Reactivity and stability synergism directed by the electron transfer between polyoxometalates and metal–organic frameworks

Abstract

The synergism between polyoxometalates (POM) and Cu(II) ions in homogeneous aerobic thiol oxidative deodorization has been realized in a more utilitarian heterogeneous catalyst: a multi-electron-capable POM captured in the pores of a metal–organic framework (MOF), HKUST-1 (POM@HKUST). The synergism between POM and the Cu(II) nodes in the MOF depends on the type of POM. Phosphovanadomolybdates, PV_xMo_12−xO₄₀^(3+x)− (x = 1–3) (PVMo) but not transition-metal-substituted polytungstates PXW₁₁ (X = V, Co, Zn and Co) result in POM@MOF materials that exhibit synergy relative to the individual structural components, the POM or MOF alone, not only for reactivity as in the case for the analogous homogeneous catalysts, but also for catalyst structural stability. The PVMo@HKUST-catalyzed reaction proceeds to essentially 100% conversion and the material is recoverable and unchanged based on FTIR spectroscopy, powder XRD data and other observations after reaction. The PXW₁₁@HKUST materials produce only limited conversions and decompose to white powders after reaction. X-ray photoelectron spectroscopy reveals that all the Cu(II) sites in the HKUST-1 become Cu(I) sites that are stable in air. Further kinetics studies show that PVMo undergoes fast multielectron transfer with intermediate Cu/RSH complexes, while PXW₁₁ show far slower and limited electron transfer ability with these Cu/RSH complexes. Limited electron transfer between Cu nodes and the encapsulated POM units not only hinders reactivity but also leads to MOF framework distortion and subsequent decomposition induced by the reduction of Cu(II) to Cu(I) sites in the framework.