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

Abstract

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, N2 adsorption, FT-IR and 13C{1H} 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 13C{1H} 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 νCO 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 μ3-OH⋅⋅⋅OCMo(CO)5 involving the μ3-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 (≈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.