Vera Bolis

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.

Hydrophilic and hydrophobic sites on dehydrated crystalline and amorphous silicas

Surface dehydroxylation of amorphous and crystalline silicas (quartz dust) has been investigated from the standpoint of the development of hydrophobicity upon thermal treatment. Hydrophobicity occurs when only siloxane bridges and isolated silanols (IR band at ca. 3750 cm–1) are present and is monitored by an enthalpy of adsorption of water lower than the latent heat of liquefaction (44 kJ mol–1). This calorimetric method allows the evaluation of the extent of hydrophilic and hydrophobic patches when both are present at the surface. All silicas develop hydrophobicity upon thermal treatment in vacuo, but quartz is much less easily dehydroxylated than amorphous materials. It is still mainly hydrophilic after outgassing at 1073 K, whereas pyrogenic silicas (Aerosil) become hydrophobic upon outgassing at T < 673 K. Quartz is also characterized by a few very reactive sites (q 90 kJ mol–1), absent on the amorphous specimens. Both these facts might be related to the specific quartz pathogenicity. Rehydroxylation at room temperature of dehydroxylated silicas occurs to a very limited extent. Hydrophilic patches exhibit a marked heterogeneity towards water with an enthalpy of adsorption decreasing from 90 to 44 kJ mol–1. The enthalpy of adsorption approaches 44 kJ mol–1 corresponding to the addition of multilayers of adsorbed water.

Lewis and Brønsted acidity at the surface of sulfate-doped ZrO2 catalysts

Both Bronsted (protonic) and Lewis (aprotic) acidic centres can exist at the surface of an homo-geneous family of sulfated zirconia catalysts, provided sulfate groups (either isolated or, better, polynuclear units) exist on the regular patches of few low-index crystal planes representing the top termination of the scale-like ZrO2 crystallites. The Bronsted/Lewis acidity ratio depends then on (at least) two variables: the surface hydration/dehydration degree, and the highest temperature reached by the system because Bronsted acidity is induced only by the thermally most labile fraction of surface sulfates.

The reactivity of oxides with water vapor

Abstract A Tian-Calvet microcalorimeter has been used to determine the heat of interaction of water vapor with a range of oxides at 303 K. New data for SiO 2 , TiO 2 , ZnAl 2 O 4 and CaO are reported which, together with already established results for Al 2 O 3 , enable an overall view to be given of the hydrophilic behavior of oxides. The change of the differential heat of adsorption with uptake for a given oxide allows the proportions of surface with high and low hydrophilicity, and with hydrophobicity if present, to be assessed. For the highly hydrophilic oxide CaO, the interaction occurs in depth, but this is suppressed when Mn ions are introduced in solid solution in CaO. The change from hydrophilic to hydrophobic behavior in oxides is correlated with the gradation from ionic to covalent bonding.

X-ray photoelectron spectroscopy and x-ray absorption near edge structure study of copper sites hosted at the internal surface of ZSM-5 zeolite: A comparison with quantitative and energetic data on the CO and NH3 adsorption

The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I) species to Cu(II) by simple exposure to the atmosphere and subsequent reduction by thermal activation in vacuo was monitored. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites was studied by means of adsorption microcalorimetry. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. Adsorption heats comprised in the 130–40 kJ mol−1 interval were obtained for the formation of both type of complexes. The perturbation induced on the Cu centers and/or on the zeolite matrix by the adsorption of the probe molecules was monitored by parallel experiments of XPS, IR, and XANES. A significant fractio...

Surface acidity of metal oxides. Combined microcalorimetric and IR-spectroscopic studies of variously dehydrated systems

Abstract The systematic use of CO adsorption at room temperature as a probe for (strong) Lewis acidity at the surface of a number of non-d/d0 metal oxides of interest in catalysis is illustrated. The advantages of the combined use of adsorption microcalorimetry and IR spectroscopy in giving an exhaustive picture of the distribution of acid surface sites are stressed. The influence of the chemical nature (TiO2, ZrO2, HfO2 and Al2O3) and of the structure (monoclinic/tetragonal for ZrO2 and γ-/δ,θ- phase for Al2O3) of the metal oxide, as well as the dehydration degree of the surface and the presence of anionic (sulfates for zirconia) or cationic (ceria for alumina) surface dopants were considered. The blue shift of the stretching frequency of adsorbed CO and the adsorption enthalpy were measured. In the case of the group IV metal oxides the two parameters were found to be correlated, whereas in the case of pure Al2O3 no correlation was found. The peculiar behavior of alumina was interpreted on the basis of different processes occurring at the CO/Al2O3 interface. An endothermic reversible reconstruction of the surface was supposed to occur upon adsorption, leading to exceptionally low heat values. The presence of small amounts of cations, other than Al3+, in the alumina matrix seemed to inhibit this effect.

Structural Characterization of Siliceous Spicules from Marine Sponges

Siliceous sponges, one of the few animal groups involved in a biosilicification process, deposit hydrated silica in discrete skeletal elements called spicules. A multidisciplinary analysis of the structural features of the protein axial filaments inside the spicules of a number of marine sponges, belonging to two different classes (Demospongiae and Hexactinellida), is presented, together with a preliminary analysis of the biosilicification process. The study was carried out by a unique combination of techniques: fiber diffraction using synchrotron radiation, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetric (DSC), Fourier transform infrared spectroscopy (FTIR), and molecular modeling. From a phylogenetic point of view, the main result is the structural difference between the dimension and packing of the protein units in the spicule filaments of the Demospongiae and the Hexactinellida species. Models of the protein organization in the spicule axial filaments, consistent with the various experimental evidences, are given. The three different species of demosponges analyzed have similar general structural features, but they differ in the degree of order. The structural information on the spicule axial filaments can help shed some light on the still unknown molecular mechanisms controlling biosilicification.

Study of the acidity of ZSM-5 zeolite by microcalorimetry and infrared spectroscopy

Microcalorimetry and i.r. spectroscopy were used to characterize the acid centres in ZSM-5 zeolite. Differential heats of adsorption of ammonia at 416 K showed that there exists a strong acidity in H–ZSM-5 zeolite and also in H–Z zeolite, which exceeds that present in H–Y zeolite. The energy distribution of strongly acid sites is more heterogeneous in H–ZSM-5 than in H–Z zeolite. A comparison of calorimetric and i.r. spectroscopic data for samples with different degrees of dehydroxylation has, moreover, shown that a fraction of Bronsted sites are weakly acidic whereas all Lewis sites are strongly acid. The spatial distribution of acidity in acid-treated ZSM-5 zeolite is not uniform; the most acidic sites, and particularly Lewis sites, are located within the channel structure of the material where they are not readily accessible to ammonia molecules at 416 K or mainly to pyridine molecules at room temperature.

Characterization of sol–gel bioglasses with the use of simple model systems: a surface-chemistry approach

The present work deals with the physico-chemical characterization of the surface features of two powdery sol–gel-synthesized bioglass specimens (58S and 77S) employed in repair of human bones and soft tissues. In-situ transmission FTIR spectroscopy and adsorption microcalorimetry (with the use of water as a probe molecule) were adopted in parallel to study on one hand bioglasses (amorphous Si, Ca and P oxides) and on the other hand pure and doped silica specimens, as reference materials. It was observed that the presence of Ca and P moieties induces the formation of surface sites that possess higher strength and higher capability towards water coordination than do the hydroxy groups present on pure silica. In particular, the presence of some irreversibly coordinated water was observed both on these bioglasses and on doped silica. The Ca/P ratio seems to play a major role: (i) an excess of Ca induces the presence of coordinatively unsaturated surface (cus) Ca2+ ions, that generate Lewis acid sites that can strongly interact with water and can yield surface carbonates; (ii) the presence of P species increases the surface hydrophilic character and leads to the formation of Si–O–P labile bridges, which exhibit an enhanced propensity to dissociate water.

The surface chemistry of crushed quartz dust in relation to its pathogenicity

Abstract The surface chemistry of finely divided quartz dust has been investigated from the standpoint of its fibrogenicity by EPR and adsorption calorimetry. Even after grinding in the atmosphere, quartz bears surface radicals, originating from homolytic cleavage of SiOSi bonds and subsequent reaction with atmospheric components. These radical sites, which readily react with both O2 and H2O, can be regarded as possible initiators of the macrophage-mediated reaction resulting in lung fibrosis. The heat of adsorption of water indicates the presence of ∼3 × 1017 very strong sites (−ΔH≥180 kJ mol−1) on all quartz samples, but these are totally abseht in chemically prepared amorphous silicas. Mechanical cleavage of the covalent SiO bond, more than crystallinity, is suggested as the primary cause of SiO2 toxicity.