Leonardo Marchese

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

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.

Revisiting MgO–CO surface chemistry: an IR investigation

The first product formed upon interaction of CO with the surface of fully dehydroxylated MgO is CO2–2. This species is generated by nucleophilic attack of a surface O2– on CO weakly σ-bonded on an Mg2+ cation in an adjacent position. The CO2–2 species generates C3O2–4 by addition of 2 CO molecules. Trimeric species (and unreacted CO2–2) are the most abundant components at 77 K. Further addition of CO molecules at room temperature generates oxygen-sensitive C2nO2–(2n+ 1) species which have an open structure and are responsible for the pink colour of the sample. Long exposure at room temperature, and moderate increase of temperature, favour disproportionation reactions, leading to adsorbed CO2(CO2–3) and C.

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.

Structure of the surface sites of δ-Al2O3 as determined by high-resolution transmission electron microscopy, computer modelling and infrared spectroscopy of adsorbed CO

Alon C is shown to be a δ-Al2O3 phase. High-resolution transmission electron microscopy (HRTEM) shows that the powder contains hexagonal particles of very regular shape. (110) faces are largely dominant. IR spectra of adsorbed CO and computer-generated models suggest that tricoordinated Al3+ ions are exposed at the surface.

A Chemical Strategy for the Relaxivity Enhancement of GdIII Chelates Anchored on Mesoporous Silica Nanoparticles

Functionalised MCM-41 mesoporous silica nanoparticles were used as carriers of Gd(III) complexes for the development of nanosized magnetic resonance imaging contrast agents. Three Gd(III) complexes based on the 1,4,7,10-tetraazacyclododecane scaffold (DOTA; monoamide-, DOTA- and DO3A-like complexes) with distinct structural and magnetic properties were anchored on the silica nanoparticles functionalised with NH(2) groups. The interaction between Gd(III) chelates and surface functional groups markedly influenced the relaxometric properties of the hybrid materials, and were deeply modified passing from ionic -NH(3)(+) to neutral amides. A complete study of the structural, textural and surface properties together with a full (1)H relaxometric characterisation of these hybrid materials before and after surface modification was carried out. Particularly for the anionic complex 2 attached to MCM-41, an impressive increase in relaxivity (r(1p)) was observed (from 20.3 to 37.8 mM(-1) s(-1), 86.2% enhancement at 20 MHz and 310 K), mainly due to a threefold faster water exchange rate after acetylation of the surface -NH(3)(+) ions. This high r(1p) value, coupled with the large molar amount of grafted 2 onto the silica nanoparticles gives rise to a value of relaxivity per particle of 29,500 mM(-1) s(-1), which possibly allows it to be used in molecular imaging procedures. Smaller changes were observed for the hybrid materials based on neutral 1 and 3 complexes. In fact, whereas 1 shows a weak interaction with the surface and acetylation induced only some decrease of the local rotation, complex 3 appears to be involved in a strong interaction with surface silanols. This results in the displacement of a coordinated water molecule and in a decrease of the accessibility of the solvent to the metal centre, which is unaffected by the modification of ammonium ions to neutral amides.

Heterogeneous catalytic epoxidation of fatty acid methyl esters on titanium-grafted silicas

Three different titanium-grafted silicates, with different morphological features were compared in the epoxidation, using tert-butylhydroperoxide as oxidant, of methyl oleate, methyl elaidate and of a mixture of methyl esters, obtained from high-oleic sunflower oil. Ti-MCM-41, a well-ordered mesoporous catalyst, showed very high conversions and excellent selectivity on all the substrates. The fatty acid methyl esters derived from renewable raw material were epoxidised selectively with very high yields in a reaction medium completely free from organic acidic compounds. The influence of the pore order of the catalysts on the catalytic performance was also examined. The interactions between the organic substrate and the ordered array of mesopores in Ti-MCM-41 were evidenced.

Physicochemical Characterization and Surface Acid Properties of Mesoporous [Al]-SBA-15 Obtained by Direct Synthesis

In this work, [Al]-SBA-15 samples were prepared by three different direct synthesis methods and one postsynthesis procedure, aiming to study the influence of the preparation procedures on their structural, textural, and physicochemical features. To this aim, samples were investigated by combining different experimental techniques (XRD, N(2) physisorption, (27)Al-MAS NMR, and IR spectroscopy). All preparation methods led to the formation of aluminum-containing SBA-15 samples. Nevertheless, depending on the preparation procedure, samples exhibited different structural, textural, and surface characteristics, especially in terms of Brønsted and Lewis acid sites content. [Al]-SBA-15(1) was synthesized by the pH-adjusting method and presented the lowest surface area and pore volumes. Its surface displayed three families of medium and one family of high strength Brønsted acid sites. The Brønsted/Lewis ratio was 3.49. [Al]-SBA-15(2) and [Al]-SBA-15(3) were synthesized by prehydrolysis of the silica and the aluminum precursors. In [Al]-SBA-15(2), ammonium fluoride was used as silica condensation catalyst. These two materials presented similar surface area, pore diameters and volumes, and Brønsted acidity. The Brønsted/Lewis acid sites ratio were 3.07 and 2.15 for [Al]-SBA-15(2) and [Al]-SBA-15(3), respectively. The [Al]-SBA-15(P) obtained by postsynthesis alumination displayed surface area similar to that of [Al]-SBA-15(3), Brønsted/Lewis acid sites ratio of 2.75, and Brønsted acidity similar to that of [Al]-SBA-15(1). The presence of extra-framework aluminum oxide was identified only on [Al]-SBA-15(3) and [Al]-SBA-15(P).

Hydroxyls on the surface of MgO powders

Abstract Water adsorption at 300 K on a MgO powder surface was studied by i.r. spectroscopy and microgravimetry, showing that a full monolayer of hydroxyls is formed which, in turn, weakly adsorbs a further monolayer of undissociated water molecules. Various bands in the i.r. spectra are related to different OH- families on extended {100} planes and on corner and edge sites.

Relaxivity modulation in Gd-functionalised mesoporous silicas.

Two hybrid materials based on mesoporous silicas SBA-15 and MCM-41 functionalized with stable and electrically neutral Gd(iii) macrocyclic complexes were prepared, and their (1)H NMR relaxometric properties investigated as a function of temperature and magnetic field strength.