Manuela Casagrande

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.

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.

Liquid phase acetophenone hydrogenation on Ru/Cr/B catalysts supported on silica

The liquid phase hydrogenation of acetophenone was studied over a series of silica supported bimetallic catalysts with various Ru/Cr atomic ratios. The catalysts were prepared by reduction of the metal salts with NaBH4 aqueous solutions and were characterised by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) (ESCA). The hydrogenation reaction was carried out in batch at a hydrogen pressure of 9 bar. The selectivity towards reduction of the carbonyl function increases with the increasing amount of Cr ions added.

An FTIR study of the accessibility of the protonic sites of H-mordenites

The interaction of different probes with two H-MOR samples has been studied by IR. In the case of the sample with Si/Al = 10 acetonitrile perturbs all the hydroxy groups while 2,2-dimethylpropionitrile (pivalonitrile) perturbs only very few. Pyridine also perturbs all the hydroxy groups but only some of them protonate the pyridine, the others only H-bond to it. n-Hexane and 2,2-dimethylbutane give the same result, perturbing only some of the bridging hydroxy groups. The results are interpreted by assuming that no OHs are located in the 8-ring channels, and that the hydrocarbons cannot interact (due to the steric hindrance of the methyl group) with the OHs located in the side pockets. On the contrary, the flat molecule pyridine can enter slightly into the side pockets and H-bond with the OHs there. Pivalonitrile interacts only with the OHs which are well exposed in the main channels. It is concluded that the active sites for alkane isomerization are likely exclusively those that are well exposed in the main channels of H-MOR and that Al substitution in the T3 sites probably does not occur. The sample with Si/Al 45, taken as an example of a dealuminated sample, presents many less bridging OHs which are entirely available for interaction with even pivalonitrile.

Synthesis and structural characterization of ordered supermicroporous MSU type silica-tin molecular sieves

Supermicroporous Sn–Si molecular sieves, with different amounts of tin, have been prepared using polyethylene oxide type surfactants as structure directing agents and a silicon alkoxide as precursor. Syntheses have been carried out in acidic media. The structural and chemical properties of the prepared materials, have been studied using a variety of techniques such as Wide Angle X-Ray Scattering (WAXS), physisorption of N2 at 77K, FT-IR spectroscopy of chemisorbed pyridine, skeletal FT-IR, Temperature Programmed Desorption of ammonia (NH3-TPD) and 119Sn Mossbauer spectroscopy.

EPR of trans-stilbene radical cations formed in zeolite cavities: a new approach to study the zeolite void space

trans-Stilbene was adsorbed in the cavities of H-ZSM-5, H-MOR and H-BEA zeolites, originating monomeric and dimeric radical cations that were studied by EPR spectroscopy. The H-BEA samples were pretreated at three progressively higher temperatures. Simulation of the spectra allowed the evaluation of the relative amounts of monomers and dimers. The nature, quantity and relative amount of monomeric and dimeric radical species were correlated to chemical and dimensional modifications of zeolite channels caused by the dealumination of the zeolite structure and extra-framework aluminium species formation.