Alexandra Fidalgo

The Sol–Gel Route to Advanced Silica-Based Materials and Recent Applications

Applications Rosaria Ciriminna,† Alexandra Fidalgo,‡ Valerica Pandarus, Franco̧is Beĺand, Laura M. Ilharco,*,‡ and Mario Pagliaro*,† †Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy ‡Centro de Química-Física Molecular and IN-Institute of Nanoscience and Nanotechnology, Instituto Superior Tećnico, Complexo I, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal SiliCycle Inc., 2500, Parc-Technologique Boulevard, Quebec City, Quebec G1P 4S6, Canada

The structural origins of superior performance in sol–gel catalysts

Why do sol-gel catalysts often show superior performance in terms of selectivity, stability and reactivity? This work is an attempt to provide a rationale which could be used as a predictive tool in the development of novel catalysts for chemical conversions that will be crucial to achieve a more sustainable development.

The Structure of Hybrid Gels by Drift and NMR Spectroscopies

Hybrid inorganic-organic gels have been prepared by the sol-gel process using tetraethoxysilane (TEOS) as precursor, mixed with a low concentration of polytetrahydrofuran (PTHF), under acid catalysis. The hybrid xerogels were characterized by DRIFTS and Solid State 1H, 13C and 29Si NMR. The DRIFT spectra indicate that the polymer is responsible for decreasing the number of free silanol groups in comparison to pure silica. Solid-state NMR spectra reveal the types of silicate structures formed and the conditions for establishing chemical bonds between the two phases, which are responsible for the silica network flexibility. We have concluded that it is possible to design a hybrid gel with tailored properties, even at very low polymer concentration, by selecting the appropriate preparation route.

Flexible hybrid aerogels prepared under subcritical conditions

A new flexible insulator was developed, consisting of a silica–latex-fiber nanocomposite aerogel that combines the lightness and insulating properties of silica aerogels with the mechanical properties of silica–latex hybrids and the flexibility of fibers. This was accomplished by coating selected insulating fibers (polyester and glass) with a silica–polymer hybrid aerogel, synthesized in situ by a two-step sol–gel process, consisting of the acidic co-hydrolysis of TEOS and trimethoxysilyl-grafted latex nanoparticles in excess water, followed by basic co-condensation. The hybrid coating has improved mechanical properties that allow its drying under subcritical conditions. The outstanding properties of the insulating blankets produced render them extremely attractive for high-technology applications, and, due to the low energy consumption process, they may even displace the commonly used mineral wool, by presenting a best value-for-money compromise.

New Catalyst Series from the Sol–Gel-Entrapment of Gold Nanoparticles in Organically Modified Silica Matrices: Proof of Performance in a Model Oxidation Reaction

Gold nanoparticles were sol–gel‐entrapped in the mesoporous structure of organically modified silica matrices, and the resulting SiliaCat Au materials were tested as oxidation catalysts in the model reaction of 1‐phenylethanol with hydrogen peroxide. The new materials were characterized by electronic microscopy, IR spectroscopy, and cryogenic N2 sorption. The results point to conclusions of general validity that allow us to advance towards the practical use of supported gold nanoparticles in a number of applications.

Nanostructured silica/polymer subcritical aerogels

The incorporation of functionalized core–shell polymer nanoparticles in a silica network, by the sol–gel process, yields a structure stiff enough to withstand capillary pressure upon subcritical drying. The resulting hybrid aerogels are machinable monoliths, stable under atmospheric conditions, and have improved mechanical performance. These combined properties make them strong candidates for insulation in high value applications.

Thickness, Morphology and Structure of Sol-Gel Hybrid Films: II—The Role of the Solvent

The influence of the solvent on the thickness, morphology and structure of silica-polytetrahydrofuran hybrid films, prepared by spin coating, has been analysed. The inorganic precursor, tetraethylorthosilicate, was hydrolysed under acid catalysis, the hydrolysis molar ratio being 4. Polymers of average molecular weight (Mn) 650 and 2900 were incorporated in the initial colloidal solutions, in a low concentration (organic/inorganic molar ratio 0.01). Two solvents were compared: ethanol, protic, and tetrahydrofuran, aprotic and a little less polar. The thickness and surface texture parameters of the films were determined by profilometry, their morphology characterized by SEM and their structure studied by FTIR. It is shown that the solvent has no effect on the molecular structure of the films, but strongly influences the surface texture and the morphology of both pure silica and hybrid films. The solutions prepared in tetrahydrofuran present shorter gelation times (tG) and allow the deposition of good quality films almost up to the gelation point (to a reduced time, t/tG, of ∼0.9). The films are thinner than those prepared from corresponding ethanolic solutions at the same reduced ageing times. For pure silica films, tetrahydrofuran is the best choice, since it reduces the fractured region on the edge of the substrate. For hybrid films, this effect is achieved by the polymer and tetrahydrofuran is responsible for a higher arithmetical mean roughness. Therefore, ethanol becomes the preferable solvent.

Encapsulation of ruthenium nitrosylnitrate and DNA purines in nanostructured sol-gel silica matrices.

The interactions between DNA purines (guanine and adenine) and the ruthenium complex Ru(NO)(NO(3))(3) were studied within nanostructured silica matrices prepared by a two-step sol-gel process. By infrared analysis in diffuse reflectance mode, it was proved that encapsulation induces a profound modification on the complex, whereas guanine and adenine preserve their structural integrity. The complex undergoes nitrate ligand exchange and co-condenses with the silica oligomers, but the nitrosyl groups remain stable, which is an unusual behavior in Ru nitrosyl complexes. In turn, the doping molecules affect the sol-gel reactions and eventually the silica structure as it forms: the complex yields a microporous structure, and the purine bases are responsible for the creation of macropores due to hydrogen bonding with the silanol groups of the matrix. In a confined environment, the interactions are much stronger for the coencapsulated pair guanine complex. While adenine only establishes hydrogen bonds or van der Waals interactions with the complex, guanine bonds covalently to Ru by one N atom of the imidazole ring, which becomes strongly perturbed, resulting in a deformation of the complex geometry.

Hybrid Silica Gel-Polytetrahydrofuran Thin Films

The sol-gel process has been used to prepare hybrid silica gel-polytetrahydrofuran (PTHF) thin films, using tetraethylorthosilicate (TEOS) as the precursor. The films were prepared by spin-coating, and characterized by profilometry, transmission FTIR and SEM. Film properties were conditioned by the preparation procedure as well as by the initial recipe, namely the solvent, polymer content and molecular weight. Evidence was obtained of the participation of the polymer in the condensation step of the process, interacting by hydrogen bonds with hydroxyl groups of the silicate network.

Polymers of Limonene Oxide and Carbon Dioxide: Polycarbonates of the Solar Economy

Limonene epoxide (1,2-limonene oxide) readily reacts with carbon dioxide inserted in a ring-opening copolymerization reaction and forms polycarbonates of exceptional chemical and physical properties. Both poly(limonene carbonate) and poly(limonene dicarbonate) can be synthesized using low-cost Zn or Al homogeneous catalysts. This study addresses selected relevant questions concerning the technical and economic feasibility of limonene and carbon dioxide polymers en route to the bioeconomy.