Esther Carbonell

Novel photocatalysts containing 2,4,6-triphenylthiapyrylium encapsulated within zeolites. Enhanced photocatalytic activity as compared to the pyrylium analogues

A series of solid photocatalysts containing either 2,4,6-triphenylpyrylium (TP+) or 2,4,6-triphenylthiapyrylium (TPTP+) encapsulated within zeolite Y or Beta, or deposited on amorphous silica or MCM-41, have been prepared. These photocatalysts have been tested for the degradation of aqueous solutions of phenol (40 ppm) or aniline (40 or 80 ppm, initial pH 3) as model reactions. TPTP+ photocatalysts were found to be more active and stable than those containing TP+, the initial efficiency order being: TPTP/MCM-41≈TPTP/SiO2>TPTP@Beta>TPTP@Y>TiO2. In the case of TPTP/SiO2 and TPTP/MCM-41 the photocatalysts are largely decomposed during the photocatalytic reaction, as compared to TPTP+ encapsulated within zeolites Y and Beta; in the latter, although the initial activity is considerably lower the dye persists after the irradiation and can be reused. In this way the final productivity of TPTP@Beta is at least four times higher than when the dye is supported on SiO2 or MCM-41. Addition of an initial concentration of H2O2 (10−3 M) accelerates the photodegradation of phenol by a factor of 6 but it does not have much influence in the case of aniline.

Direct Time-Resolved Detection of Singlet Oxygen in Zeolite-Based Photocatalysts

Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics.

Ionic Liquids as a Novel Medium for Photochemical Reactions. Ru(bpy)32+/ Viologen in Imidazolium Ionic Liquid as a Photocatalytic System Mimicking the Oxido-Reductase Enzyme†

Abstract Ionic liquids are suitable media which stabilize charged intermediates favoring those mechanisms that occur through charge separation. We have used ionic liquids to develop a photocatalytic system to perform the reduction of a carbonyl group to alcohol, thus mimicking the behavior of the reductase enzymes. The photochemical cycle is based on the well-known electron transfer from the Ru(bpy)32+ complex in its excited state, acting as electron donor to MV2+, which acts as electron acceptor. The initial electron transfer process can be promoted upon selective Ru(bpy)32+ excitation by visible light. By means of laser flash photolysis we have provided evidence of the nature and lifetimes of the intermediates involved in the photocatalytic system. Thus, the initial electron transfer between Ru(bpy)32+ triplets and viologen MV2+ forms the MV•+ radical cation, which upon accepting an H· atom from a suitable hydrogen atom donor, forms the corresponding dihydropyridine MVH+ reducing agent.

Imidazolium-Functionalized Carbon Nanohorns for the Conversion of Carbon Dioxide: Unprecedented Increase of Catalytic Activity after Recycling

Six new hybrid materials composed of carbon nanohorns (CNHs) and highly cross-linked imidazolium salts were easily synthesized using a one-step procedure based on the radical oligomerization of bis-vinylimidazolium salts (bVImiX) in the presence of pristine CNHs. The hybrid materials were characterized and employed as the sole catalysts for the conversion of carbon dioxide into cyclic carbonate by reaction with epoxides. The solids displayed excellent turnover number and productivity. Moreover, four catalysts were investigated in recycling experiments. Two catalysts containing an octyl linker between the imidazolium units and a bromide or an iodide anion showed no loss in activity after three cycles. The other two catalysts containing a p-xylyl linker and a bromide anion and different CNHs/bVImiX ratios showed an unprecedented increase of activity after recycling.

Proximity Effect using a Nanocage Structure: Polyhedral Oligomeric Silsesquioxane-Imidazolium Tetrachloro- palladate Salt as a Precatalyst for the Suzuki-Miyaura Reaction in Water

A polyhedral oligomeric silsesquioxane‐imidazolium tetrachloropalladate salt (POSS‐Imi‐PdCl4) was prepared by the reaction of a polyhedral oligomeric silsesquioxane‐imidazolium chloride salt (POSS‐Imi‐Cl) with PdCl2 and used as a pre‐catalyst for the Suzuki–Miyaura reaction in water at 100 °C at a low loading (0.08–0.16 mol %). Biphenyl compounds were isolated in high to excellent yields. A comparison of the POSS‐based catalyst with the corresponding catalyst without the nanocage structure (i.e., 1‐butyl‐3‐methylimidazolium tetrachloropalladate) highlighted the role of the POSS structure to reach higher yields in the Suzuki–Miyaura reaction. This result is ascribed to a proximity effect of the imidazolium moieties linked to the nanocage structure. Fresh and used catalytic materials were characterised by using X‐ray photoelectron spectroscopy and TEM.

Silsesquioxane–Terpyridine Nano Building Blocks for the Design of Three-Dimensional Polymeric Networks

Novel polyhedral oligomeric silsesquioxanes (POSS) decorated with eight terpyridine moieties were synthesized in a one-pot procedure via Heck coupling reaction with the aim of investigating the possible formation of three-dimensional extended supramolecular organizations. The monosubstituted analogue was also prepared and used as a model compound. Both POSS-based nanostructures were extensively characterized via 1H, 13C, and 29Si nuclear magnetic resonance (NMR), ultraviolet-visible spectroscopies and combustion chemical analysis. The assembly of these nanocaged bricks and two different metal ions (Zn2+ and Fe2+) was investigated via 1H NMR as well as absorption and emission spectroscopy. Both mono- and octa-terpyridine-functionalized POSS (O-POSS) displayed interesting photophysical properties. Moreover, under selected conditions, the O-POSS forms stable gels at room temperature and can easily be shaped in the form of a film with potential applications in nanotechnology.