Epoxidation catalysts prepared by encapsulation of molybdenum hexacarbonyl in UiO-66(Zr/Hf)-type metal-organic frameworks

The confinement and catalytic behavior of Mo(CO)(6) species inside UiO-66 (zirconium and hafnium) metalorganic frameworks (MOFs) have been investigated by means of multitechnique characterization and catalytic olefin epoxidation. Mo(CO)(6) was encapsulated in nanocrystalline MOFs by solvothermal and vapor phase impregnation methods (STI and VPI, respectively), resulting in Mo loadings of 2.0-8.8 wt% (STI) or 15 wt% (VPI). Powder X-ray diffraction, SEM-EDS, N-2 adsorption, FT-IR and C-13{H-1} CP MAS NMR collectively confirmed immobilization within the pore spaces of the hosts, which maintained their bulk crystallinity and crystal morphologies. CO resonances in the C-13{H-1} CP MAS NMR spectra were assigned to hexacarbonyl guest molecules located in the larger pore spaces, such as the octahedral cavities (for STI and VPI samples), and in more restricted environments such as the tetrahedral cavities (for the VPI sample). FT-IR spectra displayed the characteristic pattern of nu(CO) stretching modes for an octahedral symmetry molecule slightly perturbed by interaction of one of the carbonyl groups with the MOF support. The spectra evidenced the formation of a weak hydrogen bonding interaction of the type mu(3)-OH center dot center dot center dot OCMo(CO)(5) involving the mu(3)-OH groups that are exclusively present in the tetrahedral cavities of the UiO-66 structure. The Mo(CO)(6) loaded MOFs are active and reusable pre-catalysts for cis-cyclooctene epoxidation, exhibiting excellent epoxide selectivity (up to quantitative epoxide yield within 1 h reaction) and tert-butyl hydroperoxide efficiency. Catalyst recycling and retention of structural integrity were demonstrated for UiO-66(Hf) loaded with 8.8 wt% Mo by the STI method. The UiO-66(Hf) host may be a better catalyst support than its Zr counterpart due its smaller crystallite size (similar to 100 nm) and higher defectivity. The substrate scope was broadened for Mo(8.8-STI)/Hf-MOF which promoted the epoxidation of biobased olefins (methyl oleate and DL-limonene) and oxidative dehydrogenation of cyclooctanol to cyclooctanone.

Automated summary

The study focused on the confinement and catalytic behavior of Mo(CO)(6) species encapsulated in UiO-66 metalorganic frameworks, demonstrating their immobilization within the pore spaces of the hosts and their catalytic activity in olefin epoxidation. Excellent epoxide selectivity and efficiency were achieved, along with catalyst recycling and structural integrity retention. The study also highlighted the broader substrate scope and potential of UiO-66(Hf) as a catalyst support compared to its Zr counterpart.