Two-step preparation of Keggin-PW12@UIO-66 composite showing high-activity and long-life conversion of soybean oil into biodiesel

Abstract

A polyoxometalate acid can be encapsulated into a metal–organic framework to construct a novel kind of solid-acid catalyst. In this work, the two-step method —high-temperature preparation of Zr₆O₄(OH)₄(CH₃COO)₁₂ and low-temperature self-assembly—has been adopted to synthesize the PW₁₂@UIO-66 composite (PW₁₂ = H₃PW₁₂O₄₀; UIO-66 = Zr₆O₄(OH)₄(OOC–C₆H₄–COO)₁₂). The as-synthesized PW₁₂@UIO-66 composite exhibits highly crystalline, good octahedron morphology, large specific surface area (1960 m² g⁻¹) and high thermal stability (>500 °C), which clearly demonstrates the potential as a solid-acid catalyst. Additionally, the PW₁₂@UIO-66 composite may be accomplished with 85% utilization of H₃PW₁₂O₄₀ and 95% yield through this synthetic procedure. The performances of the PW₁₂@UIO-66 composite are investigated by catalyzing the simultaneous transesterification and esterification of soybean oil into biodiesel. Under the optimal conditions, the conversion of the soybean oil into biodiesel would exceed 90% over the as-synthesized PW₁₂@UIO-66 composite. As the crucial indexes for industrial prospects, the recycling and life experiments were surveyed. After 10 times recycling and 4 weeks, the structure and performance of the PW₁₂@UIO-66 composite remained unchanged and in the meantime the PW₁₂@UIO-66 composite still maintained a high activity to convert soybean oil into biodiesel.