Preparation of Hierarchically Porous Niobium(V) Oxide and Alkaline Niobate Monoliths via Sol–Gel Accompanied by Phase Separation

Hierarchically porous niobium(V) oxide monoliths were prepared through the sol–gel process accompanied by phase separation and subsequent heat treatment. Macroporous and mesoporous characteristics as well as crystal structures were investigated in detail. Macroporous structures were controlled by the starting composition to manipulate phase separation, whereas mesoporous structures were modified by heat treatment in air to crystallize niobium(V) oxide. The results showed that by heat treating the as-dried gel at 550 °C in air, a hierarchically ordered macro/mesoporous material with BET surface area of 54 m 2 g –1 was obtained. With an increase of the heat treatment temperature, an increase in the mesopore size accompanied by a decrease in the specific surface area was observed as a result of crystallite growth. Heat treatment under a N 2 atmosphere resulted in the formation of phases with lower-valence niobium such as NbO 2 and Nb 12 O 29, which were derived from the reduction by the carbothermal reaction due to the presence of carbonized organic compounds. The conversion of niobium(V) oxide into niobium-containing complex oxides was also examined. Alkaline metals, namely, Li, Na, and K, were incorporated by postgelation treatments. Niobium-rich alkaline niobate phases were obtained when alkaline chloride was incorporated, whereas the majority of lithium niobate (LiNbO 3 ) phases were obtained when samples were treated with lithium hydroxide.