Maurizio Lenarda

Textural and structural properties and surface acidity characterization of mesoporous silica-zirconia molecular sieves

Homogeneous mesoporous zirconium-containing MCM-41 type silica were prepared by supramolecular templating and their textural and structural properties were studied using powder X-ray diffraction, N2 porosimetry, atomic force microscopy, EXAFS, XPS, and UV–VIS–NIR diffuse reflectance spectroscopy. Their acid properties were also studied by using IR spectroscopy and by the use of catalytic tests such as the decomposition of isopropanol and the isomerization of 1-butene. The materials prepared show a good degree of crystallinity with a regular ordering of the pores into a hexagonal arrangement and high thermal stability. The specific surface area of the prepared materials decreases as the zirconium content rises. Zirconium atoms are in coordination 7 to 8 and located at the surface of the pores such that a high proportion of the oxygen atoms bonded to zirconium corresponds to surface non-condensed oxygen atoms. Both facts are responsible for the acid properties of the solids that show weak Bronsted and medium strong Lewis acidity.

A study of the surface acidity of acid-treated montmorillonite clay catalysts

The surface acidity of a series of commercial Sud Chemie acid-treated montmorillonite clays (K-catalysts) has been evaluated by a wide range of complementary experimental techniques. The different methods applied allow a rather complete characterisation of the surface acidity providing a complete picture of the Lewis/Bronsted acid strength/density of the surface sites. IR data show that the Bronsted sites on these catalysts are relatively weak and provide evidence for a slight increase of the strength and the density of Bronsted sites in the order K5 < K10 K20 < K30 in full agreement with the trend in iso-butene conversion, which is a measure of the strength and/or the abundancy of Bronsted sites. The apparent contradiction of these data with those obtained from the ammonia adsorption and iso-propanol conversion experiments can be explained by the structural and chemical modification of the clays upon acid treatment.

Catalytic behavior of chromia and chromium-doped alumina pillared clay materials for the vapor phase deep oxidation of chlorinated hydrocarbons

Abstract The catalytic behavior of a chromium-doped alumina pillared clay (ACrPA) and a series of chromia-pillared clay materials (XSCr) for the deep oxidation of methylene chloride has been studied. Both types of catalysts showed a high activity (conversion > 80%) at T > 350°C. The ACrPA sample showed a virtually constant catalytic activity (conversion > 99%) in the 300–400°C temperature range while the activity of the XSCr samples markedly increased with the temperature reaching an almost total conversion at 400°C. XPS (X-ray photoelectron spectroscopy) and temperature programmed reduction (TPR) studies demonstrated that ACrPA is a Cr(VI) Cr(III) mixed valence catalyst, whereas XSCr samples contain only chromium(III) ion. The high activity shown by ACrPA at low temperatures was attributed to the presence of highly dispersed Cr(VI) species, which exhibited an unusually high stability. The metal oxidation state of the two chromium based systems seemed to be important in determining the catalytic activity. The increase of the activity with the Cr content in XSCr samples suggested that the acid sites in these catalysts were located on the chromia pillars.

Preparation of hydroxy Al and AlFe pillared bentonites from concentrated clay suspensions

Aluminum and aluminumiron pillared clays (PILCs) were prepared starting from concentrated (50%) aqueous or acetone suspensions of non-purified natural calcium bentonites. The PILCs were characterized by chemical analysis, X-ray diffraction, magic angle spinning NMR, and N2 adsorption. The relative amount of Lewis and Bronsted acid sites was estimated from FT-IR spectra of adsorbed pyridine. All the PILCs prepared by this method gave highly microporous materials with micropore volumes in the range from 0.11 to 0.15 cm3/g, and with highly ordered clay sheets (d001=0.18 nm with peak widths of ca. 0.3 nm). The chemical composition of the products was not influenced by the nature of the solvent used in the pillaring process. Nevertheless, pillaring in acetone resulted in a faster and easier pillaring process. The obtained PILCs were more crystalline. The purity of the starting clays strongly influence the chemical and textural properties of the pillared clay. The scaling up of the process up to 500g batches using acetone as solvent is easily done and therefore, the method appears attractive for the industrial preparation of aluminum-based PILCs.

Hexagonal close packed nickel powder: Synthesis, structural characterization and thermal behavior

Abstract Hexagonal close packed nickel powder has been prepared upon reduction of Ni(II) with a K/B liquid alloy. This structural habit is stable at low temperature, undergoing a transition to a face centered cubic phase above 380°C.

Solid acid catalysts from clays: Evaluation of surface acidity of mono- and bi-pillared smectites by FT-IR spectroscopy measurements, NH3-TPD and catalytic tests

Alumina-pillared and double-pillared montmorillonite and saponite samples have been prepared and characterised from the point of view of their thermal stability, porosity and structure. Surface acidity was studied by ammonia TPD, iso-propanol conversion and n-butene skeletal isomerisation catalysis, and by FT-IR spectroscopy of the surface hydroxy-groups, and of adsorbed acetonitrile and pivalonitrile. The data show that the alumina pillars of pillared montmorillonite carry stronger Lewis sites than those of pillared saponite. Additionally, stronger Bronsted sites are carried by the montmorillonite layers with respect to those of saponite. Finally, pillared montmorillonite is more active in converting iso-propanol. However, it is also too active in converting n-butene, so giving rise to faster coking and more extensive cracking. So, pillared saponite has a more selective behavior in converting n-butene into iso-butene. ©2000 Elsevier Science B.V. All rights reserved.

Surface acidity modifications induced by thermal treatments and acid leaching on microcrystalline H-BEA zeolite. A FTIR, XRD and MAS-NMR study

Samples of beta-zeolite thermally treated at different temperatures and acid leached with diluted hydrochloric acid solution have been investigated by XRD, 27Al MAS-NMR spectroscopy and FTIR spectroscopy. The results have been used to interpret the behaviour of the samples as catalysts for the acylation of 2-methoxy-naphthalene. It has been shown that the as prepared sample presents two types of extraframework species. These are identified as Al hydroxo-ions highly dispersed in the internal zeolite channels and Al oxide nanoparticles. Calcination causes dealumination of the framework and progressive conversion of the Al hydroxo-ions into Al oxide nanoparticles that reduce the zeolite channels practicability, modifying the shape selectivity effect. The Bronsted acid sites present in the sinusoidal channels (νOH=3608 cm−1) can be distinguished from those located in the larger ones (νOH=3620–3612 cm−1), due to the inability of the bulky probe molecule pivalonitrile to enter the former. However, internal terminal silanols (νOH=3735 cm−1) also apparently display a significant Bronsted acidity, definitely higher than the acidity of those absorbing at 3747 cm−1, thought to be located at the external crystal surface. The strongest Lewis acidity is displayed by the aluminum hydroxo-ions, while that of alumina nanoparticles is a little weaker. Acid-leached beta-zeolite also displays a medium strength Lewis acidity, likely due to framework Al cations. Thus a partial reinterpretation of the real structure of beta-zeolite and a partial reassignment of the bands due to the surface hydroxy groups are proposed.

Hydroformylation of simple olefins catalyzed by metals and clusters supported on unfunctionalized inorganic carriers

The results, published in the open literature up to the end of 1995 on the hydroformylation of simple olefins in vapor and liquid phase catalyzed by metals or metal complexes adsorbed on inorganic oxides, clays, active carbons and zeolites, are reviewed.

Highly selective Friedel–Crafts acylation of 2-methoxynaphthlene catalyzed by H-BEA zeolite

Abstract The Friedel–Crafts acylation of 2-methoxynaphthalene with acetic and propionic anhydrides is actively catalyzed by H-BEA zeolite in batch conditions. Zeolite pre-treatment at increasing temperature leads to increasing selectivity of the less bulky 6-acyl-2-methoxy isomer. The selectivity toward the 6-isomer also increases if the catalyst/substrate ratio is increased. Catalyst activity and selectivity are correlated with the extra framework aluminum formation during thermal pretreatments of the zeolite.

Post-synthetic thermal and chemical treatments of H-BEA zeolite: effects on the catalytic activity

Abstract A beta zeolite sample (SiO 2 /Al 2 O 3 =25) was dealuminated by thermal treatment in the 773–1023 K temperature range. One sample, after the treatment at 923 K, was further washed with diluted HCl solution and rinsed with water, in order to remove the extraframework aluminum debris from its surface. Structural and morphological changes were monitored by powder X-ray diffraction, nitrogen adsorption–desorption and FT-IR spectroscopy, the surface acidity was evaluated by temperature programmed desorption of ammonia, FT-IR of adsorbed pyridine and potentiometric titration. The influence of the thermal treatments on the catalytic activity of the zeolite samples was tested in the anisole acylation and the Baeyer–Villiger oxidation of cyclohexanone. The catalytic performances were correlated with the physico-chemical and structural changes induced on the catalyst by the thermal and/or chemical pretreatments.