The controlled synthesis of metal-acid bifunctional catalysts: The effect of metal:acid ratio and metal-acid proximity in Pt silica-alumina catalysts for n-heptane isomerization

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 nanometer-scale 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.