Metal–Organic-Framework-Supported and -Isolated Ceria Clusters with Mixed Oxidation States

The formation of oxygen vacancies via reversible transitions between Ce­(IV) and Ce­(III) plays a crucial role in the propensity of cerium oxide to act as a supporting promoter in oxidative heterogeneous catalysis. An open challenge is, however, preparation of high-porosity, supported arrays of isolated ceria­(IV, III) clusters with high porosity. Herein, we report two examples of oxy-Ce­(IV, III) clusters supported and spatially isolated on an oxy-zirconium MOF, NU-1000. The clusters are introduced using either of two Ce complexes (precursors): Ce^IV(tmhd)₄ (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) or Ce^III(iPrCp)₃ (iPrCp = tris­(isopropyl-cyclopenta-dienyl), via SIM (solvothermal installation in MOFs). The prepared materials are named Ce-l-SIM-NU-1000 and Ce-n-SIM-NU-1000, respectively. X-ray photoelectron spectroscopy characterization shows that the ratio of Ce­(III) to Ce­(IV) oxidation states can be modulated. Difference envelope density analyses of X-ray scattering show that Ce_xO_yH_z clusters in Ce-n-SIM-NU-1000 are located between pairs of Zr₆ nodes, whereas in Ce-l-SIM-NU-1000, they are sited on MOF linkers throughout the micropores of NU-1000. Cluster size differences were further evaluated by pair function distribution (PDF) analyses of total X-ray scattering reveal that the node sited clusters contain of only a few cerium ions, whereas the linker-sited clusters each contain ∼90 cerium ions. The observed size appears to be defined by the size of NU-1000s triangular pores, that is, cluster formation appears to be pore templated. The Ce-SIM functionalized materials are catalytically active for hydrolysis of DMNP (dimethyl 4-nitrophenyl phosphate), a nerve-agent simulant. Conversion of a small fraction of the Ce­(IV) ions in which the presence of small fractions of the cerium­(IV) ions in Ce-l-SIM-NU-1000 to cerium­(III) significantly enhances catalytic activity–perhaps by labilizing aqua ligands and facilitating simulant binding to the clusters Lewis-basic metal ions. While not explored here, the larger clusters, when partially reduced, are, we believe, candidate catalysts for O₂ activation and subsequent selective oxidation of organic substrates.