A study of niobium-containing nanosheets by liquid state mixing and its activity in dehydration of 2-heptanol

Abstract

Nanosheet-type materials have been studied so far due to a wide range of application in the fields of secondary batteries, sensors, and catalytic reactions. Hence, they gained great attention both in academia and industrial point of view, and are capable of being used in near future. Among the reported nanosheet-type materials, HNb3O8 nanosheet is widely applied for photo-catalysis in generating hydrogen from the water, electro-catalysis such as secondary batteries, and various heterogeneous catalysis in Friedel-Crafts alkylation, esterification, hydrolysis, dehydration, and so on. In particular, the dehydration reaction to convert long carbon-chain alcohols into olefins is one of the important acid-catalyzed reactions in the petrochemical industry. However, sulfuric acid and hydrochloric acid have been generally used due to process efficiency, but they are difficult to separate from dehydration products. To circumvent such limitations, a study is required to develop a solid acid catalyst with high dehydration activity and long-term stability. Thus, I focused on niobium-containing nanosheet catalysts of atomically thin layer structure that was applied for the dehydration of 2-heptanol. All of the nanosheets in this thesis were prepared by a liquid state mixing in order to improve the exfoliation efficiency and product recovery, and enhance the catalytic activity in the title reaction. Also, Ti-containing niobate nanosheets were developed to compare and understand the characteristics of acid sites in niobate nanosheet catalysts. Traditional synthesis of exfoliated HNb3O8 nanosheet (eHNb3O8) consists of five steps such as the mixing of K2CO3 with crystalline Nb2O5, ball-milling, calcination, proton exchange with HNO3 or HCl, and final exfoliation. Among these steps, the solid state mixing was replaced by the developed liquid state mixing to contact Nb2O5 solid with a diluted K2CO3 solution. This new method induced the high exfoliation efficiency and product recovery up to 80%. The obtained eHNb3O8 showed a larger specific surface area, a higher density of total acid sites, and an improved catalytic performance in the dehydration of 2-heptanol and formic acid. Based on the above results, eHNb3O8 was additionally prepared by using amorphous niobic acid (NBA, Nb2O5·nH2O) as Nb source because the mixing of K+ with Nb source is detrimental. The NBA-derived eHNb3O8 contained a very small amount of potassium ion and showed the high stability in consecutive activity runs. The linkage between these findings was confirmed by the inferior stability of K+-deficient eHNb3O8 prepared through prolonged proton exchange. This originated from the acidity of amorphous NBA leading to the decomposition of K2CO3’s carbonate ion into CO2 in the preparation of the solid mixture K2CO3–NBA. Thus, K+ ion was more intercalated and could not be all displaced with proton by the general ion-exchange process. Consequently, the K+ ion remaining in NBA-derived eHNb3O8 acts as a ligating element to tie up a single, exfoliated nanosheet into several. Finally, Ti-containing niobate nanosheets, eHTiNbO5 and eHTi2NbO7, were prepared by the liquid state mixing in order to compare their acidity with that of eHNb3O8. As a result, eHTiNbO5 and eHTi2NbO7 samples showed the better activity in the dehydration of 2-heptanol at 513 K than eHNb3O8. In particular, nearly 100% conversion was attained with eHTi2NbO7 even at 493 K. Although I could not prove the exact location of Bønsted acid site on nanosheets, it was found that the amount of the Brønsted acid site were related with the water molecules adsorbed on the hydroxyl groups of nanosheet. Various characterization techniques suggested that the strong Brønsted acid site was formed on the hydroxyl group linked to Nb–O or Nb–O–Ti in the Nb-containing nanosheets and existed in a form of hydronium ion resulting from the adsorption of water molecules.