Structure–properties relationship in the hydronium-containing pyrochlores (H3O)1+pSb1+pTe1−pO6 with catalytic activity in the fructose dehydration reaction

Abstract

A series of defect pyrochlores of the composition (H₃O)_1+pSb_1+pTe_1−pO₆ have been prepared by ion exchange from K-containing pyrochlores K_1+pSb_1+pTe_1−pO₆ in sulfuric acid at 280 °C for 24 h. The structural characterization of the hydronium-containing pyrochlores, including the location of the H₃O⁺ units within the three-dimensional framework, was possible from neutron powder diffraction data in undeuterated samples. The crystal structure for all the compounds is defined in the Fdm space group, and consists of a covalent framework of Sb^VO₆ and Te^VIO₆ octahedra distributed at random and connected by their vertices with (Sb,Te)–O1–(Sb,Te) angles close to 136°, conforming to large cages where the hydronium species are located off-center. The absence of K⁺ ions in the ion-exchanged pyrochlores was confirmed by inductively coupled plasma optical emission spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The shape and size of the hydronium units evolve along with the series, becoming more compact as the framework covalence and Lewis-basicity decrease upon Sb enrichment of the structure (for greater p values). The amount and lability of the H₃O⁺ species also increase throughout the series, as wanted: a straightforward correlation of the catalytic activity in the fructose dehydration reaction to 5-hydroxymethylfurfural has been observed, reaching conversion rates up to 88.5% of concentrated fructose solution for the p = 0.25 catalyst. Moreover, a pseudo-first-order kinetic mechanism was simulated, and the kinetic constants obtained from diluted and concentrated enhanced reaction systems were determined and compared.