Hydrothermal synthesis and formation mechanism of controllable magnesium silicate nanotubes derived from coal fly ash

A novel controllable magnesium silicate nanotube (MSN) material derived from coal fly ash was successfully synthesized via a hydrothermal process for the first time, and the reaction conditions and mechanism of synthesizing MSN materials from magnesium oxide and sodium silicate extracted from the fly ash were studied. The optimal preparation conditions are temperature = 220 °C, pH = 13.5, and Mg: Si molar ratio = 3:2, and the tubular structure gradually appeared and showed controllable and regular growth with the increase of synthesis time. The mechanism revealed that with the gradual dissolution of brucite into the sodium silicate solution, the reaction product begins to crystallize and transform from an initial sheet-like structure to a tubular structure, and finally becomes a uniformly arranged nanotube. The formation process of MSN follows Pauling's fourth rule, Si-O tetrahedral coordination and Mg-OH octahedral coordination is further condensed to form a two-layer structure by the action of active oxygen, then the sheet is rolled into a tube under its structural stress. The growth of both outer tubular diameter and inner tubular diameter has good linear law and controllable, and the growth rate are 0.289 nm h^-1and 0.071 nm h^-1, respectively.