Solvent-stabilized colloid catalysts

A new development is biphasic hydrogenation using solvent-stabilized colloid (SSCs) catalysts [39-41]. Palladium colloid systems, especially, were proven to give high reactivity and selectivity. Best solvents are dimethylformamide and particularly the two cyclic carbonic acid esters, ethylene carbonate and 1,2-propene carbonate. In these solvents sodium tetrachloropalladate - stabilized by a sodium carbonate buffer - is reduced with hydrogen to yield the solvent-stabilized palladium colloid. Transmission electron microscopy of the palladium colloid demonstrates that the colloid particles are spherical with an average diameter of 4 nm. 

Another possibility, taking advantage of the biphasic environment, is to use fluorous organic solvents as the catalyst phase instead of water [155]. Crooks and coworkers prepared dendrimer-stabilized colloid catalysts soluble in the fluorous phase and used the catalysts in hydrogenation [156] and in a Heck reaction [157]. In both cases the colloidal catalyst in the fluorous phase was recyclable and showed some interesting selectivities and products unique to the nanoenvironment in the dendrimer interior. 

The main problem raised by these preparations is to adapt the proeedure to the solvent compatible with the embedding materials. Both the colloids and the triflate derivatives are soluble in a limited number of solvents, mostly not the alcohols generally used in such preparations. Therefore, the protocol procedure described in this contribution followed a specific sol-gel route for each hybrid catalyst. During these preparations we investigated i) the formation of the sol by hydrolysis of the alcoxide in the selected solvent ii) the addition of the stabilized colloid or triflate derivative in the formed sol iii) the addition of a surfactant in the case of triflates iv) the gelation, and finally v) the drying and calcining of the materials. 

PVP stabilized rhodium or platinum sols (3.5 nm) have been grafted onto mer-capto-derivatized silica. [254] The S-H groups ligate strongly to the metal particles. In this case the grafted catalyts are one to two orders of magnitude more active for the hydrogenation of alkenes than the unsupported colloidal catalysts in the same solvent system, and 300 times more active than commercial Rh/C and Pt/C catalysts. 

Bonnemann H ef a/1996 Nanoscale colloidal metals and alloys stabilized by solvents and surfactants preparation and use as catalyst precursors J. Organometaii. Chem. 520 143... 

Solvated metal atoms can be dispersed in excess organic solvent at low temperature and used as a source of metal particles for the preparation of both unsupported metal powders and supported metal catalysts158,161. Alternatively, metal vapor is condensed into a cold solution of a stabilizing polymer to form crystallites of the order 2-5 nm in diameters159. Equation 17 illustrates the unique activity of a colloidal Pd catalyst in the partial hydrogenation of acenaphthene. 

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