Karel Jerabek

Cross-linked polyvinyl polymers versus polyureas as designed supports for catalytically active M0 nanoclusters : Part III. Nanometer scale structure of the cross-linked polyurea support EnCat 30 and of the PdII/EnCat 30 and Pd0/EnCat 30NP catalysts

The cross-linked polyurea support EnCat 30, its related macromolecular complex Pd(II)/EnCat 30 and its related Pd(0)/EnCat 30NP nanocomposite are thoroughly investigated with SEM, TEM, ISEC and ESR in the solid state (SEM and TEM) and swollen state in THF (ISEC and ESR). Pd(II)/EnCat 30 and its related Pd(0)/EnCat 30NP are obtained by microencapsulation of palladium acetate in a polyurea framework, which is formed upon hydrolysis/condensation of mixtures of multi-functional oligo-arylisocyanates in dichloroethane. Most remarkably, both Pd(II)/EnCat and Pd(0)/EnCat 30NP turn out to be far more (nano)porous and swellable materials than the blank polyurea matrix (EnCat 30). It is proposed that there is a strong nanostructural effect exerted by Pd(II) species due to its interaction with functional groups (amines stemming from the hydrolysis of the isocyanato groups or ureido groups belonging to the polymer chains) during the growth of the cross-linked polymer framework. As a consequence, the catalytic species in both Pd(II)/EnCat 30 and Pd(0)/EnCat 30NP are much more accessible to molecules diffusing from liquid phases in contact with the materials and, hence, are better catalysts than expected from the morphology of blank polyurea EnCat 30.

Synthesis of Nanocomposites from Pd0 and a Hyper-Cross-Linked Functional Resin Obtained from a Conventional Gel-Type Precursor

Hyper-cross-linked resins stemming from a gel-type poly-chloromethylated poly(styrene-co-divinylbenzene) resin (GT) have been investigated by a multi-methodological approach based on elemental analysis, scanning electron microscopy, X-ray microanalysis, and solvent absorption. The hyper-cross-linking of the parent resin was accomplished by Friedel-Crafts alkylation of the phenyl rings of the resins with the chloromethyl groups. This produced a permanent pore system comprising both micropores (<2.0 nm in diameter) and mesopores (2.2 nm). The chloromethyl groups that did not react in the hyper-cross-linking step were transformed into methylmercaptan groups and the latter were then converted into sulfonic groups by oxidation with hydrogen peroxide. By this procedure the extensive permanent porosity of the parent unsulfonated hyper-cross-linked polymer (HGT) was retained by the sulfonated polymer (HGTS). The final exchange capacity of HGTS was determined to be 0.36 mmol g(-1). HGTS was easily metalated with Pd(II) and the subsequent reduction of the metal centers with either aqueous sodium borohydride, formaldehyde, or dihydrogen produced three Pd(0)/HGTS nanocomposites. The metal nanoparticles had diameters in the 1-6 nm range for all the nanocomposites, as determined by TEM, but with somewhat different distributions. When formaldehyde was used, more than 90% of the nanoparticles were less than 3 nm and their radial distribution throughout the polymer beads was quite homogeneous. These findings show that with this reducing agent the metal nanoparticles are generated within the pore system of the polymer matrix, hence their size is controlled by the dimensions of the pores of the polymeric support.

Nanostructure and molecular accessibility of gel-type resins for supported bio-catalysis: Part I. Poly-hydroxyethylmethacrylate-hydroxypropylmethacrylate-trimethylolpropanetrimethacrylate

Hydroxyethylmethacrylate (HEMA), hydroxypropylmethacrylate (HPMA) and trimethylolpropanetrymethacrylate (TMPTM) are copolymerised by γ-irradiation to give five potentially amphiphilic gel-type resins. The molar fraction of TMPTM (the cross-linker) is constant (4 mol%) while the five resins are featured by different HEMA/HPMA molar ratios (96/0, 68/28, 48/48, 28/68 and 0/96, respectively). The swollen state morphology of the five materials is investigated by means of inverse steric exclusion chromatography (ISEC) and electron spin resonance (ESR) spectroscopy based on the spin-probe TEMPONE (2,2,6,6-tetramethyl-4-oxo-1-oxyl-piperidine) both in water and tetrahydrofuran (THF). In THF, swelling and accessibility of the polymer network increase with the increase in HPMA content. In contrast, the five materials are poorly swollen by water and no apparent dependence of swelling on HPMA/HEMA ratio is detected.

Metal palladium dispersed inside macroporous ion-exchange resins: textural characterization and accessibility to gaseous reactants

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 under solid-liquid conditions were determined by means of ISEC, ESR and PGSE-NMR 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, thus suggesting that the metal particles are in fact embedded in a dry gel-type resin layer at the “surface” of the macropores and therefore practically unaccessible to the molecules of a gaseous phase.