Spiky-shaped niobium pentoxide nano-architecture: highly stable and recoverable Lewis acid catalyst

Abstract

Niobium pentoxide particles with a complex three-dimensional (3D) nanostructure consisting of a spiky structure have been developed as recyclable and recoverable Lewis acid catalysts. The morphology of the niobium pentoxide was successfully controlled from 1D to 3D via a bridging-ligand-assisted hydrothermal treatment, without changing the crystal structure. Compared with dispersed one-dimensional (1D) niobium pentoxide nanorods with a major-axis length and minor-axis length of 20 nm and 5–8 nm, respectively, the spiky-shaped niobium pentoxide composed of 300 nm spherical cores and nanorods with a minor-axis length of 5 nm maintained its surface nanostructure even after calcination at 400 °C in air. The 400 °C-calcined spiky particles exhibited the highest production rate of 2-((4-methoxyphenyl)amino)-2-phenylacetonitrile (0.115 mmol m−2) in a Strecker reaction, resulting in a nanoscale and ordered surface structure of spiky particles that simultaneously exhibit high specific reactivity and high structural stability. Acid site analysis and Raman spectroscopy revealed that stable nanorods that grew in the (001) orientation functioned as Lewis acid catalysts and that the origin of the acidity was a flexible Nb–O polyhedral structure in the single-nanoscale (<10 nm) niobium oxide rods. This study proposes that the spiky-shaped niobium pentoxide exhibits sintering resistivity and high activity and has potential applications as a recoverable and recyclable solid acid catalyst.

Growth Mechanism of Spiky Nb2O5 Nanoparticles and their Electrochemical Property

“…Increasing the temperature to 190 °C, the approximate major axis length increased to 30 nm, and the nanorods were separated from each other (Figure 1j–l and S4, Supporting Information). There was no change in morphology between 190 and 200 °C samples, and as in previous reports, [ 14,17,18 ] the spiky Nb 2 O 5 nanoparticles composed of a spherical core and numerous nanorods were obtained (Figure 1m–o and S5, Supporting Information). These results suggested that the nanorods grew by migration not only from a spherical core but also among primal particles, rectangular‐shaped nanoarchitectures, and nanorods.…”

“…In our previous studies, hydrothermal treatment of the ammonium niobate ( V ) oxalate solution without the acetates produced shorter Nb 2 O 5 nanorods (20 nm in major axes length) than the nanorods synthesized with acetates. [ 14 ] Furthermore, secondary particle growth was not suppressed on the shorter Nb 2 O 5 nanorods, resulting in nanorods aggregation. Thus, the acetate ions must adsorb onto the surface of Nb 2 O 5 particles and function as protecting agents to suppress the particle growth.…”

“…The details of the synthesis method for spiky Nb 2 O 5 particles are described elsewhere. [ 14,17,18 ]…”

“…[ 13 ] Our previous study showed that spiky Nb 2 O 5 particles with a rod‐to‐rod distance of 5–10 nm were thermally stable until 400 °C, even if the lengths of minor axes of nanorods are 5–8 nm. [ 14 ]…”

Synthesis and Acid Catalytic Activity of Al‐Doped Spiky‐Shaped Niobium Pentoxide

“…Recently, we have fabricated spiky-shaped Nb 2 O 5 nanoparticles composed of numerous nanorods on a sub-micro sized spherical core by hydrothermal treatment of Nb oxalate solution. [9][10][11] The 3D nanostructure has higher stability compared to dispersed nanorods. However, high catalytic activity was not observed in the spiky-shaped Nb 2 O 5 nanoparticles, even though they have a single nanoscale structure and large surface area.…”