V.B. Kazansky

Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons

Abstract Ab initio quantum chemical calculations indicated that adsorbed carbenium and carbonium ion active intermediates of acid-catalyzed transformations of hydrocarbons on zeolites are not the really existing highly reactive species but the transition states of the corresponding elementary steps. Adsorbed carbenium ion-like activated complexes can be formed both via proton addition to the double bonds of olefins or as energetically excited unstable ion pairs resulting from partial dissociation of the carbonyl bonds in more stable alkoxy species. In contrast, the highly energetically excited adsorbed carbonium ion-like transition states result only from proton attack at the C–C or C–H bonds of paraffins. The quantum chemical calculations provided the information on geometry and electronic structure of these activated complexes which depend on the elementary reactions in which these transition states are involved. The calculated heat effects and activation energies for the main elementary steps in acid catalyzed transformations of hydrocarbons on zeolites, i.e. of double bond shift, skeletal isomerization and cracking of olefins or protolytic dehydrogenation, protolytic cracking of paraffins and hydride transfer from isoparaffins to carbenium ions are in a reasonable agreement with the experiment.

Spectra of hydroxyl groups in zeolites in the near-infrared region

Diffuse reflectance ir spectra of hydroxyl groups in decationized and cationic forms of zeolites, X, Y, and mordenite have been studied for the first time in a wide spectral range, including the fundamental stretching vibrations of OH bonds, their overtones and combination bands. Surface OH groups of amorphous aluminosilicate and silica gel have also been investigated. The combination bands of stretching and bending vibrations of OH groups are shown to be more sensitive to the environment of the hydroxyls than are the stretching vibrations (fundamental and overtones). The observation has been made that in the supercages of X-zeolite there are two different types of structural hydroxyls, which are not resolved in the fundamental region of the spectra, but differ both in their bending and combined frequencies. Cations introduced into zeolites by ion exchange influence the structural OH groups, resulting in a shift of their combination bands, in comparison with decationized samples, to lower or higher frequencies, depending on the nature of the cation and on the zeolite structure. The change in the bending frequencies of different OH groups is considered to be connected with the strength of their coordination to the neighboring aluminum atoms. The Morse function parameters of the potential curves xe and ve and the isotopic effects of OH and OD groups in zeolites are calculated. The applicability of these data to characterize the acidic properties of OH groups is discussed.

Quantumchemical study of the isobutane cracking on zeolites

Abstract The ab initio quantumchemical investigation of the elementary steps of the catalytic isobutane cracking is presented. A reasonable agreement between experimental and theoretical activation energies is found. The obtained results demonstrate that the adsorbed carbenium and carbonium ions represent not the really existing reaction intermediates, but the high-energy transition states of the corresponding elementary reactions. This results in much higher activation energies than for the similar reactions in homogeneous super-acid solutions.

Comparative IR-spectroscopic study of low-temperature H2 and CO adsorption on Na zeolites

Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H—H and C—O bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H2 and CO.

A new type of acidic hydroxyl groups in ZSM-5 zeolite and in mordenite according to diffuse reflectance i.r. spectroscopy

HZSM-5 zeolites and H mordenite were studied using diffuse reflectance i.r. spectroscopy in the region of stretching vibrations of hydroxyls. A new type of acidic OH groups was detected that represents a bridged hydroxyl forming a strained intramolecular hydrogen bond with a neighboring oxygen atom. Properties of the new type of hydroxyls were investigated and are discussed.

Surface characterization of Au/HY by 129Xe NMR and diffuse reflectance IR spectroscopy of adsorbed CO. Formation of electron-deficient gold particles inside HY cavities

The Au/HY (4 wt.%) system, prepared by autoreduction of [Au(en)2]3+ in inert gas flow at 423 K, has been studied by TEM, 129Xe NMR and diffuse reflectance infrared spectroscopy of adsorbed CO. 80% of the metallic clusters were in the 1–4 nm size range, and the average linear diameter was found to be 2.6 nm. Some large particles (>5 nm) were also seen. Xenon adsorption and 129Xe NMR spectroscopy proved the presence of gold clusters inside HY cavities and showed clearly that gold clusters are weak adsorption sites for xenon. Heterogeneity in the particle size distribution was suggested by 129Xe NMR spectroscopy. Diffuse reflectance IR spectroscopy showed an intense asymmetric absorption band of CO linearly adsorbed on the gold surface. Two weak shoulders at ca. 2110–2120 and 2170 cm−1 were ascribed to neutral particles and oxidized gold sites, respectively. The main absorption band at about 2140 cm−1 is observed for the first time for CO adsorbed on a gold surface and was attributed to CO species interacting with electron-deficient gold particles Aunσ+ inside the HY lattice and in contact with protons. CO was more strongly adsorbed on these Aunσ+ particles than on other gold sites; the corresponding IR stretching vibration was observed even after evacuation for 30 min at room temperature.

A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. II. Protolytic dehydrogenation and hydrogen-deuterium hetero-isotope exchange of paraffins on high-silica zeolites

HF-21G quantum-chemical analysis of the protolytic attack of acid protons in zeolites at the C-H bonds in methane and ethane indicated that the resulting transition states depend on the sign of the bond polarization. If a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group. The case, when a proton tends to split off from the paraffin, corresponds to the hetero-isotope exchange of paraffins with surface OH groups. This is a concerted acid-base reaction with a transition state different from adsorbed carbonium ion.

Investigation of the different states of gallium in crystalline gallosilicates with pentasil structure and their role in propane aromatization

Results obtained by i.r. and e.p.r. spectroscopy show that at least two gallium-containing extraframework species differing in location, chemical nature, and acidic behavior are present in gallosilicate isostructural with ZSM-5 zeolite. Both species are formed in larger amounts by gallium release from the framework during catalyst regeneration processes. One of them, partly located on the outer crystallite surface, is found to be involved as a catalytically active site in propane aromatization. The catalytic activity is attributed to Lewis acidity.

Near infrared diffuse reflectance study of high silica containing zeolites

Abstract Decationated mordenite and high silica containing zeolites of ZSM type were studied using diffuse reflectance i.r. spectroscopy in a wide spectral range. Their hydroxyl groups and Lewis acidic centres are shown to be identical.

On two alternative mechanisms of ethane activation over ZSM-5 zeolite modified by Zn2+ and Ga1+ cations

The activation of ethane over zinc- and gallium-modified HZSM-5 dehydrogenation catalysts was studied by diffuse reflectance infrared spectroscopy. Hydrocarbon activation on HZSM-5 modified by bivalent Zn and univalent Ga cations proceeds via two distinctly different mechanisms. The stronger molecular adsorption of ethane by the acid-base pairs formed by distantly separated cationic Zn2+ and basic oxygen sites results already at room temperature in strong polarizability of adsorbed ethane and subsequent heterolytic dissociative adsorption at moderate temperatures. In contrast, molecular adsorption of ethane on Ga+ cations is weak. At high temperatures dissociative hydrocarbon adsorption takes place, resulting in the formation of ethyl and hydride fragments coordinating to the cationic gallium species. Whereas in the zinc case a Brønsted acid proton is formed upon ethane dissociation, decomposition of the ethyl fragment on gallium results in gallium dihydride species and does not lead to Brønsted acid protons. This difference in alkane activation has direct consequences for hydrocarbon conversions involving dehydrogenation.