Marcella Trombetta

Characterization and composition of commercial V2O5zWO3zTiO2 SCR catalysts

Abstract The surface and bulk structure and composition of a commercial VzWzTi oxide catalyst used for denitrification of waste gases from power plants has been investigated by XRD, XRF, ICP, TG-DTA, FT-IR and SEM-EDS analyses and by comparison with ‘model’ catalysts. The catalyst powder consists of a full ‘monolayer’ of surface complexes, mainly constituted by wolframyl species with the addition of small amounts of vanadyl and sulphate species over high-area TiO2-anatase. The characteristics of the surface vanadyl, wolframyl and sulphate species fully consist with those previously reported for model VzTi, WzTi, VzWzTi and sulphated titania catalysts. The catalyst powder is mixed with glass-like particles, playing the role of mechanical promoters. The possibility of contamination of the catalyst particles by Mg or other alkali or alkali earth cations possibly arising from the glass particles during monolith preparation and during reactor start-up and shut-off procedures as well as during catalyst operation is suggested.

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.

Characterization of alumina–titania mixed oxide supports: Part II: Al2O3-based supports

Abstract Bulk and surface characterization of alumina-rich Al 2 O 3 –TiO 2 mixed oxides with TiO 2 /(Al 2 O 3 +TiO 2 ) molar ratios equal to 0, 0.05 and 0.1, prepared by sol–gel and impregnation procedures have been performed by XRD, DTA, surface area, FTIR, UV–Vis, FT-Raman and TPD of ammonia. The results indicate that in the case of the samples prepared by the sol–gel procedure, an increase in surface area with titania loadings is obtained, in contrast to their impregnation counterparts, where a decrease is observed. The results from XRD, Raman and UV–Vis indicate that better titania dispersions are obtained by the sol–gel procedure and that at the titania loadings used here, no separate titania particles are detected. Additionally, the UV results show that Ti cations are essentially isolated, possibly into an octahedral form and with a lower coordination. In the samples prepared by the sol–gel procedure there is no evidence of titanium oxide species on the surface, suggesting that titania is probably dissolved into the alumina bulk. These results would explain the activity trends observed in alumina-rich Mo/Al 2 O 3 –TiO 2 HDS catalysts.

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.

IR study of alkene allylic activation on magnesium ferrite and alumina catalysts

The interaction of propene and butenes with a butene oxydehydrogenation catalyst, MgFe2O4, and with an isomerization catalyst, γ-Al2O3, have been studied by FTIR spectroscopy. Allyloxy species (prop-2-en-1-oxides from propene and but-3-en-2-oxide from but-1-ene) were observed over MgFe2O4, while allyl species (prop-2-en-1-yl from propene, but-3-en-2-yl from but-1-ene and 2-methylprop-2-en-1-yl from isobutene), thought to be σ-bonded to Al3+ ions, were observed over γ-Al2O3. It is proposed that in all cases the allylic C—H bond is heterolytically broken at cation–anion couples (Mn+O2–) to give rise to anionic allyls. However, when the cation is reducible, as on the Fe3+ centres of magnesium ferrite, the allyl anion is further rapidly oxidized to allyloxy species that, at high temperature, can act as cationic allyls which interact weakly with oxide anions. From propene, the cationic allyls can act as symmetric species, as is expected for acrolein synthesis.

FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0–25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Bronsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si–(OH)–Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging “ zeolitic” Bronsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework.

A study of the external and internal sites of MFI-type zeolitic materials through the FT-IR investigation of the adsorption of nitriles

Abstract The adsorption of acetonitrile (AN) and pivalonitrile (2,2-dimethylpropionitrile, PN) has been investigated on pure silicalite S1, on a ZSM5 zeolite apparently free from extraframework material and another ZSM5 zeolite apparently rich in extraframework material, on a titanium silicalite TS1 and on a boralite sample. AN enters the cavities of MFI type structures and interacts with both internal and external acid sites while PN does not enter such cavities and only interacts with the external sites. Terminal silanols, whose acidity can vary from weak to medium, are present in all cases mostly at the external surface, while very acidic bridging Si–OH–Al sites of ZSM5 zeolite are apparently exclusively located at the internal channel surface. Lewis acidic Ti cations are evident both in the internal and on the external surface of TS1. Weakly acidic BOH sites of boralite are distributed among internal and external surface. Extraframework alumina-like species of ZSM5 zeolite are apparently located in the internal cavities. Terminal silanols and hydrolized defects are located mostly at the external surface of silicalite. The mechanism of incorporation and charge balance of boron atoms as well as the structure of the hydroxy groups of boralite are still not well established.

Characterization of alumina-titania mixed oxide supports I. TiO2-based supports

Abstract The bulk and surface properties of a series of titanium-rich Al 2 O 3 -TiO 2 mixed oxides catalytic supports have been determined by different techniques ( S BET , pore size distribution, XRD, DTA, FTIR, FT-Raman, UV-vis diffuse reflectance and FTIR of adsorbed pivalonitrile and pyridine). The analysis of the results indicates that the prepared solids were fairly homogeneous, with titania showing the anatase structure and no segregation of alumina particles. Also, the addition of small amounts of alumina to titania enhances significantly its thermal stability. The UV, FTIR, Raman and XRD results suggest that part of the alumina forms a solid solution with TiO 2 and that two types of aluminum are present, one due to surface monolayer-type species and other due to octahedrally coordinated bulk species. The FT-Raman and the FTIR spectra of adsorbed probe molecules support the idea that the anatase particles are largely covered by surface alumina partial monolayers.