(220d) A General Methodology for Preparing Single Site Silica Supported Catalysts

?Next generation? catalysts will require new synthetic approaches to attain the site homogeneity necessary to achieve higher selectivities and activities than are currently observed. There are four main challenges that must be addressed in improving current approaches to heterogeneous catalyst preparation: First, a detailed knowledge defining the specific, targeted catalyst ensemble must be in hand and a designed synthesis must be developed to arrive at that ensemble. Second, the catalyst-support system must exhibit site homogeneity. To maximize atom efficiency and achieve maximum selectivity for the desired product, only the desired catalyst ensemble may be present in the system. Third, the activity of a supported catalyst scales as the number of surface sites in the system. Therefore, the highest site density on the surface of the support must also be achieved while preventing any overlap of individual sites or aggregation of the active metal and phase separation from the support to occur. Finally, access to and from the active sites in the system must be optimized for both substrates and products so that high mass transport rates are maintained in application. The requirements articulated above are finely interwoven with a working definition of nanostructuring in catalysis, i.e. the simultaneous control of structure at several different length scales. The structure of the active site involves an assembly of attachments to the surface of the support as well as arriving at the terminating ligands. The steric and electronic signature of both types of ligands must be understood and controlled to attain high activities. Thus defining the active site generally involves control of structure in the range of angstroms to a few nanometers. Maintaining site isolation while attaining high site densities is generally quite difficult. Traditional approaches to controlling the dispersion of surface binding groups and adsorbing metal site precursors from solution generally lead to a compromise involving a reduced the number of surface groups with the hope that they are well dispersed. In theory, true nanostructuring of supported catalysts should not rely on such a compromise but should have, as a part of its conceptual design, a way of achieving high site density while maintaining site isolation. High surface area supports exhibiting a distributed array of meso- and micropores to maintain high mass transport rates to and from the sites are well known design criteria for supported catalysts and constitute the final components of nanostructuring a catalysts for optimum activity. In meeting the design objectives described above we have taken a bottom up approach to the directed synthesis of nanostructured, single site catalysts. The three critical elements in the synthetic scheme are: a rigid molecular building block which will ultimately make up most of the support matrix; a family of linking reagents that will serve both to insert the active metals into the system as well as knit active sites together into a porous, cross linked matrix and, a linking reaction that relies on complementary functional groups on the building block (bb) and the linking reagents to cross link the components together. Currently we prepared several families of atomically dispersed metals (e. g. Al, Ti, V, Sn, W) within silicate building block matrices in which the connectivity of the metal to surrounding bb's is targeted and achieved. A brief overview of this work will be presented as well as our recent attempts to correlate catalytic activity with specific types of achieved nanostructuring. Finally, new directions involving polynuclear, double building block catalysts will be described.