(656c) Identifying the Morphology and Composition of Actives Sites in a Heterogeneous Metal Cation Complex Catalyst

A grand challenge within the catalyst community is developing industrially-relevant catalysts with high activity, selectivity, and recyclability. One promising solution is a metal complex (i.e., a 3d transition metal such as Nickel(II)) anchored to a porous support. Such materials comprise well-defined, uniform, and isolated active-sites. Depending on the reaction environment, these supported metal cation complexes exhibit structural changes that affect catalytic performance. In order to reach their full-potential, fundamental insights into the relationship between reaction environment, structure, and performance are required. However, the molecular-level changes are challenging to elucidate, even using state of the art in operando techniques. Hence, complementary descriptions of the system, which can be provided by simulations, are needed. In this work, we use density functional theory (DFT) calculations and ab initio thermodynamic modeling to learn how the reaction environment influences Ni(II) complex catalysts supported on the metal-organic framework (MOF) NU-1000. We investigate more than 100 different compositions and morphologies in order to reveal those that are favorable at relevant activation and reaction conditions. For example, activation with a high hydrogen partial pressure reduces the metal complex to the extent of favoring metal nanoparticle (NP) formation. However, slightly decreasing the hydrogen partial pressure expunges water molecules producing an active structure that competes thermodynamically with the NP structure. In all, we explore the influences that conditions related to catalytic activation, catalytic ethylene hydrogenation, and catalytic ethylene oligomerization have on the Ni(II) complexes. For each, we demonstrate the difference between the “as-synthesized” and the operational active site. The computational approach demonstrates how the catalyst structure is tunable using temperature and partial pressure to control the morphology and composition. The findings establish thermodynamically relevant models that require further computational catalytic investigations as well as showcasing how to tune structures using different reaction conditions.