Selectivity as a function of nanoparticle size in the catalytic hydrogenation of unsaturated alcohols.
ABSTRACT Layer-by-layer adsorption of poly(acrylic acid)-Pd(II) complexes and poly(ethylenimine) on alumina powder followed by reduction of Pd(II) with NaBH(4) yields Pd-nanoparticle catalysts embedded in multilayer polyelectrolyte films. The use of different ratios of poly(acrylic acid) to Pd(II) in deposition solutions gives a series of films with Pd nanoparticles whose average diameters range from 2.2 to 3.4 nm, and the catalytic selectivities of these nanoparticles vary dramatically with their size. Turnover frequencies (TOFs) for the hydrogenation of monosubstituted unsaturated alcohols increase with decreasing average nanoparticle size, whereas multisubstituted unsaturated alcohols show the opposite trend. Hence, the ratio of TOFs for the hydrogenation of allyl alcohol and crotyl alcohol is 39 with average particle diameters of 2.2 nm and only 1.3 with average particle diameters of 3.4 nm. Ratios of TOFs for hydrogenation of allyl alcohol and beta-methallyl alcohol are as high as 240 with the smallest nanoparticles, but substantial isomerization of beta-methallyl alcohol complicates this comparison. Increasing selectivity with decreasing average particle size occurs with both films deposited on alumina powder and nanoparticles stabilized by polyelectrolytes in solution. Presumably, high selectivities occur on the smallest nanoparticles because the active sites on the smallest Pd nanoparticles are less available for binding and hydrogenation of multisubstituted double bonds than are active sites on larger particles.
Article: Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles.[show abstract] [hide abstract]
ABSTRACT: A continuing goal in catalysis is to unite the advantages of homogeneous and heterogeneous catalytic processes. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl(2), and catalyse a range of π-bond activation reactions previously only catalysed through homogeneous processes. Multiple experimental methods are used to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, a size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared with larger, polymer-capped analogues.Nature Chemistry 01/2010; 2(1):36-41. · 20.52 Impact Factor