Solid-state transformation of single precursor vanadium complex nanostructures to V2O5 and VO2: catalytic activity of V2O5 for oxidative coupling of 2-naphthol

Abstract

A vanadium complex, [(C₅H₅N)₂V₂O₃·H₂O], of different morphologies has been obtained via a modified hydrothermal procedure using pyridine and VOSO₄ salt as the starting material. The evolved [(C₅H₅N)₂V₂O₃·H₂O] nanobelts are of 50–200 nm in width and of a length up to several millimeters. At higher temperatures (600 °C), the solid [(C₅H₅N)₂V₂O₃·H₂O] nanostructures are converted to vanadium pentoxide (V₂O₅) and vanadium dioxide (VO₂) when heated in air and nitrogen atmosphere, respectively. During growth, the mechanism of the evolution of octahedra, truncated octahedra, and hollow truncated octahedra of [(C₅H₅N)₂V₂O₃·H₂O] are reported for the first time. These types of well-structured morphology are also isolated while V₂O₅ and VO₂ are evolved. The as-grown belt-like and octahedral morphologies of [(C₅H₅N)₂V₂O₃·H₂O] are retained during the solid-state transformation, suggesting a route to evolve crystalline nanomaterials. Again, the morphological evolution of the [(C₅H₅N)₂V₂O₃·H₂O] nanostructures has been examined to be pyridine and precursor vanadyl sulfate (VS) concentration dependent. Thus, we are able to isolate truncated octahedra as an intermediate during the formation of [(C₅H₅N)₂V₂O₃·H₂O] nanobelts and nanoflowers with a high pyridine (Py) concentration. Interestingly, longer reaction times successively featured the transformation of truncated octahedra into nanobelts. Nanobelt evolution is not observed at low pyridine concentrations. However, the formation of octahedral morphology takes place at low pyridine concentration. All of the nanostructures were critically examined and characterized thoroughly by various physical techniques to ascertain their purity, structure and composition. An interesting, thermodynamically stable, single crystalline product from DMF soluble [(C₅H₅N)₂V₂O₃·H₂O] has been characterized, which indirectly supports the composition of [(C₅H₅N)₂V₂O₃·H₂O]. Selectively, vanadium pentoxide nanobelts have been found to be an efficient catalyst for the oxidative coupling of 2-naphthol to binaphthols under a molecular oxygen atmosphere.