Claude Naccache

Infrared, Microcalorimetric, and Electron Spin Resonance Investigations of the Acidic Properties of the H-ZSM-5 Zeolite

The infrared spectra of the H-ZSM-5 zeolite calcined at temperatures up to 1173 K, and the corresponding electron spin resonance and microcalorimetric data are discussed. Two types of hydroxyl groups are characterized by absorption bands at 3720 and 3605 cm −1 , a small shoulder being present at 3665 cm −1 . Infrared spectra were also recorded after pyridine adsorption, showing the presence of Bronsted acid sites and Lewis acid sites. After calcination at increasing temperatures, dehydroxylation of the zeolite is observed: above 675 K, the number of Bronsted acid sites decreases, while that of strong Lewis acid sites increases; however, a small dealumination occurs as shown by chemical analysis measurements and XPS data. Moreover, dehydroxylation enhances the constraint character of this zeolite, as observed by pyridine inability to titrate the total Lewis sites. Electron spin resonance studies of trapped hydrogen atoms, of adsorbed NO, and of adsorbed benzene radical cations formed on H-ZSM-5 at various calcination temperatures are discussed in terms of the number and strength of the acid sites. It is suggested that the acid sites which are present are very similar to those of H-mordenite although slightly stronger. A microcalorimetric study of ammonia adsorption confirms the very strong acidic character of the acid sites and shows their dependence in strength and heterogeneity upon calcination temperature.

Elucidation of the mechanism of conversion of methanol and ethanol to hydrocarbons on a new type of synthetic zeolite

13C nuclear magnetic resonance and vapor-phase chromatography have been used to investigate the conversions of methanol and ethanol to hydrocarbons on a synthetic zeolite of the type H-ZSM-5 as described by Mobil. Methanol is first dehydrated to dimethyl ether and ethylene. Then the reaction proceeds by two competitive paths: first, successive dehydration-methanolation steps to give branched aliphatics, and, second, polycondensation reactions leading to linear aliphatic and aromatic compounds. The basic mechanism is essentially the same for ethanol, with the major difference being that ethylene can also be formed by direct dehydration of ethanol. At variance to earlier proposals, a mechanism involving carbenium ions is proposed which accounts well for the high yield in branched hydrocarbons and the observation of methyl ethyl ether which is detected in the methanol conversion products.

Zeolite microporous solids : synthesis, structure, and reactivity

I. Synthesis.- Some thermodynamic and kinetic effects related to zeolite crystallization.- Organic and inorganic agents in the synthesis of molecular sieves.- Non-conventional crystalline microporous solids.- Molecular engineering of lamellar solids. I. Principles derived from the pillaring of smectite clays.- Molecular engineering of layered structures II. Synthetic approaches to some new pillared derivatives.- II. Characterization.- General overview of the characterization of zeolites.- Sorption of single gases and their binary mixtures in zeolites.- Frequency-response measurements of diffusion of sorbates in zeolites.- Diffraction Studies of zeolites.- Spectroscopic investigations of zeolite properties.- The impact of NMR spectroscopy in molecular sieve characterization I. Low Si/Al ratio materials.- The impact of NMR spectroscopy in molecular sieve characterization II. Investigations of highly siliceous systems.- III. Modification, Reactivity, and Catalytic Activity.- Modification of zeolites and new routes to ion-exchange.- Zeolite framework substitution reliable characterization methods.- Evaluation and tailoring of acid-base properties of zeolites Part 1.- Evaluation and tailoring of acid-base properties of zeolites Part 2.- Catalysis by exchanged cations and zeolite framework sites.- Zeolites in oil refining and petrochemistry.- Composition of the carbonaceous compounds responsible for zeolite deactivation. Modes of formation.- Deactivation of zeolites by coking. Prevention of deactivation and regeneration.- IV. Novel Developments and Vistas.- Molecular sieves with pore openings consisting of more than 12-T atoms.- Chemical bonding in zeolites.- Some aspects of molecular shape-selective catalysis with hydrocarbons in zeolites.- New applications of nonclassical molecular sieve catalysts.- Enzyme mimicking with zeolites.- Microporous materials in organic synthesis.- Concluding Remarks.- List of Participants.

Metal-support interaction. Catalytic properties of Ti02-supported platinum, iridium, and rhodium

Abstract The reduction of Pt TiO 2 , Ir TiO 2 , Rh TiO 2 in the range 473–773 K of temperature induces a strong metal-support interaction as evidenced by the change in catalytic properties of these metals used for ethylene, benzene, and styrene hydrogenations, cyclohexane dehydrogenation, and butane hydrogenolysis. Treatment under O 2 partially restores the usual properties of these metals. The origin of this strong metal-support interaction is discussed.

Hydroxylation of phenol over TS-1: Surface and solvent effects

Abstract Titano-Silicalite 1 (TS-1) has been used as a catalyst for the hydroxylation of phenol with hydrogen peroxide. The reaction was carried out in different organic solvents in order to investigate the solvent effect and its influence on the product distribution during the oxidation. Furthermore, the contribution of the external surface sites of TS-1 to the catalytic activity is discussed. It is suggested that the surface sites — outside the zeolite channels — as well as the gradual poisoning of these sites are responsible for the differences observed at low conversions.

Oxidizing and acidic properties of copper-exchange Y zeolite

Abstract Copper Y zeolites have been studied by ESR and ir spectroscopy. At low activation temperatures, the zeolite exhibits Bronsted acidity. Pyridine is coordinatively adsorbed on cupric ions and gives rise to an ir band at 1451 cm −1 . No true Lewis acid sites are detected. However, CO-reduced samples acquire Lewis acid centers and cupric ions are reduced to cuprous ions. Hydrogen reduction results in the formation of metallic copper, and the solid acquires acidic hydroxyl groups which are more stable than those held on HY zeolites. The oxidizing properties of the cupric Y zeolite are attributed to the cupric ions while those of reduced samples are due to true Lewis acid sites.

Electron spin resonance studies on titanium dioxide and magnesium oxide—Electron donor properties

The electron donor properties of TiO2 and MgO have been investigated by tetracyanoethylene and trinitrobenzene adsorption. The electron donor centers are associated with OH− groups present on the surfaces of the solids activated at low temperature (<300 °). During the dehydration of TiO2 and MgO at higher temperature, weakly coordinated O2- ions are formed on their surfaces, and these are responsible for the reducing properties of these solids.

Dehydrocyclization of Alkanes Over Zeolite-Supported Metal Catalysts: Monofunctional or Bifunctional Route

Abstract The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the...

ESR study of copper(II) ions in Y zeolite: Effect of water, ammonia and pyridine adsorption

Abstract The ESR technique has been used to study the localization of copper(II) ions in the zeolite framework. In the entirely hydrated samples the copper(II) ion is in the form of an hexaaquo complex localized in the centre of the supercage. Dehydration produces the migration of the ion toward the sodalite cage. Water, ammonia and pyridine coordinate with the dehydrated copper(II) ion, and a back migration toward the supercage occurs.

X-ray photoelectron spectroscopy study of Pd and Pt ions in type Y-zeolite. Electron transfer between metal aggregates and the support as evidenced by X-ray photoelectron spectroscopy and electron spin resonance

Spectroscopic study by ESCA and e.s.r. techniques was performed for Pt and Pd ions in type Y-zeolites. Pt and Pd ions were found to be ionically bonded to lattice oxygen ions. Atomically dispersed PtO and PdO were shown to give significant X.P.S. positive chemical shifts: +1.3 and +1.4 eV for Pt 4f and Pd 3d lines, respectively. These shifts were assigned to smaller electron relaxation energy.Metal aggregates of small diameter (ϕ⩽ 20 A) and narrow size distribution were prepared in zeolite or on silica. The X.P.S. chemical shift was quite significant (∼+0.7 eV) for Pt on zeolite support with respect to those (< +0.3 eV) for Pd on zeolite or Pt or Pd on silica support and did not depend on the particle size. Electron donor and electron acceptor properties of the materials were studied by following the e.s.r. formation of charge transfer complexes (radical ion) as a function of Pt or Pd loading. Pt aggregates were shown to form charge transfer complexes with Lewis acid sites and to increase electron donor properties of the zeolite no matter what the particle size. This increase was shown to arise from electron donation from the metal to the zeolite lattice. However, Pd aggregates were shown by e.s.r. not to give rise to significant electron transfer.