Modulating\nCharges of Dual Sites in Multivariate Metal–Organic\nFrameworks for Boosting Selective Aerobic Epoxidation of Alkenes

Abstract

Selective\naerobic epoxidation of alkenes without any additives\nis of great industrial importance but still challenging because the\ncompetitive side reactions including CC bond cleavage and\nisomerization are difficult to avoid. Here, we show fabricating Cu­(I)\nsingle sites in pristine multivariate metal–organic frameworks\n(known as CuCo-MOF-74) via partial reduction of Cu­(II) to Cu­(I) ions\nduring solvothermal reaction. Impressively, CuCo-MOF-74 is characteristic\nwith single Cu­(I), Cu­(II), and Co­(II) sites, and they exhibit the\nsubstantially enhanced selectivity of styrene oxide up to 87.6% using\nair as an oxidant at almost complete conversion of styrene, ∼25.8%\nselectivity increased over Co-MOF-74, as well as good catalytic stability.\nContrast experiments and theoretical calculation indicate that Cu­(I)\nsites contribute to the substantially enhanced selectivity of epoxides\ncatalyzed by Co­(II) sites. The adsorption of two O₂ molecules\non dual Co­(II) and Cu­(I) sites is favorable, and the projected density\nof state of the Co-3d orbital is closer to the Fermi level by modulating\nwith Cu­(I) sites for promoting the activation of O₂ compared\nwith dual-site Cu­(II) and Co­(II) and Co­(II) and Co­(II), thus contributing\nto the epoxidation of the CC bond. When other kinds of alkenes\nare used as substrates, the excellent selectivity of various epoxides\nis also achieved over CuCo-MOF-74. We also prove the universality\nof fabricating Cu­(I) sites in other MOF-74 with various divalent metal\nnodes.