A Green and Facile Synthesis of Ordered Mesoporous Nanosilica Using Coal Fly Ash

Ordered mesoporous materials have attracted much attention owing to their superior structural properties. In this work, we develop a green and facile method to convert coal fly ash, a cheap, abundant, and silicon-rich industrial waste, into highly ordered mesoporous nanosilica. An energy-efficient technique, the alkali-dissolution process, was systematically studied for the extraction of silica from waste materials, instead of the conventional alkaline fusion method. The extraction efficiency of silica could reach up to 46.62% within 0.5 h at 110 °C in 25 wt % sodium hydroxide solution, and the liquid–solid ratio was reduced to 1.5:1. Subsequently, simulated flue gas was introduced to precipitate the nanosilica with the assistance of a surfactant through a twice-carbonation process. A series of characterization techniques confirmed that the synthetic nanosilica (SiO<sub>2</sub>-0.16) has a high purity (99.35%), high surface area (1,157 m<sup>2</sup> g<sup>–1</sup>), large pore volume (0.95 cm<sup>3</sup> g<sup>–1</sup>), and a highly ordered hexagonal mesostructure (2.88 nm), similar to the characteristics of the material derived from silicon alkoxide. This strategy significantly decreased the energy consumption and shortened the synthesis process through the utilization of flue gas and is thus an effective and scalable approach for the synthesis of ordered mesoporous nanosilica from coal fly ash.