Opportunities in Catalysis over Metal-Zeotypes Enabled by Descriptions of Active Centers Beyond Their Binding Site

Active centers in porous solid catalysts are multifaceted in structure, comprised of primary sites that bind intermediates, secondary environments that confine intermediates and transition states, and coadsorbed intraporous molecules, clusters, and networks of reactants and solvents that interact with species along reaction coordinates. Zeotypes are a class of microporous oxides whose lattices are substituted with metal heteroatoms, which serve as primary binding sites to catalyze a variety of chemical transformations including reductions, oxidations, and condensations. Research efforts continue to develop more complete mechanistic descriptions of catalytic behavior, requiring increasingly accurate descriptions of all moieties that comprise active centers. Framework metal heteroatoms adopt various local configurations that are difficult to control synthetically and are susceptible to restructuring during reaction. These binding sites contain structural and catalytic diversity even when they are located in the nominally simplest zeotype frameworks, as is typically the case for model heterogeneous catalyst materials of purportedly well-defined structure. Secondary environments are well-documented to influence catalysis via shape selectivity and confinement effects, yet their polarity, as defined by the oxide framework and any polar binding sites it contains (e.g., heteroatoms, hydroxyl groups), also influences the structures and Gibbs free energies of intermediates and transition states, as well as coadsorbed intraporous molecules, leading to further catalytic diversity. These three aspects that describe active centers at and beyond their binding sites must be accounted for in quantitative descriptions of the kinetic and thermodynamic factors that dictate catalysis over metal-containing zeotypes. Advances in describing the structural and catalytic diversity of these materials have required harnessing state of the art computational, spectroscopic, material synthesis, and chemical characterization methods. Herein, we outline opportunities to leverage experiment and theory to interrogate these aspects of active centers with increasing fidelity, to further advance molecular descriptions of catalysis over metal-containing zeotypes and broaden their range of catalytic behavior.