The bigger picture
Transition metal ions (TMIs) hosted in zeolites have been revealed as a class of heterogeneous catalysts with remarkable activity and robustness for diverse applications. The catalytic mechanisms at play are not yet described at a molecular level, due to the lack of understanding of the speciation of TMI and subsequent formation of active sites in the zeolites.
The speciation of TMIs in zeolites is complex and depends on multiple parameters, ranging from zeolite properties and metal loadings to the mobility of TMIs under reaction conditions, and their local environment in the framework. Synthetic methods often overlook many of these factors, and this fact leads to highly heterogeneous materials.
Advances in spectroscopic and analytic methods have shed light on the structure and dynamics of TMIs in zeolites. Future research should focus on operando characterization of materials where complexity of TMI speciation is reduced to a minimum, aided by theoretical calculations.
Summary
Transition metal ions (TMIs) hosted in zeolites have potential for generating highly active and selective catalysts. The speciation of metal ions inside the microporous zeolite pore system, including geometric constraints and anchoring points, grouping, and nuclearity of species, is not well understood. Determination of the location and structure of active sites is challenging because different species co-exist in the same zeolite, and complexity is added by their dynamics under reaction conditions. Three parameters are influential on the speciation of active TMIs in zeolites: (1) zeolite properties, including framework structure, composition, and metal loadings; (2) mobility of TMI species inside the zeolite micropores; and (3) influence of other inorganic species present in the reaction spaces. In our view, molecular-level understanding of metal-ion-exchanged zeolite catalysts requires a set of characterization and computational techniques that should go hand in hand with refined synthetic methods selectively generating active species.
C. Schroeder, M. Sanchez-Sanchez, Unlocking the potential for pseudo-molecular catalysts via understanding the activity of transition metal ionic species in zeolites, Chem Catalysis, 2024, 101130. DOI: 10.1016/j.checat.2024.101130.
