Using the Non-Randomness of Grafting Sites in Siliceous Catalyst Supports to Design Active Sites

The attachment of active sites to catalyst supports such as silica, silica-alumina, and HZSM-5 via reaction of a metal complex either directly or remotely (i.e., using a tethering ligand) serves to immobilize and isolate the sites, thereby limiting bimolecular deactivation. However, recent spectroscopic and reactivity evidence suggests that the hydroxyl anchoring sites are not randomly distributed, and the mean distance between them cannot be decreased thermally. Siliceous supports display a tendency to create pairs of active sites, which can then function cooperatively. EXAFS, IR and NMR characterization of these sites will be discussed, as well as the implications for catalytic activity and molecular layer deposition.