A General Methodology for Preparing Nanostructured Silica Supported Catalysts
Craig E. Barnes, Chemistry, University of Tennessee, 552 Buehler Hall, Knoxville, TN 37996-1600
Next generation catalysts will require new synthetic approaches to achieve the site homogeneity necessary to achieve the high selectivities and activities desired in application. We are developing a new synthetic methodology by which extremely high site homogeneity is attained as the catalyst-support system is assembled. The critical elements of this approach involve first, a rigid ~nanometer sized building block (bb) which, when linked together, form the so called support matrix. The building block that we have investigated is the spherosilicate, Si8O20. The second element is a family of reagents that react with multiple building blocks to form a rigid, porous, cross linked matrix. Two types of linking groups are needed: those that will eventually become the active sites for catalysis and those that produce chemically and thermally robust linkages which stabilize the bb-linker matrix to typical reaction conditions used with metal oxide catalysts (200 – 500°C). Finally, complementary functional groups on the linking reagents and building blocks is required which allow only the desired chemical linkages to be formed and completely exclude homocondensation reactions from occurring which could lead to speciation of the matrix. To achieve precise structural control of the active sites in the matrix we have developed the method of sequential additions which, while maintaining strict atomic dispersity, allows us to control the number of Si-O-M bonds linking catalyst centers to the bb-matrix. Thus, we can produce metal sites within these matrices in which the connectivity of the catalytically active metal centers to the building blocks that surround them is synthetically designed to be from fully “embedded” (bonded to the maximum number of building blocks) to a “surface” site in which the metal is bound to exactly one building block. Our recent efforts in preparing and characterizing silicate matrices containing atomically dispersed Ti, V, Sn and Al will be described as well as their properties as solid acid catalysts.