Several series of metal-acid bifunctional catalysts with controlled metal:acid ratios and metal site-acid site proximity were evaluated for n-heptane isomerization. Through the study of metal deposition reported in the companion paper, proximity could be achieved at four distinct scales; atomic, nano-, micro-, and millimeter scales. In the first two catalyst series, atomic and nanometer scale intimacy was obtained by depositing Pt onto acidic silica-alumina supports (Pt/Al-Si). It is demonstrated that poorly dispersed Pt/silica-alumina catalysts with nanometerscale proximity displayed a greater degree of bifunctionality than highly dispersed catalysts with atomic-scale proximity. In the latter two series of catalysts the scale of intimacy was stretched to micrometers using physical mixtures, and to millimeters by separating layers of nonacidic Pt/silica and metal free silica-alumina. Good bifunctionality is maintained at micrometer-scale intimacy and breaks down only at the millimeter scale. The best bifunctionality is achieved at very high acid to metal site ratios; results indicated that a single metal site can supply several hundred acid sites. Optimized bifunctional performance of Pt/silica-alumina catalysts for n-heptane isomerization requires a high acid-to-metal site ratio with nanometer to micrometer scale site proximity. The control of the numbers and proximities of metal and acid sites achieved in this work can be extended to many other metal-acid bifunctional reactions.

The rational synthesis of metal-acid bifunctional catalysts based on aluminosilicates will involve control over which domain -alumina or silica -the metal can be deposited on, as well as the control of particle size. These factors determine the ratio and proximity of the metal and acid sites. The control of adsorption selectivity and particle size has been studied by measuring the uptake of anionic and cationic Pt precursors as a function of pH over various types of silicaalumina composites and pure oxide supports, followed by characterization of the nanoparticles resulting from reduction of the precursors.Results indicate that electrostatic adsorption can be exploited to achieve selective deposition. Ion exchange of cationic precursors also appears to occur over the acid sites. Cationic tetraammine Pt precursors, either electrostatically adsorbed onto silica domains at high pH, or ion exchanged at the acidic alumina-silica adlineation (interface between two oxides) at neutral pH, lead to small (1.7-3.0 nm) nanoparticles. The size of Pt nanoparticles resulting from anionic Pt hexachloride electrostatically deposited onto alumina domains at low pH depends on the size of the alumina domain; a critical domain size is needed to anchor the Pt precursors against sintering.In the companion paper, the factors controlling metal-acid site intimacy and ratio are demonstrated for the isomerization of n-heptane.