraframework Al and Fe cations in ZSM-5 to disproportionation and C3 alkylation of toluene

Publisher Summary This chapter describes contribution of framework and extra framework A1 and Fe cations in ZSM-5 to disproportionation and C3 alkylation of toluene. Toluene disproportionation and its C3 alkylation with isopropanol have been investigated over series of alumo- and ferrisilicates of ZSM-5 structure. It has been shown that the reaction rate for toluene disproportionation is linearly proportional to the number of both Si-OH-A1 and Si-OH-Fe groups, the latter sites exhibiting a lower activity in agreement with their lower acid site strength. A simultaneous presence of Lewis and Broensted sites does not significantly affect toluene disproportionation, but enhances toluene dealkylation, leading to a higher selectivity to benzene. On the contrary, while the rate of toluene disproportionation is controlled by the molecular sieve intrinsic activity, for toluene C3 alkylation the reaction rate is controlled by the rate of the product desorption, which is assumed to be faster on ferrisilicates, because of a lower acidity of the Si-OH-Fe groups. Therefore, all the ferrisilicates exhibit a higher toluene conversion in the alkylation reaction compared to alumosilicates.

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts

Abstract An in situ high-pressure/high-temperature FTIR spectroscopy technique was used for analysis of water adsorption on ZSM-5 and ferrierite zeolites. Adsorbed water in zeolite channels is present in neutral hydrogen-bonded adducts at low coverage, whereas at higher coverage protonated species H + (H 2 O) n are formed. The protonated species are easily stabilized in narrow zeolite channels and at higher density of aluminium atoms in the zeolite framework. Moreover, formation of hydroxonium ions is supported by increasing overall pressure. Conditions combining the temperature and pressure for manifestation of both forms of water adsorption have been analyzed.

Chromium ions within zeolites. Part 3.—The influence of the valence state of chromium on the catalytic activity of Cr zeolites in ethylene polymerization

The rate of ethylene polymerization on CrHY and Cr-stabilized Y zeolites has been measured in a static apparatus by the increase in the weight of zeolite with time. The ethylene polymerization depends particularly on the valence state of the Cr ions localized in the cation position of the zeolite. A further necessary condition for ethylene polymerization is a vacant coordination sphere in the isolated Cr ions and their localization in accessible sites. The zeolite catalytic activity increases with increasing electron-acceptor ability of the Cr ions, i.e. from CrII to CrIII and higher valence states. However, the highest catalytic activity of Cr zeolites is not connected with the presence of CrVI but with CrV ions.

N2O decomposition on iron exchanged ferrierite. A combined periodic DFT and static IN-SITU FTIR study

Abstract The decomposition of N 2 O on the iron exchanged ferrierite zeolite was studied computationally employing a periodic DFT approach and experimentally using a static and in-situ FTIR method. The calculations yielded the reaction energies and barriers for the entire course of the N 2 O decomposition. We used three distinctive cationic sites (Alpha, Beta-1 and Beta-2) to study the effects of the local arrangements of the Fe active site on the course of the decomposition. We focused on the dual role of NO in the N 2 O decomposition and furthermore on the dependence of the N 2 O decomposition on the NO concentration as well as on the nature of the iron-zeolite.