Synthesis of Fluoride-Containing High Dimensionally Structured Nb Oxide and Its Catalytic Performance for Acid Reactions.

High dimensionally structured niobium oxide (HDS-NbO) containing fluoride (F-) was prepared by a hydrothermal synthesis. F- could be introduced into HDS-NbO by replacing lattice oxygen up to a solid F-/Nb ratio of 0.55. The introduction of an appropriate amount of F- promoted the crystal growth of HDS-NbO, while niobium oxyfluoride having the hexagonal tungsten bronze structure (HTB-Nb(F,O)x) was concomitantly formed by excess F- addition. HAADF-STEM analysis suggested that the number of micropores (hexagonal and heptagonal channels) in HDS-NbO was increased by the introduction of an appropriate amount of F-. The catalytic activity for Brønsted acid reactions was evaluated by Friedel-Crafts alkylation. The catalytic activity was significantly increased by the introduction of F-, while excess introduction of F- significantly decreased the activity. Catalytic activity for the Lewis acid reaction in the presence of water was evaluated by the transformation of pyruvaldehyde into lactic acid. The catalytic activity was changed by the introduction of F- in a manner similar to that observed in the Friedel-Crafts alkylation. On the basis of the results obtained, we propose that the local catalyst structure around the micropores of HDS-NbO is crucial for the acid reactions.