Mechanistic and thermodynamic insights into binding and activation of small molecules on metallozeolites - relevance for adsorption and catalysis

Abstract

Metallozeolites exchanged with 3d transition metal ions (TMI) are versatile catalytic materials due to their well-defined framework structures, redox flexibility, and remarkable adsorption and catalytic properties. These features make them invaluable for both fundamental and applied research, underpinning numerous catalytic technologies. The binding and activation of small reactant molecules is governed by the complex mechanistic interplay of involved intrazeolite reactions, whose course is influenced by the flexible valence, spin, and coordination states of the encaged metal ions and the metal-oxo entities. Despite significant advances, the nature of active sites, confinement effects, and the complex activation mechanisms of reactant molecules, which act as both innocent and non-innocent ligands, remain subjects of ongoing debate. This has driven extensive research into the thermodynamic constraints and molecular-level insights into activation processes with orbital and spin resolution. This review critically examines the thermodynamic and molecular aspects of intrazeolite speciation of transition-metal ions and metal-oxo active sites, their structural dynamics, and reactivity toward catalytically relevant small molecules, including NH₃, H₂O, CO, N₂, O₂, NO, N₂O. Particular emphasis is placed on ligand coordination, redox activation, and the role of electronic and spin states in dictating the catalytic behaviour of metallozeolites. The discussion integrates insights from site-selective spectroscopies and computational methods to elucidate the structural, thermodynamic, and molecular aspects of metal-ligand interactions and activation pathways, with an emphasis on the role of spin states in binding and reactivity. We hope that this review can serve as a relevant and valuable reference for researchers working with zeolite catalysts, providing new insights and inspiration.