Palladium, colloidal catalysts

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

A size-selective synthesis of nanostructured transition metal clusters (Pd, Ni) has been reported166, as has the preparation of colloidal palladium in organic solvents167, the latter of which is an active and stable catalyst for selective hydrogenation. The use of microwaves in the preparation of palladium catalysts on alumina and silica resulted in hydrogenation catalysts with improved crystallite size and activity168. 

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

Although the selectivity of palladium catalysts in the hydrogenation of 1,5-COD is thus very high, the results also indicate that the hydrogenation of COE to cyclooctane (COA) does not cease after the maximum yield of COE has been attained. Hirai et al. studied the hydrogenation of 1,5-COD over a colloidal palladium catalyst, prepared by reduction of palladium(II) chloride in the presence of poly(iV-vinyl-2-pyrrolidone) in refluxing methanol with addition of sodium hydroxide, in methanol at 30°C and 1 atm H2, and obtained a mixture consisting of 0.4% 1,5-COD, 0.3% 1,4-COD, 97.8%... 

Platinum Catalyst for Reductions (CoU. Vol. i, 452) Directions are given for the preparation of a colloidal platinum (or palladium) catalyst in an anhydrous, alkaline medium. This catalyst is said to be particularly valuable for the reduction of nitriles, oximes, and nitrostyrenes to pure prinaary amines. Skiia and Keil, Ber. 63, 424 (r932). 

A variety of compounds has been reduced employing colloidal palladium catalysts supported by synthetic polymers. Table I summarizes the results of various reductions (13). 

Hydrogenation of a-chlorocodide in dilute acetic acid using a colloidal palladium catalyst proceeds slowly with production of 95 per cent. dihydrodesoxycodeine-D [ix]. Under these conditions Freund [20] had earlier reported the production of an alkali-insoluble amorphous base a-dihydrodesoxycodeine . 

Reduction of /3-chlorocodide in dilute acetic acid with a colloidal palladium catalyst gives mainly dihydrodesoxycodeine-D [ix] but also a considerable amount of tetrahydrodesoxycodeine [vm] [30, 34]. The former is presumably formed by initial 1 4-reduction of the system... 

An a-dihydrodesoxycodeine , amorphous, alkali-insoluble, and poorly characterized, was reported by Freund [4] to result from the reduction of a-chlorocodide using a colloidal palladium catalyst. A reinvestigation of this reduction, however, showed that under the conditions prescribed by Freund the product consists of 95 per cent. dihydrodesoxycodeine-D with palladized barium sulphate the results are substantially the same, but an amorphous product is obtained in 40 per cent, yield if the amount of catalyst used is large, and this can bo increased to 96-100 per cent, using palladized calcium carbonate and 100 per cent, using platinum oxide as catalyst [29]. The product appears to bo bis- 6 0 -dihydrodosoxycodeino-D [xlvj] and is probably formed... 

Mild catalytic reduction of thebaine hydrochloride in aqueous or dilute acetic acid solution using a platinum [1] or colloidal palladium catalyst [2-6] results in saturation of the 8 14-double bond and production of dihydrothebaine [i]. The yield is never high, as considerable amounts of dihydrothebainone [n] and tetrahydrothebaine [hi] are produced simultaneously, not by further reduction of dihydrothebaine, which is stable under these conditions. 

The immobilized, colloidal palladium catalyst, Si02-(C3H6SH)nPd is reported to induce the Heck reaction [14a] between ethyl iodide and ethyl acrylate. XPS data showed the presence of Pd(Il) on the surface of the colloidal Pd particles, owing to air oxidation this explains the different behavior of this and the Pd/C catalyst. Addition of BujN.HI and iodine greatly reduced the induction period. The catalytic activity of propylene carbonate-stabilized palladium colloids in the Heck reaction has been investigated [14b]. 

This universally known [66] reaction was discovered independently by Heck and Mizoroki about 30 years ago. Basically it consists of the arylation or vinylation of alkenes and is generally catalyzed in solution by palladium species (Scheme 5.7). One of the major problems of the early homogeneous systems was the precipitation of palladium black. Addition of phosphanes improves the stabiUty however oxidation of this Ugand is a drawback for easy purification of the products. Consequently, development of heterogeneous catalysis [67] through supported palladium or stabilized colloidal palladium catalysts is an area of great interest... 

In this chapter, we discuss selected literature on palladium catalysts (or catalyst precursors) for the Heck, Suzuki and Sonogashira reactions. The review covers simple homogeneous palladium complexes, Ugand-free palladium catalytic systems, stable palladium colloids and particles and supported palladium catalysts. It focuses on the role of palladium nanoparticles (as catalyst precursors or formed in situ during the course of the reaction) from a mechanistic point of view. 

The knowledge of the colloidal chemistry of gold was used by Kim and Turkevich(106) to prepare monodisperse palladium particles in diameter greater than 75 A. The number of surface atoms determined by electron microscopy was found to be equal to the number of catalytic centers determined by poison titration for the athylene hydrogenation reaction. The surface of the palladium catalyst was homogenous. The velocity of the catalytic reaction was found to be proportional to the number of surface atoms. All surface atoms were active. 

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