Preparation of activated carbon-mesoporous silica composites from coal gangue and phase transformation during its preparation

Coal gangue, as a mixture of carbon, silicon and aluminum, can be used for the preparation of composite materials. It shows a good prospect as the preparation of composite materials can avoid the difficulties of element separation and impurity interference. In this paper, activated carbon-mesoporous silica composite (AC-SiO₂) was prepared from coal gangue by alkali fusion, acid leaching and other processes. The effects of reaction conditions on the pore volume and specific surface area of coal gangue-based AC-SiO₂ were investigated; then the phase transformation during the preparation of coal gangue-based AC-SiO₂ was studied by means of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results showed that several experimental parameters of alkali fusion and acid leaching process were related to the pore volume and specific surface area of coal gangue-based AC-SiO₂, among which the KOH-impregnated concentration, roasting temperature and HCl-leached concentration were the most important factors. The kaolinite and quartz in coal gangue would be transformed into potassium nepheline and potassium silicate phases, and the solid carbon would be transformed into activated carbon, when the KOH-impregnated concentration was ≥ 10.7 mol/L and the roasting temperature was ≥ 700 °C; AC-SiO₂ was further formed by acid leaching process, when HCl-leached concentration ≥ 6.0 mol/L. At the optimized conditions, the conversion ratio of carbon and silicon in coal gangue is 90.28%, and the yield can reach 40.2%. The micropore and mesopore of AC-SiO₂ were formed by the accumulation of lamellae (the micropores and mesopores account for 1/2, specific surface area was about 835.1 m²/g, average pore diameter was about 2.97 nm and total pore volume was about 0.62 cm³/g) was prepared. The prepared AC-SiO₂ had obvious adsorption on the different molecular weight pollutants of methyl orange and Rhodamine B, and the adsorption capacity of methyl orange and Rhodamine B were exceed 99.01 mg/g and 99.87 mg/g, respectively.