Silvano Lora

Activity Enhancement by the Support in the Hydrogenation of C=C Bonds over Polymer-Supported Palladium Catalysts

Four synthetic ion-exchange resins (AH, BH, CH, DH) of different hydrophilic/hydrophobic properties were used as supports for heterogeneous palladium catalysts (A, B, C, D). The resins contained styrene (STY) and 2-(methacryloxy)ethylsulfonic acid (MESA) as the comonomers. Either divinylbenzene (DVB: CH, DH resins) or N,N-methylenebisacrylamide (MBAA: AH, BH resins) were used as the cross-linker. AH contained also N,N-dimethylacrylamide (DMAA) as the third comonomer. The catalysts (Pd 0.25-0.45% w/w) were obtained by ion-exchanging the acidic forms of the resins with [Pd(OAc)2] and reducing palladium(II) with excess sodium borohydride. The use of NaBH4 also ensured the neutralization of the acidic sites of the supports. No effect of the hydrophilic/hydrophobic properties of the supports was observed in the hydrogenation of cyclohexene and 2-cyclohexen-1-one in methanol, at 25 degrees C and 0.5, 1, and 1.5 MPa, respectively. However, catalysts A and B, containing amido groups provided by either DMAA or MBAA, proved to be more active than C and D. The observed activity enhancement was directly proportional to the nitrogen/ palladium molar ratio in the catalysts. This finding suggests that amido groups promote palladium through a direct interaction with the metal surface.

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.

Microporous poly-N,N-dimethylacrylamide-p-styrylsulfonate-methylene bis(acrylamide): a promising support for metal catalysis

Abstract The synthesis of poly- N,N -dimethylacrylamide- p -styrylsulfonate (DMAA-SS) with 4 mol% of methylene bis(acrylamide) mol% of methylene bis(acrylamide) (MBAA) as crosslinker is described. Bulk polymerization of water solution of monomers (DMAA, SS sodium salt, MBAA) was carried out under gamma irradiation. Swellability in water and various organic solvents showed mainly hydrophilic character of the resin. The ion-exchange experiments with 1 M hydrochloric acid and with [Pd(NH 3 ) 4 ] 2+ showed very good accessibility of the inner space of the material. Polymer supported [Pd(NH 3 4 ] 2+ was reduced by hydrogen in methanol, sodium borohydride in water and sodium borohydride in ethanol. Under these last conditions uniform distribution of Pd throughout the resin particles was observed. The activated material turned out to be a good catalyst for the hydrogenation of p -nitrotoluene to p -toluidine in methanol under ambient conditions.

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.

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.

Metallation of functional resins with copper acetate: control of metal speciation and catalytic activity in C-N coupling reactions

Abstract Moderately cross-linked gel-type acidic ion-exchangers (0.8–1.0xa0meq. H + /g) obtained upon copolymerization of dimethylacrylamide (DMAA), methylenebisacrylamide (MBAA) and methacrylic acid (MAA) or sulfoethyl methacrylate (SEMA) were reacted with Cu(OAc) 2 under different initial H + /Cu(II) molar ratios. The metal speciation inside the polymer networks was characterized on the basis of analytical and spectroscopic data. Depending on the kind of ion-exchanger and reaction conditions employed, different species were identified. The kind of metal speciation appears to have a profound influence on the catalytic activity of the resulting resin-supported copper species in C–N coupling reactions. Use of a resin containing the proper copper species results in a catalytic performance superior to that of unsupported copper acetate.

Interpenetrating organometallic polymer networks in heterogeneous catalysis: Mild hydrogenation of p-nitrotoluene

A cross-linked organometallic macroporous copolymer (P1) and an interpenetrating organometallic polymer network (IOPN) material (C1), both based on the organometallic monomer cis-(PdCl 2 (CN CH 2 ) 3 OC(O)CH=CH 2 ) 2 ) (1), are catalytically active and reusable in the mild and selective hydrogenation of p-nitrotoluene to p-toluidine in methanol, under ambient conditions. The IOPN catalyst C1 is generated by the dispersion of a copolymer of I inside a macroporous poly-dimethylacrylamide-methylen-ebisacrylamide matrix to give an expectedly microporous organometallic matrix which exhibits a catalytic productivity quite similar to that displayed by P1. The productivity of both catalysts is comparable to that exhibited by commercial Pd/C under the same conditions, with P1 and C1 far superior in terms of separability and handiness

Synthesis of potential hybrid catalysts upon coordination of ‘bare’ palladium(II) with crosslinked macromolecular nitriles. Catalytic dimerization of methyl acrylate

Abstract Crosslinked copolymers of cyanoethyl acrylate appear to be good ligands towards ‘Pd 2+ , in nitromethane. Thus, reaction of [Pd(NC-Me) 4 ] 2+ with macromolecular nitriles allows the transfer of the metal centre from an ‘ordinary’ coordination sphere to an unconventional one, i.e. nitrile groups pendant from polymeric backbones. The macromolecular complexes obtained appear to maintain the catalytic ability of [Pd(NC-Me) 4 ] 2+ in the dimerization of methyl acrylate in nitromethane. However, the sensitivity of the hybrid catalyst to traces of water turns out to be a limiting factor in its activity and re-usability.

A viable route to designable chiral macromolecular supports: Implications for supported metal catalysis

Chemically and structurally designed chiral microporous matrices can be obtained upon aminolysis with enantiomerically pure amino acids of an activated ester functionality pendant from the backbone of precursor cross-linked copolymers.