Stanislav Beran

Solid-state incorporation of Mn2+ ions in H-ZSM-5 zeolite

The high-temperature interaction between H-ZSM-5 zeolite and solid MnCl2, MnSO4, Mn(CH3CO2)2 and Mn3O4 has been studied by temperature-programmed desorption of ammonia, IR, EPR and mass spectrometry. It has been shown that the degree of solid-state ion exchange for MnCl2 is strongly affected by the temperature of the heat treatment. Depending on the amount of manganese cations present in the zeolite–salt mixture, at 770 K an exchange degree of 60–85% can be obtained. The solid-state reaction of MnSO4 at the same temperature leads only to 20% exchange of Mn2+ ions, while that of Mn3O4 results in ca. 50% exchange of Mn2+. Owing to its decomposition above 520 K to Mn oxides, Mn(CH3CO2)2 behaves similarly to these oxides; only part of the bridging OH groups are replaced by Mn ions. For all the Mn compounds studied, the solid-state reaction, resulting in replacement of protons of acidic skeletal OH groups by Mn2+ ions, appears to take the same course: most of the Mn2+ ions are exchanged in the initial stage of the reaction and then the reaction rate considerably decreases, levelling off to zero.

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.

Chromium ions within zeolites. Part 2.—A quantum-chemical study of the properties of chromium ions in faujasites

The CNDO (complete neglect of differential overlap) method is used for calculation of the physico-chemical characteristics of faujasites with CrII, CrIII, CrV and CrVI ions in the SII cation position, modelled by Si3Al3O6(OH)12Cr clusters. It is demonstrated that the Cr ions are bonded to the zeolite skeleton by electron donor–acceptor bonds whose formation leads to electron donation from the skeleton to the cation, thus stabilizing the valence states of the Cr ions. The most probable valence state of the Cr ions in the zeolites was found to be CrIII; however, it can readily be oxidized to CrV. In contrast, CrVI and CrV ions were found to exhibit high electron-acceptor ability and thus are readily reduced.

Automatic geometry optimization: study of the AIX3 and Al2X6 (X = F, Cl, Br, I) systems by the eht method and by the EHT method with electrostatic correction

Abstract The automatic optimization technique using analytical first derivatives of the potential energy was used to study the structural characteristics and energetics of the AIX 3 , monomers and the Al 2 ,X 6 dimers in the framework of the standard EHT method and of its modification with electrostatic corrections as recently suggested by Anderson [10]. It is demonstrated that the latter technique does not significantly improve the results compared to those obtained by the conventional EHT method. Marked nonrigidity of the AlXAIX ring in the AI 2 X 6 dimers was found.

Molecular orbital study of the catalytic oxidation of propylene on silver

Analysis of the MINDO/2 reaction coordinates concerning the decomposition of the propylene perepoxide radical anion to propylene oxide and propylene peroxide makes it possible to explain the fundamental differences of ethylene and propylene oxidation on silver catalyst.AbstractАнализ МУПДП/2 реакционных координат разложения анион-радикала перэпоксида пропилена (дающий эпоксид пропилена) дает возможность обБяснить основные различия между окислением этилена и пропилена на серебрянном катализаторе.

The Effect of Interacting Molecules and Cations on the Properties of Hydroxyl Groups in Zeolites: A Quantum Chemical Study

Non-empirical STO-3G quantum chemical calculations on the H 3 AlO X H * SiH 2 O2H A1H 3 model of the zeolite structure (X = H + , Li + or Na + ) were used to determine the effect of (i) ion exchange on the O * H group and (ii) the interaction of the group with CO, H 2 O and NH 3 molecules on the properties of an adjacent OH group. We have found that both lead to a decrease in the acidity of that group. The effect becomes more marked with the decreasing electronegativity of the cation and with the increasing basicity of the interacting molecule. The effect of the interacting molecule is always less important than the effect of cation exchange.

Quantum Chemical Study of the Characteristics of Molecules Interacting With Zeolites

Abstract The CNDO/2 method was used to study the physical characteristics of interaction complexes of carbon monoxide and water molecules with the Li, Na, Mg, Ca and Al cations situated in the S II cationic positions of faujasites, modelled by T 6 O 6 (OH) 12 clusters (T=Si or AI). Calculation demonstrate that formation of such complexes brings about transfer of electron density from the molecule to the cationic centre and, particularly, polarization of the molecule. Both these effects then result in a strengthening of the C-O bond of CO and in a weakening of the O-H bonds of water, depending on the cation type. Simultaneously, it was found that correlation can be drawn between the calculated bond strength of interacting molecules and the stretching vibrational frequencies observed for adsorption complexes of these molecules.

Quantum Chemical Study of the Properties Of Fe, Co, Ni and Cr Ion- Exchanged Zeolites

Abstract The CNDO/2 method is employed in studying the properties of Cr(II), Cr(III), Cr(V), Cr(VI), Fe(II), Fe(III), Co(II), Co(III), Ni(I), and Ni (II) cations localized in the cationic positions of zeolites modelled by clusters T 6 O 6 (OH) 12 -cat (T = Si or Al). It is shown that a strong donation of electron density from the zeolite skeleton to the cations depending on the type and valency of the cation takes place. This donation of electrons compensates the changes in the cation properties caused by the acception or donation of electrons and influences the cation redox properties, as well as other characteristics of clusters.

Electronic spectral study of the interaction of CoNaX zeolite with C1−C5 hydrocarbons

Interaction of both satured and unsaturated hydrocarbons with tetrahedral Co2+ ion complexes lodged in the zeolite skeleton is reported on the basis of diffuse reflectance spectra. While saturated hydrocarbons do not affect the coordination sphere of the Co-complex, the unsaturated hydrocarbons change considerably the distortion of the tetrahedral Co-complex.AbstractПриведены результаты изучения спектров диффузионного отражения при взаимодействии насыщенных и ненасы-щенных углеводородов с тетраэдрическими Co2+ ком-плексами внедрёнными в скелете цеолита. В то время как насыщенные углеводороды не влияют на координа-ционную сферу Co-комплексов, ненасыщенные углево-дороды сушественно изменяют искажение тетраэдри-ческого Co-комплекса.