(197bn) A Computational and Experimental Characterization of the Ligand Environment of a Ni-Oxo Catalyst Supported in the Metal?Organic Framework Nu-1000

Heterogeneous catalysts exhibit significant changes in composition due to the influence of operating conditions, and these compositional changes can have significant effects on catalytic performance. For bulk metal heterogeneous catalysts, relationships between composition and catalytic operating conditions are well documented, such as the relationship between coverage and structure with operating conditions on bulk metal surfaces. However, at present, the influence of operating conditions on the compositions of single-site heterogeneous catalysts remains largely unresolved due to the tunability and variability of the ligand environment of the active site. Single-site heterogeneous catalysts are highly attractive due to improved metal utilization and tunability; however, chemical intuition about the influence of operating conditions on compositions and structures of single-site heterogeneous catalysts hinders their design and rational optimization. To address this, we combined computational and experimental characterization of a Ni-oxo (Ni4O_xH_Y clusters) catalyst under catalytic hydrogenation conditions. Pair distribution function (PDF) analysis is combined with ab initio thermodynamic modeling to investigate ligand environments present on a Ni-oxo cluster supported in the NU-1000 metal−organic framework. Experimental PDFs provide insights into both the Ni-O coordination numbers and the Ni−O, Ni···Ni, and Ni···Zr distances, which are key in understanding the molecular structure of the catalyst. These measurements are compared directly to simulated PDFs from Density Functional Theory (DFT) optimized calculations (a total of 854 different ligand environments). Our findings suggest significant OH and H₂O content, and that different Ni ions within the cluster and/or NU-1000 structure may comprise subtly different numbers of these ligands. The observation of significant H₂O content even under H₂ (g) conditions suggests that the support (NU-1000) supplies H₂O to the cluster. The combination of simulations and experiments provides new insights into the ligand environment for Ni-NU-1000 catalysts that will be useful for understanding the ligand environments of other single-site Ni catalysts as well.