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 (eHNb(3)O(8)) 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, Nb(2)O(5)nH(2)O) as Nb source for the preparation of final eHNb(3)O(8). 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 eHNb(3)O(8) 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 eHNb(3)O(8). Consequently, the proposed model suggests that the potassium ion remaining in NBA-derived eHNb(3)O(8) acts as a ligating element to tie up a single, exfoliated nanosheet into several.