The ability to prepare supported metal catalysts in which the metal particles are both small (i.e., highly dispersed) and uniformly distributed on the support surface is important for different catalytic processes.  Classically, alumina has been the support used in the majority of petrochemical applications, but for targeted reactions where a nonacidic support is needed, silica becomes an interesting alternative.  However, with silica-supported catalysts the lack of strong interactions between conventional metal precursors and the support results in low metal dispersion and poor nanoscale homogeneity.  Since catalytic activity of many hydrogenation reactions correlates with the number of available surface metal sites, catalysts with high dispersion better utilize the available metal.  Uniformity of nanoscale metal distribution (i.e., maximizing metal particle separation) can be an important factor in minimizing coalescence.  For example, when metal particles cluster in localized areas of the support, agglomeration can be a problem and stability is poor. This has been well illustrated for supported Co in Fischer-Tropsch catalysts and for supported Ru on silica exposed to reductive atmospheres. <br/> <br/>Changes in preparation methods often affect the interaction strength between the supported oxide and the support, which in turn modifies reducibility and dispersion of the supported metal.  Some researchers have tried to use the concepts of surface charge and zeta potential to optimize supported metal catalyst preparation.  Others have incorporated water-soluble organic compounds such as organic acids into the metal salt solutions to improve dispersion.  We report on the use of bifunctional organics (namely, aminoalcohol or aminoacids), to form impregnates that interact strongly with silica, and can be used to make well dispersed, homogeneously distributed noble metal particles.  The key to making optimum Ru catalysts is to partially decompose the impregnate to form an anchored complex, which is then reductively hydrogenolyzed to remove the organic fragment.  In this way ruthenium oxide, which is mobile on the silica surface, is never formed.  We report on extension of that study to the use of bifunctional organics in preparing silica-supported Ir, Pt, and Rh catalysts.

Metal catalysts on silica: issues and opportunities

249th National Meeting (2015)

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