Encapsulation of the vanadium substituted Keggin polyoxometalates [α-PVW11O40]4− and [α-PV2W10O40]5− in HKUST-1

Abstract

Two POM@MOF hybrid materials composed of a copper-based metal–organic framework (MOF) [Cu₃(C₉H₃O₆)₂(H₂O)₃]_n (HKUST-1) encapsulating vanadium-substituted Keggin polyoxometalates (POM), [α-PVW₁₁O₄₀]⁴⁻ (PVW₁₁) and [α-PV₂W₁₀O₄₀]⁵⁻ (PV₂W₁₀), were prepared and characterized. PVW₁₁@HKUST-1 and PV₂W₁₀@HKUST-1 were synthesized hydrothermally by self-assembly of HKUST-1 in the presence of the preformed POMs, [α-PVW₁₁O₄₀]⁴⁻ and [α-PV₂W₁₀O₄₀]⁵⁻, respectively. The two POM@MOF composites were characterized by X-ray diffraction, TGA, BET surface area analysis and FT-IR and Raman spectroscopy. The electronic structure of the POM@MOF materials and their respective constituents is surveyed using solid state UV-vis reflectance spectroscopy. The UV-vis spectra order the oxidizing strength of the POM constituents ([α-PV₂W₁₀O₄₀]⁵⁻ > [α-PVW₁₁O₄₀]⁴⁻) and reveal the distinct electronic structure of the POM@MOF materials obtained by synthetic encapsulation of mono- and di-vanadium substituted Keggin polyoxotungstates in HKUST-1.