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The effectiveness of a multiphase medium including CO 2 and water was studied for the selective hydrogenation of various nitriles of 2-phenylethyl cyanide (PEC), cinnamonitrile, n -hexanenitrile (HN) and 4-cyanobiphenyl with a conventional Pd/Al 2 O 3 catalyst. Such a multiphase reaction medium was observed to have different impacts changing from one nitrile to another and the selective production of primary amine was achieved with PEC and HN but not with the other nitriles. Possible reasons for the substrate-dependent effects of CO 2 and water were discussed. [ABSTRACT FROM AUTHOR]

The features of multiphase reaction media using water (bulk water, supported thin aqueous phase) and dense phase carbon dioxide are reviewed for selective hydrogenation of carbonyl, nitro, and cyano compounds. The interface of aqueous and organic phases is important in determining the rate of hydrogenation and the product selectivity. For supported aqueous phase catalysts, the interface between the thin aqueous film and a support material (for example, porous silica gel) has also an influential impact. The pressurization of a liquid reaction mixture (organic and/or aqueous phases) with carbon dioxide shows interesting positive effects on the reaction rate, the product selectivity, and the deactivation of a conventional supported metal catalyst. In some cases the use of both water and dense phase carbon dioxide serves as an effective reaction medium. In other words, these green solvents may act as reaction promoters. Possible roles of their actions are discussed. [ABSTRACT FROM AUTHOR]

Selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) has been investigated with supported liquid-phase catalysts (SLPC), which contain Ru–TPPTS complexes in water film supported on a porous silica gel, in organic solvent (toluene). It was shown that COL can be mainly produced with small quantities of hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL). The influence of various reaction and catalyst preparation parameters on the overall rate of reaction and the product selectivity have been examined. In addition, liquid-phase adsorption of CAL on water-loaded silica samples has also been measured. It is believed that the hydrogenation of CAL to COL occurs at the interface between the water film and the organic solvent while that to HCAL at the interface between silica, water, and toluene. The hydrophilic nature of the C&z.dbnd;O bond of CAL is significant for the selective formation of COL at the water/toluene interface, to which polar C&z.dbnd;O bonds of CAL molecules are pointing. The product, COL, also has a polar OH group, and so it is difficult for COL to be hydrogenated to HCOL at this interface. At the other silica/water/toluene interface, CAL molecules may be adsorbed with their C&z.dbnd;C bonds as well, which enables the hydrogenation to HCAL. On recycling of SLPC, its activity and COL selectivity decrease. Both oxygen dissolved in toluene and change of active Ru complexes in the water film are responsible for the decrease of catalytic activity, and the change of the state of dispersion of the water film is significant for the decrease of COL selectivity. [Copyright &y& Elsevier]

The nanocrystalline TiO 2 materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO 2 catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO 2 crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H 2 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO 2 crystallite size in which it was more pronounced for Au/Pd on the larger TiO 2 (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst. [ABSTRACT FROM AUTHOR]

Abstract: The activity and selectivity of the transition metal complexes formed from Ru, Rh, Pd and Ni with triphenylphosphine (TPP) have been investigated for hydrogenation of citral in supercritical carbon dioxide (scCO2). High activities are obtained with Ru/TPP and Pd/TPP catalysts, and the overall activity is in the order of Pd≈Ru>Rh>Ni. The Ru/TPP complex is highly selective to the formation of unsaturated alcohols of geraniol and nerol. In contrast, the Pd/TPP catalyst is more selective to partially saturated aldehydes of citronellal. Furthermore, the influence of several parameters such as CO2 and H2 pressures, N2 pressure and reaction time has been discussed. CO2 pressure has a significant impact on the product distribution, and the selectivity for geraniol and nerol can be enhanced from 27% to 75% with increasing CO2 pressure from 6 to 16MPa, while the selectivity for citronellol decreases from 70% to 20%. Striking changes in the conversion and product distribution in scCO2 could be interpreted with variations in the phase behavior and the molecular interaction between CO2 and the substrate in the gas phase and in the liquid phase. [Copyright &y& Elsevier]

Highlights: [•] Water and CO2 are useful for selective hydrogenation of nitrile to primary amine. [•] Water suppresses the reaction between primary amine and imine intermediate. [•] Water prevents the catalyst from being deactivated by carbamate accumulation. [•] CO2 molecules interact with cyano group of benzonitrile decreasing its reactivity. [Copyright &y& Elsevier]

Abstract: TiO2-supported Pd–Au catalysts were prepared with Pd (0.5 wt%) by impregnation and then Au (1.0 wt%) by deposition–precipitation (Au/Pd/TiO2) and vice versa (Pd/Au/TiO2). For the former sample, the state of dispersion of Pd on TiO2 changed during the loading of Au, resulting in the formation of small Au–Pd alloy particles and thus changing the electronic properties of Pd species. The alloy formation was not clearly detected on the Pd/Au/TiO2, and the electronic properties of Pd were not so different from those of monometallic Pd/TiO2. All the catalysts were selective to 1-heptene (>95%) until close to the complete conversion of 1-heptyne. However, the selectivity of 1-heptene drastically decreased to 0% (which was further hydrogenated to heptane) in 120min for the Au/Pd/TiO2 while it still remained >60% for the Pd/Au/TiO2 similar to the monometallic Pd/TiO2 catalyst. The rate of 1-heptene hydrogenation (in the second step) did not depend on the Pd dispersion but was found to be greatly enhanced as the Pd species became electron-rich in the Au–Pd alloy particles. The modification of Pd species by Au was less significant for the first step of hydrogenation of 1-heptyne to 1-heptene. [Copyright &y& Elsevier]