Properties and characterization of red mud modified by hydrochloric, sulfuric, and nitric acid for the catalytic oxidation of toluene

Abstract

In this paper, red mud (RM), a solid waste with high alkalinity generated by the alumina production industry, was modified using three different acids. The catalytic efficiency of HNO3-MRM was the best, and the T10, T50, and T90 were 280 ℃, 325 ℃, and 361 ℃, respectively. The CO2 selectivity of HNO3-MRM was better than those of HCl-MRM and H2SO4-MRM. Further, the HNO3-MRM catalyst maintained high stability at 420 ℃. This good catalytic performance of HNO3-MRM is related to its high specific surface area, Fe3+ content, surface adsorbed oxygen content, and surface acidic sites. The reaction rate linearly increases with the number of surface acidic sites on the catalyst, indicating that surface acidity is an important factor affecting the catalytic activity of toluene. H2SO4 solution forms calcium sulfate with Ca2+ in the RM, resulting in a lower catalytic activity for H2SO4-MRM. Fe2O3, Al2O3, and SiO2 were the main components of HNO3-MRM. Fe2O3 was the active component of the catalyst, which played the role of the catalytic oxidation of toluene. Al2O3 and SiO2 were used as catalyst carriers. Moreover, the catalytic mechanism of toluene oxidation over the HNO3-MRM catalyst was studied. The catalytic oxidation of toluene on HNO3-MRM catalysts may have two simultaneous reaction paths. The first reaction path would be toluene → species benzyl→ benzaldehyde→ benzoic acid→ heptaldehyde→ CO2 and H2O. The second reaction path would be toluene → benzene →phenol → maleic acid →CO2 and H2O.