Françoise Quignard

Polymer-supported metals and metal oxide nanoparticles: synthesis, characterization, and applications

Metal and metal oxide nanoparticles exhibit unique properties in regard to sorption behaviors, magnetic activity, chemical reduction, ligand sequestration among others. To this end, attempts are being continuously made to take advantage of them in multitude of applications including separation, catalysis, environmental remediation, sensing, biomedical applications and others. However, metal and metal oxide nanoparticles lack chemical stability and mechanical strength. They exhibit extremely high pressure drop or head loss in fixed-bed column operation and are not suitable for any flow-through systems. Also, nanoparticles tend to aggregate; this phenomenon reduces their high surface area to volume ratio and subsequently reduces effectiveness. By appropriately dispersing metal and metal oxide nanoparticles into synthetic and naturally occurring polymers, many of the shortcomings can be overcome without compromising the parent properties of the nanoparticles. Furthermore, the appropriate choice of the polymer host with specific functional groups may even lead to the enhancement of the properties of nanoparticles. The synthesis of hybrid materials involves two broad pathways: dispersing the nanoparticles (i) within pre-formed or commercially available polymers; and (ii) during the polymerization process. This review presents a broad coverage of nanoparticles and polymeric/biopolymeric host materials and the resulting properties of the hybrid composites. In addition, the review discusses the role of the Donnan membrane effect exerted by the host functionalized polymer in harnessing the desirable properties of metal and metal oxide nanoparticles for intended applications.

Functionalized Chitosan as a Green, Recyclable, Biopolymer-Supported Catalyst for the 3+2 Huisgen Cycloaddition

Owing to increasing concern about environmental impact, tremendous effort has been made towards the development of new processes that minimize pollution in chemical synthesis. For this reason and others (catalyst removal, recovery, and recycling), heterogeneous catalysis is clearly on the rise, including in industry. Of the many systems that have been developed over the past decades, metallic species supported on inorganic materials (e.g. SiO2, Al2O3) or on charcoal are the most common. The immobilization of transition metals on polymer supports derived from petrochemicals (e.g. polystyrenes) has also been the focus of many efforts. Recent developments for cleaner, sustainable chemistry are being driven by a shift from petrochemical-based feedstocks to biological materials. There is considerable interest in exploiting natural polymer macrostructures, and in particular those of polysaccharides, to create high-performance and environmentally friendly catalysts. Indeed, polysaccharides present many advantages that may stimulate their use as polymeric supports for catalysis: 1) They are present in enormous quantity on earth, 2) they contain many functionalities that can be used readily for the anchoring of organometallic species, 3) they contain many stereogenic centers, and 4) they are chemically stable but biodegradable. Surprisingly, although there has been a worldwide realization that nature-derived polysaccharides can provide the raw materials needed for the production of numerous industrial consumer goods, their use as supports for catalysis is still in its infancy. Chitosan (Figure 1 A) is a particularly attractive polysaccharide for application in catalysis owing to the presence of readily functionalizable amino groups and its insolubility in organic solvents. A copolymer of b(1!4)-2-amino-2-deoxyd-glucopyranose and 2-acetamido-2-deoxy-d-glucopyranose, chitosan results from incomplete deacetylation of chitin. At least 10 gigatons of chitin are constantly present in the biosphere; thus, chitosan is a renewable green material. Of

Aerogel materials from marine polysaccharides

Hydrocolloid-forming polysaccharides are natural polyelectrolytes able to form stable hydrogels largely used in the food and pharmaceutical industry. Gelling polysaccharides derived from seaweeds or wastes of the seafood industry include polymers with several functional groups: alginates (carboxylic groups), carrageenans (sulfonic groups) and chitosan (amino groups). This article deals with suitable methods to prepare dry materials which retain the dispersion of the polymer hydrogel, namely polysaccharide aerogels. The materials whose properties are herewith described satisfy most of the appropriate requirements for heterogeneous catalysts and supports: they are stable in most organic solvents, present a high surface area and diverse accessible surface functionalities. Their application as catalysts, catalyst supports or adsorbents provide a new opportunity to obtain useful materials from one of the less energy-intensive sources of biomass.

Catalytic Cleavage of the C-H and C-C Bonds of Alkanes by Surface Organometallic Chemistry: An EXAFS and IR Characterization of a Zr-H Catalyst

The catalytic cleavage under hydrogen of the C & singlebond;H and C & singlebond;C bonds of alkanes with conventional catalysts requires high temperatures. Room-temperature hydrogenolysis of simple alkanes is possible on a well-defined and well-characterized zirconium hydride supported on silica obtained by surface organometallic chemistry. The surface structure resulting from hydrogenolysis of (≡SiO)Zr(Np)3 (Np, neopentyl) was determined from the extended x-ray absorption fine structure (EXAFS) and 1H and 29Si solid-state nuclear magnetic resonance and infrared (IR) spectra. A mechanism for the formation of (≡SiO)3Zr-H and (≡SiO)2SiH2 and the resulting low-temperature hydrogenolysis of alkanes is proposed. The mechanism may have implications for the catalytic formation of methane, ethane, and lower alkanes in natural gas.

From supported homogeneous catalysts to heterogeneous molecular catalysts

Abstract Numerous methods are now available for the synthesis of a large variety of heterogeneous molecular catalysts. Their specific domain of application, and the main advantages and drawbacks of these solids are briefly analyzed here.

Supercritical CO2 dried chitosan: an efficient intrinsic heterogeneous catalyst in fine chemistry

Chitosan microspheres are used as catalysts for the synthesis of monoglyceride by fatty acid addition to glycidol. Microspheres are obtained by drying gel beads of the natural polymer under supercritical CO2 conditions, which makes the access to the polymer functional groups easy.

Characterization of a gel in the cell wall to elucidate the paradoxical shrinkage of tension wood.

Wood behavior is characterized by high sensibility to humidity and strongly anisotropic properties. The drying shrinkage along the fibers, usually small due to the reinforcing action of cellulosic microfibrils, is surprisingly high in the so-called tension wood, produced by trees to respond to strong reorientation requirements. In this study, nitrogen adsorption-desorption isotherms of supercritically dried tension wood and normal wood show that the tension wood cell wall has a gel-like structure characterized by a pore surface more than 30 times higher than that in normal wood. Syneresis of the tension wood gel explains its paradoxical drying shrinkage. This result could help to reduce technological problems during drying. Potential applications in biomechanics and biomimetics are worth investigating, considering that, in living trees, tension wood produces tensile growth stresses 10 times higher than that of normal wood.

Structure of Alginate Gels: Interaction of Diuronate Units with Divalent Cations from Density Functional Calculations

The complexation of (1→4) linked α-L-guluronate (G) and β-D-mannuronate (M) disaccharides with Mg(2+), Ca(2+), Sr(2+), Mn(2+), Co(2+), Cu(2+), and Zn(2+) cations have been studied with quantum chemical density functional theory (DFT)-based method. A large number of possible cation-diuronate complexes, with one and two GG or MM disaccharide units and with or without water molecules in the inner coordination shells have been considered. The computed bond distances, cation interaction energies, and molecular orbital composition analysis revealed that the complexation of the transition metal (TM) ions to the disaccharides occurs via the formation of strong coordination-covalent bonds. On the contrary, the alkaline earth cations form ionic bonds with the uronates. The unidentate binding is found to be the most favored one in the TM hydrated and water-free complexes. By removing water molecules, the bidentate chelating binding also occurs, although it is found to be energetically less favored by 1 to 1.5 eV than the unidentate one. A good correlation is obtained between the alginate affinity trend toward TM cations and the interaction energies of the TM cations in all studied complexes, which suggests that the alginate affinities are strongly related to the chemical interaction strength of TM cations-uronate complexes. The trend of the interaction energies of the alkaline earth cations in the ionic complexes is opposite to the alginate affinity order. The binding strength is thus not a limiting factor in the alginate gelation in the presence of alkaline earth cations at variance with the TM cations.

From Natural Polysaccharides to Materials for Catalysis, Adsorption, and Remediation

Polysaccharides display most of the properties needed for applications in catalysis, adsorption or remediation. Requisites common to these applications are appropriate surface functions to ensure substrate-material interactions, accessibility of the functional groups, and proper shaping for easy manipulation. Natural polysaccharides are well known as supports for enzymatic catalysts and gelling agents in aqueous phase, due to the high level of dispersion of hydrocolloids. However, they suffer from diffusional limitations in the dry state, due to the low surface area of the dried materials generally used, xerogels or lyophilized solids. This chapter deals with the proper methods to prepare dry materials which retain the dispersion of the polymer hydrogel, namely polysaccharide aerogels. The materials whose properties are described here are stable in most organic solvents and present numerous and diverse surface functionalities (like hydroxy, carboxy, or amino groups). Shaping and appropriate drying of gelling polysaccharides provide a new opportunity to obtain materials from one of the less energy-intensive sources of biomass. Their application in catalysis and adsorption could open substantial markets for products of seaweed harvesting and coproducts of the seafood industry.

Palladium(0)-catalyzed substitution of allylic substrates in perfluorinated solvents

Abstract Palladium(0)-catalyzed alkylation reactions of allylic substrates can be performed using the new concept of fluorous biphasic system, allowing a very easy recycling of the catalyst.