Selective Hydrogenation of Halogenated Arenes using porous Manganese Oxide (OMS-2) and Platinum supported OMS-2 Catalysts

Porous manganese oxide (OMS-2) and platinum supported on OMS-2 catalysts have been shown to facilitate the hydrogenation of the nitro group on chloronitrobenzene to give chloroaniline with no dehalogenation. Complete conversion was obtained within 2 h at 25 [degree]C and, although the rate of reaction increased with increasing temperature up to 100 [degree]C, the selectivity to chloroaniline remained at 99.0%. Use of Pd/OMS-2 or Pt/Al2O3 resulted in significant dechlorination even at 25 [degree]C and 2 bar hydrogen pressure giving selectivity to chloroaniline of 34.5% and 77.8%, respectively, at complete conversion. This demonstrates the potential of using platinum group metal free catalysts for the selective hydrogenation of halogenated aromatics. Two pathways were observed for the analogous nitrobenzene hydrogenation depending on the catalyst used. The hydrogenation of nitrobenzene was found to follow a direct pathway to aniline and nitrosobenzene over Pd/OMS-2 in contrast to the OMS and Pt/OMS-2 catalysts which resulted in formation of nitrosobenzene, azoxybenzene and azobenzene/hydrazobenzene intermediates before complete conversion to aniline. These results indicate that for the Pt/OMS-2 the hydrogenation proceeds predominantly over the support with the metal acting to dissociate the hydrogen. In the case of the Pd/OMS-2 both the hydrogenation and the hydrogen adsorption occur on the metal sites.

Ir supported over carbon materials for the selective hydrogenation of chloronitrobenzenes

In the present work we have studied the effect of carbon supports with different graphitic character (carbon nanotubes, mesoporous graphite and activated carbon) on the catalytic performance of iridium nanoparticles on the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The effect of the oxygen groups was also evaluated by oxidizing a portion of the carbon nanotubes. The Raman and XRD spectra showed that the mesoporous graphite displayed the strongest graphitic character. The characterization of the catalysts by HR-TEM, XPS and TPR-H2, showed that the catalysts had similar particle size and that the catalysts prepared over the previously oxidized support, Ir/CNTox, was not fully reduced. The activity and selectivity achieved with the catalyst Ir/CNT was the best among the samples and the presence of irdium oxide on Ir/CNTox diminished the yield to p-chloroaniline, being the worse catalyst. The reactivity of different isomers was also studied over Ir/CNT and it followed the order m > o > p.

Electrocatalytic properties of Cu-Zr amorphous alloy towards the electrochemical hydrogenation of nitrobenzene

The Cu-Zr amorphous alloy was studied as an electrocatalyst towards the electrochemical hydrogenation of nitrobenzene. The electrocatalyst was activated by chemical etching in HF solution. Resulted changes in the morphology, chemical composition and crystalline structure of the electrocatalyst surface were characterised by scanning electron microscopy, X-ray diffraction and Auger electron spectroscopy. The electrocatalytic properties of the Cu-Zr amorphous alloy were assessed by voltammetric measurements. Due to the formation and aggregation of Zr residue modified Cu nanocrystals on the surface caused by the selective dissolution of Zr components in the chemical etching, the activated amorphous alloy is an effective electrocatalyst for the electrochemical reduction reaction of nitrobenzene with aniline as the main product. The positive shift of the peak potential and accompanying increase in the value of peak current in voltammograms with increasing Cu content and decreasing Zr content of the alloy surface in the chemical etching are indicative of improved electrocatalytic activity. (C) 2002 Elsevier Science B.V. All rights reserved.

Selective hydrogenation of nitrobenzene to aniline in dense phase carbon dioxide over Ni/gamma-Al2O3: Significance of molecular interactions

The selective hydrogenation of nitrobenzene (NB) over Ni/gamma-Al2O3 Catalysts Was investigated using different media of dense phase CO2, ethanol, and n-hexane. In dense phase CO2, the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0-100%. The phase behavior of the reactant mixture in/under dense phase CO2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions Of CO2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO2 over Ni/gamma-Al2O3 is also proposed.

An efficient approach to modify the catalyst activity for the hydrogenation of nitrobenzene

The addition of a suitable amount of PPh3 to PdCl2 or PdCl2(PhCN)(2) in situ can considerably increase the catalytic activity in the hydrogenation of nitrobenzene, while the catalytic activities of PdCl2 (reduced)+PPh3, PdCl2(PPh3)(2) and Pd(PPh3)(4) are very poor. The poisoning of catalyst by mercury indicates that the catalytically active species are composed of Pd(0) colloidal particles. Transmission electron micrographs show that the size of nanometric Pd(0) particles of PdCl2 with PPh3 added in situ is smaller than that of PhCl2(PPh3) or PdCl2 (reduced)+PPh3. A synergic effect of bimetallic catalysts such as PdCl2+nPPh(3)+NiCl2 (n= 0.5, 1) and PdCl2(PhCN)(2)+PPh3+FeCl3 gives rise to a further increase in the catalytic activity.

Effect of support redox character on catalytic performance in the gas phase hydrogenation of benzaldehyde and nitrobenzene over supported gold

Peer reviewed

Pd-coated Ni nanoparticles by the polyol method: an efficient hydrogenation catalyst

Pd-coated Ni nanoparticles of 50 +/- 15 nm size are prepared by the polyol method and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetry analysis. Surface coverage of Pd on Ni particles is less than a monolayer for 0.5 and 1 at% Pd-coated Ni. Quantitative conversion of nitrobenzene to aniline is observed over these Pd-coated Ni particles at 27degreesC under one atmospheric pressure of hydrogen. 0.5 and 1 at% Pd-coated Ni exhibits 10 times greater activity than that of typical colloidal palladium and platinum catalysts and 2.5 times higher activity than commercial 5 wt% Pd/C.

Influence of compressed carbon dioxide on hydrogenation reactions in cyclohexane with a Pd/C catalyst

The potential of CO2-expanded liquid media for chemical reactions has been examined in this work, using cyclohexane as a solvent and Pd/C as a heterogeneous catalyst for hydrogenation of styrene, citral, and nitrobenzene with H-2. The rate of hydrogenation reactions is increased, and the product selectivity is altered in the CO2-expanded cyclohexane phase. In the hydrogenation of citral, the selectivity to citronellal decreases with CO2 pressure, which changes from similar to 80% in the neat cyclohexane to similar to 65% at 16 MPa.

Selective hydrogenation of maleic anhydride to gamma-butyrolactone in supercritical carbon dioxide

The reaction rates of the hydrogenation of maleic anhydride (MAH) and succinic anhydride (SAH) were significantly accelerated and the selectivity to gamma-butyrolactone (GBL) was enhanced largely when the reaction mixture was pressurized by a non-reactant of CO2. Above 99% selectivity to GBL was achieved in 14 MPa CO2, the superior selectivity in scCO(2) was attributed to that MAH and/or SAH could be extracted to CO2 phase and separated from H2O, the hydrolysis were thus minimized and so the selectivity to GBL was improved.

Influence of metal particle size on the hydrogenation of maleic anhydride over Pd/C catalysts in scCO(2)

Hydrogenation of maleic anhydride (MAH) with Pd/C catalysts in supercritical carbon dioxide (scCO(2)) was investigated. The selectivity for gamma-butyrolactone (GBL) reached 97.3% in scCO(2) at 100% conversion of MAH, which was notably higher than that of 77.4% obtained in organic solvent of ethylene glycol dimethyl ether (EGDME). The particle size of Pd exhibited large influence on the reaction rate and selectivity of GBL. Higher selectivity of GBL was obtained with Pd/C catalyst of smaller Pd particle size, and the rate of GBL selectivity increase as a function of CO2 pressure was found to be significantly correlated with Pd particle size.

A remarkable synergic effect of water-soluble bimetallic catalyst in the hydrogenation of aromatic nitrocompounds

Hydrogenation of nitrobenzene can be catalyzed by the water-soluble catalyst PdCl2(TPPTS)(2) (TPPTS = tris(m-sulfonatophenyl)phosphine trisodium salt) under normal pressure at 65 degrees C in H2O/toluene biphasic solvent system. The exhibits higher catalytic activity and selectivity for the hydrogenation of aromatic nitrocompounds, compared with PdCl2(TPPTS)(2) or H2PtCl6 alone. The transmission electron micrographs demonstrate that the monometallic catalyst is composed of ultrafine palladium particles of almost uniform size while the particles of bimetallic catalyst are more widely distributed in size than those of the monometallic ones. (C) 1999 Elsevier Science B.V. All rights reserved.

Metallic and Carbon Nanotube-Catalyzed Coupling of Hydrogenation in Magnesium

Synergistic effect of metallic couple and carbon nanotubes on Mg results in an ultrafast kinetics of hydrogenation that overcome a critical barrier of practical use of Mg as hydrogen storage materials. The ultrafast kinetics is attributed to the metal−H atomic interaction at the Mg surface and in the bulk (energy for bonding and releasing) and atomic hydrogen diffusion along the grain boundaries (aggregation of carbon nanotubes) and inside the grains. Hence, a hydrogenation mechanism is presented.

First principle study of hydrogenation of MgB2 : an important step toward reversible hydrogen storage in the coupled LiBH4/MgH2 system

Recent experiments [F. E. Pinkerton, M. S. Meyer, G. P. Meisner, M. P. Balogh, and J. J. Vajo, J. Phys. Chem. C 111, 12881 (2007) and J. J. Vajo and G. L. Olson, Scripta Mater. 56, 829 (2007)] demonstrated that the recycling of hydrogen in the coupled LiBH4/MgH2 system is fully reversible. The rehydrogenation of MgB2 is an important step toward the reversibility. By using ab initio density functional theory calculations, we found that the activation barrier for the dissociation of H2 are 0.49 and 0.58 eV for the B and Mg-terminated MgB2(0001) surface, respectively. This implies that the dissociation kinetics of H2 on a MgB2 (0001) surface should be greatly improved compared to that in pure Mg materials. Additionally, the diffusion of dissociated H atom on the Mg-terminated MgB2(0001) surface is almost barrier-less. Our results shed light on the experimentally-observed reversibility and improved kinetics for the coupled LiBH4/MgH2 system.