Paolo Centomo

Synthesis and catalytic activity of metal nanoclusters inside functional resins: an endeavour lasting 15 years

Cross-linked functional polymers (functional resins) are versatile, designable and useful supports for metal nanoclusters that are able to provide reasonably thermally and mechanically stable multi-functional metal catalysts characterized by good activity and selectivity. The paper reviews authors’ contributions to the field from the early 1990s to the present.

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.

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.

Pd Metal Catalysts for Cross-Couplings and Related Reactions in the 21st Century: A Critical Review

Cross-couplings and related reactions are a class of highly efficient synthetic protocols that are generally promoted by molecular Pd species as catalysts. However, catalysts based on more or less highly dispersed Pd metal have been also employed for this purpose, and their use, which was largely limited to the Heck reaction until the turn of the century, has been extended in recent years to most reactions of this class. This review provides a critical overview on these recent applications of Pd metal catalysts. Particular attention is devoted to the discussion of the mechanistic pathways that have been proposed to explain the catalytic role of Pd metal. Furthermore, the most outstanding Pd metal based catalytic systems that have emerged are illustrated, together with the development of novel approaches to boost the reactivity of Pd metal. A section summarizing the current industrial applications of Pd metal catalyzed reactions of this kind concludes the review.

Template controlled synthesis of monometallic zerovalent metal nanoclusters inside cross-linked polymer frameworks: the effect of a single matrix on the size of different metal nanoparticles

Starting from the gel-type, cross-linked copolymer co-poly-{(N,N-dimethethylacrylamide/4-vinylpyridine/N,N′-methylenebisacrylamide} (DV44) we prepared nanoclusters of several noble metals (Ru, Rh, Pd, Pt, Ag, Au) inside its polymer framework through the so-called “template-controlled synthesis” approach. This procedure involves the immobilization of the relevant metal centers inside the polymer framework, followed by their reduction to M0 nanoclusters (in our case, uptake from solutions of suitable precursors and reduction with aqueous solutions of NaBH4, respectively). In this way, a set of monometallic polymer supported nanoclusters (M/DV44, M = Ru, Rh, Pd, Pt, Ag, Au) were obtained. The experimental conditions were set so as to generally obtain a homogeneous radial distribution of the metal throughout the particles of the materials. The latter were characterized by means of transmission electron microscopy for the assessment of the metal nanocluster sizes. The diameter of the nanoclusters generated by reduction inside water-swollen DV44 were compared with the size of the nanopores of the water-swollen polymer framework that had previously been determined by means of inverse steric exclusion chromatography. The diameter of the prevailing pore fractions in water-swollen DV44 was 3.2–4.3 nm and the largest available pores observed had a diameter of 8.1 nm. Nanoclusters with diameters larger than this value were observed in no cases, and in all cases the average diameter of the nanoclusters never exceeded the diameter of the prevailing pore fraction. For Pd/DV44, Ag/DV44 and Au/DV44, the average nanocluster diameter compared very well with the diameter of the prevailing pore fraction.

The influence of catalyst amount and Pd loading on the H2O2 synthesis from hydrogen and oxygen

Palladium catalysts with an active metal content from 0.3 to 5.0 wt.% and supported on a strongly acidic, macroporous resin were prepared by ion-exchange/reduction method. H2O2 direct synthesis was ...

Characterisation of Solute Mobility in Hypercross-Linked Resins in Solvents of Different Polarity: Two Promising Supports for Catalysis

Two hypercross-linked resins stemming from a gel-type poly-chloromethylated styrene-divinylbenzene resin (GT) in beaded form are investigated with a combination of spectroscopic techniques (EPR and time-domain (TD)-NMR spectroscopy) to evaluate their use as supports for the development of operationally flexible heterogeneous metal catalysts, suitable to be employed in liquid and gas phase. The first resin (HGT) is the direct product of the hypercross-linking reaction, whereas the second one (HGS) is the sulphonated analogue of HGT obtained by exchanging approximately 3 wt % of the chloromethyl groups with sulphonic groups. HGT and HGS absorb both polar and apolar solvents in the permanent nanoporosity created by the hypercross-linking, and NMR data highlight that the pore size is not affected by the different properties of the investigated liquid media. The EPR analysis of the dry resins reveals that during the hypercross-linking process paramagnetic species are formed in the HGT beads, which persist in the sulphonated resin. The mobility of solutes inside the polymers framework was investigated with EPR spectroscopy upon soaking the resins with solutions of two spin probes (2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL)) in THF, toluene, n-heptane and water. The EPR spectra show that, depending on the solvent, the two resins can act as sorbents, able to trap the solutes in the polymer framework, or as simple supports that allow free diffusion of the solutes. Our results suggest that HGT and HGS are promising supporting materials for metal catalysts, provided one chooses carefully the solvent to be employed for the catalysed reaction as this choice strongly affects the mobility of the substrates and, thus their effective reactivity.

Characterization of Synthetic Iron Oxides and their Performance as Support for Au Catalysts.

Gold(0) catalysts supported on ferric oxide are effective catalysts, and so far, the only heterogeneous ones known for the regioselective hydrogenation of α,β‐unsaturated ketones to the corresponding α,β‐unsaturated secondary alcohols. A set of ferric oxide samples, synthesized under strictly controlled conditions and thoroughly characterized (X‐ray powder diffraction, Mössbauer spectroscopy, micro‐Raman spectroscopy, transmission electron microscopy, temperature‐programmed reduction), were employed as supports of gold(0) catalysts for this reaction. The activity and selectivity of the catalysts changed in the same direction from one catalyst to another; increasing fractions of nanostructured ferric oxide phases in the support (with nanoparticles size lower than 10 nm) increased both. With a 4 % gold catalyst supported on the ferric oxide sample containing the highest proportion of nanostructured matter, we achieved a selectivity of 66 % towards the unsaturated alcohol at more than 90 % conversion in the hydrogenation of benzalacetone (ethanol, 60 °C, 103 kPa), which is one of the best performances ever reported for this reaction.

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.

Cross-linked polyvinyl polymersversus polyureas as designed supports for catalytically active M0 nanoclusters

The cross-linked polyurea support EnCat 30, its related macromolecular complex PdII/EnCat 30 and its related Pd0/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). PdII/EnCat 30 and its related Pd0/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 PdII/EnCat and Pd0/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 PdII 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 PdII/EnCat 30 and Pd0/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.