Georges Poncelet

PREPARATION AND CHARACTERIZATION OF BIDIMENSIONAL ZEOLITIC STRUCTURES OBTAINED FROM SYNTHETIC BEIDELLITE AND HYDROXY-ALUMINUM SOLUTIONS

Beidellite was synthesized hydrothermally from a noncrystalline gel at 320°C and 130 bar pressure. The beidellitic character of the product was verified by infrared spectroscopy on the NH4+-exchanged form. Intercalation was achieved with hydroxy-aluminum solutions having different OH/Al molar ratios. The solutions were investigated by several methods, including 27Al nuclear magnetic resonance. Essentially, two Al species were detected: monomelic Al and a polymerized form containing Al in four-fold coordination. This latter species was found to be selectively fixed in the interlamellar region, which resulted in a stable spacing of 18 Å at 110°C and 16.2 Å at 700°C. The pillared beidellites had specific surface areas of > 300 m2/g, mainly due to micropores. Both Brönsted and Lewis acid sites were evidenced by infrared spectroscopy using pyridine as a probe molecule.RésuméUne beidellite obtenue par synthèse hydrothermale (320°C, 130 bar) à partir d’un gel précurseur et identifiée comme telle sur base des spectres infrarouges de la forme échangée à l’ammonium, a été pontée par des solutions hydroxy-aluminiques de rapports OH/Al différents. Les solutions de pontage ont été étudiées par différentes méthodes, dont la résonance magnétique nucléaire. Deux espèces d’aluminium, l’une monomérique, l’autre polymérique contenant de l’aluminium en coordination tétraédrique coexistent dans ces solutions. On a observé que l’espèce polymérique s’adsorbe préférentiellement dans l’espace interlamellaire, conférant à l’édifice un espacement de 18 Å à 120°C et de 16,2 Å après calcination à 700°C. La surface spécifique de ces beidellites pontées est de l’ordre de 300 m2/g. La spectrométrie infrarouge de la pyridine adsorbée décèle la présence des acidités Brönsted et Lewis.

Acidity in pillared clays: origin and catalytic manifestations

Acidity in pillared clays is a broad topic, and one which has been reviewed before [1-3]. Since it is difficult to be exhaustive, the present contribution is divided in two parts. The first one tries to establish relationships between observed acidity and structural data in the only case which is sufficiently understood to date, i.e., [Al13] in smectite clays. The second part is concerned with the acid catalytic properties of pillared clays. They are classified according to the types of reactions. While most references are concerned with [Al13]@smectites, other systems will be considered too as the need arises.

Microporous 18 angstrom Al-pilared vermiculites: preparation and characterization

A new method for obtaining 18 Angstrom Al-pillared vermiculites is reported. In a first step, the vermiculite is submitted to an acid treatment followed by calcination. These treatments lead to a permanent reduction of the global negative charge of the mineral layers. The extra-framework species are removed by a subsequent leaching with a complexing acid, and the solid is converted to the sodium form by ion exchange. The charge-reduced Na (or Ca)-vermiculite is then brought in contact with the aluminum pillaring solution. The calcined Al-pillared vermiculites exhibit the characteristic features of this class of materials, namely, 18 Angstrom spacing, high surface area, microporosity, and both Bronsted and Lewis acidities. These textural properties are preserved at higher temperatures (up to 800 degrees C) compared with Al-pillared smectites, owing to the higher thermal stability of the vermiculite structure (higher dehydroxylation temperatures). The Bronsted acid content of the Al-pillared vermiculites established by IR spectroscopy of adsorbed pyridine is about twice as high as that of an Al-pillared saponite

n-heptane hydroconversion over aluminosilicate mesoporous molecular sieves

Aluminosilicate mesoporous molecular sieves (designated Al-MMS) have been prepared at room temperature using the primary amine hexadecylamine as organic templating surfactant. The materials have textural properties typical of mesoporous materials with short-range hexagonal order but exhibit higher Brønsted acidity compared to aluminium-containing MCM-41. The Pt-impregnated materials are efficient catalysts for the hydroconversion of n-heptane. At low Si/Al ratio (≤ 10) the materials have total conversions and selectivity comparable to that of USY zeolite (Si/Al = 21) and in addition exhibit considerable cyclisation at temperatures above 350°C. Our catalytic results show that the Pt-impregnated Al-MMS samples attain a good balance between the metal and acidic functions and that activity and selectivity are dependent on the Brønsted acid content and consequently on the amount of tetrahedral aluminium in the catalysts. The amount of Pt (in the range 0.25-1 wt%) mainly affects the selectivity to cyclised products which increases with Pt content at the expense of cracking; total conversion and selectivity to isomers remain largely unaffected.

Dealumination of small- and large-port mordenites: A comparative study

Large-port (LP) and small-port (SP) mordenites with different particle size and shape have been submitted under similar conditions to known dealumination procedures in view of comparing their respective behavior, and establishing the similarities and discrepancies among the dealuminated solids. The dealuminated zeolites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), acidity measurements with ammonia and X-ray photoelectron spectroscopy (XPS). The particle size of the starting mordenite markedly influenced the extraction of Al when dealumination was performed by direct acid attack or dry thermal treatment. Admission of steam during calcination noticeably improved the removal of Al from SP mordenite compared with other methods. Elimination of the extra-framework Al species produced at the calcination step required acid concentration as high as 6 M. (Hydro)thermally dealuminated LP mordenites exhibited similar surface and bulk chemical composition. Conversely, dealuminated SP mordenites showed surface enrichment in Al up to Si/Al ratios of about 40, and depletion in Al at higher ratios. These surface/bulk heterogeneous distributions were suppressed when dealumination combined preliminary acid leaching and steaming. Dealuminated small-and large-port mordenites exhibited differences in their textural characteristics, and for both, a contraction of the unit cell volume and the development of meso- and secondary microporosity

Silica from sepiolite: Preparation, textural properties, and use as support to catalysts

Silicas with high surface areas have been obtained by acid leaching of sepiolite. The influence of the preparation conditions (acid type and concentration, solid concentration, temperature and time) on the silica properties has been studied with the aim of producing silicas which may have potential uses as catalyst supports. The silicas have been characterized by chemical analysis, XRD, SEM, Si-29 MAS NMR, TGA. It is shown in this study that the silicas obtained by acid leaching of sepiolite develop high surface areas (500 m(2) g(-1)) with a high contribution of mesoporous surface (approximate to 90%). The textural characteristics after calcination at temperatures above 500 degrees C are stable, even after long aging times (>6 months). Preliminary benzene hydrogenation tests have been performed over Ni-impregnated silicas and compared with equivalent catalysts prepared with commercial silicas.

Preparation and characterisation of mesoporous zirconium oxide

Mesoporous zirconium oxides were obtained using the polymeric sol-gel procedure. The synthesis was carried out in the presence of two series of surfactants, namely, N(C-n)(4)Br and N(CH3)(3)CnBr, with n = 8-18, and C-n, the linear alkyl chain. Hydrolysis of zirconium isopropoxide was carried out both under acid and base catalysis, and also using acetylacetone (acac) as a stabiliser. In order to obtain information on the sol-gel procedure in the above conditions, the composition parameters: Zr/H2O and Zr/surfactant ratios were modified in a rather wide interval. Mesoporous textures were obtained, with surface areas and pore size distributions depending on the synthesis conditions. The evolution of the sol-gel process and the resulting oxides were characterised with several tools: nitrogen adsorption-desorption isotherms, TG-DTA, XRD. SAXS, FTIR, C-13-CP/MAS NMR, and XPS measurements. The results showed that the phase structure of ZrO2 is sensitive to the composition parameters, and confirmed that in the presence of surfactants, the synthesis occurred via a scaffolding mechanism

n-butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal and sulfate content

n-Butane isomerization has been investigated over transition metal-promoted sulfated zirconia catalysts. Fe alone was a more efficient promoter than Mn and Cr, and mixtures of Fe-Mn, Fe-Cr, and Fe-V. Promotion of a commercial sulfated zirconia (4.8% sulfate) with iron amounts in excess of 2% depressed the conversion and increased the deactivation rate. Increasing reaction temperatures improved the conversion and decreased the induction period. At 70 degreesC and above, the induction period was suppressed. The influence of activation temperature was studied over a Fe-sulfated zirconia catalyst (with 2% Fe and 4.8% SO42- pre-calcined at 600 degreesC). The best conversion was achieved when the catalyst was activated at 350 degreesC in air. This temperature was apparently sufficient to generate the redox sites active at low reaction temperature. Activation in flowing helium depressed the catalytic activity and increased the induction period. The sulfate content had a significant effect on the catalyst performance, For 2% Fe-promoted zirconia, a maximum conversion was found for 7.5% SO42-, probably related with a better balance (or synergism) of the redox and acid sites involved in a bimolecular mechanism. The time required to reach a maximum of conversion (induction period) decreased with increasing total acidity, i.e, with sulfate content. The series of catalysts with different amounts of sulfate has been characterized by X-ray diffraction, nitrogen adsorption-desorption isotherms, TGA, ammonia-TPD, DRIFT, Raman, and X-ray photoelectron spectroscopy

Hydrogenation By Hydrogen Bronzes .1. Hydrogenation of Ethylene By Hxmoo3

Hydrogenation of ethylene has been studied on hydrogen molybdenum oxide bronzes prepared by spillover of hydrogen from supported Pt particles. The temperature of the formation and of the outgassing of the bronze, as well as the Pt content, was found to influence the rate of the reaction of ethylene. Water molecules are formed at the expense of lattice oxygens and inserted H atoms when the bronze is outgassed at temperatures higher than 120 °C. The removal of water is accompanied by structural modifications and the external layers of the oxide become partially amorphous. Freshly prepared bronzes are completely oxidized upon exposure to oxygen; after back-titration with hydrogen, the bronzes exhibit higher catalytic performances than before the oxygen treatment. The hydrogenation reaction is zero order with respect to ethylene pressure and small amounts of water present in the gas phase have no significant influence on the reaction. In the presence of gaseous H2, the rate of hydrogenation increases with increasing hydrogen pressure, showing that gaseous H2 is first consumed. From the experimental data, it is reasonable to consider the Pt particles as gates through which the H atoms leave the host lattice, according to a reverse spillover process, the rate-limiting step being the motion of hydrogen toward the platinum surface.

Gallium, aluminium and mixed gallium-aluminium pillared montmorillonite: Preparation and characterization

Pillaring of montmorillonite was achieved with partially hydrolysed solutions of gallium, gallium-aluminium and aluminium. The development of 18 angstrom spcacings was investigated in relation with the OH/M3+ molar ratio, the amount of pillaring solution and the molar composition of the mixed solutions. With respect to Al-pillared clays, Ga- and Ga-Al-pillared montmorillonites exhibit similar behaviour toward heat treatment. They also have similar specific surface areas and pore volumes. IR spectroscopy of adsorbed pyridine show that both Bronsted and Lewis acidities are present, the former being slightly less developed in the Ga-pillared sample than in the aluminium analogue.