An Exceptionally Stable Metal–Organic Framework Supported–Molybdenum(VI) Oxide Catalyst for Cyclohexene Epoxidation

Noh, H.; Cui, Y.; Peters, A. W.; Pahls, D.; Ortuño, M. A.; Vermeulen, N. A.; Cramer, C. J.; Gagliardi, L.; Hupp, J. T.; Farha, O. K.
J. Am. Chem. Soc. 2016, 138, 14720 (doi:10.1021/jacs.6b08898).

Molybdenum(VI) oxide was deposited on the Zr₆ node of the mesoporous MOF NU-1000 via condensed phase deposition where the MOF is simply submerged in the precursor solution, a process named solvothermal deposition in MOFs (SIM). Exposure to oxygen leads to a monodisperse, porous heterogeneous catalyst, named Mo-SIM, and its structure on the node was elucidated both computationally and spectroscopically. The catalytic activity of Mo-SIM was tested for the epoxidation of cyclohexene. Near quantitative yield of cyclohexene oxide and the ring-opened 1,2-cyclohexanediol was observed, indicating a significantly higher activity than that of molybdenum(VI) oxide powder and comparable to that of a zirconia-supported analogue (Mo-ZrO₂) prepared in a similar fashion. Despite the well-known leaching problem of supported molybdenum catalysts (i.e. loss of Mo species thus causes deactivation), Mo-SIM demonstrated no loss in the metal loading before and after catalysis, and no molybdenum was detected in the reaction mixture. In contrast, Mo-ZrO₂ led to significant leaching and close to 80 wt % loss of the active species. The stability of Mo-SIM was further confirmed computationally, with density functional theory calculations indicating that the dissociation of the molybdenum(VI) species from the node of NU-1000 is endergonic, corroborating the experimental data for the Mo-SIM material.