Characteristics of exfoliated HNb₃O₈ nanosheet derived from amorphous niobic acid and its application to dehydration of 2-heptanol

Abstract

The synthesis of exfoliated HNb₃O₈ nanosheet (eHNb₃O₈) generally starts from the mixing of K₂CO₃ with crystalline Nb₂O₅ and involves a very routine procedure in subsequent multi-steps. Herein, we report for the first time the use of niobic acid (NBA, Nb₂O₅nH₂O) as Nb source for the preparation of final eHNb₃O₈. Different from the analogue derived from crystalline Nb₂O₅, the prepared nanosheet contains a very small amount of potassium ion and exhibits high stability in consecutive runs for the dehydration reaction. The linkage between these two features is confirmed by the inferior stability of potassium-deficient eHNb₃O₈ samples prepared via prolonged proton-exchange. When a series of niobate materials are examined, the remarkable finding is the higher K/Nb ratio in NBA-derived KNb₃O₈ than the theoretical value. This is attributed to the acidity of amorphous NBA by which the carbonate ion of K₂CO₃ is decomposed into CO₂ in the preparation of the solid mixture K₂CO₃-NBA. These more intercalated K⁺ cannot be all displaced with proton by the general ion-exchange process employed for Nb₂O₅-derived eHNb₃O₈. Consequently, the proposed model suggests that the potassium ion remaining in NBA-derived eHNb₃O₈ acts as a ligating element to tie up a single, exfoliated nanosheet into several.