Mike Robitzer

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

Nanostructure of calcium alginate aerogels obtained from multistep solvent exchange route

Ca-alginate materials were studied by small-angle X-ray scattering (SAXS) at different steps of conversion from gel to aerogel in order to determine the relation between the polymer organization at the nanoscale in the gels and the final dry aerogel. In all cases, i.e. before and after the different exchanges of solvents and after the formation of the aerogel, the SAXS patterns exhibit an asymptotic behavior at low q values (in the experimental q range 7x10(-3) up to 2.10(-2) A(-1)) close to I(q) approximately q(-1), indicative of randomly oriented rod-like scattering objects. The evolution of the diameter of such rod-like objects was thus deduced from the maxima observed on Kratky plots, i.e. I(q) q2 vs q. The results are in perfect agreement qualitatively (rod-like anisometry type of the scattering objects) and quantitatively (diameter of the rods) with direct SEM observations of the morphology of aerogels and with the results of N2 adsorption on the aerogel. This is evidence that in the chosen experimental processing conditions, the morphology of the aerogel depends on the morphology of pre-existing objects within the gel, i.e. that the structure of the aerogel provides a correct image of the structure of the parent gel.

Structural regime identification in ionotropic alginate gels: influence of the cation nature and alginate structure.

The morphologies of several ionotropic alginate hydrogels and aerogels were investigated by SAXS according to the nature of the divalent metal cation (Mn(2+), Co(2+), Zn(2+), Cu(2+)) and the guluronic fraction of the alginate. All alginate hydrogel and aerogel samples show isotropic small-angle X-ray scattering. Gelation results from cooperative associations of cations and chain segments and yields different nanostructures, that is, nanofibrillar morphology or multiple junction morphology, according to cation type and eventually mannuronic/guluronic ratio. Therefore, Mn and Cu gels present the same morphology whatever the guluronic ratio, whereas Co and Zn gels yield different nanostructures. In the size range investigated by SAXS (~10-200 Å), the structure of aerogels obtained by CO(2) supercritical drying is found to be inherited from the morphology of the parent hydrogel whatever the initial structural regime.

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.

Photoluminescent Porous Alginate Hybrid Materials Containing Lanthanide Ions

The photoluminescence features of Eu(3+)-, Tb(3+)-, Tb(3+)/Eu(3+)-alginate aerogel (hydrogel and alcogel) and Eu(3+)-alginate xerogel hybrids were investigated. The Eu(3+)-alginate aerogel and alcogel exhibit the highest (5)D0 quantum efficiencies (9.9 and 8.2%, respectively), while the hydrogel and xerogel have lower values (5.2 and 5.6%, respectively). The Tb(3+)/Eu(3+) hybrids are multiwavelength emitters in which the emission color can be tuned across the chromaticity diagram from the red toward the yellowish-green spectral regions, crossing the white area by selecting the excitation wavelength.

Synthesis, texture, and photoluminescence of lanthanide-containing chitosan-silica hybrids

Three different types of photoluminescent hybrid materials containing trivalent lanthanide (Ln(3+) = Eu(3+), Tb(3+)) ions, chitosan, and silica have been prepared with different structural features. The different silica sources lead to diverse microstructures of hybrid materials, with silica being homogeneously dispersed in the chitosan materials (LnChS-H), or forming a core-shell morphology. Postsynthesis treatment is necessary for embedding the luminescent probe. The Ln(3+)-based materials have been investigated by photoluminescence spectroscopy (12-300 K). The chitosan-Eu(3+)-related local environment is maintained in the EuChS-H hybrid material. The emission features of the core-shell materials are characterized by the presence of two Eu(3+) distinct local environments, one associated with the chitosan core and the other with the silica shell.

Chitin-Prussian blue sponges for Cs(I) recovery: From synthesis to application in the treatment of accidental dumping of metal-bearing solutions

Prussian blue (i.e., iron[III] hexacyanoferrate[II], PB) has been synthesized by reaction of iron(III) chloride with potassium hexacyanoferrate and further immobilized in chitosan sponge (cellulose fibers were added in some samples to evaluate their impact on mechanical resistance). The composite was finally re-acetylated to produce a chitin-PB sponge. Experimental conditions such as the freezing temperature, the content of PB, the concentration of the biopolymer and the presence of cellulose fibers have been varied in order to evaluate their effect on the porous structure of the sponge, its water absorption properties and finally its use for cesium(I) recovery. The concept developed with this system consists in the absorption of contaminated water by the composite sponge, the in situ binding of target metal on Prussian blue load and the centrifugation of the material to remove treated water from soaked sponge. This material is supposed to be useful for the fast treatment of accidental dumping of Cs-contaminated water.

Synthesis and study of Prussian blue type nanoparticles in an alginate matrix

A new approach for the synthesis of Prussian blue type nanoparticles containing nanocomposites in the form of beads or films, as well as their corresponding aqueous colloids, was developed by using a water-soluble alginate matrix as a template and as a stabilizing agent. This method consists of the step-by-step building of a cyanometallate network in the pores of Mn+/alginate ionotropic gels in order to obtain a large range of nanocomposites containing cyano-bridged coordination polymer nanoparticles Mn+/[M′(CN)m]3−/alginate (where Mn+ = Ni2+, Cu2+, Mn2+, Fe2+, Eu3+ and M′ = Fe3+, Cr3+ (m = 6), Mo5+ (m = 8)). The nanocomposite beads and films, as well as the corresponding aqueous colloidal solutions, were studied by infrared (IR), UV/visible spectroscopy, and transmission electron microscopy (TEM) analyses, which reveal the presence of homogeneously dispersed uniformly sized cyano-bridged coordination polymer nanoparticles of 3–7 nm. These nanocomposite beads and films present superparamagnetic, spin-glass or paramagnetic behaviour depending on the nature of the metal ions used. In addition, the Eu3+-containing nanocomposites are room temperature optically active emitters displaying a characteristic 5D0 → 7F0–4 transition.

New solid lipid microparticles for controlled ibuprofen release: formulation and characterization study.

A hot melt dispersion method was used to prepare new sustained release ibuprofen composite microparticles of a solid lipid at ambient temperature, cetyl alcohol. The dispersion of colloidal silicon dioxide nanoparticles (hydrophilic Aerosil 200 or hydrophobic Aerosil R974) either in the oily phase or in the aqueous phase led to the preparation of large (about 400 μm diameter) surfactant free free-flowing particles. Mapping-scanning electronic microscopy using silicon probe revealed that silicon was in the oily core in all cases. The nature of silica nanoparticles and the way used for their dispersion influenced the internal structure of the composite microparticles and the aggregation of nanoparticles in the core of the microparticles. Hydrophobic Aerosil R974 allowed the formation of homogeneous microparticles. Although silica nanoparticles had no influence on thermic profile, crystalline state of ibuprofen and lipid, they had an influence on the kinetics drug release related to the increase of the size of the composite solid lipid microparticles prepared.

Spin crossover polysaccharide nanocomposites

We report in this paper the synthesis, characterisation and spin-crossover (SCO) properties of a series of nanocomposite materials containing very small (∼3 nm) Hofmann clathrate [Fe(pz){M(CN)4}] (M = Ni2+, Pt2+, Pd2+) nanoparticles embedded into different biopolymer polysaccharide matrices: chitosan and alginate. With the objective to understand the influence of the matrix on the SCO properties in these systems, we have modified the internal structure of the matrix, leading to aerogel, xerogel, beads or thin film samples. We show that the cooperativity associated with the spin crossover behaviour depends not only on the cyanometallate bivalent metal, but also on the nature and morphology of the polysaccharide matrix. Moreover, we confirm that for a specific morphology of the matrix, the bistability is maintained in such 3 nm nanoparticles.