Giorgio Gatti

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).

Sulfonamide Antibiotics Embedded in High Silica Zeolite Y: A Combined Experimental and Theoretical Study of Host−Guest and Guest−Guest Interactions

A combined experimental and computational study of the interactions of three sulfonamides--sulfadiazine, sulfamethazine, and sulfachloropyridazine--embedded into the cages of high silica zeolite Y is here proposed. For all host-guest systems, the close vicinity of aromatic rings with zeolite framework was evidenced by multidimensional and multinuclear ((1)H, (13)C, (29)Si) SS-NMR measurements. Host-guest and guest-guest interactions were also elucidated by in situ FTIR spectroscopy and confirmed by ab initio computational modeling. Single molecules of sulfamethazine and sulfachloropyridazine were stabilized inside the zeolite cage by the vicinity of methyl and amino groups, respectively. Sulfadiazine is present in both monomeric and dimeric forms. Multiple weak H-bonds and van der Waals type interactions between organic molecules and zeolite are responsible for the irreversible extraction from water of all the examined sulfa drugs.

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.

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.

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.

Theoretical prediction of high pressure methane adsorption in porous aromatic frameworks (PAFs).

The adsorption isotherms of methane in four micro- and mesoporous materials, based on the diamond structure with (poly)phenyl chains inserted in all the C-C bonds, have been simulated with Grand Canonical Monte Carlo technique. The pressure range was extended above 250 bar and the isotherms were computed at 298, 313, and 353 K, to explore the potentiality of these materials for automotive applications, increasing the capacity of high-pressure tanks or storing a comparable amount of gas at much lower pressure. The force field employed in the simulations was optimized to fit the correct behavior of the free gas in all the pressure range and to reproduce the methane-phenyl interactions computed at high quantum mechanical level (post Hartree-Fock). All the examined materials showed a high affinity for methane, ensuring a larger storage of gas than simple compression in all the conditions: two samples exceeded the target proposed by U.S. Department of Energy for methane storage in low-pressure fuel tanks (180 cm(3) (STP)/cm(3) at 35 bar and room temperature).

Embedding monomers and dimers of sulfonamide antibiotics into high silica zeolite Y: an experimental and computational study of the tautomeric forms involved

This work is a second step towards the systematic study of the embedding of sulfonamide antibiotics into a synthetic high silica zeolite Y (HSZ-Y) with hydrophobic properties. In the previous paper [Braschi et al., Langmuir 2010, 31, 9524], the irreversible adsorption from water into HSZ-Y of three sulfonamides was studied by enlightening the host–guest interactions and, in the case of the smallest sized sulfadiazine, the guest–guest interactions of dimeric species inside the zeolite cage. Here the HSZ-Y was loaded with six sulfonamides, namely: sulfanilamide, sulfapyridine, sulfathiazole, sulfadimethoxine, sulfadoxine and sulfamerazine. With the exception of sulfanilamide, which showed scarce affinity for HSZ-Y (maximum loading 3% zeolite dry weight), the other sulfa drugs adsorbed at ca. 28% zeolite dry weight on average, and this is relevant for both water depollution and drug delivery issues. The low affinity of sulfanilamide for HSZ-Y was ascribed to its high hydrophilicity (water solubility 15–40 times higher than other drugs). The most stable tautomeric (amide or imide) form of each antibiotic adsorbed in zeolite Y was proposed by means of IR and solid state NMR spectroscopy augmented by computational modelling. The small dimensions and favourable stabilization energy allow the embedding of imidic and amidic dimers of sulfathiazole and sulfapyridine, respectively, inside the zeolite cage whereas the remaining sulfa drugs adsorbed in monomeric amidic forms.

One-Pot Synthesis and Physicochemical Properties of an Organo-Modified Saponite Clay

An organo-saponite clay containing intercalated cetyltrimethylammonium (CTA(+)) cations was synthesized by an efficient one-step hydrothermal method and was compared with a CTA-exchanged saponite prepared by a classical postsynthesis intercalation route. In both hybrid samples, surfactant loading up to 10% was achieved. A comparative investigation of the physicochemical properties of both solids was carried out by a multidisciplinary approach, by using a combination of spectroscopic, structural, and thermal characterization tools. Powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) data indicated that the one-pot-prepared solid showed that the presence of CTA(+) molecules in the synthesis gel did not affect the clay structure. In addition, thermal analysis suggested that the inorganic layers play an active role in stabilizing and protecting the surfactant molecules by increasing their thermal stability. A different arrangement of intercalated CTA(+) ions in the two hybrid clays was observed by solid state NMR in combination with Fourier transform infrared (FTIR) spectroscopy and assigned to a different all-trans/gauche conformation ratio of the surfactant depending on the synthetic method used to prepare the two final materials. The surfactant organization is also influenced by the lamellae charge density, which is different in the two organo-modified materials as found by (27)Al and (29)Si MAS NMR experiments.