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45592-AC5
Investigation of Catalysis by Nanoparticles and Metal-Ion Complexes Embedded in Multilayer Polyelectrolyte Films

Merlin L. Bruening, Michigan State University

Development of effective heterogeneous catalysts is vital for creating environmentally benign reactions in which expensive catalytic materials can be conveniently recycled.  This research aims at synthesizing selective, heterogeneous catalysts through immobilization of metal nanoparticles in multilayer polyelectrolyte films both on alumina particles and in membranes.  Nanoparticles are attractive catalysts because of their high surface area and tunable electronic properties, and embedding these materials in polyelectrolyte films prevents particle aggregation and restricts access to catalytic sites to impart remarkably high selectivities.
            During the past year, we demonstrated that the catalytic selectivity of Pd nanoparticles varies dramatically with particle size.  These catalysts are prepared using layer by layer deposition of a poly(acrylic acid)-Pd²⁺/polyethyleneimine film on alumina particles.  Subsequent reduction of Pd²⁺ to Pd(0) by NaBH₄ yields nanoparticles embedded in the polyelectrolyte film. Surprisingly, 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, as shown in scheme 1, the ratio of TOFs (moles hydrogenated/mole Pd/time) for the hydrogenations of allyl alcohol and 2-methyl-2-propen-1-ol is 240 with average Pd nanoparticle diameters of 2.2 nm and only 2.6 with average particle diameters of 3.4 nm.  These remarkable selectivities with the 2.2-nm particles are an order of magnitude higher than the selectivity of Wilkinson's catalyst, the prototypical homogeneous catalyst for such reactions.  Although, a number of studies showed that the activity of nanoparticles is a strong function of particle size, studies of selectivity are particularly attractive for investigating nanoparticle reactivities because comparison of relative reaction rates avoids the question of whether increased activity is simply related to an increased surface area to volume ratio.

We also investigated the use of Pd nanoparticles as catalysts for the hydrogenation of nitroaromatic compounds to the corresponding anilines.  Such reactions are important because functionalized aniline derivatives are synthetic intermediates in the production of dyes, herbicides, pesticides, and pharmaceuticals.  Unfortunately, in complex syntheses, reduction of other functional groups in nitroaromatic compounds can also lead to undesired side products.  As an initial example, we studied the hydrogenation of nitrobenzaldehyde because this reaction can potentially yield a number of different products as shown in Scheme 2.  Table

1 shows the distribution of products from the hydrogenation of nitrobenzaldehyde with both commercial catalysts and Pd nanoparticles in 3-bilayer poly(acrylic acid)/polyethyleneimine films [PAA-Pd(0)/PEI]₃.  Notably, in ethanol, the desired compound, 1a, is less than 50% of the product when using a Pd on C catalyst.  The yield of 1a is somewhat higher with commercial 5% Pd on alumina, but only the [PAA-Pd(0)/PEI]₃ catalyst gives >90% 1a in the product.   The [PAA-Pd(0)/PEI]₃ catalyst also shows nearly complete selectivity for the hydrogenation of nitro groups in the compounds shown in Scheme 3.  Future work aims at assessing the effects of nanoparticle size on a much wider range of catalytic reactions.

The funding from this grant was vital in starting this work and supporting two graduate students, Somnath Bhattacharjee and David Dotzauer.  The area of catalysis is a relatively new one in the Bruening group and support from PRF has helped us gain needed expertise to begin working in this area.  PRF funding has served as seed money for current proposals. 

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