B. Corain

Aluminium toxicity and metal speciation: established data and open questions

Histopathological findings in humans and extensive toxicological investigation in vivo and in vitro point to an unambiguous neurotoxic potency of AI(III). Experimental toxicology with aqueous AI(III) is very difficult owing to the complex and somewhat uncontrollable aqueous chemistry of the metal centre in the neutral range. The choice of neutral, hydrolytically stable synthetic toxins makes possible both the control of metal speciation and of analytical metal concentration down to about 1 mM in neutral buffered solutions. The employment of less stable complexes like Al2(citrate)2(H2O)6 and AI(lactate)3 or of ordinary salts is unavoidably complicated by the precipitation of Al(OH)3 under the same conditions. In spite of this, the choice of a carefully designed protocol, based on ensuring well defined steps, enables one to successfully control the analytical metal concentration down to 10 μM AI(III). The control of the metal speciation at these concentration levels remains an open question.

Functional resins as innovative supports for catalytically active metal nanoclusters

Functional resins are plausible candidates for supporting catalytically active metal nanoclusters alternative, or complementary to conventional supports such as amorphous carbon, metal oxides, zeolites. Mechanical and thermal stabilities are fair and perfectly suitable to application in the realm of fine chemistry. The polymer framework turns out to be a designable template for controlling nanoclusters size and size distributions.

Metal palladium dispersed inside macroporous ion-exchange resins: the issue of the accessibility to gaseous reactants

Abstract Commercial macroporous ion-exchange resins (Lewatit SPC 118 and UCP 118) were used to support Pd metal by a two step ion-exchange and reduction procedure. The textural features of the resins were determined by Inverse Steric Exclusion Chromatography (ISEC) measurements. TEM characterization of the obtained Pd/resin composite showed the presence of uniformly sized Pd crystallites located at the macropore “surface”. Pulse chemisorption analysis gave evidence for the lack of accessibility of the crystallites when the resin composite is in the dry state. This suggests that the metal particles are in fact embedded in a gel-type resin layer at the “surface” of the macropores and therefore practically unaccessible to the molecules of a gaseous phase unless the support is in the swollen state.

Metal palladium supported on amphiphilic microporous synthetic organic supports. Part I. Material preparation and textural characterization

Abstract Styrene, 2-methacryloxyethylsulfonic acid and methylenebisacrylamide are copolymerized in the presence of dimethylformamide under γ -irradiation at room temperature to give potentially amphiphilic, microporous resins. The molar fraction of methylenebisacrylamide (the cross-linker) ranges from 1 to 6 mol%. The obtained resins, after grinding and sieving, are treated with solutions of Pd(AcO) 2 and the ion-exchanged polymers are then treated with NaBH 4 in ethanol. The metal content in the materials ranges from 2.1% to 2.4% (w/w). X-ray microprobe analysis shows that Pd(0) is homogeneously dispersed throughout the particles of the polymeric materials. The polymer chain concentration and its distribution in the swollen macromolecular networks are assessed by means of Inverse Steric Exclusion Chromatography (ISEC). In spite of the higher molar proportion of styrene repeating units, the Pd(0)-resin composites swell comparably in water and tetrahydrofurane. Their amphiphilic character is the result of the different swelling properties of the hydrophilic and hydrophobic domains.

Polybenzimidazole as a promising support for metal catalysis: morphology and molecular accessibility in the dry and swollen state

Polybenzimidazole (PBI) in beaded form (250-500 microm) has been characterized in the dry state by scanning electron microscopy (SEM), BET, and nitrogen porosimetry. In the swollen state, it has been characterized by inverse steric exclusion chromatography (ISEC) in tetrahydrofuran, toluene, and water, by ESR of TEMPONE (2,2,6,6-tetramethyl-4-oxo-1-oxypiperidine), and pulse field gradient spin echo (PGSE) NMR spectroscopy, toluene, in tetrahydrofuran, ethanol and water. The dry-state results are in good agreement with the ISEC results obtained in tetrahydrofuran, toluene, and water with regard to the 40-80 nm macroporosity. The swelling-dependent surface area and pore volume detected by ISEC in toluene and water reveal the amphiphilic nature of PBI.

Polymer frameworks as templates for generating size-controlled metal nanoclusters Active and reusable metal catalysts based on organic resins and on organic/inorganic composites

Abstract Catalysts based on Pd nanoparticles supported on polyacrylic resins or on polyacrylic resins/silica composites are evaluated in terms of catalyst reusability and mechanical stability. Both technologically relevant features are satisfactory. Resin/silica/Pd composites exhibit an appreciable but not dramatic decrease of specific catalytic activity (3–4 times) when compared with their parent resin/Pd materials, but this decrease is compensated by higher mechanical stability and easier separability of the catalyst from the reaction mixture.

Metal palladium supported on amphiphilic microporous synthetic organic supports. Part II. Catalytic investigations

Abstract Two sets of resin-supported Pd(0) catalysts were compared in the hydrogenation of cyclohexene in methanol, under ambient conditions. Two lightly crosslinked poly-{styrene/2-methacryloxyethylsulfonic acid/methylenebisacrylamide} with different degrees of crosslinking (1 and 3 mol%), have been used as catalyst supports. Two catalysts with different palladium loadings (0.22 and 2.2 wt%) have been prepared from each support. When the size of the particles was relatively small, the catalysts with 2.2% and 0.22% Pd were almost equally active with respect to the amount of palladium in the reaction mixture. In contrast, when larger catalyst particles were used, catalysts with 2.2% metal produced conversion profiles typical of diffusion-controlled regimes (productivity decreased with increasing crosslinking). These catalysts were characterised by combined ESR and static gradient spin echo NMR spectrometries, which have provided information about molecular motion inside the polymer networks.

Metal palladium dispersed inside macroporous ion-exchange resins: rotational and translational mobility inside the polymer network

Abstract Commercial macroporous strongly acidic ion-exchange resin (Bayer UCP 118) in its acidic and Na + forms and a resin/Pd composite were investigated by means of electron spin resonance (ESR) and pulse field gradient spin echo-nuclear magnetic resonance (PGSE-NMR) techniques, to determine their molecular accessibility to 2,2,6,6-tetramethyl-4-oxo-1-oxypiperidine (TEMPONE ) dissolved in water and the molecular mobility of both TEMPONE and water inside their accessible domains. The paramagnetic probe and water itself occupy both the micro- and macroporous domains of the resin particles, among which a rapid exchange is observed.

High-yield γ-ray-induced polymerization of bis-2-acetoacetoxyethyl methacrylate copper. II

The copper (II) acrylic monomer Cu(aaem) 2 (aaem = 2-acetoacetoxyethyl methacrylate) undergoes high-yield y-ray copolymerization with dimethylacrylamide, styrene, and hydroxoethyl methacrylate in the presence of methylene bisacrylamide as crosslinking agent in dimethylformamide. The metalloorganic copolymers are fully characterized. Polymer-protected Cu(II) results in being extremely stable toward a number of reducing agents in different media.

Generating palladium nanoclusters inside very lipophilic gel-type functional resins: preliminary catalytic tests in the hydrogenation of 2-ethyl-anthraquinone to 2-ethylanthrahydroquinone

Abstract Four lipophilic polystyrene–methacryloylethylensulfonic acid–divinylbenzene gel-type resins, with 2 and 4% cross-linking degree, are synthesized and used as supports for “atomic” dispersion of Pd(II) finalized to the preparation of resins-supported palladium metal catalysts. Properly performed reduction of Pd(II) to Pd(0) achieves homogeneous dispersion of ca. 3xa0nm metal nanoclusters inside the supports particles in the more cross-linked resin. Resulting resin-supported palladium catalysts are efficient and moderately chemoselective in the hydrogenation of 2-ethylanthraquinone to 2-ethylanthrahydroquinone.