Nanoporous-walled silica and alumina nanotubes derived from halloysite: controllable preparation and their dye adsorption applications

• Halloysite was prepared into mesoporous-walled silica nanotubes (SBET = 414 m2/g).
• Mesoporous-walled alumina nanotubes (SBET = 159 m2/g) was also obtained.
• Monolayer adsorption capacity up to 427 mg/g for methylene blue was achieved.

Acid/alkali selective-etching chemistry cooperated with a pre-calcination treatment are proposed and demonstrated, to process natural halloysite nanotubes (Hal) into nanoporous-walled silica or alumina nanotubes. The phase transformation behavior of Hal under the different condition of calcination was investigated. Acid or alkaline aqueous solution was used to selectively remove alumina or silica in the tube wall of pre-calcined Hal with resultant developed nanopores. Microporous/mesoporous-walled silica nanotubes of specific surface area (SBET) up to 414 m2/g were obtained by acid-etching Hal pre-calcined at 850 °C. Mesoporous-walled alumina-rich nanotubes of SBET up to 159 m2/g were developed by alkali-etching Hal pre-calcined at 1000 °C. In the experiment the materials exhibit enhanced adsorption for methylene blue (MB) in aqueous solution of elevated pH. Theoretical modeling based on the concepts of Langmuir, Freundlich and Redlich–Peterson was applied for their isotherms. The optimized monolayer MB adsorption capacities of 427 mg/g for silica and 249 mg/g for alumina products were achieved, showing potential in low-cost and high-efficient adsorbents.