Supported palladium Support effect

Besson M, Gallezot P, Lahmer E, Eleche G, Euertes P (1993) Oxidation of glucose on palladium catalysts particle size and support effects. In Kosak JR, Johnson TA (eds) Catalysis of organic reactions. Marcel Dekker, New York... 

Copper based catalysts have long been considered as the only effective methanol synthesis catalysts. However, Poutsma et al. (7) showed that palladium catalysts were active in methanol synthesis from CO-H. This latter metal had been previously considered as either almost inactive or active only for methane formation (8). Furthermore it is now known that both activity and selectivity can change drastically with the support. Vannice (9) observed that the methanation activity of a Pd/Al O was enhanced eighty and forty times compared to palladium black or Pd/SiO (or Pd/TiO ) respectively. The support effect on the selectivity was pointed out by many authors even at atmospheric pressure when the reaction temperature... 

Park, C., Keane, M.A., Catalyst support effects gas-phase hydrogenation of phenol over palladium, J. Colloid Inter/. Sci. 266, 183-194, 2003... 

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

Casanovas, A., Llorca, I., Homs, N., Fierro, J.L.G., and Ramirez de la Piscina, P Ethanol reforming processes over ZnO-supported palladium catalysts Effect of alloy formation. Journal of Molecular Catalysis A Chemical, 2006, 250 (1-2), 44. 

Okumura, K. and Niwa, M. (2002) Support effect of zeolite on the methane combustion activity of palladium. Catal. Surv. Japan, 5, 121-126. 

The reaction we propose to study is the hydrogenation of 2-methyl-2-nitropropane into /-butylamine through the nitroso and the hydroxylamine intermediates (scheme 1). This sequence is called "the main reaction". A preliminary study, encompassing temperature, catalyst mass, hydrogen pressure, initial reactant concentration, metal and support effects has already been carried out [5], For this nitroaliphatic compound, palladium appears to be more active than platinum, and less active than rhodium. On the other hand, carbon seems to be more appropriate as support than alumina or calcium carbonate. 

Various supported platinum group metal systems have been tested for the SCR process.101 Among them, supported platinum systems appear to be the most active when jointly considering the NOx reduction level achieved and the temperature range at which the catalyst is active, while palladium, rhodium and iridium also show catalytic activity for the process and Rh and Ir apparently present higher selectivity to N2.101>i03-i07 Support effects are observed which generally depend on the type of hydrocarbon employed, the presence or absence of SO2 in the reactant mixture or the type of impurities present in the support.101 In this respect, a variety of materials like SiC>2, AI2O3, ZrC>2, sulphated alumina, zeolitic materials and activated carbons have been employed as supports of the metals and tested for the process.101-112... 

Irrespective of the nature of the reaction intermediate, enolic type (11) or surface carbide (12), the dechne of the turnover number for the zeolites with higher Si/Al ratio can be explained as follows. For platinum (13) and palladium (14,15) loaded zeolites, support effects are known to exist. The higher the acidity (and the oxidizing power) of the zeolite, the higher will be the electron-deficient character of the supported metal. It also is well established now (16) that the average acidity of hydrogen zeohtes increases with the Si/Al ratio. This explains why the electron deficient character of ruthenium should increase with the Si/Al ratio of the zeolite, and a stronger interaction with adsorbed CO should be expected. Vannice (19,20) reported that the N value for CH4 formation decreases when the heat of adsorption for CO increases. All this explains why the tmnover number of the methanation reaction over ruthenium decreases when the Si/Al ratio of the zeolite support increases. 

The outstanding quality of palladium in both these respects is well known but not well understood. Foremost among the complexities that have to be addressed are (i) the tendency to form hydride phases, and (ii) the frequent formation, especially with ethyne, of a carbonaceous overlayer, derived perhaps from oligomers, some of which escape into the fluid phase. Both these factors are invoked to account for palladium s remarkable properties, but both are responsive to reaction conditions and especially to particle size support effects may also operate. So many factors have to be kept in play when discussing mechanisms that one may safely conclude that none so far suggested is wholly satisfactory. The rich literature on alkyne hydrogenation deserves careful attention, and should be a fine source of inspiration for further research. 

Enache, D., Barker, D., Edwards, J., et al. (2007). Solvent-Free Oxidation of Benzyl Alcohol Using Titanic-Supported Gold-Palladium Catalysts Effect of Au-Pd Ratio on Catalytic Performance, Catal. Today, 122, pp. 407 11. 

Supported palladium, zirconium-promoted cobalt on kieselguhr, or nickel on kieselguhr can be used under relatively mild conditions to effect reduction of the nitrile function without hydrogenating the ring. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

In order to probe the influence of Au and KOAc on the vinyl acetate synthesis chemistry, four different catalysts were synthesized. All of these catalysts were prepared in a manner exemplified in prior patent technology [Bissot, 1977], and each contained the same palladium loading in an egg-shell layer on the surface of a spherical silica support. The palladium content in the catalyst was easily controlled by adjusting the solution strength of palladium chloride (PdClj) added to the porous silica beads prior to its precipitation onto the support by reaction with sodium metasilicate (Na SiOj). The other two catalyst components (Au and KOAc) were either present or absent in order to complete the independent evaluation of their effect on the process chemistry, e.g., (1) Pd-i-Au-hKOAc, (2) Pd-i-KOAc, (3) Pd-hAu, and (4) Pd only. 

The observed distribution can be readily explained upon assuming that the only part of polymer framework accessible to the metal precursor was the layer of swollen polymer beneath the pore surface. UCP 118 was meta-lated with a solution of [Pd(AcO)2] in THF/water (2/1) and palladium(II) was subsequently reduced with a solution of NaBH4 in ethanol. In the chemisorption experiment, saturation of the metal surface was achieved at a CO/Pd molar ratio as low as 0.02. For sake of comparison, a Pd/Si02 material (1.2% w/w) was exposed to CO under the same conditions and saturation was achieved at a CO/Pd molar ratio around 0.5. These observations clearly demonstrate that whereas palladium(II) is accessible to the reactant under solid-liquid conditions, when a swollen polymer layer forms beneath the pore surface, this is not true for palladium metal under gas-solid conditions, when swelling of the pore walls does not occur. In spite of this, it was reported that the treatment of dry resins containing immobilized metal precursors [92,85] with dihydrogen gas is an effective way to produce pol-5mer-supported metal nanoclusters. This could be the consequence of the small size of H2 molecules, which... 

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

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