Rational Design and Synthesis of Pt/Silica-Alumina Metal-Acid Bifunctional Catalysts

In the domain of rational catalyst design and synthesis, Strong Electrostatic Adsorption (SEA) has been applied with tremendous potential and use. In this study, SEA method has been advanced to thick slurries as well as to mixed oxide systems with a view to synthesizing stable, active and selective catalysts fine-tuned for specific applications. The main objective was to synthesize metal-acid bifunctional catalysts with controlled ratio and proximity between metal and acid sites in order to analyze their effects on a bifunctional reaction. At first, the difference in charging behavior of silica and alumina in solution was used to selectively deposit Pt onto different domains of silica-alumina mixed oxides (Al-Si). With cationic Pt precursor at neutral to basic pH range, well dispersed Pt/Al-Si catalysts with “atomic scale” intimacy between metal and acid sites were prepared. However, anionic Pt adsorption over Al-Si at low pH resulted in poorly dispersed Pt/Al-Si catalysts with “nanometer scale” intimacy between active sites. In this case it was found that as anionic Pt selectively deposited onto the alumina domains of Al-Si, a critical domain size of alumina was required to stabilize small Pt nanoparticles. These catalysts along with other physically mixed and layered catalysts constituted a series of bifunctional Pt/Al-Si catalysts with varying ratio and proximity between metal and acid sites which were tested in an n-heptane (n-C7) isomerization reaction operating at 350°C and atmospheric pressure. From the catalytic results, large (~20nm) Pt particles over acidic silica-alumina catalysts with “nanometer-scale” proximity displayed a higher degree of bifunctionality than small Pt particles (~2nm) with “atomic-scale” intimacy. Even more significant is the ratio of acid sites to metal sites. For optimum bifunctional reactivity for n-C7 isomerization, Pt and amorphous Al-Si based catalysts should be synthesized with a high enough ratio of acid-to-metal sites (\u3e10 acid sites per metal site) and an intermediate degree of intimacy (not atomic-scale but nano- or micrometer-scale) between these two active sites