Gabriella Borbély

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.

Solid-state ion exchange in zeolites: Part I. Alkaline chlorides/ZSM-5

Mixtures of solid alkaline chlorides MeCl (Me = Li, Na, K, Rb, Cs) and NH 4 -ZSM-5 or H-ZSM-5 react at elevated temperatures to evolve gaseous hydrogen chloride and form the respective cationic form of ZSM-5. The strong acidic OH groups are completely and the nonacidic ones partially removed, as evidenced by i.r. Thermogravimentic analysis, titration of the evolved gases, and temperatureprogrammed gas evolution monitored by mass spectrometry render the discrimination between a lowtemperature and a high-temperature exchange reaction possible. Evaluation of the analytical data shows good agreement between the aluminum content of the framework and the amount of neutralizing cations.

Solid-state ion exchange in zeolites: Part II. Alkaline earth chlorides / mordenite

Abstract Solid-state ion exchange of protons of acidic and non-acidic OH groups for alkaline earth cations in mordenite-type zeolites was investigated by means of TGA, temperature-programmed evolution of gases, IR and in-situ IR/GC measurements. The solid-state ion exchange occurs in two stages, i.e. via a low-temperature and a high-temperature process. Replacement of the protons of the acidic OH groups by alkaline earth cations considerably decreases the activity of hydrogene mordenite for catalysing hydrocarbon reactions such as conversion of ethylbenzene. The catalytic activity is greatly restored by treatment with water vapor subsequent to solid-state ion exchange.

Solid-state ion exchange in zeolites: Part IV. Evidence for contact-induced ion exchange between hydrated NaY zeolite and metal chlorides

Abstract Results obtained by solid-state 23Na MAS n.m.r. spectroscopy, X-ray diffractometry, and i.r. spectroscopy show that contact-induced ion exchange occurs at ambient temperature in mechanical mixtures of, for example, LiCl, KCl, BeCl2, or CaCl2, and hydrated NaY zeolite.

Physico-chemical characterization and framework topology of zeolite ZSM-20

Zeolite ZSM-20 and its aluminium-free modification (prepared through dealumination of the parent sample with SiCl4vapour) have been extensively characterized by physico-chemical methods, viz. X-ray powder diffraction, 27Al and 29Si m.a.s.n.m.r. spectroscopy, mid-infrared spectroscopy and thermal analysis. From the results, pronounced structural similarities between zeolite ZSM-20 and faujasite were deduced. Based on the information obtained, structural models for ZSM-20 were developed. Their simulated X-ray powder patterns were compared with the experimental ones. This led to a suggested framework topology of ZSM-20. Its pore system consists of supercages (also known in the faujasite structure) which are interconnected by circular 12-membered-ring windows to form large straight channels running along the c direction. The straight channels are interconnected through a two-dimensional pore system running parallel to a. The opening of the latter pores are also formed form 12-membered rings, but this time with an elliptical shape. The designation MTY is proposed for the framework topology of zeolite ZSM-20.

Zeolites H-[Ga]ZSM-5 and H-ZSM-5. A comparative study of the introduction of transition-metal cations by a solid-state reaction

A comparative electron spin resonance study of the solid-state reaction of Cu, Fe, Cr or V compounds with H-ZSM-5 and H-[Ga]ZSM-5 has been carried out. The protonic (acid) sites in H-[Ga]ZSM-5 are less powerful and less thermostable ‘traps’ for migrating cationic species than those in H-ZSM-5. In contrast to the case of H-ZSM-5, the sites in H-[Ga]ZSM-5 are not strong enough to stabilize the isolated vanadyl species. However CuII, FeIII and Crv ions which migrate from the outer surface of the zeolite crystals are coordinated in the cationic positions of H-[Ga]ZSM-5. The majority of the ions in H-[Ga]ZSM-5, like those in H-ZSM-5, are located as isolated coordinatively unsaturated cationic species accessible to gas-phase molecules. The presence of superhyperfine splitting in the e.s.r. spectra of Crv provides evidence of an electronic interaction between the unpaired electron of Crv and the nuclear spin of lattice Al3+ or Ga3+ ions.

Correlations between wavenumbers of skeletal vibrations, unit-cell size and molar fraction of aluminium of Y zeolites. Removal of non-skeletal Al species with H2Na2EDTA

The unit cell parameter (a0) and wavenumber of the asymmetric internal (ν1) and symmetric external (ν2) stretching vibrations of TO4 tetrahedra and of the double-six ring vibrations (ν3) have been studied for hydrated Na- and NH4-Y zeolites following synthesis and further modification by de-alumination with SiCl4 or by hydrothermal treatment. A set of linear equations were found for dependence of molar fraction of aluminium (α′) determined by chemical analysis and ν1, ν2, a0, respectively, when non-skeletal Al species were extracted with H2Na2EDTA. As the standard deviations Sα of α′ varied from 0.008 to 0.015, the determination of the content of Al in skeleton using i.r. and X.r.d. data is recommended for samples whose ratio Si:Al in the skeleton is below 10. The need for a well defined standard from non-modified zeolite then followed from a comparison of our results with equations already published in the literature. The linear relationship between ν1 and a0 has been found for samples both with and without non-skeletal Al species, which indicates the key importance of skeletal geometric factors to TO4 stretching vibrations.

Aluminium distribution in the bulk and on the surface of Y zeolites dealuminated with SiCl4 vapour. Influence of conditions of dealumination

Zeolites Y, dealuminated by reaction with SiCl4 at 530 and 630 K followed by washing with water, ethanol or ethanol and buffer before ammonium exchange (resulting Si/Al ratio: 3.8–10.6), have been examined using X-ray photoelectron spectroscopic measurements combined with chemical analysis. Subsequent extraction with H2Na2 EDTA allowed preferential removal of non-framework Al species which helped in the discussion of the Al location. It was found that, besides the increase of reaction temperature, the efficiency of washing (water > ethanol > ethanol and buffer) increases the Si/Al ratio both in the bulk and on the surface. Temperature increase and milder washing enhance the Al-enrichment of the surface with respect to the bulk. Non-framework Al species are responsible for the surface enrichment.

The status of chromium in aluminum-free chromosilicate with the ZSM-5 structure

Abstract It is shown that the hydrothermal synthesis of aluminum-free chromosilicate results in the formation of crystalline zeolite with the ZSM-5 structure and some Cr 3+ ions substitutes isomorphically for Si 4+ ions in the lattice of as-made samples. After oxidative calcination most chromium ions aggregate in α-Cr 2 O 3 microcrystals on the outer surface of the zeolite. However, some chromium remains stabilized strongly in the zeolitic structure as isolated low coordinated Cr(V) ions in two discrete coordinative states. It may be supposed that Cr(V) ions chemically bonded with zeolite lattice also have out of lattice ligands and are capable of interacting specifically with different molecules.

Deammoniation and dehydroxylation of calcium ammonium chabazites

The deammoniation and dehydroxylation of calcium ammonium chabazites has been studied by thermal analysis and infrared spectroscopy. At low ammonium-exchange levels dehydroxylation starts only at extremely high temperatures (ca. 1200 K) owing to the occupancy of the crystallographic SI cation sites by Ca and hence to the existence of isolated hydroxyls at 3630 cm–1 vibrating in the plane of the eight-membered rings. At higher exchange levels a second band at 3550 cm–1 and a dehydroxylation step at much lower temperatures (890–990 K) develop, pointing to the existence of hydroxyls in close proximity which vibrate in the hexagonal prisms or in the six-membered rings. Steaming and calcination results in the appearance of new i.r. bands at 3670 and 3750 cm–1 which are assigned to hydroxyls vibrating in the large cavities and to lattice-terminating OH groups, respectively. Steaming of highly ammonium-exchanged chabazite shifts the frequencies of the acidic hydroxyls to higher values but does not result (as in the case of other zeolites) in a substantial release of aluminium from the framework. The structure of the highly exchanged but steamed H-chabazites is in contrast to the non-steamed samples, which are thermostable after rehydration, indicating that during such steam treatment less acidic hydroxyl groups are formed.