Structure–properties relationship in the hydronium-containing pyrochlores (H 3 O) 1+p Sb 1+p Te 1−p O 6 with catalytic activity in the fructose dehydration reaction

• Published in: Dalton transactions : an international journal of inorganic chemistry 2020-08, Vol.49 (33), p.11657-11667
• Main Authors: Mayer, Sergio Federico, Falcón, Horacio, Fernández-Díaz, María Teresa, Campos-Martín, José Miguel, Alonso, José Antonio
• Format: Article
• Language: English
• Subjects: Catalysis; Chemical Physics; Chemical Sciences; Cristallography; Inorganic chemistry; Material chemistry; Nonlinear Sciences; Physics
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/D0DT01770A

A series of defect pyrochlores of the composition (H 3 O) 1+p Sb 1+p Te 1−p O 6 have been prepared by ion exchange from K-containing pyrochlores K 1+p Sb 1+p Te 1−p O 6 in sulfuric acid at 280 °C for 24 h. The structural characterization of the hydronium-containing pyrochlores, including the location of the H 3 O + 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 Fd 3̄ m space group, and consists of a covalent framework of Sb V O 6 and Te VI O 6 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 H 3 O + 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.