Ágnes Mastalir

Pd nanoparticles in hydrotalcite: Mild and highly selective catalysts for alkyne semihydrogenation

Pd nanoparticles incorporated in a hydrotalcite (HT) host were prepared via anion exchange between a dilute suspension of HT–nitrate and a Pd hydrosol stabilized by the anionic surfactant sodium dodecyl sulfate. Samples of two of the resulting Pd organoclays, Pd–HT1 and Pd–HT2, with metal contents of 0.1 and 0.42%, respectively, were subjected to further investigations. Characterization via ICP-AES, XRD, and TEM measurements indicated the deposition of fairly monodispersed Pd particles, predominantly on the external surface of the HT layers. The Pd–HT samples proved to be efficient catalysts for the liquid-phase semihydrogenations of both terminal and internal alkynes under mild conditions. For the transformation of phenylacetylene to styrene, a bond selectivity of 100% was obtained, and the cis stereoselectivities for the hydrogenations of the internal alkynes 4-octyne and 1-phenyl-1-pentyne reasonably approached 100%. The catalytic activity of Pd–HT increased with the Pd dispersion, whereas the alkene selectivity remained essentially unaffected.

Preparation of Pt nanoparticles in the presence of a chiral modifier and catalytic applications in chemoselective and asymmetric hydrogenations

Pt/bentonite (Pt/B) samples with 1% of Pt content were synthesized via successive cation-exchange, by using the layer-structured, synthetic cationic clay Bentolite H as a host material, the precursor [Pt(NH3)4](NO3)2 and the chiral modifier cinchonidine dihydrochloride (CD·2HCl). The samples Pt/B1 and Pt/B2, reduced before and after CD ion-exchange, respectively, were investigated, together with the unmodified sample Pt/B0. Structural characterization was performed by ICP-AES, XRD and TEM measurements. The intercalation of the chiral modifier, which took place for both Pt/B1 and Pt/B2, was less affected by the sequence of the reduction and the ion-exchange steps than the location and the size distribution of the Pt crystallites. For Pt/B1, the size distribution was considerably narrower and the amount of the interlamellar Pt particles was possibly higher than for Pt/B2. The samples were tested as catalysts for the chemoselective hydrogenation of 2-cyclohexene-1-one and for the asymmetric hydrogenation of ethyl pyruvate. It was established for the transformation of 2-cyclohexene-1-one that both the external and interlayer Pt particles participated in the reaction and the presence of the CD modifier improved both the catalytic activity and the chemoselectivity. In contrast, the chirally modified active sites participating in the asymmetric reaction were situated in the interlamellar region of the clay host, as revealed by a substantial difference between the catalytic performances of the modified samples. Pt/B1 proved to be a more active and enantioselective catalyst than Pt/B2. The enantioselectivity of Pt/B1 exceeded that of Pt/B0, which was studied in the presence of the free modifier.

Hydrogenation reactions on heterogenized Wilkinson complexes

A novel clay catalyst containing a heterogenized Rh(I) triphenylphosphine complex (Rh-bentonite) has been prepared via ion exchange of a Hungarian Na+-bentonite with Wilkinson complex [RhCl(PPh3)3]. It was established that the active species [Rh(PPh3)]+ was situated on the external surface of the catalyst, which was found to be efficient in the liquid-phase hydrogenation of 1-octene, cyclohexene, norbornadiene, 1,5-cyclooctadiene, phenylacetylene and cyclohexene-3-one.

Structure and catalytic activity of copper, nickel, and platinum graphimets prepared from graphite intercalation compounds

Abstract Commercially available copper, nickel, and platinum graphimets prepared from intercalated graphite materials were investigated by XPS, TEM, XRD, DSC and chemisorption techniques. The catalytic activity was tested in hydrogenation reactions. The XPS, TEM and chemisorption data revealed only highly-dispersed platinum particles on the surface of platinum graphimet. Copper oxide was detected on the surface of copper graphimet by means of XPS and XRD, while chemical analysis indicated the presence of residual copper chloride. Nickel graphimet consisted of intercalated nickel chloride, encapsulated metallic nickel and nickel oxide particles. Small nickel particles emerged to the surface of the nickel graphimet after pretreatment in hydrogen at 673 K and pretreatments in oxygen resulted in the aggregation of nickel oxide particles. The catalytic activity is attributed to metallic nickel on the graphite surface.

Preparation and characterization of platinum nanoparticles immobilized in dihydrocinchonidine-modified montmorillonite and hectorite

Abstract A novel synthetic method is reported for the preparation of highly dispersed platinum nanoparticles immobilized in the swelling clay minerals Na-montmorillonite and Optigel (synthetic Na-hectorite), modified by cation exchange with dihydrocinchonidine hydrochloride (DHCd·HCl). For both smectites, X-ray diffraction (XRD) data confirmed intercalation of the chiral modifier between the clay mineral layers. After ion exchange, the ability of montmorillonite to undergo swelling in organic solvents was found to be superior to that of Optigel. The surface-modified smectites were impregnated with H 2 PtCl 6 and the precursor was subsequently reduced with NaBH 4 . Transmission electron microscopy (TEM) images of Pt-montmorillonite (Pt-M) and Pt-Optigel (Pt-O) revealed the formation of ultrafine, monodispersed Pt nanoparticles, with mean crystallite sizes of 0.8 and 1.2 nm for Pt-M and Pt-O, respectively. Both samples proved to be mildly enantioselective catalysts for the asymmetric hydrogenation of ethyl pyruvate, leading to the predominant formation of ( R )-ethyl lactate. The finding that the efficiency of Pt-M was affected by the dispersion medium through disaggregation suggested that access for the reactant to the active centres was rather limited. On the other hand, the improved enantiodifferentiation and the lack of a solvent effect for Pt-O indicated easier access for the ethyl pyruvate molecules to the active centres located mainly on the external surface sites of the clay mineral lamellae. For the above reaction, the occurrence of such external active sites is considered to be responsible for the improved catalytic performance, whereas the role of the Pt particle size is regarded as less significant.

Novel clay intercalated metal catalysts: A study of the hydrogenation of styrene and 1-octene on clay intercalated Pd catalysts

Pd-hexadecylammonium montmorillonite (Pd-HDAM) catalysts have been prepared by a novel synthetic route. Sample characterization including XRD and TEM measurements confirmed the existence of interlayer Pd nanocrystallites which occupy clay particle defect sites. The catalytic activities of Pd-HDAM samples were tested by hydrogenation of 1-octene and styrene in the liquid phase. The reaction of styrene was found to be less dependent on the dispersion of Pd than that of 1-octene. The highest activities were observed for samples of low and medium Pd content. The application of various solvents made it possible to establish a correlation between the activities and the basal spacings dL of Pd-HDAM samples. When the value of dL exceeded 3 nm, interlamellar active sites became more accessible for reactants.

Hydrogenation of α,β-unsaturated ketones on metal catalysts

The hydrogenation of 3-penten-2-one on well-defined SiO2 supported Ni, Cu, Ru, Rh, Pd and Pt catalysts has been investigated under the same experimental conditions. Reactions were performed in the liquid phase in methanol at 298 K and in the gas phase at 393 and 473 K. On the basis of the turnover yields, the activity of metals was found to decrease in the order Pt>Pd≥Rh≫Ru>Ni>Cu. Selective hydrogenation of the olefinic double bond of 3-penten-2-one resulted in the formation of 2-pentanone which was further hydrogenated to 2-pentanol on all catalysts, except on Pd/SiO2.

Transformation of carbon compounds on graphimet catalysts III. Structure and catalytic investigation of Pt graphimet

Abstract The hydrogenation of cyclohexene was studied over Pt-graphimet catalysts pretreated at 298 K. The platinum atoms are situated partly on the external surface of the graphite and partly in the interlayer space. The interlayer atoms are readily accessible for hydrogen, the mass transfer resistance is greater for oxygen, and the cyclohexene cannot diffuse into the interlayer space at all. Thus, the hydrogenation of cyclohexene takes place only on the exposed atoms on the external surface of Pt graphimet, and the atoms in the interlayer space are inactive in this reaction.

Microcalorimetric and catalytic investigations of transition metal nanoparticles intercalated in graphite

Abstract The distribution of the metal content of pristine and thermally treated (medium-temperature reduction, MTR) Pt-, Pd- and Rh-graphite intercalation compounds (graphimets) was investigated by transmission electron microscopy, H 2 titration and sorption microcalorimetry. It was established that MTR resulted in a migration of interlayer metal atoms to the graphite surface accompanied by a moderate aggregation of the surface nanoparticles. Despite the increased metal content available for H 2 adsorption after MTR, the catalytic activities decreased substantially. Besides structural changes, the loss of catalytic performance was related to the increase of metal–hydrogen bond strengths suggested by the calorimetric data.

Transformation of carbon compounds on graphimet catalysts IX. Structural properties and catalytic activity of Pd-graphimet

Abstract A comparative study of pristine and thermally treated (medium-temperature reduction, MTR, 573 K, 1h) Pd-graphimet was carried out to establish the changes arising in structure and catalytic activity. The structural investigation was performed by means of BET surface area analysis, transmission electron microscopy, small-angle X-ray scattering and H 2 titration. In order to distinguish between the hydrogenation and isomerization sites of the samples, the single turnover reaction of 1-butene was applied. For pristine Pd-graphimet, the experimental evidence confirmed the presence of interlayer metal and surface metal particles of different sizes. Although the catalytic activity was clearly attributable to the 1–2 nm surface Pd crystallites, the interlayer metal content also had an influence on the reaction through deintercalation which first started at 323 K. Nevertheless, the effect of MTR was found to be far more pronounced: all the inserted Pd atoms underwent migration and a moderate sintering of the surface Pd crystallites took place. Data on the transformation of 1-butene indicated that Pd-graphimet was similar in effect to supported Pd catalysts.