Chiara Bisio

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).

Titanosilsesquioxane Anchored on Mesoporous Silicas: A Novel Approach for the Preparation of Heterogeneous Catalysts for Selective Oxidations

Polyhedral oligomeric silsesquioxanes (POSS) are a class of condensed three-dimensional organosiliceous compounds with cage frameworks that have different degrees of symmetry. These molecular materials have a general formula R8Si8O12 [1–3] and display silicon atoms bound to one-and-ahalf oxygen atoms (sesqui), the remaining valence being saturated by organic entities (R). Numerous metals have been incorporated successfully into POSS cages with the aim to obtain either specific homogeneous catalysts, or models for active sites of heterogeneous catalysts. For example, POSS cages with titanium(IV) complexes proved to be highly active in the epoxidation of olefins. Special attention was recently devoted to the heterogenisation of Ti–POSS compounds onto insoluble and easily recoverable supports. Interesting examples encompass the incorporation of Ti–POSS into mesoporous MCM-41, sol–gel matrices, polysiloxanes and the preparation of organic–inorganic hybrid materials and coatings based on polystyrene polymers containing Ti–POSS moieties. However, the literature on the direct anchoring, through covalent bonding of preformed Ti–POSS cages onto silica supports is very poor. The stabilisation of Ti–POSS compounds onto silica surfaces through covalent bonds is a relevant issue for obtaining heterogeneous catalysts with isolated active centres, because it may avoid metal leaching from the catalysts. This work is focused on the preparation of Ti–POSSbased heterogeneous catalysts, by anchoring of a functional titanium-containing silsesquioxane on the surface of an ordered mesoporous silica (SBA-15) and of a non-ordered silica (SiO2-Dav). An innovative bifunctional POSS, bearing in the structure both a Ti metal centre and a triethoxy group for grafting on the silica surfaces, was specifically synthesised for this purpose and named Ti–POSS–TSIPI (Scheme 1, 2). Ti–POSS–TSIPI was prepared by an equimolar reaction between Ti–NH2POSS (Scheme 1, 1), the synthesis of which was reported by some of us, and 3-isocyanatopropyl triethoxysilane (TSIPI) under basic conditions. The product of the reaction was monitored by both IR (Figure 1) and H NMR spectroscopy (see Experimental

On the Intercalation of the Iodine−Iodide Couple on Layered Double Hydroxides with Different Particle Sizes

Molecular iodine was intercalated from nonaqueous solution into microsized ZnAl-layered double hydroxide (LDH) in the iodide form, generating the I(3)(-)/I(-) redox couple into the interlayer region. Chloroform, ethanol, acetonitrile, or diethyl ether were used as solvents to dissolve the molecular iodine. The intercalation compounds were characterized by thermogravimetric analysis, X-ray powder diffraction, UV-vis spectroscopy, and scanning and transmission electron microscopy. The stability of iodine-solvent adducts and the iodine concentration affected the LDH iodine loading, and samples with I(2)/I(-) molar ratio ranging from 0.14 to 0.82 were prepared. Nanosized, well dispersible LDH, synthesized by the urea method in water-ethylene glycol media, were also prepared and successfully functionalized with the I(3)(-)/I(-) redox couple applying the conditions optimized for the micrometric systems.

On the hydrothermal stability of MCM-41 mesoporous silica nanoparticles and the preparation of luminescent materials

MCM-41 nanoparticles have recently attracted growing scientific interest for applications in biomedical and diagnostic fields, nevertheless their use is limited because of the low hydrothermal stability, rendering them not suitable for functionalisation (i.e. dye molecules loading, anchoring of luminescent guests, etc.) in aqueous media. In this work, nanosized MCM-41 was hydrothermally stabilised by properly adapting post-synthesis hydrothermal restructuring treatment already used for conventional MCM-41 material (particle size in the micron range). A significant improvement of the hydrothermal stability of nanosized MCM-41 was reached: the pore array of the stabilized MCM-41 was not significantly modified after hydrothermal treatment at 333 K, whereas under the same conditions the parent MCM-41 became partially amorphized. The hydrothermal stabilisation is due to pore wall restructuring occurring during post-synthesis modification, and an increase of hydrophobicity of the silica surface. The improved hydrothermal stability of nanosized MCM-41 rendered this solid suitable for dye impregnation in aqueous media and allowed the preparation of a luminescent fluorescein/MCM-41 nanocomposite material, which in aqueous suspension showed an emission efficiency 5 times higher than an equimolar fluorescein solution.

Niobium(V) Saponite Clay for the Catalytic Oxidative Abatement of Chemical Warfare Agents

A Nb(V)-containing saponite clay was designed to selectively transform toxic organosulfur chemical warfare agents (CWAs) under extremely mild conditions into nontoxic products with reduced environmental impact. Thanks to the insertion of Nb(V) sites within the saponite framework, a bifunctional catalyst with strong oxidizing and acid properties was obtained. Remarkable activity and high selectivity were observed for the oxidative abatement of (2-chloroethyl)ethyl sulfide (CEES), a simulant of sulfur mustard, at room temperature with aqueous hydrogen peroxide. This performance was significantly better compared to a conventional commercial decontamination powder.

Preparation of luminescent ZnO nanoparticles modified with aminopropyltriethoxy silane for optoelectronic applications

Highly luminescent ZnO nanoparticles have been synthesized through a co-precipitation method, starting from a solution of zinc acetate in methanol and precipitating the oxide phase in basic media in the presence of variable amounts of aminopropyltriethoxy silane (APTS). The adopted conditions led to the condensation between Zn-OH species and the alkoxy functionalities of the organosilane during the formation of nanoparticles (one-pot method) thus allowing covalent binding of organic functionalities on the ZnO surface. HR-TEM measurements indicated that samples synthesized with increasing concentration of APTS (from 1 to 10% of Si/Zn molar ratio) are made of ZnO nanoparticles of decreasing dimension, passing from ca. 6 nm for pure ZnO to ca. 3 nm for the ZnO functionalized with the highest organosilane loading. ZnO samples with reduced particle size showed a significant variation of the optical properties. In particular, the particle size reduction is associated with a significant modification of ZnO absorption properties, as studied by diffuse reflectance UV-Vis spectroscopy, and to an exceedingly high photoemission. The organo-modified ZnO nanopowder with the highest photoemission was successfully tested as a light-emitting layer in a new generation of LED devices thus proving that they also possess interesting electroluminescent properties.

Rational design of single-site heterogeneous catalysts: towards high chemo-, regio- and stereoselectivity

The main methods for the design and preparation of single-site heterogeneous catalysts on inorganic oxide supports are described and reviewed. Catalytically active metal sites can be either introduced into the framework of porous materials via direct synthesis or added to a pre-existing support by post-synthesis techniques. Particular attention is paid to selected examples where the geometry, the nature and the chemical surroundings of the active single site is a key factor to obtain catalytic systems with enhanced chemo-, regio- and stereoselectivity. The ever-increasing capabilities of ‘nanoarchitecture’ at molecular level enable chemists to build ideal catalysts for the sustainable transformation of bulky and high added-value molecules.

On the acidity of saponite materials: a combined HRTEM, FTIR, and solid-state NMR study.

Acid clays were prepared by exchanging a synthetic saponite in HCl solutions of different concentration (0.01 and 1M, respectively). A combined experimental approach (XRD, HRTEM, N2 physisorption, solid-state MAS NMR, and TGA) was used to investigate on the structural, morphological, and textural features of the samples treated under mild and strong acid conditions. FTIR spectroscopy of adsorbed probe molecules with different basicity (e.g., CO and NH3) was used to monitor the surface acid properties and acid site distribution. XRD and SS-MAS NMR indicated that the activation under mild acid conditions does not alter the clay structure, while a deep modification of the saponite framework occurred after ion exchange in 1 M HCl solution. The presence of porous amorphous silica phase after treatment under strong acid conditions was confirmed by TEM inspection augmented by SS-MAS NMR and FTIR spectroscopy. N2 and Ar physisorption measurements suggested that cavitation phenomena occurred in saponite structure. N2 physisorption confirmed that the porosity and surface area of the samples are strongly modified upon strong acid treatment. FTIR spectroscopy of adsorbed NH3 pointed out that the H-exchange in mild conditions increased the number of surface Brønsted acid sites. Conversely, these sites are significantly depleted after treatment under strong acid conditions. The use of CO as a FTIR probe molecule, which is applied for the first time to study synthetic acid clays, allowed to monitor distribution and strength of Brønsted acid sites, whose acidity is similar to that of strong acid zeolites. The Al-OH sites with medium acidity are also found in acid-activated saponites. The distribution of strong and medium acid sites is strictly dependent on the acid conditions adopted.

An efficient ring opening reaction of methyl epoxystearate promoted by synthetic acid saponite clays

The acid-catalysed reaction of methyl 9,10-epoxystearate ring opening using synthetic saponite acid clay as catalyst, has been studied for the first time. In the presence of methanol, 90% of the epoxide substrate is converted after 5 min and the main reaction product is the vicinal hydroxyether, methyl methoxyhydroxystearate, with 84% of selectivity. In the absence of alcohol the ring opening reaction proceeds slower, leading to a mixture of methyl 9- and 10-oxostearate as main products, and a 9,10-epoxystearate conversion of 66% after 1 h. The performance of acid saponite, an environmentally benign catalyst, is exceedingly higher than those of strong mineral acids, such as H2SO4, widely used for this reaction.

The interactions of methyl tert-butyl ether on high silica zeolites: a combined experimental and computational study

In this work, the interactions of methyl tert-butyl ether (MTBE) on different dealuminated high silica zeolites were studied by means of both experimental and computational approaches. Zeolites with different textural and surface features were selected as adsorbents and the effect of their physico-chemical properties (i.e. pore size architecture and type and amount of surface OH sites) on sorption capacity were studied. High silica mordenite (MOR) and Y zeolites (both with a SiO2/Al2O3 ratio of 200) and ZSM-5 solid (SiO2/Al2O3 ratio of 500) were selected as model sorbents. By combining FTIR and SS-NMR (both (1)H and (13)C CPMAS NMR) spectroscopy it was possible to follow accurately the MTBE adsorption process on highly defective MOR characterized by a high concentration of surface SiOH groups. The adsorption process is found to occur in different steps and to involve isolated silanol sites, weakly interacting silanols, and the siloxane network of the zeolite, respectively. H-bonding and van der Waals interactions occurring between the mordenite surface and MTBE molecules were modeled by DFT calculations using a large cluster of the MOR structure where two adjacent side-pockets were fused in a large micropore to simulate a dealumination process leading to silanol groups. This is the locus where MTBE molecules are more strongly bound and stabilized. FTIR spectroscopy and gravimetric measurements allowed determination of the interaction strength and sorption capacities of all three zeolites. In the case of both Y and MOR zeolites, medium-weak H-bonding with isolated silanols (both on internal and external zeolite surfaces) and van der Waals interactions are responsible for MTBE adsorption, whereas ZSM-5, in which a negligible amount of surface silanol species is present, displays a much lower amount of adsorbed MTBE retained mainly through van der Waals interactions with zeolite siloxane network.