Catalysis over Isolated and Nested Lewis Acid Centers and Noble Metal Centers Anchored by Nested Lewis Acid Centers in Zeolites

ConspectusHighly dispersed transition-metal Lewis acid centers (e.g., Zn, Co, Y, La, Fe, Sn, Hf, and Zr) and Lewis acid-anchored noble metal centers (e.g., Pt–Zn, Pt–Sn, Pt–Fe, Rh–Zn, and Rh–Co) supported on siliceous zeolites are promising catalysts for a number of industrially important reactions, such as alcohol dehydrogenation, aldol condensation, alkane dehydrogenation, and olefin hydroformylation. In this Account, we describe the preparation and characterization of Lewis acid centers grafted onto hydrogen (H)-bonded silanol groups present in zeolites as well as Lewis acid-anchored noble metal centers and discuss the mechanism and kinetics for different reactions occurring over each type of center. We show that isolated and nested Lewis acid centers can be created by the reaction of hydrated cationic species with H-bonded silanol groups on dealuminated beta (DeAlBEA) or Silicalite-1 zeolite. We then demonstrate that isolated and nested Lewis acid centers are effective catalysts for light alkane dehydrogenation. Nested Lewis acid centers can also serve as efficient anchoring sites for dispersing noble metals such as Pt and Rh to generate bimetallic centers that exhibit superior catalytic performance relative to monometallic Pt and Rh for reactions such as alkane dehydrogenation and olefin hydroformylation. Finally, we summarize our recent investigations of isolated and nested Lewis acid centers and the Pt- and Rh-based bimetallic centers as catalysts for ethanol conversion to 1,3-butadiene (ETB), acetone conversion to isobutene (ATI), propane dehydrogenation to propene (PDH), n-butane dehydrogenation to butene and 1,3-butadiene (BDH), and ethene hydroformylation to propanal. We show that the activity of Lewis acid centers for these reactions is affected by their local coordination environments. In particular, we highlight the significance of H-bonding between hydroxyl groups connected to Lewis acid centers in an open configuration (M–OH) and silanol groups on zeolite supports to generate (≡SiO)xMn+–OH···(O(H)–Si≡)y structures, which exhibit aldol condensation activities that are higher than that of (≡SiO)xMn+–OH sites. These studies demonstrate that siliceous zeolites rich in H-bonded silanol groups can be utilized to create highly dispersed Lewis acid centers and can be further employed as an anchoring platform for noble metal atoms to construct atomically dispersed bimetallic centers. Both the chemical structure and the local coordination environment of these centers significantly influence their catalytic performance.